Brief Introduction To The Working Principle Of Switching Power Supply PFC

PFC, power factor correction, is mainly used to characterize the utilization efficiency of electric energy by electronic products. The higher the power factor, the higher the utilization efficiency of electric energy.

The switching power supply adopts the traditional bridge rectification and capacitor filter circuit, which will cause serious waveform distortion of the AC input current and inject a large number of high-order harmonics into the power grid. And other electrical equipment cause serious harmonic pollution and interference. As early as the early 1980s, people have attracted attention to the harm caused by the high-order harmonic current generated by such devices. In 1982, the International Electrotechnical Commission formulated the IEC55-2 specification for limiting high-order harmonics (the later revised specification was IEC1000-3-2), which prompted many power electronics technicians to start harmonic filtering and power factor correction ( PFC) technology research. The introduction of PFC circuits in electronic power products can greatly improve the utilization efficiency of electric energy.

There are two types of PFC, one is passive PFC (also called passive PFC), and the other is active PFC (also called active PFC). Passive PFC generally adopts the inductance compensation method to reduce the phase difference between the fundamental wave current and voltage of the AC input to improve the power factor, but the power factor of passive PFC is not very high, and can only reach 0.7~0.8; active PFC consists of 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.

Active PFC circuits often use highly integrated ICs, and PC power supplies using active PFC circuits have at least the following characteristics:

1) The input voltage can be from 90V to 270V;

2) The line power factor is higher than 0.99, and has the advantages of low loss and high reliability;

3) The PFC of the IC can also be used as an auxiliary power supply, so when using an active PFC circuit, a standby transformer is often not needed;

4) The output does not change with the fluctuation of the input voltage, so a highly stable output voltage can be obtained;

5) The DC voltage ripple of the active PFC output is very small, and it is a sine wave of 100Hz/120Hz (2 times the power frequency), so the power supply using the active PFC does not need to use a large-capacity filter capacitor.

The role of PFC circuit in switching power supply

The full name of PFC in English is "Power Factor Correction", which means "power factor correction". Power factor refers to the relationship between effective power and total power consumption (apparent power), that is, effective power divided by total power consumption Quantity (apparent power) ratio. Basically, the power factor can measure the degree to which electricity is effectively utilized. When the power factor value is larger, it represents the higher the power utilization rate. The cost of switching power supply with PFC is also relatively high.
Switching power supply is a capacitor input circuit, the phase difference between its current and voltage will cause the loss of switching power, at this time, PFC circuit is required to improve the power factor. There are two types of PFC at present, passive PFC (also called passive PFC) and active PFC (also called active PFC).

Passive PFC is generally divided into "inductance compensation type" and "ValleyFill Circuit type (ValleyFill Circuit)". passive PFC and non-quiet passive PFC. The power factor of passive PFC can only reach 0.7-0.8, which is generally near the high-voltage filter capacitor.