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

Generally in a white light-emitting diode (LED), a phosphor slurry is placed around the semiconductor chip or the phosphor is conformally coated over the chip to covert the narrowband, short-wavelength radiation to a broadband white light. Over the past few years, the remote-phosphor method has provided significant improvement in overall system efficiency by reducing the photons absorbed by the LED chip and reducing the phosphor quenching effects. However, increased light output and smaller light engine requirements are causing high radiant energy density on the remote-phosphor plates, thus heating the phosphor layer. The phosphor layer temperature rise increases when the phosphor material conversion efficiency decreases. Phosphor layer heating can negatively affect performance in terms of luminous efficacy, color shift, and life. In such cases, the performance of remote-phosphor LED lighting systems can be improved by suitable thermal management to reduce the temperature of the phosphor layer. To verify this hypothesis and to understand the factors that influence the reduction in temperature, a phosphor layer was embedded in a perforated metal heatsink to remove the heat; the parameters that influence the effectiveness of heat extraction were then studied. These parameters included the heatsink-to-phosphor layer interface area and the thermal conductivity of the heatsink. The temperature of the remote-phosphor surface was measured using IR thermography. The results showed that when the heat conduction area of the heatsink increased, the phosphor layer temperature decreased, but at the same time the overall light output of the remote phosphor light engine used in this study decreased due to light absorption by the metal areas.

Keywords: light-emitting diode, remote phosphor, thermal management, solid-state lighting, down-conversion, IR thermography, extended-surface heat conduction

1. INTRODUCTION There are two main components in a phosphor-converted white LED package that convert energy: 1) the LED chip that converts electrical energy to a short-wavelength visible radiant energy, and 2) the phosphor material that down-converts the short-wavelength visible radiation to a broadband long-wavelength visible radiation [1],[2] The heat generation inside a phosphor-converted white LED package is mostly due to the inefficiencies in these conversion processes.

At present, the heat generation within an LED and the effect of this heat on LED performance is well understood [3]. With the industry moving toward higher lumen packages with smaller footprint light engines, there is significant research interest in understanding the heat production within the phosphor layer and its effects. Generally, phosphor conversion efficiency is negatively affected by an increase in temperature [4]. In addition, the temperature rise affects the binding material used in creating the phosphor layers and reduces the overall light output of the LED. The temperature rise caused by a reduced phosphor conversion efficiency and additional light absorption by the binding material accelerates lumen degradation and reduces system useful lifetime [5].

In a white LED system, the amount of phosphor layer heat buildup depends on a number factors that include: 1) the location of the phosphor, 2) the thickness and concentration of the phosphor layer, 3) the binding media used in creating the phosphor layer, 4) the LED chip and package structure, and 5) the phosphor conversion efficiency [1],[2],[6]-[8].

Past studies have observed the heat generated in the phosphor layer, caused by the conversion efficiency losses (quantum conversion losses and Stokes shift losses) and trapped photons in the phosphor layer (due to total internal reflection and Fresnel reflection), can be as high as 13% of the total input electrical power to the LED system [8]. Furthermore, studies have stated this generated heat in the phosphor layer can increase the operational temperature of the phosphor layer to greater than 150°C [2],[7].

*Corresponding author: +1 (518) 687-7100; [email protected]; http://www.lrc.rpi.edu/programs/solidstate/

LED-based Illumination Systems, edited by Jianzhong Jiao, Proc. of SPIE Vol. 8835, 883504 · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2023094

Proc. of SPIE Vol. 8835 883504-1

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Table 3 shows the effect of thermal conductivity of the heatsink material on reducing phosphor layer temperature. The aluminum heatsink reduced the geometric center temperature as the heat conduction area was increased while no reduction in the geometric center temperature was observed with acrylic heatsinks when the heat conduction area was increased. In addition, the acrylic heatsinks displayed higher phosphor operational temperature for all tested heatsink conduction area configurations. These results illustrate that the geometrical aspects of the heatsink and thermal properties of the heatsink material are important in this thermal management method.

LightTools®, a commercial ray-tracing simulation software, was used to systematically analyze the reduction of visible radiant power when the heat conduction area was increased. The phosphor layer was modeled as a non-scattering medium with a constant refractive index for these numerical simulations while the internal surface reflectivities were changed systematically. These surfaces were considered as optical absorbers when the surface reflectivities were not equal to unity. Figure 5 shows the different internal surfaces of the LED light engine setup where the surface reflectance was varied.

Figure 5. Sketch of LED light engine setup internal surfaces

Reduction in surface reflectivity decreased the visible radiant power output from the LED light engine due to the surface absorption of photons. The visible radiant power output reduction was higher for the LED mount surface, the heatsink underside surface, and the cavity wall (reflector cup) surface reflectivity reduction. The visible radiant power output reduction was lower for the heatsink conduction surface reflectivity reduction compared to the other surfaces stated above.

The visible radiant power output change was small (<5%) among the heatsink configurations for all internal LED light engine surfaces except for the heatsink conduction surface. The change in heatsink configurations was ~10-15% with respect to visible radiant output power for the heatsink conduction surface as the heatsink conduction area surface reflectivity was reduced.

These findings provide a plausible explanation for the reduced visible radiant power when the heat conduction area was increased. The increased conduction area improved heat transfer but it also increased the surface absorption of photons, thus reducing the visible radiant power. A similar trend was observed with the reflectivity of the cavity surface and the LED mount surface.



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[4] Lin, C. 1277 (2

[5] Narendgeneral

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would like to ankel, and the B

Russell Leslie, Ang Research Ce

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-C., and Liu, 2011). doi: 10.1dran, N., Maliyl illumination a

ature profile of h

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mprove overall

acknowledge tBesal Lighting Andrew Biermenter.

ov, A., and MuPIE 8550, 8550er, C., Hartmannearity of the rder different c39 Weaver, S., an

doi: 10.1117/12R. -S., “Advan1021/jz200245yagoda, N., Bapplications,” P

heatsinks #1 and

profile along arams. Heatsinkit also mainta

e profile of heaion due to bindnd binding mate

5. Dbuildup in the

veness of heat tinks used in thart of an ongolight output wh

ACKNOWthe financial suEducation Fun

man, Jean Paul

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Thermal management of the remote phosphor layer in LED systems