Patent Publication Number: US-2015078044-A1

Title: Power conversion apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2013-192041 filed with the Japan Patent Office on Sep. 17, 2013, the entire content of which is hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a power conversion apparatus. 
     2. Related Art 
     A power conversion apparatus has conventionally been known (for example, see JP-A-2011-67045). 
     The inverter apparatus (power conversion apparatus) disclosed in JP-A-2011-67045 includes a lower metal substrate and an upper dielectric substrate disposed to face each other, a MOSFET (horizontal switching element), and a snubber capacitor. The MOSFET and the snubber capacitor are disposed and held between the lower metal substrate and the upper dielectric substrate. This inverter apparatus is configured to make an electric current flow in a snubber circuit including the snubber capacitor through the lower metal substrate and the upper dielectric substrate. 
     SUMMARY 
     A power conversion apparatus includes: a horizontal switching element having a front surface and a rear surface, including a first electrode and a second electrode on the front surface, and having a first current path between the first electrode and the second electrode; a snubber capacitor electrically connected to the horizontal switching element; a first substrate on which the snubber capacitor is mounted, the first substrate being connected to the first electrode and the second electrode on the front surface of the horizontal switching element; and a second current path through which an electric current flows in a direction approximately opposite to the first current path that is a path allowing an electric current to flow between the first electrode and the second electrode of the horizontal switching element, the second current path being provided in the first substrate and disposed at a position opposite to the first current path. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram illustrating an inverter apparatus according to an embodiment; 
         FIG. 2  is a cross-sectional diagram illustrating the inverter apparatus according to the embodiment (sectional diagram taken along a line  150 - 150  of  FIG. 3 ); 
         FIG. 3  is a diagram illustrating a top surface of a first substrate of the inverter apparatus according to the embodiment; 
         FIG. 4  is a diagram illustrating an intermediate layer of the first substrate of the inverter apparatus according to the embodiment; 
         FIG. 5  is a diagram illustrating a bottom surface of the first substrate of the inverter apparatus according to the embodiment; 
         FIG. 6  is a diagram illustrating a top surface of a second substrate of the inverter apparatus according to the embodiment; 
         FIG. 7  is a diagram illustrating a bottom surface of a second substrate of the inverter apparatus according to the embodiment; 
         FIG. 8  is a planar view of a horizontal switching element according to an embodiment viewed from the front surface side; 
         FIG. 9  is a planar view of the horizontal switching element according to the embodiment viewed from the rear surface side; 
         FIG. 10  is a planar view of a control switching element according to an embodiment viewed from the front surface thereof; 
         FIG. 11  is a planar view of the control switching element according to the embodiment viewed from the rear surface thereof; and 
         FIG. 12  is a sectional diagram (sectional diagram taken along a line  150 - 150  of  FIG. 3 ) illustrating the current path of the inverter apparatus according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     A power conversion apparatus according to an aspect of the present disclosure includes: a horizontal switching element having a front surface and a rear surface, including a first electrode and a second electrode on the front surface, and having a first current path between the first electrode and the second electrode; a snubber capacitor electrically connected to the horizontal switching element; a first substrate on which the snubber capacitor is mounted, the first substrate being connected to the first electrode and the second electrode on the front surface of the horizontal switching element; and a second current path through which an electric current flows in a direction approximately opposite to the first current path that is a path allowing an electric current to flow between the first electrode and the second electrode of the horizontal switching element, the second current path being provided in the first substrate and disposed at a position opposite to the first current path. 
     In the power conversion apparatus according to the aspect of the present disclosure, since the first substrate includes the snubber capacitor mounted thereon and is connected to the first and second electrodes on the front surface of the horizontal switching element, an electric current can flow in the snubber circuit including the snubber capacitor only through the first substrate. Thus, for example, the current path of the snubber circuit including the snubber capacitor can be shortened as compared to the case where an electric current flows through the first substrate and a substrate other than the first substrate on the rear surface of the horizontal switching element. Therefore, a reduction in the wiring inductance of the snubber circuit including the snubber capacitor can be attained. 
     Further, the first substrate is configured to include a second current path. The first current path is a path allowing an electric current to flow between the first electrode and the second electrode of the horizontal switching element. In the second current path, an electric current flows in a direction approximately opposite to the first current path. The second current path is disposed at a position opposite to the first current path. Thus, the change of the magnetic flux generated in the first current path can be cancelled by the change of the magnetic flux generated in the second current path. Likewise, therefore, a reduction in the wiring inductance of the snubber circuit including the snubber capacitor can be attained. 
     The power conversion apparatus can reduce the wiring inductance of the snubber circuit including the snubber capacitor. 
     An embodiment of the present disclosure is hereinafter described with reference to the drawings. 
     Referring now to  FIG. 1 , the structure of an inverter apparatus  100  according to this embodiment is described. The inverter apparatus  100  is an exemplary “power conversion apparatus”. 
     The inverter apparatus  100  is configured to convert direct current power input from a direct current power source (not shown) through input terminals P (V+) and N (V−) into alternating current power and output the alternating current power from an output terminal. 
     The inverter apparatus  100  includes two horizontal switching elements  11  and  12 , two control switching elements  13  and  14 , which are respectively connected to the two horizontal switching elements, and snubber capacitors  15  and  16 . Note that the horizontal switching elements  11  and  12  are normally-on type switching elements. In other words, when the voltage applied to a gate electrode G 1  (G 2 ) is 0 V, an electric current flows between a drain electrode D 1  (D 2 ) and a source electrode S 1  (S 2 ) in the horizontal switching elements  11  and  12 . The horizontal switching element  11  is an exemplary “first horizontal switching element”, and the horizontal switching element  12  is an exemplary “second horizontal switching element”. 
     The control switching elements  13  and  14  are normally-off type switching elements. In other words, in each of the control switching elements  13  and  14 , an electric current does not flow between a drain electrode D 3  (D 4 ) and a source electrode S 3  (S 4 ) when the voltage applied to a gate electrode G 3  (G 4 ) is 0 V. The control switching elements  13  and  14  are cascode-connected to the horizontal switching elements  11  and  12 , respectively. Thus, an electric current flows between the drain electrode D 1  (D 2 ) and the source electrode S 1  (S 2 ) of the horizontal switching element  11  ( 12 ) while the control switching element  13  ( 14 ) is on. 
     Specifically, the gate electrode G 1  (G 2 ) of the horizontal switching element  11  ( 12 ) is connected to the source electrode S 3  (S 4 ) of the control switching element  13  ( 14 ). Thus, the control switching element  13  ( 14 ) is configured to control the actuation (switching operation) of the horizontal switching element  11  ( 12 ) by performing the switching operation based on a control signal input to the gate electrode G 3  (G 4 ). As a result, the switching circuit including the normally-on type horizontal switching element  11  ( 12 ) and the normally-off type control switching element  13  ( 14 ) is configured to be controlled as the normally-off type as a whole. 
     Next, the configuration (structure) of the inverter apparatus  100  according to the embodiment is specifically described with reference to  FIG. 2  to  FIG. 12 . 
     As illustrated in  FIG. 2  to  FIG. 7 , the inverter apparatus  100  includes a first substrate  20 , a second substrate  30 , two horizontal switching elements  11  and  12 , two control switching elements  13  and  14 , two snubber capacitors  15  and  16 , and a heat sink  40 . 
     As illustrated in  FIG. 2 , the first substrate  20  and the second substrate  30  are vertically disposed to face each other at a predetermined distance therebetween (in the Z direction). Specifically, the first substrate  20  is disposed on the upper side (in the Z1 direction) and the second substrate  30  is disposed on the lower side (in the Z2 direction). The horizontal switching elements  11  and  12  are disposed between a bottom surface  20   c  of the first substrate  20  (surface in the Z2 direction) and a top surface  30   a  of the second substrate  30  (surface in the Z1 direction). As illustrated in  FIG. 3 , moreover, the control switching elements  13  and  14  and the snubber capacitors  15  and  16  are disposed on the top surface  20   a  of the first substrate  20 . A heat conductive material  50  fills the space between the bottom surface  20   c  of the first substrate  20  and the top surface  30   a  of the second substrate  30  around the horizontal switching elements  11  and  12 . Moreover, the space except the heat conductive material  50  between the bottom surface  20   c  of the first substrate  20  and the top surface  30   a  of the second substrate  30  is filled with sealing resin (not shown). 
     As illustrated in  FIG. 3 , conductive patterns  201 ,  202 ,  203 ,  204 ,  205 ,  206 ,  207 ,  208 ,  209 ,  210 ,  211 , and  212  are provided on the top surface  20   a  of the first substrate  20 . As illustrated in  FIG. 4 , conductive patterns  221 ,  222 ,  223 ,  224 ,  225 ,  226 ,  227 ,  228 ,  229 ,  230 ,  231 , and  232  are provided in an intermediate layer  20   b  of the first substrate  20 . Moreover, as illustrated in  FIG. 5 , conductive patterns  241 ,  242 ,  243 ,  244 ,  245 ,  246 ,  247 ,  248 ,  249 , and  250  are provided on the bottom surface  20   c  of the first substrate  20 . 
     As illustrated in  FIG. 3  to  FIG. 5 , the conductive pattern  201  on the top surface  20   a , the conductive pattern  221  on the intermediate layer  20   b , and the conductive pattern  242  on the bottom surface  20   c  are connected to one another through a penetration electrode  201   a . In addition, the conductive pattern  202  on the top surface  20   a , the conductive pattern  230  on the intermediate layer  20   b , and the conductive pattern  241  on the bottom surface  20   c  are connected to one another through a penetration electrode  202   a . Further, the conductive pattern  202  on the top surface  20   a , the conductive pattern  232  on the intermediate layer  20   b , and the conductive pattern  247  on the bottom surface  20   c  are connected to one another through a penetration electrode  202   b . Furthermore, the conductive pattern  202  on the top surface  20   a , the conductive pattern  226  on the intermediate layer  20   b , and the conductive pattern  250  on the bottom surface  20   c  are connected to one another through a penetration electrode  202   c.    
     As illustrated in  FIG. 3  and  FIG. 4 , the conductive pattern  203  on the top surface  20   a  and the conductive pattern  225  on the intermediate layer  20   b  are connected to each other through a penetration electrode  203   a . As illustrated in  FIG. 3  to  FIG. 5 , the conductive pattern  204  on the top surface  20   a , the conductive pattern  222  on the intermediate layer  20   b , and the conductive pattern  246  on the bottom surface  20   c  are connected to one another through a penetration electrode  204   a . Moreover, the conductive pattern  205  on the top surface  20   a , the conductive pattern  223  on the intermediate layer  20   b , and the conductive pattern  244  on the bottom surface  20   c  are connected to one another through a penetration electrode  205   a . In addition, the conductive pattern  205  on the top surface  20   a , the conductive pattern  224  on the intermediate layer  20   b , and the conductive pattern  245  on the bottom surface  20   c  are connected to one another through a penetration electrode  205   b.    
     As illustrated in  FIG. 3  and  FIG. 4 , the conductive pattern  205  on the top surface  20   a  and the conductive pattern  228  on the intermediate layer  20   b  are connected to each other through two penetration electrodes  205   c . Further, the conductive pattern  206  on the top surface  20   a  and the conductive pattern  227  on the intermediate layer  20   b  are connected to each other through two penetration electrodes  206   a . Additionally, the conductive pattern  209  on the top surface  20   a  and the conductive pattern  231  on the intermediate layer  20   b  are connected to each other through a penetration electrode  209   a . Moreover, the conductive pattern  212  on the top surface  20   a  and the conductive pattern  229  on the intermediate layer  20   b  are connected to each other through two penetration electrodes  212   a.    
     As illustrated in  FIG. 4  and  FIG. 5 , the conductive pattern  225  on the intermediate layer  20   b  and the conductive pattern  242  on the bottom surface  20   c  are connected to each other through a penetration electrode  225   a . In addition, the conductive pattern  227  on the intermediate layer  20   b  and the conductive pattern  243  on the bottom surface  20   c  are connected to each other through two penetration electrodes  227   a . Further, the conductive pattern  228  on the intermediate layer  20   b  and the conductive pattern  246  on the bottom surface  20   c  are connected to each other through two penetration electrodes  228   a . In addition, the conductive pattern  229  on the intermediate layer  20   b  and the conductive pattern  249  on the bottom surface  20   c  are connected to each other through two penetration electrodes  229   a . Furthermore, the conductive pattern  231  on the intermediate layer  20   b  and the conductive pattern  242  on the bottom surface  20   c  are connected to each other through two penetration electrodes  231   a.    
     As illustrated in  FIG. 6 , conductive patterns  301 ,  302 , and  303  are provided on the top surface  30   a  of the second substrate  30 . The conductive pattern  301  includes an element-bonding pattern portion  301   a , and a connection pattern portion  301   b . On the element-bonding pattern portion  301   a , a rear surface  11   b  of the horizontal switching element  11  is bonded. The connection pattern portion  301   b  has the element-bonding pattern portion  301   a  connected to the first substrate  20 . As illustrated in  FIG. 7 , a ground pattern  304  is provided at the bottom surface  30   b  of the second substrate  30 . The conductive pattern  301  is an example of “potential adjustment pattern”. 
     As illustrated in  FIG. 3 , the conductive pattern  201  on the top surface  20   a  of the first substrate  20  is connected to the input terminal P (V+). The conductive pattern  202  is connected to the input terminal N (V−). The conductive pattern  204  is connected to the output terminal. The conductive pattern  208  is connected to an input terminal  17   a . Through the input terminal  17   a , a control signal is input to the gate electrode G 3  of the control switching element  13 . The conductive pattern  211  is connected to an input terminal  17   b . Through the input terminal  17   b , a control signal is input to the gate electrode G 4  of the control switching element  14 . 
     As illustrated in  FIG. 5  and  FIG. 6 , the conductive pattern  244  on the bottom surface  20   c  of the first substrate  20  is connected to the conductive pattern  301  (connection pattern portion  301   b ) on the top surface  30   a  of the second substrate  30  through a columnar electrode  18   a . Moreover, the conductive pattern  250  on the bottom surface  20   c  of the first substrate  20  is connected to the conductive pattern  303  on the top surface  30   a  of the second substrate  30  through a columnar electrode  18   b . Moreover, the conductive pattern  241  on the bottom surface  20   c  of the first substrate  20  is connected to the conductive pattern  302  on the top surface  30   a  of the second substrate  30  through a columnar electrode  18   c . In addition, the conductive pattern  248  on the bottom surface  20   c  of the first substrate  20  is connected to the conductive pattern  302  on the top surface  30   a  of the second substrate  30  through a columnar electrode  18   d.    
     As illustrated in  FIG. 8  and  FIG. 9 , the horizontal switching element  11  ( 12 ) has a front surface  11   a  ( 12   a ) and a rear surface  11   b  ( 12   b ). The front surface  11   a  ( 12   a ) of the horizontal switching element  11  ( 12 ) includes the gate electrode G 1  (G 2 ), the source electrode S 1  (S 2 ), and the drain electrode D 1  (D 2 ). In other words, in the horizontal switching element  11  ( 12 ), current mainly flows on one surface side provided with each electrode during the actuation. Therefore, the surface on the side provided with each electrode mainly generates heat. The rear surface  11   b  of the horizontal switching element  11  ( 12 ) is provided with a body electrode B 1  (B 2 ). As illustrated in  FIG. 12 , the horizontal switching element  11  ( 12 ) includes a first current path C 1  (C 4 ) between the source electrode S 1  (S 2 ) and the drain electrode D 1  (D 2 ). The first current path C 1  (C 4 ) extends in a direction parallel to the front surface  11   a  ( 12   a ) and the rear surface  11   b  ( 12   b ). The first current path C 1  (C 4 ) is a path allowing an electric current to flow between the drain electrode D 1  (D 2 ) and the source electrode S 1  (S 2 ) of the horizontal switching element  11  ( 12 ). Moreover, the first current path C 1  (C 4 ) is disposed near the front surface  11   a  ( 12   a ) of the horizontal switching element  11  ( 12 ). The source electrode S 1  (S 2 ) is an exemplary “first electrode”, and the drain electrode D 1  (D 2 ) is an exemplary “second electrode”. 
     The horizontal switching element  11  ( 12 ) is formed of a semiconductor material including GaN (gallium nitride). The horizontal switching elements  11  and  12  constitute an inverter circuit. The horizontal switching elements  11  and  12  are disposed so that each of the front surfaces  11   a  and  12   a  faces the first substrate  20 . 
     Specifically, in the horizontal switching elements  11  ( 12 ), the drain electrode D 1  (D 2 ) is connected to the conductive pattern  242  ( 246 ) on the bottom surface  20   c  of the first substrate  20  as illustrated in  FIG. 5 . In the horizontal switching element  11  ( 12 ), the source electrode S 1  (S 2 ) is connected to the conductive pattern  243  ( 249 ) on the bottom surface  20   c  of the first substrate  20 . In the horizontal switching elements  11  ( 12 ), moreover, the gate electrode G 1  (G 2 ) is connected to the conductive pattern  245  ( 247 ) on the bottom surface  20   c  of the first substrate  20 . As illustrated in  FIG. 6 , in the horizontal switching element  11  ( 12 ), moreover, the body electrode B 1  (B 2 ) is connected to the conductive pattern  301  ( 303 ) on the top surface  30   a  of the second substrate  30 . 
     Specifically, in the horizontal switching element  11  ( 12 ), the gate electrode G 1  (G 2 ), the source electrode S 1  (S 2 ), and the drain electrode D 1  (D 2 ) provided on the upper side (in the Z1 direction) are bonded to the conductive patterns on the bottom surface  20   c  of the first substrate  20  on the upper side through the bonding layer including solder or the like. 
     In the horizontal switching element  11 , the body electrode B 1  provided on the lower side (in the Z2 direction) is bonded to the element-bonding pattern portion  301   a  of the conductive pattern  301  of the second substrate  30  on the lower side through the bonding layer including solder or the like. In other words, in the horizontal switching element  11 , the body electrode B 1  on the rear surface  11   b  is connected to have the same potential as the output terminal. In the horizontal switching element  12 , the body electrode B 2  provided on the lower side (in the Z2 direction) is bonded to the conductive pattern  303  of the second substrate  30  on the lower side through the bonding layer including solder or the like. In other words, in the horizontal switching element  12 , the body electrode B 2  on the rear surface  12   b  is connected to have the same potential as the input terminal N (V−). 
     As illustrated in  FIG. 10  and  FIG. 11 , the control switching element  13  ( 14 ) includes a vertical device including the gate electrode G 3  (G 4 ), the source electrode S 3  (S 4 ), and the drain electrode D 3  (D 4 ). Specifically, in the control switching element  13  ( 14 ), the gate electrode G 3  (G 4 ) and the source electrode S 3  (S 4 ) are disposed on the upper side (in the Z1 direction) and the drain electrode D 3  (D 4 ) is disposed on the lower side (in the Z2 direction). The control switching element  13  ( 14 ) is formed of the semiconductor material including silicon (Si). 
     Here, in this embodiment, the control switching element  13  ( 14 ) is configured to control the actuation of the horizontal switching element  11  ( 12 ). The control switching element  13  ( 14 ) is mounted on a surface (top surface  20   a ) of the first substrate  20 , opposite to the surface (bottom surface  20   c ) thereof connected to the horizontal switching element  11  ( 12 ). 
     Moreover, as illustrated in  FIG. 3 , the control switching element  13  ( 14 ) is disposed on the top surface  20   a  (surface in the Z1 direction) of the first substrate  20 . Specifically, in the control switching element  13  ( 14 ), the drain electrode D 3  (D 4 ) is connected to the conductive pattern  206  ( 212 ) on the top surface  20   a  of the first substrate  20  through the bonding layer including solder or the like. In the control switching element  13  ( 14 ), the each source electrode S 3  (S 4 ) is connected to the conductive patterns  205  and  207  ( 202  and  210 ) on the top surface  20   a  of the first substrate  20  through wires including metal such as aluminum or copper. In the control switching element  13  ( 14 ), moreover, the gate electrode G 3  (G 4 ) is connected to the conductive pattern  208  ( 211 ) on the top surface  20   a  of the first substrate  20  through wires including metal such as aluminum or copper. 
     As illustrated in  FIG. 3 , the control switching element  13  ( 14 ) is disposed at a position not overlapping with the horizontal switching element  11  ( 12 ) in plan view (viewed in a direction orthogonal to the plane of the first substrate  20  (from the Z direction)). In addition, the control switching element  13  ( 14 ) is disposed at a position on the side opposite to the snubber capacitors  15  and  16  relative to the horizontal switching element  11  ( 12 ) in plan view (viewed from the Z direction). In other words, the control switching element  13  ( 14 ) is disposed close to the outer periphery of the first substrate  20  relative to the horizontal switching element  11  ( 12 ) in plan view. 
     The snubber capacitors  15  and  16  are disposed in parallel to each other so that the respective capacitors are connected to the input terminals P (V+) and N (V−) as illustrated in  FIG. 1 . The snubber capacitor  15  ( 16 ) is electrically connected to the horizontal switching elements  11  and  12  and the control switching elements  13  and  14 . Specifically, as illustrated in  FIG. 3 , the snubber capacitor  15  is disposed to connect the conductive pattern  202  and the conductive pattern  209  on the top surface  20   a  of the first substrate  20 . The snubber capacitor  16  is disposed to connect the conductive pattern  202  and the conductive pattern  203  on the top surface  20   a  of the first substrate  20 . 
     In this embodiment, the snubber capacitors  15  and  16  are mounted on the surface (top surface  20   a ) of the first substrate  20 , opposite to the surface (bottom surface  20   c ) connected to the horizontal switching elements  11  and  12 . The snubber capacitors  15  and  16  are disposed at the position not overlapping with the horizontal switching elements  11  and  12  in plan view (viewed from the Z direction). 
     As illustrated in  FIG. 2 , the heat sink  40  is provided to dissipate the heat generated during the operation of the horizontal switching elements  11  and  12 . The heat sink  40  is disposed on the ground pattern  304  side (on the lower side (in the Z2 direction)) of the second substrate  30 . 
     The heat conductive material  50  (see  FIG. 2 ) is formed of, for example, epoxy resin with excellent heat conductivity. 
     Here, in this embodiment, as illustrated in  FIG. 3 , the snubber capacitors  15  and  16  are mounted on the top surface  20   a  of the first substrate  20 . As illustrated in  FIG. 5 , the bottom surface  20   c  of the first substrate  20  is connected to the drain electrode D 1  (D 2 ), the source electrode S 1  (S 2 ), and the gate electrode G 1  (G 2 ) on the front surface  11   a  ( 12   a ) side of the horizontal switching element  11  ( 12 ). 
     In this embodiment, the first substrate  20  includes a second current path C 3  (C 6 ) as illustrated in  FIG. 12 . The first current path C 1  (C 4 ) is a path allowing an electric current to flow between the drain electrode D 1  (D 2 ) and the source electrode S 1  (S 2 ) of the horizontal switching element  11  ( 12 ). In the second current path C 3  (C 6 ), an electric current flows in a direction approximately opposite to that of the first current path C 1  (C 4 ). The second current path C 3  (C 6 ) is disposed at a position opposite to the first current path C 1  (C 4 ). Specifically, the first substrate  20  includes a third current path C 2  (C 5 ), the second current path C 3 , and the second current path C 6 . The third current path C 2  (C 5 ) is a path allowing an electric current to flow between the source electrode S 1  (S 2 ) of the horizontal switching element  11  ( 12 ) and the drain electrode D 3  (D 4 ) of the control switching element  13  ( 14 ). The second current path C 3  is a path allowing an electric current to flow between the source electrode S 3  of the control switching element  13  and the drain electrode D 2  of the horizontal switching element  12 . The second current path C 6  is a path allowing an electric current to flow between the source electrode S 4  of the control switching element  14  and the input terminal N (V−). 
     The third current path C 2  includes the conductive pattern  243  on the bottom surface  20   c  of the first substrate  20 , the penetration electrode  227   a , the conductive pattern  227  on the intermediate layer  20   b , the penetration electrode  206   a , and the conductive pattern  206  on the top surface  20   a . Moreover, the second current path C 3  includes the conductive pattern  205  on the top surface  20   a  of the first substrate  20 , the penetration electrode  205   c , the conductive pattern  228  on the intermediate layer  20   b , the penetration electrode  228   a , and the conductive pattern  246  on the bottom surface  20   c.    
     The third current path C 5  includes the conductive pattern  249  on the bottom surface  20   c  of the first substrate  20 , the penetration electrode  229   a , the conductive pattern  229  on the intermediate layer  20   b , the penetration electrode  212   a , and the conductive pattern  212  on the top surface  20   a . The second current path C 6  includes the conductive pattern  202  on the top surface  20   a  of the first substrate  20 . 
     The first current path C 1  (C 4 ) of the horizontal switching element  11  ( 12 ) and the second current path C 3  (C 6 ) of the first substrate  20  are disposed close to each other so that the changes of magnetic flux that occur due to the flow of an electric current in these current paths C 1  and C 3  (C 4  and C 6 ) can be mutually offset. Specifically, the first current path C 1  (C 4 ) and the second current path C 3  (C 6 ) are disposed to face each other vertically (in the Z direction). 
     Moreover, the second current path C 3  (C 6 ) of the first substrate  20  (including wire W 1  (W 2 ) as a fourth current path) and the third current path C 2  (C 5 ) are disposed close to each other so that the changes of magnetic flux that occur due to the flow of an electric current in these current paths C 3  and C 2  (C 6  and C 5 ) can be mutually offset. Specifically, the second current path C 3  (C 6 ) and the third current path C 2  (C 5 ) are disposed to face each other vertically (in the Z direction). 
     In this embodiment, as illustrated in  FIG. 2 , the second substrate  30  is disposed on the side opposite to the first substrate  20  (in the Z2 direction) relative to the horizontal switching elements  11  and  12 . In other words, the horizontal switching elements  11  and  12  are disposed and held between the first substrate  20  and the second substrate  30 . As illustrated in  FIG. 6 , the conductive pattern  301  on the top surface  30   a  of the second substrate  30  is formed to have the area smaller than a half of the area of the ground pattern  304  on the bottom surface  30   b.    
     The connection pattern portion  301   b  of the conductive pattern  301  is formed to have an area smaller than the element-bonding pattern portion  301   a  bonded to the rear surface  11   b  of the horizontal switching element  11 . In other words, the conductive pattern  301  has the area obtained by adding the area of the element-bonding pattern portion  301   a  bonded to the horizontal switching element  11  and the minimum area of the connection pattern portion  301   b  required configured to connect the element-bonding pattern portion  301   a  to the first substrate  20 . The conductive pattern  301  is formed to have the area smaller than or equal to the area twice as large as the horizontal switching element  11  in plan view (viewed from the Z direction). 
     In this embodiment, the effects as below can be obtained. 
     In this embodiment, the snubber capacitors  15  and  16  are mounted on the first substrate  20 . The first substrate  20  is connected to the drain electrode D 1  (D 2 ) and the source electrode S 1  (S 2 ) on the front surface  11   a  ( 12   a ) side of the horizontal switching element  11  ( 12 ). Thus, the electric current flows in the snubber circuit including the snubber capacitors  15  and  16  through the first substrate  20  without having the second substrate  30  interposed between. Therefore, the electric current path of the snubber circuit including the snubber capacitors  15  and  16  can be shortened as compared to the case in which an electric current flows through the first substrate  20  and the second substrate  30  on the rear surface  11   b  ( 12   b ) side of the horizontal switching element  11  ( 12 ). This can reduce the wiring inductance of the snubber circuit including the snubber capacitors  15  and  16 . The first substrate  20  is configured to include the second current path C 3  (C 6 ). The first current path C 1  (C 4 ) is a path allowing an electric current to flow between the drain electrode D 1  (D 2 ) and the source electrode S 1  (S 2 ) of the horizontal switching element  11  ( 12 ). In the second current path C 3  (C 6 ), the electric current flows in a direction approximately opposite to that of the first current path C 1  (C 4 ). The second current path C 3  (C 6 ) is disposed at a position opposite to the first current path C 1  (C 4 ). Thus, the change of the magnetic flux caused in the first current path C 1  (C 4 ) can be offset by the change of the magnetic flux caused in the second current path C 3  (C 6 ). This can reduce the wiring inductance of the snubber circuit including the snubber capacitors  15  and  16 . 
     In the above embodiment, the control switching element  13  ( 14 ) is mounted on the surface (top surface  20   a ) of the first substrate  20 , opposite to the surface thereof connected to the horizontal switching element  11  ( 12 ). This can suppress the conduction of heat generated in the horizontal switching element  11  ( 12 ) to the control switching element  13  ( 14 ). As a result, the deterioration in electrical characteristic of the control switching element  13  ( 14 ) due to the heat can be suppressed. 
     Moreover, in this embodiment, the control switching element  13  ( 14 ) is disposed at a position not overlapping with the horizontal switching element  11  ( 12 ) in plan view (viewed from the Z direction). Thus, the conduction of heat generated from the horizontal switching element  11  ( 12 ) disposed on the bottom surface  20   c  of the first substrate  20  to the control switching elements  13  ( 14 ) disposed on the top surface  20   a  of the first substrate  20  can be effectively suppressed. 
     In this embodiment, the control switching element  13  ( 14 ) is disposed on the side opposite to the snubber capacitor  15  ( 16 ) relative to the horizontal switching element  11  ( 12 ) in plan view (viewed from the Z direction). Thus, in plan view, the control switching element  13  ( 14 ) can be disposed outside the two horizontal switching elements  11  and  12 . For this reason, in plan view, as compared to the case in which the control switching element  13  ( 14 ) is held between the two horizontal switching elements  11  and  12 , the heat generated from the horizontal switching element  11  ( 12 ) can be less easily conducted to the control switching element  13  ( 14 ). 
     In this embodiment, the snubber capacitors  15  and  16  are mounted on the surface (top surface  20   a ) of the first substrate  20 , opposite to the side connected to the horizontal switching element  11  ( 12 ). This can suppress the conduction of heat generated from the horizontal switching element  11  ( 12 ) to the snubber capacitors  15  and  16 . 
     In this embodiment, the snubber capacitors  15  and  16  are disposed at a position not overlapping with the horizontal switching element  11  ( 12 ) in plan view (viewed from the Z direction). Thus, the conduction of heat generated from the horizontal switching element  11  ( 12 ) disposed on the bottom surface  20   c  of the first substrate  20  to the snubber capacitors  15  and  16  disposed on the top surface  20   a  of the first substrate  20  can be effectively suppressed. 
     In this embodiment, the second substrate  30  is provided with the ground pattern  304  and the conductive pattern (potential adjustment pattern)  301  connected to the rear surface  11   b  of the horizontal switching element  11 . The ground pattern  304  is provided on the surface (bottom surface  30   b ) of the second substrate  30 , opposite to the side bonded to the horizontal switching element  11 . Moreover, the conductive pattern  301  is formed to have the area smaller than a half of the area of the ground pattern  304 . This can reduce the stray capacitance (parasitic capacitance) between the ground pattern  304  and the conductive pattern  301 . As a result, the occurrence of leakage of an electric current during the high-frequency operation of the horizontal switching element  11  can be suppressed. 
     In this embodiment, the conductive pattern  301  includes the element-bonding pattern portion  301   a  and the connection pattern portion  301   b . The element-bonding pattern portion  301   a  is bonded to the rear surface  11   b  of the horizontal switching element  11 . The connection pattern portion  301   b  connects the element-bonding pattern portion  301   a  to the first substrate  20 . Furthermore, the connection pattern portion  301   b  is formed to have the area smaller than the element-bonding pattern portion  301   a . This can minimize the area of the conductive pattern  301 . As a result, the stray capacitance (parasitic capacitance) between the ground pattern  304  and the conductive pattern  301  can be reduced easily. 
     In this embodiment, the conductive pattern  301  is formed to have the area smaller than or equal to twice of the area of the horizontal switching element  11  in plan view (viewed from the Z direction). This can suppress the excessive increase of the area of the conductive pattern  301 . As a result, the stray capacitance (parasitic capacitance) between the ground pattern  304  and the conductive pattern  301  can be easily reduced. 
     In this embodiment, as described above, the inverter apparatus  100  has the heat sink  40  configured to dissipate the heat generated by the horizontal switching element  11  ( 12 ) during the operation. This heat sink  40  is disposed on the ground pattern  304  side of the second substrate  30 . Thus, the heat generated from the horizontal switching element  11  ( 12 ) can be dissipated toward the side opposite to the control switching element  13  ( 14 ). As a result, the conduction of the heat to the control switching element  13  ( 14 ) can be easily suppressed. 
     In this embodiment, the space around the horizontal switching element  11  ( 12 ) between the first substrate  20  and the second substrate  30  is filled with the heat conductive material  50 . This makes it possible to conduct the heat generated from the horizontal switching element  11  ( 12 ) to the second substrate  30  disposed on the side of the first substrate  20 , opposite to the side thereof provided with the control switching element  13  ( 14 ) through the heat conductive material  50 . Thus, the conduction of the heat to the control switching element  13  ( 14 ) can be easily suppressed. 
     In this embodiment, the control switching element  13  ( 14 ) is cascode-connected to the horizontal switching element  11  ( 12 ). Thus, by performing the switching operation based on the control signal input to the gate electrode G 3  (G 4 ) of the control switching element  13  ( 14 ), the switching operation of the horizontal switching element  11  ( 12 ) can be easily controlled. 
     In this embodiment, furthermore, the control switching element  13  ( 14 ) includes the vertical device. This can reduce the wiring inductance between the snubber capacitors  15  and  16 , and the horizontal switching element  11  ( 12 ) in the inverter apparatus  100  including the control switching element  13  ( 14 ) including the vertical device. 
     In this embodiment, the horizontal switching element  11  and the horizontal switching element  12  constituting the inverter circuit are disposed so that each of the front surfaces  11   a  and  12   a  thereof faces the first substrate  20 . Thus, an electric current flows from the snubber capacitors  15  and  16  to the front surfaces  11   a  and  12   a  of the horizontal switching elements through the first substrate  20 . Thus, the current path between the snubber capacitors  15  and  16  and the horizontal switching elements  11  and  12  can be shortened. As a result, the wiring inductance between the snubber capacitors  15  and  16  and the horizontal switching elements  11  and  12  can be reduced. 
     The embodiment disclosed herein should be considered as an example in every perspective and as not being restrictive. The range of the present disclosure is shown not by the description of the above embodiment but by the scope of claims and includes all the modifications within the meaning and the range equivalent to the scope of claims. 
     For example, the above embodiment has described the single-phase inverter apparatus as an example of the power conversion apparatus. The power conversion apparatus, however, may be other inverter apparatus (power conversion apparatus) than the single-phase inverter apparatus. For example, the power conversion apparatus may be a three-phase inverter apparatus. 
     Moreover, the above embodiment has described the normally-on type horizontal switching element as an example of the horizontal switching element. The horizontal switching element, however, may be a normally-off type horizontal switching element. 
     Further, the above embodiment has described the semiconductor material containing GaN (gallium nitride) as an example of the material of the horizontal switching element. The horizontal switching element, however, may be formed of a semiconductor material belonging to Group III-V other than GaN, or a semiconductor material belonging to Group IV such as C (diamond). Alternatively, the horizontal switching element may be formed of other semiconductor materials. 
     In this embodiment, the snubber capacitor and the horizontal switching element are disposed on opposite sides of the first substrate. However, the snubber capacitor and the horizontal switching element may be disposed on the same side of the first substrate. 
     In this embodiment, the control switching element and the horizontal switching element are disposed on opposite sides of the first substrate. However, the control switching element and the horizontal switching element may be disposed on the same side of the first substrate. 
     The above embodiment has described the example in which the actuation of the horizontal switching element is controlled by the control switching element. However, the actuation of the horizontal switching element may be controlled without the use of the control switching element. 
     The above embodiment has described the example in which the control switching element includes the vertical device. However, the control switching element may not include the vertical device. 
     The above embodiment has described the example in which the power conversion apparatus includes two snubber capacitors. However, the number of snubber capacitors included in the power conversion apparatus may be one or three or more. 
     The above embodiment has described the example in which the power conversion apparatus includes two horizontal switching elements and two control switching elements. However, the number of horizontal switching elements and control switching elements included in the power conversion apparatus may be one or three or more. 
     The power conversion apparatus of the present disclosure may be any of the following first to fourteenth power conversion apparatuses. 
     A first power conversion apparatus includes: a horizontal switching element having a front surface and a rear surface, having a first electrode and a second electrode on the front surface, and having a first current path between the first electrode and the second electrode; a snubber capacitor electrically connected to the horizontal switching element; and a first substrate on which the snubber capacitor is mounted and the first substrate is connected to the first electrode and the second electrode on the front surface of the horizontal switching element, wherein the first substrate includes a second current path through which an electric current allows in a direction approximately opposite to the first current path through which an electric current flows between the first electrode and the second electrode of the horizontal switching element, and the second current path is disposed at a position opposite to the first current path. 
     A second power conversion apparatus is the first power conversion apparatus further including a control switching element configured to control actuation of the horizontal switching element, wherein the control switching element is mounted on a surface of the first substrate, opposite to a surface thereof to which the horizontal switching element is connected. 
     A third power conversion apparatus is the second power conversion apparatus wherein the control switching element is disposed at a position not overlapping with the horizontal switching element in plan view. 
     A fourth power conversion apparatus is the second or third power conversion apparatus wherein the control switching element is disposed on a side opposite to the snubber capacitor relative to the horizontal switching element in plan view. 
     A fifth power conversion apparatus is any of the first to fourth power conversion apparatuses wherein the snubber capacitor is mounted on a surface of the first substrate, opposite to a surface thereof to which the horizontal switching element is connected. 
     A sixth power conversion apparatus is any of the first to fifth power conversion apparatuses wherein the snubber capacitor is disposed at a position not overlapping with the horizontal switching element in plan view. 
     A seventh power conversion apparatus is any of the first to sixth power conversion apparatuses further including a second substrate disposed on a side opposite to the first substrate relative to the horizontal switching element, wherein: the second substrate includes a potential adjustment pattern connected to the rear surface of the horizontal switching element, and a ground pattern provided for a surface of the second substrate, opposite to a surface thereof to which the horizontal switching element is bonded; and the potential adjustment pattern is formed to have an area smaller than a half of an area of the ground pattern. 
     An eighth power conversion apparatus is the seventh power conversion apparatus wherein: the potential adjustment pattern includes an element-bonding pattern portion to which the rear surface of the horizontal switching element is bonded and a connection pattern portion configured to connect the element-bonding pattern portion to the first substrate; and the connection pattern portion is formed to have an area smaller than the element-bonding pattern portion. 
     A ninth power conversion apparatus is the seventh or eighth power conversion apparatus wherein the potential adjustment pattern is formed to have an area smaller than or equal to the area twice as large as the horizontal switching element in plan view. 
     A tenth power conversion apparatus is any of the seventh to ninth power conversion apparatuses further including a heat sink configured to dissipate heat generated from the horizontal switching element, wherein the heat sink is disposed on the ground pattern side of the second substrate. 
     An eleventh power conversion apparatus is any of the seventh to tenth power conversion apparatuses wherein a space around the horizontal switching element between the first substrate and the second substrate is filled with a heat conductive material. 
     A twelfth power conversion apparatus is any of the first to eleventh power conversion apparatuses wherein the control switching element is cascode-connected to the horizontal switching element. 
     A thirteenth power conversion apparatus is any of the first to twelfth power conversion apparatuses wherein the control switching element includes a vertical device. 
     A fourteenth power conversion apparatus is any of the first to thirteenth power conversion apparatuses wherein: the horizontal switching element includes a first horizontal switching element and a second horizontal switching element constituting an inverter circuit; and the first horizontal switching element and the second horizontal switching element are disposed so that each of the front surfaces faces the first substrate. 
     The foregoing detailed description has been presented for the purposes of illustration and description. Many modifications and variations are possible in light of the above teaching. It is not intended to be exhaustive or to limit the subject matter described herein to the precise form disclosed. Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims appended hereto.