Led Lifetime -- EDN Digikey at LFI 2010

LED Lumen Maintenance & Lifetime

[email protected]

May 13, 2010

Copyright © 2010 Cree, Inc. pg. 2

Trivia Point: 50,000 hours is:

137 Years at 1 hour/day68.5 Years at 2 hours/day34.2 Years at 4 hours/day22.8 Years at 6 hours/day17.1 Years at 8 hours/day11.4 Years at 12 hours/day5.7 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

http://www.cree.com/products/pdf/XLamp_Reliability.pdf

pg. 5Copyright © 2010 Cree, Inc.

Lumen Maintenance Definition

40%

50%

60%

70%

80%

90%

100%

110%

0 5 10 15 20 25

Operating Time (k hrs)

Lum

en O

utp

ut (%

)

L70 = 10,000 hours

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

Lumen Maintenance: Hypothetical HID Lamp

Traditional light sources

gradually dim then fail

catastrophically (“burn out”)


LED Lumen Maintenance Critical Parameters

1. TAIRAmbient Air Temperature

2. TJ Junction Temperature

3. TSP / TC / TSSolder-Point Temperature / Case Temperature

4. IFForward 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

OSiOSiOSi

H–C–H

H

H–C–H

H

H–C–H

H


• 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)

Un-powered LEDs Depreciate at High Temp


1,000 2,000 4,0003,000 5,000 6,000 7,0000

100

85

90

105

95

% L

F

Hours on Test

Typical Behavior of LEDs on LM-80 Testing

Low Air Temp

High Air Temp


Consistently Repeatable Behavior


Cree XR-E XLamp55°C LM-80 Testing


Cree XP-E XLamp55°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.


Measured Data


When using the low temp 34,800 hour data, the L70 increases to > 200,000 hours!


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


0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

150,000

60 70 80 90 100 110 120 130 140 150 160

Mean L

70

Lif

eti

me (

hours

)

J unction Temperature (ºC)

Cree XLamp XR-E White L70 Lifetime Prediction - TAI R = 35ºC

I f = 350 mA

I f = 700 mA

I f = 1000 mA

XLamp XR-E Lumen Maintenance Projection

Given the Air (ambient) temp inside the fixture, drive current, and junction temperature, the lifetime is projected

Note: 35ºC Air Temp


0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

150,000

60 70 80 90 100 110 120 130 140 150 160

Mea

n L

70

Lif

eti

me (

hours

)



I f = 350 mA

I f = 700 mA

I f = 1000 mA




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)


0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

150,000

60 70 80 90 100 110 120 130 140 150 160

Mea

n L

70

Lif

eti

me (

hours

)


Cree XLamp XR-E White L70 Lifetime Prediction - I F = 700mA

Tair = 45C

Tair = 55C

Tair = 65C

Tair = 75C

Tair = 85C


Note: 700mA IF


0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

110,000

120,000

130,000

140,000

150,000

60 70 80 90 100 110 120 130 140 150 160

Mea

n L

70

Lif

eti

me (

hours

)


Cree XLamp XR-E White L70 Lifetime Prediction - I F = 1000mA

Tair = 45C

Tair = 55C

Tair = 65C

Tair = 75C

Tair = 85C


Note: 1000mA IF


LED Lifetime Is Irrelevant

System Lifetime is What Creates Value

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

Driver: Can be the weak link in the system if cheap components are used

Heat Sink: Linchpin of the entire system. If this is poorly designed, all the other components can be compromised


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

Led Lifetime -- EDN Digikey at LFI 2010

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