Tantalum Hybrid Capacitors For High-Power Applications

David A. Evans
Evans Capacitor Company

Abstract
Tantalum Hybrid capacitors are among the most powerful capacitors available. A new tantalum Hybrid capacitor, having a peak power density of 1.6 MW/l has been developed. The new hermetic capacitor has innovative packaging features that minimize non-active components and allow reliable assembly. The thin, flat package also tends to minimize electrical resistance while maximizing heat dissipation. This unique capacitor combines the high cell voltage capability and low resistance of an electrolytic capacitor with the increased energy density of an electrochemical capacitor.

Electrical impedance spectroscopy was used to characterize capacitor electrical properties. The new capacitor has 10X the power density of an aluminum electrolytic capacitor with similar electrical performance. The frequency response of this capacitor suits it for avionics, communications electronics, air- and surface- based radar systems, power supply, computer, and electronics applications.

Introduction
The tantalum Hybrid capacitor (Patent No. 5,369,547) is a series combination of a dielectric oxide film capacitance, Ta2O5, and a high electrochemical capacitance, a film of the conductive metal oxide, RuO2. The result is a polar capacitor; with the Ta2O5 film, the positive and the RuO2 film the negative electrodes. A high potential can be maintained across the thin electrochemically formed Ta2O5 film, while the RuO2 film remains at low potential. This allows high cell voltage without fear of reaching the electrolyte breakdown potential.

The advantages of the Hybrid capacitor can be considered with an understanding of common electrolytic capacitors. These devices employ thin oxide films on the both electrodes, but they are usually asymmetric, using a material of higher surface area at the negative electrode. The film on the positive electrode is thicker than the negative electrode film, and sets the working voltage of the capacitor. The negative electrode has a higher capacitance, but the two electrodes often have similar physical sizes.

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