Printed Circuit Design & Fab - July 2008 - (Page 26)

ThERMAL ManaGEMEnt FiGurE 3. Surface temperatures of the entire lamp. ! ! FiGurE 2. expected lifetimes for InGan luXeon rebel at various junction temperatures and drive currents at a 90 confidence level. in emerging applications such as LCD TVs where increasing the LED density improves the color variation in the image but makes the cooling more challenging. Increased junction temperature can severely affect lifetime and reliability performance of a power LED. For example, all other things being equal, a 10 degree change in junction temperature has a dramatic effect on lifetime and reliability as shown in FiGurE 2. Optimizing the thermal design may also have a major impact on product cost. For example, the effectiveness of the thermal design may determine whether or not it is necessary to use a heatsink, a decision that will substantially affect the overall cost. System codesign that brings the PCB into the thermal equation early in the design process can help off-set these costs by integrating novel cooling solutions at the circuit board level. Role of Simulation Most electronics and original equipment manufacturers (OEMs) and component suppliers have long accepted the need to identify and resolve thermal issues early in the design process. Many have adopted software that performs component- and system-level analysis to address thermal management prior to physical testing, with the goal of avoiding additional design iterations. However, manufacturers of LED systems frequently design systems built around other lighting technologies that do not provide the same thermal management challenges. These companies may not have the required knowledge and expertise to use the powerful and sophisticated computational fluid dynamics (CFD) software used by semiconductor device manufacturers, printed circuit board designers and large electronics OEMs. The CFD codes of a decade ago and even many used today require the user to have a deep understanding of the computational aspects of fluid dynamics in order to obtain accurate results. For example, users need to know how to translate their computer aided design model into the CFD environment, then “reverse” the model so that empty flow space (rather than the solid product) is modeled, to create an overall mesh with the right properties, determine boundary conditions, select the right physical 26 models, tweak solver settings to ensure convergence, as well as other tasks. Previous generations of CFD software also required a substantial amount of tuning and tweaking, such as manually modifying cells to improve the mesh quality or adjusting solver controls, or relaxation factors, in an effort to get the software to converge to a solution. But in the last few years a new generation of CFD software has been introduced that addresses the major reasons for its relative lack of use. The new software uses native 3-D CAD data, automatic detection and gridding of the flow space, and manages flow parameters as object-based features, eliminating the need for engineers to understand the computational part of CFD, allowing them to focus on the fluid dynamics of the product that is their responsibility to understand and master. The newest generation of CFD software contains sophisticated automatic control functions that ensure convergence in almost every application without the need for manual tuning. This new generation of software is well suited to the thermal design of LED systems. The skills required to operate the CFD software are simply a knowledge of the CAD system and the physics of the product, both of which the vast majority of design engineers already possess. The ability to utilize the native 3-D CAD saves time and makes it possible to capture the full geometric complexity of LED systems. The new generation of software also covers all of the possible thermal transfer mechanisms so it can be relied upon for accurate analysis. By automating the steps required in creating a CFD model, the new generation of CFD software makes it possible for LED systems designers to evaluate a large number of design alternatives very quickly. The lamp shown in FiGurES 3, 4 and 5 uses six high power LEDs with a built-in power supply that dissipates heat. Since no fans were used, design engineers could only count on conduction, natural convection and radiation to remove the heat. Using a CFD software package embedded in their CAD system, Voxdale engineers defined all the materials and their characteristics, the heat dissipation for LEDs and power supply, gravity direction for convection, etc. After automatic meshing and solving, the results were visualized on the native CAD geometry as shown in the figures. JULY 2008 printEd circuit dESign & fAB

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