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A Passive Cell Capacitor Voltage Control Method for the Current Shaping Modular Multilevel DC/DC Converter

The Current Shaping Modular Multilevel DC/DC Converter (CS-MMC) is a recently proposed converter topology for high step-down dc/dc applications. The converter structure consists of a mixture of current and voltage source submodules (CSM and VSMs) in a single-string arrangement. This topology leverages a novel energy transfer mechanism in which the CSM is employed to shape the string current which is of nearly square-wave waveshape and medium-frequency. In this paper, a novel control method is proposed for the CS-MMC in which the VSM capacitor voltages are passively controlled through the introduction of a freewheeling operating mode for the CSM. This mode ensures that the total energy of the VSM cells is balanced over a switching period. Compared to previous work, the CS-MMC with the control method of this paper features significantly lower number switching operations, partial soft-switching, and higher voltage utilization of the semiconductor devices. In this paper, the operating principles for this control method are presented along with a derivation of key system relations. Finally, simulation and experimental results from a laboratory scale converter system are provided.