Current handling and nonshorting feature of multilayer polymer (film) capacitors

Abstract

Many DC-to-DC converters are under-filtered when it comes to RFI control and must be decoupled externally. Because the input filter section sees a moderate to low series current, normally one or two amps at 48 volts, a small valued capacitor can be used on the input, provided the dissipation factor is low enough to efficiently sink the ripple current, and the high frequency impedance is low enough to bypass reflected RFI. These capacitors can be either X7R ceramic or special multilayer polymer film (MLP) capacitors. Both types are built on a "stacked" layer construction that features very low impedance and ESR extending well beyond the switching frequency of high-density converters. Developments to provide higher capacitance values in small package sizes have now led to the use of these low ESR capacitors in output filters where the load currents are at least ten times higher than on the higher voltage input bus. The selection of an electrostatic capacitor (ceramic or film) can improve the reliability of the power system to a level where the tantalum electrolytic capacitors typically used can either be reduced in number or eliminated from the output filter. MLP capacitors represent the next generation of capacitor technology. While its mechanical structure is next generation technology, it is still based on proven film capacitor fundamentals with their established track record of both durability and reliability.