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Use buck-boost LED drivers for automotive headlamp

Posted: 11 Jun 2013     Print Version  Bookmark and Share

Keywords:automobile  high brightness  HB LED  ECU  dimming 

The very definition of an automobile is in more flux today than ever before. For the last century, it has been dominated by internal combustion drive trains, mainly petrol powered with a smattering of diesel drive trains. However, we now have automotive drive trains ranging from purely electric (EV) to high efficiency internal combustion, to a myriad of combinations (commonly referred to as hybrids). All of these designs share a common goal of increasing fuel efficiency while simultaneously reducing carbon emissions. New power train designs include direct fuel injection, turbo charging, engine stop/start systems, regenerative braking, higher ethanol content fuels and cleaner diesel combustion. As more hybrids are developed, they are becoming much more dependent on cleaner electric power sources. Despite this level of progress, one aspect of their designs has remained relatively constant, and that is the need to provide forward illumination for driving at night and in less than perfect weather conditions. Furthermore, the means to generate the required light illumination has also evolved from Halogen lamps to High Intensity Discharge (HID) Lamps to most recently high brightness (HB) LED based designs, fueling yet another avenue for HB LED growth.

The market size for high brightness (HB) LEDs is expected to reach $12 billion this year and grow to $20.2 billion by 2015 with a 30.6% CAGR ramp (source: Strategies Unlimited). One of the key application areas driving this significant growth is the LEDs used in automobile designs. Applications range from headlights, daytime running lights and brake lights; to instrument cluster display backlighting, as well as all kinds of in-cabin vanity lighting. However, in order to maintain this impressive growth rate, LEDs must not only offer enhanced reliability, reduced power consumption and more compact form factors, but they must also enable innovative designs such as steerable headlights and antiglare dimming. Furthermore, in an automotive environment, all of these improvements must be optimised while also withstanding the rigors of the relatively caustic automotive electrical and physical environment. It goes without saying that these solutions must offer very low profile, compact footprints while simultaneously enhancing overall cost-effectiveness.

Although LEDs have been used in daytime running lights, brake lights, turn signals and interior lighting for several years; headlamp specific applications are relatively few. Currently a handful of production vehicles are offered with LED headlamps, including the Audi A8 and R8, Lexus's LS600h and RX450h, the Toyota Prius and Cadillac's Escalade. Some estimate that the current LED headlamp market was around $1B in 2011 and is expected to surpass $2B in 2014.

One of the biggest challenges for automotive lighting systems designers is how to optimise all the benefits of the latest generation of HB LEDs. As HB LEDs generally require an accurate and efficient DC current source and a means for dimming, the LED driver IC must be designed to address these requirements under a wide variety of conditions. As a result, power solutions must be highly efficient, robust in features and reliability while being very compact and cost effective. Arguably, one of the most demanding applications for driving HB LEDs is found in automotive headlamp applications as they are subjected to the rigors of the automotive electrical environment, must deliver high power, typically between 50W to 75W, and must fit into very space constrained enclosures, all while maintaining an attractive cost structure.

Automotive LED headlamps
Benefits, such as small size, extremely long life, low power consumption and enhanced dimming capability are the catalyst for the wide spread adoption of HB LED headlights. Several manufacturers, such as Audi, Mercedes and most recently, Lexus have used LEDs to design very distinctive driving lights or "eyebrows" around the headlights to make clear that it is in fact their brand, long before the car can be seen. Although these applications are very distinctive from a design perspective, they do not have the same level of design challenges as do both the low beam and high beam of the headlights.

We all know that the primary function of headlamps is to provide forward illumination at night or in less than ideal weather conditions such as rain, snow and fog. The need for a higher level of illumination has been the primary driver for the evolution of the headlamp. In the 1980's Halogen based lights became the industry standard, with 50W of electrical power they could deliver approximately 1,500 lumens of light which was a 50% improvement over their predecessors. This translates into an efficacy (known as light output per watt) or light delivered per watt of 30 lumens/watt (lm/W). In the mid 1990's xenon based high intensity discharge (HIDs) lamps became popular as they could deliver up to 80lm/W, enabling manufacturers to deliver even greater total light output. However, they also have shortcomings such as the need to be accurately adjusted so not to blind oncoming traffic, relatively short operational lives of 2,000 hours, the use of toxic mercury gas and are expensive to manufacture. As the efficacy of HB LEDs continues to improve they have become more desirable for headlight applications. Five years ago, production HB LEDs offered efficacies of 50lm/W which were not sufficient for headlight applications, however current LED designs offer 100lm/W with estimates that this will exceed 150lm/W in the next few years surpassing even the best HID lamps. The ability of LEDs to offer roughly the same amount of light output per watt and their other benefits such as long life, ruggedness and their environmentally friendly design makes them particularly attractive to power the next generation of head lights.

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