A new fuel cell voltage control unit (FCVCU) has been developed for a new fuel cell vehicle (FCV). In order to simultaneously reduce the electric powertrain size and increase the driving motor power, the FCVCU is needed to boost the voltage supplied from the fuel cell (FC) stack to the driving motor. The FCVCU circuit configuration has four single-phase chopper circuits arranged in parallel to form a 4-phase interleaved circuit. The intelligent power module (IPM) is a full SiC IPM, the first known use to date in a mass production vehicle, and efficiency has been enhanced by making use of the effects of the increased frequency to reduce both the size of the unit and the loss from passive parts. In addition, a coupled inductor was used to reduce the inductor size. As a result, the inductor volume per unit power was reduced approximately 30% compared to the previous VCU inductor. The control determines the number of operating phases that reduces the loss according to the input current of the FCVCU and the boost rate, and drive phase switching control is performed to switch the number of operating phases according to the input current to help increase the VCU efficiency. The establishment of these technologies achieved a size reduction of approximately 40% in terms of FCVCU volume, compared to the conventional technology, and contributed greatly to making it feasible to mount the powertrain under the front hood.