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In recent years, the LED lighting market has seen explosive growth, which is driven by many factors, among which green legislation and environmental issues are the most important factors. LEDs are much more efficient than traditional light sources (such as incandescent lamps), are more environmentally friendly than fluorescent lamps, do not contain mercury, and can adjust color and brightness. Therefore, LEDs are widely used for professional, industrial, and consumer applications. However, the design of solid-state lighting products is a complex multi-disciplinary issue. Here, we will focus on the issue of heat dissipation and how thermal simulations help development teams develop reliable, dimensionally-compliance and performance products.
In fact, the lifetime of LED lamps (usually between 25,000 and 50,000 hours) is not fully realized, and the performance of solid-state lighting products sometimes deteriorates with time. Performance (quality and quantity of light output, lifetime, color retention, and other parameters) is closely related to the temperature within the fixture or replacement lamp. For a given environment, how effectively the lamp design dissipates internally generated heat is directly related to temperature.
1 Design Challenge
Two other related factors have also affected the long-term reliability of LED lighting: aluminum electrolytic capacitors for LED drive circuit energy storage, and market demand drive lighting manufacturers to create more compact lamps.
In addition to electromechanical components such as fans, aluminum electrolytic capacitors limit the useful life in many electronic circuits. Capacitors are electrochemical devices, and during normal operation, wet electrolytes are gradually used to reform alumina dielectric layers. The capacitor will eventually dry out and cause disastrous consequences. At high temperatures, this process accelerates.
In professional applications, such as entertainment lighting, smaller lighting devices are needed, which are easier to transport and operate, and are less obtrusive in use. In retrofit applications, from street lamps to home downlights, it is necessary to keep the size and shape within the limits defined by older lighting technologies. In the case of directional lighting, it is generally desirable to include electronic driver circuitry, as well as LED emitter modules and lenses within the luminaire.
2 adaptive drive circuit
The LED driver circuit needs to convert the AC mains voltage to a low DC voltage to drive the LEDs and ensure that the maximum output is achieved in an efficient manner. Despite its high efficiency, LED chips also generate heat; components in the drive circuit, especially power transistors, also generate heat. If all the heat-generating components are placed in a confined space to meet the physical design constraints, then the heat will increase quickly, even exceeding the maximum 100°C that the LED junction can withstand.
The challenge for LED luminaire designers is how to place all these things in the available space while ensuring that the temperature of the key points inside and outside the finished product remains within an acceptable range. At this time, thermal simulation can play a role, especially during the entire design process.
3 Benefits of Thermal Simulation
In the past, most of the design and development work was based on the use of “rules of thumb†to calculate the performance of a particular component, printed circuit board (PCB), or complete component from the thermal point of view. Since the design and development process of any electronic product is iterative, calculations must be repeated throughout the entire product development process. At each stage, design errors need to be corrected and hot spots may even be missed. Each change will increase the time and cost of the project, increasing the risk of losing the opportunity in the market.
Moreover, the accuracy of this method is relatively poor, and designers have had to over-design thermal management. This may mean, for example, using larger heat sinks, increasing the size and cost of the finished product, and may even mean that fans are used without the need for a fan, greatly reducing the mean time between failures (MTBF) of the finished product. What is even more frightening is that after the finished product is produced, there is a thermal problem, and then there are warranty claims, product replacements, and damaged brand reputation.
Therefore, thermal management enables engineers to design products that are smaller, more economical, perform better, and last longer. Thermal simulations make iterative designs faster, allowing you to experiment with multiple thermal management options and ultimately reduce time-to-market.
3 Simulation during development
In the design process, the earlier the thermal simulation, the more it can reduce the risk of major design changes to overcome possible thermal problems. Throughout the project, electronic, mechanical, and thermal engineers need to work together to ensure that the results of thermal simulations are taken into account in the design process and that the thermal effects of design changes are fully recognized.
At the beginning, a very simple conceptual model - where all the electronic components are represented as a centralized thermal block - can be used to determine if it is possible to cool the lighting fixture within the specification limits. The total power consumption of the product, its size, the size of the radiator, and the airflow of the fan (if used) represent all available information at this stage.
In the next stage, when the preliminary product design has been established, the thermal modeling tool needs the following information:
• Details of the component and its position on the PCB
• Estimated consumption of the most important components
• Size outline of lighting fixture housing
After the simulation is run, the temperature profile highlights where the component may exceed its maximum allowable operating temperature.
5 Input data affects the result
The more accurate the input data, the more accurate the simulation will be. The results of preliminary simulations can guide PCB designers and mechanical engineers to make changes that may benefit the thermal performance of the luminaire. With the development of the design, this process is repeated again.
Before producing the prototype, the final design proposed earlier should be simulated again. To ensure that the results of the simulation are accurate, more detailed information is needed. This includes:
• Thermal model of the components in the luminaire, which can be obtained from the component manufacturer
• 3D CAD model of LED lamp housing, which can be imported into simulation tools in various industry standard formats
• PCB design for EDA software, these can be imported using industry standard formats such as IDF and IDX
• Details of trace element copper in the PCB layer
• Information on the characteristics of the materials used in the luminaire
• Based on engineering calculations, the latest data on the internal device power consumption of the lamp
Once the prototype is completed, the development team verifies the accuracy of the simulation by measuring the physical temperature. In assessing these data, it is important to consider the accuracy limits of the measuring equipment used. These may be thermocouples, on-chip sensors, or infrared sensors, depending on the application.
6 Accuracy of Thermal Simulation
Optimal Thermal Solutions BV's thermal design specialist Norbert Engelberts used a thermal simulation tool to develop a series of LED lighting projects. The first is to design a LED lamp for the European market to replace the 60W incandescent E27 Type A lamp. The design goal is to use convection cooling to achieve as low a heat sink temperature as possible to maximize lamp life. As the temperature increases, the operating life decreases. Thermal modeling was used to optimize the design of the heat sink and when evaluating the final product, it was found that the simulation was accurate to within 5% of the measured temperature.
The same accuracy also occurred when designing downlights. The design goal is to determine the smallest heat sink that can be used while ensuring that the LED junction temperature stays within 100°C. The overall difference between the measured temperature and the simulated temperature is only 4.6%.
Engelberts also uses thermal models in the development of street lights. The challenge here is to ensure effective thermal management within the IP66 hermetically sealed enclosure, the size and shape of which depends on the size and shape of the traditional bulb to be replaced. The weight of the lamp is a key issue, so the radiator needs to be kept to a minimum size without affecting the life of the product. From the initial design to the final design, the average temperature at each point in the lamp decreased by 19%, and some points decreased by 35%. The final product is only 13% heavier than conventional lamps, but more reliable and more energy-efficient.