High-voltage and high-power dc-dc conversion is key to dc transmission, distribution and generation, which require compact and efficient dc transformers with large step ratios. This paper introduces a dc-dc converter with the input-series-outputparallel (ISOP) arrangement of multiple high step ratio subconverter units. Each sub-converter unit is an isolated modular dc-dc converter with a stack of half-bridge cells chopping the dc down to low voltage level. The transformer provides galvanic isolation and additional step ratio. The converter achieves a large step ratio due to the combination of the series-parallel configuration, the modular cells, and the isolation transformer. The proposed dc-dc converter is analyzed in a 30 kV to 1 kV, 1 MW application to discuss the operation performance, tradeoffs, power efficiency and selection of components. Finally, the converter is validated through a laboratory down-scaled prototype.

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