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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Question about linear derating factor for MOSFETs
Date: Mon, 11 Nov 2002 07:38:35 GMT
Organization: MindSpring Enterprises
X-Server-Date: 11 Nov 2002 07:39:26 GMT
X-Newsreader: Forte Free Agent 1.21/32.243
Yea, I agree. The more I learn about this stuff, the more I realize
just how hard it is to move heat the way I want to.
I'm prototyping up a circuit using the IRFPS37N50A when they arrive
later this week for a lower powered device. But the Big Boy is going
to be using the IR FA57SA50LC MOSFET (TO-227 case). It has a fully
isolated package with a Rjc = 0.2 and a Rcs of 0.05 using just grease
(that maybe a bit optimistic IMHO), no insulator.
Hopefully I've done the math right (in another post) and it can handle
125W. I'll be using 2 of them for a 250W unit as soon as I learn more
about paralleling MOSFETS. I've reviewed a TON of app notes from IR
and Nat'l but almost all of the info is for reducing current
inbalances at much higher switching frequencies. Other than sharing a
comon heat sink and having a series gate resistor, I couldn't see much
more I could do. I certainly don't want to buy several of those
modules (at $50+ each) just to find two devices with matched Rds!
>>>The IRFPS37N50A is rated at 446 watts dissipation, and some of the
>>>TO-247s are even higher. I suppose you could get that if you mount it
>>>to an isotopic diamond block, but numbers like that don't seem to be
>>>practical using real-life heat sinking.
>>>At these power levels, insulators between the transistor and the
>>>heatsink are pretty much impossible.
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