short- and long-term performance of leds© 2008 rensselaer polytechnic institute. all rights...

© 2008 Rensselaer Polytechnic Institute. All rights reserved.

ShortShort-- and longand long--term term performance of LEDsperformance of LEDs

N. NarendranN. Narendran

Lighting Research CenterLighting Research Center

Rensselaer Polytechnic InstituteRensselaer Polytechnic Institute

Troy, NYTroy, NY

EIRT MeetingChicago, IL

April 22, 2008

2© 2008 Rensselaer Polytechnic Institute. All rights reserved.

Introduction Introduction -- LED EvolutionLED Evolution

LumiLeds Lighting


Introduction Introduction -- White LED PromiseWhite LED Promise

15 lm/W

90 lm/W

120 lm/W

150 lm/WEnergy savings

1000 hrs

20,000 hrs30,000 hrs

100,000 hrsLower maintenance cost

By 2012


Lighting


LED: Present LED: Present IndustryIndustry TrendTrend

Growing number of LEDs and LED fixturesGrowing number of LEDs and LED fixtures


Lighting Market ForecastLighting Market Forecast

Forecast from Optoelectronics Industry Development Forecast from Optoelectronics Industry Development Association (OIDA)Association (OIDA)›› Incandescent will be the worst affectedIncandescent will be the worst affected

http://images.google.com/imgres?imgurl=http://wrtassoc.com/__oneclick_uploads/2007/11/ssl-forecast.jpg&imgrefurl=http://wrtassoc.com/&h=346&w=575&sz=31&hl=en&start=7&um=1&tbnid=ziWTcKTuWLmgAM:&tbnh=81&tbnw=134&

prev=/images%3Fq%3Devolution%2Bof%2Bhigh%2Bpower%2Blight%2Bemitting%2Bdiodes%26um%3D1%26hl%3Den%26sa%3DX


Performance of Commercial Products


Commercial White LEDs Commercial White LEDs –– (March 2008)(March 2008)

Warm White

Vio Vio

OstarOstar

Z-Power P4

Acriche

XRE(P3)

Luxeon Rebel

Luxeon ILuxeon K2

Rigel

Platinum DRAGON

Golden DRAGON

Titan

Moonstone

Luxeon K2

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000Luminous flux (lm)

Lum

inou

s ef

ficac

y (lm

/W)

Cool White

Moonstone

Diamond DRAGON

Z-Power P4

Acriche

XRE (Q5)

Luxeon Rebel

Luxeon ILuxeon K2

Luxeon III

Rigel

Platinum DRAGON

Golden Dragon

Ostar

Titan

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000Luminous flux (lm)

Lum

inou

s ef

ficac

y (lm

/W)


LED: StateLED: State--ofof--thethe--artart

LumiLeds Lighting


Measuring LED Light OutputMeasuring LED Light Output

The industry practice for The industry practice for measuring light out from measuring light out from LEDs:LEDs:›› pulse testpulse test

•• such that the junction does such that the junction does not heat up during the not heat up during the measurement measurement periodperiod

•• Example: 350 mA, 20 ms, Example: 350 mA, 20 ms, 1 s duty cycle1 s duty cycle

In practice, the steady In practice, the steady state light output is state light output is needed.needed.

http://www.lumileds.com/pdfs/DS46.PDF


LED LED SystemSystemLED

Electrical ~ 80%

Optical ~ 80%

Heat ~ 85%

Final Efficiency ~ 54%

Luxeon


LED Lighting LED Lighting SystemsSystems PerformancePerformance

To the end user, it is the system performance that To the end user, it is the system performance that matters, not source performance.matters, not source performance.

0

50

100

150

200

250

2000 2004 2008 2012Year

Perf

orm

ance

(Lm

/W)

White LED

R&D Results

Liner Fluorescent Systems

CFL Systems

Incand. Systems

(2007-08)

LED Systems


Reliability


LED LifeLED Life

Many commercial LEDs in the marketplaceMany commercial LEDs in the marketplace›› All LEDs are not created the same; large All LEDs are not created the same; large

variations between them.variations between them.High power white LED

70%

75%

80%

85%

90%

95%

100%

100 1000 10000 100000Time (hours)

Rel

ativ

e lig

ht o

utpu

t

F

C

A D

E B

1W white LEDs operated at 35 deg C, 350 mA


Traffic LightsTraffic Lights

LED LED –– Value propositionValue proposition›› Uses 80% less energy than incandescent signalsUses 80% less energy than incandescent signals

›› Reduces maintenance costsReduces maintenance costs•• (typically 5(typically 5--year warranty)year warranty)

Poorly integrated systems could have Poorly integrated systems could have muchmuchshorter shorter lifetimeslifetimes

Failed LEDs


Failure MethodsFailure Methods

Catastrophic Catastrophic –– Abrupt and complete cessationAbrupt and complete cessation•• Broken stitch bondsBroken stitch bonds

•• Lifted dieLifted die--attachattach

•• Cracking epoxyCracking epoxy

•• Delamination of epoxyDelamination of epoxy--leadframeleadframe

•• Broken solder jointsBroken solder joints

Degradation Degradation –– Continuous deterioration over timeContinuous deterioration over time•• TimeTime--dependent light lossdependent light loss

•• TimeTime--dependent color shiftdependent color shift

Failure results in reduced lifeFailure results in reduced life


Market SpoilersMarket Spoilers

OverOver--promisedpromised, , underunder--delivereddelivered productsproducts

Taipei

Not good for the LED lighting industryNot good for the LED lighting industry


Thermal Issues


Ballast (Driver)

Heat Sink

Optics (Lens)

LED Array

Optics (Diffuser)

Electrical Grid

Fixture Housing

Controls

Circuit Board

LED Lighting SystemLED Lighting System


Energy output of electric light sources

70 %34 %18 %Heat(Conduction-Convection)

10 %13 %Ballast or Driver

Low29 %72 %IR

20 %23 %10 %Light

LED(estimate)

Fluorescent*(T-8 F32 rare earth)

Incandescent*(100 W)

* IESNA Handbook – 9th Edition

Why is heat management important?Why is heat management important?


Why is heat management important?Why is heat management important?

Higher temperature near the LED junctionHigher temperature near the LED junction›› ShortShort--term effect term effect

•• Lower light outputLower light output

•• Wavelength (color) shiftWavelength (color) shift

›› LongLong--term effect term effect •• Higher degradation rate (shorter life)Higher degradation rate (shorter life)

Poor heat management leads to poor system Poor heat management leads to poor system performanceperformance


Impact of Heat on LEDsImpact of Heat on LEDs

Heat at the junction affects the performance Heat at the junction affects the performance of LEDsof LEDs›› Light output reduces with increased heatLight output reduces with increased heat

›› Color shifts with heatColor shifts with heat

›› Life shortens with heatLife shortens with heat

R2 = 0.91

0

10000

20000

30000

40000

50000

60000

35 40 45 50 55 60

T pin (deg C)

Life

L70

% (h

rs)

1 Watt White LEDs

60%70%80%

90%100%110%

40 60 80 100 120Tj

Rel

ativ

e lig

ht o

utpu

t

LED A

LED B

LED C

Reference: Narendran et al., IESNA 2006


LED System Performance in Applications


Lighting ApplicationLighting Application

IncandescentHalogen CFL LED

Now, many light source technologies can Now, many light source technologies can cater to a given lighting application.cater to a given lighting application.


Ceiling tiles (Poor thermal conductor)

Halogen / Incandescent Fixture

LED Fixture

Conducted heat

Radiant heat

Insulation material above the ceiling area

Installation IssuesInstallation IssuesSystem integration and application environment System integration and application environment affectaffect LED system LED system performance.performance.


Installation IssuesInstallation Issues

InIn--groundground vs.vs. aboveabove--groundground fixturesfixtures


Application EnvironmentsApplication Environments

Generally, light sources or fixtures operate in Generally, light sources or fixtures operate in thermal environments such as:thermal environments such as:›› Fully ventilated (e.g., track lighting, openFully ventilated (e.g., track lighting, open--air)air)

›› SemiSemi--ventilated (e.g., recessed downlight, Nonventilated (e.g., recessed downlight, Non--IC)IC)

›› Enclosed (e.g., recessed downlight with ceiling insulation, Enclosed (e.g., recessed downlight with ceiling insulation, IC)IC)

http://www.ylighting.com/linear-system.htmlReference: Narendran et al., IESNA 2006


Performance of Performance of anan LED PAR Lamp LED PAR Lamp

The junction temperature of the LED array The junction temperature of the LED array changes when the PAR lamp is used in the changes when the PAR lamp is used in the three different application conditions.three different application conditions.

LED PAR30 Lamp in Application

0255075

100125150175200

0 2 4 6 8 10Time (hrs)

Tem

pera

ture

(C)

Open IC Non-IC

~ 160°C

~ 90°C

~ 60°C

Reference: Jayasinghe et al., 2006


ASSISTASSIST RecommendsRecommends

In 2007, several In 2007, several ASSIST ASSIST recommendsrecommendsdocuments were released: documents were released: ›› General guide to applicationsGeneral guide to applications›› Guide to selecting LED fixturesGuide to selecting LED fixtures›› Testing and evaluation recommendationsTesting and evaluation recommendations

ASSIST ASSIST recommendsrecommends testtest methods were methods were designed to:designed to:›› Provide more useful information for selecting Provide more useful information for selecting

and using LED directional lightingand using LED directional lighting luminairesluminaires›› Help differentiate between good and poor Help differentiate between good and poor

performing LEDperforming LED luminairesluminaires in terms of light in terms of light output and lifeoutput and life


Directional Lighting Test MethodDirectional Lighting Test Method

ASSIST ASSIST recommendsrecommends proposed three proposed three environmental conditions to test environmental conditions to test fixtures:fixtures:

›› Open air:Open air: Here the light source and the Here the light source and the driver have plenty of ventilation around driver have plenty of ventilation around them.them.

›› SemiSemi--ventilated:ventilated: Here the light source and Here the light source and the driver have limited ventilation around the driver have limited ventilation around them. them.

›› Enclosed:Enclosed: Here the light source and the Here the light source and the driver have almost no ventilation around driver have almost no ventilation around them.them.


Sphere Sphere PhotometryPhotometry

Temperature, Ts, is measured Temperature, Ts, is measured while operating the fixture in while operating the fixture in the the three environmentsthree environments..

Fixture is placed inside a Fixture is placed inside a heated enclosure which is heated enclosure which is place inside the integrating place inside the integrating sphere.sphere.

Data gathered once the Data gathered once the temperature, Ts, temperature, Ts, reachesreachesapplication application temperature.temperature. Heater

Lamp

Driver

Heated enclosure

Feedbackcontrol

Ts


LuminaireLuminaire Testing Testing

Several commercial LED fixtures are being tested in Several commercial LED fixtures are being tested in 3 environments (per 3 environments (per ASSISTASSIST recommendsrecommends))›› Open airOpen air›› NonNon--ICIC›› ICIC

ShortShort--term testingterm testing›› Flux and colorFlux and color

LongLong--term testingterm testing›› Lumen depreciation Lumen depreciation

and life (L70)and life (L70)›› Color shiftColor shift


Flux (lumens)Flux (lumens)

WellWell--designeddesigned luminaires maintain light luminaires maintain light output,output, even even in hotter in hotter environments.environments.Poorly designed luminaires have more than 30% Poorly designed luminaires have more than 30% lower light output in lower light output in ICIC--condition.condition.

236

649583

263212

678

446

223

643

396

183

0100200300400500600700800

Fixture A26W

Fixture B26W

Fixture C12W

Fixture D30W

Flux

(lum

ens)

Open air Non-IC IC


Efficacy (lm/W)Efficacy (lm/W)

Generally, system efficacy values are 30% to 50% Generally, system efficacy values are 30% to 50% lower than LED efficacy values. lower than LED efficacy values. However, However, wellwell--designeddesigned luminaires have achieved luminaires have achieved over 50 lm/W over 50 lm/W ..

10

22

54

8917

7715

5457

0

10

20

30

40

50

60

70

Fixture A26W

Fixture B26W

Fixture C12W

Fixture D30W

Effic

acy

(lm/W

)

Open air Non-IC IC


Board Temperature (Board Temperature (°°C)C)

With increasing With increasing TjTj the life shortens the life shortens ›› Generally half the life for every 10Generally half the life for every 10°°C increaseC increase

83 °C 87 °C

42 °C

80 °C95 °C

107 °C

50 °C

90 °C

115 °C 119 °C

60 °C

-0

20

40

60

80

100

120

140

Fixture A26W

Fixture B26W

Fixture C12W

Fixture D30W

Boa

rd T

empe

ratu

re (d

eg C

)

Open air Non-IC IC


Lumen Depreciation & Color ShiftLumen Depreciation & Color Shift

In the IC In the IC condition:condition:Life (L70) is less than 3000 hrsLife (L70) is less than 3000 hrsThe color shift is greater than a The color shift is greater than a 3636--step step MacAdamMacAdam ellipse ellipse (reached within 3000 hrs) (reached within 3000 hrs)

Fixture A - 26W LED Downlight

50%

60%

70%

80%

90%

100%

110%

100 1,000 10,000Time (hours)

Rel

ativ

e Li

ght O

utpu

t

Enclosed Semi-ventilated Open air

83 83 °°CC95 95 °°CC115 115 °°CCOpen airOpen airNonNon--ICICICIC

Fixture A - 26W LED Downlight

0

10

20

30

40

50

100 1,000 10,000Time (hours)

Mac

Ada

m E

llips

es



Fixture B - 26W LED Downlight

50%

60%

70%

80%

90%

100%

110%

100 1,000 10,000Time (hours)

Rel

ativ

e Li

ght O

utpu

t



In the IC In the IC condition:condition:Life (L70) is less than 3000 hrsLife (L70) is less than 3000 hrsThe color shift is greater than a The color shift is greater than a 1919--step step MacAdamMacAdam ellipse ellipse (reached within 4000 hrs)(reached within 4000 hrs)


Fixture B - 26W LED Downlight

0

10

20

30

40

50

100 1,000 10,000Time (hours)

Mac

Ada

m E

llips

es



Fixture C - 12W LED Downlight

50%

60%

70%

80%

90%

100%

110%

100 1,000 10,000Time (hours)

Rel

ativ

e Li

ght O

utpu

t



Even in the IC Even in the IC condition:condition:Life (L70) seems very longLife (L70) seems very longThe color shift is within The color shift is within a 4a 4--stepstepMacAdamMacAdam ellipseellipse (in the 3000 hrs)(in the 3000 hrs)


Fixture C - 12W LED Downlight

0

2

4

6

8

10

100 1,000 10,000Time (hours)

Mac

Ada

m E

llips

es



Fixture D - 30W LED Downlight

50%

60%

70%

80%

90%

100%

110%

100 1,000 10,000Time (hours)

Rel

ativ

e Li

ght O

utpu

t


80 80 °°CC90 90 °°CCOpen airOpen airNonNon--ICIC

Even in the IC Even in the IC condition:condition:Life (L70) seems very Life (L70) seems very longlongThe color shift is within The color shift is within a 3a 3--stepstepMacAdamMacAdam ellipseellipse (in the 3000 (in the 3000 hours)hours)

Lumen Lumen DepreciationDepreciation & Color & Color ShiftShift

Fixture D - 30W LED Downlight

0

2

4

6

8

10

100 1,000 10,000Time (hours)

Mac

Ada

m E

llips

es

Semi-ventilated Open air


LED Airport LightingLED Airport Lighting

There are several LED fixtures available in the There are several LED fixtures available in the market to replace traditional runway and taxiway market to replace traditional runway and taxiway fixtures.fixtures.

Taxiway (in-pavement, elevated) Obstruction Utility(L-852A, B, C, D, T; L-861T) (L-810)

White obstruction(L-810)

ADB-Siemens ADB-Siemens ADB-Siemens ADB-Siemens ADB-Siemens

ADB-Siemens

Dialight

Dialight


Why LEDs?Why LEDs?

Top performance criteriaTop performance criteria›› Improved visual performanceImproved visual performance

•• Intensity distribution and colorimetry specificationsIntensity distribution and colorimetry specifications

›› System reliabilitySystem reliability•• Maintain beam properties under Maintain beam properties under all weatherall weather conditions conditions

›› Reduced maintenance costReduced maintenance cost•• Life ratings under actual operating conditionsLife ratings under actual operating conditions

›› Reduced energy costReduced energy cost•• System efficiencySystem efficiency


WarmWarm--up Timeup Time

Stabilization time of LEDs is typically longer than 45 minutes, Stabilization time of LEDs is typically longer than 45 minutes, in some cases 150 minutes in some cases 150 minutes isis needed.needed.

Photometric and electrical characteristics depend on the Photometric and electrical characteristics depend on the junction temperature (Tj).junction temperature (Tj).

FAA specification requirements:FAA specification requirements:Perform tests after 15 minutes of operation.Perform tests after 15 minutes of operation.


LED Pin TemperatureALSF green threshold runway fixture

0

20

40

60

80

100

0 50 100 150 200 250Time (min)

LED

Pin

Tem

pera

ture

(ºC

)

Inside

Outside


Green Runway Threshold FixtureGreen Runway Threshold Fixture›› Measured LED pin temperature inside and out of an Measured LED pin temperature inside and out of an

enclosed spaceenclosed space


Peak Wavelength ShiftALSF green threshold runway fixture

0

1

2

3

4

5

0 50 100 150 200 250 300

Time (min)

Pea

k W

avel

engt

h S

hif

t (n

m)

Relative Light OutputALSF green threshold runway fixture

85%

90%

95%

100%

0 50 100 150 200 250 300

Time (min)

Rel

ativ

e Li

ght

Ou

tpu

t


Green Runway Threshold FixtureGreen Runway Threshold Fixture›› Measured light output and spectrumMeasured light output and spectrum



Blue OmniBlue Omni--directional Taxiway Fixturedirectional Taxiway Fixture›› Measured light output and spectrumMeasured light output and spectrum

Peak Wavelength ShiftL-861T blue omni-directional taxiway fixture

0

1

2

3

4

5

0 50 100 150 200 250 300

Time (min)

Pea

k W

avel

engt

h Sh

ift

(nm

)

Relative Light OutputL-861T blue omni-directional taxiway fixture

85%

90%

95%

100%

0 50 100 150 200 250 300

Time (min)

Rel

ativ

e Li

ght

Out

put

Life Life –– Light Output DepreciationLight Output Depreciation

Life of LEDs is better defined as light output depreciation Life of LEDs is better defined as light output depreciation over time, which depends on each LEDover time, which depends on each LED’’s technology and the s technology and the actual operating conditions.actual operating conditions.

High Power LEDs

50%

60%

70%

80%

90%

100%

100 1000 10000 100000Hours

Rel

ativ

e lig

ht o

utpu

t

White Red Blue Green

LRC Data 2004 - 350 mA; 35 ºC

FAA specification requirements:FAA specification requirements:Life is defined as a minimum number of rated hours to failure.Life is defined as a minimum number of rated hours to failure.

Life Life –– Light Output DepreciationLight Output Depreciation

Lumen depreciation of LEDs Lumen depreciation of LEDs with heatwith heat

I = Io exp - alpha (t)

70%

80%

90%

100%

0 10000 20000 30000 40000Hours

Ligh

t Out

put

Thermocouple

R2 = 0.96

010000

2000030000

4000050000

35 40 45 50 55 60T-point Temperature (deg C)

Life

(hrs

)


Dimming LEDs


DimmingDimming LEDsLEDs

Dimming methodsDimming methods›› Continuous Current Reduction (CCR) Continuous Current Reduction (CCR)

›› Pulse Width Modulation (PWM)Pulse Width Modulation (PWM)

The duty cycle (D):D = τ / T

T

τ

Ligh

t Out

put

Drive Current

Y. Gu et al., SPIE 2006


Dimming WhiteDimming White LEDsLEDs

DybleDyble etet al.al. in 2005in 2005 quantified color shift of quantified color shift of highhigh--powerpower white white LED systems due to CCR LED systems due to CCR andand PWM dimming. PWM dimming. ›› Compared to PC white LEDs, Compared to PC white LEDs, mixed color (RGB)mixed color (RGB) white LEDs had very white LEDs had very

large color large color shift undershift under both dimming methods.both dimming methods.

Dyble et al., 2005


-10-8-6-4-202468

10

0% 20% 40% 60% 80% 100%current level/duty cycle

peak

wav

elen

gth

shift

-10-8-6-4-202468

10


peak

wav

elen

gth

shift

-10-8-6-4-202468

10


peak

wav

elen

gth

shift

Peak Peak WavelengthWavelength ShiftsShiftsForFor InGaN InGaN LEDs,LEDs, the peak wavelength shifts are in opposite directions for the tthe peak wavelength shifts are in opposite directions for the two dimming wo dimming schemes.schemes.

›› Current: blue shift with increased currentCurrent: blue shift with increased current

›› PWM: red shift with increased duty cycle PWM: red shift with increased duty cycle

For For AlInGaPAlInGaP LEDs,LEDs, the peak wavelength shifts are in the same direction for the twthe peak wavelength shifts are in the same direction for the two o dimming dimming schemes.schemes.

›› Current and PWM: red shift with increased current or duty cycle Current and PWM: red shift with increased current or duty cycle

PC white (blue): 447 nmRed: 646 nm, Green: 541 nm, Blue: 450 nm

CC - R CC - G CC - BCC - W PWM - R PWM - GPWM - B PWM - W

Manufacturer A Manufacturer B Manufacturer C

PC white (blue): 459 nmRed: 639 nmGreen: 525 nmBlue: 465 nm

PC white (blue): 460 nmRed: 644 nmGreen: 518 nmBlue: 455 nm



Luminous Luminous EfficacyEfficacy

Luminous efficacy of a light source can be Luminous efficacy of a light source can be affected by several factors:affected by several factors:›› Radiant energy outputRadiant energy output

›› Power dissipated at the junctionPower dissipated at the junction

›› SPD change (wavelength shift)SPD change (wavelength shift)


Luminous Luminous EfficacyEfficacy ChangeChange

To create equal light output at dimmed levels, PWM demands To create equal light output at dimmed levels, PWM demands more power than current dimming.more power than current dimming.›› Efficacy is higher for current dimming at dimmed levels Efficacy is higher for current dimming at dimmed levels

0%

20%

40%

60%

80%

100%

0.0 0.3 0.6 0.9 1.2 1.5power (W)

rela

tive

light

out

put

0%

20%

40%

60%

80%

100%

0.0 0.3 0.6 0.9 1.2 1.5power (W)

rela

tive

light

out

put

CC - R CC - G

PWM - R PWM - G Y. Gu et al., SPIE 2006



Luminous efficacy forLuminous efficacy for LEDsLEDs from manufacturer Bfrom manufacturer B›› At dimmed At dimmed levels,levels, efficacy is always higher for current dimming, for all efficacy is always higher for current dimming, for all

colors.colors.

Manufacturer B

0%

50%

100%

150%

200%

250%

300%


rela

tive

effia

cy

CC - R CC - GCC - B CC - WPWM - R PWM - GPWM - B PWM - W




Luminous efficacy @ 20% dimming level forLuminous efficacy @ 20% dimming level for LEDsLEDs from from manufacturer Bmanufacturer B

0%

50%

100%

150%

200%

B - CCR B - PWM

rela

tive

effic

acy

R G B W



SummarySummary

ForFor InGaN InGaN LEDs,LEDs, the peak wavelength shifts are in opposite the peak wavelength shifts are in opposite directions for the two dimming schemes.directions for the two dimming schemes.›› Current: blue shift with increased currentCurrent: blue shift with increased current

•• Band filling effectBand filling effect

›› PWM: red shift with increased duty cycle PWM: red shift with increased duty cycle •• Heating effectHeating effect

For For AlInGaPAlInGaP LEDs,LEDs, the peak wavelength shifts are in the the peak wavelength shifts are in the same direction for the two dimming same direction for the two dimming schemes.schemes.›› Current and PWM: red shift with increased current or duty cycle Current and PWM: red shift with increased current or duty cycle

•• Heating effectHeating effect

At dimmed At dimmed levels,levels, luminous efficacy is always higher for luminous efficacy is always higher for current dimming, for all colors.current dimming, for all colors.


AcknowledgmentsAcknowledgments

LRC faculty, staffLRC faculty, staff, and students, and students

LRC program and project sponsorsLRC program and project sponsors

ASSIST Program SponsorsASSIST Program Sponsors


Thank YouThank You

short- and long-term performance of leds© 2008 rensselaer polytechnic institute. all rights...

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