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

• Published in: IEEE transactions on power electronics Vol. 33; no. 12; pp. 10748 - 10763
• Main Authors: Zulauf, Grayson, Park, Sanghyeon, Liang, Wei, Surakitbovorn, Kawin North, Rivas-Davila, Juan
• Format: Journal Article
• Language: English
• Published: New York IEEE 01-12-2018; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Capacitance; Cascode devices; Charging; Circuits; Conduction losses; DC–AC power conversion; dc–dc power conversion; Discharge; Electric potential; Energy storage; Frequency conversion; gallium compounds; Gallium nitride; Gallium nitrides; HEMTs; High voltages; Loss measurement; Losses; Power converters; Power measurement; Power semiconductor devices; power transistors; resonant power conversion; Switching; Temperature measurement
• ISSN: 0885-8993; 1941-0107
• DOI: 10.1109/TPEL.2018.2800533

We report losses from charging and discharging the parasitic output capacitor, <inline-formula> <tex-math notation="LaTeX">{\rm C}_{\rm OSS}</tex-math></inline-formula>, in Gallium Nitride (GaN) power devices with voltage ratings over 600 <inline-formula><tex-math notation="LaTeX">{\rm V}_{\rm DS}</tex-math></inline-formula> . 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 <inline-formula><tex-math notation="LaTeX">{\rm C}_{\rm OSS}</tex-math> </inline-formula> losses from 5-35 MHz sine, square, and Class-<inline-formula><tex-math notation="LaTeX"> \Phi _{2}</tex-math></inline-formula> 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 <inline-formula><tex-math notation="LaTeX">\mathbf{dV/dt}</tex-math></inline-formula>. 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 <inline-formula><tex-math notation="LaTeX">{\rm R}_{\rm DS,ON}</tex-math> </inline-formula> actually degrades efficiency in certain applications due to <inline-formula> <tex-math notation="LaTeX">{\rm C}_{\rm OSS}</tex-math></inline-formula> losses. In the high-voltage, high-frequency range, these reported losses must be optimized simultaneously with conduction losses on a per-application basis.