Supplier eBooks

In 2019, LED lamp sales in the U.S. reached more than 10 billion units. LEDs typically available for residential lighting have reached an efficacy of around 120 lumens per watt (lm/W). LED lamps are now available off the shelf with 220lm/W, taking about 1/12 of the power required by a filament bulb. Since 2010, the average efficacy of LEDs has improved by 6lm/W to 8lm/W each year, but that has been slowing recently. For vehicle lighting, flexibility and long-term reliability must be considered. However, engineers need to be careful with their designs. Automotive exterior or interior lighting designers need to walk somewhat carefully through the wonderland of LEDs. LED lamps are current-driven devices that are substantially affected by changes in operating conditions, such as voltage and temperature. LEDs used in the automotive environment are, of course, subject to a wide range of operating conditions, and they can degrade or permanently fail before their expected lifetime is complete. Their lifetime, if you are careful, might be 75,000 hours—almost nine years of continuous use and many times that for your typical vehicle operating scenario of only a few hours per day. But lifetime can be very much reduced unless the design engineer keeps a close eye on a few important factors. LED drive circuits fail far more often than the lamps themselves. LED taillights first appeared on automobiles in 1993 but did not see really wide adoption until around 2001. But that's 20 years, so many cars, SUVs, and trucks all have LED tail, stop, side, and turn signal lamps. If you're paying close attention, you might have noticed that a lot of cars are running around with dead taillights. All the traffic stoplights have long been converted to LED. But, if you look closely, many of those lights have dead spots that seemed to be getting better in the past two years. What gives? The reason for most of these malfunctions is the failure of the LED drive circuitry, not the lamp. If you buy an extended lifetime home LED bulb at the hardware store, you are getting a better power supply—with the same LED. LEDs are very reliable. One study found that of all automotive lamp failures, 60 percent were because of the power supply/driver, and 25 percent are caused by the circuit board or connections. LED light deprecation is another matter. LED lamps do degrade over time, and higher temperatures will aggravate that. Here is where quality semiconductor fabrication comes into play. Wafer fabrication is a tricky business. No matter how skilled manufacturers become, it is not possible to produce LED dies free of defects in the bulk crystal lattice. 14 led FAiluRe meCHAniSmS While the old incandescent or halogen or HID bulbs worked and worked and then, suddenly, stopped working, LED lamps tend to just fade away. The LED lamps main "failure" mode is a slow light output deprecation.

• Cover