Improvement on Self-Discharge Characteristic of an Electric Double Layer Capacitor

An electric double layer capacitor, SUPERCAP, has been developed to provide up to one million microfarads capacitance in less than one cubic inches. The large capacitance and low rate of discharge make the electric double layer capacitor (EDLC) useful as a non-battery, on-board backup power source which can protect circuits against temporary power interruptions. The EDLC can be charged and discharged at both high and low current rates; i.e. microamps to amps, and the backup time capability is normally determined by the backup current. In case of extremely low current backup, a few microamps or less, the self-discharge of EDLCs dominate the backup time capability. This paper describes different parameters affecting the self-discharge characteristic. The analysis made it possible for us to develop new SUPERCAPs, FZ-series, which can retain data in CMOS RAMs for more than a month---twice as long as the equivalent FA-series. Since EDLCs are based on the new capacitor concept, the operating principle, product structure and some reliability test results are also introduced.