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Old 11th September 2017, 05:30 PM   #51
doctormord is offline doctormord  Germany
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Quote:
Originally Posted by robinlawrie View Post
this sounds like you are suggesting that a larger heatsink wont keep the chip cooler.. ? in that case neither would forced air..?
My intentional saying is that the conducting area is not limited by the heatsink but the outputstage-FET-area of the TPA32XX IC. (Which is rougly ~15mm^2)

(A not unusual non-native-speaker understandingly problem - what a sentence..)
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Old 11th September 2017, 05:30 PM   #52
DPH is online now DPH  United States
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Because you're contacting a single 8 cm^2 section in the middle of the heat sink and assuming it will spread the heat out past the couple nearest fins, which is really doubtful. You'll have a massive heat gradient from the center of the heat sink to the fins at the front/back extremities.

Having two point sources and a horizontal heat spreader bar at least helps you get more out of the heat sink you have.

Edit--also what the good doctor wrote, although having a large aluminum block is going to help "absorb" transient heat better than the EVM heatsink.
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Last edited by DPH; 11th September 2017 at 05:35 PM.
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Old 11th September 2017, 05:34 PM   #53
robinlawrie is offline robinlawrie  United Kingdom
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Quote:
Originally Posted by doctormord View Post
My intentional saying is that the conducting area is not limited by the heatsink but the outputstage-FET-area of the TPA32XX IC. (Which is rougly ~15mm^2)

(A not unusual non-native-speaker understandingly problem - what a sentence..)
yes i understand that.. which is why i assumed that a thermal contact of 8cm square between aluminium bar and case heatsink would not be a limiting factor.. its already many times more conductive than the chip junction..


in any case, im going to look at increasing the spacing of the chips along the heatsink , see if i can get them to warm up different bits of the 'sink..


thanks for the input guys!
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Old 11th September 2017, 05:46 PM   #54
robinlawrie is offline robinlawrie  United Kingdom
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ok.. new version with top amp flipped round, and moved as far to back of case as possible.

this allows 2x slimmer aluminium bars. one 4cm x 1.2cm cross section bar for each amp.

heat should be spread more evenly across heatsink in this case.

in the end, even if i managed to find a magic way to mount both chips directly to heatsink, they would be very "hotspotty" due to the tiny chips!
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Old 11th September 2017, 06:12 PM   #55
DPH is online now DPH  United States
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Better!

Instead of skinny and tall, can you fit as large a square bar in between the large capacitors and other circuitry? E.g. 2 cm x 2 cm. Essentially trying to get as much horizontal coverage as possible. The giant slug of aluminum known as the bar will definitely help with thermal transients.
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Old 11th September 2017, 06:43 PM   #56
robinlawrie is offline robinlawrie  United Kingdom
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with the amps this way round, thats about the limit in the space ive got.. particularly the front amp, there is only 15 mm horizontally between the big caps on that, and the inductors on the back amp.. so i think thats about as far as i can go.

i need to wait till the amps arrive before knowing exactly how much space i have in reality.. could be a few mm out here and there!
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Old 11th September 2017, 07:32 PM   #57
strongbow60 is offline strongbow60  Sweden
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Why not use copper in the bar thats connected directly to the chip and then alu further out?
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Old 16th September 2017, 08:03 AM   #58
robinlawrie is offline robinlawrie  United Kingdom
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unless i am mistaken, its a bad idea to use copper and aluminium together in this way.. you get galvanic reactions.. heatsink is alu, so i should keep it all-alu..?

plus copper bar is rather costy!


edit: i was getting mixed up with watercooling setups, of which i have built many. there you get galvanic corrosion if you use aluminium and copper in the same water loop..

so, i guess this is doable.. but would it be worth the large extra expense?

Last edited by robinlawrie; 16th September 2017 at 08:05 AM.
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Old 16th September 2017, 08:36 AM   #59
doctormord is offline doctormord  Germany
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Almost every better CPU heatsink is made from aluminium + copper heatslug. Also most heatpipe constructions are mixed.
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Old 22nd October 2017, 07:09 PM   #60
robinlawrie is offline robinlawrie  United Kingdom
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Ok a quick update and a change of subject.

Re. Cooling, ive modified the design to use heatpipes, 2x 45w rated 6mm ones for each board. Contact along almost full length of heatsink. I had hoped to use a copper block on the chip side, but i require a machined block 2cmx5x5, and i was quoted 50 euros for it, so im going with aluminium. Hope that a) i can get a tight enough fitting hole drilled for the pipes that i can still slide them into. Guess thermal paste will help on both counts. B) that i can bend then without breaking any! Ive ordered the number i need, as they are not cheap.

Now onto grounding. Something written a lot about i know.

However i have a couple of basic questions before starting.

Im planning to star earth where possible. I assume that the pcbs should be on insulating mounts in this case, with a single earth going to the star point. However, the chips will be earthed to the case via heatsink. Should i put an insulating sheet between heatpipe block and heatsink, or keep that grounding path?

I was also planning to keep signal ground seperate, and use conductive sleeving over all twisted signal and mains cabling, (earthed to star point?) sound ok? Worth doing?

Finally, i have a minidsp and Bluetooth reciever running from 12v from main psu. I was going to isolate these from the chassis too.

Cheers!

Ps. Now i have all the bits, fitting the amps in will be a challenge! I ignored some components and case intrusions in my 3d layout. will need to remove rca jacks from amps and possibly bend some jumpers over 90 degrees. Sheesh.
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