The existence of, and rapid progress in, high-speed, high-current capacity semiconductor switches gives the option of active power factor correction and the ability to move the power factor closer to 1. Because of this, it is the strategy that is most widely applied in present-day designs. Active power factor correction uses a switching pre-regulator stage, placed in the input current path of the supply. That regulator is designed not only to maintain a constant DC voltage to feed the main converter stage of the power supply, but also to draw current from the input in-phase with the incoming AC voltage waveform. The downside of active PFC is that an additional switching stage in the supply does impose some extra losses, and some extra cost. However, there are compensating savings in the form of smaller passive filtering components, and in the supply's main converter. Also, the added pre-regulator introduces a potential source of electromagnetic interference that must be taken into account in the final design.