www.analog.com GALLIUM NITRIDE 13 www.power-mag.com Issue 3 2026 Power Electronics Europe diode drop. In the case of extended dead times, these controllers tolerate negative spikes on the switch node. With a stable high-side driver supply established, the next step is accurately measuring the highside FET’s gate-source voltage. Gate Measurements Figure 3 shows the gate and switch waveforms of a GaN-based step-down (buck) converter. Without a series gate turn-on resistor, the TOP FET’s gate-source voltage (VTOP_GS) exceeds the GaN FET’s specified +6V maximum gate-source voltage rating. Adding a 2.2Ω TGUP resistor reduces VTOP_GS and dampens gate and switch node ringing. High impedance oscilloscope probes capture the groundreferenced low-side gate (VBG) and switch node (VSW) waveforms. The TOP FET’s source (VSW) alternates between VIN and ground. The GaN’s high slew rate (over 30V/ns) and 300MHz ringing exceed the practical common-mode limit of differential probes typically used to make VTOP_GS measurements. Fortunately, available optically isolated probes make this measurement possible. These probes, pioneered by Tektronix’s IsoVu™ technology, feature unbelievably high frequency common-mode rejection ratio (CMRR) and are priced accordingly. Figure 4 shows an isolated probe in action. This type of probe connects to the oscilloscope through fibre optic cables, which not only provides galvanic isolation but also keeps common-mode input capacitance low. The probe’s attenuator tip plugs directly into an MMCX connector. The probe can also connect to PCB test points using header pins and an MMCX-tosquare-pin adapter. To maintain the probe’s full performance, the connection between the probe tip and the PCB is kept as short and well-shielded as possible. A board-mounted MMCX connector offers the best coaxial connection, but make sure to route short Kelvin traces from the FET gate and source. While the isolated probe may be the best - and only - way to measure high-side GaN gate voltages, let’s see how another common method stacks up. Passive Probes High-side gate signals can be measured with two ground-referenced passive probes, a digital oscilloscope, and math. This A - B or pseudo differential technique1 remains popular for evaluating gate timing on low voltage DC-to-DC converters, even though it suffers from limited voltage range and CMRR. Before taking high-side gate measurements, it’s best to do a quick common-mode rejection check. Connect both the probes Figure 2. GaN controller with internal smart bootstrap switch. Figure 4. Isolated probe setups. Figure 3. Turn-on waveforms with isolated probe.
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