Abstract:
A film capacitor comprises a wound body, a first terminal part electrically connected to one end of the wound body, and a second terminal part electrically connected to the other end of the wound body. The wound body is structured by winding into a laminate a first film laminate formed of a laminate of dielectric films and having therein a floating electrode and a second film laminate formed of a laminate of first and second metal films sandwiching the first film laminate and dielectric films and having therein a floating electrode ( 22 ). Each of the floating electrodes ( 22 ) is composed of integrated small electrodes independent of each other.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a film capacitor comprising a rolled assembly of stacked films, which has a stack of dielectric films and includes a floating electrode therein, a first terminal electrically connected to one end of the roll, and a second terminal electrically connected to the other end of the roll, and more particularly to a film capacitor, which has a self-healing function and a self-protective function. 
     BACKGROUND OF THE INVENTION 
     Film capacitors having a self-healing function are disclosed in Patent Documents 1 through 4, for example. 
     The film capacitor disclosed in Patent Document 1 includes a pair of capacitor electrodes, which confront each other with a dielectric film sandwiched therebetween. At least one of the capacitor electrodes is formed by metal evaporation, and is divided equally into spaced or arbitrarily spaced intervals in the longitudinal direction of the film by a plurality of insulating grooves, thereby providing a plurality of segmented electrode banks. The segmented electrode banks comprise electrodes formed by a network of insulating grooves, with a plurality of narrow current paths left therebetween. 
     The capacitor electrodes that sandwich the dielectric film serve as a metalized film having 2 to 20 independent small electrodes arrayed in the longitudinal direction of the film, and a plurality of independent small electrodes arrayed in the transverse direction of the film. The metalized film is coiled into a roll, and metal is sprayed onto opposite end faces of the roll to form terminal electrodes, thus producing a capacitor element. One or more capacitor elements are encased within the film capacitor. 
     The film capacitor disclosed in Patent Document 2 comprises a dielectric film and a metal-evaporated electrode disposed on one side of the dielectric film. The metal-evaporated electrode includes a plurality of divided electrodes arrayed in longitudinal and transverse directions thereof. The divided electrodes are segmented by non-electrode margins. Margin fuses having a fuse function are disposed within the margins. The margin fuses are superposed over other margin fuses provided on upper and lower confronting electrodes. 
     The film capacitor disclosed in Patent Document 3 is a sheet capacitor, which comprises a dielectric capacitor sheet having a thin metal film thereon and which is coiled into a capacitor roll in a direction in which the dielectric capacitor sheet extends. The film capacitor includes a metalized region, which includes the thin metal film with divisions therein. The thin metal film has a partially divided section extending transverse to the direction in which the dielectric capacitor sheet extends. 
     The film capacitor disclosed in Patent Document 4 includes a pair of evaporated electrodes having divided electrodes connected in parallel with each other by fuses, and which extend from a substantially transverse central region toward insulated margins that reside in an effective electrode area providing a capacitance. Since the fuses and the divided electrodes are disposed closely to the insulated margins, where reduced currents flow at positions spaced from components plated by sprayed metal, the heat produced by the fuses is reduced so as to lower a rise in temperature. 
     Ceramic capacitors having a floating electrode within a sintered dielectric substrate, which are different from film capacitors having floating electrodes within a stacked film assembly made up of a stack of dielectric films, are disclosed in Patent Documents 5 through 7, for example. 
     A film capacitor comprising a rolled polypropylene film, which supports thereon two electrodes disposed along the transverse direction of the polypropylene film and which are connected in series to each other, is disclosed in Patent Document 8, for example.
     Patent Document 1: Japanese Patent No. 3710873;   Patent Document 2: Japanese Laid-Open Patent Publication No. 09-199371;   Patent Document 3: Japanese Laid-Open Patent Publication No. 2002-504747;   Patent Document 4: Japanese Laid-Open Patent Publication No. 2004-134561;   Patent Document 5: Japanese Laid-Open Patent Publication No. 07-263269;   Patent Document 6: Japanese Patent Publication No. 2590357;   Patent Document 7: Japanese Laid-Open Utility Model Publication No. 60-076028; and   Patent Document 8: Japanese Laid-Open Patent Publication No. 08-288171.   

     SUMMARY OF THE INVENTION 
     It is known that the floating electrodes of ceramic capacitors may be connected in series to each other in order to provide a higher withstand voltage. However, unlike film capacitors, since the floating electrodes are disposed within the sintered dielectric substrate, the ceramic capacitors fail to have both a self-healing function and a self-protective function. 
     According to the film capacitor disclosed in Patent Document 8, the evaporated film that forms a fuse between the electrodes has an increased thickness for heat control. However, the disclosed structure is not sufficient for providing a desired heat control. 
     The present invention has been made in view of the above problems. It is an object of the present invention to provide a film capacitor, which has a high withstand voltage achieved as an advantage of providing a floating electrode. Further, the film capacitor has a heat radiation capability, and both a self-healing function and a self-protective function, and thus, the film capacitor is capable of being used in various applications. 
     A film capacitor according to the present invention comprises a roll having a stacked structure, including a first stacked film assembly, which has a stack of dielectric films including a floating electrode therein, a first metal film and a second metal film disposed in sandwiching relation to the first stacked film assembly on opposite surfaces thereof, and a second stacked film assembly, which has a stack of dielectric films including a floating electrode therein, a first terminal electrically connected to an end of the roll, and a second terminal electrically connected to another end of the roll, wherein the floating electrode comprises a cluster of small electrodes that are independent of each other, each of the small electrodes and the first metal film form a capacitor therebetween, and each of the small electrodes and the second metal film form a capacitor therebetween, and further wherein the first electrode film extends to a side edge of the first stacked film assembly so as to be electrically connected to the first terminal, and the second electrode film extends to another side edge of the first stacked film assembly so as to be electrically connected to the second terminal. 
     In the event of a fault of the film capacitor, the capacitance thereof is prevented from being greatly lowered because the capacitance can be electrically separated by the individual small electrodes. The first metal film and the second metal film, which sandwich the first stacked film assembly on opposite surfaces thereof, can serve as lead-out electrodes. Since the first metal film and the second metal film can be formed over a wide area, the films can effectively be used as a heat radiation path. 
     The film capacitor according to the present invention has a high withstand voltage, achieved as an advantage of providing the floating electrode. Further, the film capacitor has a heat radiation capability, and both a self-healing function and a self-protective function, and thus, the film capacitor is capable of being used in various applications. 
     According to the present invention, the first stacked film assembly comprises at least one stack made up from a dielectric film, which is free of the floating electrode, and a dielectric film that includes the floating electrode therein. 
     Alternatively, the first stacked film assembly comprises at least one stack made up of at least two dielectric films, each having the floating electrode therein. 
     According to the present invention, the small electrodes may be formed by evaporating a metal film onto one of the dielectric films of the first stacked film assembly. 
     Each of the small electrodes may have a rectangular shape, the small electrodes being arrayed in a matrix on the dielectric film. 
     Each of the small electrodes may have a hexagonal shape, the small electrodes being arrayed in a honeycomb pattern on the dielectric film. 
     The first metal film is evaporated onto a principal surface of a dielectric film stacked on a stack of the dielectric films, and the second metal film is evaporated onto a principal surface of another dielectric film stacked on the stack. 
     The dielectric films that make up the first stacked film assembly and the second stacked film assembly may have transversely opposite undulated edges. Such undulated edges may be of a waveform such as a sine waveform, a triangular waveform, or a semicircular waveform. 
     As described above, the film capacitor according to the present invention has a high withstand voltage achieved as an advantage of providing the floating electrode. Further, the film capacitor has a heat radiation capability, and both a self-healing function and a self-protective function, and thus, the film capacitor is capable of being used in various applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an example of an outer shape of a first film capacitor; 
         FIG. 2  is a perspective view, partially omitted from illustration, of the structure of a roll of the first film capacitor; 
         FIG. 3  is a cross-sectional view, partially omitted from illustration, of the structure of the first film capacitor; 
         FIG. 4A  is a view showing an example of the shape and layout of the small electrodes of a floating electrode; 
         FIG. 4B  is a view showing another example of the shape and layout of the small electrodes; 
         FIG. 5  is a perspective view showing another example of an outer shape of the first film capacitor; 
         FIG. 6  is a perspective view, partially omitted from illustration, of the structure of a roll according to a modification of the first film capacitor; 
         FIG. 7  is a cross-sectional view, partially omitted from illustration, of the structure of a second film capacitor; 
         FIG. 8  is a cross-sectional view, partially omitted from illustration, of the structure of a third film capacitor; 
         FIG. 9  is a cross-sectional view, partially omitted from illustration, of the structure of a fourth film capacitor; and 
         FIG. 10  is a cross-sectional view, partially omitted from illustration, of the structure of a fifth film capacitor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of film capacitors according to the present invention will be described below with reference to  FIGS. 1 through 10 . 
     As shown in  FIG. 1 , a film capacitor according to a first embodiment of the present invention (hereinafter referred to as a “first film capacitor  10 A”) comprises a roll  12 , a first terminal  14  electrically connected to an end of the roll  12 , and a second terminal  16  electrically connected to another end of the roll  12 . 
     As shown in  FIG. 2 , the roll  12  includes at least two stacked film assemblies (a first stacked film assembly  20 A and a second stacked film assembly  20 B) each comprising a stack of dielectric films  18 . 
     The first stacked film assembly  20 A and the second stacked film assembly  20 B each includes a floating electrode  22  therein. The first stacked film assembly  20 A is sandwiched between a first metal film  24  and a second metal film  26  disposed on opposite surfaces thereof. 
     The roll  12  comprises a coiled stack made up of the first metal film  24 , the first stacked film assembly  20 A, the second metal film  26 , and the second stacked film assembly  20 B. 
     As shown in  FIGS. 2 and 3 , the roll  12  of the first film capacitor  10 A is made up of the first stacked film assembly  20 A, which comprises a stack formed by a first dielectric film  28 A and a second dielectric film  28 B, the floating electrode  22 , which is evaporated onto a principal surface of the second dielectric film  28 B, a first metal foil  30  (first metal film  24 ) disposed on a principal surface (e.g., an upper surface) of the first stacked film assembly  20 A, a second metal foil  32  (second metal film  26 ) disposed on another principal surface (e.g., a lower surface) of the first stacked film assembly  20 A, and the second stacked film assembly  20 A, which comprises a stack formed by a first dielectric film  28 A and a second dielectric film  28 B. 
     The floating electrode  22  comprises a cluster of small electrodes  34 , which are independent from each other. The small electrodes  34  may be of geometrical shapes such as polygonal shapes, circular shapes, etc., or irregular shapes such as leopard spots, or a mixture of such shapes. The small electrodes  34  may be positioned at equally spaced or arbitrarily spaced intervals. In the film capacitor  10 A shown in  FIG. 4A , the small electrodes  34  are each of a rectangular shape and are arrayed in a matrix on a principal surface of the second dielectric film  28 B. Alternatively, as shown in  FIG. 4B , the small electrodes  34  are each of a hexagonal shape and are arrayed in a honeycomb pattern on a principal surface of the second dielectric film  28 B. 
     As shown in  FIG. 3 , each of the small electrodes  34  and the first metal foil  30  form a first capacitor C 1  therebetween, whereas each of the small electrodes  34  and the second metal foil  32  form a second capacitor C 2  therebetween. 
     The second stacked film assembly  20 B is of the same structure. When the components are coiled together to form the roll  12 , the first metal foil  30  is positioned on the lower surface of the second stacked film assembly  20 B. Therefore, each of the small electrodes  34  of the second stacked film assembly  20 B and the first metal foil  30  form a first capacitor C 1  therebetween, whereas each of the small electrodes  34  of the second stacked film assembly  20 B and the second metal foil  32  form a second capacitor C 2  therebetween. 
     The first dielectric film  28 A and the second dielectric film  28 B may be made of PP (polypropylene), PET (polyethylene terephthalate), PPS (polyphenylene sulfide), or the like. The first dielectric film  28 A and the second dielectric film  28 B have a width Wa (see  FIG. 2 ) in a range from 10 to 200 mm, and a thickness ta (see  FIG. 3 ) in a range from 2.0 to 30 μm. 
     The small electrodes  34  of the floating electrodes  22  may be made of aluminum, zinc, or an alloy of aluminum and zinc, or the like. The floating electrodes  22  have a thickness tb (see  FIG. 3 ) which satisfies a surface resistance within a range from 1 to 20 ohms/mm 2 . The thickness tb may be in a range from 100 to 1000 Angstroms, for example. 
     The first film capacitor  10 A has a rated capacitance value that ranges from about several tens μF to several hundreds μF, a rated voltage of several hundreds V (DC), and a rated current of several tens A. 
     The first film capacitor  10 A may have a cylindrical shape, as shown in  FIG. 1 , or a flat shape, as shown in  FIG. 5 . 
     In the event of a fault in the first film capacitor  10 A, the capacitance thereof is prevented from being greatly lowered, because the capacitance can be electrically separated by the individual small electrodes  34 . The first metal foil  30  and the second metal foil  32 , which sandwich the first stacked film assembly  20 A on opposite surfaces thereof, can serve as lead-out electrodes. Since the first metal foil  30  and the second metal foil  32  can be formed over a wide area, the foils can effectively be used as a heat radiation path. 
     The first film capacitor  10 A has a high withstand voltage achieved as an advantage of providing the floating electrode  22 . Further, the first film capacitor  10 A has a heat radiation capability, and both a self-healing function and a self-protective function, and thus, the first film capacitor  10 A is capable of being used in various applications. 
       FIG. 6  shows a film capacitor  10 Aa according to a modification thereof. The first dielectric film  28 A has undulated opposite side edges  38   a ,  38   b , whereas the second dielectric film  28 B has undulated opposite side edges  40   a ,  40   b . In  FIG. 6 , the undulated opposite side edges are shown as being shaped in a sine waveform. However, the undulated opposite side edges may also be shaped in a triangular waveform, a semicircular waveform, or the like. 
     If the side edges  38   a ,  38   b  of the first dielectric film  28 A and the side edges  40   a ,  40   b  of the second dielectric film  28 B are straight, then when the components are coiled together to form the roll  12 , the first metal foil  30  and the second metal foil  32  are less liable to become exposed from the opposite ends of the roll  12 , and therefore, the first metal foil  30  and the second metal foil  32  may not be electrically connected sufficiently to the first terminal  14  and the second terminal  16 . In addition, the ends of the roll  12  and the first and second terminals  14 ,  16  may not be held closely against each other. 
     However, since the side edges  38   a ,  38   b  of the first dielectric film  28 A and the side edges  40   a ,  40   b  of the second dielectric film  28 B are undulated, when the components are coiled together to form the roll  12 , the first metal foil  30  and the second metal foil  32  tend to be exposed from opposite ends of the roll  12 , and are electrically connected appropriately to the first terminal  14  and the second terminal  16 , as well as being held closely against the first and second terminals  14 ,  16 . Accordingly, close contact between the side edges and the terminals prevents a large local current from flowing, thereby making the film capacitor highly reliable. 
     As shown in  FIG. 7 , a film capacitor according to a second embodiment of the present invention (hereinafter referred to as a “second film capacitor  10 B”) is of essentially the same structure as the first film capacitor  10 A, but differs therefrom as follows. 
     Each of the first stacked film assembly  20 A and the second stacked film assembly  20 B comprises a stack of alternate first dielectric films  28 A and second dielectric films  28 B. In  FIG. 7 , the first stacked film assembly  20 A comprises two stacks  50  made up of first dielectric films  28 A and second dielectric films  28 B. Similarly, the second stacked film assembly  20 B comprises two stacks  50  made up of first dielectric films  28 A and second dielectric films  28 B. 
     The first metal foil  30  is disposed on the upper surface of the first dielectric film  28 A that is positioned on an upper portion of the first stacked film assembly  20 A, and the second metal foil  32  is disposed on the lower surface of the second dielectric film  28 B that is positioned on a lower portion of the first stacked film assembly  20 A. The floating electrode  22  is disposed on a principal surface of each of the second dielectric films. 
     The roll  12  comprises a coiled stack made up of the first metal foil  30 , the first stacked film assembly  20 A, the second metal foil  32 , and the second stacked film assembly  20 B. 
     In the first stacked film assembly  20 A, each of the small electrodes  34  of the floating electrode  22  on one of the second dielectric films  28 B and the first metal foil  30  form a twenty-first capacitor C 21  therebetween, whereas each of the small electrodes  34  of the floating electrode  22  on the second dielectric film  28 B together with a corresponding one of the small electrodes  34  on the other second dielectric film  28 B form a twenty-second capacitor C 22  therebetween. Each of the small electrodes  34  on the other second dielectric film  28 B and the second metal foil  32  form a twenty-third capacitor C 23  therebetween. As described above, each of the small electrodes  34  on the other second dielectric film  28 B together with a corresponding one of the small electrodes  34  on one of the second dielectric films  28 B form the twenty-second capacitor C 22  therebetween. 
     The second stacked film assembly  20 B is similar in structure. When the components are coiled together to form the roll  12 , the first metal foil  30  is positioned on the lower surface of the second stacked film assembly  20 B. Therefore, each of the small electrodes  34  of the floating electrode  22  on one of the second dielectric films  28 B of the second stacked film assembly  20 B and the first metal foil  30  form a twenty-first capacitor C 21  therebetween, whereas each of the small electrodes  34  of the floating electrode  22  on the second dielectric film  28 B together with a corresponding one of the small electrodes  34  on the other second dielectric film  28 B form a twenty-second capacitor C 22  therebetween. Each of the small electrodes  34  on the other second dielectric film  28 B and the second metal foil  32  form a twenty-third capacitor C 23  therebetween. As described above, each of the small electrodes  34  on the other second dielectric film  28 B together with a corresponding one of the small electrodes  34  on one of the second dielectric films  28 B form the twenty-second capacitor C 22  therebetween. 
     The second film capacitor  10 B also has a high withstand voltage achieved as an advantage of providing the floating electrode  22 . Further, the second film capacitor  10 B has a heat radiation capability, and both a self-healing function and a self-protective function, and thus, the second film capacitor  10 B is capable of being used in various applications. 
     As shown in  FIG. 8 , a film capacitor according to a third embodiment of the present invention (hereinafter referred to as a “third film capacitor  10 C”) is of essentially the same structure as the second film capacitor  10 B, but differs therefrom as follows. 
     Each of the first stacked film assembly  20 A and the second stacked film assembly  20 B comprises four stacks  50  made up of first dielectric films  28 A and second dielectric films  28 B. 
     In the first stacked film assembly  20 A, each of the small electrodes  34  of the floating electrode  22  on one of the second dielectric films  28 B of the first stack (the stack  50  made up of the first dielectric film  28 A and the second dielectric film  28 B closest to the first metal foil  30 ) and the first metal foil  30  form a thirty-first capacitor C 31  therebetween, and each of the small electrodes  34  of the floating electrode  22  on the second dielectric film  28 B together with a corresponding one of the small electrodes  34  on the second dielectric film  28 B of the adjacent second stack  50  form a thirty-second capacitor C 32  therebetween. 
     As described above, each of the small electrodes  34  on the second dielectric film  28 B of the second stack  50  together with a corresponding one of the small electrodes  34  on the second dielectric film  28 B of the first stack  50  form the thirty-second capacitor C 32  therebetween. Each of the small electrodes  34  on the second dielectric film  28 B of the second stack  50  together with a corresponding one of the small electrodes  34  on the second dielectric film  28 B of the adjacent third stack  50  form a thirty-third capacitor C 33  therebetween. 
     As described above, each of the small electrodes  34  on the second dielectric film  28 B of the third stack  50  together with a corresponding one of the small electrodes  34  on the second dielectric film  28 B of the second stack  50  form a thirty-third capacitor C 33  therebetween. Each of the small electrodes  34  on the second dielectric film  28 B of the third stack  50  together with a corresponding one of the small electrodes  34  on the second dielectric film  28 B of the adjacent fourth stack  50  form a thirty-fourth capacitor C 34  therebetween. 
     Each of the small electrodes  34  on the second dielectric film  28 B of the fourth stack  50  and the second metal foil  32  form a thirty-fifth capacitor C 35 . As described above, each of the small electrodes  34  on the second dielectric film  28 B of the fourth stack  50  together with a corresponding one of the small electrodes  34  on the second dielectric film  28 B of the third stack  50  form the thirty-fourth capacitor C 34  therebetween. 
     The second stacked film assembly  20 B is similar in structure. When the components are coiled together to form the roll  12 , the first metal foil  30  is positioned on the lower surface of the second stacked film assembly  20 B. As with the first stacked film assembly  20 A, the second stacked film assembly  20 B forms the thirty-first capacitor C 31  through the thirty-fifth capacitor C 35 . 
     The third film capacitor  10 C also has a high withstand voltage achieved as an advantage of providing the floating electrode  22 . Further, the third film capacitor  10 C has a heat radiation capability, and both a self-healing function and a self-protective function, and thus, the third film capacitor  10 C is capable of being used in various applications. 
     The second film capacitor  10 B has two stacks  50  formed by the first dielectric films  28 A and the second dielectric films  28 B, and the third film capacitor  10 C has five stacks  50  formed by the first dielectric films  28 A and the second dielectric films  28 B. However, the film capacitor according to the present invention may also have three stacks, or five or more stacks. 
     As shown in  FIG. 9 , a film capacitor according to a fourth embodiment of the present invention (hereinafter referred to as a “fourth film capacitor  10 D”) is of essentially the same structure as the first film capacitor  10 A, but differs therefrom as follows. 
     The first stacked film assembly  20 A comprises a stack  50  made up of a first dielectric film  28 A and a second dielectric film  28 B, wherein a first dielectric film  28 A is stacked on an upper surface of the stack  50 , and a first dielectric film  28 A is stacked on a lower surface of the stack  50 . The second stacked film assembly  20 B is similar in structure. Specifically, the first stacked film assembly  20 A includes a rolled stack made up of a first dielectric film  28 A with a first lead-out electrode  46 A (a first metal film  24 ) being evaporated on a principal surface (an upper surface) thereof, a stack  50 , and a first dielectric film  28 A with a second lead-out electrode  46 B (a second metal film  26 ) being evaporated on another principal surface (a lower surface) thereof. The fourth film capacitor  10 D also has a high withstand voltage achieved as an advantage of providing the floating electrode  22 . Further, the fourth film capacitor  10 D has a heat radiation capability, and both a self-healing function and a self-protective function, and thus, the fourth film capacitor  10 D is capable of being used in various applications. 
     In the above embodiments, each of the first stacked film assembly  20 A and the second stacked film assembly  20 B includes the stack  50  made up of the first dielectric film  28 A, which is free of the floating electrode  22 , and the second dielectric film  28 B that includes the floating electrode  22  therein. However, a film capacitor according to a fifth embodiment of the present invention (hereinafter referred to as a “fifth film capacitor  10 E”) shown in  FIG. 10  includes a stack  52  of two or more second dielectric films  28 B each having a floating electrode  22 . In the fifth film capacitor  10 E shown in  FIG. 10 , the stack  52  is stacked on the second metal foil  32 , the first dielectric film  28 A, which is free of the floating electrode  22 , is stacked on the stack  52 , and the first metal foil  30  is stacked on the first dielectric film  28 A. The structure of the stack  52  shown in  FIG. 10  also is applicable to the first film capacitor  10 A through the fourth film capacitor  10 D. 
     In the first film capacitor  10 A through the fifth film capacitor  10 E, the first stacked film assembly  20 A and the second stacked film assembly  20 B are of the same stacked structure. However, the first stacked film assembly  20 A and the second stacked film assembly  20 B may be of different stacked structures, respectively. 
     The film capacitor according to the present invention is not limited to the above embodiments, and various changes and modifications may be made to the embodiments without departing from the scope of the invention as set forth in the appended claims.