DIY repair switching power supplies

In detail: DIY repair switching power supplies from a real master for the site my.housecope.com.

Authors: Baza, NMD, plohish, mikkey, VOvan, NiTr0, ezhik97, inch, Mr. Barbara.
Editing: Mazayac.

Important links that have become difficult to find:

There is no better book on the principles of BP operation. Read to everyone! Power supplies for system modules such as IBM PC-XT / AT.

What is desirable to have to check the power supply.
a. - any tester (multimeter).
b. - bulbs: 220 volts 60 - 100 watts and 6.3 volts 0.3 amperes.
v. - soldering iron, oscilloscope, solder suction.
d. - magnifying glass, toothpicks, cotton swabs, industrial alcohol.

The safest and most convenient way to connect the repaired unit to the network is through an isolation transformer 220v - 220v.
It is easy to make such a transformer from 2 TAN55 or TS-180 (from lamp b / w TVs). The anode secondary windings are simply connected appropriately, there is no need to rewind anything. The remaining filament windings can be used to build an adjustable power supply.
The power of such a source is quite sufficient for debugging and initial testing and provides a lot of convenience:
- electrical safety
- the ability to connect the earths of the hot and cold parts of the block with a single wire, which is convenient for recording oscillograms.
- we put on the biscuit switch - we get the ability to stepwise change the voltage.

Also, for convenience, you can bypass the + 310V circuits with a 75K-100K resistor with a power of 2 - 4W - when turned off, the input capacitors are discharged faster.

If the board is removed from the unit, check for any metal objects of any kind underneath. In no case, DO NOT CUT HANDS into the board and DO NOT TOUCH the heatsinks while the unit is operating, and after turning off, wait about a minute for the capacitors to discharge. The radiator of power transistors can have 300 or more volts, it is not always isolated from the block circuit!

Video (click to play).

Principles of voltage measurement inside the block.
Please note that the ground from the board is fed to the PSU case through the conductors near the holes for the fastening screws.
To measure voltages in the high-voltage ("hot") part of the unit (on power transistors, in the duty room), a common wire is required - this is a minus of the diode bridge and input capacitors. With respect to this wire, everything is measured only in the hot part, where the maximum voltage is 300 volts. Measurements are preferably carried out with one hand.
In the low-voltage ("cold") part of the power supply unit, everything is simpler, the maximum voltage does not exceed 25 volts. For convenience, you can solder wires to the test points, it is especially convenient to solder the wire to the ground.

Checking resistors.
If the denomination (colored stripes) is still readable, we replace it with new ones with a deviation no worse than the original (for most - 5%, for low-resistance current sensor circuits it can be 0.25%). If the coating with the marking has darkened or crumbled from overheating, we measure the resistance with a multimeter. If the resistance is zero or infinity, the resistor is most likely defective and to determine its value, a schematic diagram of the power supply or the study of typical switching circuits will be required.

Diode test.
If the multimeter has a diode voltage drop measurement mode, it can be checked without unsoldering. The drop should be from 0.02 to 0.7 V. If the drop is zero or so (up to 0.005), we solder the assembly and check. If the readings are the same, the diode is broken. If the device does not have this function, set the device to measure resistance (usually the limit is 20 kOhm). Then, in the forward direction, a serviceable Schottky diode will have a resistance of the order of one to two kilo-ohms, and a conventional silicon one - of the order of three to six. In the opposite direction, the resistance is equal to infinity.

To check the power supply, you can and should collect the load.
An example of successful execution can be found here.
Pinout of the ATX 24 pin connector, with OOS conductors on the main channels - + 3.3V; + 5V; + 12V.

You can first turn on the power supply unit to the network in order to determine the diagnosis: there is no duty officer (there is a problem with the duty room, or a short circuit in the power unit), there is a duty room, but there is no start (problem with buildup or PWM), the power supply goes into protection (most often - the problem is in output circuits or capacitors), overestimated duty room voltage (90% - swollen capacitors, and often as a result - dead PWM).

Initial block check
We remove the cover and start checking, paying special attention to damaged, discolored, darkened or burnt parts.

Darkening or burnout of the printed circuit board under the resistors and diodes indicates that the components of the circuit were operating in an abnormal mode and an analysis of the circuit is required to find out the cause. The detection of such a place near the PWM means that the 22 Ohm PWM power resistor is heating up from exceeding the standby voltage and, as a rule, it is he who burns out first. Often the PWM is also dead in this case, so we check the microcircuit (see below). Such a malfunction is a consequence of the work of the "duty officer" in an abnormal mode, it is imperative to check the circuit of the standby mode.

Checking the high-voltage part of the unit for a short circuit.

We take a light bulb from 40 to 100 watts and solder it instead of a fuse or into a break in the mains wire.
If, when the unit is turned on, the lamp flashes and goes out - everything is in order, there is no short circuit in the “hot” part - we remove the lamp and work further without it (put the fuse in place or splice the mains wire).
If, when the unit is connected to the network, the lamp lights up and does not go out, there is a short circuit in the unit in the “hot” part. To detect and eliminate it, we do the following:

We solder the radiator with power transistors and turn on the power supply through the lamp without shorting PS-ON.
If it is short (the lamp is on, but not on and off), we are looking for the cause in the diode bridge, varistors, capacitors, 110 / 220V switch (if any, it is generally better to evaporate it).
If there is no short, we solder the duty room transistor and repeat the turn-on procedure.
If there is a short one, we are looking for a malfunction in the duty room.

Attention! It is possible to turn on the unit (via PS_ON) with a small load when the light is not turned off, but firstly, unstable operation of the power supply is not excluded, and secondly, the lamp will glow when the power supply is turned on with the APFC circuit.

Checking the scheme of the duty mode (duty officer).

Quick guide: we check the key transistor and its entire strapping (resistors, zener diodes, diodes around). We check the zener diode in the base circuit (gate circuit) of the transistor (in circuits on bipolar transistors, the nominal value is from 6V to 6.8V, on field ones, as a rule, 18V). If everything is normal, we pay attention to the low-resistance resistor (about 4.7 Ohm) - the power supply of the standby transformer winding from + 310V (used as a fuse, but sometimes the standby transformer also burns out) and 150k

450k (from there to the base of the standby key transistor) - start offset. High-resistance ones often go into a break, low-resistance ones - they also “successfully” burn out from current overload. We measure the resistance of the primary winding of the trance on duty - it should be about 3 or 7 ohms. If the transformer winding is open (infinity), we change or rewind the trance. There are times when, with normal primary resistance, the transformer is inoperative (there are short-circuited turns). Such a conclusion can be made if you are sure that all other elements of the duty room are in good working order.
We check the output diodes and capacitors. If available, we must change the electrolyte in the hot part of the duty room to a new one, solder in parallel to it a ceramic or film capacitor 0.15. 1.0 μF (an important revision to prevent it from "drying out"). We unsolder the resistor leading to the PWM power supply.Next, we hang a load in the form of a 0.3Ax6.3 volt light bulb at the + 5VSB (purple) output, turn on the unit to the network and check the output voltages of the duty officer. One of the exits should be +12. 30 volts, on the second - +5 volts. If everything is in order, we solder the resistor in place.

Checking the PWM chip TL494 and similar (KA7500).
About the rest of the PWM will be written additionally.

We connect the block to the network. The 12th leg should be about 12-30V.
If not, check the duty room. If there is - check the voltage on the 14th leg - it should be + 5V (+ -5%).
If not, we change the microcircuit. If there is, we check the behavior of 4 legs when the PS-ON is shorted to ground. Before closing, it should be about 3.5V, after - about 0.
We install a jumper from the 16th leg (current protection) to the ground (if not used, it is already sitting on the ground). Thus, we temporarily disable the MS current protection.
We short-circuit PS-ON to ground and observe pulses at 8 and 11 PWM legs and further at the bases of key transistors.
If there are no pulses on 8 or 11 legs or the PWM is heating up, we change the microcircuit. It is advisable to use microcircuits from well-known manufacturers (Texas Instruments, Fairchild Semiconductor, etc.).
If the picture is beautiful, the PWM and the swing stage can be considered alive.
If there are no pulses on the key transistors, we check the intermediate stage (buildup) - usually 2 pieces of C945 with collectors in the buildup trance, two 1N4148 and capacitances of 1N4148 and capacitance 1N4148 at 50V, diodes in their harness, the key transistors themselves, soldering the legs of the power transformer and isolation capacitor ...

Checking the power supply unit under load:

We measure the voltage of the standby source, loaded first by a light bulb, and then by a current of up to two amperes. If the voltage of the duty room does not sag, turn on the power supply unit, short-circuiting PS-ON (green) to ground, measure the voltages at all outputs of the power supply unit and on power capacitors at 30-50% load for a short time. If all voltages are within tolerance, we assemble the unit into a case and check the power supply unit at full load. We look at the ripple. The PG output (gray) during normal operation of the unit should be from +3.5 to + 5V.

Epilogue and recommendations for revision:

Repair recipes from ezhik97:

In the modern world, the development and obsolescence of personal computer components occurs very quickly. At the same time, one of the main components of a PC - an ATX power supply - is practically has not changed its design for the last 15 years.

Consequently, the power supply unit of both the ultra-modern gaming computer and the old office PC work on the same principle and have common troubleshooting techniques.

A typical ATX power supply circuit is shown in the figure. Structurally, it is a classic pulse unit on the TL494 PWM controller, which is triggered by a PS-ON (Power Switch On) signal from the motherboard. The rest of the time, until the PS-ON pin is pulled to ground, only the Standby Supply with a voltage of +5 V at the output is active.

Let's take a closer look at the structure of the ATX power supply. Its first element is
mains rectifier:

Its task is to convert alternating current from the mains to direct current to power the PWM controller and the standby power supply. Structurally, it consists of the following elements:

Fuse F1 protects the wiring and the power supply itself from overload in the event of a power supply failure, leading to a sharp increase in current consumption and, as a result, to a critical increase in temperature that can lead to a fire.
A protective thermistor is installed in the "neutral" circuit, which reduces the current surge when the power supply unit is connected to the network.
Next, a noise filter is installed, consisting of several chokes (L1, L2), capacitors (C1, C2, C3, C4) and a counter-winding choke Tr1... The need for such a filter is due to the significant level of interference that the impulse unit transmits to the power supply network - this interference is not only captured by television and radio receivers, but in some cases can also lead to incorrect operation of sensitive equipment.
A diode bridge is installed behind the filter, which converts alternating current into pulsating direct current. The ripple is smoothed out by a capacitive-inductive filter.

Further, a constant voltage, present all the time the ATX power supply is connected to the outlet, goes to the control circuits of the PWM controller and the standby power supply.

Standby power supply - this is a low-power independent pulse converter based on the T11 transistor, which generates pulses, through an isolation transformer and a half-wave rectifier on the D24 diode, supplying a low-power integrated voltage regulator on the 7805 microcircuit. high voltage drop across the 7805 stabilizer, which under heavy load leads to overheating. For this reason, damage to the circuits powered from the standby source can lead to its failure and the subsequent impossibility of turning on the computer.

The basis of the pulse converter is PWM controller... This abbreviation has already been mentioned several times, but has not been deciphered. PWM is pulse width modulation, that is, the change in the duration of voltage pulses at their constant amplitude and frequency. The task of the PWM unit, based on the specialized TL494 microcircuit or its functional analogs, is to convert the constant voltage into pulses of the appropriate frequency, which, after the isolation transformer, are smoothed by the output filters. The voltage stabilization at the output of the pulse converter is carried out by adjusting the duration of the pulses generated by the PWM controller.

An important advantage of such a voltage conversion scheme is also the ability to work with frequencies significantly higher than 50 Hz of the mains. The higher the current frequency, the smaller the dimensions of the transformer core and the number of winding turns are required. That is why switching power supplies are much more compact and lighter than classic circuits with an input step-down transformer.

A circuit based on the T9 transistor and the following stages is responsible for turning on the ATX power supply. At the moment the power supply is switched on to the network, a voltage of 5V is supplied to the base of the transistor through the current-limiting resistor R58 from the output of the standby power supply, at the moment the PS-ON wire is shorted to ground, the circuit starts the TL494 PWM controller. In this case, the failure of the standby power supply will lead to the uncertainty of the operation of the power supply startup circuit and the probable failure of switching on, which has already been mentioned.

The main load is borne by the output stages of the converter. This primarily concerns the switching transistors T2 and T4, which are installed on aluminum radiators. But at high load, their heating, even with passive cooling, can be critical, so the power supplies are additionally equipped with an exhaust fan. If it fails or is very dusty, the probability of overheating of the output stage increases significantly.

Modern power supplies are increasingly using powerful MOSFET switches instead of bipolar transistors, due to the significantly lower resistance in the open state, providing a higher efficiency of the converter and therefore less demanding on cooling.

Video about the computer power supply device, its diagnostics and repair

Initially, ATX computer power supplies used a 20-pin connector (ATX 20-pin). Now it can only be found on outdated equipment. Subsequently, the increase in the power of personal computers, and therefore their energy consumption, led to the use of additional 4-pin connectors (4-pin). Subsequently, the 20-pin and 4-pin connectors were structurally combined into one 24-pin connector, and for many power supplies, a part of the connector with additional pins could be separated for compatibility with older motherboards.

The pin assignment of the connectors is standardized in the ATX form factor as follows, according to the figure (the term "controlled" refers to those pins on which the voltage appears only when the PC is turned on and is stabilized by the PWM controller):

Forum of the store "Ladies' happiness"

Message dtvims Sep 25, 2014 4:51 pm

In general, it is more correct to call it: Repair of chargers for laptops, etc. for dummies! (Many letters.)
Actually, since I myself am not a professional in this field, but have successfully repaired a decent pack of power supply data, I believe that I can describe the technology as a “teapot for teapot”.
Key points:
1. Everything that you do, at your own peril and risk - it is dangerous. Starting under voltage 220V! (here you need to draw a beautiful lightning).
2. There is no guarantee that everything will work out and it is easy to make it worse.
3. If you double-check everything several times and DO NOT neglect the security measures, then everything will work out the first time.
4. Make all changes in the circuit ONLY on a completely de-energized power supply unit! Completely disconnect everything from the outlet!
5. DO NOT grab the power supply unit connected to the network with your hands, and if you bring it close, then only one hand! As the physicist used to say at our school: When you climb under tension, you need to climb there with only one hand, and hold your earlobe with the other, then when you are jerked by the current, you pull yourself by the ear and you will no longer have the desire to climb under tension again.
6. We replace ALL suspicious parts with the same or complete analogs. The more we replace, the better!

Read also: Bl1013 makita DIY repair

Another common cause of a power supply malfunction has nothing to do with a fuse. We are talking about the absence of an output voltage with a fully functional such element.
Solution:

Swollen Condenser - Desoldering and replacement required.
Failed choke - it is necessary to remove the element and change the winding. The damaged wire is unwound. In this case, the turns are counted. Then a new wire of a suitable section is wound on the same number of turns.The part is returned to its place.
Deformed bridge diodes are replaced with new ones.
If necessary, the parts are checked with a tester (if no damage is detected visually).

It is quite possible to build a hot air soldering station yourself. A fan is used as a blower, and a spiral is used as a heater. The best option for a temperature controller for a soldering iron is a thyristor circuit.

Breakdown reasons:

do not block ventilation openings;
provide optimal temperature conditions - cooling and ventilation.

Things to Remember:

The first connection of the unit is made to a 25-watt lamp. This is especially important after replacing diodes or transistor! If a mistake is made somewhere or a malfunction is not noticed, the passing current will not damage the entire device as a whole.
When starting work, do not forget that a residual discharge remains on electrolytic capacitors for a long time. Before soldering the parts, it is necessary to short-circuit the capacitor leads. You cannot do this directly. It is necessary to short-circuit through a resistance with a rating higher than 0.5 V.

Most modern consumer electronic equipment has in its design independent or located on a separate board electronic modules that reduce and rectify the mains voltage.There are several reasons for this, but the main ones are:

fluctuations in the mains voltage, for which these step-down-rectifier devices are not designed;

non-observance of operating rules;

connecting a load for which the devices are not designed.

Of course, it can be very annoying when urgent work needs to be done, and the power module of the computer is faulty, or while watching your favorite TV show, this device fails.Do not immediately panic and go to a repair shop or rush to an electronics supermarket to purchase a new unit. Often the reasons for inoperability are so trivial that they can be eliminated at home, with minimal expenditure of financial resources and nerves. Image - DIY repair switching power supplies

Of course, in order to try not only to repair the switching power supply, but also to determine its malfunction, you must have basic knowledge of electronics and have certain electrical skills.

As part of any power supply, whether built-in, like in a TV or installed as a separate device, like in a desktop computer, there are two functional blocks - high-voltage and low-voltage.

In the high-voltage side, the mains voltage is converted by a diode bridge into a constant voltage, and smoothed on a capacitor to a level of 300.0 ... 310.0 volts. Constant, high voltage is converted into a pulse voltage with a frequency of 10.0 ... 100.0 kilohertz, which makes it possible to abandon massive low-frequency step-down transformers, replacing them with small-sized pulse ones.

In the low-voltage unit, the impulse voltage is lowered to the required level, straightened, stabilized and smoothed out. At the output of this unit, there is one or more voltages required to power household appliances. In addition, various control circuits are mounted in the low-voltage unit, which make it possible to increase the reliability of the device and ensure the stability of the output parameters.

Visually, on a real board, it is quite easy to distinguish between the high-voltage and low-voltage parts. The network wires are suitable for the first, and the supply wires go from the second.

Switching regulator in the transistor power supply

A person who is going to try to repair a power supply unit for household electronic appliances must be prepared in advance that not every power supply device can be repaired. Today, some manufacturers produce electronics, the blocks of which are not subject to repair, but complete replacement.

Not a single master will undertake the repair of such a power supply unit, because initially it is intended for the complete dismantling of the old device with replacement with a new one. Often, such electronic devices are simply filled with some kind of compound, which immediately removes the question of its maintainability.

As statistics show, the main malfunctions of the power supply are caused by:

malfunction of the high-voltage part (40.0%), which are expressed by the breakdown (burnout) of the diode bridge and the failure of the filtering capacitor;
breakdown of a power field-effect or bipolar transistor (30.0%), which forms high-frequency pulses and is located in the high-voltage part;
breakdown of the diode bridge (15.0%) in the low-voltage part;
breakdown (burnout) of the choke windings of the output filter.

In other cases, the diagnosis is quite difficult and without special devices (oscilloscope, digital voltmeter) it will not be possible to perform it. Therefore, if the malfunction of the power supply is not caused by the four above-mentioned main reasons, you should not engage in home repairs, but immediately call a master for replacement or purchase a new power supply.

Faults in the high-voltage section are easy enough to detect. They are diagnosed by a blown fuse and a lack of voltage after it. The third and fourth cases can be assumed if the fuse is good, the voltage at the input of the low-voltage unit is present, and the input voltage is absent.

It is advisable to check all the details at the same time. If several electronic elements are burned out, when one of them is replaced with a serviceable one, it may burn out again due to a complex malfunction that has not been eliminated.

After replacing parts, you must install a new fuse and turn on the power supply. As a rule, after this, the power supply starts to work.

If the fuse has not blown, and there is no voltage at the output of the power supply, then the cause of the malfunction is the breakdown of the rectifier diodes of the low-voltage part, the burnout of the inductor or the output of the electrolytic capacitors of the secondary rectifier unit.

Malfunction of capacitors is diagnosed when they are swollen or fluid leaks out of their body. The diodes must be evaporated and checked with a tester in the same way as checking the high-voltage part. The integrity of the choke winding is checked by a tester. All defective parts must be replaced.

If you cannot find the desired choke, then some "craftsmen" rewind the burned-out one, picking up a wire of suitable diameter and determining the number of turns. Such work is quite painstaking and is usually performed only for unique power supplies, it is difficult to find an analogue for which.

As already mentioned, most power supplies for modern computers and TVs are built according to a typical scheme. They differ in the size of the electronic parts used and in the output power. The diagnostics and troubleshooting procedures for these devices are identical.

However, a high-quality repair requires an appropriate tool, the range of which includes:

soldering iron (preferably with adjustable power);
solder, flux, alcohol or refined gasoline (Galosha);
device for removing molten solder (desoldering pump);
Screwdriver Set;
side cutters (nippers);
household multimeter (tester)
tweezers;
100.0 watt incandescent lamp (used as ballast load).

In principle, simple TVs can be repaired without a circuit, but the main difficulty in repairing some models is that the power supply device generates the entire range of voltages - including the high-voltage used to scan the kinescope. The power supplies for household computers are made according to the same type of scheme. Let us consider separately the methodology for determining the malfunction and repairing the TV and desktop.

The failure of the TV power supply module is first of all evidenced by the absence of the "sleep" mode diode glow. The first repair operations are:

check for the integrity (absence of breakage) of the supply voltage cord;

disassembly of the television receiver and release of the electronic board;

inspection of the power supply board for the presence of externally faulty parts (swollen capacitors, burnt spots on the printed circuit board, burst cases, charred surface of resistors);

checking the soldering points, with special attention paid to soldering the contacts of the pulse transformer.

If it was not possible to visually establish the defective part, then it is necessary to sequentially check the performance of the fuse, diodes, electrolytic capacitors and transistors. Unfortunately, if the control microcircuits are out of order, their malfunction can be established only indirectly - when, with completely serviceable discrete elements, the operational state of the power supply does not occur.The most common reasons for the inoperability of television units are:

breakage of ballast resistance;

inoperability (short circuit) of the high voltage filter capacitor;

malfunction of the secondary voltage filter capacitors;

breakdown or burnout of rectifier diodes.

All these parts (except for rectifier diodes) can be checked without removing them from the board. If it was possible to identify the faulty part, then it is replaced and they begin to check the repair performed. To do this, an incandescent lamp is installed in place of the fuse and the device is connected to the network.There are several possible options for the behavior of the repaired device:

The light flashes and goes out, the sleep mode LED lights up, a raster appears on the screen. In this situation, the line scan voltage is measured first. If its value is too high, it is necessary to check and replace electrolytic capacitors with guaranteed serviceable ones. A similar situation occurs in the event of a malfunction of the optocouplers.

If the lamp flashes and goes out, the LED does not light up, the raster is absent, then the pulse generator does not start. In this case, the voltage level on the electrolytic capacitor of the filter of the high-voltage part is checked. If it is below 280.0 ... 300.0 volts, then the following malfunctions are most likely:

one of the rectifier bridge diodes is broken;
large capacitor leakage (the capacitor is "old").

If there is no voltage, it is necessary to re-check the continuity of the supply circuits and all diodes of the high voltage rectifier. If the light is high, you must immediately disconnect the power module from the network and re-check all electronic parts.The above sequence and test scheme allow you to identify the main malfunctions of the power supply device of the television receiver. Image - DIY repair switching power supplies

Today, ATX devices of various power are most widely used for powering desktop (desktop) designers. The reason for their repair should be:

the motherboard does not start (the computer is completely inoperative);
the cooling fan of the device itself does not rotate;
the block “tries” to start itself many times.

Before starting the repair of ATX devices, it is necessary to assemble the load circuit (figure). The repair is carried out in the following sequence:

the device is removed from the computer and the cover is removed from it;
a vacuum cleaner and a brush removes dust from electronic boards and surfaces of parts;
external examination of electronic elements and printed circuit boards;
a load device is connected.

If, when turned on, the lamp flashes brightly and continues to burn, then the diode bridge in the high-voltage part or the filter capacitor is out of order. Burnout of the high-voltage transformer is possible.

If the fuse is intact, then the reason for inoperability may be:

failure of the transistors of the pulse generator;
malfunction of the PWM controller.

In these cases, it is easier to purchase a new device, which, depending on the capacity, costs from 600 to 800 rubles.

Video (click to play).

With repeated self-starting of the device, the reason for inoperability is usually the failure of the reference voltage stabilizer. In this case, the computer system cannot pass the self-test mode, it turns off and turns on the power module.