B. KISELEVICH, Khatanga, Krasnoyarsk Territory
Radio, 1998, No. 4
The so-called "three-transistor" PSU 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 others.
As an example, consider such a source used in the CROWN TV - CTV5176.
The mains voltage of 220 V through the power filter is supplied to the rectifier BR601, C601 - C604 and to the demagnetization loop L2001. On the collector of the key transistor Q604, the rectified voltage passes through the winding 1-5 of the pulse transformer T601.
A blocking oscillator is made on the Q604 transistor - the positive feedback voltage is removed from the winding 7 - 8 of the transformer. The duration of the pulses generated by the blocking generator, i.e., the time the Q604 transistor is in a saturated state, is determined by the functioning 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 the winding 7 - 8 of the transformer through diode D604. When transistors Q602, Q603 are opened, 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 transistor Q604. The bias voltage to the base of transistor Q604 is applied through resistors R603, R604. The C610R617 circuit limits the pulse surges at the collector of the Q604 transistor, 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 taken from the measuring resistive divider R606VR601R607. The latter depends on the voltage on the winding 9 - 10 of the transformer, i.e., the output voltage levels of the power supply + 110 and + 12 V. The voltage across the resistor R608 - the collector load of the transistor Q601 serves as an error voltage and controls the opening moment of the PWM on transistors Q602, Q603. The trimmer resistor VR601 sets the output voltage to + 110 V.
A sawtooth voltage is removed from the resistor R605 through the C605R611 circuit to the base of the transistor O602 of the PWM shaper. It also receives the error voltage from the collector of transistor Q601. Depending on the last PWM, it opens earlier or later, counting from the moment Q604 opens. Transistors Q602, Q603 are analogous to a trinistor. The principle of its operation is similar to the operation of the trinistor in the pulsed power supply module MPZ-3.
With an increase in the mains voltage or a decrease in the load, the voltage on the winding 9 - 10 of the 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 the T601 transformer, which compensates for the increase in mains voltage.
When the mains voltage decreases, the voltage on the winding 9 - 10 of the T601 transformer will accordingly be lower. At the collector of transistor Q601, the error voltage decreases. The PWM opens at a later time, and the amount of power transferred to the secondary circuit increases to compensate for the decrease in mains voltage.
The secondary rectifiers of the block 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 feeds the line scan output stage.
On transistors Q608, Q606, Q605, a unit for turning on and off the power of the horizontal scanning output stage is assembled. Thus, the TV system is turned on or off by the remote control system, that is, it is transferred to the operating or standby mode. In standby mode, transistor Q606 is closed and +110 V is not supplied to the horizontal output stage. In some TV models, relays are used for this purpose.
For repair, the block board is removed from the TV case and placed so that there is free access to the elements. In parallel with the capacitor C604, a resistor with a resistance of 220 kOhm and a dissipation power of 0.5 W is connected. Through it, the capacitor will be discharged after the TV is turned off. Solder one of the conclusions of each of the elements L601, L602, D608, C617. In this case, the load circuits of the TV will be completely disconnected from the power supply. In parallel with the capacitor C615, a 220 V and 25 W incandescent lamp is connected, which will serve as the equivalent load of the power supply.
After repair, before connecting the power supply to the TV circuits, it is imperative to 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 nodes. One of the reasons for the failure of the power supply may be precisely these circuits.
When selecting transistors to replace failed ones, one should be guided by their characteristics indicated 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. It is permissible to replace diodes with KD209B, KD226A, KD226B.
The most common malfunction of this module is "runaway" due to a decrease in capacitance (or an 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 scheme, 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 to repair the unit with horizontal scanning disabled, using an incandescent lamp with a power of 60 ... 100 W as a load.
Note: the main part of the material from the Radio magazine, 1998, No. 4
When diagnosing television devices, it takes disproportionately more time to find a faulty component than to replace it, especially if the defect is searched for on its own, and not by a professional TV technician. Of course, it is more logical to entrust the repair to a specialist with experience and extensive practice in this kind of work, but if you have the desire, skills in handling a soldering iron and a tester, the necessary technical documentation in the form of a circuit diagram, you can try to fix the TV at home yourself.
The power supply of a modern TV, whether it is a plasma panel or LCD, LED TV, is a switching power supply with a given range of output supply voltages and a rated power delivered to the load for each of them. The power board can be made as a separate unit, which is typical for receivers of small diagonals, or integrated into the television chassis and located inside the device.
Typical symptoms of a malfunction of this unit are as follows:
Let's analyze the circuitry of a standard power supply and its typical malfunctions using the ViewSonic N3260W TV as an example.
To fully view the scheme, you can open it in a new window and enlarge it, or download it to your computer or mobile device
The first thing to start with is a thorough visual inspection of the board on the device turned off from the network. To do this, the unit must be removed from the TV by disconnecting the connectors, and it is imperative to discharge the high-voltage capacitor in the filter - C1. In the blocks of this series of TVs, the electrolytic capacitors of the filters of the secondary power supplies often fail. They are easily diagnosed by the swollen top cover. All capacitors, the appearance of which is in doubt, must be replaced immediately.
The standby unit is made on IC2 (TEA1532A) and Q4 (04N70BF) with 5V output voltage stabilization elements on the IC7 optocoupler and the ICS3 EA1 controlled zener diode. The missing or underestimated voltage at the output of this node, measured on capacitors CS22, CS28, indicates its incorrect operation. The experience of restoring this section of the circuit indicates that the most vulnerable elements are IC2, Q7, ZD4 and Q11, R64, R65, R67, which require verification and replacement if necessary. The operability of the parts is checked by the tester directly on the unit board. At the same time, questionable components are soldered and tested separately, in order to exclude the influence of neighboring circuit elements on their performance. IC2 is simply replaceable.
If there is a 5V voltage output at the output of the standby mode circuit, a red LED lights up on the front panel of the TV. On command from the remote control or a button on the front panel of the TV, the power supply must switch to operating mode. This command - Power_ON - in the form of a high potential of about 5V comes to 1 pin of the CNS1 connector, opening the keys on QS4 and Q11. At the same time, supply voltages are applied to the IC3 and IC1 microcircuits, transferring them to the operating mode. To pin 8 of IC3 directly from the collector of Q11, to pin 12 of IC1 through the Q9 switch after starting the PFC circuit. The performance of the Power Factor Correction circuit is indirectly determined by the increase in voltage from 310 to 390 volts measured across capacitor C1. If the output supply voltages of 12V and 24V appear, then the main source on IC3, Q1, Q2 is functioning normally. Practice shows the low reliability of the UCC28051 and LD6598D in critical conditions, when the filtering of secondary sources deteriorates, and their replacement is of an ordinary nature.
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 a faulty "capacity" is visible by a swollen lid, sometimes not. The consequences of poor filtering of the rectified voltage can be very different: from the loss of performance of the power source itself, to damage to the elements of the inverter or software failure in the 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, to diagnose it correctly without special tools and devices. Our advice in such cases is
This assembly eliminates current harmonics in the input circuit, which are reproduced by the rectifier diodes along with the electrolytic capacitor of the switching power supply (SMPS) mains rectifier filter. These harmonic components negatively affect the power grid, so 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 on the 8th output of the ICP801S controller simultaneously with the “working” power supply. When the standby mode is 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 quite enough. In fact, these filters are passive PFCs.
The standby power supply is a flyback converter circuit that is controlled by the ICB801S PWM controller. A converter operating at a fixed frequency of 55 ... 67 kHz generates a stabilized voltage of 5.2V at the output and has a current of up to 0.6A in the load. This voltage provides power to the control processor in standby mode, power to the PWM chips of the main source, as well as power to the PFC in operating mode. The TV goes from standby to operating mode by generating a voltage of 5.2V using a QB802 transistor switch. The supply voltage M_Vcc, at the same time, is supplied to the PWM controllers ICP801S and ICM801. At the same time, the PFC and the main power supply start up.
M. Kireev
Modern TVs use switching power supplies, the advantages of which, compared with 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 technologically placed on a chip of a PWM controller chip.
Often, with such signs of malfunction as the front panel LED lights up for 1 ... 5 s and then goes out, clicks and whistles of the power source for 1 ... 5 s and then the TV turns off, it is impossible to reliably determine the failed functional unit TV. However, from the practice of repair, it can be argued with a high probability that such external signs are a manifestation of the failure of the following TV components:
primary power supply circuits (PWM controller, switching transistor, mains rectifier, filter capacitor, damping circuit, etc.);
secondary circuits of the power source (rectifier and protective diodes, capacitors of secondary filters, elements in loads of individual voltage sources, etc.);
power supply circuits for the horizontal scanning output stage (rectifier and filter of the horizontal scanning power supply +95 ... 140 V, horizontal scanning output transistor, horizontal transformer, etc.).
Consider a technique for detecting faults in the primary and secondary circuits of switching power supplies. Troubleshooting in a device that has the above external signs of a malfunction should begin with an external inspection of the installation. In this case, special attention should be paid to the absence of traces of burnout on the cases of power transistors and microcircuits, the integrity of the cases 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, according to the indicated signs, it is possible to determine the nature of the malfunction that has occurred.
If visual inspection fails, proceed to the next troubleshooting step. Here it is necessary to carry out a little preparatory work, namely: either solder the jumpers J1, J2, J3 of the current-carrying tracks of the printed circuit board coming from the outputs of the rectifiers of the power supply, or, if there are none, carefully cut the current-carrying conductors in such a way that it is possible to reach the power supply outputs it was necessary to separately connect both loads, which can serve as various incandescent lamps, and a laboratory power supply for 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 output stage, in order to check this circuit as a whole, but this is not the case. Although the pulse source in this case will work steadily, it is possible to skip defects in rectifier diodes and filter capacitors of low-voltage rectifiers. This happened, for example, during the repair of the Vityaz 51ТЦ-420D TV. The TV did not turn on, however, when the power source was turned on separately with a load on the +135 V source, it worked steadily. The defect was hidden in the +12 V source filter capacitor and did not appear during operation without load.
Before turning on the power supply with loads, it is advisable to check all the rectifier diodes in the secondary and primary circuits for an open or breakdown, as well as oxide capacitors, which it is desirable to solder to check their parameters, since oxide capacitors operating in the power circuits of TVs quite often break tightness and electrolyte leaks out.
Most modern TVs incorporate switching power supplies, the power stages of which are made either on powerful transistors or on specialized microcircuits. If the power source under test incorporates a powerful transistor, then before connecting the power source to the network, it is necessary to check the integrity of its transitions with an ohmmeter for an open and short circuit. Possible replacements for power transistors are presented in Table. 1.
If all components, as well as the power transistor, are working, 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 with an ohmmeter, it must be connected to the network and the voltage at the PWM controller pins must be measured. The absence of one or more voltages with the remaining parts in good condition clearly indicates a faulty microcircuit, which must be replaced. Considering that on some schematic diagrams of TVs the PWM controller is drawn in the form of a "black box" (for example, "Kolon CTK-9742") or in the form of a chain of functional units ("Grundic CUC 4510"), in Table. 2
The voltage values at the outputs of the most commonly used PWM controllers in television equipment are presented. Voltage values may differ from those specified by ±10%.
After checking all the parameters of the switching power supply when working on a set of loads, you can connect the source to the rest of the TV nodes by restoring the previously removed jumpers. However, before that, you need to make sure that there are no faults in the power circuits and failed elements, for example, a short circuit or open circuit of transistors in the line scan unit, and a zener diode included in the power supply circuit of the line scan output stage, as is done in some TV models, as otherwise repeated failure of the switching power supply elements is possible. 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 desired specimen according to the magnitude of the stabilization voltage. Although in the vast majority of cases the TV starts working immediately after the restoration of a faulty power source, however, connecting a laboratory source similar to the author's version allows you to control the overall performance of the TV nodes if it is impossible to quickly restore the standard power source and the current consumption of each TV node separately, since increased consumption any node may indicate the presence of a defect, and the operation of electronic protection in any channel of the laboratory power source - directly indicate the node containing defective elements.
Usually, as follows from the practice of repair, if the malfunctions in the switching power supply are eliminated and the remaining components of the TV are operational, then the device starts working normally after being connected to the network, and if it has worked steadily for 20 ... 30 minutes, then the repair can be considered successful.
Little trick. After replacing the key transistor in the primary circuit of the power supply or PWM chip, before the first start, remove the mains fuse. Instead, a 60 W 220 Volt incandescent lamp is connected. After turning on, the lamp should flash brightly for the first moment, and then barely glow. This is an indicator of the correct operation of the B.P. If the lamp glows brightly all the time or does not burn at all, then the repair must be continued. This trick allows you to keep the key transistor in good condition, even if the P.P. out of order. (Krylov P.V.)
Literature
1. V.S. Moin. Stabilized transistor converters. Moscow: Energoatomizdat, 1986.
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