by Joshua Israelsohn, JAS Technical Media
The trend for the past decade in the automotive industry has been toward vehicular lighting that’s all solid-state. Incandescent headlights are the last hold-out, and they are on their way to the junk yard of history.
LEDs bring a number of well-chronicled advantages over traditional tungsten-filament incandescent light sources. Their improved luminous efficacy (lumens per watt) and energy savings are particularly helpful for hybrid-electric vehicles (HEVs) and electric vehicles (EVs) that place a high value on fuel economy. Vehicles built with traditional internal-combustion or diesel drivetrains are somewhat less sensitive to energy savings but still use LED lighting as both safety enhancements and differentiating elements of design style.
Incandescent bulbs are approximately 97% efficient — as heaters. Today’s white LEDs offer luminous efficacy about 5 times that of tungsten-filament bulbs, meaning they generate substantially less heat for a given light output. So the light emitters are not only small, they also operate at a lower temperature. Ordinary thermal management methods for electronics let designers pack multiple LED elements closely together, without generating temperatures high enough to reduce operating lifetime.
Solid-state lights will operate for tens of thousands of hours. Additionally, repeated on-off cycles—common in several vehicular applications—won’t degrade LED operating life. LEDs also stand up better to shock and vibration than incandescent bulbs. Long life and ruggedness both play into the trend toward more reliable vehicles that need less maintenance. The flip side is that vehicle owners have fewer components they can service with just a typical home tool kit.
For example, the changing of a headlight on some vehicle designs entails partial disassembly of front body components. LED headlights that last the life of the car would eliminate such hassles.
Finally, LEDs bring important safety advantages when used as brake lights and center high-mount stop lights. University of Michigan Transportation Research Institute studies show the rapid turn-on time of LEDs lets following drivers brake faster by 170 to 200 msec (14 to 16 ft. at 55 mph) in good lighting. The advantage rises to 300 msec (24 ft at 55 mph) with bright sunlight or high-intensity reflections on the brake light surface.
All-LED automobiles are only just now starting to emerge. Earliest applications were typically for bright red LEDs and included tail, brake and marker lights. Next came backup lights and license-plate illuminators, which could tolerate the harsh color temperature and marginal color quality that characterized early-generation white LEDs. White LEDs now have better light quality and cost less, allowing uses in instrument clusters, cabin interiors, and, more recently, daytime running lights.
Headlights are the final frontier for LEDs in automotive lighting. This application lasts because it constitutes the highest power and most complex lighting in a vehicle. It is also the lighting position subject to the most stringent federal regulation.
Smart LED ballasts
Tungsten-filament bulbs model as resistors with positive temperature coefficients of resistance. They are inherently stable when operating from power sources that don’t exceed a certain voltage level. By contrast, LEDs are current-operated devices—ideally, electrons in/photons out. So they require an electronic interface to serve, at minimum, as a regulated current source. Consequently, LEDs can’t work directly from a vehicle electric power bus.
That means there’s a power supply between the LED and the vehicle electric power bus. The power supply constitutes an additional bit of complexity compared to ordinary incandescent lights. However, the additional complexity brings with it advances in power-management ICs as well as new safety, performance, and convenience functions and enhancements. Indeed, LED headlights will offer features that are simply impractical with traditional lighting technologies.
One example is in vehicles carrying stop-start technology. Turning off the engine at stop lights can add 4 to 8% to fuel mileage in passenger vehicles simply by eliminating engine idle time. But consider that the light produced by incandescent bulbs follows their applied voltage to the 3.4 power. So tungsten-filament headlights can dim by 30% during a stop interval as the voltage on the power bus dips, assuming a fully charged battery in good condition. High starter motor current during cranking can also dim incandescent headlights by as much as 92% with a cranking pulse conforming to ISO7637-1.
In contrast, LED ballasts designed with buck-boost power regulators can track out the battery voltage and maintain constant light output during stop-start cycles.
Intelligent ballasts for LEDs can provide diagnostic information not available in tungsten-filament lighting systems. If equipped with a temperature sensor thermally coupled to the LEDs, a smart driver can compensate for temperature effects. All LEDs have a negative temperature coefficient—light intensity drops as temperature rises and the change is non-linear. For example, an LED’s light output can fall by as much as 30% as its junction temperature rises from 25 to 150° C.
Ideally, ballasts for automotive LEDs should be able to drive lights for any spot on the vehicle. The ability to provide a programmable output current handles this need. It lets the same IC drive LED strings of varying lengths and output flux targets. This approach also lets automotive designers take a platform-wide approach to lighting drive design. The precise needs of individual vehicles then get met with model-specific software. Such designs also let automakers exploit advances in emitter design and manufacturing processes without the overhead costs of redesigning driver hardware.