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Transcript of Led Lifetime -- EDN Digikey at LFI 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
Copyright © 2010 Cree, Inc. pg. 3
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…
Copyright © 2010 Cree, Inc. pg. 4
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
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”)
pg. 6Copyright © 2010 Cree, Inc.
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
Copyright © 2010 Cree, Inc. pg. 7
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
pg. 8Copyright © 2010 Cree, Inc.
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
pg. 9Copyright © 2010 Cree, Inc.
• 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
pg. 10Copyright © 2010 Cree, Inc.
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
pg. 11Copyright © 2010 Cree, Inc.
Consistently Repeatable Behavior
pg. 12Copyright © 2010 Cree, Inc.
Cree XR-E XLamp55°C LM-80 Testing
pg. 13Copyright © 2010 Cree, Inc.
Cree XR-E XLamp55°C LM-80 Testing
pg. 14Copyright © 2010 Cree, Inc.
Cree XR-E XLamp85°C LM-80 Testing
pg. 15Copyright © 2010 Cree, Inc.
Cree XP-E XLamp55°C and 85°C LM-80 Testing
pg. 16Copyright © 2010 Cree, Inc.
Using the actual low temp test data at 6,000 hours, the projected L70 value is 63,917 hours.
Projecting LED Lifetime
Measured Data
pg. 17Copyright © 2010 Cree, Inc.
Using the actual low temp test data at 10,600 hours, L70 increases to 109,903 hours.
Projecting LED Lifetime
Measured Data
pg. 18Copyright © 2010 Cree, Inc.
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
pg. 19Copyright © 2010 Cree, Inc.
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…)
pg. 20Copyright © 2010 Cree, Inc.
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
pg. 21Copyright © 2010 Cree, Inc.
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
pg. 22Copyright © 2010 Cree, Inc.
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
)
J unction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - TAI R = 45ºC
I f = 350 mA
I f = 700 mA
I f = 1000 mA
XLamp XR-E Lumen Maintenance Projection
Note: 45ºC Air Temp
pg. 23Copyright © 2010 Cree, Inc.
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
)
J unction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - TAI R = 55ºC
I f = 350 mA
I f = 700 mA
I f = 1000 mA
XLamp XR-E Lumen Maintenance Projection
Note: 55ºC Air Temp
pg. 24Copyright © 2010 Cree, Inc.
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
)
J unction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - TAI R = 65ºC
I f = 350 mA
I f = 700 mA
I f = 1000 mA
XLamp XR-E Lumen Maintenance Projection
Note: 65ºC Air Temp
pg. 25Copyright © 2010 Cree, Inc.
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
)
J unction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - TAI R = 75ºC
I f = 350 mA
I f = 700 mA
I f = 1000 mA
XLamp XR-E Lumen Maintenance Projection
Note: 75ºC Air Temp
pg. 26Copyright © 2010 Cree, Inc.
XLamp XR-E Lumen Maintenance Projection
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
)
J unction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - TAI R = 85ºC
I f = 350 mA
I f = 700 mA
I f = 1000 mA
Note: 85ºC Air Temp
pg. 27Copyright © 2010 Cree, Inc.
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)
pg. 28Copyright © 2010 Cree, Inc.
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
)
J unction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - I F = 700mA
Tair = 45C
Tair = 55C
Tair = 65C
Tair = 75C
Tair = 85C
XLamp XR-E Lumen Maintenance Projection
Note: 700mA IF
pg. 29Copyright © 2010 Cree, Inc.
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
)
J unction Temperature (ºC)
Cree XLamp XR-E White L70 Lifetime Prediction - I F = 1000mA
Tair = 45C
Tair = 55C
Tair = 65C
Tair = 75C
Tair = 85C
XLamp XR-E Lumen Maintenance Projection
Note: 1000mA IF
pg. 30Copyright © 2010 Cree, Inc.
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
Copyright © 2010 Cree, Inc. pg. 31
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
pg. 32Copyright © 2010 Cree, Inc.