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How to solve the LED heat problem

Thermal management is critical to the performance and service life of the LED , so early in the development of structural engineers must consider LED thermal problems. U.S. Department of Energy (DOE) for LED made the following assessment: no other lighting technologies like LED can have such a large energy saving potential and enhance the quality of our built environment . Since the LED 's life is a function of junction temperature , so thermal management is critical to the performance of the LED , but also as a new copper foam material starts cooling LED heat applied research .

Located in California, Rudi Hechfellner Philips Lumileds Lighting SanJose application technology manager , said: "So far LED thermal management system design is the most important aspect of LED system manufacturers by seeking to optimize radiator , efficient printed circuit boards , high thermal conductivity . rate housing and other advanced thermal design techniques to address this challenge. due to thermal simulation can be established before the physical model , from the perspective of evaluating different cooling design and optimization of system-level design, so the role of thermal simulation has become more apparent . "

Emergence of LED lighting

Solid state lighting is an emerging technology that will change the fundamental form of lighting in the future . LED design at the beginning of the power of less than 50 mW . In the past decade , LED power has increased to 40 ~ 80lm/Watt. Outside in addition to high efficiency , LED 's lifetime is long . Depending on the manufacturer and type of white LED 's life may be in the range of 6,000 to 50,000 hours , greater than 30,000 hours of fluorescent ( light ) pipe and 2,000 hours of incandescent bulbs. In addition , LED can not use a filter produces monochromatic light .

Market analysis firm Yole Developpement , said : LED solid state lighting market revenue will increase from 2007 's $ 1 billion to $ 10.3 billion in 2012 . Yole expects 2012 high brightness and high brightness LED combined revenues will reach $ 4.45 billion , which is $ 780 million in 2007 5.5 times . These solid-state light -emitting devices have become the source of choice for different applications , these applications include interior and exterior lights traffic lights, cars and trucks , the screen visible displays, small LCD back lighting and decorative lighting. The latest Isuppli report ( Reference 1 ) shows that in some occasions a succession of new uses LED.

Cooling Challenges

Compared with other light sources , high power LED heat produced serious problems , mainly because the LED is not performed by infrared heat radiation. According to U.S. Department of Energy research shows that power for driving the LED 75% to 85 % of the final converted to heat , and heat from the LED chips must be introduced into the lower portion of the printed circuit board by way of heat conduction , the heat sink , the light source housing or the structural element and so on. DOE Energy Efficiency and Renewable Energy Sector report drawn up called "Thermal Managementof WhiteLEDs" ( Reference 2) . In short , too much heat will reduce the LED light output and produce cast .

In addition , the advantages and disadvantages of thermal management will produce some long-term effects , such as reducing the light output will lead to shortened life . U.S. Department of Energy , said : Manufacturers usually at a fixed junction temperature of 25 ℃ for LED testing. However , the temperature usually is 60 ℃ node or higher under these conditions the output of the LED may be rated for 10% or less. For the tungsten lamp , the thermal path through the heat radiation mode , the heat from the heater directly into the surrounding environment . The main cooling device by way of LED chip to the system shell thermal conductivity .

LED device manufacturers provide a package-level thermal management. For manufacturers , the most concerned about is how to reduce the thermal resistance of the chip to the outside of the package . Usually some of the small plate is mounted on the LED has a plurality of leads , the leads formed in the main path of heat conduction and for the LED , the thermal resistance of the leads is critical to its chips . Package design based manufacturer and LED type change, but the concept is very similar to the package .

In this example , LED chips typically using an adhesive layer (bondlayer) is assigned to a metal paste connection layer (metalinter connectlayer), and the connection layer and the metal paste is assigned to a ceramic base (Ceramic Substrate) , and an electrically insulating thermal pad (ThermalPad). The purpose of the whole package is designed to maximize the optical output and the removal of heat from the LED chip back.

Hechfellner said: Even the most efficient LED device is also required for the design of cooling systems. As traditional light source is dissipated by radiation mode , so they do not generate such heat. Many LED manufacturers have more experience than the thermal aspects of the electrical and structural aspects . Engineers who need to change their philosophy, and after consideration should first consider electric heat . The LED system manufacturers , the challenges facing today's design 90% is caused by heat, and problems caused by electrical and structural accounted for only 10% .

Hechfellner added: maximum system-level problems faced by manufacturers is to develop a high- efficiency heat lamp holder , LED devices can be easily inserted into the holder, and the heat can be rapidly introduced into the environment. As far as I know, the current market has no such system . Improve the thermal cross-section of materials and design tools for research and development of such system is necessary . We are committed to creating a good development platform , such as LED can help accurately simulate simulation tool to facilitate our customers better thermal design.

Even the nature of the LED package is to increase the efficiency of the LED , but the heat problem persists . Since the light output decreases with increasing temperature , a greater proportion of the electric power is converted into heat to further enhance the LED temperature. Over time, LED light output will be reduced , and its heat will accelerate the aging of the LED . A conventional white LED phosphor flux attenuation indicator is yellow , which may be caused by heat or environmentally induced , but this does not mean that the chip or the low efficiency of the work to produce more heat . Thermal management solutions to meet the needs of the entire life cycle of the LED can be removed in the heat requirements.

System -level design considerations

A design consideration of each LED is different , the size and requires a clear understanding of the performance limitations and LED suffered . LED system design is the essence of effective heat transfer from the LED heat sink slug or pin into the surrounding environment . Between the slug and the printed circuit board gaskets must be reliable and valid connection . Heat by conventional PCB vias on the other layer to reach the copper block . After heat through thermal way into the enclosure or external radiator. When a shell need to remove a lot of heat , you need an external heat sink . LED heat sink material used is aluminum or copper. Because of the large thermal effects of convective heat transfer between the radiator and the air , it is necessary to optimize the geometry of the radiator .

Performance of the radiator depends on the material , the number of fins , fin thickness and base thickness and other parameters. External radiator extended heat transfer surface , so that the heat into the air. Optimized design must consider the air flow around the radiator , and air flow in this region has affected the radiator , so the design had no small challenge. Copper materials may have a high thermal conductivity, but the weight of the same volume of aluminum is lighter , but the price is cheaper. In some PCB to improve the heat transfer capacity of the substrate by using some of these substrates coated with ceramic or steel, aluminum or other materials.

LED applications, the biggest problem is the requirement to use a sealed enclosure to protect the LED. Solve this problem you can use the shell material of high thermal conductivity . Of course, using some complex methods. For example : -air (air-to-air) through the use of the internal heat exchanger fan is designed to transfer heat to the internal fins , followed by the internal heat through the heat conducting fins way into the housing . Finally, the external fins of the external fan is connected to the housing for cooling. Heat through convection - air convection into three steps - thermal conductivity . Obviously, when designing an LED system design requires consideration of many variables. There are many reasons why we need to carry out thermal design optimization.

DOE on thermal management FactSheet clearly marked: overheating would affect short-term and long-term performance LED. Short-term effects are produced and cast reduce light output. For reducing the cast LCDTVs emergency applications , such as backlighting is important in these applications LED increasing the power density of the color deviation occurs causing the image , making the heat more challenging. Sharp rise in junction temperature will seriously affect the life and reliability of the LED . Optimize thermal design may also have a significant impact on the cost of the product . For example , to determine whether the thermal design may need to use a heat sink, which will increase the cost of the product .

The role of simulation

The vast majority of electronic original equipment manufacturers and component suppliers early in the product development has long been recognized to consider thermal issues practices. Many of these manufacturers to use software to build on prior physical model for component and system-level simulation analysis , thus avoiding repeated design changes . However , LED system manufacturers are accustomed to traditional light sources angles to design the system . The problem is that these traditional light source is not necessary to consider heat. The LED system manufacturers may not have the expertise and skilled use of thermal CFD software specialists. Over the past decade , and many of today's CFD software requires the user to have a deep knowledge of fluid dynamics calculation , in order to determine the results obtained are correct.

For example , users will need to convert their CAD models into CFD software, simulation models need to set solid , mesh correctly determine the boundary conditions , choose the correct physical model is set to solve the set to ensure convergence , as well as other work. Generation CFD software requires a lot of adjustments , such as manually modify the mesh grid to improve the quality and modify the structure of relaxation to promote convergence of simulation . But in recent years there has been a new generation of CFD software engineers apply . This software uses 3DCAD model, automatic detection of flow area and mesh , so that does not have a deep knowledge of computational fluid dynamics engineers can easily use , making it easy to focus on the flow of their products . This new generation of CFD software includes a sophisticated automatic control functions without having to make manual adjustments to ensure that the results converge.

This new generation of software for the thermal design of LED systems. Operation of such software will only need to use CAD software and the understanding of the physical model , the vast majority of these engineers are already mastered . Use only original 3DCAD model saves time , but also to capture all the characteristics of LED systems possible. Such software is also included in all of the heat exchange mechanism , so it can be reliably analyzed . CFD simulation again when many processes are automated, such software makes LED systems engineers can quickly evaluate a large number of designs . Lamp shown in FIG 3, 4 and 5 using the 6 high-power LED. LED and the power of these are thermal dissipation. Since no fan , engineers calculated only conduction, convection and radiation. By using embedded into the CAD system CFD software , Voxdale engineers determined the LED and power supply all the materials and their properties.

Physical Testing

Physical changes in the study design for testing the method of expensive , time-consuming and long . However, it is very effective to validate the final design and solve manufacturing problems . Physical testing of material properties can be used to determine the value and validation DieAttach voids and other issues. Some measurements give full consideration to the relationship between temperature and specific device 's forward voltage proportionally . In determining the forward voltage drop of a particular measurement current after the application of a larger LED current, thereby heating the LED. After closing of the current of the LED by applying a smaller current.

The small current through the device characteristics can be obtained , and can achieve a small forward voltage drop. Before cooling the junction temperature can quickly measure the forward voltage. Monitoring temperature fluctuations over time to determine how the heat with the external environment through a node in each layer. This allows us to directly measure the thermal resistance of the heat flow , etc. , such as the DieAttach path. Since the LED has a fast thermal response , it requires some means to measure temperature changes microsecond interval measurement hardware . Conclusion Related Information thermal transient measurements may adopt such precision " structure function ", which helps to provide LED package , Die-Attach structural integrity failure and other problems

LED technology allows to save energy, improve lighting quality and reliability possible. Thermal design of the LED design is important in order to meet its performance requirements , and cost of the cycle . System design engineers have many options to solve the heat problem. To a new generation of embedded CAD thermal and fluid simulation software helps engineers found that thermal issues and fast optimization. The sample stage can be verified by measuring the physical model the final design to ensure the feasibility of the process. Product design and simulation experience in this area after obtaining the help .

After the automatic meshing and solution , as shown in the following figure simulation results can be observed in the original CAD model. By convection of cold air into the interior of the lamp, and hot air is discharged through the gap . Dialight PLC using CAD embedded CFD software design in LED lighting systems. Dialight is the application of LED technology leader , it focuses on the following two aspects : 1 ) Components : low brightness for electronic devices , including status display LED.2) Signal / Lighting : Using the latest LED technology for high brightness transport and track lights, obstruction lights , hazardous location lighting , and committed more applications using LED technology. Lighting products Digalight VP of Gordon Routledge said: "Although increasing the efficiency of the LED , but still have a lot of input power is converted to thermal cooling of electronic devices and LED devices is very important for their long-term reliability , thus including flow analysis . including thermal analysis helps us to complete our research and development programs . "

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