Adaptive on-time control for power factor correction stage light load efficiency

摘要

Light load efficiency of a power factor correction circuit is improved by adaptive on-time control and providing for selection between a continuous conduction mode and a discontinuous conduction mode wherein the discontinuous conduction mode increases time between switching pulses controlling connection of a cyclically varying voltage to a filter/inductor that delivers a desired DC voltage and thus can greatly reduce the switching frequency at light loads where switching frequency related losses dominate efficiency. The mode for controlling switching is preferably selected for each switching pulse within a half cycle of the cyclically varying input voltage. A multi-phase embodiment allows cancellation of EMI noise at harmonics of the switching frequency and adaptive change of phase angle allows for cancellation of dominant higher order harmonics as switching frequency is reduced.

主权项

1. A method of operating a power factor correction circuit, said method comprising steps of
applying a cyclically varying voltage to an input of said power factor correction circuit, performing switching to control conduction of pulses of current at said cyclically varying input voltage to a filter/inductor at a frequency, which is sufficiently high to supply a predetermined current at a desired output voltage to a load, said switching defining a predetermined on-time and a predetermined off-time in accordance with said predetermined current and said desired output voltage and defining a switching cycle such that current through said filter/inductor is continuous in a continuous conduction mode, reducing said predetermined current supplied to supply a reduced current to said load, determining if said reduced current is less than one half of a current ripple produced by said switching on-time and off-time, and if said reduced current is less than one-half of said current ripple, reducing said frequency of said pulses by extending said off-time during at least a portion of a cycle of said cyclically varying voltage in accordance with said reducing of said predetermined current such that current through said filter/inductor is discontinuous in a discontinuous conduction mode.