[Lf] Exploding MOSFETS]

[Andre' Kesteloot]

Tom Boucher wrote:

> Wil
>
> I cannot claim to have been anywhere near as scientific as Andy in
> investigating my exploding PA devices, but I have also recently
> installed a variometer instead of the tapped coil used before, and my
> Class D PA now appears to be quite happy and runs quite cool at the
> kilowatt input level (85 volts at 12 amps). I  have also changed the
> matching arrangement from a tapped grounded coil system to a simple
> series loading coil. I adjust the variometer for maximum antenna current
> at a low power level, then wind the Tx up to maximum. Unlike Andy, my
> long wire comes directly into the shack so I don't need any remote
> tuning arrangement. (Makes my hair stand on end though.)
>
> I do not have any SWR protection but I do have over-current protection
> (using Dave YXM's Hall Effect device idea).
>
> I have taken to installing only 2 IRFP450s instead of 4 in parallel
> push-pull. That way I only blow up 2 instead of 4! However I am on my
> last pair of IRFP450s and next time am going to install some STW20NB50s
> which are better rated and cheaper than the IRFP450. (500 volt, 0.27 ohm
> on resistance, 20 amps continuous, 80 amps peak)
>
> Not sure if the above is any help. I have yet to transmit using long
> duty cycle modes so may still have a problem.
>
> 73, Tom G3OLB