Statistical spread of LED forward voltage groups?

[treez]

What is likely to be the statistical spread of the occurrence of the various forward voltage groups of the osram oslon LED (forward voltage groups shown on page 5...)

https://www.osram-os.com/Graphics/X...CSSRM1.EC - OSLON Square (EnglishDeutsch).pdf

Are the number of leds produced in each forward voltage group likely to be the same. (over years of different production batches being produced)

 

[D.A.(Tony)Stewart]

It looks like you are trying to buy open bin LEDs and hope they will remain constant.

What is the distribution now? It is unlikely to stay constant.
Gruppe (min.) V F [V] (max.) V F [V]
K2 2.70 2.80
L1 2.80 2.90
L2 2.90 3.00
M1 3.00 3.10
M2 3.10 3.20

Quality improvements will shift the average down and reduce the spread, while cost reductions may increase the average and spread. But this is just a guess. I think this information is proprietary internal info only.

What do you hope to save? benefit?

On the other hand, I can supply LEDs from other factories with constant voltage +/-0.1 for as long as required along with tighter specs on colour temperature.
Also when I get specs from some factories on my orders of say 10k pcs each batch is matched to within 1mV when it comes from same wafers.
What exactly are your requirements?

 

[treez]

we have parallel array with leds tight on mcpcb. (8 strings of 15-in-series)....no equalisation resistors. 48V is rail voltage for buck switcher. The high vf ones mean our buck runs out of duty cycle.....so we want only the low vf ones

 

[FvM]

This seems to be a serial topic. I'm not sure if new facts came up in the meantime.

https://www.edaboard.com/threads/288757/
https://www.edaboard.com/threads/287807/

 

[KlausST]

Hi,

no equalisation resistors

usually LEDs have a very low ohmic series resistance (dU/dI), voltage may drift with time and temperature.
even with voltage binned LEDs i expect problems with constant brightness
good luck...

Klaus

 

[chuckey]

If it was economic the factory would make their LEDs within say 2.7 - 2.8 Vf. So you would expect a normal distribution within this tolerance with say 1 in 10^5 falling outside it. So when they make them with an overall voltage of 2.7 - 3.2V with again 1 in 10^5 falling outside it, then select them for the various bins. The centre of the spread would be 2.95, so the group, 2.9 -3.0 would be equally spread +- around 2.95, but the 3.0 - 3.1 bin would have more 3.0V then 3.1 and the 3.1 - 3.2V would have more 3.1 Vfs then 3.2V. This used to be the way of batching transistors on Hfe. The problem is that if you order 10K of the high Hfe type the factory would have to make 10^6 transistors most of which would be mid range of the Hfe so the price would rise !!
Frank

 

[Audioguru]

I have a cheap ($3.99) Chinese flashlight that has 24 white LEDs connected directly in parallel, a 1.8 ohm series current-limiting resistor and three AAA cells for power. I replaced the poor quality "super heavy-duty" carbon-zinc cells with name-brand alkaline cells. All the LEDs appear to have the same brightness so they must be matched pretty well. Maybe you should buy some of these cheap flashlights and use their matched LEDs for your project.

 

[treez]

I alsohave one of thise lights.....mine was £7 from asda. So I was ripped off by the middle man because they don't pay the Chinese that much for them.
Yes they are in parallel...but those leds have resistance infused into them so they can be paralleled....

 

[D.A.(Tony)Stewart]

They don't add precision low R resistors, but the gold bond wire in a 5mm LED may be a couple ohms.

The ESR of the chip in a 5mm LED is related to chip size and process tolerances. This results in. +/--30% ESR tolerance which results in the Vf variations at rated current over time and between wafer suppliers. As I stated before a single wafer batch can yield extreme tight tolerances, much tighter than needed for parallel operation. I suspect they sort their batches to combine them.

 

[Audioguru]

I bought a few of those $3.99 Chinese flashlights for my son-in-law, my son and me from a store, not from China. The store probably bought them from a wholesaler for about $3.00 each. The wholesaler probably bought them from China for $2.00 each. You got ripped off because 7 British Pounds is $12.63 Canadian dollars.

I don't think they have resistance in the LEDs because they are very bright and the series current-limiting resistor for all of them is only 1.8 ohms. If they had resistance inside then the 24 LEDs would not need a series 1.8 ohm resistor.

 

[treez]

the bulk semiconductor resistance is increased in each led...just enough to enable parallel operation. Because the leds are in parallel, each one carries very little current. So they infuse the semiconductor deliberately to increase its resistance....then they don't have to think about having all the parallel leds for a lamp having to come from the same wafer.....and box of leds, from the warehouse, will do.

My Asda light had 72 leds ...three groups of 24 in parallel...and each of the three had a 10 ohm resistor in series with it....running off 2 series 1.5V cells.

 

[Audioguru]

My cheap Chinese flashlight has 24 white 5mm LEDs in parallel and a 1.2 ohm resistor in series with the group. Its three Energizer AAA alkaline battery cells measure 4.65V before use. When the 24 LEDs are turned on the battery voltage drops to 4.48V and the 1.2 ohm series resistor has 0.546V across it so the total current is 0.546V/1.2 ohms= 455mA and the current in each LED averages at 455mA/24= 19.0mA.

The flashlight also has 3 white 5mm LEDs in parallel at one end with a 2.2 ohm series resistor. When the 3 LEDs are turned on the voltage across the resistor is different each time (cheap switch) but is usually about 0.46V then the total current in the 3 LEDs is 0.46V/2.2 ohms= 209mA and the current in each LED averages at 209mA/3= 70.0mA. These LEDs produce no smoke (yet).

The forward voltage of the white LEDs is 3.9V to 4.1V.

Your Asda flashlight does not have enough battery voltage to light 3.5V to 3.9V white LEDs. Does it have 1.8V red ones?

 

[treez]

yes youre right...it was 3 1v5's in series ,not two.

Your white leds have vf = 3v9 to 4v1 and only carry 19mA...that proves they have internal infused resistance in the bulk semiconductor in them....because your vf is just too high for a "normal" led.

-remember the Chinese can do this, because the Gov't owns everything , and can order a led foundry to make ten million "doctored" leds, and then order ten companies to make lights out of them.

 

[Audioguru]

The Chinese can pay a little girl a bowl of rice each day for her to sort the forward voltage of LEDs into groups.
My 19mA LEDs have almost the same forward voltage as my 70mA LEDs.
I think my 70mA LEDs will age quickly or will burn out soon.