Semiconductor Device-metal Oxide Varistor-heat Sink Assembly

Abstract

A single or plurality of metallic plates arranged in parallel relationship along the length of a body of metal oxide material in contact therewith provide a means for removal of heat developed in the bulk of the metal oxide material. The metal oxide material exhibits highly nonlinear resistance characteristics to provide transient voltage protection for semiconductor devices connected on or between the metallic plates or between the metallic plates and terminals mounted along the body of metal oxide material at selected voltage points. Our invention relates to a transient voltage suppressor-heat sink assembly, and in particular, to an assembly wherein the voltage suppression is provided by sintered metal oxide material and the heat sink by a plurality of metallic plates. Semiconductor devices such as silicon controlled rectifiers, triacs, diacs, diodes, transistors and the like are often subject to transient voltages which can cause temporary nondestructive or even destructive breakdown of the semiconductor device. These transients may result from various means including a spurious input signal or voltage surge, or be generated on a cycle-by-cycle basis during commutation and switching of the semiconductor device. Thus, it is readily apparent that excessive peak of voltage transients must be reduced to levels within the rating of the particular semiconductor device in order to assure satisfactory semiconductor circuit operation. A recently developed material which exhibits highly nonlinear resistance characteristics, and will be described in greater detail hereinafter, has been found to have exceptional voltage limiting characteristics as well as many other advantages such that it is the basis of a new class of improved transient voltage suppressors. This material, a sintered metal oxide, has a relatively high energy handling capability, however, this thermal capacity may not be adequate in some higher power circuit applications of the metal oxide varistor such as phase controlled rectifier and inverter circuits. Therefore, a principal object of our invention is to provide a heat sink for metal oxide varistors. Another object of our invention is to provide a rigid assembly for the metal oxide varistors and heat sink. A further object of our invention is to provide a semiconductor circuit within the assembly wherein the heat sink also removes heat generated in the semiconductor devices. Another object of our invention is to fabricate the assembly in compact form for minimizing the inductance of interconnecting leads. A still further object of our invention is to utilize the heat sink as voltage tap points along the metal oxide varistor body for connection to the semiconductor devices. In accordance with our invention, we provide an improved transient voltage suppressor-heat sink assembly for semiconductor devices by utilizing a body of sintered metal oxide material exhibiting highly nonlinear resistance characteristics as the transient voltage suppressing means. A single or plurality of metallic plates disposed in parallel relationship along the length of the body of metal oxide varistor and in contact therewith provide a means for removal of heat developed in the bulk of metal oxide material. A voltage is applied across the body of metal oxide material and the metallic plates are located at selected voltage points along the body. One or more semiconductor devices may be interconnected in the metal oxide body-metallic plate assembly and at least one of the electrodes of each semiconductor device is connected directly to a metallic plate for providing a means for removal of heat developed in the semiconductor device. The highly nonlinear resistance characteristics of the metal oxide material limit the voltage appearing between the plates and thereby provide transient voltage suppression across the semiconductor devices interconnected therewith. The semiconductor devices can be interconnected in various type circuits for single phase and polyphase applications, and the assembly is of compact form for minimizing the inductance of interconnecting leads.

Description

The features of our invention which we desire to protect herein are pointed out with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like parts in each of the several figures are identified by the same reference character and wherein:

FIG. 1 is a graphical representation of the nonlinear resistance and resultant voltage limiting characteristics of metal oxide and silicon carbide material for different values of the exponent alpha plotted in terms of volts vs. amperes on a log-log scale;

FIG. 2a is a schematic diagram of a circuit application of the metal oxide varistor connected across a load in a circuit supplied by 600 volts with a 1,000 volt transient superimposed thereon;

FIG. 2b is a plot of volts vs. amperes on a log-log scale for depicting a graphical comparison of the steady state power dissipation in metal oxide and silicon carbide varistors in the FIG. 2a circuit when clamping the load voltage at 1,200 volts;

FIG. 2c is a plot of volts vs. amperes on a log-log scale for depicting a graphical comparison of the voltage clamped across the load for the metal oxide and silicon carbide varistors for a maximum steady state power dissipation of one watt in the varistors;

FIG. 3 is a perspective view, partly broken away, of a typical semiconductor device-metal oxide varistor-heat sink assembly constructed in accordance with our invention;

FIG. 4a is a schematic diagram of a phase controlled rectifier circuit utilizing metal oxide varistors connected across each semiconductor device for transient voltage suppression;

FIG. 4b is a side view, partly in section, of an assembly incorporating the circuit of FIG. 4a in a semiconductor device-metal oxide varistor-heat sink assembly in accordance with our invention;

FIG. 5a is a schematic diagram of a direct current to three phase alternating current inverter circuit utilizing metal oxide varistor devices connected across the semiconductor devices for transient voltage suppression;

FIG. 5b is a side view, partly in section, of the assembly incorporating the circuit of FIG. 5a in a semiconductor device-metal oxide varistor-heat sink assembly in accordance with our invention;

FIG. 5c is an enlarged view of a first means for supporting the assembly of FIG. 5b;

FIG. 5d is an enlarged view of a second means for supporting the assembly of FIG. 5b;

FIG. 6 is a schematic diagram of a metal oxide varistor heat sink assembly providing direct current dynamic braking in a motor application of the inverter circuit illustrated in FIG. 5a in accordance with our invention;

FIG. 7a is a schematic diagram of a metal oxide varistor heat sink assembly providing alternating current dynamic braking in a motor application of the inverter circuit illustrated in FIG. 5a in accordance with our invention;

FIG. 7b is a side view, partly in section, of the alternating current dynamic braking metal oxide varistor-heat sink assembly illustrated in FIG. 7a;

FIG. 8a is a top view of a circular staircase arrangement of a semiconductor device-metal oxide varistor-heat sink assembly in accordance with our invention;

FIG. 8b is a side view of the assembly illustrated in FIG. 8a;

FIG. 9 is a side view of a semiconductor device-metal oxide varistor-heat sink assembly wherein the varistors are in an alternate staggered arrangement in accordance with our invention; and

FIG. 10 is a perspective view of a bridge circuit arrangement of a semiconductor device-metal oxide varistor-heat sink assembly in accordance with our invention.

There are a few known materials which exhibit nonlinear resistance characteristics and which require resort to the following equation to relative quantitatively current and voltage by the power law:

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 nonmetallic varistors typically exhibit an alpha exponent of no more than 6. This relatively low value of alpha represents a nonlinear resistance relationship wherein the resistance varies over only a moderate range. Due to this moderate range of resistance variation, the silicon carbide varistor is often connected in series with a gap when used in a circuit for transient voltage suppression since continuous connection of the varistor could exceed the power dissipation capabilities thereof unless a relatively bulky body of such material is used in which case the steady state power dissipation is a rather severe limitation. An additional drawback is the ineffectiveness of the voltage clamping action as a result of the limited value of silicon carbide alpha exponent. The moderate range of resistance variation results in voltage limitation which may be satisfactory for some applications, but is generally not satisfactory when the transient voltage has a high peak value.

A new family of varistor materials having alphas in excess of 10 within the current density range of 10.sup.-.sup.3 to 10.sup.2 amperes per square centimeter has recently been produced from metal oxides although few applications have been disclosed for this new metal oxide varistor material, also referred to herein as MOV, a trademark of the General Electric Company. 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(s) 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 (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 (10,000 megohms has been measured) 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 as shown in FIG. 1. Thus, the voltage versus current characteristics of the MOV is similar to that of the Zener diode with the added characteristic of being bidirectional and over more decades of current. The conduction mechanism of the MOV is not yet clearly understood but it is completely unlike the avalanche mechanism associated with the Zener diodes, a possible theroretical explanation of its operation being that of space charged limited current. The rated voltage and the voltage range over which the varistor effect occurs are determined by the particular composition of the MOV material and the thickness to which it is pressed in the fabrication process. The MOV includes 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 also permits a much higher energy handling capability as compared to junction devices. Thus, since an MOV device can be built 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 as seen in FIG. 1 for the plot designated .alpha. = 4 and thus the silicon carbide varistor has a much smaller voltage operating range thereby limiting its applications as described hereinabove. The thermal conductivity of MOV material is fairly high (approximately one-half that of alumina) whereby it has a much higher power handling capability than silicon carbide, and its 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, soldered, and operated at very low voltages, capabilities not possible for the larger grained silicon carbide.

The voltage versus current characteristics plotted in FIG. 1 of the drawings illustrate the nonlinear or exponential resistance characteristics exhibited by varistor material, and in particular, the increasing nonlinearity and enhanced voltage limiting obtained with increased values of the exponent alpha where the top line .alpha. = 4 is typical for silicon carbide varistors and the three lines .alpha. = 10, 25 and 40 apply to varistors fabricated of MOV material. The VOLTS abscissa is in terms of the voltage appearing across the terminals of a specific MOV device in response to current flowing through the bulk of the MOV material and represented along the CURRENT ordinate. 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. Extension of the plots to lower and higher current levels would obviously indicate correspondingly much higher and lower resistances, respectively, and operation of the MOV device may transiently reach such levels depending upon the particular circuit application of the device. For purposes of comparison, each of the volts versus amperes plots passes through the point identified by 100 volts and one milliampere. It should be understood that metal oxide materials (zinc oxides) are available having alpha exponents even greater than 40 which thereby obtains even greater enhanced voltage clamping action than that exhibited for the alpha = 40 line.

Referring now to FIG. 2a, there is shown a simple direct current circuit utilizing a varistor connected across a load and serving the basis for the graphs of FIGS. 2b and 2c to be described hereinafter. In particular, the circuit includes a load 20 connected in series with a 600 volt D.C. source and a source 21 producing a 1,000 volt transient. The voltage transient is assumed to be a square wave pulse of 10 millisecond duration. The internal impedance of the 600 volt source is represented by series resistor 22 of 10 ohms. A varistor device 23 is connected across load 20 for clamping or limiting the voltage thereacross to a desired value. The D.C. is used for convenience to simplify the example, and is also valied for A.C. operation when properly analyzed.

Referring now to FIG. 2b, there are drawn the resistance lines which illustrate the operating characteristics of the circuit illustrated in FIG. 2a. In particular, the upper 10 ohm source impedance line indicates the voltage developed across load 20 with increasing current flow therethrough, the maximum 1,600 volts being the summation of the steady state 600 and transient 1,000 volts. For a current flow of 40 amperes, the load voltage is 1,200 volts due to the 400 volt drop across the 10 ohm resistor 22. In the case wherein a silicon carbide varistor 23 is connected across load 20 for clamping the voltage at 1,200 volts for a current flow of 40 amperes, the load line for such varistor having an alpha exponent equal to 6 crosses the steady state 600 volt line at 0.6 ampere resulting in a steady state power dissipation of 360 watts in the varistor device. In comparison, a metal oxide varistor fabricated of zinc oxide material having an alpha exponent of 25 has a steady state power dissipation of a mere 90 milliwatts. Thus, the metal oxide varistor employed in our invention has a steady state power dissipation which is in the order of 1/4,000 of the dissipation in a silicon carbide varistor. Obviously, a metal oxide varistor having an alpha exponent in excess of 25 would have a steady state power dissipation even less than the 90 milliwatts. It can be appreciated that the 360 watt steady state power dissipation in a silicon carbide varistor is virtually unbearable and would quickly result in destruction of the varistor device unless such device was of such massive volume that it would have the requisite energy handling capability, a completely unpractical consideration. Also, it should be noted that the more typical, commercially available silicon carbide varistors have alpha exponents of 3 to 4 whereby the problem associated therewith is even worse.

Another convenient manner in comparing the characteristics of the metal oxide and silicon carbide varistor devices is illustrated in FIG. 2c wherein a maximum allowable steady state power dissipation of 1 watt is maintained for both varistor devices. In the case of the silicon carbide varistor, (.alpha. = 6) the volt-ampere characteristic line intersects the 10 ohm source impedance line at the voltage level of 1,594 volts, that is, the silicon carbide varistor is capable of clamping the load voltage at only 1,594 volts for a maximum applied voltage of 1,600. In comparison, the metal oxide varistor having an alpha exponent of 25 and identical 1 watt steady state power dissipation is capable of clamping the voltage across the load at 940 volts, an improvement of 654 volts over the silicon carbide varistor. Thus, the silicon carbide varistor has suppressed the applied 1,600 volts by a mere 6 volts whereas the metal oxide varistor has suppressed it by 660 volts. The FIGS. 2b and 2c graphs clearly indicate the superior steady state power dissipation and voltage clamping characteristics obtained by the metal oxide material as compared to a silicon carbide varistor. These superior characteristics are utilized in the semiconductor device-varistor heat sink assemblies to be described hereinafter in accordance with our invention.

Referring now to FIG. 3, there is shown a typical assembly of a semiconductor device-metal oxide varistor-heat sink structure constructed in accordance with our invention. Basically, the assembly consists of at least one body of sintered metal oxide material, which as one example, may consist primarily of zinc oxide and a small percentage of bismuth oxide, one or a plurality of metallic plates disposed in parallel relationship along the length of each body of metal oxide material and in contact therewith, and at least one semiconductor device mounted on the plate or connected between two of the plates in each stack. The metallic plate(s) function to remove heat generated in the bulk of the metal oxide material during circuit operation in which the metal oxide varistor functions as a transient voltage suppressor. Although the thermal conductivity of MOV material is relatively high, it is often not sufficient for the relatively high power circuits to be described hereinafter. The plate(s) also provide the heat sink for the semiconductor devices mounted thereon and employed in the circuit. The semiconductor devices may be of the packaged (stud) type as illustrated, or of the glassivated chip type, conventional potting techniques being utilized with the latter devices. Forced air can be provided over the plates to increase the heat transfer, if necessary. In particular, the assembly illustrated in FIG. 3 consists of a plurality of metallic plates 30-34 fabricated of a metal having a relatively high heat conductivity such as copper or aluminum. The body 53 of sintered metal oxide material may comprise a single body passing through aligned holes in metallic plates 30-34 as illustrated in FIG. 5b or may be a plurality of smaller bodies as illustrated in the other FIGURES, being rigidly retained in place between the metallic plates. The single or plurality of MOV bodies may be solid as illustrated in FIGS. 5b, 7b and 10 or may be hollow as illustrated in FIGS. 3, 4b, 8b and 10 wherein FIG. 4b is a side view of a phase controlled rectifier circuit shown without the input-output lines in FIG. 3.

The typical assembly as shown in FIG. 3, in accordance with our invention, includes suitable mounting means such as brackets 36 connected near the ends along at least one side of the assembly, and preferably, on two sides to assure a rigid support. In the particular embodiment of the assembly illustrated in FIGS. 3 and 4b, the MOV is a plurality of hollow members 53a and 53d positioned in alignment between adjacent metallic plates 30-34. The MOV bodies may have any of a number of shapes, the most common being circular right cylinders. This assembly of MOV members and metallic plates is rigidly retained by any suitable means, one manner illustrated in FIGS. 3 and 4b being a bolt 40 or other suitable threaded member passing through all of the plates and MOV bodies and through brackets 36. Bolt 40 is electrically insulated from the MOV bodies, to prevent short circuiting thereof, by means of a hollow cylindrical ceramic spacer 41 positioned around bolt 40. Spacer member 41 is a single body having ends thereof terminating at the adjacent inner surfaces of brackets 36 and is keyed along its length. Keyed holes in plates 30-34 are aligned with insulating spacer member 41 and are approximately the same dimension to prevent rotation of the plates upon assembly of the structure. Additional hollow MOV bodies 42 may be located between the outermost plates 30 and 34 and the brackets 36, or alternatively, such members 42 may be ceramic or other type electrically insulating members. A nut 43 is tightened on a lockwasher 44-spacing washer 45 assembly in contact with the outer surface of bracket 36 for retaining the elements of the assembly as a rigid structure. A second spacing washer 46 may be utilized on the opposite side of the bracket and may be keyed with spacing member 41.

Plates 30-34 have a portion of their upper and lower surfaces overlapped by the MOV bodies and in contact therewith and thus provide a means for removal of heat developed in the bulk of the metal oxide material, that is, provide a heat sink for the MOV bodies as well as assuring continuity of a metal oxide varistor across each of the silicon controlled rectifiers (SCRs). The plates also provide a heat sink for semiconductor devices as well as a circuit interconnection and terminal means for the devices and electric power input and output lines. As one example of a circuit application of our invention, FIG. 4a illustrates a phase controlled rectifier circuit employing SCRs which are controllably gated on and turned off to provide a predetermined direct current output voltage (D.C. OUTPUT) in response to a three phase alternating current input voltage (30 A.C. INPUT). In particular, SCR devices 47, 48 and 49 have their cathode electrodes connected to the positive polarity (+) output voltage terminal of the circuit and have their anode electrodes respectively connected to the three phase A.C. input lines (AC.sub.1, AC.sub.2, AC.sub.3). In like manner SCRs 50, 51 and 52 have their cathodes connected to the three phase A.C. input lines and their anodes connected to the negative polarity (-) output terminal. A metal oxide varistor 53 is connected across each of the SCRs 47-52 for transient voltage suppression purposes and in this case protects the three A.C. input lines, the six semiconductor devices and the D.C. output lines (bus) against transient voltages.

The semiconductor device-metal oxide varistor-heat sink assembly illustrated in FIG. 4b is a structural embodiment of the phase controlled rectifier circuit illustrated in FIG. 4a. Thus, the three phase A.C. input is supplied to the central metallic plate 32, 32a and 32b by means of conductors AC.sub.1, AC.sub.2, AC.sub.3 which are soldered or otherwise suitably connected to such plates. It should be understood that the assembly is adapted for any number of alternating current phases and would include a stack of intermediate metallic plates, bodies of MOV material and semiconductor devices for each phase. The outermost plates 30 and 34 are of sufficient length to retain all of the stacks of intermediate metallic plates, MOV bodies and semiconductor devices therebetween. The intermediate metallic plates 31 and 33 may be of square shape as illustrated, or circular, or of other shapes as may be necessitated by the particular application. The SCRs 47, 48 and 49 have their anodes soldered or in any other suitable manner directly connected to the upper surfaces of central plates 32, 32a and 32b, respectively, for heat sink purposes. In like manner, the anodes of rectifier devices 50, 51 and 52 are soldered to the upper surfaces of common bottom plate 34. Ideally, the overall length of the SCRs from the anode to the cathode would equal the spacing between the central plate 32 and the outermost common plates 30 and 34. Such ideal situation is illustrated in FIG. 4b whereby the terminal end of the cathodes may be soldered or in other suitable manner directly connected to the bottom surface (or alternatively, to the top surface after passage through small holes 55) of common top plate 30 in the case of rectifiers 47, 48 and 49 and to the surfaces of central plates 32, 32a and 32b for rectifiers 50, 51 and 52, respectively, and thereby avoid use of interconnecting leads which introduce unwanted inductance. However, in most practical applications, the voltage rating of the MOV bodies, which is determined by the thickness dimension (height dimension in FIG. 4b) will not be equal to the overall length of the semiconductor devices being protected against transient voltage, and thus in the more general case as depicted in FIG. 5b, electrically conductive leads will be connected between the cathodes and metallic plates. However, due to the compactness of the assembly, the lengths of such leads will be short thereby minimizing the added inductance. In the FIG. 4b embodiment, the semiconductor devices 47-52 pass through holes in the intermediate plates 31, 31a, 31b and 33, 33a and 33b, such holes 54 being depicted in FIG. 3 in plates 33 and 33a. The holes 54 are of diameter slightly larger than the diameter of the semiconductor device passing therethrough to provide some spacing therebetween. The three phase A.C. input leads and the D.C. output leads can be connected to the plates by means of terminals soldered to the plates, or alternatively, terminal lugs may be fitted through small holes in the plates and the leads soldered thereto.

As can be seen most clearly in the FIG. 4a schematic representation of the FIG. 4b phase controlled rectifier assembly, a body 53 of MOV material is connected across the two power electrodes of each semiconductor device which in this case is a silicon controlled rectifier. The thickness of each single or plurality of MOV bodies connected across each rectifier is selected for a voltage rating at a desired level above the circuit rated voltage at which the voltage clamping or suppression action will occur. Thus, the thickness of the MOV body (or bodies) may be selected for a voltage rating which is in the order of 10 percent above the circuit rated voltage, as one example. The principal advantage of our invention is, of course, the superior transient voltage suppression obtained with the use of a metal oxide varistor. This superior voltage suppression is obtained primarily due to the following three exceptional properties of MOV material: (1) the resistance characteristics are highly nonlinear (alpha greater than 10) over a wide range of current and result in a high degree of voltage limiting, (2) the response time is negligible, and (3) the relatively high thermal conductivity permits rapid dissipation of heat developed in the MOV material due to the voltage transient. The body of MOV material may be selected for a voltage rating which provides voltage suppression only for intermittent transients which may appear on the three phase A.C. input lines and, or the D.C. output lines, or, may be selected to also provide for voltage suppression of transients occurring in the semiconductor circuit due to turn-on or commutating off of the SCR devices, it being understood that FIG. 4a is a simplified circuit not showing the turn-on and commutating elements if employed. As can be seen from the FIG. 1 graph, during the transient (high) voltage condition, the MOV material provides a relatively low resistance path for the current which thence decays at a rate determined primarily by the LR time constant of the circuit, the resistance of the MOV increasing substantially as the voltage, and current, are decreasing. During steady state circuit operation, the MOV exhibits a relatively high resistance and low power dissipation and has a negligible effect on the operation of the SCRs. The MOV body may be visualized as having first and second opposed major surfaces which are flat and parallel to each other. The thickness of the MOV device is determined primarily by the voltage rating thereof and, as examples, MOV bodies having voltage ratings of 240 and 480 volts have thicknesses of 0.10 and 0.20 inches, respectively. Metallized surfaces may be formed on the opposed parallel major surfaces of the MOV bodies for providing good electrical and mechanical contact with the MOV material. The metallized surface is obtained by a suitable bonding process which may be accomplished by low temperature soldering or by pressure contact, as two examples. The metallized surface may be obtained by firing a thin layer of silver-glass frit (silver and glass particles) on the MOV surface. Ohmic contact is utilized in order to take advantage of the bulk phenomenon operation of the MOV material. In the application herein described, the metallized surface need be applied only along the portions of the major surfaces of the MOV bodies which are in contact with the metallic heat sink plates.

The metal oxide varistor connection across each semiconductor device also functions to reduce the rate of change of a transient voltage impressed across the power electrodes of the semiconductor device to thereby prevent false triggering and resultant unwanted current conduction therethrough or other breakdown of the device. This rate of change of voltage protection is obtained because a simplified equivalent circuit representation of a metal oxide varistor consists of an ideal varistor in parallel with a geometry-dependent capacitor and a shunt resistor.

Referring now to FIG. 5a, there is shown a schematic diagram of conventional D.C. to three phase A.C. inverter circuit which utilizes bodies of MOV material for transient voltage suppression. In particular, the anodes of SCRs 47, 48 and 49 are connected to the positive D.C. input terminal and the cathodes of SCRs 50, 51 and 52 are connected to the negative input terminals. Cathode and anode of rectifiers 47 and 50, respectively, are connected to a first (AC.sub.1) of the three phase output of the inverter. In like manner, the cathode and anode electrodes of SCRs 48 and 51, respectively, are connected to a second (AC.sub.2) output and so on. Feedback diodes 54 are connected in parallel with the silicon controlled rectifiers for conducting reactive commutation current. It should be obvious that other elements of the inverter circuit both of the active and passive type such as linear resistors, capacitors and inductors may be mounted on our assembly, as desired, for purposes of minimizing the lengths of interconnecting leads.

The semiconductor device-metal oxide varistor-heat sink assembly embodiment of the FIG. 5a circuit is illustrated in FIG. 5b which is similar to that illustrated in FIG. 4b with the following exceptions. Alternate metallic plates 31 and 33 are of smaller dimension than the same plates in the FIG. 4b assembly for permitting portions of the SCR and feedback diode devices to extend beyond such plates in spaced apart relationship therefrom. That is, alternate plates 31 and 33 in the FIG. 5b embodiment do not include the holes 54 utilized in the FIG. 4b embodiment for passage of the rectifier devices therethrough. The FIGURE 5b assembly also illustrates the more general sizing of the MOV and semiconductor elements whereby the cathode electrodes of the SCRs and diodes are connected by means of electrically conductive leads to the appropriate metallic plates. It must be stressed that the compactness of the FIG. 5b assembly, and all of the other assemblies described herein, permits use of short lengths of interconnecting leads since the other circuit elements may also be mounted on the metallic plates, and thereby minimizes the added inductance. This inductance of interconnecting leads is especially important for the satisfactory operation of high frequency power circuits such as inverters, as well as for both high and low frequency circuits subjected to microsecond voltage transients resulting from causes such as charge storage effects in the semiconductor device.

Due to the reverse polarity connections of each SCR and its feedback diode, the anode electrodes of each pair of such devices are connected to alternate metallic plates for improved heat sink efficiency. The primary difference between the FIGS. 4b and 5b assemblies are the use of a single solid body 53 of MOV material in the FIG. 5b embodiment which passes through the center of each of the metallic plates 30-34 and is in good mechanical contact therewith to assure electrical circuit continuity between each plate and the MOV body. The metallic plates may be rigidly attached to the MOV body in any suitable manner such as by a press fit or by being soldered thereto. In this application wherein a voltage is impressed across the full length of the MOV body, the metallic plates also function as voltage tap points.

Suitable mounting means for the assembly may be provided on the outermost metallic plates 30 and, or 34 for supporting the metal oxide body - metallic plate assembly on a second assembly (not shown) associated therewith. As one example of mounting means, FIG. 5c illustrates mounting bracket 36 retained at the end portion of MOV body 53 by means of a pair of speed nuts 55. Top metallic plate 30 (and the other plates) are rigidly retained on MOV body 53 by means of a dogged configuration of the plate in the region of the hole through which body 53 passes. In the FIG. 5d example of a mounting means, a second pair of speed nuts 57 are used to retain the metallic plates on the MOV body. Also, a phenolic or other electrically insulating member 58 is rigidly retained on MOV body 53 and the metallic mounting bracket 36 is connected to member 58 remote from MOV body 53. This latter mounting means permits the ground bracket 36 to be at ground potential whereas bracket 36 in FIG. 5c is at some voltage to ground different from the top plate 30 voltage in the presence of a transient voltage. FIGS. 5c and 5d also indicate that our assembly may utilize only a single metallic plate as the heat sink for both the MOV body and any semiconductor device and other circuit elements mounted thereon. The mounting bracket 36 which can also be described as a ground bracket by being in contact with MOV body 53 in FIG. 5c permits current flow to ground during a transient.

Referring now to FIG. 6, there is shown a three phase alternating current motor 60 supplied with three phase A.C. power from the output of a D.C. to three phase A.C. inverter of the type illustrated in FIGS. 5a and 5b and also includes an MOV direct current dynamic braking network 63 which may be connected to the input of the inverter circuit. The system operation is controlled by means of a ganged switch which includes a first set of contacts 61 in the D.C. power input line and a second set of contacts 62 for interconnecting the D.C. input of the inverter to the MOV network 63 which functions as a variable resistor for D.C. dynamic braking. Alternatively, separate switches 61 and 62 may be employed. The MOV D.C. dynamic braking network 63 consists of a plurality of MOV bodies which are connected across the input terminals of the inverter circuit of FIG. 5b and provide the load during pump-back operation when braking the motor. The plurality of MOV bodies are interconnected to form a plurality of parallel MOV paths since the pump back mode of operation may involve a high initial current flow. Each parallel MOV path includes an equal number of one or more MOV bodies of voltage level consistent with the rated voltage appearing across the inverter input terminals. For purposes of simplicity, MOV network 63 is illustrated as comprising three parallel paths each having two MOV bodies serially connected, it being understood that the number of parallel paths and cross sectional area of each MOV is determined by the energy to be absorbed in the braking operation. The structural assembly for this metal oxide varistor D.C. dynamic braking network is illustrated in FIG. 7b which additionally includes a variable resistor that does not form a part of the D.C. dynamic braking assembly 63. For convenience, the structure of the assembly 63 will be described in the discussion of the assembly of FIG. 7b.

FIG. 7a illustrates a second motor system similar to that of FIG. 6 in that a three phase A.C. motor is supplied with power from the output of a D.C. to three phase A.C. inverter of the type illustrated in FIGS. 5a and 5b. In FIG. 7a, however, the system is further provided with an alternating current dynamic braking network 71 utilizing metal oxide varistors and being connected to the juncture of the motor 60 input terminals and inverter output terminals by means of ganged switch 70. The A.C. dynamic braking network 70 also includes a plurality of parallel MOV paths each of which may include a plurality of MOV bodies 53 as does the D.C. dynamic braking network 63 of FIG. 6. One of the distinctions between networks 63 and 71 is that the input to the D.C. dynamic braking network is across each of the parallel paths of MOV bodies whereas the A.C. dynamic braking network 71 has the input connected to the center point of each of the parallel MOV paths. Also, since the A.C. dynamic braking network 71 is connected directly across the motor 60 terminals, a variable resistor 72 is connected across the parallel paths of MOV bodies for purposes of controllably varying the LR time constant of the dynamic braking circuit. Resistor 72 may be a passive device or an active circuit equivalent to allow more versatile control operation, one example of the active circuit being a semiconductor diode bridge circuit with a transistor connected across the bridge. The networks 63, 71 and assemblies thereof are thus substantially the same except for the use of an additional variable resistor 72 in the A.C. network and the manner of interconnection of the two networks with respect to the inverter.

The structure of the A.C. dynamic braking network 71 is illustrated in FIG. 7b wherein the bodies of MOV material 53a, 53b, 53c and 53d are depicted as separate solid bodies as distinguished from the separate hollow bodies in FIG. 4b and the single solid body in FIG. 5b to illustrate further examples of types of MOV bodies that may be employed in our invention. The intermediate metallic plates 31, 32 and 33 and the longer outermost plates 30 and 34 are utilized in the A.C. dynamic braking assembly as the heat sink for the MOV bodies as in the case of a similar assembly for the D.C. dynamic braking assembly 63. Variable resistor 72 is illustrated as being directly connected across plates 30 and 34 and the power input connections are provided at the plates defining the center point of each stack of MOV bodies for the A.C. dynamic braking assembly. Alternatively, resistor 72 may be connected across plates 30, 34 but be physically located remote from the assembly. Also, resistor 72 may be another body of MOV material connected between plates 30 and 34 and provided with a suitable pick-off for selecting the desired resistance value. In the case of the D.C. dynamic braking assembly, the power input connections are across plates 30 and 34. As a result, the D.C. dynamic braking assembly 63 may include any number of parallel paths of MOV bodies whereas the three input lines to the three phase A.C. dynamic braking assembly 71 restricts such plurality of center-tapped parallel paths of MOV bodies to multiples of the number of phases to be protected.

Referring now to FIGS. 8a and 8b, there are shown a top and side view, respectively, of a high voltage rectifier assembly comprising a plurality of serially connected high voltage rectifiers each of which is protected against transient voltage surges by means of a separate (one or more) solid body of MOV material. Due to the higher voltage impressed across the string of rectifiers, the metallic plates 80-89 are arranged in a circular staircase pattern around central electrically insulating members 90 which provide the support for the metallic plates. As most clearly seen in FIG. 8a, adjacent metallic plates are in complete overlapping relationship in the region of the MOV bodies 53a-i and are in partial overlapping relationship in the region of the location of high voltage rectifiers 91a-i. The circular staircase prevents arcing across the entire assembly due to the high voltage impressed thereacross. Each insulating member 90 is provided with a top shoulder portion for supporting a metallic plate between adjacent insulating members. The insulating members 90 are provided with a central aligned hole through which is passed a suitable bolt or other threaded metal rod secured by nuts and washers for fastening the insulating members 90 and metallic plates into a rigid assembly. High voltage terminals are connected to the outer surfaces of the outermost plates 80 and 89 and insulated high voltage leads 93 and 92 are respectively connected thereto. The staircase pattern is utilized to solve the voltage gradient problem inherent in high voltage applications and it should be understood that the MOV bodies need not necessarily be all of the same size and voltage rating in order to obtain a desired uniform voltage grading. Thus, the MOV bodies may provide any desired voltage distribution along the assembly, and specific voltages may be tapped off at the intermediate plates 81-88, as desired.

Referring now to FIG. 9, there is illustrated yet another assembly of the MOV bodies, and in particular, an assembly wherein the MOV bodies are alternately staggered for purposes of providing both a separating and supporting means for the metallic plates. Thus, the FIG. 9 embodiment need not utilize an external clamping or connecting means for obtaining a rigid assembly since the staggered arrangement of MOV bodies provide the necessary rigidity. As in the case of the FIGS. 7b and 8b embodiments, the MOV bodies may be soldered, clamped, riveted or in any other suitable means connected between adjacent metallic plates to provide a rigid connection thereto. The semiconductor devices 95 may conveniently be connected in an aligned arrangement at the centers of the metallic plates, or alternatively, may also be in alternate stacked relationship (not shown) as desired. The staggered arrangement of the MOV bodies breaks up a continuous path across the MOV bodies to thereby avoid creepage or other breakdown across a string of serially connected MOV bodies. This creepage may be very important in particular environments of operation of our semiconductor device-varistor-heat sink assemblies in which the MOV bodies may have their outer surfaces coated with a material that induces the creepage.

The hereinabove described embodiments have all been illustrated with MOV bodies being directly connected between the adjacent metallic plates. It should be obvious that the metallic plates may not be required at both major surfaces of each MOV body in all cases and thus may be required to be located only along at least one of such major surfaces to provide the heat sink function. In FIG. 10 there is shown an assembly in accordance with our invention wherein four diodes are connected in bridge circuit relationship and only three metallic plates 31, 32 and 33 are utilized. The supporting structure is of the same type as illustrated in FIG. 3 except that the outermost top metallic plate 30 is replaced by a terminal lug 96 and the intermediate metallic plate 33 adjacent the outermost lower metallic plate 34 is replaced by a second terminal lug 97. The cathode electrodes of the first 98, second 99, third 100 and fourth 101 diodes are respectively connected to terminal lug 96, 96 and plate 31, 31 and the anode electrodes are respectively connected to plate 34, plate 32, 32 and plate 34. Shielded conductor 102 provides electrical continuity between plates 31 and 34 and passes through a hole in the tab portion of plate 32. Leads 104 and 105 respectively connected to plates 32 and 34 provide the input to the bridge whereas leads 106 and 107 respectively connected to terminal lug 96 and plate 31 provide the bridge output. The outermost bodies 103 connected between terminal lug 96 and bracket 36 and between metallic plate 34 and bracket 36 may also be MOV bodies or electrically insulating bodies depending upon whether it is desired to insure that the brackets are not at any potential above ground. Obviously, other circuit elements could also be interconnected between the metallic plates or the metallic plates and terminal lugs, as required. Two additional metallic plates, replacing terminal lugs 96 and 97, would be utilized for obtaining maximum heat sink capabilities wherein each MOV body major surface is in contact with a metallic plate.

Having described several embodiments of a power semiconductor device-metal oxide varistor-heat sink assembly which provides transient voltage suppression across the semiconductor devices, it should be obvious that the bodies of MOV material can assume any of a number of shapes and may be connected in various arrangements. The MOV bodies in each assembly may have equal dimensions or unequal dimensions depending upon the voltage tap points to be defined by the metallic plates, and/or terminal lugs which are connected along the MOV body. The high resistance of the MOV bodies during circuit steady state operation permits only a small current flow through the bodies and therefore results in very low power dissipation as indicated in FIGS. 2b and 2c. In view of the low steady state power dissipation, the MOV body can be made physically small for the energy handled and permits the mounting of the semiconductor devices and other circuit elements very close thereto thereby minimizing additional inductance of connecting leads which usually is a very restrictive factor in achieving the real clamping or voltage suppression desired, that is, the physical limitations of the other component sometimes used in conjunction with semiconductor devices require remote location and substantial interconnections. The use of the metal oxide varistor for voltage suppression provides an assembly having many advantages over one wherein other components such as selinium barrier layer rectifiers, Zener diode junction devices or silicon carbide devices are employed since each of such components have at least one of the following limitations which is not present in the metal oxide varistor: the nonlinear resistance relationship varies only a moderate range, the power dissipation in the steady state operation of the circuit is excessive, the voltage clamping action is not adequate, the component does not have bulk properties and therefore is not applicable for power circuit applications, the component has polarity restrictions which are not usually favorable for mounting in conjunction with a semiconductor device, the component is not available for high voltage applications. Thus, while our invention has been particularly shown and described with reference to specific embodiments thereof, it should be obvious by those skilled in the art that obvious changes in form and detail may be made without departing from the scope of the invention as defined by the following claims.

Claims

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A semiconductor device-metal oxide varistor-heat sink assembly comprising

a body of sintered metal oxide material exhibiting highly nonlinear resistance characteristics,

a plurality of metallic plates disposed in parallel relationship along the length of said body of metal oxide material and in contact therewith for providing a means for removal of heat developed in the bulk of the metal oxide material,

means for supply voltage across said body of metal oxide material, and

at least one semiconductor device provided with two electrodes, a first of the two electrodes connected directly to a first of said metallic plates whereby the metallic plates also provide a means for removal of heat developed in the semiconductor device and also provides support therefor, the second electrode connected to a predetermined point along said body of metal oxide material corresponding to a selected voltage, the highly nonlinear resistance characteristics of the metal oxide material limiting the voltage appearing between the metallic plates and the selected voltage point in accordance with the thickness dimensions of the metal oxide material therebetween to thereby provide transient voltage protection for the semiconductor device.

2. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

said metallic plates are oriented perpendicular to the longitudinal axis of the body of metal oxide material.

3. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

the second electrode of said semiconductor device is connected to a terminal lug in contact with the body of metal oxide material at the selected voltage point.

4. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

the second electrode of said semiconductor device is connected to a second of said metallic plates in contact with the body of metal oxide material at the selected voltage point.

5. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

a plurality of said semiconductor devices are interconnected in the assembly and at least the first electrodes of each of said semiconductor devices are directly connected to the metallic plates.

6. The semiconductor device-varistor-heat sink assembly set forth in claim 5 wherein

said plurality of semiconductor devices are connected in series circuit relationship.

7. The semiconductor device-varistor-heat sink assembly set forth in claim 5 wherein

said plurality of semiconductor devices are connected in bridge circuit relationship.

8. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

said body of metal oxide material is a single right circular cylindrical body,

said metallic plates provided with aligned holes for passage of said body of metal oxide material therethrough and having the sides of the holes in contact with the side of said body of said metal oxide material at the selected voltage points.

9. The semiconductor device-varistor-heat sink assembly set forth in claim 8 wherein

said plates are equally spaced along said body of metal oxide material.

10. The semiconductor device-varistor-heat sink assembly set forth in claim 8 wherein

at least one of said plates is unequally spaced from the other plates along said body of metal oxide material.

11. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

said body of metal oxide material is a plurality of right circular cylindrical bodies.

12. The semiconductor device-varistor-heat sink assembly set forth in claim 11 wherein

said plurality of cylindrical bodies of metal oxide material are each of equal thickness whereby each body provides the same level of voltage suppression.

13. The semiconductor device-varistor-heat sink assembly set forth in claim 11 wherein

at least one of said cylindrical bodies of metal oxide material is of thickness unequal to the thickness of the other bodies to provide a different level of voltage suppression, said bodies of metal oxide material being separated from each other by the metallic plates passing therebetween.

14. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

said metallic plates are each of equal dimension.

15. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

at least one of said plates is of unequal dimension compared to the other plates.

16. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

selected of said metallic plates are provided with holes for accommodating a semiconductor device having its first electrode connected directly to another of said metallic plates.

17. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

selected of said metallic plates are provided with holes for passage of electrical conductors from the second electrode of the semiconductor device for interconnection to the predetermined point along said body of said metal oxide material, the interconnecting conductors being of short length due to the compactness of the assembly whereby unwanted inductance introduced by the interconnecting conductors is minimized.

18. The semiconductor device-varistor-heat sink assembly set forth in claim 11 wherein

said bodies of metal oxide material are in alignment and retained in rigid contact with said plates.

19. The semiconductor device-varistor-heat sink assembly set forth in claim 11 wherein

said bodies of metal oxide material are in alignment and at least one end surface of each body of metal oxide material is in rigid contact with a corresponding one of said plates.

20. The semiconductor device-varistor-heat sink assembly set forth in claim 11 wherein

said bodies of said metal oxide material are in alignment and the intermediate bodies have both end surfaces thereof in rigid contact with said plates.

21. The semiconductor device-varistor-heat sink assembly set forth in claim 11 and further comprising

means for retaining said bodies of metal oxide material and said plates in a single rigid assembly.

22. The semiconductor device-varistor-heat sink assembly set forth in claim 11 wherein

said bodies of metal oxide material are positioned in a staggered relationship between the metallic plates to form a pattern wherein alternate bodies are in alignment to provide a structurally rigid assembly.

23. The semiconductor device-varistor-heat sink assembly set forth in claim 11 wherein

said plates are staggered in a circular staircase pattern whereby said assembly is especially adapted for high voltage applications.

24. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

said body of metal oxide material comprises at least one body for each phase of a polyphase semiconductor circuit, the bodies in each phase being arranged in spaced apart parallel relationship to the bodies in the other phases,

a first and second of said plurality of metallic plates being of greater length dimension and forming the outermost plates of the assembly and retaining end portions of said bodies of metal oxide material,

the remainder of said plurality of metallic plates connected along the lengths of said bodies of metal oxide material intermediate said outermost plates, said intermediate plates being of smaller length than said outermost plates and arranged in a plurality of separate stacks equal to the number of phases in the semiconductor circuit.

25. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

the second electrode of said semiconductor device is connected to a second of said plates positioned along said body of metal oxide material at the selected voltage point.

26. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

the second electrode of said semiconductor device is connected to a terminal lug positioned along said body of metal oxide material at the selected voltage point.

27. The semiconductor device-varistor-heat sink assembly set forth in claim 11 wherein

said bodies of metal oxide material are hollow.

28. The semiconductor device-varistor-heat sink assembly set forth in claim 11 wherein

said bodies of metal oxide material are solid.

29. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

said body of metal oxide material has an alpha exponent in excess of 10.

30. The semiconductor device-varistor-heat sink assembly set forth in claim 1 wherein

said body of metal oxide material is composed primarily of zinc oxide.

31. A metal oxide varistor-heat sink assembly comprising

a plurality of bodies of sintered metal oxide material exhibiting highly nonlinear resistance characteristics,

first and second metallic plates disposed in parallel relationship to each other and in contact with the outermost end portions of said bodies of metal oxide material, said bodies comprising at least one body for each phase of a polyphase circuit to be protected against transient voltages, the bodies in each phase being arranged in spaced apart parallel relationship to the bodies in the other phases,

a plurality of metallic plates of smaller size than said first and second plates, said plurality of plates disposed along the lengths of said bodies of metal oxide material at predetermined points thereof and in contact therewith intermediate said first and second plates and parallel therewith, said first, second and plurality of plates oriented perpendicular to the longitudinal axes of said bodies of metal oxide material, said intermediate plates arranged in a plurality of separate stacks equal to the number of the phases to be protected, said plates providing a means for removal of heat developed in the bulk of the bodies of metal oxide material, and

electrical conductor means connected to selected of said metallic plates for connecting said selected plates to a source of voltage which may be subjected to transient voltages, the highly nonlinear resistance characteristics of the metal oxide material limiting the voltage appearing between the metallic plates in accordance with the thickness dimensions of the metal oxide material therebetween.

32. The metal oxide varistor-heat sink assembly set forth in claim 31 wherein

said electrical conductor means are connected to said first and second metallic plates.

33. The metal oxide varistor-heat sink assembly set forth in claim 31 wherein

said electrical conductor means are connected to centermost plates in each stack of plates.

34. The metal oxide varistor-heat sink assembly set forth in claim 33 and further comprising

a variable resistor connected across said first and second metallic plates whereby said assembly forms an alternating current dynamic braking network with means for varying the LR time constant of the dynamic braking circuit.

35. The metal oxide varistor-heat sink assembly set forth in claim 34 wherein

said variable resistor is an active semiconductor circuit means for providing variable impedance.

36. The metal oxide varistor-heat sink assembly set forth in claim 31 and further comprising

a plurality of semiconductor devices each provided with at least two electrodes, said semiconductor devices connected in a desired circuit relationship and each device having at least a first of its electrodes connected directly to a corresponding one of said metallic plates whereby said plates also provide a means for removal of heat developed in the semiconductor devices and mechanical support therefor, the second electrodes of said semi-conductor devices connected to predetermined points along said bodies of metal oxide material corresponding to selected voltages, the nonlinear resistance characteristics of the metal oxide material limiting the voltages appearing between the metallic plates and selected voltage points in accordance with the thickness dimensions of the metal oxide material therebetween to thereby provide transient voltage suppression protection for the semi-conductor devices.

37. A metal oxide varistor-heat sink assembly comprising

a body of sintered metal oxide material exhibiting highly nonlinear resistance characteristics,

at least one metallic plate disposed along the length of said body of metal oxide material and normal to the longitudinal axis of said body,

means for supporting said body of metal oxide material on said plate whereby said plate provides a heat-sink for the body of metal oxide material, and

means for supporting the metal oxide body-metallic plate assembly on a second assembly associated therewith.

38. The metal oxide varistor-heat sink assembly set forth in claim 37 and further comprising

at least one semiconductor device mounted on said metallic plate whereby said plate also provides a heat-sink for said semiconductor device, a second electrode of said semiconductor device connected to a point on said body of metal oxide material longitudinally displaced from the point of connection of said plate whereby the highly nonlinear resistance characteristics of the metal oxide material limit the voltage appearing across said semiconductor device to provide transient voltage protection therefor, the interconnection of the second electrode to the body of metal oxide material being of short length to minimize the unwanted inductance introduced thereby.