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

The Changhong LT3263X LCD TV uses the power supply and high-voltage integrated board (board number: FSP150P-3HF02) manufactured by Quanhan Enterprise Co., Ltd., along with the logic board type T315XW04V3 TEST BD. The circuit includes mains input and a three-stage bidirectional common mode filter, AC-DC conversion, +5V auxiliary power supply, PFC (power factor correction) and boost circuit, DC-DC backlight power supply conversion circuit, protection circuit, on/off control circuit, and more. The PCB layout can be seen in Figure 1, and the power supply circuit block diagram is illustrated in Figure 2.


Firstly, let's explore the working principles of each section of the circuit: 1. Mains input and the three-stage bidirectional common mode filter circuit are constructed as shown in Figure 3. The 220V AC voltage passes through the fuses R100 and FU100, entering a three-level common mode filter consisting of L100, L101, and L003. This setup helps eliminate interference pulses from the power grid and prevents local high-frequency pulses from affecting other devices. R100 is a varistor, which reduces its resistance significantly when the grid voltage surges, blowing the fuse to protect the power circuit. Additionally, R101-R103 serves as a discharge resistor. After passing through the three-stage filtering, the 220V AC voltage is sent to the next stage. 2. Rectification and Filtering: The AC voltage output by the three-stage bidirectional common mode filter is sent to the BD100 bridge rectifier for rectification, producing a pulsating DC voltage of around 200V. One channel is directly sent to the next stage via diode D121, while the other path goes through the C106 and L102 π-type filter, composed of C107, to produce a DC voltage of approximately 300V. This is then supplied to the 5V sub-power supply circuit and the PFC power factor correction and boost circuit. However, since C106 and C107 have small capacities, the output voltage resembles a nearly pulsating full-wave waveform. 3. Formation of the 5V Sub-Supply Voltage: As shown in Figure 4, this part of the circuit uses the STRA6252 power supply thick film, with parameters listed in Table 1. The 300V ripple voltage from the rectifier circuit is sent directly through diodes D121 and D120 to the primary 4-5 winding of the switching transformer T600 and to STR6252 pin 1 (the D-pole of the internal FET). External capacitors C602 and C603 are charged by the internal circuit. When the voltage on the capacitor reaches the chip’s startup voltage, the sub-power supply begins to operate. At this point, STR6252 initiates oscillation internally, causing the primary current of T600 to flow, generating an induced electromotive force at the primary end. This induces an electromotive force at the secondary 4, 5 pins, which is rectified by D601, charging C607. When the positive voltage on C607 reaches 14.5V, STR6252 achieves a stable operating state. At this stage, the positive voltage VCC0 on C607 is 17.9V, preparing the PFC and PWM chips for power supply. The positive voltage on C602 is 14.6V.


When the circuit reaches a stable working state, the AC voltage from the secondary output of T600 is filtered by D610 rectification, C623, L620, and C621, producing a 5V DC voltage that is sent to the main CPU and other circuits as standby power. In this circuit, an error sampling circuit comprising R625, R623, R624, R622, R621, PC600 (PC817), IC620 (TL431), etc., regulates the +5V voltage. Sampling resistors R624 and R621 divide the +5V voltage and send it to the R pole of IC620. The resistance values of the A and K poles are automatically adjusted based on changes in the R pole voltage, thus regulating the luminous intensity of the internal light-emitting diode of the optocoupler PC600. It adjusts the resistance between the collector and emitter of the phototransistor, feeding back the output voltage change information to pin 6 of STR6252, controlling the internal excitation signal duty cycle to achieve regulated power supply output. 4. Power-On Control and Chip Power Supply Circuit: As shown in Figure 5, once the auxiliary power supply operates normally, the +5V voltage powers the CPU and reset, awaiting the user’s command. When the user presses the power button, the CPU outputs the power-on voltage, setting the PC-ON terminal voltage to 5V, turning on Q291 via R290. The current through the internal LED of PC102 increases, reducing the equivalent resistance between the photocoupler collector and emitter, cutting off Q181, and losing control of Q180. The 18V voltage formed by R183 and ZD180 creates a reference voltage regulator circuit, ensuring the Q180 collector output is 15V (VCC1), serving as the power source for the PFC chip. Additionally, VCC1 outputs a 12V voltage through Q190, supplying the PWM chip (VCC3). Resistors R190–R193 are sampling resistors, and the voltage regulator circuit will not be discussed further.
5. PFC Power Factor Correction and Boost Circuit: So-called power factor correction essentially removes the filter capacitor after the 220V rectifier bridge stack (eliminating current waveform and phase changes caused by capacitor charging), replacing it with a chopper circuit (equivalent to a switching power supply). The pulsating DC is converted to a high-frequency (around 100 kHz) AC and then rectified and filtered, outputting a DC voltage to the PWM switching power supply, following the process AC→DC→AC→DC. For the power supply circuit, the output of the rectifier circuit is not directly connected to the filter capacitor, presenting a purely resistive load to the power supply line, aligning the voltage and current waveforms in phase. This improves load capacity. The B+PFC voltage output from the chopper ranges from 380V to 420V. The PFC power factor correction circuit is shown in Figure 6. FAN6961 is an 8-pin critical mode PFC control-specific chip that provides controlled on-time to adjust the output voltage for natural power factor adjustment. It features a built-in multi-vector error amplifier for rapid transient response, achieving an accurate output voltage clamp. L120 is the primary boost inductor, the secondary zero-current sensing inductor, D121 is the isolation diode, D120 is the freewheeling diode, Q105 is the injection tube, and Q120 is the chopper tube. R131 is the discharge resistance of Q120’s G pole, while R126, R127, and R128 form the chopper overcurrent detection circuit. R132, R133, R134, R135, and R136 form a PFC output voltage sampling feedback circuit, supplying the PFC output voltage to the inverting input terminal of the IC’s internal error amplifier at pin 120. R121, R122, R123, R124, and R125 form a 300V sampling circuit, inputting the rectified output voltage into the 3-pin internal multiplier of IC 120 at a certain ratio, comparing 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 the two inputs, output by pin 7.
Overall, this power supply design ensures efficient and stable operation, meeting the demands of modern LCD TVs.