U.S. patent number 3,810,063 [Application Number 05/229,362] was granted by the patent office on 1974-05-07 for high voltage current limiting fuse including heat removing means.
This patent grant is currently assigned to Westinghouse Electric Corporation. Invention is credited to Donald D. Blewitt.
United States Patent |
3,810,063 |
Blewitt |
May 7, 1974 |
HIGH VOLTAGE CURRENT LIMITING FUSE INCLUDING HEAT REMOVING
MEANS
Abstract
A current limiting fuse structure comprising a plurality of
elements of fusible material adapted for higher voltage circuit
operation. Each fuse element has a plurality of areas of reduced
cross-section to facilitate the generation of a plurality of
arclets at the time of melting to limit electrical current in the
circuit to be protected by the fuse during the blowing of the fuse.
The fuse elements and/or adjacent pulverulent arc quenching
material are enclosed by a housing or casing which may be formed
from ceramic material. Edges of each fuse element are disposed in
intimate physical or structural contact with the associated casing
to facilitate the removal or transfer of heat which may be
generated in the areas of reduced cross section. The material from
which the casing is made is such a composition or type that is a
relatively good electrical insulator so as to withstand the voltage
of the protected circuit once the fuse has blown and is also a good
conductor of heat so as to efficiently remove heat from the
enclosed fuse elements. The casing may be formed from a plurality
of sections which may be joined to each other and the terminals of
the fuse by high temperature epoxy cement. A metallic heat
exchanger or heat sink may be assembled or disposed in intimate
contact with an outside surface of the casing either by bolting or
by using a suitable bonding material cement so that the heat which
may be generalized in the enclosed fuse and which may be conducted
away from the fuse elements by the casing may be further conducted
to the air or similar environment through the associated heat
exchanger.
Inventors: |
Blewitt; Donald D. (Pittsburgh,
PA) |
Assignee: |
Westinghouse Electric
Corporation (Pittsburgh, PA)
|
Family
ID: |
22860882 |
Appl.
No.: |
05/229,362 |
Filed: |
February 25, 1972 |
Current U.S.
Class: |
337/166; 337/159;
337/293 |
Current CPC
Class: |
H01H
85/47 (20130101) |
Current International
Class: |
H01H
85/47 (20060101); H01H 85/00 (20060101); H01h
085/04 () |
Field of
Search: |
;337/166,185,222,159,161,276,293,202 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
23,514 |
|
Feb 1913 |
|
GB |
|
1,300,348 |
|
Jun 1962 |
|
FR |
|
Primary Examiner: Gilheany; Bernard A.
Assistant Examiner: Bell; Fred E.
Attorney, Agent or Firm: Stratton; A. T.
Claims
1. An electrical fuse structure comprising spaced electrically
conducting terminals, at least one elongated fuse element for
conducting and interrupting electrical current interposed and
connected therebetween, said fuse element being formed from fusible
material having at least two longitudinal edges, said edges
comprising a relatively small portion of the total surface area of
said fuse element, a casing of substantially solid, substantially
electrically insulating heat conducting material enclosing said
fuse element, said casing abutting, on the inner surface thereof,
both said longitudinal edges along substantially the entire length
of said elongated fuse element for transversely removing large
quantities of heat from said fuse element through said casing at
significantly different locations thereon by way of said abutting
junction of said edges of said fuse element and said casing during
nonfusing conditions of said fuse element when electrical current
is flowing therethrough relative to the quantity of heat which is
removed generally longitudinally to said spaced terminals during
similar conditions, pulverulent arc quenching material, said latter
material substantially abutting that portion of said fuse element
other than said substantial portion of said edges of said fuse
element, said latter material substantially contributing to the
removal of heat from said fuse element during a fusing operation of
said fuse element, said pulverulent material being at least
partially contained by said casing, said fuse element conducting a
larger amount of electrical current for a fixed cross section
thereof during a nonfusing condition than a similar fuse element
having only one edge thereof abutting said inner surface of said
outer casing.
2. The combination as claimed in claim 1 wherein said fuse element
is
3. The combination as claimed in claim 1 wherein said fuse element
is
4. The combination as claimed in claim 3 wherein said silver-based
fuse element comprises at least one region of reduced
cross-section, said
5. The combination as claimed in claim 4 comprising a heat
exchanger means wherein a portion of said casing is disposed in
intimate structural
6. The combination as claimed in claim 5 wherein said heat
exchanger means comprises substantially metallic material, said
metallic heat exchanger
7. The combination as claimed in claim 1 wherein a plurality of
said fuse elements are disposed within said casing and spaced from
each other
8. The combination as claimed in claim 1 wherein the outer
perimeter of a transverse cross section of said casing comprises at
least one apex, said casing having a central opening wherein at
least one said fuse element and
9. The combination as claimed in claim 1 wherein the outer
perimeter of a transverse cross section of said casing comprises at
least one straight line segment, said casing having a central
opening wherein at least one said fuse element and said pulverulent
arc quenching material are
10. The combination as claimed in claim 1 wherein the outer
perimeter of a transverse cross section of said casing
substantially comprises the shape of a rectangle, said casing
having a central opening wherein at least one said fuse element and
said pulverulent arc quenching material are
11. The combination as claimed in claim 1 wherein the outer
perimeter of a transverse cross section of said casing
substantially comprises the shape of a square, said casing having a
central opening wherein at least one of said fuse element and said
pulverulent arc quenching material are
12. The combination as claimed in claim 1 wherein at least one of
said elongated edges comprises a linear edge having at least one
notched portion, said linear edge abutting said casing along
substantially the entire length of said elongated fuse element
exept at said notched portion.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
Certain inventions relates to those disclosed in the present
application are disclosed in copending application Ser. No. 229,363
now U.S. Pat. No. 3,732,515 filed concurrently by D. D. Blewitt and
W. G. Shaw and assigned to the present assignee.
BACKGROUND OF THE INVENTION
This invention relates generally to high voltage current limiting
fuses, and in particular relates to such fuses of the type which
employ means for removing heat generated within the parallel fuse
elements during normal operation.
Electrical current limiting fuse links or elements may be formed
from fusible material having areas of reduced cross-section. The
areas of reduced cross-section may be formed in the fusible
material so that a predetermined plurality of arclets or arcs may
be produced in the fusible material as it blows or melts due to
electrical overload. The arclets in effect provide regions of
equivalent high resistance in the protected circuit so that
overload current flowing in the melting fuse elements may be
quickly limited and decreased rather than allowed to continue or
increase because of the available current in the circuit to be
protected. A current limiting fuse may be formed by providing
electrical terminals or electrodes at the opposite ends of a length
of the previously described fusible material and electrically
connecting these terminals to the protected electrical circuit. The
fuse may then be completed by surrounding the fusible material and
part of each terminal with a suitable insulating enclosing means
such as a glass melamine cylindrical housing. The volume when
formed within the melamine glass enclosure may be filled with an
arc quenching medium or material.
In low voltage applications, an element of fusible material may be
relatively short when compared to higher voltage fuse elements and
the number of areas of reduced cross section may be relatively few
in number. The time it takes a given fuse element to melt or blow
is predictable because there is a relationship between the time a
given fuse takes to melt and the amount of current flowing in the
fuse. For example, if the amount of current flowing in a given fuse
element is 150 percent of normal rated current, and is continuously
applied it may be reasonably or accurately predicted that the fuse
will blow or disintegrate in approximately 10 minutes. As a further
example, if the amount of overload current flowing in the same fuse
section amounts to 600 percent of normal rated current, the fuse
section may melt in as short a time as 0.1 seconds.
Correspondingly, the areas of reduced cross section which are areas
of relatively high electrical resistance produce heat. It is this
heat which results from the flow of electrical current and which
eventually may cause the fuse element or links to blow or
disintegrate usually at an area of reduced cross-section.
Specifically the heat generated at an area of reduced cross-section
may cause the fuse element to blow or disintegrate at a
significantly lower rated current than normal or more quickly than
normal if the heat is not removed, that is, if the heat is allowed
to build up. Usually other physical properties require the fusible
material to be of such composition and size that it is relatively
ineffective for removing and dissipating large amounts of heat to
the environment surrounding the fusible material. Consequently, a
decrease or degrading of other fuse characteristics due to
excessive heat generation may occur especially in higher voltage
fuses.
In low voltage fuses, such as those rated 600 volts and below the
distance or spacing between the previously mentioned terminals or
electrodes and consequently the length of the fuse link is
relatively small compared to a high voltage fuse. In addition, the
number of regions of reduced cross section along the length of the
fusible material is relatively small, perhaps only one area of
reduced cross section may be needed. In such cases, the heat
generated by the flow of current through the area of reduced cross
section may be efficiently removed longitudinally through the
fusible material and into the electrodes or terminals which are
disposed adjacent to the opposite ends of the fuse link. Therefore,
in low voltage current limiting fuses, the effect of heat buildup
or generation due to electrical current flowing through an area of
reduced cross section is not too great.
In high voltage current limiting fuses, however, the terminals of
the fuse may be required to be spaced further apart than in a low
voltage current limiting fuse for two reasons. First, the integrity
of the circuit to be protected must be maintained by forming an
electrically insulating gap between the previously mentioned
terminals after the fuse has blown. The distance between terminals
or the size of the insulating gap must therefore be generally
larger for high voltage fuses. Second, high voltage fuses may
require fuse elements having a large number of areas of reduced
cross sections or notches. This is to simultaneously generate a
predetermined number of voltage arcs or arclets to reduce or limit
current during the blowing or fusing operation of the fuse. A
problem arises because the longer the fuse link and the more areas
of reduced cross section present in a link, the more difficult it
is to remove heat from the hot points or hot spots. Near the ends
of the fusible material or fuse link heat flows into the respective
electrodes or terminals rather easily. But towards the center of
the fuse link, the heat, in order to be removed to the terminals at
either end must travel through other areas of reduced
cross-sections which themselves are at a high heat level due to the
current flowing through them. Heat generated in high voltage
current limiting fuses during normal operation may be great enough
to require the use of additional parallel high voltage fuse
elements or links between the terminals of the fuse. By placing
parallel fuse sections in the fuse, each fuse link may conduct less
current and consequently, the heat generated at restricted areas
may be reduced. However, in many high voltage applications, it is
necessary to place so many parallel fuse elements or links in the
fuse that it becomes structurally difficult to accommodate all of
them within a single fuse housing of an acceptable or reasonable
size. In addition, a point is reached where the heat generated
along the length of each parallel fuse link affects the heat
dissipating characteristics of each proximate or adjacent fuse link
because the more fuse elements that are placed in a restricted
area, the more difficult it is to remove heat by convection or
conduction from any one of them. This problem may be solved by
increasing the structural size or diameter of the fuse structure.
However, this gives a much larger fuse which may be more expensive
to manufacture and more difficult to handle for any given voltage
and current rating. Another problem that may result from using
parallel elements or links of fusible material in a single fuse is
that peak let-through current (I.sub.pk) during the blowing or
melting operation of a fuse may increase in proportion to or with
the number of parallel fuse elements added so that the clearing
ability of the fuse structure may be reduced. Electrical equipment
or appliances to be protected during the fusing operation may in
fact be damaged or destroyed by the increased peak-let-through
current which may develop during a fusing operation.
Current limiting fuses may be used to protect static, solid state
or semiconductor devices, such as thyristors or silicon controlled
rectifiers. In low voltage applications, the heat generating and
heat dissipating characteristics of a protective fuse structure as
previously mentioned and those of a silicon controlled rectifier or
semiconductor device are likely to be similar or analogous. That
is, the semiconductor device may be represented by a thin layer of
semiconductor material bonded to adjacent electrodes or terminals
which may be called anode and cathode terminals. Heat generated by
current flow through the semiconductor material which often
exhibits a high resistive property may be dissipated by the
adjacent anode and cathode terminals relatively easily. In a
similar manner as mentioned a low voltage current limiting fuse may
be represented as a relative short length of fusible material with
perhaps only one region of reduced cross-section, disposed between
two relatively closely spaced adjacent terminals. The heat
generated by current flowing through the area of reduced cross
section may be easily dissipated by the close adjacent terminals.
However, advances in the semiconductor art have reached the point
where some semiconductor or static apparatus such as silicon
controlled rectifiers or diodes may be manufactured for high
voltage applications. It has been found in such cases, that it is
only necessary to increase the size of the semiconductor material
slightly to achieve proper high voltage insulating characteristics.
That is the distance between the adjacent anode terminal and
cathode terminal may be only slightly larger for a high voltage
semiconductor device than for a low voltage semiconductor device.
However, as was mentioned previously, the characteristics of known
types of high voltage current limiting fuses which may be used to
protect the latter mentioned high voltage semiconductor device may
be drastically different from its low voltage counterparts, that is
the distance between spaced output terminals may be increased by a
much greater amount proportionally for a low voltage fuse than the
high voltage semiconductor device and the need for adding
additionally parallel fuse elements develops.
This has led to the conclusion that it is relatively easy to
protect a low voltage semiconductor device with a low voltage
current limiting fuse because in terms of heat dissipation, they
may be thought of as similar analogous devices, that is the time it
takes to damage a low voltage semiconductor device and to melt a
low voltage current limiting fuse with respect to the amount of
current flowing continuously through them may be similar over a
very large current range. However, this is not necessarily true for
high voltage applications where the respective relationships of
destruction time versus continuously flowing current may not be
similar over a wide range of current. For example, a given current
such as one which is 150 percent of rated fuse current may cause a
high voltage fuse of the type previously discussed to melt or fuse
in 10 seconds but the semiconductor device for which the fuse was
provided to protect may be able to withstand the same amount of
current for 30 to 40 seconds without being destroyed. At a higher
current, however, such as 500 percent of rated current the fuse may
in fact disintegrate or melt at approximately the same time as the
semiconductor device. This means that at low values of overload
currents, known types of high voltage fuses may be relatively
inefficient in protecting the semiconductor device because the
semiconductor device withstands more overload current for a longer
period of time when the usual type of protective fuse allows, but
at high values of overload current the semiconductor device and the
fuse may have similar analogous heat conducting properties so that
the fuse in fact may be protecting the semiconductor device in an
efficient manner. This leads to the conclusion that the circuit
including the fuse and the semiconductor device is effectively
derated because of the inability of known types of high voltage
current limiting fuse to match the thermal characteristics of the
semiconductor device which it protects over a wide current range.
Consequently, it would be advantageous to provide a high voltage
current limiting fuse the thermal properties of which are more
closely analogous to those of the high voltage semiconductor device
for which the fuse is used to protect similarly to the manner in
which a low voltage fuse approximates the thermal characteristics
of a low voltage semiconductor device.
One method for removing heat from the hot spots or areas of reduced
cross section of fuse elements was disclosed by K.W. Swain et al,
in U.S. Pat. No. 2,871,314 issued Jan. 27, 1959. In that patent it
is proposed to separate or aportion single elements of fusible
material having multiple notches serially into multiple elements of
fusible materials having single notches or areas of reduced cross
section. One element of fusible material for every notch is then
interposed between heat dissipating means such as air cooled fins.
This construction provides longitudinal migration of heat along the
silver or similar fusible material towards immediately adjacent
cooling means in a manner similar to that provided by a low voltage
fuse where heat may move longitudinally to adjacent terminals.
However, for high voltage applications, which by nature may require
relatively long fuse elements with many notches, this may mean an
interposing of many heat exchanger means or cooling means in series
with the numerous notched elements of fusible material thus
creating a fuse which may be larger than a standard high voltage
current limiting fuse.
It is important to note in the last-mentioned proposed fuse
structure, that effective removal of heat from areas of heat
generation, such as areas of reduced cross section is accomplished
by conducting heat away primarily in a longitudinal direction, as
opposed to a radial or lateral direction from the areas or paths of
electrical current conduction.
SUMMARY OF THE INVENTION
In accordance with the invention, a novel means for removing heat
from an element of fusible material is disclosed in which enhanced
radial or lateral movement or migration of heat from an element of
fusible material is employed. This construction alleviates the
necessity for serially interposing heat removing means and thus
allows for the construction of a high voltage fuse without the need
for substantially extending the length of the fuse further than is
required to meet for previously mentioned electrically insulating
gap characteristics of a high voltage fuse. It also allows for the
generation of the same number of voltage arcs or arclets. Upon
melting or fusing heat may be removed in a lateral or radial
direction into an associated electrically insulating casing. The
casing may be formed from a ceramic material and may axially
overlap the terminals of the fuse and may either engage or make
direct physical contact with at least one edge or one portion of a
fuse link or be situated proximate to a fuse link. The casing may
serve the dual purpose of enclosing pulverulent arc quenching
material such as silica sand or quartz sand and removing heat from
the fuse link laterally or radially from the fuse elements
outwardly into the casing rather than predominantly from the fuse
elements longitudinally into spaced, disposed end terminals. This
construction provides the regions between notches which are points
of high heat generation with a path for conducting heat into the
environment outside of the fuse structure without the necessity of
using a heat conducting path through other areas of relatively high
heat generation. The material from which the casing is formed may
be a relatively high heat conducting material which is also a good
electrical insulator. Consequently, the electrical properties of a
blown fuse are not reduced or degraded substantially but the heat
dissipating properties of an operating fuse are enhanced. In a
further embodiment of the invention a metallic or extremely
efficient heat exchanging member or means may be placed in
proximity with the outer surface of the casing to facilitate the
removal of heat from the casing at an even higher rate than if the
outer surface of the casing were merely exposed to an environment
such as air.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, reference may be had
to the preferred embodiments exemplary of the invention shown in
the accompanying drawings, in which:
FIG. 1A shows a schematic diagram of an electrical circuit
including a known or prior art current limiting fuse;
FIG. 1B shows a schematic diagram of another electrical circuit
including a known current limiting fuse;
FIG. 2 shows a known low voltage semiconductor device and a known
current limiting fuse structure;
FIG. 3 shows a plot of current versus time for a known low voltage
semiconductor device and for a known low voltage fuse
structure;
FIG. 4 shows a known high voltage semiconductor device and a known
high voltage current limiting fuse structure;
FIG. 5 shows a plot of current versus time for a known high voltage
semiconductor device and a known high voltage current limiting fuse
structure;
FIG. 6 shows an isometric drawing of a high voltage current
limiting fuse structure embodying the principles of this invention
with heat dissipating casing means and separate heat sink;
FIG. 7 is a partially broken away sectional view of the apparatus
shown in FIG. 6 taken in the direction of lines VII--VII;
FIG. 8 is a view of the apparatus depicted in FIG. 7, taken at
section VIII--VIII; and
FIG. 9 is a view of the apparatus shown in FIG. 7 taken at section
IX--IX.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings and FIG. 1A in particular, a known
type of electrical circuit 10 comprising a voltage source 12, a
solid state device 14, a load 16 and a fuse 18 is shown. Voltage
source 12 may be a source of alternating electrical current capable
of providing a current I1 at a voltage potential V, or it may be a
similar direct current source. Current I1 energizes load 16 through
the solid state device 14 which may be a silicon controlled
rectifier (SCR) or a thyristor and which may be gated. Current I1
must also flow through fuse 18. Ideally, the thermal properties of
fuse 18, that is, the capability of fuse 18 to remove or dissipate
heat generated by the flow of current I1 through it, should match
the thermal properties of the solid state device 14 for which the
fuse 18 is provided to protect. If the fuse 18 and solid state
semiconductor device 14 are properly matched in a thermal sense the
fuse 18 will blow or interrupt the current I1, in a protective
manner before the structure of the semiconductor device 14 is
damaged by the flow of current I1. Said in another way fuse 18
should blow only before semiconductor device 14 itself reaches the
threshold of thermal destruction.
Referring now to FIG. 1B, another known type of electrical circuit
20 is shown comprising a source of alternating current voltage 22,
a load 24 connected in circuit relationship with three parallel
connected semiconductor diodes 26a, 26b and 26c. The semiconductor
diodes are protected by fuses 28a, 28b and 28c respectively. A
voltage V provided by voltage source 22 may produce a current 12
flowing in the single path comprising diode 26a and fuse 28a. As
was the case in electrical circuit 10 shown in FIG. 1A, the thermal
or power dissipating properties of semiconductor diode 26a and fuse
28a are preferably matched.
In electrical circuit 10, electrical fuse or current limiting fuse
18 operates most efficiently if it fuses immediately before
semiconductor device 14 is in danger of being damaged by the heat
therein which results during operation. In electrical circuit 20,
semiconductor device 26a may be destroyed before fuse 28a responds.
This is because of the parallel relationship of the branch circuits
which includes three diodes 26a, 26b and 26c. In other words, the
failure of diode 26a may not render the circuit 20 inoperative
because diodes 26b and 26c are still operative to conduct current.
However, it may be important in a situation where diode 26a fails
to a short circuit that fuse 28a blows or melts soon after thus
creating an open circuit between its terminals so that no
significant amount of short circuit current may bypass the parallel
diodes 26b and 26c. Fuse 28a shown in FIG. 1B and fuse 18 shown in
FIG. 1A, may be, but are not limited to, current limiting type
fuses, such that electrical currents I1 and I2, respectively, may
be limited by the voltage arcs generated within fuses 18 and 28a
respectively.
Referring now to FIG. 2, a more detailed view of a semiconductor
device 30, which may be similar to semiconductor device 14 or
semiconductor device 26a as disclosed or as described in connection
with FIGS. 1A and 1B respectively, is shown. In addition, a portion
of a known electrical fuse structure 40 which may be similar to
electrical fuse 18 or electrical fuse 28a shown in FIGS. 1A and 1B
is also shown. Semiconductor or solid state, static device 30 and
fuse structure 40 are shown in proximity in FIG. 2 to aid in
indicating the physical thermal analogy between them. Semiconductor
device 30 comprises a relatively thin layer of semiconductor
material 32, such as silicon or germanium or compositions of
either, disposed as a wafer between an anode 34 and a cathode 36.
The anode 34 and cathode 36 may be metallic, electrically
conducting, heat conducting, supporting means for accommodating the
flow of electrical current through the series circuit arrangement
of anode 34, semiconductor material 32 and cathode 36. Because
semiconductor region 32 acts as a relatively high impedance or high
resistance when compared with the adjacent metallic electrodes 34
and 36 heat may be generated therein. In addition, semiconductor
material 32 may be a relatively poor heat dissipating means because
of its physical properties and because of its relatively small
size. Consequently, heat generated within region 32 must be
conducted longitudinally to metallic, heat radiating or conducting
regions or electrodes 34 and 36 as shown by arrows 35T and 35L,
respectively. In such a situation, the heat generated in region 32
may be dissipated easily by the adjacent heat conducting masses 34
and 36. In similar or analogous fashion, fuse structure 40 may be
thought of as comprising elements of silver alloyed or tin alloyed
fuse links 42A and 42B which are mounted or disposed in parallel
electric arrangement between spaced electrodes or terminals 44 and
46. The silver ribbons of fusible material 42A and 42B may include
notched sections or sections of reduced cross section 45A and 45B
respectively. These regions of reduced cross-section area because
of their relatively small size and thermal properties may be heated
to a greater degree when electric current flowing through fuse
structure 40 is channeled through them. The heat generated in
regions 45A and 45B may be relatively poorly dissipated because of
the presence of the pulverulent arc quenching material 47. In such
a case, provided electrodes or terminals 44 and 46 are spaced
relatively close to each other, heat as indicated by arrows 48TA
and 48TB, 48LA and 48LB may be conducted to the large metallic
masses 44 and 46 respectively where heat dissipation is more easily
accomplished. When a semiconductor device 30 and protective fuse
structure 40 are used in the same electrical circuit so that the
fuse acts in a protective manner with respect to the semiconductor
device the thermal electric properties of each are analogs or
similar to one another.
Referring now to FIG. 3, a graph or plot 50 of the thermal or heat
dissipating properties of a semiconductor device similar to device
30 shown in FIG. 2 and a current limiting fuse structure similar to
structure 40 shown in FIG. 2 are shown. A standard way of showing
the thermal properties of elements like 30 and 40 versus electrical
current is a graph where time in seconds is plotted against current
in amperes on a log-log scale. Curve 52 shows the reaction of a
semiconductor device such as device 30 in terms of current and
time. Curve 54 shows a similar reaction for a fuse structure such
as structure 40. Curve 56 is a curve similar to curve 54 for a fuse
structure such as structure 40 shown in FIG. 2.
Curve 52 shows the ideal relationship of a fuse structure 30 to
semiconductor device 40 for a series protective circuit such as
circuit 10 shown in FIG. 1A. The relationship of curve 52 to curve
56 shows an ideal protective situation for a parallel arrangement
of semiconductor 30 and a fuse 40 such as shown in the circuit 20
in FIG. 1B. Lines 58, 60 and 62 indicate the normal rated current
values or 100 percent load current values for the three devices or
electrical elements represented by the curves 52, 54, 56
respectively. Most importantly, graph 50 shows that the
relationship between the heating characteristics of curves 52 and
54 are similar over a relatively large range of currents, that is,
the spacing between curves 52 and 54 as depicted by distances D1
and D2 respectively at two relatively diverse current values are
approximately the same. The same may be said for the distances D3
and D4 between curves 52 and 56. This shows that if semiconductor
device 30 and fuse structure 40 as shown in FIG. 2 are connected
together in the same electrical circuit it is relatively easy to
maintain a proper and efficient protective function over a
relatively large range of electrical currents.
Referring now to FIG. 4, a semiconductor device 70 and fuse
structure 80 seimilar to semiconductor device 30 and fuse structure
40 shown in FIG. 2, is depicted. The thermally analogous
semiconductor device 30 and fuse structure 40 shown in FIG. 2 are
best adapted for low voltage circuit application that is, where the
voltage stress applied to semiconductor section 32 and fuse
elements 42A and 42B are similar and relatively low. Consequently,
the lengths of fuse elements 42A and 42B may be relatively short
and the spacing between terminals or electrodes 44 and 46 may be
small. However, semiconductor device 70 and fuse structure 80 are
adapted for high voltage circuit applications. Semiconductor device
70 comprises a relatively thin wafer, wedge or segment of
semiconductor material 72 interposed between electrically and heat
conducting metallic anode and cathode electrodes 74 and 76,
respectively. Semiconductor material 72 is adapted to be stressed
at a relatively high electrical voltage, such as a thousand volts
or greater. Semiconductor device 30 as shown in FIG. 2 may be
converted or its structure modified for the higher voltage
application by increasing the longitudinal size of the semconductor
material 72 of FIG. 2 by a relatively small increment 73 as shown
in FIG. 4. The adaption of fuse structure 40 shown in FIG. 2 to
high voltage application requires a more radical change in
structure, however. This can be seen by reference to fuse structure
80 shown in FIG. 4. High voltage current limiting fuse 80 may have
elongated electrically parallel fuse elements 82A and 82B each with
a plurality of notched sections or areas of reduced cross sectional
areas 85A1, 85A2, 85A3 and 85A4; and 85B1, 85B3 and 85B4
respectively. The elongated fuse 82A and 82B may be disposed and
extended between two electrically conducting and heat conducting
terminal members or electrodes 84 and 86. Terminal members 84 and
86 are spaced further from each other than terminals or electrodes
44 and 46 shown in FIG. 2. The larger spacingis required primarily
for two reasons. First, the electrically insulating characteristics
of a blown fuse must be such that the insulating gap existing
between the remaining electrodes is sufficient to withstand the
high voltage impressed upon the circuit by an external voltage
source after fusing, and second, the space between the terminals 84
and 86 must be large enough to accommodate one or more fuse
elements each with a plurality of notches to generate multiple
current limiting arcs during high voltage fusing. In fuse structure
80, an enclosing outer housing casing 83 is shown with silica sand
87 interposed between the fuel elements and the housing as a
pulverulent arc quenching filler.
In semiconductor device 70, heat generated in semiconductor region
72 is removed rather easily as shown by arrows 75T and 75L to the
relatively large adjacent heat dissipating electrodes 74 and 76,
respectively. On the otherhand, heat may not be removed as easily
axially from inner portions of the fuse elements 82A and 82B, as
shown in fuse structure 80. Heat generated in the vicinity of
notches or areas of reduced cross sections 85A1 and 85B1 as well as
85A4, and 85B4 may be removed as shown by arrows 88TA, 88TB and
88LA, 88LB respectively to the relatively larger heat conducting
electrodes or end terminals 84 and 86. But heat generated within
axially inner areas of reduced cross-section 85A2, 85A3 and 85B2,
85B3 may only be removed as shown by arrows 88m1a, 88m2a and 88m1b,
88m2b to electrodes or end terminals 84 and 86, respectively. This
heat is removed by traversing a relatively long distance, since
terminals or electrodes 84 and 86 are not adjacent to or as close
to the last mentioned heat generating areas of reduced
cross-section, and by flowing through other areas of reduced cross
section such as 85A1 and 85A4 and 85B1 and 85B4 before it is
conducted into the heat dissipating electrodes or end terminals 84
and 86. As an example, heat generated in the area of reduced
cross-section 85B3 as shown by arrow 88m2b must move through
another heat generating area or area of reduced cross-section 85B4
before it can be conducted to heat dissipating terminal or
electrode 86.
Referring now to FIG. 5, the thermal relationship between
semiconductor device 70 and fuse structure 80 can be shown
graphically by graph 90. Graph 90 has the same coordinate system as
graph 50 shown in FIG. 3. The semiconductor device 70 may be
represented by a semiconductor thermal-electric or power
dissipating curve or time-current curve 92 which is very similar to
curve 52 as shown in FIG. 3. Curve or plot 94 may be related to or
correspond to curve 54 as shown in FIG. 3. It will be noted,
however, that the curve or plot 94 may be thought of as the curve
54 pivoted or rotated in the direction shown by the arrow of
rotation A around point 97 consequently resulting in a relatively
small spacing or distance D2 between curves 94 and 92 in the
relatively high current range near point 97 and a relatively large
spacing distance D1 between curves 94 and 92 in the relatively low
current range. This indicates that the heat dissipating or power
dissipating characteristics of a high voltage fuse device such as
80 and a high voltage semconductor device such as 70 both depicted
in FIG. 4, are not well matched for a relatively wide range of
current. That is, if the high current ranges of the two electrical
elements 70 and 80 are relatively well matched in heat dissipating
properties, the low current ranges are not, or as curve 96 shows,
if the low current ranges are relatively well matched, then the
high current ranges are poorly matched in heat dissipating
characteristics. Curve or plot 96 is in effect merely curve 94
moved in the direction of arrow B so that the spacing or distance
D3 between curve 96 and 92 is relatively small, thus resulting in a
spacing or distance D4 between curve 96 and 92 which is relatively
large. The movement of curve 94 to the right as indicated by arrow
B is accomplished in a fuse structure such as structure 80 by
providing additional parallel fuse elements, similar to fuse
elements 82a and 82b as shown in fuse structure 80, between the
terminals or electrodes 84 and 86. The addition of electrically
parallel fuse elements means that any given current flowing into or
out of an external circuit to conducting terminals or electrodes 84
and 86 is divided up among a larger number of parallel paths.
Consequently, less current flows in each path and since the current
necessary to cause an interrupting operating of the overall fuse is
a function of the current necessary to break down or melt one path
or one fuse element, the total amount of time the fuse structure
may remain in a normally conductive state for any given amount of
continuously flowing current is increased. As a compromising
measure to achieve a better thermal analog between a fuse structure
such as 80 and a semiconductor device such as 70, parallel fuse
elements may be added in parallel to fuse links 82A and 82B in fuse
structure 80 to cause curve 94 to be moved to the right to the
position of curve 98. Curve 98 intersects or crosses curve 92 at
point 99. This minimizes the distance D5 and D6 at both ends of the
current range of the fuse structure 80 and semiconductor element
70. But as can be seen, in the lower current range, the fuse
structure will blow or melt first to protect the semiconductor
device and in the higher current range, the fuse structure will
blow later than the semiconductor device, with the semiconductor
device not being protected by the fuse structure. This is an
accepted but relatively poor way to protect high voltage
semiconductor devices. In addition, the adding of parallel fuse
elements to existing fuse links such as 82A and 82B reaches a point
of diminishing return since the amount of current which can be
conducted by each added fuse element becomes diminishingly less
because of the structurally crowded conditions within the fuse
structure which tends to prevent dissipation of heat in an axial
direction. Finally, the addition of a larger number of fuse
elements in parallel in a single fuse structure increases the
peaklet-through current I.sub.pk and thus may defeat the original
purpose of the current limiting fuse to limit current flow during
the fusing cycle.
Referring now to FIG. 6, the primary embodiment of the invention is
illustrated in a fuse structure 100. Fuse structure 100 comprises
terminals of electrically conducting heat dissipating metallic
material 102 and 104 which are spaced at a suitable distance from
each other depending upon the rated voltage. The spacing or
distance allows fuse 100 to be used in higher voltage application.
Terminals 102 and 104 may have provided therein holes or openings
103 and 105 respectively where bolts or similar attaching means may
be used to connect or mount the fuse structure in an external
electrical circuit. Conductor or heat dissipating terminal 102 may
be maintained at a voltage V1 and terminal 104 may be maintained at
a substantially different voltage V2 after fusing. Interposed
between terminals 102 and 104 are a plurality of electrically
conducting fuse elements, such as primarily silver alloy fuse
elements 106, 108, 110, 111 and 113, which are all connected in
parallel circuit relationship with respect to each other. Enclosing
fuse elements 106, 108, 110, 111 and 113 may be a generally
U-shaped enclosing housing 112 which may be ceramic and which is
preferably a good electrical insulator and a good conductor of
heat. Placed adjacent to the U-shaped insulating member 112 may be
a platelike similar insulating member 114 which may be made or
formed of similar ceramic material. Cover member 114 may be
assembled in position with base member 112 and be secured thereto
by some suitable means such as cementing with a high temperature
epoxy resin which may be also heat conducting and substantially
electrically insulating. The combination of joined members 112 and
114 forms a hollow generally tubular shape which may partially
enclose the previously mentioned ribbons or elements of notched
sections of fusible material such as 106, 108, 110, 111 and 113. It
is preferable to have the ends of the combination of enclosure 112
and 114 axially overlap terminals 102 and 104, as shown in the
region 118 for example, where a similar or appropriate securing
means, such as epoxy resin, may, be used to secure the ceramic
members 112 and 114, to electrical conductors 102 and 104. The
completely enclosed chamber formed by members 112 and 114,
conductor terminal 102 and conductor terminal 104 is filled with a
pulverulent arc quenching material 136 such as silica and/or quartz
sand which may fuse or melt into a solid conglomerate or fulgurite
upon the fusing or melting of a fuse element such as 106. The fuse
106 may be the same as all fuse elements shown in FIG. 6 and will
be used for explanatory purposes to show the effects of heat
buildup and removal in the primary embodiment of the invention.
Notched sections or areas of reduced cross-section such as 120B are
placed or cut into fuse element 106. These form a region such as
region 121 of reduced cross section where heat may be generated
during electrical current flow to a great degree. The portion of
fuse element 106 adjacent to terminal 102 provides a path for heat
as shown by arrow 124 to be easily dissipated into terminal 102 as
shown by the spreading out of the heat as indicated arrows 126a,
126b and 126c in terminal 102. A second set of notches or areas of
reduced cross section 122T and 122B provide an area of reduced
cross-section 123 which is spaced away from the ends of the fuse
elements 106. The heat which longitudinally moves away from the hot
spot at 123 is severely limited in conduction. However, the
addition of the heat conducting material from which the casing is
formed and which may be placed or disposed in direct contact with
the edges of the fuse ribbon or element 106 or close to the edges
or portions of fuse element 106 allows heat to move generally
laterally or transversely with respect to the axis or longitudinal
dimension of the fuse element 106 as shown by arrows 125T and 125B
to the ceramic enclosure formed by the integral U-shaped members
114 and 112 whereupon the heat may be conducted through the ceramic
material to surfaces 132 and 134 respectively, for example, where
it may be dissipated to the outside environment such as air, forced
gas or a heat exchanging means.
In one embodiment, of the invention, a heat exchanging means 135
which may be electrically conducting and which should be comprised
of a very good heat conductor such as aluminum or brass, may be
provided to support the fuse structure 100 and aid in the removal
of heat. Heat exchanger 135 comprises fins such as 142 and valleys
146 between fins 142 thus providing the relatively large amount of
heat dissipating surface necessary in a heat exchanging means. Heat
exchanging means 135 may be secured to the top surface 132 of cover
114 by a bolt 150 or by a suitable high temperature epoxy resin
cement which may be applied between surfaces 148 and 132.
In operation, as is shown by arrows 125T and 125B, heat may be
removed in a transverse manner from the regions of reduced cross
section of the fusible material, such as the areas of reduced cross
sections 121 and 123. Consequently, the fusible material for any
given amount of time may be maintained at a cooler temperature. Put
another way, the time prior to destruction or blowing of the fuse
element may be delayed by cooling, thus allowing for continuous
conduction of a relatively larger amount of current through a
fusible element such as 106 before it melts or blows.
Referring now to FIG. 7, a sectional view of the fuse structure
shown in FIG. 6 is depicted showing fuse elements 106, 108, 110,
111 and 113 as well as U-shaped ceramic electrically insulating
heat conducting members 112 and similar slab shaped member 114. The
heat dissipating terminal 102 is shown with heat conducting or
spreading outwardly in the conductor 102 for heat dissipating
purposes, as indicated by arrows 126a, 126b and 126c.
Referring now to FIG. 8, which shows the sectional view as
indicated in FIG. 7, a complete longitudinal section of fuse
element 110 with areas of reduced cross sections such as formed by
notches 110T1 and 110B1 as well as formed by notch 110T2 and 110B2
is shown. In the region of notches 110T2 and 110B2 which may be a
hot spot in the fuse, arrows indicate the outward flow of heat. It
will be noted that arrows H5 and H3 show heat flowing
longitudinally. However, the heat sink or heat conducting members
114 and 112 mounted in close proximity to or touching fuse element
110 allows heat to flow from fuse link 110 into the heat conducting
members 114 and 112 as indicated by arrows H1, H2, H4 and H6.
Consequently, more heat can be removed from the hot spot formed by
the notches 110B1, 110T1, 110T2 and 110B2 and therefore the area of
reduced cross section will be maintained at a cooler temperature
for a given amount of time.
Referring now to FIG. 9, fuse structure 100 is depicted showing the
structural relationship between U-shaped member 112 and slab 114. A
cross section of fuse material 106 is shown with the heat removing
arrows or paths 125a and 125b shown for dissipating heat from the
silver ribbon 106 through the conducting ceramic members 114 and
112 to surfaces 132 and 134, respectively, for dissipation.
It is to be understood that any number of parallel fuse elements
may be employed in the primary embodiment of the invention,
although as illustrated only five are shown. It is also to be
understood that the voltage applied such as indicated by V1 and V2
may be an alternating current voltage or direct current voltage and
may be of a relatively high value. It is also to be understood that
the number of notches or areas of reduced cross-section may be
predetermined for any given length of fusible material. It is also
to be understood that the fusible material need not necessarily be
of a flat or ribbon shape with V-shaped notches but may be tapered
in cross-section or of fusible material with other than V-shaped
notches placed in it to provide the areas of reduced cross section
necessary. It is also to be understood that heat exchanging means
135 need not be provided on fuse structure 100 for fuse structure
100 to operate properly in a particular application. Heat
exchanging means 135 is merely a more efficient means of removing
heat where desired but not necessarily the only means. It is also
to be understood that the ceramic member may be formed other than
by using a U-shaped enclosing member added to a cover shaped
member. As an example two U-shaped members may be joined, or two
L-shaped members may be joined, or four plates may be joined. Other
combinations are also possible. The U-shaped member joined to the
cover is merely shown as one way of fabricating the enclosing
means. It is also to be understood that although the ceramic
material forms a rectangular opening as viewed in a cross section a
generally circular or similar means may also be used to accomplish
the same purpose, and it is also to be understood that although the
disclosure is primarily adapted for relatively higher voltage
current limiting fuses the same principles may be used for low
voltage fuses for more efficient heat removal. It is also to be
understood that the fusible material may be any suitable fusible
material and is not limited to silver nor is the enclosing member
such as 112 limited to ceramic material.
The apparatus embodying the teachings of this invention has many
advantages. Primarily, a high voltage semiconductor device may be
protected over a relatively wide range of electrical current by a
high voltage current limiting fuse having the improved heat
removing means without the necessity of extending the size of the
fuse or of comprising the time versus current characteristics of
the fuse. This means that for a given current carrying capacity,
less parallel fuse elements need be used and thus a smaller fuse
may be created and consequently, the peak-let-through current may
be reduced. This more efficiently protects the semiconductor
device. The semiconductor device to be protected need not be
derated to be used with a fuse having poorer heat removing
qualities. This also means that the heat removing qualities and the
current carrying capacity of the fuse may be made similar to those
of a low voltage fuse by more efficient removal of heat.
Consequently, the improved fuse structure will blow, fuse or
breakdown at a time which is closely coordinated with the time
which the protected semiconductor device would otherwise be
destroyed by the buildup of heat due to excessive current.
* * * * *