The isolated DC/DC stage in solid state transformer (SST) is a very critical stage in the whole system. It converts the high DC voltage to low DC voltage with the galvanic isolation after the high voltage AC/DC rectifier and before the low voltage DC/AC inverter. In previously developed solid state transformers, dual active bridge (DAB) and dual half bridge (DHB) are the most common topology for this part. In this paper, a bidirectional LLC resonant converter is presented to replace the traditional DAB or DHB in SST application. Compared with DAB, the bidirectional LLC converter can achieve higher efficiency with wider range of zero voltage switching (ZVS), zero current switching (ZCS) and simpler control strategy, which makes it highly attractive for solid state transformer applications. The converter operation modes are presented and analyzed in this paper. For achieving higher efficiency, a methodology of hardware design optimization is introduced to minimize the power losses. Analysis, simulation and experimental results based on a scaled down prototype are shown and discussed.

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