In this paper, the modular multilevel dc/dc converters, by integrating the full-bridge converters in series, are provided for high power and high voltage dc-based systems. Each circuit module includes a full-bridge converter and a half-bridge LLC resonant converter to implement the wide range of soft switching and low conduction losses. The ZVS condition of the power switches at the leading leg is implemented by the energy stored on the output inductor. The ZVS condition of power switches at the lagging leg is implemented by the LLC resonant converter. Therefore, all switches can be turned on at ZVS from light load. A passive snubber is adopted on the secondary side of full-bridge converter to reduce the conduction loss during the freewheeling interval especially at high input voltage and full load conditions. The outputs of the full-bridge converter and resonant converter are series connection. Therefore, energy is transferred by both converters at whole switching period. Two circuit modules are connected input-series and output-parallel to effectively limit the voltage rating of switches at Vin/2. Flying capacitor is adopted in the proposed circuit to balance input voltages in each switching cycle without the complexity control scheme. Finally, Experiments with a 1920W prototype are provided to demonstrate the performance of the proposed circuit.
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