Patent Publication Number: US-2021185817-A1

Title: Circuit device and power conversion apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a circuit device and a power conversion apparatus. 
     BACKGROUND ART 
     Japanese Patent Laying-Open No. 2017-41998 (PTL 1) discloses a power conversion apparatus including a transformer. The transformer includes a core, a primary coil pattern, and a secondary coil pattern. A first substrate with the primary coil pattern and a second substrate with the secondary coil pattern are stacked on each other. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Laying-Open No. 2017-41998 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the transformer and the power conversion apparatus disclosed in PTL 1, the first substrate with the primary coil pattern and the first substrate with the secondary coil pattern are stacked on each other while being spaced away from each other. As a current is flowed through the primary coil pattern and the secondary coil pattern to operate the transformer and the power conversion apparatus, thus, it is difficult to dissipate heat generated in the primary coil pattern and the secondary coil pattern to outside of the transformer and the power conversion apparatus. The temperature of the transformer and the temperature of the power conversion apparatus rise, leading to an increased power loss in the transformer and the power conversion apparatus. The present invention has been made in view of the above problem. An object of the present invention is to provide a circuit device and a power conversion apparatus that can prevent or reduce a temperature rise and a power loss during operation. 
     Solution to Problem 
     A circuit device of the present invention includes a core, a first circuit board, a second circuit board, a heat dissipation member, a first heat transfer member, and a second heat transfer member. The first circuit board includes a first substrate and a first coil pattern. The first coil pattern surrounds at least a part of the core. The second circuit board includes a second substrate and a second coil pattern. The second coil pattern surrounds at least a part of the core. The heat dissipation member supports the core, the first circuit board, and the second circuit board. The first heat transfer member is disposed between the first circuit board and the heat dissipation member and is in surface contact with the first circuit board and the heat dissipation member. The second heat transfer member is disposed between the first circuit board and the second circuit board and is in surface contact with the first circuit board and the second circuit board. 
     A power conversion apparatus of the present invention includes a circuit device of the present invention and an inverter circuit that controls a current flowing through the first coil pattern. 
     Advantageous Effects of Invention 
     As a current is flowed through the first coil pattern and the second coil pattern to operate the circuit device and the power conversion apparatus, the first coil pattern and the second coil pattern generate heat. Heat generated in the first coil pattern is transferred through the first heat transfer member to the heat dissipation member with a relatively low thermal resistance. Heat generated in the second coil pattern is transferred through the second heat transfer member, the first circuit board, and the first heat transfer member to the heat dissipation member with a relatively low thermal resistance. A temperature rise and a power loss during operation of the circuit device and the power conversion apparatus can thus be prevented or reduced in the circuit device and the power conversion apparatus. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram of a power conversion apparatus according to Embodiment 1. 
         FIG. 2  is a schematic perspective view of a circuit device according to Embodiment 1. 
         FIG. 3  is an exploded schematic perspective view of the circuit device according to Embodiment 1. 
         FIG. 4  is a schematic plan view of the circuit device according to Embodiment 1. 
         FIG. 5  is a schematic sectional view of the circuit device according to Embodiment 1, which is taken along the sectional line V-V shown in  FIG. 4 . 
         FIG. 6  is a schematic plan view of a circuit device according to Embodiment 2. 
         FIG. 7  is a schematic sectional view of the circuit device according to Embodiment 2, which is taken along the sectional line VII-VII shown in  FIG. 6 . 
         FIG. 8  is a schematic plan view of a first circuit board (with a first electronic component omitted) included in a circuit device according to Embodiment 3. 
         FIG. 9  is a schematic bottom view of the first circuit board included in the circuit device according to Embodiment 3. 
         FIG. 10  is a schematic plan view of a second circuit board (with a second electronic component omitted) included in the circuit device according to Embodiment 3. 
         FIG. 11  is a schematic bottom view of the second circuit board included in the circuit device according to Embodiment 3. 
         FIG. 12  is a schematic sectional view of a circuit device according to Embodiment 4. 
         FIG. 13  is a schematic plan view of a first circuit board included in the circuit device according to Embodiment 4. 
         FIG. 14  is a schematic plan view of a second circuit board included in the circuit device according to Embodiment 4. 
         FIG. 15  is a schematic sectional view of a circuit device according to Embodiment 5. 
         FIG. 16  is a schematic sectional view of a circuit device according to Embodiment 6. 
         FIG. 17  is a schematic plan view of a first circuit board included in the circuit device according to Embodiment 6. 
         FIG. 18  is a schematic plan view of a second circuit board included in the circuit device according to Embodiment 6. 
         FIG. 19  is a schematic sectional view of a circuit device according to Embodiment 7. 
         FIG. 20  is a schematic plan view of a first circuit board included in the circuit device according to Embodiment 7. 
         FIG. 21  is a schematic plan view of a second circuit board included in the circuit device according to Embodiment 7. 
         FIG. 22  is a schematic sectional view of a circuit device according to Embodiment 8. 
         FIG. 23  is a schematic plan view of a circuit device according to Embodiment 9. 
         FIG. 24  is a schematic sectional view of the circuit device according to Embodiment 9, which is taken along the sectional line XXIV-XXIV shown in  FIG. 23 . 
         FIG. 25  is a schematic plan view of a circuit device according to Embodiment 10. 
         FIG. 26  is a schematic sectional view of the circuit device according to Embodiment 10, which is taken along the sectional line XXVI-XXVI shown in  FIG. 25 . 
         FIG. 27  is a schematic plan view of a first circuit board included in a circuit device according to Embodiment 11. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will now be described. Like components are designated by like reference numerals, description of which will not be repeated. 
     Embodiment 1 
     An example circuit configuration of a power conversion apparatus  1  of the present embodiment will be described with reference to  FIG. 1 . Power conversion apparatus  1  of the present embodiment is, for example, a DC-DC converter. Power conversion apparatus  1  includes an inverter circuit  2 , a transformer circuit  3 , a rectifier circuit  4 , a smoothing circuit  5  including a coil device  100 , and a control circuit  6 . Power conversion apparatus  1  converts a direct-current (DC) voltage V i  supplied to an input terminal  110  to a DC voltage V o  and outputs DC voltage V o  from an output terminal  111 . 
     Inverter circuit  2  includes switching elements  7   a ,  7   b ,  7   c ,  7   d . Each of switching elements  7   a ,  7   b ,  7   c ,  7   d  is, for example, a metal-oxide semiconductor field-effect transistor (MOSFET), an insulated gate bipolar transistor (IGBT), or the like. Each of switching elements  7   a ,  7   b ,  7   c ,  7   d  is formed of a semiconductor material such as silicon (Si), silicon carbide (SiC), or gallium nitride (GaN). 
     Transformer circuit  3  includes a transformer  101 . Transformer  101  includes a primary-side coil conductor  120 , a core  10  (see  FIGS. 2 to 5 ), and a secondary-side coil conductor  121 . For example, primary-side coil conductor  120  is a high-voltage-side coil conductor, and secondary-side coil conductor  121  is a low-voltage-side coil conductor. Primary-side coil conductor  120  is connected to inverter circuit  2 . Secondary-side coil conductor  121  is connected to rectifier circuit  4 . Secondary-side coil conductor  121  is magnetically coupled to primary-side coil conductor  120  with core  10  therebetween. 
     Rectifier circuit  4  includes diodes  8   a ,  8   b ,  8   c ,  8   d . Each of diodes  8   a ,  8   b ,  8   c ,  8   d  is formed of a semiconductor material such as Si, SiC, or GaN. Smoothing circuit  5  includes coil device  100  serving as a smoothing coil and a capacitor  9   a.    
     Power conversion apparatus  1  includes, at the stage preceding inverter circuit  2 , a coil device  102  serving as a smoothing coil and a capacitor  9   b . Power conversion apparatus  1  includes a coil device  103  serving as a resonance coil between inverter circuit  2  and transformer circuit  3 . 
     Power conversion apparatus  1  receives, for example, a DC voltage V i  of not less than 100 V and not greater than 600 V. Power conversion apparatus  1  outputs, for example, a DC voltage V o  of not less than 12 V and not greater than 16 V. Specifically, DC voltage V i  supplied to input terminal  110  is converted to a first alternating-current (AC) voltage by inverter circuit  2 . The first AC voltage is converted to a second AC voltage lower than the first AC voltage by transformer circuit  3 . The second AC voltage is rectified by rectifier circuit  4 . Smoothing circuit  5  smoothes a voltage output from rectifier circuit  4 . Power conversion apparatus  1  outputs, from output terminal  111 , DC voltage V o  output from smoothing circuit  5 . 
     Input terminal  110 , output terminal  111 , and at least one of switching elements  7   a ,  7   b ,  7   c ,  7   d , diodes  8   a ,  8   b ,  8   c ,  8   d , and capacitors  9   a ,  9   b  are mounted on, for example, a circuit board (first circuit board  15 , second circuit board  16  (see  FIGS. 2 to 5 )). The circuit board is attached to a heat dissipation member  60  (see  FIGS. 2 to 5 ). Heat dissipation member  60  is, for example, a housing of power conversion apparatus  1 . Any other electronic component may be mounted on the circuit board. Input terminal  110 , output terminal  111 , and an electronic component including at least one of switching elements  7   a ,  7   b ,  7   c ,  7   d , diodes  8   a ,  8   b ,  8   c ,  8   d , and capacitors  9   a ,  9   b  may be mounted in the housing of power conversion apparatus  1 . 
     A circuit device  105  of the present embodiment will be described with reference to  FIGS. 2 to 5 . Power conversion apparatus  1  includes circuit device  105 . Note that power conversion apparatus  1  may include any of circuit devices  105   b  to  105   i  of Embodiments 2 to 11 in place of circuit device  105  of Embodiment 1. Circuit device  105  is, for example, transformer  101  included in power conversion apparatus  1 . Circuit device  105  may be any of coil devices  100 ,  102 ,  103 . Circuit device  105  includes core  10 , first circuit board  15 , second circuit board  16 , heat dissipation member  60 , a first heat transfer member  50 , and a second heat transfer member  51 . 
     Core  10  includes a magnetic material. Core  10  is, for example, a ferrite core of manganese-zinc (Mn—Zn)-based ferrite or nickel-zinc (Ni—Zn)-based ferrite, an amorphous core, or an iron dust core. Core  10  includes, for example, a first core portion  10   a  and a second core portion  10   b . For example, core  10  is an EI-type core with first core portion  10   a  having an I-shape and second core portion  10   b  having an E-shape. Second core portion  10   b  has a first leg  11   a , a second leg  11   b , and a third leg  11   c . Second leg  11   b  is located between first leg  11   a  and third leg  11   c . First core portion  10   a  is disposed in a recess  60   b  of heat dissipation member  60 . Second core portion  10   b  is stacked on first core portion  10   a . The shape of core  10  is not particularly limited, and core  10  may be an EE-type core, a U-type core, a UU-type core, an EER-type core, or an ER-type core. 
     First circuit board  15  includes a first substrate  30  and a first coil pattern  20 . First circuit board  15  is, for example, a printed circuit board. First substrate  30  includes a first main surface  30   a  facing a surface  60   a  of heat dissipation member  60  and a second main surface  30   b  opposite to first main surface  30   a . First substrate  30 , which is formed of an electrically insulating material, is an insulated substrate. First substrate  30  is formed of, for example, glass-reinforced epoxy resin, phenolic resin, polyphenylene sulfide (PPS), polyetheretherketone (PEEK), or the like. First substrate  30  may be formed of a ceramic material such as aluminum oxide (Al 2 O 3 ) or aluminum nitride (AlN). First substrate  30  may be provided with a first through hole  30   h  extending from first main surface  30   a  to second main surface  30   b . Second leg  11   b  of core  10  is inserted into first through hole  30   h . Second leg  11   b  of core  10  passes through first circuit board  15  (first substrate  30 ). 
     First coil pattern  20  corresponds to primary-side coil conductor  120  (see  FIG. 1 ). First coil pattern  20  is provided on first main surface  30   a , on second main surface  30   b , or in first substrate  30 . First circuit board  15  is a single-sided wiring board with first coil pattern  20  provided on first main surface  30   a  or on second main surface  30   b . First coil pattern  20  is made of a material having a higher electrical resistivity and a lower thermal conductivity than those of first substrate  30 . First coil pattern  20  is formed of a conductive material such as copper (Cu), silver (Ag), gold (Au), tin (Sn), copper (Cu) alloy, nickel (Ni) alloy, gold (Au) alloy, silver (Ag) alloy, or tin (Sn) alloy. First coil pattern  20  is, for example, a thin conductor layer having a thickness of not less than 1 μm and not greater than 5000 μm. 
     First coil pattern  20  surrounds at least a part of core  10 . First coil pattern  20  surrounds, for example, at least one of first leg  11   a , second leg  11   b , and third leg  11   c . Particularly, first coil pattern  20  surrounds second leg  11   b  of core  10  through a space between first leg  11   a  and second leg  11   b  and a space between second leg  11   b  and third leg  11   c . First coil pattern  20  surrounding at least a part of core  10  means that first coil pattern  20  is wound a half of a turn around at least a part of core  10 . Herein, a coil pattern having the number of turns, which is one, means that the coil pattern passes through all the spaces around second leg  11   b , which are surrounded by first core portion  10   a  and second core portion  10   b , in a single winding operation. A part of first coil pattern  20  may be located between first core portion  10   a  and second core portion  10   b.    
     As shown in  FIGS. 2 to 4 , first electronic components  40  forming inverter circuit  2  (see  FIG. 1 ) are mounted on at least one of first circuit board  15  and second circuit board  16 . Particularly, first electronic components  40  may be mounted on second main surface  30   b  of first circuit board  15 . First electronic components  40  are, for example, switching elements  7   a ,  7   b ,  7   c ,  7   d  (see  FIG. 1 ). First electronic components  40  are electrically connected to first coil pattern  20 . 
     Second circuit board  16  includes a second substrate  31  and a second coil pattern  21 . Second circuit board  16  is, for example, a printed circuit board. Second substrate  31  includes a third main surface  31   a  facing first circuit board  15  and a fourth main surface  31   b  opposite to third main surface  31   a . Second substrate  31 , which is formed of an electrically insulating material, is an insulated substrate. Second substrate  31  is formed of, for example, glass-reinforced epoxy, phenolic resin, polyphenylene sulfide (PPS), polyetheretherketone (PEEK), or the like. Second substrate  31  may be formed of a ceramic material such as aluminum oxide (Al 2 O 3 ) or aluminum nitride (AlN). Second substrate  31  may be provided with a second through hole  31   h  extending from third main surface  31   a  to fourth main surface  31   b . Second leg  11   b  of core  10  is inserted into second through hole  31   h . Second leg  11   b  of core  10  passes through second circuit board  16  (second substrate  31 ). 
     Second coil pattern  21  corresponds to secondary-side coil conductor  121  (see  FIG. 1 ). Second coil pattern  21  is provided on third main surface  31   a , on fourth main surface  31   b , or in second substrate  31 . Second circuit board  16  is a single-sided wiring board with second coil pattern  21  provided on third main surface  31   a  or on fourth main surface  31   b . Second coil pattern  21  is provided on a substrate different from that of first coil pattern  20 . Second coil pattern  21  can thus be designed easily and independently of first coil pattern  20  in terms of shape, thickness, the number of turns, and the like. Second coil pattern  21  is made of a material having a lower electrical resistivity and a higher thermal conductivity than those of second substrate  31 . Second coil pattern  21  is formed of a conductive material such as copper (Cu), silver (Ag), gold (Au), tin (Sn), copper (Cu) alloy, nickel (Ni) alloy, gold (Au) alloy, silver (Ag) alloy, or tin (Sn) alloy. 
     Second coil pattern  21  is a thin conductor layer having a thickness of, for example, not less than 1 μm and not greater than 5000 μm. The thickness of second coil pattern  21  may be different from the thickness of first coil pattern  20 . For example, when power conversion apparatus  1  is a step-down DC/DC converter, a first current flowing through first coil pattern  20  corresponding to primary-side coil conductor  120  is smaller than a second current flowing through second coil pattern  21  corresponding to secondary-side coil conductor  121 . The thickness of first coil pattern  20  may thus be smaller than the thickness of second coil pattern  21 . 
     Second coil pattern  21  surrounds at least a part of core  10 . Second coil pattern  21  surrounds, for example, at least one of first leg  11   a , second leg  11   b , and third leg  11   c . Particularly, second coil pattern  21  surrounds second leg  11   b  of core  10  through, for example, the space between first leg  11   a  and second leg  11   b  and the space between second leg  11   b  and third leg  11   c . Second coil pattern  21  surrounding at least a part of core  10  means that second coil pattern  21  is wound a half or more of a turn around at least a part of core  10 . A part of second coil pattern  21  may be located between first core portion  10   a  and second core portion  10   b.    
     Regarding at least a part of core  10  (e.g., second leg  11   b  of core  10 ), second coil pattern  21  is wound in a direction different from that of first coil pattern  20 . Second coil pattern  21  is magnetically coupled to first coil pattern  20  with core  10  therebetween. Second circuit board  16  covers at least a part of first circuit board  15  and mechanically protects first circuit board  15 . 
     As shown in  FIGS. 2 to 4 , second electronic components  41 ,  42  forming rectifier circuit  4  (see  FIG. 1 ) are mounted on at least one of first circuit board  15  and second circuit board  16 . Particularly, second electronic components  41 ,  42  may be mounted on fourth main surface  31   b  of second circuit board  16 . Second electronic components  41 ,  42  are, for example, diodes  8   a ,  8   b ,  8   c ,  8   d  (see  FIG. 1 ). Second electronic components  41 ,  42  are electrically connected to second coil pattern  21 . 
     Heat dissipation member  60  supports core  10 , first circuit board  15 , and second circuit board  16 . Heat dissipation member  60  further supports first heat transfer member  50  and second heat transfer member  51 . Heat dissipation member  60  has surface  60   a  facing first circuit board  15 . Recess  60   b  is provided in surface  60   a  of heat dissipation member  60 . A part (first core portion  10   a ) of core  10  is housed in recess  60   b . Heat dissipation member  60  is in surface contact with core  10  (first core portion  10   a ). As a current is flowed through first coil pattern  20  and second coil pattern  21  to operate circuit device  105 , an energy loss due to a magnetic loss occurs in core  10 , thus causing core  10  to generate heat. The heat generated in core  10  can be transmitted to heat dissipation member  60  with a low thermal resistance. A temperature rise of core  10  and a power loss in core  10  during operation of circuit device  105  can be prevented or reduced. 
     Core  10 , first circuit board  15 , and second circuit board  16  may be fixed to heat dissipation member  60  with a fixing member  70  (see  FIG. 25 ) such as a screw, a machine screw, or a rivet. Core  10 , first circuit board  15 , and second circuit board  16  may be pressed toward heat dissipation member  60  with a spring (not shown) to be fixed to heat dissipation member  60 . 
     Heat dissipation member  60  is, for example, a part of the housing of power conversion apparatus  1  which houses core  10 , first circuit board  15 , and second circuit board  16 . Circuit device  105  (transformer  101 ) can thus be mounted in power conversion apparatus  1  by merely fixing core  10 , first circuit board  15 , second circuit board  16 , first heat transfer member  50 , and second heat transfer member  51  to heat dissipation member  60 . Since circuit device  105  does not need to be assembled before circuit device  105  is mounted in power conversion apparatus  1 , the manufacturing cost of power conversion apparatus  1  can be reduced. Further, since a housing of circuit device  105  per se is not necessary, power conversion apparatus  1  including circuit device  105  can be miniaturized. Heat dissipation member  60  has a thermal conductivity of not less than 0.1 W/(m·K). Heat dissipation member  60  may have a thermal conductivity of not less than 1.0 W/(m·K) or may have a thermal conductivity of not less than 10.0 W/(m·K). Heat dissipation member  60  may be electrically grounded. 
     Heat dissipation member  60  is formed of, for example, a metallic material such as copper (Cu), aluminum (Al), iron (Fe), ferric (Fe) alloy (e.g., SUS304), copper (Cu) alloy (e.g., phosphor bronze), or aluminum (Al) alloy (e.g., ADC12). Heat dissipation member  60  may be formed of a resin material containing a thermally conductive filler. The resin material used in heat dissipation member  60  is, for example, polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), polyetheretherketone (PEEK), or the like. Heat dissipation member  60  is formed of, for example, a nonmagnetic material. Heat dissipation member  60  is manufactured by a method such as cutting, die casting, forging, or molding using a die. 
     First heat transfer member  50  is disposed between first circuit board  15  and heat dissipation member  60  and is in surface contact with first circuit board  15  and heat dissipation member  60 . First circuit board  15 , first heat transfer member  50 , and heat dissipation member  60  are stacked on each other. First heat transfer member  50  thermally connects first circuit board  15  to heat dissipation member  60  with a relatively low thermal resistance. First heat transfer member  50  is a first heat transfer sheet. 
     First heat transfer member  50  may have electrical insulating properties. First heat transfer member  50  having electrical insulating properties may be in surface contact with first coil pattern  20 . First heat transfer member  50  may be in contact with core  10 . 
     First heat transfer member  50  may be formed of a resin material such as silicone, urethane, epoxy, acrylonitrile butadiene styrene (ABS), polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), phenol, or polyimide, a fibrous material such as glass fiber or aramid fiber, or a ceramic material such as aluminum oxide or aluminum nitride. First heat transfer member  50  may be a silicone rubber sheet or a urethane rubber sheet. First heat transfer member  50  may be formed of a silicone gel, a silicone grease, or a silicone adhesive. 
     First heat transfer member  50  has a thermal conductivity higher than that of each of first substrate  30  and second substrate  31 . First heat transfer member  50  has a thermal conductivity of not less than 0.1 W/(m·K). First heat transfer member  50  may have a thermal conductivity of not less than 1.0 W/(m·K) or may have a thermal conductivity of not less than 10.0 W/(m·K). First heat transfer member  50  may have elasticity. First heat transfer member  50  may be crushed by pressing first circuit board  15  toward heat dissipation member  60 . 
     Second heat transfer member  51  is disposed between first circuit board  15  and second circuit board  16  and is in surface contact with first circuit board  15  and second circuit board  16 . First circuit board  15 , second heat transfer member  51 , and second circuit board  16  are stacked on each other. Second heat transfer member  51  is a second heat transfer sheet. Second heat transfer member  51  thermally connects second circuit board  16  to first circuit board  15  with a relatively low thermal resistance. 
     Second heat transfer member  51  may have electrical insulating properties. Second heat transfer member  51  having electrical insulating properties may be in surface contact with at least one of first coil pattern  20  and second coil pattern  21 . Second heat transfer member  51  may be in contact with core  10 . 
     Second heat transfer member  51  may be formed of a resin material such as silicone, urethane, epoxy, acrylonitrile butadiene styrene (ABS), polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), phenol, or polyimide, a fibrous material such as glass fiber or aramid fiber, or a ceramic material such as aluminum oxide or aluminum nitride. Second heat transfer member  51  may be a silicone rubber sheet or a urethane rubber sheet. Second heat transfer member  51  may be formed of a silicone gel, a silicone grease, or a silicone adhesive. Second heat transfer member  51  may be formed of the same material as that of first heat transfer member  50  or may be formed of a material different from that of first heat transfer member  50 . 
     Second heat transfer member  51  has a thermal conductivity higher than that of each of first substrate  30  and second substrate  31 . Second heat transfer member  51  may have the same thermal conductivity as that of first heat transfer member  50  or may have a thermal conductivity different from that of first heat transfer member  50 . Second heat transfer member  51  has a thermal conductivity of not less than 0.1 W/(m·K). Second heat transfer member  51  may have a thermal conductivity of not less than 1.0 W/(m·K) or may have a thermal conductivity of not less than 10.0 W/(m·K). Second heat transfer member  51  may have elasticity. Second heat transfer member  51  may be crushed by pressing second circuit board  16  toward first circuit board  15 . 
     The effects of circuit device  105  and power conversion apparatus  1  of the present embodiment will be described. 
     Circuit device  105  of the present embodiment includes core  10 , first circuit board  15 , second circuit board  16 , heat dissipation member  60 , first heat transfer member  50 , and second heat transfer member  51 . First circuit board  15  includes first substrate  30  and first coil pattern  20 . First coil pattern  20  surrounds at least a part of core  10 . Second circuit board  16  includes second substrate  31  and second coil pattern  21 . Second coil pattern  21  surrounds at least a part of core  10 . Heat dissipation member  60  supports core  10 , first circuit board  15 , and second circuit board  16 . First heat transfer member  50  is disposed between first circuit board  15  and heat dissipation member  60  and is in surface contact with first circuit board  15  and heat dissipation member  60 . Second heat transfer member  51  is disposed between first circuit board  15  and second circuit board  16  and is in surface contact with first circuit board  15  and second circuit board  16 . 
     As a current is flowed through first coil pattern  20  and second coil pattern  21  to operate circuit device  105 , first coil pattern  20  and second coil pattern  21  generate heat. The heat generated in first coil pattern  20  is transferred through first heat transfer member  50  to heat dissipation member  60  with a relatively low thermal resistance. The heat generated in second coil pattern  21  is transferred through second heat transfer member  51 , first circuit board  15 , and first heat transfer member  50  to heat dissipation member  60  with a relatively low thermal resistance. A temperature rise and a power loss of circuit device  105  during operation of circuit device  105  can thus be prevented or reduced. 
     Heat generated in first electronic components  40  during operation of circuit device  105  is transferred through first circuit board  15  and first heat transfer member  50  to heat dissipation member  60  with a relatively low thermal resistance. Heat generated in second electronic components  41 ,  42  during operation of circuit device  105  is transferred through second circuit board  16 , second heat transfer member  51 , first circuit board  15 , and first heat transfer member  50  to heat dissipation member  60  with a relatively low thermal resistance. A temperature rise and a power loss of circuit device  105  during operation of circuit device  105  can thus be prevented or reduced. 
     Power conversion apparatus  1  of the present embodiment includes circuit device  105  and an inverter circuit that controls a current flowing through first coil pattern  20 . As a current is flowed through first coil pattern  20  and second coil pattern  21  to operate power conversion apparatus  1 , first coil pattern  20  and second coil pattern  21  generate heat. The heat generated in first coil pattern  20  is transferred through first heat transfer member  50  to heat dissipation member  60  with a relatively low thermal resistance. The heat generated in second coil pattern  21  is transferred through second heat transfer member  51 , first circuit board  15 , and first heat transfer member  50  to heat dissipation member  60  with a relatively low thermal resistance. A temperature rise and a power loss of power conversion apparatus  1  during operation of power conversion apparatus  1  can thus be prevented or reduced. 
     Embodiment 2 
     Circuit device  105   b  according to Embodiment 2 will be described with reference to  FIGS. 6 and 7 . Circuit device  105   b  of the present embodiment is similar in configuration to circuit device  105  of Embodiment 1 and is different mainly in the following respects. 
     In circuit device  105   b , first circuit board  15  includes a third coil pattern  22  surrounding at least a part of core  10  (e.g., second leg  11   b  of core  10 ). Third coil pattern  22  is spaced away from first coil pattern  20  in the thickness direction of first substrate  30 . Third coil pattern  22  is provided on first main surface  30   a , on second main surface  30   b , or in first substrate  30 . First circuit board  15  is, for example, a double-sided wiring board with first coil pattern  20  provided on second main surface  30   b  and third coil pattern  22  provided on first main surface  30   a . In a plan view of second main surface  30   b , third coil pattern  22  may have the same shape as that of first coil pattern  20  or may have a shape different from that of first coil pattern  20 . 
     Third coil pattern  22  is electrically connected to first coil pattern  20  with a first via electrode  27  therebetween. First via electrode  27  extends in the thickness direction of first substrate  30 . First via electrode  27  may extend from first main surface  30   a  to second main surface  30   b . First via electrode  27  may be formed by filling holes extending in the thickness direction of first substrate  30  with a conductive material (e.g., metallic material) or may be formed by depositing a conductive film (e.g., metallic film) on surfaces of the holes extending in the thickness direction of first substrate  30 . 
     In circuit device  105   b , second circuit board  16  includes a fourth coil pattern  23  surrounding at least a part of core  10 . Fourth coil pattern  23  is spaced away from second coil pattern  21  in the thickness direction of second substrate  31 . Fourth coil pattern  23  is provided on third main surface  31   a , on fourth main surface  31   b , or in second substrate  31 . Second circuit board  16  is, for example, a double-sided wiring board with second coil pattern  21  provided on fourth main surface  31   b  and fourth coil pattern  23  provided on third main surface  31   a . In a plan view of fourth main surface  31   b , fourth coil pattern  23  may have the same shape as that of second coil pattern  21  or may have a shape different from that of second coil pattern  21 . 
     Fourth coil pattern  23  is electrically connected to second coil pattern  21  with a second via electrode  28  therebetween. Second via electrode  28  extends in the thickness direction of second substrate  31 . Second via electrode  28  may extend from third main surface  31   a  to fourth main surface  31   b . Second via electrode  28  may be formed by filling holes extending in the thickness direction of second substrate  31  with a conductive material (e.g., metallic material) or may be formed by depositing a conductive film (e.g., metallic film) on surfaces of the holes extending in the thickness direction of second substrate  31 . 
     First heat transfer member  50  having electrical insulating properties may be in surface contact with third coil pattern  22  and heat dissipation member  60 . Second heat transfer member  51  having electrical insulating properties may be in surface contact with first coil pattern  20  and fourth coil pattern  23 . 
     It suffices that circuit device  105   b  includes at least one of third coil pattern  22  and fourth coil pattern  23 . First circuit board  15  may include three or more layers of coil patterns. Second circuit board  16  may include three or more layers of coil patterns. For example, first circuit board  15  may further include a coil pattern (not shown) inside first substrate  30 . Second circuit board  16  may further include a coil pattern (not shown) inside second substrate  31 . 
     Circuit device  105   b  of the present embodiment achieves the following effects similar to those of circuit device  105  of Embodiment 1. 
     In circuit device  105   b  of the present embodiment, first circuit board  15  includes third coil pattern  22  surrounding at least a part of core  10 . Third coil pattern  22  is spaced away from first coil pattern  20  in the thickness direction of first substrate  30  and is electrically connected to first coil pattern  20  with first via electrode  27  therebetween. 
     As a current is flowed through first coil pattern  20 , second coil pattern  21 , and third coil pattern  22  to operate circuit device  105   b , first coil pattern  20 , second coil pattern  21 , and third coil pattern  22  generate heat. The heat generated in first coil pattern  20  and third coil pattern  22  is transferred through first heat transfer member  50  to heat dissipation member  60  with a relatively low thermal resistance. The heat generated in second coil pattern  21  is transferred through second heat transfer member  51 , first circuit board  15 , and first heat transfer member  50  to heat dissipation member  60  with a relatively low thermal resistance. Since the heat generated in first coil pattern  20  is transferred to third coil pattern  22 , overheating of first coil pattern  20  can be prevented or reduced. A temperature rise and a power loss of circuit device  105   b  during operation of circuit device  105   b  can thus be prevented or reduced. 
     In circuit device  105   b  of the present embodiment, second circuit board  16  includes fourth coil pattern  23  surrounding at least a part of core  10 . Fourth coil pattern  23  is spaced away from second coil pattern  21  in the thickness direction of second substrate  31  and is electrically connected to second coil pattern  21  through second via electrode  28 . 
     As a current is flowed through first coil pattern  20 , second coil pattern  21 , and fourth coil pattern  23  to operate circuit device  105   b , first coil pattern  20 , second coil pattern  21 , and fourth coil pattern  23  generate heat. The heat generated in first coil pattern  20  is transferred through first heat transfer member  50  to heat dissipation member  60  with a relatively low thermal resistance. The heat generated in second coil pattern  21  and fourth coil pattern  23  is transferred through second heat transfer member  51 , first circuit board  15 , and first heat transfer member  50  to heat dissipation member  60  with a relatively low thermal resistance. Since the heat generated in second coil pattern  21  is transferred to fourth coil pattern  23 , overheating of second coil pattern  21  can be prevented or reduced. A temperature rise and a power loss in circuit device  105   b  during operation of circuit device  105   b  can thus be prevented or reduced. 
     Embodiment 3 
     A circuit device according to Embodiment 3 will be described with reference to  FIGS. 8 to 11 . The circuit device of the present embodiment is similar in configuration to circuit device  105   b  of Embodiment 2 and is different mainly in the following respects. 
     In the present embodiment, a first amount of heat generation in first circuit board  15  is larger than a second amount of heat generation in second circuit board  16 . Herein, the first amount of heat generation in first circuit board  15  means an amount of heat generated in all the coil patterns (e.g., first coil pattern  20  and third coil pattern  22 ) formed in first circuit board  15 . The second amount of heat generation in second circuit board  16  means an amount of heat generated in all the coil patterns (e.g., second coil pattern  21  and fourth coil pattern  23 ) formed in second circuit board  16 . First coil pattern  20  to fourth coil pattern  23  are designed such that the first amount of heat generation in first circuit board  15  is larger than the second amount of heat generation in second circuit board  16 . Compared with second circuit board  16 , first circuit board  15  has a large amount of heat generation and is disposed close to heat dissipation member  60 . A temperature rise of the circuit device of the present embodiment can thus be reduced. 
     The turn ratio between primary-side coil conductor  120  and secondary-side coil conductor  121  is determined depending on the specifications of the circuit device and the power conversion apparatus. A ratio between a voltage applied to primary-side coil conductor  120  and a voltage applied to secondary-side coil conductor  121  and a ratio between a current flowing through primary-side coil conductor  120  and a current flowing through secondary-side coil conductor  121  are determined in accordance with the turn ratio. The shape and the number of layers of coil patterns formed in each of first circuit board  15  and second circuit board  16  are determined in accordance with the turn ratio. In other words, the lengths of the coil patterns formed in first circuit board  15  and second circuit board  16  are determined in accordance with the turn ratio. At least one of a width, a thickness, or an electrical resistivity of a coil pattern formed in each of first circuit board  15  and second circuit board  16  is determined such that the first amount of heat generation in first circuit board  15  is larger than the second amount of heat generation in second circuit board  16 . 
     A case where primary-side coil conductor  120  is formed in first circuit board  15  and secondary-side coil conductor  121  is formed in second circuit board  16  will be considered below. As shown in  FIGS. 8 and 9 , each of first coil pattern  20  and third coil pattern  22  is wound seven-eighths of a turn around second leg  11   b  of core  10 . First coil pattern  20  and third coil pattern  22  are electrically connected in series with each other by first via electrode  27 . First coil pattern  20  and third coil pattern  22  are wound two turns around second leg  11   b  of core  10  as a whole. In other words, primary-side coil conductor  120  is a series conductor of two turns. 
     As shown in  FIGS. 10 and 11 , each of second coil pattern  21  and fourth coil pattern  23  is wound one turn around second leg  11   b  of core  10 . Second coil pattern  21  and fourth coil pattern  23  are electrically connected in parallel with each other by second via electrode  28  and a third via electrode  28   a . In other words, secondary-side coil conductor  121  is two-parallel-connected conductor of one turn. The turn ratio between primary-side coil conductor  120  and secondary-side coil conductor  121  is 2:1. 
     An amount of heat generation W 1  (W) of primary-side coil conductor  120  is given by Equation (1) 
         W   1   =I   1   ×V   1   =I   1   2   ×R   1   (1)
 
     where V 1  denotes a first voltage (V) applied to primary-side coil conductor  120 , I 1  denotes a first current (A) flowing through primary-side coil conductor  120 , and R 1  denotes a first resistance value (Ω) of primary-side coil conductor  120 . 
     An amount of heat generation W 2  (W) of secondary-side coil conductor  121  is given by Equation (2) 
         W   2   =I   2   ×V   2   =I   2   2   ×R   2   (2)
 
     where V 2  denotes a second voltage (V) applied to secondary-side coil conductor  121 , I 2  denotes a second current (A) flowing through secondary-side coil conductor  121 , and R 2  denotes a second resistance value (Ω) of secondary-side coil conductor  121 . 
     First coil pattern  20  has a width b 1  (m), a length L 1  (m), a thickness t 1  (m), and an electrical resistivity ρ 1  (Ω·m). Second coil pattern  21  has a width b 2  (m), a length L 2  (m), a thickness t 2  (m), and an electrical resistivity ρ 2  (Ω·m). Third coil pattern  22  has a width b 3  (m), a length L 3  (m), a thickness t 3  (m), and an electrical resistivity ρ 3  (Ω·m). Fourth coil pattern  23  has a width b 4  (m), a length L 4  (m), a thickness t 4  (m), and an electrical resistivity ρ 4  (Ω·m). First resistance value R 1  of primary-side coil conductor  120  is given by Equation (3). Second resistance value R 2  of secondary-side coil conductor  121  is given by Equation (4). 
     
       
         
           
             
               
                 
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     Since the turn ratio between primary-side coil conductor  120  and secondary-side coil conductor  121  is 2:1, V 1 :V 2 =2:1 and I 1 :I 2 =1:2. Here, when ρ 1  to ρ 4  are equal to each other, b 1  to b 4  are equal to each other, L 1  to L 4  are equal to teach other, t 1  and t 3  are equal to each other, and t 2  and t 4  are equal to each other, a ratio of amount of heat generation W 1  of primary-side coil conductor  120  to amount of heat generation W 2  of secondary-side coil conductor  121  is given by Equation (5). Thickness t 1  of first coil pattern  20  and thickness t 2  of second coil pattern  21  are determined such that W 1 /W 2  is greater than one. 
         W   1   /W   2   =t   2 /2 t   1   (5)
 
     Also for a case where t 1  and t 3  are different from reach other and t 2  and t 4  are different from each other, thickness t 1  of first coil pattern  20 , thickness t 2  of second coil pattern  21 , thickness t 3  of third coil pattern  22 , and thickness t 4  of fourth coil pattern  23  are determined such that W 1 /W 2  is greater than one. When amount of heat generation W 2  of secondary-side coil conductor  121  is larger than amount of heat generation W 1  of primary-side coil conductor  120 , secondary-side coil conductor  121  is formed in first circuit board  15 , and primary-side coil conductor  120  is formed in second circuit board  16 . 
     First circuit board  15  may include three or more layers of coil patterns. Second circuit board  16  may include three or more layers of coil patterns. For example, first circuit board  15  may further include a coil pattern (not shown) inside first substrate  30 . Second circuit board  16  may further include a coil pattern (not shown) inside second substrate  31 . 
     Embodiment 4 
     Circuit device  105   c  according to Embodiment 4 will be described with reference to  FIGS. 12 to 14 . Circuit device  105   c  of the present embodiment is similar in configuration to circuit device  105  of Embodiment 1 and is different mainly in the following respects. 
     In circuit device  105   c , first circuit board  15  includes a heat transfer via  29  passing through first substrate  30 . Heat transfer via  29  is in contact with first heat transfer member  50  and second heat transfer member  51 . Heat transfer via  29  extends in the thickness direction of first substrate  30  and extends from third main surface  31   a  to fourth main surface  31   b . Heat transfer via  29  may be formed by filling holes extending from third main surface  31   a  to fourth main surface  31   b  with a thermally conductive material (e.g., metallic material) or may be formed by depositing a thermally conductive film (e.g., metallic film) on surfaces of the holes extending from third main surface  31   a  to fourth main surface  31   b . Heat transfer via  29  has a thermal conductivity higher than that of first substrate  30 . 
     First circuit board  15  may further include third coil pattern  22  provided on first main surface  30   a . First circuit board  15  may further include a first conductive pattern  26   a  provided on first main surface  30   a . First conductive pattern  26   a  is formed of the same material as that of third coil pattern  22 . First conductive pattern  26   a  may be electrically insulated from the coil patterns (e.g., first coil pattern  20 , second coil pattern  21 , and third coil pattern  22 ). 
     First circuit board  15  may further include a second conductive pattern  26   b  provided on second main surface  30   b . Second conductive pattern  26   b  is formed of the same material as that of first coil pattern  20 . Second conductive pattern  26   b  may be electrically insulated from the coil patterns (e.g., first coil pattern  20 , second coil pattern  21 , and third coil pattern  22 ). Heat transfer via  29  may be in contact with first conductive pattern  26   a  and second conductive pattern  26   b . Heat transfer via  29  may function as a third via electrode electrically connecting second conductive pattern  26   b  to first conductive pattern  26   a.    
     Circuit device  105   c  of the present embodiment achieves the following effects similar to those of circuit device  105  of Embodiment 1. 
     In circuit device  105   c  of the present embodiment, first circuit board  15  includes heat transfer via  29  passing through first substrate  30 . Heat transfer via  29  is in contact with first heat transfer member  50  and second heat transfer member  51 . As a current is flowed through first coil pattern  20 , second coil pattern  21 , and third coil pattern  22  to operate circuit device  105   c , first coil pattern  20 , second coil pattern  21 , and third coil pattern  22  generate heat. The heat generated in first coil pattern  20  and third coil pattern  22  is transferred through first heat transfer member  50  to heat dissipation member  60  with a relatively low thermal resistance. The heat generated in second coil pattern  21  is transferred through second heat transfer member  51 , heat transfer via  29 , and first heat transfer member  50  to heat dissipation member  60  with a lower thermal resistance. A temperature rise and a power loss in circuit device  105   c  during operation of circuit device  105   c  can thus be prevented or reduced. 
     Embodiment 5 
     Circuit device  105   d  according to Embodiment 5 will be described with reference to  FIG. 15 . Circuit device  105   d  of the present embodiment is similar in configuration to circuit device  105  of Embodiment 1 and is different mainly in the following respects. 
     Heat dissipation member  60  includes a first projection  62  projecting from surface  60   a  toward second circuit board  16 . First projection  62  may be a member separate from a portion of heat dissipation member  60  other than first projection  62 . First projection  62  may be made of a material different from that of heat dissipation member  60 . 
     Circuit device  105   d  further includes a third heat transfer member  52 . Third heat transfer member  52  is disposed between second circuit board  16  and first projection  62  and is in surface contact with second circuit board  16  and first projection  62 . Second circuit board  16 , third heat transfer member  52 , and first projection  62  are stacked on each other. Third heat transfer member  52  thermally connects second circuit board  16  to first projection  62 . Third heat transfer member  52  is a third heat transfer sheet. Third heat transfer member  52  may have electrical insulating properties. 
     Third heat transfer member  52  may be formed of a rubber material such as silicone or urethane, a resin material such as acrylonitrile butadiene styrene (ABS), polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), epoxy, phenol, or polyimide, or a ceramic material such as aluminum oxide or aluminum nitride. Third heat transfer member  52  may be formed of a silicone gel, a silicone grease, or a silicone adhesive. 
     Third heat transfer member  52  has a thermal conductivity of not less than 0.1 W/(m·K). First heat transfer member  50  may have a thermal conductivity of not less than 1.0 W/(m·K) or may have a thermal conductivity of not less than 10.0 W/(m·K). Third heat transfer member  52  may have elasticity. Third heat transfer member  52  may be crushed by pressing second circuit board  16  toward heat dissipation member  60 . 
     Circuit device  105   d  of the present embodiment achieves the following effects in addition to the effects of circuit device  105  of Embodiment 1. 
     Circuit device  105   d  of the present embodiment further includes third heat transfer member  52 . Heat dissipation member  60  includes surface  60   a  facing first circuit board  15  and first projection  62  projecting from surface  60   a  toward second circuit board  16 . Third heat transfer member  52  is disposed between second circuit board  16  and first projection  62  and is in surface contact with second circuit board  16  and first projection  62 . 
     The heat generated in second coil pattern  21  in flowing a current through first coil pattern  20  and second coil pattern  21  to operate circuit device  105   d  is transferred through a first heat dissipation path including second heat transfer member  51 , first circuit board  15 , and first heat transfer member  50  and a second heat dissipation path including third heat transfer member  52  to heat dissipation member  60  with a lower thermal resistance. A temperature rise and a power loss of circuit device  105   d  during operation of circuit device  105   d  can thus be prevented or reduced. 
     Second circuit board  16  is supported by first projection  62  with third heat transfer member  52  therebetween. Deformation of and mechanical damage to second circuit board  16  due to vibrations or an impact applied to circuit device  105   d  can thus be prevented or reduced. 
     Embodiment 6 
     Circuit device  105   e  according to Embodiment 6 will be described with reference to  FIGS. 16 to 18 . Circuit device  105   e  of the present embodiment is similar in configuration to circuit device  105  of Embodiment 1 and is different mainly in the following respects. 
     Heat dissipation member  60  includes first projection  62  projecting from surface  60   a  toward second circuit board  16 . First projection  62  may be a member separate from a portion of heat dissipation member  60  other than first projection  62 . First projection  62  may be made of a material different from that of the portion of heat dissipation member  60 . 
     A part of core  10  (e.g., second leg  11   b  of core  10 ) and first projection  62  are inserted into first through hole  30   h . A part of core  10  (e.g., second leg  11   b  of core  10 ) is inserted into second through hole  31   h , but first projection  62  is not inserted into second through hole  31   h . First projection  62  passes through first circuit board  15  (first substrate  30 ) but does not pass through second circuit board  16  (first substrate  30 ). 
     Second heat transfer member  51  is disposed between second circuit board  16  and first projection  62  and is in surface contact with second circuit board  16  and first projection  62 . Second circuit board  16 , second heat transfer member  51 , and first projection  62  are stacked on each other. Second heat transfer member  51  thermally connects second circuit board  16  to first projection  62 . In a plan view of surface  60   a  of heat dissipation member  60 , first projection  62  may overlap a part of second coil pattern  21 . 
     Circuit device  105   e  of the present embodiment achieves the following effects in addition to the effects of circuit device  105  of Embodiment 1. 
     In circuit device  105   e  of the present embodiment, heat dissipation member  60  includes surface  60   a  facing first circuit board  15  and first projection  62  projecting from surface  60   a  toward second circuit board  16 . Second heat transfer member  51  is disposed between second circuit board  16  and first projection  62  and is in surface contact with second circuit board  16  and first projection  62 . First substrate  30  is provided with first through hole  30   h . A part of core  10  (second leg  11   b  of core  10 ) and first projection  62  are inserted into first through hole  30   h.    
     The heat generated in second coil pattern  21  in flowing a current through first coil pattern  20  and second coil pattern  21  to operate circuit device  105   e  is transferred through the first heat dissipation path including second heat transfer member  51 , first circuit board  15 , and first heat transfer member  50  and the second heat dissipation path including second heat transfer member  51  and first projection  62  to heat dissipation member  60  with a lower thermal resistance. A temperature rise and a power loss of circuit device  105   e  during operation of circuit device  105   e  can thus be prevented or reduced. 
     Second circuit board  16  is supported by first projection  62  with second heat transfer member  51  therebetween. Deformation of and mechanical damage to second circuit board  16  due to vibrations or an impact applied to circuit device  105   e  can thus be prevented or reduced. 
     A part of core  10  (second leg  11   b  of core  10 ) and first projection  62  are inserted into first through hole  30   h . First circuit board  15  and core  10  can thus be aligned with respect to heat dissipation member  60 . First circuit board  15  and core  10  can be prevented from being displaced with respect to heat dissipation member  60  in the direction along surface  60   a  of heat dissipation member  60  due to vibrations or an impact applied to circuit device  105   e.    
     In circuit device  105   e  of the present embodiment, first projection  62  may overlap a part of second coil pattern  21  in a plan view of surface  60   a  of heat dissipation member  60 . The heat generated in second coil pattern  21  in flowing a current through first coil pattern  20  and second coil pattern  21  to operate circuit device  105   e  is thus transferred through the second heat dissipation path including second heat transfer member  51  and first projection  62  to heat dissipation member  60  with a lower thermal resistance. A temperature rise and a power loss of circuit device  105   e  during operation of circuit device  105   e  can be prevented or reduced. 
     Embodiment 7 
     Circuit device  105   f  according to Embodiment 7 will be described with reference to  FIGS. 19 to 21 . Circuit device  105   f  of the present embodiment is similar in configuration to circuit device  105   e  of Embodiment 6 and is different mainly in the following respects. 
     Heat dissipation member  60  further includes a second projection  63  projecting from surface  60   a  toward second circuit board  16 . Second projection  63  may be a member separate from a part of heat dissipation member  60  other than first projection  62  and second projection  63 . Second projection  63  may be made of a material different from that of the portion of heat dissipation member  60 . 
     A part of core  10  (e.g., second leg  11   b  of core  10 ) and second projection  63  are inserted into first through hole  30   h . A part of core  10  (e.g., second leg  11   b  of core  10 ) is inserted into second through hole  31   h , but second projection  63  is not inserted into second through hole  31   h . Second projection  63  passes through first circuit board  15  (first substrate  30 ) but does not pass through second circuit board  16  (second substrate  31 ). A part of core  10  (e.g., second leg  11   b  of core  10 ) is disposed between first projection  62  and second projection  63 . 
     Second heat transfer member  51  is disposed between second circuit board  16  and second projection  63  and is in surface contact with second circuit board  16  and second projection  63 . Second circuit board  16 , second heat transfer member  51 , and second projection  63  are stacked on each other. Second heat transfer member  51  thermally connects second circuit board  16  to second projection  63 . 
     Circuit device  105   f  of the present embodiment achieves the following effects in addition to the effects of circuit device  105   e  of Embodiment 6. 
     In circuit device  105   f  of the present embodiment, heat dissipation member  60  further includes second projection  63  projecting from surface  60   a  toward second circuit board  16 . Second heat transfer member  51  is disposed between second circuit board  16  and second projection  63  and is in surface contact with second circuit board  16  and second projection  63 . First substrate  30  is provided with first through hole  30   h . A part of core  10  (second leg  11   b  of core  10 ), first projection  62 , and second projection  63  are inserted into first through hole  30   h.    
     The heat generated in second coil pattern  21  in flowing a current through first coil pattern  20  and second coil pattern  21  to operate circuit device  105   f  is transferred through the first heat dissipation path including second heat transfer member  51 , first circuit board  15 , and first heat transfer member  50 , the second heat dissipation path including second heat transfer member  51  and first projection  62 , and a third heat dissipation path including second heat transfer member  51  and second projection  63  to heat dissipation member  60  with a lower thermal resistance. A temperature rise and a power loss of circuit device  105   f  during operation of circuit device  105   f  can thus be prevented or reduced. 
     Second circuit board  16  is supported by first projection  62  and second projection  63  with second heat transfer member  51  therebetween. Deformation of and mechanical damage to second circuit board  16  due to vibrations or an impact applied to circuit device  105   f  can thus be prevented or reduced. 
     A part of core  10  (second leg  11   b  of core  10 ), first projection  62 , and second projection  63  are inserted into first through hole  30   h . First circuit board  15  and core  10  can thus be aligned with respect to heat dissipation member  60 . First circuit board  15  and core  10  can be prevented from being displaced with respect to heat dissipation member  60  in the direction along surface  60   a  of heat dissipation member  60  due to vibrations or an impact applied to circuit device  105   f.    
     Embodiment 8 
     Circuit device  105   g  according to Embodiment 8 will be described with reference to  FIG. 22 . Circuit device  105   g  of the present embodiment is similar in configuration to circuit device  105  of Embodiment 1 and is different mainly in the following respects. 
     In circuit device  105   g , second heat transfer member  51  is composed of a plurality of heat-transfer partial layers (e.g., a first heat-transfer partial layer  57  and a second heat-transfer partial layer  58 ). The plurality of heat-transfer partial layers are stacked on each other. For example, second heat transfer member  51  is formed by stacking first heat-transfer partial layer  57  and second heat-transfer partial layer  58 . First heat-transfer partial layer  57  is in surface contact with first circuit board  15 . Second heat-transfer partial layer  58  is in surface contact with second circuit board  16 . Each of the plurality of heat-transfer partial layers (e.g., first heat-transfer partial layer  57  and second heat-transfer partial layer  58 ) has electrical insulating properties. The plurality of heat-transfer partial layers may have the same thickness or a different thickness. The plurality of heat-transfer partial layers may be made of the same material or may be made of a different material. 
     Second coil pattern  21  is provided on third main surface  31   a . Second heat transfer member  51  having electrical insulating properties is in surface contact with first coil pattern  20  and second coil pattern  21 . 
     Circuit device  105   g  of the present embodiment achieves the following effects in addition to the effects of circuit device  105  of Embodiment 1. In circuit device  105   g  of the present embodiment, second heat transfer member  51  is composed of a plurality of heat-transfer partial layers. The plurality of heat-transfer partial layers are stacked on each other. Each of the plurality of heat-transfer partial layers has electrical insulating properties. Accordingly, even if a partial layer of a plurality of heat-transfer partial layers includes a void and a dielectric breakdown occurs in the partial layer of the plurality of heat-transfer partial layers during operation of circuit device  105   g , the other layers of the plurality of heat-transfer partial layers can maintain electrical insulation between first coil pattern  20  and second coil pattern  21 . The occurrence of an electrical discharge (e.g., partial discharge or corona discharge) between first coil pattern  20  and second coil pattern  21  due to the void can be prevented or reduced during operation of circuit device  105   g . Second coil pattern  21  can be electrically insulated from first coil pattern  20  more reliably. 
     Embodiment 9 
     Circuit device  105   h  according to Embodiment 9 will be described with reference to  FIGS. 23 and 24 . Although circuit device  105   h  of the present embodiment is similar in configuration to circuit device  105   g  of Embodiment 8 and achieves effects similar to those of circuit device  105   g  of Embodiment 8, circuit device  105   h  is different from mainly in the following respects. 
     In circuit device  105   h  of the present embodiment, second heat transfer member  51  includes first heat-transfer partial layer  57 , second heat-transfer partial layer  58 , and a third heat-transfer partial layer  59 . First heat-transfer partial layer  57 , second heat-transfer partial layer  58 , and third heat-transfer partial layer  59  are stacked on each other. First heat-transfer partial layer  57  has electrical insulating properties and is in surface contact with first circuit board  15 . Second heat-transfer partial layer  58  has electrical insulating properties and is in surface contact with second circuit board  16 . Third heat-transfer partial layer  59  is disposed between first heat-transfer partial layer  57  and second heat-transfer partial layer  58 . Third heat-transfer partial layer  59  has a thermal conductivity higher than that of each of first heat-transfer partial layer  57  and second heat-transfer partial layer  58 . Third heat-transfer partial layer  59  has, for example, a thermal conductivity of not less than 10.0 W/(m·K). 
     Third heat-transfer partial layer  59  may have conductivity or may have electrical insulating properties. Third heat-transfer partial layer  59  is formed of, for example, a metallic material such as copper (Cu), silver (Ag), gold (Au), tin (Sn), iron (Fe), copper (Cu) alloy, nickel (Ni) alloy, gold (Au) alloy, silver (Ag) alloy, tin (Sn) alloy, or iron (Fe) alloy. Third heat-transfer partial layer  59  may be formed of a non-metallic material such as graphite or ceramic. Third heat-transfer partial layer  59  is electrically insulated from first coil pattern  20  by first heat-transfer partial layer  57 . Third heat-transfer partial layer  59  is electrically insulated from second coil pattern  21  by second heat-transfer partial layer  58 . Third heat-transfer partial layer  59  is not magnetically coupled to first coil pattern  20  and second coil pattern  21 . Third heat-transfer partial layer  59  thus does not generate heat during operation of circuit device  105   h.    
     The effects of circuit device  105   h  according to the present embodiment will be described. Circuit device  105   h  of the present embodiment achieves the following effects in addition to the effects of circuit device  105   g  of Embodiment 8. 
     In circuit device  105   h  of the present embodiment, second heat transfer member  51  includes first heat-transfer partial layer  57 , second heat-transfer partial layer  58 , and third heat-transfer partial layer  59 . First heat-transfer partial layer  57 , second heat-transfer partial layer  58 , and third heat-transfer partial layer  59  are stacked on each other. First heat-transfer partial layer  57  has electrical insulating properties and is in surface contact with first circuit board  15 . Second heat-transfer partial layer  58  has electrical insulating properties and is in surface contact with second circuit board  16 . Third heat-transfer partial layer  59  is disposed between first heat-transfer partial layer  57  and second heat-transfer partial layer  58  and has a thermal conductivity higher than that of each of first heat-transfer partial layer  57  and second heat-transfer partial layer  58 . Third heat-transfer partial layer  59  thus diffuses the heat generated in first coil pattern  20  and second coil pattern  21  in the direction in which third heat-transfer partial layer  59  extends (the direction along surface  60   a  of heat dissipation member  60 ). Local overheating of circuit device  105   h  can be prevented during operation of circuit device  105   h . A temperature rise and a power loss of circuit device  105   h  during operation of circuit device  105   h  can be prevented or reduced. 
     Embodiment 10 
     Circuit device  105   i  according to Embodiment 10 will be described with reference to  FIGS. 25 and 26 . Circuit device  105   i  of the present embodiment is similar in configuration to circuit device  105  of Embodiment 1 and is different mainly in the following respects. 
     In circuit device  105   i , a second thickness d 2  of second circuit board  16  is larger than a first thickness d 1  of first circuit board  15 . When first coil pattern  20  is formed on first main surface  30   a  or on second main surface  30   b  in first circuit board  15 , first thickness d 1  of first circuit board  15  is defined as the sum of the thickness of first substrate  30  and the thickness of first coil pattern  20 . When first coil pattern  20  is formed inside first substrate  30  in first circuit board  15 , first thickness d 1  of first circuit board  15  is defined as the thickness of first substrate  30 . 
     When second coil pattern  21  is formed on third main surface  31   a  or on fourth main surface  31   b  in second circuit board  16 , second thickness d 2  of second circuit board  16  is defined as the sum of the thickness of second substrate  31  and the thickness of second coil pattern  21 . When second coil pattern  21  is formed inside second substrate  31  in second circuit board  16 , second thickness d 2  of second circuit board  16  is defined as the thickness of second substrate  31 . First circuit board  15  and second circuit board  16  are fixed to heat dissipation member  60  with fixing member  70  such as a screw, a mechanical screw, or a rivet. 
     Circuit device  105   i  of the present embodiment achieves the following effects in addition to the effects of circuit device  105  of Embodiment 1. In circuit device  105   i  of the embodiment, second thickness d 2  of second circuit board  16  is larger than first thickness d 1  of first circuit board  15 . Second circuit board  16  thus has rigidity higher than that of first circuit board  15 . Circuit device  105   i  can be prevented from being mechanically damaged due to vibrations or an impact applied to circuit device  105   i.    
     Embodiment 11 
     A circuit device and a power conversion apparatus according to Embodiment 11 will be described with reference to  FIG. 27 . The circuit device and the power conversion apparatus of the present embodiment have configurations similar to those of circuit device  105  and power conversion apparatus  1  of Embodiment 1 and are different mainly in the following respects. 
     In the circuit device and the power conversion apparatus of the present embodiment, control circuit  6  that controls first electronic components  40 ,  43  forming inverter circuit  2  (see  FIG. 1 ) is disposed on at least one of first circuit board  15  and second circuit board  16 . For example, control circuit  6  is disposed on first circuit board  15  (particularly, second main surface  30   b ). 
     At least one of first circuit board  15  and second circuit board  16  includes a third conductive pattern  25 . Third conductive pattern  25  electrically connects control circuit  6  to first electronic components  40 ,  43 . For example, first circuit board  15  includes third conductive pattern  25  provided on first substrate  30  (particularly, on second main surface  30   b ). A current flowing through third conductive pattern  25  is smaller than a current flowing through first coil pattern  20  and second coil pattern  21 . The thickness of third conductive pattern  25  may thus be smaller than the thickness of first coil pattern  20 . The thickness of third conductive pattern  25  may be smaller than the thickness of second coil pattern  21 . 
     The circuit device and the power conversion apparatus of the present embodiment achieve the following effects in addition to the effects of Embodiment 1. In the circuit device and power conversion apparatus  1  of the present embodiment, control circuit  6  that controls first electronic components  40 ,  43  is mounted on at least one of first circuit board  15  and second circuit board  16 . At least one of first circuit board  15  and second circuit board  16  includes third conductive pattern  25 . Third conductive pattern  25  electrically connects control circuit  6  to first electronic components  40 ,  43 . 
     Therefore, a cable and a connector for electrically connecting control circuit  6  with first electronic components  40 ,  43  can be omitted, leading to miniaturization of the circuit device and the power conversion apparatus. Further, the length of third conductive pattern  25  connecting control circuit  6  with first electronic components  40 ,  43  can be reduced, which may reduce the influence of electromagnetic noise on first electronic components  40 ,  43 . 
     It should be understood that Embodiments 1 to 11 disclosed herein are illustrative and non-restrictive in every respect. At least two of Embodiments 1 to 11 disclosed herein may be combined unless there is inconsistency. For example, power conversion apparatus  1  includes any of circuit devices  105 ,  105   b ,  105   c ,  105   d ,  105   e ,  105   f ,  105   g ,  105   h ,  105   i  of Embodiments 1 to 11. The scope of the present invention is defined by the terms of the claims rather than the description above and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     REFERENCE SIGNS LIST 
       1  power conversion apparatus;  2  inverter circuit;  3  transformer circuit;  4  rectifier circuit;  5  smoothing circuit;  6  control circuit;  7   a ,  7   b ,  7   c ,  7   d  switching element;  8   a ,  8   b ,  8   c ,  8   d  diode;  9   a ,  9   b  capacitor;  10  core;  10   a  first core portion;  10   b  second core portion;  11   a  first leg;  11   b  second leg;  11   c  third leg;  15  first circuit board;  16  second circuit board;  20  first coil pattern;  21  second coil pattern;  22  third coil pattern;  23  fourth coil pattern;  25  third conductive pattern;  26   a  first conductive pattern;  26   b  second conductive pattern;  27  first via electrode;  28  second via electrode;  28   a  third via electrode;  29  heat transfer via;  30  first substrate;  30   a  first main surface;  30   b  second main surface;  30   h  first through hole;  31  second substrate;  31   a  third main surface;  31   b  fourth main surface;  31   h  second through hole;  40 ,  43  first electronic component;  41 ,  42  second electronic component;  50  first heat transfer member;  51  second heat transfer member;  52  third heat transfer member;  57  first heat-transfer partial layer;  58  second heat-transfer partial layer;  59  third heat-transfer partial layer;  60  heat dissipation member;  60   a  surface;  60   b  recess;  62  first projection;  63  second projection;  70  fixing member;  100 ,  102 ,  103  coil device;  101  transformer;  105 ,  105   b ,  105   c ,  105   d ,  105   e ,  105   f ,  105   g ,  105   h ,  105   i  circuit device;  110  input terminal;  111  output terminal;  120  primary-side coil conductor;  121  secondary-side coil conductor.