Automotive LED light source application and design

(Author: Luo Qingyue)

The lighting system on the car not only allows the driver to actually understand the condition of the car, but also provides the conditions that must be met during the safe driving process. The new generation of light source - LED , which is not unfamiliar to many consumers, has even been further expanded into automotive applications, and select the appropriate LED products according to the different needs of each application. Basically, due to the difference in appearance between the LED and the conventional light source, the light output and the performance output are also very different. Therefore, the LED should be perfectly applied to the automobile headlight, including optical design and heat dissipation design. It will be different from the design concept of traditional car lamps. Once the car can successfully integrate with the LED and overcome the technical threshold, it will create a new design principle for the car design.

Light control system on the car

From the cars around you can see that LEDs have been widely used in cars, including: turn indicator lights, displays or related lighting in the car, such as car display dashboards, roof lights, taillights to the outside, Front and rear indicator lights, reversing lights, third brake lights, etc., more or less can see the LED in the car light source related applications. From a system perspective, the lighting control on the car is a subsystem in the vehicle's main control architecture, which mainly includes: door control and dashboard display system, while the basic architecture is to collect the state quantity of each switch on the car and further The onboard light source is driven according to its state and loads the required action power.

It can be seen that the control of the vehicle body control system must be designed according to the light source of different power requirements. From another angle, from the application of LED in the car, although the LED is applied in the third brake light, to the taillights of the car, the turn signal to the brake light, the LED light source has been set in the car as lighting for several years. Use, step by step to create the imagination of the LED as a lighting inside and outside the car.

Figure 1: The future development trend of headlights for advanced vehicles in the world will be based on the development of Adaptive Front-lighting System (AFS) and LED white lighting products. (Source: )

Different LED packaging technologies must be used for different application requirements.

Different LED components have different application paths and environmental scopes, so the packaging methods are also different. If the LED chip and packaging technology can be further improved, the brightness of the product can be doubled. When the application level is different, the car center factory must choose different LED light source and packaging technology to face the different environmental requirements of the car. In general, the application of LED in the automotive industry is based on the difference in brightness, which is simply classified as three types of requirements, such as indicator number, projection source and illumination source, as follows:

• Application of the indicator number - Since the lumen value of the LED light source used in this range is not high, the power consumption is also low (about 70 ∼ 200 mW), and the generated heat source has less influence on the package, so many manufacturers When designing the package, the consequences of the heat source may be neglected. Therefore, most of the resin is directly coated with the resin material, and then the package is moved, and the resin material is used for heat conduction. The coefficient is low (W/mK), which is prone to poor heat dissipation, and the thermal resistance coefficient of the interface between the LED component and the heat dissipation system is increased.

Figure 2: SMT surface-encapsulated LED, suitable for central high-position brake lights, front turn signals, rear combination turn signals, tail lights and brake lights, mirror steering signal indicators, automotive exterior lighting, instrument panels, control backlights, central control Interior lighting such as backlighting, navigation, and sound systems. (Source: )

• In-vehicle lighting source application - In addition to the above-mentioned in-vehicle indicator light source, it can also be used for interior lighting, fog lights and front and rear direction indicators with high brightness requirements. As the brightness demand increases, the package power must also increase relatively. However, as a result, LEDs are likely to affect the color attenuation problem due to increased power, so the heat dissipation problem has to be taken into consideration. In the package design, in addition to resin-based material packaging, a metal block can be designed to derive the heat source generated by the LED at the first time in order to maintain the luminous efficiency and thermal resistance of the LED.

• Automotive projection light source application - this is currently the highest requirement for LED package brightness in automotive applications. Previous lighting systems were mainly used, including fog lamps, near lamps, and high lamps. The package of a single body must reach 4W or more. The thermal resistance must be less than 5K/W, in order to maintain the heat dissipation capability of the LED body in a high temperature environment, and keep the output efficiency of the LED light source within the specified range.

Different application levels also have different brightness requirements

Basically, in terms of the demand for flow brightness, the lighting equipment used in automobiles usually requires about 80 lumens of brightness, and most of them are packaged in Surface Mount Technology (SMT). The package is about 2 lumens in a single package. With a large output, the luminous efficiency can reach 15∼20lm/W. Secondly, the third brake light source for the vehicle requires about 30 lumens of brightness. Generally, a 5mm diameter barrel packaging technology is used to enhance the design of the illumination angle and intensity, and then mounted on the light-emitting component through the resin lens. To achieve the adjustment of light, the brightness of the single package is about 4 lumens, and the luminous efficiency can reach 20∼40lm/W. As for the brightness requirements of the car taillights, about 300 ∼ 500 lumens, generally using 1W SMT packaging technology, the brightness of the single package is about 10 ∼ 20 lumens, the efficiency can reach 15 ∼ 40 lm / W.

Figure 3: Limited by the lack of brightness, in the past, LEDs were mostly used as indicators or lighting in the car. In recent years, they have only been used in the third brake lights, tail lights, and direction auxiliary lights. (Source: )

The above is the light source measurement data installed on the car as the car body. At present, the LED factory and the car factory are actively cooperating to try to introduce the LED into the front lighting system (headlights, fog lights), in which the car factory is responsible for the brightness of the headlights. Demanding about 2,000 lumens of white light, the LED factory currently uses a high wattage SMT LED package architecture, output 100 ∼ 200 lumens per single package, efficiency is expected to increase to 50 ∼ 100 lm / W, currently used in the car's light source Can be divided into incandescent bulbs, halogen bulbs, gas discharge bulbs and LED light sources.

How to start designing LED car headlights

•requirements

Before starting to design headlights, you should first consider the relevant regulations, including optical brightness, environmental testing, brightness attenuation, etc., further consider the relevant optical design, mechanism design, heat-resistant design and electronic control design, etc. In terms of optical design considerations, in addition to the reflector design, it is necessary to consider the light-emitting type of the LED itself. Different package types will produce different light-type outputs, which will further affect the requirements of the reflector or imaging. Traditional headlamp designs need to be considered similar to different bulbs (H1, H4, H7, H11, etc.).

In the traditional headlight design, the photon release of the bulb itself comes from heating the tungsten filament, which does not affect the brightness output due to the heat generated by itself or the high temperature from the engine room. The heat dissipation focuses on the uniform temperature design of the entire headlight cavity. Non-bulb heat dissipation, but the choice of headlight material should consider whether it can withstand the high temperature from the bulb. For example, the headlight cavity of the car can withstand a temperature of 100 °C, and the temperature inside the fog lamp can be as high as 300 °C, so The materials selected are generally based on heat-resistant materials. However, for LEDs, the photon release comes from the energy level jump of the PN interface, which is negatively correlated with temperature. The higher the temperature, the weaker the light source output. Therefore, heat dissipation becomes an important issue in LED design.

•optical design

In the optical design, the regulatory requirements are considered first, and the relationship between the angle of view and the intensity is discussed. For the case of the near lamp, the special 15 degree angle design must be designed. In the traditional luminaire design , the angle and intensity control using the reflector and lens engraving was changed to become the direct control of the intensity angle by the reflector, and the fisheye lens design method using the imaging method was also developed. Regardless of the design method, you must first consider the characteristics of the light source, especially the angle and intensity of the Beam pattern. For traditional light sources, most of them are columnar light sources, which can produce a butterfly-like light pattern. Output, and then develop optical components such as lenses, reflectors, baffles, lenses, etc. When using LED as a light source to design a luminaire, it is necessary to reconsider its optical characteristics from a traditional cylindrical light source to a planar light source, and then with external optical components to produce different combinations for different products, according to the design of German car manufacturer HELLA Classification, the light source can be divided into eight categories.

LEDs currently emit less light per unit area than halogen bulbs and discharge bulbs. To get the same lumen output, LEDs require a larger package area. As the output area of ​​the light source increases, the difficulty of optical design increases. Therefore, in the existing concept car, the modular single optical lamp design is replaced by a modular optical design, and multiple sets of light sources are used to achieve the traditional lighting. The level of illumination, in addition to reducing the difficulty of optical design, also increases the sense of design of the car body.

Figure 4: Air flow through a flat heat sink for computational fluid dynamics simulation. The air in the heat sink area is circulated and exported to increase the heat dissipation of the LED. (Source: )

•cooling problem

Since about 90% of the input heat of the LED's input energy must be discharged, this is much higher than that of the conventional light source. In addition, the LED die is classified into a semiconductor material and cannot withstand high temperatures (<120 ° C). The heat removal from the grain to the atmosphere only allows a temperature difference of about 50 °C, which is much lower than that of the conventional light bulb. Therefore, the heat dissipation design is one of the subjects that distinguishes the LED light source from the conventional light source, and the lamp cannot use the fan to dissipate heat, and the lamp Products should be able to be popularized in the automotive market, and further consideration must be given to the appearance design and optical design principles of the lamps, which will be extremely troublesome for high-power LED lighting equipment.

Strictly speaking, the heat source generated by the traditional lamps is actually much higher than the LED; however, the traditional lamps will not reduce the output power of the light source because of the high temperature, but the light output of the LED will cause the LED due to its own interface problem. Luminous efficiency is affected. How the heat generated is dissipated to the external environment is closely related to its packaging structure materials, involving the use of heat-dissipating materials and related appearance. The concept of thermal resistance, when representing the input W power, needs to increase how much K temperature is enough to dissipate heat. With the existing packaging technology, the maximum allowable LED operation is 185 ° C (Lumiled K2), but generally because of the relationship between the package adhesive, the allowable operating temperature is about 125 ° C, in addition to considering the light source output efficiency, it must also be considered. The deterioration of the encapsulating material, for example, the aging phenomenon which is likely to occur at a high temperature when the resin-based material is used as a coated LED crystal.

At present, some manufacturers have developed the effect of “heat-temperature uniform temperature superconducting technology”, which can heat the heat generated by LEDs; that is, when the temperature difference of LED lamps is between plus and minus 1 degree, The forced cooling technology uses the dust-shielded semi-open space designed by the luminaire to perform the air venting action, that is, the convection of the air can be achieved, and the heat dissipation effect can be achieved. This development concept is like the heat dissipation of the human body. In addition to relying on the skin to dissipate heat, it also relies on the lungs of the second space to breathe for forced heat dissipation.

in conclusion

In the LED industry, the epitaxial process is prone to cause uneven chip conditions, and the so-called process of classified sales, especially for chips with higher wattage, is required in the LED industry to be fully inspected before shipment. Then, according to the LED wavelength, the next step of the sub-class, although after the package, each LED individual will still have subtle differences in brightness, color temperature, reliability. The use of LEDs as automotive headlamps will use multiple chip designs to deliver sufficient luminous efficiency for optical design of automotive luminaires. In addition, the need to pay attention to the quality of LED detection and quality control, in order to ensure that LEDs have the same quality of light source output.

Figure 5: Higher brightness and lower power consumption are trends in the LED chip industry, and future LED chips must be able to meet both of these requirements. In addition, low-power LEDs can significantly extend battery life. (Source: ST)

Since 2004, at least 13 automakers in the market have demonstrated the concept of LED headlights in the relevant auto show. Even in 2007, Lexus will launch a mass-produced car with LED as the headlight. I believe that the problems encountered by the LED light source will inevitably be relieved under the expectation of consumers. . LEDs have the advantage of small size in the first place. When used in front-mounted luminaires, they can reduce the size of the entire set of lamps, further giving valuable engine space and other related equipment, and designing with existing halogen bulbs or discharge bulbs. The total length of the lamps is about 300mm, and in many concept car designs, the LED lamps are only 125mm long, and with the advantage of small size, they can also be designed with different styles to create different visual perceptions for the car body. To get rid of the circular design concept of the past car lamps.