This paper presents a 55-kW three-phase inverter based on soft-switching modules for hybrid electric vehicle drives at high-temperature conditions. The main switch of the module is composed of the hybrid switch, which is composed of parallel IGBT and MOSFET. Highly integrated soft-switching modules have been employed to achieve switching loss as well as conduction loss reduction. The operation principle of the proposed inverter is analyzed in detail. Experimental evaluations of the inverter have been conducted through both inductive load and motor-dynamometer load at coolant temperatures ranging from 25°C to 90°C. Efficiency measurement using power meter showed that the peak efficiency is around 99%, and it drops slightly at lower speed and higher temperature conditions. To ensure measurement fidelity, a double-chamber differential calorimeter system was designed and calibrated for the inverter testing. Through long-hour testing, the measured efficiencies consistently showed 99% and higher. The soft-switching inverter has been operated reliably and demonstrated high efficiency at different temperature and test conditions.