Improving LED Performance: Thermal Analysis of LED Phosphor Layer

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2. New York City Section Further Improving White LED Performance : Thermal Analysis of LED Phosphor Layer Ukwatte L. Indika U. Perera Lighting Research Center Rensselaer Polytechnic Institute, Troy, NY

3. Motivation • For phosphor converted white LEDs to reach 226 lm/W by 2020 further improvements are needed. • The down-conversion and LED encapsulant materials are key areas that require improvement in achieving this target. • Improvement in encapsulant material identified as a key area of research only in the 2015 U.S. DoE R&D roadmap • Heat generated in the LED package affects both lumen output and lumen degradation. Source: U.S. DoE R&D roadmap, 2015

4. Motivation Phosphor heat The heat generated in an LED package • LED chip • Phosphor layer • Increased optical power LEDs, the heat in the phosphor layer is becoming a significant problem. Thermal management • LED: Studied extensively • Phosphor layer: Studies are scarce Thesis motivation: • To study phosphor layer heat transfer to improve LED system performance phosphor layer LED heat optics LED LED heat sink dissipated via heat sink In 2014 U.S. DoE SSL R&D Roundtable meeting identified thermal management of the down-conversion material layer as a key area for research

5. Background (Y2.93Ce0.07) Al5O12 under 460 nm Heat is generated within a phosphor layer due to • Phosphor conversion efficiency losses, Stokes shift losses, and trapped photons within the phosphor layer The increase in temperature affects LED performance • Reduced light output • Phosphor quenching • Reduced lifetime • LED and encapsulant degradation • Chromaticity shift • At the chip • At the phosphor Barton et al. 1998; Tamura et al. 2000; Narendran 2005; Tsai et al. 2009; Setlur 2009; Chen et al. 2010; Wenzl et al. 2012, 2013; Huang et al. 2013 Source: Chen et al. (2010) Source: Tamura et al. (2000)

6. Background … cont. Phosphor layer heat buildup depends on • Distance between LED and phosphor Arik et al. 2003; Fan et al. 2007; Hwang et al. 2010; Yan et al. 2011; Hu et al. 2012; Dong et al. 2013 • Thickness of the phosphor layer Yan et al. 2011; Wenzl et al. 2013; Kolodeznyi et al. 2014; Mou et al. 2014 • Concentration of phosphor Arik et al. 2003; Yan et al. 2011; Hu et al. 2012; Huang et al. 2013; Luo and Hu 2014; Mou et al. 2014 • Binding media used for encapsulating phosphor Arik et al. 2003; Wiesmann et al. 2012 • LED package structure Arik et al. 2003; Narendran et al. 2004, 2005; Kim et al. 2005; Zhu 2006; Hoelen et al. 2008; Yan et al. 2011; Hu et al. 2012; Luo et al. 2013 • Conversion efficiency of phosphor Mueller-Mach et al. 2002; Arik et al. 2003; Setlur et al. 2004, 2006; Wenzl et al. 2012; Luo et al. 2013; Luo and Hu 2014

7. Summary: literature review Methods to lower phosphor temperature • Binding materials and or fillers to improve thermal conductivity • Limited experimental conductive binding material studies • Computer modeling with limited experimental validation Arik et al. 2003 ; Huang et al. 2004; Lin et al. 2010; Chung et al. 2012; Wiesmann et al. 2012; Luo et al. 2013 • Use of modified drive current • Computer modeling with limited validation • Reduced light output and perceivable color shift Wenzl et al. 2013 • Use of composite structures to dissipate heat • Limited studies quantifying performance of these methods • Reduced light output and limited analysis of the reasons • No studies looking into reliability of these type of composite heat sink structures Allen et al. 2010; Lowery et al. 2011; Tong et al. 2011; Perera and Narendran 2013

8. Problem statement and thesis goal Short-term effects Problem: • Heat buildup in phosphor layer negatively affects phosphor-converted white LED performance • Short-term and long-term effects Thesis goal: • To better understand the heat transfer in the phosphor layer to reduce phosphor operating temperature and improve system performance. • Heat induced lumen degradation in short and long term Source: Lin et al. (2010) Long-term effects Source: Narendran et al. 2004

9. Knowledge gap • Limited studies on how heat generation in the phosphor layer affects • The temperature and the temperature distribution • The light output of the LED system (short- and long-term) • Only a few studies have considered thermal management of the phosphor layer to improve system performance • Most of these studies have not quantified or experimentally validated their findings. • There are no models that predicts temperature of the phosphor layer and the resulting light output of the LED system for different system configurations.

10. Thesis objectives • To analyze and quantify the contribution of heat transfer mechanisms in the thermal management solution used in my MS thesis study. • To develop a theoretical model that can predict phosphor layer temperature profile and the light output for an LED system. • To validate the model by comparing the model results with observed past experiment results.

11. Theoretical model Light propagation model • Objective: To predict phosphor layer temperature and total light output Thermal model

12. Literature survey Thermal modeling of phosphor layer heat Heating profile from light propagation Predefined heating profile and FEM Arik et al. 2003; Fan et al. 2007; Hwang et al. 2010; Krivic et al. 2012; Dong et al. 2013; Luo et al. 2013; Kolodeznyi et al. 2014 Ray-trace and FEM FD/FEM Yan et al. 2011; Wiesmann et al. 2012; Wenzl et al. 2013 Kang et al. 2006; Hu and Luo 2012; Huang et al. 2013; Luo and Hu 2014

13. Phosphor layer analysis with FD/FEM

14. Theoretical model development

15. Combined light propagation and heat model Optical model parameters Thermal model parameters Geometrical model parameters Model equations ** SPIE 2014: Mathematical model to analyze phosphor layer heat transfer of an LED system One of the most downloaded papers from the SPIE Digital Library in February 2015. Boundary conditions

16. Experiment validation of models • The developed model was used to estimate the total light output and phosphor layer top surface temperature of the experimental setup. dc power supply dc power supply

17. Experiment validation of models • Validated the model • Illustrated the developed model’s improved ability of estimation compared to other models. • Temperature dependence of light propagation • Absorption of light in other directions to the direction of light propagation

18. Further validation of thesis model • It was hypothesized that model developed in this thesis will predict light output and temperature results of past experiments • Study A : Heat transfer mechanism contribution • Study B : Study verifying light output • Study C : Model verification for phosphor concentration and thickness of phosphor layer

19. Study A: Quantify contribution of mechanisms • Objective: • To compare the results from the thesis model to past experiment results. • To determine the contribution of heat transfer mechanisms in dissipating phosphor layer heat dc power supply for fan dc power supply for LEDs

20. Study A … contd. • Validation of the temperature profile and quantification of mechanisms predicted from model ** ITHERM 2014: Understanding Heat Dissipation of a Remote Phosphor Layer in an LED System

21. Study B: Verify light output • Objective: • To check the ability of the developed thesis model to capture the reduction in light output as the interface area is increased in the phosphor layer heat sink

22. Study B … contd. • The model captured the reduction in light output observed in experimental studies

23. Study C: Verify the model for concentration • To predict the top surface temperature of remote phosphor plates of different thicknesses and phosphor concentrations Source: Mou et al. (2014)

24. Study C … contd. The model predicted light output of the LED system and phosphor layer top surface temperature • Phosphor layer thickness and phosphor weighting

25. Study on long-term effects of heat sinking • Objective was to investigate the long-term effects of optical radiation on phosphor layer with heat sink • Light output emitted from the phosphor layer was measured • The surface temperature of the phosphor layer on the emitting side was measured

26. Long-term lumen depreciation study The degradation of the remote phosphor layer was significantly lower due to the reduced phosphor layer temperature

27. Knowledge contribution – e.g., identification of key research tasks

28. Knowledge contribution – e.g., identification of key research tasks … cont’d

29. Knowledge contribution – e.g., design tool 50.8 mm 6 mm 25.4 mm

30. Knowledge contribution – e.g., design tool … cont’d • Phosphor layer diameter constraint by reflector geometry • Not possible to maintain the operating temperature with 10% by weight

31. Knowledge contribution – e.g., design tool … cont’d • Design constraint achieved with 6 mm thick phosphor layer with 2% by weight concentration

32. Conclusion of dissertation • New knowledge contribution to the field: • Development of a theoretical model for estimating radiant power and temperature profile of a LED phosphor layers • Useful for assessing factors that affect LED system performance. • Useful in identifying key research tasks and as a design tool

33. Acknowledgement • Advisor: Nadarajah Narendran, Ph.D. • Committee members: Mark Rea Ph.D., Russell Leslie AIA, FIES, Theodorian Borca-Tasciuc Ph.D. • LRC faculty, staff and students • Jean Paul Freyssinier, Yiting Zhu, Andrew Bierman, Martin Overington, Yi-wei Liu, John Bullough, Nick Skinner, Howard Ohlhous, Robert Wolsey, Jennifer Taylor, Lenda Lyman, Bonnie Westlake • Funding • ASSIST(# H11014), • FAA, RPI/LRC (IRA), Henkel, The Besal Lighting Education Fund (Acuity Brands Lighting) • My family and friends

34. Acknowledgement Alliance for Solid-State Illumination Systems and Technologies Members

35. Thank You !