Notes on the diagnosis and repair of small Switchmode Power Supplies V1.01

Contents:


1) About the Author

Here are the current version of my 'Notes on the diagnosis and repair of small switchmode power supplies'

Author: Samuel M. Goldwasser
E-Mail: sam@stdavids.picker.com
Corrections/suggestions: [Feedback Form] [mailto]
Copyright (c) 1994, 1995
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.

2) Safety:

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 300 V or greater DC when powered (often, even if the supply does not work correctly) and for some time after being unplugged.

There is also risk of instantly destroying expensive parts of the supply (like the switchmode power transistor) if your probe should slip and short something.

General Safety Guidelines when working on line powered equipment including: TVs, monitors, and microwave ovens.

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!


3) Power Supply Fundamentals

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 Linear Power Supply (LPS) such you would find in most audio or precision analog equipment includes a power transformer which converts the 115/230 VAC 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 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 hybrid regulators for VCRs. The regulator circuit essentially compares the output with a reference and adjusts the current flow to make the output 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%.

Notable characteristics of LPSs are excellent regulation and low output noise.

4) What is a Switch Mode Power Supply?

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 Thyristers (SCRs or Triacs) to take directly rectified line voltage and convert it to a pulsed waveform. 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 230 VAC. Up to this point, there is no line isolation as there is no power transformer.

A 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 in less critical applications, just a capacitor.

This high frequency transformer provides the isolation barrier.

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.

Probably the most common topology for small switchers is the flyback circuit shown below: 

             CR1                              CR2           L
   o---------|>|---+----+-------_  T1  _------|>|-----+---UUUUU---+---+----o V+
       line rect.  |    |       _) || (_             _|_         _|_  |
                   |    \ R1    _) || (_           C ---       C ---  |
  AC               |    /       _) || (_______________|___________|__ | ___o V-
 Line             _|_   \       |                                     |
  in       filter ---   |     |/       +-------+   +-----------+   +-----+
            cap    |    +-----+--------|  PWM  |<--| Isolation |<--| REF |
                   |       Q1 |\       +-------+   +-----------+   +-----+
                   |            |
   o---------------+------------+

The input to the supply is the AC line which may have RFI and surge protection (not shown). There may be several Ls and Cs to minimize the input as well as the radiation of radio frequency interference. There may be MOV type of surge suppressors across the three input leads (H, N, G).

Rectification is usually via a voltage doubler or bridge. One common circuit uses a bridge rectifier as a doubler or normal bridge by changing a jumper. The voltage across the switching transistor is usually designed to be around 150-300 V.

When Q1 turns on, current increases linearly in T1 based on the voltage applied and the inductance of the transformer. When Q1 turns off, the field collapses and 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 pie filter on the output smoothes the pulses provided by CR2. Sometimes, a full wave configuration is used with a center tapped transformer secondary.

A reference circuit monitors the primary output and controls the duty cycle of the switching pulses to maintain a constant output voltage. (Secondary outputs are not shown.)

R1 is the startup resistor (some startup circuits are more sophisticated) and provides the initial current to the switchmode transistor base. The PWM circuit guarantees that Q1 will not be turned on continuously.

Most small SMPSs use opto-isolators for the feedback. An opto-isolator is simply an LED and a photodiode in a single package. Typically, a 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.

Where additional regulation is needed, small linear regulators may also be included at the output(s).

There are many other topologies for switching power supplies but 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.

The advantages of implementing the switch mode operation are with respect to size, weight, and efficiency.

Since the transformer and final filter(s) run at a high frequency (we are talking about 10KHz to 1 MHz or more), they can be much smaller and lighter than the big bulky components needed for 50/60 Hz operation. 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% with improvements being made in technology continuously. Since the advent of the laptop computer, portable 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.

5) Where are SMPSs uses?

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, laptops and their power packs, external peripheral boxes, X-terminals, TVs, some VCRs, Camcorder AC adapters, other video equipment.

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.

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.

6) Troubleshooting

The diagnosis is often difficult 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.

7) Notes on SMPSs in TVs and Monitors

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 basic comments below 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.

8) SMPS Failure Modes

SMPS fail in many ways but the following are common:


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.

All other parts are readily available from places like MCM Electronics, Dalbani, Premium Parts, and other national distributors.

Also, while it is tempting to suspect any ICs or hybrid controllers, these parts are pretty robust unless a catastrophic failure elsewhere sent current where it should not have gone.

9) Repair Comments

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. It only takes a few more minutes. However, for other problems like an open startup resistor or dried up capacitor, this excessive caution is unnecessary as these are usually isolated failures.

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. 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.

10) Flameproof Resistors in Switch Mode Power Supplies

'FR' means 'Flameproof Resistor' or 'Fusable Resistor'. They are the same. 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.

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 emitter circuit of a bipolar switchmode transistor. The value will usually be a fraction of an ohm. For testing ONLY, a normal resistor may be substituted but the proper replacement MUST be installed before returning the supply to service.

In TVs and monitors, these are often found in the hot supply side 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.

11) Unusual Components

The following are some other types of parts that you may find in a SMPS and may not be familiar to you:

MOVs (Metal Oxide Varisters)
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.

NTC (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.

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. Very reliable.

Bypass Capacitors
High quality plastic dipped or rectangular molded capacitors as part of RFI filter. Rarely fail.

High Frequency Transformer
The large transformer which provides line isolation and voltage conversion from the line. These are usually custom and replacements are only available from the manufacturer. However, some distributors will stock replacements for a few TVs and computer monitors.

Optoisolator
Either a 4 or 6 pin DIP or a 4 pin cylindrical object. 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.

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.

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.

12) Initial Post-Repair Testing

Once defective parts have been replaced, if possible remove the normal load from the supply 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 primary 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. If you do not have a Variac, put a lightbulb 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 lightbulb 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 lightbulb 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.

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.

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