COB Thermal Management

Transcription

1 Lextar.com Mar Revision 1.0

2 Description & Content Description In order to take advantage of the LED performance benefits, thermal management is the precondition and must be understood and employed. This document explains the heat dissipation design and the importance of thermal management. Include discussion of basic fundamentals and recommendations for thermal design. This document applies to Lextar Nimbus Series and Core Series COB Core Series Nimbus Series 500/ / Contents 1 Introduction 2 Structure & Thermal Resistance 3 Thermal Design 4 Thermal Simulation

3 Introduction Introduction The chip diode in LED package radiates light and also heat as sources according to the input power. One of main cause of LED failures is improper thermal management. Many performance characteristics of LED package are influenced by the operating temperature. Besides, the surface area of LED package is usually small as a little heat generator, and is expected to release the heat into the environment. So, such as a heat dissipation structure like heat sink is thus required. Way of control the junction temperature (T j ) of the LED diode is important. Basically, the T j must be kept from exceeding the maximum rating in the specifications under any conditions. The heat generated at the LED diode can be conducted to the external radiator efficiently because the package structure for the Lextar COB (Nimbus & Core series) minimizes the thermal resistance.

4 Structure & Thermal Resistance Structure & Thermal Resistance For example, the package of the COB is connected to an external heat sink with TIM (thermal interface material), is shown in Fig. 1. The heat generated from COB in the junction section of the LED diode is transferred mainly to the heat dissipation structure by thermal conduction. The thermal resistance R j-s is between the junction section and the substrate side of the package outer shell, and also the specific thermal resistance value of the COB (R j-s = R COB ). T j = (R j-s P d ) + T s = ((R j-s + R s-a ) P d ) + T a T j : junction temperature [ ] T s : case temperature, bottom of package [ ] T a : ambient temperature [ ] P d : input power of LED package [W] R j-s : thermal resistance from junction to case [K/W or /W] R s-a : thermal resistance from case to ambient [K/W or /W] Fig. 1 Example of cross-section Additional, the thermal resistance of the TIM (adhesive thermal grease or thermal pad) outside the package is R TIM, the thermal resistance of the heat dissipation structure is R h, and the ambient temperature is T a. The thermal resistances outside the package R TIM and R h could be integrated into the thermal resistance R s-a (R s-a = R TIM + R h ).

5 Thermal Design Thermal Design Thermal management of COB LEDs must be considered during the design phase of applications. The driving conditions and the temperature surrounding the LEDs in the application should be conformed to the maximum ratings in specification. The definition of package thermal resistance R j-s is from junction to the bottom of substrate. As shown in Fig. 2, the de-rating curve gives the maximum driving current versus case (or substrate) temperature (T s ) for different operating limits and was based on the thermal resistance. From Fig.2, the maximum driving current I F of package is 720mA. When the operating condition setting is base on it, the substrate temperature should be under 67 as shown of the horizontal line. Fig.2 also shows that the limitation of operating temperature of T s is 100, and the driving current should be under 450mA in the condition. Fig. 2 De-rating curve The operating limits The operating limitation of each COB data sheet shows that the maximum current and T s conditions under which the COB operates successfully. In order to design for an LED system, even choices one model of light engine as COB, the demonstrated T s and T a must be different due to use different heat dissipation structures at same input power. And the junction temperature must be re-evaluated. For an LED module system, the definition of total thermal resistance R j-a is from junction to ambient and the R j-a = R j-s + R s-a. De-rating curve gives the maximum driving conditions for different system thermal resistances and the T j also kept at temperature not exceeding the maximum rating.

6 Thermal Simulation Thermal Simulation In package component level, good thermal conduction in the structure is most important. When heat generated from chip diode, it transfer through the component into assembly module outside the package and dissipated to the ambient environment. When performing the thermal simulations as assumptions must be determined and the operating boundary conditions must be checked. Computational fluid dynamics (CFD) analysis is one of simulation tools for solving thermal conduction, convection, and radiation to evaluate the thermal design. This method proposes for quick and inexpensive for design adjustments. Simulations usually show the answers and depending to the approach taken, just like due to the boundary condition setting of interface between materials and heat spreading path assuming, it may not always be equal to actual situation. After simulating or optimizing the design, building a prototype to double confirmed the performance is always recommended. Lextar Electronics Corp. is the leading LED (Light Emitting Diode) maker integrating upper stream epitaxial, middle stream chip, and downstream package, SMT and LED lighting applications. Founded in May, 2008, Lextar is a subsidiary of AU Optronics, the leading TFT-LCD and solar PV manufacturer. Lextar s product applications include lighting and LCD backlight. Lextar s manufacturing sites include Hsinchu and Chunan in Taiwan, and Suzhou in China. The company turnover in 2010 is 266 million USD.