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Old 14th December 2011, 12:57 PM   #11
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Thanks, I usually get the heat paste all over my clothes, instead of the heatsink.

Anyway, are there any advances to 40W without a fan for a MOSFET?

(30 deg C ambient)
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Old 14th December 2011, 01:05 PM   #12
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If you can electrically insulate the heatsink from everything else then you can lose the mica insulator which will improve the thermal conductivity.
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Old 14th December 2011, 01:29 PM   #13
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A typical 175degC 200W FET mounted on a large passive heatsink can be operated at far above 40W.
The sink certainly does not need to be infinite, nor fan cooled.
A FET with Tc = 100degC (175degC - 25degC) is rated at 50% of Pmax, i.e. 100W.
A 0.2C/W sink operating at Ta=20degC will run with delta T ~ 100W * 0.2 * DF = 50*DF.
For 50Cdegrees DF ~1.1
Expect that sink to run ~ 20+60 = 80degC.
Tc = 100degC
Ts = 80degC
Delta Tc-s = 20Cdegrees.
The required Rth c-s <= 100W / 20Cdegrees <= 5C/W.
Any sensible 200W device will give <5C/W for Rth c-s

Now substitute your own numbers, to find what limiting value of Rth c-s is required to ensure the 200W device can operate within spec when dissipating well over 40W.
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Last edited by AndrewT; 14th December 2011 at 01:32 PM.
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Old 14th December 2011, 02:32 PM   #14
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If you have a fet dissipating in the neighborhood of 40W or beyond without a fan, you'll tend to have ambient temperature around the heatsink in excess of 30C.
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Old 14th December 2011, 02:42 PM   #15
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That is not the way to use, nor interpret, the datasheets and app notes that give guidance of the use and cooling of FETs.
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Old 14th December 2011, 03:05 PM   #16
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Well you can't just take the ambient temperature outside of the entire chassis. Assuming this PSU is enclosed you have to account for a temperature rise to some extent and 30C ambient would be a fairly small rise. I'm actually thinking the more conservative figure would be closer to 50C ambient.

For example we can assume that if this were an open frame PSU sitting out on a desk, the fet is going to be cooler running than if it were in a metal box with a few vent slits here and there.

Last edited by !; 14th December 2011 at 03:07 PM.
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Old 14th December 2011, 10:13 PM   #17
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The applications engineer from powerint.com thinks 5W is the maximum you can do with a heatsinked TO220 FET..............


When is a fan needed to cool a FET? | Power Integrations

i am thinking about heatsinks as the five on page 4 of this....

http://www.powerint.com/sites/defaul...les/rdr203.pdf

...a FET on that kind of heatsink could only do 5W max?
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Old 14th December 2011, 10:35 PM   #18
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^ Can't tell how tall those heatsinks on the PDF are, too lazy to see if there's a BOM in the document to look up heatsink part #'s, but yes that's a roughly 5W capable heatsink... with a poor design putting electrolytic capacitors so nearby, I would not want 5W dissipation that close as it will probably fail prematurely as a result unless... you add a fan.
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Old 14th December 2011, 11:20 PM   #19
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Thanks

I have picked the Aavid Thermalloy 530002B00000G heatsink for TO220 which is Rth_sa = 2.6C/W.

My FET is STF11NM80

http://docs-europe.electrocomponents...6b80dbfe7e.pdf

This has Rth_jc = 3.6C/W

So therefore, with 30degC ambient, if i want to keep the FET junction below 100degC then i can dissipate 11.3W in the FET.

Would you agree with this?
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Old 15th December 2011, 12:28 AM   #20
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I agree with those numbers, but I would design in more margin, not wanting 100C die or assuming only 30C ambient at that power level... this is where chassis design and passive-active cooling come into play and even then, the finished product may end up in a non-ideal environment (room temperature, stacking components, dust may accumulate, etc.) hurting thermal dissipation.
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