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In 2012, the market size of high-brightness (HB) LEDs is expected to reach US $ 12 billion. By 2015, the market will grow to US $ 20.2 billion, with a compound annual growth rate of 30.6% (data source: Strategies Unlimited). One of the key application areas driving this significant growth of LEDs is the use of LEDs in automotive design. The applications of LEDs in automobiles include headlights, daytime running lights, brake lights and turn signals, instrument panel displays and backlighting, as well as all types of interior lights. Because customers have style preferences and safety requirements, almost all new models provide LED daytime running lights (DRL). Most Honda Accord 2013 models use LED daytime running lights, and LED headlights are provided in travel and hybrid models (see Figure 1). In order to maintain this amazing growth rate, LEDs must not only provide stronger reliability, lower power consumption and more compact form factors, but also must achieve innovative designs such as turnable headlights and anti-glare dimming. In addition, in the automotive environment, all of these LED improvements must be optimized while also withstanding the relatively harsh automotive electrical and physical environments. It goes without saying that these solutions must also provide a very flat and compact footprint while increasing the overall cost performance.
Although LEDs have been used in daytime running lights, brake lights, turn signals and interior lighting for several years, applications specific to headlights are relatively new. At present, only a small number of production vehicles provide LED headlights, including Honda Accord, Audi A8 and R8, Lexus LS600h and RX450h, Toyota Prius, Cadillac Escalade and Porsche Cayenne. Some industry estimates indicate that the LED headlamp market was approximately US $ 1 billion in 2012. It is expected that by 2014, the market will exceed US $ 2 billion and continue to grow exponentially.
One of the biggest challenges for automotive lighting system designers is: how to optimize all the advantages of the latest generation of high-brightness LEDs. High-brightness LEDs require an accurate and efficient DC current source, have a dimming method, and must provide various protection functions. In addition, the driver ICs for these LEDs must be designed to meet the above requirements under a variety of conditions. Therefore, the power supply solution must be very efficient, provide rugged functionality and reliability, while being very compact and economical. It can be said that in terms of driving high-brightness LEDs, the most demanding applications are automotive headlight lighting applications, including daytime running lights and headlights, because these applications are in the harsh automotive electrical environment and must provide high power , Generally between 15W and 75W, it must also be placed in a housing with very limited space. While meeting all of these requirements, it must also maintain an attractive cost structure.
Figure 1: LED daytime running lights and headlights for the 2013 Honda Accord Travel / Hybrid model
LED daytime running lights and headlights
The advantages of small size, extremely long life, low power consumption, and stronger dimming ability are the catalysts for the widely used high-brightness LED daytime running lights and headlights. Several car manufacturers (such as Audi and Mercedes-Benz, as well as Lexus and Toyota) have designed very unique daytime running lights with LEDs, making these daytime running lights the "eyebrows" or "bottom line" of the headlights. Highlight the unique charm of the brand. Not only are these applications unique from a design perspective, but they also have some design challenges when providing reliable, cost-effective solutions. As high-brightness LEDs are gradually used in low-beam and high-beam headlights, these challenges have become more significant.
It is well known that the main function of the headlights is to provide forward lighting at night or under less than ideal weather conditions (such as rain, snow and fog). The need for higher illuminance has been the main driving force for the development of headlamps. In the 1980s, halogen lamps became the industry standard. With 50W of electric power, these lamps can provide about 1500 lumens of light, which is 50% higher than the previous tungsten filament lamp. Such luminous quantity conversion is successful (that is, the well-known light output per watt) or the light provided per watt is 30 lumens per watt (30lm / W). In the 1990s, high-intensity discharge (HID) xenon lamps became mainstream because these lamps can provide 80 lm / W, allowing manufacturers to provide greater overall light output. However, xenon lamps also have shortcomings, such as: they need to be adjusted accurately to avoid the roads on the opposite side from being unclear; the working life is relatively short, only 2,000 hours; the use of toxic mercury gas and expensive manufacturing costs. As the efficacy of high-brightness LED lights continues to increase, these types of lights have become more ideally suited for headlight applications. Five years ago, mass-produced high-brightness LED lamps provided 50lm / W, which is not enough for headlight applications. However, the current LED design can provide 100lm / W light output, and it is estimated that in the next few years, The light output will exceed 150lm / W, which exceeds even the best high-intensity discharge lamps. LEDs can provide approximately the same amount of light output per watt, and there are other advantages, including long life, ruggedness and environmentally friendly design, so the use of LEDs to form a new generation of headlights has become particularly attractive.
The use of LEDs to construct car headlights and daytime running lights has many advantages and has several positive effects. First of all, LED lights never need to be replaced, because their reliable life span is more than 100,000 hours (the service life is 11.5 years), which exceeds the life of the vehicle. This allows car manufacturers to permanently embed LED lights into the body without having to leave room for replacement. LED lights also help to greatly change the style of vehicles, because Led Lighting systems do not require the depth or area required for high-intensity discharge lamps or halogen lamps. When providing light output (in lumens) from input electrical power, high-brightness LED lamps are also more efficient than halogen lamps (and will soon exceed high-intensity discharge lamps). This has two positive effects. First of all, it can consume less electric power of the car bus, which is especially important in electric vehicles and hybrid vehicles. Equally important, high efficiency can also reduce the heat that needs to be dissipated in the housing, thereby eliminating the need for any bulky and expensive Radiator. Finally, by using a high-brightness LED array in the headlight array, and electronically turning and dimming, the LED array can be easily designed and optimized for many different driving situations.
Design Parameters
In order to ensure the best performance and long working life, LED needs an effective driving circuit. This means that no matter how the input voltage source changes, the driver IC must efficiently provide accurate DC current and provide accurate LED voltage regulation. Secondly, the driver IC must provide a dimming method and also provide multiple protection functions to prevent the LED from being open or shorted. In addition to relying on the automotive power bus to work reliably in a harsh electrical environment, the driver IC must also be economical and save space.
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