Boosting confidence in de-rated DC caps for AC circuits

A key feature of Syfer’s stand at Electronica 2010, is the firm’s latest range of fully characterised X7R and C0G dielectric multilayer chip capacitors (MLCCs). “These devices are ideal for de-rating in a range of AC applications,” Ellis added. They 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.”

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, with a consistent and predictable response. These characterised devices are appealing especially to designers of power supplies, AC/DC converters and other power circuits where there is no danger of shock, surges or lightning strikes. They are also useful for designs that are intentionally overrated and rarely operating at AC voltage levels.

The MLCC range is available in case sizes from 0805 to 2220, and in both C0G and X7R dielectrics. As expected, the ultra-stable 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, UK facility, and are readily available in production quantities on a standard lead time of less than 9 weeks. Components in this range are normally RoHS-compliant, although tin-lead terminated versions are available for RoHS-exempt applications. Prices depend on model and volume, but typically cost significantly less than the fully safety-approved devices.

This range is complementary to Syfer’s certified ranges of surge and safety capacitors. Y2/X1, Y3/X2 and X2 rated components are available in case sizes 1808, 1812, 2211, 2215 and 2220 with certifications from TÜV and UL for standard terminations, together with the FlexiCap™ flexible polymer termination option.