U.S. patent number 3,771,091 [Application Number 05/302,539] was granted by the patent office on 1973-11-06 for potted metal oxide varistor.
This patent grant is currently assigned to General Electric Company. Invention is credited to John D. Harnden, Jr..
United States Patent |
3,771,091 |
Harnden, Jr. |
November 6, 1973 |
POTTED METAL OXIDE VARISTOR
Abstract
A metal oxide varistor has a high thermal conductivity potting
material encapsulating completely the metal oxide varistor and a
portion of the leads therefrom, a pair of metal plates spaced in a
generally parallel reslationship to the opposite major surfaces of
the varistor and bonded thereto by the potting material, and one of
the plates having at least a pair of mounting holes therein or
other suitable means.
Inventors: |
Harnden, Jr.; John D.
(Schenectady, NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
23168177 |
Appl.
No.: |
05/302,539 |
Filed: |
October 31, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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185723 |
Oct 1, 1971 |
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Current U.S.
Class: |
338/20; 338/22SD;
29/613 |
Current CPC
Class: |
H01C
7/102 (20130101); Y10T 29/49087 (20150115) |
Current International
Class: |
H01C
7/102 (20060101); H01c 007/10 () |
Field of
Search: |
;338/13,20,21 ;317/235Q
;29/610,613 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Albritton; C. L.
Parent Case Text
This application relates to metal oxide varistors, and more
particularly, to such varistors which are encapsulated with high
thermal conductivity potting material. This application is a
continuation-in-part of my copending application, Ser. No. 185,723,
filed Oct. 1, 1971, now abandoned, and assigned as herein.
Claims
THE INVENTION CLAIMED IS:
1. A metal oxide varistor comprising a metal oxide substrate having
first and second opposed major surfaces and having an alpha in
excess of 10 in the current density range of from 10.sup..sup.-3 to
10.sup.2 amperes per square centimeter, a pair of electrodes having
an electrical potential in excess of 20 volts applied therebetween
in non-rectifying contact with the respective opposite major
surfaces of the substrate, a high thermal conductivity potting
material encapsulating one major surface and the edges of the
substrate and its associated electrode, and a pair of metal plates
spaced in a generally parallel relationship to the opposite major
surfaces of the substrate, at least one of said plates being bonded
in position by the potting material.
2. A metal oxide varistor as in claim 1, in which each said
electrode has an electrical lead in contact therewith and wherein:
the potting material encapsulates additionally the second major
surface of the substrate, its associated electrode, and a portion
of the associated leads thereby encapsulating completely the
substrate, and the second plate is bonded in position by the
potting material.
3. A metal oxide varistor as in claim 1, in which the potting
material is a high thermal conductivity epoxy material.
4. A metal oxide varistor as in claim 1, in which the potting
material has a high thermal conductivity ceramic incorporated
therein.
5. A metal oxide varistor as in claim 4, in which the ceramic is
boron nitride.
6. A metal oxide varistor as in claim 1, in which the plates are
copper.
7. A metal oxide varistor comprising a metal oxide substrate having
first and second opposed major surfaces and having an alpha in
excess of 10 in the current density range of from 10.sup..sup.-3 to
10.sup.2 amperes per square centimeter, a pair of electrodes in
non-rectifying contact with the respective opposite major surfaces
of the substrate, a high thermal conductivity epoxy potting
material including a ceramic selected from the group consisting of
beryllium oxide, boron nitride, and aluminum nitride encapsulating
one major surface and the edges of the substrate and its associated
electrode, and a pair of metal plates spaced in a generally
parallel relationship to the opposite major surfaces of the
substrate, at least one of said plates being bonded in position by
the potting material.
8. A metal oxide varistor as in claim 7, in which each of said
electrodes has an electrical lead in contact therewith and wherein:
the potting material encapsulates additionally the second major
surface of the substrate, its associated electrode, and a portion
of the associated leads thereby encapsulating completely the
substrate, and the second plate is bonded in position by the
potting material.
9. A metal oxide varistor as in claim 7, in which the ceramic is
boron nitride.
10. A metal oxide varistor as in claim 7, in which the plates are
copper.
Description
There are a few known materials which exhibit non-linear resistance
characteristics and which require resort to the following equation
to relate quantitatively current and voltage:
I = (V/C) .sup..alpha.
where V is the voltage between two points separated by a body of
the material under consideration, I is the current flowing between
the two points, C is a constant and .alpha. is an exponent greater
than 1. Both C and .alpha. are functions of the geometry of the
body formed from the material and the composition thereof, and C is
primarily a function of the material grain size whereas .alpha. is
primarily a function of the grain boundary. Materials such as
silicon carbide exhibit nonlinear or exponential resistance
characteristics and have been utilized in commercial silicon
carbide varistors, however, such varistors typically exhibit an
alpha (.alpha.) exponent of no more than 6.
Metal oxide varistor materials, also referred to herein as MOV, a
trademark of the General Electric Company, having alphas in excess
of 10 within the current density range of 10.sup.-.sup.3 to
10.sup.2 amperes per square centimeter are described, for example,
in Canadian Pat. No. 831,691, issued Jan. 6, 1970. Although the
alpha of the MOV materials in which range the alpha remains
substantially constant, are identified by the current density range
of 10.sup..sup.-3 to 10.sup.2 amperes per square centimeter, it is
appreciated that the alphas remain high also at higher and lower
currents although some deviation from maximum alpha values may
occur. The MOV material is a polycrystalline ceramic material
formed of a particular metal oxide with small quantities of one or
more other metal oxides being added. As one example, the
predominant metal oxide is zinc oxide with small quantities of
bismuth oxide being added. Other additives may be aluminum oxide,
iron oxide, magnesium oxide, and calcium oxide as other examples.
The predominant metal oxide is sintered with the additive oxide or
oxides to form a sintered ceramic metal oxide body. Since the MOV
is fabricated as a ceramic powder, the MOV material can be pressed
into a variety of shapes of various sizes. Being polycrystalline,
the characteristics of the MOV are determined by the grain or
crystal size, grain composition, grain boundary composition, and
grain boundary thickness, all of which can be controlled in the
ceramic fabrication process.
The nonlinear resistance relationship of the MOV is such that the
resistance is very high (up to approximately 10,000 megohms) at
very low current levels in the microampere range and progresses in
a nonlinear manner to an extremely low value (tenths of an ohm) at
high current levels. The resistance is also more nonlinear with
increasing values of alpha. These nonlinear resistance
characteristics result in voltage versus current characteristics
wherein the voltage is effectively limited, the voltage limiting or
clamping action being more enhanced at the higher values of the
alpha exponent. Thus, the voltage versus current characteristics of
the MOV is similar to that of the Zener diode with the added
characteristic of being symmetrically bidirectional and over more
decades of current. The breakdown mechanism of the MOV is not yet
clearly understood but is completely unlike the avalanche mechanism
associated with Zener diodes, a possible theoretical explanation of
its operation being that of space charge limited current. The
"breakdown" voltage of an MOV device is determined by the
particular composition of the MOV material and the thickness to
which it is pressed in the fabrication process. The MOV involves
conduction changes at grain boundaries resulting in the advantage
of bulk phenomenon allowing great flexibility in the design for
specific applications simply by changing the dimensions of the body
of MOV material. That is, the current conduction in the absence of
closely spaced electrodes along one surface of the MOV body is
through the bulk thereof. The bulk property of the MOV permits a
much higher energy handling capability as compared to junction
devices. Thus, since an MOV device can be built up to any desired
thickness, it is operable at much higher voltages than the Zener
diode junction device and can be used in a range from a few volts
to several kilovolts. The voltage changes across a silicon carbide
varistor device are much greater than across an MOV device for a
given current change and thus the silicon carbide varistor has a
much smaller voltage operating range thereby limiting its
applications. The thermal conductivity of MOV material is fairly
high (approximately 1/2 that of alumina) whereby it has a much
higher power handling capability than silicon carbide, and it
exhibits a negligible switching time in that its response time is
in the subnanosecond domain. Finally, the MOV material and devices
made thereof can be accurately machined and can be soldered,
capabilities not possible for the larger grained silicon
carbide.
It is known in the art that the encapsulation or "potting" of
semiconductor devices, such as silicon carbide varistors, silicon
transistors, and germanium devices, in plastic potting materials
such as, for example, epoxy materials, frequently results in
undesirable degradation in the performance characteristics of the
semiconductor devices. The degradation in performance
characteristics has two origins. The first of these is a chemical
reaction occurring between the plastic encapsulant and the
semiconductor device. The second is a mechanical stress upon the
semiconductor device resulting from the substantially different
coefficients of expansion of the semiconductor material and the
plastic encapsulating material.
Subsequently, the chemical reaction causing performance degradation
was determined to be in fact an electrochemical reaction. Several
choices of materials are now known in the art to be available,
which will permit the plastic encapsulation of low voltage
semiconductor devices wherein the voltage stresses are on the order
of a few volts and in any event do not exceed ten volts, and the
performance degrading electrochemical reaction does not take place.
However, heretofore, it has not been possible to successfully
encapsulate a high voltage semiconductor device in a plastic
potting material directly.
My present invention is directed to providing an improved potted
metal oxide varistor.
The primary object of my invention is to provide a metal oxide
varistor with increased power capacity and improved thermal
capability, especially at high operating voltages and at the same
time providing the very desirable electrical isolation between the
heat cooling members and the electrical connections.
In accordance with one aspect of my invention, a metal oxide
varistor has a high thermal conductivity potting material
encapsulating completely the metal oxide varistor and a portion of
the leads therefrom, a pair of metal plates spaced in a generally
parallel relationship to the opposite major surfaces of the
varistor and bonded thereto by the potting material, and one of the
plates having at least a pair of mounting holes therein or other
suitable means.
These and various other objects, features and advantages of the
invention will be better understood from the following description
taken in connection with the accompanying drawing in which:
FIG. 1 is a graphical representation of the nonlinear resistance
and resultant voltage limiting characteristics of the MOV material
for different values of the exponent alpha plotted in terms of
volts vs. amperes on a log-log scale;
FIG. 2 is a top elevation view partially in section of a metal
oxide varistor made in accordance with my invention;
FIG. 3 is a sectional view of the metal oxide varistor shown in
FIG. 2 which is taken along line 3--3 in FIG. 2; and
FIG. 4 is a sectional view of a portion of a modified metal oxide
varistor.
The volts versus amperes characteristics plotted in FIG. 1 of the
drawing illustrate the nonlinear or exponential resistance
characteristics exhibited by MOV material, and in particular,
indicate the increasing nonlinearity and enhanced voltage limiting
obtained with increased values of the exponent alpha (.alpha.). The
volts abscissa is in terms of voltage and the amperes ordinate is
in terms of current density. Although the use of linear scales on
the graph would show the decreasing slopes (decreasing resistance
values) with increasing currents, such curves can be readily
manipulated by the choice of scales, and for this reason, log-log
scales are chosen to obtain a family of lines each of which remains
substantially straight within the indicated current range. It can
be seen from the FIG. 1 plots that the resistance exhibited by the
MOV material is quite high at low current levels and becomes
increasingly smaller in a nonlinear exponential manner with
increasing current levels, and such nonlinearity is greater for
greater values of the exponent alpha (.alpha.). Extension of the
plots to lower and higher current levels would obviously indicate
correspondingly much higher and lower resistances, respectively,
and operation of the subject machines may transiently reach such
levels. The "leakage" current through the MOV material is
negligible.
In FIGS. 2 and 3 of the drawing, there is shown generally at 10 a
metal oxide varistor embodying my invention. Varistor 10 has a
metal oxide substrate 11 with first and second opposed major
surfaces 12 and 13 and having an alpha in excess of 10 in the
current density range of from 10.sup..sup.-3 to 10.sup.2 amperes
per square centimeter. A pair of electrodes 14 and 15 are in
nonrectifying contact with the respective opposite major surfaces
12 and 13 of substrate 11. A pair of electrical leads 16 and 17 are
in electrical contact with electrodes 14 and 15, respectively. In
operation, the varistor of this invention is intended to be
connected into an electrical circuit by leads 16 and 17 for
protecting the circuit, or components thereof, against voltage
transients. Electrodes 14 and 15 have, in operation, typically a
steady state voltage stress in excess of 20 volts between them, and
are subjected to transient voltage stresses on the order of
thousands of volts. A high thermal conductivity potting material 18
encapsulates completely substrate 11, electrodes 14 and 15, and a
portion of leads 16 and 17. A pair of high thermal conductivity
plates 19 and 20 are spaced in a generally parallel relationship to
the opposite major surfaces 12 and 13 of the substrate 11. Plates
19 and 20 are bonded in position by potting material 18. Base or
bottom plate 19, which is of larger dimensions than plate 20 has at
least a pair of mounting holes 21 therein. Plates 19 and 20 could
carry fins to increase further the available surface area resulting
in a reduced thermal impedance and thus even greater power
capacity.
The metal oxide varistor of my invention has improved power
capacity over the conventional lead mounted metal oxide varistor by
at least an order of magnitude.
In FIG. 4 of the drawing, there is shown a sectional view of a
portion of a modified metal oxide varistor. As opposed to affixing
a pair of leads 16 and 17 to the exterior surfaces of electrodes 14
and 15, respectively, as in FIGS. 2 and 3, a pair of electrical
leads or connectors 22 and 23 in FIG. 4 are affixed to and in
electrical contact with the respective edges of electrodes 14 and
15. In this manner, the exterior surface of each lead is initially
approximately flush with the exterior surface of the respective
electrode thereby reducing the thickness of the potting material 18
between each associated electrode and plate. The respective
varistors of FIGS. 1, 2, and 3 can be further modified by bonding
plate 19 directly to electrode 14, for example, by soldering,
without potting material therebetween. The varistor is otherwise
similar in construction.
Advantages are obtained from a metal oxide varistor primarily due
to the following three exceptional properties of MOV material (1)
the resistance characteristics are highly nonlinear (.alpha.>10)
over a very wide range of current and result in a high degree of
voltage limiting, (2) the response time is negligible and
relatively nonvarying, (3) the high thermal conductivity permits
rapid dissipation of heat developed in operation, and (4) the metal
oxide varistor material does not react chemically with epoxy
potting components, even when subjected to very high voltage
stresses, and it is not subject to mechanical damage resulting from
differential expansion coefficients because the MOV material is
mechanically very strong. MOV material limits voltage build-up and
provides a relatively low resistance path for the current which
thence decays at a rate determined primarily by the LR time
constant of the associated device or until a current zero is
reached, the resistance of the MOV body increasing substantially as
the voltage, and primarily the current, are decreasing.
My varistor provides the unique advantage of increased heat
dissipation over a conventional varistor thereby producing improved
power capacity. Such increased heat dissipation is accomplished by
encapsulating the metal oxide substrate, its associated electrodes
and a portion of the respective leads with a high thermal
conductivity potting material of which I prefer epoxy resin
material. Such high thermal conductivity epoxy resins are
commercially available. I found that this advantage is increased
further by adding a high thermal conductivity ceramic to the epoxy
resin prior to encapsulation of the metal oxide substrate. Of
various suitable and available ceramics, including beryllium oxide,
boron nitride, and aluminum nitride, I prefer to employ boron
nitride.
This advantage is increased still further by bonding a pair of high
thermal, conductivity plates in spaced relationship to the opposite
major surfaces of the metal oxide substrate with the high thermal
conductivity potting material, which has preferably added thereto
the above discussed ceramic material. Of suitable and available
plates for this purpose, metal plates, and particularly copper
metal plates, are preferred. One of the plates is provided with at
least a pair of mounting holes therein and is preferably of larger
dimensions than the other plate.
Thus, while my invention has been particularly shown and described
with reference to the above illustrated embodiment thereof, it
should be understood by those skilled in the art that various
changes in form and detail may be made therein without departing
from the scope of the invention as defined by the following
claims.
* * * * *