What is the difference between a passive pfc circuit and an active pfc circuit?

PFC circuits are usually divided into active PFC and passive in the actual application process, and their application effects and operation modes are also different.

1. Passive PFC circuit
A correction circuit that does not use active devices such as transistors is generally composed of passive devices such as diodes, resistors, capacitors, and inductors.

2. Active PFC circuit
The active PFC circuit has a good effect. It can basically completely eliminate the distortion of the current waveform, and the phase of the voltage and current can be controlled to be consistent. It basically completely solves the problems of power factor, electromagnetic compatibility, and electromagnetic interference, but The circuit is very complicated.

The basic idea is to remove the filter capacitor after the 220V rectifier bridge stack (to eliminate the current waveform distortion and phase change caused by the capacitor charging), and a "chopper" circuit converts the pulsating DC into a high frequency (about 100kHz) AC passing After rectification and filtering, the DC voltage is then supplied to the conventional PWM switching regulated power supply, the process is AC→DC→AC→DC.

The basic principle of the active PFC circuit is to add a DC-DC chopper circuit between the rectifier circuit of the switching power supply and the filter capacitor, as shown in the figure (the chopper circuit is equal to an additional switching power supply). For the power supply line, the output of the rectifier circuit is not directly connected to the filter capacitor, so it presents a purely resistance load to the power supply line, and its voltage and current waveforms are in the same phase and the same phase. The work of the chopper circuit is also similar to a switching power supply, so the active PFC switching power supply is a dual switching power supply switching power supply circuit, which is composed of a chopper (we will call it "PFC switching power supply") and a voltage regulator. Switching power supply (we will call it "PWM switching power supply" from now on).

The difference between passive pfc circuit and active pfc circuit
In the process of power supply design and power system debugging, there are two common PFC application methods, one is passive PFC, this power factor correction method is also called passive PFC, and the other is active PFC, also called It is active PFC. In terms of working principle, the difference between the two is that passive PFC technology generally uses inductance compensation to reduce the phase difference between the fundamental current and voltage of the AC input to improve the power factor, but the power factor of passive PFC is not very good. High, it can only reach about 0 7-0 8. Active PFC is composed of inductors, capacitors and electronic components. It is small in size and can achieve a high power factor, but the cost is higher than that of passive PFC.

In addition to the differences in working principles, active and passive PFC circuits also have some other characteristics that can be distinguished. The most typical one is that high-integration ICs are often used in active power factor correction circuits. Source PFC circuit PC power supply, the input voltage can be from 90V to 270V, and the circuit has a line power factor higher than 0 99, and has the advantages of low loss and high reliability. And because of the advanced level of IC, this kind of PFC circuit can also be used as an auxiliary power supply. Therefore, in the use of active PFC circuit, the output does not change with the fluctuation of the input voltage, the output voltage is highly stable, and the output DC voltage ripple is small And it is 100HZ/120HZ sine wave, often does not need a standby transformer.