Core Tip : This article describes methods that can help the automotive lighting industry achieve optimal thermal management. We discussed the selection and measurement of LED thermal characteristics and the selection of the most suitable LED for a specific application. Since overheating may damage the stability of the LED system, we also discussed the thermal simulation of complex shaped lighting systems such as headlights and taillights, and the use of synchronous computational fluid dynamics technology to design higher quality products Develop automotive lighting systems in an efficient and economical way.
Industry Trends
According to McKinsey & Company's insights on the global lighting market, the automotive lighting market is currently about 18 billion US dollars (13 billion euros), accounting for about 20% of the entire lighting market, and is expected to increase to 25 billion US dollars by 2020 (18 billion euros). With the development of LEDs (Light Emitting Diodes), LEDs in automotive applications are expected to grow significantly in the next 10 years. An article published in "LEDs Magazine" in November 2012 stated that all lighting systems of Daimler's upcoming S-series Mercedes will use LEDs. From 2010 to 2020, the price of LEDs will be reduced to one-tenth of the current price, so LEDs will be more competitive than traditional light sources.
Unlike traditional automotive lighting sources, LEDs are more sensitive to temperature, not only need to have a sufficient understanding of the structure and characteristics of the LEDs used in the design, but also need to understand the entire thermal management system from radiator to cooling fluid. With these skills, lighting designers can optimize their designs to ensure long-lasting LED life, minimum emission wavelength shift, or minimum light output loss. They can also use LEDs as a light source more effectively and promote the overall popularity of LEDs in the automotive industry.
Challenges of using LEDs in automotive lighting
With the transformation of light source design from incandescent lamps to LEDs, the traditional thermal management concepts are outdated and new ways of thinking need to be developed. About 83% of the electric energy of most incandescent lamps forms heat radiation, and about 12% forms heat loss, and will not face the problem of heat dissipation of the light source. LEDs mostly transfer heat losses through conduction (about 60-85%) and are very sensitive to thermal management. The electro-optical conversion efficiency of 100-watt incandescent lamps is only about 5%, while the conversion efficiency of LEDs can reach about 15-40%, and is still improving.
The main thermal challenge of LEDs is to maintain high color stability and life expectancy. LEDs in the automotive industry need to have lifetime tolerance. Not only are LEDs more efficient, but their higher visibility is also valuable, so they are safer. The European Economic Commission (ECE) stipulates that all new cars must be equipped with daytime running lights (DRL) from 2011.
Because exterior lights such as headlights and taillights are almost completely sealed systems (except for extremely small airflow inlets and outlets and small openings in ordinary incandescent lamps), replacing LEDs due to a defect is impractical. When multiple LEDs in the headlights or taillights fail, they can only be solved by replacing the entire headlight. Therefore, not only LEDs, the entire luminaire design must have high reliability and quality, because replacing the entire headlight is expensive; if it is still under warranty, then the original equipment manufacturer (OEM) and supplier of the system will want to It costs a lot.
Analysis of the characteristics of thermal and radiant behavior ensures high reliability
Original supplier data sheets do not always provide the data needed to produce accurate and reliable simulation results from fluid or structural analysis; manufacturers also do not often provide guarantees or instructions for measurement data errors. Therefore, you will need to test and measure the characteristics of your car before installing applications to ensure the reliability of components and materials.
Thermal characteristics
The thermal resistance (Rth) of an LED can affect the product's life, efficiency, and operation of multiple domains, as well as electrical, thermal, and optical performance (Figure 1). The LED kit, like all other semiconductor device kits, can be characterized by thermal resistance to achieve stable operation. The thermal resistance (Rth) value tells us how much the unit heat source will increase the temperature when applied to the device.
Figure 1: Thermal issues affect all aspects of the LED kit.
The most basic method is to measure the temperature-dependent voltage of the component. The LED turns on or off from a stable state, and after a period of time, it reaches another stable state (hot / cold, and vice versa). During this process, transient measurements are continuously performed to provide a thermal transient response curve at a small measurement current. With the help of the measured temperature difference and power difference (for switching components) (Figure 2), the structural function (Figure 3) can be derived.
Figure 2: Mentor Graphics' T3Ster thermal transient tester can record the LED's transient response after just 1 microsecond (1x10-6 seconds) with a temperature resolution of 0.01 ° C.
Figure 3: Through the transient response, we can automatically determine the structure function of the LED kit sample. This R / C mode can be directly used in thermal simulation software.
In November 2010, the Joint Electron Devices Engineering Council (JEDEC) released the standard JESD51-14 for measuring the thermal resistance of the case (RthJC) using the dual thermal interface method. The standard requires two measurements: that is, without additional layers and with additional layers, the deviation position can reflect the thermal resistance of a component. This method is suitable for power semiconductor components with exposed cooling surfaces and one-dimensional heat flow paths. This situation is also valid for power LEDs.
The structure function shown in Figure 3 allows us to determine the thermal resistance crust (RthJC), which is very important for accurate thermal simulation. The structure function can not only help to determine the thermal resistance, but also be used to compare different LEDs, solder / adhesive quality, defects and defect locations, cooling efficiency and temperature dependence of different PCB / MCPCB types. Everything between the grain and the surrounding environment can be seen in the structure function, and the changes due to defects and aging can also be seen by comparison with normal or ideal assembly.
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