Repair of switching power supply for dummies. Power supply repair

A little about the application and design of the UPS

An article has already been published on the site, which talks about the design of the UPS. This topic can be somewhat supplemented with a short story about repairs. The abbreviation UPS is often referred to. To avoid any discrepancies, let us agree that in this article this is a Switching Power Supply.

Almost all switching power supplies used in electronic equipment are built according to two functional circuits.

Fig.1. Functional diagrams switching power supplies

As a rule, fairly powerful power supplies, such as computer ones, are made using a half-bridge circuit. By push-pull circuit Power supplies for powerful stage UMZCHs and welding machines are also manufactured.

Anyone who has ever repaired amplifiers with a power of 400 watts or more knows very well how much they weigh. We are talking, naturally, about UMZCH with traditional transformer block nutrition. UPSs for televisions, monitors, and DVD players are most often made according to a circuit with a single-ended output stage.

Although in reality there are other types of output stages, which are shown in Figure 2.

Fig.2. Output stages of switching power supplies

Only power switches and primary winding power transformer.

If you look closely at Figure 1, it is easy to see that the entire circuit can be divided into two parts - primary and secondary. The primary part contains a network filter, a network voltage rectifier, power switches and a power transformer. This part is galvanically connected to the AC network.

In addition to the power transformer, switching power supplies also use decoupling transformers, through which control pulses of the PWM controller are supplied to the gates (bases) of power transistors. In this way, galvanic isolation from the secondary circuit network is ensured. In more modern schemes, this decoupling is carried out using optocouplers.

The secondary circuits are galvanically isolated from the network using a power transformer: the voltage from the secondary windings is supplied to the rectifier, and then to the load. Voltage stabilization and protection circuits are also powered from the secondary circuits.

Very simple switching power supplies

They are performed on the basis of a self-oscillator when there is no master PWM controller. An example of such a UPS is the Taschibra electronic transformer circuit.

Fig.3. Electronic transformer Taschibra

Similar electronic transformers are produced by other companies. Their main purpose is . A distinctive feature of this scheme is its simplicity and small number of parts. The disadvantage is that without a load this circuit simply does not start, the output voltage is unstable and has high level pulsations. But the lights still shine! In this case, the secondary circuit is completely disconnected from the supply network.

It is quite obvious that repairing such a power supply comes down to replacing transistors, resistors R4, R5, sometimes VDS1 and resistor R1, which acts as a fuse. There is simply nothing else to burn in this scheme. Given the low price of electronic transformers, more often than not, a new one is simply purchased, and repairs are done, as they say, “for the love of art.”

Safety First

Since there is such a very unpleasant juxtaposition of the primary and secondary circuits, which during the repair process you will definitely have to touch with your hands, even if by accident, then some safety rules should be recalled.

You can touch the switched-on source with only one hand, and in no case with both at once. Anyone who works with electrical installations knows this. But it is better not to touch at all, or only after disconnecting from the network by pulling the plug from the socket. Also, you should not solder anything while the source is on or simply twist it with a screwdriver.

In order to ensure electrical safety on power supply boards, the “dangerous” primary side of the board is outlined with a fairly wide stripe or shaded with thin strips of paint, more often white. This is a warning that touching this part of the board with your hands is dangerous.

Even a switched-off switching power supply can be touched with your hands only after some time, at least 2...3 minutes after turning off: the charge on high-voltage capacitors is retained for quite a long time, although in any normal power supply there are discharge resistors installed in parallel with the capacitors. Remember how in school they offered each other a charged capacitor! Killing, of course, will not kill, but the blow is quite sensitive.

But the worst thing is not even this: well, just think, it stung a little. If you immediately test the electrolytic capacitor with a multimeter after turning it off, then it is quite possible to go to the store for a new one.

When such a measurement is anticipated, the capacitor must be discharged, at least with tweezers. But it is better to do this using a resistor with a resistance of several tens of kOhms. Otherwise, the discharge is accompanied by a bunch of sparks and a fairly loud click, and such a short circuit is not very useful for the capacitor.

And yet, during repairs you have to touch the switched-mode power supply, at least to take some measurements. In this case, an isolation transformer, often called a safety transformer, will help protect your loved one as much as possible from electric shock. You can read how to make it in the article.

In a nutshell, this is a transformer with two windings for 220V, with a power of 100...200W (depending on the power of the UPS being repaired), the electrical diagram is shown in Figure 4.

Fig.4. Safety transformer

The winding on the left in the diagram is connected to the network; a faulty switching power supply is connected to the right winding through a light bulb. The most important thing with this connection is that you can safely touch any end of the secondary winding with ONE hand, as well as the entire element of the primary circuit of the power supply.

About the role of the light bulb and its power

Most often, repairs to a switching power supply are carried out without an isolating transformer, but as an additional safety measure, the unit is turned on through a 60...150W light bulb. By the behavior of the light bulb, you can, in general, judge the state of the power supply. Of course, such inclusion will not provide galvanic isolation from the network; it is not recommended to touch it with your hands, but it may well protect against smoke and explosions.

If, when plugged into the network, the light bulb lights up at full intensity, then you should look for a fault in the primary circuit. As a rule, this is a broken power transistor or rectifier bridge. During normal operation of the power supply, the light bulb first flashes quite brightly (), and then the filament continues to glow faintly.

There are several opinions about this light bulb. Some say that it does not help get rid of unforeseen situations, while others believe that the risk of burning a newly sealed transistor is much reduced. We will adhere to this point of view and use a light bulb for repairs.

About collapsible and non-demountable housings

Most often, switching power supplies are made in cases. Enough to remember computer blocks power supply, various adapters plugged into the outlet, charging device for laptops, mobile phones and so on.

In the case of computer power supplies, everything is quite simple. Several screws are unscrewed from the metal case, the metal cover is removed and, please, the entire board with the parts is already in your hands.

If the case is plastic, then you should look on the back side, where the power plug is located, for small screws. Then everything is simple and clear, unscrew and remove the cover. In this case, we can say that we were just lucky.

But lately everything has been moving along the path of simplifying and reducing the cost of designs, and the halves of the plastic case are simply glued together, and quite firmly. One friend told me how he took a similar block to some workshop. When asked how to disassemble it, the craftsmen said: “Aren’t you Russian?” Then they took a hammer and quickly split the body into two halves.

In fact, this is the only way to disassemble plastic glued cases. You just need to hit it carefully and not very fanatically: under the influence of blows to the body, the tracks leading to massive parts, for example, transformers or chokes, can be broken.

It also helps to insert a knife into the seam and lightly tap it with the same hammer. True, after assembly traces of this intervention remain. But even if there are minor marks on the case, you won’t have to buy a new unit.

How to find a diagram

If in the old days almost all devices domestic production circuit diagrams were attached, then modern foreign electronics manufacturers do not want to share their secrets. All electronic equipment is equipped only with a user manual, which shows which buttons to press. Schematic diagrams are not included with the user manual.

It is assumed that the device will work forever or that repairs will be carried out at authorized service centers where repair manuals, called service manuals, are available. Service centers They do not have the right to share this documentation with everyone, but, praise the Internet, it is possible to find these service manuals for many devices. Sometimes this can be done free of charge, that is, for nothing, and sometimes the necessary information can be obtained for a small amount.

But even if you couldn’t find the required circuit, you shouldn’t despair, especially when repairing power supplies. Almost everything becomes clear upon careful examination of the board. This powerful transistor is nothing more than an output switch, and this chip is a PWM controller.

In some controllers, the powerful output transistor is “hidden” inside the chip. If these parts are large enough, then they have full markings, from which you can find the technical documentation (data sheet) of the microcircuit, transistor, diode or zener diode. It is these parts that form the basis of switching power supplies.

It is somewhat more difficult to find datasheets for small-sized SMD components. Full markings do not fit on a small case; instead, a code designation of several (three, four) letters and numbers is placed on the case. Using this code, using tables or special programs, again found on the Internet, it is possible, although not always, to find reference data for an unknown element.

Measuring instruments and tools

To repair switching power supplies, you will need the tool that every radio amateur should have. First of all, these are several screwdrivers, side cutters, tweezers, sometimes pliers and even the hammer mentioned above. This is for plumbing and installation work.

For soldering work, of course, you will need a soldering iron, preferably several, of varying power and dimensions. A regular soldering iron with a power of 25...40 W is quite suitable, but it is better if it is a modern soldering iron with a thermostat and temperature stabilization.

To solder multi-lead parts, it’s good to have on hand, if not a super expensive one, then at least a simple inexpensive soldering gun. This will allow you to solder multi-pin parts without much effort and destruction of printed circuit boards.

To measure voltages, resistances and, somewhat less frequently, currents, you will need a digital multimeter, even if not a very expensive one, or a good old pointer tester. That pointer device It’s too early to write off what he gives additional features, which modern digital multimeters do not have, can be read in the article.

Can provide invaluable assistance in repairing switching power supplies. Here, too, it is quite possible to use an old, even not very broadband, cathode-ray oscilloscope. If, of course, it is possible to purchase a modern digital oscilloscope, then that is even better. But, as practice shows, when repairing switching power supplies you can do without an oscilloscope.

Actually, when repairing, there are two possible outcomes: either repair it or make it even worse. Here it is appropriate to recall Horner’s law: “Experience grows in direct proportion to the number of equipment disabled.” And although this law contains a fair amount of humor, in the practice of repairs things are exactly this way. Especially at the beginning of the journey.

troubleshooting

Switching power supplies fail much more often than other electronic equipment components. The first effect is that there is a high mains voltage, which after rectification and filtering becomes even higher. Therefore, the power switches and the entire inverter cascade operate in very difficult conditions, both electrically and thermally. Most often, faults lie in the primary circuit.

Faults can be divided into two types. In the first case, failure of a switching power supply is accompanied by smoke, explosions, destruction and charring of parts, sometimes of printed circuit board tracks.

It would seem that the option is the simplest, you just need to change the burnt parts, restore the tracks, and everything will work. But when trying to determine the type of microcircuit or transistor, it turns out that the part markings have disappeared along with the housing. It is impossible to find out what was here without a diagram, which is often not at hand. Sometimes the repair ends at this stage.

The second type of malfunction is quiet, as Lyolik said, without noise and dust. The output voltages simply disappeared without a trace. If this switching power supply is a simple network adapter like a charger for a cell phone or laptop, then first of all you should check the serviceability of the output cord.

Most often, a break occurs either near the output connector or at the exit from the housing. If the unit is connected to the network using a cord with a plug, then first of all you should make sure that it is in working order.

After checking these simplest circuits, you can already go into the wilds. For these wilds, let's take the power supply circuit of the 19-inch LG_flatron_L1919s monitor. Actually the fault was quite simple: it turned on yesterday, but today it doesn’t turn on.

Despite the apparent seriousness of the device - after all, a monitor, the power supply circuit is quite simple and clear.

After opening the monitor, several swollen electrolytic capacitors (C202, C206, C207) were discovered at the output of the power supply. In this case, it is better to change all the capacitors at once, six in total. The cost of these parts is cheap, so you shouldn’t wait for them to swell too. After this replacement, the monitor started working. By the way, such a malfunction is quite common in LG monitors.

Swollen capacitors triggered the protection circuit, the operation of which will be discussed a little later. If after replacing the capacitors the power supply does not work, you will have to look for other reasons. To do this, let's look at the diagram in more detail.

Fig 5. Power supply of the LG_flatron_L1919s monitor (click on the picture to enlarge)

Surge filter and rectifier

The mains voltage is supplied to the rectifier bridge BD101 through the input connector SC101, fuse F101, and filter LF101. The rectified voltage through the thermistor TH101 is supplied to the smoothing capacitor C101. This capacitor produces a constant voltage of 310V, which is supplied to the inverter.

If this voltage is absent or much less than the specified value, then you should check the mains fuse F101, filter LF101, rectifier bridge BD101, capacitor C101, and thermistor TH101. All of these details can be easily checked using a multimeter. If you suspect capacitor C101, then it is better to replace it with a known good one.

By the way, the mains fuse doesn't just blow. In most cases, replacing it does not lead to recovery normal operation switching power supply. Therefore, you should look for other reasons leading to the blown fuse.

The fuse should be installed at the same current as indicated on the diagram, and in no case should the fuse be “powered up”. This may lead to even more serious problems.

Inverter

The inverter is made according to a single-cycle circuit. The PWM controller chip U101 is used as a master oscillator, to the output of which the power transistor Q101 is connected. The primary winding of transformer T101 (pins 3-5) is connected to the drain of this transistor through inductor FB101.

Additional winding 1-2 with rectifier R111, D102, C103 is used to power the PWM controller U101 in steady state operation of the power supply. The PWM controller is started when turned on by resistor R108.

Output voltages

The power supply produces two voltages: 12V/2A to power the backlight inverter and 5V/2A to power the logical part of the monitor.

From winding 10-7 of transformer T101 through the diode assembly D202 and filter C204, L202, C205, a voltage of 5V/2A is obtained.

Winding 8-6 is connected in series with winding 10-7, from which, using the diode assembly D201 and filter C203, L201, C202, C206, C207, a constant voltage of 12V/2A is obtained.

Overload protection

Resistor R109 is connected to the source of transistor Q101. This is a current sensor, which is connected through resistor R104 to pin 2 of the U101 chip.

When there is an overload at the output, the current through transistor Q101 increases, which leads to a voltage drop across resistor R109, which is supplied through resistor R104 to pin 2CS/FB of microcircuit U101 and the controller stops generating control pulses (pin 6OUT). Therefore, the voltage at the output of the power supply disappears.

It was this protection that was triggered when the electrolytic capacitors were swollen, which were mentioned above.

Protection level 0.9V. This level is set by the reference voltage source inside the microcircuit. A Zener diode ZD101 with a stabilization voltage of 3.3V is connected in parallel with resistor R109, which protects the 2CS/FB input from high voltage.

A voltage of 310V from capacitor C101 is supplied to pin 2CS/FB through a divider R117, R118, R107, which ensures that protection against increased network voltage is triggered. The permissible range of mains voltage at which the monitor operates normally is in the range of 90…240V.

Output voltage stabilization

Made on an adjustable zener diode U201 type A431. The output voltage of 12V/2A through the divider R204, R206 (both resistors with a tolerance of 1%) is supplied to the control input R of the zener diode U201. As soon as the output voltage becomes 12V, the zener diode opens and the PC201 optocoupler LED lights up.

As a result, the optocoupler transistor opens (pins 4, 3) and the controller supply voltage through resistor R102 is supplied to pin 2CS/FB. The pulses at the 6OUT pin disappear, and the voltage at the 12V/2A output begins to drop.

The voltage at the control input R of Zener diode U201 drops below reference voltage(2.5V), the zener diode is locked and turns off the PC201 optocoupler. Pulses appear at the 6OUT output, the 12V/2A voltage begins to increase and the stabilization cycle is repeated again. The stabilization circuit is built in a similar way in many switching power supplies, for example, in computer ones.

Thus, it turns out that three signals are connected to the input 2CS/FB of the controller using a wired OR: overload protection, protection against overvoltage of the network and the output of the output voltage stabilizer circuit.

This is where it’s appropriate to remember how you can check the operation of this stabilization loop. For this purpose it is enough to turn OFF!!! from the power supply network supply 12V/2A voltage from adjustable block nutrition.

It is better to connect to the output of the PC201 optocoupler with a pointer tester in resistance measurement mode. While the output voltage regulated source below 12V, the resistance at the optocoupler output will be high.

Now we will increase the voltage. As soon as the voltage exceeds 12V, the arrow of the device will sharply drop in the direction of decreasing resistance. This indicates that the zener diode U201 and optocoupler PC201 are working properly. Therefore, output voltage stabilization should work fine.

In exactly the same way, you can check the operation of the stabilization loop of computer switching power supplies. The main thing is to understand what voltage the zener diode is connected to.

If all of the above checks were successful, and the power supply does not start, then you should check transistor Q101 by removing it from the board. If the transistor is working properly, the U101 chip or its wiring is most likely to blame. First of all, this is the electrolytic capacitor C105, which is best checked by replacing it with a known good one.

If you have repaired a UPS, then you have probably encountered this situation: all faulty elements have been replaced, the remaining ones seem to have been checked, but you turn on the TV and... bam... and everything has to start all over again! There are no miracles in radio engineering, and if something doesn’t work, there’s a reason for it! Our task is to find her!

The UPS is the most unreliable component in modern radio devices. This is understandable - huge currents, high voltages - after all, all the power consumed by the device passes through the UPS. At the same time, let’s not forget that the amount of power supplied by the UPS to the load can change tens of times, which cannot have a beneficial effect on its operation.

Most manufacturers use simple circuits UPS. This is understandable. The presence of several levels of protection can often only complicate repairs and have virtually no effect on reliability, since the increase in reliability due to an additional protection loop is compensated by unreliability additional elements, and during repairs we have to spend a long time figuring out what these parts are and why they are needed. Of course, each UPS has its own characteristics, differing in the power supplied to the load, the stability of the output voltages, the range of operating mains voltages and other characteristics that play a role during repairs only when you need to choose a replacement for a missing part.

It is clear that when making repairs it is advisable to have a diagram. Well, if it’s not there, simple TVs can be repaired without it. The operating principle of all UPSs is almost the same, the only difference is in the circuit designs and types of parts used.

I use a technique developed over many years of repair experience. More precisely, this is not a technique, but a set of mandatory actions for repairs, proven by practice.

The proposed method assumes that you are at least somewhat familiar with how a TV works. For repairs, you need a tester (avometer) and, preferably, but not necessarily, an oscilloscope.
So, let's repair the power supply.

They brought you a TV or yours got damaged.

* Turn on the TV, make sure that it is not working, that the standby indicator is not lit. If it is on, then the problem is most likely not in the UPS. Just in case, you will need to check the horizontal scan supply voltage.

* Turn off the TV and take it apart.

* External inspection of the TV board, especially the area where the UPS is located. Sometimes swollen capacitors, burnt resistors, etc. may be found. It will be necessary to check them in the future.

* Carefully review the soldering, especially the transformer, key transistor/microcircuit, chokes.

* Check the power circuit: check the power cord, fuse, power switch - if there is one, chokes in the power circuit, rectifier bridge. Often, with a faulty UPS, the fuse does not burn out - it simply does not have time. If the key transistor breaks through, the ballast resistance is more likely to burn out than the fuse. It happens that the fuse burns due to a malfunction of the posistor, which controls the demagnetizing device (demagnetization loop). Be sure to check on short circuit filter capacitor leads mains power, without unsoldering it, since in this way you can often check for breakdown the collector-emitter terminals of a key transistor or microcircuit if a power switch is built into it. Sometimes power is supplied to the circuit from the filter capacitor through ballast resistors, and if they break, it is necessary to check for breakdown directly at the switch electrodes.

* Take a moment to check the remaining parts of the unit - diodes, transistors, some resistors. First, we check without desoldering the part; desoldering it only when there is a suspicion that the part may be faulty. In most cases, such a check is sufficient. Ballast resistors often break. Ballast resistances have a small value (tenths of an Ohm, units of Ohm) and are designed to limit pulse currents, as well as for protection as fuses.

* We need to see if there are any short circuits in the secondary power circuits - to do this, we check the terminals of the capacitors of the corresponding filters at the outputs of the rectifiers for short circuits.

After completing all checks and replacing faulty parts, you can perform a live test. To do this, instead of the mains fuse, we connect a 150-200 Watt 220 Volt light bulb. This is necessary so that the light bulb protects the UPS in case the malfunction is not resolved. Disconnect the degaussing device.

Turn it on. There are three options:

1. The light flashed brightly, then went out, and a raster appeared. Or the standby mode indicator lights up. In both cases, you need to measure the voltage supplying the drain scanner - it varies for different TVs, but not more than 125 Volts. Often its value is written on the printed circuit board, sometimes near the rectifier, sometimes near the TDKS. If it is increased to 150-160 Volts, and the TV is in standby mode, then switch it to operating mode; some TVs allow voltages to be increased by Idling(when horizontal scanning does not work). If the voltage is too high in operating mode, check the electrolytic capacitors in the power supply only by replacing them with a known good one. The fact is that often the electrolytic capacitors in the UPS lose frequency properties and at the generation frequency they cease to perform their functions, despite the fact that when checked by a tester using the charge-discharge method, the capacitor seems to be in good order. The optocoupler (if there is one) or the optocoupler control circuit may also be faulty. Check whether the output voltage is regulated by internal regulation (if any). If it is not adjustable, then you need to continue searching for faulty parts.

2. The light flashed brightly and went out. Neither the raster nor the standby mode indication appeared. This indicates that the UPS does not start. It is necessary to measure the voltage on the surge protector capacitor, it should be 280-300 Volts. If it is not there, sometimes they put a ballast resistor between the mains rectifier bridge and the capacitor. Check the power supply and rectifier circuits again. If the voltage is too low, one of the diodes of the mains rectifier bridge may be broken or, what is more common, the mains power filter capacitor may have lost capacity. If the voltage is normal, then you need to once again check the rectifiers of the secondary power supplies, as well as the starting circuit. The triggering circuit for simple TVs consists of several resistors connected in series. When testing a circuit, you need to measure the voltage drop across each of them, measuring the voltage directly at the terminals of each resistor.

3. The light is on at full brightness. Turn off the TV immediately. Recheck all items. And remember - there are no miracles in radio engineering, it means you missed something somewhere, you didn’t check everything.

95% of the faults fit into this diagram, but there are more complex faults when you have to rack your brains. For such cases, you cannot write methods and you cannot create instructions.

Depending on the causes and types of breakdowns that occur, different types of tools may be required; you must have:

• a set of screwdrivers with different types of working tips and sizes;
• insulating tape;
• pliers;
• knife with a sharp blade;
• soldering machine, solder and flux;
• braid designed to remove unnecessary solder;
• tester or ;
• tweezers;
• wire cutters;

In the most difficult cases, when it is not possible to determine the exact cause of the problem, an oscilloscope may be needed.

Repair of major faults

After performing diagnostics and identifying the causes of incorrect operation, you can start repairing it:

1. Dust accumulated inside the power supply can be simply eliminated using a regular household vacuum cleaner.
2. If the cause was a faulty fuse, then you need to purchase new part, which is available in all relevant stores. After this, the old element is removed and a new fuse is soldered. If this sequence of actions does not help, and the power supply still does not work, then all that remains is to take it to a workshop for diagnostics using professional equipment, or simply purchase a new device.
3. If the problem was in the capacitors or, then the malfunction is corrected using the same algorithm: new parts are purchased and soldered into the circuit instead of the old elements.
4. If the fault problem was in the throttle, then it is not necessary to replace it, since this element can be repaired using a fairly easy method. The inductor is removed from the power supply, after which you will need to disassemble it and begin to wind up the burnt wire, while it is important to carefully count the winded turns. Then you need to select a similar wire with the same diameter and wind it instead of the damaged conductor, making the same number of turns that were wound. After these steps are completed, the throttle is installed back in its place and, if everything was done correctly, the device should function.
5. Thermistors cannot be repaired, they are simply replaced with new elements, most often this is done together with fuses.
6. For prevention, during repairs, you can remove the cooler from the device and lubricate it with machine oil, and then install it in place.
7. If cracks are found on the surface of the board, which have damaged the connection of the contacts, they must be closed by soldering. In the same way, any contact failure in a resistor, inductor or is corrected.

Device

Power supplies of this type are essentially a type of voltage stabilizer, the design of which looks like this:

1. Mains rectifier is one of the main elements that is necessary to smooth out the resulting pulsations. Also, it is required to maintain the charge of the filter capacitors in the on mode and to continuously transfer electricity to the load if the voltage in the main supply network has dropped below acceptable parameters for operation. Its design includes special types of filters that allow it to suppress most of the resulting interference.
2. Voltage transformer, main components which are the converter and the controller of the control device.
3. Converter also has a complex structure, which includes a pulse-type transformer, an inverter, a number of rectifiers and stabilizers that provide secondary recharge and supply voltage to the load. An inverter is needed to change the shape of the DC output voltage, which after the conversion process becomes alternating voltage with a rectangular shape. The presence of a transformer operating at high frequencies with a value above 20 kHz is due to the need to maintain the operating state of the inverter in self-generator mode, as well as to obtain voltage that is used to feed the controller, load circuits and a number of protective circuits.
4. Controller performs the functions of controlling the transistor switch, which is part of the inverter. In addition, it stabilizes the voltage parameters supplied to the load and protects the device as a whole from possible overloads and unwanted overheating. If the power supply has an additional feature that provides remote control device, then the controller is also responsible for its implementation.
5. Power supply controller this type consists of a number of functional units, such as a source that provides it with uninterruptible power; protective system; pulse duration modulator; a logical circuit for signal processing and a generator of a special type of voltage intended for supply to transistors located in the converter.
6. Most modern models contain optocouplers used as isolation. They are gradually replacing transformer types of isolation, this is due to the fact that they take up less free space and have the ability to transmit signals in a much wider frequency spectrum, but at the same time require a significant number of intermediate amplifiers.

Main faults and their diagnosis

Sometimes switching power supplies break down and their faults can be of a very different nature, but there are a number of similar cases, on the basis of which a list of the most common types of faults has been compiled:

1. Unwanted ingestion dust devices, especially construction dust.
2. Fuse failure, most often this problem is caused by another malfunction - burnout diode bridge.
3. No output voltage with a functional and good fuse. This problem can be caused by various reasons, the most common being a breakdown of the rectifier diode, or a burnt-out filter choke in the low-voltage region of the circuit.
4. Failure of capacitors, most often this happens for the following reasons: loss of capacitance, leading to poor filtering of the output voltage and increased operating noise; excessive increase in series resistance parameters; short circuit inside the device or broken internal leads.
5. Broken contact connections, which is most often caused by cracks in the board.

If the power supply fails for any reason, then before independently conducting Any troubleshooting work requires a thorough diagnosis to identify the causes.

Depending on the different situations, this procedure has its own characteristics:

1. Inspect the power supply generally for the presence of dust accumulated in it, which may cause its incorrect operation.
2. Check main board for the presence of cracks on its surface.
3. Conducting a visual inspection The main board of the power supply allows you to determine the status of the fuses. It will be quite easy to notice a breakdown; this element of the device will swell or completely collapse in the event of a breakdown. It is also recommended to immediately carry out a comprehensive check of the power bridge, filter capacitor and all power switches.
4. If the fuse is in good condition, then it is necessary to check the inductor and electrolytic capacitors; faults can also be easily identified by a visual method based on the resulting deformations or swellings. It is more difficult to diagnose a diode bridge or individual diodes, they will need to be removed from the circuit and checked separately using a tester or multimeter.
5. Capacitor test is also carried out visually, since the resulting overheating could melt the electrolyte and destroy their cases, or using a special device designed to measure the level of their capacity, if external faults was not identified.
6. Inspect the thermistor, which is susceptible frequent breakdowns due to power surges or overheating. If its surface has turned black, and it itself is destroyed by light touches, then this is the cause of the problem.
7. Check contacts all remaining elements (resistor, transformer, inductor) for possible connection failures.

Additionally, when diagnosing or repairing switching power supplies, it is recommended to follow the following tips:

1. Carrying out self-repairs such devices is a rather complex process that requires certain skills and knowledge, even if available detailed instructions. Therefore, if you lack confidence in your abilities, it is better to contact a qualified technician so as not to cause even more serious damage to the power supply.
2. Before starting any actions with the switching power supply, it must be disconnected from the power supply. At the same time, pressing the corresponding key on the device itself does not guarantee complete safety during repairs, so it is necessary to disconnect the power cord.
3. After the power supply has been completely de-energized, you need to wait about 10-15 minutes before starting any work. This time is required to completely discharge the capacitors on the board.
4. If soldering work is required, then they must be carried out extremely carefully, since overheating of the soldering area can cause the tracks to peel off, and there is also a risk of them being shorted with solder. Soldering machines with a power setting in the range of 40-50W are best suited for these purposes.
5. Assembling the power supply after completion of the repair, it is allowed to carry out only after a careful inspection of the soldering areas, in particular, checking the short circuit with solder between the tracks is required.
6. It is recommended to provide the switching power supply high-quality ventilation and cooling, which will protect it from contamination and overheating, which minimizes possible breakdowns. Also, blocking the ventilation holes on the device is not allowed.

In any modern TV There is a switching power supply.

A power supply is a whole unit designed to provide the TV with supply voltages of a certain power necessary for the normal functioning of the electrical appliance.

When the pulse unit is faulty, all sorts of problems with the television receiver are observed, including the fact that it does not work at all or stops turning on.

Possible power supply malfunctions

When VseRemont24 technicians come to a client’s home, they most often encounter a faulty power supply. This is the most common malfunction of televisions of all models, brands and types.

The power supply may be in general scheme TV or as a separate module.

Power supplies are unique in each TV, each has its own circuit. But their performance is equally negatively affected by:

• violation by the owner of operating rules (especially temperature conditions),
• relatively simple circuits,
• unprofessional equipment repair.

Malfunctions typical for most power supplies:

1. Fuse blown.
2. The power supply does not start, there is voltage on the rectifier, the key elements are working.
3. The power supply does not start because the protection is triggered.
4. The power (key) transistor burns out.
5. Under- or over-voltage in primary or secondary circuits.

Obviously, only an experienced TV technician can figure out the breakdown and repair the TV. DIY repair extremely undesirable, but possible.

Checking and repairing the power supply

If you have some experience, all the necessary knowledge and tools (in particular, a multimeter and a soldering iron), try to repair a television receiver.

Algorithm of actions when checking the TV power supply:

1. Turn off the TV (unplug from the socket).
2. Discharge the high voltage capacitor.
3. Remove the board from the TV case.
4. Inspect the board (visual diagnostics).
5. Check resistors, capacitors, diodes, transistors, etc. with a multimeter.
6. Inspect the back of the board. Check for cracks, gaps between tracks, and reliability of soldering of parts.

Resistors can:

• darken
• crack,
• The quality of pin soldering deteriorates.

If all this is noticeable visually, it makes sense to replace the resistors with new ones with a deviation from the original of no more than plus or minus 5%.

If nothing is noticeable externally, you should check the resistors with a multimeter. The resistor is faulty if resistance = 0 or?.

Faulty electrolytic capacitors appear swollen. Their capacity is also checked. Permissible deviations- plus or minus 5%.

A working silicon diode has a resistance in the forward direction of 3-6 kOhm, and in the reverse direction - ?.

To measure resistance, you need to unsolder the diode. To check, set the multimeter to resistance measurement mode with a limit of 20 kOhm.

The second option is to check with a multimeter without soldering the diode. In this case, the multimeter must be set to the voltage drop measurement mode (should be up to 0.7 V). If the multimeter shows 0 or near zero, the diode will still have to be unsoldered and checked again. If the readings do not change, a breakthrough has probably occurred. A part needs to be replaced.

Bipolar transistors are tested in both directions (forward and reverse) at the transitions:

• collector base,
• base-emitter.

The test involves measuring the voltage drop in the transistors. It is also important to check that there is no breakdown in the collector-emitter junction.

Serviceable transistors behave like diodes, faulty ones need to be rechecked completely - the entire “piping”:

• diodes,
• resistors,
• capacitors.

To check the supply voltage of a switching power supply, you will need:

• his diagram
• two incandescent lamps? 100W.

Algorithm of actions:

1. Using the diagram, find the output to the horizontal scanning stage.
2. Disable output.
3. Connect the incandescent lamp.
4. Connect the power supply through the second lamp.

If the lamp comes on and is bright, the power supply is faulty. If the light comes on and goes out or shines dimly, the input circuits of the power supply are working properly.

To determine which element is broken (which is why the light comes on), you need to refer to the diagram.

The test voltage measurement is carried out with a connected light bulb on the B+ load. The diagram shows what the voltage should be. Usually it is 110-150V. If it is correct, the power supply is working.

If the voltage is increased (200V), check the elements of the primary circuit of the power supply. If it is lowered - secondary circuits.

All faulty parts are soldered off and new ones are soldered in their place.

Remember! It is impossible to repair the TV power supply yourself without knowledge and experience. Even more important is that handicraft and amateur repairs are a direct threat to the health and even lives of people!

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 breakdown 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:

1. the TV does not turn on and the LED does not light up;
2. 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;
3. 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.

It is also worth mentioning separately possible breakdowns other components of the TV that are not caused by problems with the power supply, but directly affect its operation. These include power supply units, circuits feedback, PSU 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:

1. Unstable voltage. If the voltage in the outlet constantly “jumps”, then it can not only worsen the performance of the TV, but also lead to wear and tear of its components.
2. Short circuit. Causes burnout of the power supply or other components of the TV.
3. Burnt out mains fuse. In this case, the standby indicator will not light up.
4. 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:

1. 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.

1. 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.

1. 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 exclude the indicated phenomena, it is used special block PFC (Power Factor Correction), 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 there is any doubt about own strength 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.