Getting started with PWM controlled switching power supplies
Basic Principles of Switching Power Supply PWM
The switching frequency of PWM is generally constant, and the control sampling signals include output voltage, input voltage, output current, output inductor voltage, and peak current of switching devices. These signals can form a single loop, double loop, or multi loop feedback system, achieving the purpose of stabilizing voltage, current, and constant power. At the same time, it can also achieve some additional functions such as overcurrent protection, anti bias magnetic, and current equalization. There are currently five main types of PWM feedback control modes.
Switching power supply PWM feedback control mode
Generally speaking, the forward type main circuit can be simplified using the step-down chopper shown in Figure 1, where Ug represents the PWM output drive signal of the control circuit. According to the selection of different PWM feedback control modes, the input voltage Uin, output voltage Uout, switch device current (led from point b), and inductor current (led from point c or 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 by the circuit shown in Figure 2 to obtain the voltage signal Ue, which is then processed or directly sent to the PWM controller. The voltage operational amplifier (e/a) in Figure 2 has three functions: ① Amplify and provide feedback on the difference between the output voltage and the given voltage Uref to ensure stable voltage 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 wideband switching noise components attached to the output terminal 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 switch noise, if the attenuation of high-frequency switch noise is not sufficient, the steady-state feedback will be unstable; If the high-frequency switch noise attenuation is too large, the dynamic response will be slow. Although contradictory, the basic design principle for voltage error operational amplifiers is still "high low-frequency gain and low high-frequency gain" Calibrate the entire closed-loop system to ensure stable operation.
Characteristics of Switching Power Supply PWM
1) Different PWM feedback control modes have their own advantages and disadvantages. When designing a switching power supply, the appropriate PWM control mode should be selected according to the specific situation.
2) The selection of various control modes and PWM feedback methods must be combined with consideration of specific input and output voltage requirements of the switching power supply, main circuit topology and device selection, high-frequency noise level of the 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.
