Patent Publication Number: US-2007103951-A1

Title: Power converter apparatus

Description:
BACKGROUND OF THE INVENTION  
      This invention relates to a power conversion apparatus comprising a smoothing capacitor for smoothing rectified voltage, a power module containing a plurality of switching devices operable at high temperatures, and a drive circuit for controlling the turn-on and turn-off of the switching devices.  
      The upper limit of temperature at which the IGBT module used in an IGBT inverter incorporating silicon (Si) semiconductor devices therein can operate reliably, is around 125 degrees centigrade (125° C.). As compared with such a silicon semiconductor device, power semiconductor devices using SiC (silicon carbide), GaN (gallium nitride) or diamond as their semiconductor substrates are known as operable at temperatures higher than the temperature upper limit for the silicon semiconductor device. The Japanese patent document, JP-A-10-294471 (paragraphs [0015] through [0018]), discloses a junction type SiC transistor having no gate oxide layer. Since this junction type SiC transistor is a power device which does not uses a gate oxide layer, it can be operated at relatively higher temperatures. On the other hand, around 125 degrees centigrade (125° C.) is the upper limit of temperature at which such parts incorporated in the inverter as the smoothing capacitor for smoothing the rectified voltage or the parts (power transformer and photo-coupler) of the drive circuit for controlling the turn-on and turn-off of switching devices, can operate reliably. If those parts having the temperature upper limit of around 125 degrees centigrade (125° C.) are located within the housing which encases therein power semiconductor devices using SiC, GaN or diamond as their substrates, then the parts may be exposed to temperatures far exceeding the upper limit.  
      The Japanese patent document, JP-2004-350360 (paragraphs [0022] through [0030]), discloses the provision of the power conversion apparatus wherein the cooling mechanism is simplified by using semiconductor devices having a range of operating temperatures higher than the operating temperatures for ordinary silicon devices, and the parts layout is designed in such a manner that the conduction of heat from the power conversion area to the control area is reduced by separating the former from the latter.  
     SUMMARY OF THE INVENTION  
      SiC devices have an advantage over ordinary Si devices since the former can reliably operate at higher temperatures than the latter. Also, SiC devices are mainly of unipolar type to which junction type SiC devices and MOSFET-SiC devices belong. Unipolar devices are characterized by their very high switching speed. The very high switching speed leads advantageously to very low switching loss (very low power loss in turn-on or turn-off).  
      However, the unipolar devices, too, have a disadvantage that the surge voltages generated due to the switching action of the inverter are superposed on the output voltage of the inverter. Therefore, the surge voltages are applied to the motor terminals, too. If the surge voltages are high enough, they adversely affect the insulation of the motor windings, thereby degrading the insulation.  FIG. 2  graphically shows the relationship between the length of the wiring cables from the output terminals of the inverter to the motor terminals and the magnitude of the surge voltage, with the switching speed (rise time in turn-on: tr) varied as parameter. This result has been borrowed from the Journal of the Institute of Electrical Engineers of Japan, Vol. 107, Nov. 7, 1987. While the rise time in turn-on is 0.1˜0.3 μS for IGBT devices using conventional Si switching devices, the corresponding rise time for unipolar type SiC devices is less than 0.1 μS. Thus, the latter is faster than the former in switching. With this improved devices, therefore, the inverter and its load, i.e. motor, must be located close to each other.  
      The object of this invention is to provide an inverter apparatus which secures the reliability of motor winding insulation, with which a simple cooling mechanism can be used, and which can operate at relatively higher temperatures.  
      According to the inverter apparatus embodying this invention, the power module containing a plurality of switching devices operable at high temperatures is cooled by being attached to the housing for the transmission, the engine or the motor. Also, the inverter apparatus is provided with a soft switching circuit or a snubber circuit for suppressing surge voltages generated by the main inverter circuit.  
      According to this invention, the inverter can be operated at high temperatures while high reliability of motor winding insulation is being secured, and further the size of the inverter itself can be reduced.  
      Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  schematically shows an inverter as a first embodiment of this invention, applied to a gasoline engine system used on an automobile;  
       FIG. 2  graphically shows the relationship between wiring conductor length vs. surge voltage, with switching speed varied as parameter;  
       FIG. 3  is a circuit diagram of an inverter as a second embodiment of this invention;  
       FIG. 4  is a circuit diagram of an inverter as second embodiment of this invention, wherein the soft switching circuit incorporated therein is depicted in detail;  
       FIG. 5  is a circuit diagram of an inverter as a third embodiment of this invention;  
       FIG. 6  is a circuit diagram of an inverter as a fourth embodiment of this invention;  
       FIG. 7  is a circuit diagram of an inverter as a fifth embodiment of this invention;  
       FIG. 8  schematically shows an inverter as a sixth embodiment of this invention, applied to a gasoline engine system used on an automobile; and  
       FIG. 9  schematically shows an inverter as a seventh embodiment of this invention, applied to a gasoline engine system used on an automobile; 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
      Embodiments of this invention will now be described in reference to the attached drawings.  
     Embodiment 1  
       FIG. 1  shows the structure of a system to which an inverter as a first embodiment of this invention is applied. In  FIG. 1 , reference numeral  31  indicates an engine as a prime mover which is, for example, a water-cooled internal combustion engine such as a gasoline engine. A starter  32  serves to start the engine  31 . The engine  31  has an air intake pipe in which an electronically controlled throttle  33  is installed to control the intake air flow. The fuel injector injects amount of fuel which suitably corresponds to the intake air flow. The signal representing the air-to-fuel ratio defined on the basis of the intake air flow and the amount of the fuel to be injected, and the signal representing the rotational speed of the engine, determine the ignition timing at which the ignition module causes the spark plugs to be fired.  
      A transmission  41  is provided with an input shaft  42  and an output shaft  43 . The input shaft  42  of the transmission  41  is furnished with mesh type gears  44 , gears  45  and a hub sleeve  46 . The gears  45  are fixedly mounted on the input shaft  42  while the mesh type gears  44  are so mounted on the input shaft  42  as not to move in the axial direction of the input shaft  42 . The hub sleeve  46  is mechanically coupled to the input shaft  42  by an engaging mechanism which can move in the axial direction of the input shaft  42  but which is restrained in the rotation about the input shaft  42 . The output shaft  43  of the transmission  41  is furnished with mesh type gears  44 , gears  45  and hub sleeves  46 . The gears  45  are fixedly mounted on the output shaft  43  while the mesh type gears  44  are so mounted on the output shaft  43  as not to move in the axial direction of the output shaft  43 . The hub sleeves  46  are mechanically coupled to the output shaft  43  by an engaging mechanism which can move in the axial direction of the output shaft  42  but which is restrained in the rotation about the output shaft  43 . The gears on the input shaft  42  are engageable with the gears on the output shaft  43 , and when the torque. generated by the engine is transmitted from the input shaft  42  to the output shaft  43 , different transmission ratios can be achieved. Those ratios correspond to, for example, first speed gear through fifth speed gear and reverse gear.  
      A clutch  35  is interposed between the input shaft  42  and the crank shaft  34  of the engine  31 . The engagement of the clutch  35  causes the driving force generated by the engine  31  to be transmitted from the crank shaft  35  to the input shaft  42 . The disengagement of the clutch  35 , on the other hand, breaks off the transmission of the driving force being transmitted from the engine  31  to the input shaft  42 . This type of clutch  35  is widely used on various automobiles on which gasoline engines are installed. As the clutch  35  is engaged gradually, the automobile can be started. The same effect can be obtained if a torque converter is interposed between the engine  31  and the transmission  41 . The output shaft  43  of the transmission  41  is provided with a final gear  36 , and the final gear  36  is mechanically coupled to wheels  37  by a driving axle  38 .  
      The output shaft  52  of a motor  51  has a gear  53  mounted fixedly thereon. The gear  53  is engaged with one of the gears  45  mounted on the input shaft  42  of the transmission  41 . With this structure, the torque generated by the motor  51  can be transmitted to the input shaft  42 . This motor  51  is an AC motor driven by a variable-voltage, variable-frequency, three-phase electric power.  
      In this embodiment, a power module  11 , which incorporates therein a plurality of power semiconductor switching devices operable at high temperatures, is attached to the housing of the transmission  41  in contact with the outer surface thereof. The housing is filled with oil, and through the circulation of the oil is cooled the power module  11  which contains the power semiconductor switching devices operable at high temperatures. Moreover, since the power module  11  containing the power semiconductor switching devices operable at high temperatures is located near the motor  51 , the wiring conductors  12  connecting the motor  51  with the power module  11  should be made short so that the surge voltages developed across the terminals of the motor  51  can be suppressed to a low level. Consequently, the high insulation of the motor windings can be secured. A smoothing capacitor  14  for smoothing rectified voltage and wiring conductors  15  connecting the smoothing capacitor  14  with the power module  11 , constitute the main circuit for the power module  11 . If the wiring conductors  15  between the smoothing capacitor  14  and the power module  11  is long, a surge voltage (ΔV) as given by the following expression (1) is generated at the time of switching taking place in the power module  11 . Therefore, the length of the wiring conductors  15  should be made as short as possible. 
 
ΔV= L ( di/dt )  (1)
 
 where L indicates the inductance of the wiring conductors  15  between the smoothing capacitor  14  and the power module  11 , and di/dt represents the change in current taking place when each of the power switching devices turns off. 
 
      Power semiconductor switching devices using SiC (silicon carbide), GaN (gallium nitride) or diamond, all of which are high temperature-resistive semiconductor materials, as their semiconductor substrates should preferably be used as the power semiconductor switching devices operable at high temperatures, contained in the power module  11 . With these semiconductor materials, SiC (silicon carbide), GaN (gallium nitride) and diamond, the band gap energy is greater than that of Si (silicon), i.e. 2 eV (electron volts). In order to secure a highly reliable operation at high temperatures, a junction type transistor made of SiC which uses no gate oxide layer is most preferably recommended of all these wide band gap semiconductors.  
      The drive circuit for controlling the on/off operation of the power module  11  containing the power semiconductor switching devices operable at high temperatures, comprises such electronic parts as a control circuit PCB  22 , a resistor  23 , a capacitor  24  and a driver IC  25 . Control signals and driving signals for the power module  11  containing the power semiconductor switching devices operable at high temperatures, are transmitted through wiring conductors  21  connecting the control circuit PCB  22  with the power module  11 . As described above, according to this embodiment, the power module  11  is cooled by putting itself in contact with the housing of the transmission  41  while the drive circuit is located at a place which is separate from a high temperature zone and in a moderate temperature condition.  
      Thus, with this structure described above, this embodiment enables an inverter to be operated at high temperatures while highly reliable insulation of the windings of the motor driven by the inverter can be secured with the employment of a simple cooling system, with the result that the size of the inverter can be reduced.  
     Embodiment 2  
       FIG. 3  is a circuit diagram of an inverter as a second embodiment of this invention. In  FIG. 3 , components equivalent to those shown with the first embodiment are indicated by the same reference numerals as in  FIG. 1 . A power module  11  containing a plurality of power semiconductor switching devices operable at high temperatures includes power semiconductor switching devices  61  operable at high temperatures. While the length of wiring conductors  12  between the power module  11  and a motor  51  is kept short, the length of wiring conductors  21  between the power module  11  and a capacitor  14  for smoothing a rectified voltage is left relatively long. Consequently, there is generated a surge voltage (ΔV) as given by the above expression (1), equated to the product of the inductance L of the wiring conductors  12  and the current reduction rate (di/dt) associated with the turning-off of the power semiconductor switching device  61 .  
      In this second embodiment of the invention is provided a soft switching circuit  71  for suppressing this surge voltage. The soft switching circuit  71  comprises a switching device  73  for soft switching at high operating temperatures and a soft switching control circuit  72 .  
       FIG. 4  shows the detail of the soft switching circuit  71  incorporated as a part in the power module  11  shown in  FIG. 3 . The soft switching circuit  71  comprises a switching device  76  for soft switching at high operating temperatures, resistors  74  and a capacitor  75 . The switching device  76  for soft switching at high operating temperatures turns on in timing with the turn-off of the power semiconductor switching devices  61  so that the magnitude of the surge voltage is rendered low which is given by the product of the inductance L of the wiring conductors and the current reduction rate di/dt associated with the turn-off of the power semiconductor switching devices  61  operable at high temperatures. Thereafter, the switching device  76  for soft switching is softly turned off.  
      The provision of the soft switching circuit  71  enables the magnitude of the surge voltage to be rendered low which is given by the product of the inductance. L of the wiring conductors and the current reduction rate di/dt associated with the turn-off of the power semiconductor switching devices  61 , and also the length of the wiring conductors  12  between the power module  11  and the motor  51  to be reduced, with the result that the surge voltages developed across the terminals of the motor  51  can be suppressed to a low level. With this embodiment, too, highly reliable insulation of the motor windings can be secured.  
     Embodiment 3  
       FIG. 5  is a circuit diagram of an inverter as a third embodiment of this invention. In  FIG. 5 , again, components equivalent to those shown with the first and the second embodiments are indicated by the same reference numerals as in  FIGS. 1 and 3 . A power module  11  containing a plurality of power semiconductor switching devices operable at high temperatures, which is attached to the housing of the transmission in contact with the outer surface thereof, includes power semiconductor switching devices  61  operable at high temperatures. In this third embodiment of the invention, a snubber capacitor  81  is provided to suppress the surge voltage. Since the snubber capacitor  81  is located near or mounted within the power module  11 , the surge voltage can be effectively suppressed. A ceramic capacitor or a film capacitor which has high resistances to high temperatures and vibrations should preferably be used as the snubber capacitor  81 .  
      The provision of the snubber capacitor  81  enables the magnitude of the surge voltage to be rendered low which is given by the product of the inductance L of the wiring conductors and the current reduction rate di/dt associated with the turn-off of the power semiconductor switching devices  61 , and the length of the wiring conductors  12  between the power. module  11  and the motor  51  to be reduced, with the result that the surge voltages developed across the terminals of the motor  51  can be suppressed to a low level. Accordingly, highly reliable insulation of the motor windings can be secured.  
     Embodiment 4  
       FIG. 6  is a circuit diagram of an inverter as a fourth embodiment of this invention. In  FIG. 6 , components equivalent to those shown with the first through third embodiments are indicated by the same reference numerals as in  FIGS. 1, 3  and  5 . In this embodiment, too, a power module  11  containing a plurality of power semiconductor switching devices operable at high temperatures, which is attached to the housing of the transmission in contact with the outer surface thereof, includes power semiconductor switching devices  61  operable at high temperatures. In this fourth embodiment of the invention, a snubber capacitor  81  and a snubber resistor  82  connected in series with the snubber capacitor  81  are provided to suppress the surge voltage. Since the snubber capacitor  81  and the snubber resistor  82  are located near or mounted within the power module  11 , the surge voltage can be effectively suppressed. The snubber capacitor  81  and the snubber resistor  82  used in this embodiment should preferably have high resistance to both high temperatures and vibrations.  
      The provision of the snubber capacitor  81  and the snubber resistor  82  enables the magnitude of the surge voltage to be rendered low which is given by the product of the inductance L of the wiring conductors and the current reduction rate di/dt associated with the turn-off of the power semiconductor switching devices  61 , and also the length of the wiring conductors  12  between the power module  11  and the motor  51  to be reduced, with the result that the surge voltages developed across the terminals of the motor  51  can be suppressed to a low level. Accordingly, highly reliable insulation of the motor windings can be secured.  
     Embodiment 5  
       FIG. 7  is a circuit diagram of an inverter as a fifth embodiment of this invention. In  FIG. 7 , components equivalent to those shown with the first through fourth embodiments are indicated by the same reference numerals as in  FIGS. 1, 3 ,  5  and  6 . In this embodiment, too, a power module  11  containing a plurality of power semiconductor switching devices operable at high temperatures, which is attached to the housing of the transmission in contact with the outer surface thereof, includes power semiconductor switching devices  61  operable at high temperatures. In this fifth embodiment of the invention, a snubber capacitor  81 , a snubber resistor  82  connected in series with the snubber capacitor  81  and a snubber diode  83  connected in shunt with the snubber resustor  82  are provided to suppress the surge voltage. Since the snubber capacitor  81 , the snubber resistor  82  and the snubber diode are located near or mounted within the power module  11 , the surge voltage can be effectively suppressed. The snubber capacitor  81 , the snubber resistor  82  and the snubber diode used in this embodiment should preferably have high resistance to both high temperatures and vibrations.  
      The provision of the snubber capacitor  81 , the snubber resistor  82  and the snubber diode  83  enables the magnitude of the surge voltage to be rendered low which is given by the product of the inductance L of the wiring conductors and the current reduction rate di/dt associated with the turn-off of the power semiconductor switching devices  61 , and also the length of the wiring conductors  12  between the power module  11  and the motor  51  to be reduced, with the result that the surge voltages developed across the terminals of the motor  51  can be suppressed to a low level. Accordingly, highly reliable insulation of the motor windings can be secured.  
     Embodiment 6  
       FIG. 8  shows the structure of a system to which an inverter as a seventh embodiment of this invention is applied. In  FIG. 8 , components equivalent to those shown with the first through fifth embodiments are indicated by the same reference numerals as in  FIGS. 1, 3 ,  5 ,  6  and  7 .  
      In this embodiment, a power module  11  containing a plurality of power semiconductor switching devices operable at high temperatures is attached to the body of the engine  31 . The engine body  31  is cooled by cooling water. Through the circulation of the cooling water is cooled the power module  11  containing a plurality of power semiconductor switching devices operable at high temperatures. Since the power module  11  containing a plurality of power semiconductor switching devices is located near the motor  51 , the length of the wiring conductors  12  between the power module  11  and the motor  51  can be reduced with the result that the surge voltage developed across the terminals of the motor  51  can be suppressed.  
      With this structure, while highly reliable insulation of the motor windings is secured, the inverter can be operated at high temperatures with a relatively simple cooling mechanism. Consequently, according to this embodiment, the size of the inverter can be reduced.  
     Embodiment 7  
       FIG. 9  shows the structure of a system to which an inverter as a seventh embodiment of this invention is applied. In  FIG. 9 , components equivalent to those shown with the first through sixth embodiments are indicated by the same reference numerals as in  FIGS. 1, 3 ,  5 ,  6 ,  7  and  8 . In this embodiment, a power module  11  containing a plurality of power semiconductor switching devices operable at high temperatures is attached in contact with the outer surface of the housing of a motor  51  as an electric load. The motor housing is usually made of iron or aluminum which has a large heat capacity. The large heat capacity helps cool the power module  11  containing a plurality of power semiconductor switching devices operable at high temperatures. Also, since the power module  11  containing a plurality of power semiconductor switching devices operable at high temperatures is located near the motor  51 , the length of the wiring conductors  12  between the power module  11  and the motor  51  can be reduced with the result that the surge voltage developed across the terminals of the motor  51  can be suppressed. With this structure, while highly reliable insulation of the motor windings is secured, the inverter can be operated at high temperatures with a simple cooling mechanism. Consequently, according to this embodiment, the size of the inverter can be reduced.  
      It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.