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Led Lifetime -- EDN Digikey at LFI 2010






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    Led Lifetime -- EDN Digikey at LFI 2010 Led Lifetime -- EDN Digikey at LFI 2010 Presentation Transcript

    • LED Lumen Maintenance & Lifetime
      May 13, 2010
    • Trivia Point: 50,000 hours is:
      137 Years at 1 hour/day
      68.5 Years at 2 hours/day
      34.2 Years at 4 hours/day
      22.8 Years at 6 hours/day
      17.1 Years at 8 hours/day
      11.4 Years at 12 hours/day
      5.7 Years at 24 hours/day
    • Semiconductor Reliability Testing
      Reliability test methods and acceptance criteria for semiconductor components have been standardized (JEDEC, EIAJ, others…) and practiced for decades
      Think: processors, regulators, microcontrollers, etc..
      If you’ve ever flown in an airplane, driven in a car, or spoken on a cell phone, you’ve depended on this body of scientific work and testing…
    • LED Reliability Testing
      LEDs are semiconductor components that happen to emit light
      Most LED manufacturers conduct the traditional standardized semiconductor component reliability testing on their LED lamps (http://www.cree.com/products/pdf/XLamp_Reliability.pdf)
      The Illumination Engineering Society of North America published IES LM-80-2008 18 months ago to characterize the Lumen Maintenance aspect of LED semiconductor components
      Note: Lumen Maintenance ≠ LED Lifetime. Another standards committee – TM-21 – is working on that aspect of the problem
    • Lumen Maintenance Definition
      Definition: change in light output of a light source over operational life, relative to initially measured light output
      Lxx = time to xx% of original light output
      • L70 = time to 70% of original light output
      • L50 = time to 50% of original light output
      Traditional light sources gradually dim then fail catastrophically (“burn out”)
      Lumen Maintenance: Hypothetical HID Lamp
      L70 = 10,000 hours
    • LED Lumen Maintenance Critical Parameters
      1. TAIR
      Ambient Air Temperature
      2. TJ
      Junction Temperature
      3. TSP / TC / TS
      Solder-Point Temperature / Case Temperature
      4. IF
      Forward Current /
      Drive Current
    • Low temp (25ºC) testing is a good surrogate for the LED chip depreciation – 1-2% @ TJ = 65ºC
      LEDs Last Forever! [under ideal conditions]
      At lower ambient air temperatures, LEDs hardly depreciate at all
    • High Air Temperature Degrades Encapsulant
      • Cree now understands that the silicone-based encapsulants used in the industry degrade when exposed to high temperatures
      • Degradation comes from organic pendant groups (e.g. CH3, C6H5, -OH) that can off-gas or be trapped in the matrix
      • The higher the air temperature, the more the encapsulant will degrade, the more light lost
    • Un-powered LEDs Depreciate at High Temp
      • The out-diffusion of volatiles from the silicone encapsulant causes the refractive index of the encapsulant to decrease
      • As the refractive index decreases the critical angle increases allowing less light to be emitted from the lamp (Snell’s Law)
    • Typical Behavior of LEDs on LM-80 Testing
      Low Air Temp
      High Air Temp
      % LF
      Hours on Test
    • Consistently Repeatable Behavior
    • Cree XR-E XLamp
      55°C LM-80 Testing
    • Cree XR-E XLamp
      55°C LM-80 Testing
    • Cree XR-E XLamp
      85°C LM-80 Testing
    • Cree XP-E XLamp
      55°C and 85°C
      LM-80 Testing
    • Using the actual low temp test data at 6,000 hours, the projected L70 value is 63,917 hours.
      Projecting LED Lifetime
      Measured Data
    • Using the actual low temp test data at 10,600 hours, L70 increases to 109,903 hours.
      Projecting LED Lifetime
      Measured Data
    • When using the low temp 34,800 hour data, the L70 increases to > 200,000 hours!
      Projecting LED Lifetime
      Measured Data
      Exponential Curve-fit is inaccurate and exceedingly conservative
    • So, How to Translate Lumen Maintenance to Lifetime…?
      • Cree has accumulated millions of XLamp XR-E LED lamp device hours of long-term data under both LM-80-compliant conditions and other test configurations
      • The effects of TAIR, TJ, TSP and IF on long-term lumen maintenance in this system have been closely studied and are well understood
      • Cree has observed that the lumen maintenance characteristics of the XLamp XR-E white LED lamps are different in the first 5,000 hours (called Period A) than in the time period following 5,000 hours (called Period B)
      • A “best fit” algorithm was developed to accurately model this lumen maintenance behavior, based on the critical parameters TAIR, TJ, TSP and IF
      • This algorithm is likely to be different for every LED lamp system (e.g. XLamp XP, MC, Rebel, Dragon, NS6, etc…)
    • Cree LED Lifetime Model
      • Degradation in first 5,000 hours is mostly due to degradation in the silicone encapsulant
      • After 5,000 hours, this mechanism drops out and the slower chip degradation dominates
      • We see no “B10” early life failures or catastrophic failures in our XLamp systems
    • XLamp XR-E Lumen Maintenance Projection
      Note: 35ºC Air Temp
      Given the Air (ambient) temp inside the fixture, drive current, and junction temperature, the lifetime is projected
    • XLamp XR-E Lumen Maintenance Projection
      Note: 45ºC Air Temp
    • XLamp XR-E Lumen Maintenance Projection
      Note: 55ºC Air Temp
    • XLamp XR-E Lumen Maintenance Projection
      Note: 65ºC Air Temp
    • XLamp XR-E Lumen Maintenance Projection
      Note: 75ºC Air Temp
    • XLamp XR-E Lumen Maintenance Projection
      Note: 85ºC Air Temp
    • XLamp XR-E Lifetime Projection
      There’s Nothing Magic about 50,000
      • Models err on the conservative side
      • As much as we’d like it, there is no “one-number” answer (like CRI)
    • XLamp XR-E Lumen Maintenance Projection
      Note: 700mA IF
    • XLamp XR-E Lumen Maintenance Projection
      Note: 1000mA IF
    • LED Lifetime Is Irrelevant
      System Lifetime is What Creates Value
      Heat Sink: Linchpin of the entire system. If this is poorly designed, all the other components can be compromised
      Driver: Can be the weak link in the system if cheap components are used
      LED Lamps: Practically never fail; depreciate very slowly in a well-designed system
      Optical Components: Can (rarely) yellow over time and lose light; system design choice
    • LED Lumen Maintenance & Lifetime Summary
      Testing and characterizing LED lumen maintenance is an evolving science based on decades of work from the component semiconductor industry
      LED Lumen Maintenance ≠ LED Lifetime
      We have fully characterized the XLamp XR-E LED lamp and have developed an accurate predictive model for both the Lumen Maintenance and Lifetime of this lamp system
      Key input variables to Lumen Maintenance and Lifetime are:
      LED junction temperature (TJ) – calculated from TSP
      Forward drive current (IF)
      Air temperature around the lamp (TAIR)
      From these, L70 and Lifetime of the LED lamp can be predicted with confidence
      Different LEDs will likely have different lifetime models (TM-21)
      LED Lifetime ≠ Luminaire Lifetime: need systems approach