US3810063A

US3810063A - High voltage current limiting fuse including heat removing means

Info

Publication number: US3810063A
Application number: US00229362A
Authority: US
Inventors: D Blewitt
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1972-02-25
Filing date: 1972-02-25
Publication date: 1974-05-07
Anticipated expiration: 1991-05-07
Also published as: JPS48112942U

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.

Description

United States Patent 1191 Blewitt 14 1 May 7,1974

[ HIGH VOLTAGE CURRENT LIMITING FUSE INCLUDING HEAT REMOVING MEANS [75] Inventor: Donald D. Blewitt, Pittsburgh, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

22 Filed: Feb. 25, 1972 21 App]. No.: 229,362

Primary Examiner-Bernard A. Gilheany Assistant Examiner-Fred E. Bell Attorney, Agent, or Firm--A. T. Stratton [5 7] 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 melt ing 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.

12 Claims, 10 Drawing Figures HIGH VOLTAGE CURRENT LIMITING FUSE INCLUDING HEAT REMOVING MEANS 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 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 ma terial 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 u wi l 9w 9 d sin egrat ia rqzs sly. 1.9 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 sig nificantly 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 es pecially 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. e

In high voltage current limiting fuses, however, the terminals of the fuse may be required to be spaced fur ther 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,,,,.) 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 shortlength 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 forhigh 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 cathodeterminal-may be only slightly larger fora 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 percent of rated fuse current may cause a high voltage fuse of the type previously discussed to melt or fuse in IQ 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 'closelyanalogous 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 his 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 cooledfms. This construction provides longitudinal migration of heat along thesilver 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 numerousnotched ele- 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 arelets. 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 servethe 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 betterfhnderstanding of the invention, reference may behad 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 withheat 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 VIIVII;

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 IXIX.

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 II, 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 260. 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 18 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 3ST and L, respectively. In such a situation, the heat generated in region32 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 453 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 protec tive 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 and fuse structure 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 428 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 metal lic 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 increas ing 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 828 each with a plurality of notched sections or areas of reduced cross-sectional areas A1, 85A2, 85A3 and 85A4; and 85B1, 8583 and 85B4 respectively. The elongated fuse 82A and 828 may be disposed and ex tended between two electrically conducting and heat conducting terminal members or

electrodes

84 and 86.

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 8584 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 8582, 85B3 may only be removed as shown by arrows 88mla, 88m2a and 88mlb, 88m2bt0 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 crosssection, and by flowing through other areas of reduced crosssection such as 85A1 and 85A4 and 85B] and 8584 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 crosssection 85B4 before it can be conducted to heat dissipating terminal or electrode 86.

Referring now to FIG. 5, thethermal relationship between semiconductor device 70 andfuse 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 o'r 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 depictedin 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,

10 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 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 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 appli-' cation.

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 1 1 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, I11 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 1208 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 126'c in terminal 102. A second set of notches or areas of re duced cross section 122T and 1228 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 1258 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 gasor a heat exchanging means.

In one embodiment, of the invention, a heat exchanging means l 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 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 1258, 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, 1261) and 126C.

Referring now to'FIG. 8, which shows the sectional view as indicated in FIG. 7, a complete longitudinal section of fuse element with areas ofreduced cross sections such as formed by notches 110Tl and 11081 as well as formed by notch 110T2 and 11082 is shown. In the region of notches 110T2 and 11082 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 110B], I10Tl, 110T2 and 11082 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 a 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 j thatthe 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. lt 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.

I claim as my invention:

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 substantially ribbon shaped.

3. The combination as claimed in claim 1 wherein said fuse element is substantially silver-based in composition. I

4. The combination as claimed in claim 3 wherein said silver-based fuse element comprises at least one region of reduced cross-section,-said casing being substantially formed from ceramic material.

5. The combination as claimed in claim 4 comprising a heat exchanger means wherein a portion of said casing is disposed in intimate structural contact with at least a portion of said heat exchanger means.

6. The combination as claimed in claim 5 wherein said heat exchanger means comprises substantially metallic material, said metallic heat exchanger means at least partially structurally supporting said casing.

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 substantially only by pulverulent arc quenching material.

8. The combination as claimed in claim 1 wherein the outer perimeter of a transverse cross section of said casing comprisesat least one apex, said casing having a central opening wherein at least one said fuse element and said pulverulent arc quenching material are disposed.

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 disposed.

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 disposed.

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 disposed.

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.

Claims

3. The combination as claimed in claim 1 wherein said fuse element is substantially silver-based in composition.

4. The combination as claimed in claim 3 wherein said silver-based fuse element comprises at least one region of reduced cross-section, said casing being substantially formed from ceramic material.

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 said pulverulent arc quenching material are disposed.