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EETE NOVEMBER 2012

DESIGN & PRODUCTS AUTOMOTIVE ELECTRONICS Automotive headlamps demand buck-boost LED drivers By Jeff Gruetter nowadays, automotive drive trains range 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. 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. In that area, high brightness LEDs are becoming a recurrent feature in automobile designs, as they can be found in applica- tions ranging from headlights, daytime running lights and brake lights; to instrument cluster display backlighting, as well as all kinds of in-cabin vanity lighting. In order to remain competitive with other light sources, 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 Fig. 1: Production LED headlamp optimized while also withstanding the rigors of the relatively caustic automotive electrical and physical environment. these applications are very distinctive from a design perspec- tive, they do not have the same level of design challenges as Although LEDs have been used in daytime running lights, do both the low beam and high beam of the headlights – see brake lights, turn signals and interior lighting for several years; figure 1. headlamp specific applications are relatively few. Currently a handful of production vehicles are offered with LED headlamps, We all know that the primary function of headlamps is to pro- including the Audi A8 and R8, Lexus’s LS600h and RX450h, the vide forward illumination at night or in less than ideal weather Toyota Prius and Cadillac’s Escalade. Some estimate that the conditions such as rain, snow and fog. The need for a higher current LED headlamp market was around $1B in 2011 and is level of illumination has been the primary driver for the evolution expected to surpass $2B in 2014. of the headlamp. In the 1980’s Halogen based lights became the industry standard, with 50W of electrical power they could One of the biggest challenges for automotive lighting deliver approximately 1,500 lumens of light which was a 50% systems designers is how to optimize all the benefits of the improvement over their predecessors. This translates into an latest generation of HB LEDs. As HB LEDs generally require efficacy (known as light output per watt) or light delivered per an accurate and efficient DC current source and a means for watt of 30 lumens/watt (lm/W). In the mid 1990’s xenon based dimming, the LED driver IC must be designed to address these high intensity discharge (HIDs) lamps became popular as they requirements under a wide variety of conditions. As a result, could deliver up to 80lm/W, enabling manufacturers to deliver power solutions must be highly efficient, robust in features and even greater total light output. However, they also have short- reliability while being very compact and cost effective. Arguably, comings such as the need to be accurately adjusted so not to one of the most demanding applications for driving HB LEDs is blind oncoming traffic, relatively short operational lives of 2,000 found in automotive headlamp applications as they are subject- hours, the use of toxic mercury gas and high manufacturing ed to the rigors of the automotive electrical environment, must costs. As the efficacy of HB LEDs continues to improve they deliver high power, typically between 50W to 75W, and must fit have become more desirable for headlight applications. Five into very space constrained enclosures, all while maintaining an years ago, production HB LEDs offered efficacies of 50lm/W attractive cost structure. 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 Automotive LED headlamps HID lamps. Several manufacturers, such as Audi, Mercedes and most re- cently, Lexus have used LEDs to design very distinctive driving The benefits of using LEDs in automotive headlights have lights or “eyebrows” around the headlights to make clear that it several positive implications. First, they never need to be re- is in fact their brand, long before the car can be seen. Although placed, since their solid state longetivity of up to 100K+ hours (11.5 service years) surpasses the life of the vehicle. This allows Jeff Gruetter is Sr. Product Marketing Engineer, Power Products automobile manufacturers to permanently embed them into at Linear Technology Corporation –www.linear.com headlight designs without requiring accessibility for replace- 20 Electronic Engineering Times Europe November 2012 www.electronics-eetimes.com


EETE NOVEMBER 2012
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