Active Power Factor Correction Technology for Switching Power Converters

　　Since there are rectifier components and filter capacitors at the input end, many electronic devices powered by rectifier power supply have a power factor of only 0.6 to 0.65 at the input end of the mains. Active power factor correction technology (APFC for short) can increase the input power factor to 0.95 to 0.99, making the THD of the input current less than 10%, which can not only control the harmonic "pollution" of the mains power grid, but also improve the overall efficiency of the switching power supply  . Single-phase APFC was developed earlier at home and abroad, and the technology is relatively mature; three-phase APFC has more types, and further research and development is needed.

　　A general high power factor AC/DC switching power supply consists of two main circuits. Between the rectifier and the DC/DC converter , a pre-PFC converter is added to increase the power factor of the AC input end to close to 1, while making the output DC voltage adjustable. A two-stage high power factor AC/DC switching power supply requires at least two main switches and two sets of control drive circuits. For low-power switching power supplies, a two-stage main circuit structure is used, which has low overall efficiency and high cost.

　　If the input power factor requirement is not particularly high, the PFC converter and the subsequent DC/DC converter are combined into a main circuit to form a single-stage high power factor AC/DC switching power supply. Only one main switch tube can correct the power factor to above 0.8 and make the output DC voltage adjustable. The converter with this main circuit structure is called a single-tube single-stage (Sing1e SwitchSing1e Stage, S4) converter, that is, S4PFC converter. For example, an isolated S4PFC AC/DC converter, the front PFC converter uses a Boost converter running in DCM, and the main circuit of the rear voltage regulator is a flyback converter, which runs in CCM or DCM; the two-stage circuits share one main switch tube. Of course, if active clamping or other soft switching technology is added, an auxiliary switch tube is also required, which is called a single-stage (Sing1e Stage, S2) isolated power factor correction soft switching power supply.

　　The Hong Kong Polytechnic University reported the experimental results of S2 isolated power factor correction soft switching power supply: 80V/W·150 kHz, efficiency 86.5%, power factor 0.98, THD 13%, input/output voltages 110V AC and 280V DC respectively.