At present, there is a big difference in market forecasts for high-brightness LEDs (light-emitting diodes). Although the forecast data is different, the trend is clear: the high-brightness (HB) LED market is growing at an alarming rate. Some forecast data have a compound annual growth rate of 15%, while others have a compound annual growth rate of 35%. In 2007, LED lighting in the automotive sector was in its infancy, with sales of US$670 million, but is expected to grow at a compound annual growth rate of 15%, reaching US$1.2 billion in 2011 (1). If non-automotive applications are taken into account, the compound annual growth rate is close to 35%, and by 2011, the total high-brightness LED market is likely to easily exceed $2.5 billion.
What drive is this high growth potential? First, LEDs produce light 10 times more efficiently than incandescent lamps and almost twice as much as fluorescent lamps, thus greatly reducing the amount of electrical power required to deliver a specific amount of light output (in lumens). Second, incandescent lamps need to be replaced after 1,000 hours of use. Fluorescent lamps can last for 10,000 hours, while LEDs last for more than 100,000 hours. In most applications, this allows the LED to be permanently embedded in the final application without the need for an attachment. Examples include display screens for car bodies or LCD screens for HDTVs, as these LEDs do not need to be replaced during the life of a car, HDTV or sports stadium display. Third, LEDs are orders of magnitude smaller than other lighting products, and the form factor can be made very flat so that they can be permanently embedded in automotive interior and exterior applications. In addition, countless colors can be achieved by using red, green and blue LED configurations. Finally, the LEDs are dimmable, and their turn-on and turn-off times are fast and undetectable by the human eye, which greatly improves the backlighting of HDTVs and other types of displays. Without these LEDs, it is impossible to have a high-resolution LCD HDTV with extremely high contrast and sharpness.
Still, one of the biggest challenges for lighting system designers is how to take advantage of all the benefits of the latest generation of LEDs. Because they typically require an accurate DC current source and require dimming, the LED driver IC must be designed to meet the needs of a variety of applications. Power solutions must be efficient, reliable, and as always, very compact and economical. There is reason to believe that one of the most demanding LED driver applications is in automotive applications, including LED-based interior lighting, LCD display backlighting, brake/steering indicator lights, and even headlights.
Automotive LED lighting
The advantages of small size, low power consumption, and short turn-on time have made high-brightness LEDs widely available in today's automobiles. The original LED application was the Central High Brake Light (CHMSL), which uses red LEDs to provide a very small array of illumination that is easy to install and never needs to be replaced.
Figure 1: Audi R8 / Lexus LED headlights / turn signal lights / driving lights.
Traditionally, incandescent lamps are the most economical source of light, and many cars are still in use. However, the reduced space available for lighting components and the need for long working life, combined with LEDs for bright natural colors and simplified design, have quickly replaced incandescent lamps in many applications. White LED arrays replace traditional cold cathode fluorescent lighting (CCFL) TFT-LCD backlighting is becoming more and more common. White LEDs provide more accurate and adjustable backlighting, and the working life will easily exceed the life of the car itself. Even the headlights, which are primarily used in halogen/氙 filament design applications, are gradually adopting electronic “controllable†high-current LED arrays (see Figure 1). All automotive lighting applications, including internal/external and backlighting applications, are almost turning to LEDs. There are several advantages to using LEDs in this environment. First, they never need to be replaced because their reliable life span of more than 100,000 hours (11.5 years of service) exceeds the life of the car itself. This allows automakers to permanently embed them in "in-cabin" lighting systems without the need for a corresponding design for easy replacement. The style can also vary greatly, as the LED lighting system requires less depth and area than incandescent lamps.
Figure 2 shows the various LED lighting applications in modern cars. Inside the car, there are several “standard built-in†lighting modules that use various types of LEDs. Some are single LEDs, while others such as navigation dashboard backlighting require an LED array. LEDs are also recognized for external lighting. More than 40% of central high-position brake lights now use red LEDs. In addition, the Audi 2008 A8 uses a high current LED array as the daytime running light (DRL). All external “forward lighting†including the Lexus 600 sedan and the Audi R8, including the headlights, are done by LEDs. Similarly, even more mid-range cars and many motorcycles use color LED arrays as brake/turn signal indicators.
Figure 2: Typical application of LED lighting in modern cars.
Automotive LED lighting design parameters
Table 1: Forward voltage drop and drive current for high current white LEDs.
To ensure optimum performance and long life, LEDs require an efficient drive circuit. These drive circuits must be able to adapt to the most demanding automotive power bus and be both economical and space efficient. To ensure a longer life, the first thing is to not exceed the current and temperature limits of the LED. Table 1 shows the typical forward voltage and drive current for high current white LEDs.