Thermal Challenges and Solutions for Industrial Solid State Lighting ApplicationsPeter RescaSr. Director of Product DevelopmentAdvanced Thermal SolutionsPresented at: APEC 2016 Long Beach CA

Agenda1.Challenges of thermal management in Industrial lighting2.Implications of temperature stress on the system3.Tools to manage the thermal stress4.Application Example5.Conclusion

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Comparison of LED to Incandescent Lamp

8%

73%

19%

Incandescent

Visible LightIRHeat

20%

80%

LED

Visible LightIRHeat

• Most of the energy is infrared• Energy losses are mainly lost by Radiation heat transfer• No radiation• Conduction heat transfer dominant

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LED - inside view

• The efficiency is dependent on many parameters• Not all of these parameters can be influenced by the user• The most important parameter that can be influenced is the junction temperature of the LED, by applying effective thermal managementLuxeon K2 power LED (courtesy of Lumileds) 4

Conduction - junction to heatsink

Metal core 1.6mmPrepreg

150umCopper70um

HeatslugSolderpaste 150um

Rslug-solder padRjunct-heatslug

Rmetal coreInterface material

Heatsink baseRinterface

Rhs, (material and spreading resistance)Ths

Tjunct.

P

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LED Parameters vs. Temperature• Light output is strongly dependent on temperature• Temperature has an effect on forward voltage• Temperature reduces lifetime• Exceeding Tj, max may permanently damage LED=> Thermal Management for LED based solutions is imperative!

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Light Output vs. Temperature

http://cree.com/~/media/Files/Cree/LED-Components-and-Modules/XLamp/Data-and-Binning/XLampMCE.pdf

Temperature can directly impact the output of the emitting light

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Lifetime vs. Temperature

(B50, L70) lifetimes against junction temperature for LUXEON Rebel LED

Temperature can directly impact the life of the LED

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Thermal Analysis Process

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DFM Solution

Analytical Modeling • Fundamental based modeling for quick results• Generate models for what-if scenarios• Physics based results

Computational Modeling• Flotherm• Cfdesign• Icepak• CAD tools

Empirical Modeling• Liquid & air flow testing• JEDEC testing• IR & LC thermography• Temperature measurement• Velocity measurement• Pressure measurement

Validated Prototype

Ready for high volume, low cost production

Custom Heat SinksStandard Heat Sinks

Results

What Does Thermal Management Entail?Objective: To maintain device junction below specified temperature for worst case environment.

Hierarchy (level) of modelling:• Environment: Where the system resides• Enclosure: Houses the electronics• Board: Housing the components• Component: Housing the dies (chip)• Chip: Housing the electronics parts

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Example Thermal Managementof LED Based Down-lighter

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LED Based DownlighterHeat Sink

LEDsLens

Lens housing and protective cover

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Three modes of heat transfer• Conduction• Convection• Radiation

Convection

RadiationRadiation

Internally conduction

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Impedance Model

Metal core 1.6mmPrepreg

150umCopper70um

HeatslugSolderpaste 150um

Rslug-solder padRjunct-heatslug

Rmetal core

9 K/W (see spec)0.13 K/W (k=50W/mK, 22.5mm1.8 K/W (spreading resistance + material)

Interface materialHeatsink base

Rinterface 0.2 K/WRhs, base, spreading0 K/W

Ths

Tjunct.

P

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Conduction and Spreading

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Heat

LED

Good x-y Thermal conductivity(3 W/m-K Dielectric)Poor x-y Thermal Conductivity (0.3 W/m-K Dielectric)

DielectricCopper Metal Base

Conduction and Spreading

Spreading in copperSpreading in prepreg

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Convection/ Radiation)*(

1* rrcc

hs AhAhR

KmWhc 210

KmWhr 23.6

WKR

RRRtotalhs

rctotalhs

6.424.038.4,

int,

WK

EERhs 38.422.5*3.696.1*101

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BaseFins

Interface base to fins

Phs

RconvRrad

Tambient

Rbase bot-top 0.006 K/WContact resistance base to fins 433 mm2h' screw contact 1.1 [K-cm2/W]Rinterface 0.25 K/W DThs-air@9.6W =

44.4K17

Convection/ Radiation

WKWRhsKWT

/86.5)6.9(3.56)6.9(

No. of Fins 12Weight (grams) 45

Length (L) 45Base thickness (t) 7

Heat Sink Configuration Dimensions [mm]Diameter (D) 45

Thermal Performance Graph

0.0010.0020.0030.0040.0050.0060.0070.0080.00

4.00 5.00 6.00 7.00 8.00 9.00 10.00 11.00 12.00Power Dissipation [W]

Ths-a

mbien

t [K]

Vertical mounted

• Performance based on analytical DThs-air@9.6W = 44.4K • Assume free inflow of air to fin area18

Analytical Analyses LED Downlighter

Ths, base64/ 84°C

Tpcb =72/ 92°C

Tcore66/ 86°C

Rj-pcb = 9.1K/W

Rspr, pcb = 1.8K/W

Rinterface0.2K/WRsp, hs0K/W

P=3.20W (3x)

Tj =101/ 121°C

Pc+r=9.6W

20/ 40°C

Tj, max = 121°C < 139°C => OK

All resistances are calculated by using Analytical based formula

Rhs. Conv + rad4.6K/W

Light efficiency LED 1000mA@108°C ~9.4% => 3.53x0.91 = 3.20W dissipationTj, life = 101°C < 114°C => OK

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Computional Analyses (CFD)

heatsink

base

MCPCBCompact

ModelLED

Interfacematerial3D Modeling downlighter in free air.Solved for Conduction/ convection/ radiation

Side of housing not shown

Troom 20°CTj, max =113°C

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Experimental Test Setup

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Measurement of junction temperature based on Forward Voltage

Test setup

Infrared picture downlighter

Results

Paramater Units

Analytical, with

experimental hs-data

Analytical, only CFD Experiment

Tambient °C 20 20 20 20Iforward mA 1000 1000 1000 1000Light efficiency % 9% 9% 9% 9%Tdissipated total °C 9.6 9.6 9.6 9.6Theatsink base °C 76 66 75 71Tboard, copper led °C 84 74 84 78Tj, led °C 113 103 113 107Comparison methods 106% 96% 105% 100%

Results of different solution methods show good comparison, within 6%.22

Other Considerations• Power Dissipation – LED components, multichip modules and fixtures• Solar Loading• Heat Sink Materials - Copper, Aluminum• Interface Material – Grease, Phase Change, Gap Pad• Process – Cast, Skiving, Extrusions

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Conclusion• LED’s by their construction and application pose unique thermal challenges.• Proper thermal management a critical variable that can be established and controlled in the design.• Impedance diagrams are a helpful tool for understanding, modeling and analyzing a thermal management problem.• Always calculate/measure the junction temperature; Use the three step approach to the analyze the problem (Analytical/ Computational/ Experimental)• Measure the light efficiency or ask the supplier for input to have the real dissipated power.• Example did show good comparison between three methods providing a solution right the first time.

LEDs can never be cool enough!!

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Thank You• For more information:• www.qats.com• http://www.qats.com/Applications/LED-Applications• Peter Resca – presca@qats.com

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