Recently, I had the opportunity to repair a Panda LCD TV. As there was limited available information on this model, I decided to document my findings by drawing the PFC (Power Factor Correction) circuit and the switching power supply circuit on the combined board. Given that these schematics were drawn directly from the physical device, the component designations and models are authentic and reliable, providing valuable first-hand information for fellow technicians. However, due to the sheer number of chip capacitors on the board and the difficulty in individually measuring their capacities, the chart does not include standard capacitance values for these components.
The Panda L32A7031 LCD TV employs an integrated power supply design, meaning the switching power supply circuit and the backlight inverter circuit are combined on a single board, commonly referred to as an IP board. Since the inverter circuit converts the approximately 380V DC output from the PFC into an AC sinusoidal high voltage, the burden on the switching power supply is significantly reduced. Here, we’ll focus on introducing the PFC circuit and the switching power supply circuit located on the IP board.
1. The PFC circuit is illustrated in Figure 1. Based on the L6562D chip, Q4 serves as the power output transistor, PL1 acts as the energy storage inductor (and also functions as a zero-current detection transformer), D5 is the rectifier diode, and C12 is the PFC output filter capacitor. Note: This PFC circuit is unique and lacks protective diodes.
During operation, the 8th pin of the L6562D receives a 14V DC voltage, initiating the PFC circuit. When Q4 turns on, the inductor PL1 stores energy; when Q4 turns off, the induced voltage across PL1 becomes negative on the left and positive on the right, adding to the 300V input voltage, resulting in approximately 400V DC on C12.
In standby mode, the 8th pin voltage of the L6562D is 0V, halting the PFC circuit’s oscillation. Consequently, the 300V pulsating DC voltage passes through C12 after being rectified by D5, making the PFC output voltage around 300V during standby.
2. The switching power supply circuit on this unit lacks an auxiliary power supply. Therefore, upon connecting the AC mains, the switching power supply begins functioning. As depicted in Figure 2, the switching power supply centers around the 62807 chip, with Q2 as the push-pull transistor and Q6 as the power switch transistor. Additionally, P1 optocoupler and IC4 form part of the voltage regulation feedback loop.
Upon powering on, regardless of whether the PFC output is 300V or 400V, the 5807 of 62807 receives the startup voltage (provided by Z4 voltage regulation through R39, R40, R41), initiating the power supply and enabling the voltage regulation circuit. D6 performs rectification, and the filtering by EC5 generates a DC voltage of about 16V, supplying power to 62807 via D2 and R43 instead of the startup voltage.
In standby mode, when the STB terminal of the power supply board is at a low level (around 0V), Q8 and Q10 turn off, disabling the optocoupler P2, causing Q5 to turn off, resulting in an emitter voltage VCC of 0V. Consequently, the 8th pin of the L6562D in the PFC circuit receives no voltage. Hence, when the PFC circuit halts in standby mode, the PFC output terminal provides 300V for power supply.
The cutoff of Q10 lowers the gate voltage of the P-channel FET IC10 to approximately 24V, with the gate-source voltage at 24V, thus turning IC10 off, eliminating the 24V output. Similarly, the cutoff of Q10 also sets the gate voltage of the P-channel FET IC5 to 12V, with gate-source voltage at 12V, turning IC5 off, and eliminating the 12V output. With no output at the 12V terminal, the gate voltage of the N-channel FET IC6 becomes 0V, turning IC6 off, and eliminating the 5V output. It’s evident that during standby, only the 5VSB terminal has voltage output, with no output from the other three terminals.
In the powered-on state, the STB terminal of the power supply board goes high, saturating and turning on Q8 and Q10. This enables the optocoupler P2, turning on Q5, generating an emitter voltage of about 14V, and setting VCC to about 14V, allowing the PFC circuit’s L6562D’s 8th pin to receive voltage, thereby initiating the PFC circuit's oscillation. Consequently, the DC voltage at the PFC output terminal increases to approximately 400V during the powered-on state, supplying the switching power supply and the backlight circuit (backlight circuit output is 400V).
The conduction of Q10 reduces the gate voltage of the P-channel FET IC10 to about 4.2V, while the source voltage remains at 24V. This results in a gate-source voltage VGS = 4.2V - 24V = -19.8V, a negative VGS, turning IC10 on, and enabling the 24V terminal to have an output.
When Q10 turns on, the gate voltage of the P-channel FET IC5 drops to 0.6V, with the source voltage at 12V. This yields a gate-source voltage VGS = 0.6V - 12V = -11.4V, turning IC5 on and enabling the 12V terminal to have an output.
The presence of voltage at the 12V terminal raises the gate voltage of the N-channel FET IC6 to 12V, turning IC6 on, and producing a 5V voltage output at the source S, i.e., the +5V terminal has an output. Once IC6 turns on, the source voltage becomes 5V. At this point, the gate voltage remains at 12V, giving a gate-source voltage VGS = 12V - 5V = 7V, meaning the gate voltage is 7V higher than the source, ensuring IC6 continues to conduct, maintaining the 5V output.
Summary: For a P-channel FET to turn on, the gate voltage must be lower than the source voltage (i.e., VGS is negative) and reach a specific threshold. The current flow in a P-channel FET is from source S to drain D; for an N-channel FET, the gate voltage must be higher than the source voltage (i.e., VGS is positive) and reach a certain threshold. The current flows from drain D to source S. It is clear that the gate-source voltage and current direction differ between N-channel and P-channel FETs—don’t confuse them.
Description: 1. The Zener diodes are measured values, where Z3, Z4, and Z5 are three-legged Zener diodes, while Z2 and Z6 are standard. 2. IC5 and IC6 are 8-pin surface-mount FETs.
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