Automotive LEDs: Adapting For The Future

One of the key early markets for high-brightness LEDs, the automotive sector first adopted solid-state lighting technology for internal uses, as well as rear and signal lighting functions. That was because, in the early days of HB-LEDs, red and orange emitters were far brighter and more advanced than blue and white devices based on the troublesome gallium nitride (GaN) material system.

However, once developers got to grips with GaN and delivered sufficiently bright white emitters, all that began to change. From 2003, LEDs started to appear in more technically-demanding front-lighting systems, starting with the now-familiar daytime running lights (DRLs) seen in Audi vehicles.

Today, full-LED headlamps incorporating both the main lighting functions (low and high beam) are available, but until very recently they have been restricted to high-end vehicles, and even then are often sold only as “luxury” options. Things are changing on two fronts. First, LED headlamps now appear to be transferring quickly into mid-range and compact cars. Second, the new technology of adaptive lighting is beginning to emerge.

As the cost of the technology drops, the mainstream adoption of LED headlamps alongside the additional complexity afforded by adaptive lighting are two key trends in the market for automotive headlamps. Both have implications in terms of both volume demand and performance for makers of both light emitters and the driver ICs that are their essential partners.

In this article, we review current developments in LED headlamps and adaptive automotive lighting, and discuss how the requirements of adaptive functionality impacts LED and control components.

Audi’s head start

Until recently, the front ends of most vehicles have remained a largely LED-free zone. The exceptions to that rule have always been most evident in vehicles from Audi, whose early adoption of multi-LED daytime running lights (DRLs) in cars like the A8 rapidly became the “signature” of these high-end models.

Following Audi’s lead, Lexus, Jeep and Buick were among the first to introduce similar white LED DRLs, while their more recent appearance in vehicles as popular as the Honda Accord and Toyota Corolla have seen LED front lighting suddenly become a familiar feature. However, cars with full-LED headlamps are still a relative rarity, typically restricted to options on luxury models. That is now changing as the technology becomes both more affordable and functional, with full-LED headlamps now slated for mid-range models including the Peugeot 308, Seat Leon, Volkswagen Passat and, perhaps most mainstream of all, even the Ford Mondeo.

At the same time, top-of-the-range models are using more LEDs than ever. According to a report in Automotive News Europe,¹ the sixth-generation Mercedes S-class will be the first vehicle to feature LEDs exclusively in all of its exterior lighting functions. Peugeot’s new 308 hatchback sees the French auto maker become the first in Europe to offer full-LED headlamps as standard in the majority of trims on a compact car.


Figure 1: The Peugeot 308 hatchback is one example of the impending proliferation of LED headlamps into mid-range vehicles. This full-LED headlamp unit has been designed for the latest model by Magneti Marelli’s Automotive Lighting (AL) division.

That trend will mean growing demand for LEDs targeted at automotive forward-lighting applications, including various high-brightness emitters from Philips Lumileds and Nichia, Cree’s XLamp XB-D range, and Luminus Devices’ “big chip” SST-50 range of LEDs which are suitable for spot lighting. Osram Opto Semiconductors is regarded as a key player within automotive lighting, selling some LEDs specifically for headlamp functions, and others such as the Golden Dragon and Advanced-Power TOPLED devices for DRLs.

On top of this migration from the luxury end to the mid-range and even some budget models, the other key trend in LED headlamps is the emergence of new “adaptive” forward-lighting systems that respond automatically to the environment of the car. The adaptive technology allows drivers to keep their high beam on at all times, by automatically dimming or switching-off specific emitters when necessary.

Not surprisingly, Audi, the carmaker that has pioneered new LED lighting functions, is again pushing the boundaries. It showed off its “Matrix” adaptive LED headlamps at the 2013 International Consumer Electronics Show (CES) in Las Vegas; while at the recent 2014 event it unveiled a new hybrid system featuring not just LEDs, but also laser diodes for the high-beam function, which it claims can light the road up to 500 meters ahead.

At the Frankfurt Motor Show in September 2013, more details of the Matrix system, appearing in the new A8 model for 2014, were forthcoming. It comprises a high beam featuring 25 individual light emitters, with the adaptive system even hooked up to the car’s navigation system to predictively steer light towards the edge of the road as corners are approached.


Figure 2: Audi’s new “Matrix” adaptive headlamps, which feature 25 emitters in the high beam, are available as an option on the new A8 model. The German company has pioneered the use of LEDs in forward automotive lighting systems, and its LED daytime running lights have become a familiar and much-imitated signature in recent years.

LED headlamp component development

Since summer 2013, the German LED maker Osram Opto Semiconductors has been coordinating a research project backed by Germany’s Federal Ministry of Education and Research (BMBF) that features car firm Daimler, driver IC manufacturer Infineon Technologies and automotive lighting specialist Hella. It is aimed at advancing the adaptive technology even further; for example, extending the range of headlamps by applying a higher current as car speed increases, and spreading a wider beam to illuminate the additional hazards encountered in urban driving. Shortly after the Frankfurt Motor Show last year, Osram said that it had developed the first multi-die LED component designed specifically for adaptive automotive headlamps.

A variant within the company’s series of Ostar products, and the latest version of a chip already used in LED headlamps, it is based on no fewer than five individual emitters. Each of those die can be controlled independently via intelligent control based on a car’s sensor feedback, pitching the product directly at the next generation of adaptive systems and demanding a new level of control from driver circuitry.


Figure 3: Multi-die components with individually-addressable emitters have been designed specifically for adaptive headlamps (Courtesy Osram OS).

Like Osram, Philips Lumileds also offers a multi-die option for adaptive systems in the form of both a two- and four-die component. Although suitable for both low- and high-beam headlamp functions, unlike Osram’s latest innovation, the emitters are not individually addressable.

Adaptive lighting units

At the September 2013 International Symposium on Automotive Lighting (ISAL) conference, where Osram’s new multi-chip component was unveiled, the auto lighting specialists Hella and Valeo were also present. Both delivered presentations on the range of functions now possible with adaptive LED headlamps.

Using signals from sensors such as cameras, navigation tools and steering components, these headlamps are designed to respond when the vehicle is cornering, or being approached or overtaken by another vehicle, and to direct beams down or towards the edge of the road as necessary.

Hella, whose designers first came up with a full-LED headlamp for series production in 2009, says that the future will see LED arrays featuring more than ten individually-addressable high-intensity emitters used in adaptive headlamp systems. In those systems, the chips would be actuated with the aid of a modulator, thus switching individual chips on and off, as well as controlling the intensity of the overall light distribution.

In Frankfurt, Valeo had revealed that its “BeamAtic Premium LED” adaptive lighting system would be used in both mainstream and premium vehicles starting in 2014, with a next-generation design scheduled for series production in 2016.

At this level of the supply chain, the other key players include Magneti Marelli’s Automotive Lighting (AL) business unit, Visteon, and Koito Manufacturing. AL, which launched an LED headlamp as long ago as 2007 and is now supplying units that will feature in Peugeot’s new 308 hatchback, indicated just how many different chips are needed in each headlamp. Its low-beam illumination is achieved by combining ten medium-power LEDs delivering 610 lumens, alongside a 2 x 4 array for the high beam and no fewer than thirteen devices for the DRL.

Driver IC requirements

With the LED headlamps performing far more complex functions in these new adaptive configurations, and control of individual die within the same component required, driver ICs become an even more crucial part of the story. In the past, the key function of the driver IC has been to maintain a consistent current flowing through the LEDs to keep light flux at a constant level, with devices such as Texas Instruments’ LM3406HVMHX/NOPB and Infineon Technologies’ BCR 321U E6327 fitting the bill. However, the new adaptive functionality is more complex, and individual LEDs must be dimmed or switched off according to the local conditions encountered.

In Audi’s Matrix system, when the on-board camera detects other vehicles approaching, the relevant sections of the headlamp’s high beam are masked by either dimming or shutting off entirely individual diodes while retaining very bright illumination in the remaining zones.

As driver IC supplier ROHM Semiconductor has explained in a previous TechZone article,² the most straightforward driver function can be provided by linear regulators. They provide simple control and have the advantage of not requiring electromagnetic interference (EMI) filters. However, their power dissipation can be a problem for higher power applications, and because of that, buck DC-DC converters are regarded as a better option in a headlamp. When the driver IC is required to control several LEDs in series, a boost converter topology is typically used, exploiting its more versatile capability.

LED drivers can also be designed to offer a combination of both series and parallel LED control, and this approach provides circuit designers with the kind of flexibility needed to control LEDs that are being used for multiple functions with a single driver. For adaptive headlamps, where the same LEDs would be used to deliver both high and low beams, that ability to dim the light output can be likened to some interior car lighting functions, where dimming is already a standard requirement. ROHM’s BD8381EFV-M LED driver IC has been designed with multiple HB-LEDs, in both high- and low-beam headlamps, as well as DRL applications in mind. Capable of withstanding high input voltages of up to 50 V, and featuring a current-mode buck-boost DC-DC controller, it offers stable operation over varying voltage inputs and removes constraints on the number of LEDs that can be connected in series.


Figure 4: More advanced driver ICs capable of dimming and switching off individual emitters within an LED array are required for adaptive lighting systems like Audi’s new Matrix. This circuit diagram indicates the capabilities of ROHM’s BD8381EFV-M driver (Courtesy ROHM Semiconductor).

Other ICs suitable for driving LED-based DRLs and headlamps include Infineon Technologies’ TLD5098EL-ND and TLD5095EL-ND DC-DC buck-boost components, both of which are available from DigiKey.

Legislative matters

One bump in the road that is still holding up automakers and their suppliers as they look to deploy their new adaptive lighting systems is safety legislation in the United States. Legislators there have not yet caught up with their European counterparts, something that is proving a source of frustration for companies such as Koito Manufacturing.

As Japan’s largest maker of automotive headlamps, it is a key supplier for several of the best-selling cars in the US. They believe that LEDs will become ubiquitous in forward lighting by the end of this decade as the cost of the technology decreases. However, achieving that market dominance will first require moves by the US National Highway Traffic Safety Administration (NHTSA), which as of late 2013 had not yet allowed adaptive lighting systems on its roads.

Already asked by Toyota to modify its lighting standards to permit adaptive lighting, NHTSA says that it is reviewing the request following meetings with representatives of the lighting committee representing the Society of Automotive Engineers. It has also been conducting its own research into the technology during 2013, and if that outcome is positive, it will likely prove to be a key step in the auto lighting industry as adaptive front-lighting systems based on highly-controllable LEDs move further into the mainstream.

Summary

Two key trends in exterior automotive forward lighting – the proliferation of LED headlamps into mid-range and compact cars, and the emergence of adaptive lighting systems – have important implications for both LEDs and driver ICs. As well as a growing demand for white LEDs and driver ICs overall, headlamp unit designers require multi-chip components with individually-dimmable emitters, and the kind of versatile driver ICs that can handle such additional complexity. Assuming that US legislators comply with the wishes of the industry, LED technology finally appears set to change the face of forward automotive lighting systems.

References:

1. John Stanley, Automotive News Europe June 17, 2013: Peugeot, Seat, Ford push LED headlamps into European mass market
2. DigiKey TechZone (ROHM Semiconductor): LED Drivers for Lighting Control Applications