I have a bank of x6 MOSFETs (SUG90090E) connected in parallel, which I am using to switch a current of maximum 30A through a coil (resistance 1R and inductance 1mH), and am looking for some general advice with regards to the "flyback" section of the circuit. The current will not be switched at high frequency - only once per second or so. My schematic at present looks as below (each MOSFET has its own dedicated driver chip):
My goal is to have the current reduce to zero in several tens of microseconds. Based on pieces of designs I have seen elsewhere, I have tried to create an arrangement of components which I believe may do the job. The idea is that when the MOSFETs are switched off, the large induced voltage that appears at the bottom of the load is allowed to rise until it reaches the clamping voltage of the TVS diode D1. When this happens, the TVS allows a small amount of current to flow to the base of Q7, switching it on, and allowing the large circulating current to dissipate away. As far as I understand, the fact that a large voltage is allowed to build up means that the rate of decrese of current can be greater, due to V = -L*dI/dt, when compared to the case of using a simple single "flyback diode" (which would allow it to build up only to a diode drop or so).
I have tried to choose the components suitably - for example, the clamping voltage of the TVS diode 1.5KE100A is maximum 178V: and so this should be enough to protect the SUG90090E MOSFETs, which have a VDS(max) of 200V. I have also chosen the transistor Q7 BUF420AW and diode D2 IDW100E60 such that their forward currents are at least greater than the steady state current through the load when switched on (their max rated values are 30A and 150A, respectively). I have added a small capacitor C1, but am sure if it will make any difference in practice?
My question is whether this is generally a reasonable approach to switching off the current fast in this type of situation, and am looking for reassurance that it's wise to continue down this route? Are the components chosen suitably enough, or is there something that I have not accounted for? Any advice or tips would be helpful.
At the suggestion of @Bimpelrekkie , I have tried to make a simplifed LTSpice model of this situation. The simulation also compares the case when the transistor Q7 is removed, leaving only a TVS diode across the coil (as suggested by @Andyaka).
There is little difference between the two cases - the current is reduced from 35A down to zero in ~250us, and the back EMF spikes are capped below safely below the 200V rating of the SUG90090E MOSFET. The difference is that in the case with the BJT, it is this transistor which absorbs the power of P = I*V = (170V - 35V)*35A ~ 4.7kW (peak value, when current is maximum). With only a single TVS, then it is the TVS which takes the 4.4kW peak.
I feel now that @Andyaka suggestion is reasonable, and as long as I choose a TVS which is able to handle this power then it could be the only snubber element necessary.