Thermal modelling boosts LED performance

This means that design of LEDs and LED systems needs to be more sympathetic to thermal issues, and simulation and modelling tools exist specifically tailored to such applications. This article will discuss two of them. Heat sink design This heat issue stands in stark contrast to traditional lighting sources, in which heat is actually needed in order for the device to produce a significant amount of visible light. LEDs operate best at the lowest possible temperature. Most of the high-power LED modules used in general lighting are built using a large number of LEDs mounted on a substrate. This solution is the most efficient from a thermal point of view, since the substrate itself conducts heat well. When this substrate is mounted effectively on an efficient heat sink, the heat is released into the ambient air. Since the adjacent surfaces of the LED module and the heat sink are not entirely smooth, some amount of thermal interface material (TIM), typically thermal grease, is added between the surfaces. This is exactly the same solution used between microprocessor components and the heat sink in a computer. The use of grease (or any other TIM) is vitally important because any air gap between the hot surface and the heat sink creates extremely high thermal resistance between the surfaces. Since the management of heat in LEDs is regulated largely by the convection of heat into the ambient air, the design of the heat sink is crucial. While aluminium is excellent at conducting heat, it is an expensive material to use in LED applications. The shape of the heat sink becomes even more critical to ensuring efficient heat management when less efficient conductors are used. It is in this context that research manager Aulis Tuominen and researcher Mika Maaspuro at Business and Innovation Development Technology (BID) at the University of Turku in Finland set out to design the most efficient heat sink for an LED lighting device. Critical to their research was the simulation software COMSOL's Multiphysics simulation tools. "The overall size of the heat sink or any geometrical dimension of some structure or material parameter can easily be changed in the simulation model," explained Maaspuro. "Repeating the simulation of the model while using different geometrical dimensions or other design parameters provided important data about the thermal behaviour of the LED lighting device. This information cannot be found otherwise. Building prototypes is too expensive and time-consuming." Getting the simulations and models to match the physical prototypes as accurately as possible is critical to making the development of new LED systems fast while reducing costs. In addition to the heat-sink design Maaspuro worked on, he and his colleagues have also looked into the effects that TIMs have in LED lighting devices and how these in turn impact the junction temperature of the LEDs. "There are new materials, especially nanomaterials, that have much higher thermal conductivity than widely used silicone-based materials," says Maaspuro. "By using simulations, the effect of these new materials on the thermal behaviour of the LED lighting device can be figured out before any sample of that new material is even available for testing." Sharp solution Often design teams have to cope with a wide variety of component types and technologies, and this means product developers must draw on multiple scientific and engineering disciplines right from the outset of a project to meet functionality, quality, cost, and time-to-market goals. A good example of this multidisciplinary approach is in the R&D laboratories of Sharp Corporation - one of the world's largest producers of televisions and liquid crystal displays and a major force in LED lighting systems, solar cells, multifunction business machines, and a variety of other electronics-based products. Sharp Laboratories of Europe (SLE), Sharp's wholly owned affiliate in Oxford, has approximately 100 employees whose primary focus is to conduct R&D on electronics hardware and devices. The lab has active projects in display technology, semiconductor devices, lighting, health, and energy systems. SLE developed technology has gone into Sharp's mobile phones, smart cards, personal computers, laptops, and automotive displays. "A common feature of much of our work is its multidisciplinary nature, as reflected by the broad range of scientific specialties across our research staff, including materials scientists, chemists, physicists, optical designers, electronic engineers, and software developers," said Chris Brown, research manager for SLE's Optical Imaging and Display SystemsHealth and Medical Devices Group. "Activities tended to be driven by depth of knowledge in just one technical specialty, such as optics or electronic circuit design. More recently, though, there has been a shift in focus to systems or products as a whole, such as health systems and energy systems. By their nature, these activities are broader, and the research is driven by understanding how all the parts fit together," he said. Brown says SLE uses COMSOL Multiphysics® in a number of projects across the lab, for purposes ranging from early-stage research to product development. The main areas and some typical projects include LED devices, displays, labs-on-a-chip, and energy systems. SLE supports Sharp's LED business by providing technical analysis and design modifications to improve the performance of its LED devices. One example is optimisation of LED electrode designs for improved wallplug efficiency. As mentioned above a major issue with LED devices is that high operating temperatures can cause a reduction in the efficiency at which they convert electricity to light. The relationship between optical efficiency and temperature in an LED is not linear, however, meaning that any hot spots in the LED chip will disproportionately reduce the efficiency of the entire device. The key goal, therefore, is to create a uniform temperature distribution. This is done by designing the LED's electrodes so that no hot spots occur. The resulting uniform temperature distribution will also tend to maximise heat dissipation from the LED chip and will result in a lower average temperature. The structure of a typical LED chip is shown in Figure 3X, with a Multiphysics simulation of the LED shown in Figure 4X1. Simulation of the LED's electrical and thermal performance allows the electrode design to be optimised. The lab originally used specialised LED design and simulation software for this project, but it was limited in functionality and didn't offer Multiphysics' analysis capability. "Now we also use LiveLink for SolidWorks in COMSOL Multiphysics to simplify the process of design translation and minimise the risk of translation errors," Brown commented Sarah Mitchell, a researcher at SLE specialiszing in LED simulation. "The gradually increasing complexity of our simulations means we must take into account multiple physics-based processes. By allowing simulation of electrical and thermal aspects, we can achieve a much more accurate match between simulation and experimental data, and as a result, we are able to optimise LED designs for improved performance and reduced time to market."