Repair of switching power supply. Anyone with basic radio-electronic skills can repair a power supply or voltage converter on their own. Take action, identify the problem and fix it. (10+)
We repair the switching power supply ourselves, with our own hands. Malfunctions
Attention! Some elements of the power source are under mains voltage during operation. Make sure you have the necessary qualifications to safely perform repairs on the switching power supply.
Diagnostics and repair of a switching power supply in most cases can be performed with basic skills in radio electronics.
Power supply device, step-down mains voltage converter
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Nowadays, almost all household electrical appliances have special devices called pulse units. They can take the form of either a separate module or a board located in the device design.
Impulse power block
Since pulse units are designed to rectify and reduce the mains voltage, they can often fail. Therefore, in order not to buy a new expensive household device, knowledge of how to fix it yourself will be quite in demand. This article will tell you how to identify malfunctions in the operation of a given device or board, as well as how to repair it yourself.
Description of the voltage converter
The switching power supply can be in the form of a board or an independent remote module. It is intended, as already mentioned, to reduce and rectify the mains voltage. Its necessity is based on the fact that the standard power supply has a voltage of 220 volts, and for the operation of many household appliances a much lower value of this parameter is required.
Today, instead of standard buck-rectifier circuits assembled on the basis of a diode bridge and a power transformer, pulsed voltage conversion power supplies are used.
Note! Despite the presence of high circuit reliability, switching power supplies often break down. Therefore, nowadays it is very important to repair these elements of electrical circuits.
Scheme pulse block nutrition
All types of pulsed power supply (built-in or external to the device) have two functional blocks:
- high voltage. In such a power supply, the mains voltage is converted to DC using a diode bridge. Moreover, the voltage is smoothed to a level of 300.0...310.0 volts on the capacitor. As a result, high voltage is converted into pulsed voltage with a frequency of 10.0...100.0 kilohertz;
Note! This design of the high-voltage unit made it possible to abandon massive low-frequency step-down transformers.
- low voltage. Here the pulse voltage is reduced to an unnecessary level. In this case, the tension is smoothed out and stabilized.
As a result of this structure, at the output of the switching type power supply, several or one voltage is observed, which is needed to power household appliances.
It is worth noting that the low-voltage unit can contain a variety of control circuits that increase the reliability of the device.
Switching power supply (board). Colors are shown in the diagram
Since power supplies of this type have a complex structure, their proper DIY repair must rely on some knowledge in electronics.
When repairing this device, do not forget that some of its elements may be under mains voltage. In this regard, even when conducting an initial inspection of the unit, extreme caution must be observed.
Repair in most cases will not cause complications, because... Switching power supplies have a standard design. Therefore, their faults will also be similar, and do-it-yourself repairs look like a completely feasible task.
Possible causes of failure
Malfunctions that render a switching power supply inoperative can occur for a variety of reasons. Most often breakdowns occur due to:
- presence of mains voltage fluctuations. A malfunction can be caused by vibrations for which these buck-rectifier modules are not designed;
- connecting to the power supply loads that household appliances are not designed for;
- lack of protection. By not installing protection, some manufacturers simply save money. If such a problem is detected, you just need to install the protection in a specific place where it should be;
- failure to comply with operating rules and recommendations specified by manufacturers for specific models.
At the same time, recently common cause Breakdown of voltage converters is due to manufacturing defects or the use of low-quality parts during assembly. Therefore, if you want your purchased switching power supply to work as long as possible, you should not buy it in dubious places and not from trusted people. Otherwise, it might just be wasted money.
After diagnosing the unit, the following faults are often discovered:
- 40% of cases – malfunction of the high-voltage part. This is evidenced by the burnout of the diode bridge, as well as the breakdown of the filter capacitor;
- 30% - breakdown of a bipolar (forming high-frequency pulses and located in the high-voltage part of the device) or power field-effect transistor;
- 15% - breakdown of the diode bridge in its low-voltage part;
Diode bridge
- Burnout (breakdown) of the inductor windings on the output filter is rare.
All other breakdowns can only be determined using special equipment, which is unlikely to be kept at home by the average person. For a deeper and more accurate check, a digital voltmeter and an oscilloscope are required. Therefore, if the breakdown does not lie in the four options above, then you will not be able to repair a power supply of this type at home.
As you can see, repairs carried out in this situation with your own hands can take a wide variety of forms. Therefore, if your computer or TV has stopped working due to a broken power supply, then you do not need to run to a repair service, but you can try to solve the problem on your own. At the same time, home repairs will cost significantly less. But if you cannot cope with the task on your own, then you can already go to the specialists from the repair service.
Failure detection algorithm
Any repair always begins with finding out the cause of the malfunction of the switching power supply.
Note! To repair and troubleshoot a switching power supply, you will need a voltmeter.
Voltmeter
In order to identify it, you must adhere to the following algorithm:
- disassemble the power supply;
- using a voltmeter we measure the voltage that is present on the electrolytic capacitor;
Measuring the voltage on an electrolytic capacitor
- if the voltmeter produces a voltage of 300 V, then this means that the fuse and all elements of the electrical network (power cable, line filter, input chokes) associated with it are working normally;
- in models with two small capacitors, the voltage indicating their serviceability, which is given by the voltmeter, should be 150 V for each device;
- if there is no voltage, then it is necessary to test the diodes of the rectifier bridge, fuse and capacitor;
Note! The most insidious elements in the electrical circuit of a switching type power supply are fuses. There are no external signs indicating their breakdown. Only dialing will help you identify their malfunction. If burned, they will produce high resistance.
Switching Power Supply Fuses
- if a faulty fuse is detected, then you need to check the remaining elements of the electrical circuit, since they rarely burn out alone;
- From the outside it is quite easy to identify a damaged capacitor. It usually swells or collapses. Repair in this case will consist of desoldering it and replacing it with a functional one.
- It is imperative to check the following elements for serviceability:
- rectifier or power bridge. It has the form of a monolithic block or is organized from four diodes;
Switching power supply power bridge
- filter capacitor. It may look like one or more blocks that are connected to each other in series or in parallel. Typically the filter capacitor is located in the high-voltage part of the block;
- transistors placed on the radiator.
Pay attention! When carrying out repairs, you need to immediately find all the faulty parts of the switching power supply, since they should be desoldered and replaced at the same time! Otherwise, replacing one element will lead to burnout of the power unit.
Features of repair work and tools for them
For a standard type of device, the above diagnostic and diagnostic steps repair work will be identical. This is due to the fact that they all have a typical structure.
Soldering parts to the board
Also, in order to carry out high-quality independent repairs of a pulse voltage converter, you need a good soldering iron, as well as the ability to operate it. In this case, you will also need solder, alcohol, which can be replaced with purified gasoline, and flux.
In addition to a soldering iron, the following tools will definitely be needed for repairs:
- Screwdriver Set;
- tweezers;
- household multimeter or voltmeter;
- incandescent lamp. Can be used as ballast load.
With such a set of tools, anyone can do simple repairs.
Carrying out repair work
When planning to repair a damaged pulse voltage converter with your own hands, you need to understand that such manipulations are not carried out for products intended for complex replacement. They are not designed for repair and no technician will undertake to repair them, since it requires complete dismantling of the electronic filling and replacing it with a new working one.
Switching power supply board
In all other cases, repairs at home and with your own hands are quite possible.
Proper diagnosis is half the repair. Faults associated with the high-voltage part can be easily detected both visually and using a voltmeter. But a fuse malfunction can be detected if there is no voltage in the area after it.
If faults are detected with its help, all that remains is to simply replace them simultaneously. When carrying out repair work, it is necessary to rely on the appearance of the electronic board. Sometimes, in order to check each part, it is necessary to unsolder it and test it with a multimeter. It is advisable to check all details. Despite the difficulty of such a process, it will allow us to identify all damaged elements of the electrical circuit and replace them in time to prevent burnouts of the device in the foreseeable future.
Replacing burnt out parts
After all the burnt out parts have been replaced, you need to install a new fuse and check the repaired power supply by turning it on. Usually, if everything was done correctly, and all standards and instructions for repair work were followed, the converter will work.
Typically, diagnosing a problem with a TV takes much more time than fixing the identified problem. Of course, you can always entrust this work to a professional, but in the end the whole procedure will be delayed even more. Therefore, users often try to repair the TV power supply with their own hands. Is it worth doing? How to proceed when repairing it yourself? What nuances do you need to pay attention to in order to be sure that the power supply is faulty and not cause even more damage to the TV? You will find answers to all these and numerous other questions in this material.
Manifestation of PSU malfunction
Unlike other TV components, any failure of the power supply immediately affects the performance of the TV as a whole. This means that after turning on the TV to the network, the activity indicator will not even light up, not to mention the output of sound, pictures or other signs of life. The breakdown may be as follows:
- the TV does not turn on and the LED does not light up;
- the device does not function due to protection in the power supply, which is usually accompanied by a whistling noise from the pulse transformer. This manifestation may also indicate the necessary;
- The output voltage is too low or too high from the power supply.
If the device can turn on and simply exhibits some kind of defect in its operation, then this is most likely caused by another component of the TV, and not the power supply. However, there are also a number of exceptions in which the problem is still associated with the power supply:
- the device does not turn on, although the standby LED is on;
- the image appears some time after the sound;
- To get a normal picture and sound, the TV must be turned on and off several times.
Separately, it is also worth mentioning possible breakdowns of other components of the TV, which are not caused by problems with the power supply, but directly affect its operation. These include power supply units, feedback circuits, power supply loads, and so on.
Main reasons
Failure of the power supply is one of the most common breakdowns of modern devices. The cause of this malfunction can be caused by many factors, but among them there are 4 main ones:
- Unstable voltage. If the voltage in the outlet constantly “jumps,” then it can not only degrade the performance of the TV, but also lead to wear and tear of its components.
- Short circuit. Causes burnout of the power supply or other components of the TV.
- Burnt out mains fuse. In this case, the standby indicator will not light up.
- Capacitors wear out over time. A very common problem that does not depend on external factors. Worn capacitors can be identified by their swelling.
Analysis and determination of the problem
The first step is to disassemble the TV by removing the back cover of the device, which is secured with screws around the perimeter. Depending on the model and manufacturer of your TV, you can gain access to the power supply after this stage.
If you did not notice this part after removing the cover, then it is protected by a protective metal casing. In some models, it is also possible to install another protection specifically for the power supply. At each stage, you will need to unscrew the screws in a circle that secure the component to be removed.
What does a power supply unit and its components look like?
Before you start repairing the TV power supply, you need to understand what this component looks like. All modern models have several power supplies, but they are all placed on one board. It is not at all difficult to distinguish it from others, because in addition to capacitors and other components, this board also contains three transformers (painted black and yellow).
As for the components of the power supply, they are as follows:
- Standby power supply. In order for the device to be in standby mode (the LED is lit) and await any command, it must receive a voltage of 5V. It is the duty power supply unit that supplies it to the TV.
- Inverter block. If the TV tries to turn on, but then immediately goes back to standby mode, then the problem is related to this part. It is responsible for powering the corresponding component, so if there is no power, the processor cannot receive confirmation of functionality from the inverter and replaces the mode with standby mode.
- BlockPFC. Power is divided into active and reactive. The first one does useful work, and the reactive one simply passes from the generator to the load and back. The second type can be inductive or, as is typical for TVs, capacitive (capacitors). Reactive power is needed for the operation of the TV, but it can significantly increase energy consumption and also wear out capacitors faster, which negatively affects the durability of the power supply as a whole.
To eliminate these phenomena, a special PFC (Power Factor Correction) block is used, which, as its name suggests, deals with power factor correction.
Checking the voltage in the socket on the control unit
The TV may not turn on due to unstable voltage, so this problem is solved with the help of a stabilizer. Also, the cause is often a malfunctioning extension cord or socket. Moreover, different power sources in the apartment can be connected to different machines in the panel, so the presence of light in the house does not mean that the outlet you need is supplied with electricity. If there are no problems with the power supply, then you need to use a tester to ring the output of the standby power supply.
The result should be 5V, and if you get a lower value or no voltage at all, then the problem is worn capacitors. They can be identified by visual inspection, because such components will be swollen.
In the same case, when no problem is found here either, it is necessary to check the fuse. To do this, you also need to ring it, checking if there is a short circuit somewhere. Additionally, you need to inspect the back of the board by removing it from the frame.
Troubleshooting
First of all, it is necessary to discharge the input capacitors. If this is not done, then during the repair process there may be a short circuit or other problems that will lead to more serious damage. To discharge, you can use a low-resistance resistor, a tester or a regular light bulb, brought to the contacts for a few seconds. After this, you can unsolder the damaged capacitors and replace them with working ones with the same power.
Important! Any repair of a power supply is associated with a number of risks. If you act carelessly, you can cause even greater harm to the TV or even your own health. If you have any doubts about your own abilities, you should trust the repair procedure to an experienced technician.
A video lesson from a master will tell you in detail about the entire process of repairing a power supply:
Conclusion
Repairing a TV power supply is one of the most popular services in workshops. It is this component that most often fails in modern TVs. If you encounter a similar situation, you can fix the problem yourself. Our detailed guide, containing the necessary recommendations and explanations.
The Complace service center repairs switching power supplies in a wide variety of devices.
Switching power supply circuit
Switching power supplies are used in 90% of electronic devices. But to do this you need to know the basic principles of circuit design. Therefore, we present a diagram of a typical switching power supply.
Switching power supply operation
Primary circuit of a switching power supply
The primary circuit of the power supply circuit is located before the pulse ferrite transformer.
There is a fuse at the input of the unit.
Next comes the CLC filter. The coil, by the way, is used to suppress common-mode interference. Following the filter is a rectifier based on a diode bridge and an electrolytic capacitor. To protect against short high-voltage pulses, a varistor is installed after the fuse in parallel with the input capacitor. The resistance of the varistor drops sharply at increased voltage. Therefore, all excess current goes through it to the fuse, which burns, turning off the input circuit.
The protective diode D0 is needed in order to protect the power supply circuit if the diode bridge burns out. The diode will not allow negative voltage to pass into the main circuit. Because the fuse will open and burn.
Behind the diode there is a 4-5 ohm varistor to smooth out sudden jumps in current consumption at the moment of switching on. And also for the initial charging of capacitor C1.
The active elements of the primary circuit are as follows. Switching transistor Q1 and PWM (pulse width modulator) controller. The transistor converts the rectified DC voltage of 310V into alternating voltage. It is converted by transformer T1 on the secondary winding into a reduced output.
And one more thing - to power the PWM regulator, the rectified voltage taken from the additional winding of the transformer is used.
Operation of the secondary circuit of the switching power supply
In the output circuit after the transformer there is either a diode bridge or 1 diode and a CLC filter. It consists of electrolytic capacitors and a choke.
Optical feedback is used to stabilize the output voltage. It allows you to galvanically decouple the output and input voltages. Optocoupler OC1 and integrated stabilizer TL431 are used as feedback actuators. If the output voltage after rectification exceeds the voltage of the TL431 stabilizer, the photodiode is turned on. It includes a phototransistor that controls the PWM driver. The TL431 regulator reduces the duty cycle of the pulses or stops altogether. Until the voltage drops to the threshold.
Repair of switching power supplies
Malfunctions of switching power supplies, repair
Based on the circuit diagram of the switching power supply, we will move on to its repair. Possible malfunctions:
- If the varistor and the fuse at the input or VCR1 have burned out, then we look further. Because they just don't burn.
- The diode bridge burned out. Usually this is a microcircuit. If there is a protective diode, then it usually lights up. They need to be replaced.
- The 400V capacitor C1 is damaged. Rarely, but it happens. Often its malfunction can be identified by its appearance. But not always. Sometimes a seemingly good capacitor turns out to be bad. For example, by internal resistance.
- If the switching transistor burns out, then unsolder it and check it. If faulty, replacement is required.
- If the PWM regulator burns out, then replace it.
- Short circuit, as well as breakage of the transformer windings. The chances of repair are minimal.
- An optocoupler malfunction is an extremely rare case.
- Malfunction of the TL431 stabilizer. For diagnostics, we measure the resistance.
- If there is a short circuit in the capacitors at the output of the power supply, then we unsolder it and diagnose it with a tester.
Examples of repair of switching power supplies
For example, consider the repair of a switching power supply for several voltages.
The malfunction was the absence of output voltage at the output of the block.
For example, in one power supply two capacitors 1 and 2 in the primary circuit were faulty. But they weren't swollen.
On the second one the PWM controller did not work.
All the capacitors in the picture appear to be working, but the internal resistance turned out to be high. Moreover, the internal ESR resistance of capacitor 2 in the circle was several times higher than the nominal one. This capacitor is in the PWM regulator circuit, so the regulator did not work. The functionality of the power supply was restored only after replacing this capacitor. Because PWM worked.
Repair of computer power supplies
An example of repairing a computer power supply. An expensive 800 W power supply came in for repair. When it was turned on, the circuit breaker was knocked out.
It turned out that the short circuit was caused by a burnt-out transistor in the primary power circuit. The repair price was 3,000 rubles.
It makes sense to repair only high-quality, expensive computer power supplies. Because repairing a power supply may be more expensive than a new one.
Prices for repair of pulsed power supplies
Prices for repairing switching power supplies vary greatly. The fact is that there are a lot electrical diagrams switching power supplies. There are especially many differences in circuits with PFC (Power Factor Correction). ZAS increases efficiency.
But the most important thing is whether there is a circuit diagram for the burnt-out power supply. If such an electrical diagram is available, then repairing the power supply is greatly simplified.
The repair price ranges from 1000 rubles for simple power supplies. But it reaches 10,000 rubles for complex expensive power supplies. The price is determined by the complexity of the power supply. And also how many elements burned in it. If all new power supplies are the same, then all faults are different.
For example, in one complex power supply, 10 elements and 3 tracks burned out. Nevertheless, it was possible to restore it, and the cost of repair was 8,000 rubles. By the way, the device itself costs about 1,000,000 rubles. Such power supplies are not sold in Russia.
Chinese device chargers for laptops described .
D-Link power supply
Introduction.
We have already considered the classic; we deliberately omitted some points for a simpler presentation of the material. Practice has shown that some specialists have questions even after reading the publication; we will try to correct this gap. The material is independent and strictly focused on repairing the power supply with PWM UC3843 (3842,3844,3845). As an example, we will consider the already discussed power supply D-Link JTA0302D-E (5V*2A) made on PWM 3843 due to its classic design.
Circuit design.
Although some of the power supplies we repair do not have native circuits, we carry out most repairs of power supplies based on PWM 3843 (3842,3844,3845) according to the circuit diagram below.
Power supply circuit D-Link JTA0302D-E (5V*2A), this circuit design is typical for canonical circuit options.
Although such a circuit does not meet the standards, it is as close as possible to the canonical version of the design of electrical circuits. Some signs indicate that the circuit was copied from a ready-made power supply, which means that this is how the author sees it. If we had drawn this diagram, we would have gotten a slightly different option, which would be easier to repair, the diagram is from a slightly different power supply, the feedback circuits are drawn somewhat chaotically, cold and hot ground, but still it is easier to do diagnostics using it.
D-Link 5V*2A power supply circuit, this circuit design is typical for visual repair aids.
The difference between these two circuits in the element base is small, but there are serious differences in execution, if the first circuit is oriented to GOST, then the second circuit was drawn by a specialist who had previously repaired a similar power supply.
Terminology.
Since the material is designed for a specialist who rarely repairs switching power supplies, searching through related resources or answers from more experienced colleagues sometimes leads to a dead end, instead of helping in solving the problem. This comes from the specific terminology used among specialists when repairing power supplies. It is worth noting that terminology may vary from region to region, for example a grifflic may be called a snubber, and a starting capacitor may be called a first strike capacitor.
D-Link 5V*2A power supply diagram, with minor adjustments, for ease of reading.
Block diagram of the D-Link 5V*2A power supply
To avoid ambiguity, we will specifically write down each element of the block diagram; we will consider the functionality and diagnostic features later.
1.Input filter
Fuse F1 (2.25A) there may be a typo or an unfortunate abbreviation here, most likely it means 2A * 250V, functionally it does not filter, but we classified it as an input filter circuit
The thermistor TR (5 Ohm) is necessary for the “soft start” of the power supply at the moment of switching on, and although functionally it does not filter, we classified it as an input filter circuit.
X-capacitor XC1 (100 pF*250B), here it is worth paying attention - this is an X capacitor.
Choke L1 - as a rule, this is a wire choke on ferrite (not permalloy), made in the form of a transformer.
2.Input rectifier
Diode bridge DB1-DB4(1N4007)
Input rectifier capacitor C1 (33uF*400V)
3.High frequency transformer
T1.1 High voltage (primary) winding
T1.2 Winding for PWM power supply
T1.3 Low voltage (secondary) winding
4. Griflik.
Resistor R1 (39 kOhm) is rarely available in planar design, since it dissipates significant power
Capacitor C2 (4700 pF * 2 kV) the use of a low-voltage capacitor in this circuit is unacceptable.
High-speed diode VD1(PS1010R) - despite the operating voltage of the capacitor is 2kV, the operating voltage of this diode is usually 1kV, with a good current of 1A.
5. Output rectifier.
Schottky diode VD5-VD6 (SB340) The use of Schottky diodes allows you to do without additional cooling elements at low powers.
Capacitors LowESR C9, C10 (680 µF * 10V) the use of conventional capacitors is acceptable, but it sharply reduces the life of the power supply, since these capacitors operate in a very harsh mode.
Choke L2 performs a double function - it is a storage device for capacitor C20, and is also a filter element.
Capacitor C20 (220 µF * 10V) - thanks to inductor L2, it operates in normal mode and there are no special requirements for this capacitor, except for weight and size parameters.
Resistor R21 (220 Ohm) is not formally an element of the output rectifier, but serves for quick discharge of C9, C10, C20, L2.
6. Power key.
MOS transistor with n-channel VT1(P4NK60Z), field-effect transistor with which PWM UC3843 is designed to work
7. Current sensor.
Resistor R2 (1.5 Ohm), despite the fact that it dissipates significant power, is found in both planar and wire versions. In the case of planar design, it is dialed by connecting several planar resistors in parallel.
Resistor R8 (300 Ohm), R3 (750 kOhm) and C4 (10 nF) we did not want to add these elements to the current sensor section, since they create some confusion in terminology, because The term current sensor refers specifically to resistor R2 (1.5 Ohm) and only he, but you can’t erase the word from the song, since formally these elements are also circuits of the current sensor, we are forced to mention them, thereby creating some confusion in the terminology of the current sensor.
8. Start circuit.
Resistor R4 (300 kOhm), despite its simplicity, is one of the most complex elements of the power supply, as it determines possible replacements of PWM with analogues, it is this element that looks like a faulty element, since it dissipates significant power, it is when replacing this resistor that they forget to look at the operating voltage resistor, but it must be at least 400 V; for example, a planar resistor of size 1206 has a maximum operating voltage of 250V.
9. Working food
10.Starting capacitor.
Capacitor C6 (47 µF * 25V) - without exaggeration, can be called the main element of a switching power supply. Indirectly, as soon as the mechanic begins to see this capacitor just by looking at the power supply, we can talk about the qualifications of this repairman. Note that this element must always be replaced during any repair of a switching power supply; neglecting this recommendation turns the repair into a fight against windmills.
11. PWM.
U2(UC3843) – needs no introduction, let’s just note that this is the easiest to implement and most reliable PWM for its time.
12. Power key driver.
Resistor R5 (150 Ohms), the circuit under consideration is the most unfortunate example for considering a power switch driver, since for the most part, the driver has a radical difference from the one under consideration, usually it is a resistor with a nominal value of 15-30 Ohms.
13. External generator circuits.
Resistor R11 (3 kOhm) and capacitor C5 (10 nF) set the generation frequency.
14. Feedback.
Divider on resistors R22 (5.25 kOhm) and R23 (4.87 kOhm)
Current limiting resistor R17 (470 Ohm)
Galvanic isolation optocoupler U1.1, U1.2
Adjustable Zener diode U3(KA431AZ)
Feedback circuit correction elements capacitors C12 (1uF*50V), C3(10nF)
Separately, it is worth noting the noise-suppressing Y capacitor YC2 (2200pF), but not so much because of its functionality, but thanks to it it is possible (and necessary) to distinguish between “hot” and “cold” ground.