What is the difference between active PFC and passive PFC in switching power supply?

The full English name of PFC is "Power Factor Correction", which means "Power Factor Correction", and in layman's terms, it is to increase the proportion of useful power. The switching power supply

is a capacitive input circuit, and the phase difference between the current and voltage will cause the loss of switching power, so the PFC circuit is required to improve the power factor. There are currently two types of PFCs, which are divided into passive PFC and active PFC.

1. Characteristics of active PFC: Compared with the simple and intuitive appearance of passive PFC, active PFC circuits are much more complex. The active PFC circuit is a step-up circuit composed of inductance

coils, filter capacitors, switch tubes, control ICs and other components, which can increase the input voltage of the switching power supply and reduce the power loss of the current. Active PFCs can achieve a high power factor, typically more than 98%, but the cost of switching power supplies is relatively high due to the complexity of the circuit and the large number of components.

An active PFC circuit consists of a high-frequency inductor, a switch, a capacitor, and a control IC. The circuit is characterized by a complex structure, but it has many advantages: power factor up to 0.99, low

loss and high reliability, input voltage can be from 90V to 270V (wide input), etc., because the output DC voltage ripple is small, so the switching power supply using active PFC does not need to use a large capacity filter capacitor.

There are four characteristics of active PFC

1. The design of active PFC is complex and the cost is high.

2. Active PFC has better performance than passive PFC and is more energy-saving.

3. The power factor of active PFC can exceed 99% at most.

4. Active PFC can adapt to higher voltage range.

Second, passive PFC characteristics: first look at the difference between passive PFC and active PFC in design, passive PFC is generally a large inductor, the switching power supply is copper wire wound silicon steel sheet covered with insulating tape, the shape is very similar to the small transformer.

The passive PFC uses the inductance compensation method to reduce the phase difference between the fundamental current and the voltage of the AC input to improve the power factor, and the power factor of the passive PFC can only reach 0.6~0.75, which is generally placed near the high-voltage filter capacitor. The simple structure of the passive PFC solution also forms its biggest feature - low cost. A passive PFC

is usually a large inductor consisting of multiple pieces of silicon steel with copper wires wound around the outside. The power factor is not very high, it can only reach 0.7~0.8, so the efficiency of its switching power supply is relatively low, and the heat generation is relatively large. Passive PFC is not useless, its structure is simple, the stability is good, and it is more suitable for low-end switching power supplies.