# Protecting fat gates for over voltage with a clamping diode

#### MikeJacobs

Joined Dec 7, 2019
226
So I have a need to protect a few gate from over voltage conditions with a clamping diode.
I want to clamp the gate directly to the source. So it can never exceed to the zener voltage.

I tried to put this in the sim but for some reason I am not seeing any clamping action.

Any ideas?

All I see at the gate is the input sign wave. I am expecting to see the gate clamp at the zener voltage with is 10V

#### Papabravo

Joined Feb 24, 2006
21,264
Do fat gates consume large amounts of junk food?
No clamping action, because. at 18V(pk) on the gate, there is 10V across the zener and 8V across R2?
Are you aware that a zener adds a capacitative load to the gate?

#### ericgibbs

Joined Jan 29, 2010
19,015
hi Mike,
The MOSFET is a Source follower, what do you expect the Source voltage to be when a positive drive over voltage is applied to the Gate.?
E

#### ElectricSpidey

Joined Dec 2, 2017
2,839
Try it with the load on the drain.

Or connect the Zener to ground instead of across the gate.

#### MikeJacobs

Joined Dec 7, 2019
226
Sorry I drew it wrong
Here is what I meant it to be
Also, sorry about the typo, not enough coffee I guess

#### ericgibbs

Joined Jan 29, 2010
19,015
hi Mike,
E

#### MikeJacobs

Joined Dec 7, 2019
226
It seems to work now in this simplified model. My real model of my real circuit still doesn't work. I need to examine it a bit more I think. hmmm

#### Papabravo

Joined Feb 24, 2006
21,264
As I mentioned, the zener looks like a capacitor and will discharge the gate very slowly, if at all. Could a pulldown resistor in parallel with the zener hurt you?

#### MikeJacobs

Joined Dec 7, 2019
226
As I mentioned, the zener looks like a capacitor and will discharge the gate very slowly, if at all. Could a pulldown resistor in parallel with the zener hurt you?
Thank you, that is very interesting, I did not know that. I saw that effect in the sim the other day. It almost looked like the FET was latching on it was interesting

#### dl324

Joined Mar 30, 2015
17,011
So I have a need to protect a few gate from over voltage conditions with a clamping diode.
Why do you think you need to do that? You control the maximum gate voltage when you design a circuit.

BTW, what is a "fat gate"?