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Budapest University of Technology and Economics

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Department of Electron Devices

Diagnostics of LED-based streetlighting luminaires by means of thermal transient method Gábor Marosy, Zoltán Kovács, András Poppe

The 16th THERMINIC Workshop, 6-8 October 2010, Barcelona, Spain

© BME Department of Electron Devices, 2010.

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6 October 2010 G. Marosy, Z. Kovács, A. Poppe: Diagnostics of LED-based streetlighting luminaires ... 2

Introduction ►Similar trends in SSL like Moore’s Law in microelectronics

►Continuous decrease of price of light [$/lm] More application fields, streetlighting is among the early birds

Low voltage levels controllability


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Introduction ►LED based light sources still more expensive than traditional

ones, but they provide some benefits Longer life-times expected – if junction temperatures are kept low

Controllability (in case of DC driving)

DC power distribution networks for indoor applications?

Easy dimming (e.g. by PWM)

Possibility of integration with communications networks

Possibility of built-in intelligence, including diagnostic functions

Streetligting: present safety standards allow low voltage only inside an LED luminaire DC driven LEDs with driver/control circuitry

50% liminous flux 80% limous flux


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Introduction: vision of intelligent streetlighting ►Control, diagnostics, sensing and communications functions

can be built in.


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What is the real issue? Keep Tj low! ► In case of LEDs, heat-loss is mainly by conduction

Power LED

light: ~15..40%

heat loss: ~85-60%,

mostly by conduction

- ►Heat-conduction is the initial part of the dissipation

►The luminaire is used as a heat-sink, like in case of retrofit LED lamps

►There are different thermal interfaces present in the heat-flow path

active layer to submount classical die attach (TIM1) solder/glue to MCPCB

classical TIM to heat-sink (TIM2)

Thermal transient testing proved to

be a good testing tool for TIMs…


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The luminaire acts as a heat-sink ►Quality of the TIM between the LEDs and the luminaire is

critical

LED package models identified

by T3Ster+TERALED, simulation

by FloTHERM CAD model by courtesy of

HungaroLux Ltd. (Budapest,

Hungary)


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Data from thermal transient tests of LEDs

1e-5 1e-4 0.001 0.01 0.1 1 10 100

0.5

1

1.5

2

2.5

3

3.5

Time [s]

Pu

lse

th

erm

al re

sis

tan

ce

[K

/W]

T3Ster Master: Pulse Rth Diagram

CREE_MCE_AL_2_25_MY_F1_T25_I0350 - 0.50

CREE_MCE_AL_2_25_MY_F1_T25_I0350 - 0.25

CREE_MCE_AL_2_25_MY_F1_T25_I0350 - 0.10

CREE_MCE_AL_2_25_MY_F1_T25_I0350 - 0.05

h(t) a(t)

Junction temperature

time

time

Power

0.5 1 1.5 2 2.5 3 3.5 4

0.001

0.01

0.1

1

10

100

Rth [K/W]

Cth

[W

s/K

]

T3Ster Master: cumulative structure function(s)

CREE_MCE_AL_2_25_MY_F1_T25_I0350 - Ch. 0

Pulse thermal resistance diagram

Structure function

Package dynamic model

► The h(t) step-wise change in heating is applied at the junction (abrupt switching)

► The a(t) temperature response at the junction is being measured (unit-step response function) while linearity is assumed

► All available information is extracted from a(t) using sophisticated mathematical procedures structure functions – ideal to characterize TIM compact dynamic thermal models pulsed thermal resistance / complex locus (frequency domain representation)


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The goal is: assess quality of TIMs in the field ►How feasible is it?

Case studies of different assemblies for streetligting luminaires analyzed by thermal transient measurements

• Temperature dependent change of TIM observed (Poppe et al, SEMI-THERM’10)

Long term stability studies have been performed to see what one may expect

• Changes of classical TIM observed frequently (Poppe et al, SPIE SSL’10)

• In case of some LEDs DA degradation and degradation of other thermal interfaces (such as glue/solder between the LED package and MCPCB was observed)

►Which change can be measured in a luminaire? At PWM-based dimming

• Short time windows only, e.g. 10..100ms

• Would allow DA testing, but requires time and VF measurement resolution of a lab equipment

Daytime, planned on/off sequences • No testing time limitations

• With smaller time resolution classical TIM issues can be detected

• Timing requirements: for time constant measurement between 0.1 and 10 is necessary (Székely, THERMINIC’08) realistic with cheap circuitry

►Problems: no option for K-factor calibration / computation


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Case study – for the KÖZLED project ►3 different assemblies with high-end 10W white LEDs

FR4 PCB, TIM between the heat-slug and the Cu block

FR4 PCB, heat-slug soldered to the Cu block

MCPCB-s made of Al and Cu, heat-slug soldered

CAD images by

courtesy of

OptimalOptik Ltd.


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FSF52

AL

TG2500

RthJC real ≈ 2 K/W

Results for 10W white LEDs ►Measured at 700 mA and 85oC

Thermal impedance of 3 samples, power corrected with Popt


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►Measured at 700 mA and 85oC Structure functions of 3 samples, power corrected with Popt

RthJC is identified in a way similar to the transient double interface method, standardized by the JEDEC JC15 committee

FSF52 AL TG2500

RthJC real ≈ 2 K/W

Results for 10W white LEDs


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Results for 10W white LEDs ►Measured at 700 mA & between 15oC and 85oC

Structure functions of sample AL-2, power corrected with Popt

As temperature changed, TIM quality changed

LED package:

no variation

Al MCPCB &TIMs:

temperature

dependence


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LM80 test chamber with

all the LEDs assembled

In-situ light output

measurement

In-situ thermal transient

measurement

All measurements are

done in-situ to eliminate

any Rth change which is

NOT due to ageing

LM80 Tests @ Univ. of Pannonia, Hungary


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4.Osram LUWV5AM 350mA

85%

90%

95%

100%

105%

110%

0 200 400 600 800 1000 1200

41

42

43

44

45

46

aver.

Vendor O

Structure functions taken at

0h, 500h, 1000h

Relative

luminous flux

Time [h]

Results from LM80 test of different LEDs ► In cooperation with University of Pannonia, Veszprém

(Hungary), within the KöZLED project

►8 different kinds of LEDs from 4 vendors, so far 4000h burning time

No change inside

the LED package

Ligh output drop likely due

to increased Rth caused by

TIM degradation, not by

LED degradation


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Results from LM80 tests: with accurate dVF/dT ►Results when accurate temperature sensitivity of the forward

voltage is considered in thermal transient testing:

►Problems: dVF/dT sensitivity also changes in time

We have no means to calibrate

0 2 4 6 8 10 12

1e-5

1e-4

0.001

0.01

0.1

1

10

100

Rth [K/W] C

th [

Ws/K

] T3Ster Master: cumulative structure function(s)

15.1

Vendor O, sample #44, 0h




0.9

With real values of dVF/dT

1e-6 1e-5 1e-4 0.001 0.01 0.1 1 10 0

2

4

6

8

10

12

14

16

t [s]

T

J [

°C] T3Ster Master: Smoothed response






LED package: small variation

after 500h

TIM: +0.06∙RthJA


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Results from LM80 tests: changing dVF/dT ►Results for the changes of the temperature sensitivity of the

forward voltage for samples of vendor O:

►E.g. for sample #44 4.3% change in the temperature sensitivity was observed This change “compensates” the effect of TIM ageing…

Vendor O, samples 41-46: dVF/dT Sensitivity Change

-2.4

-2.3

-2.2

-2.1

-2

-1.9

-1.8

-1.7

-1.6

0 500 1000 1500 2000 2500 3000 t [hour]

Sen

sit

ivit

y [

mV

/K]

O_41

O_42

O_43

O_44

O_45

O_46


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►Considering a constant sensitivity value (-2mV/K) the aging of TIM is screened:

Results from LM80 tests: without real dVF/dT

1e-6 1e-5 1e-4 0.001 0.01 0.1 1 10 t [s] 0

2

4

6

8

10

12

14

16

T

J [

°C] T3Ster Master: Smoothed response





dVF/dT = -2mV/K

0 2 4 6 8 10 12 14 16

1e-5

1e-4

0.001

0.01

0.1

1

10

100

1000

Rth [K/W]

Cth

[W

s/K

]


GoldenD_44_0h - Ch. 0

44_54_64_500h - Ch. 0

14-24-34-44_2000h - Ch. 3

14_24_34_44_3000h - Ch. 3

With an assumed constant sensitivity even structure functions appear to be identical, though in a quick diagnostic system in the field probably there will be no possibility to implement structure function calculations.

We may expect to see drastic changes only!


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Drastic ageing observed at another vendor… ►Results with real sensitivities:

Vendor HK, samples 61-65: dVF/dT Sensitivity Change -1.7

-1.6

-1.5

-1.4

-1.3

-1.2

-1.1

-1

-0.9 0 500 1000 1500 2000 2500 3000

Se

ns

itiv

ity [

mV

/K]

HK_61 HK_62 HK_63

HK_64 HK_65

t [hour]

0 2 4 6 8

5.6 4.7 0.6

1e-5

1e-4

0.001

0.01

0.1

1

10

100

Cth

[W

s/K

] T3Ster Master: cumulative structure function(s)

Rth [K/W]


Vendor HK, sample #61, 0h Vendor HK, sample #61, 500h



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Drastic ageing observed at another vendor… ►Results considering an assumed constant sensitivity only:

1e-6 1e-5 1e-4 0.001 0.01 0.1 1 10 0

1

2

3

4

5

6

t [s]

T3Ster Master: Smoothed response

T

J [

°C]

dVF/dT = -2mV/K



0 1 2 3 4 5 6

1e-4

0.001

0.01

0.1

1

10

100

1000

Rth [K/W]

Cth

[W

s/K

]


HK_61_0h - Ch. 0

41_51_61_500h - Ch. 2

51-61-91-101_2000h - Ch. 1

51_61_71_3000h - Ch. 1

Delamination of the LED/MCPCB interface as well as TIM ageing can be observed even from the measured raw forward voltage transients.

30 s measurement time is sufficient for this test, sufficient time resolution can be 0.1ms or even 1ms

Real life conditions are even harsher: vibration humidity temperature cycling


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Conclusions ►We made an analysis of possible structural integrity issues

in the junction to ambient heat-flow paths of LEDs to be used in streetlighting luminaires.

►Preliminary results of long term stability studies of LEDs suggest that in harsh environment we should expect degradation of the different thermal interfaces.

► If the degradation exceeds a given limit (resulting thermal resistance change bigger then variation of temperature sensitivity of the forward woltage), with built-in thermal transient measurement based diagnosis early warnings can be provided and maintenance of luminaires can be scheduled to avoid fatal breakdown. Requirements for such testing circuitry are not too high, ~1ms time

resolution seems to be sufficient ti detect LED/MCPCB or TIM2 delamination/ageing

No sophisticated data processing is required for such tests, it seems, raw forward voltage transients are sufficient


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Acknowledgements ►This work was partially supported by the KÖZLED

TECH_08-A4/2-2008-0168 project of the Hungarian National Technology Research and Development Office.

►Further support of the Hungarian Government through the TÁMOP-4.2.1/B-09/1/KMR-2010-0002 project at the Budapest University of Technology and Economics is also acknowledged.

►We are grateful for the help of KÖZLED partners: University of Pannonia (Veszprém, Hungary) for sharing their LM80

test facility and test results with us.

OptimalOptic Ltd. for LED samples with different thermal management solutions and their CAD images

Hungarolux Ltd. for CAD images of their luminaires

►Help of Mentor Graphics MicReD Division is also acknowledged for some CFD simulations as well as helping us perform some of our measurements

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