Abstract:
The present invention aims to provide a power converter with an arm including switching devices connected in parallel, realizing long lifespans of switching devices. An inverter includes an upper and a lower arm, and gate drive circuits each driving the corresponding arm according to a gate control signal Gup_s indicating ON/OFF periods. Each arm includes switching devices connected in parallel. Each gate drive circuit includes: a switching gate control circuit  230   u  bringing a switching device  210   u  into conduction at the beginning of the ON period and bringing the same out of conduction within the ON period; and a conduction gate control circuit  231   u  bringing switching devices  211   u  and  212   u  within a period from when the switching device  210   u  is brought into conduction until the same is brought out of conduction, wherein the switching device  210   u  has a lower parasitic capacitance than the switching devices  211   u  and the  212   u.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a power converter such as an inverter and a DC/DC converter having an arm composed of a plurality of switching devices connected in parallel. 
       BACKGROUND ART 
       [0002]    In recent years, power converters such as inverters and DC/DC converters have increased their output power for the use in hybrid electric vehicles, electric vehicles, and the likes. In a high-output power converter, switching devices constituting an arm are required to manage high voltage and high current. To support high voltage, the switching devices are connected in series, and to support high current, the switching devices are connected in parallel. For example, Patent Literature 1 discloses an inverter having an arm composed of a plurality of switching devices connected in parallel. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         [Patent Literature 1] Japanese Patent Application Publication No. 2005-6412 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0004]    When switching devices are connected in parallel, it is likely that a large amount of current flows in a particular device due to variations in the electrical properties of the devices. Hence, the device in which a larger amount of current flows generates a large amount of heat, and the lifespan of the device is likely to be short. In particular, parallel connection is more problematic during the switching operation than during the regular operation. That is, in addition to the variations in the electrical properties (e.g. on-resistance, threshold voltage), the difference in the junction temperature, the difference in the wiring inductance, and variations in properties of the gate drive circuits have an influence during the switching operation, and when compared with the case of a single device, the problem of variations is more significant in the case of the switching operation. 
         [0005]    The present invention is made in view of the problems above, and aims to provide a power converter having an arm composed of a plurality of switching devices connected in parallel and realizing long lifespans of the switching devices. 
       Solution to Problem 
       [0006]    To achieve the aim, the present invention provides a power converter comprising: an upper arm; a lower arm; and gate drive circuits each configured to drive a corresponding one of the arms according to a reference signal, the reference signal having a first-potential period and a second-potential period, wherein each arm includes a set of switching devices connected in parallel, each gate drive circuit includes: a switching gate control circuit configured to bring a first switching device among the corresponding set of switching devices at the beginning of the first-potential period, and to bring the first switching device out of conduction at a point within the first-potential period; and a conduction gate control circuit configured to bring a second switching device among the corresponding set of switching devices into conduction at a point within a period from when the first switching device is brought into conduction, which corresponds to the beginning of the first-potential period, until the first switching device is brought out of conduction, and the first switching device has a smaller parasitic capacitance than the second switching device. 
       Advantageous Effects of Invention 
       [0007]    In the power converter pertaining to the present invention with the structure described in Solution to Problem, among the switching devices included in the arm, the switching loss occurs in the first switching device which performs the switching operation, and the conduction loss occurs in the second switching device which performs the conduction operation. 
         [0008]    Hence, in the first switching device, heat caused by switching loss is generated, but heat caused by conduction loss is suppressed. In the second switching device, heat caused by conduction loss is generated, but heat caused by switching loss is suppressed. Therefore, the switching devices are prevented from having a short lifespan due to heat generation. 
         [0009]    Conduction loss and switching loss, which are power loss occurring in switching devices and are recognized as common properties of switching devices, have a trade-off relationship. That is, a switching device with a reduced conduction loss has a relatively large switching loss, and a switching device with a reduced switching loss has a relatively large conduction loss. 
         [0010]    In the power converter pertaining to the present invention, however, switching loss and conduction loss occur in different switching devices. Hence, switching loss and conduction loss, which conventionally have a trade-off relationship, can be each reduced in terms of the entire apparatus. That is, the present invention provides a highly efficient power converter that can reduce both types of loss in terms of the entire apparatus. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  shows the entire structure of a synchronous motor drive system using a power converter pertaining to the present invention. 
           [0012]      FIG. 2  is a waveform diagram showing switching operations performed by an upper arm and a lower arm of an inverter. 
           [0013]      FIG. 3  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Embodiment 1. 
           [0014]      FIG. 4  is a timing chart illustrating operations performed by a gate control circuit. 
           [0015]      FIG. 5  shows the current-voltage characteristic of a switching device. 
           [0016]      FIG. 6  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Modification 1 of Embodiment 1. 
           [0017]      FIG. 7  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Modification 2 of Embodiment 1. 
           [0018]      FIG. 8  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Modification 3 of Embodiment 1. 
           [0019]      FIG. 9  is a timing chart illustrating operations performed by a gate control circuit pertaining to Modification 3. 
           [0020]      FIG. 10  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Modification 4 of Embodiment 1. 
           [0021]      FIG. 11  illustrates mechanism by which a lower arm  22   u  malfunctions due to a turning on of a upper arm  21   u.    
           [0022]      FIG. 12  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Embodiment 2. 
           [0023]      FIG. 13  is a timing chart illustrating operations performed by a gate control circuit of an inverter pertaining to Embodiment 2. 
           [0024]      FIG. 14  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Embodiment 3. 
           [0025]      FIG. 15  shows operations of gate control circuit in an inverter pertaining to Embodiment 3 and switching operations thereof. 
           [0026]      FIG. 16  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Embodiment 3. 
           [0027]      FIG. 17  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Embodiment 4. 
           [0028]      FIG. 18  shows the entire structure of a synchronous motor drive system using an inverter pertaining to Embodiment 5. 
           [0029]      FIG. 19  shows a positional relationship between an arm and a capacitor of the inverter pertaining to Embodiment 5. 
           [0030]      FIG. 20  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Embodiment 6. 
           [0031]      FIG. 21  compares voltage-current characteristics in saturation regions of IGBT and MOSFET. 
           [0032]      FIG. 22  shows the entire structure of a synchronous motor drive system using a DC/DC converter pertaining to the present invention. 
           [0033]      FIG. 23  shows detailed structures of an arm  81  and a gate drive circuit  83  depicted in  FIG. 22 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]    The following describes embodiments of a power converter pertaining to the present invention, with reference to the drawings. 
       Embodiment 1 
       [0035]      FIG. 1  shows the entire structure of a synchronous motor drive system using a power converter pertaining to the present invention. 
         [0036]    The synchronous motor drive system shown in the drawing includes a battery  1 , an inverter  2  as a power converter pertaining to the present invention, a motor  3 , and a control circuit  4 . 
         [0037]    The battery  1  is a DC power source, and supplies DC power to the inverter  2 . 
         [0038]    The inverter  2  is a three-phase inverter that converts DC power supplied by the batter  1  to AC power and supplies three-phase AC power to the motor  3 . 
         [0039]    The motor  3  is rotatably driven by three-phase AC power supplied from the inverter  2 . 
         [0040]    The control circuit  4  controls the inverter  2  so that the motor  3  operates in a desired manner. 
         [0041]    The following describes the details of the inverter  2  pertaining to the present embodiment. 
         [0042]    An inverter has the same number of legs as AC power outputs. The inverter  2  pertaining to the present embodiment includes a leg  25   u , a leg  25   v  and a leg  25   w.    
         [0043]    The leg  25   u  includes: an upper arm  21   u  and a lower arm  22   u , which are connected in series between the positive terminal and the negative terminal of the battery  1  (the upper arm  21   u  is connected to the positive side, and the lower arm  22   u  is connected to the negative side); and an upper arm-side gate drive circuit (upper arm drive circuit)  23   u  and a lower arm-side gate drive circuit (lower arm drive circuit)  24   u , which respectively correspond to the upper arm  21   u  and the lower arm  22   u . Similarly, the legs  25   v  and  25   w  respectively include upper arms  21   v  and  21   w , lower arms  22   v  and  22   w , upper arm-side gate drive circuits (upper arm drive circuits)  23   v  and  23   w , and lower arm-side gate drive circuits (lower arm drive circuits)  24   v  and  24   w.    
         [0044]      FIG. 2  is a waveform diagram showing basic operations of the upper arm-side gate driver circuit and the lower arm-side gate drive circuit for the U phase. 
         [0045]    The symbol Is_u represents a current-instruction signal, which is externally input to the control circuit  4  and indicates a waveform of a current to be applied to the U phase. This waveform matches the waveform of the current to be applied to the U-phase coil of the motor  3 . The symbol fc represents a carrier signal used for PWM operations, which is generated by a carrier signal generation circuit included in the control circuit  4 . The control circuit  4  compares the carrier signal fc with the current-instruction signal and thereby generates a gate control signals Gup_s and Gun_s, and outputs the gate control signals Gup_s and Gun_s to the upper arm-side gate drive circuit and the lower arm-side gate drive circuit, respectively. 
         [0046]    Although only the U-phase among the three phases is described above, the control circuit  4  also generates and outputs gate control signals Gvp_s and Gvn_s and gate control signals Gwp_s and Gwn_s for the V phase and the W phase, respectively. In the following, only the U-phase will be explained. However, the inverter  2  has the same structure for the V phase and the W phase. 
         [0047]      FIG. 3  shows the details of the upper arm  21   u  and the upper arm-side gate drive circuit  23   u  depicted in  FIG. 1 . 
         [0048]    The upper arm-side gate drive circuit  23   u  includes a switching gate control circuit  230   u , and a conduction gate control circuit  231   u . The upper arm  21   u  includes three switching devices  210   u ,  211   u  and  212   u . Here, each of the switching devices  210   u ,  211   u  and  212   u  is made up of a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET). The switching gate drive circuit  230   u  is connected to the control terminal of the switching device  210   u , and the conduction gate control circuit  231   u  is connected to the respective control terminals of the switching devices  211   u  and  212   u . The switching devices  210   u ,  211   u  and  212   u  are connected in parallel, and their respective high electrical potential-side primary terminals are connected to one another, and their respective low electrical potential-side primary terminals are connected to one another. 
         [0049]    The gate control signal Gup_s output from the control circuit  4  is input to each of the switching gate control circuit  230   u  and the conduction gate control circuit  231   u . Based on the gate control signal Gup_s, the switching gate control circuit  230   u  outputs a switching gate drive signal Gup_sw to the control terminal of the switching device  210   u . The conduction gate control circuit  231   u  outputs a conduction gate drive signal Gup_on to the respective control terminals of the switching devices  211   u  and  212   u.    
         [0050]    The following describes the details of the switching gate drive signal Gup_sw output by the switching gate control circuit  230   u  and the conduction gate drive signal Gup_on output by the conduction gate control circuit  231   u , with reference to  FIG. 4 . 
         [0051]    The gate control signal Gup_s output by the control circuit  4  has a first-potential period, for which the electrical potential is at the High level, and a second-potential period, for which the electrical potential is at the Low level. The gate control signal Gup_s serves as a reference signal used by the switching gate control circuit  230   u  and the conduction gate control circuit  231   u  to operate. 
         [0052]    The switching gate control circuit  230   u  raises the switching gate drive signal Gup_sw at time t 1 , which is when a rise is detected in the gate control signal Gup_s input by the control circuit  4 . Then, the switching gate control circuit  230   u  drops the switching gate drive signal Gup_sw within a shorter period than the first-potential period for which the electrical potential of the gate control signal Gup_s is at the High level. That is, the switching gate control circuit  230   u  drops the switching gate drive signal Gup_sw at time t 3 . Furthermore, the switching gate control circuit  230   u  raises the switching gate drive signal Gup_sw again at time t 4 , which is when a drop is detected in the gate control signal Gup_s. Then, the switching gate control circuit  230   u  drops the switching gate drive signal Gup_sw within a shorter period than the second-potential period for which the electrical potential of the gate control signal Gup_s is at the Low level. That is, the switching gate control circuit  230   u  drops the switching gate drive signal Gup_sw at time t 6 . 
         [0053]    Through the operations described above, the switching gate control circuit  230   u  outputs, as the switching gate drive signal Gup_sw, a square wave twice within the period of one cycle of the gate control signal Gup_s. 
         [0054]    On the other hand, the conduction gate control circuit  231   u  raises the conduction gate drive signal Gup_on at time t 2  in the period of one cycle of the gate control signal Gup_s. The time t 2  indicates a point within the period between t 1  and t 3  for which the switching gate drive signal Gup_sw is raised to the High level along with the rise of the gate control signal Gup_s. Then, the conduction gate control circuit  231   u  drops the conduction gate drive signal Gup_on at time t 5 . The time t 5  is a point within the period between t 4  and t 6  for which the switching gate drive signal Gup_sw is raised to the High level along with the drop of the gate control signal Gup_s. 
         [0055]    By the operations described above, the upper arm  21   u  will be conductive during the period between t 1  and t 6 , for which at least one of the switching gate drive signal Gup_sw and the conduction gate drive signal Gup_on is at the High electrical potential. In the upper arm  21   u , however, the operation for switching is performed by the switching device  210   u , and the operation for establishing electrical conduction is performed by the switching device  211   u  and the switching device  212   u . As a result, among losses occurring in the upper arm  21   u , switching loss occurs in the switching device  210   u , and conduction loss occurs in the switching device  211   u  and the switching device  212   u.    
         [0056]    As described above, in the inverter pertaining to the present embodiment, the location where switching loss occurs and the location where conduction loss occurs are separated. Specifically, the farmer occurs in the switching device that performs the switching operation and the latter occurs in the switching devices that perform the conduction operation. Conduction loss and switching loss, which are power loss occurring in switching devices and are recognized as common properties of switching devices, have a trade-off relationship. That is, a switching device with a reduced conduction loss has a relatively large switching loss, and a switching device with a reduced switching loss has a relatively large conduction loss. 
         [0057]    In the inverter pertaining to the present embodiment, however, switching loss and conduction loss occur in different switching devices. Hence, switching loss and conduction loss, which conventionally have a trade-off relationship, can be each reduced in terms of the entire apparatus. That is, the present invention provides a highly efficient inverter that can reduce both types of loss in terms of the entire apparatus. 
         [0058]    Note that the switching devices  210   u ,  211   u  and  212   u , which constitute the upper arm and are connected in parallel, do not necessarily have the same maximum voltage rating and the same maximum current rating. For example, the switching device  210   u , which mainly performs the switching operation, may have a relatively small maximum current rating compared to the switching devices  211   u  and  212   u  each of which mainly performs the conduction operation. Generally, the level of the maximum current rating depends on the chip area of the switching device. Therefore, as the maximum current rating decreases, the conduction loss increases, but the switching loss decreases. Regarding the switching device  210   u , which mainly performs the switching operation, it is not very necessary to consider the conduction loss. Therefore, in order to reduce the switching loss, it is preferable that the maximum current rating is low. It is however necessary to conduct the maximum current, and to allow the occurrence of loss for a short period. 
         [0059]    Generally, switching devices using MOSFET can bare approximately three to ten times the current rating for a short period. For example, as can be seen from  FIG. 5  which shows the current-voltage characteristic of a MOSFET having a current rating of 20 A, a MOSFET can operate within the saturation region when the current is at 60 A. Therefore, a device having a maximum current rating that is approximately ⅓ of the switching device  211   u  and the switching device  212   u  can be used as the switching device  210   u , which mainly performs the switching operation. 
         [0060]    It is preferable that the switching devices  211   u  and  212   u , which mainly perform the conduction operation, have a relatively large maximum current rating compared to the switching device  210   u  which mainly performs the switching operation. Regarding the switching devices  211   u  and  212   u , it is not very necessary to consider the switching loss, and the maximum current rating of a switching device and conduction/switching loss have the relationship as described above. Therefore, in order to reduce the conduction loss, it is advantageous that the maximum current rating is low. 
         [0061]    However, if the maximum current rating is unnecessarily high, it means that the chip area is large. This leads to increase in size of the apparatus. Hence, it is preferable that the maximum current rating is approximately twice to four times the maximum amount of current applied to the upper arm. 
         [0062]    In the above, switching devices suitable for the conduction operation and the switching operation are described focusing on the relationship between the maximum current rating and the conduction loss and the switching loss. However, the upper limit of the maximum current rating may be determined not only by the current carrying capacity as the characteristic of the device, but also by the thermal resistance of the entire package. For example, when the switching device  210   u  and the switching devices  211   u  and  212   u  are all arranged on a single substrate and sealed with mold resin to form a single module, the maximum current rating is determined by the thermal resistance of the entire package. The maximum current rating determined by the thermal resistance of the entire package has essentially no correlation with the amounts of conduction loss and switching loss. 
         [0063]    On the other hand, parasitic capacitance is determined by the properties of a switching device per se, and when the same material and the same structure are adopted in switching devices, parasitic capacitance and current carrying capacity generally correlate with each other in each switching device. That is, parasitic capacitance increases as the chip area increases. In other words, among switching devices made up from the same material and have the same structure, a switching device with a higher parasitic capacitance exhibits a smaller amount of conduction loss and a larger amount of switching loss. Therefore, which switching device is suitable for the conduction operation and which is for the switching operation can be determined based on the parasitic capacitance. Specifically, it is preferable that the switching devices  211   u  and  212   u , which mainly perform the conduction operation, have a relatively large parasitic capacitance compared to the switching device  210   u , which mainly performs the switching operation. In addition, a switching device increases its speed of performing the switching operation as the parasitic capacitance decreases. For this reason, there is a merit to select a switching device with a low parasitic capacitance as the switching device  210   u  which is required to perform the switching operation at high speed. 
         [0064]    Note that the parasitic capacitance of a switching device is “input capacitance” occurring between a source and a gate, “output capacitance” occurring between a source and a drain, or “feedback capacitance” occurring between a drain and a gate. Generally, each type of capacitance is not determined independently, and when one of them is doubled, the other two are also approximately doubled. Therefore, switching devices to be used may be selected based on any type of capacitance. 
         [0065]    Here, as described above, when the material and the structure of switching devices are the same, current carrying capacity and parasitic capacitance correlate with each other. However, when the material and the structure are different, there can be no correlation. For example, in a comparison between a switching device using silicon carbide (SiC) and a switching device using silicon (Si), the SiC device has a smaller chip area and a smaller parasitic capacitance even with the same current carrying capacity. Therefore, switching loss can be further reduced when a switching device  210   u  which mainly performs the switching operation is made up from a SiC device with low parasitic capacitance which can perform the switching operation at high speed. 
         [0066]    Moreover, although SiC devices are costly, they have higher thermal resistance than Si devices. When the switching device  210   u , the switching devices  211   u  and  212   u  are enclosed within a single package, the temperature of the entire package depends largely on heat generated by the switching devices  211   u  and  212   u  performing the conduction operation, and the temperature of the switching device  210   u  further rises locally when performing the switching operation. Considering the above, in the case the switching device  210   u  and the switching devices  211   u  and  212   u  are enclosed within a single package, it is preferable that Si devices, which are relatively cheap, are used as the switching devices  211   u  and  212   u , and a SiC device, which has high thermal resistance, is used as the switching device  210   u  whose temperature temporarily rises when performing the switching operation. 
       Modification 1 of Embodiment 1 
       [0067]    The example described above has s structure in which the upper arm  21   u  made up from a plurality of MOSFETs connected in parallel. However, the present invention is not limited to this. The switching devices may be insulated gate bipolar transistors (IGBTs), field effect transistors (J-FETs), bipolar transistors, or their combinations.  FIG. 6  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Modification 1 of Embodiment 1. The structure shown in this figure is the same as the structure shown in  FIG. 3  except that the upper arm  21   u  is replaced with an upper arm  31   u , and performs the same operation. The upper arm  31   u  includes switching devices  310   u ,  311   u  and  312   u , which are all IGBTs. As with Embodiment 1, when the switching devices are IGBTs, switching loss and conduction loss can be reduced separately. That is, both of them can be reduced, and this realizes a high-efficiency inverter. 
         [0068]    Here, the present modification, in which the switching devices included in the upper arm are not MOSFETs but IGBTs, is advantageous when the conduction loss of the inverter is larger enough than the switching loss of the same. 
         [0069]    Generally, MOSFETs have lower switching loss than IGBTs, but have higher conduction loss than IGBTs. Hence, when conduction loss is major among loss occurring in an inverter, it is advantages that the switching devices of the inverter to which the present invention is applied are IGBTs. 
         [0070]    In the description of Embodiment 1 above, it is stated that when all the switching devices are MOSFETS, it is preferable that the switching device  210   u  which mainly performs the switching operation has a lower maximum current rating than the switching devices  211   u  and  212   u . However, since IGBTs have higher switching loss and lower breakdown resistance than MOSFETs, the reliability of the inverter is degraded when the switching device which mainly performs the switching operation is made up from an IGBT and has low current carrying capacity (Generally, breakdown resistance and current carrying capacity have a trade-off relationship). Therefore, in the case of an inverter using IGBTs, it is preferable that the switching devices which mainly perform the switching operation are also made up from devices that have approximately the same current carrying capacity as the switching devices which mainly perform the conduction operation. 
       Modification 2 of Embodiment 1 
       [0071]    Next, a modification in which the upper arm includes different types of switching devices is described. 
         [0072]      FIG. 7  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Modification 2 of Embodiment 1. The structure shown in this figure is the same as the structure shown in  FIG. 3  except that the upper arm  21   u  is replaced with an upper arm  41   u , and performs the same. The upper arm  41   u  includes switching devices  410   u ,  411   u  and  412   u . The switching device  410   u  is a MOSFET, which is a unipolar device, and the switching devices  411   u  and  412   u  are IGBTs, which are bipolar devices. 
         [0073]    In the present modification, as with Embodiment 1, switching loss and conduction loss can be reduced separately. That is, both of them can be reduced, and this realizes a high-efficiency inverter. 
         [0074]    Generally, MOSFETs, which are unipolar devices, have a larger conductance resistance and a larger chip area than IGBTs, which are bipolar devices. Therefore, MOSFETs have higher conduction loss than IGBTs. On the other hand, since MOSFETs can perform the switching operation at higher speed than IGBTs, and can reduce the switching loss. Therefore, when the switching devices included in the upper arm are the combination of MOSFETs and IGBTs as with the present modification, that is, when the switching devices which mainly perform the switching operation are MOSFETs and the switching devices which mainly perform the conduction operation are IGBTs, the advantages of both switching devices can be maximized, and further effect of loss reduction can be achieved. 
       Modification 3 of Embodiment 1 
       [0075]    The following describes another modification of Embodiment 1. 
         [0076]      FIG. 8  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Modification 3 of Embodiment 1. The structure shown in the figure is different form  FIG. 3  in that the conduction gate control circuit  231   u  of the upper arm-side gate drive circuit  21   u  is replaced with a conduction gate control circuit  231   u _ 1  and a conduction gate control circuit  231   u _ 2 . 
         [0077]    The upper arm-side gate drive circuit  23   u  includes the switching gate control circuit  230   u , a conduction gate control circuit  231   u _ 1  and a conduction gate control circuit  231   u _ 2 . The upper arm  21   u  includes three switching devices  210   u ,  211   u  and  212   u . The switching gate drive circuit  230   u  is connected to the control terminal of the switching device  210   u , and the conduction control circuits  231   u _ 1  and  231   u _ 2  are connected to the respective control terminals of the switching devices  211   u  and  212   u . The switching devices  210   u ,  211   u  and  212   u  are connected in parallel, and their respective high electrical potential-side primary terminals are connected to one another, and their respective low electrical potential-side primary terminals are connected to one another. 
         [0078]    The gate control signal Gup_s output from the control circuit  4  is input to each of the switching gate control circuit  230   u  and the conduction gate control circuits  231   u _ 1  and  231   u _ 2 . Based on the gate control signal Gup_s, the switching gate control circuit  230   u  outputs a switching gate drive signal Gup_sw to the control terminal of the switching device  210   u . The conduction gate control circuits  231   u _ 1  and  231   u _ 2  respectively output a conduction gate drive signals Gup_on 1  and Gup_on 2  to the control terminals of the switching devices  211   u  and  212   u.    
         [0079]    The following describes the details of the switching gate drive signal Gup_sw output by the switching gate control circuit  230   u  and the conduction gate drive signals Gup_on 1  and Gup_on 2  respectively output by the conduction gate control circuits  231   u _ 1  and  231   u _ 2 , with reference to  FIG. 9 . 
         [0080]    The switching gate drive signal Gup_sw output by the switching gate control circuit  230   u  is the same as the example shown in  FIG. 4 . The conduction gate drive signal Gup_on 1  output by the conduction gate control circuit  231   u _ 1  is the same as the conduction gate drive signal Gup_on shown in  FIG. 4 . 
         [0081]    The conduction gate control circuit  231   u _ 2 , which is newly added to the present modification, raises the conduction gate drive signal Gup_on 2  at time t 7  in the period of one cycle of the gate control signal Gup_s. The time t 7  indicates a point within the period between t 1  and t 3  for which the switching gate drive signal Gup_sw is raised to the High level along with the rise of the gate control signal Gup_s. Then, the conduction gate control circuit  231   u _ 2  drops the conduction gate drive signal Gup_on 2  at time t 8 . The time t 8  indicates a point within the period between t 4  and t 6  for which the switching gate drive signal Gup_sw is raised to the High level along with the drop of the gate control signal Gup_s. 
         [0082]    Here, either one of the conduction gate control signal Gup_on 1  or Gup_on 2  may be raised prior to the other as long as it is raised in the period between t 1  and t 3 , and they may be raised at the same time. Similarly, either one of the conduction gate control signal Gup_on 1  or Gup_on 2  may be dropped prior to the other as along as it is dropped in the period between t 4  and t 6 , and they may be dropped at the same time. 
         [0083]    With the structure of the present modification, the switching operation is performed by the switching device  210   u , and the conduction operation is performed by the switching devices  211   u  and  212   u . Thus, with the stated structure of the upper arm  21   u , as with the structure shown in  FIG. 3 , the location where switching loss occurs and the location where conduction loss occurs can be separated. 
         [0084]    Furthermore, since the same number of conduction gate control circuits as the switching devices which perform the conduction operation are provided and each switching device is driven by different one of the gate control circuits, the amount of current for the gate drive signal is smaller than in the structure shown in  FIG. 3 , and drive loss occurring in the upper arm-side gate drive circuit  21   u  is smaller. 
         [0085]    Also, since each switching device is driven by a different gate control circuit, each switching device can be controlled according to its properties, and this makes it easy to increase the amount of current and output power of the inverter. Moreover, since each switching device can be controlled according to its state, it is possible to control a high-temperature switching device to reduce losses in the device, for example. Specifically, by lowering the gate drive voltage applied to a high-temperature switching device, it is possible to reduce the output current from the switching device and reduce losses. 
       Modification 4 of Embodiment 1 
       [0086]    The following describes yet another modification of Embodiment 1. 
         [0087]      FIG. 10  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Modification 4 of Embodiment 1. The structure shown in this figure is different from the structure shown in  FIG. 3  in that the control terminals of the switching gate control circuit  230   u  and the switching device  210   u  are connected via a switching gate resistor  1000 , and the control terminals of the conduction gate control circuit  231   u  and the switching devices  211   u  and  212   u  are connected via a conduction gate resistor  1001 . 
         [0088]    Here, the resistance value of the switching gate resistor  1000  is smaller enough than the conduction gate resistor  1001 . 
         [0089]    With this structure, the electrical potential of the control terminal of the switching device  210   u  which performs the switching operation can be immediately changed to the potential for turning ON, and thus the structure realizes a switching operation with reduced switching losses. Furthermore, since the control terminals of the switching devices  211   u  and  212   u  which perform the conduction operation are prevented from receiving excessive electrical field stress, it is possible to improve the reliability of the switching devices  211   u  and  212   u . Furthermore, the amount of current for the gate drive signal can be reduced, and accordingly the drive loss occurring in the upper arm-side gate drive circuit  21   u  can be reduced. 
         [0090]    Furthermore, the present modification achieves an effect of suppressing malfunctions occurring when dv/dt is high. Since the switching devices which mainly perform the switching operation perform the switching operation at a higher speed than the switching devices which mainly perform the conduction operation, they have a smaller parasitic capacitance. Therefore, the switching devices which mainly perform the switching operation are susceptible to disturbance and often cause malfunction. 
         [0091]      FIG. 11  illustrates mechanism by which the lower arm  22   u  malfunctions due to turning on of the upper arm  21   u . Generally, the switching devices in the inverter alternately turn on the upper arm  21   u  and the lower arm  22   u  so that the upper arm  21   u  and the lower arm  22   u  do not cause short circuit, and a pause (usually called “dead time”) for which both the upper arm  21   u  and the lower arm  22   u  are turned off is interposed. When the upper arm  21   u  is turned on after the pause, a DC voltage Vdc from the battery  1  is applied between the drain and the source of the lower arm  22   u . At this moment, the parasitic capacitance  200  of the lower arm  22   u  is rapidly charged according to the switching speed of the upper arm  21   u , and current Ig flows via a gate resistor  1002  and the arm drive circuit  24   u  connected to the gate terminal of the lower arm  22   u . Due to the flow of the current Ig, potential difference occurs between the ends of the gate resistor  1002  according to the resistance value of the gate resistor  1002 . Here, when the parasitic capacitance between the drain and the gate of the lower arm  22   u  is Cgd and the gate resistor  1002  is Rg, the potential difference occurring between the ends of the gate resistor  1002 , namely potential difference Vgs between the gate and the source of the lower arm  22   u , can be represented by the following formula. 
         [0000]        Vgs=Rg×Cgd ×( dVdc/dt )
 
         [0092]    That is, Vgs increases as the resistance Rg of the gate resistor  1002  which determines the switching speed of the lower arm  22   u  increases, or the parasitic capacitance Cgd of the lower arm  22   u  increases, or the switching time dVdc/dt of the upper arm  21   u  increases, and accordingly the lower arm  22   u  will be likely to malfunction. 
         [0093]    The parasitic capacitance Cgd of the lower arm  22   u  is determined according to the internal structure of the lower arm  22   u , and cannot be changed freely. Also, if the switching time dVdc/dt of the upper arm  21   u  at turning on is decreased, the switching speed of the upper arm  21   u  decreases, and the switching loss increases. Thus, it is not preferable to change the switching time dVdc/dt. 
         [0094]    With the present modification, however, by reducing the gate resistor connected to the control terminal of the switching devices that perform the switching operation, high-speed switching is realized and the combined resistance between the gate terminal of the lower arm  22   u  and the arm drive circuit  24   u  is reduced. Thus, malfunctions due to high dv/dt can be suppressed. 
       Embodiment 2 
       [0095]      FIG. 12  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Embodiment 2 The structure shown in the figure is different form  FIG. 3  in that the upper arm-side gate drive circuit  23   u  is replaced with an upper arm-side gate drive circuit  33   u.    
         [0096]    The upper arm-side gate drive circuit  33   u  includes a turn-on gate control circuit  330   u , a conduction gate control circuit  331   u , and a turn-off gate control circuit  332   u . The upper arm  21   u  includes three switching devices  210   u ,  211   u  and  212   u . The turn-on gate drive circuit  330   u  is connected to the control terminal of the switching device  210   u , and the conduction gate control circuit  331   u  is connected to the control terminal of the switching device  211   u , and the turn-off gate drive circuit  332   u  is connected to the control terminal of the switching device  212   u . The switching devices  210   u ,  211   u  and  212   u  are connected in parallel, and their respective high potential-side primary terminals are connected to one another, and their respective low potential-side primary terminals are connected to one another. 
         [0097]    The gate control signal Gup_s output from the control circuit  4  is input to each of the turn-on gate control circuit  330   u , the conduction gate control circuit  331   u , and the turn-off gate control circuit  332   u . Based on the gate control signal Gup_s, the turn-on gate control circuit  330   u  outputs a turn-one gate drive signal Gup_tr to the control terminal of the switching device  210   u . Similarly, the conduction gate control circuit  331   u  outputs a conduction gate drive signal Gup_on to the control terminal of the switching device  211   u , and the turn-off gate control circuit  332   u  outputs a turn-off gate drive signal Gup_tf to the control terminal of the switching device  212   u.    
         [0098]    The following describes the details of the turn-on gate drive signal Gup_tr output by the turn-on gate control circuit  330   u , the conduction gate drive signal Gup_on output by the conduction gate control circuit  331   u , and the turn-off gate drive signal Gup_tf output by the turn-off gate control circuit  332   u , with reference to  FIG. 13 . 
         [0099]    The turn-on gate control circuit  330   u  raises the turn-on gate drive signal Gup_tr at time t 1 , which is when a rise is detected in the gate control signal Gup_s input by the control circuit  4 . Then, the turn-on gate control circuit  330   u  drops the turn-on gate drive signal Gup_tr within a shorter period than the first-potential period for which the electrical potential of the gate control signal Gup_s is at the High level. That is, the turn-on gate control circuit  330   u  drops the turn-on gate drive signal Gup_tr at time t 3 . 
         [0100]    The turn-off gate control circuit  332   u  raises the turn-off gate drive signal Gup_tf at time t 4 , which is when a drop is detected in the gate control signal Gup_s. Then, the turn-off gate control circuit  332   u  drops the switching gate drive signal Gup_tf within a shorter period than the second-potential period for which the electrical potential of the gate control signal Gup_s is at the Low level. That is, the turn-off gate control circuit  332   u  drops the switching gate drive signal Gup_tf at time t 6 . 
         [0101]    The conduction gate control circuit  331   u  raises the conduction gate drive signal Gup_on at time t 2 . The time t 2  is a point within the period between t 1  and t 3  for which the turn-on gate drive signal Gup_tr is raised to the High level. After that, the conduction gate control circuit  331   u  drops the conduction gate drive signal Gup_on at time t 5 . The time t 5  is a point within the period between t 4  and t 6  for which the turn-off gate drive signal Gup_tf is raised to the High level. 
         [0102]    Through the operations described above, each of the gate control circuits, namely the turn-on gate control circuit  330   u , the conduction gate control circuit  331   u  and the turn-off gate control circuit  332   u , outputs, as a gate drive signal, square wave once within the period of one cycle of the gate control signal Gup_s. 
         [0103]    With such operations, turning on in switching is performed by the switching device  210   u , conduction is performed by the switching device  211   u  and turning off in switching is performed by the switching device  212   u . As a result, losses occurring in the upper arm  21   u  can be separated into turn-on losses, conduction losses, and turn-off losses. Therefore, switching loss (the sum of turn-on loss and turn-off loss) and conduction loss, which conventionally have a trade-off relationship, can be reduced separately. That is, both of them can be reduced, and this realizes a high-efficiency inverter. 
         [0104]    Note that the switching devices which constitute the upper arm and are connected in parallel do not necessarily have the same threshold voltage. Generally, threshold voltage has a relationship with the on-resistance (a characteristic of a switching a device, which is a primary cause of conduction loss), and on-resistance decreases as threshold voltage decreases. Therefore, even more efficient inverter can be realized by making up the upper arm from switching devices having different threshold voltages according to their respective operations. 
         [0105]    Specifically, in comparison between a switching device performing the turn-on operation and a switching device performing the conduction operation, it is preferable that the switching device performing the conduction operation has a lower threshold voltage than the switching device performing the turn-on operation. With such a structure, the conduction operation will be performed by a switching device having a low on-resistance. 
         [0106]    In comparison between a switching device performing the turn-on operation and a switching device performing the turn-off operation, it is preferable that the switching device performing the turn-off operation has a higher threshold voltage than the switching device performing the turn-on operation. With such a structure, when turning on is performed, the switching device operates at a relatively low threshold voltage, and this increases the speed of the turning on. Also, when turning off is performed, the switching device operates at a relatively high threshold voltage, and this increases the speed of the turning off. Therefore, switching loss can be further reduced. 
         [0107]    Switching devices are more likely to malfunction when performing the switching operation than when performing the conduction operation, and are also more likely to malfunction when the potential difference between the on-voltage and the off-voltage of the switching device is smaller. Hence, in the structure shown in  FIG. 3 , it is preferable that the potential difference between the on-voltage and the off-voltage output by the switching gate control circuit  230   u  is set to be larger than the potential difference between the on-voltage and the off-voltage output by the conduction gate control circuit  231   u . With such a structure, malfunctions in switching can be suppressed. In the case of the structures shown in  FIG. 12 , it is preferable that the potential difference between the on-voltage and the off-voltage increases in the following order: the turn-on gate control circuit  330   u , the conduction gate control circuit  331   u  and the turn-off gate control circuit  332   u . As the on/off potential difference of the turn-on gate control circuit  330   u  is large, the switching device is unlikely to malfunction, and as the on/off potential difference of the turn-off gate control circuit  332   u  is small, the turning on can be performed at high speed. 
       Embodiment 3 
       [0108]      FIG. 14  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Embodiment 3 
         [0109]    The upper arm-side gate drive circuit  53   u  includes a switching gate control circuit  430   u , a conduction gate control circuit  431   u , a primary voltage detection circuit  432   u , a first primary voltage determination circuit  533   u , and a second primary voltage determination circuit  534   u . The upper arm  21   u  includes three switching devices  210   u ,  211   u  and  212   u.    
         [0110]    The switching gate drive circuit  430   u  is connected to the control terminal of the switching device  210   u , and the conduction gate control circuit  431   u  is connected to the respective control terminals of the switching devices  211   u  and  212   u . The switching devices  210   u ,  211   u  and  212   u  are connected in parallel, and their respective high potential-side primary terminals are connected to one another, and their respective low potential-side primary terminals are connected to one another. 
         [0111]    The upper arm-side gate drive circuit  53   u  includes the primary voltage detection circuit  432   u , the first primary voltage determination circuit  533   u  and the second primary voltage determination circuit  534   u  in addition to the components of the upper arm-side gate drive circuit  23   u  shown in  FIG. 3 . The following describes specific operations of only the primary voltage detection circuit  432   u , the first primary voltage determination circuit  533   u  and the second primary voltage determination circuit  534   u , which are different from the structure shown in  FIG. 3 . 
         [0112]    The primary voltage detection circuit  432   u  detects the primary voltage, which is the voltage between the primary terminals of the upper arm  21   u , and outputs a detection signal corresponding to the primary voltage of the upper arm to the first primary voltage determination circuit  533   u  and the second primary voltage determination circuit  534   u.    
         [0113]    The first primary voltage determination circuit  533   u , upon receipt of the detection signal, compares the input signal with a predetermined voltage Vt 1 , and when it is lower than the predetermined voltage Vt 1 , the first primary voltage determination circuit  533   u  outputs a first determination signal to the conduction gate control circuit. Here, the predetermined voltage Vt 1  is, as shown in  FIG. 15 , the voltage under that condition that the switching operation performed by the switching device  210   u  according to the switching gate drive signal Gup_sw has been completed. Specifically, Vt 1  is the on-voltage of the switching device  210   u  performing the switching operation. 
         [0114]    With such a structure, the switching device  210   u  performing the switching operation detects that the switching has been completed, and the switching devices  211   u  and  212   u  performing the conduction operation are turned on. Thus, the period between the completion of the switching performed by the switching device  210   u  and the start of the conduction can be shortened. Therefore, this structure realizes quick switching between the operations, without putting excessive load on the switching device  210   u  performing the switching operation. 
         [0115]    The second primary voltage determination circuit  534   u , upon receipt of the detection signal, compares the input signal with a predetermined voltage Vt 2 , and when it is lower than the predetermined voltage Vt 2 , the second primary voltage determination circuit  534   u  outputs a second determination signal to the switching gate control circuit. Here, the predetermined voltage Vt 2  is, as shown in  FIG. 15 , the voltage under the condition that both the switching operation performed by the switching device  210   u  according to the switching gate drive signal Gup_sw and the conduction operation performed by the switching devices  211   u  and  212   u  according to the conduction gate drive signal Gup_on have been completed. Specifically, Vt 2  is the on-voltage at which the switching device performing the switching operation and the switching devices performing the conduction operation are all turned on. The second primary voltage determination circuit  534   u  therefore functions as a switching completion primary voltage determination circuit which determines whether the voltage has the value at which the conduction of the switching device  210   u  that performs the switching operation is to be stopped. 
         [0116]    With this structure, the switching device  210   u  performing the switching operation is turned off at detection of the completion of the conduction operation performed by the switching devices  211   u  and  212   u  performing the conduction operation. Therefore, the structure realizes quick switching between the operations, without putting excessive load on the switching device  210   u  performing the switching operation. 
       Modification of Embodiment 3 
       [0117]    In Embodiment 3 described above, the gate control is performed by using the voltage between the primary terminals of the switching devices. Alternatively, it is possible to use the primary current flowing between the primary terminals of the switching devices. 
         [0118]      FIG. 16  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Modification of Embodiment 3. 
         [0119]    The upper arm-side gate drive circuit  63   u  includes a switching gate control circuit  430   u , a conduction gate control circuit  431   u , a switching current detection circuit (SW current detection circuit)  630   u  and a switching current determination circuit (SW current determination circuit  631   u ). The upper arm  21   u  includes three switching devices  210   u ,  211   u  and  212   u . The switching gate drive circuit  430   u  is connected to the control terminal of the switching device  210   u , and the conduction gate control circuit  431   u  is connected to the respective control terminals of the switching devices  211   u  and  212   u . The switching devices  210   u ,  211   u  and  212   u  are connected in parallel, and their respective high potential-side primary terminals are connected to one another, and their respective low potential-side primary terminals are connected to one another. 
         [0120]    The upper arm-side gate drive circuit  63   u  includes the switching current detection circuit  630   u  and the switching current determination circuit  631   u  in addition to the components of the upper arm-side gate drive circuit  23   u . The following describes specific operations of only the switching current detection circuit  630   u  and the switching current determination circuit  631   u.    
         [0121]    The switching current detection circuit  630   u  detects the primary current of the switching device  21   u  performing the switching operation, and outputs a detection signal corresponding to the value of the primary current of the switching device  210   u  to the switching current determination circuit  631   u . The detection signal can be easily handled if it is generated by converting the amount of the current to a voltage value, for example. 
         [0122]    The switching current determination circuit  631   u , upon receipt of the detection signal, compares the input with a predetermined voltage, and when it is higher than the predetermined voltage, the switching current determination circuit  631   u  outputs a determination signal to the conduction gate control circuit. Here, the predetermined voltage is the voltage at the end of the switching. Specifically, the predetermine voltage depends on the amount of the current to be applied by the upper arm. 
         [0123]    With this structure, the switching devices  211   u  and  212   u  performing the conduction operation is turned on at detection of the completion of the switching operation performed by the switching device  210   u  performing the switching operation. Therefore, the structure realizes quick switching between the operations, without putting excessive load on the switching device  210   u  performing the switching operation. 
       Embodiment 4 
       [0124]      FIG. 17  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Embodiment 4. 
         [0125]    The upper arm-side gate drive circuit  73   u  includes a first gate control circuit  730   u , a second gate control circuit  731   u , a third gate control circuit  732   u , a temperature detection circuit  733   u  and a temperature determination circuit  734   u . The upper arm  21   u  includes three switching devices  210   u ,  211   u  and  212   u . The first gate drive circuit  730   u  is connected to the control terminal of the switching device  210   u . The second gate control circuit  731   u  is connected to the control terminal of the switching device  211   u . The third gate control circuit  732   u  is connected to the control terminal of the switching device  212   u . The switching devices  210   u ,  211   u  and  212   u  are connected in parallel, and their respective high potential-side primary terminals are connected to one another, and their respective low potential-side primary terminals are connected to one another. 
         [0126]    The following explains specific operations. The temperature detection circuit  733   u  detects the temperatures of the switching device  210   u ,  211   u  and  212   u  (specifically, the junction temperatures of the chips constituting the switching devices), and outputs detection signals respectively corresponding to the temperatures of the switching devices  210   u ,  211   u  and  212   u  to the temperature determination circuit  734   u.    
         [0127]    The temperature determination circuit  734   u , upon receipt of the detection signals, compares the temperatures of the switching devices  210   u ,  211   u  and  212   u . The temperature determination circuit  734   u  causes the gate control circuit connected to the control terminal of the switching device with the lowest temperature to perform the same operation as the switching gate control circuit  230   u  described above with reference to  FIG. 3 . With respect to the gate control circuits connected to the control terminals of the other switching devices, the temperature determination circuit  734   u  causes them to perform the same operation as the conduction gate control circuit  231   u  described above with reference to  FIG. 3 . 
         [0128]    It is generally known that the values of avalanche resistance and short circuit capacity, which indicate the reliability of a switching device, are significantly decreased with increased temperature of the switching device. 
         [0129]    Hence, as described above, the switching device with the lowest temperature among the switching devices  210   u ,  211   u , and  212   u  included in the upper arm  21   u  is caused to perform the switching operation, so that the avalanche resistance and the short circuit capacity of the upper arm  21   u  can be maintained at relatively high levels. This leads to an inverter with high efficiency and high efficiency. 
         [0130]    Note that when the switching devices connected in parallel have the same electrical properties, the switching devices show almost no difference in temperature rise due to the switching. Hence, if this is the case, the switching devices perform the switching in turn without considering the temperatures. 
         [0131]    With such a structure, the number of switching operations performed by each of the switching devices connected in parallel can be averaged, and this prevents switching load from being concentrated to a particular switching device. Therefore, the stated structure contributes to extension of the life spans of the switching devices. 
       Embodiment 5 
       [0132]      FIG. 18  shows the entire structure of a synchronous motor drive system using an inverter pertaining to Embodiment 5. The following explains an example in which MOSFETs are used as the switching devices. 
         [0133]    An inverter  2  pertaining to the present embodiment includes the same number of legs as the outputs of AC power, namely legs  25   u ,  25   v  and  25   w . The legs  25   u ,  25   v  and  25   w  respectively include: upper arms  21   u ,  21   v  and  21   w  and lower arms  22   um    22   v  and  22   w , which are connected in series between the positive terminal and the negative terminal of the batter  1  (the upper arms are connected to the positive side, and the lower arms are connected to the negative side); capacitors  101   u ,  101   v ,  101   w ,  102   u ,  102   v  and  102   w , which are connected to the upper arms  21   u ,  21   v  and  21   w  and the lower arms  22   u ,  22   v  and  22   w ; and upper arm-side gate drive circuits (upper arm drive circuits)  23   u ,  23   v  and  23   w  and lower arm-side gate drive circuits (lower arm drive circuits)  24   u ,  24   v  and  24   w , which correspond to the upper arms  21   u ,  21   v  and  21   w  and the lower arms  22   u ,  22   v  and  22   w    
         [0134]    The capacitors  101   u ,  101   v ,  101   w ,  102   u ,  102   v  and  102   w  are respectively connected in parallel to and are respectively adjacent to the upper arms  21   u ,  21   v  and  21   w  and the lower arms  22   u ,  22   v  and  22   w.    
         [0135]      FIG. 19  schematically shows a positional relationship between the switching devices included in the upper arm  21   u  and the capacitor  101   u . The upper arm  21   u  and the capacitor  101   u  are adjacent to each other. In particular, among the switching devices  210   u ,  211   u  and  212   u  included in the upper arm  21   u , the switching device  210   u  performing the switching operation is the closest to the capacitor  101   u.    
         [0136]    With this structure, considering the wiring inductance of each of the capacitor  101   u  and the switching devices  210   u ,  211   u  and  212   u , the wiring between the capacitor  101   u  and the switching device  210   u  performing the switching operation can be the shortest. The structure therefore suppresses surge voltage, which occurs when the switching device  210   u  performs the switching operation at high speed. This leads to an inverter with high efficiency and high efficiency. 
       Embodiment 6 
       [0137]      FIG. 20  shows detailed structures of a gate control circuit and an arm of an inverter pertaining to Embodiment 6. 
         [0138]    In the present embodiment, an upper arm-side gate drive circuit  83   u  includes a switching gate control circuit  830   u , a conduction gate control circuit  831   u , a low-current gate control circuit  832   u , and a current-instruction determination circuit  833   u . The upper arm  41   u  includes switching devices  410   u ,  411   u  and  412   u . The switching device  410   u  is a MOSFET, and the switching devices  411   u  and  412   u  are IGBTs. 
         [0139]    The switching gate control circuit  830   u  and the conduction gate control circuit  831   u  have the same structure as the switching gate control circuit  230   u  and the conduction gate control circuit  231   u  shown in  FIG. 3 . 
         [0140]    The low-current gate control circuit  832   u  a low-current gate drive signal Gup_low, which have the same waveform as the gate control signal Gup_s output by the control circuit  4 , to the control terminal of the switching device  410   u.    
         [0141]    The current-instruction determination circuit  833   u  monitors a current-instruction signal Is_u input to the control circuit  4 . When the current-instruction signal Is_u indicates that the inverter is instructed to drive with current falling within a rage where MOSFETs require a lower voltage than IGBTs to obtain a desired current value, the current-instruction determination circuit  833   u  instructs the low-current gate control circuit  832   u  to output a gate drive signal. When the current-instruction signal Is_u indicates that the inverter is instructed to drive with current falling within a range where IGBTs require a lower voltage than MOSFETs to obtain a desired current value, the current-instruction determination circuit  833   u  instructs the switching gate control circuit  830   u  and the conduction gate control circuit  831   u  to output a gate drive signal. 
         [0142]    Generally, voltage-current characteristics in saturation regions of IGBT and MOSFET have the relationship as shown in  FIG. 21 . Within the first range shown in the drawing, MOSFET requires lower voltage than IGBT to obtain a desired current value. Therefore, when the same current is applied, the loss is lower in MOSFET. In contrast, within the second range, IGBT requires lower voltage than MOSFET to obtain a desired current value. Therefore, the loss is lower in IGBT. 
         [0143]    With the stated structure, as with Embodiment 1, the switching loss and the conduction loss occurring while the inverter operates with a current falling within the second range shown in  FIG. 21  can be separated according to the gate drive signals output by the switching gate control circuit  830   u  and the conduction gate control circuit  831   u . On the other hand, with a current falling within the first range shown in  FIG. 21 , the inverter operates only with the switching devices made up from MOSFETs with lower losses, according to the low-current gate drive signal Gup_low output by the low-current gate control circuit  832   u.    
       Other Modifications 
       [0144]    The inverter pertaining to the present invention has been described above based on Embodiments. However, the present invention is not limited to Embodiments. For example, the following modifications may be applied. 
         [0000]    (1) In each embodiment, a three-phase inverter is taken as an example. However, the present invention is applicable to any power converter in which arms are composed of a plurality of switching devices connected in parallel. 
         [0145]    For example, the present invention is applicable to DC/DC converters including arms composed of a plurality of switching devices. 
         [0146]      FIG. 22  is a modification of the synchronous motor drive system shown in  FIG. 1  in which a DC/DC converter  8  is added. 
         [0147]    The DC/DC converter  8  includes an arm  81 , an arm  82 , a gate drive circuit  83 , a gate drive circuit  84  and an inductor  85 . The gate drive circuit  83  and the gate drive circuit  84  correspond to the arm  81  and the arm  82 , respectively. 
         [0148]      FIG. 23  shows the details of the arm  81  and the gate drive circuit  83  depicted in  FIG. 22 . The gate drive circuit  83  includes a switching gate control circuit  830  and a conduction gate control circuit  831 . The arm  81  includes three switching devices  810 ,  811  and  812 . The switching gate drive circuit  830  is connected to the control terminal of the switching device  810 , and the conduction gate control circuit  831  is connected to the respective control terminals of the switching devices  811  and  812 . The switching devices  810 ,  811  and  812  are connected in parallel, and their respective high electrical potential-side primary terminals are connected to one another, and their respective low electrical potential-side primary terminals are connected to one another. The arm  82  and the gate drive circuit  84  have similar structures to the structures shown in  FIG. 23 . 
         [0149]    In the DC/DC converter  8  having such a structure, the switching gate control circuit  830  outputs a switching gate drive signal similar to the example shown in  FIG. 4 , and the conduction gate control circuit  831  outputs a conduction gate drive signal similar to the example shown in  FIG. 4 . Thus, as with the embodiments of the inverter  2  described above, the locations where the switching loss and the conduction loss occur in the DC/DC converter  8  can be separated between the switching devices performing the switching operation and the switching devices performing the conduction operation. 
         [0000]    (2) All or some of the switching devices constituting the arms may be made up from wide bandgap semiconductors such as a SiC (silicon carbide) device and a GaN (gallium nitride) device. 
         [0150]    SiC devices and GaN devices have higher breakdown resistance than Si devices, and lead to reduction in chip size. Therefore, such devices can reduce the size and the cost of the inverter, and furthermore improve the reliability. 
         [0151]    Although the amount of loss occurring in these devices is smaller than Si devices, they cost higher. Thus, instead of using SiC devices or GaN devices for all the switching devices, they may be used for only the switching devices that contribute to the reduction of the loss. 
         [0152]    For example, in a system that is largely affected by the switching loss rather than the conduction loss, the switching device  210   u  shown in  FIG. 3 , which primarily performs the switching operation, may be replaced with a SiC device or a GaN device. Similarly, in a system that is largely affected by the conduction loss rather than the switching loss, each of the switching devices  211   u  and  212   u  shown in  FIG. 3 , which primarily perform the conduction operation, may be replaced with a SiC device or a GaN device. Such a structure further reduces the loss while suppressing the cost. 
         [0000]    (3) Any of Embodiments and Modifications may be combined with one another. 
       INDUSTRIAL APPLICABILITY 
       [0153]    The present invention realizes a high efficiency inverter. Furthermore, the present invention further improves the efficiency and the reliability by appropriately and individually controlling switching devices connected in parallel. Therefore, the present invention is applicable to, for example, motor drive systems that are required to achieve high efficiency, including hybrid electric vehicles and electric vehicles, fuel-cell electric vehicles, electrically-driven compressors, electrically-driven power steering systems and elevators, and to power generation systems that are also required to achieve high efficiency, such as wind power generators. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1 : battery 
               2 : invertor 
               3 : motor 
               4 : control circuit 
               8 : DC/DC converter 
               21   u - 21   w ,  31   u : upper arm 
               22   u - 22   w : lower arm 
               23   u - 23   w ,  33   u ,  53   u ,  63   u ,  73   u ,  83   u : upper arm-side gate drive circuit 
               24   u - 24   w : lower arm-side gate drive circuit 
               25   u - 25   w : leg 
               81 ,  82 : arm 
               83 ,  84 : gate drive circuit 
               85 : inductor 
               101   u ,  101   y ,  101   w ,  102   u ,  102   v ,  102   w : capacitor 
               200 : parasitic capacitance 
               210   u ,  211   u ,  212   u ,  310   u ,  311   u ,  312   u ,  410   u ,  411   u ,  412   u : switching device 
               230   u ,  430   u ,  830   u : switching gate control circuit 
               231   u ,  431   u ,  831   u : conduction gate control circuit 
               231   u _ 1 : conduction gate control circuit  1   
               231   u _ 2 : conduction gate control circuit  2   
               330 : turn-on gate control circuit 
               331   u : conduction gate control circuit 
               332   u : turn-off gate control circuit 
               432   u : primary voltage detection circuit 
               433   u : primary voltage determination circuit 
               533   u : first primary voltage determination circuit 
               534   u : second primary voltage determination circuit 
               630   u : switching current detection circuit 
               631   u : switching current determination circuit 
               730   u : first gate control circuit 
               731   u : second gate control circuit 
               732   u : third gate control circuit 
               733   u : temperature detection circuit 
               734   u : temperature determination circuit 
               830 : switching gate control circuit 
               831 : conduction gate control circuit 
               810 ,  811 ,  812 : switching device 
               832   u : low-current gate control circuit 
               833   u : current-instruction determination circuit 
               1000 : switching gate resistor 
               1001 : conduction gate resistor 
               1002 : gate resistor of lower arm  22   u