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In 2012, the market for high-brightness (HB) LEDs is expected to reach $12 billion, and by 2015, the market will grow to $20.2 billion, with a compound annual growth rate of 30.6% (source: Strategies Unlimited). One of the key application areas that has led to this significant growth in LEDs is the use of LEDs in automotive design. LED applications in automobiles include headlights, daytime running lights, brake lights and turn signals, dashboard displays and backlighting, as well as all types of interior lighting. Due to customer style preferences and safety requirements, LED daytime running lights (DRL) are available in almost all new models. Most of the Honda Accord's 2013 models feature LED daytime running lights and LED headlights in travel and hybrid models (see Figure 1). In order to maintain this amazing growth rate, LEDs must not only provide greater reliability, lower power consumption and a more compact form factor, but must also achieve innovative designs such as steerable headlights and anti-glare dimming. In addition, in the automotive environment, all of these improvements must be optimized for LEDs while still being subjected to relatively harsh automotive electrical and physical environments. It goes without saying that these solutions must also provide a very flat, compact footprint while increasing overall price/performance.
Although LEDs have been used in daytime running lights, brake lights, turn signals, and interior lighting for several years, headlamp-specific applications are relatively new. At present, only a small number of cars provide LED headlights, including the 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 in 2012 was approximately $1 billion, and it is expected that by 2014, the market will exceed $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, high-efficiency DC current source with dimming methods and must provide a variety of protection features. In addition, the driver ICs for these LEDs must be designed to meet these requirements in a wide variety of conditions. Therefore, power solutions must be very efficient, provide rugged functionality and reliability, while being very compact and affordable. It can be said that the most demanding application for driving high-brightness LEDs is automotive headlight lighting applications, including daytime running lights and headlights, because such applications are in a harsh automotive electrical environment and must provide high power. It is generally between 15W and 75W and must be placed in a very limited space to meet all of these requirements while maintaining an attractive cost structure.
Figure 1: LED Honda Accord Travel/Hybrid LED Daytime Running Lights and Headlights
LED daytime running lights and headlights
The advantages of small size, long life, low power consumption, and stronger dimming ability are the catalysts widely used for 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 that make these daytime running lights the "eyebrows" or "bottom lines" of the headlights. Highlight the unique charm of the brand. These applications are not only unique from a design perspective, but also have some design challenges when providing a reliable, cost-effective solution. As high-brightness LEDs are gradually used in low beam and high beam headlights, these challenges are becoming more pronounced.
It is well known that the main function of the headlights is to provide forward illumination at night or in less than ideal weather conditions (eg rain, snow and fog). The need for higher illumination has been a major driver of headlamp development. 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 lamp. Such luminescence conversion success (known as the light output per watt) or light provided per watt is 30 lumens per watt (30 lm/W). In the 1990s, high-intensity discharge (HID) xenon lamps became mainstream because they provided 80 lm/W, allowing manufacturers to provide greater overall light output. However, xenon lamps also have disadvantages, such as the need to accurately adjust to avoid obscuring the road to the opposite vehicle; the working life is relatively short, only 2000 hours; the use of toxic mercury gas and the high cost of manufacturing. As the efficacy of high-brightness LEDs continues to increase, such lamps have become more ideal for headlamp applications. Five years ago, mass-produced high-brightness LEDs provided 50 lm/W, which was not enough for headlamp applications, but current LED designs offer 100 lm/W of light output, and it is estimated that in the next few years, The light output will exceed 150 lm/W, which even exceeds the best high intensity discharge lamps. LEDs offer roughly 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 create a new generation of headlights has become particularly attractive.
The use of LEDs to form automotive headlights and daytime running lights has many advantages and has several positive effects. First of all, LED lights never need to be replaced because they have a reliable life of more than 100,000 hours (11.5 years of service life), exceeding the life of the vehicle. This allows automakers to permanently embed LED lights in the body without leaving room for replacement. LED lights also help to dramatically change the style of the vehicle, as LED lighting systems do not require the depth or area required for high-intensity discharge lamps or halogen lamps. High-brightness LEDs are also more efficient than halogen lamps when they provide light output (in lumens) from input electrical power (and will soon exceed high-intensity discharge lamps). There are two positive effects. First, it can consume less car bus power, which is especially important in electric and hybrid cars. It is also important that high efficiency also reduces the amount of heat that needs to be dissipated in the housing, eliminating the need for any bulky and expensive Radiator. Finally, by using high-brightness LED arrays in headlamp arrays and electronically steering and dimming, LED arrays can be easily designed and optimized for many different driving situations.
Design Parameters
To ensure optimum performance and long operating life, LEDs require an efficient drive circuit. This means that regardless of how the input voltage source changes, the driver IC must efficiently deliver accurate DC current and provide accurate LED voltage regulation. Second, the driver IC must provide a dimming method and a variety of protection features to protect against LED open or short-circuit faults. In addition to relying on the automotive power bus with a very harsh electrical environment to work reliably, the driver IC must be economical and space efficient.
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