LED life

LED lumen maintenance and mortality needs to be understood in similar terms to those of conventional incandescent and gas-discharge lamps.

Incandescent lamps display little change in light output until the bulb fails catastrophically. In large lighting installations, each lamp failure reduces the overall light output of the system, and in many such installations, a minimum illumination level is mandated by prevailing codes. When the light level falls below the code minimum, relamping is required. This is an expensive process involving administrative and manual labor. The total cost of relamping can be 16 times higher than the purchase cost of the lamp, particularly for high bay or municipal lighting installations. The issue is magnified when both lamps and ballasts are systemic.

090327-ave-rated-life-all-lamp-types-defines-b10-and-b50-valuesThe graph above is generic to conventional lamp types and illustrates the definition of B10 and B50 lamp failure for any multiple lamp installation.

LED sources do not tend to fail catastrophically, the light output degrades gradually over time. The level of degradation can be controlled by two variables available to the design engineer, input current (how hard the chip is driven), and junction temperature at the LED, a function of heat sink design and other variables. The power source can be selected to control the forward DC current, and resulting thermal management controls the operating temperature, yielding extended LED life.

The useful operating life of a power LED is extremely long, and often exceeds the lifetime of the product in which it is embedded. In a large installation, the effect of lumen degradation over time may be to reduce the overall light output level below a specified minimum. However, as relamping is so infrequent, the total cost of ownership is reduced.

The power LED industry group, the Alliance for Solid-State Illumination Systems and Technologies (ASSIST), has found that 70% lumen maintenance is close to the threshold at which the human eye can detect a reduction in light output for isolated light sources. For side-by-side lamps compared one to the other, the ASSIST threshold figure is 80%. ASSIST maintain that 30% reduction in light output is acceptable to the majority of users for general lighting applications. ASSIST propose that two co-ordinates be used to express the useful life of an LED component. These are L70, the time to 70% lumen maintenance, and L50, the time to 50% lumen maintenance.


The graph above illustrates how varying the forward current from the power source will affect LED life. 700 mA is a good reference for power sources used to drive retrofit LED fixtures, although in the graph below, we see the junction temperature stress tests have been run at 1500 mA. Note that driving the LED at a level below its maximum rated forward current will extend its useful life, thereby increasing the quoteable L70 and L50 lifetimes.

090327-how-changes-in-junction-temp-affect-useful-life-of-an-led1In the graph above we see that limiting junction temperature to 115C will greatly impact LED life. Using the ASSIST L70 parameter, the LED lasts 80,000 hours without detectable lumen degradation.

To ensure that designers meet established criteria for light output and relamping intervals, ASSIST recommend understanding that LED performance depends on many parameters, including heat sink design, ambient temperature, thermal resistance of package and leads, and voltage and current applied to the fixture.

To enable a simplified graphical approach, more information than that expressed by the L70 or L50 figures is required. An equivalent to the traditional B-lifetime figure (illustrated at the top of today’s post) is necessary for engineers to understand the percentage of LED in which the lumen output falls below threshold, remembering that an LED exhibiting “lumen failure” below L70 still provides illumination.

To ease the challenge for design engineers who need to factor drive current and thermal management needs, testing data should be expressed in terms of L-lifetimes and B-lifetimes and presented graphically with reference to driving current and junction temperature. That graph is illustrated below, the B50 (equivalent) limiting the LED “life” to 60,000 hours.

090327-b50-l70-how-changes-in-both-tj-and-current-affect-life-of-ledFrom a design engineer’s perspective as it relates to product design for real-world customers, the data is most usefully expressed as functions of LED life as junction temperature and forward current are varied. The heat sink design and the AC-DC power supply choices become critical. Factors of thermal performance, first-cost, and weight and size of both heat sink and power supply become key to successful fixture design, low cost of installation, and low life-cycle cost.

The photo at the top of Dean Kamen’s folly illustrates an RGB LED application; testing was all performed on white LED.

ASSIST is sponsored by NYSERDA and Sylvania.



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