05-25-17 How do you measure gate drive current?

On behalf of Ryan Schnell.

The first step I would take is to determine whether the gate drive current during operation needs to be measured. The easier measurement is to look at the drain-to-source (or collector-to-emitter) voltages of the power switches and seeing if their waveforms look good. If there is an issue, then possibly we could look into the gate voltage and current waveforms.

Since there are high currents in the system, the measurement will get distorted easily with larger measurement loops. Using a very tight, differential voltage probe, you could measure across the external series gate resistor connecting the gate driver to the power switch (assuming you have one). Then, by dividing the voltage seen by the resistance, you can know the current in the gate drive. The Tektronix scope, TIVM1, mentioned in the webinar could definitely do this. If you don’t have access to such a new, and expensive scope, any differential probe should work, as long as the common mode range is not exceeded.

You can also check the gate currents with the bus voltage off, which allows you to use lower voltage probes. Even though the Cgd capacitance does change during fully loaded conditions, the gate drive signals will be representative of what full operation waveforms can look like.

Good luck, and I hope your measurements come out cleanly!