This paper describes the principles and characteristics of a novel bidirectional superposed dual active bridge dc-dc converter with an ac-link transformer, which has a mechanism formed from two bridge-type converters that are linked through superposition in additive polarity in series. Conventionally, for an isolated dual active bridge dc-dc converter, the rated voltage of its switching elements is decided according to the dc power source voltage and load current. Therefore, as the voltage and current specifications of a dc-dc converter become higher, physical size becomes larger, conduction and switching losses increase and power efficiency reduces. To solve this problem, the authors devised the proposed dc-dc converter capable of lowering the rated voltage of switching elements, by means of sharing dc power source voltage and load current between two converters. Further, the capacity of the high frequency transformer becomes small, making the dc-dc converter more efficient and smaller. We designed and constructed a 1kW dc-dc converter prototype, and conducted performance evaluation testing. As a result, a conversion efficiency of 96.6% at a rated output of 1kW was obtained. A detailed analysis of power flow was also carried out, to identify the characteristics of the converter developed.
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