LED Life and Forward Current

As discussed in our section on “LED light bulb case and heat sink temperature” the life of the actual LED which produces the light is partly determined by the operating temperature of the LED (LED junction temperature) such that the lower the temperature the longer the life of the LED. The other main factor affecting LED life relates to the forward current which as mentioned is proportional to the brightness of the LED. Generally, forward current is not a major problem as LED manufacturers will specify safe operating ranges, although upper ranges do require better heat sink design. If the LED runs very hot lower forward currents can extend the life of the LED chip, although generally if the LED chip is kept relatively cool with good heat sink design (below 85°C), the life time will not vary too much.

L70 LED Life Expectancy

LED light bulbs rarely fail catastrophically like there incandescent predecessors. When manufacturers specify the life of a LED or life of the whole LED light bulb in hours they are referring to the L70 data which represents a reasonably accurate theoretical estimate of the time taken for the LED to lose 30% of its brightness or reduced down to 70% of its original brightness, hence L70. This is typically specified in the range of 30,000 to 40,000 hours and is often referred to as Lumen maintenance or Lumen depreciation. 

However, the LED light will not suddenly fail but continue to produce light well beyond the L70 life point, albeit at a lower brightness until the light becomes too dim to be useable. Estimates suggest the LED light will last for up to 100,000 hours or more before it finally shuts down. In many applications that is “light for life”! Therefore, LED life is usually plenty long enough and does not really become an issue except during extended operating hours or high operating temperatures, which as mentioned can result from poor design.

Since the life of the LED is actually very long, potentially 100,000 hours at lower brightness, there is potentially a good rational to using external power supplies given the power supply will give in well before the LED has declined to faint light levels. 

Calculation of L70 Life Expectancy using LM-80 test data and TM-21 Extrapolation

Calculation of the L70 LED life point is a relatively simple but time consuming process utilising the LM-80 test data which requires the brightness or lumens testing of multiple LED samples at three different temperatures, 55°C, 85°C and one other, and determining the loss in light intensity at multiple time points over 6000 to 8000 hours. Yes, the testing can take almost one year to do!

As dicussed below, once we have the LM-80 test data for the LED chip we can then extrapolate using the TM-21 method which is basically an exponential function and formula to ascertain the L70 life in hours of the LED chip when it declines to 70% of its output.

Do actual LED light bulbs have LM-80 test data?

Most LED light bulb manufacturers do not have LM-80 test data for their bulbs but in general they can obtain that test data from the LED manufacturer. Estimating the life of the actual LED light bulb or "Rated Lamp Life" is then a relatively simple process of measuring the LED case temperature in the actual light bulb and then using that temperature data to extrapolate from the LM-80 curve. The TM-21 formula is then uses that data to estimate the life time or L70 life of the LED light bulb.

References:

Lay-Ekuakille AG et al (2009). "Led-based Public Lighting System Reliability for a Reduced Impact on Environment and Energy Consumption." Energy and Environment Research Center, Dipartimento d’Ingegneria dell’Innovazione, Università Degli Studi del Salento (2008): 8. Web. 30 Dec

Sebitosi AB & Pillay P (2003). "White LEDs for Rural Lighting." IEEE- Institute of Electrical and Electronics Engineers: 2619-2623.

Extend the life of LED lighting systems with thermal management, Rick Zarr Texas Instruments, EDN Network, September 2012

Lifetime of White LEDs. Sept 2009. Building Technologies Program. US Department of Energy. www.eere.energy.gov