Audi uses LEDs in for the headlights of a serial-built car, the A8. The German car maker proably is not the only one incorporating LED headlights, but at electronica, ON Semi as well as NXP used an Audi A8 headlight assembly to demonstrate their respective LED driver designs. Actually, it is impressive how LED headlights widen the spectrum of options for car designers: Intelligent cornering light can be implemented just by switching certain LEDs on and off, doing away with fault-prone, clumsy motors. Cameras mounted at the rear view mirror can identify head-on traffic and selectively dim the headlight beam to avoid dazzling the other drivers. When the car speeds up on the highway, the shape of the beam changes in order to providing better vision to the driver.
All these effects are controlled by an ECU (of course), and the headlight LEDs are driven directly by tiny ICs – no bulky power transistors, no heat sinks. ON Semiconductor demoed several of these devices in a LED headlight assembly with some 15 high-power LEDs and certainly two dozens of smaller LEDs for turn indicators and daytime running light. Amazing: Even when the headlight LEDs were in high-speed highway mode with full power, the driver chips did not heat up; you could always touch them with the hand without burning your fingers.
This is not the end of the story; ON Semi is working on even more powerful solutions, based on novel semiconductors. Unlike competitor Infineon (and other power semiconductor manufacturers), ON Semi does not focus its developments on SiC, but instead on GaN, another wide-gap semiconductor. “Silicon Carbide is too expensive”, explained CEO Keith Jackson. “GaN is much more interesting. With its higher band-gap energy, it allows higher power density. And since it is already used in LEDs, it is a proven technology”. However, the technology currently is not yet ready for roll-out, and ON Semi declined to provide any hint regarding the timeline.
An almost identical set-up was showcased by NXP. Again, an Audi headlight assembly was illuminated by LEDs which in turn were driven by apparently identical tiny integrated circuits; only minor differences in the functionality distinguished both set-ups. According to NXP Senior Vice President Kurt Sievers, the chip maker's automotive business unit has gotten know-how transfusion from the company's LED lighting division. “The challenge is that the temperature range for automotive applications is much wider than for home lighting devices,” Sievers explained. With the physical temperature, the light temperature varies, and in order to maintain an even light color, automotive lighting engineers are forced to take countermeasures. Audi, for instance, has implemented a color control loop with sensors and a microprocessor for this purpose.
NXP developers are currently working on semiconductor-inherent color control, utilizing a patented technology they call “sensorless sensing” which will make the control loop redundant. According to Sievers, the technology will make LED headlights much more competitive. “Today, a LED headlight costs about 2.5 times more than a normal headlight assembly. With our sensorless sensing, this factor will shrink to 1.5”, Sievers said. The technology could speed up the roll-out for LED headlights. Around the years 2015/2016, Sievers expects the break-through across the board. “At that time, we expect that some 50 percent of all new cars will be equipped with LED headlights,” Sievers said.