The Current Shaping Modular Multilevel DC/DC Converter with Current Doubling

In this paper, a modular multilevel dc/dc converter is proposed for high step-down DC applications. The converter consists of a single string of cascaded voltage source submodules (VSMs) and a current source submodule (CSM) of current doubler type. The converter is based on a recently proposed energy transfer mechanism whereby the CSM of the converter directs its current, through control action, to either the positive or negative terminal of the DC grid. Therefore, the CSM provides "shaping" of the current of the string. Due to the current doubling action by the CSM, the string current has an amplitude equal to approximately 1/2 the DC load current. This topology therefore offers improved current utilization of the VSMs as well as reduced VSM cell capacitances compared to that of the previously proposed topology based on this energy transfer mechanism. In this paper, the structure and operating principles of the converter are presented along with a steady-state circuit analysis. Simulation results are provided to validate the operation of the topology.