The basic working principle of PWM switch regulated or current stabilized power supply is to provide closed-loop feedback through the difference between the controlled signal and the reference signal in the control circuit in the event of input voltage changes, internal parameter changes, or external load changes, to adjust the conduction pulse width of the main circuit switching device, so as to stabilize the output voltage or current of the switching power supply and other controlled signals.
Generally speaking, the forward type main circuit can be simplified by the buck chopper shown in Figure 1, and Ug represents the pWM output driving signal of the control circuit. According to the different pWM feedback control modes selected, the input voltage Uin, output voltage Uout, switching device current (led out from point b), and inductance current (led out from point c or point d) in the circuit can all be used as sampling control signals. When the output voltage Uout is used as a control sampling signal, it is usually processed through the circuit shown in Figure 2 to obtain the voltage signal Ue, which is then processed or directly sent to the pWM controller. The function of the voltage operational amplifier (e/a) in Figure 2 is twofold: ① Amplify and feedback the difference between the output voltage and the given voltage Uref to ensure stable voltage regulation accuracy in steady-state. The DC amplification gain of this operational amplifier is theoretically infinite, but in reality it is the open-loop amplification gain of the operational amplifier Convert the DC voltage signal with a wider frequency band switch noise component attached to the output end of the switch main circuit into a relatively "clean" DC feedback control signal (Ue) with a certain amplitude, which retains the DC low-frequency component and attenuates the AC high-frequency component. Due to the high frequency and amplitude of switching noise, if the attenuation of high-frequency switching noise is not enough, the steady-state feedback will be unstable; If the high-frequency switch noise attenuation is too large, the dynamic response is slower. Although contradictory, the basic design principle for voltage error operational amplifiers is still 'high low-frequency gain and low high-frequency gain' Correct the entire closed-loop system to ensure stable operation.
1) Different pWM feedback control modes have their own advantages and disadvantages. When designing a switching power supply, it is necessary to choose the appropriate pWM control mode based on the specific situation.
2) The selection of various control mode pWM feedback methods must be combined with specific input and output voltage requirements of the switching power supply, main circuit topology and device selection, high-frequency noise level of output voltage, and duty cycle variation range.
3) The pWM control mode is evolving and interconnected, and can be transformed into each other under certain conditions.
