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From: Spehro Pefhany
Subject: Re: Selecting a heatsink?
X-Newsreader: Forte Agent 1.92/32.572
Date: Tue, 14 Jan 2003 22:03:04 GMT
NNTP-Posting-Date: Tue, 14 Jan 2003 17:03:04 EST
On Tue, 14 Jan 2003 15:45:42 -0500, the renowned Glenn Ashmore
>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?
Right up to here: ^^^^^^
It's 4.7 degrees C above the heat sink temperature. You have to find
a heat sink that will hold the junction temperature to an acceptable
level under the maximum dissipation and ambient temperature. Most
of the rise is typically between the heat sink and ambient. The heat
may also increase the ambient temperature the heat sink sees.
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