Fixed-Frequency K+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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$K$</tex-math></inline-formula> and adjusting width <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$D$</tex-math></inline-formula>. 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.