Modular Multilevel Resonant DC Transformer With Inherent Balancing Capability

Abstract

As the demand for dc/dc converters in medium voltage applications is increasing in recent years, more and more attention has been paid to modular multilevel technology to reduce device stress. Derived from modular multilevel converter and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> converter, this article proposes a modular multilevel resonant dc transformer. Compared with the multistack structures, the proposed dc transformer has a simpler single-stack structure, which reduces the number and cost of sub-modules (SMs). Moreover, through the co-design of the boost inductor and the transformer, the proposed scheme can realize zero voltage switching of the primary switches and zero current switching of the secondary rectifier. With circulant modulation, not only a flexible step-down ratio is obtained, but also the SM capacitor voltages can be self-balanced without adding the control complexity. In addition, a high operating frequency is synthesized through a low switching frequency of the SM, thereby reducing the size of passive components and switching losses. The operating principle and theoretical analysis of the dc transformer are presented. A downscaled hardware prototype is built, and experimental results validate the correctness of the theoretical analysis and the feasibility of the converter.