A Highly Reliable Single-Phase High-Frequency Isolated Double Step-Down AC-AC Converter with Both Noninverting and Inverting Operations

In this paper, the switching cell concept is extended to isolated ac-Ac converters, and a highly reliable double step-down ac-Ac converter is proposed with high-frequency transformer (HFT) isolation. By using the switching cell structure and coupled inductors, the proposed converter has no commutation problem as it is immune from both short-circuit and open-circuit problems, even when all the switches are turned on or turned off, simultaneously. Therefore, it does not require pulse width modulation dead times along with bulky and lossy RC snubbers or voltage-sensing circuitry to implement soft-commutation strategies, resulting in high reliability and high quality output waveforms. The HFT in the proposed converter provides electrical isolation and safety which is required in applications such as dynamic voltage restorer (DVR), solid-state transformer (SST), etc., without the need for external bulky line frequency transformer. Moreover, all the passive components experience twice the switching frequency; therefore, their size can be reduced. The proposed converter is very suitable for application as DVR, to compensate both voltage sags and swells, owing to its ability to provide both inverting and noninverting outputs. A detailed theoretical analysis and operation of the proposed ac-Ac converter are provided, and its applications as DVR and SST are also discussed. Experimental results with scaled-down prototype are also provided to verify its performance.