Below is the circuit I used to implement an active load which use an op-amp to adjust the mosfets' resistance based on DAC voltage at the op-amp input.
VIN_NANO is about 10VDC.
Mosfet is IXYS IXFH28N60P3, Vds: 600V, On-resistance: 260mOhm @ 14A,10V Vgs, power dissipation: 695 Watt.
The scheme works well with Battery String voltage of 12VDC. When I increase it to 125 VDC, the mosfets die almost instantaneously. What would be the reason behind this?
Test load current looks like this: