Protecting fat gates for over voltage with a clamping diode

Thread Starter

MikeJacobs

Joined Dec 7, 2019
123
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
Screen Shot 2020-04-25 at 9.14.52 AM.png
 

Papabravo

Joined Feb 24, 2006
13,743
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
10,055
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
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
123
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
13,743
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?
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
123
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
 
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