Thermal performance of cooling system for red, green and blue LED light source for rear projection TV Sung Ki Kim*, Seo Young Kim** and Young Don Choi***, Member, IEEE *Thermal Management Team, Digital Display Research Center, Samsung Electronics, 416, Maetan3-Dong, Paldal-Gu, Suwon, Korea E-mail:
[email protected] **Thermal/Flow Control Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea *** Department of Mechanical Engineering, Korea University, 1, Anam-Dong, Sungbuk-Gu, Seoul, Korea ABSTRACT Due to the various advantages on illumination, high-power light emitting diode (LED) system has been focused recently. Because the light source that consist of red, green and blue LEDs can provide full high definition color gamut with advantage of longer lifetime, LED light source starts to be applied in digital display system. However, much heat is dissipation from LED and operating temperature severely influences the characteristics of LEDs including dominant luminosity wavelength, luminosity and lifetime. Therefore careful thermal management is crucial to ensure accurate image quality of display device as well as guaranteed lifetime. The present study is conducted to investigate the performance of thermal management system for LED light source in rear projection TV. The result shows that the reducing thermal resistance between LEDs and substrate is the most effective way to dissipate heat and the applicable limit of thermal resistance is existed for various heat-dissipating condition of LEDs. And it is also suggested that the efficacy of heat transport system from LED to ambient applied in red, green and blue LED light system to ensure the product quality. KEY WORDS: Light Emitting Diode (LED), Thermal management, Rear Projection TV. NOMENCLATURE Tc Te Tj
Temperature of center point in ceramic case [oC] Temperature of edge point in substrate [oC] Temperature of LED junction [oC] INTRODUCTION
Recently light emitting diode (LED) system as a illumination system has been focused as an alternative of conventional lamp. With the advantage of variable color point and color gamut increase, application for light source has been increased in display devices such as projection and LCD backlit. In projection display devices, luminosity from LED light source should be in the range of 30~50 lm/W in 0-7803-9524-7/06/$20.00/©2006 IEEE
minimum and much heat is dissipated from LEDs because of low luminous efficiency. Because performance of LED device is strongly dependent on the operating temperature [1], and the operation beyond temperature limit could cause the luminosity decrease, wavelength shift and lifetime decrease including failure of electrode soldering in package. Such effect of operation temperature could cause variation of color point in RGB-LED system. Therefore proper thermal management is crucial for product development of high quality. In this study, it is discussed the thermal characteristics of RGB-LED light source and the management scheme to be applied in optical module of 56” rear projection TV set. It is shown that thermal resistance within LED package affect significantly cooling performance of the system and applicable limit of thermal resistance is proposed. RESULTS AND DISCUSSION The efficacy of high-powered LEDs enables compact and high intensity white light source. Because of the low luminous efficiency and loss through optical part along light path in illumination module, the luminosity from light source (LEDs) should be over 30lm/W and power consumption is inherently high. However, the operating temperature is also increased as the heat flux is increased. It is known that dominant wavelength is shifted and the luminosity is decreased by 2~10 % for every 10°C increase of operating temperature of LED junction [2]. This variation cause significant effect on display performance. Therefore the junction temperature of LED is one of key parameters to determine the product quality using LED illumination system. Figure 1 shows typical configuration of LED light source system. Generally light and temperature sensors are necessary to control color point and operating temperature because of deviation of LED characteristics. Due to the variable characteristic of LED as semiconductor, active control system can be applied to achieve the required color point regarding the dependence of luminosity and wavelength on temperature. A feedback control system using temperature and color point compensation [2] could achieve the required color control. Especially AlInGaP is relatively more sensitive than InGaN LED on the operating temperature [2] and active cooling scheme is inevitable in some case.
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Fig. 1 Typical configuration of LED light source system Active cooling system, i.e. thermo electric cooler (TEC) with controller, enables the operation in designated constant temperature, luminosity and color point can be guaranteed. However, the application is feasible in case that thermal management system could cope with additional heat generation from TEC. Figure 2 exhibits the schematic view of LED package. LED package consists of dies, ceramic case and substrate. Heat generated from LED dies, in which conducted to substrate, transferred to heat sinks and then dissipated by heat rejection part. Dies are attached on ceramic case with epoxy adhesive and the thermal resistance is affected significantly by void fraction in bonding process. Therefore it is crucial to reduce the void fraction in bonding of dies for high performance thermal management system. Spreading of heat transported from ceramic case is occurred in substrate and the thermal design of substrate is important parameter to reduce thermal resistance in the LED package. A numerical simulation is conducted to optimize the thermal performance of substrate as a heat spreader. Figure 3 shows the effect of heat generation from LEDs on thermal performance in LED package. Numerical simulation is conducted by FLOTHERM on LED package and heat rejection part with various power consumption conditions. It is calculated that the temperature increases as the input power into LED increases. And the difference between junction and edge of substrate is augmented as power consumption from LED is increased. Therefore, it is conjectured that the heat spreading performance should be considered in high power LED system. Hot spot on substrate could cause degradation of heat transport performance resulting dry out in heat pipe by uneven heat distribution or low convective coefficient within cold plate.
Fig. 3 Temperature of components in LED package according to heat generation from LEDs. Figure 4 exhibits the CFD model of LED cooling system for rear projection TV set and the thermal resistance map. It is applied a scheme of remote heat sink in current cooling system. The heat generated in LEDs is transported to heat sinks via heat pipe array and rejected to ambient by fan duct system. Fan-duct system ensures the hot air to be exhausted to outlet vent without increasing the air temperature in cabinet. The heat rejection part consists of heat pipes and fin stack. Heat pipes are used to transfer heat from LEDs to a fin stack to convect the heat to the supplied airflow. Heat pipes and fin stack have little constraint on space and position within cabinet and can isothermalize the fin temperature reducing conduction loss. Table 1 exhibits the thermal resistance in LED cooling system. Over 50 % of thermal resistance in LED cooling system is induced in LED package. Thermal resistance in heat rejection part is relatively small and the value between heat collector and heat sinks with 100W heat generation from LEDs is 0.09°C /W. The remote heat sinks dissipate heat of 100 W with resistance around 0.33°C /W depending on the available airflow. Airflow rate, however, is related to noise production and the thermal design should consider the parameter. In current system, the design conditions are set to noise production of 25 dBA from experiment. Therefore, the thermal resistance in LED package should not exceed 0.5°C /W regarding junction temperature limit of LED in current system.
Fig. 2 The schematic view of LED package.
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Table 1. Thermal resistance in LED cooling system of 100W power consumption. Parameter LED junction~Substrate TIM Heat collector~Heat sinks Heat sinks~Ambient Total
Thermal resistance (°C /W) 0.51 0.02 0.09 0.33 0.95
CONCLUSION
(a)
It is discussed the thermal issues of LED light source for optical module in rear projection TV set. In spite of advantage on gamut control, thermal characteristics of LED require careful consideration on operating temperature management. It is shown that the most thermal resistance is occurred in LED package in high power LED illumination system. Therefore, the effort to reduce thermal resistance is crucial to increase thermal performance of system to ensure high quality display. The optimal design in LED package with high conductive material or direct bonding circuit technology can be potential scheme for high power LED system. REFERENCES [1] N. Narendran, Y. Gu, J. P. Freyssinier, H. Yu and L. Deng, “Solid-state lighting: failure analysis of white LEDs,” J. Crystal Growth, vol. 268, pp 449-456, 2004. (b) Fig. 4 The schematic configuration of high power LED. (a) CFD model of LED cooling system (b) the thermal resistance map.
[2] S Muthu, F. J. Schuuramus and M. D. Pashley, “Red, Green and Blue LED based white light generation: Issues and control,” IEEE Proc. Industry Applications Conf., pp. 327-333.
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