Panda 1.32A7031 LCD TV power supply principle - Power Circuit - Circuit Diagram

MOS power IC full range

One-piece inductor

At present, there is very little information on the Panda LCD TV. For this reason, after repairing a Panda LCD TV, I will draw the PFC circuit and switching power supply circuit on the 2-in-1 board. Since it is drawn according to the real object, the number of the component is The model is authentic and reliable, and it is a rare first-hand information for the reference of maintenance peers. Among them, the chip capacitor does not indicate the capacity, and the number of chip capacitors is too large. It is inconvenient to take each chip capacitor to measure its capacity. Therefore, the chip capacitor on the graph has no standard capacity.
Panda L32A7031 LCD TV power supply is integrated, that is, the switching power supply circuit and backlight inverter circuit are integrated on one board, also called IP board. Since the inverter circuit changes the true current voltage of about 380V of the PFC output into an AC sinusoidal high voltage, the burden of the switching power supply is greatly reduced. Only the PFC circuit and the switching power supply circuit on the IP board are introduced here.
1. The PFC circuit is shown in Figure 1. The circuit is based on L6562D, Q4 is the power output tube, PL1 is the energy storage inductor (also the zero current detection transformer), D5 is the rectifier diode, and C12 is the PFC output filter capacitor. Note: This PFC circuit is unique and has no protection diodes.

In the power-on state, the 8th pin of the L6562D gets 14V DC voltage, and the PFC circuit starts. When Q4 is turned on, the inductor PL1 stores energy; when Q4 is turned off, the induced voltage direction of both ends of PL1 is left negative right positive, superimposed with 300V, and generates about 400V DC voltage on C12.
In the standby state, the 8-pin voltage of the L6562D is 0V, and the PFC circuit stops vibrating. Thus, the 300V pulsating DC voltage is filtered by C12 after D5, so the voltage at the PFC output is about 300V during standby.
2. Switching power supply circuit This machine has no auxiliary power supply, so when the AC mains is connected, the switching power supply starts to work. As shown in Figure 2, the switching power supply is centered on 62807, Q2 is the push tube, Q6 is the power switch tube, and P1 optocoupler and IC4 are part of the voltage regulator loop.

When the mains is turned on, regardless of the PFC output is 300V or 400V, the 5807 of 62807 will get the starting voltage (Z4 voltage regulation is provided by R39, R40, R41), the power supply starts, the voltage regulator circuit works, D6 rectification, EC5 The DC voltage generated by the filtering is about 16V, and the power is supplied to 62807 via D2 and R43 instead of the starting voltage.
In the standby state, when the STB terminal of the power supply board is at a low level (about 0V), Q8 and Q10 are turned off, and the optocoupler P2 is turned off, causing Q5 to be turned off, and the emitter voltage VCC is 0V, so the 8 pin of the L6562D in the PFC circuit has no voltage. Therefore, when the PFC circuit stops in standby, the PFC output terminal is 300V, which is used for power supply.
The cutoff of Q10 makes the gate voltage of IC10 P-channel FET 24V, the gate and source voltages are 24V, IC10 is cut off, then 24V terminal has no output voltage; Q10 is cut off, also makes P-channel FET IC5 The gate is 12V, the gate and source are both 12V, then IC5 is turned off, the drain D has no output, and the 12V terminal has no output. There is no output at the 12V terminal, so that the gate voltage of the N-channel FET IC6 is 0V, the IC6 is turned off at the output, and the output is not output at the 5V terminal. It can be seen that only 5VSB has voltage output for several output terminals of the power supply during standby, and no output is provided for the other three terminals.
In the power-on state, the STB terminal of the power supply board becomes high level, then Q8 and Q10 are saturated and turned on, the optocoupler P2 is turned on, Q5 is turned on, the emitter generates a voltage of about 14V, and VCC is about 14V, so that the PFC circuit is in the PFC circuit. The L6562D's 8 pin gets the voltage, so the PFC circuit starts to oscillate. Therefore, the DC voltage of the PFC output terminal is boosted at the power-on state, about 400V, for the switching power supply and the backlight circuit (the backlight circuit power output is 400V).
The conduction of Q10 causes the gate voltage of P-channel FET IC10 to drop to about 4.2V, and the source voltage is 24V. Then the gate-source voltage is VGS=4.2V-24V=-19.8V, VGS is negative, IC10 Turned on, the drain D has 24V, then the 24V terminal has an output.
When Q10 is turned on, the gate voltage of P-channel FET IC5 is 0.6V, and the source is 12V. The gate-source voltage VGS=0.6V-12V=-11.4V, IC5 is turned on, and drain D has 12V. The 12V terminal has an output.
The 12V terminal has a voltage, so that the gate voltage of the N-channel FET IC6 is 12V, the N-channel IC6 is turned on, and the source S has a 5V voltage output, that is, the +5V terminal has a voltage output. After IC6 is turned on, the source voltage becomes 5V. At this time, the gate voltage is still 12V, then the gate-source voltage VGS=12V-5V=7V, that is, the gate voltage is 7V higher than the source, so IC6 can continue to conduct. The 5V terminal continues to have an output.

Summary: P-channel FET should be turned on. The gate voltage must be lower than the source voltage (ie, VGS is negative) and must reach a certain value. The P-channel FET current direction is from source S to drain D; the N-channel FET is turned on, the gate voltage must be higher than the source voltage (ie, VGS is positive), and a certain value is reached. The N-channel FET current direction flows from the drain D to the source S. It can be seen that the N-channel and P-channel gate-source voltage and current directions are opposite, so don't get confused.

Description: 1. The Zener diode is the measured value, where Z3, Z4, and Z5 are three-legged Zener diodes, while Z2 and Z6 are normal. 2. IC5 and IC6 are 8-pin patch FETs.