Reliability criterion of film capacitor based on moisture diffusion in encapsulation

Abstract The current reliability criterion (RC) of the film capacitor (FC) is usually a fixed value, which does not consider the staged capacitance loss caused by moisture diffusion under high temperature and/or high humidity. The expected lifetime of capacitors would deviate far from the actual one. The moisture diffusion process of the encapsulated FC is analysed based on Fick's second law. In order to investigate the capacitance loss and moisture diffusion characteristics, 4 types of film capacitors are stressed with AC voltage under different temperatures and humidities. Based on the staged analysis of the capacitance and weight gain curves, the effect of moisture diffusion on capacitance loss is revealed. Results indicate that the linear stage of capacitance loss corresponds to the Fickian moisture diffusion stage of weight gain. The effect of voltage on FC aging is the most significant. The capacitance loss rate increases by 301% when the voltage rises by 30%. An RC calculation method of the FC is further proposed, which could also screen the encapsulation quality and shorten the aging test duration by at least 80%