An isolated bi-directional soft-switched DC-AC converter using wide-band-gap devices with novel carrier-based unipolar modulation technique under synchronous rectification

A novel carrier-based unipolar-SPWM-oriented modulation technique with synchronous rectification for isolated bi-directional soft-switched high-frequency-AC (HFAC) link DC-AC converter using SiC MOSFET is presented in this paper. The DC-AC converter is composed of a full-bridge (FB) inverter cascaded with a cycloconverter through a high-frequency transformer. In order to increase the efficiency and power density, we proposed to utilize SiC MOSFETs for the converter. A carrier-based unipolar-SPWM-oriented modulation technique is proposed to realize zero-voltage-switching (ZVS) for FB and zero-current or zero-voltage-switching (ZVS/ZCS) for cycloconverter in all load range, and to suppress the voltage spikes introduced by the transformer leakage inductance as well. Synchronous rectification is implemented to further increase the converter efficiency. With the novel modulation technique, 1/4 of the AC switches in the cycloconverter are always on which eliminates 1/4 of the switching loss. Simulation model and a 400 VDC to 240 VAC, 1.2 kW prototype have been developed to validate the effectiveness and performance of the proposed unipolar soft-switching modulation technique and SiC converter.