Switch mode power supply - pWM Basics
The switching frequency of pWM is generally constant, and the control sampling signals are: output voltage, input voltage, output current, output inductor voltage, and peak current of switching device. By these signals can constitute a single-loop, dual-loop or multi-loop feedback system, to achieve the purpose of regulating voltage, current and constant power, and at the same time can achieve some incidental overcurrent protection, anti-bias, equalisation of current and other functions. Now there are five main pWM feedback control modes.
Switching power supply pWM feedback control mode
Generally speaking, the main forward circuit can be used to simplify the buck chopper shown in Figure 1, Ug indicates that the control circuit of the pWM output drive signal. According to the choice of different pWM feedback control mode, the circuit of the input voltage Uin, output voltage Uout, switching device current (led by point b), inductor current (led by point c or point d) can be used as a sampling control signal. When the output voltage Uout is used as a control sampling signal, it is usually processed by the circuit shown in Fig. 2 to obtain the voltage signal Ue, which is then processed or fed directly to the pWM controller. The role of the voltage operational amplifier (e/a) in Fig. 2 is threefold: (1) to amplify and feedback the difference between the output voltage and the given voltage Uref to ensure the accuracy of the voltage regulation in the steady state. The DC amplification gain of the op-amp is theoretically infinite, which is actually the open-loop amplification gain of the op-amp. The DC voltage signal with broadband switching noise at the output of the main switching circuit is transformed into a relatively "clean" DC feedback control signal (Ue) with a certain amplitude, i.e., retaining the DC low-frequency components and attenuating the AC high-frequency components. Because the switching noise of higher frequency, amplitude, high-frequency switching noise attenuation is not enough, then the steady state feedback is not stable; high-frequency switching noise attenuation is too large, then the dynamic response is slower. Although contradictory, but the basic design principle of the voltage error operational amplifier is still "low-frequency gain should be high, high-frequency gain should be low". The whole closed-loop system is corrected to make the closed-loop system work stably.
Switching power supply pWM characteristics
1) different pWM feedback control mode has its own different advantages and disadvantages, in the design of switching power supply selection should be based on the specific circumstances of the choice of the appropriate pWM control mode.
2) The selection of various control modes pWM feedback method must be combined with the consideration of specific switching power supply input and output voltage requirements, the main circuit topology and device selection, the output voltage of the high-frequency noise size, duty cycle change range.
3) pWM control mode is the development of change, is interconnected, under certain conditions can be converted to each other.
