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Design and breakthrough of automobile LED lighting system

时间:2017-04-06 10:34:29    来源:

All along, the car's brake lights, signal lights, reversing lights, and rear fog lamps are used 21W to 27W, brightness of 280 to 570 lumens of tungsten filament bulb as a light source. The rear lights, parking lights, side display lights and turn signals flashing lights are 4W to 10W, tungsten filament bulbs with brightness from 40 to 130 lumens, and high brightness xenon fluorescent tube (HID) used in automobile headlamps. But in automobile lighting, more and more LED is used as light source. The rear central high stop light is the earliest use of LED car lights. In addition, other lights and lights, such as brake lights, turn signals and car lights, have been switched to LED bulbs in the light market. In recent years, the car lights, such as daytime running light (DRL), and front of the beginning to use LED bulbs. Auto headlights are expected to switch to high brightness LED soon. In addition, inside the car interior, the back light of a few car monitors also begin to use LED as illuminant, be like instrument board and TFT monitor.
Advantages of automotive LED lighting systems
White LED color temperature is 6000K, the vision is almost equivalent to the natural light. The ability to distinguish colours from human eyes is best under natural background conditions. In the evening, the human eye is only capable of distinguishing the scenery or the edges of the road only in natural colors.
The average life of a LED lamp is longer than that of a tungsten filament lamp and a high brightness fluorescent tube, and it is more stable and reliable. If you can reduce the chance of failure without changing the bulb frequently, it saves you a lot of trouble. And using LED can save more energy. The LED lamp is smaller in shape and easier to fit into the lamp holder, making it easier to match the stylish lighting design while taking up less space. More importantly, the LED brake lamp starts faster. The LED brake light is only about 50ms lighter than the tungsten filament bulb. The starting time is about 250ms, which reduces the risk of being hit by the trailing vehicle. In addition, the LED is a solid-state light source that can withstand greater vibration.
Technical requirements for driving LED lights
Since the brightness of the LED is proportional to the current flowing through it, a constant current source is needed to drive the LED. In any case, the current flowing through the LED remains constant to ensure consistency of LED brightness. In addition, ripple current can be controlled at an acceptable level in all cases. Therefore, the design of the LED driver belongs to the design of the power conversion circuit, which is characterized by constant current output rather than constant voltage output.
In the design of LED drives, blocking circuits must be added to provide protection against power reverse operation. Another challenge we will encounter is to ensure that the LED works properly when the car is cold activated or the load is off. In normal operation, the battery voltage of the car is between 9V and 16V (for example, the 12V system bus), and the battery voltage of the large truck is between 18V and 32V (for example, the 24V system bus). When a power failure or cold start occurs, the input voltage range of the battery is very different from the normal range.
In the generator operation period, if the battery power supply is suddenly cut, the opportunity to continue to use power generation, generator power supply capacitance electric part there will be a sudden increase in voltage phenomenon, if you do not take protective measures, the use of electrical appliances will be damaged. If the LED driver is not a constant current drive, the LED current will change, resulting in LED brightness changes, which is why the current car lights flashing phenomenon exists. The following is a definition of a load outage test to simulate some of the load break ups. Different car manufacturers use different standards, so the definition of load test is also different.
Most of the advanced AC generators are equipped with a centrally controlled load power off clamp circuit, although load sags may cause load voltage rise. The reference level of the 12V bus system is limited to between 35V and 42V, while the reference level of the 24V bus system is clamped between 50V and 60V.
When the weather is cold, the boot device causes a significant drop in the battery power supply voltage. Figure 2 shows typical waveforms of "cold start" tests.
Because the input power LED drive end is connected with an input end of the battery, so the car lights like brake lights to driving safety must be affected by the load power and cold start, and must be able to continue normal operation in this case. In typical cases, the input range of the 12V bus system is 6V to 42V, and the input voltage range of the 24V bus system is 12V to 60V.
LED drive technology
Resistance current limiting is one of the driving methods of LED. Its advantage is low cost and simple design. The disadvantage is that the current will change with the forward voltage and the driving voltage, so the brightness of LED will change. When the battery voltage is relatively high, the drive efficiency is poor, because most of the power will be consumed on the resistor. In addition, the current limiting resistor generates a large amount of heat, leading to heat dissipation. So it's a simple, but not the most efficient way.
The second method is a linear current stabilizer, also called a constant current source. Its advantages are simple design and constant current. The drawback is that the efficiency is very low, and with the increase of the input voltage, the voltage drop of the linear chip is greater, so the linear chip will produce a lot of heat. But it improves the disadvantage of current fluctuation in the resistance current limiting method.
In addition to the above two methods, there is a switching power driven method. Table 1 is a comparison of three different methods using a switching power supply as the LED driver

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