Switching power supply troubleshooting. Switching power supply: repair and refinement

Repair of switching power supply. Any person with basic electronic skills can repair the 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. Faults

Attention! Some elements of the power supply are under mains voltage during operation. Make sure you are qualified to safely repair 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 converter of mains voltage

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Very often my clients contact me with the problem that the power supply does not work on any device. Power supplies I divide into two categories: "simple" and "complex". By "simple" I refer to antennas, power supplies from any game consoles, from portable TVs and other similar ones that are directly plugged into the outlet. In a word - remote, i.e. separate from the main device. "Complex" in my distribution scheme are the power supplies that are in the device itself. Well, we will leave the "complex" ones alone for now, but let's talk about the "simple" ones.

There are not very many reasons for the failure of remote power supplies. I will list them all:

1. Break in the windings of the transformer (primary and secondary);

2. Short circuit in the transformer windings;

3. Failure of the voltage rectifier (diode bridge, capacitor, stabilizer and related radio elements).

If, when the unit breaks down, there are no voltages at its output at all, then most likely the reason is in the transformer. If there is an undervoltage at the output, then the problem is with the rectifiers. You can check the transformer by measuring the resistance on its windings. On the primary winding, the resistance must be more than 1 kOhm, on the secondary or secondary - less than 1 kOhm. In some power supplies, on the primary winding, under the wrapper that wraps the winding itself, a fuse is placed. To get to it, you need to break the wrapper on this winding. Most often, such a protection mechanism is present in Chinese-made transformers. So if the primary winding does not ring, then check if a fuse can be installed on it.

Done with the transformer. Now let's move on to checking the voltage rectifier and its components. The most common failure in power supplies is the failure of one or more elements, of which, in fact, the voltage rectifier consists. These are the reasons we will discuss in this article. We will produce do-it-yourself power supply repair.

Let's consider this using the example of an antenna power supply with output voltage 12 V.

On this power supply, the output voltage is too low: instead of the prescribed 12 Volt, it outputs 10 Volt. So let's start fixing this problem. First, of course, you need to disassemble the block itself. After we make sure that the transformer in this device is intact, we proceed to check the rectifier elements.

First of all, we check the diode bridge - these are four diodes, to which there are contacts from the secondary winding of the transformer. I told how to check diodes in the video that you will find at the end of this article. In our block, the diode bridge is intact. Now we look at the capacitor: it happens that the capacitors "swell". Our capacitor is not "bloated". If the diode bridge and capacitors are intact, we inspect the rectifier board for blackening or burning of the elements on the board.

If visually everything is in order, then safely solder the voltage regulator. This rectifier has a voltage stabilizer 12 Volt- 78L12. Almost always it is this element that fails. Before removing this part from the board, remember how this part was installed on the board so that you do not reverse the polarity when replacing. Together with the stabilizer, I also recommend replacing the capacitor, this is for reliability, since most often it also fails.

After replacing these parts, check to see if the wires coming from the transformer have soldered off during the repair process.

If everything is fine, we collect ours. The measurements made after our repair of this power supply showed the output voltage 12 Volt which is basically what we needed. Everything!

Complace service center repairs switching power supplies in a variety of devices.

Switching power supply circuit

Switching power supplies are used in 90% of electronic devices. But you need to know the basic principles of circuitry. Therefore, we present a diagram of a typical switching power supply.

Operation of a switching power supply

Primary circuit 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.

Then there is the CLC filter, and the coil is used to suppress common mode noise. Following the filter is a rectification circuit based on a diode bridge and an electrolytic capacitor. Often, to protect the circuit from 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 out, 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 blow.

Behind the diode is a 4-5 ohm varistor to smooth out sudden surges in current consumption at the time of switching on and initial charging of the capacitor C1.

Active elements of the primary circuit: switching transistor Q1 with PWM (pulse width modulator) control controller. The transistor converts the 310V DC rectified voltage into AC, which is converted by the T1 transformer on the secondary winding into a reduced output.

And yet - to power the PWM controller, a rectified voltage is used, taken from the additional winding of the transformer.

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, consisting of electrolytic capacitors and a choke.

Optical feedback is used to stabilize the output voltage. It allows you to decouple the output and input voltage galvanically. Optocoupler OC1 and integral stabilizer TL431 are used as feedback actuating elements. When the output voltage after rectification exceeds the voltage of the TL431 stabilizer, the photodiode turns on, which turns on the 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 of the switching power supply, let's move on to its repair. Possible malfunctions:

1. If the varistor and fuse at the input or VCR1 burned out, then we look further. Because they don't light up that easily.
2. Broken diode bridge. Usually it is a microchip. If there is a protective diode, then it usually burns. They need to be replaced.
3. Damaged capacitor C1 at 400V. Rarely, but it happens. Often its malfunction can be identified by appearance, but not always.
4. If the switching transistor burned out, then we solder and check it. In the event of a malfunction, replacement is required.
5. If the PWM controller burned out, then we change it.
6. Short circuit or open circuit of the transformer windings. The chances of repair are minimal.
7. Optocoupler failure is extremely rare.
8. Malfunction of the TL431 stabilizer. To diagnose, we measure the resistance.
9. If there is a short circuit in the capacitors at the output of the power supply, then we solder it and diagnose it with a tester.

Examples of repair switching power supplies

For example, consider the repair of a switching power supply for several voltages.

The malfunction consisted in the absence of output voltages at the output of the unit.

For example, in one power supply, two capacitors 1 and 2 in the primary circuit were faulty. But they weren't bloated.

On the second, the PWM controller did not work.

In appearance, all the capacitors in the picture are working, but the internal resistance turned out to be large. Moreover, the internal resistance ESR of the capacitor 2 in the circle was several times higher than the nominal one. This capacitor is in the binding circuit of the PWM regulator, so the regulator did not work. After replacing this capacitor, PWM started working and the power supply was restored.

Prices for the repair of switching power supplies

Prices for the repair of switching power supplies are very different. The fact is that there are a lot of electrical circuits according to which switching power supplies are made. There are especially many differences in circuits with PFC (Power Factor Correction, otherwise the power correction factor), which increase efficiency. The most important thing is whether there is a circuit for a burned-out power supply. If such an electrical circuit is available, then repairing the power supply is greatly simplified.

The repair price ranges from 1,000 rubles for simple power supplies to 10,000 rubles for complex expensive PSUs. The price is determined by the complexity of the power supply, as well as how many elements burned out in it. If all new PSUs are the same, then all the faults are different.

For example, in one complex power supply, 10 elements and 3 tracks burned out. Nevertheless, it was restored, and the repair cost was 8,000 rubles. The device itself costs about 1,000,000 rubles. Such power supplies are not sold in Russia.

The device of Chinese laptop chargers is described.

The reason for the failure of the power supply, or why the equipment stops working. Recently, I began to notice more and more often that people began to apply, and I myself find myself, for a strange and monotonous repair of equipment. It all starts according to approximately the same scenario - the device worked for itself for a year or two, and then suddenly it started to turn on slowly, or not to start at all, or when turned on it turns off abruptly, or it tries to turn on but does not turn on! In general, we take a tester and measure the voltage on it, more precisely at the output terminals, it is usually within the acceptable range, or alternatively it differs by 0.3-0.4 volts down, for example, for 12 volt power supplies it is usually 11.4 volts.

But if you check with an oscilloscope, or a simple tester from the speaker, you can hear high-frequency ripples, so this equipment with such power cannot work without smoothing!

Such capacitors, as a rule, swell outwardly noticeably on the cover or explode at all, when checking they can show a noticeable decrease in capacitance - instead of 1000 microfarads there will be 120-150 microfarads, or even less, or in the tester the capacitor can be defined altogether as another element.

With such a miracle, when the capacitor suddenly becomes a resistor or a diode, the power supply tries to turn on, but the currents become high and in large branded TVs such blocks go into protection. When you try to turn it on again, everything repeats in a circle ...

Often, the filter capacitor can be replaced with an increased capacity, for example, instead of a battery of three capacitors of a rare capacity of 1500 microfarads, it can be set to 4000 microfarads. The main thing is to check the stability of the device and the level of ripples, so that everything is normal, and so that the capacitor is at the right voltage, or better with a voltage margin, then it will be additionally protected from surges.