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LED-levensduur - artikel

#1
Een artikel dat ik enige tijd geleden heb geschreven - sorry is in het Engels

gepubliceerd op: https://www.abyssware.eu/led-lifetime.html

Anytime I am looking on leaflets or web pages of LED based light fixtures I am puzzled to see (if there is such) - LIFETIME - 50 000 hours note. Doesn't matter how cheap or expensive a fixture is, what LED brand use, what currents or what temperatures, it is always the same.
Is this a reality, a myth or some kind of curse on LEDs? I think wizards are too busy with their own stuff to have time to curse the LED technology, reality cannot be either as there are too many variables involved to have always (since more than 15 years) exactly same number. In the end I think is a myth. Should we try to bust it? Or at least to confirm it?
Yes, I think worth the effort.
NOTE - As I don't own a fancy test center I relied on official data published by LED manufacturers and/or certified laboratories. Data used in this material was valid in 2017.

High-power LED lamps typically do not fail catastrophically, i.e., fail to emit light, but slowly decrease in light output over time. To characterize this gradual light loss, manufacturers uses Illuminating Engineering Society (IES) LM-80 compliant test configurations and procedures. Because many high-power LED lamps do not reach their L70 (the duration of time until the LED light output has decreased to 70% of its initial light output) lifetime even after thousands of hours of testing, there are used methods recommended in IES TM-21 to project long-term lumen maintenance behavior of its LEDs.
But as there are no certain rules, some manufacturers use as lifetime definition of LEDs the L85 (85% of initial light), L70 and even L50.
We can conclude that light fixtures data sheets and / or webpage where the LIFETIME - 50 000 hours appears, should be completed by the L value, whatever it is. I think would be nicer to see LIFETIME - 50 000 hours (L70) - right??
Despite my efforts I was able to find lifetime test data only from 3 manufacturers (Cree, Osram and Lumileds), but nothing for cheaper LEDs, quite popular in hobby (like SemiLeds for ex). If someone can provide me such extra data, I would be glad to upgrade this article.

CONSTRUCTION & DEGRADATION
MECHANISMS OF LEDS


Operating temperature and drive current are two well‑known variables that affect the long‑term lumen maintenance of high‑power LED chips. Over thousands of hours of use, high temperatures reduce the efficiency of the quantum wells in the chip causing a slow loss in light output. This is why precise temperature control during LM‑80 testing is so important. In addition to the LED chip, the manufacturing methods and the materials used in the construction of the LED lamp components are also critical variables that are affected by temperature and drive current and are the primary factors that impact the lumen maintenance behavior of LEDs. As all of these variables are interrelated, the topic of LED lumen maintenance is quite complex.

fig-1_4.jpg

The primary factors that can influence the lumen maintenance of this type of product include (but may not be limited to) the following:
• The silicone material used as the lens on the LED lamp
• The LED chip materials and fabrication technology
• The phosphor used and the phosphor application method
Taken as individual sub‑components within the LED lamp, they each are influenced by the operating conditions (temperature and current) and each might degrade differently over time. The lower the junction temperature Tj, the higher the expected lifetime of the LED. It is therefore important that a good thermal management system is implemented not only within the LED, but also by the system in the application.

REAL DATA
Even if I stopped on above mentioned 3 manufacturers, the amount of available data is too large for such an article which should be rather short and easy to swallow. That's why I will post here only the final graphs for some of the most popular LEDs from each.



CREE - data for CREE XPE2 and XPG2 LEDs:
The following 2 tables (Fig 2 and Fig 3) provide the official data from Cree. At different temperatures (Ts and Ta), LEDs are driven at different currents (If). Some more explanations may be needed for the last column, where values are in L70(7k)>39 300 hrs format.
L70 means that the LED will have 70% light output after 39 300 hours and this is calculated based on a (7k)= 7000 hours measurement and then extrapolated.
Some inconsistency appear on FIG 3 data set 9 and 10 for ex, where for 85°C L70(6k)>36 300 hrs, but at 105°C L70(9k)>51 400hrs. Also on FIG 3 data set 4 and 5. Looks like higher temperature, longer the lifetime of the LED, but actually is the measurement range which give a different extrapolation base (6 000 hours or 9 000 hours). I hope Cree will solve the inconsistency in their data.


fig-02_9.jpg fig-03_5.jpg

LUXEON - data for REBEL:
For Luxeon there are 3 graphs available (Fig 4, 5 & 6) for the same LED type (Rebel white 6000k). In this case data is more consistent, as all extrapolations were made based on 9 000 hours tests. The graphs use an exponential base for horizontal axis, so may be a bit tricky to understand.
Fig 4 graph shows the behavior of the LED at 500 mA (50% of max allowed current) and different junction (Tj) temperatures. Here is easier to see the temperature / lifetime dependency, but even at Tj 120°C LEDs go beyond 100 000 hours limit. Figure 5 shows similar values but for 700 mA current. For figure 6 LED is powered at 1000mA, so at maximum allowed current and still lifetime is > 100 000 hours, even at high temperature.


fig-4_3.jpg fig-05_4.jpg
 
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#2
fig-06_3.jpg


OSRAM - data for Oslon SSL and Oslon Square:
Osram provide for each LED type 2 graphs, somehow easier to understand than the ones above mentioned.
Fig 7 (Oslon Square) and Fig 9 (Oslon SSL) provide info about max limits of the light engine in order to have the 70K at > 100 000 hours. It is clearly visible that for the L70 limit is dependent by current and temperature.
Fig 8 and 10 shows the light depreciation over time at different currents and Ts=85°C.



fig-07_2.jpg fig-08_2.jpg fig-09_2.jpg fig-10_2.jpg

CONCLUSION
- LED lifetime is very dependent by temperature and current; Most tests are run at lower current and temperature than maximum allowed according to datasheets, where probably lifetime is reduced considerably
- The L70 values goes up to >100 000 hours for Osram and Luxeon LEDs for quite demanding test conditions, but Cree stays lower even on less demanding test conditions. Extrapolating the curves for Cree LEDs, the L70 limit is reached at ~60 000 hours. But is not quite clear for which LED type and what test conditions. So we should consider the values on table 36-54 000 hours.
- 50 000 k seems to be not a correct or well documented value; myth busted.
In my opinion, in order to have a fixture with a lifetime of 50 000 hours or more it is critical that good quality LEDs to be used, kept at low temperature (as far as possible from max junction temp) and never drive them at max allowed current, but 10-20% less than that.


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DEFINITIONS & CONCEPTS
Forward current (IF)
: The amount of current flowing through an LED lamp operating in forward bias, typically measured in milliamps (mA).
Forward voltage (VF): The voltage potential across an LED lamp operating in forward bias, typically measured in volts (V).
Lumen depreciation: The luminous flux output lost (expressed as a percentage of the initial output) at any selected elapsed operating time. Lumen depreciation is the converse of lumen maintenance.
Lumen maintenance: The luminous flux output remaining (expressed as a percentage of the initial output) at any selected elapsed operating time. Lumen maintenance is the converse of lumen depreciation.
Lumen maintenance life: The elapsed operating time at which the specified percentage of lumen depreciation or lumen maintenance is reached, expressed in hours. The elapsed operating time does not include the time when the light source is cycled off or periodically shut down.
Rated lumen maintenance life (Lρ): The elapsed operating time over which the LED light source maintains a given percentage of its initial light output. This is expressed as Lρ where ρ is the percentage value. For example,
• L50 = Time to 50% lumen maintenance, in hours.
• L70 = Time to 70% lumen maintenance, in hours.
• L85 = Time to 85% lumen maintenance, in hours.
For LED lamps, lumen maintenance is often shown as curves of relative lumen output over time for the LED under various operating conditions, such as drive current and junction temperature.
Temperature, ambient air (TAIR): The temperature of the air immediately surrounding the LED. In general, this temperature should be measured outside the full-width half-maximum (FWHM) beam angle of the LED and within the enclosure that contains the LED.
Temperature, junction (TJ): The temperature of the junction of the LED die inside the LED lamp. Measuring the LED die temperature by direct mechanical means is difficult and attempting to do so may lead to erroneous results. It is recommended to determine TJ indirectly through measurement of TSP, VF and IF and using the following equation.
TJ = TSP + ( Rth j-sp x VF x IF )
Note: Rth j-sp is the thermal resistance between the LED junction and the solder point of the LED lamp.
Temperature, solder point (TSP): The temperature of the thermal pad on the bottom of the LED lamp. TSP is also called case temperature (TC).
 

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