A Zero-Current-Switching High Conversion Ratio Modular Multilevel DC–DC Converter

As the demand of dc-dc converter for medium and high voltage application is increasing in recent years, more and more attention has been paid to modular multilevel converters (MMC). This paper proposes a novel transformerless MMC to achieve high-conversion-ratio DC-DC power conversion purpose. This converter can be used in Medium Voltage Direct Current (MVDC) system, as a high step-up ratio solid-state transformer (SST). The proposed converter possesses scalability, simplicity as well as inherent voltage balancing capability. No complex control algorithm is needed to balance the voltages in each submodule. By using the MMC concept, N low voltage submodules are connected in series to generate a high voltage output. The conversion ratio can be regulated by the phase-shifted control. Two kinds of submodules are outlined to build the MMC. By utilizing stray inductance distributed on wire, the bulky inductors with high power loss can be removed from the system. Resonant operation between wire stray inductance and submodule capacitors assists the converter to work under ZCS mode at high efficiency. A design example with 11-time step-up ratio is analyzed and simulated. And a 200W scaled down lab prototype with 7-time voltage step-up capability has been built to verify the proposed concept.