The Current-Shaping Modular Multi-Level DC/DC Converter

Solid-state transformers (SSTs) enable the interconnection of ac and/or dc networks with medium-frequency galvanic isolation. In comparison to the conventional ac transformer, SSTs can be a relatively expensive solution for transforming voltages, requiring two sequential power conversion stages and a high-voltage transformer. Addressing drawbacks of the SST, in this paper a new modular multi-level dc/dc converter topology is presented which enables power exchange between dc networks at high conversion ratios. The topology consists of a cascade connection of current source and voltage source sub-modules (CSM and VSMs) in a single string. The power inductor of the converter is separated from the high-voltage string of VSM cells by the CSM cell. This enables the CSM cell to shape a string current that is of nearly square-wave shape at medium switching frequencies. The shape of the string current enables simultaneous balancing of the VSMs and near constant power delivery to the lower-voltage dc network. This paper presents the structure and main operating principles of the topology. Supporting simulation and experimental results are also provided.