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DC load using a CPU cooler

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microbug:
Oh, good!

Kevin.D:
You will get about max 150W from 2 of those if your Heatsink has a Rth of about .25 K/W (allowing NFet junction temp to reach ~130 C) .
150W is plenty  anyway  to test most things you will come across ( a typical bench supply 30V@5 A =150W) .Why do you need 300W ? thats alot of power .

p.s I have made a 150 Watt one and if I ever needed to extend the power range then it's easy enough to make a power range extender .(Say 4 * 50W 1 ohm resistors fixed on to the side of a can you can fill with water ) .
You could then put these resisitors  in series with your eload and the DUT so that they dissipate 50% of the total power and your eload drops the other half  .(i.e the resitors drop ~ half the total voltage ,( thats why youd have say 4* 1 ohm resistors permanetly fixed fixed to your water can so then you could wire them to have the exact required  resistance you needed to drop 50% of the total dut voltage . (i.e with 4 *1 ohm resistors  you can have 1/4 ,1/2,1 ,2,4 ohm  values by wiring them in various ser/par combinations.)
 

microbug:
The heatsink is rated at 320W power dissipation. Unfortunately, it doesn't specify efficiency in K/W, but I'd expect better performance than that! The SOA covers 300W operation easily; I'd expect some thermal derating, but that's why there will be two. I chose a 300W limit because the extra 20W of heatsink capacity gives a little headroom should things go wrong.

Anyone else care to weigh in?

Kevin.D:
Well I dont know about the heatsink you have chosen (does it's spec give a max temp at 320W dissipation ? if so then to get it's Rth is just   (temp@320W-25)/320.
I used 2 standard amd K6 sinks+fans  on mine they each had a Rth of .45 K/W each when I measured them .

Forget about these soa graphs they dont contain much usefull info for dc operation(they simply reflect the  Rth j-c if you look),manufacturers use them to produce nice banner spec headlines on there product's like '300W dissipation ' .They  produce them by emersing the whole transistors in a cooling fluid to hold the case temp at 25 and then measure junction temp rise wiht power dissipation .
 You can't do that of course  you have to cool your transitors in a conventional fasion .
So you have to take into account the extra thermal impedance of the Case to Sink(Rth c-s) an also + any insulating material you have between case tab and sink (which can add another ~ .2 C/W to thermal resistance ).

So  looking  at your data sheet for to264 case style we  have
Rth j-c   .28 C/W      (notice if we only had this one Rth we could get the SOA graph)
Rth c-s  .2 C/W + another .2 C/W if we use tab insulation  .
total Rth j-s =  ~ .7 C/W
now your thermal calculation is easy .lets say you have 150W power into that ,so that would be
150 * .7 = 105 C temp rise at fet junction above whatever your heatsink temp is at 150W dissipation .
since I dont know what the  sink you using is rated at , lets guess and say it's rated at ~ .25 C/W
so at 150W this will reach a temp of  = 150*.25 =37 + 25 ambient temp =62 C

So with a single fet at 150W your junction temp will reach 62+ 105 = 167 C (to HOT)
with two fets the j-s temp rise would be 105/2 = 52 C
so now your junction temp at 150W is 62+52 =114  C (so at least it's now it'sunder your 150 C absolute max for junction temp)  .

DONT take to much notice of manufacturers headline grabbing specs and soa graphs done under  liquid cooling conditions ,  Just gather your various Rth's and do the simple calculations yourself.

Spikee:
I tested two ixys75L2 mosfets on a arctic freezer 7 pro (150w max rated). and i was able to get the full wattage. The A30 heatsink should be twice as good. Around 0,01 - 0,005deg c/w should be possible.

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