Shaping switching waveforms in a 650 v GaN FET bridge-leg using 6.7 GHz active gate drivers
The application of active gate driving to 40 V GaN FETs has previously been shown to reduce ringing and EMI-generating spectral content in the switch-node voltage waveforms. This paper, for the first time, shows active gate driving applied to 650 V GaN FETs, and the shaping of device voltages and currents during switching transients. A custom integrated active gate driver is used, which can dynamically vary its output resistance from 0.12 to 64 Ω, with a 150 ps timing resolution. At 200 V DC link and 10 A load current, a significant degree of control over the active-switch drain current and switch-node voltage is demonstrated, for both buck and boost mode operation. The current overshoot and ringing in the power waveforms due to circuit parasitics are actively reduced and the voltage oscillations in the DC link are damped. The timing of resistance sequences is shown to be critical to the success of active shaping methods, thus justifying the unparalleled 150 ps resolution of the driver. Under continuous operation and at reduced ratings of 100 V and 2 A load current the significant control of the switch node voltage and voltage spectra is also demonstrated. The switching delay is reduced, and parts of the spectrum are reduced by up to 9 dB, equivalent to the effect of tripling the gate resistance but without any reduction in the overall switching speed.