Re: TESTED works, Flyback driver plans

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Pyrotrons-at-aol-dot-com>

Hi Justin,
           
> Have you put a scope probe on the MOSFET drain to see
> what kind of flyback voltage is appearing across the 
> primary? I suspect the substrate zener in the MOSFET 
> is saving it from destruction and is probably 
> dissipating a lot of the power which is warming up 
> the heatsink. ?
>
> Regards,
> malcolm

Yup ; )

There is about 150V p-p on the primary of the flyback. The IRF540 MOSFET is
rated 100V. I knew something was up with that...

You are referring to the MOSFET "body" diode? I knew this was a diode that
was inherent with the MOSFET silicon design, I did NOT know that it was a
zener!

So you think that is what's saving the MOSFET? You're correct in that it
gets hotter than that hot place. I'm using about a 2" x 5" heatsink, fins
spaced every 1/4" or so that are about 1/2" tall. I've got really good
thermal conduction between the device and sink (no mica, lots of compound,
tightly bolted in).

It will burn your fingers after about 30 seconds of continous power. Inside
the enclosure, I've made room for a small DC fan, I'm looking around to
yank on out of old equipment somewhere because they're so expensive.

So if I used a properly rated MOSFET (IRF740 or 840), it will run cool? My
guess, I'm no EE. 

I'll put an IRF840 in it when I get home from class, to see what it does.
It seems as if I've done this before, and performance decreased as well as
MOSFET heating.

Later on,

Justin Hays
KC5PNP
G-3 #1150
Email: pyrotrons-at-aol-dot-com
Website: www.hvguy-dot-com