Author: Samuel M. Goldwasser
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Copyright (c) 1994, 1995
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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!
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.
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.
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.
Symptoms: totally dead supply, fuse blows instantly (unless fusable resistor has opened). Measuring across C-E or D-S of switchmode transistor yields near ohms even when removed from circuit.
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 as the supply attempts to restart and then shuts down.
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).
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. There will be full voltage on the main filter capacitor(s) (1x or 2x peak - around 150 or 300 VDC depending on design.)
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 across main filter capacitor(s). If 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.
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.
In almost all cases, when in doubt parallel a known good capacitor of similar capacitance and at least equal voltage rating.
For Panasonic (and other) VCR power supplies, it has been suggested 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.
Symptoms: almost any kind of intermittent behavior is possible. Visually inspect 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.
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 optocoupler and its associated circuitry. A weak optocoupler may allow for excessive output voltage. A shorted photodiode in the optocoupler may prevent startup. An open photodiode may lead to a runaway condition. WARNING: probe these circuits with care both as a result 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.
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.
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.
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.
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.