Notes on the Troubleshooting and Repair of
Small Switchmode Power Supplies
Version 2.77
Copyright © 1994-2003
Samuel M. Goldwasser
--- All Rights Reserved ---
For contact info, please see the
Sci.Electronics.Repair FAQ Email Links Page.
Reproduction of this document in whole or in part
is permitted if both of the following conditions
are satisfied:
-
This notice is included in its entirety at
the beginning.
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There is no charge except to cover the costs
of copying.
An example schematic of a 200W PC ATX Power
Supply
Table of Contents
Back to SMPS Repair FAQ
Table of Contents.
Preface
Author: Samuel M. Goldwasser
For contact info, please see the
Sci.Electronics.Repair FAQ Email Links Page.
Copyright © 1994-2003
All Rights Reserved
Reproduction of this document in whole or in
part is permitted if both of the following
conditions are satisfied:
1. This notice is included in its entirety at
the beginning.
2. There is no charge except to cover the costs of
copying.
Careless troubleshooting of a line powered
switchmode power supply can result in severe
electrical shock or electrocution. This is
potentially more lethal than the high voltage
section of a TV or monitor due to the high current
availability. Even the charged on the main filter
capacitors with the unit unplugged can kill.
We will not be responsible for damage to
equipment, your ego, county wide power outages,
spontaneously generated mini (or larger) black
holes, planetary disruptions, or personal injury
or worse that may result from the use of this
material.
Back to SMPS Repair FAQ
Table of Contents.
Introduction
Until the 1970s or so, most consumer electronic
equipment used a basic power
transformer/rectifier/filter capacitor type of
power supply for converting the AC line into the
various voltages needed by internal circuitry.
Even regulation was present only where absolutely
needed - the high voltage supplies of color TV
sets, for example. Remember those old TVs with
boat anchor type power transformers? (Of course,
if you recall those, you also recall the fond days
of vacuum tube sets and the corner drugstore with
a public tube tester!)
Switchmode supplies had been commonplace in
military and avionic equipment long before they
found their way into consumer electronics. I have
some DC-DC and DC-AC converter modules from a
Minuteman I missile from around 1962 as one
example. I suppose that the cost of the switching
transistors wasn't as big a deal with a $100
million missile as a $300 TV (even in 1960s
dollars).
Nowadays, all TVs, monitors, PCs; most laptop
and camcorder power packs; many printers, fax
machines, and VCRs; and even certain audio
equipment like portable CD players use this
technology to reduce cost, weight, and size.
Unlike PC system boards where any disasters are
likely to only affect your pocketbook, power
supplies, especially line connected switchmode
power supplies (SMPSs) can be dangerous. Read,
understand, and follow the set of safety
guidelines provided later in this document
whenever working on line connected power supplies
as well as TVs, monitors, or other similar high
voltage equipment.
Having said that, repairing a power supply
yourself may in fact be the only economical
option. It is very common for service centers to
simply replace the entire power supply board or
module even if the problem is a 25 cent capacitor.
It may simply not pay for them to take the bench
time to diagnose down to the component level. Many
problems with switchmode power supplies are easy
to find and easy and inexpensive to fix. Not all,
but surprisingly many.
This document will provide you with the
knowledge to deal with a large percentage of the
problems you are likely to encounter with the
common small switchmode power supplies found in
many types of consumer electronic equipment
including PCs, printers, TVs, computer monitors,
and laptop or camcorder power packs. It will
enable you to diagnose problems and in many cases,
correct them as well. With minor exceptions,
specific manufacturers and models will not be
covered as there are so many variations that such
a treatment would require a huge and very detailed
text. Rather, the most common problems will be
addressed and enough basic principles of operation
will be provided to enable you to narrow the
problem down and likely determine a course of
action for repair. In many cases, you will be able
to do what is required for a fraction of the cost
that would be charged by a repair center -
assuming they would even bother.
Should you still not be able to find a
solution, you will have learned a great deal and
be able to ask appropriate questions and supply
relevant information if you decide to post to
sci.electronics.repair. It will also be easier to
do further research using a repair text such as
the ones listed at the end of this document. In
any case, you will have the satisfaction of
knowing you did as much as you could before taking
it in for professional repair. With your new-found
knowledge, you will have the upper hand and will
not easily be snowed by a dishonest or incompetent
technician.
The following probably account for 95% or more
of the common SMPS ailments:
- Supply dead, fuse blown - shorted switchmode
power transistor and other semiconductors, open
fusable resistors, other bad parts. Note: actual
cause of failure may be power
surge/brownout/lightning strikes, random
failure, or primary side electrolytic
capacitor(s) with greatly reduced capacity or
entirely open - test them before powering up the
repaired unit.
- Supply dead, fuse not blown - bad startup
circuit (open startup resistors), open fusable
resistors (due to shorted semiconductors), bad
controller components.
- One or more outputs out of tolerance or with
excessive ripple at the line frequency (50/60
Hz) or twice the line frequency (100/120 Hz) -
dried up main filter capacitor(s) on rectified
AC input.
- One or more outputs out of tolerance or with
excessive ripple at the switching frequency (10s
of kHz typical) - dried up or leaky filter
capacitors on affected outputs.
- Audible whine with low voltage on one or
more outputs - shorted semiconductors, faulty
regulator circuitry resulting in overvoltage
crowbar kicking in, faulty overvoltage sensing
circuit or SCR, faulty controller.
- Periodic power cycling, tweet-tweet,
flub-flub, blinking power light - shorted
semiconductors, faulty over voltage or over
current sensing components, bad controller.
In all cases, bad solder connections are a
possibility as well since there are usually large
components in these supplies and soldering to
their pins may not always be perfect. An excessive
load can also result in most of these symptoms or
may be the original cause of the failure.
Some manufacturers have inexpensive flat rate
service policies for power supplies. If you are
not inclined or not interested in doing the
diagnosis and repair yourself, it may be
worthwhile to look into these. In some cases, $25
will get you a replacement supply regardless of
original condition. However, this is probably the
exception and replacements could run more than the
total original cost of the equipment - especially
as in the case of most TVs and many computer
monitors, where the power supply is built onto the
main circuit board.
Nothing really degrades in a switchmode power
supply except possibly the electrolytic capacitors
(unless a catastrophic failure resulted in a total
meltdown) and these can usually be replaced for a
total cost of a few dollars. Therefore, it usually
makes sense to repair a faulty supply assuming it
can be done reasonably quickly (depending on how
much you value your time and the down time of the
equipment) and, of course, assuming that the
equipment it powers is worth the effort. Most
replacement parts are readily available and kits
containing common service components are also
available for many popular power supplies (such as
those found in some terminals, MacIntosh and other
Apple computers, various brands of video monitors,
and some TVs and VCRs).
Where an exact replacement power supply is no
longer available or excessively expensive, it may
be possible to simply replace the guts if space
allows and the mounting arrangement is compatible.
For example, for an older full size PC tower, the
original power supply may be in a non-standard box
but the circuit board itself may use a standard
hole configuration such that an inexpensive
replacement may be installed in its place.
Alternatively, many surplus electronics
distributors have a wide selection of power
supplies of all shapes, sizes, output voltages,
and current capacities. One of these may make a
suitable replacement for your custom supply with a
lot less hassle than attempting to repair your
undocumented original. It will likely be much
newer as well with no end-of-life issues like
dried up electrolytic capacitors to deal worry
about. Of course, you must know the voltage and
current maximum current requirements of each of
the outputs in order to make a selection.
For the specific case of SMPSs for standard
computers (PC, Macs, workstations, servers), it
often doesn't make sense to spend much time or
money on repair. The cost of replacement of power
supplies for PCs in particular is so low, that
just buying a new power supply may be the best
course of action. Furthermore, the risk of a
faulty repair causing expensive or fatal damage to
the mainboard and peripherals including total loss
of all data stored on disk, makes repair a risky
endeavor unless thorough testing can be performed
before installation. However, it won't hurt to
check for obvious problems like bad connections.
Put the dead one aside and considering trying to
repair it if there isn't anything better to do.
Realistically, this will be never. :)
See the manuals on "Failure Diagnosis and
Repair of TVs" and "Failure Diagnosis and Repair
of Computer and Video Monitors" for problems
specific to that type of equipment. For computer
power supplies and other general info, also see:
"PC Switchmode Power Supplies". These are all
available at this site under the Repair Menu.
Back to SMPS Repair FAQ
Table of Contents.
Switchmode Power Supplies
A typical line connected power supply must
perform the following functions:
- Voltage conversion - changing the 115/230
VAC line voltage into one or more other voltages
as determined by application.
- Rectification - turning the AC into DC.
- Filtering - smoothing the ripple of the
rectified voltage(s).
- Regulation - making the output voltage(s)
independent of line and load variations.
- Isolation - separating the supply outputs
from any direct connection to the AC line.
A typical linear power supply of the type found
in most audio equipment includes a line power
transformer which converts the 115/230 VAC 50/60
Hz to other (usually lower) voltages (now that
most equipment has done away with vacuum tubes
except for CRTs, more on that later). The power
transformer also provides the isolation between
the load and the line. The outputs are rectified
by a diode bridge or other solid state
configuration. Filtering is accomplished with
electrolytic capacitors and sometimes inductors or
resistors arranged as a low pass filter C-L-C (pi)
or C-R-C or other configuration.
Where regulation is important - that is, it is
desirable for the output voltage to be relatively
independent of line or load variations, a
regulator stage is added. This may take the form
of a Zener diode if the current requirements are
modest, discrete transistor circuit, or an
integrated 3 terminal regulator like an LM317
(variable), 7805 (+5), or 7912 (-12). There are
many more as well as linear regulators for higher
voltages such as +115 VDC or +125 VDC for TV power
supplies and multiple output (e.g., +5.1 VDC, +12
VDC) hybrid regulators for VCRs.
The regulator circuit essentially compares the
output (possibly only one if there are multiple
outputs in the same package) with a reference and
adjusts the current flow to make the output(s) as
nearly equal to the desired voltage as possible.
However, a significant amount of power may be lost
in the regulator especially under high line
voltage/high load conditions. Therefore, the
efficiency of linear power supplies is usually
quite low - under 50% overall is typical.
Notable characteristics of LPSs are excellent
regulation and low output ripple and noise.
Also called switching power supplies and
sometimes chopper controlled power supplies, SMPSs
use high frequency (relative to 50/60 Hz)
switching devices such as Bipolar Junction
Transistors (BJTs), MOSFETs, Insulated Gate
Bipolar Transistors (IGBTs), or Thyristors (SCRs
or triacs) to take directly rectified line voltage
and convert it to a pulsed waveform.
Most small SMPSs use BJTs or MOSFETs. IGBTs may
be found in large systems and SCRs or triacs are
used where their advantages (latching in the on
state and high power capability) outweigh the
increased complexity of the circuitry to assure
that they turn off properly (since except for
special Gate Turn Off (GTO) thyristors, the gate
input is pretty much ignored once the device is
triggered and the current must go to zero to reset
it to the off state.)
The input to the switches is usually either
150-160 VDC after rectification of 115 VAC, or
300-320 VDC after doubling of 115 VAC or
rectification of 220-240 VAC. Up to this point,
there is no line isolation as there is no line
connected (large, bulky, heavy) power transformer.
A relatively small high frequency transformer
converts the pulsed waveform into one or more
output voltages which are then rectified and
filtered using electrolytic capacitors and small
inductors in a 'pi' configuration C-L-C, or for
outputs that are less critical, just a capacitor.
This high frequency transformer provides the
isolation barrier and the conversion to generate
the multiple voltages often provided by a SMPS.
Feedback is accomplished across the isolation
barrier by either a small pulse transformer or
opto-isolator. The feedback controls the pulse
width or pulse frequency of the switching devices
to maintain the output constant. Since the
feedback is usually only from the "primary"
output, regulation of the other outputs, if any,
is usually worse than for the primary output.
Also, because of the nature of the switching
designs, the regulation even of the primary output
is usually not nearly as good both statically and
dynamically as a decent linear supply.
DC-DC converters are switchmode power supplies
without the line input rectification and
filtering. They are commonly found in battery
operated equipment like CD players and laptop
computers. They have similar advantages to SMPSs
in being compact, light weight, and highly
efficient.
Probably the most common topology for small
switchers is the flyback circuit shown below and
in
Block Diagram of Basic Flyback Switchmode Power
Supply.
CR1 CR2 L :::::
H o-------|>|---+----+---------+ T1 +-----|>|------+---^^^^^---+---+----o V+
line | | )||( Main +_|_ +_|_ | Main
rect. | / )||( output C ___ LC Pi C ___ | Output
| \ R1 )||( rect. - | filter - | |
AC HV +_|_ / +-+ +--------------+-----------+---|----o V-
Line filter ___ \ | |
in cap - | | |/ +-------+ +-----------+ +-----+
| +-----+--------| PWM |<--| Isolation |<--| REF |
| Q1 |\ +-------+ +-----------+ +-----+
| |
N o-------------+------------+
The input to the supply is the AC line which
may have RFI and surge protection (not shown).
There may be several inductors, coupled inductors,
and capacitors to filter line noise and spikes as
well as to minimize the transmission of switching
generated radio frequency interference back into
the power line. There may be MOV type of surge
suppressors across the three input leads (H, N,
G). A line fuse is usually present as well to
prevent a meltdown in case of a catastrophic
failure. It rarely can prevent damage to the
supply in the event of an overload, however.
Line rectification is usually via a voltage
doubler or diode bridge. One common circuit uses a
bridge rectifier as a doubler or normal bridge by
changing one jumper. The voltage across the
switching transistor is usually around 160-320 V.
Some universal supplies are designed to accept a
wide range of input voltages - 90-240 VAC
(possibly up to 400 Hz or more) or DC - and will
automatically work just about anywhere in the
world as long as a suitable plug adapter can be
found.
When Q1 turns on, current increases linearly in
T1 based on the voltage applied and the leakage
inductance of T1's primary winding. Little power
is transferred to the secondary during this phase
of the cycle. When Q1 turns off, the field
collapses and this transfers power to the output.
The longer Q1 is on, the more energy is stored
(until saturation at which point it blows up).
Thus, controlling the pulse width of the Q1
on-time determines the amount of power available
from the output.
The output rectifier, CR2, must be a high
efficiency, high frequency unit - a 1N400X will
not work. The pi filter on the output smooths the
pulses provided by CR2. Sometimes, a full wave
configuration is used with a center tapped
transformer secondary.
Note that the transformer, T1, is a special
type which includes an air gap in its core (among
other things) to provide the inductive
characteristics needed for operation in flyback
mode.
Multiple output windings on T1 provide for up
to a half dozen or more separate (and possibly
isolated as well) positive or negative voltages
but as noted, only one of these is usually used
for regulation.
A reference circuit monitors the main output
and controls the duty cycle of the switching
pulses to maintain a constant output voltage.
(Secondary outputs are not shown in the above
schematic.)
R1 is the startup resistor (some startup
circuits are more sophisticated) and provides the
initial current to the switchmode transistor base.
In the old days, SMPS controllers were designed
with discrete components. Assuring stable
operation is a challenge with any SMPS but
particularly with the flyback topology where
leaving the drive on for too long will result in
transformer core saturation and instant smoke.
Nowadays, an IC PWM controller chip is almost
always used. The block diagram of a one very
popular PWM controller IC is shown below.
Many small SMPSs use opto-isolators for the
feedback. An opto-isolator is simply an LED and a
photodiode in a single package. As its name
implies, an opto-isolator provides the isolation
barrier (between the low voltage secondary outputs
and the line connected primary) for the feedback
circuit. Typically, a reference circuit on the
output side senses the primary output voltage and
turns on the LED of the opto-isolator when the
output voltage exceeds the desired value. The
photodiode detects the light from the LED and
causes the pulse width of the switching waveform
to be reduced enough to provide just the right
amount of output power to maintain the output
voltage constant. This circuit may be as simple as
putting the photodiode across the base drive to
the BJT switch thus cutting it off when the output
voltage exceeds the desired value. The reference
is often a TL431 or similar shunt regulator chip
monitoring a voltage divided version of the
primary output. When the shunt regulator kicks in,
the opto-isolator LED turns on reducing the
switchmode transistor drive. There may be an
adjustment for the output voltage.
Other designs use small pulse transformers to
provide isolated feedback.
Where additional regulation is needed, small
linear regulators may be included following the
output(s).
There are many other topologies for switching
power supplies. However, the basic principles are
similar but the detail differ depending on
application. The flyback topology described above
is one of the most common for small multi-output
supplies. However, you may find other types of
circuits in TVs and monitors. Some are downright
strange (to be polite). I sometimes wonder if
engineers are given bonuses based on the
uniqueness and difficulty level of understanding
their designs!
The benefits provided by implementing switch
mode operation are with respect to size, weight,
and efficiency.
- Size and weight - since the transformer and
final filter(s) run at a high frequency (we are
talking about 10 kHz to 1 MHz or more), they can
be much smaller and lighter than the big bulky
components needed for 50/60 Hz operation. Power
density for SMPSs compared to LPSs may easily
exceed 20:1.
- Efficiency - since the switching devices are
(ideally) fully on or fully off, there is
relatively little power lost so that the
efficiency can be much higher for SMPSs than for
LPSs, especially near full load. Efficiencies
can exceed 85% (compared to 50-60% for typical
LPSs) with improvements being made continuously
in this technology.
Since the advent of the laptop computer,
cellular phone, and other portable devices, the
importance of optimizing power utilization has
increased dramatically. There are now many ICs for
controlling and implementing SMPSs with relatively
few external components. Maxim, Linear Technology,
and Unitrode (now part of Texas Instruments) are
just a few of the major manufacturers of
controller ICs.
Switch mode power supplies are commonly used in
computer and other digital systems as well as
consumer electronics - particularly TVs and newer
VCRs though audio equipment will tend to use
linear power supplies due to noise considerations.
You will find SMPSs in:
- PCs, workstations, minicomputers, large
computers.
- Laptop and notebook computers, PDAs - both
internal DC-DC converters and their AC power
packs.
- Printers, fax machines, copiers.
- Peripheral and expansion boxes
- X-terminals and video terminals, point of
sale registers.
- TVs, computer and video monitors.
- Many VCRs.
- Camcorder AC adapters.
In additional, you will find DC-DC converters
which are SMPSs without the AC line connection,
internally in an increasing number of consumer and
industrial applications including things like
portable CD players.
The up side is that they are usually quite
reliable, efficient, and cool running.
The down side is that when a failure occurs, it
may take out many parts in the supply, though not
usually the equipment being powered unless the
feedback circuitry screws up and there is no
overvoltage protection.
Back to SMPS Repair FAQ
Table of Contents.
Switchmode Power Supply Troubleshooting
The primary danger to you is from the input
side of the supply which is directly connected to
the AC line and will have large electrolytic
capacitors with 320 V or greater DC when powered
(often, even if the supply does not work
correctly) and for some time after being unplugged
(especially if the power supply is not working
correctly but does not blow fuses).
WARNING: The filter capacitors used in many
switchmode power supplies can store an amount of
energy that can kill - always discharge and
confirm this before touching anything.
There is also risk of instantly destroying
expensive parts of the supply (and any attached
equipment as well) like the switchmode power
transistor if your probe should slip and short
something either directly or by killing the
feedback circuit.
These guidelines are to protect you from
potentially deadly electrical shock hazards as
well as the equipment from accidental damage.
Note that the danger to you is not only in your
body providing a conducting path, particularly
through your heart. Any involuntary muscle
contractions caused by a shock, while perhaps
harmless in themselves, may cause collateral
damage - there are many sharp edges inside this
type of equipment as well as other electrically
live parts you may contact accidentally.
The purpose of this set of guidelines is not to
frighten you but rather to make you aware of the
appropriate precautions. Repair of TVs, monitors,
microwave ovens, and other consumer and industrial
equipment can be both rewarding and economical.
Just be sure that it is also safe!
- Don't work alone - in the event of an
emergency another person's presence may be
essential.
- Always keep one hand in your pocket when
anywhere around a powered line-connected or high
voltage system.
- Wear rubber bottom shoes or sneakers.
- Don't wear any jewelry or other articles
that could accidentally contact circuitry and
conduct current, or get caught in moving parts.
- Set up your work area away from possible
grounds that you may accidentally contact.
- Know your equipment: TVs and monitors may
use parts of the metal chassis as ground return
yet the chassis may be electrically live with
respect to the earth ground of the AC line.
Microwave ovens use the chassis as ground return
for the high voltage. In addition, do not assume
that the chassis is a suitable ground for your
test equipment!
- If circuit boards need to be removed from
their mountings, put insulating material between
the boards and anything they may short to. Hold
them in place with string or electrical tape.
Prop them up with insulation sticks - plastic or
wood.
- If you need to probe, solder, or otherwise
touch circuits with power off, discharge
(across) large power supply filter capacitors
with a 2 W or greater resistor of 5-50 ohms/V
approximate value (e.g., for a 200 V capacitor,
use a 1K-10K ohm resistor). Monitor while
discharging and/or verify that there is no
residual charge with a suitable voltmeter. In a
TV or monitor, if you are removing the high
voltage connection to the CRT (to replace the
flyback transformer for example) first discharge
the CRT contact (under the insulating cup at the
end of the fat red wire). Use a 1M-10M ohm 1W or
greater wattage resistor on the end of an
insulating stick or the probe of a high voltage
meter. Discharge to the metal frame which is
connected to the outside of the CRT.
- For TVs and monitors in particular, there is
the additional danger of CRT implosion - take
care not to bang the CRT envelope with your
tools. An implosion will scatter shards of glass
at high velocity in every direction. There is
several tons of force attempting to crush the
typical CRT. Always wear eye protection.
- Connect/disconnect any test leads with the
equipment unpowered and unplugged. Use clip
leads or solder temporary wires to reach cramped
locations or difficult to access locations.
- If you must probe live, put electrical tape
over all but the last 1/16" of the test probes
to avoid the possibility of an accidental short
which could cause damage to various components.
Clip the reference end of the meter or scope to
the appropriate ground return so that you need
to only probe with one hand.
- Use a proper high voltage probe or high
voltage meter to measure voltages which are
potentially beyond the capabilities of your DMM
or VOM - not something cobbled together from 1/4
watt resistors! Note that fault conditions or
even testing at *reduced* input voltage may
result in greatly excessive voltage on one or
more outputs due to lack of regulation.
- It may be possible to perform some of the
tests at greatly reduced voltage (e.g., 30 VDC
to the chopper instead of 300 VDC) by supplying
external power to the controller chip (if used)
and injecting base/gate drive from a signal
generator. This would greatly reduce the shock
hazard as well as equipment damage from a
slipped probe or missed faulty component.
- Perform as many tests as possible with power
off and the equipment unplugged. For example,
the semiconductors in the power supply section
of a TV or monitor can be tested for short
circuits with an ohmmeter.
- Use an isolation transformer if there is any
chance of contacting line connected circuits. A
Variac(tm) is not an isolation transformer! The
use of a GFCI (Ground Fault Circuit Interrupter)
protected outlet is a good idea but will not
protect you from shock from many points in a
line connected TV or monitor, or the high
voltage side of a microwave oven, for example.
(Note however, that, a GFCI may nuisance trip at
power-on or at other random times due to leakage
paths (like your scope probe ground) or the
highly capacitive or inductive input
characteristics of line powered equipment.) A
fuse or circuit breaker is too slow and
insensitive to provide any protection for you or
in many cases, your equipment. However, these
devices may save your scope probe ground wire
should you accidentally connect it to a live
chassis.
- Don't attempt repair work when you are
tired. Not only will you be more careless, but
your primary diagnostic tool - deductive
reasoning - will not be operating at full
capacity.
- Finally, never assume anything without
checking it out for yourself! Don't take
shortcuts!
The diagnosis of problems in switchmode power
supplies is sometimes made complicated due the
interdependence of components that must function
properly for any portion of the power supply to
begin to work. Depending on design, SMPS may or
may not be protected from overload conditions and
may fail catastrophically under a heavy load even
when supposedly short circuit proof. There is
particular stress on the switching devices (they
are often 800 V transistors) which can lead to
early or unexpected failure. Also, SMPS may fail
upon restoration of power after a blackout if
there is any kind of power spike since turn-on is
a very stressful period - some designs take this
into account and limit turn on surge.
However, the cause of many problems are
immediately obvious and have simple fixes - the
blown chopper transistor or dried up main filter
capacitor. Don't assume your problem is complex
and convoluted. Most are not. You should not avoid
attempting a repair just because there is a slight
chance it will be more challenging!
A low power (e.g., 25 W) fine tip soldering
iron and fine rosin core solder will be needed if
you should need to disconnect any soldered wires
(on purpose or by accident) or replace soldered
components. A higher power iron or small soldering
gun will be needed for dealing with larger
components. Never use acid core solder or the type
used for sweating copper pipes!
CAUTION: You can easily turn a simple repair
(e.g., bad solder connections) into an expensive
mess if you use inappropriate soldering equipment
and/or lack the soldering skills to go along with
it. If in doubt, find someone else to do the
soldering or at least practice, practice,
practice, soldering and desoldering on a junk
circuit board first! See the document:
Troubleshooting and Repair of Consumer Electronic
Equipment for additional info on soldering and
rework techniques.
The most valuable piece of test equipment (in
addition to your senses) will be a DMM or VOM.
These alone will suffice for most diagnosis of
faulty components (like shorted semiconductors or
open fusable resistors).
CAUTION: If the SMPS (or any other piece of
equipment) is capable of producing voltages beyond
1,000 V (or the max range on your meter), make
sure you use a proper high voltage probe or high
voltage meter - fault conditions could easily
result in voltages in the system that are way
beyond those that are expected, even if run at
reduced input voltage and/or with a series current
limiter.
In designs using controller ICs, an
oscilloscope comes in handy when there are startup
or overcurrent/voltage shutdown or cycling
problems. Since everything runs at a relatively
low frequency, almost any scope will do.
These are the little gadgets and homemade
testers that are useful for many repair
situations. Here are just a few of the most basic:
- Series light bulb for current limiting
during the testing of TVs, monitors, switching
power supplies, audio power amplifiers, etc. I
built a dual outlet box with the outlets wired
in series so that a lamp can be plugged into one
outlet and the device under test into the other.
For added versatility, add a regular outlet and
'kill' switch using a quad box instead. The use
of a series load will prevent your expensive
replacement part like a switchmode power
transistor from blowing if there is still some
fault in the circuit you have failed to locate.
(Now, if I would only remember to do this more
often!). See the section:
The series light bulb trick.
- A Variac. It doesn't need to be large - a 2
A Variac mounted with a switch, outlet and fuse
will suffice for most tasks. However, a 5 amp or
larger Variac won't hurt. If you will be
troubleshooting 220 VAC equipment in the US,
there are Variacs that will output 0-240 VAC
from a 115 VAC line (just make sure you don't
forget that this can easily fry your 115 VAC
equipment.) By varying the line voltage, not
only can you bring up a newly repaired monitor
gradually to make sure there are no problems;
you can also evaluate behavior at low and high
line voltage. This can greatly aid in
troubleshooting power supply problems. WARNING:
a Variac is an autotransformer - not an
isolation transformer and does not help with
respect to safety. You need an isolation
transformer as well.
Note: Some SMPS designs require power to be
applied instantly to provide the startup voltage
to the controller. If this is the case with
yours, it won't be possible to bring up the
voltage slowly (unless you power that chip
separately). However, it should still be
possible to run the unit somewhat reduced line
voltage. Also, running any SMPS at reduced line
voltage is stressful. It may also result in
outputs that are not properly regulated and go
much higher than normal. Thus, a Variac should
be used with caution - with the outputs
connected to dummy loads instead of the powered
equipment and a series current limiter (e.g.,
light bulb) in the input.
- Isolation transformer. This is very
important for safely working on live chassis
equipment like line connected switchmode power
supplies (primary side). You can build one from
a pair of similar power transformers
back-to-back (with their highest rated
secondaries connected together. I built mine
from a couple of similar old tube type TV power
transformers mounted on a board with an outlet
box including a fuse. Their secondary high
voltage windings were connected together to
couple the two transformers together. The unused
low voltage windings can be put in series with
the primary or output windings to adjust
voltage. Alternatively, commercial line
isolation transformers suitable for TV
troubleshooting are available for less than $100
- well worth every penny.
There is absolutely no imaginable reason not
to use an isolation transformer for
troubleshooting SMPSs except possibly for the
final test where confirmation is needed that the
inrush from a direct line connection (which will
have virtually unlimited instantaneous current
capability) will not damage the newly repaired
supply.
- Variable isolation transformer. You don't
need to buy a fancy combination unit. A Variac
can be followed by a normal isolation
transformer. (The opposite order also works.
There may be some subtle differences in load
capacity.).
A working SMPS may discharge its capacitors
fairly quickly when it is shut off but DO NOT
count on this. The main filter capacitors may have
bleeder resistors to drain their charge relatively
quickly - but resistors can fail and the term
'quickly' may be relative to the age of the
universe. Don't depend on them.
The technique I recommend is to use a high
wattage resistor of about 5 to 50 ohms/V of the
working voltage of the capacitor. This isn't
critical - a bit more or less will be fine but
will affect the time it takes to fully discharge
the capacitor. The use of a current limiting
resistor will prevent the arc-welding associated
with screwdriver discharge but will have a short
enough time constant so that the capacitor will
drop to a low voltage in at most a few seconds
(dependent of course on the RC time constant and
its original voltage).
Then check with a voltmeter to be double sure.
Better yet, monitor while discharging.
Obviously, make sure that you are well
insulated!
For the power supply filter capacitors which
might be 400 uF at 200 V, a 2 K ohm 10 W resistor
would be suitable. RC=.8 second. 5RC=4 seconds. A
lower wattage resistor (compared to that
calculated from V^^2 / R) can be used since the
total energy stored in the capacitor is not that
great (but still potentially lethal).
The discharge tool and circuit described in the
next two sections can be used to provide a visual
indication of polarity and charge for TV, monitor,
SMPS, power supply filter capacitors and small
electronic flash energy storage capacitors, and
microwave oven high voltage capacitors.
Reasons to use a resistor and not a screwdriver
to discharge capacitors:
- It will not destroy screwdrivers and
capacitor terminals.
- It will not damage the capacitor (due to the
current pulse).
- It will reduce your spouse's stress level in
not having to hear those scary snaps and
crackles.
A suitable discharge tool for each of these
applications can be made as quite easily. The
capacitor discharge indicator circuit described
below can be built into this tool to provide a
visual display of polarity and charge (not really
needed for CRTs as the discharge time constant is
virtually instantaneous even with a multi-M ohm
resistor.
- Solder one end of the appropriate size
resistor (for your application) along with the
indicator circuit (if desired) to a well
insulated clip lead about 2-3 feet long. For
safety reasons, these connections must be
properly soldered - not just wrapped.
- Solder the other end of the resistor (and
discharge circuit) to a well insulated contact
point such as a 2 inch length of bare #14 copper
wire mounted on the end of a 2 foot piece of PVC
or Plexiglas rod which will act as an extension
handle.
- Secure everything to the insulating rod with
some plastic electrical tape.
This discharge tool will keep you safely clear
of the danger area.
Again, always double check with a reliable
voltmeter or by shorting with an insulated
screwdriver!
Here is a suggested circuit which will
discharge the main filter capacitors in switchmode
power supplies, TVs, and monitors. This circuit
can be built into the discharge tool described
above.
A visual indication of charge and polarity is
provided from maximum input down to a few volts.
The total discharge time is approximately 1
second per 100 uF of capacitance (5RC with R = 2 K
ohms).
Safe capability of this circuit with values
shown is about 500 V and 1000 uF maximum. Adjust
the component values for your particular
application.
(Probe)
<-------+
In 1 |
/
\ 2 K, 25 W Unmarked diodes are 1N400X (where X is 1-7)
/ or other general purpose silicon rectifiers.
\
|
+-------+--------+
__|__ __|__ |
_\_/_ _/_\_ /
| | \ 100 ohms
__|__ __|__ /
_\_/_ _/_\_ |
| | +----------+
__|__ __|__ __|__ __|__ Any general purpose LED type
_\_/_ _/_\_ _\_/_ LED _/_\_ LED without an internal resistor.
| | | + | - Use different colors to indicate
__|__ __|__ +----------+ polarity if desired.
_\_/_ _/_\_ |
In 2 | | |
>-------+-------+--------+
(GND Clip)
The two sets of 4 diodes will maintain a nearly
constant voltage drop of about 2.8-3 V across the
LED+resistor as long as the input is greater than
around 20 V. Note: this means that the brightness
of the LED is NOT an indication of the value of
the voltage on the capacitor until it drops below
about 20 volts. The brightness will then decrease
until it cuts off totally at around 3 volts.
Safety note: always confirm discharge with a
voltmeter before touching any high voltage
capacitors!
Whereas a multimeter is intended to measure
voltages (and other things), a checker is used
mostly to just produce a quick indication of the
presense of voltage, its polarity, and other basic
parameters. One use is a quick, but reliable
indication of the status of the charge on a BIG
capacitor. An, example of a simple version of such
a device is the "capacitor discharge indicator
circuit" described above.
(From: Ian Field (ionfieldmonitors@ic24.net).)
The version of the checker that I have, also
contains a miniature 12 V battery for continuity
checking - any resistance less than about 22K will
produce some glow. It's handy for quick checks of
semiconductor junctions - in general if it
produces a slight glow it's leaky, but transistor
B/E junctions have an inherent zener voltage, so
there is usually some glow. Also schottky-barrier
diodes give a reverse leakage glow - this does not
mean they're faulty, check the Vf with the
diode-check on a DMM before binning! Any zener
diode above 10-11 V can be given a quick test for
S/C, lower Vz will produce some glow - again check
Vf before binning.
These checkers are getting hard to obtain, most
of the component stockists here only carry vastly
over complicated (and expensive) versions with
built-in measurement computer and LCD - these
wouldn't last 5 min's around flyback circuitry!
Some Automotive accessory shops have a simpler
version with no battery - always check that it's
stated to be capable of measuring AC or DC at 4 to
380 V before parting with money! The internal
circuit should contain the LED's, a 15 ohm
resistor to limit the maximum surge current when
the PTC is cold and the special PTC film-thermistor.
The battery can be added with a button from a VCR
front panel - but don't blame me if you kill
yourself because you didn't insulate the added
components properly! There is a more complicated
non-battery version with 2 LED's close to the
front of the handle to indicate polarity and a row
of LED's along the length of the handle to
indicate the voltage-range. This version contains
2 special PTC's and a discrete-transistor bargraph
circuit - there might be room to add a battery
inside the case. As for the special PTC this is
the only place I've seen them - one possibility
that might be worthy of looking into is the
Siemens PTC SMPSU startup thermistor for TDA4600
control chips, this usually has a series resistor
of at least 270 ohms and is more likely to turn-up
in European TV set's, but I have seen it in early
Matsushita IBM displays and a few others (possibly
Tandon) the PTC thermistor is always blue and
looks like a very-miniature copy of the Philips
white-plastic PTC degauss thermistor.
When powering up a monitor (or any other modern
electronic devices with expensive power
semiconductors) that has had work done on any
power circuits, it is desirable to minimize the
chance of blowing your newly installed parts
should there still be a fault. There are two ways
of doing this: Use of a Variac to bring up the AC
line voltage gradually and the use of a series
load to limit current to power semiconductors.
Actually using a series load - a light bulb is
just a readily available cheap load - is better
than a Variac (well both might be better still)
since it will limit current to (hopefully)
non-destructive levels.
CAUTION: Running any SMPS at greatly reduced
line voltage will be stressful for it, especially
if the output load is a significant fraction of
its full load ratings. In addition, at some range
of line voltage, the output regulation may not
work properly and the output(s) may go much higher
than expected. Use dummy loads in place of the
valuable equipment if possible when doing such
testing!
What you want to do is limit current to the
critical parts - usually the switchmode (chopper)
power transistor of an SMPS or horizontal output
transistor (HOT) of a TV or monitor. Most of the
time you will get away with putting it in series
with the AC line. However, sometimes, putting a
light bulb directly in the B+ circuit will be
needed to provide adequate protection. In that
location, it will limit the current to the HOT
from the main filter capacitors of line connected
power supplies. This may also be required with
some switchmode power supplies as they can still
supply bursts of full (or excessive) current even
if there is a light bulb in series with the AC
line.
Actually, an actual power resistor is probably
better as its resistance is constant as opposed to
a light bulb which will vary by 1:10 from cold to
hot. The light bulb, however, provides a nice
visual indication of the current drawn by the
circuit under test. For example:
- Full brightness: short circuit or extremely
heavy load - a major fault probably is still
present.
- Initially bright but then settles at reduced
brightness: filter capacitors charge, then lower
current to rest of circuit. This is what is
expected when the equipment is operating
normally. There could still be a problem with
the power circuits but it will probably not
result in an immediate catastrophic failure.
- Pulsating: power supply is trying to come up
but shutting down due to overcurrent or
overvoltage condition. This could be due to a
continuing fault or the light bulb may be too
small for the equipment.
Note: for a TV or monitor, it may be necessary
(and desirable) to unplug the degauss coil as this
represents a heavy initial load which may prevent
the unit from starting up with the light bulb in
the circuit.
The following are suggested starting wattages:
- 40 W bulb for VCR or laptop computer
switching power supplies.
- 100 W bulb for small (i.e., B/W or 13 inch
color) monitors or TVs.
- 150-200 W bulb for large color monitors or
projection TVs.
A 50/100/150 W (or similar) 3-way bulb in an
appropriate socket comes in handy for this but
mark the switch so that you know which setting is
which!
Depending on the power rating of the equipment,
these wattages may need to be increased. However,
start low. If the bulb lights at full brightness,
you know there is still a major fault. If it
flickers or the TV (or other device) does not
quite come fully up, then it should be safe to go
to a larger bulb. Resist the temptation to
immediately remove the bulb at this point - I have
been screwed by doing this. Try a larger one
first. The behavior should improve. If it does
not, there is still a fault present.
Note that some TVs and monitors simply will not
power up at all with any kind of series load - at
least not with one small enough (in terms of
wattage) to provide any real protection. The
microcontroller apparently senses the drop in
voltage and shuts the unit down or continuously
cycles power. Fortunately, these seem to be the
exceptions.
TVs and monitors have at least one SMPS - the
horizontal deflection flyback circuit and may have
an additional SMPS to provide the low voltages or
the DC for the horizontal output transistor. Most
of the theory of operation and troubleshooting
techniques apply to these as well. However,
manufacturers of TVs and monitors tend to be
really creative (can you say, obscure?) when it
comes to these designs so a little more head
scratching is often necessary to decipher the
circuit and get into the mind of the designer.
However, the basic failure modes are similar and
the same test procedures may be used.
(From: Ian Field (ionfieldmonitors@ic24.net).)
At a pinch, discharging BIG electrolytic
capacitors with a test lamp (230 V, 60 W in the
UK; 115 V, 25 W in series in the US) will do, but
if the lamp has blown you are in for a nasty
surprise! While I am not criticising the use of
spare high-wattage resistors, I tend to find that
these get tidied away, so there's none about when
you need one!
The lamp sometimes get's used if I can't find
an NTC, but I always check with a voltage checker
because of the risk! - power resistors can go O/C
as well whereas NTC thermistors generally fail S/C
- which usually happens as a result of some
transient phenomenon such as a lightning-strike
near the underground power line.
This is unlikely with the energy dump of
discharging an off-line electrolytic (unless the
equipment is still powered at the time!). My bench
isn't the tidiest in the world, so gadgets tend to
get misplaced - including power resistors with
added discharge-progress LED indicators. This is
where an inrush-suppressor NTC comes into it's
own, even without selecting the type - it will
discharge a capacitor almost instantaneously with
the minimum of arc-burn on the solder pads.
Obviously the energy causes some heating - in the
case of large electrolytics direct-off-line
rectifier smoothing/reservoir the amount of
heating is just sufficient to give an idea of the
condition of the capacitor - capacitor failure is
comparatively rare, so it's not often anticipated
and can cause misleading symptoms - so making this
double as a routine check occasionally saves a
hell of a lot of time!
The trick I have found works even better is to
use a NTC inrush-current suppressor thermistor.
These items can be salvaged from a scrap monitor
or PSU, and careful selection may reveal some
types with a "room temperature resistance" of
several kohms - with the line-voltage on a
capacitor discharging through them, self-heating
reduces the resistance to a few ohms. This reduces
the welding-sputter as the contact current is only
a few milliamps - this rises to a few amperes as
the capacitor "dumps" its charge as the NTC
resistance fall's with self-heating.
One point I would disagree on is that not all
of the many electrolytics need discharging! Most
SMPSU's of any appreciable power have high energy
electrolytics in the secondary - whether this is
expressed as high voltage or high current. In the
case of monitors, the post PWM-B+ rail has a large
storage electrolytic which can do appreciable
damage in the event that line-drive failure has
prevented use of it's energy. The NTC thermistor
method helps here; after "dumping" the line
voltage electrolytics - the NTC is at a lower
resistance and ready for following up on the lower
charge electrolytics.
There is a point concerning "test-lamp dummy
loads" this has more to do with monitors than
SMPSU boxes. As well as the suggested use to limit
inrush current to a safe value on SMPSU boxes - I
also use this method on line-O/P stages especially
to verify that flyback-transformer failure was not
the original cause of B+PWM or SMPSU blow-up.
Recently I have been caught-out a few times
because some "energy-star" designs are so
efficient that the inrush current of the lamp
itself is ample to cause catastrophic damage! The
UK 220/230 V 60 W test lamp I have here has a
calculated operating resistance of 806.7/881.7
ohms compared to a measured cold resistance of
about 67 ohms so the PTC effect of the filament
tends to limit the advantage!
To clarify my comment on confirming whether a
faulty flyback transformer has damaged the B+PWM;
older circuits use a MOSFET buck-regulator, in
which S/C failure of the MOSFET feeds unregulated
B+ to the line-O/P stage - This invariably
destroys the HOT and sometimes the transformer,
but either could as easily be the original cause.
In any event - bypassing the B+PWM MOSFET via the
test lamp passes just enough current to see if the
transformer is operable - with the price of
replacements, very few quotes are accepted - so
it's well worth making sure before ordering an
expensive replacement or doing too much repair
work! More recently, the trend has been for
flyback - step up B+ regulators. When the B+
MOSFET fails S/C it simply stalls the main SMPSU
(sometimes destroying the rectifier!). Since this
type of B+PWM is step-up, the operability of the
flyback transformer can be checked by simply
removing the S/C MOSFET. The most recent designs
appear to be based on semi-resonant SMPSU topology
- they resemble buck-regulator PWM controllers,
but the PWM MOSFET is at chassis potential and the
transformer primary is at full PSU-rail, the
line-O/P transistor is between the two with the
drive transformer connected to provide an
emitter-coupled blocking oscillator configuration,
to add "regen" to the base drive. The boost diode
often includes the buck-regulators "ringing-choke"
in the "net inductive component" that it recovers
energy from! As the later configuration most
closely resembles the buck-regulator type, the
test lamp is required to confirm transformer
operability - but the weird and wonderful circuit
arrangements can make it lots of fun working out
where to connect it!
A voltage checker that I find indispensable is
the Steinel Master check 3 from; Steinel GmbH &
Co. (KG Dieselstrabe 80-86 D-4836 Hertzebrock 1,
Germany). The version I have consists of a pair of
"inverse-parallel" LED's in series with a
metal-film PTC thermistor on a tiny ceramic tube
former - this has a very low thermal inertia so
the PTC thermistor limits the current to a safe
value for the LED's for any applied voltage
between 4 & 380 V the combination of 2 LED's give
a clear indication of AC or DC polarity. When this
checker is used around SMPSU's, you can clearly
see the effect of minority carrier transition time
losses in the rectifiers, because the leading-edge
of the waveform pushes the rectifiers Vf well in
excess of 0.7 V for the minority carrier injection
delay - before the rectifier begins to conduct.
Where a PSU rectifier has two electrolytics and a
choke in a Pi filter, the checker will often
reveal negative transients on the electrolytic
closest to the rectifier - which is a clear sign
of capacitor ESR failure.
Also see the section:
Sounds that SMPSs make.
SMPS fail in many ways but the following are
common:
- Faulty primary-side components. Rectifier
diodes, filter capacitor(s), MOVs, and other
parts located before the switchmode (chopper)
transistor(s) may short due to a surge or
lightning, or for no apparent reason. Main fuse
will blow instantly. Symptoms: Totally dead
supply, fuse blows instantly (vaporizes or
explodes) even if switchmode transistor is
removed unless a fusable resistor has blown to
protect the fuse. :) Test all components on line
side of high frequency transformer for short
circuit failures with a multimeter.
- Shorted switchmode transistor - may take out
additional parts such as fusable flameproof
resistors in collector or emitter circuits (or
source or drain circuits for MOSFETs). Main fuse
will blow unless protected by fusable resistors
and one or more of them blows first.
Symptoms: Totally dead supply, fuse blows
instantly (vaporizes or explodes unless fusable
resistor has opened). Measuring across C-E or
D-S of switchmode transistor yields near zero
ohms even when removed from circuit.
- Shorted rectifier diodes in secondary
circuits - these are high frequency high
efficiency diodes under a fair amount of stress.
Symptoms: In a very basic supply without
overcurrent protection, the failure of one or
more of these diodes may then overload the
supply and cause a catastrophic failure of the
switchmode power transistor (see above) and
related components. Thus, these should be
checked before reapplying power to a supply that
had a shorted switchmode transistor.
On short circuit protected supplies, the
symptom may be a periodic tweet-tweet-tweet or
flub-flub-flub as the supply attempts to restart
and then shuts down. Any power or indicator
lights may be blinking at this rate as well.
Test with an ohmmeter - a low reading in both
directions indicates a bad diode. Sometimes
these will test OK but fail under load or at
operating voltage. Easiest to replace with known
good diodes to verify diagnosis. Rectifiers
either look like 1N400X type on steroids -
cylinders about 1/4" x 1/2" (example: HFR854) or
TO220 packages (example: C92M) with dual diodes
connected at the cathode for positive supplies
or the anode for negative supplies (the package
may include a little diagram as well). These may
either be used with a center-tapped transformer,
or simply parallel for high current capacity. If
in doubt, remove from the circuit and test with
the ohmmeter again. If not the output used for
regulation feedback, try the supply with the
rectifier removed. As noted, a test with an
ohmmeter may be misleading as these rectifiers
can fail at full voltage. When in doubt,
substitute a known good rectifier (one half of a
pair will be good enough for a test).
- Bad startup circuit - initial base (gate)
drive is often provided by a high value, high
power resistor or resistors from the rectified
AC voltage. These can simply open for no good
reason.
Symptoms: In this case the supply will appear
totally dead but all the semiconductors will
check out and no fuses will blow. Check the
startup resistors with an ohmmeter - power
resistors in the AC line input section. WARNING:
there will be full voltage on the main filter
capacitor(s) - 1X or 2X peak or around 160 or
320 VDC depending on design. Discharge before
probing.
- Dried up capacitors - either input or output
side.
Symptoms: The main filter capacitor may dry
up or open and cause the output to be pulsing at
60 (50) or 120 (100) Hz and all kinds of
regulation problems. Measure voltage across main
filter capacitor(s). If the reading is low and
drops to a much lower value or 0 instantly upon
pulling the plug, then one of these capacitors
may be open or dried up. If you have an
oscilloscope, monitor for ripple (use an
isolation transformer!!). Excess ripple under
moderate load is an indication of a dried up or
open capacitor.
In extreme cases, a main filter capacitor
with greatly reduce capacity or that is totally
open may result in failure of the switchmode
transistor and a dead supply that blows fuses or
fusable resistors. Therefore, it is always a
good idea to test the electrolytic capacitors
whenever repairing a SMPS that has blown its
switchmode transistor.
Capacitors in the low voltage section may
fail causing regulation problems. Sometimes
there are slew rate limiting capacitors which
feed from the primary output to the regulator
controller to limit initial in-rush and
overshoot. A failure of one of these may mess up
regulation at the very least. For example,
excess leakage may reduce the output of the main
output (and as a consequence, all the others as
well).
Where a controller like a UC3842 is used, a
failure of the capacitor on its Vcc pin may
result in a aborted startup or cycling behavior
as it is starved for juice each time it pulses
the switchmode power transistor:
(From: John Hopkins (bugs71@ptdprolog.net).)
"I have encountered a bad cap (10uf 35v) on
the Vcc input of a UC3842 IC in the power
supply. Turn unit on, get very short burst of
power supply output, then nothing. Every time
the 3842 output a pulse, it ran out of VCC.
Small part, big problem."
In almost all cases, when in doubt parallel a
known good capacitor of similar capacitance and
at least equal voltage rating (except for these
slew rate limiting capacitors where substitution
is the only sure test).
For Panasonic (and other) VCR power supplies,
it is common - almost assured after a few years
- that one or more the output filter capacitors
commonly fail and replacing all of them, while
perhaps a brute force solution, will fix a
whining supply or one having bad regulation or
noise. However, check the semiconductors as well
before applying power. See the section:
Panasonic VCR SMPS.
- Bad connection/cold solder joints - as with
all other mass produced power systems (including
TVs and monitors), cracked or defective solder
connections are very common especially around
the pins of high power components like
transformers, power resistors and transistors,
and connectors.
These are particularly common with portable
equipment. Universal AC adapters for camcorders
and laptop computers are often abused to the
point of failure. Large components like the line
filter choke and high frequency transformer are
prone to crack the solder bond at their pins or
even break loose from the circuit board.
Symptoms: almost any kind of behavior is
possible. The unit may be erratic, intermittent,
or totally dead. Visually inspect the solder
side of the circuit board with a bright light
and magnifying glass if necessary. Gently prod
or twist the circuit board with an insulating
stick to see if the problem can be made to
change. Note that a one-time intermittent can
blow many components so inspecting for
intermittents is a really good idea even you
believe that all bad components have been
replaced.
- Regulation problems - outputs high or low.
Symptoms: voltage has changed and adjustment
pot if one exists has no effect or is unable to
set voltage to proper value. Check components in
the feedback regulator, particularly the opto-isolator
and its associated circuitry. A weak opto-isolator
may allow for excessive output voltage. A
shorted photodiode in the opto-isolator may
prevent startup. An open photodiode may lead to
a runaway condition. WARNING: probe these
circuits with care both because of the safety
issues but also since any slip of the probe may
lead to a runaway condition and catastrophic
failure of the switchmode transistor and its
related parts as well as damage to any attached
equipment.
Note that the high frequency transformer does
not make the top 10 list - failure rates for these
components are relatively low. You better hope so
in any case - replacements are usually only
available from the original manufacturer at
outrageous cost.
Most other parts are readily available from
places service parts distributors like MCM
Electronics as well as general electronics
distributors like DigiKey and Mouser.
Rebuild kits are available for many common
supplies used in VCRs, monitors, terminals. See
the section: Repair parts
sources.
Also, while it is tempting to suspect any ICs
or hybrid controllers since it is thought that
replacements are difficult and expensive to
obtain, these parts are pretty robust unless a
catastrophic failure elsewhere sent current where
it should not have gone. And, ICs at least, are
usually readily available.
Most switchmode power supplies when operating
normally produce little or no detectable sound.
The switching frequencies are usually well above
the range of human hearing, but your dog or pet
dolphin might be driven nuts!
However, under various fault conditions, and
sometimes when lightly loaded, there may be
tell-tail audible indications of the SMPS's state
of happiness. The cause may be in the SMPS itself
or its load.
- Tweet-tweet-tweet or chirp-chirp-chirp
(sometimes flub-flub-flub) - Short circuit or
current overload. This is usually an indication
of a shorted secondary-side rectifier and/or if
in a TV or monitor using an SMPS, a shorted
horizontal output transistor. The power supply
is in a repeating cycle attempting to start up,
being dragged down by the overload, and shutting
down.
- High pitched, but audible, whine - Excessive
load. Like (1), this may be caused by shorted
components. For example, a common failure a
Panasonic VCR power supply is for the 18 V zener
diode across the 15 V output to short due to
dried up electrolytic capacitors. The result in
an overload and whine, but usually not a
catastrophic failure of anything else.
Which of (1) or (2) actually present will
depend on the particular design of the SMPS and
the severity of the overload. If the design uses a
hard SCR crowbar, an overvoltage condition may
trigger one of the symptoms!
- Tick-tick-tick - Too little or no load. In a
TV or monitor using an SMPS, this is likely due
to a lack of horizontal drive or a blown fuse or
fusable resistor in the horizontal deflection
system. The power supply is in a repeating cycle
of starting up, being unable to limit the
regulated voltage to a safe value, and shutting
down. Attempting to power up an SMPS without an
adequate dummy load may result in these
symptoms.
- Hissing or white noise - possibly from
inductors - this may be normal under certain
conditions:
(From: Charlie Allen (charlie.allen@usa.net).)
"Some switchmode power supply inductors
will make a hissing or white noise sound,
typically when the circuit is lightly loaded
and running in a "pulse skip" or PFM mode. I
have heard it on many DC/DC circuits. You
could try removing the coil and pouring in
some epoxy."
The following sections provide a set of
guidelines for attacking SMPS problems. Those in
the next 5 paragraphs are common to SMPS using
both discrete and integrated controllers:
- First, determine that it is not something
trivial like a blown fuse due to a legitimate
overload (that has since been removed). I have a
SCSI peripheral box that will blow its SMPS fuse
if the SCSI cable is inserted live.
- Categorize the problem into: startup
problem, catastrophic failure, incorrect
outputs, or excessive ripple or noise.
- Determine what the proper output voltages
should be. Identify the main (regulated) output.
- Disconnect the supply from the equipment it
is powering if possible. This will prevent the
possibility of expensive damage should the
output voltages soar to stratospheric levels for
some reason. If this is not possible, you will
need to be extra careful - always use a Variac
to bring up the input slowly and monitor the
main output at all times.
Note: Some SMPS designs require power to be
applied instantly to provide the startup voltage
to the controller. If this is the case with
yours, it won't be possible to bring up the
voltage slowly (unless you power that chip
separately). However, it should still be
possible to run the unit somewhat reduced line
voltage.
CAUTION: Running any SMPS at greatly reduced
line voltage will be stressful for it,
especially if the output load is a significant
fraction of its full load ratings. In addition,
at some range of line voltage, the output
regulation may not work properly and the
output(s) may go much higher than expected. Use
dummy loads in place of the valuable equipment
if possible when doing such testing!
- Determine an appropriate load for the
outputs (if not connected to the equipment). A
typical SMPS will want a minimum of 5% to 20% of
full load current at least on the main output to
regulate properly. Others may not need any load
- it depends on the design or they may have an
internal load. Here are some typical load
currents:
- VCR - 0.2 A on +5 V and +12 V outputs.
- PC - 2 A on +5 V, 1 A on +12 V. A dual
beam auto head light works well.
- Monitor - 0.2 A on +60 V to +120 V output.
- Typical 40 W switcher = 1 A on +5 V and
+12 V.
The following paragraphs apply mainly to SMPSs
using discrete circuitry (no ICs) for pulse width
control. For those using integrated controller
chips, see the next section:
Troubleshooting SMPSs using integrated controllers.
- Startup problems - check the power on the
switchmode transistor and work back from there
if there is none. Check for open fusable
resistors in the return as well. Determine if
there is startup base/gate drive. Check for open
startup resistors, bad connections, blown parts
in the controller circuitry.
- Blows fuses - check primary side components,
switchmode transistor(s), and all other
semiconductors for shorts. Then check for open
fusable resistors and bad connections. Finally,
check the electrolytic capacitors for reduced
capacity and leakage.
- Power cycling - monitor current and voltage
sensing signals to determine if they are
actually signaling a fault. Open or out of
tolerance resistors may result in incorrect
sensing. With the series light bulb and/or
Variac, disable each of these inputs by
bypassing the appropriate components. If one of
these experiments prevents the cycling behavior,
either that circuit has a faulty component or
the controller circuit is not functioning
properly.
- Regulation or ripple/noise problems - check
main HV filter capacitor and other filter
capacitors for decreased value or opens. Check
regulation components including shunt regulators
and zener diodes.
Since there are usually several fault
conditions that can result in an aborted startup
or cycling behavior, the basic troubleshooting
procedure needs to be modified when dealing with
SMPS using controller ICs like the UC3840 or
UC3842.
Also see the section:
Typical controller ICs found in small switchmode
power supplies for descriptions of two common
integrated controller ICs.
The following paragraphs apply to SMPSs using
integrated controllers. For those using discrete
components only (no ICs), see the previous
section: Troubleshooting
SMPSs using discrete controllers.
- Startup problems - check the power on the
switchmode transistor and work back from there
if there is none. Check for open fusable
resistors in the return as well. Check for power
to the controller. Determine that no fault
condition inputs have abnormal voltages during
startup. Check for drive out of the controller
IC and see if it reaches the switchmode
transistor. You will probably need to power
cycle the line input and monitor each of the
relevant signals as you do so. (It may not be
possible to bring up the voltage slowly with a
Variac if the startup circuit uses capacitive or
inductive coupling to generate the intial supply
voltage for the controller chip. However, a
Variac can still be used to run it at reduced
voltage if this is the case.) Determine if the
supply is shutting down abnormally due to a
legitimate or bogus over-current or over-voltage
condition or is never actually starting up due
to a lack of a voltage or a stuck-at fault on a
sense line. Monitor its power to determine if it
is stable during startup - a bad capacitor or
diode could result in insufficient or decreasing
voltage which causes the controller to give up.
CAUTION: Running any SMPS at greatly reduced
line voltage will be stressful for it,
especially if the output load is a significant
fraction of its full load ratings. In addition,
at some range of line voltage, the output
regulation may not work properly and the
output(s) may go much higher than expected. Use
dummy loads in place of the valuable equipment
if possible when doing such testing!
Powering the controller separately may aid in
troubleshooting of these and related problems.
This will decouple the chopper drive from the
voltage usually derived via a winding on the
high frequency transformer to power the
controller once the supply is running.
- Blows fuses - check primary side components,
switchmode transistor(s), and all other
semiconductors for shorts. Then check for open
fusable resistors and bad connections. There is
a chance that a blown transistor took out the
controller chip as well. Under normal
conditions, controllers like the UC3840 or
UC3842 should current limit on a PWM
cycle-by-cycle basis. Therefore, a blown fuse
indicates a failure of either the switchmode
transistor, controller or both.
- Power cycling - First make sure you are
providing the minimum load if one is required.
Many SMPSs will cycle on overvoltage if there is
none. Some may blow up! Assuming the load
conditions are normal, monitor current and
voltage sensing and Vcc inputs to controller to
determine which, if any, are at fault. Open or
out of tolerance resistors may result in
incorrect sensing. Check for faulty reference
setting components like zener diodes. With the
series light bulb and/or Variac, disable each of
the sense inputs by bypassing the appropriate
components. If one of these experiments prevents
the cycling behavior, either that circuit has a
faulty component or the controller IC's input
characteristics have changed and it will need to
be replaced. It should be possible to determine
if these sensing reference levels are correct
from the controller specifications and thus
should be ignored by the controller as within
normal limits.
- Regulation or ripple/noise problems - check
main HV filter capacitor and other filter
capacitors for decreased value or opens. Check
regulation feedback components to controller
including any reference voltage output and zener
diodes. Determine if the controller is
responding to error voltage. If possible,
monitor both error and PWM drive signals on a
dual trace scope.
Once defective parts have been replaced, if
possible remove the normal load from the supply if
you have not already done so just in case it
decides to put excessive voltage on its outputs
and replace with a dummy load. For a multiple
output supply, the most important output to have a
load on is the one that is used for regulation but
some modest load on all the outputs is preferred.
You should be able to determine a suitable value
by considering the application. For something like
a VCR, a few hundred mA on the main output is
probably enough. This would require something like
a 25 ohm 2 W resistor for a 5 or 6 volt output or
50 ohm 5 W resistor for a 12 volt output
(depending on which is the primary output). For a
PC power supply, a couple of amps may be needed -
a 2 or 3 ohm 15 W resistor on the +5 output. The
minimum load is sometimes indicated on the
specification sticker. In the case of a TV or
monitor, disconnecting the load may not be
possible (or at least, easy).
If available, use a Variac to bring up the
input voltage slowly while observing the main
output. You should see something at about 50% of
normal input voltage - 50 or 60 V for a normal 115
VAC supply. With a small load, the output should
very quickly reach or even exceed its normal
value. Regulation at very low line voltage may be
far off - this is often normal. Just make sure
you're using dummy loads so your equipment can't
be damaged.
Note: Some SMPS designs require power to be
applied instantly to provide the startup voltage
to the controller. If this is the case with yours,
it won't be possible to bring up the voltage
slowly (unless you power that chip separately. So,
if nothing happens when doing this, don't panic -
it may be a feature, not a bug. :) It should still
be possible to run the unit somewhat reduced line
voltage on the Variac.
If you do not have a Variac, put a light bulb
in series with the line (this is desirable in any
case). Use a 100 W bulb for a TV or PC, 40 W for a
VCR typical. The light bulb should limit the
current to a non-destructive value long enough to
determine whether everything is OK. It may not
permit normal operation under full load, however.
When power is first applied, the light bulb will
flash briefly but may just barely be glowing once
the output has stabilized. If it is fairly bright
continuously, there is likely still a problem in
the supply. See the section:
The series light bulb trick.
Once you are finished, save your schematic and
notes for the future. For example, multiple models
of VCRs even from different manufacturers use the
same basic design, maybe even the same supply.
Any time the switchmode transistor requires
replacement, check all semiconductors for shorts
and fusable resistors for opens. even if you
locate **the** problem early on. Multiple parts
often fail and just replacing the transistor may
cause it to fail as a result of something else
still being bad. In particular, check primary side
electrolytic capacitors for reduced capacity or
opens. These conditions can result in a blown
switchmode transistor as it attempt to supply
adequate current during the troughs of the
rectified high voltage DC. It only takes a few
more minutes. For other problems like an open
startup resistor this excessive caution is
unnecessary as these are usually isolated
failures. However, if any dried up electrolytics
are found, it is good practice to test them all -
or just replace them all since the cost and time
will be minimal. As they say, 'peas in a pod fail
at nearly the same time'.
It is often helpful to trace the circuit by
hand if a service manual is not available. You
will gain a better understanding of this supply
and be able to put the knowledge to use when the
next one shows up on your bench - there is a lot
of similarity even between different
manufacturers. A bright light behind the circuit
board may help to make the foil runs and jumpers
more visible. The only difficult part will be
determining how the transformer windings are
hooked up. An ohmmeter will help but even if you
cannot entirely determine this, just make a note.
For most purposes, the exact topology of the
windings is not critical for diagnostic
procedures.
These are of the form: tweet-tweet-tweet or
flub-flub-flub or some other similar variation.
Any LEDs may be flashing as well and in the case
of something like a monitor or TV, there may be HV
static or even a partial raster in synchrony with
the sounds. These types of problems are more
common with sophisticated implementations - the
simple ones just blow up!
As noted elsewhere, shorted secondary
components are a very likely cause of this
behavior. These include diodes, capacitors, and
overvoltage SCRs. The fact that there is some
output suggests that the main switchmode (chopper)
transistor is working. There would likely be no
output at all if it were bad.
Note that an underloaded supply may be cycling
due to overvoltage and there may actually be
nothing wrong! Many SMPSs require a minimum load
to maintain stability and to provide proper
regulation. This is typically 20 percent of
maximum on the primary output (the one which
drives the feedback loop). However, minimum loads
may also be needed on other outputs depending on
design. The only way to be sure is to check the
manufacturer's specs.
Other possibilities for periodic or pulsing
outputs:
- One of the diodes is failing at volage -
quite possible. As long as you do not remove
both from the output that is used for feeback,
it should be safe to take them out one at a time
and then substitute for the one remaining in the
feedback voltage. Use a Variac and series light
bulb when testing in this manner and constantly
monitor the main output.
- Some other cause of excessive current -
shorted capacitor, transformer (though not
likely), etc.
- Faulty current sense circuit - open or
increased value resistor.
- Faulty voltage sense circuit - detecting
overvoltage or regulation defective and it is
shutting down (correctly).
- Faulty component in the startup circuit.
This could be a bad diode, resistor, or even an
electrolytic capacitor that has changed value or
is open at low voltage (when the controller is
just waking up).
- Faulty controller IC (if applicable).
Where an SMPS doesn't start and it isn't
obvious why, it might help to drive the chopper
from an external signal source to see what then
works. The only time this is really practical is
where a single transistor or MOSFET is used -
generating a push-pull waveform probably isn't
worth it.
This will always be a risky procedure both for
you and the power supply. The switching frequency
is likely unknown but for these tests you can
assume it is in the 10 to kHz range. You can
reduce the risk somewhat (to the supply at least)
by using a series light bulb load and/or running
on reduced line voltage. The most important thing
to avoid is putting in an excessively long drive
pulse which will result in the high frequency
transformer saturating, huge amounts of current,
and likely a dead transistor and possibly other
parts if there is nothing to limit the current. If
you have the option, start with a narrow pulse
waveform to minimize on-time and don't push your
luck! :)
Similarly, where a power supply attempts to
start but cycles or shuts down, consider powering
the controller chip from a separate supply to
eliminate any issues of the transformer derived
voltage that normally runs it after startup.
Back to SMPS Repair FAQ
Table of Contents.
Components Found in Switchmode Power Supplies
Most of the components used in switchmode power
supplies are common and easily identified.
However, some may be unfamiliar and
unrecognizable. Others could be totally custom
parts - ASICs or hybrid circuits - developed
specifically for a particular model or product
line. However, these rarely fail despite your
temptation to blame them specifically *because*
locating a replacement is difficult and most
likely expensive.
Common components like transistors, diodes,
capacitors, and resistors, can usually be tested
with a multimeter at least for total failure. Also
see the documents: "Testing of Bipolar Transistors
with a VOM or DMM" and "Testing Capacitors with a
Multimeter and Safe Discharging".
Of course, with catastrophic failures, no
equipment beyond your eyeballs and nose may be
needed.
- Bipolar power transistors (often BU or
2SC/2SD numbers) - high voltage power types are
used for the main switchmode (chopper)
transistor.
Test for shorted and open junctions. These
are the most common failures for the power
transistors. Partial failure where there is some
leakage or various parameters change value are
unlikely.
Substitution of a transistor with at least
equal voltage and current ratings should be fine
for testing as long as you use a series light
bulb to limit the current should something still
be wrong elsewhere in the circuit. A not-exact
match may run hotter than normal. Always use a
heatsink.
- Power MOSFETs (2SK numbers) - many newer
supplies are using these rather than the bipolar
type. In some ways they are more robust but
testing is more difficult.
Testing for shorts is still possible but
anything more requires additional equipment
beyond a multimeter. However, the original
problem did not blow a fuse or fusable resistor,
if the MOSFET is not shorted, there is an good
chance that it is still fine and you should look
elsewhere for the problem. It may be a problem
with the startup circuit or controller.
Note: if your supply produces any output (say,
more than 10% of rated voltage), it is unlikely
that the chopper transistor is bad as it must be
working to some extent and, as noted, these
usually blow totally.
- Small bipolar transistors - these may be
found in feedback and control functions.
Test for shorted and open junctions with a
multimeter. Substitute with similar known good
transistor is best, however. I have seen little
silicon transistors that had developed enough
leakage to prevent a 400 W supply from coming
up!
- Diodes and rectifiers - a bridge or set of 2
or 4 discrete diodes is usually used for the AC
line rectification and/or doubling. High
efficiency and/or fast recovery types are used
in the secondary side for rectification of the
various output voltages. These may look like
normal axial lead diodes or may be combined in
pairs inside TO220 type packages.
Test for shorted and open junctions. However,
sometimes, diodes will only fail with full
voltage in-circuit but test good with a
multimeter.
Replacements for the primary side rectifiers
are very inexpensive and readily available. If
the unit blows fuses with the switchmode
transistor and main filter capacitors pulled,
the rectifiers may indeed be bad.
It is usually safe to remove secondary
rectifiers one at a time to see if the supply
will come up. As long as you do not remove all
diodes for the output that provides the feedback
for the regulation, this should be relatively
low risk. (However, do this with a dummy load -
not your expensive laptop computer just in
case.) Even removing those diodes is usually
safe if you can power the supply using a Variac
since you will be able to limit the input (while
monitoring the main output) should the outputs
go overvoltage.
- SCRs - small SCRs may be found in the
overvoltage protection circuitry of some
supplies. Note that SCR type of crowbars are
used across the output as a way to guarantee
that an overvoltage condition will kill the
output regardless of the reason for the
overvoltage condition. Hopefully, the supply's
overcurrent protection will kick in rather than
having the supply blow up. This is not always
the case, unfortunately.
Test for shorts if output on which SCR is
connected is not coming up. Remove the SCR. Now,
using a Variac to bring up the voltage slowly,
see if the relevant output is going over
voltage, is still clamped at a low level, or is
the correct voltage (under load). A momentary
overvoltage spike at turn-on could also trip the
crowbar. This could be due to a faulty
inrush/slew rate limiting circuit.
- TL431 or similar shunt regulator IC - either
a TO92 or 8 pin DIP. Has 3 active terminals - A,
C, and R. Current will flow from C to A if R-A
is greater than 2.5 V.
Test for shorts but substitution is best.
However, with care (using a Variac AND series
light bulb to limit the input current, it is
possible to determine if the circuit in which
these are connected is working. Short across
TL431 - supply should either turn off or run at
greatly reduced output. Remove the TL431 - there
should be no regulation - outputs should
continue to climb as Variac is increased. By
monitoring input to TL431 it should be possible
to determine if it is doing its job.
- Optoisolator - either a 4 or 6 pin DIP or a
4 pin cylindrical object. This provides the
regulator feedback across the isolation barrier.
Replacements are readily available.
Test by putting 10-20 mA through LED and
measuring decrease in resistance of reverse
biased photodiode. However, this will not
identify a weak optoisolator. Swapping is best.
- Filter capacitors - electrolytic type are
used for filtering of the rectified (possibly
doubled) AC line input and for filtering of the
various outputs of the power supply.
If no capacitor checker is available, test
for opens, shorts, and leakage with a multimeter.
For electrolytics, this is straightforward.
Inspect the capacitor for any discoloration, a
bulging case, or other evidence of trauma.
An ESR meter is a convenient device for
rapidly checking the health of electrolytic
capacitors. The ESR (Effective Series
Resistance) of a capacitor increases as the
capacitor deteriorates ('dries up'). Even a
capacitor that tests good on a capacitor checker
may not work properly due to excessive ESR.
When in doubt, the best approach is to
substitute a known good capacitor of at least
equal working voltage and similar uF rating.
Also see the document:
Capacitor Testing, Safe Discharging and Other
Related Information.
(From: Steve (libertytek@aol.com).)
"When you find a position that eats
electrolytic caps, replace them but add a
parallel .22 to .47 uF ceramic monolithic.
Whenever these crappy caps are used with
even small high frequency currents passing
through them, they break down chemically
causing other failures also. Even the "high
ripple current" rated caps won't tolerate what
they should and are often rated at 1,000 -
2,000 hours.
I also often find too little heat sinking
and will add more surface to improve cooling."
- Bypass Capacitors - high quality plastic
dipped or rectangular molded capacitors as part
of RFI filter. These rarely fail.
Test for shorts - your multimeter will
probably not be able to detect the small
capacitance. Substitute if in doubt.
Note that many of these are special high
quality low loss types with regulatory approval
for use across the power line in the line
filter. Exact replacements are required for
safety.
- Resistors - test for correct value with a
multimeter. If measured in-circuit, value may
read low if shunted by other components. If a
higher than normal reading is obtained
in-circuit, the resistor is bad. Metal film
types like to go open circuit - especially very
high value resistors.
Startup resistors in particular tend to go
open-circuit resulting in a dead supply but no
blown fuses or fusable resistors. These are
usually high value (100K typical) medium wattage
and run hot since they are across the full
rectified line voltage.
- Flameproof or fusable resistors (They are
the same) - these are often designated 'FR'.
They will look like power resistors but will be
colored blue or gray, or may be rectangular
ceramic blocks. They should only be replaced
with flameproof resistors with identical
ratings. They serve a very important safety
function: they cannot catch fire when overheated
and will open rather than changing value which
implements an overload protection function.
These usually serve as fuses in addition to
any other fuses that may be present (and in
addition to their function as a resistor, though
this isn't always needed). If an FR type
resistor has blown, you probably have shorted
semiconductors that will need to be replaced as
well. Check all the transistors and diodes in
the power supply with an ohmmeter. You may find
that the main switch mode transistor has decided
to turn into a blob of solder - dead short.
Check everything out even if you find one bad
part - many components can fail or cause other
components to fail if you don't locate them all.
Check resistors as well, even if they look OK.
The most common location for these in a small
SMPS is in the return circuit of a the
switchmode transistor. However, they may be in
the power feed as well. The value may be a
fraction of an ohm but can be larger.
In TVs and monitors, these are often found in
the hot power feed to the main low voltage power
supply and in various secondary supply feeds as
well. For the main supply, they will be 5-25 W
rectangular ceramic power resistors. For the
secondary supplies, they may be the 1/2-2 W blue
or brown tubular variety.
Test for opens. Those in the return circuits
are usually very low value - a fraction of an
ohm to a few ohms - if in the return of the
switchmode (chopper) transistor. The type in the
power feeds may be anywhere from a fraction of
an ohm to several K ohms depending on the
circuit load.
For testing ONLY, a normal resistor may be
substituted but the proper replacement MUST be
installed before returning the supply to
service.
Since they function as fuses, flameproof
resistors should not be replaced with higher
wattage types unless specifically allowed by the
manufacturer. These would not blow at the same
level of overload possibly resulting in damage
to other parts of the circuitry and increasing
the risk of fire.
- MOVs - Metal Oxide Varistors - look like
brightly colored plastic coated disk capacitors
but not marked with capacitance. These are surge
suppressors. A severe surge or lightning strike
may obliterate one or more of these. There will
usually be either 1 between the Hot and Neutral
or 3 across H, N, and safety ground.
If they are visibly damaged in any way, just
remove (for now) or replace. Test with an
ohmmeter - resistance should be nearly infinite.
- NTC resistors (thermistors) - Negative
Temperature Coefficient resistors act as inrush
surge limiters. There may be one or two of these
in series with the AC input. These are a high
value when cold but drop to a low value once
they heat up due to current flow into the
supply. These often look like fat black disk
capacitors.
Test when cold and hot (use a hot air gun or
hair dryer if not in-circuit). Resistance should
drop from 10s of ohms to a very low value.
- High frequency transformers - these include
the switchmode power transformer and any
feedback (toroidal or E-I core type)
transformers.
The main transformer which provides line
isolation and generates the multiple output
voltages from the 150-320 VDC input rail. These
are usually custom wound for each model power
supply and replacements are only available from
the manufacturer. However, some distributors
will stock replacements for a few TVs and
computer monitors.
Testing for opens is usually easy since
connections to the input (chopper) and output
rectifiers are fairly obvious. However, feedback
windings may be involved and these are not
readily determined without a schematic or
tracing the circuit (and, possibly not even
then.) The good news is that failures of these
transformers is less common than one might fear.
Some supplies use small transformers for
feedback rather than optoisolators. These can be
tested for opens but rarely cause problems.
There may also be transformers in series with
the input that can be similary tested.
Identifying shorted turns requires a 'ring
test' or measurement of the Q. See the document:
Testing of Flyback (LOPT) Transformers.
- Control/standby power transformers - used to
provide power to the controller of some units
when the main chopper is idle. These may be
damaged by a power surge, usually with an open
primary winding. Test with an ohmmeter. A
shorted winding will result in overheating.
- Inductors - test for opens. Identifying
shorted turns requires a 'ring test' or
measurement of the Q. See the document:
Testing of Flyback (LOPT) Transformers.
AC line input inductors can just be bypassed
if they test open.
Output 'pi' filter inductors rarely fail but
if you suspect one, just remove it and jumper
across the pads for testing - ripple just won't
be quite as good.
- Coupled Inductors - used as part of the Pi
type RFI filter in the AC input circuit. These
look like small transformers but the windings
are in series with the AC line. There are
usually 1 or 2 of these on better supplies and
they are very reliable.
Test for opens. Identifying shorted turns
requires a 'ring test' or measurement of the Q.
See the document:
Testing of Flyback (LOPT) Transformers.
These inductors can just be removed and bypassed
during testing if they are open since they only
affect input line noise filtering.
Many small SMPSs don't have any fans built in
but expect there to be a fan or fans elsewhere in
the equipment designed draw air over the power
supply. Most computer power supplies do have a fan
inside - and these are high failure items due to
how cheaply they are made.
A bad or tired fan, or even clogged air
filters, can result in overheating and outright
failure, or at the very least, increased stress on
components and reduced life expectancy. Thus,
periodic maintenance is highly recommended.
Inspection of the fan(s) and filter(s) should be
one of the first steps in any testing procedure.
The most common problem with fans is dry/gummed
up/worn bearings. Ball bearings are rarely found
in PC power supplies (the manufacturer saved 25
cents). Even on expensive workstation computers,
mediocre fans may be used (Sun Microsystems had to
replace a whole bunch of fans on state-of-the-art
Ultra-Sparc systems because of bad bearings).
Quick test: With the power off, give the fan a
spin. If it continues to coast for at least a
couple of seconds, the bearings are probably good.
If it stops instantly, they are gummed up. If in
doubt, replace the fan with a good quality ball
bearing type. It's really not worth attempting to
disassemble and oil the bearings unless you have
nothing better to do.
Fan motors do go bad but this is much less
common than bad bearings. With modern brushless DC
motors, one phase could be defective resulting in
sluggish operation and/or failure to start if
stopped in just the wrong position.
On more sophisticated equipment with
temperature sensing to adjust fan speed, the speed
control circuitry could also be bad.
WARNING: Replacement of the fan on SMPSs
requires access to the interior. Make sure the
equipment is unplugged and the large filter
capacitors are fully discharged before doing
anything inside the case - both for your safety
and to prevent damage to the supply.
For more on fans, fan motors, and lubrication,
see the document:
Notes on the Troubleshooting and Repair of Small
Household Appliance and Power tools.
(From: Clive Cooper (clpc@cooperware.com).)
I spent 3 days searching for a problem on a
SMPS. It turned out to be a simple fault that
eluded me for some time.
The SMPS worked fine for about 10 minutes and
then the output voltages dropped slowly and
eventually the supply went dead.
It turned out that the fan, although it
appeared to be working fine was only getting 60%
of the supply voltage it needed. This was due to a
faulty temperature sensor that just told the fan
that the supply was cold even when it was hot.
Conclusions: A fan that is blowing is not
necessarily blowing what it should be blowing and
the fact that it seems to be working doesn't mean
its working at maximum efficiency.
Back to SMPS Repair FAQ
Table of Contents.
Items of Interest
The same power supply design is used with minor
variations in a wide variety of Panasonic (and
clone) VCRs from the 1980s and 1990s (and may
continue to this day). Depending on the specific
model, there may slightly different output
voltages and number of outputs but the general
organization is identical. These use discrete
components throughout with feedback from the
primary output (5 to 5.2 V depending on model)
using an optoisolator to essentially short out the
drive to the main chopper transistor (Q1) when the
output equals the desired voltage. The most common
problems found with any of these supplies is dried
up electrolytic capacitors. Generally, the first
to go will be C16 and C17 on the +5.1 VDC line
and/or C21 in the feedback path (actual part type
and number may vary slightly with model). Symptoms
will be either that the primary output is somewhat
low (4 to 4.5 VDC) or that the supply has gone
overvoltage and blown the protection zener (D15)
resulting in a high pitched whine as the chopper
struggles to drive current into a short circuit
(this usually doesn't damage any other parts if
caught in a reasonably timely manner). If any
capacitor related problems are found, it is a good
idea to replace all the electrolytics in the
supply. Model specific capacitor kits as well as
total rebuild kits are available from places like
Studio
Sound Service and
MCM
Electronics.
The Panasonic VCR power supply schematic is
available in both PDF and GIF format:
Here is some information on the Unitrode UC3840
programmable off-line PWM controller and its
simplified cousin, the UC3842. These are typical
of the types of sophisticated inexpensive
integrated SMPS controller ICs that are now
readily available.
The information below is just a summary.
These devices generate the PWM pulse control to
the switchmode (chopper) transistor as well as
various fault sensing and other control functions.
Parts such as these are now found in many small
switchmode power supplies and provide much more
precise control during startup and normal
operation, and better handling of fault conditions
compared to most implementations using discrete
circuitry.
However, they also result in additional head
scratching when troubleshooting since many faults
or incorrectly detected faults can shut down the
unit or cause a power cycling type of behavior.
Therefore, a datasheet for the controller chip
will prove essential. In many cases a scope will
be needed to monitor the various sense, control,
and drive signals. A systematic troubleshooting
approach must be used to eliminate power, startup,
sensing, and control components one at a time once
obvious shorted or open parts or bad connections
have been eliminated from consideration.
Features of the Unitrode UC3840 include:
- Fixed frequency operation set by user
selected components.
- A variable slope ramp generator for constant
volt-second operation. providing open-loop line
regulation and minimizing or in some cases, even
eliminating the need for feedback control.
- A drive switch for low current start-up off
of the high voltage line.
- A precision reference generator with
internal over-voltage protection.
- Complete over-voltage, under-voltage, and
over-current protection including programmable
shutdown and restart.
- A high current single-ended PWM output
optimized for fast turn-off of an external power
switch.
The following pin descriptions for the Unitrode
UC3840 were derived from a Unitrode application
note. Errors in interpretation are quite possible.
Pin 1: Compensation Error amplifier (op amp) compensation network.
Pin 2: Start/U.V. lockout This comparator performs three functions. With
an increasing voltage, it generates a turn-on
signal at a start threshold. With a decreasing
voltage, it generates an under-voltage fault
signal at a lower level separated by a 200uA
hysteresis current. At the under-voltage
threshold, it also resets the Error Latch if
the Reset Latch has been set.
Pin 3: OV sense Over-voltage input from power supply output(s).
Pin 4: Stop (Ext stop) External logic signal to inhibit power.
Pin 5: Reset External logic signal to reset error condition
caused by (1) over-voltage, (2) over-current, (3)
input under-voltage detect, (4) external stop.
Pin 6: Current threshold This voltage input sets the over-current trigger
levels for the internal comparators.
Pin 7: Current sense This is the pulse-by-pulse PWM current control.
The input is a voltage taken across a series
resistor in the switchmode transistor's return.
There are two internal comparators with a
difference in threshold of 400 mV. The one
with the lower threshold limits the current
for each PWM cycle. The one with the higher
threshold sets the error flop-flop and shuts
down the supply if its threshold is ever
exceeded.
Pin 8: Slow start This input limits the maximum PWM duty cycle.
During power-on, an RC delay can therefore
control the rate at which the output ramps up.
The final value limits the maximum PWM duty cycle
during normal operation.
Pin 9: Rt/Ct R and C determine the constant PWM oscillator
frequency.
Pin 10: Ramp Ramp generator output.
Pin 11: Vi sense This voltage is normally derived from the DC
input and controls the slope of the ramp.
Pin 12: PWM output This is the drive signal to the switchmode
transistor. This is an open collector output
and will normally be used in conjunction with
the Driver bias (Pin 14) signal to provide
total drive to the switchmode transistor.
Pin 13: Ground Signal and drive common.
Pin 14: Driver bias Supplies drive current to external power switch
to provide turn-on bias and pullup during normal
operation. Disabled for shutdown if the Error
Latch is set.
Pin 15: Vcc UC3840 chip supply derived from the DC input rail
during startup and secondary winding on high
frequency transformer during normal operation.
Pin 16: 5 V reference Stable voltage reference (output) for regulation
control.
Pin 17: Inv input Error amplifier inverting input.
Pin 18: Non inv input Error amplifier non-inverting input.
The difference between the inputs on Pins 17 and
18 control PWM duty cycle. These will generally
be derived by comparing the main output with
the desired voltage reference.
The UC3842 provides the necessary functions to
implement an off-line fixed frequency current mode
control schemes with a minimal external parts
count. Note how most of the pin functions are
subsets of those found in the more sophisticated
UC3840. The UC3842 retains most of the features of
the UC3840 but requires fewer external components
and comes in a much smaller package (8 vs. 18
pins).
The following pin descriptions for the Unitrode
UC3842 were derived from a Unitrode application
note. Errors in interpretation are quite possible.
Pin 1: Compensation Error amplifier (op amp) compensation network.
Pin 2: Vfb Error amplifier (non-inverting) input for
regulation feedback.
This input is used to control PWM duty cycle
and is normally derived from the main regulated
output voltage. It is similar in function to
The non-inverting input, Pin 18, of the UC3840.
Pin 3: Current sense This is the pulse-by-pulse PWM current control.
The input is a voltage taken across a series
resistor in the switchmode transistor's return.
Pin 4: Rt/Ct R and C determine the constant PWM oscillator
frequency.
Pin 5: Ground Signal and drive common.
Pin 6: PWM output This is the drive signal to the switchmode
transistor. It uses a totem pole output which
has a high current drive capability both high
and low.
Pin 7: Vcc UC3842 chip supply derived from the DC input rail
during startup and secondary winding on high
frequency transformer during normal operation.
Pin 8: 5 V reference Stable voltage reference (output) for regulation
control.
Depending on the particular circuit design, a
variety of fault conditions can result in cycling
or shutdown of an SMPS controlled by a chip like
the UC3842. And, an underloaded supply may be
cycling due to overvoltage!
In addition to the overload condition described
below, a dried up electrolytic capacitor on the
Vcc line can also result in this cycling behavior
since it is unable to hold up the voltage between
output pulses. In addition, the sense inputs can
trigger shutdown. In all, an often complex
difficult to understand and troubleshoot situation
- sometimes too much so for its own good!
(Portions from: Yves Houbion (yves.houbion@fundp.ac.be).)
Pin 7 is the power supply (Vcc). The oscillator
inside the 3842 begins to work above 16 V on Vcc
and stops working when this voltage drops below 11
V. With a stopped oscillator, the current
consumption is very low, around 1 mA; with a
working oscillator, the current is much higher,
about 12 mA. (The specific voltages and currents
are typical values for one particular version of
the 3842 and can vary from device to device and
depending on model.)
Vcc is generally powered in two ways: a high
value power (startup) resistor connected to the
main bridge (e.g., +300V) and a from a winding off
the transformer (via a rectifier/filter
capacitor). The value of the startup resistor is
selected such that there is more than 16 V with 1
mA but less than 11 V at 12 mA. So the oscillator
can't continue to work with only the startup
resistor supplying power.
Suppose we apply AC power to the supply. The
+300V comes on. First, the 3842 consumes only 1 mA,
Vcc reaches 16 V, and the oscillator starts up. If
all is well (no overloads), the transformer
provides the necessary 12 mA current to maintain
Vcc at more than 11 V.
However, if the transformer is overloaded, Vcc
falls under 11 V and the oscillator stops working.
The current decreases to 3 mA, the voltage
increase (coming from the +300V) the oscillator
start again, ad-infinitum.
Tweet-tweet-tweet....
Assuming it is not a wide compliance 'universal
type', a common way to do this is with a jumper
(or switch) in the line input circuitry below
(also shown in
Typical SMPS Input Voltage Select Circuit):
D1
AC o-----+----|>|-------+---------+-----o DC (+)
~| D2 |+ |
+----|<|----+ | +_|_
D3 | | C1 ---
+----|>|----|--+ - |
| D4 | +--o-o--+ +320 VDC to chopper
AC o---+-+----|<|----+ - | J1 |
|~ | | +_|_
+-------------|----+ C2 ---
| - |
+------------+-----o DC (-)
- With the jumper, J1, installed, the circuit
is a voltage doubler for use on 115 VAC. (D3 and
D4 never actually conduct because they are
always reverse biased.)
- With the jumper, J1, removed, the circuit is
a simple bridge rectifier for use on 230 VAC.
Would it be possible to modify a power supply
designed for operation on 120 VAC for use overseas
where the power is 240 VAC?
I don't advise it. There are many factors
involved in changing a power supply unless it is
designed for dual voltage or autoswitching. They
saved a few cents if it is not easily switched,
what can I say?
The problem is that it is probably a flyback
converter and these are pretty finicky about
changes. In addition to the caps, and switching
transistor, the transformer would probably
saturate at the higher voltage unless the
switching frequency were also doubled. Getting
these things to work normally without blowing up
is touchy enough. To change one without a thorough
understanding of all the design parameters would
be really risky.
Going the other way may be more realistic if
(and this is a big if) you will not be running at
anywhere near full capacity. Many switchmode power
supplies will run on much lower than their rated
input voltage. However, regulation may be poor and
the switchmode transistor will need to be passing
much higher current to maintain the same power
output. To maintain specifications could require
extensive changes to the circuitry and replacement
of the switchmode transistor and possibly
transformer and other parts as well. Again, I do
not recommend this.
Use a small stepup or stepdown transformer
instead. The only exceptions are:
- If there are clearly marked jumpers to
select the input voltage.
- You can reverse engineer enough of the input
circuitry to add/remove the needed connection
with confidence.
CAUTION: As they say in wood-working:
"Measure twice, cut once". Make sure you are
dealing with the correct jumper AND you are
going the right way (increasing or decreasing as
needed). If the manufacturer didn't include this
feature, there may be a good reason!
- The supply is clearly marked as being
autoswitching or having universal power input.
Also see the secton: "Switching between 115 VAC
and 230 VAC input".
Surplus PC power supplies are widely available
and inexpensive. However, what do you do if 5 V
isn't exactly what you need for a project?
(From: Winfield Hill (hill@rowland.org).)
Some of the PC power supplies I've dissected do
have pots, by they have a limited
voltage-adjustment range. One interesting thing,
every design used a TL431 chip, which is a 3-pin
TO-92 regulating IC, as the voltage reference and
opto-feedback component. Find this chip and trace
out the resistors connected to it to determine
which part to change to make a higher voltage.
But, watch out for the SCR over-voltage circuit
in some supplies. This is usually set to trip
around 6 to 6.5 volts, and its trip point would
need to be modified as well.
As far as the step-down transformer turns
ratio, there's little trouble one will encounter
here, because the power supply is no doubt
designed to function properly with reduced AC line
voltages. The penalty one will pay for turning up
the output voltage is a higher minimum AC voltage.
In most designs, the +12 and -12 V supplies
merely track the 5V supply, and are not separately
regulated. They may soar to higher voltages anyway
if unloaded, but will be additionally increased in
voltage by the ratio of 5V output increase. Even
though the rating of the 5V electrolytic may not
be exceeded, and still have a sufficient safety
margin, this may not be the case for the 12 V
outputs. So that issue should be examined as well.
Finally, a reminder for any reader tempted to
break open the box and start experimenting.
Voltages of up to 320 V are present, so be
careful. Know what you're doing. For safety, stay
away from open supplies when plugged in, or always
keep one hand behind your back when probing.
Remember a the AC bridge and HV DC and flyback
transformer portion of all these supplies is
operating straight from the AC line, so don't
connect the ground of your oscilloscope to any of
that circuitry. A battery-operated multimeter is
best.
Should you always use a surge suppressor outlet
strip or line circuit? Sure, it shouldn't hurt.
Just don't depend on these to provide protection
under all circumstances. Some are better than
others and the marketing blurb is at best of
little help in making an informed selection.
Product literature - unless it is backed up by
testing from a reputable lab - is usually pretty
useless and often confusing.
Line filters can also be useful if power in you
area is noisy or prone to spikes or dips.
However, keep in mind that most well designed
electronic equipment already includes both surge
suppressors like MOVs as well as L-C line filters.
More is not necessarily better but may move the
point of failure to a readily accessible outlet
strip rather than the innards of your equipment if
damage occurs.
Very effective protection is possible through
the use of a UPS (Uninterruptible Power Supply)
which always runs the equipment off its battery
from the internal inverter (not all do). This
provides very effective isolation power line
problems as the battery acts as a huge capacitor.
If something is damaged, it will likely be the UPS
and not your expensive equipment. Another option
is to use a constant voltage transformer (SOLA)
which provides voltage regulation, line
conditioning, and isolation from power spikes and
surges. Manufacturers of these products may even
provide equipment damage warranties which will
reimburse for surge damage to the powered
equipment while using their products. I am not
sure how one proves that the UPS was being used at
the time, however!
It is still best to unplug everything if the
air raid sirens go off or you see an elephant
wearing thick glasses running through the
neighborhood (or an impending lightning storm).
Ground Fault Circuit Interrupters (GFCIs) are
very important for minimizing shock hazards in
kitchens, bathrooms, outdoors and other
potentially wet areas. They are now generally
required by the NEC Code in these locations.
However, what the GFCI detects to protect people -
an imbalance in the currents in the Hot and
Neutral wires caused possibly by someone touching
a live conductor - may exist safely by design in 3
wire grounded electronic equipment and result in
false tripping of the GFCI. The reason is that
there are usually small capacitors between all
three wire - Hot, Neutral, and Ground in the RFI
line filters of computer monitors, PCs, and
printers. At power-on and even while operating,
there may be enough leakage current through the
capacitors between Hot and Ground in particular to
trip the GFCI. Even for ungrounded 2 wire devices,
the power-on surge into inductive or capacitive
loads like switching power supplies may falsely
trip the GFCI. This is more likely to happen with
multiple devices plugged into the same GFCI
protected outlet especially if they are controlled
by a common power switch.
Therefore, I do not recommend the use of a GFCI
for computer equipment as long as all 3 wire
devices are connected to properly grounded
circuits. The safety ground provides all the
protection that is needed.
Startup is the most stressful time for a
typical switchmode power supply. The output filter
capacitors as well as the load must be driven
while the input voltage is changing - possibly
wildly. With careful design, these factors can be
taken into consideration. Not all power supplies
are designed carefully or thoroughly tested under
all conditions. When power is restored, surges,
dips, brownouts, and multiple on-off cycles are
possible. This is why it is always recommended
that electronic equipment be unplugged until power
has been restored and is stable.
Supplies that are autoselecting with respect to
input power are vulnerable to voltages at an
intermediate value between their low and high
ranges. At some values, they may autoselect the
incorrect input range:
(From: Mike Diack (moby@kcbbs.gen.nz).)
A subject dear to my heart due to a recent
unpleasant experience - Was using a Picturelel
videoconference ISDN codec on a job when, because
of a powerline fault, the line voltage dropped to
170 volts. The PicTel has a big Onan switchmode
PSU which is autoswitching between 100-120 and
200-240 volts. It got confused, and (regrettably)
chose the former.... with very smelly results.
Moral: turn off things with cunning PSUs when
brownouts occur (oh yes the airconditioner units
got very hot and tripped out, too)
Two common causes are (1) loose transformer (or
other cores) vibrating at a subharmonic of the
switching frequency and (2) dried up electrolytic
capacitors (primary side) introducing 120 Hz hum
under load.
(From: Ray Hackney (rhackney@unicomp.net).)
Simplistically speaking, the sound comes from
something moving.
With non switch mode power supplies (SMPS), it
may be ferrous material (like a metal cover) being
drawn toward the power transformer. That's obvious
since pushing on the cover will soften the hum.
The frequency is usually 60Hz or 120Hz.
The only time you should hear a "noise" from a
SMPS is during a period of "unstable" operation
(i.e. their "loop" isn't stable and in
regulation.) That's why you may hear them "chirp"
or whistle when you first turn them on or off. It
may also indicate a PC type power supply that's
overloaded. In years gone by, I've seen a quiet PC
become a whistler after having a new, big (30 meg,
full height!) hard disk added. Sometimes the pitch
of the whistle would change depending on what
parts of the system were being accessed or what
software was being executed. (Usually, when the
old Intel AboveBoard was being accessed in this
'286, the audible pitch was lower indicating
greater current draw.)
For all power supplies, it may be the windings
on the "magnetics" (inductor or transformer). If
they're not wound tightly and secured they can
vibrate. Many video monitors exhibit this problem
when their flyback transformer emits a whistle. It
may be the windings themselves moving or the
winding assembly may be loose on the core.
Sometimes the capacitors in a SMPS will emit
sound. Caps in SMPS' frequently have high AC
current levels. If the supply is supposed to have
what's known as "continuous current" and goes into
"discontinuous current" mode, the capacitor plates
get stressed pretty heavily and move in the
capacitor body (but only with some types). Since
the SMPS will go into and out of discontinuous
mode at a rate < 10kHz, it's audible. I've run
into this on breadboards I've built for 200W and
2.5kW SMPS'.
(From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)
Electrolytic capacitors like to be kept cool!
If there's anything that these capacitors can't
stand, it's heat. It causes them to dry out.
Electrolytic capacitors exist in (at least) two
different temperature ratings: 85 C and 105 C. The
latter are obviously more temperature resistant.
Unfortunately they also tend to have a higher ESR
than their 85 C counterparts. So in an application
where the heat is due to I^2 * ESR dissipation,
the 105 C type may actually be a *worse* choice!
If the heat is due to a nearby hot heatsink then
105 C is indeed a better choice.
ESR is usually something to be minimized in a
capacitor. However, where the original design
depended (probably by accident) on a certain ESR,
this may not always be the case:
(From: Lee Dunbar (dunbar@unitrode.com).)
Substitutions of low ESR caps into circuits
which had lousy caps is not always the good idea
that it appears to be.... Caution is advised, as
low ESR caps will not limit surge currents.
The circuits' series impedance drops (compare
substituted capacitors ESR when new with the
original capacitor's ESR was when it was a new
capacitor), which, in turn, lets the surge
magnitude rise, the higher currents destroy can
semiconductors and other components.
I guess what the industry needs is a good
capacitor cross reference guide for aluminum
electrolytics!
(From: Alan Liefting (aliefting@ihug.co.nz).)
- Replace all faulty components and check all
semiconductors and resistors.
- Remove switching MOSFET or disconnect its
supply voltage.
- Connect variable DC PSU to main secondary
voltage (one which is monitored).
- Connect an oscilloscope to the gate of the
switching MOSFET. As the DC voltage from the
external PSU is increased to the value of what
the SMPS output should be, a change in the
mark-space ratio of the PWM waveform can be
observed. Only a small change in the supplied
voltage around the SMPS output voltage is
required to observe the effect as the power
supply attempts to regulate.
If the supplied voltage is increased further
the PWM waveform will cease as the control
circuit will detect an overvoltage.
With this technique the whole power supply can
be tested without fear of destroying MOSFETs and
it can be determined if the SMPS is regulating to
the correct voltage (if known!).
This will not work with all SMPS's. It depends
on the design.
(From: John Croteau (croteau@erols.com).).
Switchmode power supply repairs can be
difficult. The problem is manufacturers don't
usually give you an easy test set up. They should
tell you if it will run at no load or what dummy
load to use. Secondly they should tell you what
voltage or resistance to use to replace the opto-isolator
(or transformer) for that load. The SMPS hot side
is a high frequency oscillator whose 'on time' is
varied by feedback supplied through the opto-isolator.
The troubleshooting procedure should normally be
in this order.
- First eliminate external causes such as
shorts or no load as the cause of the shutdown.
- Eliminate the secondary side shorted diodes,
capacitors, etc.
- After eliminating overloads on the outputs
check the DC supply to the power device.
- Check the bias coming from the feedback.
Trace the bias supplied by the feedback and try
to determine what is the correct bias for that
situation (usually no power same as start-up).
- If the bias is as on the schematic then
troubleshoot the hot (primary) side as any
oscillator.
- If the bias is wrong and there is no short
on the output then concentrate on why the
feedback doesn't supply the expected voltage
to bias the scillator on.
- If you work on many of the same type SMPS:
- Determine the normal load and make a dummy
load.
- Determine the value of resistance that is
created at the output of the opto-isolator
(hint: use Ohm's law). Then remove one leg of
the output of the opto-isolator and replace it
with a resistor as calculated.
By using a fixed load and cutting out the
feedback it is very easy to troubleshoot. Don't
forget to check the voltages and waveforms in
your test set-up and record them for future
reference.
(From: Russell Houlton (71101.2454@CompuServe.com))
I wanted to pass on some comments on repair of
switchmode power supplies. I've fixed a few
myself.
- I see quite a few where the filter
capacitors have failed. Not all electrolytic
capacitors are the same. You should get
capacitors that are rated for high frequency
service. Use of "normal" caps that one finds in
the local electronics stores are likely to go
bad in about a year. Not something a
professional who values his reputation wants to
see happen. In fact, I suspect that some
manufacturer fail to understand this and use the
wrong caps causing common failures in their
units. Especially units that may be subjected to
use in warmer areas. I see this mostly with
specialized devices rather then mass consumer
items.
I highly recommend the Panasonic HFS series
cap that can be bought from DigiKey (and other
places I'm sure). These unit are specially
designed for good size as well as use in
switching supplies. They are also rated for 105
degrees C as opposed to the more common 85
degrees C temperature rating. I have never had
to replace a HFS cap I installed, where I've had
to replace "common" caps in repair situations.
(No I don't sell the HFS or have stock in
DigiKey,
I'm just passing some info that has worked
well for me.) (Note: The HFS series caps have
been discontinued by Panasonic. They recommended
using the EEU-FC (FC series) as a replacment.
--- Sam.)
- SMPS usually try to regulate one of the
output voltages by using the switcher, usually
it's the output with the most power, but might
be the one that's most voltage critical. If the
filter caps go bad in the main output voltage,
the auxiliary output voltages will go high. The
SMPS may also start to make high-pitched sounds
as the ripple messes with the feedback system.
The aux output voltages may go so high that the
secondary regulator may go into foldback to
protect itself. I found this out the hard way.
It's really something that can kick you in the
pants because normally one would not check the
5V supply if the problem seems to be a bad 23.5
V regulator.
- Noisy (whining or buzzing) SMPSs that still
work tend to be either bad main output voltage
capacitors or bad electrolytics in the power
oscillator circuit. See the section:
Buzzing or other sounds from
SMPSs.
- Most SMPSs have also have a *minimum* power
draw requirement from their loads. This is
especially true of the main output voltage. If
not enough power is drawn from the supply, the
supply may not be stable and can not supply full
power on the auxiliary voltages. An example of
this is using a 250 W PC power supply just to
drive a disk drive. Without the heavy 5 V power
draw of a motherboard, the supply may not start
up reliable or provide the needed 12 V power for
the hard drives.
- I've also seen cases where one of the
voltage doubler caps will open up causing
failure in one of the switching transistor(s).
It will short out a single transistor unit, but
in a dual switching unit, the transistor
associated with the good cap will over-work
itself and open. An easy way to test is to
remove the other switching transistor (in a dual
unit) and apply power for 2 seconds. (Take all
prudent precautions for working around a live
and open unit!) Disconnect power and use your
voltmeter to check the voltage across each of
the caps. They should discharge at a roughly
equal rate. A bad cap will lose all it's voltage
in less then 2 seconds. A good cap will hold
it's charge much longer.
- Lastly, some unit that have the switch mode
power supply in the same enclosure as the CRT
will have a sync signal that comes from the
horizontal flyback transformer. This keeps the
SMPS in sync with the display so that the small
magnetic fields that are created by the SMPS
don't create a wavy pattern on the screen.
Something to remember if a newly re-assembled
unit shows a window screen like interference
pattern on the display.
(From: Bob Wilson (rfwilson@intergate.bc.ca).)
I really suggest you refer to a handbook on
basic switchmode power supply design for the
nitty-gritty.
I have a schematic of a 200 Watt PC power
supply, and I assure you that there are enough
cost-saving clever shortcuts in the design, that
unless you know a fair amount about the design of
switchers, it will just totally and completely
baffle you.
Nearly all 200 W PC power supplies are
*identical* knock offs of one-another (except for
the power-good comparator section). The
transformer has a +5 V output which is what is
regulated. It also has a +12 V output and a -12 V
output. The -5 V output is derived from the -12 V
output using a 7905 regulator. All transformer
outputs are related in voltage by the transformer
turns ratio. The power supply topology is a Half
Bridge, which normally requires a "buck section"
in each output (namely an inductor, catch diode
and capacitor). To vastly improve the cross
regulation between windings, a common core is used
to wind all the 3 output inductors on.
Basically, however, a 200 W PC power supply is
a half-bridge design, with a bridge-type voltage
doubler in front which simply rectifies 220 V, or
doubles 110 V to 220 V. So the thing is basically
a 220 Volt design.
The controller is typically a TI TL494 that
operates off the output of the supply. This means
that in order to start, there must already be an
output voltage present! How they do this is really
really clever, and also extremely confusing. The
power transformer is itself, self oscillating.
This generates a rudimentary output voltage that
allows the thing to bootstrap up to normal
operation, and the controller chip to take over.
The +12 Volt output is what is used to power
the PWM chip. Thus, the supply runs off its own
output. This is done to eliminate the need for
troublesome opto-coupler feedback. To boot itself
up (after all, there is no initial +12 V to allow
itself to start), the driver transformer is
modified (very cleverly) to form part of a
blocking oscillator. Thus the unit initially self
oscillates in a crude fashion until there is
enough voltage on the +12 V output to allow the
PWM to start, which then swamps out the self
oscillation and normal operation commences.
Since the controller resides on the output side
of the transformer, the drive to the 2 half-bridge
NPN bipolar transistors is by a driver transformer
(a direct connection cannot be made because the
transistors are on the primary side).
Frequency of operation is 50 kHz, which is low
by today's standards, but this means lower cost
transformer winding (Litz wire is not needed, for
example).
(From: Steve Bell (service@bell-electronics.freeserve.co.uk).)
There are not normally schematics available for
PC power supplies.
If the PC's are very old, some service manuals
included the PSU schematics - but these will be
very very old, like the early Apricot that used
the Astec AC9335, the Olivetti XT, and some PC's
that ran CPM like the LSI Octopus.
Most AT class computers never had schematics
available for their power supplies, the
manufacturers simply intended them to be swapped
out. Schematics were produced by the power supply
manufacturers for companies such as Compaq, Sun
Microsystems etc, but these where only released to
their authorized service centres. These are far
and few between and sign non disclosure
agreements, so schematics are not available to
anyone else. I used to work for one such company,
they repaired all of Sun's power supplies from
throughout Europe.
Here in the UK, and also in the USA, if a
'standard' AT or ATX power supply fails it's often
cheaper to replace. Many PC power supplies aren't
standard - we repair many from Compaq, Elonex, HP,
Apple, Sun etc that are non standard and can be
repaired far cheaper than the manufacturers
exchange unit. Also in many countries a swap out
isn't available or can be costly.
Even without schematics power supplies can be
repaired. The same type of circuitry repeats
itself. Older power supplies often used the TL494C
pwm IC, newer use the UC3842/3/4/5 series driving
MOSFETs. Some older still used the NE5560, SG3524
etc., many didn't use a PWM IC, but instead
discrete components. Data sheets on the IC's used
are very useful.
Considering the 'standard' 200 W AT PSU, these
usually use 2 T0220 power transistors, and at
power on a resistor, usually around 270 - 330 K
turn on the top transistor, and current flows in
the inverter transformer. A winding on the
secondary supplies a voltage that is rectified and
smoothed to feed the PWM IC and its drivers. These
then drive the 2 transistors via a transformer and
the power supply is running. The 5 V output
voltage is then sensed and fed back to the PWM IC
to maintain regulation. Most faults on these types
of power supplies involve the high value resistors
failing associated with the inverter transistors,
the transistors and associated components, bridge
rectifier shorted - also check inrush thermistor
for cracks or pinholes. Secondary rectifiers can
fail, and electrolytic caps can fail if the fan
stops. The PWM circuitry can always be fed with an
external DC supply and checked independently.
The other type of power supply, less common in
older power supplies, uses the UC3842 series PWM
IC driving a single MOSFET. This IC sits on the
primary side, so its ground is floating high. An
isolating transformer is needed when scoping
anything on the primary, with the scope ground
clip to primary ground.
Sourcing substitute components isn't normally
difficult. I'd start by obtaining some decent
transistor, diode and MOSFET data books so you can
relate faulty parts to what's available locally.
If the power supplies are all 200 W, a simple
resistive load will suffice for testing made from
large ally clad resistors on a substantial heat
sink. These will toast when running for longer, so
will need fan cooling - a cheap desk fan is
sufficient. If you are going to be repairing a lot
of varying power and type over a prolonged period,
it may be better to build an active load. This is
basically banks of 2N3055's (for 5V & 3.3V) and
2N3773 (for 12 V upwards) in series with low value
power resistors than can have their conduction
varied and current monitored by other circuitry.
You simply have a lead for each power supply type
you test/repair. Also don't forget to check the PG
o/p on the power supply.
When testing or operating a common PC
(computer) power supply without being connected to
its mainboard and peripherals, a substitute load
must be provided. This would be the case if you
wanted to determine whether a supply was good or
wanted to use the supply for other purposes.
To test the supply, you want to:
- Remove all of the (expensive) stuff -
mainboard, drives, etc. Unplug all of the power
supply connectors.
- Provide a dummy load to +5 and +12 outputs.
- Typical (but not always) color codes for PC
power supplies:
- Red: +5, Yellow: +12, Black: Gnd (Probably
case as well).
- White: -5, Blue: -12, Orange: Power_good
(output).
(Some newer supplies may have a +3.3 output
as well which may be green).
- PC power supplies (as well as most other
switchers) need a minimum load on +5 and
possibly on +12 as well. An amp (e.g., 5 ohms on
+5) should be enough.
I use an old dual beam auto headlight. It
adds a touch of class as well to an otherwise
totally boring setup. :-) You can also use auto
tail light bulbs or suitable power resistors or
old disk drives you don't really care about (you
know, those boat anchors).
- There are no sense lines. There is a 'Power_Good'
line which is an output from the power supply to
the mainboard and can be ignored unless you want
to connect it to an indicator to let you know
all the outputs are within specs (it may need a
pullup and I don't know its drive capability).
- Pinout for the standard PC and clone
connector (some companies like Compaq do NOT use
this type of connector, however.). Black (Gnd)
wires together for the P8 and P9 connectors when
installed to mainboard.
J8: Pin 1 = Power_Good J9: Pin 1 = Gnd
Pin 2 = +5 Pin 2 = Gnd
Pin 3 = +12 Pin 3 = -5
Pin 4 = -12 Pin 4 = +5
Pin 5 = Gnd Pin 5 = +5
Pin 6 = Gnd Pin 6 = +5
Note: for an XT only, J8-Pin 1 is Gnd,
J8-Pin 2 is no connect.
The peripheral connectors are: Pin 1: +12,
Pin 2 and 3: Gnd, Pin 4 = +5.
PC power supplies are often ideal for other
purposes but the required loads represent wasted
power. So, it would be nice to be able to
eliminate them. Unfortunately, it probably isn't
easy to modify a PC power supply so less/no load
is needed for regulation. However, it is worth
testing a supply to see how low you can actually
go on the loads - many WILL regulate the +5 with
no load on the +12 but probably not the reverse.
While 20 percent load is often recommended, 5
percent or less may work just fine. And, some
don't need any additional loads on either output
(they will probably include a minimal load
resistor internally).
CAUTION: Usually, a PC power supply will just
shut down with too little load. However, some may
be much more unhappy. Thus attempting to determine
a safe minimum load does entail some risk of
letting the smoke out.
The reason that you need a load is that the PWM
controller can't do down to really small duty
cycles needed for low loading. The design would
need to be changed and differs from unit to unit.
You don't need a fancy "mainboard simulator" or
"special ATX test tool" to run an ATX power supply
on the bench.
(From: Stefan Krommes (stkro@gmx.net).)
There is a wire (Power_Supply_On) on the ATX
connector that will turn on the main supply when
pulled low. This color of that wire is mostly
labeled on the side of the supply - or (as with
fortron supplies) it is the one that is not
labeled (green). If you look on top of the ATX
connector as if it was plugged into the board the
wire in question is the 4th from the right hand
side on the top row (where there is the clip) - a
look into your mainboard manual might give you a
visual idea since the ATX connector is often
depicted there with pins labeled.
As said if you fire up your supply it would be
wise to load it. Check the label on the supply and
load the main +5V and +12 V line to about 15-20%
of their max. current capability. - Some
automotive bulbs (headlight, brakelight, etc.)
come handy not only for the 12 V but for the main
5 V line too. Below about 20% load there is the
chance of the supply not regulating the voltage
properly but it should start.
The 5 V standby line does not need any load -
it should read a clean 5 VDC as soon as the supply
is plugged in and the mains power switch is
switched on.
The main supply should get on-line as soon as
the Power_Supply_On wire gets pulled to ground and
the voltages should measure within 5% if loaded to
about 20% of maximum power.
(From: Sam.)
CAUTION: I have seen one case where an ATX
supply actually blew up without a load using this
"hot wire" technique. I don't know if there was
already a problem with the unit or it really
absolutely required a load. But almost immediately
after grounding the green wire, multiple
electrolytic caps on the secondary side exploded
and spilled their guts, one of the MOSFETS
shorted, and then the power fuse blew. :(
It may be that the designers of this supply in
their infinite wisdom assumed that since power is
enabled via the mainboard, there would never be a
circumstance where there wouldn't be at this that
as a load!
(From: Arthur Jernberg (stubbie45@hotmail.com).)
Here is the pinout for an ATX mainboard:
Left to right: First row voltage; Second row
current/color:
[+3.3V] [+5V] [+12V] [-12V] [-5V] [+5VSB] [COM] [P-ON] [PG]
[14A] [22A] [9A] [1A] [.5A] [PUR] [BLK] [GRN] [GRY]
(From: Sam.)
IMPORTANT CAUTION: Apparently, Dell, who has
followed industry standards in most respects,
changed the ATX power supply pinout on their PCs
sometime in 1998 and may still be using this
proprietary pinout. See:
Dell Proprietary (Non-Standard) ATX Design.
Since there is absolutely no valid technical
reason for doing this, one can only assume it is
due to some, shall we say, shady business decision
to prevent people from going to a third party for
upgrades or replacement mainboards or power
supplies. Worse, installing a non-Dell power
supply with a Dell mainboard will result in a
destroyed power supply and possible damage to the
mainboard. Thanks Dell. :( It's straightforward
though not trivial to change the power supply
pinout back to standard (but it isn't a matter of
just moving pins around in the connector since the
number of wires for some signals has also
changed). But there should be no need. Companies
should compete by selling a better product, not a
closed system.
(From: Tony Duell (ard@p850ug1.demon.co.ku).)
In case you have been lucky enough not to have
come across the beast, the basic idea is:
- Rectify/smooth mains, giving 340V DC.
- Put that into a non-isolated step-down
switching converter using a fat power
transistor, a freewheel diode and an inductor.
The control IC for this is a 723 (!) with
feedback applied via an optocoupler from the
secondary side. Oh, and there's a current-limit
shutdown from a 0R15 resistor in series with the
'load' (section 3, etc). The output of this
stage is about 150V
- The output of that feeds a pair of
transistors that run as a free-running push/pull
oscillator. That drives the primary of the main
chopper transformer (which provides isolation
from the mains).
- The secondaries of that transformer are
rectified to provide output voltages. There's a
feedback to (2) as described, and also a
crowbar.
Now for the failure mode (seen it happen):
- The first chopper goes short-circuit because
it feels like it. The 150 VDC line jumps to 340
VDC (remember, no isolation, and the
inductor/transistor are in series from the 340
VDC line to the 150V line).
- The crowbar fires, shorting the output. The
primary-side current goes up as well.
- The overcurrent trip operates, and the 723
tries to shut down. It removes the drive from
the *first* chopper. But that's shorted, so
removing the drive doesn't do a darn thing.
- The 2 transistors in the push/pull circuit
short as well.
- Rectified mains is now connected acros : 3
shorted transistors, a few small windings
(oscillator feedback transformer, inductor,
etc), and the 0R15 resistor. The latter
explodes.
- 340 VDC is now applied across the
connections of the ex-0R15 resistor. A few small
transistors and the 723 expire, along with some
PCB tracks.
- The fuse (10 A, FF blow) fails.
Repairing that was an entertainment. I still
have my photocopy of the schematic with all the
dud components circled!
(From: Kevin Beeden (kevin.beeden@rrl.co.uk).)
I was fighting with a Hitachi PSU (SP-13A unit
from UK VT-F860E VCR).
Symptoms: Following line power disconnect,
wouldn't start up. Traced circuit, found broken Tr
in LV side, replaced, no good. No gate drive.
Trouble figuring gate drive cct. Applied 30 VDC to
HV side and drove gate from 10 kHz signal
generator (5 V squarewave via 1K resistor). The
MOSFET switcher was okay, output active but won't
start. Startup drive is via a 1 uF, 250 V cap.
Can't see any drive on gate when power applied
from cold. Read these notes. Doh! Capacitor gone
open circuit. Replace cap and now get gate drive
until output settles.
I was happily surprised to find that the
circuit worked off of 30 VDC, as I'm not keen on
debugging live mains or 400 VDC! This may be a top
tip for fellow debuggers. I simply connected 30
VDC to the post-rectification smoothing cap, hit
the gate with the signal generator, and away it
went. Note: this is under no load conditions; the
PSU was removed from the VCR.
I also found a devious little component
masquerading as a resistor. It appears to actually
be back-to-back (Schottky? 1V3) diodes and a 220
ohm resistor in some series or parallel
arrangement. Marked externally as 220R, identical
appearance to other resistors.
On a typical SMPS (or piece of equipment like a
TV, monitor, or VCR using an SMPS), the nameplate
current rating may be much much smaller than the
fuse rating (or equivalently, the nameplate power
rating may imply a much smaller current). Why is
this the case?
There are actually many reasons for the fuse to
be much larger than the maximum current specified
on the nameplate. Some of these include:
- The peak current during power on is actually
much higher due to the degauss coil and charging
of the SMPS caps.
- ?The power factor is much smaller than unity
for a typical SMPS and this increase the average
current - which is what the fuse sees.
- The fuse for the SMPS is really there to
protect against catastrophic failure so it can
be much larger than the average expected
current. If that fuse blows, it's usually the
result of the failure of the main chopper
transistor, not a slight overload (for which the
SMPS should shut down).
Replacing the fuse with a smaller one really
won't make the equipment any safer but may result
in nuisance blowing.
Back to SMPS Repair FAQ
Table of Contents.
Service Information
If the solutions to your problems have not been
covered in this document, you still have some
options other than replacement.
(Also see the related document:
Troubleshooting and Repair of Consumer Electronic
Equipment.
Manufacturer's service literature: Service
manuals may be available for for your equipment.
Once you have exhausted other obvious
possibilities, the cost may be well worth it.
Depending on the type of equipment, these can
range in price from $5-100 or more. Sometimes,
these may even be free (yes, even in this day and
age where you have to pay for free air at your
local gas station!) Some are more useful than
others. However, not all include the schematics so
if you are hoping to repair an electronic problem
try to check before buying.
Inside cover of the equipment: TVs often have
some kind of circuit diagram pasted inside the
back cover. In the old days, this was a complete
schematic. Now, if one exists at all for a
monitor, it just shows part numbers and location
for key components - still very useful.
Sams Photofacts: These have been published for
over 45 years mostly for TVs and radios. There are
some for VCRs and a few for some early PC monitors
and other pre-Jurassic computers. However, for the
power supplies in TVs, there will nearly always be
a Sams with complete schematics.
Whatever the ultimate outcome, you will have
learned a great deal. Have fun - don't think of
this as a chore. Electronic troubleshooting
represents a detective's challenge of the type hat
Sherlock Holmes could not have resisted. You at
least have the advantage that the electronics do
not lie or attempt to deceive you (though you may
beg to differ at times). So, what are you waiting
for?
Many companies now have very extensive
information available via the World Wide Web. Here
are a few company sites:
The following site has a variety of information
and links to SMPS related sites:
There is a nice detailed article on PC power
supply repair from Nuts and Volts Magazine on-line
at:
PC Power Supply Repair by T. J. Byers. This is
probably a good place to start if you are
specifically interested in the common PC power
supply. It looks like this article is pre-ATX but
most of the information still applies.
Also see the document:
PC Switchmode Power Supplies.
Texas Instruments has an application note which
is sort of the "short" version of SMPS repair.
See:
Off-Line SMPS Failure Modes PWM Switchers and
DC-DC Converters.
Here are some suggested books with information
relating to SMPS and DC-DC converter design,
testing, troubleshooting, repair, etc.:
- Power Supplies, Switching Regulators,
Inverters & Converters, 2nd Edition.
Irving Gottleib
TAB Books, 1994
ISBN 0-8306-4404-0
- Modern DC-to-DC Switchmode Power Converter
Circuit
Rudolf P. Severns and Gordon E. Bloom
Van Nostrand Reinhold
ISBN 0-442-21396-4
- Principles of Solid State Power Conversion
Ralph E. Tarter
Howard W. Sams & Co., Inc.
ISBN 0-672-22018-0
- Advances in Switched-Mode Power Conversion
R.D. Middlebrook & Slobodan Cuk
Contact: TeslaCo, Pasadena, CA 91107 (last known
address)
- Simplified Design of Switching Power
Supplies
John D. Lenk
Butterworth-Heinemann
ISBN 0-7506-9821-7.
- Power Electronics, 2nd ed.
B.W. Williams
McGraw-Hill, 1992
ISBN 0-07-070439-2
- Switching Power Supply Design
Abraham Pressman, Second Edition
McGraw-Hill, 1998
ISBN 0-07-052236-7.
ISBN 0-07-050806-2 (First Edition, 1991).
- Switch Mode Power Conversion
K. Kit Sum
- Switchmode Power Supply Handbook
Keith Billings
McGraw-Hill, 1989
ISBN 0-07-005330-8
- Power Supply Cookbook
Marty Brown
ISBN 0-7506-9442-4
- Transformer and Inductor Design Handbook
Colonel Wm. T. McLyman (yes, his 1st name is
"Colonel" - not military)
Marcel Dekker, Inc.
ISBN 0-8247-6801-9
- Magnetic Core Selection for Transformers &
Inductors
Colonel Wm. T. McLyman
Marcel Dekker, Inc.
ISBN 0-8247-1873-9
- Magnetic Components, Design and Applications
Steve Smith
ISBN 0-442-20397-7.
- Soft Ferrites, 2nd ed.
E.C. Snelling
Butterworth, 1988
ISBN 0-408-02760-6
- High Frequency Switching Power Supplies,
Theory and Design
George Chryssis
McGraw-Hill Co., 1984
- Switched Mode Power Supplies - Design and
Construction
Whittington, Flynn and Macpherson published
Research Studies Press, Ltd.
ISBN 0-863-80203-6
In addition, many smeiconductor manufacturers
publish extensive information on switchmode
technology. Mostly, this is in connection with
their product lines but will also contain a lot of
general information. Much of this is available on
Internet via the World Wide Web. Companies
include: Maxim, Linear Technology, and Unitrode.
(From: OneStone (OneStone@bigpond.com).)
"Try the
Linear Technologies Website.
Look for their App notes:
- AN25 Switching regulators for Poets.
- AN19 LT1070 Design manual
- AN29 Some thoughts on DC-DC converters
- AN30 Switching regulator circuit
collection.
- AN31 Linear Circuits for digital systems.
Then try one of their new data sheets, such
as the LT1370, for some modern circuit
configurations, such as SEPIC converters. The
above APP notes are all contained in their
Linear Applications handbook, Volume 1, 1990. If
you are a designer they also have a CD-ROM
available, which includes some switcher and
filter design software. It's a bit limited, but
a great starting point if you don't need to
stretch the boundaries."
For diagnosing power problems in TVs and
Computer or Video monitors, here is one book that
includes many illustrations and case histories.
- Troubleshooting and Repairing Solid State
TVs
Homer L. Davidson
2nd Edition, 1992
TAB Books, Inc.
Blue Ridge Summit, PA 17214
(From: Ernst C. Land, Jr. (a6mech@ionet.net)
and Mark Zenier (mzenier@eskimo.com or mzenier@netcom.com).)
The September 1996 (VOL. 17 NO. 9) issue of
Nuts & Volts Magazine has a great article on
theory, troubleshooting, and repair of PC power
supplies. Their web site is: http://www.nutsvolts.com.
When you get there, click on [more], then [back
issues]
I have found one of the most useful single
sources for general information on semiconductors
to be the ECG Semiconductors Master Replacement
Guide, about $6 from your local Philips
distributor. STK, NTE, and others have similar
manuals. The ECG manual will enable you to look up
U.S., foreign, and manufacturer 'house' numbers
and identify device type, pinout, and other
information. Note that I am not necessarily
recommending using ECG (or other generic)
replacements if the original replacements are (1)
readily available and (2) reasonably priced.
However, the cross reference can save countless
hours searching through databooks or contacting
the manufacturers. Even if you have a wall of
databooks, this source is invaluable. A couple of
caveats: (1) ECG crosses have been known to be
incorrect - the specifications of the ECG
replacement part were inferior to the original.
(2) Don't assume that the specifications provided
for the ECG part are identical to the original -
they may be better in some ways. Thus, using the
ECG to determine the specifications of the parts
in your junk bin can be risky.
Other cross reference guides are available from
the parts source listed below.
Many manufacturers are now providing extensive
information via the World Wide Web. The answer to
you question may be a mouse click away. Perform a
net search or just try to guess the manufacturer's
home page address. The most obvious is often
correct. It will usually be of the form "http://www.xxx.com"
where xxx is the manufacturers' name,
abbreviation, or acronym. For example, Hewlett
Packard is hp, Sun Microsystems is sun, Western
Digital Corp. is wdc. NEC is, you guessed it, nec.
It is amazing what is appearing freely accessible
via the WWW. For example, monitor manufacturers
often have complete information including detailed
specifications for all current and older products.
Electronic parts manufacturers often have detailed
datasheets and application notes for their product
offerings.
Here is a non-commercial Web site that has some
information and sample circuits (not necessarily
tested!) on using Maxim parts in various DC-DC
converter applications as well as a bunch of links
to relevant SMPS chip manufacturers:
The question often arises: If I cannot obtain
an exact replacement or if I have a monitor, TV,
or other equipment carcass gathering dust, can I
substitute a part that is not a precise match?
Sometimes, this is simply desired to confirm a
diagnosis and avoid the risk of ordering an
expensive replacement and/or having to wait until
it arrives.
For safety related items, the answer is
generally NO - an exact replacement part is needed
to maintain the specifications within acceptable
limits with respect to line isolation, X-ray
protection and to minimize fire hazards. Typical
parts of this type include flameproof resistors,
some types of capacitors, and specific parts
dealing with CRT high voltage regulation. However,
during testing, it is usually acceptable to
substitute electrically equivalent parts on a
temporary basis. For example, an ordinary 1 ohm
resistor can be substituted for an open 1 ohm
flameproof resistor to determine if there are
other problems in the the SMPS chopper before
placing an order as long as you don't get lazy and
neglect to install the proper type before
considering the repair complete.
For other components, whether a not quite
identical substitute will work reliably or at all
depends on many factors. Some deflection circuits
are so carefully matched to a specific horizontal
output transistor that no substitute will be
reliable.
Here are some guidelines:
- Fuses - same type (usually normal or fast
blow), exact same current rating and at least
equal voltage rating. I have often soldered a
normal 3AG size fuse onto a smaller blown 20 mm
long fuse as a substitute.
- Resistors, capacitors, inductors, diodes,
switches, potentiometers, LEDs, and other common
parts - except for those specifically marked as
safety-critical - substitution as long as the
replacement part fits and specifications should
be fine. It is best to use the same type - metal
film resistor, for example. But for testing,
even this is not a hard and fast rule and a
carbon resistor should work just fine.
- Rectifiers - many of these are high
efficiency and/or fast recovery types.
Replacements should have equal or better PRV,
Imax, and Tr specifications. However, the AC
input bridge can usually be replaced with
anything with at least equal voltage and current
ratings.
- Main filter capacitor(s) - use replacements
with at least equal working voltage and similar
uF rating. For testing, even something with half
the capacity will be fine. For the final
replacement bigger is not always better and even
using a smaller one (uF) will be fine as long as
you are not running the supply near full load
capacity. Use of a higher temperature rated
capacitor than the original may be desirable as
its life may have been shorteded by a hot
environment. This practice will never hurt.
- Transistors and thyristors (except SMPS
choppers or HOTs) - substitutes will generally
work as long as their specifications meet or
exceed those of the original. For testing, it is
usually OK to use types that do not quite meet
all of these as long as the breakdown voltage
and maximum current specifications are not
exceeded. However, performance (like regulation
specifications) may not be quite as good. For
power types, make sure to use a heatsink.
- SMPS chopper (or horizontal output)
transistors - exact replacement is generally
best but except for very high performance
monitors, generic HOTs that have specifications
that are at least as good will work in many
cases. Make sure the replacement transistor has
an internal damper diode if the original had
one. For testing with a series light bulb, even
a transistor that doesn't quite meet
specifications should work well enough (and not
blow up) to enable you to determine what else
may be faulty. The most critical parameters are
Vceo/Vcbo, Ic, and Hfe which should all be at
least equal to the original transistor. I have
often used by favorite BU208D as a temporary
substitute for other HOTs and SMPS (chopper)
transistors. Make sure you use a heatsink (with
insulating washer if applicable) and thermal
grease in any case - even if you have to hang
the assembly with a cable-tie to make it fit.
Also see the section:
Replacement power transistors while testing.
The following are usually custom parts and
substitution of something from your junk box is
unlikely to be successful even for testing:
flyback (LOPT) and SMPS transformers, interstage
coils or transformers, microcontrollers, and other
custom programmed chips.
Substituting entire circuit boards and other
modules from identical models is, of course,
possible and an excellent troubleshooting aid even
if it is only used to confirm or identify a bad
part. However, if the original failure was
catastrophic, you do run some risk of damaging
components on the substitute circuit board as
well.
During testing of horizontal deflection
circuits or switchmode power supplies,
particularly where the original failure resulted
in the death of the HOT or chopper, overstress on
replacement transistors is always a possibility if
all defective components have not be identified.
Therefore, using a part with better
specifications may save you in the long run by
reducing the number of expensive blown parts. Once
all other problems have been located and repaired,
the proper part can be installed.
However, this is not always going to work. In a
TV and especially a high performance monitor, the
HOT may be closely matched to the drive and output
components of the deflection circuits. Putting in
one with higher Vce, I, or P specifications may
result in overheating and failure due to lower Hfe.
Where possible, a series load like a light bulb
can be used limit the maximum current to the
device and will allow you to power the equipment
while checking for other faults. Some designs,
unfortunately, will not start up under these
conditions. In such cases, substituting a 'better'
device may be the best choice for testing.
(From: Glenn Allen (glenn@manawatu.gen.nz).)
I been repairing SMPS of all types but when I
started on those using MOSFETs I was blowning a
few of them when replaced because something else
was faulty.
Ever since I have been using a BUZ355 on a heat
sink I haven't blown it. It is rated at 800 V, 6
A, and 220 W. it is a TO218 case bigger than a
T0220. It seems the higher ratings allows you to
do repair where as a something like a 2SK1117 or
MTP6N60 will just blow.
The following is useful both to confirm that a
substitute replacement chopper transistor is
suitable and that no other circuit problems are
still present. However, this will not catch single
shot events that may blow the transistor instantly
without any increase in temperature.
It was written with TV and monitor horizontal
output transistors in mind but applies to the
switchmode/chopper transistors found in line
powered SMPSs as well.
(From: Raymond Carlsen (rrcc@u.washington.edu).)
After installing a replacement HOT in a TV set
or monitor, I like to check the temperature for
awhile to make sure the substitute is a good match
and that there are no other problems such as a
weak H drive signal. The input current is just not
a good enough indicator. I have been using a WCF
(well calibrated finger) for years. For me, the
rule of thumb, quite literally, is: if you can not
hold your finger on it, it's running too hot, and
will probably fail prematurely. Touching the case
of the transistor or heat sink is tricky....
Metal case transistors will be connected to the
collector and have a healthy pulse (>1,200 V
peak!) and even with plastic case tab transistors,
the tab will be at this potential. It is best to
do this only after the power is off and the B+ has
discharged. In addition, the HOT may be hot enough
to burn you.
A better method is the use of an indoor/outdoor
thermometer. I bought one recently from Radio
Shack for about $15 (63-1009). It has a plastic
'probe' on the end of a 10' cable as the outdoor
sensor. With a large alligator clip, I just clamp
the sensor to the heat sink near the transistor
and set up the digital display near the TV set to
monitor the temperature. The last TV I used it on
was a 27" Sanyo that had a shorted H. output and
an open B+ resistor. Replacement parts brought the
set back to life and the flyback pulse looked OK,
but the transistor was getting hot within 5
minutes... up to 130 degrees before I shut it down
and started looking for the cause. I found a 1 uF
160 volt cap in the driver circuit that was open.
After replacing the cap, I fired up the set again
and monitored the heat sink as before. This time,
the temperature slowly rose to about 115 degrees
and stayed there. I ran the set all day and
noticed little variation in the measurement. Test
equipment doesn't have to cost a fortune.
For general electronic components like
resistors and capacitors, most electronics
distributors will have a sufficient variety at
reasonable cost. Even Radio Shack can be
considered in a pinch.
However, for modern electronic equipment
repairs, places like Digikey, Allied, and Newark
do not have the a variety of Japanese
semiconductors like ICs and transistors or any
components like flyback transformers or degauss
Posistors.
See the document:
Major Service Parts Suppliers for some
companies that I have used in the past and others
that have been recommended.
In addition, specifically for VCR SMPS repair:
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