Power factor is given by dividing effective power (W) by apparent power (VA).
(Power factor) = (effective power) / (apparent power)
The first stage of a typical switching power supply is a rectifying circuit that rectifies an AC input voltage with a diode bridge and smoothes the output of the diode bridge with capacitors. The rectifying circuit is nonlinear with respect to AC input, and therefore generates low-frequency harmonic currents in inputs.
These harmonic currents cause not only failures directly to equipment, such as equipment malfunction and abnormal heat generation in equipment parts, but also a social issue that a power factor reduction due to an increase in the total amount of harmonic currents. Thus it leads to an increased loss in electric power plants and transmitting installations.
For this reason, countries and regions around the world have been establishing standards and regulations pertaining to harmonic currents following the international standard (IEC61000-3).
Abiding by these standards and regulations requires that the power factor of equipment be closer to 1 (= 100%). General circuit schemes for improving power factor include an active filter method based on electronic control and a passive filter method using passive components. Circuits based on these schemes are referred to as PFC (Power Factor Correction) circuits.