Understanding Power Factor in AC Circuits

Power factor (PF) is the ratio of real power (W) to apparent power (VA). A PF of 1.0 means all the power drawn from the supply does useful work. A PF below 1.0 means the supply is delivering reactive power that returns to the source, creating additional current load.

Switching power supplies without PFC (power factor correction): a typical SMPS without PFC draws current in short pulses at the peaks of the AC cycle, resulting in PF of 0.5-0.7. This means the supply draws 1.5-2x more apparent current than the real power would suggest, loading the wiring and transformer.

PFC correction: active PFC (a boost converter stage before the main conversion stage) shapes the input current to be sinusoidal, achieving PF above 0.99. The IEC 61000-3-2 standard requires active PFC for electronic equipment above 75W in the EU. For a DIY bench supply or single-unit project, PFC is optional. For any product sold in the EU above 75W, PFC is mandatory.