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From: "Phil Allison"
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
Subject: Re: Question about linear derating factor for MOSFETs
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Mon, 11 Nov 2002 19:02:45 +1100
NNTP-Posting-Date: Mon, 11 Nov 2002 18:52:56 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"John Muchow" wrote in message
> >>> ** That 0.05 figure will take some doing IMO. Maybe allow 0.2 as a
> >>>worst case.
> Ahh...OK, this is the good stuff I need to know. It's so easy to just
> trust the data sheets for the thermal compounds and MOSFETs for this
> kind of stuff.
> >>> ** I think you have not allowed for the small area of the mounting
> >>>surface that you will be actually using. The maker's degrees / watt
> >>>is likely based on an evenly spread over the mounting surface heat
> >>>and not a small area in the middle.
> >>> BTW What fraction of the mouting surface are you coverig with
> >>>SOT -227 ?
> You're right, I assumed even coverage (I do remember seeing that in
> one of the specs) when I started and never went back and factored in
> the device being smaller thanthe heat sink. Are there rough guides as
> to how to take device size vs. sink size into account? If it matters,
> there is a fan mounted directly over the bonded-fin heat sink I was
> considering, with a rated CFM of 2x over what was called for in the
> datasheet to give the 0.34 theta-sa value.
> I guess if the derating (for a large difference in device vs. sink
> size) makes a big difference then multiple smaller devices spread
> across the sink is a better idea. I really like the idea of the screw
> lug terminals of the SOT-227 case...no need for a PCB for the high
> power part of the circuit.
> The SOT-227 is approx. 1x1.5" and the heat sink mounting surface is
> 4x4". I think this is 9.4% coverage.
** I hope the mounting base is VERY thick.
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