Bright future for energy efficient lighting

Leland Teschler
Executive Editor

Get ready for new rules from the Dept. of Energy. Next year, DOE will implement regulations effectively making it impossible to equip some kinds of fluorescent lamps with transformer-style ballasts.

The DOE regs have energy consumption in mind. Lighting accounts for about 25% of all U.S. electrical power. No wonder, then, the DOE wants lamps to get more efficient. So its new regulations dictate 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.

The regulations apply mainly to longtube fluorescent lamps. They don't apply to low-power-factor ballasts as used in residential settings. Nevertheless, the handwriting is on the wall. Lamps of all kinds will have to be more efficient in the future.

This future is coming quickly in Europe. The EU will ban all magnetic ballasts in 2006, forcing a move to solidstate ballasts for fluorescent bulbs sold there.

Small wonder the emphasis on energy efficiency has fostered more interest in solid-state ballasts. These use switching circuitry instead of transformers to generate the high voltages (about 500 V) needed to initially energize fluorescent lights and the lower voltages used to sustain lamp operation. In so doing they can provide energy efficiency by controlling the lamp waveform more precisely. One drawback to the electronic approach, however, has been higher cost.

Recent developments indicate that the cost differential may soon be a thing of the past. International Rectifier, a maker of integrated circuits with U.S. offices in El Segundo, Calif., recently developed a chip designed specifically to handle ballast functions for compact fluorescent (CFL) bulbs. (The term CFL applies to families of smaller-diameter fluorescent lamps designed as replacements for incandescent bulbs.)

Though energy efficiency was one motivation for IR's new chips, ballasts using the ICs will also be more reliable, space efficient, and cost less than older solid-state devices. The company has also devised new ICs targeting ballasts for halogen lights. And high-intensity discharge (HID) lamps may eventually get ballast ICs using similar technology, say IR officials

INSIDE A LAMP
Fluorescent lamps consist of a tube filled with inert gas and a small amount of mercury vapor. The ballast energizes the gas by applying a high voltage to heat the filaments. Once the mercury starts to conduct, 200 V or less is enough to maintain it. When the lamp is on, the ballast also serves as a current limiter.

The mercury produces most of its energy in the ultraviolet (UV) range. The UV activates a white phosphor coating on the tube, which then emits visible light. Fluorescents are physically larger than incandescents of the same light output because it takes more surface area to emit an equivalent amount of light. Fluorescents are more efficient, however. Incandescent bulbs produce about 15 lumens/W, Energy Star-qualified CFLs, 40 to 60 lumens/W.

Similarly, solid-state ballasts are more energy efficient than magnetic (transformer-style) ballasts. The primary reason is that magnetic ballasts operate at ac line frequency while solidstate versions work at much higher rates, usually 20 kHz and above to stay out of the audio range. Fluorescent bulbs are most efficient when operating at these higher frequencies. Operation at higher frequencies also lets ballast components be physically smaller and makes for a more compact package.

After April 1 of next year, manufacturers are barred from producing ballasts that don't meet the minimum efficacy ratings put out by the DOE. After next July 1, ballasts not meeting these requirements can only be sold as replacements for existing units. April 1, 2006 is the cutoff date for fixture makers to stop incorporating ballasts into new fixtures that don't meet the new requirements. And in 2010, ballasts lacking the mandated minimum efficiencies can no longer be sold even as replacements for existing units.

Manufacturers say they could devise magnetic ballasts to meet the new regs only by incorporating higher-grade laminated steel in the transformer and by making the whole ballast physically bigger. Neither option is economical, they say. But magnetic ballasts will still be found in specialized applications that DOE regs don't cover.

Solid-state ballasts thus far have had some well-known drawbacks. For example, they employ discrete components rather than ICs, making them bulky. Circuit operation depends to some degree on ambient temperature and on lamp impedance, which itself varies with operating temperature. The ballast circuit is basically an oscillator that employs two bipolar transistors and is not self-starting. The circuits use positive-temperature-coefficient thermistors for preheat and have no protection against lamp overcurrent or open filament conditions. Such factors can make components in the ballast output stage fail catastrophically.

Also, lamp impedance changes with age. This can move the oscillation frequency away from its most-efficient operation point. The beta of the bipolar transistors in the circuit can drift as well, also affecting operating frequency.

International Rectifier says its new ballast-control IC eliminates such difficulties. Called the IR2520D, it cuts ballast component count by 20%. This could help squeeze ballast circuits into smaller spaces, perhaps into an ordinary lamp base rather than the polycarbonate housings used today. The chip is designed to drive two MOSFETs that power the lamp. These are less sensitive to temperature changes than the bipolar transistors in ordinary CFL ballasts. The new chip includes a zero-voltageswitching circuit that maintains soft switching regardless of supply voltage, frequency, and lamp conditions. Included as well is internal overcurrent protection.

Nevertheless, the ICs won't cure all potential temperature problems. "The biggest factor in temperature performance is the electrolytic capacitor in the power-supply portion of the circuit," explains Tom Ribarich, director of International Rectifier's Lighting IC Design Center. "That capacitor should have a 105°C, 5,000-hr rating. But some companies cut corners and use 85°C caps. That can be a problem in a recessed fixture mounted with the base up, because heat collects in the top of the fixture. You can easily see 100°C on all the ballast components there."

IR's chip powers the lamp at about 50 kHz. This puts its operation above the band used by infrared remote controls so there is no interference. There is a voltage-controlled oscillator input pin on the device that also lets it dim CFLs.

The chip measures lamp operating qualities, primarily crest factor (the ratio of peak to rms current), to check for faults. A normal crest factor is less than two. When it exceeds four, the chip shuts down the ballast. This condition generally indicates the lamp is at its end of life.

One ballast chip can also run several CFLs simultaneously, says IR, thus providing economies for multiunit installations. And IR's Ribarich thinks the new chips will likely find their way into ballasts for linear fluorescent tubes as well. In this case, ballasts would need external power factor correction, he says.

HALOGEN ICs
Another ballast chip for halogen lamps uses the same basic technology as the CFL driver, but incorporates more sophisticated fault detection. Called the IR2161, it can adapt itself to changing supply voltage, frequency conditions, and lamp conditions.

"This chip must be more sophisticated to catch all the possible fault conditions," explains IR's Ribarich. "That's because the user can connect low-voltage halogen lamps to a track indiscriminately. We've seen cases where buildings burn down because someone hung too many lamps on a converter recessed in a ceiling. It can overheat and fail violently."

IR claims its new halogen ballast chip detects all such fault conditions. If it detects a shorted track, for example, it shuts down the ballast and repeatedly checks to see if the short has disappeared. If it detects too many lamps attached to the track, it blinks the lamps as a signal.

Like the CFL device, the halogen lamp chip reduces ballast part count by 20%, says IR. It also has an active dead time circuit that maintains soft switching regardless of the external conditions. Softstart limits inrush current to the lamp filament to boost lamp life. The chip also is compatible with triac wall-switch light dimmers.

Halogen lamps are not inherently energy efficient, so the motivation for using the ballast ICs is space savings and reliability. But odds are that there will be more ICs in the offing for ballasts. IR's Ribarich says the company is examining the concept of ballast chips for high-intensity discharge lamps. Most HID ballasts are magnetic, partly because they generate a lot of heat. Electronic ballasts for these devices must sit relatively far away from the lamp. And electronic ballasts for HID tend to handle 400 W or less. Today, higher power lamps of 1 kW and above are strictly magnetic.

MAKE CONTACT:
For technical papers on lighting, visit
www.irf.com/ productinfo/lighting/tplighting.html
Reference design ballast kits for single 26-W CFLs and a nondimming mini-ballast for single 25-W CFL using the IR2520D are available at
www.irf.com/forms/eltdk.html
Free ballast design software is at
www.irf.com/ productinfo/lighting/indexsw.html