Lifetime of an LED
Thursday, July 2, 2009 in Energy Efficiency
By Richard Chung
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!
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About the author:
Director, World Wide PCIA Marketing
Tags: efficient lighting, Fairchiild Semiconductor, HID, LED, LFL, lighting
August 19th, 2009 at 4:48 am
I heard that capacitor is a key component to affect on the lifetime of LED products. I think both CFL, LFL & LED also have light output degradation.
August 28th, 2009 at 8:20 pm
Assuming an LED is driven to get the same brightness by using a pulsed and constant current (as in scan and constant drive modes), its lifetime seems to be longer to a good extent in case of the constant current.
August 28th, 2009 at 9:48 pm
Sir,
I find in many LED display boards, the track is very narrow which means heating of the track which also
generates heat and if enclosed in a tight environment, lack of ventilation can cause degradation leading to ultimate failure which will be baffling.
As a person in the LED lighting side working on 5mm mostly, I keep the tracks as 50 mills which is considered sufficient for a load of 500 ma without heating. Providing a transparent acrylic cover affects light (light deviates from lighter to denser medium and back to lighter medium)as well as creating suffocating heat for the LEDs, I keep ventilation without any acrylic front cover.
Also, the LEDs must be operated around 18 ma only and in case of LEDs with lesser mcd value, no doubt, emitted light will be less. Moral, adopt healthy practices for the longevity of leds
September 3rd, 2009 at 8:12 am
A pixel, in economical monocolor LED moving info panels, is usually formed by more than one LED.
So based on what Mr Adithyan has pointed out earlier and for the static drive mode, driving LEDs of a pixel in series as possible decreases appreciably the track heat around the LED pins (for the same width). Less heat gives a more chance for a longer life. But this trend will likely need the addition of a voltage regulator to supply the driver ICs seperately.
For instance, connecting LEDs in series is no more risky due the uniform high grade quality that most new LEDs are produced.