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Mystery led heat transfer POWER LED

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Davidbarrel

Davidbarrel

Industrial
May 9, 2016
21
hi everyone
i made a test, i assembled two different type LED (citizen/cree) on the same heat sink, the power of the LED is same (36W) but in the first case i got a temperature (TC) of 70 °c in the second case i got 90 °c....so what the reason of this gap??

thanks
have a good dday
 
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Best to start swapping things between the two.

Also, check the Vf of each.

It might be something we can't see over the internet.
 
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thanks for the reply
i double checked, there are the same Vf, they work at 1000mA and have around 35.6 Vf.
you can find anything in internet :)

best
 
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I meant measure the Vf. As per your mystery, something funny seems to be going on. So you'll need to start checking and double-checking everything. In the physical space.

Swapping things between the two can be very helpful. When the 'fault' moves, you've got it.





 
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Perhaps one is more efficient at producing photons?
 
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Thank you, Dave... I was beginning to wonder if everyone had forgotten the basics.

Efficiency... no two LEDs have the same.

Dan - Owner
URL]
 
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"you can find anything in internet :)"

Does that mean you found the Vf on the internet or did you measure it in your test?
 
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What is your "mistery" LED part number? Your supposed Vf suggests it's a multiple LED stack, AND that there's probably a dropping resistor in series. All of them have variability.

Additionally, you say, " i assembled two different type LED (citizen/cree) on the same heat sink," so how did you ensure that the thermal conductances were identical, if at all?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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Efficiency wasn't forgotten. In my case, I discounted it somewhat based on the info presented. Or the detail not mentioned.

If there is a difference in efficiency, and given that they're both being fed with the same 36w of power, wouldn't the more efficient LED be noticeably brighter?

If one LED was noticeably brighter, presumably the OP would have mentioned it. Well, hopefully...

Furthermore, given the low overall efficiency ratio of LEDs in general, any % reduction in waste heat might be a several times larger % increase in light output. Because of the efficiency ratio.

This also ties into the fairly large temperature difference between the reported 90°C and 70°C, which seemed to me to be more than could be explained away by variations in efficiency; assuming that both LEDs are recent products of similar design. And again, given that the OP didn't mention different light output.

Still, you never know... Efficiency might be the answer. Or part of the answer.

Troubleshooting over the internet is often dependent on eliciting the unmentioned details from the OP.

 
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Yes i think like you Ve1bll, maybe the efficency is a part of the answer but the gap is too big.
The cree led is cxb2530 It has an output of 4200 lumen at 1050 mA and the vf misured is 35.6 v.
I used the same thermal pad for both
Also the cri is the same
 
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The eye's luminous response is logarithmic, and small changes in luminance are not readily discernible.

How are you "measuring" the Vf? If they both have the same Vf AND If, then they are running the same power, and then only other variable are your thermal pads

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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It looks like the Cree device is very sensitive to temperature. Not a surprise. Check page 6 of for current to maintain voltage drop as dependent on temperature.

See It indicates around half the power input into a Cree module generates heat; therefore losing about 18 watts.

If that is being dissipated at 70C into a 20C environment (because why would that be mentioned in the problem statement?) that's a 50C rise vs a 70C rise in the other case. That scales to 25 W or an efficiency difference of 25/36 or 30% efficiency.
 
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Mr. IRstuff are you ok?

i made a simulation......after i made thermal test in laboratory with the real Led!
 
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Davidbarrel,

You haven't supplied enough information. Is there an engineer at the company you are working with who can help?
 
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Is that the LED device temperature or the heat sink temperature?
 
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What you measured is Tc, case (package) temperature. Even if both LEDs had exactly the same power and efficiency, and thus die junction temperature Tj would have been identical, differences in the heatsink design would make Qjc different, hence the variation in Tc.
This tutorial from Maxim can help:
 
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