A bidirectional isolated DC-DC converter as a core circuit of the next-generation medium-voltage power conversion system

This paper describes a bidirectional isolated dc-dc converter considered as a core circuit of 3.3-kV/6.6-kV high-power-density power conversion systems in the next generation. The dc-dc converter is intended to use power switching devices based on silicon carbide (SiC) and/or gallium nitride, which will be available on the market in the near future. A 350-V, 10-kW and 20 kHz dc-dc converter is designed, constructed and tested. It consists of two single-phase full-bridge converters with the latest trench-gate insulated gate bipolar transistors and a 20-kHz transformer with a nano-crystalline soft-magnetic material core and litz wires. The transformer plays an essential role in achieving galvanic isolation between the two full-bridge converters. The overall efficiency from,the dc-input to dc-output terminals is accurately measured to be as high as 97%, excluding gate drive and control circuit losses from the whole loss. Moreover, loss analysis is carried out to estimate effectiveness in using SiC-based power switching devices. Loss analysis clarifies that the use of SiC-based power devices may bring a significant reduction in conducting and switching losses to the dc-dc converter. As a result, the overall efficiency may reach 99% or higher.