Repeatedly exceeding MOSFET breakdown voltage - any damage?

[brett]

Hi
My questions are:
1. will repeatedly exceeding the breakdown voltage of a power MOSFET cause any damage?

2. if the answer to 1 is yes, then will a MOSFET with low breakdown voltage be any more prone to damage than one with a high breakdown voltage?

thanks for any help

PS
The background is that I'm switching about 2 amps through a low impedance transmitter coil (it's a non-audio project, but driving a large audio relay would be similar, so I feel that it's a relevant question here).
I'm using a power MOSFET as the switch on the ground side of the coil (It's an MTP3055 for it's ruggedness, but I might use an IRF540/830 or similar and save a dollar).
When the MOSFET turns "off", the inductor voltage "spikes" in the reverse direction.  I calculate the initial voltage as about 1000 V (maybe higher).
Various MOSFETs have drain-source breakdown voltages of 60V (MTP3055) to 500V(IRF840), so the MOSFET will "break over" each time I switch it off (20 times a second).

[brett]

Hi
after doing some more research, it seems that many MOSFETs are rated for "Avalanche energy", which I ussume is the energy of a breakdown event.

Using E = 1/2 LI^2, where E is the energy in Joules, L is the inductance in Henries and I is the current in amps, it seems that my coil holds about 2 mJ.

A PHP6N60E is rated for repeated avalanches of 10mJ and the MTP3055 is rated for single avalanches of 50mJ.

So I *think* that these MOSFETs are rugged enough to handle the breakdown/avalanche from my 2A and 0.8mH coil.

Is this correct, or am I barking up the wrong tree?

[R.G.]

You are correct. MOSFETs do not have some of the failure mechanisms of bipolar transistors because current hot spot formation is not an event in the normal physics of the device. Hot spots tend to spread out and get less intense rather than the opposite as in bipolars. You're reading the datasheet correctly, I think. The only other thing to watch for is that even with a repeated avalanche rating, if the breakovers cause excess power dissipation by being so frequent, the dissipated heat can still destroy the device, so watch the avalances per second too.

[brett]

Thanks RG.   
It is amazing to think how quickly I've got a handle on this.  Access to a guru like RG, and technical reports and datasheets is just fantastic.  We sure live in the information age.

Here's a useful link for others that are "messing with MOSFETs".http://www.irf.com/technical-info/appnotes/mosfet.pdf

Have a great day.