High Performance Thermal Solution for High Power eGaN® FET Based Power Converters

Higher power density wafer level chipscale (WLCS) GaN FETs have traditionally relied on cooling from junction-to-board into the PCB. Some work has been published on adding backside cooling using silicon pads or gel based thermal interface materials (TIM) [1, 2, 3]. Past thermal solutions for GaN FETs required careful assembly to minimize mechanical stresses that complicated implementation. This paper explores a thermal cooling method suitable for WLCS GaN FETs that eliminates mechanical stresses during assembly and yields excellent thermal performance using a liquid thermal interface material. A 140 V to 28 V Buck converter, capable of delivering 34 A (1 kW) was designed and built. Tests were performed using forced air cooling on the PCB and repeated with the proposed thermal solution that resulted in more than a four times increase in output current capability compared to a converter without a heatsink.