The Modular Multilevel Converter for High Step-Up Ratio DC-DC Conversion

High step-up ratio DC-DC converters with megawatt ratings are of interest in wind turbine interfaces and high-voltage direct-current systems. The modular multilevel DC-DC converter based on the standard boost converter topology but with the normal single switches replaced by a number of capacitor-clamped sub-modules. The converter is operated in resonant mode with resonance between sub-module capacitors and the arm inductor. A phase-shifted switching arrangement is applied such that there is a constant number, i.e., N, of sub-modules supporting the high voltage at a time. In this operationmode, the step-up ratio is dependent on the number of sub-modules and the inductor charging ratio. The converter exhibits scalability without using a transformer and is capable of bidirectional power flow. The output is simulated using MATLAB R2014a. The experimental verification of the concept using a lab-scale prototype is provided.