LED Automotive Lighting Technology and Status Quo Analysis
As a fourth-generation vehicle light source, LEDs have many advantages over other conventional light sources. This paper analyzes the feasibility and advancement of automotive LED lighting, introduces its typical driving circuit, and focuses on the application status of LED and AFS on automotive headlamps. The problems faced by automotive LED lighting and their countermeasures were introduced, and their future development was prospected.
1 The feasibility and advancement of automotive LED lighting
The use of lighting sources in cars began around the early 20th century. The kerosene lamp and acetylene lamp were the first to be used. The electric light source was used in 1910. It has undergone incandescent lamps, tungsten halogen lamps, and HID (Intensity Discharge Lamp) lamps. It has been used in LED cars since 1985. Lamp era. At the same time, the technology of LED lights applied to adaptive front lighting system AFS (Adaptive Front Lighting System) appeared.
At present, LED has been used by many automotive manufacturers to create a variety of lamp models. In order to increase their overall competitiveness, BMW, Ford, Honda, Toyota, Mercedes-Benz, Audi and other famous brand cars have launched new models of cars equipped with a variety of LED headlights to attract customers. LEDs have many advantages over other light sources: (1) Long life and good shock resistance. The theoretical life of the LED can reach 50,000 hours, and the actual life can reach 20,000 hours (ordinary halogen bulbs are only about 150 to 500 hours), which generally exceeds the life of the automobile itself. In addition, there is no vulnerable moving part in the basic structure of the LED, so the seismic performance is very good. (2) Energy saving and environmental protection. LED can obtain sufficient brightness under the condition of low voltage and low current. Its power consumption is only 10% to 20% of the incandescent lamp of the same brightness; LED light source does not contain mercury which is harmful to human health, and the production process and waste will not Cause environmental pollution. (3) Fast response. Compared with incandescent lamps, the response time of LED lamps has reached several tens of nanoseconds. In this way, when LED is used as a taillight of a car, subsequent car drivers can react earlier to reduce traffic accidents. (4) Small size. The compact LED makes the design of the car style more free and diversified, which makes the model more fashionable. Compared with the traditional light source, the installation depth of the LED signal light system can be reduced by 80 mm, which is of great significance for the car styling and interior parts arrangement. .
At present, the automobile industry is still a pillar industry in the global economy, and it is in a critical period of rapid development. It will certainly drive the development of automotive lighting and provide a vast market space for LED applications in automobiles.
2 Vehicle LED Lighting Drive Circuit
The LED is a current-controlled semiconductor device. Figure 1 shows the volt-ampere characteristics of the LED. From Figure 1, we can see that this curve is steep, and almost no current flows through the LED before the forward conduction. When the forward voltage exceeds the turn-on voltage, the current rises sharply, and the light emission brightness L is approximately proportional to the forward current IF: L = KIF, where K is a scaling factor, can be controlled by controlling the LED's IF. Therefore, in order to ensure the consistency of its brightness, a constant current source driving circuit is generally used.
The operating voltage range of the car battery is 9 V to 16 V, usually 12 V, but the voltage of the battery can fall to 4 V when the car cold start, and when the battery defect is directly powered by the generator, this voltage can reach 36 V high pressure. Therefore, for automotive LED luminaires, to reliably drive constant-current LED strings, the drive controller must have precise voltage and current regulation, protection circuitry, and dimming capabilities. Therefore, it is necessary to design a driving circuit with good regulation performance and constant current output.
Currently, automotive LED drivers generally use two methods to control the forward current. (1) Use the LED's V-I curve to determine the voltage required to apply the LED to the desired forward current. The disadvantage is that any change in the forward voltage of the LED causes a change in the LED current, where the voltage drop and power dissipation of the ballast resistor wastes power and reduces battery life. (2) Drive the LED with a constant current source. Because this method requires the LEDs to be connected in parallel in the circuit, driving the shunt LEDs requires placing a ballast resistor in each LED string, which leads to reduced efficiency and current mismatch. Therefore, neither of these two methods fully reflects the superiority of LED. In order to overcome the shortcomings of the existing automotive LED drivers, an efficient and intelligent driving method of LED arrays for automobiles has emerged. The method adopts half-bridge DC-DC conversion technology, full-wave rectification technology and opto-electronic coupling technology to ensure the working efficiency of the entire driver circuit. An intelligent control scheme based on embedded system is proposed. This solution adopts intelligent PWM steady flow. Control and dimming control with load open/short protection and overcurrent and overvoltage protection. Figure 2 shows the LED array smart driving experiment circuit.
As shown in Figure 2, the CPU outputs two PWM signals A and B that are completely inverted and symmetrically applied to the switching devices to make them turn on. The energy is coupled to the secondary through the high-frequency transformer T, and then the fast recovery is achieved. Diodes D1 and D2 are full-wave rectified to drive the LED array. The optocoupler of the LED array drive circuit completes the monitoring of the LED array drive current and feeds back to the CPU to form a closed-loop control system of intelligent current negative feedback to ensure the stable reliability of the drive current.
Automotive LED driver circuits are becoming more and more integrated and intelligent. The miniaturization of chips and packages like PMUs (Power Management Units) will gradually replace the multiple single-function circuit combination methods to accommodate very limited onboard applications in the board space. At the same time, thanks to the constant development of microcontrollers, DSPs and other control chips and embedded technologies, automotive lighting systems can be automated through software technology, so that LED constant current drive accuracy and automatic brightness adjustment will be more accurate. Intelligent control has become the design concept of a new generation of automotive LED drivers.
3 Automotive LED Headlamps
As the headlamps of automobiles play an important role in driving safety, LED headlights are the most difficult and finally put into use. In the past, LED headlamps were only used in concept cars. With the continuous development of LED lighting technology and the automotive industry, the application of LED headlamps has transitioned from concept cars, luxury cars to mid-range cars and even general models, and the lighting The intensity has reached the level of incandescent lamps.
Car headlights include high beam and low beam lights. When driving at night, the high beam should ensure that the objects in the range of 100 m in front and 2 m in height are illuminated with even brightness; The low beam not only ensures that the driver can see the obstacles in front of the car but also can not let Glare occurs on the oncoming driver or pedestrian to ensure the safety of the car at night traffic.
Traditional car headlights output light beams with both low beam and high beam, and each beam distribution pattern is statically distributed. The specific light distribution also conforms to national standards. However, in practical applications, the beam emitted by the system is distributed in a limited range of angles, and visually blind areas are easily generated in some complicated road conditions (such as cornering). In addition, the conventional automotive front lighting system does not have the function of automatically adjusting the beam distribution, and the conversion between the low beam beam and the high beam beam needs manual operation by the driver, so that vehicles are likely to be dizzy between vehicles in a frequent driving environment. look. In order to overcome the above shortcomings of the traditional car headlamps, the adaptive front-lighting system AFS emerged.
AFS is a headlamp system that enables drivers to better adapt to changes in speed, road type, and weather conditions to improve driving safety. Its working principle is as follows: When the car enters a special road condition (such as a curve), the signal transmitted from the angle sensor and the speed sensor to the electronic control unit (ECU) changes correspondingly due to changes in the steering wheel and the speed. The ECU captures these signal changes, and at the same time determines which corner the vehicle has entered and issues a corresponding command to the headlight control unit. The control unit manipulates the micromotor mounted inside the AFS lamp body to drive the light emission according to the received command. The elements rotate around the corresponding axis of rotation, thus changing the lighting when traveling on unconventional roads and in the weather, providing better security.
With the development of white LED technology and the demand for aerodynamics and automotive styling, the position of the front of the car is getting lower and lower streamlined, leaving less room for headlamps. In order to meet the needs of automotive lighting intelligence and humanization, the combination of AFS and LED lamps has become the development trend of modern automotive headlamps.