The difference between active and passive power supplies

PFC English is called "Power Factor Correction". The computer switching power supply is a capacitive input type circuit. Since the maximum value and the maximum voltage value of the AC current do not occur at the same time, the product of the power calculation formula “current x voltage” does not equal the current work in this case. The power, at this time we also need to multiply by a factor, this coefficient is PFC namely "power factor correction" Its value range is between 0 and 1. Basically, the power factor can measure the extent to which electricity is effectively used. When the power factor is larger, it means that the power utilization is higher. There are two kinds of PFC at present, one is passive PFC (also known as passive PFC) and active PFC (also called active PFC).

Active PFC is mainly composed of high-frequency inductors, switches, capacitors and control ICs and other components, can be simply summarized as step-up switching power supply circuit, this circuit is characterized by a complex structure, but the advantages are many: power factor up to 0.99, Low loss and high reliability, the input voltage can be from 90V to 270V (wide input), etc. Since the output DC voltage ripple is very small, the power supply using an active PFC does not require the use of a large-capacity filter capacitor.

Passive PFC is usually a bulky inductor. Its inside is composed of multiple pieces of silicon steel wire wrapped with copper wire. Its principle is to use inductance compensation method to reduce the phase difference between the fundamental current and voltage of AC input. To increase the power factor, the power factor of the passive PFC is not very high and can only reach 0.7 to 0.8. Therefore, the efficiency is relatively low, and the heat output is relatively large. Passive PFC is not only useless, its structure is simple, stability is good, more suitable for low-end power supply.

However, it is also worth noting here that "power factor" does not equal "conversion efficiency." Some businesses now interpret the active 0.99 power factor as 99% power conversion efficiency, which is obviously not correct. Although both describe the concept of power saving, the meaning of the two concepts is different for individuals. High PFC "power factor" is for the country to save money, and high "conversion efficiency" is for the user to save money.