Changing the PWM feedback control mode on the power supply
The fundamental working principle of PWM switching or constant current power supplies is that 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 when the input voltage changes, the internal parameters change, and the external load changes. The output voltage or current of the switching power supply and other regulated signals are stabilized by the conduction pulse width.
Switching power supply pWM fundamentals
The control sampling signals for pWM include output voltage, input voltage, output current, output inductor voltage, and peak current of switching devices. The switching frequency of pWM is typically constant. To achieve the goals of voltage stabilization, current stabilization, and constant power, these signals can be combined to build a single-loop, double-loop, or multi-loop feedback system. Additionally, it is possible to realize some extra features like current sharing, anti-bias magnetic fields, and overcurrent protection. pWM feedback control modes currently come in five primary categories.
changing the pWM feedback control mode of the power supply
In general, the step-down chopper in Figure 1 can simplify the forward-type main circuit, with Ug standing in for the control circuit's pWM output drive signal. 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 employed as sampling control signals depending on the choice of various pWM feedback control modes. The circuit in Figure 2 is typically used to transform the output voltage Uout into a voltage signal Ue, which is subsequently processed or delivered directly to the PWM controller when the output voltage Uout is utilized as a control sampling signal.
Three tasks are involved:
① To guarantee accurate voltage regulation in the steady state, the difference between the output voltage and the specified value Uref is amplified and sent back. Although the operational amplifier's open-loop amplification gain is theoretically limitless, it is actually the DC amplification gain.
2 Retain the DC low-frequency components and attenuate the AC high-frequency components to create a relatively "clean" DC feedback control signal (Ue) with a certain amplitude from the DC voltage signal with switching noise components of a wider frequency band at the output of the switch main circuit. The steady-state feedback will be unstable if the attenuation of high-frequency switching noise is not sufficient, and the dynamic response will be slow if the attenuation of high-frequency switching noise is excessive due to the high frequency and large amplitude of switching noise. The fundamental design tenet of the voltage error operational amplifier is still that "low frequency gain should be high, high frequency gain should be low," despite their apparent contradictions.
To make the closed-loop system operate steadily, make the necessary corrections to the entire system.
characteristics of the power supply while switching
1) Each pWM feedback control mode has advantages and disadvantages of its own. The proper pWM control mode should be chosen when constructing a switching power supply depending on the circumstances.
2) When choosing pWM feedback techniques for different control modes, it is important to consider the switching power supply's unique input and output voltage requirements, the main circuit topology and device choices, the high-frequency noise of the output voltage, and the range of duty cycle changes.
3) The pWM control mode evolves and changes, is connected, and can change into one another under specific circumstances.
