Functions like high beam, low beam, fog lights, and daytime running lights (DRLs) can all be implemented with LEDs (Figure 1). Powered from a car battery, the input voltage of LEDs is typically 12V but can be as high as 16V on a fully charged battery. Vehicles employing start/stop technology experience large voltage dips when the engine starts, so the lower limit for the power source can be well below the typical 12V and can often be as low as 5V.
The switching regulators that power LEDs must have high efficiency, operate over the entire input voltage range provided by the automobile battery and must survive 60V ‘dump’ voltage transients. At the output of the voltage regulator, there may be a switched string of diodes that can be fully engaged with 12 LED diodes (42V) or dimmed down to a single LED diode (3.5V). Accordingly, the voltage regulator’s input voltage may vary from 5V to 60V while the output voltage may go from 3.5V to 42V and be above or below the input at any given time.
This design solution reviews the challenges of efficiently powering switched LED strings from a car battery and proposes a novel buck-boost, average current control solution that overcomes these challenges.
Typical Headlight System
Typical headlight systems utilize a boost converter as a front-end to manage the variabilities of the input voltage. The boost converter delivers a well-regulated and sufficiently high output voltage (Figure 2).
Dedicated buck converters, working from this stable input supply, control the lamp’s intensity and position. Each buck converter controls a single function such as high beam, low beam, fog lights, daytime running lights (DRLs), or light position, etc. The matrix manager switches the string diodes in or out, with each buck converter output adjusted accordingly.
The drawback of this configuration is that each buck regulator works with a boosted input voltage well above the battery voltage even when its output voltage is low. This is the worst-case scenario for the power train switching losses and duty cycle.