High Brightness LED lighting Applications in Auto

High brightness LED lighting applications in automotive key issues

Needless to say, the use of high-brightness LED lighting will be the main features of future cars, thanks to the LED relative to traditional incandescent lighting scheme that has many of the basic advantages. In addition, the use of LED lighting can also be driven by automotive design technology and design style changes. However, just as any innovative technologies, LED are widely used in automotive lighting, before still many difficulties to be overcome.

 Key Features

1. Reliability and life expectancy of the LED life of 50,000 hours, while the halogen is 20,000 hours, tungsten incandescent three one thousand hours. Relative to incandescent lamps, LED's rugged, less susceptible to vibration, use with radiation output brightness will not be decreased. A plurality of LED based lighting solutions also have "redundancy" benefits, even if one LED fails, you can still continue to use the lighting device. Proper use of LED (especially the right to control the LED temperature), which can effectively extend the LED life expectancy. Conversely, if the temperature is too high, LED can be easily damaged. LED applications in automotive lighting also involves many legal definition. Most countries of the brake lights or headlights failure - lights on or off with a clear definition. But for the use of multiple LED lamps, lighting is difficult to accurately define whether the damage. Manufacturers and legislative bodies are defined LED is used.

2, Efficiency / lumens per watt compared to standard incandescent lamps, LED consumption per unit of electricity can produce more light output. However, when compared with the halogen lamp, LED light output of advantage is not obvious. Latest LED lumens per watt with outstanding value, but some values ​​are obtained under the optimized conditions, and usually not at the highest output conditions obtain. In general, when the LED current increases, the light output does not increase linearly. Therefore, even if the LED current output 0.5A x lumens, it is not at 1.0A current output 2x lumens.

3, speed of response to brake lights and direction indicator tube, for example, assume that a vehicle speed of 125 km / h, or 35 m / s, incandescent hot start time is about 250 milliseconds, and responsive to early about LED 8 m from the brake warning issued, so as to effectively prevent car collision. Indicator is true.

4, directional Another key feature is the LED light way. With incandescent lamps, LED only through a surface light, which lights headlights and aeronautical charts application is good, but may not be suitable compartment lighting other lighting applications (Figure 1). Figure 1: the incandescent lamps, LED emits light having directivity. Control the LED 1. Current Control
 A basic problem of LED, LED is controlled by the current device, the voltage drop is relatively low. The easiest way is to use a resistor to limit the LED current, but the method is not suitable for the rated voltage of 12V or 24V battery system because the actual battery voltage from 6V to 18V or 12V to 36V. Therefore, if necessary to maintain the brightness, it must be the constant current control.

5, The current linear control linear control refers to a linear regulator to maintain a constant current through the LED. Linear control efficiency is very low, in some cases, for example, a single forward voltage is 3.5V 1A (3W) LED, 1A to the current regulator, while maintaining the nominal 12V power supply drops to 8.5V, so used 3W LED will waste 8.5W of power. Linear current control technique is the least noise, and from an EMC point of view, the linear current control is the safest.

6, Switching regulator inductive switching constant current technology, while the electrical noise generated more, but it's more efficient. According to the number of LED use, you can use buck or buck / boost regulator.

 7. EMC issues must minimize radiated and conducted noise, the noise control within the allowable limits. Although fixed frequency PWM method, and relatively easy to filter, but because the LED load more stable, if appropriate measures, hysteresis controllers and PFM is the right choice. Switching regulator trend is the frequency will be higher, in order to reduce the inductance / capacitance volume. This automotive applications, it is always the best solution. The frequency is maintained at a low level to help avoid interference problems. Fundamental frequency of the "jitter" or "extended" technology really helps meet the requirements of the EMC test similar peak, but the best way is to not produce any radiation, and any switch-mode regulators are difficult to achieve this. Radiant heat, thermal conductivity and thermal management using a high-brightness LED users (especially in the automotive industry) to the key issues facing the biggest challenge with one of the LED's self-heating problem. LED lumens per watt has made great improvements, but in fact the majority of LED energy are transformed into heat conduction. LED compartment lighting can produce radiant heat for less, but in cold climates, the radiant heat headlight able to effectively melt snow on the lens. Therefore, the thermal management is critical and reliable control of the LED. Thermal management mainly refers to the temperature increases to reduce the current. The advantage of using a high-brightness LED is a current change is large, the eye can not detect changes in brightness. In general, the current decreased by 25%, a single LED brightness did not change significantly. However, LED will change with temperature and current change color, this will affect the automotive lighting applications remains to be explored. LED for lighting spectrum is, in general, will affect night vision effect under the driver's sense of distance, these problems may be more important. Using PWM method to reduce the brightness ratio, rather than the direct current control, enjoy greater proportion of light and dark, and the color temperature does not change, thus reducing the brightness using PWM method is a better approach. However, the frequency selection is also important. Generally considered better frequency of 200Hz, 200Hz because the human eye will not feel blinking light, in addition to ensure that the lower frequency is lower than the switching type regulator switching frequency. However, it must foresee headlights potential problems exist stroboscopic effect. A more suitable method is to use a higher frequency to adjust the LED brightness, and thus avoid the "yaw" effect. In addition, the inductor must be chosen carefully to avoid car produces audible noise. LED temperature sensing is also a need to solve the problem. Thermistor is a widely used method, but using the thermistor must be very careful temperature control response should be set for the LED need to reduce the temperature corresponding to a current limit. When the ambient temperature decreases, the simple temperature control can cause the LED current increases. Figure 2 shows the ambient temperature of the LED's typical response requirements. Figure 2: LED typical response of the ambient temperature requires the application of LED in automotive applications, LED is mainly used for external and internal lighting. Exterior lighting equipment and EMC problems involving thermal limits, as well as many of the complex load testing unloading standard, for example, the voltage is 40V, 60V, 80V, or 100V LED drive circuit must meet the stringent requirements of automotive EMC specifications. For through efficient, inductive switching regulator drives the LED, there is a certain degree of difficulty meeting the above requirements, all precautions will increase the overall cost of the program. In exterior lighting applications, headlights, fog lights and indicator is the target application. Since the LED has the following benefits can be widely used to create LED interior comfort, such as instrument type lights, pedal lights, map lights, rear fog lamps, rear brakes and lights, and for the show car "entertainment information "flat panel display backlighting color mixing and mood lighting LED applications will further increase. Zetex LED products for simple switch LED load BSP75 device is equipped ESD protection, overheating, load sags, over-current and over-voltage protection, reliable FET, suitable for currents up to 1.2A LED on the switch. If you need to monitor the current (such as display LED light clusters brake failure), Zetex current monitor ZXCT1081 can adapt to 60V voltage transients and 125 ℃ operating temperature. For automotive LED control, Zetex offers a range of switching regulator, drive current up to 1A. ZXLD1350, ZXLD1360 and ZXLD1362 is a hysteretic buck converter can drive 3W LED 1W LED strings and strings. These devices use a few components to ensure reliable control of LED, and can be a single pin, achieve closure, soft start or PWM dimming and temperature control and other functions

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