Image courtesy of KYOCERA AVX. Metallized film capacitors exhibit a self-healing property that significantly improves their lifetime reliability characteristics. Figure 4 depicts the basic process wherein a dielectric defect results in a high current, high-temperature short circuit that quickly demetallizes the surrounding area.
Unfortunately, this mechanism can be dificult to control, and in the worst case, a run-away process can result, causing the destruction of the entire capacitor in short order. To avoid this, KYOCERA AVX developed a controlled self-healing process in 1974 based on the segmentation of overall capacitance into elementary cells protected by fuse gates.
The main reasons of metallized film capacitors failure depending on electrodes type were determined. Experimental results confirmed efficiency of metallized film capacitors with segmented metallization. It can be used in numerical simulation of self-healing processes and efficiency evaluation of different segmentation patterns.
Abstract: Metallized film capacitors (MFCs) have been widely commercialized, and the insulation failure has become an important issue under high electric field. However, due to the self-healing characteristic, the MFCs offer a notable advantage in electrical insulation.
Figure 1: Conventional aluminum electrolytic capacitor. Image courtesy of KYOCERA AVX. On the other hand, metal film capacitors rely on a metallized dielectric film to form the capacitive structure. Many film materials are available - most commonly, polypropylene and polyester.
Capacitors made of metallized polypropylene films suffer partial discharges, called self-healing, due to weak electrical defects. Those defects are destroyed by an electrical arc that extinguishes when enough metal of the electrodes is vapourized around this point.