High current varistor

Details High current varistor High current varistor

2000-10-27

2001-06-26

Gellner, Michael L. (Department: 2832)

Electrical resistors

Resistance value responsive to a condition

Current and/or voltage

C338S021000, C361S126000

Reexamination Certificate

active

06252493

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to metal oxide varistors. More particularly, this invention relates to a novel configuration for such a varistor that greatly increases the current carrying capabilities over the disc or “hockey puck” shaped metal oxide varistors.
BACKGROUND OF THE INVENTION
Polycrystalline metal oxide varistors, commonly known as MOV's, are well known in the art. MOV's include metal electrodes separated by sintered ceramics comprising a variety of metal oxides, zinc oxide being the predominant ceramic with lesser quantities of other oxides added in, including but not limited to oxides of bismuth, manganese, cobalt, antimony and/or tin. The metal electrodes may be made of any conductive material and are typically disposed on opposed major surfaces of the ceramic substrate.
MOVs commonly have the geometry of a circular disc shape with a thickness much smaller than the radius of the disc. A generic embodiment of a prior art MOV is shown in
FIG. 1
, wherein a ceramic substrate
11
in the shape of a disc separates a circular shaped first electrode
14
from a circular shaped second electrode
18
. Such disc-type MOV's are typically coated with a non-conductive material to prevent arcing between the electrodes about the cylindrical sides of the disc.
MOV's are provided in electrical parallel with a parent electrical circuit. Current travels, if at all, from one electrode to the other through the ceramic substrate, which acts as a variable resistor (varistor). The principal advantage of MOV's is that the electrical conductivity of the ceramic substrate changes non-linearly with respect to the voltage applied. The voltage at which an MOV's electrical conductivity dramatically changes is referred to as the clamping or breakdown voltage. When the applied voltage is below the threshold or clamping voltage of the MOV, the device acts as an open circuit and virtually does not conduct. When the device is electrically connected in parallel with a parent circuit, and an over-voltage condition occurs (as often happens during a surge), the voltage may rise well over the nominal operating voltage of equipment located in the parent circuit. When this surge exceeds the clamping or breakdown voltage, the MOV's ceramic substrate will breakdown electrically, thus creating a virtual short circuit in parallel with the load; conducting the surge away from the parent circuit and associated protected equipment. MOVs behave electrically much like two Zener diodes facing each other in series. Like such an arrangement, MOVs are bi-directional.
The electrical properties of MOV's may be described by the following equation:
I
=
(
V
C
)
α
wherein:
I is the current through the MOV,
V is the voltage across the electrodes,
C is a constant dictated by the substrate material and its geometric configuration, and
&agr; is a constant for a particular range of current across the electrodes.
Regarding the constant C in the above equation, the clamping voltage of a particular MOV is a function of the thickness of the particular substrate material interposed between the electrodes. Thicker substrates exhibit higher clamping and breakdown voltages. However, the amount of surge current that a particular MOV can effectively dissipate also is a function of the surface area of the electrode/substrate juncture. If the surge current is too great for this surface area and for the mass of the varistor substrate, the device will be destroyed due to its inability to dissipate the surge energy and the high impedance that may be posed by the insufficient surface area of the electrode/substrate juncture. This destruction often results in a catastrophic failure of the varistor device, and depending on the mode of failure may also result in a condition known as thermal runaway. While prior art MOV's encompass a wide variety of clamping voltages, many are limited in their ability to carry significant current capacities. In order to carry higher currents, the radius of disc-shaped MOVs must be increased. This is undesirable because of the extra space such an MOV would occupy in a circuit board for example. Thus, what is needed in the art is a metal oxide varistor of more compact shape that can dissipate higher currents without undergoing thermal runaway and/or catastrophic failure.
SUMMARY OF THE INVENTION
The present invention comprises an MOV with significantly increased surface area per unit volume, thus yielding an MOV with a greater current carrying capability. Specifically, a metal oxide varistor assembly comprises a hollow ceramic substrate, or body, having a generally concave interior surface and a generally convex exterior surface that are substantially complementary to each other. The hollow body has at least one opening therethrough. A first electrode is in electrical contact with the interior surface and has a portion that extends through the hollow body opening. If the hollow body defines more than one opening, the extension of the first electrode penetrates only one such opening. A second electrode is in electrical contact with the exterior surface.
The term ‘generally concave’ is not limited to curved surfaces, but also encompasses a plurality of planar surfaces that define a hollow. The term ‘generally convex’ is similarly broad, not limited to curved surfaces but also encompassing a plurality of planar surfaces whose normals diverge. For example, the interior and exterior surfaces of a pyramid formed by four planar triangles fall within the generally concave and generally convex descriptors, respectively. The term ‘substantially complementary’ surfaces refers to surfaces that are substantially similar in shape but not necessarily parallel. The hollow ceramic body may have a uniform thickness t between the interior and exterior surfaces in which case the surfaces are parallel. Alternatively, there may be instances where areas of reduced thickness are desired to control overshoot and upturn through the varistor, in which case the opposed surfaces will not be parallel but will still be substantially complementary. A spherical body defining a non-concentric and nearly spherical cavity exhibits substantially complementary surfaces since the interior and exterior surfaces are geometrically very similar. Conversely, a cube defining an internal spherical cavity does not exhibit substantially complementary surfaces.
The most practical embodiments of the present invention are those wherein the interior and exterior surfaces of the ceramic body are defined by body radii and the cross sections of the ceramic body include plane regions, of which some fully enclose and some partially enclose a hollow. The volumes of many solid or hollow bodies can be defined by the ‘method of slicing’. Suppose for example that the body is bounded by two parallel planes perpendicular to the x axis at x=a and x=b. Imagine the body to be cut into thin slices of thickness &Dgr;x by planes perpendicular to the x axis. Then the total volume of the body (enclosed by the exterior surface) can be defined as the sum of the volumes of these slices. Similarly, the volume of the hollow interior portion of the body can be defined as the sum of the volumes of the hollow of these slices. These bodies of the more practical embodiments are defined by the radii whose origin(s) is/are enclosed by the hollow body, such as a sphere, a cone, an ellipse, and variations thereof stretched or compressed along one or more axes.
In the preferred embodiment of the present invention, a metal oxide varistor assembly comprises a ceramic substrate formed into a hollow spherical body. Other hollow and partially hollow shapes, whether or not that hollow shape is a body of revolution, are included within the concept of this invention. A sphere is an ideal shape for maximizing the amount of surface area per unit volume. By the arrangement described herein, it will be appreciated that this spherical shape is employed to maximize the unit volume surface area between the electrodes and the ceramic

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