Light-emitting diode (LED) driver electronics improve automotive headlight style

Prior to 1990, standard automotive headlamp assemblies contained only incandescent light sources and were mostly independent to create all the functions required. Whether these functions are created by sealed beam lights or by bulbs, reflectors, and optical assemblies, they all operate in a similar manner. The light mode is static and is primarily defined by the reflector and optics. The supported standard lighting modes are high beam, low beam, turn signal, position light and fog light modes. For decades, these features have only provided the basic light distribution options needed to drive safely in most situations.

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In the early 1990s, a new source of high-intensity discharge (HID) emerged. This power supply produces more efficient, brighter lighting and supports low beam and high beam functions. However, after the transition to HID, a simple battery connection was not enough to control the lights. In contrast, HID sources require advanced power electronics to convert the DC battery voltage source into a resonant AC power source that properly regulates the light output.

This change brings design that once involved only mechanical and optical systems to the design of advanced electronics designers. Over the next 20 years, the HID system was further improved by engineers into a cost-effective solution for simultaneous high beam/low beam functions. . However, it still does not have more advanced lighting features because a single light source is subject to a fixed size.

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In 2007, the first light-emitting diode (LED)-based headlamps were used in new cars. LED-based systems give designers greater flexibility because of their extremely small light source. Of course, the adjustment of the LED also depends on the conversion of a certain power supply. In particular, LED drivers are indispensable for converting battery voltage sources into constant current drive sources suitable for series or series/parallel LED arrays.

Similar to HID electronics, LED drivers also increase the cost and complexity of automotive headlamp systems. Despite this, the inherent size of the LED driver is reduced, the controllability of the intensity and color is significantly improved, and the efficiency is also improved. The benefit to car manufacturers is that the system has been improved to be more suitable for sale. In addition, designers can take advantage of the aesthetically pleasing and innovative form factor that also creates effective brand recognition for automakers.

Throughout the past eight years, LED car headlights have evolved from a single LED function option, such as daytime running lights (DRL) or fog lights, to a complete LED headlamp system that is versatile. Today, most mid- to high-end cars have a fully LED-based car headlight option. Now let's take a closer look at the LED car headlight system.

LED car headlight architecture

Due to the inherent power levels of the associated components, LED automotive headlamp assemblies are typically designed as a single stage switch mode power supply. In general, buck-boost topologies are preferred for providing regulation during load dump and cold start. During load dump, the battery voltage may rise to 60V (and sometimes even higher); during cold start, the battery voltage may drop to 4.5V or even lower! Under any extreme input conditions, buck - The boost converter regulates the output current to the LED string (total forward voltage above or below the battery voltage).

1. This typical automotive headlamp system implements a single stage power architecture in which each converter can adjust a different function in the system.

A complete front lighting system often consists of multiple converters, each of which is responsible for adjusting different parts of the system (Figure 1). Each car headlight function is typically supported by a separate LED string. Usually only DRL and position lights are multiplexed into one string. In this case, the position lamp is created by dimming the DRL string with a duty cycle of about 10% by a pulse width modulation (PWM) function.

Most existing LED automotive headlamps have two basic electronic components: an array of LEDs in the headlights of the car, along with associated optical and mechanical components, and a lighting control unit that is typically externally connected within the weather-resistant enclosure ( LCU). The LCU printed circuit board (PCB) contains power converters such as current regulators and microprocessors and transceivers that can communicate with other electronic control units (ECUs) in the system. A body control unit (BCU) located near the cockpit sends commands to the LCU to manage all body functions in the car.

The LED array itself is located on a metal core PCB that is connected to a certain type of heat sink system. The board typically contains LEDs, temperature compensation, and current binning information that can be programmed for the desired system output current. Automotive headlamps will contain multiple LED array PCBs, including separate boards that support multiple, if not all, functions simultaneously.

Adaptive front lighting system

Various vehicles have more sophisticated functions in headlights, commonly referred to as adaptive front lighting systems (AFS). Current HID-based AFS systems have an automatic leveling motor that calibrates the vertical change of the vehicle's position relative to the terrain. This ensures that the lamp is precisely aligned on the vertical axis to avoid violating the regulations regarding the high beam and low beam modes.

In addition, some AFS can also change the horizontal position of the HID source (relative to the steering wheel position), speed, and sometimes the camera input. This feature maximizes light output when the driver needs to view the road or in a potentially dangerous situation. However, stepper motors require the use of HID sources to achieve such functions. Stepper motors limit the ability to react to multiple conditions at once, and can create reliability issues throughout the life of the car.

The same AFS function can be easily implemented in LED car headlights, with the added advantage of greater, more comprehensive controllability and higher reliability. For high dynamics, a front-view camera (usually found in the rearview mirror assembly) can be used to control the dynamic light output for the entire AFS. This is especially useful for glare-free high beam systems that allow the driver to use both high beam and low beam lights at any time. In such a system, the camera can control the headlight system to turn off the lights in the relevant area when detecting an oncoming car. In fact, the camera has more advanced features such as pedestrian detection, lane lighting, collision avoidance and other safety-critical features.

Adaptive car headlight architecture

The enhanced dynamic capabilities required for adaptive car headlights require a different type of power electronics architecture. Two-stage topologies are often preferred because of the need to support fast-changing output conditions in the event of load dumps at the input and cold-start. The most common solution is to use a step-up regulator to convert the battery input into a stable high-voltage DC rail, and then use a separate buck converter to drive each series-type LED string in the system.

2. The adaptive car headlight system can be powered by a two-stage power architecture – one that calms the battery's input transients and the other that ensures consistent output regulation.

The system is more capable of precise adjustment over a wide range of conditions, with the principle that the first stage can quell the input transient events of the car battery, while the second stage guarantees that the output adjustments are consistent at all times ( figure 2). In addition, this topology is more effective at reducing conducted and radiated electromagnetic interference (EMI) than single-stage buck-boost systems because both input and output currents are continuous waveforms. For the most dynamic systems (such as glare-free high beam), a buck output is typically required to achieve the desired dimming resolution and contrast when performing PWM dimming.