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Spanning less than 15 years, the application of solid-state light-emitting diodes (LEDs) in automotive design is still a new phenomenon. Yet, most industry observers agree that LEDs are the lighting solution of the future for vehicle interiors and exteriors. OSRAM and other manufacturers are setting a rapid pace for technological advancement in LED light sources. These gains in LED lighting are benefiting both manufacturers and consumers.
Durability, long life and energy efficiency are chief among the many advantages offered by LED lighting solutions. The low power consumption of LEDs means vehicle electronic systems can achieve higher levels of performance. With an operating life of as long as 100,000 hours, LEDs can last the life of a vehicle, lowering warranty claims and increasing customer satisfaction. In addition, the compact size of LED packages provides original equipment manufacturers (OEMs) increased design flexibility and new styling options that appeal to consumers. As manufacturers pay more attention to the environmental impact of their products, LEDs have gained visibility as an environmentally friendly technology that is free of lead and mercury.
The brief history of automotive LEDs is marked by technological breakthroughs and rapid adoption in the marketplace. In 1990, the first LED package for automotive instrument panels, TOPLED, was introduced by OSRAM. That year marked the beginning of automotive LED applications. Volkswagen led the adoption of automotive LED technology during the 1990s and was soon followed by several other European manufacturers.
By the end of the decade, LEDs expanded from vehicle interiors to exterior applications. Automakers in Europe and North America used LEDs as a third brake lamp, also known as a center high-mounted stop lamp (CHMSL). Exterior LED lighting expanded to include complete rear cluster lamp (RCL) applications in early 2000, giving new freedom to vehicle designers. These RCLs, which include tail, stop and turn-signal lights, enabled OEMs to develop low-profile chassis without drill holes, which provided greater resistance to vibrations
In 2003, proprietary thin-film technology allowed engineers to remove the light-absorbing gallium arsenide (GaAs) substrate from LED semiconductor chips. First-generation thin-film LEDs achieved much greater lumen efficiency or brightness, and enabled the first full-color heads-up display on the 2003 BMW 5 series.
New technologies continue to change the paradigm for solid-state lighting. OSRAM has developed second-generation thin-film technology to create powerful LEDs that can deliver the performance many emerging automotive applications will require.
Lumen efficiency of 34 lumens per watt can be achieved with new indium gallium aluminum phosphate-based (InGaAIP) thin-film technology. This innovation is used as the rear exterior light source on the 2004 Aston Martin DB9, as well as the 2004 BMW 6 series. LED technology gives these taillights a faster rise time. As a result, drivers following these vehicles have improved braking ability at a distance of 20 feet when traveling 60 miles per hour.
In addition to safety benefits on the outside of the car, new LED innovations deliver styling improvements to vehicle interiors. The 2005 Ford Mustang features the industry's first adjustable color instrument cluster. At the push of a button, drivers can adjust red, blue and green LEDs inside the instrument panel to achieve one of 127 custom colors.
In August 2004, the company achieved an unprecedented lumen efficiency of 108 lumens per watt using a thin-film LED in a TOPLED package. This record-setting technology will likely be commercialized by late 2005.
While much attention is paid to improving the performance of LED packages themselves, for leading LED manufacturers, scalability and surface-mount technology (SMT) compatibility are key factors in developing LED solutions for interior and exterior automotive applications.
Scalability allows manufacturers to choose an LED package and circuit board to create an array that optimizes the performance, reliability and value of a specific application. Designers can choose a scalable LED based on lumen efficiency, color control and package space, among other factors. These LEDs can be mounted on the most cost-effective material that meets the thermal, electrical and packaging constraints set by the application design.
SMT technology allows automakers to gain cost efficiencies by using industry-standard assembly equipment with LED packages. Among the materials available with SMT processing are standard FR4 printed circuit boards made from glass fiber epoxy laminate, that allow the low-cost assembly of rigid lighting arrays. Standard flexible circuitry for 2-D and 3-D designs allows LED arrays to be folded and thinly packaged (Figure 1). The dramatic look of 3-D -lamp design was demonstrated on the Ford Focus concept car at the Auto China Motor Show in June 2004.
The flexibility gained through scalability and SMT compatibility enhances the possibilities for the newest generation of high-performance LEDs, such as the Golden DRAGON, the first high-powered SMT LED (Figure 2).
No matter how innovative the technology, supplying components to the automotive industry is never easy. It is a struggle to balance the demand for more performance and new features against the never-ending grind of cost containment. What worked yesterday won't be good enough for today, and what is successful today won't carry you far into the future. Successful suppliers are able to give customers choices without going broke in the process.
One proven supplier strategy is to design a basic standardized product platform and add line extensions to provide the new features and performance that customers want. Effective line extensions introduce an appealing new feature that requires modest development effort, uses the same tooling with only minor modifications, and does not significantly drive up production costs. As an additional benefit, quality improvements are integrated throughout the product line in a cumulative manner over a long period of time.
In automotive lighting, the traditional S8 wedge incandescent signal lamp is an example of this approach. Introduced nearly 20 years ago, the S8 wedge lamp became the industry standard for RCLs. Over the years, this design platform has supported dozens of line extensions to meet automakers' demands for new appearance, output, operating life and durability characteristics.
In contrast, each LED RCL system developed since early 2000 is unique to the model for which it was designed. Each system has a different LED array, electronic driver and vehicle interface. To achieve the high-tech look of the LED RCL, automakers have had to increase spending for the vehicle lighting assembly.
The downside of the custom LED RCL approach is that it limits the opportunity for long-term product improvements and makes a line extension strategy impossible. This drives cost at each link in the value chain. New designs are needed for each model change, along with new tooling, validation, manufacturing plans and, ultimately, spare parts inventories. Any problem with the system assembly, LEDs or electronic drivers requires the complete replacement of all components. With new system designs taking place every three years to four years, it is impossible to reap the same long-term quality improvement benefits that are inherent in a standardized product line.
A higher cost for LED RCL systems will likely limit market acceptance in the long run. Cost isn't a barrier for some automakers, who are willing to pay for the custom appearance and performance this approach offers. For others, however, this is a significant barrier. Clearly, there is opportunity for another solution.
The answer could be Joule, a standardized automotive LED system that blends the benefits of a standardized light source with the benefits of LEDs. This technology has the packaging attributes of a traditional light source but is powered by LEDs. As a “plug-and-play” replaceable solution, Joule does not require supplemental electronic or thermal management components in either the RCL or the vehicle. Automakers can specify a single light source across different model lines. At the time of a model update, the plastic RCL housing can be changed and still use the same light source, which drives system cost efficiencies.
The Joule standardized solution also introduces new styling opportunities. Using one basic product platform, different configurations — or line extensions — can accommodate different customer requirements. This technology is currently available in red and amber, for tail, stop and turn signal lights. It will eventually be available with white LEDs for back-up lighting or other automotive applications.
The LED light source supports the latest clear lens, “invisible bulb” design trend, as well as direct and indirect optical systems. Because the light source is packaged like a conventional light bulb, it can be used to distinguish high-line platforms from entry-level models. An incandescent bulb system with standard colored lenses could be used for one trim line and, with only a change to the reflector, an automaker could offer Joule with a colorless appearance and upscale look on high-line models (Figure 3).
Costs and benefits exist for both the customized and standardized approach to LED systems. In the end, some customers will opt for a custom LED array that meets their requirements for performance, features and appearance. Other users will find that a standardized automotive LED system is the most logical choice.
VEHICLE ELECTRONICS INTERFACE
As OEMs increasingly use lighting to distinguish high-line vehicle platforms and differentiate their products from the competition, there will be an increased use of existing vehicle electronics systems for lighting controls. This utilization will be a key factor in the development of cost-effective LED systems in the automotive industry.
LEDs require a constant, regulated electrical current supply to ensure consistent luminous flux or light output. A consistent current also extends the life of an LED over a wide range of operating conditions. LED driver electronics in a vehicle must provide a constant current supply through a wide range of input voltages, typically 9 Vdc to 16 Vdc. System-level function integration between vehicle electrical systems and LED driver electronics will allow simplification and cost savings for OEMs.
Existing vehicle lighting circuit electronics can power condition output signals. As a result, power conditioning features can be removed from LED driver circuitry. However, vehicle electronics capabilities are specific to each OEM. Vehicle platforms with a high level of electronics integration are better positioned to assume additional LED function control.
The Joule standardized automotive LED system lends itself to future integration with vehicle electronics. In addition to outage detection and communication, this standardized system meets the electromagnetic interface (EMI) requirements of OEMs. The importance of EMI performance will only grow, as vehicle electronics systems are required to control more content with increased integration.
LED FORWARD LIGHTING
LED lighting has evolved over the last 15 years from interior instrument panels to higher-powered exterior tail, stop and turn-signal lights. Advancements in white LED technology have led to the development of some forward lighting applications, including fog lamps and daytime running lamps. The most anticipated LED innovation continues to be vehicle headlamps.
Many technical challenges must be resolved to make white LED headlamps a reality. Chief among these challenges is obtaining the necessary luminance from white LED packages. Another significant hurdle is thermal roll-off management — that is, minimizing the drop in light output that occurs with an LED package as it reaches thermal stability or heats up in operation.
The first LED headlamps are anticipated to be introduced by 2008. These headlamps will debut on high-line vehicles in limited production and will likely be unique, vehicle-specific solutions. Current OEM plans anticipate large-scale production of white LED headlamps around 2012.
ABOUT THE AUTHORS
David Hulick is global product manager for OSRAM Sylvania.
Frederick Peterson is engineering manager at OSRAM Sylvania.
Michael Godwin is product marketing manager for OSRAM Sylvania.
Vipin Madhani is engineering manager for OSRAM Sylvania.