Do UV power LEDs come in different forward voltage groups?

[treez]

My boss says that our UV LEDs always have a forward voltage of 3.2V

I explained to him that that will be a LED from a particular forward voltage group, and he disagreed, and said there is only one forward voltage group. He said this is what the Chinese UV LED vendors told him, and that he never had a datasheet for this LED.

Do UV power LEDs come in different forward voltage groups, like white power leds?

 

[Ogu Reginald]

Forward voltage is the amount of voltage “lost” in
the LED when operated at a certain reference
current, VF depends primarily
on the color of the LED, but actually varies a bit
from LED to LED, sometimes even within the same
bag of LEDs. Standard red, orange, yellow and
yellow-green LEDs have a VF of about 1.8 V, while
pure-green, blue, white, and UV LEDs have a VF of
about 3.3 V. So, the voltage drop from our yellow
LED will be about 1.8 V.

 

[Dr.Che]

Forward voltage Vf is internal property of p-n junction and depends from energy band structure.

 

[Audioguru]

Some LED manufacturers have selected forward voltages and slightly different colors in different "bins". The details are on the datasheet.
I guess the Chinese vendors do not care about it.

 

[treez]

All the UV LED datasheets I have seen do not seem to have different voltage bins for the UV Leds...I am wondering if UV LEDs just don't get Vf binned?

 

[betwixt]

Maybe not binned but the differences are still there.

Perhaps it's because it is unlikely anyone will try color matching or precisely luminance matching UV LEDs.

Brian.

 

[treez]

thanks, but UV leds are often used in high current arrays, and they need to be paralleled, and hence forward voltage matching is essential

 

[D.A.(Tony)Stewart]

There is a lot of confusion over LEDs and diodes in casual experts between threshold , forward and binned voltage. Some factories test every part and bin them with tolerances for Vf, xy colour coordinates etc and bin every part automatically. Bins are then combined for different customers according to part numbers details. WHite LEDs can have in theory over 100 bins at the factory but reduced to a dozen part numbers by combining bins. ( Some specify there must be 3 adjacent bins for intensity and 3 for wavelength and 2 for voltage. Single wavelength emitters will have fewer variables and thus fewer bins because the White is a balance of the ratio of atomic thin phosphor over a thick blue substrate which affects both colour warmth or XY coordinate, intensity and then there is the ESR variable that affects Vf.

All diodes have a threshold voltage, Vth which is basically the voltage at very dim illumination, like a silicon diode is 0.6V and may be selected at say 1% of the rated current to measure estimate ESR. I usually extrapolate the curve to the axis at rated current to measure ESR.

The Vf is always at rated current (not max) which is exponential but a linear regression to extrapolate Vf is useful. where Vf ~ Vth + I*ESR

The binned voltage is a design and process control by the wafer manufacturer where they buy the wafer and slice and dice into chips, assemble, test and then sort into bins. If they stated 3.2, they obviously didn't state the tolerance as all production has tolerances. but one batch has chips is very controlled can be matched within 1 mV and have a standard deviation, but then gets mixed with other batches that might be different by 10, 50, 100 mV, 200mV or 400mV. The difference is all due to the bulk resistance of the junction or ESR. The threshold is always the same but not useful at low current.

Voltage bins may be sorted into 100mV bins but upon customer agreement, may be 1 bin ,2, 3 or open bins ( more than 6) A manufacturer is free to choose epi-wafers from different sources, stock can get mixed results unless agreement is made with supplier or distributor.

If the packages are not labeled with the min-max voltage, then you have no idea, what you are getting, especially without a spec. (bad form) except based on trust.

The Vth always increases with wavelength with the lowest threshold for IR emitters and highest for UV.
White and Blue have the same because they are the same inside, where some blue energy is converted to a wide spectrum of yellow phosphors to balance the RG reception in RGB with more or less phosphor to make it warm or cool.

Generally the bigger the chip power, the lower the ESR where W*ESR~<1 as a rule of thumb for whites. This will be different for UV and Green etc. and often higher with poor quality parts. A tiny 50mW SMT white chip might be 3.6V with 20 Ohms ESR whereas a 3W white chip could be anywhere from 2.9V ( best) to 3.6V (worst) at 1000mA ( actually 3.6W)

Poor quality parts also have a wide tolerance, which is normal, so they are sold open bin, but binned parts, the factory might scrap or sell the higher ESR or Vf parts to other customers.

I would make sure the Vf tolerance stated on the PO until agreed, if you don't have a spec. Otherwise, for parallel operation in high power, you WILL get thermal runaway with large mismatch.

The other thing in high power diodes is they are always tested with pulses at room temp and will have a negative temperature coefficient (Shockley Effect) which can vary by more than a couple hundred mV.

I have a secret formula for running LEDs with CV sources in parallel operation where matching voltage is critical. The critical point is factored by ESR, and junction ambient thermal resistance and drive current, If a correction is needed, it is often no more than the variation of ESR between devices e.g. a 15 Ohm resistor for 5mm LEDs. which results in very efficient current regulation at the right Vf.

 

[treez]

Hello again,
Please find attached a datasheet for the LEUV-V512A6 UV LED.
The forward voltage is quoted as being between 3.2v and 3.8v.
Do you know what the likelihood of occurrence of any particular forward voltage will be?...for example in any given batch,what is the percentage of 3.8v leds that you would expect to get?

The forward voltages are recorded in the datasheet at 350mA current, but we are running these at 440mA.

We have them on a piece of MCPCB IN series parallel banks, as you would expect.

 

[FvM]

The datasheet also says that the UV LEDs are binned in 0.1V ΔVf groups.

 

[D.A.(Tony)Stewart]

​

Hello again,
Please find attached a datasheet for the LEUV-V512A6 UV LED.
The forward voltage is quoted as being between 3.2v and 3.8v.
Do you know what the likelihood of occurrence of any particular forward voltage will be?...for example in any given batch,what is the percentage of 3.8v leds that you would expect to get?

The forward voltages are recorded in the datasheet at 350mA current, but we are running these at 440mA.

We have them on a piece of MCPCB IN series parallel banks, as you would expect.

You can expect them sorted in 0.1V bins then 0.2V bin from 3.6~3.8 . You can expect 10% in the upper bin, but it could also be 0 or 20%. The distribution of parts can be anything.

If mature parts >3yrs, & in high volume, the distribution may be lower in your favour, or they may exceed 3.8 and get rejected at factory. if you rely on reliability based on Tj, of your design, then you must consider the worst case with 440mA current source.

Thermal protection is advised but useless unless on each chip, unless current share is guaranteed by design.

This is an exceptionally small and thin chip with 1sq.mm putting440mA*3.5V Watts into it and limits your ambient assuming Water cooled! MCPCB may not be adequate to cool an array.

the array must be assembled from the same bins unless 100-200mΩ silver epoxy is used,or small Rs is used.

Epoxy, Ag high fill content, is desirable considering the solder limit of <5 sec @ liquidus .

Rs or ESR of gang connections , also may become critical for direct Parallel array to prevent Thermal Runaway and self destruction of one part, then the next... Etc.

You can specify 3.6V max but expect a cost increase to cover parts not used in batch order.

You can also expect Vf to drop as Tj rises towards 125'C the absolute max in worse case environment.

FWIW, ESR is approx 0.56 Ω ± 0.1 above 350mA from my calculations.

Tjc is 69 deg C/W and MCPCB Tca is unknown with ambient from adjacent LED unknown.

 

[treez]

The distribution of parts can be anything

Thanks, though this is bad news......the high forward voltage leds cause us problems with the bUck converter running out of duty cycle to drive them from our vin....are you sure the distribution can be anything?, we were hoping that it was like a bell curve with most in the 3.5V group (3.45-3.55)and very few at the edge, especially the upper edge, ie 3.8V?

 

[D.A.(Tony)Stewart]

It depends on wafers. most parts in the wafer will be the same. Wafer to wafer variation depends on process controls

. It is unlikely 100% will be high, but not on a bell curve, more skewed towards lower side with random batches that are high. 3.2-3.3 is a perfect part for UV at 350mA and 0.3V lower for White/Blue. If you want a second source just ask.

AreU running strings.of 13 or 14 off 51V or 48?