I am trying to stabilize the current in coils in my experiment. For that I am using a circuit as in the picture:
First section is a fast switch. It is also galvanically isolated from the TTL signal with opto-coupler (not in the picture)
Second section is fast discharging of the coils. When we suddenly stop the current from the supply, the coils respond in reverse voltage build-up opening the diode. Then in quarter of a cycle of the LC circuit the energy flows from the coils to the capacitor and the diode closes. Then it slowly discharges through the resistor. Coils inductance is 350 uH.
Third section is linearized MOSFET. I am using OPA277 here. It linearizes the response of the MOSFET by comparing the voltage on the sense resistor with the input and adjusting the gate voltage. 10R resistor is added to reduce gate ringing of the MOSFET. It is connected to the PID control (I am currently using RedPitaya for that). Current measurement for the PID is done independently with a current transducer. In the circuit there are 4 such sections connected in parallel to lower the power dissipation.
What my problem is, that when I am sending larger currents (a few amps) after some time the MOSFET responsible for current stabilization burns. Firstly I thought that it is due to heating up, but it is placed on a water cooled aluminum casing and its temperature doesn't rise. Then I added additional clamping Shottky diodes bypassing the body diode of the MOSFETs (each transistor got one, directly on the terminals of their casings) because I thought it is due to high reverse voltage. The fast discharging setup lets the current discharge in around 100 us and thus large reverse voltage is generated (I calculated it to be around 414 V at 70 A - largest current I want to use).
I have run out of ideas what could be the problem. I would be glad for some explanation what can cause the burning of the MOSFETs and possibly how to protect them.