Electronic Design

LED Illumination Provides Constant (Current) Opportunities

What do ac-dc power supplies, lighting ballasts, and driver ICs have in common? They all represent immediate growth opportunities for new designs based on current regulation, specifically for various applications in LED lighting. I’m not talking about general illumination applications producing white light, as LEDs are only beginning to be adopted there, and opportunities are developing differently. But if you look toward signs, billboards, and specialty applications for color lighting, you’ll see growing markets for LEDs.

Current regulation requires different loop control and new designs. It generally isn’t possible to adopt a conventional voltage regulation type of ac-dc power supply (the most common form of ac-dc power conversion) for the new current-driven systems. Also, while voltage-sourcing power supplies are typically required to provide a constant, well-regulated output voltage, current-sourcing power supplies are often required to supply varying current levels (e.g., for dimming).

In addition to changes in the control loop and overall design, the new applications demand different communications protocols. The types of (mostly digital) systems that employ ac-dc voltage regulation also use communications protocols such as I2C or PMBus. In the case of lighting systems that need current regulation, communications such as DALI or CAN bus are more common.

Channel letter lighting is an excellent example. The use of LEDs has surpassed neon in these signs. Red, green, yellow, and other color LEDs bring a number of important advantages to this segment of the sign market. The two most obvious are lower energy consumption (up to 80% more efficient than neon) and longer operating lives compared with neon or other illumination alternatives.

Also important is the greater flexibility offered by strings of LEDs compared with rigid neon tubes. The LED strings are much more rugged than the tubes as well. LEDs have a faster turn-on and turn-off time compared to neon for more dynamic displays, and ease of dimming and control favor LEDs over neon. Not only do LEDs use safety extra-low-voltage (SELV) power sources (12 to 24 V dc typically) compared with 15 kV for neon, they also enable new powering architectures and improved mechanical designs for the signs.

With high-voltage neon, the power source must be literally attached to the letters. It is not generally safe to run high-voltage power over significant distances. On the other hand, the SELV power sources used with LEDs can be located at a convenient location (such as a nearby ac outlet), and a relatively small gauge (e.g., 16 to 18 AWG) wire can carry the power safely and efficiently to the sign.

What’s Next

The emergence of ac-dc power supplies for LEDs will require power-supply makers to develop new designs. In addition, many of the initial applications for LED illumination are in outdoor applications such as signs. This will demand rugged, industrial-grade power supplies. LED power-supply designs differ from computer-grade power supplies in three important ways: they must regulate output current, they must be sealed against the elements, and they must be more thermally rugged. LED lighting provides for greater resistance to shock, vibration, and wear, significantly increasing its lifespan.

While various types of signs present the best opportunity today for the constant-current power demanded by LEDs, general illumination applications will be the largest opportunity in the long run. In fact, the infrastructure to support large-scale deployment of LED lighting is being developed today by the EMerge Alliance, which was established in late 2008.

“The EMerge Alliance was established to promote the rapid adoption of safe, low-voltage dc power distribution and use in commercial building interiors,” the organization says. “EMerge is focused on developing a global standard that integrates interior infrastructures, power, controls, and a wide variety of peripheral devices, such as lighting, in a common platform.”

While a lot of attention is being paid to the potential use of high-voltage dc distribution in large facilities such as data centers, there are even larger opportunities for the use of low-voltage dc distribution as part of a hybrid ac and dc power structure for industrial, commercial, government, and even residential buildings.

The use of dc distribution can complement other trends in building power, including the growth of “green” energy sources, the use of wireless building automation systems, demand-side management, the implementation of high-efficiency lighting, and more. And, it can reduce construction and operating costs, improve flexibility, and enhance sustainability.

According to Don Mulvey, executive vice president with Roal Electronics USA, dc-powered lighting systems will also be significantly more reliable than today’s ac-powered systems.

“We compared the expected reliability of a lighting system with ac distribution versus a lighting system with dc distribution. It was a real eye-opener and showed through simple statistics the benefits of dc distribution as related to predicted MTBF (mean time before failure) of electronics,” Mulvey says.

Of course, if you’re running dedicated low-voltage dc distribution wiring in a building for lighting applications, you probably would prefer not to run separate wires down to individual switches. Those added wires would add cost and take time to install. A better solution would be wireless lighting controls. Complementing emerging dc power distribution technologies are wireless building automation technologies such as the ZigBee wireless communications protocol, mesh networking architectures, energy-harvesting technologies, thin-film batteries, and other technologies, making them even more valuable and easier to install.

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