Cooling heat from Power LED to metal bar

[vivitern]

I'm playing around with workplace illumination and have developed a 20 V "“> 38 V PWM'able constant current source to drive my power LEDs (max power about 64W). So far, so good. However, I've nearly thermally killed one LED by fixing it on a significantly undersized heat sink ("luckily", the wire contacts unsoldered themselves just in time, stopping the process).

Now, I'm considering cooling options. Wanting to avoid active cooling (i.e. the humming of a fan), I was considering the "lazy" way out (dimension far from final, I don't have a heatsink candidate yet):

I'd like to mount the 19 x 19 mm LED directly onto an aluminum bar or profile. Now, I'm already playing around with thermal simulation software, but that seems over the top (and so far, it mostly crashes, plus I have a lot of theory to catch up on).
One 60W LED is a thermal challenge because the heat source is small and very powerful. Therefore, you will need thick metal to spread the heat laterally into a large enough heatsink.

This is similar to a desktop PC cpu: small surface area, lots of power. Many desktop PC heat sinks use heat pipes to solve the heat spreading problem. A fanless PC heatsink should work.

However, this doesn't solve the other problem, which is that one 60W LED is a very bright point source, and it is not ideal for workplace illumination. It will be blindingly bright and will cast harsh shadows.

So:

Is there a well-known analytic model for heat distribution when attaching a constant-power heat source to a piece of metal?
if not, is there a go-to simulation software? So far, I'm playing with Elmer.
Is simulation the way to go here, at all, or is passive cooling damned for 60W LEDs?
Can I solve both problems by using LED strips like this TLHB4201, datasheet: http://www.kynix.com/uploadfiles/pdf9675/TLHB4201.pdf



Junction-Case Thermal Resistance 0.8 K/W
19x19 mm
max rated power 64.2 W
continuous power I'm planning to use: 36.6 V · 0.72 A = 26.352 W

 

[Zephrant]

CPU heatsinks are commonly available for 100watts or more and many have good specs posted for them.

I'd certainly start there.

I don't have any thermal sim software at the moment, so can't help you there. Post up what you find though, would be interesting to play with. I'm considering doing a DIY fixture for my 72g using 8020 extrusions and water cooling, so am keeping my eyes open for any similar information.

 

[AzMechE]

It sounds like you already have a solution. Go with the led strips.

Your assumption of needing a thick piece of metal is not entirely accurate. There are fundamentally two problems you have to solve. Getting the heat away from the chip and dumping that heat into the air. A thick piece of high thermal conductivity metal can get the heat away from the chip but in doing so also inhibits getting the heat into the air.

FEA/CFD would be a good option to optimize assuming you know what you are doing. If not it's likely going to be a steep learning curve and still have the issue of "garbage in = garbage out".

The "easiest" way to test might just be getting a couple different thickness of copper sheet to attach between the led and the heatsink. I would start around the 2mm thickness range.

Also, this is all assuming your heatsink can dissipate the power efficiently which I'n not going to dive into.

If this is for a professional project I would recommend consulting a Mechanical Engineer who deals with microelectronic packaging as this is exactly the type of problem they generally work on.

 

[oreo57]

I really need to pay more attention to specifications. Like....per se, dimensions....
Length 10"
Width 8". Or weight.....10bs!

72 Watts theoretical..Drivers on board..

 

[lingwendil]

Any more output and that thing could generate lift!

 

[oreo57]

Any more output and that thing could generate lift!

Photon warp drive..

 

[oreo57]

If this is for a professional project I would recommend consulting a Mechanical Engineer who deals with microelectronic packaging as this is exactly the type of problem they generally work on.

First they would design it correctly, then the bean counters would strip it down...

Secondly then its built and analyzed to death..

Thirdly, real world proves analysis is wrong, mostly due to parts suppliers fudging..

 

[blasterman789]

Unless you need stupid directional light I strong advise DIY'ers to look at things like Philips Fortimo lights or similiar because they negate the need for heat sinking. I just did a project where I retrofitted five 12x24' faux light bays that previously had been running 8' fluorescents. I just built wood frames and mounted the Fortimo's in the frames five at a time. No screwing around with heat sinking. With lumen requirements in the hundreds of thousands the savings on heatsinking for that project was HUGE.
For dealing with high wattage arrays like Bridgelux the Radial heatsink that Oreo57 showed is per mass the most efficient type of heat sink you can use for passive cooling of down firing LEDs. Wakefield has some about the size of a coffee can that can handle 30-40 watts, and I've cut them down with a band saw. Plate type heat sinks don't well for high wattage LEDs because there's little vertical convection. They are great when actively cooled, but radial sinks are by far the best design if you want passive.
Just built some directional spots using Cree XPH70s, and those used low RPM fans on beer can sized radials. Even a sink that size can't passively cool a XPH70 running at 25watts and requires some help.