The total headlight system is usually composed of multiple converters, each regulating a different part of the system. Sometimes the high-beam and low-beam functions are combined into one string. In this case, the high beam is implemented when all LEDs in that string are fully on. The low beam is implemented by pulse-width modulation (PWM), dimming the LEDs in the string to provide a lower-lumen output. More frequently, however, the low beams are implemented by one or two strings at full brightness, and the high-beam function combines the low-beam strings with another string or two at full intensity. That approach allows the manufacturer to use the additional strings to increase the beam spread of the high beam by mechanically positioning the lights facing an adjacent angle.
The LED-based headlight mainly includes two basic electronic components: the LED array and the lighting control unit (LCU). The LCU usually houses all the current regulators and other power converters, as well as the microprocessor, which communicates with other electronic control units (ECUs) throughout the automobile. Upstream, the body control unit (BCU) controls the LCU and manages all the body functions in the car. The LED array is located on a heavily heat-sinked assembly that contains the LEDs, thermistors for measuring temperature, and coding resistors, which are a simple, cost-effective way to program the LCU power output.
Future headlight systems
This system most likely will become even more complex as additional functionality is added to the headlight, including dynamic fading for cornering lights, dynamic anti-glare systems, and increased safety functionality. Current headlight systems already use camera input to run auto-leveling motors, which respond to changes in the position of the car relative to the terrain. These cameras can be used to control dynamic light output, as well.
From a power electronics point of view, it makes sense to use a two-stage topology for a dynamic front lighting system. In a DC link topology, for example, a boost converter takes the 12V battery input and provides a stable high-voltage DC rail. Then, independent buck converters can be used to drive each series LED string separately in the system. This boost plus buck system (see photo below) is inherently more efficient and provides better mitigation of electromagnetic interference than the single-stage buck-boost systems, since the input and output current are both continuous. Additionally, the buck converter is an excellent choice for regulating output current, due to its high output impedance, which makes it look like a true current source.
Future headlight systems will use two-stage power architectures.
The cascaded DC link approach has better dynamics, as well, since the first stage can ballast the input transients of the car battery to provide a well-regulated DC rail with significant energy storage capability, while the second stage can ensure consistent regulation at all times. It is also much easier to perform high-speed PWM dimming on a buck stage with little or no output capacitance, allowing for much more resolution and a higher contrast ratio when PWM dimming. This amounts to a higher level of controllability in the system, providing the possibility for dynamic fading of the different parts of the system.
Since the automotive market is not as cost-sensitive as consumer electronics, this move to two-stage LED headlight drivers could have staying power in the market due to performance improvement and additional safety functionality.