Switching Power Supply Circuit with Reduced Total Harmonic Distortion

摘要

A switching power supply circuit has a control circuit for current-mode on-off control of a primary switch connected to an inductor in a voltage boost topology for operation in boundary-conduction mode. The time at which the primary switch is opened is determined by magnitude of current flowing through the primary switch together with the instantaneous voltage present on an AC input to the power supply circuit. The time at which the primary switch is closed is determined by demagnetization of the inductor. An improvement to the foregoing switching power supply circuit comprises a maximum-on-time enforcement circuit to limit the maximum possible primary switch on-time to a predetermined maximum period of time. The enforcement circuit provides a signal to the control circuit to cause termination of the primary switch on-state if and only if the primary switch has been turned on for more than the predetermined maximum period of time.