Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2001-05-25
2003-04-08
Nguyen, Chau N. (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S303000
Reexamination Certificate
active
06545854
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally concerns ceramic filter capacitors such as are most commonly discoidal in form.
The present invention particularly concerns the organization of internal electrodes within a discoidal feed-through ceramic filter capacitor so as to, by use of the electrodes' fringing fields, realize a compact capacitor of very high breakdown voltage.
2. Description of the Prior Art
The present invention will be seen to concern the size and placement of the internal electrodes within a multi-layer feed-through ceramic filter capacitor. Such a capacitor is commonly discoidal in shape, passing wires upon which are carried electrical signals to be filtered through one or more holes that are most commonly aligned to the axis of the disc. (Other geometries are, however, possible. See, for example, U.S. Pat. No. 5,822,174.) Typically multiple metal plate regions of each of a first, and of a second, electrode are typically located in partially overlapping positions upon alternating ceramic layers, thereby to create capacitance between these interleaved metal plate regions, and between the electrodes that are formed from the plate regions.
Capacitors so constructed are of particular use to filter electrical signals upon the wires and leads of implanted cardiac pacemakers and cardiac defibrillators. These latter devices use high voltages, commonly about 750 volts.
When the electrodes of a filter capacitor are subjected to high voltages—on the order of hundreds and, with safety margins, even thousands of volts—then their partially-overlapped metal plate regions are subject to develop voltage breakdown paths (1) through the ceramic to each other, and/or (ii) to oppositely-charged regions of the outside surface of the capacitor where electrical connections are made—all as is more thoroughly discussed in the DESCRIPTION OF THE PREFERRED EMBODIMENT section of this specification. According to this propensity, the alternating polarity metal plate regions of the two electrodes must be separated both (i) from each other, and (ii) from oppositely-charged regions at the exterior of the capacitor, by a reasonable thickness of ceramic.
Because (i) capacitance between spaced-parallel plate regions is a function of their separation, and (ii) plate density cannot be particularly high in a multi-layer capacitor where only an adequately thick ceramic buffer can insure such a high breakdown voltage as is desired, it has heretofore been believed that the metal plate regions of alternating polarity should be positioned (i) spaced-parallel (ii) in at least partial overlap along the axis of the discoidally-shaped ceramic body and of any hole(s) in this body. The simple reason that the metal plate regions are so located overlapping spaced parallel is so as to create capacitance along the elementary model of two parallel plate electrodes. Indeed, the formula for the capacitance of the conventional parallel-plate ceramic capacitor is:
Cap
=
kA
d
where Cap is the capacitance in farads, k is the dielectric constant in farads per meter, A is the area of electrode overlap in square meters and d is the distance of separation between plates in meters.
The present invention will be seen to show that capacitance suitable for filtering can be realized by an alternative metal plate configuration; a configuration that is much improved for voltage breakdown. A hallmark of this configuration, and of the present invention, will be seen to be the plate regions of each first and each second electrode pair (being that some filter capacitors can contain multiple electrodes) will be non-overlapping along the axis of the discoidally-shaped ceramic body, and along the axis of any hole(s) in this body.
Both the overlapping electrode plate regions of prior art multi-layer ceramic filter capacitors, and the multi-layer ceramic filter capacitors of the present invention totally lacking any such overlap in the, are highly visually distinctive (in views of the capacitor interiors). The overlapping of electrode plate regions is not much commented upon in the prior art, most likely because such has previously been deemed a fundamental and immutable basis of the construction of ceramic capacitors of all types—including feed-through filter capacitors. Accordingly, the next-following written descriptions of selected prior art patents, while amply showing the state of the art in multi-layer ceramic feed-through filter capacitors, do not deal directly with the present invention. The figures of these prior art patents do, however, clearly show that overlap of electrode plate regions that is obviated by the present invention.
For example, U.S. Pat. No. 4,148,003 to Colburn, et al. for a SERIES FEED-THROUGH CAPACITOR concerns a feed-through capacitor of the type for use in filters and the like and including discoidal ceramic capacitors arranged in stacked relationship and electrically connected in series. Series arrangement of the discoidal capacitors prevents shorting through the feed-through capacitor in the event of failure of one of the discoidal capacitors therein and results in substantially improved insertion loss characteristics when compared to feed-through capacitors having a single discoidal capacitor. Internal electrode metal plate regions are overlapping.
U.S. Pat. No. 5,333,095 to Stevenson, et al. for a FEED-THROUGH FILTER CAPACITOR ASSEMBLY FOR HUMAN IMPLANT concerns a feed-through filter capacitor assembly and related installation method are provided for shielding a conductive terminal pin or lead of the type used, for example, in an implantable medical device such as a heart pacemaker or defibrillator, against passage of external interference signals. The feed-through assembly includes a terminal pin subassembly having at least one terminal pin supported within a cylindrical conductive ferrule by a hermetically sealed insulator ring. The ferrule is adapted for mounting onto a conductive pacemaker housing, as by welding or brazing, to support the terminal pin for feed-through passage to the housing interior. A co-axial filter capacitor is mounted at an inboard side of the pacemaker housing, with capacitor electrode plate sets coupled respectively to the pacemaker housing and to the terminal pin by a conductive adhesive or the like. In one preferred form, multiple filter capacitors are provided in a substantially co-planar array within a common base structure, with each capacitor in association with a respective terminal pin. Internal electrode metal plate regions are again overlapping.
U.S. Pat. No. 5,751,539 also to Stevenson, et al. for an EMI FILTER FOR HUMAN IMPLANTABLE HEART DEFIBRILLATORS AND PACEMAKERS concerns an improved ceramic feed-through capacitor design which results in distinct advantages in EMI (electromagnetic interference) filtering and therapeutic wave form management for implantable defibrillators and pacemakers and the like. The invention provides ceramic capacitor electrode plate designs which provide both low impedance de-coupling for EMI suppression, and, at the same time provide an isolated common ground point through a separate coupling capacitor for electrical isolation of the filtered circuit from the metal case (usually titanium) of the defibrillator or the like. Such an arrangement allows the defibrillator HV (high voltage) output pulse to the heart to be referenced lead to lead (including reverse polarity), or from either lead to a common floating ground point or to the titanium case. The primary application of the invention is directed to implantable defibrillators where the output pulse to the heart is typically high voltage (up to 750 volts) and of short duration (typically in the 10
−2
seconds, or 10 millisecond, range). The invention is equally applicable to implantable pacemakers. The invention is stated to be suited for a combination defibrillator (tachycardia) and pacemaker (bradycardia) unit where there are sense (heart monitoring), low voltage (pacing) and high voltage (defibrillator) output leads. Internal elec
Devoe Daniel F.
Trinh Hung
Presidio Components, Inc.
Thomas Eric
Wood Herron & Evans L.L.P.
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