Quasi resonant AC/DC forward converter with sinusoidal input current

Offline switching power supplies usually employ a rectifier bridge with a simple capacitor input filter to draw power from the AC line. The filter capacitor charges to nearly the peak AC line voltage, supporting an unregulated DC bus powering the switching power converter. Since the filter capacitor is charged only on the top of the peak line voltage, the capacitor has to be very large depending on hold up requirements. The line current waveform is (with a capacitor input filter) a nonsinusoidal, narrow pulse with a very high peak current. The input power factor is only 0.5-0.7 and the high harmonics of the current causes line noise. The RMS value of the input current may be twice the equivalent RMS sinusoidal waveform. The purpose of this paper is to present a cost effective AC/DC power converter where both functions are performed with a single switching transistor, transformer and chokes with output power up to 100 W.<>