PFC power factor correction circuit - Home Appliances

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Hey everyone, newbies, feel free to ask questions and experienced folks, don’t hesitate to share your knowledge!

PFC stands for Power Factor Correction, which is a technique used to improve the efficiency of electrical systems. According to regulations, LCD TVs with a total power exceeding 75W must include a PFC circuit. The main goal is to optimize how much power is drawn from the grid and to reduce interference in switching power supplies.

A PFC circuit is also known as a boost post-regulation circuit. It's essentially a parallel switching power supply that's placed between the rectifier and the filter capacitor. This helps stabilize the output voltage and make it more efficient.


When VT1 is turned on, the voltage from the bridge rectifier BD1 is grounded through the energy storage inductor L1. During this time, electrical energy is stored in the inductor in the form of magnetic energy, and the induced electromotive force has a positive polarity on the left and negative on the right.

When VT1 turns off, the polarity of the induced electromotive force in the inductor L1 reverses, converting the magnetic energy back into electrical energy. At this point, the left side becomes positive and the right side becomes negative. This voltage is added to the output voltage from the bridge rectifier, creating a voltage of approximately +380V that charges the storage capacitor C1.

When VT1 turns on again, the process repeats. However, at this stage, the voltage stored in capacitor C1 is higher than the voltage at the anode of diode D1, so D1 remains off. As a result, the operation of D1 becomes intermittent. Since both the bridge rectifier and the large inductor L1 are supplying power to the load simultaneously, the output voltage can reach as high as +375V to +400V. When the input voltage is 220V, the typical output of the PFC circuit is around +380V, which is why it's commonly referred to as the +380V rail.