Author: Colin Davies
Global application manager
Zetex Semiconductors
Undoubtedly, the use of high-brightness LED lighting will become a major feature of future cars, thanks to many of the basic advantages of LEDs over traditional incandescent lighting solutions. In addition, the use of LED lighting can also drive changes in automotive design techniques and design styles. However, just like any innovative technology, LEDs still have to overcome many difficulties before they are widely used in automotive lighting.
Key characteristics
1. Reliability and service life
The expected life of the LED is 50,000 hours, while the tungsten halogen lamp is 20,000 hours and the tungsten incandescent lamp is 3,000 hours. Compared with incandescent lamps, LEDs are structurally strong and are not susceptible to vibration. The brightness of the light output during use is not significantly reduced. Lighting solutions based on multiple LEDs also have the "redundancy" benefit, and even if one LED fails, the lighting can continue to be used. Proper use of LEDs (especially the temperature of the LEDs properly controlled) can effectively extend the life expectancy of the LEDs. Conversely, if the temperature is too high, the LED is easily damaged. LED applications also involve many legal definition issues in automotive lighting. Most countries have a clear definition of brake lights or headlights - lights on or off. However, for lamps with multiple LEDs, it is difficult to accurately define whether the lamps have been damaged. Manufacturers and legislatures are defining ways to use LEDs.
2. Efficiency / lumens per watt
Compared to standard incandescent lamps, LEDs consume more light output per unit of electrical energy. However, the advantages of the actual light output of the LED are not significant when compared to halogen lamps. The latest LEDs have excellent lumens per watt, but some values ​​are obtained under optimized conditions and are usually not obtained under the highest output conditions. In general, when the current of the LED increases, the amount of light output does not increase linearly. Therefore, even if the LED outputs x lumens at a current of 0.5 A, it does not output 2x lumens at 1.0 A.
3. Response speed
Take the brake light and the direction indicator tube as an example. If the vehicle has a speed of 125 km/h, that is, 35 m/s, the hot start time of the incandescent lamp is about 250 milliseconds, and the fast-reacting LED can be sent about 8 meters earlier. Brake warnings to effectively avoid car collisions. The same is true for the indicator light.
4. Directionality
Another key feature is the way LEDs are illuminated. Unlike incandescent lamps, LEDs emit light through only one surface, which is good for headlamps and aeronautical light applications, but may not be suitable for other lighting applications for car lighting (Figure 1).
Figure 1: Unlike incandescent lamps, LEDs emit light with directivity.
Method of controlling LED
Current control
A fundamental problem with LEDs is that LEDs are current controlled devices with relatively low voltage drops. The easiest way is to use a resistor to limit the current of the LED, but this method is not suitable for systems with a rated voltage of 12V or 24V, because the actual voltage of the battery is from 6V to 18V or 12V to 36V. Therefore, if it is necessary to maintain brightness, constant current control must be performed.
2. Linear control of current
Linear control refers to the constant current held by the LED through the linear regulator. Linear control is inefficient in some cases. For example, a single 1A (3W) LED with a forward voltage of 3.5V requires the regulator to reduce the rated 12V supply to 8.5V while maintaining 1A, thus using 3W LEDs. A power of 8.5W will be wasted. Linear current control is the least noisy technique, and from the EMC point of view, linear current control is the safest.
3. Switching regulator
Inductive switching constant current technology produces more electronic noise, but it is more efficient. Depending on the number of LEDs used, a buck or down/boost regulator can be used.
4. EMC issues
Radiated and conducted noise must be minimized to keep the noise within acceptable limits. Although the frequency of the PWM method is fixed and relatively easy to filter, since the LED load is relatively stable, the hysteresis controller and the PFM are suitable choices if appropriate measures are taken. The trend in switching regulators is that the frequency will be higher to reduce the size of the inductor/capacitor. This is always the best solution for automotive applications. Keeping the frequency low helps to avoid interference problems.
The “jitter†or “expansion†technique of the fundamental frequency does help to meet similar peak EMC testing requirements, but the best approach is to not generate any radiation, which is difficult to achieve with any switching regulator.
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Radiant heat, conducted heat and heat management
One of the key issues and biggest challenges for users of high-brightness LEDs (especially the automotive industry) is the self-heating of LEDs. The lumens per watt of LEDs have been greatly improved, but in fact most of the electrical energy of LEDs is converted into conduction heat. LEDs produce less radiant heat for compartment lighting, but in cold climates, the radiant heat of the headlights effectively melts the snow on the lens. Therefore, thermal management is the key to reliable LED control.
Thermal management mainly refers to reducing current when temperature increases. The advantage of using a high-brightness LED is that the eye does not perceive a change in brightness when the current changes greatly. In general, the current is reduced by 25%, and the brightness of a single LED does not change significantly.
However, LEDs change color with changes in temperature and current, and whether this will affect automotive lighting applications remains to be explored. Whether the spectrum of the LED is suitable for illumination and whether it affects the driver's sense of distance under normal night vision effects may be more important.
Using the PWM method to reduce the brightness ratio instead of the DC control, a larger ratio of light to dark can be obtained, and the color temperature does not change. Therefore, it is better to use the PWM method to reduce the brightness. However, the choice of frequency is also important. It is generally considered that the frequency is 200 Hz, because the human eye does not feel the flicker of 200 Hz light, and the lower frequency ensures that the switching frequency is lower than that of the switching regulator. However, the potential problem of stroboscopic effects in headlamps must be foreseen. A more appropriate method is to use a higher frequency to adjust the brightness of the LED to avoid the "yaw" effect. In addition, the inductor must be carefully selected to avoid audible noise in the car.
The temperature sensing of LEDs is also a problem that needs to be solved. Thermistors are widely used, but care must be taken when using thermistors. The temperature control response should be set to the upper temperature limit for the current that the LED needs to reduce. When the ambient temperature is lowered, simple temperature control can cause the current of the LED to increase. Figure 2 shows the typical response of an LED to ambient temperature.
Figure 2: Typical response requirements for LEDs to ambient temperature.
LED application range
In automotive applications, LEDs are primarily used for exterior and interior lighting. External lighting equipment involves thermal limits and EMC issues, as well as many complex standards for unloading load testing. For example, drive circuits with voltages of 40V, 60V, 80V, or 100V LEDs must meet the stringent requirements of automotive EMC specifications. For LEDs driven by high efficiency, inductive switching regulators, it is difficult to meet the above requirements, and all precautions will increase the cost of the overall solution. In exterior lighting applications, headlights, fog lights and indicator lights are targeted applications.
Due to the advantages of LEDs, LEDs can be widely used to create a comfortable environment in the car, such as instrument lighting, pedal lighting, aerial lights, rear fog lights, rear brakes and lights, and used to display car entertainment. The "color" backlighting and ambient lighting of the flat panel display will further increase the application of LEDs.
Zetex LED products
The BSP75 device for simple switching of LED loads is a reliable FET with ESD protection, overtemperature, load sags, overcurrent and overvoltage protection for switching LEDs up to 1.2A.
If it is necessary to monitor the current at the same time (such as the failure of the cluster of brake LEDs), Zetex's current monitor ZXCT1081 can accommodate 60V voltage transients and 125°C operating temperature. For automotive LED control, Zetex offers a range of switching regulators with drive currents up to 1A. The ZXLD1350, ZXLD1360 and ZXLD1362 are hysteresis step-down converters that can drive 1W LED strings and 3W LED strings. These devices ensure reliable LED control with few components and can be turned off, soft-started or PWM dimmed and temperature controlled from a single pin.
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