Working Principle of COM Flyback PFC Converter

FIG1 is a block diagram of a peak current controlled CCM flyback PFC converter. The controller includes a multiplier and voltage and current comparators.

　　Figure 1 Schematic diagram of a peak current controlled CCM flyback PFC converter

　　Assume that at the beginning of the cycle, the switch V is turned on, t=0, iv=Iv1; when t=ton=DuTs, iv=Ip, at this time, the switch V is turned off, and the output iD changes from zero to Ipo. During the period when the switch tube V is turned on, the change law of the switch current from Iv1 to Ip is:

　　In a switching cycle, the average value of iv ivav is related to the rectified input voltage Udc
 

　　Where L is the primary inductance of the transformer.

　　According to the principle of volt-second balance within a cycle, it can be proved that the duty cycle

　　Therefore, the current peak value of the circuit in Figure 2 follows Udc. The current average value is a function of Du and Udc, and Du is related to the input voltage ui, so the current average value is a nonlinear function of Udc. The advantages of the CCM flyback PFC converter are: low noise, power factor can be corrected to close to 1, high efficiency, and low peak current.

　　The CCM flyback PFC converter can also use charge control. Figure 2 shows the principle circuit diagram of the CCM flyback PFC converter using charge control. In addition to the main switch tube v, there is also a signal switch tube ⒕ in the system. It is also a dual-loop control system, including a charge control loop and a voltage loop. The error voltage of the voltage loop and the rectified input voltage (divided by Κ') are multiplied by a multiplier to obtain the reference signal Z of the charge control loop. The voltage uT on the capacitor CT (proportional to the charge of CT) is compared with Z to control the main switch tube V1. At the beginning of each switching cycle, the switch tube V1 is turned on and V2 is turned off. The current signal iVT detected by the current transformer charges the capacitor CT. When its voltage UT reaches Z, the switch tube V1 is turned off, V2 is turned on, and the capacitor CT discharges. The average current iVlav of the switch tube V1 and the voltage uT on the capacitor CT are shown in the following formula

　　It can be seen that if Du and Ts are constants, ivlav is proportional to uT.

　　Figure 2: Schematic diagram of a charge-controlled CCM flyback PFC converter