So I have been researching about protecting a MOSFET. I have used MOSFETs for basic on/off switches but never really thought about fully protecting them. I have used them for on/off switches to engage relay coils. The only thing I did was place a fly back diode across the the inductive load.
Then I came across Self Protected MOSFETs. Here is a link to a diagram: https://www.google.com/search?q=self+protected+mosfet&espv=2&biw=1920&bih=955&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiY7M3j0MrKAhVIVT4KHWQvD_kQ_AUIBigB#imgrc=uUMrOVnQMAn6oM%3A
I am only really interested in the ESD and Over Voltage protection sections to implement in my own designs. If I were to make a robust design to drive, say, a DC motor, here are my thoughts:
Protect The Gate: I would place a series resistor of about 100 ohms to prevent too much charge the gate layer to puncture.
Ensure MOSFET Turns Off: I would place a 100k ohm resistor between the gate and source (assuming source is tied to circuit ground or battery negative) to ensure the MOSFETs gate capacitor like behavior discharges so that if the input floats, the MOSFET turns off.
Vgs Protection: I would place a TVS diode before the 100 ohm resistor so I could catch transients. Whether it is uni or bi directional I think depends on the gates tolerance for these voltages. I am going to assume that Vgs should only be positive and thus use a unidirectional TVS. And pick it so that the clamping voltage is not beyond the absolute maximum of Vgs for the MOSFET. Reverse voltage should clamp right away.
>>>Vds Protection:<<< This is where my confusion arises. Do I need this? Most MOSFETs have a body diode and should protect from voltages due to inductors being switched off quickly. I don't like taking risks so I would use a faster diode or maybe even a TVS across drain and source and place a flyback diode on the inductor. But in the diagram I linked to, the TVS is across drain and gate. Why?
If I assume a large voltage builds up on the drain, the TVS would conduct (before max Vds I assume of the transistor), go through the gate resistor, and go to both the postive rail driving the gate and through the 100k ohm bleed resistor to ground? But doesn't this also turn on the MOSFET? This is where things get murky and despite researching the answer on the internet, I couldn't find a clear answer.