interesting topic
with the switch, whatever it may be, in the negative (earth) side of the coil the spike
will be negative, yes negative with respect to ground. MOSFET's dont like it! you can put
a fast doide in series with the MOSFET to bear the brunt of the reverse bias but it must turn
off faster than the MOSFET. parasitc capacitance becomes critical in the sharing of the reverse
bias.
easier to use highvoltage BJT's. a 1000-1500V BJT will probably have a reverse bias tolerance of
400V.