PWM technique utilisation in the quasiresonant convertor

The quasiresonant forward convertor topology has a high power conversion efficiency due to the favourable switching conditions for the transistors and output rectifier diodes. In addition, this topology can employ output transformer parasitics in the resonant tank which makes it attractive for high voltage switched mode power supply applications. Unfortunately, a very broad range of operating frequency is required to control the output power, and therefore the highly efficient pulse width modulation (PWM) control technique in this resonant topology looks very promising. The adverse effects of the associated nonzero current turn-off switching can be reduced by using nondissipative snubbers. The resulting, PWM-controlled quasi-resonant convertor topology combines the advantages of both resonant power conversion and pulse width modulation techniques. The paper reports the analysis and PWM control characteristics of this hybrid convertor topology. An optimum operating mode of the quasiresonant convertor is identified and expressions for the optimum convertor parameters are derived. A 2.5 kW, 4 kV, 50 kHz convertor prototype for microwave heating was built to verify the analytical results