Abstract:
In a switching circuit, at least two bus bars are fixed with an insulator interposed therebetween and secured to a substrate of the switching circuit. Being combined with the bus bars, the insulator serves not only to insulate those bus bars but also to enhance the bending rigidity and bending strength of the switching circuit. As current flows in one of the bus bars in a direction opposite to a direction in which current flows in the other bus bar, the inductance of the bus bars is reduced, resulting in the decreased fly-back voltage generated upon switching operation.

Description:
INCORPORATION BY REFERENCE 
   The disclosure of Japanese Patent Application No.2001-181345 filed on Jun. 15, 2001, including the specification, drawings and abstract are incorporated herein by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to a switching circuit provided with a plurality of bus bars serving as a conductor for coupling a switching element to a power source or a load. 
   2. Description of Related Art 
   Generally a metallic bus bar is well known as a conductor having a cross section area large enough to allow electric current to flow between circuit elements or between a circuit element and a power source or between a circuit element and a load in a power circuit such as a switching circuit for an inverter motor.  FIG. 8  is a circuit diagram showing a generally employed switching circuit  40  serving as an inverter.  FIG. 9  shows a plan view of the switching circuit  40 . 
   Referring to  FIG. 8 , the switching circuit  40  takes the form of a three-phase inverter having switching elements S 1  to S 6 , such as IGBT (Insulated Gate Bipolar Transistor), and diodes D 1  to D 6  connected in parallel with the corresponding switching elements S 1  to S 6 . Each of the switching elements S 1  to S 6  is opened and closed in accordance with a control signal sent from a control unit (not shown). In response to opening or closing of the switching elements S 1  to S 6 , the direct current applied from a direct-current power source  42  is converted into a three-phase alternating current. The three-phase alternating current, then, is supplied to a three-phase inverter motor  44  serving as the load. 
   Referring to  FIG. 9 , the switching circuit  40  has power source side bus bars  46  ( 46 P,  46 N) between the switching elements S 1  to S 6  and the power source (not shown), and has load side bus bars  48  ( 48 U,  48 V,  48 W) between the switching elements S 1  to S 6  and the respective phases (U-phase, V-phase, W-phase) at the load side. Each of the power source side bus bars  46  is formed into a frame-like shape defined by longitudinal members  46 C and lateral members  46 R. The longitudinal members  46 C are inserted into a substrate  50 , which is then subjected to resin molding, so that the longitudinal members  46 C are embedded in the substrate  50 . Like the longitudinal members  46 C, the lateral members  46 R are embedded in the substrate  50  except the upper surface. The upper surface of the lateral member  46 R is exposed on the substrate  50  as a surface to which wires are bonded for connection with the switching elements S 1  to S 6 . Embedding of the bus bars  46 P and  46 N in the substrate  50  makes it possible to enhance a flexural rigidity and flexural strength of the switching circuit  40 . Furthermore, insulation between adjacent bus bars, for example, bus bars  46 P and  46 N, or the bus bars  46 P,  46 N and  48 U,  48 V,  48 W can be secured, respectively. 
   Each of the load-side bus bars  48  is formed as a strip member that laterally extends on the substrate  50 . The switching members S 1  to S 6  are provided on the substrate  50  between the lateral members  46 R of the power source side bus bars  46  and the load side bus bars  48 . The respective ends, upper and lower ends shown in  FIG. 9 , of the switching members S 1  to S 6  are connected to the lateral members  46 R and the load-side bus bars  48  via wires  52 . 
   The wires  52  are bonded to the respective bus bars  46 ,  48  through ultrasonic bonding. More specifically, in the state where the metallic wires are pressed onto the bus bars  46 ,  48 , each portion of contacts between the metallic wires and the bus bars  46 ,  48  is oscillated using ultrasound. Accordingly the wires  52  are bonded onto the surface of the bus bars  46 ,  48 . 
   However, the aforementioned switching circuit formed by insert-molding of the bus bars (power source side) into the substrate has caused various problems. For example, as a flow path of the resin becomes complicated during the insert molding, the resultant switching circuit may be deformed or cracked. Furthermore the inserted bus bars may be deformed by the temperature rise during the resin molding. 
   Conventionally, adjacent bus bars are kept insulated by inserting the bus bars into an insulating resin, that is, covering the peripheral area of the bus bars with the resin material. However, this may increase the size of the substrate. 
   When a certain failure occurs during resin molding of the substrate, a gap may be generated in a portion of the substrate on which the lateral member of the inserted bus bar ( 46 R shown in  FIG. 9 , for example) abuts. This may prevent the bus bars from being insulated, or cause difference in the oscillating state or the temperature rise during ultrasonic bonding between the portion with the gap and the portion with no gap. As a result, the bonding strength or the resistance value may vary depending on the bonded portion. 
   SUMMARY OF THE INVENTION 
   A switching circuit according to an embodiment of the invention includes a plurality of switching elements and a plurality of bus bars each serving as a conductor. The bus bars connect the switching elements to one of a power source and a load. Two or more of the plurality of bus bars are fixed to each other with an insulator interposed between adjacent ones of the two or more bus bars. The two or more of the plurality of bus bars with the insulator interposed therebetween are secured to a substrate of the switching circuit. As the aforementioned circuit requires no insert molding of the bus bars, the process for producing the switching circuit may be simplified, reducing the time and cost for producing the circuit. As the resin used for producing the circuit can be reduced, the resultant circuit may be compact and light. A plurality of bus bars are fixed with insulators interposed therebetween, thus preventing deformation of the bus bar itself. Furthermore, the combined bus bars and the insulators may serve as a reinforcing member of the substrate. Accordingly, the flexural rigidity and flexural strength of the switching circuit can be enhanced. 
   Two of the plurality of bus bars are fixed to each other with the insulator interposed therebetween. Current flows in one of the two bus bars in a direction opposite to a direction in which current flows in the other bus bar. In the switching circuit provided with the bus bars, an induced voltage (fly-back) generated during switching is increased by inductance distributed in the bus bars. This may prevent the switching circuit from conducting high-speed switching. According to the aforementioned aspect of the invention, however, the magnetic fields generated around the adjacent bus bars can be offset by current flowing along the bus bar in the opposite directions. The inductance of the bus bars, thus, can be reduced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a switching circuit of an embodiment of the invention; 
       FIG. 2  is a perspective view of the switching circuit illustrating how electrode-side bus bars and insulators are fixed to a substrate of the switching circuit; 
       FIG. 3  is a sectional view along line A—A of  FIG. 1 , illustrating a cross-section of the electrode-side bus bar of the switching circuit: 
       FIG. 4  is a sectional view along line B—B of  FIG. 1 , illustrating another cross-section of the electrode-side bus bar of the switching circuit; 
       FIG. 5  is a sectional view illustrating bus bars and insulators of the switching circuit according to another embodiment of the invention; 
       FIG. 6  is a side elevation view illustrating a portion to which wire bonding is performed on the bus bar of the switching circuit; 
       FIGS. 7A and 7B  show the bus bars and the insulator of the switching circuit of still further embodiment of the invention; 
       FIG. 8  is a circuit diagram of a conventional switching circuit formed as an inverter; and 
       FIG. 9  is a plan view illustrating a whole structure of the conventional switching circuit. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Embodiments of the invention will be described referring to the drawings.  FIG. 1  is a plan view showing an essential portion of a switching circuit  10  of an embodiment of the invention. The direction of y-axis in figure such as  FIG. 1  will be hereinafter called as a longitudinal direction, and the direction of x-axis in the figure will be hereinafter called as a lateral direction. 
   The switching circuit  10  serves to convert direct current into three-phase current (U-phase, V-phase, W-phase) as a circuit equivalent to an inverter as shown in FIG.  6 . Referring to  FIG. 1 , the switching circuit  10  includes a plurality of bus bars  14  ( 14 P,  14 N) connected to a power source (not shown), and a plurality of bus bars ( 16 U,  16 V,  16 W) connected to a load (not shown). The bus bar  14  includes longitudinal members  14 PC,  14 NC that extend in the longitudinal direction, and lateral members  14 PR,  14 NR that extend in the lateral direction. Each bus bar  16  is formed into a strip-like shape extending in the lateral direction. The lateral members  14 PR,  14 NR and the bus bars  16  are arranged in parallel at predetermined intervals. In  FIG. 1 , arranged in order from the upper side to lower side along the y-axis direction of the figure are the lateral member  14 PR of the positive electrode side bus bar  14 P, the U-phase side bus bar  16 U, the lateral member  14 NR of the negative electrode side bus bar  14 P, the V-phase side bus bar  16 V, the lateral member  14 PR of the positive electrode side bus bar  14 P, the W-phase side bus bar  16 W, and the lateral member  14 NRof the negative electrode side bus bar  14 N, respectively. The longitudinal member  14 PC of the positive electrode side bus bar  14 P and the longitudinal member  14 NC of the negative electrode side bus bar  14 C are stacked via an insulator  18  in the direction vertical to the switching circuit. The stacked structure will be described in detail later. Each of the aforementioned bus bars  14 ,  16  may be formed of a metallic member, for example, copper. The insulator  18  is formed of, for example, PPS (polyphenylene sulfide), PBT (polybutyleneterephthalate) or the like. 
   Switching elements  20  are disposed on gaps between the lateral member  14 PR and the bus bar  16  and between the lateral member  14 NR and the bus bar  16 . Referring to  FIG. 1 , arranged in order from the upper side to the lower side of the switching circuit in the figure are a switching element  20 PU between the positive electrode and the U-electrode at load side, a switching element  20 NU between the negative electrode and the U-electrode at load side, a switching element  20 NV between the negative electrode and the V-electrode at load side, a switching element  20 PV between the positive electrode and the V-electrode at load side, a switching element  20 PW between the positive electrode and the W-electrode at load side, and a switching element  20 NW between the negative electrode and the W-electrode at load side. 
   Each of the switching elements  20  is connected to the bus bars  14 ,  16  via wires  22  bonded to the bus bars  14 ,  16  through ultrasonic bonding. In this embodiment, each rear surface of the bus bars  14 ,  16  is brought into tight contact with an upper surface of a substrate  12 . Rear surfaces of the bus bars  14 ,  16  and the upper surface of the substrate  12  are formed into flat faces. Then adhesive is applied to the whole flat faces of the rear surfaces of the bus bars  14 ,  16  and the upper surface of the substrate so as to be bonded. This makes it possible to make a state of contact between the bus bars  14 ,  16  and the substrate  12  uniform, and an oscillating state and the temperature rise caused by ultrasonic bonding uniform. Accordingly, variation in the state of contact between the wire  22  and the bus bars  14 ,  16  may further be reduced. 
   Provided on the bus bars  14 ,  16  are control terminal bases  24  (shown in dashed line) each formed of a thin frame-like resin having an opening above the switching element  20  and a bonding position. The switching element  20  is connected to a control terminal (not shown) provided on the control terminal base  24  via a wire (not shown). Opening and closing of the switching element  20  is controlled in accordance with a control signal sent from a control unit (not shown) via the control terminal and the wire. 
   Referring to  FIGS. 2  to  4 , the description about assembling of the bus bars  14  and the insulator  18  to the substrate  12 , and the structure of the assembly will be explained in detail.  FIG. 2  is a perspective view showing assembling of the bus bars  14  and the insulator  18  to the substrate  12 .  FIG. 3  is a sectional view along line A—A of  FIG. 1 , illustrating a section of the power source-side bus bar of the switching circuit.  FIG. 4  is a sectional view along line B—B of  FIG. 1 , illustrating another section of the power source-side bus bar of the switching circuit. 
   Referring to  FIG. 2 , a plurality of bus bars (two bus bars  14 P,  14 N in this embodiment) are fixed to the substrate  12  in the state where the insulator  18  is interposed between the bus bars  14 P and  14 N. In this embodiment, a plurality of positioning pins  28 M provided on the upper surface of the substrate  12  are inserted into corresponding holes  28 F formed on the bus bars  14  such that the bus bars  14  are positioned with respect to the substrate  12 . The surface of the substrate  12  on which the bus bars  14  abut is applied with the adhesive so that the bus bars  14  are adhered to the substrate  12 . 
   Referring to  FIG. 3 , the insulator  18  is interposed between the longitudinal member  14 NC of the negative electrode side bus bar  14 N as the upper side and the longitudinal member  14 PC of the positive electrode side bus bar  14 P as the lower side. The insulator  18  serves to prevent electric current from flowing between the bus bars  14 P and  14 N. Positions of the bus bars  14 P and  14 N are aligned with a positioning pin  30  that is formed of the insulating material and inserted into the insulator  18 . The bus bars  14 P,  14 N and the insulator  18  are adhered with the adhesive. Referring to  FIG. 4 , both ends of the insulator  18  are provided with protrusions  18 E,  18 E each extending vertically in an opposite direction with respect to the insulator  18 . The positive electrode-side bus bar  14 P and the negative electrode-side bus-bar  14 N are fitted in the portion above and below the isolator  18 . Accordingly the protrusions  18 E,  18 E serve to align positions of the bus bars  14 P,  14 N. 
   In the aforementioned structure of the bus bars  14 P,  14 N having the insulator  18  interposed therebetween, the direction of electric current that flows through the bus bar  14 P is different from that of electric current flowing through the bus bar  14 N. In the embodiment, the aforementioned bus bars  14 P and  14 N are structured to extend in the same direction (longitudinal direction) so as to be connected to the power source at the same side (left side in  FIG. 3 , lower side in FIG.  1 ). As a result, the electric current flows through the negative electrode side bus bar  14 N in the direction designated as n (from right to left), and the electric current flows through the positive electrode side bus bar  14 P in the direction designated as p (from left to right). The magnetic field generated around the bus bar  14 P has a rotating direction opposite to that of the magnetic field generated around the bus bar  14 N. The aforementioned structure may reduce inductance generated in the bus bars  14 P,  14 N, thus minimizing fly-back voltage upon switching. Accordingly this may realize higher switching speed. 
   The invention is not limited by the aforementioned embodiment in which one insulator is interposed between two bus bars over a whole length of those bus bars. Alternatively, the insulator may be interposed between two bus bars only partially, or a plurality of insulators are intermittently interposed between the bus bars. 
   In the embodiment, a plurality of bus bars are stacked on the substrate via the insulator in a vertical direction thereto. Alternatively, the bus bars  14 P,  14 N may be arranged on the substrate in parallel via the insulator  18  as shown in FIG.  5 . In this case, the second moment of area of the bus bar may be enhanced by forming the section of the bus bar into an L-like shape so as to improve flexural rigidity and flexural strength. 
   In the aforementioned embodiment, each rear surface of the bus bars ( 14 PR,  14 NR,  16 U,  16 V or  16 W) subjected to the ultrasonic bonding is brought into tight contact with the upper surface of the substrate  12 . Alternatively, as shown in  FIG. 6 , the bus bar  32  may have portions that are not subjected to the ultrasonic bonding, for example, both ends of the bus bar  32 , supported with the substrate  12  so as to form a gap between the bus bar  32  and the substrate  12 . This structure allows the oscillating state or the temperature rise at a plurality of bonded portions to be uniform, reducing variation of the bonding strength and resistance value of the wire. The structure shown in  FIG. 6  may be applied to the bus bars ( 14 PR,  14 NR,  16 U,  16 V, or  16 W) of the switching circuit  10  of the embodiment according to the invention. 
   In the aforementioned embodiment, the bus bar  14  has two different portions, the portion subjected to the wire bonding (longitudinal members  14 PC,  14 NC), and the portion to be stacked via the insulator ( 14 PR,  14 NR). Alternatively, the same members of the bus bar may be subjected to wire bonding at a position close to the fixation.  FIG. 7A  is a plan view showing an example of stacking the  34 L and  34 R via the insulator  36 .  FIG. 7B  is a sectional view of the bus bars to be fixed via the insulator as shown in FIG.  7 A. In this example, the bus bars  34 L,  34 R are fixed with a bolt  37  and a nut  38  which are inserted through the insulator  36  at a position close to the surface subjected to the wire bonding, that is, the wire bonding area. In this example, a washer  39  formed of an insulating material and a spacer (not shown) formed of the insulating material and inserted into a hole of the bolt that fixes the bus bars  34 L and  34 R may bring the nut  38  in contact with the bus bars  34 L,  34 R so as to prevent electric current from flowing between the bus bars  34 L and  34 R. In this example, the bus bars  34 L,  34 R are on the same plane, and positioned at the same height from the substrate  12  (the surface on which the switching elements are placed). This structure may allow the wire bonding of the bus bars  34 L and  34 R to be performed in substantially the same conditions, thus reducing variation in the bonded state of a plurality of positions of the bus bars  34 L,  34 R. In this example, the bus bars  34 L and  34 R are bent so as to enhance the second moment of area to bending in the longitudinal direction. As the direction of the electric current flowing through the bus bar  34 L is opposite to that of the electric current flowing through the bus bar  34 R, the inductance may be reduced. The structure may be realized by combining the aforementioned examples. 
   The aforementioned embodiment requires no embedding of the bus bars in the resin for insulating purpose. Additionally the embodiment allows enhancement of flexural rigidity and flexural strength, thus making the bus bar further light and compact.