U.S. patent number 3,745,505 [Application Number 05/191,168] was granted by the patent office on 1973-07-10 for semiconductor device-metal oxide varistor-heat sink assembly.
Invention is credited to John D. Harnden, Jr., Fred C. Turnbull.
United States Patent |
3,745,505 |
Turnbull , et al. |
July 10, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
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.
Inventors: |
Turnbull; Fred C. (Scotia,
NY), Harnden, Jr.; John D. (Schenectady, NY) |
Family
ID: |
22704394 |
Appl.
No.: |
05/191,168 |
Filed: |
October 21, 1971 |
Current U.S.
Class: |
338/20; 257/723;
165/80.3; 257/722 |
Current CPC
Class: |
H01L
25/03 (20130101); H01L 2924/00 (20130101); H01L
2924/3011 (20130101); H01L 2924/0002 (20130101); H01L
2924/0002 (20130101) |
Current International
Class: |
H01L
25/03 (20060101); H01c 007/10 () |
Field of
Search: |
;378/13,20,21 ;317/234A
;165/80 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Albritton; C. L.
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.
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.
* * * * *