MLC Discoidal Capacitors for EMI-RFI Filters Employing Non- Overlapping Electrodes Yield Substantial Performance Improvements

摘要

Multilayer ceramic discoidal capacitors used in EMI/RFI filter feedthroughs have traditionally been designed with overlapping electrodes. The problem arises when there is a need to increase the voltage rating by an order of magnitude from, 100VDC to 1000VDC, while maintaining similar capacitance. This is especially challenging when the discoidal capacitor's overall size is small, approximately .080", and the capacitance requirement is relatively large, approximately 1500pF. Due to volume constraints of the device, the traditional method of increasing dielectric thickness in order to increase voltage rating would yield a part with marginal reliability. In this application, the solution is to employ non-overlapping electrodes. The electrodes can be arranged in a way that takes advantage of the fringe effect capacitance that is produced at the edges of the electrodes. This electrode arrangement resembles a small washer sitting entirely inside the hole of a larger washer and not touching the larger washer. When the sets of washers are stacked and interleafed with layers of dielectric, they effectively form a small cylinder inside a larger cylinder. Since the electrodes are non-overlapping, the voltage rating of the capacitor is no longer determined by the dielectric thickness; instead it is determined by the gap, g, between the outside of the small cylinder and the inside of the large cylinder. The active area of the capacitor is thus determined by the outer surface area of the small cylinder, 2πrh, where, h, is the height of the cylinder. The traditional equation for capacitance kA/d is replaced by k2πrh/g. The non-overlapping electrode (NOE) construction results in a variety of performance improvements over the traditional design. Improvements include increased capacitance density, increased voltage rating as a result of higher voltage breakdown, and substantially improved insertion loss performance. Other improvements are higher Q and lower ESR.