High brightness LED in automotive lighting applications

Needless to say, the use of high brightness LED lighting will become the main feature of the future car, thanks to the LED compared with the traditional incandescent lighting program has many of the basic advantages. In addition, the use of LED lighting can also lead to changes in automotive design technology and design style. However, just like any innovative technology, LED is still widely used in automotive lighting, still need to overcome many difficulties.

Key characteristics

1 reliability and service life

LED's life expectancy is 50 thousand hours, while the halogen lamp is 20 thousand hours, 3 thousand hours of tungsten incandescent lamp. Compared with incandescent lamp, the structure of LED is strong, and it is not easy to be affected by vibration. Based on multiple LED lighting program also has the advantages of redundancy, even if a LED failure, you can continue to use the lighting device. The correct use of LED (especially the correct control of the temperature of LED), can effectively extend the life expectancy of LED. On the contrary, if the temperature is too high, LED is easy to damage. LED application also involves many legal issues in automotive lighting. In most countries, there is a clear definition of the brake light or headlight failure - the light is on or off. However, the use of multiple LED lights, it is difficult to accurately define whether the lamp has been damaged. Manufacturers and legislators are defining the use of LED.

Efficiency / 2 lumens per watt

Compared with incandescent lamp with standard, LED consumption per unit of electricity can produce more light output. However, compared with the halogen lamp, the actual LED light output advantage is not obvious. The latest LED has an excellent lumen per watt value, but some values are obtained under optimized conditions, which are usually not obtained at the highest output conditions. Generally speaking, when the current of LED increases, the light output does not increase linearly. Therefore, even if the LED output current in the X lumen 0.5A, it will not be in the 1.0A current output 2x lumens.

3 response speed

To brake lights and direction indicator lamp as an example, assume that the vehicle speed of 125 km / h, which is 35 meters / sec, hot start time of incandescent lamp is about 250 milliseconds, and responsive LED to about 8 meters distance from the brake warning, so as to avoid collision. So is the indicator.

4 direction

Another key feature is the light of LED. Unlike incandescent lamp, LED only through a light emitting surface, which is good for the headlights and chart lights, but may not be suitable for other car lighting lighting applications.

Method for controlling LED

1 current control

One of the basic problems LED, LED is a current control device, the voltage drop is relatively low. The simplest method is to use the resistor to limit the current of the LED, but the method is not suitable for the system with rated voltage of 12V or 24V battery, because the actual voltage of the battery is from 6V to 18V or 12V to 36V. Therefore, if you need to keep the brightness, you must carry out constant current control.

Linear 2 current control

Linear control to maintain the current through the LED is constant through the linear regulator. Linear control in some cases, the efficiency is very low, for example, the forward voltage to single 1A 3.5V (3W) LED, 1A current regulator in the need to maintain the rated 12V power supply will be reduced to 8.5V, 3WLED will use this power waste 8.5W. Linear current control is the technology that produces the least noise, and from the EMC point of view, linear current control is the most secure.

3 switching regulator

Inductive switching constant current technology although produce electronic noise more, but it is more efficient. According to the number of LED use, can be used for buck or buck / boost regulator.

4.EMC

Must minimize the radiation and conduction noise, noise control will be within the permissible limits. Although the frequency of the PWM method is fixed, and it is relatively easy to filter, but because of the LED load is stable, if appropriate measures, hysteresis controller and PFM is the appropriate choice. The development trend of the switching regulator is that the frequency will be higher in order to reduce the inductance / capacitance volume. This is always the best solution for automotive applications. Keeping the frequency at a lower level helps avoid interference.

Frequency "jitter" or "expansion" technology really helps with similar peak EMC test requirements, but the best way is not to have any radiation, and any switching regulator are difficult to achieve.

Heat transfer, heat transfer and thermal management

Use high brightness LED users (especially in the automotive industry) the key problem to be faced with one of the biggest challenges, is the problem of self heating LED. LED has made a lot of improvement in the lumen per watt, but in fact most of the LED power is converted into heat conduction. LED can produce a small amount of radiation that is suitable for car lighting, but in a cold climate, the radiant heat of the headlights can effectively melt the snow on the lens. Therefore, thermal management is the key to reliable control of LED.

Thermal management mainly refers to the increase of temperature when the current is reduced. The advantage of using high brightness LED is that when the current changes, the eye can not detect the brightness change. In general, the current decreased by 25%, and the brightness change of individual LED was not obvious.

But LED will change with temperature and current change color, this will affect the automotive lighting applications remains 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.

Using PWM method to reduce