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Boosting confidence in de-rated dc caps for ac circuits

Boosting confidence in de-rated dc caps for ac circuits

UK capacitor manufacturer, Syfer Technology says it has addressed the issue of understanding precisely how a de-rated dc capacitor will behave in normal ac operational conditions. "This has the great advantage of enabling designers to specify these components with much greater confidence," said Matthew Ellis, senior engineer at Syfer.

A key feature of Syfer's stand at Electronica 2010, is the firm's latest range of X7R and C0G dielectric multilayer chip capacitors (mlccs). The devices are said to be suitable for de-rating in a range of ac applications. They are designed to offer capacitances of up to 120nF, for continuous use at up to 250Vac 60Hz. "And we'll have a number of technical experts on hand at Munich to advise engineers and specifiers," added Ellis.

In non safety-critical ac circuits, engineers have long been using de-rated dc mlccs to meet the need for higher capacitance than is available with safety rated components. The problem has been the lack of information on how their characteristics change in ac environments. Syfer has tested and analysed its devices, and now offers a range of capacitors for ac applications. The characterised devices are especially suitable for designers of power supplies, ac/dc converters and other power circuits where there is no danger of shock, surges or lightning strikes.

The mlcc range is available in case sizes from 0805 to 2220, and in both C0G and X7R dielectrics. According to Syfer, the C0G devices have negligible capacitance shift with applied voltage. However, they also have the lowest dielectric constant, typically 20 to 100, which means only relatively low capacitance values are achievable (470pF to 10nF).

The company has grouped the X7R devices into three categories, offering: +/-30; 30 to 50 and 30 to 80% maximum capacitance shift over the 0V to 240Vac range. In these cases, factors such as self heating, instability with temperature, and the piezoelectric effect, need to be considered. It is not uncommon for significant capacitance loss to occur at rated voltage. Piezoelectric effects tend to be an issue for large components and high capacitance values at frequencies higher than the 50/60Hz of mains supplies.

The MLCCs are manufactured at Syfer's Norwich facility, and are available in production. Components in the range are normally RoHS compliant, although tin-lead terminated versions are available for RoHS-exempt applications.

Author
Chris Shaw

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