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Old 25th February 2012, 11:21 PM   #1321
udailey is offline udailey  United States
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try making the plate thicker.
The placement of the mosfets could be poor to but you can change that by extending their reach with wires.
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Old 25th February 2012, 11:25 PM   #1322
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For starters : 0.45 C/in2/W makes >1 C/W for the body size of a TO-247

1 C/W is in the order of what you get for a TO-220 mica.

(for high fin density, you might enjoy this one : http://frigprim.x10.mx/articels2/parallel_plchim.html )
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Old 26th February 2012, 12:44 AM   #1323
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jerfer - that sounds about right, bro. it's tough. I suggest you head to the market section of these forums and track down the Keratherm pads. They're super high-tech german blah blah blah but the bottom line is that once your transistor is torqued down they have a thermal resistance of 0.07 C/W (per square cm maybe? it's still exceptionally low.)
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Old 26th February 2012, 01:10 AM   #1324
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SilPad isn't That bad.

Suppose the software assumes 0.45 C/in2/W for the SilPad, it can calculate the thermal resistance of the insulator by taking the total contact cross sectional area of the power device, or just the metal surface of the mounting side.

125C junction -51C heatsink = 74C between junction and heatsink.
74C divided by 30W makes 2.47 C/W thermal resistance from junction to heatsink.
2.47 minus 0.89 is 1.58 C/W for the SilPad.
0.45 C/in2/W divided by 1.58 C/W makes ~0.3 square inches.
Pretty much corresponds to the area of the metal surface.
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Old 26th February 2012, 02:07 AM   #1325
jerfer is offline jerfer  Canada
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Quote:
Originally Posted by udailey View Post
try making the plate thicker.
Do you mean the heatsink's mounting plate? I'm interested in some sinks that are more efficient than the Rtheta profile I simulated. They have thicker base-plates (1/2" instead of 0.230") and higher fins (3.5" instead of 1.7"). I wanted to see how they would perform knowing that the simulation would be more conservative using the Rtheta profile.

So, do you think that with the thicker base-plate I'll get lower junction temps? If so, cool (yeah, that was a pun alright).

Quote:
Originally Posted by udailey View Post
The placement of the mosfets could be poor to but you can change that by extending their reach with wires.
I thought about and simulated with the FETs placed all in a row and ended up with worse performance: junction temps were ~4 degrees higher and heatsink temp under devices was 4.5 higher. To be honest I didn't expect that since I've only ever read that the ideal placement is in a row and not stacked.
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Old 26th February 2012, 02:24 AM   #1326
jerfer is offline jerfer  Canada
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Quote:
Originally Posted by jacco vermeulen View Post
For starters : 0.45 C/in2/W makes >1 C/W for the body size of a TO-247

1 C/W is in the order of what you get for a TO-220 mica.
So, do you think the SilPad can do better than what R-Tools assumes?

Quote:
Originally Posted by jacco vermeulen View Post
(for high fin density, you might enjoy this one : Natural Convection and Chimneys )
That what I was talking about with my initial question! I had posited that the sink be mounted laying flat but it was the stack or chimney effect I was trying to describe. The nice thing about this is that you can get away with less heatsink. Although you still end up with the total height because of the chimney.
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Old 26th February 2012, 02:25 AM   #1327
jerfer is offline jerfer  Canada
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Originally Posted by derwhalfisch View Post
jerfer - that sounds about right, bro. it's tough. I suggest you head to the market section of these forums and track down the Keratherm pads. They're super high-tech german blah blah blah but the bottom line is that once your transistor is torqued down they have a thermal resistance of 0.07 C/W (per square cm maybe? it's still exceptionally low.)
Will do. Thanks for the pointer.

--edit
I just modeled it using the Keratherm's Rth and WOW! Junction temps are down to 80 C or below. So, how can anyone build a J or anything with this much bias per FET and mount it using anything but this?
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Last edited by jerfer; 26th February 2012 at 02:40 AM. Reason: Updated simulation
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Old 26th February 2012, 02:35 AM   #1328
jerfer is offline jerfer  Canada
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Quote:
Originally Posted by jacco vermeulen View Post
SilPad isn't That bad.

Suppose the software assumes 0.45 C/in2/W for the SilPad, it can calculate the thermal resistance of the insulator by taking the total contact cross sectional area of the power device, or just the metal surface of the mounting side.

125C junction -51C heatsink = 74C between junction and heatsink.
74C divided by 30W makes 2.47 C/W thermal resistance from junction to heatsink.
2.47 minus 0.89 is 1.58 C/W for the SilPad.
0.45 C/in2/W divided by 1.58 C/W makes ~0.3 square inches.
Pretty much corresponds to the area of the metal surface.
I modeled it using only the metal surface area of the mounting side. I was trying to be as conservative as possible while remaining reasonable. Ignore my last question to you. You just answered it.
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Old 2nd May 2012, 01:12 PM   #1329
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Default heat sinks used

Quote:
Originally Posted by ma_coule View Post
... again ... and again...
& for fun!
Hi,

Sorry I have not been looking here for a few weeks. The heatsinks I am useing are two for each mono Aleph J-X, each of the two with a value of 0.33 degC/W, these also have one 5in. fan each attached to the each heatsink, the two heatsink fans are fed by two additional 5inch fans cooling the power supply components by having the two fans mounted inside the enclosures first & second barrier and the air is sucked by the first chaimber power supply components, these fans then blow the air through the three next air tight compartments and finally becomeing the source air for the power transistor heatsinks quietly keeping the output transistors with a tempiture of below 60degree Celcius while using a high bias.

here is the heatsink I am useing:
http://canada.newark.com/aavid-therm...ts/dp/10WX1020

Thank You For Your Interest

Ian

Last edited by SCAIFF001; 2nd May 2012 at 01:14 PM. Reason: forgot to include the adderss to my heatsinks for review
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Old 2nd May 2012, 01:24 PM   #1330
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Default mounting the front end board

Hi everyone,

I am just about finished and am now completeing the final wiring of the amp, the inputs of the front end are for ballanced input ( IN+, GND,IN-),
is the "SG" input for the RCA input ground ?

Thanks For Any Help

Ian
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