Lifetime of an LED
Thursday, July 2, 2009 in Energy Efficiency
By Richard Chung
LED options are advertised as longer lifetime and more efficient lighting options to the traditional CFL, LFL, and HIDs. Is this really true? What are the factors that determine LED product life?
LED lifetime is measured in lumens depreciation. It is percentage of light output degradation from initial light output. LED Industry standard from IESNA (Illumination Engineering Society of North America) such as LM-80 specifies procedure for determining lumens depreciation. Researching the different LED vendors and their datasheets, the common theme is proper heat sink and thermal management of silicon junction temperature extends the lifetime of the LEDs. Look for lifetime versus junction temperature graphs in LED vendor datasheets that accounts for lumens depreciation. For example, L70 notation means that light output is 30% less than initial output. It is the percentage decrease that a “typical” eye starts to detect a decrease in light output. Different end applications can determine suitable lumens depreciation levels.
Thermal management of LEDs is half the effort to ensure the advertised lifetime. The LED power supply or driver design is the other factor for lifetime. When the LED does not work, the end user does not care if it is caused by LED’s lumens depreciation or the LED power supply that failed.
Because LEDs are non-linear (varying forward voltage versus forward current) devices, a constant current LED driver or power supply is required. Because non-linear LEDs produce power factor less than 1, PFC (Power Factor Correction) LED drivers are needed when it exceed a certain power level and/or harmonic current limits. Requirement to light high brightness LEDs is a power supply. There are components in a power supply that can decrease the life. More details next time!
About the author:
Director, World Wide PCIA Marketing