MOSFET gate protection diodes 2N7002

[Jester]

I'm thinking of using a 2N7002 FET in a circuit where the gate voltage could exceed the +/-20V VGS rating of the above mentioned FET, however the schematic symbol of this part shows internal back-to-back Zeners between the gate and source AND there will be a 10k series resistor between the gate drive signal and the actual gate.

I don't see a rating for the internal Zeners, other than VGS = +/-20V in the MAXIMUM RATINGS

The gate signal, will typically be < 10V, however there could be at times a repetitive 50V peak signal (see below), this signal will be current limited by a 10k resistor.

Will the internal back-to-back Zeners protect the FET or do I need to include additional protection?

Link to data sheet: https://www.onsemi.com/pub_link/Collateral/2N7002W-D.PDF

 

[Audioguru]

ON Semi shows the zener diodes and a series gate resistor. They allow the 2000V electrostatic gate-source voltage spike.
Philips NXP and Fairchild do not show the zeners and resistor and do not have the electrostatic rating.

 

[Jester]

ON Semi shows the zener diodes and a series gate resistor. They allow the 2000V electrostatic gate-source voltage spike.
Philips NXP and Fairchild do not show the zeners and resistor and do not have the electrostatic rating.

The Fairchild part, shows +/-20V continuous, or 40V pulsed.... interesting

My question is, can I take advantage of the internal clamping Zeners to clamp the peaks on the waveform above to safe limits assuming I have a 10k series resistor to limit the current to the Zeners?

 

[Vbase]

It is impossible that the internal resistor is 10K, it must be 10 ohm. 10K will slow down the mosfet so much that it will be useless.
I recommend you use external zener, 1 is enough if your drive is positive only. A series resistor too of 100 to 470 ohm, depending on the frequency.

 

[FvM]

Reading the datasheet carefully will clarify that the internal zener diodes have a higher clamping level than +/- 20V. So they are not suited to guarantee a gate-source voltage within the maximum ratings. They have the purpose to limit rare ESD events, Driving it into breakdown in regular operation will result in fast device degradation and early end of life.

 

[Jester]

It is impossible that the internal resistor is 10K, it must be 10 ohm. 10K will slow down the mosfet so much that it will be useless.
I recommend you use external zener, 1 is enough if your drive is positive only. A series resistor too of 100 to 470 ohm, depending on the frequency.

I did not state that the 10k resistor was internal, I stated that my circuit has a 10k resistor between the gate drive signal and the actual gate.
"AND there will be a 10k series resistor between the gate drive signal and the actual gate"

- - - Updated - - -

Reading the datasheet carefully will clarify that the internal zener diodes have a higher clamping level than +/- 20V. So they are not suited to guarantee a gate-source voltage within the maximum ratings. They have the purpose to limit rare ESD events, Driving it into breakdown in regular operation will result in fast device degradation and early end of life.

FvM, thank you

 

[Vbase]

I did not state that the 10k resistor was internal, I stated that my circuit has a 10k resistor between the gate drive signal and the actual gate.
"AND there will be a 10k series resistor between the gate drive signal and the actual gate"

Yes, you did say that it is your resistor.
Internal or external 10K is the wrong value for gate resistor.

 

[Jester]

I did not state that the 10k resistor was internal, I stated that my circuit has a 10k resistor between the gate drive signal and the actual gate.
"AND there will be a 10k series resistor between the gate drive signal and the actual gate"

Yes, you did say that it is your resistor.
Internal or external 10K is the wrong value for gate resistor.

If I have no concern with speed, what is the issue with using a 10k gate resistor?

 

[Vbase]

If I have no concern with speed, what is the issue with using a 10k gate resistor?

If speed isn't important use transistor and save the zener.
The mosfet gets hotter and wastes energy with 10K, but I guess you aren't concerned with that too, neither do I.