Using distributed circuits to model power capacitor behavior

Distributed circuit modeling of stacked and spiral capacitors is described. The modeling, executed on a spreadsheet, allows what-if analysis of the effects of changes in capacitor specifications such as metal film resistivity and changes in geometric dimensions on dissipation factor, equivalent lumped capacitance, film current density, film voltage and power distribution profile. The system provides a complete range of data including equivalent series resistance and complex impedance. The effects of varying degrees of edge disconnection are provided in the model system. It provides a tool not only for power circuit designers, but for manufacturers concerned with design optimization and understanding of the effects of varying degrees of in-use degradation. In this study, a simple capacitor model with varying degrees of edge disconnection is explored. The results correspond to common experience whereby degraded capacitors exhibit large increases in dissipation factor with little change in nominal capacitance. The results predict the existence of hot spots in a capacitor degraded by partial edge disconnection.