Controlling gate current with a trimmer?

[eTobey]

Hi,

would it create a big issue, if i use a trimmer resistor  to control the gate current of a mosfet in a test circuit?

I could imagine, it would create some ringing or some other problems?

[Andy Chee]

As a strict test mockup, I don't see any particular issue.

But if your production design is dependent on finely tuned gate current, and this test rig is intended to determine such values for production, then I might suggest you may possibly have a broader design flaw.

Is your design a MOSFET linear amplifier of some sort?  Please elaborate on the broader purpose for testing.

[eTobey]

Just a BLDC controller.

[Siwastaja]

With a physically small trimmer, extra inductance probably isn't an issue. However for a BLDC controller are you going to use 6 trimmers and match all of them in value?

Really, you would use one value of Rg and then spend possibly an hour taking measurements (ringing, efficiency, etc.), and then choose another value of Rg and redo the measurements. Maybe a third value. So it would be simplest by far to just desolder and resolder different values of fixed resistors, this is what I always do. Added benefit is you can commit to as final as possible PCB layout from the start and don't need to waste board area for those trimmers.

You can really ballpark-calculate the range of Rg you want to work with (e.g., simple equation matching Qg(tot) = time * gate current), and then fine-tune with measurement. For example, if calculation says 3 ohms, you may want to test with 0, 2.2 and 4.7 ohms.

[David Hess]

would it create a big issue, if i use a trimmer resistor  to control the gate current of a mosfet in a test circuit?

I could imagine, it would create some ringing or some other problems?

It will work fine.  The resistance will dampen any ringing.

[Terry Bites]

A ferrite bead on the gate also helps reduce ringing.