A new fuel cell voltage control unit (FCVCU) was developed for a new fuel cell vehicle (FCV). In order to simultaneously reduce the electric powertrain size and increase the driving motor power, an FCVCU is needed to boost the voltage supplied from the fuel cell stack (FCSTK) 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 and loss of 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 to approximately 30% compared to the conventional VCU inductor. The heat generated by the smoothing capacitor increases together with the current. To reduce the temperature rise of the capacitor element, an indirect water-cooling structure is used that radiates the heat of the capacitor and the bus bar terminal block built into the capacitor to the FCVCU case via heat spreading sheets. 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 mounting the powertrain inside the front hood.