1. Current control
A fundamental problem with leds is that they are a device controlled by current and have a relatively low voltage drop. The simplest method is to use resistance limit the LED current, but this method is not suitable for the rated voltage of 12 v or 24 v battery system, because the actual voltage is 6 v battery to 18 v or 12 v to 36 v. Therefore, if the luminance needs to be maintained, constant current control must be carried out.
LED car light
LED car light
2. Linear control of current
Linear control is the use of a linear regulator to keep the current flowing through the LED constant. Linear control, in some cases, the efficiency is very low, for example, the forward voltage of 3.5 V 1 a single (3 W) LED, need regulator while maintaining the current 1 a will be rated 12 V power supply to 8.5 V, so use 3 W LED will waste the power of 8.5 W. Linear current control is the least noisy technology, and from the perspective of EMC, linear current control is the safest.
3. Switch type regulator
Although inductor switching constant current technology produces more electronic noise, its efficiency is higher. Depending on the number of leds used, you can use a step-down or step-down regulator.
EMC problem 4.
Radiation and conduction noise must be minimized and controlled within the tolerance limits. Although the frequency of PWM method is fixed and it is relatively easy to filter, due to the stable LED load, if appropriate measures are taken, hysteresis controller and PFM are suitable choices. The development trend of the switching regulator is that the frequency will be higher to reduce the inductance/capacitance volume. This is always the best solution for automotive applications. Keeping frequencies low helps avoid interference problems.
Fundamental frequency "jitter" or "expanding" technology did help to conform to the requirements of the EMC testing similar peak, is the best way but does not produce any radiation, and any switching regulator are difficult to achieve this.
Radiant heat, conduction heat and heat management one of the key problems and one of the biggest challenges for users of high-brightness leds, especially in the automotive industry, is the self-heating of leds. LED lumens per watt have been greatly improved, but in fact most of the LED's electricity is converted to conduction heat. Leds generate less radiant heat for car lighting, but in cold weather, the heat from the headlights effectively melts snow on the lens. Therefore, thermal management is the key to reliable LED control.
Heat management mainly refers to the reduction of current when the temperature increases. The advantage of using high brightness LED is that when the current changes greatly, the eye cannot detect the brightness change. In general, the current drops by 25 percent and the brightness of a single LED does not change significantly.
However, it remains to be seen whether leds change color with temperature and current changes that affect lighting applications in cars. Whether the LED spectrum is suitable for lighting and whether it will affect the driver's sense of distance under the general night vision effect may be more important.
Using PWM method to reduce the luminance ratio, instead of direct current control, a larger light to dark ratio can be obtained, and the color temperature will not change, so PWM method to reduce the luminance is a better method. But frequency selection is also important. It is generally believed that the frequency of 200Hz is better, because the human eye will not feel the flash of 200Hz light, in addition, the lower frequency can ensure that the conversion frequency is lower than the switching regulator. However, it is necessary to anticipate the potential flashover effects of the headlights. A more appropriate method is to use a higher frequency to adjust the brightness of the LED to avoid the "pendulum" effect. In addition, the inductor must be carefully selected to avoid audible noise in the car.
LED temperature sensing is also a problem to be solved. Thermistor is a widely used method, but the use of thermistor must be very careful, the temperature control response should be set to the temperature upper limit corresponding to the reduced current of LED. When the ambient temperature drops, simple temperature control can lead to increased LED current. Figure 2 shows the typical response requirements of LED to the ambient temperature.
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