Heat Sink Help?

Doer

Doer

Well-Known Member
I am trying to understand heat sink needs for LED. But, some of them are sold per inch like at heatsinkusa.com and some are quoted by Watts, and some calculators I've found end up in cubic vol. some end up with total area including fins. Then I get number swim.

The concept is you don't want to exceed 90% of Junction Temp on the Spec.

That is balanced by how cool can you keep the heat sink. That is often given as distance per second of airflow.
Well, fans are measured in Cubic Feet per Minute.

To do this right I know you have to add up all the C per watt (C/W) including the thermal pad, Junction and Heat Sink, and then you get the answer. And you have to apply that answer to the airflow converted....swim...swim..

But, I how about the other way? I ask for your help.

This heat sink has the bottom plate room for several of the Cree CXA3590.

Rosewill RCX-Z200 92mm Ball CPU Cooler
by Rosewill



So, my question is the advisability of running 2 3950s (72vF) @ 100w each, on this heat sink?

I really don't think I am exceeding junction temp since it will be in a Cool tube type tunnel, and I will add a lot of exhaust CFM on one end and cool air on the other.

Thoughts?
 
JavaCo

JavaCo

Well-Known Member
Not enough room for more then one cxa3590 on that heat sink. You would be hard pressed to get two CXA3070's on it. Thats a Pentium 4 heat sink so the max wattage they made Pentium 4 was 115 watts on the extreme edition. Kinda doubt it will be able to handle 200 watts
 
REALSTYLES

REALSTYLES

Well-Known Member
Doer said:
I am trying to understand heat sink needs for LED. But, some of them are sold per inch like at heatsinkusa.com and some are quoted by Watts, and some calculators I've found end up in cubic vol. some end up with total area including fins. Then I get number swim.

The concept is you don't want to exceed 90% of Junction Temp on the Spec.

That is balanced by how cool can you keep the heat sink. That is often given as distance per second of airflow.
Well, fans are measured in Cubic Feet per Minute.

To do this right I know you have to add up all the C per watt (C/W) including the thermal pad, Junction and Heat Sink, and then you get the answer. And you have to apply that answer to the airflow converted....swim...swim..

But, I how about the other way? I ask for your help.

This heat sink has the bottom plate room for several of the Cree CXA3590.

Rosewill RCX-Z200 92mm Ball CPU Cooler
by Rosewill



So, my question is the advisability of running 2 3950s (72vF) @ 100w each, on this heat sink?

I really don't think I am exceeding junction temp since it will be in a Cool tube type tunnel, and I will add a lot of exhaust CFM on one end and cool air on the other.

Thoughts?
Click to expand...
Just get two of them and be on the safe side. One CXA each 2 on 1 heat sink I see heat issues. I have 8 on a 7.29" wide and 24" long with a 200mm fan and it get to 36c take in mind I'm running 413 watts so that little heat sink would get hot and why not use the Arctic 11 like Supra and others have used?
 
Doer

Doer

Well-Known Member
The arctic 11?

Well, it will only hold one, for sure.



I guess what I am looking for is a 250w heat sink with a pad large enough for 2 x CXA3590.

My other idea is a get a longer heat sink (5 feet long?) And put 3 pairs of 3590s on it Since that will the bottom of a high speed wind tunnel can be the main structure the square metal conduit is attached to.

The problem is, as you said, these are CPU coolers and they don't have large pads. Even a 400w cooler will only have small pad.
 
Doer

Doer

Well-Known Member
O I C!! A 200 mm fan? That is different, at over twice the size.

I will plow thru this, the old fashion way. It is just rocket science, I know, but numbers hurt my head. It is why I prefer Binary.
I may as well, thrash it here in this thread. Each of the pieces in the system, add heat. That was a big confusion for me, but it now makes sense. So, for example, the gap filler will only pass most of the heat, it retains some.. So, the total retention is the heat that must be radiated including the retention of the heat sink itself.

Hypothesis:

6 x CXA3590 can be driven at 600w on the same heat sink but is it practical?

Need to calculate:
- length and width of heatsink (hs)
- profile of hs (hsP)

From:
- thermal resistance of hs (Rh)
- thermal resistance of gap filler (Rb)
- thermal resistance of COB module (Rj-c)
- max case temp (Tc)
- max junction temp (Tj)

Verses:
- Average ambient Temp
- Output Temp of exhaust flow
- Air mass over heat sink per minute

From there I will use charts like this one to see where I am at to 1/2 life my Emitters. (better emitter coming soon)




The thing is, Air has a Celsius rise per watt rating, the C/W, also, that must be corrected for density and humidity. Lucky for me I am at sea leave and low humidity.

So, the entire heat engine is calculated so the Air Speed through the tunnel is always fast enough to keep the case and junction Temps in the bounds I choose from the chart. The idea is to only get to 90% of that value else my fail safe cuts power to the array.

http://www.mechatronix-asia.com/LED_heat_sink_calculation_simulation_thermal_design.html



Here is the example of how the C/W math works. At this point you multiply by the Watts of work you intend to dissipate.

The only thing missing now is the needed thermal resistance of the heat sink Rh.


Choose a heat sink with an Rh value of < 4.65°C.

OK. Let me get back to you on this. :)
 
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