A Power Factor Corrector (PFC), typically used as the first stage of switched mode power supplies, particularly suited for Universal Mains inputs, is disclosed, along with methods for controlling a switched mode power supply having power factor correction. In order to increase efficiency, particularly under low load conditions, without undue degradation of the Power Factor, the switching of the PFC circuit is confined to one or more operating windows within each half-cycle. In example embodiments, the operating window comprises a small time window centered around the peak of the mains voltage. The higher the power level, the wider the switching window.