Are PTC Fuses for MOSFET gates a good idea?

[LDMattei]

Hello,
I wanted to make a circuit in order to control some indutive loads in a automotive envioroment, so I was thinking if it would be a good idea to add a PTC fuse to protect the MOSFET gate. I do know that it won't act fast enough in order to save the MOSFET, but I thought it was a good ideia to prevent a fire or anything like that, thus I designed it in other to trigger at the maximum current my LVS diode can handle before thermal shutdown.
So I wanted some feedback to know if this is actually a good ideia.

My circuit diagram is attached below (sorry for my ugly writting). F1 is the PTC fuse of 30mA, R1 is 1KΩ, R2 is 10kΩ and the the LVS triggers at 16.7V, everything is powered by a car battery.

[ajb]

What are you protecting against?  The only real hazard to the gate is an overvoltage event on the input signal here, so the level of protection depends on the sorts of faults that may occur on that node.  As drawn, the MOSFET gate is not actually protected by the fuse--it's protected by the TVS diode, and the TVS diode is protected by the fuse and the resistor.  This is a fairly sensible arrangement, but the additional 1k resistor in series is also limiting current, so a 30mA PTC is only going to kick in if the input voltage hits 30V above the TVS voltage--so something like 47V, but it depends on the clamping characteristics of the diode.  It may not be necessary to have both the PTC and the resistor, since both are effectively doing the same thing.  A PTC may be better if you expect DC faults, which would otherwise cause the TVS diode or resistor to cook itself, but for transients the resistor on its own may be fine or even preferable.  The lower initial resistance of the PTC would be an advantage if you need to drive the gate hard for faster switching, but if you need fast switching then you need local gate drive with a tight loop and therefore you wouldn't need this kind of protection in the first place.

As an aside, TVS diodes begin to conduct at a particular voltage, but as the current increases the diode voltage can rise several volts above that.  So a 16.5V TVS diode may clamp at something like 20V with a sufficiently energetic fault, which is into the territory of Vgs(max) for a lot of MOSFETs.  So worth considering that you may need a lower voltage TVS diode to adequately protect the transistor depending on the specific parts and the fault conditions you're designing for.

Anyway, if the input to this circuit is coming from the outside world, then a PTC and/or resistor in combination with the TVS diode is a good idea, but if it's generated by some other circuitry on board it's likely not necessary, or can be better protected using other methods like not driving the gate directly from the upper voltage rail.