Throughout the history of automobiles light sources have always played an important role. Originally cars merely had front lights to find a way in the dark. Soon such other light sources as taillights, indicators, etc., were added for security and better organization in growing traffic. Warning and fog lights increased functionality for extreme conditions. Interior light sources such as cluster, dome, map or courtesy lighting added convenience for the driver and passengers.
Except for the earliest models, these light sources were electrically powered and used incandescent light bulbs. Only in recent years have such other light sources as light emitting diodes (LED) and cold cathode florescent lamps (CCFL) become common light sources in automobiles. The key advantages of these new light sources are long lifetime and improved efficiency. Typical life spans of incandescent bulbs are specified at 10,000h while florescent bulbs can reach 50,000h and LEDs 100,000h.
This extended lifetime is a clear improvement of reliability and necessary maintenance effort, which ultimately reduces service costs. Other LED advantages include low voltage operation, low electromagnetic radiation, an immunity to mechanical stress, thinner form factors, and wide operating temperature range and wide dimming range. With the improvements in LED technology it seems that eventually this technology will replace the others. Currently, however, CCFL definitely still has clear advantages in certain applications like backlighting of large areas and wherever high power unfocussed light is needed.
CCFL and LED light sources need a power supply, and each technology has its special requirements. The power supplies must also perform additional application specific functions. This article presents power supply solutions for LED and CCFL light sources. Design approaches are discussed for various automotive interior and exterior applications.
Let there be light: Interior lighting
Interior lighting applications include cluster or instrument backlighting, dome or map reading lights, courtesy lights at doors or in the trunk and display backlighting. Since display backlighting has special requirements it will be discussed separately later in the article.
All the interior light applications can use LEDs as a light source. While map and dome lighting use one high-brightness LED, cluster and courtesy lighting fixtures normally require more than one LED connected in series. The series connection is preferred to avoid current (and therefore color) mismatch between different LEDs. All applications need a regulated constant current source with an integrated dimming function. Just such a combination of features is available in a chip such as the MAX16800 from Maxim, which incorporates all the control and current sourcing circuits (Figure 1a).
Figure 1a. Functional diagram of MAX16800 dimming controller
Figure 1b. : Typical dimming control circuit implementation using the MAX16800. It operates over a 6.5-to-40-V input voltage range and delivers up to a total of 350 mA to one or more strings of high-brightness LEDs
