Design of an Internal Fuse for a High-Frequency Solid Tantalum Capacitor

A miniature fuse module was designed and incorporated into a solid tantalum capacitor for high-frequency bypass applications. The fuse consists of a fine bimetallic wire which reacts exothermically upon reaching a critical initiation temperature. It is housed in an electrically and thermally insulative body to minimize fusing energy. The desirability of having fused devices is discussed. Design constraints, in particular those design considerations which minimize inductance and series resistance while optimizing fusing characteristics, are presented. Construction techniques to produce fuse assemblies and join them to capacitors are reviewed, and parametric measurements of fused and unfused devices are compared. Factors affecting the amount of energy required to actuate the fuse and the reliability of actuation are identified. Typically, fuse actuation occurs within 25 ms of short circuit capacitor failure and requires between 1.5 and 2.5 A to initiate with the total energy used being less than 150 mJ. A device can now be produced that withstands switching and transient surges without incident while protecting circuits and equipment from disruption and damage caused by short circut capacitor failures.