Power unit. We repair the TV power supply 3 x transistor TV power supply

Secrets of the TV master

B. KISELEVICH, Khatanga village, Krasnoyarsk Territory
Radio, 1998, No. 4

The so-called “three-transistor” power supply is a fairly common switching power supply that was used in many models of CRT TVs - PHILIPS - 2021, AKAI - ST-1407, AKAI - 2107, SHERION, CROWN - STA/ 5176, ELEKTA - CTR-1498EMK, RECOR and many more.

Power supply circuit

As an example, consider such a source used in the CROWN TV - CTV5176.
The 220 V mains voltage is supplied through the power filter to the rectifier BR601, C601 - C604 and to the demagnetization loop L2001. To the collector of the key transistor Q604, the rectified voltage passes through winding 1-5 of the pulse transformer T601.

A blocking generator is made on transistor Q604 - the positive feedback voltage is removed from winding 7 - 8 of the transformer. The duration of the pulses generated by the blocking generator, i.e., the time the transistor Q604 is in a saturated state, is determined by the operation of the pulse width modulator (PWM).

A capacitor C607 is connected to the base of transistor Q604, which, during the closed state of the transistor, is charged by a voltage pulse of winding 7 - 8 of the transformer through diode D604. When transistors Q602, Q603 open, the PWM capacitor C607 is connected to the emitter junction of the saturated transistor Q604, and the discharge current of the capacitor, flowing through the transistors and resistor R616, quickly closes the transistor Q604. The bias voltage is applied to the base of transistor Q604 through resistors R603, R604. Circuit C610R617 limits pulse surges on the collector of transistor Q604, thereby protecting it from breakdown.

To power the DC amplifier on transistor Q601, the alternating voltage from winding 9 - 10 is rectified by diode D603 and charges capacitor C606. The voltage at the emitter of transistor Q601 is stabilized by a parametric stabilizer on elements D601, R609, and the voltage to the base of the transistor is removed from the measuring resistive divider R606VR601R6 07. The latter depends on the voltage on winding 9 - 10 of the transformer, i.e., the output voltage levels of the power supply + 110 and +12 V. The voltage on resistor R608 - the collector load of transistor Q601 serves as an error voltage and controls the opening moment of PWM on transistors Q602, Q603. Trimmer resistor VR601 sets the output voltage to + 110 V.

A sawtooth voltage is removed from resistor R605 through circuit C605R611 to the base of transistor O602 of the PWM driver. The error voltage comes to it from the collector of transistor Q601. Depending on the last one, the PWM opens earlier or later, counting from the moment the transistor Q604 opens. Transistors Q602, Q603 are an analogue of a thyristor. The principle of its operation is similar to the operation of the thyristor in the MPZ-3 pulse power module.

When the network voltage increases or the load decreases, the voltage on winding 9 - 10 of transformer T601 increases. As a result, transistors Q602, Q603 open earlier, closing the output transistor Q604 at an earlier time. This reduces the energy stored in transformer T601, which compensates for the increase in network voltage.

When the network voltage decreases, the voltage on winding 9 - 10 of transformer T601 will be correspondingly lower. At the collector of transistor Q601, the error voltage decreases. The PWM opens at a later time, and the amount of energy transferred to the secondary circuit increases, compensating for the decrease in mains voltage.

The secondary rectifiers of the unit are made according to a half-wave circuit. Winding 4 - 2 transformers and elements D606, C612, L601 form a +12 V voltage source used to operate the remote control system and other low-current circuits. Winding 4 - 3 and elements D607, L602 are included in the +110 V voltage source that powers the horizontal scanning output stage.

Transistors Q608, Q606, Q605 are used to assemble a unit for turning on and off the power supply for the horizontal scanning output stage. Thus, the TV is turned on or off by the remote control system, i.e. it is switched to operating or standby mode. In standby mode, transistor Q606 is closed and the +110 V voltage is not supplied to the horizontal scan output stage. Some TV models use relays for this purpose.

For repairs, the unit board is removed from the TV case and placed so that there is easy access to the elements. A resistor with a resistance of 220 kOhm and a dissipation power of 0.5 W is connected in parallel to capacitor C604. The capacitor will discharge through it after the TV is turned off. Solder one of the terminals of each of the elements L601, L602, D608, C617. In this case, the TV's load circuits will be completely disconnected from the power supply. An incandescent lamp of 220 V and 25 W is connected in parallel to the capacitor C615, which will serve as the equivalent load of the power supply.

After the repair, before connecting the power supply to the TV circuits, you must check the horizontal output transistor and the secondary circuits of the horizontal transformer. Voltage is often taken from the secondary windings of the latter, rectified and smoothed to power the TV components. One of the reasons for the failure of the power supply may be precisely these circuits.

When selecting transistors to replace failed ones, you should be guided by their characteristics listed in table. 1.

Transistors 2SC1815Y can be replaced with KT3102B, 2SB774T with KT3107B, and 2SD820, BU11F with KT872A. The latter is mounted on a heat sink with an insulating gasket. Diodes can be replaced with KD209B, KD226A, KD226B.

The most common malfunction this module is “going into disarray” due to a decrease in capacity (or increase in ESR) of electrolytic capacitors. Moreover, the reason for this trouble is not even the quality of the parts used: the main problem is that modern switching power supplies operate at high frequencies (15 kHz or even higher...), and conventional electrolytes are simply not designed for such high frequencies and in During operation they begin to heat up.
If the filter capacitor (according to the diagram this is C606) more or less copes with its duties, then C607 works in a very difficult mode (it has to pass high-frequency pulses through itself).
Therefore, when repairing this SMPS, it is imperative to pay attention first of all to these capacitors, and repair the unit with the horizontal scan turned off, using an incandescent lamp with a power of 60...100 W as a load.

Note: most of the material is from Radio magazine, 1998, No. 4

When diagnosing television devices, disproportionately more time is spent on finding a faulty component than replacing it, especially if the search for the defect is carried out on your own and not by a professional TV technician. Of course, it is more logical to entrust the repair to a specialist who has experience and extensive practice in this type of work, but if you have the desire, skills in handling a soldering iron and tester, and the necessary technical documentation in the form of a circuit diagram, you can try to repair the TV at home yourself.

The power supply of a modern TV, be it a plasma panel or LCD, LED TV, is a switching power supply with a given range of output supply voltages and the rated power supplied to the load for each of them. The power board can be made in the form of a separate block, this is typical for small diagonal receivers, or integrated into the television chassis and located inside the device.

Typical symptoms of a malfunction of this unit are the following:

• The TV does not turn on when you press the power switch
• The standby LED is on, but there is no transition to operating mode
• Noise in the image in the form of kinks and stripes, background sound
• There is sound, but no image, which may appear after a while
• It takes several attempts to turn on for normal picture and sound to appear.

Let's look at the circuitry of a standard power supply and its typical faults using the ViewSonic N3260W TV as an example.

To fully view the diagram, you can open it in a new window and enlarge it, or download it to your computer or mobile device

The first thing you should start with is a thorough visual inspection of the board with the device disconnected from the network. To do this, the unit must be removed from the TV by disconnecting the connectors, and be sure to discharge the high-voltage capacitor in the filter - C1. In units of this series of TVs, the electrolytic capacitors of the secondary power supply filters quite often fail. They are easily diagnosed by a swollen top cover. All capacitors whose appearance is questionable must be replaced immediately.

The standby mode unit is made on IC2 (TEA1532A) and Q4 (04N70BF) with elements for stabilizing the 5V output voltage on the optocoupler IC7 and the controlled zener diode ICS3 EA1. Missing or low voltage at the output of this unit, measured on capacitors CS22, CS28, indicates its malfunction. The experience of restoring this section of the circuit shows that the most vulnerable elements are IC2, Q7, ZD4 and Q11, R64, R65, R67, which require checking and replacement if necessary. The performance of the parts is checked by a tester directly on the unit board. In this case, questionable components are soldered off and tested separately to eliminate the influence of neighboring circuit elements on their performance. IC2 chip simply needs to be replaced.

If there is a 5V voltage at the output of the standby circuit, a red LED lights up on the front panel of the TV. Upon command from the remote control or a button on the front panel of the TV, the power supply should go into operating mode. This command - Power_ON - in the form of a high potential of about 5V comes to pin 1 of the CNS1 connector, opening the keys on QS4 and Q11. At the same time, supply voltages are supplied to microcircuits IC3 and IC1, putting them into operating mode. To the 8th pin of IC3 directly from the collector of Q11, to the 12th pin of IC1 through the Q9 switch after starting the PFC circuit. The performance of the Power Factor Correction circuit is indirectly determined by an increase in voltage from 310 to 390 volts, measured at capacitor C1. If output supply voltages of 12V and 24V appear, then the main source on IC3, Q1, Q2 operates in normal mode. Practice shows the low reliability of the UCC28051 and LD6598D in critical conditions, when the filtering of secondary sources deteriorates, and their replacement is routine.

Summarizing the experience of repairing television power supplies, it should be noted that the weakest link in their composition is the filter capacitors, which lose their properties and nominal parameters over time. Sometimes the faulty “container” is visible from the swollen lid, sometimes not. The consequences of poor filtering of rectified voltage can be very different: from loss of functionality of the power supply itself, to damage to the inverter elements or software failure of memory chips on the motherboard.

It is very difficult to independently understand all the causes and consequences when repairing the power supply of a modern TV, and to diagnose it correctly without special tools and devices. Our advice in such cases is

Hi all!

In this article we will look at LCD TV power supply Samsung BN44-00192A , which is used in devices with screen diagonals of 26 and 32 inches. We will also look at some typical malfunctions of this module.

All components of this power supply located on one board. The appearance of the board is shown in the figure:

Power module diagram BN44-00192A can be found on this site.

This module is functionally divided into several nodes:

— Power Factor Correction (PFC) or power factor corrector (PFC);

— “standby” power supply;

— power supply “working”.

Let's look at each node separately.

Power factor corrector

This unit eliminates the harmonic components of the current in the input circuit, which are reproduced by the rectifier diodes together with the electrolytic filter capacitor of the mains rectifier of the switching power supply (SMPS). These harmonic components negatively affect the power grid, which is why manufacturers of household appliances are required to equip their products with PFC devices. Depending on the power, these devices are active and passive. In the BN44-00192A power supply we are considering, the PFC device is active.

Here PFC is turned on by switching the voltage M_Vcc at pin 8 of the ICP801S controller simultaneously with the “working” power source. When the standby mode is turned on, the active PFC does not work, since the +311V voltage from the diode bridge through the DP801 diode is supplied to the filter capacitor. To filter harmonics at low loads, the installed input filters are sufficient. Essentially, these filters are passive PFCs.

Standby power supply

The standby power supply is a flyback converter circuit, which is controlled by an ICB801S PWM controller. The converter operating at a fixed frequency of 55...67 kHz generates a stabilized voltage of 5.2V at the output and has a load current of up to 0.6A. This voltage provides power to the control processor in standby mode, power to the PWM chips of the main source, and power to the PFC in operating mode. The TV switches from standby mode to operating mode by generating a voltage of 5.2V using a QB802 transistor switch. The supply voltage M_Vcc, in this case, is supplied to the PWM controllers ICP801S and ICM801. At the same time, the PFC and the main power supply are started.

Power supply "working"

The operating power supply is implemented using a forward converter circuit, which is made using a half-bridge circuit. This output source generates stabilized voltages:

24V (power supply for the backlight inverter), 13V, 12V and 5.3V for powering the lane.

Typical faults

Now let's look at the most popular defects of this power supply.

These include:

M.Kireev

Modern televisions use switching power supplies, the advantages of which compared to transformer ones are quite well described in the literature. The power supply functionally consists of primary and secondary circuits (Fig. 1).

The power switch VT1 is either made in the form of a separate transistor, or is technologically located on the chip of a PWM controller chip.

Often, with such signs of a malfunction as the front panel LED lighting up for 1...5 s and its subsequent extinguishing, clicks and whistling of the power supply for 1...5 s and the subsequent shutdown of the TV, it is impossible to reliably determine the failed functional unit TV. However, from repair practice, it can be said with a high probability that this kind of external signs are a manifestation of the failure of the following components of the TV:
primary circuits of the power supply (PWM controller, key transistor, mains rectifier, filter capacitor, damping circuit, etc.);
secondary circuits of the power source (rectifier and protective diodes, secondary filter capacitors, elements in the loads of individual voltage sources, etc.);
power circuit of the horizontal scan output stage (rectifier and filter of the horizontal scan power supply +95...140 V, horizontal scan output transistor, horizontal transformer, etc.).

Let's consider a technique for detecting faults in the primary and secondary circuits of switching power supplies. Troubleshooting in a device that has the external signs of malfunction listed above should begin with an external inspection of the installation. In this case, special attention should be paid to the absence of burn marks on the housings of power transistors and microcircuits, the integrity of the housings of oxide capacitors, the absence of signs of destruction of powerful low-resistance resistors that serve as current-limiting elements, and places of “cold” soldering of the leads of fuel elements. Sometimes visually, based on the indicated signs, it is possible to determine the nature of the malfunction that has occurred.

If the visual inspection does not produce results, you must proceed to the next stage of troubleshooting. Here you should carry out a little preparatory work, namely: either unsolder the jumpers J1, J2, J3 of the current-carrying paths of the printed circuit board coming from the outputs of the power supply rectifiers, or, if there are none, carefully cut the current-carrying conductors so that the outputs of the power supply can be It was necessary to separately connect both the loads, which can serve as various incandescent lamps, and the laboratory power supply of the main components of the TV (Fig. 2).

The appearance of the power supply is shown in Fig. 4.

At first glance, it is enough to connect the load to one rectifier, for example, feeding the horizontal scan output stage, in order to check this circuit as a whole, but this is not so. Although the pulsed source will work stably in this case, it is possible to miss defects in the rectifier diodes and filter capacitors of low-voltage rectifiers. This happened, for example, during the repair of the Vityaz 51TTs-420D TV. The TV did not turn on, but when the power source was turned on separately with a load on the +135 V source, it worked stably. The defect was hidden in the filter capacitor of the +12 V source and did not appear during operation without load.

Before turning on the power supply with loads, it is advisable to check all rectifier diodes in the secondary and primary circuits for breaks or breakdowns, as well as oxide capacitors, which it is advisable to unsolder to check their parameters, since oxide capacitors working in the power supply circuits of televisions quite often lose their tightness and electrolyte leaks.

Most modern televisions include switching power supplies, the power stages of which are made either of powerful transistors or of specialized microcircuits. If the power source being tested contains a powerful transistor, then before connecting the power source to the network, it is necessary to check the integrity of its transitions using an ohmmeter for open circuits and short circuits. Possible replacements for power transistors are presented in table. 1.

If all components, as well as the power transistor, are in good condition, the source can be connected to the network. If the pulse source is made using a PWM controller, then due to the impossibility of checking the microcircuit using an ohmmeter, it must be connected to the network and the voltages at the terminals of the PWM controller must be measured. The absence of one or more voltages while other parts are in good working order clearly indicates a faulty microcircuit that needs to be replaced. Considering that on some circuit diagrams of TVs the PWM controller is drawn as a “black box” (for example, “Kolon CTK-9742”) or as a chain of functional units (“Grundic CUC 4510”), in Table. 2

The voltage values ​​at the terminals of the most commonly used PWM controllers in television equipment are presented. Voltage values ​​may differ from those indicated by ±10%.

After checking all the parameters of the switching power supply when operating on a set of loads, you can connect the source to the remaining components of the TV by restoring the previously removed jumpers. However, before this, you need to make sure that there are no malfunctions in the power circuits or failed elements, for example, short circuits or broken transistors in the horizontal scanning unit, and a zener diode connected to the power supply circuit of the horizontal scanning output stage, as is done in some TV models, since otherwise the elements of the switching power supply may fail again. Possible options for replacing imported Zener diodes with domestic ones are given in Table. 3.

When replacing a zener diode, it may be necessary to select the correct one based on the stabilization voltage. Although in the vast majority of cases the TV starts working immediately after restoring the faulty power supply, connecting a laboratory source, similar to the author’s version, allows you to check the overall performance of the TV components if it is impossible to quickly restore the standard power source and the current consumption of each TV unit separately, since the increased consumption any node can indicate the presence of a defect, and the activation of electronic protection in any channel of a laboratory power source can directly indicate a node containing defective elements.

Usually, as follows from repair practice, if the faults in the switching power supply are eliminated and the remaining components of the TV are operational, then the device begins to work normally after being connected to the network, and if it worked steadily for 20...30 minutes, then the repair can be considered successful.

A little trick. After replacing the key transistor in the primary circuit of the power supply or PWM chip, the mains fuse should be removed before turning it on for the first time. Instead, connect a 60 W 220 Volt incandescent lamp. After switching on, the lamp should flash brightly at first, and then barely glow. This is an indicator of proper operation of the B.P. If the lamp glows brightly all the time or does not light at all, then repairs need to be continued. This trick allows you to keep the key transistor in good condition, even if the B.P. faulty. (Krylov P.V.)

Literature
1. V.S. Moin. Stabilized transistor converters. M.: Energoatomizdat, 1986.
[email protected]