Green Technology: How to power an energy-efficient light

Tag : Filament Transformer

It isn’t just more-efficient lighting that is driving downenergy bills. Compact fluorescent lamps (CFLs) and LEDswouldn’t be found in many light sockets today if itweren’t for electronics able to economically drive thesebulbs. And there are still lessons being learned about how to getthe cost out of illumination systems.
Take fluorescent lighting, for example. The old-time ballastspowering fluorescent tubes were little more than transformers thatenergized the tube gas by applying a high voltage to heat thefilaments. The ballast also serves as a current limiter when thelamp is on. The problem with old-style ballasts was one of bothbulk and inefficiency.
CFLs only took off with the advent of electronic-ballast circuitsthat were both economical and compact enough to fit in the base ofa lamp holder. Today’s CFL drivers are basically switch-modepower-supply circuits that include power-factor correction andprotection against such conditions as shorts and openbulbfilaments. These use switching circuitry instead of transformers togenerate the high voltages (about 500 V) that initially energizefluorescent lights and the lower voltages (about 200 V) thatsustain lamp operation. Fluorescent bulbs are most efficient whenoperating at the 20 kHz and higher frequencies that electronicswitchers generate. Operation at higher frequencies also letsballast components be physically smaller and makes for amore-compact package.
It isn’t just CFLs that have electronic ballasts. Linearfluorescents have gone electronic as well. As of 2006, DOEregulations dictated what are called ballast-efficacy ratings— basically a measure of energy efficiency. The ratings aresuch that transformer-style ballasts aren’t efficient enoughfor many of the most common fluorescents used in shop and factorylighting. In the same year, the EU banned all magnetic ballasts,forcing a move to electronic ballasts for fluorescent bulbs soldthere.
Ballasts may be going electronic but not all of them have the samelevel of integration. Some manufacturers still design their own.“Cost has been a barrier to the use of singlechipballasts,” says Fairchild Semiconductor Director of Marketing Claudia Innes. But there are subtleties todriving a fluorescent bulb that can be a learning process for somemanufacturers. “Compared to powering an incandescent bulb,you have to account for more conditions and provide safety featuresfor different kinds of failures,” she says. “A lot ofdesigners don’t know how to do this. So theelectronic-ballast chips build in a lot of failure protection tomake sure a problem doesn’t damage the entire ballast.”
For example, lamp impedance changes with age. This can move theoscillation frequency away from its most-efficient operation point.To check for faults, ballast circuits must watch the crest factor(the ratio of peak to rms current). A crest factor exceeding fourgenerally indicates the lamp is at its end of life.
Dimming is another issue. Ballast circuits usually adjust avoltage-controlled oscillator to dim CFLs, but “If you put adimmable CFL next to a dimmed incandescent, you’ll noticethey don’t dim to the same extent and they don’t dimthe same way. From a design point of view, there are several morethings you have to account for,” says Innes.
A typical electronic ballast first rectifies ac, then converts theresulting dc to a signal in the range of 50 kHz through a MOSFET orIGBT switch. This switching action can generate harmonics in thecurrent and voltage. These distortions cause radiated interferenceand put a damper on efficiency. So electronic ballasts generallyincorporate power-factor-correction (PFC) circuits to compensate.PFC chips basically keep the switch on time at a fixed relationshipto the input line voltage so the load appears resistive to the acline.
A ballast-control chip then handles preheating and ignition,watches for conditions that indicate an open filament, andimplements zero-voltage switching of the final high-voltage stage.The high-voltage stage that actually connects to the lamp isusually a half-bridge powering either MOSFETs or IGBTs.
Whether to use one or several chips to implement these functionsoften depends on how manufacturers view trade-offs between the costof components versus the total system. “Every connection is apoint 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. Thatall changed in 2006 with the advent of superbright LEDs. “Nowit takes under 100 LEDs to generate the equivalent to ahigh-pressure sodium light,” says Cree Inc . Director of Business Development Mark McClear.
Key to this turn of events was CREE’s development of itsEZBright LED power chip. Since then, other manufacturers havebrought out versions of high-output LEDs. But CREE has come up witha new LED topology that it says is more efficient than earlierchips by a factor of two and figures it is perhaps a year ahead ofits closest competitors.
Current research by LED makers focuses on bettering powerefficiency and lumens/ dollar spent. Today these figures are about100 lumens/W and 40 lumens/dollar. Expectations are that the year2010 will see 150 lumens/W with costs down substantially.“Every time we improve efficiency, it makes possible anotherwave of new applications,” says McClear.
It turns out that the benefits of LEDs are not limited toefficiency. “It costs a municipality as much to change a bulbas to buy a new lamp. Because LEDs last two to five times longerthan incumbent bulbs, they avoid a lot of maintenance costs,”says McClear. And there is a sleeper benefit to using them foroutside lighting: “When you replace a yellow sodium lightwith LEDs, people think you have cleaned up the place,”McClear says. “That’s because the eye has more visualacuity in the LED’s light range. Surveillance cameras workbetter with LED light and people actually feel safer in parkingdecks 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 aspecific output. The firm says its design is protected by over 20patents.
“Our biggest issue was thermal management. Once we solvedthat, we optimized the optical design to get the most out of theLED,” says Beta Sales Director Kevin Orth.
Though LED-powered streetlamps are more expensive than theconventional 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 tobe 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 wantto drive 100 LEDs to get the maximum amount of light, there aremany ways to do it — so many, in fact, that a lot of peopleget 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 drivethe LEDs in series from a dc supply. The problem with this approachis that it can involve voltages high enough to be classified ashazardous by UL. The high-voltage components involved can beexpensive. “It is okay if you really understand all thesafety codes and are willing to double insulate and isolate. But itgets ugly in terms of safety testing and I don’t recommendit,” says Richardson.
A second slightly different approach also uses a singlestage powersupply but incorporates galvanic isolation, usually in the form ofa transformer. This gets around some of the safety issues and hasthe advantage of availability as commercial off-the-shelf units.The problem is that this approach is only practical for drivingstrings of about eight LEDs at most, says Richardson. “Youmight produce at most 1 A this way,” he explains. “Itis expensive because you pay a premium for the engineering thatgoes into the supply.”
The third way is the most widely used. It employs a commercialac/dc converter that produces an output below 60 V, thus stayingbelow hazardous voltages. The output goes to multiple dc/dcconverters, each driving an LED string. Besides avoiding dangerousvoltage levels, the approach guarantees some of the LEDs stay litin the event one fails open.
“You need more engineering time to design this sort ofcircuit, but the result is the most flexible and reliable of thethree possibilities,” says Richardson. By eliminating theneed to work at high voltages, it may also be the least difficultto realize for most engineering staffs. “I haven’t metmany power-supply engineers well versed in both high-voltage ac andlow-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