LED has become a new star in the selection of automotive lighting materials
How LED lights work
LED is a special diode, which is a semiconductor device that forms a PN junction by means such as doping. When the diode conduction conditions are met, current flows through the LED, releasing energy in the form of light and heat. LED is a current-controlled current-type element. Its luminous intensity mainly depends on the magnitude of the current passing through. When it is in forward conduction, its voltage drop is very high, and it has a certain fluctuation range.
Since the LED does not have infrared and ultraviolet radiation, the energy it consumes is almost all thermal energy except converted into light energy, and can only be transmitted in the form of heat conduction. Therefore, when the LED is working, the junction temperature will gradually increase. LEDs are devices with a negative temperature coefficient. The current flowing through the LED will increase as the temperature rises. This forms positive feedback, resulting in a further increase in junction temperature. If not controlled, the LED will be burned. The thermal parameters of LED have a great relationship with the junction temperature of the PN junction. The relationship between the dominant wavelength and the temperature is as follows:
mp (Tl) = m0 (T0) + 3Tg # 0.1nm / ° C
It can be seen from the above formula that every time the LED junction temperature rises by 10 ° C, the dominant wavelength (which can be observed by the human eye) drifts towards the long wave by 1nm (1nm = 10-9m), which causes the LED brightness to decrease and light decay. Therefore, the overheating of individual LEDs will cause the uniformity of the LED array's light emission to deteriorate.
Significant advantages of LED lighting
LED is called a new light source, because LED has the characteristics of point light source and solid state light source, so it has the advantages that other lighting sources can not match.
(1) The LED lifespan can theoretically reach 100,000 hours, and the actual lifespan can also reach more than 20,000 hours, which is more advantageous than the 1,000 hours of ordinary incandescent bulbs and 10,000 hours of fluorescent lamps, and generally does not need to be during the life of the car. replace.
(2) There is no delay in lighting, and the response time is faster. The startup time of LEDs is only a few tens of nanoseconds, which is much shorter than incandescent bulbs.
(3) In the case of high light brightness and low visibility of natural light, the incidence of automobile accidents is greatly reduced; basically no radiation, which belongs to "green light source".
(4) The LED occupies a small volume, has a simple structure, and is highly shock-resistant. Designers can freely change the lamp mode to diversify the shape of the car to meet the needs of different consumers.
(5) The LED light source is far less affected by the voltage change than the incandescent bulb, showing excellent safety and reliability. At the same time, the energy consumption is 80% less than the incandescent lamp with the same light effect, which is very energy-saving.
Based on the above advantages, LEDs can be widely used in automotive lighting, but a single LED can not meet the requirements of automotive lighting intensity, must be multiple series, parallel or series-parallel LED array
Design and characteristics of LED driver
The LED drive mode can adopt three types of resistance current limiting, linear regulator and switching converter. The resistor current limiting scheme is suitable for low efficiency applications, so this method is not used in automotive lighting with extremely high efficiency requirements and a wide input voltage range; linear regulators are suitable for low current or LED forward voltage drop slightly lower than the power supply In the case of voltage, there are also problems of efficiency and small input voltage range; the switching converter has the characteristics of flexible circuit topology, high efficiency and wide input voltage. Therefore, after comprehensively considering factors such as working efficiency, installation size, quiescent current, working voltage, noise and output regulation, the drive circuit mostly uses a switching converter. The topology of the switching converter is divided into Buck, Boost and Buck-Boos. At present, LED is used in automotive lighting, and its driving power must be lead-acid batteries.
Because the input voltage range of the battery will be very different from the normal range, the drive circuit generally uses a Buck-Boost topology to meet the voltage requirements of the LED array. The DC gain (ratio of output voltage to input voltage) of this circuit topology is related to the duty cycle D (the ratio of turn-on time to cycle within one switching cycle). When the battery voltage is lower than the required voltage of the LED, adjust D> 0.5 to make the circuit in a boosted state; when the battery voltage is higher than the required voltage of the LED, adjust D <0.5 to make the circuit in a reduced voltage state. LEDs are current-controlled current-type components, and their brightness is proportional to the current flowing through them. If the LED is not driven by a constant current, when the passing current fluctuates, even if the voltage is constant, the brightness of the LED will change. In order to ensure stable and reliable brightness, the LED needs a constant current to drive, and it is also necessary to control the ripple current to an acceptable level under any circumstances. Therefore, the output of the LED drive circuit must be a constant current output rather than a constant voltage output.
The cost of LED
If a car replaces all the lighting with LEDs, about 200 to 300 LEDs are needed for the inside and outside. Compared with other lighting sources, the cost is quite high. Although the advantage of long life can make up for its costly disadvantage, on the whole, its cost is still much higher than other light sources. This is the main reason why LED performance is outstanding but not occupying a large market share.
However, governments of all countries attach great importance to LED technology and invest a lot of money every year in research. I believe this problem will be successfully solved in the near future.
Due to the relatively high cost of LEDs and the relatively complicated driving technology, they have not yet entered the automotive market comprehensively and extensively. China has already stipulated that all cars on the highway must be equipped with LED fog lights. In the future, as the price of LEDs gradually decreases, its car light modules will gradually use high-brightness LEDs as light sources. Since the launch of Audi's first R8 model with LED headlights, internationally renowned car companies such as Cadillac, BMW, Toyota, Mercedes-Benz and Ford have launched new cars equipped with colorful LED lamps to attract customers. With the continuous development of the automotive industry, LED will be widely used in the automotive market in the next few years. On the whole, the LEDization of automotive lamps is a major trend in the development of automotive lamps, and it will definitely be a leap in the development history of automotive lighting systems.
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