Application Of Soft-switching Techniques In PWM Converter

ABSTRACT:Power electronic devices in traditional PWM converters usually work in hard-switching mode, which causes high switching loss and serious EMI problems especially in high power applications. This paper introduces Zero-Current Transition (ZCT) soft switching technique. ZCT is achieved by controlling the resonance between resonant inductors and capacitors when the devices need to turn on or off. Further more, the introduction of auxiliary circuit does not influence on traditional Pulse-Width-Modulation (PWM) strategy. With the application of ZCT techniques, converter can work in a higher switching frequency, which would help to improve the performance of the converter.Firstly, topology and operating principle of ZCT soft switching PWM converters are introduced in this paper. Losses models in hard-switching mode and soft-switching mode are compared. The application of ZCT increases device current stresses, and also introduces extra losses in the converters. As long as the extra losses of the auxiliary circuit are less than the reduced switching losses, ZCT-PWM converter would raise the efficiency. Based on the proposed losses models, this paper introduces a design method of resonance parameters, which would achieve minimize losses of power electronic devices in ZCT-PWM converters.A 500kVA ZCT soft switching converter prototype is built to verify the feasibility of ZCT in high power applications. The design of main circuit, control system and software is introduced in details. Load experiments and efficiency comparison between hard switching and soft switching have been carried out. The experimental results indicate ZCT of power electronic devices is achieved; resonant process is consistent with the theoretical analysis; the relation of converter efficiency, resonant peak current and resonant period is verified. In addition, the effect of ZCT in reducing losses is weaken by diode reverse recovery.