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Subject: Re: Selecting a heatsink?
Date: Tue, 14 Jan 2003 22:57:30 -0000
NNTP-Posting-Date: 14 Jan 2003 22:57:35 GMT
X-Newsreader: Microsoft Outlook Express 6.00.2720.3000
Glenn Ashmore wrote:
> How do you properly size a heatsink for a MOSFET? What spec do you
> look at on the MOSFET data sheet to compare with the heatsink specs?
> For example: I want to run 6.5 amps through a MOSFET with a drain to
> source on resistance of .04 Ohms. I think the formula to get watts is
> I^2*R so I am generating about 1.7 watts. The thermal resistance is
> 2.2ēC/W junction to case and .5ēC/W case to sink for a total of
> 2.7ēC/W times 1.7 watts is 4.7ēC above ambient. Am I correct so far?
> That sounds mighty low.
Say you will settle for a 50C rise over ambient, and taking your 1.7W
50/1.7 - (2.2 + 0.5) = 29.4 - 2.7 = 26.7C/W, so any heatsink better
(lower) than that will do.
I'd be tempted to double that 1.7W, heat is generated in switching, as
somebody said. A 12C/W heatsink is pretty small & cheap.
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