COSS Losses in 600 V GaN Power Semiconductors in Soft-Switched, High- and Very-High-Frequency Power Converters

We report losses from charging and discharging the parasitic output capacitor, C-OSS, in Gallium Nitride (GaN) power devices with voltage ratings over 600 V-DS. These losses are of particular importance in soft-switched circuits used at MHz switching frequencies, where the output capacitance of the device is charged and discharged once per switching cycle during the device's off-time. This process is assumed lossless. We measure C-OSS losses from 5-35 MHz sine, square, and Class-Phi(2) waveshapes in enhancement-mode and cascode devices, and find that losses are present in all tested devices, equal or greater than conduction losses at MHz frequencies, and exponentially increasing with dV/dt. The cascode device outperforms the e-mode devices under 300 V, but the e-mode devices are preferred above this operating voltage. Furthermore, we show that, within a device family, losses scale linearly with output energy storage. Packaging appears to have only a minor effect on these losses. Finally, we demonstrate 10 MHz, 200 W dc-dc converters with varying device configurations, showing that, even with constant circulating currents, moving to larger devices with lower R-DS, O-N actually degrades efficiency in certain applications due to C-OSS losses. In the high-voltage, high-frequency range, these reported losses must be optimized simultaneously with conduction losses on a per-application basis.