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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Question about linear derating factor for MOSFETs
Date: Mon, 11 Nov 2002 07:55:55 GMT
Organization: MindSpring Enterprises
References: <email@example.com> <firstname.lastname@example.org>
X-Server-Date: 11 Nov 2002 07:56:45 GMT
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>>> ** That 0.05 figure will take some doing IMO. Maybe allow 0.2 as a
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 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 ?
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.
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