The basic principle of switch mode 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 the switching device. These signals can form single loop, double loop, or multi loop feedback systems to achieve stable voltage, current, and constant power, while also providing additional functions such as overcurrent protection, anti bias, and current sharing. There are currently five main pWM feedback control modes.
Switching power supply pWM feedback control mode
Generally speaking, the forward type main circuit can be simplified by a buck chopper, and Ug represents the pWM output driving 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 out from point b), and inductor 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 voltage operational amplifier (e/a) has three functions:
① Amplify and feedback the difference between the output voltage and the given voltage Uref to ensure stable voltage regulation accuracy during 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.
② Transform the DC voltage signal with a wide 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 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 slower. Although contradictory, the basic design principle for voltage error operational amplifiers is still "high low-frequency gain and low high-frequency gain.".
③ Correct 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 switching power supplies, it is necessary to choose the appropriate pWM control mode according to 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 range of duty cycle variation.
3) The pWM control mode is evolving and interconnected, and can be transformed into each other under certain conditions.
