Very low LED currents have disproportionately very poor, low light output?

Hi

We want to drive 350mA (average) through a Cree XT-E white LED. We want as much light output as possible.

What will be the difference in light output if we drive the LED with…..
..a) 350mA constant DC current (as in the red waveform attached)
..b) 350mA average but it’s a 500Hz triangle wave (as in the green waveform attached.)

The light_Flux vs Current graph at the top of page 27 of the LED datasheet (below) appears to suggest that the maximum “Light flux per Amp” comes at around 100mA.

Do you know what this curve looks like from zero Amps to 100mA?

Datasheet: Cree XT-E Lamp (350mA LED)
https://www.cree.com/led-components/media/documents/XLampXTE.pdf

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i have read that due to "Auger recombination", then when below a certain level of LED current, the light photons just cant all properly escape from the LED silicon, and so at low currents, there is proportionally very much less light output....so in other words, if you view page 27 of the Cree LED datasheet ......the unshown bit below 100mA is a region of very very low light flux output....thats why they dont show it.

Would you agree......if the bit below 100mA was just a plain extrapolation...then they would have shown it...but because it is in fact a region of (proportionally) very poor light output...they dont show it...for marketing reasons. Would you agree?

My experience is that you have to go to extremely low currents, certainly less than 1% of the rated current, before you see the light output drop off.

The effect varies between LEDs in the same batch, and varies with temperature. It is consistent with a leakage resistance across the LED.

I've got a desk light that I converted from tungsten halogen to LED, with 5 white LEDs in series. When I turn it off, the smoothing capacitor is quickly discharged to near the threshold voltage of the LEDs, so the current is tiny, and the LEDs carry on glowing for some time. Under those conditions one of the LEDs goes out before the others, but they are only just glowing at that point, while when turned on they are blinding if I look straight at them

Also, when I turn off and measure the voltages, the voltage across the dim LED is less than the voltage across the ones that are glowing. The voltage is such that the LED is off, and the current is being taken by whatever defect is taking the leakage current.

At any reasonable LED current, the leakage current will be insignificant.

I suspect that the manufacturers don't quote the light output at really low currents because there is inconsistency, and there is no commercial advantage to quoting it.

I would regard the light output per mA as being virtually unchanged from about 1% to about 40% of maximum current. Temperature and piece-to-piece variation will be larger.

Flyback said:

Do you know what this curve looks like from zero Amps to 100mA?

Click to expand...

first, what kind of gyrations do you propose the line would make between 0 and 100mA? See red line below and let me know why you even want to operate below 10% power

second, I don't suggest you look for tiny differences in an angle on a Datasheet. If the intern engineer that is assisting the Corporate Communications team to format the datasheet selects, in Excel, a Scatter plot with smoothed data rather than connect-the-dots scatter plot, the ends of the graphed data will have some unexplainable curves.

Third, an LED is usually given a "test current" at the highest current level in the most efficient region of the output vs current graph. Again, see red line. For this LED, 350mA.