Detailed explanation of single-stage PFC, primary-side constant current control and thyristor dimming LED driver IC

1. EMI filtering is provided by L1-L3, C2 and Y capacitor (C7). The Y capacitor (C7) is connected across the primary side and the secondary side.

2. The LC filter circuit filters the rectified AC mains output. The filter capacitor capacity here should not be too large to avoid affecting the circuit power factor correction ( PFC ) effect.

3. The input peak value of the rectified AC input mains power charges C8 through D6. The current then passes through R4, R7 and R8 and is injected into the voltage monitoring pin of U1. The VOLTAGE MONITOR pin current and FEEDBACK pin current are used internally to control the average output load LED operating current. The total resistance value of R4, R7, and R8 (3.909 MΩ total), as well as R11 and R12 connected to the voltage monitor pin (1.402 MΩ total), provides excellent voltage regulation performance over the entire input voltage range of 90VAC-265VAC. U1 can also use the voltage monitor pin current to set the input overvoltage and undervoltage protection thresholds.

4. Clamping circuit, due to the influence of leakage inductance, diodes D1 and VR1 clamp the drain voltage to a safe level.

5. The function of diode D5 is to prevent reverse current from flowing through U1 during the period when the AC input mains voltage input voltage is lower than the reflected output voltage (VOR).

6. The reference pin R is connected to ground (source) through the external resistor R19. The selected value sets the internal reference, which determines dimming (LNK403-410) or non-dimming (LNK413-420) operation, as well as the input undervoltage and overvoltage thresholds for the voltage monitor pin. For non-dimming or PWM dimming applications using the LNK413-420, the external resistor should be 24.9kΩ±1% for high input voltage and universal input voltage designs and 49.9kΩ±1% for low input voltage designs. . For phase-controlled dimming applications using the LNK403-410, the external resistor should be 49.9kW±1%. Since resistor tolerance directly affects output tolerance, a resistor tolerance of 1% is recommended. Do not use other resistor values.

7. Flyback transformer.

8. Bypass pin BP power supply circuit, D4 has the function of preventing current backflow.

9. Output rectifier filter circuit.

10. The output bleeder resistor is used to provide an output load for the circuit output when the circuit load is open, and to provide a discharge circuit for the output filter capacitors C4 and C5.

11. Bypass pin BP rectifier filter circuit, resistors R18 and R9 are bleeder resistors. The minimum output power of the circuit can be selected through a 10μF bypass pin capacitor C12, so a smaller device current limit can be selected, thereby reducing conduction losses. For open designs or designs with a certain space for heat dissipation, refer to the maximum output power. Maximum output power can be selected with a 100µF BYPASS pin capacitor, except for the LNK4x3, which has only one power setting. In all cases, for optimal output current tolerance, the device temperature should be maintained below 100°C.

12. Output overvoltage and load cutoff protection are provided by D8, C14, R24, VR3, C15, R23 and Q2. If the LED output load is removed, the output voltage will increase, causing the bias winding voltage on C14 to increase accordingly. Once this voltage exceeds the voltage rating of VR3, Q2 will conduct, thereby pulling the U1 feedback pin voltage low and initiating auto-restart operating mode. Once in auto-restart mode, a low duty cycle (~3%) and a small false load on the output prevent the output voltage from rising to a higher value. Once the output load is reconnected, the power supply will resume normal operation.

13. Y capacitor, which provides isolation for the input and output loops of the circuit. Y capacitor also has EMI filtering function.