This study presents a two-switch continuous conduction mode pulse width modulation flyback converter that employs an LC snubber circuit. The snubber circuit is used to achieve zero voltage switching (ZVS) operation for the main switches during the turn-off transition and soft switching for power diodes. With the proposed LC snubber, the magnetic energy in the transformer leakage inductance can be fully recycled and transferred to the input side. The resonant circuit consists of a resonant inductor, a resonant capacitor and two diodes. The operating principles, theoretical analysis and the design methodology of ZVS two-switch continuous conduction mode (CCM) flyback converter are presented. A 200 W (50 V/4 A) laboratory prototype of the proposed converter, operating at a switching frequency of 300 kHz is built to verify the theoretical analysis. At full load the efficiency is 94%. 1 Introduction Low element count, isolated, design simplicity and simple control circuit are advantages of the flyback converter. However the main drawbacks of the single switch flyback converter are: † The high voltage spike on switch at turn-off state due to resonance of the transformer leakage inductance and the switch output capacitance that causes losses and electromagnetic interference (EMI) noise. † The high voltage stress on switch at turn off, that is the sum of input voltage, reflected output voltage and voltage spike that cause selecting the high voltage switch, increases conduction losses. † The switch is operated at hard switching. † The rectifier diode is operated at hard switching and the reverse recovery loss is high.

In this study, an improved two-switch zero-current zero-voltage switching pulse-width modulation (ZCZVS-PWM) forward converter, which employs a simple resonant lossless snubber circuit, is introduced. A simple resonant snubber circuit consists of a capacitor, an inductor and two diodes. In proposed converter, switch Q 1 operates under exactly zero-current switching at turn-on, and exactly zero-voltage switching (ZVS) at turn-off, and switch Q 2 operates under exactly ZCZVS at turn-on, and ZVS at turn-off, and the all-passive semiconductor devices operate under soft-switching at turn-on and turn-off. The proposed converter has no current and voltage spikes in the switches in comparison with the hard-switching forward converter counterpart and is suitable for high switching frequency and high-power operation. The proposed converter is analysed and various operating modes of the improved two-switch ZCZVS-PWM forward converter are discussed. Analysis and design considerations are presented and the prototype experimental results of a 160 W (32 V/5 A) proposed converter operating at 300 kHz switching frequency, confirm the validity of theoretical analysis.

A regenerative-passive snubber (RPS) for pulse-width modulation (PWM) two-switch forward converter (TSFC) is presented. The RPS circuit is recycled the magnetic energy stored in the transformer leakage inductor and is achieved zero voltage switching turn off of power switches. Moreover, the RPS provides the resonant path formed by snubber capacitors and the snubber inductor to achieve zero current switching (ZCS) turn on of switches. In addition, the output diodes turn on/off under ZCS. The proposed RPS-PWM TSFC is analysed and experimental result for a 160 W, 300 kH prototype at the output voltage of 32 V are obtained to verify the theoretical analysis.

A dual-passive snubber (DPS) for dual-switch forward converter is presented. Snubber networks provide zero-current turn-on and zerovoltage turn-off conditions for dual switches. DPS achieves soft switching conditions for power diodes at secondary side of transformer. The detailed circuit operation of converter with proposed passive snubber, theoretical analysis and design example are presented. The measured result taken from a laboratory prototype rated at 160 W (32 V/5 A), input voltage of 150 V-DC and switching frequency of 300 kHz. The peak efficiency is 93%.