Green Technology: How to power an energy-efficient light

May 4, 2009 4:01 PM,

Leland E. Teschler

New lamps are more efficient, but what about the electronics? Designers need to learn a few new tricks about driving LEDs and CFLs.

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Whether to use one or several chips to implement these functions often depends on how manufacturers view trade-offs between the cost of components versus the total system. “Every connection is a point of failure and every component picked-and-placed has a cost. Still, some people design their own,” says Innes.

Spotlight on LEDs
Two years ago, there was no such thing as an LED streetlight. That all changed in 2006 with the advent of superbright LEDs. “Now it takes under 100 LEDs to generate the equivalent to a high-pressure sodium light,” says Cree Inc. Director of Business Development Mark McClear.

CREE Inc. invented its superbright LED architecture in 2006 and the rest is history. LEDs are now going into street lights, office lighting, and other general illumination uses where reduced maintenance and energy costs offset their higher price tag.

Key to this turn of events was CREE’s development of its EZBright LED power chip. Since then, other manufacturers have brought out versions of high-output LEDs. But CREE has come up with a new LED topology that it says is more efficient than earlier chips by a factor of two and figures it is perhaps a year ahead of its closest competitors.

Current research by LED makers focuses on bettering power efficiency and lumens/ dollar spent. Today these figures are about 100 lumens/W and 40 lumens/dollar. Expectations are that the year 2010 will see 150 lumens/W with costs down substantially. “Every time we improve efficiency, it makes possible another wave of new applications,” says McClear.

It turns out that the benefits of LEDs are not limited to efficiency. “It costs a municipality as much to change a bulb as to buy a new lamp. Because LEDs last two to five times longer than incumbent bulbs, they avoid a lot of maintenance costs,” says McClear. And there is a sleeper benefit to using them for outside lighting: “When you replace a yellow sodium light with LEDs, people think you have cleaned up the place,” McClear says. “That’s because the eye has more visual acuity in the LED’s light range. Surveillance cameras work better with LED light and people actually feel safer in parking decks illuminated with LEDs.”

Several lamp makers now make outdoor fixtures incorporating LEDs. One of these, Beta Lighting in Sturtevant, Wis., employs CREE LEDs configured as light bars, each containing 20 LEDs. Beta adds light bars to get fixtures of a specific output. The firm says its design is protected by over 20 patents.

“Our biggest issue was thermal management. Once we solved that, we optimized the optical design to get the most out of the LED,” says Beta Sales Director Kevin Orth.

National Semiconductor recommends driving banks of LEDs by means of a basic setup where LEDs are divided into substrings, each powered by a separate driver. This keeps the voltage across the LEDs below that considered hazardous and which would demand special insulation and safety measures. In addition, a single LED going open circuit would not kill the entire light output.

Though LED-powered streetlamps are more expensive than the conventional lights they replace, they cost less to own, Orth says.

How to do Drivers
LEDs may be the wave of the future, but there doesn’t seem to be a consensus about how best to configure their source of power. “So far, there is no set topology for driving LEDs,” says National Semiconductor Corp. Senior Application Engineer Chris Richardson. “If you want to drive 100 LEDs to get the maximum amount of light, there are many ways to do it — so many, in fact, that a lot of people get intimidated by the task.”

There are three general approaches to driving banks of LEDs today, Richardson says. The first, and most efficient, is to simply drive the LEDs in series from a dc supply. The problem with this approach is that it can involve voltages high enough to be classified as hazardous by UL. The high-voltage components involved can be expensive. “It is okay if you really understand all the safety codes and are willing to double insulate and isolate. But it gets ugly in terms of safety testing and I don’t recommend it,” says Richardson.

A second slightly different approach also uses a singlestage power supply but incorporates galvanic isolation, usually in the form of a transformer. This gets around some of the safety issues and has the advantage of availability as commercial off-the-shelf units. The problem is that this approach is only practical for driving strings of about eight LEDs at most, says Richardson. “You might produce at most 1 A this way,” he explains. “It is expensive because you pay a premium for the engineering that goes into the supply.”

The third way is the most widely used. It employs a commercial ac/dc converter that produces an output below 60 V, thus staying below hazardous voltages. The output goes to multiple dc/dc converters, each driving an LED string. Besides avoiding dangerous voltage levels, the approach guarantees some of the LEDs stay lit in the event one fails open.

“You need more engineering time to design this sort of circuit, but the result is the most flexible and reliable of the three possibilities,” says Richardson. By eliminating the need to work at high voltages, it may also be the least difficult to realize for most engineering staffs. “I haven’t met many power-supply engineers well versed in both high-voltage ac and low-voltage dc,” says Richardson.

Make Contact

Beta LED, (800) 236-6800, betaled.com
CREE Inc., (919) 313-5300, cree.com
Fairchild Semiconductor,
(207) 775-8100, fairchildsemi.com
National Semiconductor, (800) 272-9959, national.com