Power supply switch mode PWM feedback control mode
The basic working principle of PWM switching voltage stabilization or current stabilization power supply is that when the input voltage changes, 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 turn-on pulse width stabilizes the controlled signal such as the output voltage or current of the switching power supply.
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 switching device peak current. These signals can form a single-loop, double-loop or multi-loop feedback system to achieve voltage stabilization, stable current and constant power. At the same time, some accompanying overcurrent protection, anti-bias magnetization, current sharing and other functions can be achieved. There are currently five main pWM feedback control modes.
Switching power supply pWM feedback control mode
Generally speaking, the forward main circuit can be simplified by the buck chopper shown in Figure 1, and Ug represents the pWM output drive signal of the control circuit. Depending on 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 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 feed back the difference between the output voltage and the given voltage Uref to ensure the voltage stabilization accuracy in the steady state. The DC amplification gain of this op amp is theoretically infinite, but is actually the open-loop amplification gain of the op amp. ② Convert the DC voltage signal with a wider-band switching noise component at 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. Because the switching noise has a higher frequency and larger amplitude, if the high-frequency switching noise is not attenuated enough, the steady-state feedback will be unstable; if the high-frequency switching noise is attenuated too much, the dynamic response will be slow. Although contradictory, the basic design principle for voltage error operational amplifiers is still "the low-frequency gain should be high and the high-frequency gain should be low." ③ Calibrate 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 different advantages and disadvantages. When designing and selecting a switching power supply, the appropriate pWM control mode must be selected according to the specific situation.
2) The selection of pWM feedback methods for various control modes must take into account the input and output voltage requirements of the specific switching power supply, the main circuit topology and device selection, the high-frequency noise of the output voltage, the duty cycle variation range, etc.
3) pWM control modes are evolving, interconnected, and can be transformed into each other under certain conditions.
