Advantages of automotive LED lighting systems
White LEDs have a color temperature of approximately 6000K and are visually equivalent to natural light. The ability to distinguish colors by the human eye is best under natural background conditions. In the evening, the human eye only has a good ability to distinguish between the scene or the edge of the road in the natural color.
The average life of LED lamps is longer than that of tungsten lamps and high-intensity fluorescent tubes, and it is more stable and reliable. If you can reduce the chance of failure, and do not need to change the bulb frequently, it saves a lot of trouble. And using LEDs can save more energy. LED lights are smaller and easier to fit into the lamp holder, making it easier to work with stylish lighting designs while taking up less space. More importantly, LED brake lights start faster. The LED brake light has a lighting time of only about 50ms, which is about 250ms faster than the start time of the tungsten bulb, thus reducing the risk of collision with the trailing vehicle. In addition, LEDs are solid-state light sources 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 required to drive the LED. In any case, the current flowing through the LED should be kept constant to ensure the brightness of the LED. It is also necessary to control the ripple current to an acceptable level under any circumstances. Therefore, the design of the LED driver belongs to the power conversion circuit design, which is characterized by a constant current output instead of a constant voltage output.
In LED driver designs, blocking circuits must be added to protect against reverse power operation. Another challenge we will encounter is to ensure that the LEDs work properly in the event of a cold start or load outage. In normal operation, the car's battery supply voltage is between 9V and 16V (for example, 12V system bus), while the large truck's battery supply voltage is between 18V and 32V (for example, 24V system bus). In the event of a load outage or cold start, the battery's input voltage range will vary greatly from the normal range.
During the operation of the generator, if the power supply of the battery is suddenly cut off, the generator will continue to generate electricity at this time, and the power consumption of the capacitor-powered part will suddenly increase. If no protective measures are taken, it will be damaged. Electrical appliances. If the LED driver is not driven by a constant current, the current of the LED will change, causing the brightness of the LED to change, which is why the current car light will flicker. The following is a definition of a load power-down test that mimics some cases of load breaks. Different car manufacturers use different standards, so the definition of load power-off test is also different. Figure 1 is only one case.
Although the load dump causes an increase in the load voltage, most advanced alternators are equipped with a centrally controlled load-carrying clamp circuit. 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.
In cold weather, the starter device will cause the battery supply voltage to drop significantly. Figure 2 shows the typical waveform of the "cold start" test.
Since the input supply end of the LED driver is connected to the input of the battery, a car light such as a brake lamp that is safe for driving must be unaffected by load power interruption and cold start, and must even be able to continue normal operation in this case. In a typical case, the 12V bus system has an input range of 6V to 42V, and the 24V bus system has an input voltage range of 12V to 60V.
LED drive technology
Resistor current limiting is one of the LED driving methods, which has the advantages of low cost and simple design. The disadvantage is that the current will change with the forward voltage and the driving voltage, so the brightness emitted by the LED will change. When the battery voltage is relatively high, the efficiency of the drive is very poor, because most of the power is consumed at this time. In addition, the current limiting resistor generates a large amount of heat, causing heat dissipation problems. So this is a simple but not the most efficient method.
The second method is a linear current regulator, also called a constant current source. Its advantages are simple design and constant current; the disadvantage is that the efficiency is very low, and as the input voltage increases, the voltage drop carried by the linear chip is larger, so the linear chip also generates a large amount of heat. However, it improves the shortcomings of current fluctuations in the resistor current limiting method.
In addition to the above two methods, there is also a Switching Power Supply type driving method. Table 1 compares the three different methods. The switching power supply is used as the LED driving method with high efficiency and constant current. Because of the high efficiency of the open-light power converter, it is used by more and more car manufacturers. The biggest drawback is that the design line is more complicated and the cost is relatively high.
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