Abstract: A parallel resonant circuit for use in a series resonant converter. The parallel resonant circuit is located between the clamp diodes and the dominant series resonant capacitors of the converter. The parallel resonant circuit has a predetermined resonant frequency which is set to be above the telephone voice band. The circuit limits the operating frequency of the converter for light loads to be no lower than the parallel resonant frequency.

Abstract: A series resonant converter is disclosed, in which a series resonant circuit consisting of a resonant capacitor and a resonant inductor is connected in series with AC input terminals of a rectifier. Positive current and negative current are alternately caused to flow from a power supply through the series combination of the series resonant circuit and rectifier through on-off control of a plurality of switch elements, thereby obtaining a DC voltage from the rectifier. A parallel resonant circuit is provided in series with the series resonant circuit. The resonant frequency of the parallel resonant circuit is lower than the resonant frequency of the series resonant circuit and higher than the audible frequency range.