Re: TESTED works, Flyback driver plans

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

Original poster: "Steve Greenfield by way of Terry Fritz <twftesla-at-qwest-dot-net>" <alienrelics-at-yahoo-dot-com>

> I've got really good
> thermal conduction between the device and sink (no
> mica, lots of compound,
> tightly bolted in).

Careful, if you bolt it in too tightly you can distort
the heat sink or the MOSFET body and make a smaller
area for heat to flow through. And too much heat sink
paste will hold them apart, it should be just enough
to fill in the roughness.

I had to retrofit some switching power supplies, they
came with a TO220 inside that had a short life due to
too much heat and I had to mount a TO3 on the outside
to replace it. The finish was a very deep ground
aluminum, very rough. I used sheets of sandpaper on a
thick chunk of aluminum mounted in a vise to grind
down the area, using finer sandpaper down to 400 to
get a mirror flat finish. I used those silicone
insulators that don't use heat sink paste. 

BTW, do -not- do a belt and suspenders thing with
those silicone rubber insulators, heat sink paste will
cause them to degrade and just add more thickness to
transfer heat through.

Before that, the other techs had been just using mica
insulators and way too much heat sink paste, and they
had been no more reliable after their retrofits than
before.

> It will burn your fingers after about 30 seconds of
> continous power.

Way too hot. Semiconductors are normally rated to run
at a max temp of 75C, but that doesn't mean you
should. If a wet finger sizzles, yow!

> 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.

If you look at the specs, the 840 has a much higher ON
resistance which could reduce the Q.

The nice thing about MOSFETs is that you can just add
more in parallel. IE, stick a few IRF840's in series
to lower the ON resistance. As always, watch out for
parasitic VHF oscillations.

Steve Greenfield


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