To qualify for Class A status, the dead angle of the current waveform must be less than 45 degrees. The rectifier must conduct for at least 315 degrees of each 360 degree input cycle. A 315 degree conduction angle yields a power factor between .85 and .90. Significantly higher than the .65 that is typical of an uncorrected switcher. As the underlying cause of low power factor and high circulating currents created by switch mode power supplies is the discontinuous input-filter charging current, the solution lies in introducing elements to increase the rectifier's conduction angle.
Passive correction is simply the use of an inductor in the input circuit.
We used to call this an inductive input filter. If the inductor is sufficiently large, it stores sufficient energy to maintain the rectifiers in conduction throughout the whole of their half cycle. Today we do the same to reduce the harmonic distortion caused by discontinuous conduction of these same rectifiers. Of course, the inductors used in passive correctors are large since they work at the mains frequency. A variety of topologies can be used including the boost converter and the buck converter.