Thermal Management for LED Applications

The ultimate goal of system thermal design is not the prediction of component temperatures, but rather the reduction of thermally associated risk to the product.

Hence, the objectives of a designer are not in the first place to calculate or measure temperatures, but to keep the lifetime beyond x years, to keep the color point within margin y, and to raise the efficiency to z %. And indeed, these objectives, determining the quality of LED-based products, are linked to the junction temperature. This is the main reason why a book on LED thermal management starts with an introductory chapter on LED reliability issues.

Parts of this chapter have been sourced from a chapter in a book on Solid State Lighting Reliability [Pecht and Chang, Solid state lighting reliability: components to systems, Springer, New York, pp. 43–110, 2013].

This chapter provides the basics on the physics of light-emitting diode (LED) operation: band structures, carrier transport, different recombination mechanisms, etc. and presents the Shockley model of ideal semiconductor diodes. Device construction techniques, LED packaging styles —all affecting LED efficiency/efficacy are also discussed.

This chapter addresses the key thermal mechanisms active in light-emitting diode (LED) packaging and describes a big-picture approach to thermal design of the LED package. It also highlights differences in thermal phenomena between LED chips and integrated circuit (IC) semiconductor chips. Best-practice thermal design activities are identified.

In this chapter, after a generic discussion of thermal testing techniques used to characterize packaged semiconductor devices; the latest practical test methods widespread in thermal testing of LED components and SSL luminaires are discussed. Thus, the focus is on the latest, power semiconductor and LED-specific test procedures, environments and thermal metrics—all derived from the classical JEDEC JESD51 family of testing standards. Detailed discussion is devoted to the transient extension of the so-called static test method and the differential measurement principle in its practical realization.

Different representations of the thermal impedance are presented starting from the classical Z _th (t) functions ending with the so-called structure functions. These are discussed in depth because they became the de facto standard in laboratory testing of thermal properties of LED components, in reliability analysis and in quality assurance at leading LED manufacturers. The basic concepts are introduced through practical examples.

Light-emitting diodes (LEDs) are solid-state sources, where the emitted light depends on the characteristics of the semiconductor material. It is usual to call an LED a light-emitting diode prepared from inorganic semiconductors, in contrast to organic light-emitting diode (OLEDs), prepared from organic compounds. In this chapter, devoted to the measurement of optical properties, we will deal only with inorganic semiconductor LEDs.

The chapter discusses the increasing need for a more sophisticated thermal characterization of light emitting diodes (LEDs) and LED-based products. It goes without saying that the LED business is growing exponentially, in fact, much faster than analysts predicted 5 years ago. Unfortunately, until recently the progress in thermal characterization has not kept pace. The situation was a serious problem until the first component-level LED thermal testing standards were published. As these standards are relatively new, there are still manufacturers who are not yet aware of the new testing procedure and think they can publish whatever thermal information they want. The problem also still exists for the experienced user because the thermal data that are published are often rather useless in practice when accuracy is at stake, and accuracy is needed for an educated guess of expected performance and lifetime. This situation has much in common with the one the integrated circuit (IC) world was facing almost 20 years ago. Provided the manufacturers want to cooperate, it is relatively easy to build upon the experience gained in the IC business and their standard protocols that are in use worldwide.

In 2008, the JEDEC JC15 committee on thermal standardization of semiconductor devices decided to take action and created a task group to deal with thermal standardization issues of power LEDs. International Commission on Illumination/Commision Internationale de l’Eclirage (CIE) has also created new technical committees (e.g., TC2-63, TC2-64), which also aim to address thermal issues during measurement of high-brightness/-power LEDs. The chapter describes novel test methods, which form the basis of new measurement guidelines including combined thermal and radiometric measurement of LEDs. In 2012, some new thermal testing standards were published, which are also discussed in this chapter. Initiatives to arrive at compact thermal modeling standards are also covered.

A Light Emitting Diode (LED) is a semiconductor that converts electrical energy into light and heat. Typically, energy conversion efficiencies, i.e., the percentage of input energy converted to light, are in the 20–40 % range, resulting in a significant amount of heat being generated in the pn junction of the LED. In most applications, this heat has to be conducted away from the junction and then convected and/or radiated to the ambient air. The convective part of the heat transfer usually requires extended surfaces or heat sinks and in many cases, a method of creating airflow over the heat sink to transport the heat away into the ambient, and is generally referred to as air-side heat transfer. This chapter will discuss the various methods that can be employed for air-side heat transfer. The chapter is divided into five major sections. Section 2 deals with the system-level thermal management of LEDs and discusses the importance of cooling LEDs as well as the various thermal paths and resistances involved in a typical LED system. Section 3 describes the fundamentals of both natural and forced convection heat transfer, including some basic relations and equations used in convective heat transfer. Section 4 describes the different technologies that exist today for air-side heat transfer. Section 5 compares the different technologies with respect to the system-level metrics such as acoustics, power consumption, reliability, etc., required for designing a cooling solution. Finally, Sect. 6 summarizes the chapter.

Thermal interface materials (TIMs) serve a critical function in light-emitting diode (LED) assemblies and electronic systems, although the cost of some is very modest. The basic function is to provide an effective thermal path between two dissimilar surfaces, often the base of the LED array and a heat sink or a metal heat-dissipating surface. The TIM material in the simplest definition is intended to reduce air gaps between mating metal surfaces, given the very poor thermal conductivity of air. Proper selection and application of a TIM will compensate for relative surface roughness and surface imperfections on one or both mating surfaces that create loss of surface-to-surface contact for the transmission of heat.

Several other chapters (especially, Chap. 1 and 2) focus on the complexity of the physics that determines the performance of light-emitting diode (LED)-based systems. It is evident that temperature plays a significant role in optimizing light output and reliability. In many cases, the area available for cooling for a medium to high power LED is insufficient and hence area extension is required, usually by means of a local heat sink (using heat pipes to transfer the heat to where sufficient area is available is another option). Hence, an important way of controlling the temperature is by a proper choice of a heat sink. While in many applications the design freedom is limited, such as in retrofit LED lamps, the designer is still confronted with many questions: shape, number of fins, fin thickness, gap between fins, base dimensions, material, etc. Since literally thousands of heat sinks are available, many designers are confronted with the question: which one? Very often, the designer’s choice is based on cost and manufacturer’s data. Unfortunately, these data cannot be used with confidence because they are almost exclusively based on measurements in a closed duct, thereby disregarding bypass effects and inflow conditions.

This chapter on heat sinks is devoted to examples of industrial LED applications and offers ways to arrive at an optimal choice for a heat sink given a certain application. The selection of cooling solutions for LED applications should be based on an overall system-board-component level investigation rather than on solving individual component problems. From a total system perspective, this approach will result in cheaper or better thermal management solutions. Computational fluid dynamics (CFD) analyses are a very valuable tool to arrive at the best possible solution at the system, board, and component levels. Within this process, compact heat sink modeling is required and a method is presented how to realize this for various geometries.

Power to a light-emitting diode (LED) is converted to light and heat. However, these are not independent and affect each other in complex ways. Knowing the thermal state of the LED chip is essential to understanding the light output properties of the LEDs. Controlling the temperature of thermal contacts is used in the testing of LEDs, where possible. Changes to some LED optical and electrical properties with temperature are described with examples.

The setups used in testing LEDs for total flux and averaged LED intensity are discussed with practical equipment illustrations. The configurations and conditions for laboratory and production lines are compared. The relationships are explored so that the correction of production test results to laboratory quantities can be made.

The effects of measurement equipment on the results are examined; in particular, the requirements of nonequilibrium testing with short pulses. Short pulses are routinely employed in production testing and results “corrected” to equilibrium conditions.

The role of uncertainties and tolerances and their differences are discussed. Measurement precision, result distribution, and their relationship to actions such as rejection and binning protocols are explored. In particular, the effect of traceability paths on the observed differences between production lines is explained. Comparison of results depends on the number of variables involved in tests and the scope of the intercomparison.

Light-emitting diodes (LEDs) are already widely applied in general lighting applications. For almost all lamps and luminaires based on conventional technologies, an LED equivalent has been developed and put on the market. The application space of LEDs ranges from small systems, using a few watts of power to systems that generate more than 10,000 lumens and dissipate more than 200 W of power. Over this wide application area, many different system architectures are used and many different thermal solutions are applied. This chapter focuses on the application of LEDs; it is split-up in sections that cover different application areas. First, it describes the application of LEDs in lamps. These retrofit LED lamps have a prescribed form factor and replace existing lamps, based on conventional technologies (incandescent, halogen, or compact fluorescent). Next, the application of LEDs in new systems is described for several application fields. Finally, LEDs in some special applications are discussed. These applications have special requirements that need more advanced thermal solutions. This chapter concludes with a section on remaining challenges.

This section discusses a variety of thermal challenges in LED-driven displays, especially in early development phase of the LED displays. The unique fundamentals of the displays are their wide screen size, flat and thin envelop, light weight for a wall mount, and the requirements of bright and high contrast on screens. A historical background in the introduction is followed by the technological challenges of the system primarily based on a passive air convection cooling for LED displays. Even with active air cooling solution, this is still important for the initial design aimed for energy efficient and low acoustic noise systems. The following two sections “Thermal Spreading and Transient for Design” and “Active Cooling of Laser LED for Projection Displays” discuss the technologies to handle the device-focused thermal issues in conjunction with thermal imaging characterization of the LED device.

Recent years have shown an explosion in different LED–LCD displays, and a corresponding proliferation of LED TVs in the consumers’ living rooms. They offer a great picture, a great styling with a thin-form factor, increased functionality such as 3D and internet access, and great value for money. Likewise, the technical product design of LED–LCD TVs also made great strides in the past years. One of the key aspects of LED TV design is cooling of the LEDs, which also has evolved to cater for steadily increasing power densities in ever thinner product enclosures. In the lighting and computer industry, large finned heat sinks are used to address comparable thermal challenges. In the thin-form factor of a modern TV set, this is not feasible, and the only large surface areas available for cooling are located on the front and the back of the TV set. The role of in-plane heat spreading in LED–LCD thermal management has not been addressed so far. The scope of this subchapter is to clarify the thermal significance of heat spreading in the in-plane direction in the set, especially in relation to cost down initiatives using less LED packages at equal total light output.

Light-emitting diodes (LEDs) are increasingly used in applications with extreme temperatures, pressures, dynamic loading, and conditions, which can degrade materials, such as solar loading and fluids. This chapter discusses a number of these harsh environment LED applications that include military/aerospace vehicles, automotive lighting, and outdoor lighting. It also describes test methods used to assess the reliability of LEDs and system integration issues, with an emphasis on thermal management, related to harsh environments.

This chapter describes possible future development directions of solid-state lighting from a high-level materials, devices, and system applications perspective. Since solid-state lighting is likely the highest volume future LED application and is a completely new form of lighting technology, advances in semiconductor and packaging materials, LED devices, and new lighting systems designs are convolved with the evolution of business models and new capabilities brought by new corporate entrants to the rapidly evolving global solid-state lighting business. Examining the development of future LED applications is, therefore, complicated, and it is helpful to briefly set the stage with a brief review of current and emerging LED technology trends and applications.