On the evaluation of ferroelectric nonlinear capacitors for application in power circuits

A method for selecting ferroelectric capacitors as nonlinear components for application in turn-off snubber circuits for power electronic switches is investigated. By initiating, without destroying, localized events in the dielectric layer of a multilayer capacitor, such capacitors can be selected (and some of them also stabilized) by observing the number of events and the rate of events. This is achieved by exposing them to a high field strength by means of the application of a high voltage from ahigh-impedance source. The observed phenomena confirm that the conduction is mainly by electrons. An explanation for the instabilities in the steady-state DC current under increasing voltage as observed by other authors is proposed. Components which have been selected in this manner allow the usage of these capacitors at extremely high voltages (approximately 4 to 6 times the rated voltage for linear applications). An apparatus for the selection and stabilization is developed. The selected capacitors have a ratio of capacitance at zero voltage to that at the maximum operating voltage of approximately 10:1. The criteria for selecting capacitors for nonlinear applications (these applications are at high field strengths) are discussed. Preliminary results are given for applications of these capacitors in power electronic switching circuits.<
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