Design Steps Toward a 40-kVA SiC JFET Inverter With Natural-Convection Cooling and an Efficiency Exceeding 99.5%

This paper describes the concept, design, construction, and experimental investigation of a 40-kVA inverter with silicon carbide junction field-effect transistors (JFETs). The inverter was designed to reach an efficiency exceeding 99.5%. The size of the heat sink is significantly reduced in comparison to silicon insulated-gate bipolar transistor designs, and the high efficiency makes it possible to use free-convection cooling. This could potentially increase reliability compared with solutions with fans. A very low conduction loss has been achieved by parallel connecting ten 85-mΩ normally-ON JFETs in each switch position. A special gate-drive solution was applied, forcing the transistors to switch very fast (approximately 10 kV/μs), resulting in very low switching losses. As output power is almost equal to input power, special effort was done to precisely determine the amount of semiconductor power losses via comparative thermal measurements. A detailed analysis of the measurements shows that the efficiency of the inverter is close to 99.7% at 40 kVA.