general photo-electro-thermal theory for led systems the led...

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General Photo-Electro-Thermal Theory for LED Systems &

the LED Revolution for General Lighting

Ron HuiCenter for Power ElectronicsCity University of Hong Kong

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Content

• Introduction – Lighting Systems and Power Systems

• The LED Revolution– Problems & Solutions

• General Photo-Electro-Thermal Theory• Conclusions

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NASA

2 Billion people have no electric lighting

Lighting consumes ~ 20% of total electrical power

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Road Lighting systems “today” use High-Intensity-Discharge Lamps

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Energy saving of 30%

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Examples of Saving Achieved by Outdoor System

鶴山 28% (10,000 street lamps in Sept. 2006)高明 49%東莞 30%梧州 30%Hong Kong 25% (1500 street lamps in 2005-2006)

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48 MW

26 MW

26 MW

Power

15 MW240,000Shanghai

9 MW130,000Shenzhen

9 MW130,000Hong Kong

e.g. 30% change

No. of street lamps

City

Public lighting systems in car parks, corridors and hallway can also be grouped together

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Why LED?

• High-Intensity-Discharge (HID) lamps cannot be re-ignited immediately after turn-off unless “hot-igniter” of 20kV is available.

• HID lamps can only be dimmed over “tens of minutes” to avoid sudden breaking of discharge arc.

• LEDs are semiconductor devices capable of switching at Mega-Hertz (A Highly Dynamic Load – micro-second response time)

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Content




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120HID lamps

20-80LEDs

50-60CFLs

10Incandescent lamps

1candles

Lumen/Watt

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US$ 10US$ 7Cost /10000 hrs

1 piece √4 piecesE-waste / 40000 hrs

US$ 40US$ 7Cost / unit

No √YesMercury

40,000 hrs √10,000 hrsLifetime

80 lm/W √60 lm/W Efficacy

LED bulbe-CFL

√ √X

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LED Current Status

• Successful in– Decorative – Display– Signaling– Signage

• Not so successful in – General/Public Lighting

All major LED Road Lighting projects in China in 2006-2009 failed.

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3 remaining hurdles to General/Public Lighting

• High Temperature greatly affects the LED performance because

– Efficacy (Lumen/Watt) degradation with junction temperature rise

1. LED System Thermal Design not optimised.2. LED Device structure not optimised. (junction thermal

resistance is too high)3. Lifetime of LEDs (<8 years) and LED Driver (<5 years)

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Heat remains a limiting factor in LED Technology

From 2008 IEEE APEC Tutorial Note

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Photograph of the setup

Simple methodology for heat measurementin an open system

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Temperature data analysis and heat dissipation factor

Pheat = Heat dissipation from the lamp in Watts Plamp = Total electrical input power of the lamp in Watts.

With the heat dissipation measurement obtained, one can define a heat-dissipation factor kh(Plamp) for a lighting device, where

This kh(Plamp) factor is an indicator of the amount of heat energy emitted from a lighting device for a given electrical input power of the lamp.

lamp

heatlamph P

PPk

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Practical tests and heat dissipation comparison

Thermal measurements have been conducted in

(i) one 18W T8 fluorescent lamp,

(ii) one 14W T5 fluorescent lamp,

(iii) six Philips 3W white high-brightness LEDs

(iv) eighteen Philips 1W white LEDs

(v) six CREE 3W LEDs

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Reference temperature-time curves & reference temperature-power curvebased on a “Reference Resistor”

Temperature Rise - Time curves of 12W, 14W, 16W, and 18W of the reference-resistor Temperature Rise - Power reference curve

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Temperature measurements of the three types of lighting device

Temperature rise curves for one 18W T8 fluorescent lamp working at 12W, 15W & 18W

Heat dissipation factor kh(Plamp) of 18W T8 lamp as a function of lamp power.

i) One 18W T8 fluorescent lamp

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Independent Confirmation

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The Temperature Rise – Time curves of the one 18W T8 lamp,Eighteen Philps Luxeon K2 1W LEDs, six Philps K2 3W LEDs

and six CREE LEDs – with all systems at 18W operation

18W

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Comparison of luminous efficacy and heat dissipation of LEDs and fluorescent lamps

0.870.890.90.730.77HeatDissipation

Factor

78.5 at 3W(heatsink

temperature of 76oC)

30 at 3W(heatsink


31 at 1W(heatsink


96.760.3MeasuredEfficacy

Lumen/Watt

107At 25oC

Junctiontemperature

40At 25oC

Junctiontemperature

45At 25oC

Junctiontemperature

9661.1RatedEfficacy

Lumen/watt

LED(CREE)

3W LED(PhilipsLuxoen)

1W LED(PhilipsLuxeon)

14W T5Fluorescent

lamp(Philips)

18W T8Fluorescent

lamp(Osram)

At Full Power

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Solutions

• Optimal LED Devices • Optimal LED Integrated System Design

• LED Drivers with long (>15 years) lifetime

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Content




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LED Characteristics3 - 4 oC/W

2009

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Efficacy degradation with temperature

125

)25(exp

TCT

IIo

jCo

ojeo TTkEE 1

oj TT 0E

ke is negative

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Basic General Photo-Electro-Thermal Theory

ojeo TTkEE 1

Pheat

Rjc

Rjc

Pheat

Rhs

Ta N LED devices

Ths Tj

1

N

dv PEN

Light and Electric Power

Light and Heat

dhhsaheathsahs PNkRTNPRTT

dhjchsheatjchsj PkRTPRTT

Heat and Electric Power

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Basic General Photo-Electro-Thermal Theory

ojeo TTkEE 1

Pheat

Rjc

Rjc

Pheat

Rhs

Ta N LED devices

Ths Tj

1

N

dv PEN

Light and Electric Power

Light and Heat

dhhsaheathsahs PNkRTNPRTT

dhjchsheatjchsj PkRTPRTT

Heat and Electric Power

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21 dhsjchedoaeov PNRRkkPTTkNE

221 ddv PP

v

dP*dP

Peak luminous output may not occur at the rated power !!!

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d

jcdhe

d

hdhsjce

d

edhsjchdoa

dhsjcheoae

od

v

dPdR

PkkdPdkPNRRk

dPdkPNRRkPTT

PNRRkkTTkNE

dPd

222

21

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Simplified model with constant Rjc

hsjche

oaed NRRkk

TTkP

2

1*

21 dhsjchedoaeov PNRRkkPTTkNE

ke is –ve

Rjc =10oC/W

Choices:

Rhs= 3.9oC/W

N=?

v

dP*dP

Increasing sizes of heatsink

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Vigorous model with temperature-dependent Rjc

djcjcojc PkRR 1

321 djcojchedhsjcohedoaeov PRkkkPNRRkkPTTkNE

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221 dddv PPP

Extra 3rd term

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Thermal resistance RhsoC/W

The better the cooling effect, the lower the thermal resistance Rhs becomes.

Practical Verification

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Tests on Luxeon 3W LEDs

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Test-1:

Mounting 8 Luxeon 3W White LEDs on a “Small” heatsink

with thermal resistance of 6.3oC/W

Pd=1.95W @ 315 lm

Problem !!!2

21 ddv PP

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221 dddv PPP

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Test-2:

Mounting 8 Luxeon 3W White LEDs on a “Medium” heatsink


Pd=2.45W @420 lm

Problem !!!

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Test-2:

Mounting 8 Luexon 3W White LEDs on a “Large” heatsink


Pd=3.65W @ 560 lm

Problem !!!

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Effects of Thernal Design on Light Output

0

200

400

600

800

1000

1200

1400

0 1 2 3 4 5

Power per LED (W)

Tota

l Lum

inou

s Fl

ux (L

umen

s)

measured light output of6.3oC/Wcalculated light output of 6.3degree/W

measured light output of 4.5degree/Wcalculated light output of 4.5degree/Wmeasured light output of 2.2degree/Wcalculated light output of 2.2degree/W

calculated light output of 0degree/W

Rated LED Power = 3W

Luexon 3W

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Tests on Cree 3W LEDs

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Mounting 2 CREE 3W LEDs on heatsinkwith Rjc = 6.3oC/W

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Mounting 2 CREE 3W LEDs on heatsinkwith Rjc = 4.5oC/W

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Optimal Design Method

Increasing cooling effect (e.g. thermal resistance Rhs decreasing)

Pd (W)

A B

C D

Theoretical limit: Rhs=0

Rated Pd

Per LED Power

Total Luminous flux/N

o. of LED devices

(lumens)

a

bc

d

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Increasing cooling effect(e.g. thermal resistance Rhs decreasing)

Pd (W)

A B

C D

Theoretical limit: Rhs=0

Rated Pd

Per LED Power

Total Luminous flux/N

o. of LED devices

(lumens)

a

bc

d

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Implications of the theory

hsjche

oaed NRRkk

TTkP

2

1*

Device Level: for LED manufacturers

[for the same rated power]

Multi-chip LED structure is better than single-chip structure

System Level: for the LED system designers

[for the same system power]

Distributed system with more low-power LEDs is better than concentrated system with less high-power LEDs

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Multi-chip

Single-chip

Device Level

Multi-chip

Single-chip

Multi-chip

30 x 0.1W = 3W

Single-chip

1 x 3W = 3W

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Using “Distributed” five 1W LEDs or “Concentrated” one 5W LED ?

System Level

Distributed

Concentrated

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Conclusions• LED technology is at a critical point of entering the market

of General/Public Lighting• LED improvements continue.• LED Systems with lifetime > 8 years to be ready in 2010• LED Drivers with lifetime of >15 years to be ready in 2010.

• Large-scale LED lighting systems are dynamic loads for emerging power grids.

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Caution• Theoretically, LEDs are being

promoted as energy-efficient lighting devices.

• Practically, they have opened a door to widespread lighting energy abuses & light pollutionproblems.

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