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, COSS , in Gallium Nitride (GaN) power devices with voltage ratings over 600 VDS. 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 COSS losses from 5-35 MHz sine, square, and Class-Φ2waveshapes 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 RDS,ON actually degrades efficiency in certain applications due to COSS losses. In the high-voltage, high-frequency range, these reported losses must be optimized simultaneously with conduction losses on a per-application basis.