How fast is the world’s fastest GaN switch?  It is an interesting question, and maybe is mostly wanted for bragging rights, but the measurement does present some challenges.  With the GaN switching 100 V at an expected bandwidth of above 1 GHz, the selection of the probe is critical.  GaN is very low capacitance, so the probe must be near-zero capacitance.  The probe also needs to have a flat response up to a few GHz and provide a usable dynamic range of more than 100 V.  Unfortunately, no such probe exists, and if it did, the PCB trace inductance would still impact the measurement.

In an earlier post, How Can You Determine the Turns Ratio of a Power Transformer in a Sealed DC-DC Module?, we showed that it is possible to recreate a switching waveform using the E and H fields surrounding the switch.  Since this is an unusual measurement, the method should first be validated, performing the measurement on a known switching speed, of similar magnitude.  Using a Picotest VRTS3 demo board, which has 50 Ohm signals at about 375 ps, this is about the closest measurement we have that is reasonably close. Using an SMA connector to provide a direct measurement of the edge speed and a waveguide-field probe to sample the fields from the PCB trace, two results are obtained for comparison.  The direct measurement and the measurement recreated by integrating the E-fields data.

This measurement, shown in Figure 1, provides remarkably similar looking results, within 10% of each other for the edge speed (fall time).

Figure 1  Direct voltage measurement and integrated E-field measurement of a 375ps edge available on the VRTS3 demo board.

Feeling confident with the method, it is applied to the GaN device.  Placing the E-field probe directly above the GaN FET, the same method is applied.  The resulting measurement, shown in Figure 2, indicates a switching speed of 138.9 ps or an equivalent bandwidth of 2.5 GHz. 

Figure 2  Applying the same E-field probe method to the GaN FET results in a fall time of 138.9ps or 2.5GHz.

Subsequently, this measurement was also performed by Langer EMV using their own specialized equipment designed for this purpose.  Langer EMV provided their measurement of 136ps [1].

This measurement was originally presented at DesignCon 2021.

Reference:

[1] Steven M. Sandler, Just How Fast is GaN Fast?, Signal Integrity Journal, March 12, 2020