A multi-port DC/DC converter having DC48V and DC12V ports is proposed for next generation HV sub system. The converter integrates an isolated DC/DC converter and a non-isolated DC/DC converter in one circuit. A low voltage side full-bridge and an inductor are shared by two topologies, and the number of switches and magnetic components can be saved. In the present study, a duty modulation method is proposed to reduce losses in light load condition. Moreover, to achieve compatibility of high efficiency and high power density, the switching frequency is increased and 600 V cascode type GaN FET is used for high voltage side switches. A 175 kHz, 1.0 kW prototype is constructed, and power density of 55 W/in₃ is achieved. The loss reduction in light load condition using the proposed duty modulation is verified. The measured efficiency is over 90% for a wide output range, and the maximum value achieves 93.9% for DC12V, and 95.3% for DC48V, respectively.