This article was published in the September 2013 issue of LEDs Magazine.
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As soon as the driver opens the car door, LEDs are all around. They are used in interior lighting, to illuminate the dashboard console, and occasionally — at least for the moment — in the vehicle's headlamps. While some promising development projects could bring OLEDs out of their present domain of interior screens and into the exterior of the car, this is still at the conceptual stage. As for the rise of the vehicular LED, broader deployment may not be restricted by technology or performance but by cost and legislation.
In the interior instrument panel, or dashboard, predominantly low- to mid-power LEDs are used. They are mainly small, SMD-packaged LEDs, which are standard reflow-soldered onto printed circuit boards (PCBs). Osram Opto Semiconductors, for example, recently announced a new RGB MultiLED family with boosted blue energy for auto cabin applications (Fig. 1). In the case of exterior automotive lighting, however, high-power LEDs are typically used.
Many companies have a dedicated portfolio of automotive LEDs that meet the industry's specific needs, such as higher operating temperatures ranges. A vehicle's rear lighting has to operate at 85°C and front lighting has to be able to operate at 105°C. For comparison, LEDs for many general-illumination applications operate in conditions of 25°–65°C, although temperatures can hit 85°C in some cases such as in recessed ceiling applications.
In vehicles, LEDs must also be able to demonstrate extended lifetimes and reliability even in the face of vibration and a general rugged environment. Philips Lumileds, for example, qualifies its automotive LEDs to the AECQ-101 and IEC 60810 standards. Automakers often require components that are qualified to such standards.
LED features match application needs
The precise nature of the point of light from the LED has led to increased adoption of LEDs for exterior applications such as daytime running lights (DRLs) — the front lights on a vehicle that automatically switch on when the vehicle is moving forward. The transition to LEDs, however, has required LED-centric participants to adapt to the auto environment and the automotive engineers to adapt to the world of solid-state lighting (SSL).
For example, the LED driver IC industry has had to modify its architectures to meet the increased integration and performance demands of more complex electronic monitoring and control systems. The lower current consumption of LEDs also warrants a change to the conventional ECU (electronic control unit) as the diagnosis thresholds vary from incandescent to LED lighting. The circuitry must adapt regulation parameters to the binning of LEDs and their operating temperature.
LED-based vehicle lights are usually configured in the form of a string, which ensures that the current is the same across the series-connected LEDs. A single string may include 6, 8, 10, 15, or as many as 20 LEDs. The series configuration is preferable to parallel LEDs, explained Bryan Legates, director of design engineering for power products at Linear Technology. The alternative, parallel formation does not inherently share current equally, resulting in some lights being brighter than others. In the case of parallel LEDs, a filter is needed to make all lights equal, adding to cost and complexity. The constant current afforded by LED strings maintains brightness regardless of the input voltage or LED forward voltage variations.
Generally, a boost-based driver architecture is used, as the nominal automotive system voltage is 13.8V or lower in transient conditions. Protecting boost architectures against short circuits is particularly important in automotive design. In a collision, electrical arcing is a potential hazard because it could ignite spilled fuel. Boost LED drivers are being introduced with robust short-circuit protection; they are challenging the pervasive use of SEPIC (single-ended primary-inductor converter) topologies, which can be expensive and complex, increasing the cost of the vehicle.
It is improvements in LED drivers and DC/DC converters that will add functionality, but not cost, to LED systems in vehicles to make them more accessible, argued Legates. Reflecting this trend, the company introduced the LT3795 boost DC/DC LED driver earlier this year. It can be configured in boost, buck-boost, SEPIC, and buck mode architectures.
Texas Instruments seems to agree that drivers will create more LED opportunities. The company recently introduced a multi-topology DC/DC LED driver, the TPS92690. It can also be used for boost, SEPIC, Ćuk, and flyback topologies, and has low EMI. It is destined for headlamps but also for fog lights and general-purpose area lighting (Fig. 2).
European DRL legislation
While you can find a number of reasons why LEDs are a good match for automotive applications, legislation will also provide strong influence for the adoption of LEDs in some cases, and rejection in others. The decision by the European Commission (EC) for all new passenger cars to be fitted with DRLs in February 2011, followed by a mandate for the same on all trucks and buses from August last year, has seen the use of LEDs increase. The low energy budget of LEDs makes them the obvious choice for a light source that is on all the time.
Some parts of the motor industry question the eco-friendliness of having a vehicle's lights on during daylight hours. Supporters counter the energy consumption objection with statistics that daytime running lights use only 25–35% of the energy that driving lights consume. When the DRLs use LEDs, this number further reduces to just 10% of the energy that driving lights use.
There are also question marks over the distractions that a light from a car can cause in daylight and claims that the glare of running lamps can mask other road users, such as pedestrians or cyclists. There are also claims from dissenters that the glare in daylight can distort distance perception, making it difficult for road users to discern how far away a vehicle is. Whatever the doubts, the EC has pressed ahead and DRLs are now to be fitted in all new vehicles. The EC says it has reduced accident rates in Scandinavian countries where the daylight hours in winter months are limited to just two to four hours in the north.
Although LEDs have been used in brake lights, reverse lights, signal indicators, and interior lighting for years, the conventional headlight is only now yielding ground. Fig. 3 depicts an Osram Ostar LED designed for headlamp use. The headlamp application is moving at a slow pace, largely due to the cost penalty associated with LED headlights in favor of a conventional bulb and its metal surround. At present only high-end cars from luxury automakers use LED headlights in place of halogen or incandescent bulbs. Current examples include the Audi A8 and R8, the Lexus LS600h and RX450h, and the Porsche Cayenne.
In Europe, LEDs are seen as part of the aesthetics of a vehicle, with the distinctive blue light of LEDs being a distinguishing, attractive feature. Every industry observer remarks on the curve of the headlights and the aesthetic appeal — with accompanying price tag — that they represent.
The same attributes that make LEDs so popular in interiors — i.e., small form factors, allowing them to fit into space-restricted dashboards, and low power consumption, as well as high luminosity — are making them attractive for use in a vehicle's headlights. Used here, they eliminate the bulb and minimize the surrounding metal reflector. By their nature, they provide precise light in a sharp beam, rather than the broad spread of light produced by a bulb. LEDs are low maintenance and are designed to last the lifetime of the average car. The light produced by LEDs used in headlamps or DRLs is 5500K, which resembles daylight, reducing eyestrain.
LEDs also require less energy and that ultimately leads to better fuel economy. Legates estimates that the differential between traditional bulbs and LED lighting is a factor of 8 — a 12W LED is roughly equivalent to a 100W bulb and the typical headlamp is approximately 200W for an incandescent bulb or 30W for an LED. This energy reduction is beneficial for electric vehicles. Unlike gas vehicles, the electricity consumption in an electric vehicle directly relates to the engine battery and thus the range of a journey.
However, there are practical constraints to using LEDs, especially in forward lighting in both DRLs and headlamps. They have to operate in the harsh automotive electrical environment and must operate at relatively high power levels, typically 15–75W, all in the confines of the headlamp enclosure space. DRLs typically account for ~15W of LED power, with low beams at ~20W and high beams at ~30W of LED power. A Matrix system can have 75W of total LED power, although typically this is 30-45W as not all LEDs will be on at the same time.
Miro Adzan, EMEA power marketing manager at Texas Instruments, identifies electromagnetic interference (EMI) as a typical challenge to LED design. "SMPS (switch-mode power supply) LED drivers are avoided for low-power uses like tail lights, but for efficiency reasons they are a must for running lights and headlamps. That is when the topic of EMI reduction typically arises," he said. A reduction in EMI is a benefit to the overall power bus design of the vehicle.
Headlamp affinity and legislation
Highlighting a divide between Europe and North America and Japan, Adzan, who is based in Germany, praised LED headlights for their reactivity. "LED headlights will provide added driver safety by adapting the beam brightness based on the proximity of other vehicles. In other words, an oncoming driver should not be blinded if your high beams are on. Cars with this capability will sense this happening and adjust the beam angle and intensity around the oncoming car."
The intelligent dimming of headlights is hotly contested, with European car manufacturers advocating the benefits for safe driving, whereas North American, Japanese, and Korean car manufacturers remain largely unimpressed.
In the US, it is a legal requirement that a vehicle's beams have to be able to switch from high to low. The introduction of automated headlights, pioneered by Audi's recently announced Matrix headlamp, could be stopped in its tracks.
The LED headlamp market was estimated to be worth $1 billion in 2012 and set to double to $2 billion by 2014. Such a fast-growing market is bound to excite companies, but it seems that a US law has also agitated many.
Audi, together with fellow German car manufacturers BMW and Mercedes-Benz as well as America's General Motors, is challenging the 1968 US law mandating that headlights must switch between high and low settings. The companies claim that this does not reflect the changes in lighting technology in the last 45 years.
Particular urgency is sensed for Audi with the Matrix beam planned for its high-end A8 sedan, unveiled with the Coupe Concept at the Paris Motor Show in 2012. It groups five clusters of five LEDs within a reflector (Fig. 4). It uses small cameras to sense other vehicles and a control system to dim or turn off individual LED bulbs. It can illuminate around corners or adjust to road conditions, with some beams on and some beams off (Fig. 5).
Legates reasoned that this is a sensible feature in European cars, which may have to negotiate curving mountainous roads, but it is superfluous on America's straight highways. Another drawback he mentioned is the introduction of microcontrollers and cameras that increase complexity and reduce reliability by introducing more components and a software element. It also adds considerable cost — around $2,000 per vehicle.
Despite this, European manufacturers are undeterred in the pursuit of intelligent vehicle lighting. Osram Opto Semiconductors is coordinating an integrated microphotonics project, sponsored by the German FMER (Federal Ministry of Education and Research) to create adaptive forward lighting, or glare-free camera-controlled headlamps that react to conditions.
The project, which began in 2013 and is scheduled to run until January 31, 2016, aims to develop the technical framework for energy-efficient LED headlamps that produce a glare-free high beam and a low beam that adapts to the speed of the vehicle. In slow-moving city traffic, a wider light can illuminate the margins of the road to show pedestrians and potential hazards, whereas when the vehicle is moving fast, the range of light is automatically increased.
The future of OLEDs
OLEDs in vehicle lighting are still at the conceptual phase, confirms Shai Dewan, Philips Lighting. Philips, Audi, and Merck are working on a project with the University of Cologne, Germany, where OLED panels can be used in the curve of the car exterior for seamless lighting design.
Audi has also shown an OLED concept, the Swarm, where OLEDs coat the back of a vehicle, acting in a graceful arc as brake lights or indicators. They can also light up when the car approaches another in the dark or to illuminate handles and other features in darkness.
Today, OLEDs are commonly used in phones and tablets but are finding their way into vehicles in headrest screens or dashboard consoles. Tesla has used a 17-inch display as its central console, replacing dials and knobs with a sleek screen. Vital for an electric vehicle, this reduces the energy budget, which is sourced from the engine battery.
The common goal of car manufacturers is to reduce weight, energy consumption, and bill of materials cost, and this has been preserved in the pursuit of LEDs used in vehicles. The industry-leading technology will become economically viable lower down the range in time, and small improvements, such as adjustable interior light colors to match a mood or paintwork, can continue to advance the driving experience.