Posted on 23 April 2012

Calculating the Useful Life of Capacitors

 

Useful life (also termed service life or operational life) is defined as the life achieved by the capacitor without exceeding a specified failure rate. Total failure or failure due parametric variation is considered to constitute the end of the useful life.

Depending on the circuit design, device failure due to parametric variation does not necessarily imply equipment failure. This means that the actual life of a capacitor may be longer than the specified useful life. Data on useful life has been obtained from experience gained in the field and from accelerated tests.

The useful life can be prolonged by operating the capacitor at loads below the rated values (e.g. lower operating voltage, current or ambient temperature) and by appropriate cooling measures. In addition to the standard type series, EPCOS types are available with useful life ratings specially matched to customer specifications.

The tables in the individual data sheets list the rated ripple current IAC,R for the upper category temperature (+85°C, +105 °C or +125 °C) and for a frequency of 100 Hz. The useful life for known ripple current loads and ambient temperatures is determined on the basis of the useful life graphs as follows: Determine the quotient IAC/IAC,R of the required ripple current at the given ambient temperature and the rated ripple current at the upper category temperature. The corresponding useful life value is given by the curve passing through the respective ambient temperature and the current quotient coordinates, or it can be interpolated if none of the useful life curves passes directly through these coordinates.

The frequency dependence of the ripple current has not been taken into account in the procedure described above. This must be introduced into the calculation in the form of an additional factor. For each series, precise curves for conversion factor IAC,f/I AC,100 Hz versus frequency f are given in the individual data sheets.

The EPCOS Design Tool AlCap 6.0 can be used to calculate the useful life of an aluminum electrolytic capacitor.

The tool's possible applications are:

  • Calculations of life expectancies for individual types of aluminum electrolytic capacitors,
  • Refined search to include parameters such as size limits or minimum required life expectancies in order to quickly find optimal solutions,
  • Defining of profiles that occur during operation and represent different loads,
  • Saving of solutions for further processing in a subsequent session,
  • Product recommendations containing a link to the respective data sheets.

 

For further information on aluminum electrolytic capacitors please read the following articles:

Mounting Positions of Aluminum Electrolytic Capacitors with Screw Terminals

Capacitor Bank Design

Film Capacitors for DC-Link

Equivalent Series Resistance (ESR) for Capacitors

 

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