12 GALLIUM NITRIDE www.analog.com Issue 3 2026 Power Electronics Europe www.power-mag.com Low Voltage GaN Converter Gate Drive and Measurement Kurk Mathews, Senior Applications Manager, and Luis Onofre Lazaro, Applications Engineer, Analog Devices Abstract This article discusses the need for precise gate voltage control and accurate gate measurements when using gallium nitride field-effect transistors (GaN FETs). Highside gate measurements taken with advanced isolated probes are compared to those obtained using alternative equipment and techniques. Introduction Gallium nitride field-effect transistors (GaN FETs) offer faster switching speeds, smaller packages, and lower power losses than silicon FETs. These features allow power converters to run at higher frequencies, reducing overall solution size while maintaining high efficiency. While the basic DC-to-DC converter design remains the same, GaN introduces additional design and testing challenges. One of the more important challenges is precise control of gate voltage and timing. This can be difficult because the switching times can exceed what traditional controllers and test equipment can manage. Fortunately, GaN-specific controllers and measurement techniques can solve these problems and ensure a reliable power supply design without adding extra complexity. GaN Performance and Characteristics Figure 1 shows a 12V step-down converter based on the LTC7891 synchronous controller driving 100V GaN FETs. Operating at 500kHz, it achieves 97% efficiency with a 20A load and 48V input. This is about 2% higher efficiency - 40% less power loss - than with current generation 100V silicon FETs.* To match this performance with silicon, the switching frequency would need to be less than half, requiring a larger inductor and overall solution size. This device is part of the new family of controllers specifically designed to meet GaN’s gate drive and switching requirements without the need for additional components. Analog Devices’ portfolio of GaN-optimised controllers also includes the LTC7890, a 100V dualchannel buck controller, along with the LTC7893 and LTC7892, which serve as 100V boost and dual-channel boost controllers, respectively. Gate Voltage Silicon FETs are typically specified with a gate-source voltage between 4.5V and 10V and an absolute maximum rating of ±20V. In contrast, a 100V GaN FET might be specified with a gate-source voltage of 5V limited to +6V/-4V for long-term reliability. Meeting these GaN specifications requires a tightly regulated gate supply and minimal high frequency overshoot or undershoot. While a precise 5V supply is sufficient for the low-side FET, additional circuitry or a GaN-specific controller is needed to limit the high-side FET’s gate-source voltage. Gate Driver Supply In Figure 2, a bootstrap capacitor and diode (CBOOT and DBOOT) implement a traditional high-side gate drive supply. When the high-side (TOP) switch turns off, the switch node pulls low - either due to the inductor current or the low-side (BOT) switch turning on. With both switches off, the switch node’s voltage is limited to approximately 1V below ground by the silicon FET’s body diode. In contrast, GaN FETs conduct in the reverse direction, performing like a 2V to 3V body diode. When using a bootstrap diode, the negative switch voltage adds to the bootstrap capacitor’s voltage, increasing the high-side FET’s gate-source voltage. Alternatively, a smart switch can prevent the driver from overcharging without the need for extra clamping diodes. This active switch turns on after the BOT gate is on, resulting in a regulated high-side gate drive voltage that does not depend on the body Figure 1. The LTC7891 step-down (buck) converter schematic. *The LTC7891 controller features 4V to 5.5V gate drive suitable for GaN and logic level silicon FETs. The LTC7897 controller, with 5V to 10V gate drive, is suitable for standard level silicon FETs.
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