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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. [ABSTRACT FROM AUTHOR]

High-power-density bidirectional dc–dc converters consisting of buck-and-boost converters are widely used in hybrid electric vehicles due to their smaller size and low cost. With a conventional constant-value inductor, the light-load efficiency of these converters is greatly reduced because of the large constant peak-to-peak current swing and relatively constant conduction losses that result in the entire load range. This paper proposes a new type of nonlinear inductor using magnetorheological (MR) fluid to improve the intermediate- and light-load efficiencies of these converters. The use of the proposed inductor does not require any additional circuit or complex control, and yet, it has benefits of decreasing conduction losses and improving efficiency due to its increasing inductance when the load is changed from heavy to light. A 3.7-kW prototype converter is built, and the efficiency from heavy to light load is measured separately using the proposed MR fluid-gap inductor and conventional air-gap inductor, which validates the advantages of the proposed MR fluid-gap inductor. [ABSTRACT FROM PUBLISHER]