A New Soft-Switching AC-DC Converter Based on Coupled Inductors for Onboard Charging Applications

This article proposes a soft-switching full-bridge ac-dc converter consisting of its hard-switching counterpart and simple active auxiliary branch. The auxiliary branch is composed of an inductor and two power switches. The proposed converter features zero voltage switching (ZVS) turn-on for main switches and zero current switching (ZCS) condition for auxiliary switches. The ZVS condition is created by turning on auxiliary switches ahead of the main switch turn-off signal to reversely excite the auxiliary inductor. The two mutually coupled inductors connected in series in opposing magnetic field manner are designed as the main filter inductor and to provide positive voltage for the auxiliary inductor to degauss it as well. Meanwhile, the auxiliary inductor contributes ZCS turn-on and turn-off to the auxiliary switches because it slows down the change rate of the current through the switches. Considering the parasitic capacitors of the auxiliary switches inevitably resonate with the auxiliary inductor, a clamping technique is given to alleviate the additional voltage stresses caused by the resonance. Then, the mathematical parameter design is deduced to guide the converter realization. Finally, a laboratory prototype with 1000 W rated power operating at 50 kHz switching frequency is completed and tested to demonstrate the soft-switching features. The efficiency of the proposed converter is up to 97.67% at full load and the maximum efficiency improvement is 5.3% at light load, which further highlights the outstanding performance.