Illuminate the front-LED automotive headlamp design features
LED no stranger For the consumer, has been widely used in the car, according to different performance requirements choose the appropriate LED products, LED light source in comparison to the traditional appearance of different things in addition to performance-based output and optical output is also greatly the same applies to the headlights when faced with optical design and thermal design is different from the traditional concept of lighting design. However, there are also different from the traditional sources because of their characteristics, when the successful integration with the body to overcome the technical difficulties, the model will create a new design concept.
LED has been widely used in vehicle lighting or instructions related to the use of light source, we can see from Figure 1, from the car dashboard lights, car dome light, make-up lights, lamps, etc. Welcome to the taillights of the vehicle, before and after the turn lights, down lights, third brake light, etc. can see the relevance of the application of LED.
Figure 1 on board the LED application
Source: Toyota Gosei
LED Applications from the historical point of view, as early as 1992 have been applied to the third brake light LED's precedent; and in 2000 was further applied to taillights, turn signal and brake lights; to take the lead in the 2002 Audi A8 LED light source will be placed on front line as a lamp lights, open the designer and engineer of the lamps in the front room for imagination. After many in the international auto show can be seen on the LED as the source of the concept car headlamp, as shown in Figure 2.
Figure 2 LED headlamp concept car
Source: Detroit Auto Show
LED on the inherent advantages of small size, when applied to pre-lighting can reduce the size of the entire group of lamps to enable the engine of a valuable space and other related equipment to the existing halogen bulb or a discharge type bulb design lamps of about 300mm, and in many of the design concept cars, LED lighting is only 125mm long. And by the advantages of small size, but also with the design of a number of different design, and then for different body shapes to create visual perception, beyond the traditional circular design of the lamp.
The different needs of the application of LED package
With the application of different levels, the depot also choose a different package of LED light source corresponding to different environmental requirements, in accordance with the needs of different brightness can be divided into simple instructions, and lighting and projection needs of three different. Light source directions can be found in the third brake lights, taillights group (rear lights, brake lights, turn lights, etc.), side lights, the light output lumens value of the low, low power requirements, about 70mW ~ 200mW, the heat generated by the closure body is less loaded and therefore will be neglected in the package on the impact of this heat, and direct the use of resin-coated material to the overall package, and because the resin of low thermal conductivity (W / mK), so its related thermal Thermal resistance is not easy due to the increased 50 ~ 200K / W; and lighting package with its power source will be relative to these countries, in addition to the instructions with the light source can be applied to categories of products, higher brightness can be used for the day line lights, fog lights, turn signals, etc. before, but also because of increasing loss of power (power at about 1 ~ 5W), heat release part of the light source is not as if the instructions do not consider heat issues, in addition to resin packaging material need to use outside the metal Export block heat in order to maintain the efficiency of the light, the resistance remained at 15K / W following; and projection light source is a light source with brightness demand package to the highest bidder, according to their applications before the system (far from lights, the last lights, fog lights, etc.) mainly in the single package to be more than 4W, and in the resistance to be less than the 5K / W, to ensure that the engine room to maintain the high temperature heat capacity, and maintain the efficiency of light output in the range of available.
Application of different levels, the total demand brightness also different in terms of internal lighting approximately 80 lumens of brightness, the general choice of surface-mount type (SMT) package, single package about 2 lumens output, efficiency up to 15 ~ 20lm / W. Third brake light in terms of approximately 30lm brightness, the general choice of the structure of shell-type package (φ = 5mm), single package about 4 lumens brightness, efficiency up to 20 ~ 40lm / W.
And taillights group demand for the brightness of about 300 ~ 500lm, the general choice of the SMT package 1W structure, monomer brightness package of about 10 ~ 20lm, efficiency up to 15 ~ 40lm / W. All of the above has been applied to the body of the light source, the current LED factory and the depot is active cooperation, and try to import the former according to the system LED (headlights, fog lamps), in which the brightness of headlights depot demand for about 2,000 lumens white light, LED plant is currently the use of high wattage of SMT LED package structure, the output of each single package can be 100 ~ 200lm, efficiency is expected to be raised to 50 ~ 100lm / W.
Currently used light sources in the vehicle can be divided into incandescent bulbs, halogen bulb, gas discharge lamps and LED light source, the comparison shown in Figure 4.
In the design of LED headlights
Started the design of the headlights, we should first consider the relevant provisions of laws and regulations, including the brightness of light-based, environmental testing, such as brightness attenuation needs further consideration related to optical design, mechanical design, thermal design of electronic control and design details for the LED , the optical design considerations in addition to cover the design of reflection, the need to consider the LED-ray of their own type, different types of packaging will have a different type of light output, further reflection will affect the cover or the requirements of imaging through competition, with the traditional design would have to consider a different headlight bulbs (H1, H4, H7, H11, etc.) similar. Headlights in the traditional design of the bulb itself, the release of photons from the heated tungsten filament, will not be issued their own heat or heat from the engine room and the impact of light output, heat focused on the entire cavity of both headlights design temperature of the heat rather than light bulbs, but the choice of materials in the headlights on the need to consider whether it can withstand heat from the bulb (about headlight cavity temperature under 100 ℃, fog cavity temperature up to 300 ℃ ), so in this material is generally selected in order to heat the main timber; LED, however, its release from the PN-photon interface can beat bands, a negative correlation with temperature, the higher the temperature the weaker the light output , so as cooling LED light source designed to become an important issue.
Figure 5 regulations in the European Union in recent optical light type
Optical design needs to consider legislation to discuss the relationship between angle and intensity, light as an example to be nearly unique for its 15 degrees pt design, as shown in Figure 5. Designed in the traditional lamp by the use of pre-reflective lens hood with the marking for the control of the angle and intensity, into a direct control over the use of reflective shield strength point of view, but also the development of imaging methods using the fish-eye lens design. The design of whatever mode of selection shall consider the characteristics of light sources, especially the angle and intensity of light-based output (Beam pattern), in terms of traditional light sources, most of the light source for the column may have a similar appearance of the optical type Butterfly output, and then developed them with the lens, reflex enclosures, baffles, lenses and other optical components. The use of LED lamps as a light source design, the need to re-consider the optical properties of the column from the traditional light source light source into a plane (the design of different packages have different type of light output), and with external optical components to produce different combinations of applied to different products, according to Germany HELLA lights manufacturers design the classification of light sources can be divided into eight categories, as shown in Figure 6. LED current per unit area and not the volume of light-emitting halogen lamp and discharge lamp, want the same lumen output, LED area the need for a larger package. With the increase in area of light output, optical design also enhance the degree of difficulty, so the concept of the existing board are to replace the existing modular optical design of a single lamp room design, the use of multiple light sources to achieve the traditional lamp lighting levels, in addition to reducing the difficulty of optical design, but also increase the body shapes the design of flu.
Figure 6 headlamp design technology
Figure 7 LED lighting design
Thermal design of LED light source from one of the topics of traditional light sources. Heat generated by the traditional lighting is much higher than LED, but the traditional heat lamps will not reduce the light output capacity, but the LED's light output will interface with their own (Junction, PN interface) drop in temperature, Figure 8. And the heat generated by how casual the environment in addition to structural materials and their packaging is closely related to the heat involved in the use of appearance and related materials, the relationship as shown in Figure 9, in which the concept of resistance, on behalf of W power input, there is a need increase the number of K before the temperature enough to heat. Existing packaging technology to the maximum allowed LED operating at 185 ℃ (Lumiled K2), but because the relationship between packaging of plastic material that allows the operating temperature at about 125 ℃, in addition to considering the efficiency of light output but also to consider deterioration of plastic packaging materials (resin-like materials are aging at high temperature).
Figure 8 interface temperature on the impact of light output
The temperature of the engine room near the maximum in the lamp to 85 ℃, with Lumiled K2 will have space for heat dissipation of 100 ℃, but in general only the LED heat space 40 ℃ were observed related to thermal resistance, R_Junction-Slug, R_Slug -Board has decided to package body, only for the purposes of the designer R_Board-Ambient efforts, including how the body will be fixed in the cooling package substrate (followed by the quality), thermal structure design, the main passive dispersion thermal considerations, such as the external environment and conditions. So here should be associated with the analog design agencies to obtain lamps in the engine room of the flow field and temperature conditions and then consider the required cooling mode, if selected to be scattered and moving hot heat a larger space, the engine is is not a small burden, if the choice of active cooling, although the smaller the required cooling the environment, but because of an increase of movable parts such as fans, but this may need to consider the possibility of moving pieces through the lights on the relevant laws and regulations , including vibration, dust, moisture and other corrosive and harsh environments.
Figure 9 the relationship between packaging and heat resistance
The application of LED in the headlight problems encountered
LED headlights used in many of the auto show to display various depots are related to the concept of cars, but some have not yet seen in the headlights have finished there, there are still many problems need to be resolved. The main issues which should be put on the light output, thermal design problems and errors.
The brightness of headlights needs of nearly 900 lumens lamp, 1100 lumens lamp far, the overall demand for 2000 lumens, or about 40 1W production Lumiled package Luxeon emitter body 25 ℃ in the total light output, but when the outside temperature to 50 ℃ , the efficiency will be reduced to below 80 percent, and the light output area of about 90_2 (Gas Discharge bulbs about 13_2), the design would rise with difficulty.
In the LED industry, to deal with non-uniform epitaxial chips caused by the development of the classification of the so-called process of sales, especially high-wattage chips in the LED industry is seized ship, in accordance with the classification of light wavelength, so Packaging individual after the gap there is a minor, this gap may exist in brightness, color temperature or the reliability. However, the application of LED in the headlights, the inevitable will make use of multiple chips to output sufficient brightness for optical design, at this time should pay particular attention to LED quality testing and quality control aspects in order to ensure the same quality of light output.
Down since 2003, more than more than 13 depots in the relevant show on display to the demands of LED headlamps for the concept car. For example, Lexus announced the depot, in 2007, will be introduced to LED for the mass production car headlights. Believe exist in the LED of the associated works will be in the nextconsumers a solution, let us wait and see.
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