If the ballast consumes more than 25 W in most countries it is mandatory that the power factor of the current drawn from the mains is > 0.9. This power factor is an indication of how closely the current drawn from the mains resembles a sine wave. A perfect sine wave gives a PF of 1. A ballast running on rectified mains will have a PF of approximately 0.6. A PFC works by shorting an inductor to ground and then disconnecting it at its peak as shown in the graph by the red curve (1). The current (3) in the inductor will keep on flowing through the diode D1 into the capacitor C1. The resulting average current is shown as the blue curve (2).  A power factor of close to one can be achieved by applying the right pattern on the PFC FET. The operating frequency of a PFC is from 25 to 120 kHz.