--- In randrdesulfatorforum@yahoogroups.com, "particeps.criminis"
<particeps.criminis@...> wrote:
>
> 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.
>
Mosfets might be rated 200 Ampere (Like the irf 1405)
http://www.irf.com/product-info/datasheets/data/irf1405.pdf
But as you can see at the end of page 2 the package limits it to 75
Ampere .
With sufficient wire leading to and from the fet you might manage to
pulse 150 or 200 through it .
But as the datasheet tells 121 (out of 202) Ampere is with maximum
pulslength of 8 uS (400uS with 2% duty cycle).
With a switching frequency like this ringing is almost inevitable and
probably means instant death of the mosfet.
Limiting the current by using a mosfet in its linear range with powers
like this is the immediate destruction of the mosfet .
Ohm's law says that even half a ohm already makes 100 Watt of heat !
Even with a low duty cycle it would be hard to dispose of the heat
created .
Last week i found this forum (so i'm a newbie too)
But i been making a few simulations on it and might be designing
something simular .
(My design was with a pic877 12 volt charging ,30 Khz pulses
&measurement of internal resistance)
So far it looks good but first i want it to work real life before i
trust it :)

Good luck .