Unlike many other semiconductor devices, LEDs have a limited operation lifespan or lifetime.
The LED lifespan or LED lifetime is not infinite and performance degrades over time with light levels falling gradually.
In view of their long lifespan, LEDs are considered as reliable light sources, both as indicators and for lighting, but their lifespan as well as other factors like failures and the MTBF need to be considered when using them in any design.
For any component or system, the MTBF is the Mean Time Between Failures. The MTBF is the elapsed time which is predicted between inherent failures of a component or system during its operation.
The MTBF is a figure used in calculations for the reliability of items of equipment. In order to be able to calculate the MTBF of the equipment, it is necessary to know the MTBF of the individual components, e.g. the LED MTBF in this case.
The failure rate for a component, and the MTBF are linked. MTBF can be calculated as the inverse of the failure rate if it is assumed that there is a constant failure rate, which is not unreasonable as a first order assumption.
The MTBF figures are often quoted in the manufacturers data sheets. However the MTBF can be considerably reduced by operating components close to their rated limits. Hostile environments such as high temperature and vibration also reduce the MTBF.
However when run within their limits, the LED lamps have a long lifetime, and do not fail very often.
Expected LED lifetime
LED light reduces over time. This form of LED lifespan or LED lifetime is particularly important for applications such as lighting where it is important that the light is maintained above a certain level.
A term called lumen deprecation is used to describe this aspect of the LED performance.
The LED lifetime or LED lifespan is the time to when the light output falls to a given level. The LED life expectancy may be quoted in the format L70 or L50, for the life to when the light output falls to 70% or 50% of the original value respectively.
The LED lifetime, i.e. the LED operational life is generally defined in the following terms:
L70% = Time to 70% of illumination (lumen maintenance)
L50% = Time to 50% of illumination (lumen maintenance)
The L70 value was chosen because a power LED industry group called the Alliance for Solid-State Illumination Systems and Technologies, ASSIST, undertook tests which demonstrated people generally did not notice a gradually diminishing LED light output until it had dropped by 30% of its original brightness, i.e. to 70% of initial light output. This then gave rise to the L70 figure. However for non-critical areas the L50 figure may also be used.
As a rough guide, most LEDs intended for lighting applications offer L70 values of 50,000 to 60,000 hours, although with improved manufacture of the high brightness LEDs used for lighting, some manufacturers are now quoting figures of 100 000 hours.
Factors affecting LED lifespan
There are a number of factors that affect the useful LED lifetime. By ensuring that the LED is protected from adverse conditions it is possible to ensure the maximum lifetime is maintained.
Temperature: One of the major issues in ensuring the maximum life is obtained from a LED is keeping the temperature down. Excess temperature will considerably shorten the life. To prevent the LED chip running over temperature there are a number of elements that can be included within the design
Good thermal path from LED chip to mount: It is necessary to ensure that the heat can be effectively removed from the LED semiconductor itself. This is the first step in ensuring the LED junction temperature does not rise to high and adversely affect the LED lifetime.
Good bonding between LED and external mount: It is necessary to ensure that the LED package is effectively bonded to the element on which it is mounted. The thermal resistance should be as low as possible, possibly using thermal mounting grease t ensure complete contact.
Good heatsink: In order that heat is removed effectively from the overall assembly the heatsink on which the LED is mounted should have a low thermal resistance. It should also be located so that heat will flow away from the heatsink. For LED lighting, this is particularly important because often lamps will be located within small light fittings and this will not aid cooling, and hence the LED lifetime will be reduced.
LED drive level: To obtain the best LED lifetime, the LED should be driven well within its ratings. Overdriving a LED will drastically reduce its lifetime, although it will increase the light output.
Power supply: The power supply needs to match the light emitting diode for optimum LED life expectancy. Not only should the voltage be regulated, but the current also needs to be closely controlled to ensure the LED does not run outside its ratings, or even too close to its maximum ratings.
Environment: General conditions such as vibration, and temperature extremes - even when not operating - place mechanical stresses on the diode which will reduce the LED lifetime. Ideally, a LED should be operated within a stable dry environment. When this is not possible, a shorter LED lifetime should be anticipated.
Although it may appear obvious at first sight that the LED life should be as long as possible, this may not always be the main requirement. It is possible that in some cases light output is more important than LED lifetime, and in this case it may be permissible to overdrive the LED to obtain the additional light. Additionally budgetary constraints may limit the inclusion of more effective thermal management, and in this case a decision can be made to balance LED life expectancy against the cost.