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A converter intended to be used for the interconnection of battery-based energy storage systems with the cells of a multilevel converter is addressed in this paper. High efficiency at light and medium loads is important in these applications and it can be achieved using soft switching techniques. Two control techniques with fixed and variable switching frequency are proposed and compared. The use of silicon carbide (SiC) mosfets provides a higher attainable switching frequency, which is especially interesting in variable frequency control techniques, allowing the operation at high voltages and high switching frequencies, with high efficiencies over a wide power range. A synchronous boost DC/DC converter rated for 400 to 800 V and 10 kW is designed and developed with SiC mosfets obtaining efficiencies higher than 97% from 100% to 3.5% of full load using a variable switching frequency (up to 200 kHz) control. Significant efficiency improvement is achieved at medium and light loads. [ABSTRACT FROM PUBLISHER]