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Abstract
Tantalum Hybrid capacitors have been selected for use in military laser power supplies and airborne radar systems. These applications require very high specific power, high energy density, and high reliability. A common question is, how long will this capacitor last in a given application. The question of reliability can be answered by observing changes to electrical properties occurring over time in capacitors operating under conditions of voltage and temperature existing in the intended use. A failure can be defined as a change in ESR or capacitance which exceeds some predetermined limit, such as the specified tolerance range. Because these changes happen very slowly under normal conditions, it can take years before a sufficient number of failures has been accumulated to draw any conclusion about reliability. In order to decrease the time period, normal ageing is accelerated by making the operating conditions more severe.

This work focuses on optimization and life testing of THQA2016502, a hermetic tantalum Hybrid capacitor with a 5mF, 16 volt rating. Ageing was accelerated by exposing the capacitors to voltage above the rated voltage in an 85°C environment. The original capacitor was modified based on test results, leading to a more optimal, more reliable design. The test also yielded valuable general information concerning Hybrid capacitor engineering. Capacitance and ESR data over 5,040 hours of testing are presented.

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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