MOSFET protection scheme for voltage spikes

Question

I would like to understand an overvoltage protection scheme for low-side MOSFETS that seems counter intuitive to me, but is referenced in several sources.

Here is the link to the protection scheme: https://www.eetimes.com/wp-content/uploads/media-1072846-fig3.gif

Apologies for not uploading the picture, I receive a server error

All the protection elements make sense to me except the over-voltage clamping Zener/TVS diode connected between the drain and the gate of the MOSFET.

The notes state the following:

"Over-voltage protection is implemented via a Zener diode stack operating as an active clamp that turns the gate on when the VDS exceeds 65V. Active clamping ensures that the whole of the MOSFET is turned on and the voltage energy dissipated across the entire area of the MOSFET. For example, when a relay deactivates, any generated transient would be clamped at 65V for the duration of the transient and the energy dissipated. Once the transient's energy has been dissipated the device would revert to normal off state."

I understand why turning on the MOSFET to dissipate energy more efficiently during an inductive spike makes sense, however, if the TVS/Zener diode conducts in an overvoltage event, would the gate source voltage not be pulled above 20 V (Which would damage the MOSFET)?

Especially considering that the TVS diode is connected directly between the gate and the drain with no current limiting resistor, the ESD protection TVS/Zener diode will do little to keep the gate current limited to 20 V (since there is also a resistor between the ESD protection device and the MOSFET gate)

This scheme is referenced in several places, so I assume the design is sound and will not damage the FET. Can someone please explain to me why this design will not damage the FET during an inductive spike?

Sources: https://www.eetimes.com/more-protection-for-less/ http://www.icbase.com/File/News/download/ON_Trainning_10.PDF (Page 11) https://www.diodes.com/design/support/technical-articles/self-protecting-mosfets-deliver-improved-reliability-in-the-harsh-environment-of-automotive-applications/

Answer 1

The scheme protects against inductive spikes when switching off an inductive load. Those are limited in current to the on-current; you just need to provide a path for that current. The circuit opens the MOSFET when VDS exceeds the sum of the Zener diode voltage, the forward biasing voltage of the normal diode, and the threshold voltage of the MOSFET. The voltage will not rise significantly above VGS since a switching MOSFET has a high transconductance and the gate voltage will just rise high enough to admit the limited induction current at the comparatively high flyback voltage.

Answer 2

That is an active clamp used many times for example in low side driver ICs. What it does is turn on the output device when the voltage gets to high partially or fully turning it on depending on the amplitude and capacity of the surge. This normally only activates during a voltage surge and tries to keep the surge voltage to a tolerable level. It will dump that energy directly into the load. Caution needs when using this technique as the load can and will turn on. You would not want this to happen with an vehicle air bag. This is commonly used technique used for Load Dump protection in automotive devices. There are many other methods that will work.