thanks paul. I just ordered this guy:
Arcotronics | Passives | Capacitors | Capacitors, RC Networks | IGBT (Snubbers) C4BS Series |C4BSNBX4150ZA0J
seat of the pants engineering as I don't get on well with maths. I find in general that the theory can't take into account real world conditions and is best used for indication purposes. Have you scoped the b- to m- on your fet based controllers?