Analysis of Phosphor Heat Generation and Temperature Distribution in Remoteplate Phosphor-Converted Light-Emitting Diodes

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1 Proceedings of the Asian Conference on Thermal Sciences 2017, 1st ACTS March 26-30, 2017, Jeju Island, Korea ACTS-P00328 Analysis of Phosphor Heat Generation and Temperature Distribution in Remoteplate Phosphor-Converted Light-Emitting Diodes Yupu Ma, Xingjian Yu, Bin Xie, Run Hu, Xiaobing Luo* School of Energy and Power Engineering, Huazhong University of Science and Technology, 1037 Luoyu Rd. Hongshan District, Wuhan , China Presenting Author: * Corresponding Author: ABSTRACT In this study, phosphor heat generation distribution function versus light invasion depth for remote-plate pc-led was calculated by a modified Kubelka-Munk theory. And then phosphor temperature distribution was obtained accurately by finite-element simulation, in which the function was input as a non-uniform distributed volume heat source for phosphor layer. We found that the thermal power density decreased with the invasion depth and phosphor temperature increased first and then decreased with invasion depth. The effects of geometric and physical parameters of phosphor layer on the thermal behavior of remote-plate pc-led were thoroughly studied. Results revealed that the hotspot tended to shift from LED chip to phosphor layer with decreasing phosphor quantum efficiency or increasing phosphor concentration. In addition, phosphor temperature dropped when quantum efficiency increased, or phosphor concentration and layer thickness decreased. KEYWORDS: Light-emitting diodes, Phosphor temperature, Phosphor heat generation distribution. 1. INTRODUCTION White phosphor-converted light-emitting diodes (pc-leds) have already been applied in general lighting in our daily life [1]. Besides heat generation in active layer of LED chip, part of blue light power converted into heat when penetrating through the phosphor layer [2]. Although the phosphor layer heat generation is quite small compared with the heat generation in chip layer, phosphor temperature can be higher than junction temperature in some situation [2-4] for lacking of good heat dissipation path for phosphor layer. Luo et al. observed the highest temperature of the phosphor particles can reach ºC, resulting in the phosphor quenching or even the silicone carbonization [4]. Therefore, besides junction temperature, phosphor temperature is another vital parameter to characterize thermal performance of pc-leds [3]. At such circumstance, it is necessary to analyze phosphor temperature distribution and seek for thermal design or packaging process aimed at phosphor temperature reduction. So far, there exist some literatures covering phosphor temperature analysis. Yan et al. [3] and Hu et al. [5] studied the temperature field of phosphor-converted LED packages by combining the Monte Carlo optical simulation and finite element simulation together. Kim et al. [6] obtained the heat load of phosphor layer experimentally, and then acquired phosphor temperature distribution by inputting the heat load as a boundary condition in the thermal simulation. However, they regarded the phosphor layer as a uniform volume heat source and did not consider the heat load distribution in the phosphor layer, which led to inconsistent phosphor temperature distribution with actual situation. In this study, phosphor heat generation distribution function verse invasion depth for remote pc-led was calculated based on Kubelka-Munk theory [7,8]. Then the function was input as the non-uniform distributed phosphor heat load and the phosphor temperature filed was obtained by finite element simulation. The factors affecting the heat load and temperature distribution of the phosphor layer were thoroughly analyzed and discussed. 2. MODEL DESCRIPTION In our study, we built the same remote pc-led model as that of ref. [3,5]. As shown in Fig. 1, the LED chip (1 mm 1 mm mm) is mounted on a copper slug via die attach adhesive. The copper slug is embedded in a 1

2 polyphthalamide reflector cup and bonded onto the MCPCB via solder. A conventional hemispherical lens is residing on the top of PPA cup. The phosphor layer is a circular thin plate and separated from the chip by silicone encapsulant. The thermal properties and geometry dimension of all parts were chosen according to ref. [3]. Fig. 1 The schematic cross-sectional view of the remote-plate phosphor coating LED package. In the thermal simulation conducted by the commercial software COMSOL, one-quarter of the model was simulated due to its symmetry. The boundary conditions applied in our model were also determined referring to [3]. The ambient temperature was stable at 25 ºC. Forced convection on top and bottom surfaces of MCPCB board with a constant heat transfer coefficient of 30 W/(m 2 K ) and natural convection on all other surfaces of LED package with a constant heat transfer coefficient of 10 W/(m 2 K) were applied. The driving current was 0.35 A with a corresponding electrical power of 1 W. The light extraction efficiency of blue LED chip was assumed to be 30%. At such circumstance, the heat generation within chip layer was 0.7 W. The heat load of phosphor layer is calculated according to the modified Kubelka-Munk theory [9]. It is assumed that the light propagation only occurs in z direction, which is exactly suitable for the remote thin-plate phosphor coating package. When the blue light penetrates through the thin phosphor layer, the general forms of forward scattering light energy E(z) and backscattering light energy F(z) for both blue light and yellow light can be expressed as follows. For blue light 1 z z E z A e B 1 e (1) For yellow light B B 1 z 1 z (2) F z A e B e 2 EY z C 1 e D1 e A e B e z z con z z FY z C 1 e D1 e A e B e z z con z z 2 2 The phosphor heat generation function Qphos(z) verse invasion depth z can be calculated as Qphos z 1con B EB z FB z Y EY z FY z (5) 3. RESULTS AND DISCUSSIONS Fig. 2 shows the normalized phosphor heat generation function Qphos(z)/E0 versus normalized invasion depth z/d with varying (a) phosphor quantum efficiency ηqe, (b) phosphor concentration c, and (c) phosphor layer thickness d. E0 denotes the output optical energy power from the blue LED chip, which is 0.3 W according to the set boundary condition. We can see that Qphos(z) drops constantly along the invasion depth z. Qphos(z) decreases with rising ηqe, because a higher ηqe means more converted yellow light and less generated heat. When concentration rises, Qphos(z) increases sharply in the bottom part of layer, but reduces slightly in the upper layer. It should be noted that although the Qphos(z) versus z/d is decreasing with rising thickness, the total phosphor generation increases as thickness rises. In order to express the phosphor heat load distribution precisely, the normalized phosphor heat generation curves were fitted. Fig. 3(a) illustrates the fitted curve of phosphor heat generation function when ηqe=0.7, c=0.3 g/cm 3, and d=0.2 mm, which can be expressed as: (3) (4)

3 3 2 Qphos z z z z (6) E0 d d d In view of the thermal power density qphos(z) applied in the simulation, we can further obtain qphos(z) as follows: Qphos z E0 3 qphos z W m E S (7) where S is the cross-sectional area of phosphor layer, i.e., S=π r 2. Fig. 3(b) demonstrates the thermal power density distribution in the phosphor layer. It can be seen that qphos(z) does not vary along the horizontal direction and gradually decreases along the vertical direction. It should be noted that chip thermal power density is W/m 3, which is two orders of magnitude more than maximum qphos(z). In the following part, we thoroughly discussed the effects of phosphor quantum efficiency, concentration, and layer thickness on phosphor temperature distribution. 0 Fig. 2 The normalized phosphor heat generation function versus normalized invasion depth with varying (a) phosphor quantum efficiency, (b) phosphor concentration, and (c) phosphor layer thickness. Fig. 3 (a) The calculated and fitted curves of phosphor heat generation verse invasion depth; (b) the thermal power density distribution in phosphor layer. 3.1 EFFECT OF PHOSPHOR QUANTUM EFFICIENCY Fig.4 shows surface temperature field with varying phosphor quantum efficiency. It can be found that phosphor temperature is obviously higher than chip temperature when ηqe is 0.7, which agrees with the findings that phosphor temperature can exceed junction temperature in some situations [2,3]. When ηqe increases from 0.7 to 1.0, phosphor temperature decreases but junction temperature increases instead. Moreover, phosphor temperature becomes even lower than junction temperature when ηqe is 1.0. It can be explained that a higher ηqe means a lower qphos(z), which leads to lower phosphor temperature. Therefore, enhancement of ηqe is of great importance for phosphor temperature reduction. It can also be found that the temperature of internal layer is higher than that of outer side in the horizontal direction and the maximum temperature is always located at the center. Fig. 5 illustrates temperature distribution along z-axis in the phosphor layer when ηqe =0.7 and the red mark represents the hotspot, at which maximum temperature is located. Th and zh denote hotspot temperature and location, respectively. It is clear that hotspot is located at slightly higher than the bottom of phosphor layer which has the maximum qphos(z). Hotspot location shifts from the phosphor layer to chip layer with rising ηqe from 0.7 to 1.0, which can be seen in Fig. 4. 3

Fig. 4 Surface temperature field with varying phosphor quantum efficiency. Fig. 5 Temperature distribution along z-axis in the phosphor layer when ηqe =0.7. 3.2 EFFECT OF PHOSPHOR CONCENTRATION Fig.

1 g/cm 3, phosphor temperature is lower than junction temperature, i.e., hotspot is located at chip layer. However, hotspot is shifted to phosphor layer as c increases to 0.

4 Fig. 4 Surface temperature field with varying phosphor quantum efficiency. Fig. 5 Temperature distribution along z-axis in the phosphor layer when ηqe = EFFECT OF PHOSPHOR CONCENTRATION Fig. 6(a) shows the surface temperature field with varying phosphor concentration c. Phosphor temperature gradually increases as concentration rises. When c is 0.1 g/cm 3, phosphor temperature is lower than junction temperature, i.e., hotspot is located at chip layer. However, hotspot is shifted to phosphor layer as c increases to 0.2 g/cm 3, which is consistent with the finding of [5]. Fig. 6(b) illustrates hotspot temperature and location versus varying c from 0.2 g/cm 3 to 0.6 g/cm 3. Th and zh both increase with rising c, because a higher a leads to a higher qphos(z), thus higher phosphor temperature. And we can conclude that the hotspot shifts further from LED chip with rising phosphor concentration. Fig. 6 (a) Surface temperature field and (b) hotspot temperature and location with varying phosphor concentration. 3.3 EFFECT OF THICKNESS OF PHOSPHOR LAYER A thicker phosphor layer means more absorbed light energy and more converted thermal energy in the phosphor layer, thus higher phosphor temperature is expected, as shown in Fig. 7(a). Fig. 7(b) shows hotspot temperature and location versus varying phosphor layer thickness d from 0.1 mm to 0.4 mm. It can be found that Th and zh increases as d rises, which implies that the thinner phosphor layer is preferred in view of lower phosphor temperature. 4

5 Fig. 7 (a) Surface temperature field and (b) hotspot temperature and location with varying phosphor layer thickness. 4. CONCLUSIONS In this work, we obtained the phosphor temperature distribution by finite element method considering phosphor heat generation distribution versus invasion depth for remote-plate pc-leds. Results revealed that the highest thermal power density was located at bottom of phosphor layer (i.e., z=0), but the highest temperature within phosphor layer was usually located at slightly above z=0. Although the highest thermal power density of chip layer was nearly 100 times more than that of phosphor layer, the phosphor temperature could be 15 C higher than junction temperature. We found that the hotspot tended to shift from LED chip to phosphor layer with decreasing quantum efficiency or increasing concentration. It can be concluded that in order to realize lower phosphor temperature for better optical and thermal performance of pc-leds, higher phosphor quantum efficiency or lower phosphor concentration and layer thickness are preferred. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China ( , ) and by the Fundamental Research Funds for the Central Universities (2016JCTD112). NOMENCLATURE a Absorption coefficient of phosphor (mm -1 ) c Phosphor concentration (g cm -3 ) d Phosphor layer thickness (mm) E(z) Forward scattering light energy (W mm -1 ) F(z) Backscattering light energy function (W mm -1 ) Qphos(z) Phosphor heat generation function (W mm -1 ) qphos(z) Phosphor thermal power density (W mm -3 ) s Scattering coefficient of phosphor (mm -1 ) z Invasion depth (mm) ηcon Light energy conversion efficiency ( - ) ηqe Quantum conversion efficiency ( - ) REFERENCE [1] X.B. Luo, R. Hu, S. Liu, K. Wang, Heat and fluid flow in high-power LED packaging and applications, Prog. Energ. Combus. 56 (2016) [2] M. Arik, S. Weaver, C. Becker, M. Hsing, A. Srivastava, Effects of localized heat generations due to the color conversion in phosphor particles and layers of high brightness light emitting diodes., in: Proc. IEPTCE, 2003, pp [3] B. Yan, N.T. Tran, J.P. You, F.G. Shi, Can junction temperature alone characterize thermal performance of white LED emitters? IEEE Photonics Technol. Lett. 23 (2011) [4] X.B. Luo, X. Fu, F. Chen, H. Zheng, Phosphor self-heating in phosphor converted light emitting diode packaging, Int. J Heat Mass Transf. 58 (2013) [5] R. Hu, X.B. Luo, H. Zheng, Hotspot location shift in the high-power phosphor-converted white light- emitting diode packages, Jpn. J. Appl. Phys. 51 (2012) 09MK05. [6] J.H. Kim, M.W. Shin, Thermal behavior of remote phosphor in light-emitting diode packaging structures, IEEE Electron Dev. Lett. 36 (2015) [7] D.Y. Kang, E. Wu, D.M. Wang, Modeling white light-emitting diodes with phosphor layers, Appl. Phys. Lett., 89 (2006) [8] M. Huang, L. Yang, Heat generation by the phosphor layer of high-power white LED emitters, IEEE Photonics Technol. Lett. 25 (2013) [9] X.B. Luo, R. Hu, Calculation of the phosphor heat generation in phosphor-converted light-emitting diodes, Int. J Heat Mass Transf. 75 (2014)

Thermal management of the remote phosphor layer in LED systems

Thermal management of the remote phosphor layer in LED systems Indika U. Perera and Nadarajah Narendran* Lighting Research Center, Rensselaer Polytechnic Institute, 21 Union St., Troy, NY 12180 USA ABSTRACT