Optimization And Life Testing Of A Tantalum Hybrid Capacitor

Tag : Hybrid Capacitor
David A. Evans, Evans Capacitor Company
Abstract
Tantalum Hybrid capacitors have been selected for use in militarylaser power supplies and airborne radar systems. These applicationsrequire very high specific power, high energy density, and highreliability. A common question is, how long will this capacitorlast in a given application. The question of reliability can beanswered by observing changes to electrical properties occurringover time in capacitors operating under conditions of voltage andtemperature existing in the intended use. A failure can be definedas a change in ESR or capacitance which exceeds some predeterminedlimit, such as the specified tolerance range. Because these changeshappen very slowly under normal conditions, it can take yearsbefore a sufficient number of failures has been accumulated to drawany conclusion about reliability. In order to decrease the timeperiod, normal ageing is accelerated by making the operatingconditions 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 abovethe rated voltage in an 85°C environment. The originalcapacitor was modified based on test results, leading to a moreoptimal, more reliable design. The test also yielded valuablegeneral 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 seriescombination of a dielectric oxide film capacitance, Ta2O5, and ahigh electrochemical capacitance, a film of the conductive metaloxide, RuO2. The result is a polar capacitor; with the Ta2O5 film,the positive and the RuO2 film the negative electrodes. A highpotential can be maintained across the thin electrochemicallyformed Ta2O5 film, while the RuO2 film remains at low potential.This allows high cell voltage without fear of reaching theelectrolyte breakdown potential.
The advantages of the Hybrid capacitor can be considered with anunderstanding of common electrolytic capacitors. These devicesemploy thin oxide films on the both electrodes, but they areusually asymmetric, using a material of higher surface area at thenegative electrode. The film on the positive electrode is thickerthan the negative electrode film, and sets the working voltage ofthe capacitor. The negative electrode has a higher capacitance, butthe two electrodes often have similar physical sizes.