Hello all,

Newcomer to the group here, and I am migrating my 555 based desulfator
design over to a PICAXE based platform. Along with this change, I
will be adding several features and demossing is one I would like to
include. I have seen that both the PIC based desulfator supported
here and 'desulfator2' over on the ezboard have experimented with the
demossing circuit.

My concerns are in the longevity of a MOSFET under the extreme current
pulses experienced by the demossing pulse. The devices I am using are
60V 50A N channel, and are rated for 200A pulsed. What I would like
to do is limit the current pulse to around the 200A figure to
hopefully increase the circuit reliability. Since I gather we want a
high current, short duration pulse, any means of current limiting I
would use should be non-inductive, or very low inductance.

I was thinking a wirewound resistor, either in a ceramic/cement
package or an aluminum housing would NOT be ideal unless it was a low
inductance variety. I have been reading datasheets on other power
resistors, and thinking of purchasing a thick film, TO-220 package
device. Of course, the largest of these types are only rated to about
50 watts, which would be exceeded by a huge margin at say 12.6V, 200A.
However, with such a short duration I was hoping the device would
survive and not deteriorate the film inside.

Does anyone have experience with large momentary overloads on these
devices, either the resistor, mosfets, or both, in long term
operation? Perhaps this is not even an important consideration, as
the Rds does increase rapidly when approaching 200A in my device
according to the datasheet, although the graph is not complete to the
200A level. Could it be that limiting gate voltage <10V is another
and better solution to the current limit, as the resistance required
to maintain 200A or less is very small, but this would increase the
dissipation in the MOSFET.