A Quasi-Square-Wave Modular Multilevel Resonant DC/DC Converter With ZCS and Current-Shaping Capacity for High Step-Ratio Application

High step-ratio dc/dc converter is one of the key equipments for dc grids to satisfy the requirement of interconnecting low voltage dc and medium voltage dc buses. This article proposes a high step-ratio modular multilevel resonant dc/dc converter, where two full bridges are connected in parallel at LV side, and two submodules (SMs) strings are connected in series at MV side. Due to the series connection of two SM strings, the number of SMs and the size of the passive filter at MV terminal can be significantly reduced. Moreover, with the quasi-square-wave (QSW) modulation, the proposed converter can achieve flexible current-shaping capability. On this basis, through an internal phase-shifted angle of QSW modulation, the output voltage can be regulated with a fixed switching frequency, simplifying the design procedure of magnetic components. And zero current switching turn- off of full-bridge switches at LV side can be realized in forward and backward mode. In addition, based on the analysis of the operation principle with the time-domain method, the basic control strategy and parameter design procedure are introduced in detail. Finally, the feasibility of the proposed converter is validated by a 100 V/1 kV/4 kW prototype.