Green Technology: How to power an energy-efficient light

It isn’t just more-efficient lighting that is driving down energy bills. Compact fluorescent lamps (CFLs) and LEDs wouldn’t be found in many light sockets today if it weren’t for electronics able to economically drive these bulbs. And there are still lessons being learned about how to get the cost out of illumination systems.

Take fluorescent lighting, for example. The old-time ballasts powering fluorescent tubes were little more than transformers that energized the tube gas by applying a high voltage to heat the filaments. The ballast also serves as a current limiter when the lamp is on. The problem with old-style ballasts was one of both bulk and inefficiency.

A typical electronic ballast for driving a compact fluorescent lamp today takes the form of a switch-mode power supply. A power-factorcontrol chip is used in the first stage of switching to make ensuing stages appear resistive to the ac line, thus reducing energy loss from harmonics. Bulb failure-mode detection takes place through use of a ballast-control chip which, in turn, drives the final stage of high-voltage switching at about a 50-kHz rate to power the lamp.

CFLs only took off with the advent of electronic-ballast circuits that were both economical and compact enough to fit in the base of a lamp holder. Today’s CFL drivers are basically switch-mode power-supply circuits that include power-factor correction and protection against such conditions as shorts and openbulb filaments. These use switching circuitry instead of transformers to generate the high voltages (about 500 V) that initially energize fluorescent lights and the lower voltages (about 200 V) that sustain lamp operation. Fluorescent bulbs are most efficient when operating at the 20 kHz and higher frequencies that electronic switchers generate. Operation at higher frequencies also lets ballast components be physically smaller and makes for a more-compact package.

It isn’t just CFLs that have electronic ballasts. Linear fluorescents have gone electronic as well. As of 2006, DOE regulations dictated what are called ballast-efficacy ratings — basically a measure of energy efficiency. The ratings are such that transformer-style ballasts aren’t efficient enough for many of the most common fluorescents used in shop and factory lighting. In the same year, the EU banned all magnetic ballasts, forcing a move to electronic ballasts for fluorescent bulbs sold there.

Ballasts may be going electronic but not all of them have the same level of integration. Some manufacturers still design their own. “Cost has been a barrier to the use of singlechip ballasts,” says Fairchild Semiconductor Director of Marketing Claudia Innes. But there are subtleties to driving a fluorescent bulb that can be a learning process for some manufacturers. “Compared to powering an incandescent bulb, you have to account for more conditions and provide safety features for different kinds of failures,” she says. “A lot of designers don’t know how to do this. So the electronic-ballast chips build in a lot of failure protection to make sure a problem doesn’t damage the entire ballast.”

For example, lamp impedance changes with age. This can move the oscillation 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 four generally indicates the lamp is at its end of life.

Dimming is another issue. Ballast circuits usually adjust a voltage-controlled oscillator to dim CFLs, but “If you put a dimmable CFL next to a dimmed incandescent, you’ll notice they don’t dim to the same extent and they don’t dim the same way. From a design point of view, there are several more things you have to account for,” says Innes.

A typical electronic ballast first rectifies ac, then converts the resulting dc to a signal in the range of 50 kHz through a MOSFET or IGBT switch. This switching action can generate harmonics in the current and voltage. These distortions cause radiated interference and put a damper on efficiency. So electronic ballasts generally incorporate power-factor-correction (PFC) circuits to compensate. PFC chips basically keep the switch on time at a fixed relationship to the input line voltage so the load appears resistive to the ac line.

A ballast-control chip then handles preheating and ignition, watches for conditions that indicate an open filament, and implements zero-voltage switching of the final high-voltage stage. The high-voltage stage that actually connects to the lamp is usually a half-bridge powering either MOSFETs or IGBTs.