Changhong LT3263X LCD color TV switching power supply principle and maintenance (on) - Power Circuit - Circuit Diagram

MOS power IC full range

KKST patch plug-in active passive crystal oscillator hot sale 24.576000MHZ

inductance

Changhong LT3263X LCD TV adopts the power supply + high voltage integrated board (board number: FSP150P-3HF02) produced by Quanhan Enterprise Co., Ltd., and the logic board type T315XW04V3 TEST BD. The circuit consists of mains input and three-stage bidirectional common mode filtering, AC-DC conversion, +5VS auxiliary power supply, PFC power factor correction and boost circuit, DC-DC backlight power supply voltage conversion circuit, protection circuit, on/off control circuit, etc. Composition, PCB board as shown in Figure 1, power supply circuit block diagram shown in Figure 2.




First, the working principle of each part of the circuit
1. The mains input and the three-stage bidirectional common mode filter circuit are composed as shown in Figure 3. The 220V AC voltage is sent to the three-level common mode filter consisting of L100, L101 and L003 through the fuse R100 and the fuse resistor FU100 to eliminate The influence of the grid interference pulse on the power supply and the influence of the local high frequency pulse on other equipment.
R100 is a varistor. When the grid voltage rises, the resistance value is drastically reduced to blow the fuse and protect the safety of the power circuit. In addition, R101-R103 is a discharge resistor, and is sent to the next-stage circuit through the three-stage filtered 220V AC voltage.
2. Rectification and filtering The AC voltage output by the three-stage bidirectional common mode filter is sent to the BD100 bridge stack for rectification, and the pulsating DC voltage of about 200V is output. One channel is directly sent to the subsequent stage via the diode D121, and the other path is passed through the C106 and L102. The π-type filter composed of C107 is filtered to obtain DC power of about 300V, and is supplied to the 5V sub-power supply circuit and the PFC power factor correction and self-boosting circuit. However, the C106 and C107 have small capacity, and the output voltage is close to the pulsating full-wave waveform.
3. 5V sub-supply voltage forms this part of the circuit as shown in Figure 4, using the power supply thick film STRA6252, the parameters are shown in Table 1, the 300V ripple voltage output by the rectifier circuit is directly sent through the diode D121, D120 and the switching transformer T600 primary 4-5 winding To the STR6252 pin 1 (the D-pole of the internal FET), the external capacitors C602 and C603 are charged by the internal circuit. When the voltage on the capacitor rises to the chip startup voltage, the sub-power supply starts to work. At this time, the STR6252 starts from the inside. Vibration, T600 primary current has passed, generating induced electromotive force at the primary end, at the same time generating induced electromotive force at the secondary 4, 5, 4 feet of induced electromotive force is rectified by D601, charging C607, when C607 positive voltage When the voltage rises to 14.5V, the chip STR6252 reaches a stable working state. At this time, the C607 positive voltage VCC0 is 17.9V, which prepares the PFC and the PWM chip to supply power. The C602 positive voltage is 14.6V.



When the circuit enters the stable working state, the AC voltage of the secondary output of T600 is filtered by D610 rectification, C623, L620, C621, and the 5V DC voltage is sent to the main CPU and other circuits as standby power.
In this circuit, an error sampling circuit composed of R625, R623, R624, R622, R621, PC600 (PC817), IC620 (TL431), etc., regulates the +5V voltage. The sampling resistors R624 and R621 divide the +5V and send them to the R pole of the IC620. The resistance values ​​of the A and K poles are automatically adjusted by the change of the R pole voltage, thereby adjusting the luminous intensity of the internal light emitting diode of the photoelectric scrambler PC600. Adjust the resistance between the collector and the emitter of the phototransistor, feed back the output voltage change information to the 6th pin of the STR6252, and control the internal excitation signal duty cycle to achieve the regulated output of the power supply.
4. The power-on control and chip power supply circuit are shown in Figure 5. After the auxiliary power supply is working normally, the +5V voltage is supplied to the CPU and reset, waiting for the user's command. When the user presses the power button, the CPU outputs the power-on voltage, the PC-ON terminal voltage is 5V, and the Q291 is turned on via R290. The internal light-emitting diode current of PC102 increases, the equivalent resistance between the photocoupler collector and the emitter decreases, Q181 is cut off, and the control of Q180 is lost. The 18V voltage is composed of R183 and ZD 180 to form a reference voltage regulator circuit, so that the Q180 collector output is 15V. The voltage (vcc1) is used as the power source for the PFC chip. In addition, VCC1 outputs 12V voltage through Q190 as the power supply of PWM chip (VCC3). R190～R193 are sampling resistors, and the voltage regulator circuit will not be described again.

5. PFC power factor correction and boost circuit The so-called power factor correction is essentially to remove the filter capacitor after the 220V rectifier bridge stack (to eliminate the current waveform and phase change caused by capacitor charging), by a chopper circuit (斩The wave circuit is equal to a switching power supply. The pulsating DC is changed to a high frequency (about 100 kHz) AC and then rectified and filtered. The DC voltage is output to the PWM switching power supply, and the process is AC→DC→AC→DC.
For the power supply circuit, the output of the rectifier circuit is not directly connected to the filter capacitor, so it presents a purely resistive load for the power supply line, and the voltage and current waveforms are in phase and phase. Improve load capacity. The B+PFC voltage output from the chopper is 380V to 420V.
PFC power factor correction circuit shown in Figure 6, FAN6961 is a PFC controlled 8-pin critical mode dedicated chip, providing controlled on-time to adjust the output voltage for natural power factor adjustment, built-in multi-vector error amplifier to provide fast transients The response reaches an accurate output voltage clamp. L120 primary is boost inductor, secondary zero current sense inductor, D121 is isolated diode, D120 is freewheeling diode, Q105 is perfusion tube, Q120 is chopper tube. R131 is the discharge resistance of Q120 G pole, and R126, R127 and R128 form the chopper overcurrent detection circuit. R132, R133, R134, R135, and R136 form a PFC output voltage sampling feedback circuit, which supplies the PFC output voltage to the inverting input terminal of the IC 120 pin internal error amplifier. R121, R122, R123, R124, R125 form a 300V sampling circuit, input the rectified output voltage into the 3-pin internal multiplier of IC 120 according to a certain ratio, and compare it with the reference voltage VREF on the non-inverting input of the error amplifier to generate a DC error voltage. VEAO is also input to the multiplier. The output of the multiplier is the result of two inputs output by pin 7.