Detailed explanation of the working principle of PFC circuit

Detailed explanation of the working principle of PFC circuit

The output voltage Vo of the main circuit is compared with the reference voltage Vref and then input to the voltage error amplifier A; the detection value Vf of the rectified voltage Vin and the output signal Ve of the voltage error amplifier A are added to the input of the multiplier M; the output lref of the multiplier is used as the reference signal for current feedback control, and after being compared with the input current sampling value If, ​​it is added to the PWM control chip and driver through the current error amplifier to control the on and off of the main MOS tube.

Here we take the BOOST circuit as an example.

The BOOST driving method is the simplest (source grounding, no need for bootstrapping and isolation), and it can also provide a certain boost capability. This circuit is still used in most switching power supplies, and its peak efficiency is 97%. Other topologies have been used in high-power or higher-demand application scenarios.

An inductor is connected after the full-bridge output to act as an energy storage device. When the switch is turned on, the AC current only passes through the inductor, and the current in the inductor rises. When the switch is turned off, the current stored in the inductor supplies power to the capacitor and the load through the diode. Therefore, whether the Mosfet is turned on or off, there is always current. There is no current interruption.

Simply put, the inductor and switch tube can ensure that there is always current in the inductor branch, and there will be no current interruption in the full-bridge rectifier circuit, which improves the power factor.

PFC control circuit diagram

1. Input RC damping to absorb input high-frequency interference signals;

2. Anti-surge input current resistor when starting up;

3. Input common mode interference filtering inductor;

4. Output voltage boost inductor. This circuit is a widely used output voltage boost active power factor correction circuit.

5. Output start diode. When the circuit output has not reached the normal output voltage, the AC input mains rectifier outputs the DC voltage to supply power to the load through this diode. Once the circuit works normally and the output voltage reaches the normal value, this diode is turned off and does not work.

6. The power factor correction current sampling resistor, through the control of the circuit, makes the AC input current waveform a sine wave with the same frequency and phase as the AC input mains voltage, thus achieving the power factor correction effect;

7. The RD driving circuit of the power factor correction power switch tube Q1, when the driving signal turns on Q1, the diode D7 does not work. When Q1 is turned off, the input capacitor of Q1 is turned on by D7, so that the input capacitor of Q1 is discharged faster through the R10 and R9 loop, so that Q1 smoothly enters the next switching working cycle;

8. The RC compensation circuit of IC makes IC work stably;

9. Output voltage boost power switch tube;

10. RC absorption circuit, used to reduce the power consumption of the output voltage boost diode;

11. D3 is the output voltage boost diode;

12. Output voltage sampling resistor divider circuit, the output voltage sampling voltage is sent back to the 6th pin of IC for output voltage feedback to make the output voltage stable;

13. Output overvoltage protection voltage sampling resistor divider circuit. Once the circuit has an output overvoltage fault, the circuit protection shuts off the output;

14. Output filter capacitor: a large-capacity capacitor and a small-capacity capacitor are connected in parallel. The large-capacity capacitor plays the role of output voltage filtering and stabilizes the output voltage. The small-capacity capacitor is used to filter out high-frequency interference. Generally, the high-frequency characteristics of large-capacity electrolytic capacitors are not good. The small-capacity capacitor is used to improve the high-frequency filtering characteristics of large-capacity electrolytic capacitors.

15. PFC correction input filter capacitor is used to filter out input high-frequency interference. The capacity should not be too large, otherwise it will affect the effect of power factor correction;

16. Output discharge resistor, used to discharge the charge on the output filter capacitor once the output load is open.