A New Resonant Modular Multilevel Step-Down DC-DC Converter with Inherent-Balancing

Modular multilevel converters (MMCs) have become increasingly interesting in dc-dc applications, as there is a growing demand for dc-dc converters in high voltage applications. Power electronics transformers (PETs) are commonly used for high step-down ratio dc-dc power conversion, with high power rating and efficiency achieved. However, this arrangement requires a large number of high isolation voltage transformers and a complicated balancing control scheme. To provide a simple solution with inherent voltage balancing, this paper presents a new resonant MMC topology for dc-dc conversion. The proposed converter achieves high voltage step-down ratio depending on the number of sub-modules. The converter also exhibits simplicity and scalability with no necessary requirement of high voltage isolation transformers. By using phase-shift control, a much higher converter operating frequency is achieved compared to the switching frequency. Resonant conversion is achieved between the series inductor and sub-module capacitors. The operation principle and theoretical analysis are presented in this paper, which have been verified by experimental results based on a bench scale prototype.