I have the below circuit (link to simulation). It represents the low-voltage interlock and high-voltage precharge & discharge circuit for my Formula SAE team's EV car. While most other FSAE teams use a normally-closed relay for the discharge circuit, our alumni have opted for a "self-closing" n-MOSFET circuit, since it has a smaller footprint. When the interlock opens, the capacitor activates the MOSFET's gate until voltage drops down to VGS(th), which is sufficient on paper. The Zener diode is meant to limit the gate voltage below VGS(max) during this process.
The discharge circuit's purpose is to allow the high-voltage capacitor to discharge thru a chassis-mount resistor. As per FSAE rule EV.8.2.2, the high-voltage side must drop below 60 V within 5 seconds of opening the interlock. In the simulation, this is all fine and dandy, that voltage being reached within 3 seconds.
However, in real life, it takes nearly 10 seconds. I think I have narrowed down the issue to the MOSFET gate failing, for an unknown reason. On 2 removed MOSFETs, I have measured internal RGS values of 4 kΩ and 38 kΩ, which seems to indicate part damage.
I tried 3 different models of MOSFET, but the problem remains. The part I currently have is AOD7N65, which seemed to satisfy all requirements when I picked it. Peak discharge current is about 170 mA, which is within that part's safe operating area for 400 V.
So, the big question; why is the MOSFET failing? Am I missing something? Any help is appreciated.