A simplified three-phase zero-current-transition inverter with three auxiliary switches

Most existing three-phase soft-switching inverters with fewer than six auxiliary switches have fundamental drawbacks in performance. There exist a few soft-switching inverters with six auxiliary switches that can potentially achieve desirable performance, but are penalized with the high cost and large size associated with the auxiliary switches. This paper proposes a zero-current-transition (ZCT) inverter topology that requires only three auxiliary switches. Each phase of the proposed circuit employs one auxiliary switch and one LC resonant tank to assist switching transitions. With considerable reduction in device count, cost, and size, the proposed topology realizes zero-current turn-off for all main switches and auxiliary switches, and provides soft commutation for all diodes. Meanwhile, it requires no modification to normal pulsewidth modulated (PWM) algorithms. The operation principles, design and control guidelines, and an analysis using the state-plane technique are presented. Based on the proposed topology, a 50-kW three-phase prototype inverter has been developed for electric vehicle propulsions, and tested to the full power level with a closed-loop induction motor drive system. Experimental results on the 50-kW prototype are provided to verify the proposed concept in high-power AC adjustable speed drive applications.