# Design guidelines for arrays of 10 or more rebels



## snarfer (Feb 21, 2008)

I'm designing some lights that will have relatively large numbers of Luxeon rebels in them. These are lights for film/theatrical use and have to have very high CRI, 92+. So I'm doing a little bit of color mixing, at least three different colors on the board, and at least ten LEDs total per board, but eventually a lot more. I need a lot of light!

I am pretty confident in my controller designs, however now I need to design PCBs for mounting arrays. I've seen a lot of discussion here about mounting rebels on little disks of three and so forth, but haven't found any hard information about larger arrays, as in, "this is what I did and the parts starting smoking and magic dust came out" or "it worked and was great."

According to the Philips datasheet it is possible to mount the rebels as close as 4mm apart, or 2mm apart with fewer vias. Well OK, maybe that's alright with two or three, but what happens when you try to mount 100 rebels at 4mm spacing? Has anyone tried it?

I really need the LEDs to be pretty close together, as I want to mix colors without using extra optics.

I will be monitoring ambient temperature and forward voltage on each string with a microprocessor (dsPIC). However it would kind of suck if I ended up being limited to 300mA each or something.

Then there's the whole metal core pcb versus FR-4 thing. After everything I've read on these forums I had to conclude that the best, and certainly most cost-effective, approach is to go with FR-4 with as little epoxy and as much copper foil as possible. I've found a supplier that, if you believe their website, stocks FR-4 down to 0.1mm thickness (no, I'm not going to actually order it with 0.1mm thickness. That sounds ridiculous) and foil up to 127um thick. They also can handle the very small vias in the pattern.

OK, but will that be enough? These are still going to be expensive boards and they need to be right the first time. Also should I plan on additional heatsinking? Are fins really necessary? 

Ideally I'd like to be able to forgo the heatsink and just place my control circuitry behind the array, with a suitable airspace for cooling. Is that realistic? Or should I plan on building a separate box for the controller?

Anyway these are the issues I'm confronted with right now. Experimentation on this scale is pretty expensive, so I'm hoping that some of you guys might have a little experience to share.

Thanks,

David


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## IMSabbel (Feb 21, 2008)

If you mount those rebels at 4mm spacing, you will get at 1A driving current about 20W/cm^2 heat dissipation.
Modern CPUs have had Values up to 5 times that amount, _and_ have much lower maximum junction temperatures. 

Its all a matter of heatsink design. If you take a _real_ one, you could handle 100 of them easily.
(with real one i am talking about a copper plate with large surface aluminium fins mountet on top of it (or better connected by heatpipes going through perpenticular fins) + forced air cooling. Not just a metal puck like people use in flashlights.

As long as you can get as many joules/s away as the leds produce, there is no limit to scalability.

(and yes, you can simlpy forget about putting 10 highly driven rebels on a metal core PCB without a heatsink)


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## jtr1962 (Feb 21, 2008)

I had these made to mount single Rebels:



























These are made of 0.04" thick FR-4. The Rebels were soldered to the boards, and the boards were thermal epoxied to a heat sink. I'm not the end user, but the end user tells me there are no issues running them at 1 amp.


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## snarfer (Feb 22, 2008)

jtr1962: Wow! Those look great. What software did you use to design the PCB in these pictures? 

IMSabbel: Thanks for putting the heat issues in perspective. I hadn't thought of comparing it to a CPU. In fact this leads me believe that maybe using an actual CPU cooler would be a valid approach, especially because these things are mass-produced and therefore cheap.


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## knabsol (Feb 22, 2008)

Hi!
I´m building a pant light and have 42 rebels in an array. They are mounted on a 0.5mm aluminium plate and about 1½" away from each other. They are mounted with arctic silver epoxy and they are currently driven at about 350mA. First I had them driven at 700mA but the heatsink felt a little bit too warm after a couple of hours of use. At 350mA was a huge different in temperature but not that huge difference in light output.

My lesson from this has been that I would rather mount more of them and drive them at a lower current.


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## snarfer (Feb 22, 2008)

An inch and a half is a lot more than 4mm! But it sounds like in your case the problem is less in heat getting out of the LEDs and into the heatsink, as the heatsink itself not having sufficient surface area, right?

I found some old Pentium II heatsinks, They are about 2 inch by 5 inch with lots of fins, more than an inch high, and have a nice flat back for mounting on the board. The P-II dissipates approximately 30 watts, so I guess that these would be designed for that.

With 4mm spacing and a little extra for running traces I get 12 rebels into an approximately 14 cm squared area. So at 1 amp that would be 36 watts. But not all of that is turned into heat. I think something like 60 percent is actually emitted as light, depending on which bin. so really i would only need to dissipate 14 watts of heat. Maybe in fact I could mount twice as many rebels on that one heatsink. Correct me if I'm wrong on that one...I'm not sure how the 20 watts/cm square was derived in the message earlier in this thread.

In fact, now that I think about it, it should be possible to find a p4 heatsink designed to dissipate over 100 watts, although I would really prefer to do without the fan.


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## jtr1962 (Feb 22, 2008)

snarfer said:


> jtr1962: Wow! Those look great. What software did you use to design the PCB in these pictures?


Nothing special-just the old DOS version of Protel Autotrax which is currently available for free download in several places. Even though the software is over 15 years old, it's still surprisingly useful.


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## SemiMan (Feb 22, 2008)

Those CPU heat sinks are highly effective because of the fan. Without the fan, the effectiveness goes down considerably.

If you want to have your LEDS really tight and drive them hard, your FR4 design needs to be really good and you need to do things like vias filled with solder\arctic silver in order to get the heat transfer as you essentially would need to pull it from right under the part with little room for thermal vias outside the part. That is where metal core boards comes in. It has advantages where space is at a premium.

Solder is about 7-10 times more conductive than Arctic Silver by the way for the same thickness.

Are you planning to mix in Amber / Red? Keep in mind their output drops a lot with temperature.

What are you planning to do for the optics?

Semiman


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## snarfer (Feb 22, 2008)

Unfortunately I have one very important design requirement: complete silence. So I can't put in a fan. I just found an interesting article on heatsinks here: http://sound.westhost.com/heatsinks.htm

The advice to use more parts at lower amperage sounds like a good idea too. I think the rebel is still pretty much the best $/lumen value among LEDs, even at 350mA.

The reason I ask about the software is that I've been trying to do this in Eagle and having a hard time with creating the complex curves in the soldermask layer. Actually I have Protel already, but just never went to the trouble of learning it, so maybe this will be a reason to use it. 

Initially I had planned to make the lights compatible with the triple rebel optics sold by Polymer Optics. However, really high CRI color changing has to be RGBA, four colors instead of three. I saw another optic that mixes color from five rebels, but is designed as a snap-in for a mr-16 replacement. It didn't seem like it would fit well with a flat panel design. So I decided to forgo optics at this stage and just cluster the parts really closely. They will be much closer than in those optics anyway. Also for my purposes wide angle illumination may be a good thing. And most likely there will be some diffusion applied as well.

Even though I might not always need to mix four colors, it makes sense to build the arrays and controllers with four color mixing in mind, because then I can just keep it to one controller design.

Semiman: I've seen this written in another thread, that it helps to actually fill the vias with solder. I was a little confused though, when I read in the Philips datasheet "The two smaller via's inside the thermal land have a finished diameter of 0.25mm. The drill hole for a 35 micron PTH thickness will be 0.32mm. The solder mask area around the vias is needed to avoid solder to flow through the vias to the backside. This would lead to a reduced heat sink contact of the PCB bottom surface. " 

I guess they are just talking about solder leaking out the backside during reflow. They also mention the thickness of the plating on the inside of the vias as a parameter with positive correlation to thermal conductivity, so filling the vias entirely with solder seems like it would help.

And yes, I will definitely be soldering the rebels to the PCB. Actually it seems like reflowing the parts would really be the only efficient way to put this together.


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## frenzee (Feb 23, 2008)

When this becomes available, you really should do some thermal modeling before spending the bucks.


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## IMSabbel (Feb 24, 2008)

SemiMan said:


> Those CPU heat sinks are highly effective because of the fan. Without the fan, the effectiveness goes down considerably.



Not necessarily true.
You can discern those types easily by looking at the fin distance: 
small distances and rather thick fins-> only usable with forced air
distances>5mm, heatpipes getting heat to all fin levels->no fan really needed.

I currently run my Core2Duo (>60W max heat) without a fan, on one of those coolers:
http://www.silentpcreview.com/article251-page1.html
So i guess the thermal resistance from the copper base is (without fans) better than 0.3 K/W.

This would allow for about 20 Rebels to be put on it.


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## SemiMan (Feb 24, 2008)

IMSabbel said:


> I currently run my Core2Duo (>60W max heat) without a fan, on one of those coolers:
> http://www.silentpcreview.com/article251-page1.html
> So i guess the thermal resistance from the copper base is (without fans) better than 0.3 K/W.
> .



Your Core2Duo is >60W max, but unless you are gaming (which you may be), it often runs at lower power. That said, it is also sitting inside a computer case. Likely that computer case has a 50-150cfm fan (computer case and the power supply) which is evacuating not that big of a case. That means you still have pretty reasonable air flow.

Here is a data sheet of the type your describe ... at least close http://www.aavidthermalloy.com/products/microp/AAVID_037888.pdf You can see the data sheet starts at a fairly high air flow. With no air flow, the heat sinking capability drops a lot.

Here is another example http://www.aavidthermalloy.com/cgi-...gth=3&airflow=57.2&CType=Natural&AirUnits=LFM

The spacing between the fins is large, and they are somewhat thick so heat will make it to the end of the fins... notice the difference that air flow makes.

Semiman


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## NoahLD (Feb 24, 2008)

I'm also interested in creating a fixture for film/ theatre and understand many of the design issues. At LDI (entertainment industry lighting convention)in Orlando this year, a company called Selador showed a prototype fixture that used seven different colors of Rebels packed into a cluster of at least 100. The fixture had on board electronics, passive cooling, and dmx control. It was the first theatrical fixture I've seen that had Rebels. This is an interesting project. Thanks


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## IMSabbel (Feb 24, 2008)

SemiMan said:


> Here is a data sheet of the type your describe ... at least close http://www.aavidthermalloy.com/products/microp/AAVID_037888.pdf You can see the data sheet starts at a fairly high air flow. With no air flow, the heat sinking capability drops a lot.
> 
> Here is another example http://www.aavidthermalloy.com/cgi-...gth=3&airflow=57.2&CType=Natural&AirUnits=LFM
> 
> ...



Sorry,but those two coolers have next to nothing in common.
Look at the dimensions:is 9x6cm, with 11cm height, using 3 heatpipes, and less than 2mm fin spacing (if i counted them correctly). 
The scythe ninja his 12x12x15cm, has 12 heatpipes, and (iirc) 5mm fin spacing. the wider spacing makes for a aerodynamic resistence thats at least an order of magnitude lower (allowing effectice convection cooling). Sure it has copper, but the tight space requirements for server cases just dont make this design competitive for non-forced air cooling. just like i said in my original post.



And yeah, i know my cpu heat dissipation. With an open case, cutting airflow from casefans drastically, and running both cores at 100%, the tips of the heatpipes are _slightly_ warm.

But to cut this long talk short: 
There _are_ very efficient heatsinking solutions out there, for prices that are far lower than any custom build solution could ever be, thanks to economy of scale in the cpu buisness.
You just have to select the right one for the job you want.


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## snarfer (Feb 25, 2008)

I knew I couldn't be the only one working on this. The Selador.net site is an interesting read. I'm not sure that mixing with all 7 possible colors would be all that efficient, but it's most likely a good way to get around the Color Kinetics patents. 

However, addressing the subject at hand, Selador x7extra fixtures pack 32 3 watt LEDS into a 6 inch by 12 inch space with only passive cooling. That's a concrete example of what's possible. Thanks for a great find on that one.

Regarding the heatsinks. In fact the Pentium II heatsinks appear to be designed for passive cooling, based on the pin spacing and structure, and of course the lack of any provision for mounting a fan. That said, I think the QLED application looks like something worth waiting for. Any idea on when that will be available?


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## Stillphoto (Feb 25, 2008)

I haven't been actively working on this recently, but after my first exposure to the Lightpanels lights I asked around here about reverse engineering them for less. Also started to figure out some alternative led light systems for the film industry.

As far as silent cooling, I've seen many a technocrane on set with a ton of little silent fans running to cool the hydraulics...That said there has to be an active cooling system quiet enough to at least aid in a standard passive heatsink. Especially with the large panel design, it should be somewhat easy to integrate an array of fans into the rear panel.

Keep me posted, sounds like fun. I'm going to assume you're working your device being dimmable without the use of pwm, or at least at a rate that won't cause flicker.


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## IMSabbel (Feb 25, 2008)

snarfer said:


> Regarding the heatsinks. In fact the Pentium II heatsinks appear to be designed for passive cooling, based on the pin spacing and structure, and of course the lack of any provision for mounting a fan. That said, I think the QLED application looks like something worth waiting for. Any idea on when that will be available?



I would be careful with heatsinks of CPUs that old.
Because back then, many were only in the 15-30W range, with larger heat emmittnig surfaces than modern PCs (the original klamath P-II had 203mm^2 die size, plus 2 external chips for the L2 cache. 
Also, those Slot 1 moduler were very wide to begin with, and the fin spacing could also be just the result of a quick "how much surface area do we need" calculation.

(also important for passive cooling is the orientation of the fins, of course. Anything but vertical will hurt cooling efficiency.)


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## snarfer (Feb 25, 2008)

It appears, from looking at some "silent PC" sites, that larger fans are quieter than smaller fans, for moving same amount of air. Even so, fans at the base of the technocrane are one thing, fans built into lights that might be right next to the talent, or, worse yet, mounted on the camera, are another thing entirely...

As far as flicker-free LED dimming is concerned, I think that is fairly well-known technology at this point. There are plenty of interesting and royalty-free techniques to be found, for example:

[SIZE=-1]www.*artistic**licence*.com/app%20notes/appnote011.pdf 

As long as the PWM or PFM or BAM operates at a high enough frequency the camera shouldn't see any flicker. I'm sure that anything over 20kHz would be safe, but that is probably excessive. Plenty of cheap microcontrollers can do that with more than 10 bit resolution. Some with much higher resolution. 

Here is one DIY project with DMX and 9 dimming channels for less than ten dollars:

http://www.hoelscher-hi.de/hendrik/english/led.htm

My thoughts in beginning my investigation into this were exactly the same, how to build my own Litepanels for cheap. I had already built my own Kino-Flo Diva lights and I wanted something similar but less bulky, and lighter. Unfortunately, what I found out is that Litepanels really aren't that great. My dimmable Fluorescents are putting out over 15,000 lumens on a 250 watts of power, but the Litepanels can barely put out 800, and they cost twice as much as the Kinos. Well 8 times, if you compare them to my hack versions.

However, with new LEDs coming out that rival fluorescents in efficiency, it's beginning to seem like this might really be something worth building.

There are of course a lot of other unknowns to consider, for example what is the best way to create high CRI white light with LEDs? 


[/SIZE]


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## SemiMan (Feb 26, 2008)

While the micro may be able to generate a 20 KHz 10 bit PWM, where you are likely going to run into trouble is on your current source. A simple linear current source can turn on/off and regulate at very high speeds, but a switch mode current source can not. You can get it to turn on and off quickly, but regulating with some level of accuracy at high speed is another issue. On the Artistic License document, though BAM requires in theory less processing power, the resolution in time in which one must be able to control the current source is still the same. If you use a switch mode current source, make sure you use one that has a high switching frequency and go for plain PWM. Avoid controllers that have PFM modes or hysteretic control. The other thing you will want to do is use a small output capacitor.

On the patent front, if you ever consider to commercialize, NEOPAC has a patent on heat pipes to cool LEDS. I have not read it in a while so I am unsure of the breadth of it.

IMSabbel, look at page 4 of the review of the heat sink you recommended: http://www.silentpcreview.com/article251-page4.html . With no fan, and vertical orientation of the heat sink (not ideal, but real world and perhaps outside your control in theatre lighting), at a 70watt load (approximately), the temperature rise was over 49C and then their PC locked. With just a little bit of forced air flow (which is far more than you will get convection) the thing performed stellar. It is a great heat sink and with a near silent fan, probably one almost impossible to hear more than a few feet away, it would be ideal for lighting. However, without some forced air flow, you need to be cautious.


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## IMSabbel (Feb 26, 2008)

SemiMan said:


> IMSabbel, look at page 4 of the review of the heat sink you recommended: http://www.silentpcreview.com/article251-page4.html . With no fan, and vertical orientation of the heat sink (not ideal, but real world and perhaps outside your control in theatre lighting), at a 70watt load (approximately), the temperature rise was over 49C and then their PC locked. With just a little bit of forced air flow (which is far more than you will get convection) the thing performed stellar. It is a great heat sink and with a near silent fan, probably one almost impossible to hear more than a few feet away, it would be ideal for lighting. However, without some forced air flow, you need to be cautious.


a) " Without a fan, and with the motherboard sitting horizontal on the test bench" -> very bad for convection.
b) Ok, the K/W may be worse than my guess, but mine is build in vertically. And 50C temperature rise aint that bad for LEDS (they dont have a maximum junction temperature of 70C like intel chips).


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## SemiMan (Feb 26, 2008)

IMSabbel said:


> a) " Without a fan, and with the motherboard sitting horizontal on the test bench" -> very bad for convection.
> b) Ok, the K/W may be worse than my guess, but mine is build in vertically. And 50C temperature rise aint that bad for LEDS (they dont have a maximum junction temperature of 70C like intel chips).



For me, the killer has been keeping Reds\Ambers somewhat cool so that I could actually get enough light out of them. I use thermal feedback in my application to compensate for the light drop.


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## snarfer (Feb 27, 2008)

> While the micro may be able to generate a 20 KHz 10 bit PWM, where you are likely going to run into trouble is on your current source. A simple linear current source can turn on/off and regulate at very high speeds, but a switch mode current source can not. You can get it to turn on and off quickly, but regulating with some level of accuracy at high speed is another issue. On the Artistic License document, though BAM requires in theory less processing power, the resolution in time in which one must be able to control the current source is still the same. If you use a switch mode current source, make sure you use one that has a high switching frequency and go for plain PWM. Avoid controllers that have PFM modes or hysteretic control. The other thing you will want to do is use a small output capacitor.



I had to read this a couple times before I realized what you were talking about. A PFM controller with a constant on (or off) time might not regulate predictably. On the other hand, I just got an LED controller from Maxim that has a built in 20 kHz dimming circuit. I don't see why that wouldn't work, given that it seems to be designed for exactly this application.

In my case I am building a circuit right now based on a DSP with four current control PWM loops, and then BAM on top of that. Well I don't know if it will necessarily work that way, but at least it will be hardware enough that I can test some different approaches. Also it seems like the DSP approach has some advantages in terms of being able to do some overall temperature compensation by changing the comparator threshold voltage on the fly.


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## snarfer (Feb 29, 2008)

*The New QLED app*

The QLED software for thermal design of LED arrays is now available. I got a download link for the "trial version." Out of curiosity I clicked the purchase link just to see how much the full version costs. Get this:

"*Purchasing Instructions:*
The price of a single annual license for QLed is US$10,000. Floating and other licensing options are also available. "

I think I can afford to melt a lot of LEDs for that kind of money.


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## SemiMan (Feb 29, 2008)

Believe Flotherm costs us about $25K, not to mention the cost of someone who can actually use it...


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## LukeA (Feb 29, 2008)

SemiMan said:


> For me, the killer has been keeping Reds\Ambers somewhat cool so that I could actually get enough light out of them. I use thermal feedback in my application to compensate for the light drop.



Is active thermoelectric cooling an option?


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## snarfer (Mar 1, 2008)

I think with active thermocooling you would end up with an LED lamp with the efficiency of an incandescent. Peltier coolers take a lot of power.


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