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High step-up nonisolated multicellular dc-dc converter has been newly proposed for PV microconverters. The multicellular converter consists of the nonisolated step-up cell converters using bidirectional semiconductor switches, and these cell converters are connected in Input Parallel Output Series (IPOS). The voltage transformation ratio of the step-up converter is /(1 − ) in case all the transistors in cell converters are operated at the duty ratio of . The proposed multicellular dc-dc converter also accomplishes high efficiency because of no magnetic coupling such as the high frequency transformer and the coupled inductor. Laboratory prototype has been fabricated to show the feasibility of the proposed converter. Design consideration for the 20 V-40 V to 384 V, 240 W nonisolated multicellular dc-dc converter has been also conducted, and the potential to achieve the efficiency of 98% has been shown. The proposed multicellular converter contributes to realizing the environmentally aware data centers for future low carbon society.

This paper proposes a new capacitively isolated multicell dc-dc transformer (dcx). The proposed dcx consists of dc-dc cell converters with pairs of isolation capacitors that substitute for high frequency transformers. The capacitively isolated cell converter achieves higher efficiency compared with the conventional inductively isolated dc-dc converter because of no transformers. The multicell approach also accomplishes the various voltage transformation ratios based on the input-series-and-output-parallel and input-parallel-and-output-series connections of the cell converters, overcoming the drawback of the capacively isolated converter with the fixed voltage transformation ratio. The analysis of the isolation voltage-sharing across the capacitors was conducted to show the feasibility of the proposed multicell dcx. The prototype of the 96 V-24 V, 200 W dcx using two 48 V-24 V, 100 W capacitively isolated dc-dc cell converters was also fabricated to confirm the validity of the analysis. The proposed approach contributes to the achievement of highly efficient and scalable power converters to realize future low-carbon societies.

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An active gate controlled semiconductor protection switch using SiC-MOSFET is proposed to achieve the fault tolerant operation of ISOP (Input Series and Output Parallel) connected multicell dc-dc converter. The SiC-MOSFET with high temperature capability simplifies the configuration of the protection circuit, and its on-resistance control by the active gate controller realizes the smooth protection without the voltage and the current surges. The first laboratory prototype of the protection switch is fabricated by using a SiC-MOSFET with a high frequency buck chopper for the active gate controller. The effectiveness of the proposed protection switch is verified, taking the impact of the volume reduction into account.