LED-Driver Electronics Enhance Headlight Style, Safety, and Reliability

A complete front-lighting system usually comprises multiple converters, each regulating a different part of the system (Fig. 1). Typically, each headlight function is supported with separate LED strings. Usually only DRL and position lights are multiplexed into one string. In this case, the position light is created by pulse-width modulation (PWM) dimming the DRL string at roughly 10% duty cycle.

Most existing LED headlights have two basic electronic components: the LED array inside the headlight along with associated optical and mechanical components, and the lighting control unit (LCU) that’s typically attached externally in a weather-resistant housing. The LCU printed circuit board (PCB) contains the current regulators and other power converters, as well as the microprocessor and transceivers that communicate to the other electronic control units (ECUs) in the system. The body control unit (BCU), located near the cabin sends commands to the LCU and ultimately manages all of the body functions in the car.

The LED array itself resides on a metal-core PCB connected to some type of heat-sinking. This board typically contains the LEDs, temperature compensation, and current-binning information that programs the desired system output current. The headlight will contain multiple LED-array PCBs, including separate boards for several, if not all, of the functions.

Adaptive Front-Lighting Systems

Various automobiles already implement more complex functionality in the headlight, commonly referred to as adaptive front-lighting systems (AFSs). Current HID-based AFS systems have auto-leveling motors that calibrate vertical changes in the position of the car relative to the terrain. This ensures that the light is aimed correctly in the vertical axes, as not to violate the regulations around high- and low-beam patterning.

In addition, certain AFSs change the horizontal position of an HID source relative to the steering wheel position, speed, and sometimes camera input. This functionality is intended to maximize the light output when the driver needs to see the road or possible hazards. However, stepper motors are used to make such functionality possible with an HID source. Stepper motors limit the capability to react to multiple conditions at one time, and can become a reliability problem over the lifetime of the car.

The same AFS features can be easily realized in LED headlights, with the added advantages of better, more complex controllability and better reliability. For highly dynamic features, the front-view camera, usually found inside the rear-view mirror assembly, can be utilized to control dynamic light outputs for full AFS. This is especially useful for glare-free high-beam systems, which allow the driver to use both the high and low beam concurrently at all times. In such a system, the camera detects the oncoming cars and tells the headlight to turn off only the light in that area. In fact, the camera can be used for even more advanced functions like pedestrian detection, lane illumination, collision avoidance, and other safety-critical features.

Adaptive Headlight Architectures

The enhanced dynamics necessary for an adaptive headlight call for a different type of power-electronics architecture. Given the need to support quickly changing output conditions in the presence of load-dump and cold-crank conditions at the input, a two-stage topology is generally preferred. The most common solution employs a boost voltage regulator to convert the battery input into a stable high-voltage dc rail. Then, independent buck converters are used to separately drive each series LED string in the system.

This system is more capable of accurate regulation over a wide range of conditions, since the first stage can ballast the input transients of the car battery, while the second stage can ensure consistent regulation of the output at all times (Fig. 2). Furthermore, this topology provides better mitigation of conducted and radiated electromagnetic interference (EMI) than the single-stage buck-boost systems, since the input and output current are both continuous waveforms. For the highest dynamic systems, such as glare-free high beam, a buck output stage is usually required to achieve the desired dimming resolution and contrast ratio when PWM dimming.

LED Matrix Manager

The adaptive headlight architecture seems promising, but how do you effectively achieve the dynamics? If we look at the system as a controllable light pattern, we can break it down into a pixel-level control. The definition of a pixel can vary, of course, but ideally controlling each and every LED individually gives the most design flexibility.  For this two-stage boost-buck architecture, the TPS92661-Q1 LED matrix manager can provide these functions.

With the TPS92661-Q1, pixel-level control is possible in systems incorporating up to 96 LEDs (Fig. 3). The device provides separate PWM control for each and every LED. This function, given the 1000:1 dimming ratio and programmable dimming frequency, enables most any level of dynamic control. Glare-free high beam, dynamic bending beam, and any other type of AFS function are easier to add to any two-stage LCU design. The matrix manager mounts directly to the metal-core PCB where the LED array resides, helping to optimize thermal connection and system performance.

The device has 12 series switches designed to shunt current around the LED at a given duty cycle and frequency. Serial communications with the microcontroller on the LCU provide the path for high-speed dimming commands to be received and acted upon nearly instantly.

One other benefit concerns inherent fault coverage. Unlike typical LED string drivers, the TPS92661-Q1 touches each LED individually, allowing it to detect and protect against any individual fault. This adds an extra layer of coverage that helps extend the lifetime of the headlight. If a single LED fails either in an open or shorted condition, the string is able to be protected and surrounding LED outputs can be increased to maintain the desired light level.