How to measure the fastest power switches

December 01, 2014 // By Steve Sandler, Picotest
If you're designing power circuits with GaN devices, you need a grasp of the device's switching speed. To measure that, your oscilloscope, probes, and interconnects must be fast enough to minimise their effect(s) on the measurements.

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, your oscilloscope, probes, and interconnects must be fast enough to minimise their effect(s) 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 (from Efficient Power Conversion Corp.). 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.

Figure 1. The test boards are shown with the half bridge configuration only on the left and the complete DC/DC converter on the right. The banana sockets allow connection of the board to an electronic load. BNC connectors provide access to an external pulse generator.

Instrument and probe requirements

To ensure that the instrument and probe don't significantly impact the measurement, we can assume that the rise times of the probe, oscilloscope, and the half-bridge can be added using root sum squares. This isn't always true, but for our initial estimates we'll assume this relationship holds.

The measured rise time of the half-bridge including the rise time of the oscilloscope and the rise time of the probe is:

The actual rise time of the half-bridge is determined as follows:

To restrict the measurement error to some percentage, K, the rise time of the instrumentation can be related to the actual rise time:

Solving for K, the ratio of the instrument rise time to the actual half-bridge rise time is:

So for the two examples, if we wish the measured result to be less than 5% or less than 10%, then the rise time of the oscilloscope and the probe needs to be less than 32% or 46% of the FET rise time, respectively. Stating it differently, the instrumentation should be 3.1 or 2.2 times faster than the FET rise time, respectively.

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