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Brighter LEDs drive automotive lighting

Posted: 01 Sep 2005 ?? ?Print Version ?Bookmark and Share

Keywords:automotive? leds? luminosity? oma? gaas?

LEDs have been manufactured since the early 1970s, but in recent years, advances in this technology have notably accelerated. The maximum available brightness for LEDs has increased by several orders of magnitude, starting with less than 0.01 lumen at 20mA and today reaching more than 1 lumen. With these improvements, target applications for LEDs have also broadened. At luminosity levels above 0.3 lumens or so, LEDs start to become practical replacements for incandescent lamps. As such they provide many benefits such as reliability. In general, today's LEDs will long outlive any of the systems in which they are used.

This characteristic is most useful in automotive systems, where reliability is a matter of safety and avoiding expensive repairs. It is not surprising that the automotive industry has moved quickly to embrace LED technology wherever practicable and is driving further innovation in this technology because of particular automotive requirements.

Evolution of LED brightness

Between 1970 and 1995, LEDs evolved gradually to offer higher levels of brightness. Since the mid-1990s, however, the pace of innovation has accelerated with the invention of blue and white LEDs and a doubling of average brightness for the devices.

Improvements in LED brightness are mainly due to advances in substrate materials. Beginning with the first GaAs phosphide products, the industry turned to nitrogen-doped GaAsP and GaP to achieve the first yellow and green LEDs in the late 1970s, and then used single- and double-hetero GaAlAs to achieve luminosities of over 0.1 lumen in the early 1990s. Since then, various combinations of indium and gallium have served as \substrates for newer, brighter LEDs in colors including blue.

Despite these advances, several problems remained, including the fact that the substrate tends to absorb much of the light generated by the LED. Several approaches have been taken to work around this issue. Lumileds Lighting attacked the problem by using a patented, transparent AlInGaP substrate. Another approach was to add a Bragg reflector grating layer above the substrate. This provides twice the brightness of LEDs with an absorbent substrate, but any light that is emitted at a 90 angle is lost. Vishay has improved on this solution with an organic mirror adhesion (OMA) technology in which a mirror surface is grown on a silicon substrate. All the light that goes down to the mirror comes out of the front of the device, thus achieving the same level of brightness as with the transparent substrate approach for about a four times an improvement over standard LEDs.

Tech efficiency

LEDs don't just need to be bright; they also need to work efficiently. This means not only converting electric power into light with minimal losses, but also controlling the effect of the heat generated by the current running through the device. A huge challenge of LEDs is that their junction temperature, which goes up when more current is applied, has a direct effect on the wavelength of the light they produce. Simply increasing the forward current can merely change the color of the device without making it any brighter. Thus, die and packaging technologies are needed that allow more brightness with less current.

LEDs are steadily replacing incandescent lamps in automobiles, both for vehicle interior and exterior lighting. Stringent quality requirements are the rule, not only for device failure rates (where the maximum allowed is typically 0.1 per 10 million pieces), but also for brightness and color. Consistency in color and brightness is obviously important when LEDs are brought together in an assembly for dashboard illumination, instrument clusters or entertainment system controls. The brightness of the illuminated areas always needs to be homogeneous, without any shadows. The need for consistency goes beyond purely aesthetic concerns, since LEDs have become part of the color scheme of car interiors and how each manufacturer brands its cars. This means that the wavelength of LEDs produced and delivered for a given make of car is allowed to vary only by an amount that is invisible to the human eyetypically just a few nanometers.

The desire of automotive manufacturers for distinctive LED colors to brand their products has created the need for expertise in delivering "color on demand," which requires increasingly complex LED products where dies of different colors are combined in the same package. This creates its own set of design challenges, since the human eye's perception of color depends on its intensity and colors on the higher end of the spectrum (e.g. yellow) tend to overpower colors on the lower end of the spectrum (e.g. blue) at the same level of intensity. To create a true green LED, for example, the luminous intensity of the blue component must be about double the intensity of the yellow component.

For exterior lighting, very high levels of brightness are requiredsince brake lights and center high-mounted stoplights (CHMSLs) must be visible not only after dark, but in broad daylightand it wasn't until the advent of OMA technology that Vishay was able to deliver the required levels of brightness. Mandated requirements now range from 17cd/m? for taillights to 980cd/m? for daylight operation of turn signals.

In each case, multiple LEDs are used to implement each function. For example, 148 LEDs are needed to implement the taillights and CHMSL in a recent Volkswagen model. In the taillight assembly for this car, the same LEDs switch from red to yellow, depending on whether they are being used for the turn-signal or as part of the brake light. Another popular LED application that has recently been approved for Europe is adaptive brake lights, where a smaller or larger portion of the brake light area is lit, depending on whether the driver brakes normally or forcefully slams on the brakes.

Low maintenance

Compared to incandescent lamps, LEDs are low-maintenance devices with a typical operating lifetime of 50,000hrs. For drivers, this means lower maintenance cost. The use of LED lighting units instead of conventional headlamps further contributes to the overall health of a car by eliminating the need for additional openings into the chassis that can introduce unwanted moisture.

Europe is leading the way in the use of LEDs for automotive applications, with more than 80 percent of new cars, including some LED components. The comparable figure for Japan is 30 percent, with only 5 percent for North America. Five years from now, however, it is likely that we will see the same level of penetration in the United States and Japan as in Europe.

Meanwhile, LEDs are continuing to improve. Devices introduced this year are once again doubling the maximum brightness per device. This means that for applications such as automotive exterior lighting, fewer devices will be needed to achieve the same overall level of luminosity. Although the brightness of LEDs is increasing rapidly, the price of these devices is relatively stable, so the increased use of LEDs will remain highly cost-effective.

- Horst Lengning

Sr. Manager, Product Marketing

Vishay Intertechnology Inc.

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