US3771091A - Potted metal oxide varistor - Google Patents

Potted metal oxide varistor Download PDF

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Publication number
US3771091A
US3771091A US00302539A US3771091DA US3771091A US 3771091 A US3771091 A US 3771091A US 00302539 A US00302539 A US 00302539A US 3771091D A US3771091D A US 3771091DA US 3771091 A US3771091 A US 3771091A
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United States
Prior art keywords
metal oxide
substrate
potting material
oxide varistor
plates
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Expired - Lifetime
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US00302539A
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English (en)
Inventor
J Harnden
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General Electric Co
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General Electric Co
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01C—RESISTORS
    • H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
    • H01C7/10—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material voltage responsive, i.e. varistors
    • H01C7/102—Varistor boundary, e.g. surface layers
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T29/00—Metal working
    • Y10T29/49—Method of mechanical manufacture
    • Y10T29/49002—Electrical device making
    • Y10T29/49082—Resistor making
    • Y10T29/49087—Resistor making with envelope or housing

Definitions

  • ABSTRACT Continuation-impart of Ser. No. 185,723, Oct. 1,
  • a metal oxide varistor has a high thermal conductivity potting material encapsulating completely the metal Cl 317/235 Q oxide varistor and a portion of the leads therefrom, a [51 Int. Cl "016 7/10 pair of metal plates spaced in a enerall arallel h- [58] Field of Search 338/13, 20, 21; ti nshi to the o osite major surfaces of the varistor /235 Q; 613 and bonded thereto by the potting material, and one of the plates having at least a pair of mounting holes [56] References Cited therein or other suitable means.
  • 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 a is an exponent greater than 1.
  • Both C and a 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 a 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 analpha (a) 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 to 10 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" to 10 amperes per square centimeter, it is appreciated that the alphas remain high also at higher and lower currents although some deviation from maxi mum 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.
  • 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 approxi+ mately 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.
  • 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 alongone 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.
  • 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% 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.
  • the MOV material and devices made thereof can be accurately machined and can be soldered, capabilities not possible for the larger grained silicon carbide.
  • 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.
  • FIG. 1 is a graphical representation ofthe 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;
  • 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 (a).
  • the volts abscissa is in terms of voltage and the amperes ordinate is in terms of current density.
  • FIGS. 2 and 3 of the drawing there is shown generally at 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 to 10 amperes per square centimeter.
  • a pair of electrodes 14 and 15 are in nonrcctifying 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.
  • 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 l4 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 1 1, electrodes 14 and 15, and a portion of leads l6 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 l9 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.
  • FIG. 4 of the drawing there is shown a sectional view of a portion of a modified metal oxide varistor.
  • 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.
  • MOV material is mechanically very strong.
  • the resistance characteristics are highly nonlinear (01 l 0) 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 nietal 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.
  • a high thermal conductivity ceramic including beryllium oxide, boron nitride, and aluminum nitride, I prefer to employ boron nitride.
  • a metal oxide varistor comprising a metal oxide substrate having first and second opposed major surfaces and having an alpha in excess of in the current density range of from 10* to 10 amperes per square centimeter, a pair of electrodes having an electrical potential in excess of 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.
  • 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
  • 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 to l0 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.
  • 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.

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  • Engineering & Computer Science (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Thermistors And Varistors (AREA)
  • Details Of Resistors (AREA)
US00302539A 1972-10-31 1972-10-31 Potted metal oxide varistor Expired - Lifetime US3771091A (en)

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US30253972A 1972-10-31 1972-10-31

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US (1) US3771091A (GUID-C5D7CC26-194C-43D0-91A1-9AE8C70A9BFF.html)
JP (1) JPS5534566B2 (GUID-C5D7CC26-194C-43D0-91A1-9AE8C70A9BFF.html)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4371860A (en) * 1979-06-18 1983-02-01 General Electric Company Solderable varistor
WO1992005679A1 (en) * 1990-09-13 1992-04-02 Crout Samuel B Encapsulation method for electrical components
US6407411B1 (en) * 2000-04-13 2002-06-18 General Electric Company Led lead frame assembly
US20100319744A1 (en) * 2009-06-23 2010-12-23 Laird Technologies, Inc. Thermoelectric modules and related methods

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS6062666U (ja) * 1983-10-06 1985-05-01 東陶機器株式会社 湯水混合栓
EP0217021B1 (de) * 1985-09-02 1990-04-18 BBC Brown Boveri AG Ueberspannungsableiter und Verfahren zu seiner Herstellung

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2751477A (en) * 1952-07-15 1956-06-19 Pittsburgh Plate Glass Co Electrical resistive device
US3503029A (en) * 1968-04-19 1970-03-24 Matsushita Electric Ind Co Ltd Non-linear resistor
US3564109A (en) * 1967-08-24 1971-02-16 Siemens Ag Semiconductor device with housing
US3609471A (en) * 1969-07-22 1971-09-28 Gen Electric Semiconductor device with thermally conductive dielectric barrier

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2751477A (en) * 1952-07-15 1956-06-19 Pittsburgh Plate Glass Co Electrical resistive device
US3564109A (en) * 1967-08-24 1971-02-16 Siemens Ag Semiconductor device with housing
US3503029A (en) * 1968-04-19 1970-03-24 Matsushita Electric Ind Co Ltd Non-linear resistor
US3609471A (en) * 1969-07-22 1971-09-28 Gen Electric Semiconductor device with thermally conductive dielectric barrier

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4371860A (en) * 1979-06-18 1983-02-01 General Electric Company Solderable varistor
WO1992005679A1 (en) * 1990-09-13 1992-04-02 Crout Samuel B Encapsulation method for electrical components
US6407411B1 (en) * 2000-04-13 2002-06-18 General Electric Company Led lead frame assembly
US20100319744A1 (en) * 2009-06-23 2010-12-23 Laird Technologies, Inc. Thermoelectric modules and related methods
US8193439B2 (en) 2009-06-23 2012-06-05 Laird Technologies, Inc. Thermoelectric modules and related methods

Also Published As

Publication number Publication date
JPS5534566B2 (GUID-C5D7CC26-194C-43D0-91A1-9AE8C70A9BFF.html) 1980-09-08
JPS4995173A (GUID-C5D7CC26-194C-43D0-91A1-9AE8C70A9BFF.html) 1974-09-10

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