A 10-kW (53V/200A), forced-air-cooled DC-DC converter has been developed for fuel cell systems. This converter uses new high-voltage bipolar-mode static induction transistors (BSIT), a new driving method, a zero-voltage-switched pulse-width-modulation technique, and a new litz wire with low AC resistance. It weighs only 16.5kg, has a volume of 26,000cm3, operates at 40kHz, and has a power conversion efficiency of about 95%. The power loss of this converter is 20% less than that of conventional natural-air-cooled DC-DC converters, and the power density is 3 times as high.