Design a TVS diode?

[Krister Persson]

I want to protect a MOS FET for ESD with a bidirectional TVS diode. The MOS FET I have has not a built-in TVS diode. The max gate-source voltage (Vds) is 30V. Shall I chose a TVS diode that has a clamping voltage (Vcl) exactly 30V or to be on the safe side a little under like 27V?

 

[mainline]

I want to protect a MOS FET for ESD with a bidirectional TVS diode. The MOS FET I have has not a built-in TVS diode. The max gate-source voltage (Vds) is 30V. Shall I chose a TVS diode that has a clamping voltage (Vcl) exactly 30V or to be on the safe side a little under like 27V?

The clamping voltage should be higher than the normal operating voltage of the signal line, but low enough to protect the MOSFET.

 

[Krister Persson]

The clamping voltage should be higher than the normal operating voltage of the signal line, but low enough to protect the MOSFET.

I understand that. But since the max gate-source is 30V, over that the MOS FET can fail so the clamping voltage must be a little under 30V in order to protect it. I don't know the operation voltage but I suspect it's significant under 30V.

 

[Krister Persson]

The clamping voltage should be higher than the normal operating voltage of the signal line, but low enough to protect the MOSFET.

And to make it even more complicated..
Another MOS FET I have has a built-in TVS diode for EMD protection. This MOS FET has also max 30V gate-source. But the built-in TVS has Breakdown voltage (Vbr) at 30V. So the Vcl must be a couple of more volts. Strange. Vbr is when the diode begins to conduct. And Vcl when it conduct fully. See table below. You might think that the built-in TVS should have Vcl at 30V in order to protect the MOS FET?