Accelerated Power Cycling of GaN HEMTs using Switching Loss and Fast Temperature Measurement

Power cycling is typically performed by periodically self-heating a power device using a DC current. This paper demonstrates a technique to boost the heating power to shorten the heating phase, by the addition of switching loss. This power cycling technique is demonstrated on 190 mOmega, 600 V Gallium Nitride (GaN) discrete devices, where it achieves 240,000 thermal cycles per week with a junction temperature swing Δ ΤJ of 100 deg C, and where the device remains integrated in a switching converter. The device under test is heated rapidly from 30 deg C to 130 deg C in 0.5 s, by hard-switching at 1 MHz, at rated current and 400 V. An optimized thermal path cools the junction back to 30 deg C in 2 s. The junction temperature is closedloop controlled to maintain an approximately constant temperature swing. This requires junction temperature sensing with low ms-scale latency, implemented here using peak turn-on di/dt as the junction temperature indicator. The inferred temperature is fed into a control system that governs the heating and cooling durations. The resulting closed-loop-controlled heating time is shown to be a sensitive real-time indicator of device change. The paper discusses the practicality of temperature calibration methods, in light of temperature-sensitive electrical parameters’ known drift and sensitivity to bias temperature instability, and the problem of self-heating during calibration. Experimental results show one GaN device surviving 400,000 cycles with a DeltaTJ of 102 deg C with no apparent thermal degradation, and another GaN device cycling at a DeltaTJ of 136 deg C, whose heating duration reduces from 500 to 10 ms over the course of 30,000 cycles, indicating an apparent degradation of the device’s thermal properties.