Bright future for energy efficient lighting

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.

The typical solid-state ballast circuit for a compact fluorescent lamp (CFL) uses discrete components. The operating point depends on lamp electrical properties and temperature. So the operating point may change as the lamp ages, potentially leading this self-oscillating circuit into resonance and failure. A new IC reduces component count by 20% and keeps the operating point stable over temperature and lamp variations. Chip developer International Rectifier says the resulting ballasts will be more reliable and should fit in a smaller space.

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.