Analysis of Natural Convection Cooling Solutions for GaN HEMT Transistors

GaN HEMT power transistors have become an attractive option for high switching frequency converters in the 600 -650 V breakdown voltage range. The high voltage and current variation rates involved in the commutation process of such devices in combination with the package terminals stray inductance introduce unwanted and dangerous over-voltages and oscillations. For this reason TO-247 or TO-220 packages typically used in the medium power range (for example for IGBTs and Super Junction MOSFETs) are avoided for discrete GaN HEMTs, which are mainly packaged in different surface mount (SMD) formats. In SMD packages, the stray inductances are in the range of the nH and below, but the heat extraction plane (typically a Cu thermal pad) is often at the same level than the electrical terminals, hindering the simultaneous optimum design of the electrical and thermal circuit layouts. This work analyses the high speed switching (i.e., optimum circuit layout) versus thermal management (i.e., low thermal resistanece) trade-off, comparing different options for cooling p-GaN HEMT transistors in a PG-DSO SMD package. Two approaches based only on natural convection have been considered: board-level and heatsink-based solutions. Comparison among the different cooling options has been quantified by means of the HEMT junction to ambient thermal impedance, evaluated with the devices assembled in test-vehicle PCB boards. The results provide information for practicing engineers who would like to have a point of reference for passive cooling solutions of discrete GaN HEMTs.