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Old 13th January 2012, 12:43 AM   #101
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Quote:
Originally Posted by opc View Post
What I'm still struggling to find is a decent gap filling thermal pad that doesn't cost a fortune. I had two scrap pieces of 0.2" thick high compliance stuff from work and it's pretty much perfect as it forms around the buffers and makes good contact with the PCB which is where all the heat goes.
@454Casull see bolded section above ...

The buffer is 0.18" high and opc wants to couple heat in pcb to the heatsink

Last edited by hochopeper; 13th January 2012 at 12:49 AM.
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Old 13th January 2012, 01:06 AM   #102
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probably there is something on the pcb thats .2" higher than the buffers

edit: nope, just checked, yes opc, why is it there is a thermal gap that needs filling? from the standoffs if you use them between pcb and heatsink?
the thermal surface is facing the board instead of the heatsink. He's just trying to get heat out of the board by bonding the board to the heatsink using some kind of a "sponge" type thermal pad.

Do
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Old 13th January 2012, 01:24 AM   #103
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Hi Guys,

I've attached a picture that shows the basis of the problem.

The thermal resistance from the die to the outside of the case on a TO-263 pack is very high. It's of minimal use heatsinking to the top surface of the packages since the drop across the molding compound itself is so high.

The tab and copper plane it's soldered to, on the other hand, has very low thermal resistance.

Ideally, we should aim for a gap filling pad that actually has the TO-263 packages cut out of it, and then apply a small amount of thermal paste to the top of the packages and let the gap filling pad conduct heat from the top copper layer up to the heatsink.

To be honest, it's not too big of a deal unless you plan on running these full out for long periods of time. I'm running my pair without any heatsink and they're just fine (if not a little toasty at about 58C) even during moderate use.

The reason for the 0.2" height is that the buffer packages themselves are 0.180" high and the 0.2" height allows the gap filling pad to conform well around the edges and down onto the PCB itself.

Cheers,
Owen
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File Type: jpg IMG_7022.jpg (141.6 KB, 522 views)
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Old 13th January 2012, 01:25 AM   #104
qusp is offline qusp  Australia
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Doh, of course. hey Owen, love the pimped out half brick recommendation. i use their BMR-C1 on individual to252, great stuff

i like the ribbed HC6158, but perhaps the rods have better thermal performance i'll have to check. GB anyone? i'm also thinking of contacting Richard at sumR Magnetics about a custom potted transformer to suit, as i know he has a few cans he can use to suit and will have one that will fit. we would need someone in Canada or the US to take delivery of the initial bulk order and send them out. i'm going to start a thread in the GB area in a moment for all the potential group buys we can do for this build.

the buffers and lme49990 are an obvious addition and i mean the lme49610 as i cant see a downside for using these, even if you dont need the extra swing right now. pretty sure we could buy direct given the numbers we could possibly get. with 40 boards even if only 30 of us threw in thats 480 buffers and 180 lme49990 i would buy some extras of both at the price we would get. then we have heatsinks, transformers and the main 1210 size ceramics. also LT317/337 or LT1085/1033 and the power supply caps

so who would we contact about getting a gap filler custom Die? and would it become a realistic solution with a number of us to get the cost down? you may be ok running in open air in Canadian winter, but boxed up in a 40c summers day in Australia doesnt sound like fun

Last edited by qusp; 13th January 2012 at 01:31 AM.
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Old 13th January 2012, 02:22 AM   #105
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What about custom machined heatsinks that have pockets machined out for the buffers?

Edit: Like this:
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File Type: jpg LPUHP Heatsink.par.jpg (174.6 KB, 494 views)

Last edited by HYPERTUNE; 13th January 2012 at 02:34 AM.
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Old 13th January 2012, 02:29 AM   #106
opc is offline opc  Canada
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Quote:
What about custom machined heatsinks that have pockets machined out for the buffers?
Even better! Count me in

I would actually get in on a GB for the heatsinks and the LME49610 parts. Possibly transformers as well.

I like the initiative! Keep me posted.

Cheers,
Owen
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Old 13th January 2012, 02:35 AM   #107
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Originally Posted by qusp View Post
Doh, of course. hey Owen, love the pimped out half brick recommendation....so who would we contact about getting a gap filler custom Die? and would it become a realistic solution with a number of us to get the cost down? you may be ok running in open air in Canadian winter, but boxed up in a 40c summers day in Australia doesnt sound like fun
How about installing the buffers vertically and soldering them to copper bars. The heat sink will go on top. You can double the buffers this way by soldering them in 4 rows.

On a second thought, the design is great as is
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Old 13th January 2012, 02:39 AM   #108
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Concept model attached above.

Give me some dimensions and I'll get on it.

What do you think about having the heatsink threaded and screwed on from below the board?

Edit: Rev1.1
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File Type: jpg LPUHP Heatsink V1.1.par.jpg (302.5 KB, 469 views)

Last edited by HYPERTUNE; 13th January 2012 at 02:57 AM.
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Old 13th January 2012, 03:07 AM   #109
opc is offline opc  Canada
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What you have modeled would honestly be the ideal heatsink for this. Good contact with both the top of the buffer case and the PCB would allow the heatsink to provide the highest possible benefit.

The idle power is actually extremely low considering the performance (~4W for the output stage), and even at full tilt the dissipation isn't that bad. I personally would sleep better at night knowing that the buffers were as cool as possible though.


Quote:
What do you think about having the heatsink threaded and screwed on from below the board?
Again, this would be ideal... threading from the bottom makes for much easier installation and a cleaner look.

I can send you the dimensions for the board, screw locations, and buffer height/location.

Would you be willing to take on making 30-50 of these? I would certainly be in for 6 of them.

Cheers,
Owen
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Old 13th January 2012, 03:21 AM   #110
qusp is offline qusp  Australia
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Originally Posted by exa065 View Post
How about installing the buffers vertically and soldering them to copper bars. The heat sink will go on top. You can double the buffers this way by soldering them in 4 rows.

On a second thought, the design is great as is
sounds fiddly wrt lead bending and you lose the effectiveness of the pcb, which already does a fairly good job. interesting idea, but construction would be a pita and perhaps expensive. glad someone mentioned under the board as a possibility, but i was going to wait till i had the boards before thinking about that. the solution Owen and Mark are talking about sounds ideal, but how do we avoid shorting the conductive parts and components? by anodising the heatsink? this is regarding the underside mounting

perhaps a slab of keratherm isolator between pcb and heatsink and just avoid the very front where the lme49990 are?

the only problem i see with the bottom is it will increase the height of the completed unit
I do LOVE the copper heatsink look

Last edited by qusp; 13th January 2012 at 03:27 AM.
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