Leading edge modulator for post regulation of multiple output voltage power supplies

Abstract

A multiple output voltage power supply includes a transformer having a primary winding and a plurality of secondary windings. The primary winding is adapted to have an input voltage coupled thereto. A primary side switch is coupled to the primary winding and is adapted to control power applied to the transformer in a power cycle responsive to a peak current mode control signal. A plurality of output circuits are coupled to respective ones of the plurality of secondary windings and provide respective output voltages. The plurality of output circuits each further includes a forward rectifier adapted to rectify power during a positive portion of the power cycle and a free-wheeling rectifier adapted to rectify power during a negative portion of the power cycle. One of the plurality of output circuits further includes a bypass switch adapted to interrupt operation of the respective forward rectifier. A secondary side post regulation circuit controls operation of the bypass switch. The post regulation circuit further includes a signal generator adapted to receive a synchronization signal corresponding to the power cycle. The signal generator generates a sawtooth waveform that rises during a positive portion of the synchronization signal and falls during a zero portion of the synchronization signal. A Schmidt trigger coupled to the signal generator has an enabled state when a leading edge of the sawtooth waveform reaches a threshold level and a disabled state when the synchronization signal falls to zero. A driver coupled to the Schmidt trigger generates a drive signal used to control the bypass switch in accordance with the enabled and disabled states of the Schmidt trigger.