Study On An Improved Full Bridge Soft-Switching Converter

Recently, phase-shift control full-bridge ZVS-PWM converter attracts more attention. This converter employs phase-shift control to provide ZVS conditions to the primary switches utilizing circuit parasitic such as the switch junction capacitance and the transformer leakage inductance. Many advantages such as simple circuit and ZVS without additional resonant components make this converter widely adopted for many applications where MOSFETs are used as switching devices. However, the ZVS range is limited unless the leakage inductance is large. The large leakage inductance results in effective duty cycle loss, which increases the conduction loss caused by the freewheeling current. Moreover, it is not suitable for high power application using IGBTs, since IGBTs have high turn-off switching losses due to the current tail characteristics, even though the turn-off switching losses can be reduced by adding an external capacitor to the IGBTs. To solve these problems, FB-ZVZCS-PWM converter is proposed. In the ZVZCS full bridge converter, one leg operates in ZVS mode while the other leg operates in ZCS mode.This dissertation provides the analysis and comparison of these converters; it also refers to the improvement of these converters. The ZCS condition can be obtained by introducing an auxiliary circuit into the primary or secondary side. However, these converters introduce other questions too. The auxiliary circuit which consists of loss component or active switch costs the overall efficiency. Converter with tapped inductor or transformer makes the converter somewhat complex and the core loss is inevitable. Converter with snubber circuit seems a good choice and brings a duty cycle boost effect; however, it increases the voltage stress of the secondary rectifier diodes.Based on the analysis of principles of ZVS and ZVZCS converter, a new topology with two diodes and a capacitor in the auxiliary circuit is proposed to overcome these questions. The operation principle and equivalent circuit structures in different modes are analyzed. The ZVS range of leading-leg and ZCS range of lagging-leg and the operation modes of secondary rectifier are analyzed and compared with other converters. The results show that the converter achieves soft switching easier than conventional converters with fewer components in the auxiliary circuit and without increasing the secondary voltage stress.The small signal model of the proposed ZVZCS converter is derived based on the small signal model of buck converter incorporating the effects introduced by the auxiliary circuit which consists of a transformer leakage inductance and auxiliary capacitors to achieve ZVZCS. The analysis results and design consideration are verified by a simulation circuit.