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How to measure the world’s fastest power switch

Gallium Nitride (GaN) FETS are poised to replace silicon power devices in voltage regulators and DC-DC power supplies. Their switching speeds are significantly faster and their RDS(on) is lower than silicon MOSFETS. That can lead to higher power efficiency power sources, which is good for all of us. If you’re designing power circuits with GaN devices, you need a grasp of the device’s switching speed. To measure that, you’re oscilloscope, probes, and interconnects must be fast enough to minimize their effect on the measurements.

One of the most frequent questions I receive on the subject of device performance is “how fast are they, really?” My general response is that they are blazingly fast but that we just don’t know quantitatively how fast. To find out, I made some measurements using a 33-GHz real-time oscilloscope and a high-speed transmission-line probe. I’ll discuss the design limitations that mask the device’s speed, and what’s in store for the future. With these measurements, I believe we’ll be designing power supplies switching at 250 MHz before long.

Figure 1 shows two evaluation boards used to perform the measurements. Both boards include a gate-voltage regulator, driver, pulse conditioner, and two eGaN switches. The board on the right is a complete DC-DC converter, which includes a Gen4 monolithic half-bridge (both switched on one die) and includes an L-C output filter. The board on the left uses individual Gen3 eGaN devices in a half-bridge configuration, lacking the L-C output filter. In both cases, an external pulse generator provides a PWM (pulse-width modulated) signal through a BNC connector soldered to the test board’s PWM input. The switch rise time is measured on each board at input voltages of 5 V and 12 V.

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