Skyworth 84E99UD LCD TV power supply circuit principle and troubleshooting ideas (below) - Power Circuit - Circuit Diagram

The circuit diagram for the motherboard's DC-DC converter power supply is shown in Figure 5. IC7 (FSFR1700XSL) is an integrated circuit for a half-bridge LLC resonant converter that employs zero-voltage soft-switching technology; TR5 is a pulse transformer; C103 is a resonant capacitor (which forms LLC resonance together with the inductor in TR2); R143 is a current-limiting resistor; D35 (UF4007) is a bootstrap boost diode; C102 is a bootstrap boost capacitor; R104 is an overcurrent detection sampling resistor; R150, R151, R153 are sampling resistors; PC4 (FOD817) is a photocoupler; IC8 is a precision voltage reference regulator IC; R148, C108 are used to prevent parasitic oscillations.

The basic operational process works as follows: When the PFC circuit functions correctly, Q2 turns on, sending the PWM1 VCC voltage from Q2's output to pin 7 of IC7 to provide the operating voltage. Once the undervoltage lockout detection circuit within IC7 detects that the voltage at input pin 7 exceeds the typical gate voltage of 12.5V, the undervoltage lockout detection circuit outputs the enable control signal, and the voltage regulator circuit supplies the relevant circuit with its operating voltage. When the internal oscillation circuit at pin 3 of IC7 is properly powered, the oscillation circuit begins to oscillate. The signal generated by the oscillation passes through the frequency divider, the delay device, the NOT gate, the electrical phase shift/balance delay device, and the gate driver to the high-side control gate of the low-side power switch transistor. This initiates the operation of the half-bridge LLC resonant converter circuit. The primary 2-6 winding of the pulse transformer TR5 induces an electromotive force through the current, and each secondary winding senses a corresponding electromotive force, which is then rectified and filtered to produce 12V and 24V, achieving DC-DC conversion.

The voltage regulation operational process is as follows: When the voltage at the 12V or 24V output is high, the high voltage is sampled by the resistor, causing the input voltage of IC8 to rise, which in turn reduces the output voltage. The current passing through the internal LED of PC4 increases, and as the phototransistor current increases, the feedback voltage obtained at pin 2 of IC7 becomes lower than normal. Consequently, the on-time of the switch transistor is shortened, reducing the induced voltages on TR5, restoring the output voltage at 12V and 24V to normal values. When the voltage at the 12V or 24V output is low, the regulation process operates in reverse.

The in-chip overvoltage protection process occurs when the 7-pin supply voltage of IC7 exceeds the typical gate voltage of 23.5V. The overvoltage protection circuit immediately activates, forcing the oscillator to stop oscillating, preventing the internal power switch from being driven and turning it off. This achieves overvoltage protection.

The overcurrent protection process starts when the current in the power output loop becomes excessively large due to some reason. This excessive current is sampled by R140, R139, and C98, which integrate and feed the feedback signal back to pin 4 of IC7. If the voltage at pin 4 of IC7 falls below 0.58V and the duration exceeds 1.5μs, the overcurrent protection circuit activates, forcing the oscillator to stop oscillating and turning off the internal power switch transistor without a drive signal, thus achieving overcurrent protection.

The overheat protection process begins when the temperature of the power switch tube exceeds 130°C. The overheat protection circuit activates, forcing the oscillator to stop oscillating and turning off the internal power switch transistor without a drive signal, thereby achieving overheat protection.

U9 constitutes a 12V/24V overcurrent detection circuit. The pin parameters of FSFR1700XSL are shown in Table 1.

The circuit diagram for the backlight-powered DC/DC converter is shown in Figure 6. U1 (FAN7631) is an LLC control integrated circuit introduced by FAIRCHILD. TR1/TR2/TR3 is a pulse transformer; C18/C19 is a resonant capacitor (which forms LLC resonance together with the inductance in TR1/TR2/TR3); R10 is a current limiting resistor; D1 (UF4007) is a bootstrap boost diode; C10 is a bootstrap boost capacitor; R11 is overcurrent detection sampling resistor; R33, R32, R34 are sampling resistors; PC1 (FOD817) is a photocoupler; IC1 (KA431) is a voltage regulator integrated circuit with a precise voltage reference; R20, C16 are used to prevent parasitic oscillations. U4, U5, and U6 are LLC secondary synchronous rectification driver ICs.

The basic operational process works as follows: When the PFC circuit functions correctly, Q1 turns on, sending the PWM2 VCC voltage from Q1's output to pin 12 of U1 to provide the operating voltage. When the internal oscillation circuit at pin 2 of U1 is properly powered, the oscillation circuit begins to oscillate. The signal generated by the oscillation passes through the frequency divider, the delay device, the NOT gate, the electrical phase shift/balance delay, and is sent to the high-side and low-side control gates of the power switch transistors. The half-bridge LLC resonant converter begins to operate. The primary windings 2-6 of the pulse transformers TR1, TR2, and TR3 pass the current to induce an electromotive force, and the secondary windings sense the corresponding electromotive force, which is then rectified and filtered to produce a 24V voltage, thereby implementing DC-DC conversion.

The voltage regulation operational process is as follows: When the voltage at the 24V output terminal is high, the high voltage is sampled by the resistor, causing the input voltage of IC1 to rise, which in turn reduces the output voltage. The current passing through the internal LED of PC1 increases, and as the phototransistor current increases, the feedback voltage obtained at pins 1 and 2 of U1 becomes lower than normal. Consequently, the on-time of the switch transistor is shortened, reducing the induced voltages on TR1, TR2, and TR3, restoring the output voltage at 24V to normal values. When the voltage at the 24V output is low, the voltage regulation process operates in reverse.

Secondly, the common troubleshooting process:

1. 5V high/low: For a 5V high/low fault, first check if the voltage feedback circuit is normal (if necessary, disconnect PC3 and judge using a 3kΩ resistor between pins 3 and 4 of PC3). Then try replacing PC3 for diagnosis.

2. No PFC voltage: For the failure of PFC voltage, first consider whether there is an overcurrent component. Then check if the working condition of U3 is available, whether the PFC boost circuit is normal, whether the signals output to the gates of switch Q16/Q20 from pins 12/13 of U3 are normal, and whether the external current and voltage feedback loops (pins 8, 9, 10, 15, and 16) are functioning properly. Finally, consider replacing U30.

3. PFC voltage high/low: For a PFC voltage high/low fault, first consider whether the input voltage is high or low. Then consider whether the 8-pin voltage feedback loop is normal. Finally, consider replacing U3.

4. 12V, 24V no output: For a 12V, 24V no output fault, first check whether there is a standby control level issue. Secondly, consider whether Q6, Q7, and related components are damaged, whether the oscillation capacitors C18, C9 are deteriorated or have poor soldering, and whether the bootstrap capacitor C10, D1, etc., are damaged. Then compare and repair the secondary rectifier circuit components. Finally, consider replacing U1.

5. 12V, 24V high/low: For a 12V, 24V high/low fault, mainly check whether the feedback circuit components composed of IC1 and PC1 are faulty.