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Old 9th February 2011, 01:40 PM   #21
Hiten is offline Hiten  India
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Don't know if this works and hope no one laughs, here is a basic concept.

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Old 9th February 2011, 02:16 PM   #22
marce is offline marce  United Kingdom
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What devices are you looking at, as they may be available in newer more thermally efficient packages?
For FETs etc there is a variety of new packages designed for heat removal from the die, for IC's and drivers QFN packages with thermal pads are becoming more prevalent.
Unfortunately the newer thermal packages are mostly SMD. The point of these packages is to create the lowest thermal resistance between the die and the outside world where the heat can be removed.
PCB layout and orientation when completed can have a big influence on the thermal requirements of a design.

International Rectifier - The Power Management Leader - DirectFET Home Page
TI DualCool™ NexFET™ Power MOSFET Technology -
PCB Thermal stuff.
EMS007 Designing In Thermal Management
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Old 9th February 2011, 03:27 PM   #23
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Default Why ???.

Originally Posted by uncle_leon View Post
I'm working on a new heat transfer idea for my future amp and I was wondering if anyone can help me with this.

In short, I want to get rid of everything I can, that sits between the heat source and the "heatsinkable" surfaces, on ALL sides of LM3875 except the one with leads. It doesn't matter if there will be voltages present on any of the exposed surfaces.
Dumb proposition from the outset.
Removing encapsulation will render the device photo sensitive and liable to damage to the bonding wires.
Just mount the standard device directly to a decent slab of copper which is then thermally coupled but electrically isolated from a standard large heatsink.
Fan cool as an extra if you have to.
Be sure to lap all surfaces before assembly to ensure best thermal coupling.

I believe not to believe in any fixed belief system.
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Old 9th February 2011, 07:31 PM   #24
michal is offline michal  Canada
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Originally Posted by uncle_leon View Post
The objective I'm aiming for is a way of dissipating heat that:
- will not use any highly (electrically) conductive materials, to avoid unnecessary Eddy currents
- will not use any high dielectrics, because they accumulate static charge
- will have good vibrational properties, i.e. will shield the chip from outside vibrations, while allowing it to release its own vibrations freely (see 47 Laboratory philosophy)
- will be capable of maintaining constant low temperature of the chip (obviously)

This may seem like an impossible combination, but I believe it can be achieved.
A diamond heat-sink should do it. With graphene layer in between.
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Old 9th February 2011, 08:03 PM   #25
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Just the other day I viewed a video (on YouTube?) where I guy was making a demo heat pipe from soldered copper tubing with a bottom reservoir partially filled with acetone. This should make a very nice phase change medium, because the boiling point is 134F (56.5C). The acetone won't heat above the boiling point until all of it has boiled away, and this provides a nice constant temperature sink for heat (phase changes suck up a lot of heat).

The problem with something that uses a lower boiling temperature fluid is that (a) these are not widely available to the public and (b) you will need a refrigeration system to cool down the condensing end of the heat pipe because indoor summer room temperature will be too close to, or above, the boiling point!

You could build a completely passive system using acetone, place something to transfer heat out of that system (like a heat sink) in to the environment, making sure to locate that where the acetone vapor will rise to.

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Old 10th February 2011, 12:37 AM   #26
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@marce - I have built numerous custom cooling solutions for PCs, most of them passive or semi-passive - including a fluid cooling system, and a sub-zero system. You obviously know your stuff and I see your points - but this is about an audiophile device, which means that numerous factors come into play, that are normally not a consideration - I listed them in my previous post. By vibration I meant vibration on microscopic level - either generated within the device, or carried through air/floor from the "outside world" (the speakers, most importantly).

The chip in question is the legendary LM3875; it does have a newer version (LM3886), but it comes in the same package, and I prefer the older one anyway

I like the idea of submersion cooling, but I think it may be difficult to find a fluid that: is neither too conductive nor too insulating, has both good heat conducting properties and low boiling point. And is reasonably accessible, stable, non-toxic and safe for handling and for the environment The best I could think of is 96% ethanol/4% water. This would be very weakly insulating, have reasonable cooling properties (worse than water), and a reasonable boiling point of 78C. It is also relatively harmless, although flammable.

I considered acetone, and to be honest, I would much rather use methyl acetate instead, because it smells a lot nicer Both have similar physical properties otherwise - including, unfortunately, very high flammability.

Phase-change coolant, while very efficient, presents a problem of variable pressure. An elegant answer to that would to leave some empty space in the container and evacuate air from it. This would reduce pressure, boiling point and the risk of explosion .
A diamond heat-sink should do it. With graphene layer in between.
Diamond heatsinks actually do exist (diamond composite, to be precise); they are just not very widely available. In fact, this is a part of the mysterious heat-sinking solution I mentioned in post#1 I'm waiting for a reply from the company that makes them If they could do a sample/short run for me at an acceptable price, I'll definitely go for it. It's not that expensive, apparently. I mean, we are talking tens of dollars for a small one, not thousands.

Last edited by uncle_leon; 10th February 2011 at 12:39 AM.
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Old 10th February 2011, 12:51 AM   #27
michal is offline michal  Canada
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How about graphite? Should be cheaper than diamond, however more fragile. I was looking for different way to heat-sink transistors of a Class A amp. I've settled on heat-pipes.
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Old 10th February 2011, 01:33 AM   #28
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There do exist graphite heatsinks (actual heatsinks; the diamond ones I mentioned above are more like heat-spreaders really). Apparently they have pretty decent thermal performance. Graphite is still somewhat electrically conductive, but nowhere near as conductive as copper, so it should be a good choice. Unfortunately, I haven't yet succeeded in finding a company that actually sells them...

Your amp looks kinda cool actually with all them coolers Although LM3875 dissipates relatively little power (some 30W p.c. at loudest setting) and normally only requires a moderate passive heatsink.

Last edited by uncle_leon; 10th February 2011 at 01:40 AM.
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Old 10th February 2011, 09:33 AM   #29
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Silver is the highest heat conductivity metal, but the base plate of the chip is still just copper.
you also still have the interface between the chip and the metal.

One thing that comes to mind is low temperature soldering, using a metal similar to bismuth.
You solder your amp chip to a plate of silver, and then cool that silver plate with liquid or thermoelectric cooling.

This letting the chip vibrate itself sounds a bit bogus though, along the lines of wooden volume knobs.

There is no way to get good thermal conductivity without electrical conductivity. Diamond is your only material thats magic like that.

Last edited by Rainwulf; 10th February 2011 at 09:36 AM.
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Old 10th February 2011, 05:21 PM   #30
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Default Beryllium Oxide

Originally Posted by Rainwulf View Post
There is no way to get good thermal conductivity without electrical conductivity. Diamond is your only material thats magic like that.
You have apparently overlooked Beryllium oxide (BeO).

Among non-metallic materials, BeO is second only to diamond in thermal conductivity, with k approaching 280 W/m/K. By comparison, metallic aluminum has a k of only 240 W/m/K, and copper slightly over 400 W/m/K. It is also a superb electrical insulator exhibiting vanishingly small dissipation factor, which makes it especially useful at gighertz frequencies. BeO is most commonly used for critical aerospace / military applications where electrical, thermal and mechanical performance are of paramount importance.

These BeO insulators would be perfect for an LM1875 or LM3886 based chip amp:

629879-01 BeO - 1.4" x 1" x 0.064" Beryllium Oxide Insulator

Not exactly inexpensive, but combined with a copper heat sink and appropriate thermal compound, they would be extremely effective.
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