# Thermal Management Demystified?



## DBO6 (Jul 14, 2012)

I've been toying around with some side-lit fibre optics and high powered LEDs (Cree XML-T6). I've resorted to making my own light engine enclosures as seen below. 















I'm really struggling with thermal management and was hoping for some advice on what I can do to get temps under control. 

The modules are running from regulated power at 2.5a and the temps are getting very high (almost too hot to touch). I've recorded around 60-70 deg.C after about 15minutes of operation. I haven't pushed the boundaries much more than that but was hoping for a lower overall temp so I could run these for an extended time. 

I've even tried adding extra heat sinks (2nd pic above) and putting in a variable PCM controller to see if I could get any improvement but nothing major has shifted. 
I've read the Cree thermal management guidelines but I'm just mystified about what the best options are to get temps back down. Does pulse modulation play a big role? Should I go with bigger heat sinks? etc.

As I said, any help or advice on the topic is much appreciated.


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## Walterk (Jul 14, 2012)

Wow, nice effect those fibres!

Looking again, I think they are just quite small. They get hotter then I expected for the size.
Does it run cooler when leaving the endcaps for the fibres off?
If it does, it tells you you need more exposed surface, fins.

Make cuts and thermal glue alu strips or plates to attach fins to the body?
Or bore hole holes and place copper or alu pins in it, seems you have some spare volume under the star.

Paint it matt black when its finished.

In the end its all about watts going in, and say 80% percent of this energy has to be radiated out of the aluminium. Maybe more because you have less transparancy in the front window.


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## DBO6 (Jul 14, 2012)

Thanks for the tips. The thing that has me confused is a comparison to an XML torch I've got. I took the light module out of that and it runs cooler at 2.8a than my drivers at 2.5. The light engines are machined from billet alloy and are 35mm diameter with another 40mm heat sink on the base. They have way more thermal mass than what's in the torch 

The only major difference between the torch and these enclosures would be the driver and that's why I thought it could be some kind of active thermal management like better PWM control. Other than using regulated power I've got no driver running the modules so could it be the lack of one that's causing my issues perhaps?


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## moozooh (Jul 14, 2012)

Take some pages from CPU cooling industry's book:
— use materials with better thermal conductivity (copper);
— make heatsink's surface area larger (preferably by making fins thinner and more numerous);
— heat can be delivered to farthest parts of use heatsink by heat pipes (this is tricky though);
— contact surfaces must be polished and covered with thermal paste or thermal glue.


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## Lon (Jul 14, 2012)

My first thought was with moozooh, use copper with fins milled in. Somewhere around 300% increase in thermal sinking I think.
Very cool effect by the way.


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## Walterk (Jul 14, 2012)

If you can take reliable measurements from the current for the torch and current for the fibre-lights...
Every set-up has different losses in wires, DMM, cells, driver, etc.
Looking forward to see the big copper finned housing  .


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## DBO6 (Jul 14, 2012)

Don't have any 35+mm round bar copper handy. Its faily pricey stuff in that size from memory so I might have to keep that as a later option. The contact seats between pcb and the housing are already polished and coated with arctic silver paste. 

I was just so surprised given the torch performance and figured I must be going wrong somewhere.

I was going to try an adjustable strobe circuit next to see if bringing the frequency of the lights down to 80 or 70 herz provides some improvement. A small peltier between the housing and the rear heat sink was another concept gnawing at my mind. I've even got another great circular heatsink I bought from eBay which supports a 10w led running 24/7 at close to 3 amps. Only downside is its size (about 75mm dia.). I'd like to keep the light housings as small as possible.


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## Yoda4561 (Jul 14, 2012)

you'll still need to heatsink the hot side of the peltier, and due to efficiency losses it'll need to be bigger than a heatsink adequate for direct attachment to the light engine. A small 40-60 mm cpu heatsink/fan combo might be a better option, look for chipset and older CPU coolers. Getting some active airflow over the heatsink unit is the best option if you want to keep things compact.


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## DBO6 (Jul 14, 2012)

Any ideas on what would be an acceptable long term operating max temp? I remember reading that junction temps of 90 deg C were still Ok but that sounds very high.


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