Study Of Quasi-resonant Soft-switched Two-switch Flyback Converter

The conventional flyback converter is widely used in various auxiliary power supply and low power supply because of its simple structure,electrical isolation and low cost.Nevertheless,the voltage stress of switches is very high.At the same time,the switches of conventional flyback converter are operated at hard switching,with large switching loss and Electromagnetic Interference(EMI).In contrast,Two-switch flyback converter overcomes the problem of high voltage stress,and the voltage stress on the switches is confined to input voltage and the leakage inductance energy can feed back to input voltage.However,the switches are still operated at hard switching,which limits the performance of the converter.The resonant network is introduced into two-switch flyback converter,and a quasi-resonant soft-switched two-switch flyback converter is proposed in this paper.The proposed converter not merely retains the advantages of the two-switch flyback converter that the voltage stress of switches are clamped at the input voltage,but also achieves zero-voltage-switching(ZVS)of all switches and zero-current-switching(ZCS)of rectifier diode due to the resonance between the resonance inductance and the blocking capacitor,which can reduce the power loss.According to the operation principle of switches and rectifier diode,the quasi-resonant soft-switched two-switch flyback converter can be divided into the ZVS mode and ZVS-ZCS mode.Firstly,the operating principle and characteristics are studied,and the voltage stress and current stress of switches and requirement of the ZVS of the proposed converter working in ZVS mode are analyzed in detail.The simulation and experimental results are presented to verify the theoretical analysis of the proposed converter.Compared with the ZVS mode,the ZVS-ZCS mode can not only realizes ZVS of all switches,but also achieves ZCS of rectifier diode.Besides,it further improves the efficiency of the converter due to less resonance loss.According to the difference of resonant current,the ZVS-ZCS mode can be divided into the ZVS-ZCS standard mode and ZVS-ZCS low loss mode.The operating principle and characteristics are studied in detail,and the design principles of key parameters are provided.Finally,a prototype which operates on the ZVS-ZCS standard mode in heavy load and operates on the ZVS-ZCS low loss mode in light load is designed.The experimental results show that the proposed converter can achieve high efficiency at both the heavy load and light load.A large part of the loss in the quasi-resonant soft-switched two-switch flyback converter is from the secondary rectifier diode,when the output voltage is low.In order to further improve the efficiency,the synchronous rectifier is introduced to reduce the conduction loss of the rectifier diode.The common synchronous rectifier driving strategies are introduced,and summarize the merits and drawbacks of external-driven,voltage-mode self-driven and current-mode self-driven strategies.Finally,based on the characteristics of the converter,voltage-mode self-driven strategy is selected.The theoretical analysis is verified by experimental results.