Fixed-Frequency K plus D Modulation of Modular Multilevel Resonant DC-DC Converters for Output Voltage Overshoot Optimization

Modular multilevel resonant dc-dc converters (MMRDCs) are emerging as a competitive solution for medium wide input voltage-low voltage conversion, yet they often encounter output voltage overshoot when submodules insert or extract. In this article, a novel K+D control scheme of MMRDCs is proposed, which combines both inserted submodule number K and adjusting width D. Output voltage overshoot could be greatly suppressed and soft switching characteristics could also be improved enormously based on the proposed K+D modulation strategy. The design of magnetic components would benefit from fixed-frequency control. Continuous dc voltage gain is analyzed which could effectively simplify the control loop. Simulations on a 200 kW MMRDC that converts 9-18 kV to 750 V have been presented to verify the effectiveness of theoretical analysis and control strategies. Experimental results from a prototype are also provided to demonstrate its feasibility.