Improving automotive lighting designs with high-performance LED drivers

LED DRLs & 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 DRLs and headlights. Several manufacturers, such as Audi, Mercedes and most recently, Lexus and Honda have used LEDs to design very distinctive DRLs as “eyebrows” or “underlines” around the headlights as part of their unique branding. Not only are these applications very distinctive from a design perspective, they also create several design challenges to offer a reliable and cost effective solution. As HB LEDs are adopted into both the low beam and high beam of the headlights, these challenges become even more pronounced.

It is well known 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 tungsten filament predecessors. This translates into an efficacy, (known as light output per watt) or light delivered per watt, or 30 lumens/watt (lm/W). In the mid 1990s, 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 as 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 of long life, ruggedness and environmentally friendly design, makes them particularly attractive to power the next generation of head lights.

The benefits of using LEDs in automotive headlights and DRLs have several positive implications. First, they never need to be replaced, since their solid state longetivity of up to 100K+ hours (11.5 service years) surpasses the life of the vehicle. This allows automobile manufactures to permanently embed them into the vehicles bodywork without requiring accessibility for replacement. This also enables styling to be dramatically changed as LED lighting systems do not require the depth or area as HID or Halogens do. HB LEDs are also more efficient than Halogen bulbs (and are soon to surpass HIDs) at delivering light output (in lumens) from the input electrical power. This has two positive effects. First, it drains less electrical power from the automotive bus, which is especially important in EVs and hybrids, and equally important, it reduces the amount of heat that needs to be dissipated in the housing eliminating any requirement for bulky and expensive heat sinking. Finally, by using arrays of HB LEDs in headlight arrays and electronically steering or dimming them, they can easily be designed to optimize lighting for many different driving conditions.

Design Parameters
In order to ensure optimal performance and long operating life, LEDs require an effective drive circuit. This means that the driver ICs must deliver an accurate and efficient DC current as well as accurate LED voltage regulation regardless of the variations of the input voltage source. Secondly, they must offer a means of dimming and also provide a wide array of protection features just in case a LED open or short circuit is encountered. In addition to operating reliably from the electrically caustic automotive power bus, they must also be both cost and space effective.

Stop/Start, Cold Crank & Load Dump Conditions
In order to maximize fuel mileage while minimizing carbon emissions, alternative drive technologies are continuing to evolve. Whether these new technologies incorporate electric hybrids, clean diesel or a more conventional combustion engine designs, the chances are that they will also need to incorporate a stop-start motor design. Already prevalent in virtually all hybrid designs throughout the world, many European and Asian and car manufacturers have been incorporating these designs into conventional gas and diesel vehicles as well. In the USA, Ford recently announced that it will incorporate stop-start systems into many of its forthcoming domestic models.

The concept of a stop-start system for the engine is straight-forward, the engine is shut off when the vehicle comes to a stop and then restarted immediately before the vehicle is required to move again. This eliminates the fuel used, and emissions generated, while the car is stopped in traffic or at a stop light. This stop-start design can reduce fuel consumption and emissions between 5% and 10%. However, the biggest challenge for these designs is making the entire stop-start scenario imperceptible to the driver.