The dc transformer (DCX) is a dc-dc converter with a fixed voltage conversion ratio, enabling optimized design under constant voltage gain conditions and delivering superior performance. When employed in the dc-dc stage of a three-phase (3-ϕ) ac-input unfolding-based converter architecture, the DCX not only provides these performance benefits but also facilitates quasi-single-stage ac-ac power conversion. However, conventional topologies encounter difficulties when interfacing the DCX with the time-varying dc voltages generated by an unfolder. This paper examines these challenges and presents a series bridge DCX (SB-DCX) topology specifically designed for operation within unfolding architectures. Analytical models are developed to support accurate analysis and informed design. Hardware results from a 600 V,5 kW SB-DCX prototype experimentally validate the proposed topology, demonstrating a peak efficiency of 98.5%. Overall, the SB DCX enables practical DCX operation with an unfolder, supporting quasi-single-stage 3-ϕ ac-ac conversion for applications such as MV-LV solid-state transformers.
