An investigation of current distribution over four GaN HEMTs in parallel configurations

GaN HEMTs are desirable for parallel configurations due to (i) relatively steady threshold voltage over a wide temperature range and (ii) the positive temperature coefficient of their on-resistances. However, their operations are sensitive to the circuit parasitics and dissimilarities between devices. This paper investigates the current distribution among four parallel GaN HEMTs, considering two symmetrical circuit layouts of a power cell. For switching experiments, 25 V input voltage and a maximum load current of 7 A are considered. Current through each GaN HEMT is measured using stud-type, high bandwidth, coaxial shunt current sensors (CSs). The same circuit is further investigated by removing the current sensors. A second circuit is built by eliminating the CSs as to achieve high current. The circuit is tested up to 27 A/ 80 V. Here, only the voltage measurements and thermal images are obtained. Thermal images are used to evaluate the power dissipation between the four HEMTs and to correlate them with the current measurements. Further, the current waveforms are simulated using a SPICE model. The parasitic elements of the layout are calculated by ANSYS Q3D.