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Old 12th November 2004, 12:58 PM   #1
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Default Dividing heatsinks?

Hello!

I'm building an Alpeh-X and I've following question: Is it possible to divide the heatsink of the output MOSFETs into several heatsinks, so that each FETT will be mounted on an individual heatsink?
Is it right to calculate the heatsink as follows:
Assuming there are 100W to dissipate, dividing it by the number of FETs (e.g. four FETs), then each FET will dissipate 25W. Then the thermal resistance of each individual heatsink should be 1.2K/W. Is it right?

AudioAngel
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Old 12th November 2004, 04:08 PM   #2
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AudioAngel,

Yes you can... see:

http://www.passdiy.com/images/gallery/a75-p11-f3.jpg

my A75 was built that way, and it works great.
It's done with 2+2 IRF 232/ IRF 9231
on each heatsink .... with a total of 2*24 MosFets
There are no difference in temperature the only thing is to use angles or some sort of fixation to attach the mosfet's to, and be able to keep the
MosFet at the same temperature. There is also a very good example from Peter Daniel, find his design.

I need to know a little more of your Aleph-X for each heatsink calculation

Mono, or stereo, Number of fet's by channel.... Transfo VA ?

Is it 2*50 watts or 1*100 watts.... MosFet Types: xxx andvoltage of supply ?

Regards.

Alain.
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Old 12th November 2004, 05:14 PM   #3
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use the AXE spreadsheet on the wiki to calculate your junction temperatures. with four heat sinks, divide the K/W by four to get the net effective heat sink thermal resistance, like parallel resistors.

using your example, four 1.2 deg. K/W sinks and four fets at 25 watts and .7 on lines 66 and 67 (what I measured with silpads) gives a junction temperature of 106 C - a little too hot for reliable operation. You'd need four .8 deg. K/W sinks to get the junction temps down under 100C if you only use 4 transistors. doubling up transistors with the same total bias means that you can get away with smaller heat sinks. Four 2 deg.K/W sinks with two fets keeps junction temps under 100C. the sinks will be hotter, but the junction temp is what really matters.

Your proposed sinks will do nicely with 2 FETs at 12.5 W each per sink - 80 deg. junctions and 52 degree sinks.
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