Conventional offline switch-mode power converters create nonsinusoidal input currents with a high harmonic content. This causes stress for the power wiring, circuit breakers, and electric utility. In addition, these harmonics can affect other electronic equipment connected to the same power line. Active power factor correction (PFC) that reshapes the input current before its application to the switch-mode supply solves this problem.

PFC has become important since the European Union established the EN61000-3-2 Standard and Amendment A14 for electronic equipment. This standard limits allowable ac line current harmonics. Limits depend on the input power, type of product, and specific harmonic. See the table for a list of the original equipment classifications and the Amendment A14 classifications. Class D limits are of major interest because they cover PCs, computer monitors, and television receivers. Other devices must only meet Class A limits.