PWM TECHNIQUE UTILIZATION IN THE QUASI-RESONANT CONVERTER

The quasiresonant forward convertor topology has a high power conversi on efficiency due to the favourable switching conditions for the trans istors and output rectifier diodes. In addition, this topology can emp loy output transformer parasitics in the resonant tank which makes it attractive for high voltage switched mode power supply applications. U nfortunately, a very broad range of operating frequency is required to control the output power, and therefore the highly efficient pulse wi dth 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. T he resulting, PWM-controlled quasi-resonant convertor topology combine s the advantages of both resonant power conversion and pulse width mod ulation techniques. The paper reports the analysis and PWM control cha racteristics of this hybrid convertor topology. An optimum operating m ode of the quasiresonant convertor is identified and expressions for t he optimum convertor parameters are derived. A 2.5 kW, 4 kV, 50 kHz co nvertor prototype for microwave heating was built to verify the analyt ical results.