Design and Testing of a Soft-Switching Solid-State Transformer Module

The soft-switching technique has the potential to be used in solid-state transformers (SSTs) to increase the efficiency and power density. In conventional two-stage SST modules, the first-stage ac-dc or dc-dc converter usually operates as hard switching, while soft switching is typically achieved only in the second-stage dc-dc converter. This article proposes a two-stage fully soft-switching SST module featuring zero-voltage switching (ZVS) of both the first full-bridge (FB) stage and the second isolated dc-dc stage across the entire load range. The previously reported soft-switching SST module utilizing ZVS modulation with variable switching frequency for the first-stage converter cannot be used in common input-series output-parallel (ISOP) SST architecture. The proposed soft-switching FB stage maintains a fixed switching frequency, facilitating the cascading of the SST module in an ISOP SST system. To further improve the efficiency and power density, an optimization design of the magnetic components including the soft-switching resonant inductor and isolation transformer are conducted. Finally, a high-modularity SST module is constructed and experimentally validated. The proposed two-stage SST module obtains a full-load efficiency of 98.1% and a peak efficiency of 98.4% for bidirectional power flow.