Comprehensive solution to system-level LED thermal management problems

Thermal management is crucial to the performance and service life of LEDs , so structural engineers must consider the heat dissipation of LEDs in the early stages of research and development.

The U.S. Department of Energy (DOE) has made the following evaluation of LEDs: No other lighting technology has the potential to save energy and improve the quality of our built environment as much as LEDs. Since the service life of LEDs is a function of junction temperature, thermal management is critical to the performance of LEDs.

“Thermal management is by far the most important aspect of LED system design,” said Rudi Hechfellner, application technology manager at Philips Lumileds Lighting in San Jose, California. “LED system manufacturers are meeting this challenge by seeking optimized heat sinks, efficient printed circuit boards, high thermal conductivity housings and other advanced thermal design techniques. Thermal simulation is becoming increasingly important because it can evaluate different design options and optimize system-level designs from a thermal perspective before physical models are built.”

Figure 1: Solving the LED thermal management challenge at the system level

The emergence of LED lighting

Solid-state lighting is an emerging technology that will fundamentally change the form of lighting in the future. LEDs were originally designed with a power of less than 50 milliwatts. Over the past decade, the power of LEDs has risen to 40-80 lm/Watt. In addition to high energy efficiency, LEDs also have a long life. Depending on the manufacturer and type, the life of white LEDs can be in the range of 6,000 to 50,000 hours, which is longer than the 30,000 hours of fluorescent tubes and 2,000 hours of incandescent bulbs. In addition, LEDs can produce monochromatic light without the use of filters.

Market analysis firm Yole Development says LED revenues in the solid-state lighting market will rise from $1 billion in 2007 to $10.3 billion in 2012. Yole expects revenues for high-brightness and ultra-high-brightness LEDs to reach $4.45 billion in 2012, 5.5 times the $780 million in 2007. These solid-state light-emitting devices have become the light source of choice for a variety of applications, including traffic lights , interior and exterior lights for cars and trucks, screen displays , small LCD backlighting, and decorative lighting. The latest Isuppli report (reference 1) shows that LEDs are being adopted in new applications.

Thermal Challenges

Compared with other light sources, high-power LEDs have serious heat dissipation problems, mainly because LEDs do not dissipate heat through infrared radiation . According to research by the U.S. Department of Energy, 75% to 85% of the power consumption used to drive LEDs is ultimately converted into heat energy, and the heat must be conducted from the LED chip to the lower printed circuit board, heat sink, housing or light source structure components , etc. The U.S. Department of Energy's Energy Efficiency and Renewable Energy Division has produced a report called "Thermal Management of White LEDs" (Reference 2). In short, too much heat will reduce the light output of LEDs and produce color casts.

In addition, poor thermal management can have some long-term effects, such as reduced light output leading to a shortened lifespan. The U.S. Department of Energy says manufacturers typically test LEDs at a fixed junction temperature of 25°C . However, in normal cases, the junction temperature is 60°C or higher, and in these cases the output of the LED lamp may be only 10% or less of the rated value. For tungsten bulbs, the heat dissipation path is through thermal radiation, and the heat is directly transferred from the filament to the surrounding environment. The main heat dissipation path for LED devices is heat conduction from the chip to the system housing.

Figure 2: Surface temperature distribution of the entire LED (click on the image to view the original image)

Figure 3: Temperature distribution inside an LED lamp (click on the image to view the original image)

Manufacturers of LED devices provide thermal management at the package level. For manufacturers, the main concern is how to reduce the thermal resistance from the chip to the external package. Usually some small LED lamps mounted on a flat panel have many leads, which form the main heat conduction path, and for these LEDs, the thermal resistance from the chip to the leads is critical. The package design varies according to the manufacturer and LED type, but the concept of the package is very similar.

In this case, the LED chip is usually attached to a metal interconnect layer using a bond layer, which is in turn attached to a ceramic substrate and an electrically insulating thermal pad. The entire package design is designed to maximize optical output and remove heat from the back of the LED chip.

Hechfellner said: Even the most efficient LED fixtures require a cooling system to be designed for them. Because traditional light sources dissipate heat by radiation, they do not have such thermal issues. Many LED manufacturers have more experience with electrical and structural aspects than thermal aspects. Engineers need to change their mindset and consider thermal issues first and electrical issues second. For LED system manufacturers, 90% of the challenges faced in today's designs are caused by thermal issues, while electrical and structural issues only account for 10%.

Figure 4: High power LED package Sketch (click on the image to view the original image)

Hechfellner added: "The biggest challenge facing system-level manufacturers is developing a heat-sinking holder that the LED device can be easily inserted into and the heat can be quickly transferred to the environment. As far as I know, there is no such system on the market today. Improved thermal cross-section materials and design tools are necessary to develop such systems. We are committed to creating a good development platform, such as simulation tools that can help accurately simulate LEDs, so that our customers can make better thermal designs."

The nature of LED packaging is that even as the efficiency of the LED increases, the problem of heat dissipation remains. Since light output decreases with increasing temperature, a greater proportion of electrical power converted to heat will further increase the temperature of the LED. Over time, the light output of the LED will decrease, and its heat will accelerate the aging of the LED. A common indicator of white LED flux attenuation is the yellowing of the phosphor, which can be caused by heat or environmental factors, but does not mean that the chip is working less efficiently or generating more heat. The thermal management solution needs to meet the requirements of removing heat throughout the life of the LED.

System-Level Design Considerations

Design considerations for each LED are different and require a clear understanding of the size constraints and performance of the LED. The essence of LED system design is to effectively transfer heat from the LED heat sink, metal block or pins to the surrounding environment. A reliable and effective connection must be made between the metal block and the printed circuit board pad. Usually the heat is transferred through the thermal vias in the PCB to the copper block on another layer. The heat is then transferred to the housing or external heat sink by thermal conduction. When a large amount of heat needs to be removed from a housing, an external heat sink is required. Common materials for LED heat sinks are aluminum or copper. Since the convective heat transfer resistance between the heat sink and the air is very important, it is necessary to optimize the heat sink geometry.

The performance of the heat sink depends on parameters such as the material, the number of fins, the thickness of the fins and the thickness of the base. The external heat sink expands the heat transfer surface and facilitates the entry of heat into the air. The optimal design must take into account the air flow around the heat sink, and the air flow in this area is affected by the heat sink, so it poses a considerable challenge to the design. The material copper can have a high thermal conductivity, but aluminum is lighter and cheaper in the same volume. In some PCBs, the heat transfer capacity is improved by using some substrates, which are made of ceramic or covered with iron, aluminum or other materials.

(Click on the image to view the original image)

Figure 5: Lifetime of InGaN LUXEON Rebel at different junction temperatures and drive currents at 90% confidence level

The biggest challenge in LED applications is the requirement for a sealed housing to protect the LED. This problem can be solved by using housing materials with high thermal conductivity. Of course, some complex methods are also used. For example: air-to-air heat exchange design uses an internal fan to transfer heat to the internal fins, and then the heat enters the housing through thermal conduction from the internal fins. Finally, the external fins connected to the housing are cooled by an external fan. The heat enters the air in three steps: convection-conduction-convection. Obviously, there are many design variables to consider when designing an LED system. There are many reasons for us to optimize thermal design.

The DOE's FactSheet on thermal management clearly states: Overheating can affect the short-term and long-term performance of LEDs. The short-term impact is color cast and reduced light output. Reducing color cast is important for backlighting in critical applications such as LCD TVs, where increasing LED power density causes image color deviations, making heat dissipation more challenging. Rapidly rising junction temperatures can seriously affect the life and reliability of LEDs. Optimizing thermal design may also have a significant impact on the cost of the product. For example, thermal design may determine whether a heat sink is needed, which will increase the cost of the product.

The role of simulation

The vast majority of electronics, OEMs and component suppliers have long recognized the practice of considering heat dissipation in the early stages of product development. Many of these manufacturers use software to perform component and system-level simulation analysis before the physical model is established, thereby avoiding repeated design changes. However, LED system manufacturers are accustomed to designing systems from the perspective of traditional light sources. The problem is that these traditional light sources do not have to consider heat dissipation. These LED system manufacturers may not have expertise in heat dissipation and experts who are proficient in using CFD software. In the past decade and many CFD software today require users to have deep expertise in computational fluid dynamics in order to determine whether the results obtained are correct.

For example, users need to convert their CAD models into CFD software, set the solid model to be simulated, mesh correctly, determine boundary conditions, select the correct physical model, set the solution settings to ensure convergence, and other tasks. The previous generation of CFD software required a lot of adjustments, such as manually modifying the mesh to improve the mesh quality and modifying the relaxation to make the simulation structure converge. But in recent years, a new generation of CFD software suitable for engineers has emerged. This software uses 3D CAD models, automatically detects flow areas and divides meshes, so that engineers who do not have deep knowledge of computational fluid dynamics can also use it easily, so that they can focus on the flow of the product. This new generation of CFD software includes mature automatic control functions, which can ensure the convergence of results without manual adjustments.

Figure 6: Visualization of flow trace distribution (click on the image to view the original image)

This new generation of software is suitable for thermal design of LED systems. Operating this type of software only requires the use of CAD software and understanding of physical models, which most engineers already have. Using the original 3D CAD model not only saves time, but also makes it possible to capture all the characteristics of the LED system. This type of software also includes all the heat exchange mechanisms, so it can be analyzed reliably. Many processes are automatically carried out when performing CFD simulations, and this type of software allows LED system engineers to quickly evaluate a large number of design options. The lamp shown in Figures 3, 4 and 5 uses 6 high-power LEDs. These LEDs and the power supply will dissipate heat. Since no fans are used, engineers only calculate heat conduction, natural convection and radiation. By using CFD software embedded in the CAD system, Voxdale engineers determined all the materials and their characteristics for the LEDs and the power supply.

Physical Testing

Physical testing is an expensive, time-consuming and long way to investigate design changes. However, it is very effective for verifying the final design and solving manufacturing problems. Physical testing can determine the material property values ​​used and verify issues such as gaps in the DieAttach. Some measurement methods take into account that the temperature of a particular device will be proportional to the forward voltage. After determining the forward voltage drop at a specific measurement current, a larger current is applied to the LED, thereby heating the LED. This current is then turned off and a smaller current is applied to the LED.

The characteristics of the device can be obtained through this small current, and a small forward voltage drop can be achieved. The forward voltage can be measured quickly before the junction temperature cools down. Monitoring the temperature fluctuations over time can determine how the heat flow passes through each layer of the node and the external environment. This allows us to directly measure the thermal resistance on heat flow paths such as DieAttach. Because LEDs have a fast thermal response, some measurement hardware that can measure temperature changes in the device within microsecond time periods is required. This type of thermal transient measurement may use a high-precision "structure function", which helps provide relevant information such as LED packaging, Die-Attach failure, and other structural integrity issues.

in conclusion

LED technology makes it possible to save energy and improve lighting quality and reliability. Thermal design is very important in the LED design process in order to meet its performance, life cycle and cost requirements. System design engineers have many options to solve heat dissipation problems. The new generation of thermal and fluid simulation software embedded in CAD helps engineers find heat dissipation problems and quickly optimize solutions. At the sample stage, the final design can be verified by measuring the physical model to ensure the feasibility of the process. The experience gained in this regard will help future product design simulations.

After automatic meshing and solving, the simulation results can be observed on the original CAD model as shown in the figure below. Cool air enters the interior of the lamp by convection, while hot air is exhausted through the gaps. Dialight PLC uses CFD software embedded in CAD to design LED lighting systems. Dialight is a leader in applied LED technology, focusing on the following two areas: 1) Components: including low-brightness LEDs for electronic equipment status display. 2) Signal/Lighting: Using the latest high-brightness LED technology for traffic and track signals, obstruction lights, hazardous area lighting, and is committed to using LED technology in more applications. Gordon Routledge, VP of lighting products at Digalight, said: "While LEDs are becoming more and more efficient, a large amount of input power is still converted to heat. Cooling of electronic devices and LED devices is very important for their long-term reliability, so thermal analysis, including flow analysis, helps us complete our research and development plans."