A degradation model for metallized polyester capacitors

Abstract

Till now it was usual to think that the physical degradation of the end-spray contacts of metallized polyester capacitors, accelerated by exposure to high temperature, only implied an increase of the equivalent series resistance. This paper reports that together with this resistance increase, a contribution due to a parasitic capacitance must be taken into account to describe this degradation mechanism correctly. A degradation model is presented that enables fairly good fitting of experimental measurements of capacitance, dissipation factor and impedance vs. frequency of capacitors submitted to long-term tests, that exhibit an electrical behaviour deviating from the one described by the usual equivalent series circuit. The paper illustrates a technological analysis method that enables the consistence of the proposed model to be confirmed and that can currently be used to make quick estimations of the reliability performance of components by different manufacturers and to carry out periodic quality control. It was also seen that, in well manufactured capacitors, the described degradation mechanism is rather slow, with respect to the high temperature exposure time, so that the reliability level of these components is not affected.