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Old 24th March 2006, 09:21 AM   #1
yoke is offline yoke  Croatia
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Default Fischer Elektonik SK41 Heatsinks

Hi
I have couple of those heat sinks but I don't know should I trust this spreadsheet from Fischer

It says that at 150mm C/W should be around 0.28, so that one heat sink should be enough for 100W ( 100x0,28=28*C above ambient so it would be 24+28=52*C )

But I have some bigger heat sink then those and they get 60-65*C at 100W so I’m not shure should I trust this spreadsheet?...does any one have experience with SK41?

Thanks
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Old 24th March 2006, 01:53 PM   #2
yoke is offline yoke  Croatia
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It's fischer elektronik not elektonik :-(

here is pdf file

http://www.fischerelektronik.de/fisc...l_PDF/A_70.pdf
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Old 26th March 2006, 01:13 PM   #3
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Default Fischer

You can trust the Fischer Data, as they rate conservatively.

Uli

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Old 26th March 2006, 01:49 PM   #4
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Fischer assumes in their calculation vertically placed profiles, and 80 degC temperature difference between ambient and heatsink. Efficiency goes down as the temperature difference drops, but not by a factor of 2.

Patrick
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Old 27th March 2006, 06:17 AM   #5
yoke is offline yoke  Croatia
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Thanks for replay

If I understand this right this 0.28 is for 20*C ambient and 100*C on heatsinks !!!
To get this for normal operating temperature I should use some factor which is less then 2...

I have read some place that people use 1.3 for Conrad heatsinks !?!

So in my case if I use factor of 2 it will get me to 2x0.28=0.56 and for 75W it will be 42 above ambient ... whell that’s hot
But if that could be like 1.3 then it's 0.37 , 75Wx0.37=28*C above ambient and that is accessible, I hope !

Will have to see when I place mosfets on them

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Old 27th March 2006, 09:43 AM   #6
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Aavid quotes a factor of 1.3 :

http://www.aavidthermalloy.com/technical/correct.shtml

But for the heatsink I am using, if I go from dT = 80° to dT =40°, I shall have to use factor of 2. This depends on the heatsink geometry, especially the length and the spacing between the fins.

The safest is to ask the manufacturer, quotes the amount of heat you want to dissipate, the the desired dT.

I did, for my heatsinks, and the measured temperature is within 2° to calculated. Worth asking.


Patrick

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Old 27th March 2006, 10:16 AM   #7
yoke is offline yoke  Croatia
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I have wrote to Fischer couple days ago, and recive replay todey

This si my question:

I have buy some of those heatsinks and have doubth do I have right spreadsheat for this profile.
It say that SK41 for 150mm have koeficijent of 0.28*C/W so for 100W of disipation it would have 28*C above ambient, is this right ???
I ask this bacause I have similar heatsink which is biger then SK41 and it gets realy hot when it nead to desipate 100W, it heats at about 60*C ( 20*C ambient ) ( at 20*C ambient SK 41 should rich 48*C ).

Replay:

According to our technician Mr. xxx, I can tell you that your calculation is absolutely correct.


So now I don't know...I think I will have to build it and see...
Patrick, thanks for the info, I didn't know before that C/W in datasheet depend on dT...
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Old 27th March 2006, 10:46 AM   #8
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The relationship between difference in dT and difference in thermal factor of a heatsink is 4th order.

Suppose the thermal factor A is measured at dT=80 C.
You want the thermal factor B at dT=40 C.

=> ln (80/40) = 4 ln(B/A)
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