Highlight LED will become the main features of the next generation of cars

There is no doubt that the highlight of the LED will become the main features of the next generation of cars. This is because LED has many important advantages over the traditional incandescent lighting scheme. At the same time, the use of LED can also lead to changes in technology, and even car design style. However, as with any innovative technology, LED is widely used in automotive lighting, there are still many difficulties to overcome.

Reliability and durability

The relative life expectancy of LED is 50 thousand hours, while halogen tungsten lamp is about 20 thousand hours, halogen incandescent lamp is about 3 thousand hours. Compared with the incandescent lamp, the solid structure of the LED is not easy to be affected by vibration, and the brightness of the light output can not be obviously decreased during use. Using multi LED lighting program, with the advantages of redundant parts, even if a LED failure, the lighting device can continue to run. The correct use of LED (especially the correct control of the temperature of LED), can effectively extend the life expectancy of LED. In contrast to the use of the temperature is too high, LED will be very easy to damage. The application of automotive lighting involves many legal definitions. Most countries have clearly defined the LED lighting, it is difficult to accurately define whether the lamp has been damaged. Manufacturers and legislators are defining the use of LED.

Efficacy / lumens per watt

Compared with standard incandescent bulbs, LED has a higher output per unit of power. However, compared with the halogen lamp, the actual light output advantage is not obvious. The latest LED has an excellent lumen per watt, but some values are obtained under optimized conditions and are usually not obtained at high output conditions. Generally speaking, when the current of LED increases, the light output does not increase linearly. Therefore, even if the LED in the 0.5A power generation X lumens, when the 1.0A power does not produce 2X lumens.

response speed

For example, for the brake and direction indicator lamp, the vehicle speed of 125 km, that is, 35 m / s, incandescent bulb thermal start time is about 250 milliseconds. LED with immediate response can be issued about 8 meters ahead of the brake warning, so as to avoid collision. So is the indicator.

Directional

LED light emitting mode, is a single point characteristics. Unlike incandescent lamps, LED is only exposed to a single surface. This is more suitable for the headlights and chart lights, but for other lighting applications (such as car lighting) the use of LED will be.

Method for controlling LED

Current control

LED is a relatively low voltage drop current control device, which is the main problem to be solved. The easiest way is to use resistors to limit the current of LED. But the method does not apply to rated 12V or 24V battery systems, because the actual voltage is from 6V to 18V or 12V to 36V. Therefore, if you need to maintain the lighting, you must carry out constant current control.

Current linear control

Linear control refers to the linear constant current device, to maintain the current stability of LED. Linear control in some cases with lower efficiency, for example, with a single 1A 3.5V positive voltage (3W) LED, the regulator will need to reduce the 8.5V 12V power 1A rated power, the use of 3WLED will be a waste of 8.5W! Linear current control is the lowest noise of electronic technology, and from the Perspective of EMC, linear current control the safe current control method.

Switching current regulator

Inductive switch constant current technology, although a higher electronic noise, but it is a more efficient solution, according to the number of LED use, you can use buck or buck / boost technology. LED faced challenges in the field of application in automobile

EMC problem

Radiation and conduction noise must be minimized, and noise is controlled within the allowable limits. PWM (pulse width modulation) technology provides a fixed frequency, and relatively easy to filter. However, the LED load is relatively stable, and therefore, if appropriate measures are taken, the hysteresis loop ("Bangbang") controller and the PFM (pulse frequency modulation) are suitable for selection. There is a tendency to increase the frequency of the switching current regulator to reduce the size of the inductor / capacitor, which is not the best solution for automotive applications. Keeping the frequency at a lower level helps to avoid interference.

The fundamental frequency of "jitter", or "spread spectrum" technology, does help to conform to the quasi peak EMC test. However, the best method is not to produce any radiation, and any switching current regulator is difficult to achieve this requirement.

Thermal radiation, heat conduction and heat management

One of the key issues facing the use of the highlighted LED user (especially in the automotive manufacturing industry) is one of the hot issues of LED. LED has made a lot of improvement in the lumen per watt, but the fact is that most of the LED's electrical energy is converted into heat transfer. LED has the advantage of low radiant heat, suitable for car lighting. On the contrary, in cold climates, the radiant heat of the headlights can effectively melt the snow on the lens. Therefore, thermal management is the key to effective control of LED.

Thermal control mainly refers to the increase in temperature when the current. Highlight the advantages of using LED, when the current is larger, the eye can not detect changes in brightness. In general, when the current is decreased by 25%, the brightness change of single LED is not obvious. However, LED will change with temperature and current changes in color characteristics, whether this will affect the application of automotive lighting is still to be explored. Whether the spectrum of LED is suitable for lighting, and whether it will affect the driver's sense of distance under the general night vision, may be more important.

PWM method to reduce the brightness ratio, rather than direct current control, can get a greater proportion of light and dark and color temperature will not change, so PWM is a good way to reduce the brightness. However, frequency