Integrated Polycrystalline Varistor Surge Protective Device For High Frequency Applications

Abstract

A polycrystalline varistor of the bulk effect zinc oxide base type adapted for use in voltage surge suppression on VHF signal lines is disclosed. The device comprises a unitary body including a spiral inductor electrically in series with the varistor element to prevent capacitive loading of the protected signal line.

Description

BACKGROUND OF THE INVENTION

I. scope

This invention relates to polycrystalline metal oxide varistors. More particularly, this invention relates to adaptation of such varistors to provide protection for VHF signal lines.

Ii. prior Art

It is important to provide protection for equipment connected to VHF signal cables, particularly CATV cables, from high amplitude voltage surges, particularly atmospheric lightning produced surges. In present CATV systems, a single coaxial conductive pair is employed to simultaneously transmit VHF signal energy and operating power between successive repeater amplifiers of the system. Within a typical repeater amplifier enclosure, filters are employed to separate the VHF signal from the low frequency operating power. The incoming transmission line is separated into two parallel lines. A high pass filter is connected to one of the parallel lines to pick off signal energy. A low pass filter is connected to the other of the parallel lines to pick off operating power. Protection against lightning surges is provided by shunting the output of the high pass signal filter with a spark gap, and shunting the output of the low pass power filter with a Zener diode. Zener diodes have limited power handling capability; however, because lightning produced voltage surges have very fast rise times, the low pass filter greatly attenuates the voltage surge appearing on the power pick-off line, and brings it within the capability of a Zener diode. It follows that most of the lightning surge energy appears on the signal pick-off line and must be suppressed by the spark gap. The use of a spark gap surge suppressor in this application is less than ideal for two reasons. Firstly, there is a volt-time firing lag which is inherent in the operation of spark gaps. This results in significantly incomplete suppression of the voltage surge. Secondly, once the spark gap has fired, a plasma conduction path exists between the electrodes thereof and a portion of signal energy is lost unnecessarily in the period of time between the end of the lightning pulse and the resetting of the spark gap by extinguishment of the plasma path.

It might occur to one skilled in the art that the aforementioned shortcomings of spark gaps and Zener diodes as surge suppressors might be obviated by substituting varistor devices for them. However, such a direct substitution is ineffective in VHF applications for reasons set forth hereinafter.

There are a few known substances whose resistance characteristic is non-linear and is expressed by the equation

I = (V/C) .sup..alpha.

where

I is the current flowing through the material,

V is the voltage applied across the material,

C is the constant which is a function of the physical dimensions of the body, its composition, and the parameters of the process employed to form the body, and

.alpha. is a constant for a given range of current and is a measure of the non-linearity of the resistance characteristic of the body.

A well-known example of such varistor materials is silicon carbide. Silicon carbide and other non-metallic varistor materials are characterized by having an alpha exponent of less than 6. Recently, a family of polycrystalline metal oxide varistor materials have been produced which exhibit an alpha exponent in excess of 10. These new varistor materials comprise a sintered body of zinc oxide crystal grains, including additionally an intergranular layer of other metal oxides and/or halides, as, for example, beryllium oxide, bismuth oxide, bismuth fluoride, or cobalt fluoride, and are described, for example, in U.S. Pat. No. 3,682,841, issued to Matsuoka et al on Aug. 8, 1972 and U.S. Pat. No. 3,687,871, issued to Masuyama et al on Aug. 29, 1972.

In basic structure, both the silicon carbide and metal oxide varistors comprise a body, usually disk shaped, of varistor material having a pair of electrodes applied to opposite major faces thereof. This structure provides devices which are inherently capacitive.

The magnitude of the capacitance of these varistor devices is such that they would significantly load a VHF signal line if connected in shunt therewith. Furthermore, with respect to the silicon carbide varistor, there is an additional problem resulting from the high steady state ohmic leakage current thereof. In other words, because of the inherent characteristics thereof, neither of these varistors may be directly substituted in the CATV signal line application discussed above. The silicon carbide varistor could not be substituted for the Zener diode in the power line example whereas the metal oxide varistor could advantageously be substituted for the Zener diode.

Iii. cross-reference to Related Copending Application

This invention is related to copending application of Martzloff, Ser. No. 374,933, filed of even date herewith and assigned to the assignee of this invention.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of this invention to provide apparatus including a polycrystalline metal oxide varistor for suppression of voltage surges on VHF transmission lines.

It is another object of this invention to provide such apparatus which is capable, as a unitary device, of simultaneously protecting a CATV system with respect to both VHF frequency and low frequency power components.

It is another object of this invention to provide such apparatus which has a high power dissipation capability.

It is another object of this invention to provide such apparatus having improved switching speeds with comparison to spark gaps.

It is another object of this invention to provide such apparatus which is readily and inexpensively producible by integrated circuit techniques.

Briefly, and in accordance with one embodiment of this invention, a body of polycrystalline metal oxide varistor material has a pair of electrodes on opposite faces thereof. One of said electrodes has a protruding portion thereof. A layer of insulating material is applied over said electrode and is penetrated by said protruding portion. A spiral inductor is printed or otherwise applied over the insulating layer with one terminus thereof in electrical contact with the protruding portion of said one electrode. The other terminus of the spiral inductor and the second of said electrodes serve as terminals for connection of the inventive device across a VHF signal line to provide protection against voltage surges thereon.

The novel features of this invention sought to be patented are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may be understood from a reading of the following specification and appended claims in view of the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a sectional elevation view of one embodiment of the VHF signal line protective apparatus in accordance with this invention.

FIG. 2 illustrates a modification of the apparatus in FIG. 1 wherein the effective inductance thereof is increased by the inclusion of ferrite material in the substrate of the inductor coil.

FIG. 3 illustrates a further modification in which a second layer of ferrite including material overlies the inductor windings.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 a body of polycrystalline metal oxide varistor material 11 has a pair of metallic electrodes 12 and 13 on opposite faces thereof. Varistor material 11 has the characteristic that when an electrical potential which does not exceed a magnitude known as the varistor voltage of the varistor device and is a function of the thickness, composition, and fabrication process of body 11, is applied between electrodes 12 and 13, the resistance to a flow of electric current between electrodes 12 and 13 is on the order of several thousands megohms. Accordingly, it may be seen that with low voltages applied between electrodes 12 and 13, the device behaves electrically essentially as a capacitor. While very little steady state d-c leakage current flows through these new polycrystalline metal oxide varistor devices, at very high frequencies, because of the capacitive nature of the varistor devices, substantial capacitive leakage currents flow. As a point of comparison, in the case of prior art varistor devices, such as silicon carbide varistor devices, the low voltage resistance of the device is typically less than one-tenth megohm. Accordingly, d-c leakage is a problem with prior art varistors. Structurally, however, the silicon carbide varistors are similar to the polycrystalline metal oxide varistors and so additionally, capacitive leakage is a problem at very high frequencies for both varistor types.

In accordance with this invention, the VHF leakage problem across a polycrystalline metal oxide varistor is solved by depositing a layer 14 of insulating material, for example, glass, alumina, or other suitable material over one electrode 12. Insulating layer 14 serves as a substrate on which a spiral inductor 15 is applied in any convenient method known in the art, as for example, by screen printing. One terminus 16 of inductor 15 is electrically connected to electrode 12 by a quantity 17 of conductive material. Conductive material 17 may be formed as a projecting portion of electrode 12 about which insulating layer 14 is applied, or may be formed of the material of and in the process of deposition of inductor 15 through an aperture provided for this purpose in insulating layer 14. A second terminus 18 of inductor 15 has secured thereto an electrical lead 19 for connection of the device to a first conductor of a conductor pair to be protected. A second electrical lead 20 is similarly connected to electrode 13 for connection to the other conductor of said conductor pair.

Electrically, the device thus provided consists of the varistor comprising elements 11, 12, and 13 in series with inductor 15 between terminal 19 and 20. At low voltages the devices thus provided constitute an LC series resonant circuit. At high voltages, exceeding the varistor voltage of the varistor element of the device, the device thus provided comprises a small inductor in series with a small resistance. The device is so constructed, as will hereinafter be more fully described, to exhibit series resonance at a frequency below the VHF range. Accordingly, at VHF, the device presents a high impedance and so may be connected across a VHF signal line without introducing any significant losses in the VHF signal.

The energy coupled into a transmission line from an atmospheric lightning stroke typically has almost all of its energy in the spectral region below several megahertz. Typically, most of the energy is contained in the spectral segment between 100 kilohertz and 3 megahertz. It might be assumed, therefore, that the preferred design would call for providing series resonance in the neighborhood of 1 megahertz. However, it should be remembered that the varistor switching time is very short, on the order of a few nanoseconds, and that once the varistor element has become conductive, the capacitive reactance of the varistor element is no longer in the circuit. Therefore, the resonant frequency of the device is not critical so long as it is sufficiently below VHF for the device to present a high impedance to the VHF signal frequencies.

For example, a device which was constructed in accordance with this invention, employing a four-turn air core inductive element, exhibited a resonant frequency of 18 megahertz and functioned as intended in accordance with the above recited objects of this invention in a VHF signal line.

FIG. 2 illustrates a first alternative embodiment of this invention in which insulating layer 21 replaces insulating layer 14 of FIG. 1. Insulating layer 21 includes a quantity of magnetic material as for example ferrite material and may either consist of ferrite material or comprise ferrite or other magnetic material in a binder such as, for example, epoxy. The use of ferrite material in the insulating layer substrate for inductor 15 increases the effective inductance thereof and lowers the resonant frequency of the device.

FIG. 3 is a second alternative of the embodiment in which a complete ferrite, or ferrite loaded, core is provided for inductor 15. This embodiment further increases the effective inductance of inductor 15.

In a CATV cable protection application, the device of this invention provides both superior performance as outlined above, and an economic advantage. Because the device of this invention simultaneously performs the functions provided in the prior art by Zener diode in the low frequency power pick-off line and a spark gap in a high frequency signal pick-off line, a single device in accordance with this invention may be employed in preference to the two devices used in the prior art. Accordingly, the device in accordance with this invention is to be connected across a CATV cable at the junction thereof with the input terminals of the VHF signal pick-off filter and the low frequency power pick-off filter. Economic advantage is obtained because the cost of one device in accordance with this invention is less than the combined costs of the two devices employed in the prior art and also because CATV system fabrication costs are reduced by having only one two-terminal device to connect in place of two. As previously mentioned, the economic advantage is combined with the operational advantage of having a device which is faster in switching than a spark gap and sturdier than a Zener diode.

While this invention has been described with reference to particular embodiments and examples, other modifications and variations will occur to those skilled in the art in view of the above teachings. Accordingly, it should be understood that within the scope of the appended claims, the invention may be practiced otherwise than is specifically described.

Claims

1. Apparatus for suppression of voltage surges on VHF signal cables comprising:

a body of varistor material having a pair of opposed faces and an electrode on each of said faces;

a first non-conductive coating overlying one of said electrodes;

an inductor overlying said non-conductive coating; and

means for electrically connecting said one of said electrodes to said

2. Apparatus in accordance with claim 1 wherein said inductor comprises conductive material deposited over said first non-conductive coating in a

3. Apparatus in accordance with claim 1 wherein said first non-conductive

4. The apparatus of claim 3 wherein said magnetic material is ferrite

5. The apparatus of claim 3 including additionally:

a second non-conductive coating including magnetic material therein, said second non-conductive coating overlying said first non-conductive coating

6. Apparatus in accordance with claim 1 wherein:

said inductor has first and second terminals; and

said means for electrically connecting connects said one of said electrodes to said first of said terminals to produce a series resonant circuit between the other of said electrodes and said second of said terminals.

7. The apparatus of claim 6 including additionally:

means for connecting said second of said terminals to a first conductor of a VHF signal cable; and

means for connecting said other of said electrodes to a second conductor of

8. Apparatus in accordance with claim 1 wherein said varistor material

9. The apparatus of claim 8 wherein said varistor material comprises zinc oxide as a major constituent and a minor constituent selected from the group consisting of other metal oxides and halides.