Block diagram of voltage-controlled switching power supply in frequency domain

The voltage-controlled switching power supply is a single-loop automatic control system. Figure 1 (a) shows its block diagram. The main circuit of the switching power supply is powered by the rectified DC voltage Ui from the mains power grid to the DC/DC PWM converter. The output voltage uo(t) is detected to obtain k1uo(t). The error value obtained by comparing k1uo(t) with the reference voltage Ur is amplified by the amplifier (hereinafter referred to as the voltage controller) and the output control signal uC(t) is given to the PWM pulse width modulator (PWM). PWM consists of a sawtooth wave generator with a period of Ts and an amplitude of Um, a comparator and a trigger. uC(t) is compared with the sawtooth wave to form a switching function q(t) to control the main switch tube. Figure 1 (b) shows its working waveform. The duty cycle continuous function d(t) is the average value of the switching function, and the discrete value is the duty cycle of the first cycle. The system block diagram can be easily obtained from Figure 1.

Figure 2 shows the block diagram of a voltage-controlled switching power supply, i.e., the frequency domain model of the switching power supply system. Assuming that the given voltage Ur is a constant, its disturbance ur = 0. The output of the switching power supply y(s) = uo(s).

In Figure 2, G(s) is the forward channel transfer function, and G(s) = uo(s)/d(s) is the control-output transfer function of the switching converter (operating in CCM). H(s) is the transfer function of the feedback channel (including voltage detection, controller, PWM and other components). E(s)d(s) is a non-independent voltage source controlled by the duty cycle d(s). zo(s) is the output impedance of the switching converter. ui(s) and Io(s) represent the disturbances of the mains grid voltage and load current, respectively.

It can be seen that the open-loop and closed-loop transfer functions of the switching power supply system can be obtained by only calculating the transfer functions of each block of the system.

Figure 1 Voltage-controlled switching power supply system

Figure 2 Block diagram of voltage-controlled switching power supply