Description

To solve some problems caused by the AC control method used for the existing modular multilevel DC-DC converter (MMDC), this paper has proposed a new MMDC topology with a wide range of output voltage by means of DC control. By reconstructing the submodule structure, the converter has a power branch and an auxiliary balance branch, which are used to transmit DC power and balance the capacitor voltage respectively. Both DC analysis and control method are adopted to establish a mathematical model so as to deduce the mathematical relationships between the key electrical parameters. Moreover, what have been done include analyzing the self-balancing principle of the capacitor voltage and the mechanism of the auxiliary balance branch, calculating the related parameters of the capacitor voltage fluctuation, and presenting a DC closed-loop control strategy based on a small signal model. The analyses of the steady and dynamic states in combination with simulation and experiment show that the proposed converter can stably operate in the mode of DC control, with a large adjustable range of output voltage, a small fluctuation in voltage of the capacitor, and the ability of the inductor to suppress the spike of the auxiliary balance current effectively so as to reduce the impact on the device.