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Old 12th October 2001, 04:12 AM   #1
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Looking at Pass' figures on the dissipation of an Aleph 5 I figure I would need a heat sink or mulitple heat sinks equaling .083 C/W to run at 50 degrees celcius. I found some big surplus listed at $42.50 but they're really $35.00. How many of these would I need (power consumtion is 300 watts)? If I buy new sinks how much should I be prepared to spend on them. Thanks for the imput.
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Old 12th October 2001, 04:15 AM   #2
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Sorry the link to the heatsink is

http://www.aaaim.com/cgi-local/shop9...HUN.htm#HD9800
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Old 12th October 2001, 06:40 AM   #3
paulb is offline paulb  Canada
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Keep in mind you have to add the thermal resistance of the heatsink (Theta_sa, sink-to-ambient) to the resistance of the junction to the case (Theta_jc, shown on datasheets) and the case to the heat sink (Theta_cs: depends on how you mount it, won't be much less than 0.5 C/W). Even an infinite heatsink would not effectively give you .083 C/W.

When using parallel transistors, calculate for one individual transistor. Divide the temperature rise (25 C?) by (300 watts divided by the number of devices): this will give the TOTAL thermal resistance (C/W). Subtract the Theta_jc and the Theta_cs. If you're left with a ridiculously small or negative number, you need more devices.
Other folks browsing this site will undoubtedly have some experience to say what they get for Theta_cs.

More info is at http://sound.westhost.com/heatsinks.htm.
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Old 12th October 2001, 07:15 PM   #4
Eric is offline Eric  United States
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I've looked at that same heatsink, and it seems that each one may be sufficient to dissipate somewhere near 125 watts. A rough eyeball indicates the heatsink is #61570 from R-Theta and is rated at 0.8 c/w per 3" length.

Get yourself some SilPad 2000 or SilPad k-10 from Bergquist to insulate your mosfets from the heatsink. Its rated at about 0.2 c/w which is excellent. Search the Digikey web site for SilPad.

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Old 13th October 2001, 12:40 PM   #5
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try to get the best isolating pads because you need the best to dissipate so much power and have your mosfets at low temperature. The best are kapton and have 0,07 thermal resistance.
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Old 13th October 2001, 02:24 PM   #6
Eric is offline Eric  United States
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Promitheus: Any idea where to find kapton insulators with this low of a thermal rating? The Bergquist silpads are the best rated that I've seen so far at 0.20 c/w...

Thanks, Eric

[Edited by Eric on 10-13-2001 at 10:18 AM]
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Old 14th October 2001, 11:06 AM   #7
djk is online now djk
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'promitheus' is in error.Kapton by itself makes a very poor isolating pad."The Bergquist silpads are the best rated that I've seen so far at 0.20 c/w... " I think you are correct here.These are a boron-nitride filled elastomeric. The only thing better would be BeO2 ceramic.But one thing to remember is that the 0.20*C/W is based on a TO-3 case with high mounting pressure.The highest performing of the Bergquist sil-pad series, the 2000, at 100PSI for a TO-220 case ranges from 1.65*C/W ~ 2.0*C/W depending on how thick it is.This kind of clamping pressure should not be applied with a single mounting screw but rather with a bar and spring clamp.The larger TO-247 packages can get by with a large bell washer. http://www.bergquistcompany.com/ther..._2000_2001.pdf
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Old 14th October 2001, 08:13 PM   #8
hifiZen is offline hifiZen  Canada
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hmm, i tend to disagree on that. Kapton is one of the best insulators known to man. It has by far the highest breakdown voltage of anything I've ever come across. Consequently, it can be made extremely thin, which is the secret to it's low thermal constant.

I think the numbers speak for themselves...
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Old 14th October 2001, 10:17 PM   #9
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I agree that Kapton is one of the best isulating material for power transistors. But use of very thin films needs perfectly machined, and perfectly cleaned heat sinks. Because of this, mass production uses elastomeric films, the elasticity of which is supposed to cancel surface irregularities. But thermal resistance increase dramatically.

Regards, P.Lacombe.
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Old 14th October 2001, 10:17 PM   #10
djk is online now djk
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"hmm, i tend to disagree on that. " Gee, you must have a really good lab.I mean better than the people that produce these things.Note that I gave a direct link to the spec sheet.Kapton film insulators by themselves, without the filled elastomer, have 0.3*C/W for a TO-3 package.The BQ sil-pad 2000 is 0.2*C/W and BeO2 is 0.17*C/W .Thermal compound with no insulator is 0.09*C/W ."I think the numbers speak for themselves..." They certainly do.promitheus said: "The best are kapton and have 0,07 thermal resistance." This is clearly incorrect for any case sized transistor used in audio.While kapton has good high voltage insulation properties it has poor conformal or gap filling properties, the secret to good thermal performance.Some of the filled elastomer insulators use a very thin piece of kapton film for voltage isolation but it is the binder/filler that makes them conduct heat well, not the kapton.The search for something to replace grease type thermal compounds was driven by two main items: 1)eliminating the grease itself because it boils off in space applications and contaminates wash solutions 2)it is expensive to re-finish the heatsink every time it is dis-assembled.The ZnO2 in filled heatsink compound is abrasive.In high performance situations wih hocky-puck devices, SCR driven inverters and energy management, two approaches are taken: 1)Gap filling compound, the white stuff 2)clear grease, It costs more to use clear grease because the heatsink must be milled to a TIR of 0.001" before mounting the device.ZnO2 filled compound fills the gaps well enough to use the the extrusion as received from the mill.It is abrasive enough that just assembling, dis-assembling once, and subsequent re-assembly degrades the thermal performance enough to justify re-finishing the assembly from a mean time before failure standpoint.I like the Grafoil washers from Avid where voltage isolation is not required.I like the Sil-Pad products from Bergquist where insulation is required.I use BeO2 where the circuit demands low capacitance to the sink.
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