Switching power supply PWM feedback control mode
The basic working principle of PWM switching or constant current power supply is that when the input voltage changes, the internal parameters change, and the external load changes, the control circuit performs closed-loop feedback through the difference between the controlled signal and the reference signal to adjust the switching device of the main circuit. The conduction pulse width makes the output voltage or current of the switching power supply and other controlled signals stable.
Basic principle 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 to achieve the purpose of voltage stabilization, current stabilization and constant power. At the same time, some additional functions such as overcurrent protection, anti-bias magnetic field, and current sharing can be realized. Now there are mainly five pWM feedback control modes.
Switching power supply pWM feedback control mode
Generally speaking, the forward-type main circuit can be simplified by the step-down chopper shown in Figure 1, and 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, switching device current (derived from point b), and inductor current (derived from point c or point d) in the circuit can 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 a 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 feed back the difference between the output voltage and the given voltage Uref to ensure the accuracy of voltage regulation in a steady state. The DC amplification gain of the operational amplifier is theoretically infinite, but it is actually the open-loop amplification gain of the operational amplifier. ② Transform the DC voltage signal with switching noise components of a wider frequency band at the output of the switch main circuit into a relatively "clean" DC feedback control signal (Ue) with a certain amplitude, that is, retain the DC low-frequency components and attenuate the AC high-frequency components. Because the frequency of switching noise is high and the amplitude is large, if the attenuation of high-frequency switching noise is not enough, the steady-state feedback will be unstable; if the attenuation of high-frequency switching noise is too large, the dynamic response will be slow. Although contradictory to each other, the basic design principle of the voltage error operational amplifier is still "low frequency gain should be high, high frequency gain should be low". ③ Correct the entire closed-loop system to make the closed-loop system work stably.
Switching power supply pWM characteristics
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 pWM feedback methods for various control modes must take into account the specific input and output voltage requirements of the switching power supply, the main circuit topology and device selection, the high-frequency noise of the output voltage, and the range of duty cycle changes.
3) The pWM control mode develops and changes, is interrelated, and can be transformed into each other under certain conditions.
