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From: Glenn Ashmore
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Subject: Re: Selecting a heatsink?
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NNTP-Posting-Date: Tue, 14 Jan 2003 17:58:07 EST
Date: Tue, 14 Jan 2003 18:00:15 -0500
Walter Harley wrote:
> The reason it probably sounds low to you (other than the fact,
> observed by others, that you've not considered sink-to-ambient
> resistance) is that the steady-state fully-turned-on situation you
> describe is the best case. Where things get hot is when that
> MOSFET has to switch back and forth between "on" and "off"; if it
> does that a lot, e.g., in a switching power supply, it dissipates a
> lot more power.
> In other words, if you really are only dissipating 2W with a nice
> big power MOSFET that's probably capable of handling 100W or more,
> you're right, it's not going to get all that hot.
That was really my question. I can find junction to ambient (62ēC/W
Max) but not heatsink to ambient. The Wakefield catalog just says 65ēC
@6W at "typical load" for natural convection for the biggest heatsink I
can fit on the board. I don't know how to translate that.
This is a marine lighting application. Typically the MOSFET will turn
on once a day and stay on for 6 to 8 hours. I am driving it at close to
max Vsg so I am not so worried about rapid heat rise. Just long term
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
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