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From: "Phil Allison"
References: <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 17:44:42 +1100
NNTP-Posting-Date: Mon, 11 Nov 2002 17:34:56 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"John Muchow" wrote in message
> Thanks for the feedback. In my first repsonse to Phil, I mentioned
> that I'm limiting my load to 125-150 watts (I've since settled on 125
> watts) and was only setting up an example to ask if I understood the
> math for derating calculations. Sorry, I didn't make that clear
> The actual numbers I have for my application and the SOT-227 cased
> device I'm considering are:
> Max power dissipation = 625W
> Max operating junction temperature = 150-degrees C.
> Linear Derating Factor = 5.0W/degree C
> theta-jc, junction to case = 0.20-degrees C./watt
> theta-cs, case-to-sink (flat, greased surface) =.05-degrees C./watt
** That 0.05 figure will take some doing IMO. Maybe allow 0.2 as a
> theta-sa, heat sink-to-air = 0.34-degrees C./watt (assuming 18 CFM
** 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 spec
is likely based on an evenly spread over the mounting surface heat source
and not a small area in the middle.
BTW What fraction of the mouting surface are you coverig with the
SOT -227 ?
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