Here’s the simple truth… White LED’s (for the most part) are really Royal Blue LEDs which are passed through a phosphor to convert the blue light into white light. As with ANY conversion, there is loss involved in the conversion.

So, lets start with a Cree Royal Blue LED like the XT-E.

Since, in this application, we’re talking about fairly light drive-currents, I’ll use the minimum published (most efficient) (350mA) numbers in this example.

Vf at 350mA: 2.85

Current: 350mA

That gives us 0.9975 Watts of input power.

Even the highest bin only has 550mW of output power (light) which means that 447.5mW of heat is generated.

The three bins of Royal-Blue output 500, 525 and 550mW respectively. It’s fairly safe to assume that those same power levels are in the White LED’s with a phosphor on top.

So, even the highest rated Royal-Blue LED is only 55% efficient at converting electricity into light. The phosphor on top will further decrease that efficiency (quite substancially, in fact). It’s a fairly safe bet that AT BEST, a white LED is about 35% efficient at making light from electricity. Hence, 12Watts of LED’s will probably produce about 7.8 Watts of heat.

And that’s at VERY low drive strengths. At higher currents, it get’s worse and fast.

At 1500mA (full drive), the same XT-e now figures like this:

Vf @ 1500mA: 3.37V

Current: 1500mA

That gives us 5.055 Watts of input power.

But output has only gone up by about 310%, so the highest binned Royal-Blue is putting out 1705mW of light.

Now the efficiency is calculated as: 1.705/5.055 = 33.7% efficient.

Add the phosphor on top, and you’re going to be darn lucky to be left with 25% light and 75% heat.

So an XT-E driven at 1500mA will generate about 3.8 Watts of Heat from about 5 Watts of power.

PPtk