MOSFET gate protection

[aheid]

So I was just fooling around in LT Spice with a silly idea, and I got a bit surprised about the behavior of the gate voltage.

The circuit is of a very simple electronic load. When looking a the transient response, I was confused about current going into the gate voltage source. So I put the diode D1 there, and sure enough, the voltage rose above that of the gate voltage source.

It took me a little while before I recalled that a MOSFET has parasitic capacitors all over, and I assume this behavior is the result of the gate-drain parasitic capacitor.

As such I assume that this behavior would occur if I were to build the circuit as shown, which means that there's a possibility for the gate-source voltage to go beyond the maximum rated value.

If so, is it advisable to connect a Zener diode between the gate and ground to ensure the gate-source voltage stays within the rated values? Or is there a better way to protect the gate? Or is it a non-issue in real life?

[T3sl4co1l]

Yup, Cdg is a real thing, particularly on old transistors under most conditions, and newer transistors at low Vds.

What are you protecting against?  Are you expecting a very rapidly changing load voltage?

Which, actually, you should -- mechanical contacts are amazingly fast, in the sub-nanosecond range, so the dV/dt from hot-plugging a source to a load can be quite exciting indeed!

You'll want some inductance between the terminals and the transistor, and some extra capacitance at the transistor to help filter it.  Probably an R+C as well, to dampen everything and help ensure it doesn't oscillate.  Basic filtering stuff -- only use what bandwidth you need.

That will probably leave enough change at the transistor that you'll still want some protection, and yes, a zener from ground to gate will do a fine job.

Tim

[aheid]

Thank you for the confirmation and details!

Well currently I'm just exploring and learning. I'm going to make my own electronic load, but not based on the shown circuit. It'll be a more "standard" one using an opamp for gate control.

While I'm not expecting large dv/dt, that is also a function of me not being very experienced and don't know all that much about how real circuits behave yet.

So if I understood you correctly, you mean I should put a small inductor (ferrite bead thing?) in front of the drain?