PWM signal generation circuit diagram

The separate excitation control methods of switching power supply include pulse width modulation ( PWM ) and pulse frequency modulation (PFM). Most of them adopt PWM method. Therefore, the principle of PWM control is briefly introduced. The figure shows the PWM signal generation circuit block diagram and working waveform. The working process is as follows: to the controlled voltage Uo. The feedback voltage Ur obtained by the detection is added to the non-inverting input terminal of the amplifier A1, and the fixed reference voltage Uo is added to the inverting input terminal of A1. The DC error voltage Ue amplified by A1 is added to the inverting input terminal of comparator A2, and the sawtooth wave signal Usa generated by the fixed frequency oscillator is added to the non-inverting input terminal of A2. A2 outputs a square wave signal, and its duty cycle changes with the error voltage , which realizes pulse width modulation. For a single-tube converter, the PWM signal output by A2 can be used as a switching signal to control the power transistor. For power conversion circuits such as push-pull or bridge, the PWM signal should be divided into two groups of signals, that is, phase separation. The phase split circuit consists of a flip-flop and two " AND " gates. The clock signal of the flip-flop corresponds to the falling edge of the sawtooth wave. The A terminal and the B terminal output two sets of PWM signals that are 180° different.

(a) PWM signal generation circuit block diagram; (b) Working waveform

Figure PWM signal generation circuit block diagram and working waveform