DCM Flyback PFC Converter

Like the Bootst PFC converter, the inherent characteristics of the flyback PFC converter when operating in DCM mode are: when the output voltage is regulated by voltage-type PWM control, the steady-state duty cycle Du is constant (that is, the on-time TON is constant), and the input current is close to a sine wave. Therefore, power factor correction can be achieved without multipliers and current control in the control circuit.

Figure 1 (a) shows the schematic diagram of the DCM flyback PFC converter, which is a single-loop voltage feedback PWM control system. Figure 1 (b) shows the input current waveform under high-frequency PWM switch control within a half-cycle of the power frequency. The switch current iv is a triangular waveform, the dotted line is the envelope of the current peak value iP, and the solid line is the switch current average value iV (av) curve within a switching cycle.

Where Udc is the rectified input voltage.

From formula (8-22), it can be seen that the average value of the switch tube current iVav of the DCM flyback PFC converter is linearly related to the input rectified voltage Udc.

It can be proved that the ideal DCM flyback PFC converter shown in Figure 1 (a) can be equivalent to a loss-free resistor (Loss Free Resistor) controlled by the duty cycle Du for the input, as shown in Figure 25. Therefore, the circuit in Figure 1 (a) can achieve an input power factor of approximately 1 without the need for a current controller.

In Figure 2, Udc is the rectified input voltage, which is added to the DC input terminal of the DCM flyback PFC converter through the EMI filter. The transformer voltage ratio of the flyback PFC converter is n:1, and iv is the input current U. , iP are the output voltage and output current respectively, and ip represents the equivalent input (lossless) resistance of the flyback PFC converter.

Figure 1 DCM flyback PFC converter

Figure 2 Equivalent circuit of a DCM flyback PFC converter