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; narenn2@rpi.edu; 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 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/03/2013 Terms of Use: http://spiedl.org/terms