Voltage Balancing in Series-Connected GaN Devices Using Scalable Transformer-Coupled Gate Driver

Abstract

This article presents a scalable transformer-coupled open-loop gate-driving technique that enables voltage balancing across series-connected GaN devices. High-voltage pulse generation with short rise times at kilovolt levels is a significant challenge. Conventional solid-state devices such as insulated-gate bipolar transistors and SiC metal–oxide–semiconductor field-effect transistors, though capable of high blocking voltages, are limited in switching speed and cannot reach the nanosecond regime. Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) due to their lateral structure offer low gate charge and ability to switch faster than 50 V/ns, are an attractive potential candidate. However, due to their lateral architecture, GaN HEMTs also have limited voltage-blocking capability, with most commercially available GaN power devices rated up to approximately 650 V, limiting their direct use in fast high voltage waveform generation at kilo-volts level. Series-connecting GaN devices can overcome this limitation but introduces severe voltage-balancing challenges, as even minor gate-signal mismatch at nanosecond timescale can cause destructive imbalance. Conventional closed-loop balancing methods, are difficult to implement at GaN switching speeds due to feedback latency. This article presents a simple open loop technique for driving GaN devices in series with a transformer-coupled gate driver. The proposed gate drive ensures simultaneous turn-on/off with identical gate signals across all devices. An ultrafast full-bridge GaN based inverter excites wideband gate-drive transformers designed to preserve the ultrafast transition speeds of individual devices, while providing high-voltage isolation and near-equal voltage sharing establishing a scalable solution. Experimental results with two series-connected GaN HEMTs confirm nearly balanced voltage sharing at 1 kV across varying loads and currents