Zener diodes in the PC power supply. Do-it-yourself computer power supply repair

- in many ways, our electrical networks deliver problems to the computer power supply. It is no secret that the stability of the AC voltage in the network leaves much to be desired, this situation most often leads to negative consequences with household appliances. Power surges also adversely affect the PC power supply, even if it is in standby mode.

This publication is dedicated to radio amateurs who have skills in electronics repair, and gives advice on how to do it. There is an available method for checking the health of the voltage source. Before proceeding with troubleshooting, it should be disconnected from the system board, of course, with a de-energized computer. The connectors with wires coming from the power supply to the motherboard are simply disconnected. For different models of the ATX PSU, the main connecting connectors are both 20-pin and 24-pin, plus auxiliary power wires of 4 or 6 pins. These additional wires are designed to provide +12v to the processor and video card. After all components are disconnected from the unit, the process of checking the device itself begins.

To do this, take the largest wiring harness and find two contacts on its connector marked with numbers 15 and 16 with green and black wires. On different connectors, the numbering may differ, but the main guideline is the green and any black wire. Then connect the test model to the 220v network, and close these two contacts with a small piece of wire. As a result of this short circuit, a signal is sent to the motherboard and the PSU starts. Here, this piece of ground wire simply plays the role of an ordinary switch. If the fan started to work after a short circuit, then it is more likely that the power supply is in working condition. Therefore, the problem must be looked for elsewhere.

Repair sequence

Therefore, starting a step-by-step repair of a computer power supply with your own hands, it is tedious to understand that the capacitors installed in the power circuits have a large capacity. It is they who accumulate a huge supply of energy for its subsequent transfer to the load. Therefore, you must always be careful when working with the power part, so that before you start checking the device, you must definitely discharge the capacitors. Otherwise, you can get such a discharge that it won’t seem small, besides, the accumulated energy in the capacitors is stored for a long time.

I had a case when I remembered a 10000uf 400v capacitor lying around in a shed for half a year. And when I wanted to clean it from dust, I received such a discharge that it darkened in my eyes and the skin on my fingers burst from the burn. So always be extremely careful when working with devices where capacitors with a large capacity are installed. Discharging the conder is very simple, take (depending on the capacitance) a 1 kΩ resistor with a power of 10 W, or an ordinary light bulb and a soft discharge occurs.

Device disassembly

First of all, the case cover is naturally removed and the entire internal space is necessarily brought into proper form, that is, all the dust accumulated there is removed. The stratification formed there from dust plays its negative role in terms of heat removal coming from power elements. Therefore, excessive contamination of the computer power supply can also be one of the factors for its failure. Then it actually starts do-it-yourself computer power supply repair step by step.

One of the reasons for the failure of the device may be a banal blown fuse 5A. So it is checked for an open with a multimeter first of all, and if it shows an open, then replace it with a new one or make a “bug” from a burnt one. To do this, solder a copper wire Ø 0.16 mm over the glass cylinder of the fuse, then apply mains voltage to the unit - if the fan is working, then everything is fine. Now this “bug” needs to be removed, and a new, factory-made one should be put in its place.

Finding faulty capacitors

As a rule, computer power supplies are mounted using electrolytic capacitors with a significant capacity. But at the same time, there are unscrupulous PSU manufacturers who, in order to save money, install conduits with a lower permissible voltage value. Such devices in most cases belong to the category of cheap products and fail more often than others. It is these electrolytes that are made without a voltage margin that become the main problem in power supplies.

At the slightest jump in the voltage in the network, the capacitance does not withstand this burst of energy. In this case, either the shell breaks, as a result of strong heating of the electrolyte, or the radio component swells and the electrolyte flows out of it. Naturally, such elements are no longer suitable for further use and need to be changed.

Attention! Bad job fan causes the capacitors to swell. The thing is that the fan must cool the capacitors, which are heated due to the accumulation of voltage in them. Therefore, experts recommend periodically lubricating the fan bearings and cleaning the entire cooler.

In some cases, visual defects of the capacitor were not found, but it is best to play it safe and test them with an ohmmeter in order to detect internal resistance. If the resistance is large relative to the nominal, then most likely there is no contact between the lining of the electrical energy storage device and the output, that is, a break.

Continuing the topic of electrolytic energy storage devices, it is worth explaining such a point. Replacing such “inflated” components with new ones will be premature if you do not first localize the problem that led to their swelling. Otherwise, well, you replace them with new ones, and after a while they will become “pregnant” again)), and all over again. As practice shows, the reason for such a malfunction lies in incorrect stabilization of the supply voltage or its absence at all. Therefore, until you find out why this is happening, it is not necessary to replace swollen ones with new ones.

Once again I want to warn everyone who does not have a certain experience in the repair of such devices - do not undertake to do do-it-yourself computer power supply repair step by step. This can cost you much more than giving the power supply to specialists for repair. Among other things, professional equipment is needed to repair such equipment.

Control transistors and powerful switches

Any transistor installed in the circuit is a semiconductor device, which is also subject to extreme processes occurring in it. That's why, do-it-yourself computer power supply repair step by step and sequentially. After the capacitors, these semiconductors are also subject to verification. To determine the state of the transistor, it is necessary to check the base-collector and base-emitter junctions in both directions with a multimeter. This is done in order to detect an open or short circuit at these transitions.

The same should be done on the collector-emitter junctions, while it is desirable to unsolder one end of the resistor installed in the emitter circuit. After that, a conclusion is already made about the suitability of this element. Then we turn to checking rectifier diodes, we check them in the same way as transistors - the diode shows high resistance in one direction, and shows nothing in the other direction, that is, the transition is closed.

Power Supply Upgrade

What can improve the computer power supply? Upgrading refers to some alteration of the device, in particular, the replacement of certain electronic components with better ones to increase the reliability of the circuit. The concept of a small alteration includes the replacement of capacitors installed in the power path with branded capacitors with great value rated voltage. Why branded ones? Because among the imported ones, you can choose the sizes corresponding to the place of installation on the board, moreover, with a higher voltage than the original.

Attention! Replacing the capacitor is associated with its correct installation on the plateau. So pay attention to the negative output strip. It is wide vertical and light. So the new device must be installed in exactly the same position so that the strip gets to the old installation site.

Now that you have changed all the suspicious and obviously failed elements to serviceable ones, the PSU should turn on without any problems. One of the main performance indicators of the device is the start and stable work fan, no obvious overheating of parts at idle. There is another method for checking the readiness of the block for work, more professional. This method consists in testing all the electrical parameters of the radio elements installed in the circuit. On the contacts in the connectors, the voltage value must correspond to 12v and 5v.

From the above, it follows: repairing a computer power supply is not as simple as it might seem at first. However, as mentioned above, if you have at least basic knowledge in radio electronics, then you can take on independent repairs. In this case, it is desirable to have a schematic diagram of the device at hand and study it thoroughly.

Cases of failure of power supplies in a computer are not uncommon. The reasons for this are:

1. Voltage surges in the power grid;

2. Poor workmanship, especially for cheap power supplies and system blocks;

3. Unsuccessful design and circuit solutions;

4. The use of low-quality components in the manufacture;

5. Overheating of the elements due to the unsuccessful location of the system unit, contamination of the power supply, stopping the cooling fan.

What are the "symptoms" of a malfunctioning power supply in a computer?

Most often, this is the complete absence of signs of life of the system unit, that is, nothing is buzzing, the indication LEDs are not lit, there are no sound signals.

In some cases, the motherboard does not start. In this case, fans may spin, indications may be lit, drives may make sounds, and HDD but nothing appears on the monitor screen.

Sometimes the system unit, when turned on, begins to show signs of life for a few seconds and immediately turns off due to the operation of the power supply overload protection.

In order to finally make sure that the power supply is malfunctioning, you need to open the right cover of the system unit, when viewed from behind. Pull the main plug of the main connector of the power supply, which has 20 or 24 pins, from the motherboard socket, and close the contacts with the green (sometimes gray) and the nearest black wire. If at the same time the power supply starts up, then the motherboard is most likely to blame.

The start of the power supply can be determined by the rotation of the power supply fan, if it is working, and the clicks of the drives, but for reliability it is better to check the voltage at the connector. Between contacts with black and red wires - 5v, between black and yellow - 12v, between black and pink - 3.3v; between black and purple - 5v standby voltage. Minus on black, plus on color. In order to make sure that the power supply is running, it is enough to measure one of the voltages, except for the “duty” 5v on the purple wire.

Sometimes users start looking for a fuse. Do not look, they are not outside. There is one inside, but changing it in most cases is not only useless, but dangerous and harmful, as this can lead to even more problems.

If it turns out that the power supply is faulty, then in most cases it is better to replace it, but it is also possible if it is economically feasible.

When buying a new power supply, you must first take into account the power, which should not be less than the previous one. It is also necessary to pay attention to the output connectors so that it is possible to connect all the devices of the system unit, although, if necessary, connection problems can be solved with the help of adapters. You can read about how to choose a power supply of the right quality.

Do I need to repair the power supply myself? If you do not have at least elementary knowledge and skills in the field of electronics, definitely not. Firstly, you most likely will not be able to do this, and secondly, it is dangerous to life and health if you do not follow the safety rules.

For those who nevertheless decided to start repairing the power supply, there is an opportunity to get acquainted with my personal experience and thoughts about it.

The performance of a personal computer (PC) not least depends on the quality of the power supply unit (PSU). If it fails, the device will not be able to turn on, which means that you will have to replace or repair the computer's power supply. Whether it's a modern gaming computer or a weak office computer, all PSUs work. on a similar basis, and the troubleshooting methodology for them is the same.

The principle of operation and the main components

Before you start repairing a PSU, you need to understand how it works, to know its main components. Repair of power supplies should be carried out very carefully and remember about electrical safety during work. The main nodes of the PSU include:

• input (mains) filter;
• additional stabilized signal driver 5 volts;
• main driver +3.3 V, +5 V, +12 V, as well as -5 V and -12 V;
• line voltage stabilizer +3.3 volts;
• high-frequency rectifier;
• voltage generation line filters;
• node of control and protection;
• a block for the presence of a PS_ON signal from a computer;
• voltage driver PW_OK.

The inlet filter is used for interference suppression generated by the BP in ​ electrical circuit. At the same time, it performs a protective function during abnormal operation of the PSU: protection against exceeding the current value, protection against voltage surges.

When the PSU is connected to a 220 volt network, a stabilized signal with a value of 5 volts is supplied to the motherboard through an additional driver. The operation of the main driver at this moment is blocked by the PS_ON signal generated motherboard and equal to 3 volts.

After pressing the power button on the PC, the PS_ON value becomes zero and the starting the main converter. The power supply begins to generate the main signals to the computer board and protection circuits. In the event of a significant excess of the voltage level, the protection circuit interrupts the operation of the main driver.

To start the motherboard at the same time, from the power device, a voltage of +3.3 volts and +5 volts is applied to it to form the PW_OK level, which means food is normal. Each color of the wire in the power device corresponds to its voltage level:

• black, common wire;
• white, -5 volts;
• blue, -12 volts;
• yellow, +12 volts;
• red, +5 volts;
• orange, +3.3 volts;
• green, PS_ON signal;
• grey, PW_OK signal;
• purple, standby food.

The power supply device is based on the principle pulse width modulation(PWM). The mains voltage converted by the diode bridge is supplied to the power unit. Its value is 300 volts. The operation of transistors in the power unit is controlled by a specialized PWM controller chip. When a signal arrives at the transistor, it opens, and a current appears on the primary winding of the pulse transformer. As a result of electromagnetic induction, voltage also appears on the secondary winding. By changing the pulse duration, the opening time of the key transistor is regulated, and hence the signal magnitude.

The controller, which is part of the main converter, starts up from enable signal motherboard. The voltage enters the power transformer, and from its secondary windings it enters the remaining nodes of the power source, which form a number of necessary voltages.

PWM controller provides output voltage stabilization by using it in a feedback loop. With an increase in the signal level on the secondary winding, the feedback circuit reduces the voltage at the control output of the microcircuit. At the same time, the microcircuit increases the duration of the signal sent to the transistor switch.

Before proceeding directly to the diagnosis of a computer power supply, you need to make sure that the problem is in it. The easiest way to do this is to connect known to be serviceable block to system block. Troubleshooting in the computer power supply can be carried out according to the following method:

1. In case of damage to the PSU, you must try to find a manual for its repair, a circuit diagram, and data on typical malfunctions.
2. Analyze the conditions under which the power source worked, whether the electrical network was working.
3. Using your senses, determine if there is a smell of burning parts and elements, if there was a spark or flash, listen to whether extraneous sounds are heard.
4. Assume one malfunction, highlight the faulty element. Usually this is the most time-consuming and painstaking process. This process is even more laborious if there is no circuit diagram, which is simply necessary when searching for "floating" faults. Using measuring instruments, trace the path of the fault signal to the element on which there is a working signal. As a result, conclude that the signal disappears on the previous element, which is inoperative and needs to be replaced.
5. After repair, it is necessary to test the power supply with its maximum possible load.

If you decide to repair the power supply yourself, first of all, it is removed from the system unit case. After the fixing screws are unscrewed and the protective cover is removed. Having blown and cleaned from dust, they begin to study it. Practical repair DIY computer power supply step by step can be represented as follows:

1. Visual inspection. With it, special attention is paid to blackened places on the board and elements, the appearance of capacitors. The top of the capacitors should be flat, the bulge indicates its uselessness, at the bottom at the base there should be no smudges. If there is a power button, it will not be superfluous to check it.
2. If the inspection did not arouse suspicion, then the next step would be to check the input and output circuits for the presence of a short circuit (short circuit). In the presence of a short circuit, a punched semiconductor element is detected, standing in a circuit with a short circuit.
3. The mains voltage is measured on the capacitor of the rectifier unit and the fuse is checked. In the case of a voltage of 300 V, we proceed to the next step.
4. If there is no voltage, the fuse burns out, the diode bridge is checked, the key transistors for a short circuit. Resistors and protective thermistor for open circuit.
5. The presence of the standby voltage, stabilized five volts, is checked. Statistics show that when the power device does not turn on, one of the most common reasons is a malfunction of the standby power circuit, with operable power elements.
6. If stabilized five volts are present, the presence of PS_ON is checked. When the value is less than four volts, the cause of the low signal level is looked for. Usually PS_ON is formed from the standby voltage through a 1 kΩ pull-up resistor. The supervisor circuit is checked, first of all, for compliance in the circuit with capacitor capacitance values ​​and resistor values.

If the cause is not found, the PWM controller is checked. To do this, you need a stabilized 12 volt power supply. On board the foot of the microcircuit is turned off, which is responsible for the delay (DTC), and the source power is supplied to the VCC leg. The oscilloscope looks at the presence of signal generation at the outputs connected to the collectors of transistors, and the presence of a reference voltage. If there are no pulses, the intermediate stage is checked, which is most often assembled on low-power bipolar transistors.

Typical malfunctions and element checks

When restoring the PC power supply, you will need to use various kinds of devices First of all, it is a multimeter and preferably an oscilloscope. Using the tester, it is possible to measure for a short circuit or open circuit of both passive and active radio elements. The performance of the microcircuit, if there are no visual signs of its failure, is checked using an oscilloscope. In addition to measuring equipment for repairing a PC power supply, you will need: a soldering iron, solder suction, washing alcohol, cotton wool, tin and rosin.

If the computer's power supply does not start, possible faults can be represented in the form of typical cases:

1. The fuse in the primary circuit blows. Broken diodes in the rectifier bridge. The separation filter elements are called for a short circuit: B1-B4, C1, C2, R1, R2. Breakage of varistors and thermistor TR1, the transitions of power transistors and auxiliary Q1-Q4 are short-circuited.
2. DC five volts or three volts too low or too high. Violations in the operation of the stabilizing circuit, microcircuits U1, U2 are checked. If it is not possible to check the PWM controller, then the microcircuit is replaced with an identical one or an analogue.
3. The output signal level differs from the working one. Fault in the feedback circuit. The PWM chip and the radio elements in its piping are to blame, special attention is paid to capacitors C and low-power resistors R.
4. No PW_OK signal. The presence of the main voltages and the PS_ON signal is checked. The supervisor responsible for monitoring the output signal is being replaced.
5. No PS_ON signal. The supervisor's microchip burned out, the elements of tying its circuit. Check by replacing the chip.
6. The fan does not spin. Measure the voltage supplied to it, it is 12 volts. Ring the thermistor THR2. Measure the resistance of the fan leads for a short circuit. Carry out mechanical cleaning and lubricate the seat under the fan blades.

Principles of measurement of radioelements

The PSU case is connected to the common wire of the printed circuit board. The measurement of the power part of the power supply is carried out relative to the common wire. The limit on the multimeter is set to more than 300 volts. In the secondary part, there is only a constant voltage, not exceeding 25 volts.

Resistors are checked by comparing the readings of the tester and the markings applied to the resistance housing or indicated on the diagram. Diodes are checked by a tester, if it shows zero resistance in both directions, then a conclusion is made about its malfunction. If it is possible in the device to check the voltage drop across the diode, then you can not solder it, the value is 0.5-0.7 volts.

Capacitors are tested by measuring their capacitance and internal resistance, which requires a specialized ESR meter. When replacing, be aware that capacitors with low internal resistance (ESR) are used. transistors call for performance p-n junctions or in the case of outfield, the ability to open and close.

Checking a Repaired Power Supply

After the ATX unit has been repaired, it is important to correctly switch it on for the first time. At the same time, if not all problems have been eliminated, failure of the repaired and new components of the device is possible.

Starting the power supply device can be done autonomously, without using a computer unit. To do this, the PS_ON contact is bridged with a common wire. Before turning on the fuse, a 60 W light bulb is soldered in place, and the fuse is removed. If, when turned on, the light starts to shine brightly, then there is a short circuit in the unit. In the event that the lamp flashes and goes out, the lamp can be unsoldered and a fuse installed.

The next stage of the PSU check takes place under load. First, the presence of a standby voltage is checked; for this, the output is loaded with a load of about two amperes. If the duty room is in order, the power supply is turned on by shorting PS_ON, after which the output signal levels are measured. If there is an oscilloscope - the ripple looks.

Sent yuri11112222- Power supply circuitry: ATX-350WP4
Power supply circuitry: ATX-350WP4

The article provides information on circuit solutions, recommendations for repair, replacement of analog parts for the ATX-350WP4 power supply. Unfortunately, the author could not establish the exact manufacturer, apparently, this block assembly is quite close to the original, presumably Delux ATX-350WP4 (Shenzhen Delux Industry Co., Ltd), appearance block is shown in the photo.

General information. The power supply is implemented in the ATX12V 2.0 format, adapted to the domestic consumer, so it does not have a power switch and a switch for the type of variable network. Output connectors include:
connector for connecting to the system board - the main 24-pin power connector;
4-pin connector +12 V (P4 connector);
removable media power connectors;
Serial ATA hard drive power. It is assumed that the main power connector
can be easily converted to 20-pin by dropping the 4-pin group, making it compatible with older format motherboards. The presence of a 24-pin connector allows for maximum connector power using standard terminals of 373.2 W.
Operational information about the ATX-350WP4 power supply is given in Table.

Structural scheme. The set of elements of the block diagram of the ATX-350WP4 power supply is typical for pulse-type power supplies. These include a two-section mains noise suppression filter, a low-frequency high-voltage rectifier with a filter, a main and auxiliary pulse converters, high-frequency rectifiers, an output voltage monitor, protection and cooling elements. A feature of this type of power supply is the presence of mains voltage at the input connector of the power supply, while a number of elements of the block are energized, there is voltage at some of its outputs, in particular, at the outputs + 5V_SB. The block diagram of the source is shown in Fig.1.

Power supply operation. The rectified mains voltage of about 300 V is the supply voltage for the main and auxiliary converters. In addition, from the output rectifier of the auxiliary converter, the supply voltage is supplied to the control chip of the main converter. In the off state (signal PS_On has a high level) of the power supply, the main converter is in the "sleep" mode, in this case, the voltage at its outputs is not recorded by the measuring instruments. At the same time, the auxiliary converter generates the power supply voltage of the main converter and +5V_SB output voltage. This power supply acts as a standby power supply.

The inclusion of the main converter into operation occurs according to the principle of remote switching, according to which the Ps_On signal becomes equal to zero potential ( low level voltage) when you turn on the computer. Based on this signal, the output voltage monitor issues a permission signal for the formation of control pulses of the PWM controller of the main converter of maximum duration. The main converter wakes up from sleep mode. From high-frequency rectifiers through appropriate smoothing filters, voltages of ±12 V, ±5 V and +3.3 V are supplied to the output of the power supply.

With a delay of 0.1 ... 0.5 s relative to the appearance of the PS_On signal, but sufficient for the end of transients in the main converter and the formation of supply voltages of +3.3 V. +5 V, +12 V at the output of the power supply, monitor output voltages, an RG signal is generated. (food is normal). P.G signal is informational, indicating the normal operation of the power supply. It is issued to the motherboard for the initial installation and launch of the processor. Thus, the Ps_On signal controls the power supply, and the P.G. is responsible for starting the motherboard, both signals are part of the 24-pin connector.
The main converter uses a pulse mode, the converter is controlled by a PWM controller. The duration of the open state of the converter keys determines the magnitude of the voltage of the output sources, which can be stabilized within the allowable load.

The state of the power supply is monitored by the output voltage monitor. In case of overload or underload, the monitor generates signals that prohibit the operation of the PWM controller of the main converter, putting it into sleep mode.
A similar situation arises under conditions of emergency operation of the power supply associated with short circuits in the load, which are controlled by a special control circuit. To facilitate thermal conditions in the power supply, forced cooling is used, based on the principle of creating negative pressure (ejection of warm air).

The schematic diagram of the power supply is shown in Fig.2.

The mains filter and the low-frequency rectifier use elements of protection against mains interference, after passing through which the mains voltage is rectified by a bridge-type rectifier circuit. Protection of the output voltage from interference in the AC network is carried out using a pair of sections of the surge filter. The first link is made on a separate board, the elements of which are CX1, FL1, the second link is made up of the elements of the main board of the power supply CX, CY1, CY2, FL1. Elements T, THR1 protect the power supply from short circuit currents in the load and voltage surges in the input network.
The bridge rectifier is made on diodes B1-B4. Capacitors C1, C2 form a low-frequency network filter. Resistors R2, R3 - elements of the discharge circuit of capacitors C1, C2 when the power is turned off. Varistors V3, V4 limit the rectified voltage when the mains voltage surges above the accepted limits.
The auxiliary converter is connected directly to the output of the mains rectifier and schematically represents a self-oscillating blocking oscillator. The active elements of the blocking oscillator are transistor Q1, a p-channel field effect transistor (MOSFET) and transformer T1. The initial gate current of transistor Q1 is generated by resistor R11R12. At the moment of power supply, the blocking process begins to develop, and current begins to flow through the working winding of the transformer T1. The magnetic flux created by this current induces an EMF in the positive feedback winding. In this case, capacitor C7 is charged through the diode D5 connected to this winding, and the transformer is magnetized. The magnetization current and the charging current of the capacitor C7 lead to a decrease in the gate current of Q1 and its subsequent blocking. The damping of the surge in the drain circuit is carried out by the elements R19, C8, D6, the reliable locking of the transistor Q1 is carried out by the bipolar transistor Q4.

The main power supply converter is made according to a push-pull half-bridge circuit (Fig. 3). The power part of the converter is transistorized - Q2, Q3, the diodes D1, D2 that are turned back on provide protection of the converter transistors from "through currents". The second half of the bridge is formed by capacitors C1, C2, which create a rectified voltage divider. The diagonal of this bridge includes the primary windings of transformers T2 and TK, the first of them is rectifier, and the second one functions in the control and protection circuit against "excessive" currents in the converter. To eliminate the possibility of asymmetric bias of the transformer TZ, which can occur during transients in the converter, an isolation capacitor SZ is used. The operation mode of the transistors is set by the elements R5, R8, R7, R9.
The control pulses are fed to the converter transistors through a matching transformer T2. However, the converter starts in a self-oscillating mode, with the transistor 03 open, the current flows through the circuit:
+U(B1...B4) -> Q3(k-e) -> T2 - T3 -> SZ -> C2 -> -U(BL..B4).

In the case of an open transistor Q2, the current flows through the circuit:
+U(B1...B4) -> С1 -> С3 -> Т3 -> Т2 -> Q2(k-e) -> -U(B1...B4).

Through the transition capacitors C5, C6 and limiting resistors R5, R7, control signals enter the base of the key transistors, the R4C4 rejector circuit prevents the penetration of impulse noise into the alternating electrical network. Diode D3 and resistor R6 form a discharge circuit for capacitor C5, and D4 and R10 form a discharge circuit Sat.
When current flows through the primary winding of the TK, the process of energy accumulation by the transformer occurs, this energy is transferred to the secondary circuits of the power source and the capacitors C1, C2 are charged. The steady-state operation of the converter will begin after the total voltage on the capacitors C1, C2 reaches +310 V. In this case, the U3 chip (pin 12) will receive power from the source made on the elements D9, R20, C15, C16.
The converter is controlled by a cascade made on transistors Q5, Q6 (Fig. 3). The load of the cascade is the symmetrical half-windings of the transformer T2, at the connection point of which a supply voltage of +16 V is supplied through the elements D9, R23. The operation mode of transistors Q5 and Q6 is set by resistors R33, R32, respectively. The cascade is controlled by pulses of the U3 PWM driver chip coming from pins 8 and 11 to the bases of the cascade transistors. Under the influence of control pulses, one of the transistors, for example Q5, opens, and the second, Q6, respectively, closes. Reliable locking of the transistor is carried out by the D15D16C17 chain. So, when current flows through an open transistor Q5 through the circuit:
+ 16V -> D9 -> R23 -> T2 -> Q5(k-e) -> D15, D16 -> housing.

A voltage drop of +1.6 V is formed in the emitter of this transistor. This value is sufficient to turn off transistor Q6. The presence of capacitor C17 helps to maintain the blocking potential during the "pause".
Diodes D13, D14 are designed to dissipate the magnetic energy accumulated by the half-windings of the T2 transformer.
The PWM controller is based on the AZ7500BP chip (BCD Semiconductor) operating in push-pull mode. The elements of the generator timing circuit are capacitor C28 and resistor R45. Resistor R47 and capacitor C29 form an error amplifier correction circuit 1 (fig.4).

To implement the push-pull mode of operation of the converter, the output stage control input (pin 13) is connected to a reference voltage source (pin 14). From pins 8 and 11 of the microcircuit, control pulses enter the base circuits of transistors Q5, Q6 of the control stage. A voltage of +16 V is supplied to the power output of the microcircuit (pin 12) from the auxiliary converter rectifier.

The “slow start” mode is implemented using error amplifier 2, the non-inverting input of which (pin 16 U3) receives a supply voltage of +16 V through the divider R33R34R36R37C21, and the inverting input (pin 15) receives voltage from the reference source (pin 14 ) from the integrating capacitor C20 and resistor R39.
The non-inverting input of the error amplifier 1 (pin 1 U3) through the adder R42R43R48 receives the sum of the voltages +12 V and +3.3 V. The opposite input of the amplifier (pin 2 U3) through the divider R40R49 is supplied with voltage from the reference source of the microcircuit (pin. 14 U3). Resistor R47 and capacitor C29 are elements of the frequency correction of the amplifier.
Chains of stabilization and protection. The duration of the output pulses of the PWM controller (pin 8, 11 U3) in the steady state is determined by the feedback signals and the sawtooth voltage of the master oscillator. The time interval during which the "saw" exceeds the feedback voltage determines the duration of the output pulse. Consider the process of their formation.

From the output of the error amplifier 1 (pin 3 U3), information about the deviation of the output voltages from the nominal value in the form of a slowly changing voltage is fed to the PWM shaper. Further, from the output of the error amplifier 1, the voltage is supplied to one of the inputs of the pulse-width modulator (PWM). A sawtooth voltage with an amplitude of +3.2 V is supplied to its second input. Obviously, if the output voltage deviates from the nominal values, for example, in the direction of decrease, the feedback voltage will decrease at that value of the sawtooth voltage supplied to the pin. 1, which leads to an increase in the duration of the output pulse cycles. At the same time, more electromagnetic energy is accumulated in the transformer T1, which is transferred to the load, as a result of which the output voltage rises to the nominal value.
In emergency operation, the voltage drop across the resistor R46 increases. In this case, the voltage at pin 4 of the U3 microcircuit increases, and this, in turn, leads to the operation of the “pause” comparator and the subsequent decrease in the duration of the output pulses and, accordingly, to limiting the current flow through the converter transistors, thereby preventing Q1, Q2 from building.

The source also has short-circuit protection circuits in the output voltage channels. The short circuit sensor on channels -12 V and -5 V is formed by elements R73, D29, the midpoint of which is connected to the base of transistor Q10 through resistor R72. The voltage from the +5 V source is also supplied here through the resistor R71. Therefore, the presence of a short circuit in the -12 V (or -5 V) channels will lead to the opening of the transistor Q10 and an overload on terminal 6 of the voltage monitor U4, and this, in turn, will stop the converter from output 4 of the converter U3.
Management, control and protection of the power supply. Almost all computers, in addition to high-quality performance of its functions, require easy and quick on / off. The task of turning on / off the power supply is solved by implementing the principle of remote on / off in modern computers. When the I/O button located on the front panel of the computer case is pressed, the PS_On signal is generated by the processor board. To turn on the power supply, the PS_On signal must be at low potential, i.e. zero, when turned off - high potential.

In the power supply, the tasks of control, monitoring and protection are implemented on the U4 chip of the output voltage monitor of the power supply LP7510. When a zero potential (signal PS_On) arrives at pin 4 of the microcircuit, a zero potential is also formed at pin 3 with a delay of 2.3 ms. This signal is triggering for the power supply. If the signal PS_On high level or the chain of its receipt is broken, then a high level is also set at pin 3 of the microcircuit.
In addition, the U4 chip monitors the main output voltages of the power supply. So, the output voltages of 3.3 V and 5 V power supplies should not go beyond the established limits of 2.2 V< 3,3В < 3,9 В и 3,5 В < 5 В < 6,1 В. В случае их выхода за эти пределы более чем на 146 мкс на выходе 3 микросхемы U4 устанавливается высокий уровень напряжения, и источник питания выключается по входу 4 микросхемы U3. Для источника питания +12 В, контролируемого по выводу 7, существует только контроль над его превышением. Напряжение питания этого источника не должно превышать больше чем 14,4 В. В перечисленных аварийных режимах основной преобразователь переходит в спящий режим путем установления на выводе 3 микросхемы U4 напряжения высокого уровня. Таким способом осуществляется контроль и защита блока питания от понижения и повышения напряжения на выходах его основных источников (рис.5).

In all cases of a high voltage level on pin 3, the voltage on pin 8 is normal, PG is low (zero). In the case when all supply voltages are normal, a low PSOn signal is set at pin 4, and a voltage not exceeding 1.15 V is present at pin 1, a high level signal appears at pin 8 with a delay of 300 ms.
The thermal management circuit is designed to maintain temperature regime inside the power supply case. The circuit consists of a fan and a THR2 thermistor, which are connected to the +12 V channel. Maintaining a constant temperature inside the case is achieved by adjusting the fan speed.
Surge voltage rectifiers use a typical full-wave, mid-point rectifier circuit to provide the required ripple.
The +5 V_SB power supply rectifier is made on a D12 diode. The two-link output voltage filter consists of capacitor C15, inductor L3 and capacitor C19. Resistor R36 - load. Stabilization of this voltage is carried out by microcircuits U1, U2.

The +5 V power supply is made on a D32 diode assembly. The two-link output voltage filter is formed by the winding L6.2 of a multi-winding inductor, inductor L10, capacitors C39, C40. Resistor R69 - load.
The +12 V power supply is similarly executed. Its rectifier is implemented on a D31 diode assembly. The two-link output voltage filter is formed by the winding L6.3 of a multi-winding inductor, inductor L9, capacitor C38. Power supply load - thermal control circuit.
Voltage rectifier +3.3 V - diode assembly D30. The circuit uses a parallel-type stabilizer with a regulating transistor Q9 and a parametric stabilizer U5. The voltage is supplied to the control input U5 from the divider R63R58. Resistor R67 - divider load.
To reduce the level of interference radiated by pulse rectifiers into the electrical network, resistive-capacitive filters are connected in parallel to the secondary windings of the transformer T1 on the elements R20, R21, SU, C11.
Negative voltage power supplies -12 V, -5 V are formed in a similar way. So for a source - 12 V, the rectifier is made on diodes D24, D25, D26, smoothing filter L6.4L5C42, resistor R74 - load.
A voltage of -5 V is formed using diodes D27, 28. The filters of these sources are L6.1L4C41. Resistor R75 - load.

Typical malfunctions
Mains fuse T blown or no output voltages. In this case, it is necessary to check the health of the elements of the barrier filter and the mains rectifier (B1-B4, THR1, C1, C2, V3, V4, R2, R3), and also check the health of transistors Q2, Q3. Most often, if the wrong AC network is selected, the varistors V3, V4 burn out.
The serviceability of the elements of the auxiliary converter, transistors Q1.Q4 is also checked.
If a malfunction is not detected and the failure and failure of the elements considered earlier was not confirmed, then the presence of a voltage of 310 V on the series-connected capacitors C1, C2 is checked. In its absence, the serviceability of the elements of the network rectifier is checked.
Voltage + 5 \ / _ZV is above or below normal. Check the stability of the stabilization circuit U1, U2, the defective element is replaced. As a replacement element for U2, you can use TL431, KA431.
Output supply voltages are above or below normal. We check the health of the feedback circuit - U3 microcircuit, U3 microcircuit piping elements: capacitors C21, C22, C16. If the items listed above are in good condition, replace U3. As analogues of U3, you can use TL494, KA7500V, MB3759 microcircuits.
No P.G signal. You should check the presence of the Ps_On signal, the presence of supply voltages +12 V, +5 V, +3.3 V, +5 B_SB. If present, replace the U4 chip. As an analogue of the LP7510, you can use the TPS3510.
There is no remote power supply activation. Check the presence of the housing potential (zero) on the PS-ON contact, the serviceability of the U4 chip and its binding elements. If the piping elements are in good condition, replace U4.
No fan rotation. Make sure that the fan is working, check the elements of its switching circuit: the presence of +12 V, the serviceability of the thermistor THR2.

D. Kucherov, Radioamator Magazine, No. 3, 5 2011

ADDED 07/10/2012 04:08

I'll add on my own:
Today I had to make myself a power supply unit to replace the burned out again (I think I will not repair it soon) Chieftec 1KWt. I had a 500w Topower silent.

In principle, a good European PSU, with honest power. The problem is the protection works. Those. during normal duty, only a short start. Derg with a valve and everything.
I did not find a short circuit on the main tires, I began to investigate - miracles do not happen. And finally I found what I was looking for - a -12v bus. A banal defect is a broken diode, I didn’t even consider which one. Just replaced with HER207.
I installed this PSU into my system - the flight is normal.

So, they gave a 350-watt Power Man power supply for repair

What do we do first? External and internal inspection. We look at the "offal". Are there any burnt radioelements? Maybe somewhere the board is charred or the capacitor exploded, or it smells like burnt silicon? All this is taken into account during the inspection. Be sure to look at the fuse. If it burned out, then we put a temporary jumper in its place for about the same number of amperes, and then we measure it through two network wires. This can be done on the power supply plug with the “ON” button turned on. It should NOT be too small, otherwise, when you turn on the power supply, it will happen again.

We measure voltage

If everything is OK, we turn on our power supply to the network using the network cable that comes with the power supply, and do not forget about the power button if you had it in the off state.

My patient showed 0 volts on the purple wire. I take and ring the purple wire to the ground. Ground is black wires labeled COM. COM is short for "common", which means "general". There are also some types of "lands":

As soon as I touched the ground and the purple wire, my multimeter beeped meticulously “ppeeeeeeeeeeep” and showed zeros on the display. Short circuit, definitely.

Well, let's look for a circuit for this power supply. Googling the Internet, I found a diagram. But found only on Power Man 300 watts. They will still look the same. The differences in the circuit were only in the serial numbers of the radio components on the board. If you can analyze printed circuit board to match the schema, then it won't be a big problem.

And here is the schematic for Power Man 300W. Click on it to enlarge it in full size.

Looking for the culprit

As we see in the diagram, the standby power, hereinafter referred to as the duty room, is denoted as + 5VSB:

Directly from it comes a zener diode with a nominal value of 6.3 volts to the ground. And as you remember, a zener diode is the same diode, but it is connected in reverse circuits. The zener diode uses the reverse branch CVC. If the zener diode were alive, then our + 5VSB wire would not short to ground. Most likely the zener diode burned out and destroyed.

What happens during the combustion of various radio components from a physical point of view? First, their resistance changes. For resistors, it becomes infinite, or in other words, it goes into a break. With capacitors, it sometimes becomes very small, or in other words, goes into a short circuit. With semiconductors, both of these options are possible, both a short circuit and an open circuit.

In our case, we can only check this in one way, by removing one or both legs of the zener diode at once, as the most likely culprit in the short circuit. Next, we will check whether the short circuit between the duty room and the mass has disappeared or not. Why is this happening?

Here are some simple tips:

1) When connected in series, the greater than larger rule works, in other words, the total resistance of the circuit is greater than the resistance of the largest of the resistors.

2) With a parallel connection, the opposite rule works, less than the smaller one, in other words, the final resistance will be less than the resistance of the resistor of the smaller rating.

You can take arbitrary values ​​​​of the resistances of the resistors, calculate it yourself and see for yourself. Let's try to think logically, if we have one of the resistances of parallel-connected radio components equal to zero, what readings will we see on the multimeter screen? That's right, also equal to zero ...

And until we eliminate this short circuit by soldering one of the legs of the part that we consider problematic, we will not be able to determine in which part we have a short circuit. The thing is that with a sound continuity, ALL parts connected in parallel with a part that is in a short circuit will ring shortly with a common wire!

We are trying to solder the zener diode. As soon as I touched it, it fell apart. No comments…

It's not about the stabilizer.

We check whether the short circuit in the duty room and mass circuits has been eliminated, or not. Indeed, the short circuit is gone. I went to the radio store for a new zener diode and soldered it. I turn on the power supply, and ... I see how my new, just bought zener diode emits magic smoke) ...

And then I immediately remembered one of the main rules of the repairman:

If something burned out, first find the cause of this, and only then change the part to a new one or you risk getting another burned part.

Swearing obscenities to myself, I bite the burned-out zener diode with side cutters and turn on the power supply again.

So it is, the duty room is too high: 8.5 volts. The main question is spinning in my head: “Is the PWM controller still alive, or have I already burned it safely?”. I download the datasheet for the microcircuit and see the maximum supply voltage for the PWM controller, equal to 16 Volts. Uff, it seems like it should carry ...

Checking capacitors

I start to google on my problem on special sites dedicated to the repair of ATX power supplies. And of course, the problem of the overvoltage of the duty room turns out to be a banal increase in the ESR of electrolytic capacitors in the duty room circuits. We look for these capacitors on the circuit and check them.

I remember my assembled ESR meter

It's time to test what he can do.

I check the first capacitor in the duty circuit.

ESR is within normal limits.

Finding the culprit

Checking out the second

I'm waiting for a value to appear on the multimeter screen, but nothing has changed.

I understand that the culprit, or at least one of the culprits of the problem, has been found. I solder the capacitor to exactly the same one, at face value and operating voltage, taken from the donor board of the power supply. I want to go into more detail here:

If you decide to put an electrolytic capacitor in the ATX power supply not from a donor, but a new one from the store, be sure to buy LOW ESR capacitors, not ordinary ones.Ordinary capacitors do not work well in high-frequency circuits, but in the power supply, just such circuits.

So, I turn on the power supply and again measure the voltage at the duty room. Taught by bitter experience, I am no longer in a hurry to install a new protective zener diode and measure the voltage on the duty room, relative to the ground. The voltage is 12 volts and a high-frequency whistle is heard.

Again I sit down to google on the problem of overvoltage at the duty room, and on the site rom.by dedicated to both the repair of ATX power supplies and motherboards, and in general all computer hardware. I find my malfunction by searching in typical malfunctions of this power supply. It is recommended to replace the 10uF capacitor.

I measure the ESR on the capacitor .... Ass.

The result, as in the first case: the device goes off scale. Some say, they say, why collect some devices, such as swollen non-working capacitors, so you can see - they are swollen, or opened with a rose

Yes, I agree with this. But this only applies to large capacitors. Capacitors of relatively small denominations do not swell. In their upper part there are no notches on which they could open up. Therefore, it is simply impossible to determine their performance visually. It remains only to change them to known working ones.

So, having gone through my boards, I also found the second capacitor I needed on one of the donor boards. Just in case, his ESR was measured. It turned out to be normal. After soldering the second capacitor into the board, I turn on the power supply with a key switch and measure the standby voltage. What was required, 5.02 volts ... Hooray!

I measure all other voltages at the power supply connector. All are within the norm. Operating voltage deviations less than 5%. It remains to solder the 6.3 volt zener diode. I thought for a long time why the zener diode is exactly 6.3 Volts when the duty voltage is +5 Volts? It would be more logical to put it on 5.5 volts or similar if it stood to stabilize the voltage on the duty room. Most likely, this zener diode is here as a protective one, so that if the voltage on the duty room rises above 6.3 Volts, it burns out and short circuits the duty room, thereby turning off the power supply and saving our motherboard from burning when it enters her overvoltage through the duty room.

The second function of this zener diode, you see, is to protect the PWM controller from overvoltage. Since the duty room is connected to the power supply of the microcircuit through a fairly low-resistance resistor, therefore, almost the same voltage is supplied to the 20th leg of the power supply of the PWM microcircuit as is present in our duty room.

Conclusion

So, what conclusions can be drawn from this repair:

1) All parts connected in parallel influence each other during the measurement. Their values ​​of active resistances are calculated according to the rule of parallel connection of resistors. In the event of a short circuit on one of the radio components connected in parallel, the same short circuit will be on all other components that are connected in parallel with this one.

2) To identify faulty capacitors, one visual inspection is not enough and it is necessary either to change all faulty electrolytic capacitors in the circuits of the problem node of the device to obviously working ones, or to reject them by measuring with an ESR meter.

3) Having found any burnt part, we are not in a hurry to change it to a new one, but we are looking for the reason that led to its combustion, otherwise we risk getting another burnt part.