Among PFC switching power supplies, the switching power supply regulated power supply is a very important part. The switching power supply function in the PFC is not much different from the ordinary switching power supply, but there is a difference in power supply. Ordinary switching power supply requires 220V rectifier power supply, while PFC power supply is powered by B+PFC.
After rectification, no filter capacitor is added, and the unfiltered pulsating positive half-cycle voltage is used as the power supply of the chopper. Since the positive voltage of the chopper is “chopped” into a current waveform, the characteristics of the waveform are:
1. The current waveform is discontinuous, and its envelope is the same as the voltage waveform, and the phase of the envelope and the voltage waveform is the same.
2. Due to the effect of chopping, the half-pulsating DC power becomes high-frequency (decided by the chopping frequency, about 100khz) “AC” power. This high-frequency “AC” power must be rectified again before it can be regulated by the subsequent PWM switch Power usage.
3. From the general view of external power supply, the power system achieves that the AC voltage and AC current are in phase and the voltage and current waveforms are in line with the sinusoidal waveform, which not only solves the problem of power factor compensation, but also solves the problems of electromagnetic compatibility and electromagnetic interference.
The high-frequency “alternating current” power is rectified by the rectifier diode and filtered into a direct current voltage and then supplied to the subsequent PWM switching power supply. This DC voltage is also called B+PFC. The B+PFC voltage output by the chopper is generally higher than +300V after the original 220 AC rectification and filtering. The reason is that the high-voltage line is selected, the line diameter of the inductor is small, and the line voltage drop small. The capacity of the filter capacitor is small, and the filtering effect is good, and there are many advantages such as low requirements for the downstream PWM switch tube.
At present, in the PFC switching power supply part, the chopper tube that functions as a switch has two working modes:
1. Continuous conduction mode (CCM): The operating frequency of the switching tube is constant, and the duty cycle of conduction changes with the amplitude of the chopping voltage.
2. Discontinuous conduction mode (DCM): The operating frequency of the chopper switch tube changes with the size of the chopping voltage.
The PFC switching power supply part and the excitation part of the PWM switching power supply part in the power factor correction switching power supply are all completed by an integrated circuit, and a single IC can complete the design.
Post time: Dec-27-2021