lumen/watt goes down?

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mercrazy
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lumen/watt goes down?

using Cree’s product characterization tool, lumens per watt goes down as current goes up.
why is that?
thanks

mercrazy

Old Mate
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Heat

More current equals more heat.

For example take a XML2 u2 bin running at 700 mah at 25 degrees c will produce 342 lumens.
When its at 85 degrees c at the same current its rated at 300 lumens.

Heat is the enemy .

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Enderman
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More current = more heat
More heat = lower output
This applies to literally every LED, not just cree.

mercrazy
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sounds like DTP is a good idea for all LEDs.
how much does DTP add to cost of aluminum circuit boards?
when they publish their data, do they account for DTP or regular boards?
thanks.

mercrazy

Enderman
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mercrazy wrote:
sounds like DTP is a good idea for all LEDs.
how much does DTP add to cost of aluminum circuit boards?
when they publish their data, do they account for DTP or regular boards?
thanks.

The data for LEDs is measured at a certain junction temp, it will say on the datasheet.
It has nothing to do with what type of PCB it is.
You could have a non-DTP PCB with excellent cooling or a copper DTP PCB with horrible cooling, what matters is the temperature right at the LED.
mercrazy
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how can i be sure what i’m getting from my board manufacturer?
do i just take their W/m-K rating since i don’t have a way to test?
thanks

mercrazy

Enderman
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mercrazy wrote:
how can i be sure what i’m getting from my board manufacturer?
do i just take their W/m-K rating since i don’t have a way to test?
thanks

There really is no exact way to know how it will perform without testing.
Copper DTP is the best, which is why most people here use these: http://www.international.mtnelectronics.com/index.php?route=product/cate...
ggf31416
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It isn’t just a matter of heat.
Even if temperature is keep constant the efficiency goes down as the current increases. It’s called droop