How BIFRED Converter Works

Figure 1 shows three equivalent circuits of the BIFRED converter in one switching cycle. Assuming that the output filter capacitor C is very large, it can be equivalent to a voltage source Uo. Figure 2 shows the operating waveform of the BIFRED converter.

TON＝DuTs is the time when the switch is on, Du is the duty cycle, and Ts is the switching period;

Toff＝（1-Du）Ts is the switch off time;

Du1Ts is the time when the current iL decreases but is not zero; (1-Du-Dut)Ts is the time when the current iL is equal to zero.

(1) 0≤t≤DuTs, switch S1 is turned on, D2 is turned off. Input current iL rises, and the slope of the rise is Udc/L1, which is the rectified input voltage. L1 stores energy, and capacitor CB discharges. The transformer's excitation inductance Lm stores energy, and the excitation current rises, and the slope of the rise is UB/nLm. The equivalent circuit is shown in Figure 1 (a).

Figure 1 Equivalent circuit of a BIFRED converter during one switching cycle

Figure 2: Working waveform of BIFRED converter

(2) DuTs≤t≤（Du＋Dul）Ts, switch S1 is turned off, and D2 is turned on. The energy stored in L1 charges the capacitor CB, iL decreases, and its decreasing slope is -（UB＋nUo-μdc）／L1. The energy stored in the excitation inductor Lm is released to the load, causing the excitation current im to decrease, and its decreasing slope is -Uo/Lm. The equivalent circuit is shown in Figure 1(b).

(3) t≥(Du, +Du1)Ts, switch S1 is still off, current iL is equal to zero, and excitation current im continues to decrease linearly with the same slope. The equivalent circuit is shown in Figure 1(c).

As mentioned above, the inductor current peak value iLPK of the DCM Boost PWM converter is proportional to sln|ωt| and has a PFC function.

The experimental results of a BIFRED converter prototype are as follows: Uo = 5V, P = 90W, fs = 50kHz, η = 71.12%, the third harmonic of the input current is 16.84%, the fifth harmonic is 2.715%, and the seventh harmonic is 2.3%.