In almost no other application are aluminum electrolytic capacitors exposed to such high stress as in electronic ballasts: high ripple current capability must be combined with an extended temperature range in a minimum of space. EPCOS has specially developed a new series for this critical application.

Lighting installations are currently being re-equipped as part of a worldwide energy-saving program. Whereas conventional incandescent lamps have a luminous efficacy of only 5%, the figure for fluorescent tubes is as high as 25%. Until recently, these tubes were still operated with magnetic ballasts with chokes, which are now being successively replaced by electronic ballasts - not least because of statutory requirements. They save 25% more energy than magnetic ballasts and offer added benefits. When operated at high frequencies, electronic ballasts have three advantages. First, they reduce flicke (allowing immediate starting). Second, they prevent stroboscopic effects (safety, health compliance). And third, they enable lamps to be dimmed.

Two classes of ballast

Electronic ballasts demand very high performance and reliability from aluminum electrolytic capacitors. They are exposed to a high ripple current with various frequencies, operate with great voltage fluctuations and must ensure long service life. In some cases they are also exposed to surge voltages. Electronic ballasts can basically be divided up into two classes: those with and without power factor correction.

Ballasts without power factor correction are used predominantly for low-power applications. In these circuits, the capacitor acts directly as a filter for the rectified voltage. A low-frequency current is applied to it during charging and a high-frequency current during discharging. In this type of circuit, the capacitor is designed for a rated voltage of 350 or 400 V at the usual 230 V line voltage, whereas a rated voltage of 250 V is specified for a 110/127 V line voltage.

Electronic ballasts with power factor correction contain an active harmonic filter between the diodes and the capacitor filter. The power factor is close to one if the harmonics content is very small. The filter circuit eliminates the high voltage peaks in the charging circuit and blocks the low-frequency content by means of the capacitor. Thanks to the active harmonic filters, AC voltage peaks cannot impair the capacitor. As a rule, the capacitor in this circuit is designed for a rated voltage of 400 to 450 V.

Aluminum electrolytic capacitors with improved technology

Aluminum electrolytic capacitors consist of two electrically conducting layers arranged opposite each other, the anode and cathode. A dielectric layer is inserted between them. The anode consists of an aluminum foil whose surface has been roughened and thus increases in area. An oxide layer (Al₂O₃) deposited on it forms the dielectric. The cathode also consists of an aluminum foil whose large contact area is used to conduct the electrical charge to the electrolyte.

The electrolyte is a liquid which fills the finely etched recesses of the anode's structured foil. The various layers of foil are separated by paper spacers. They contain the electrolyte, prevent short circuits and ensure the required dielectric constant between the anode and cathode foil 1.

The latest technological improvements relating to the purity of the aluminum foil, the etching process and a new electrolyte have opened a wide range of applications to aluminum electrolytic capacitors. If aluminum electrolytic capacitors are to be used in electronic ballasts, the anode must have a particularly stable oxide layer, the electrolyte must remain stable even at high voltage peaks, the paper design must be suitable and the sealing very effective 2.

However, these properties alone do not ensure long useful life and the desired charging and discharging characteristics. In recent years, advances have been made in production technology, e.g. in winding, impregnation, sealing and tempering.

Analyses have shown that a combination of innovative materials with matching product and process design makes considerable improvements possible. This is the only way of satisfying the extreme demands of electronic ballast operation.

The right capacitors for all requirements

Apart from being able to withstand ambient temperatures up to 140 °C, the new single-ended capacitors developed by EPCOS have a very long service life up to 200,000 hours at 40 °C. They feature high charging current and high dielectric strength without any loss in reliability. The compact dimensions of these capacitors offer advantages over conventional types, especially for spacesaving design of electronic ballasts.

Their mechanical properties are also noteö-worthy. The internal structure of these capacitors has been optimized for maximum capacitance. This product family offers a variety of sizes and terminal configurations. Highlights include particularly low-profile versions with a height of 20 mm for electronic ballasts with a low insertion height; these are available with diameters of 10, 12.5, 16, 18 and 20 mm. These capacitors can be supplied in various configurations, such as in bulk, taped and with protection against polarity reversal (PAPR). In the PAPR design, one of the terminals is flattened and broadened so that it can only be mounted on the board in the correct position 5.

High performance, compact dimensions, a high ambient temperature ceiling and choice of operating voltages make the new series of single-ended capacitors developed by EPCOS ideal for electronic ballasts 3 4.