Abstract:
An object is to reduce a discharging time required for discharging residual charges of a capacitor. A power conversion device according to the present invention includes: an inverter circuit unit; a step-up circuit unit; a smoothing capacitor; and a step-up circuit control unit which controls the step-up circuit unit, wherein the step-up circuit unit has a first switching element, a second switching element connected electrically in series to the first switching element, and a reactor having a conducting current controlled by switching operation of the first switching element and the second switching element, and the step-up circuit control unit has a first control mode of controlling the switching operation of the first switching element and the second switching element by changing a duty command value and outputting a stepped up voltage from the step-up circuit unit, and a discharge control mode of discharging electric charges stored in the smoothing capacitor into the reactor, with the duty command value fixed to a predetermined value.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a power conversion device, and more particularly, an on-vehicle power conversion device which realizes discharge operation of a smoothing capacitor provided for an inverter and a step-up converter. 
       BACKGROUND ART 
       [0002]    As environmentally-friendly vehicles, a hybrid vehicle and an electric vehicle are vehicles using, as a power source, a direct-current power supply, an inverter and a motor driven by the inverter in addition to a conventional engine. Specifically, a power source is obtained by driving an engine, as well as converting a direct-current voltage from a direct-current power supply into an alternating-current voltage by an inverter and rotating a motor by the converted alternating-current voltage. Additionally, an electric vehicle is a vehicle which uses, as a power source, a direct-current power supply, an inverter, and a motor driven by the inverter. 
         [0003]    This inverter is provided with a capacitor for smoothing direct-current power from the direct-current power supply. The inverter needs to be provided with a discharging circuit function of discharging charges remaining in the capacitor after the inverter stops. The inverter is also provided with a step-up converter for stepping up a direct-current voltage from the direct-current power supply. 
         [0004]    PTL 1 discloses that in a control device  30 , residual charges of a capacitor C 1  connected between a step-up converter and a direct-current power supply are consumed by step-up operation by the step-up converter and residual charges of a capacitor C 2  connected between the step-up converter and an inverter circuit are consumed by step-down operation by the step-up converter. 
         [0005]    However, it demanded to further reduce a discharging time required for discharging residual charges of a capacitor. 
       CITATION LIST 
     Patent Literature 
       [0006]    PTL 1: JP 2004-201439 A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    An object of the present invention is to reduce a discharging time required for discharging residual charges of a capacitor. 
         [0008]    Another object of the present invention is to further improve reliability when in discharging residual charges of a capacitor by using a reactor of a step-up converter. 
       Solution to Problem 
       [0009]    In order to solve the above issue, a power conversion device according to the present invention includes: an inverter circuit unit which converts a direct current into an alternating current; a step-up circuit unit which steps up a voltage to be applied to the inverter circuit unit; a smoothing capacitor connected electrically in parallel to the inverter circuit unit and the step-up circuit unit; and a step-up circuit control unit which controls the step-up circuit unit, wherein the step-up circuit unit has a first switching element, a second switching element connected electrically in series to the first switching element, and a reactor having a conducting current controlled by switching operation of the first switching element and the second switching element, and the step-up circuit control unit has a first control mode of controlling the switching operation of the first switching element and the second switching element by changing a duty command value and outputting a stepped up voltage from the step-up circuit unit, and a discharge control mode of discharging electric charges stored in the smoothing capacitor into the reactor, with the duty command value fixed to a predetermined value. 
       Advantageous Effects of invention 
       [0010]    The present invention enables reduction in a discharging time required for discharging residual charges of a capacitor. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a diagram for explaining a configuration of a power conversion system according to a first embodiment of the present invention. 
           [0012]      FIG. 2  is a diagram for explaining a configuration of a control device  530  according to the first embodiment of the present invention. 
           [0013]      FIG. 3  is a flow chart for explaining a flow from start to stop of the control device  530  according to the first embodiment of the present invention. 
           [0014]      FIG. 4  illustrates a waveform varying with time when processing from start to stop of the control device  530  according to the first embodiment of the present invention is executed. 
           [0015]      FIG. 5  is a diagram for explaining a configuration of a carrier generation unit  520  according to the first embodiment of the present invention. 
           [0016]      FIG. 6  is a diagram for explaining a processing procedure of a carrier frequency setting unit  590  of a step-up converter  100  according to the first embodiment of the present invention. 
           [0017]      FIG. 7  is a diagram for explaining a processing procedure of a duty command generation unit  450  of the step-up converter  100  according to the first embodiment of the present invention. 
           [0018]      FIG. 8  illustrates temperature characteristic of a coil and a core of a reactor when the step-up converter is driven. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0019]    In the following, an embodiment or the present invention will be described with reference to the drawings. 
       Embodiment 1 
       [0020]      FIG. 1  is a diagram for explaining a power conversion system according to a first embodiment of the present invention. 
         [0021]    A power conversion system  0  has a battery  1 , a system relay  2 , a step-up converter  100 , an inverter  190 , an inverter  260 , a smoothing capacitor  110 , a discharge resistance  115 , a voltage sensor  120 , a current sensor  270  and a current sensor  280 , a control device  530  which controls switching of a switching element  80  and a switching element  90  of the step-up converter  100 , switching elements  130 ,  140 ,  150 ,  160 ,  170  and  180  of the inverter  190 , switching elements  200 ,  210 ,  220 ,  230 ,  240  and  250  of the inverter  260 , and a gate drive circuit  540  which generates gate voltages of the switching elements  80  and  90  of the step-up converter  100 , the switching elements  130 ,  140 ,  150 ,  160 ,  170 ,  180  of the inverter  190 , and the switching elements  200 ,  210 ,  220 ,  230 ,  240 ,  250  of the inverter  260 . 
         [0022]    For the battery  1 , a nickel hydride battery or a lithium ion battery is used. The system relay  2  is arranged between the battery  1  and the step-up converter  100 , and when the system relay  2  is off (cut off state), the battery  1  and the step-up converter  100  are electrically cut off and when the system relay  2  is on (conductive state), the battery  1  and the step-up converter  100  are electrically connected to supply power of the battery  2  to the step-up converter  100 . 
         [0023]    The step-up converter  100  has its primary side connected to the battery  1  and its secondary side connected to the smoothing capacitor  110 . Additionally, the step-up converter  100  changes a ratio of on to off of the switching element  80  of an upper arm and the switching element  90  of a lower arm of the step-up converter  100  according to an output voltage command vcs* of the step-up converter  100  received from a vehicle control device (not shown) to control voltage values at both ends of the smoothing capacitor  110  to be not less than a voltage value of the battery  1 . 
         [0024]    Thus, by increasing input voltages of the inverter  190  and the inverter  260  to be high voltages by using the step-up converter  100 , outputs of a motor generator  320  and a motor generator  360  can be increased. 
         [0025]    The smoothing capacitor  110  smooths an output of the step-up converter  100  and inputs of the inverter  190  and the inverter  260 . The discharge resistance  115  is connected between the voltage sensor  120 , and the inverter  190  and the inverter  260 . 
         [0026]    The voltage sensor  120  is connected in parallel to the smoothing capacitor  110  to detect voltage values at both ends of the smoothing capacitor  110 . The voltage sensor  120  is configured by combining a voltage-dividing resistance and a non-inverting amplification circuit using an operational amplifier, or the like. 
         [0027]    The inverter  190  has its direct current side connected to the smoothing capacitor  110  and its three-phase alternating current side connected to a three-phase coil  290  wound around a stator of the motor generator  320 . Then, according to a torque command Trq 1 * of the motor generator  320  received from the vehicle control device (not shown), the inverter  190  converts a direct-current voltage of the smoothing capacitor  110  to a three-phase alternating current voltage, which is a variable voltage of a variable frequency. The inverter  190  further applies the converted three-phase alternating current voltage to the three-phase coil  290  of the motor generator  320  to control the three-phase alternating current flowing through the three-phase coil  290  of the motor generator  320 . 
         [0028]    The inverter  260  has its direct current side connected to the smoothing capacitor  110  and its three-phase alternating current side connected to a three-phase coil  330  wound around a stator of the motor generator  360 . Then, according to a torque command Trq 2 * of the motor generator  360  received from the vehicle control device (not shown), the inverter  260 , similarly to the inverter  190 , converts a direct-current voltage of the smoothing capacitor  110  to a three-phase alternating current voltage, which is a variable voltage of a variable frequency. The inverter  260  further applies the converted three-phase alternating current voltage to the three-phase coil  330  of the motor generator  360  to control the three-phase alternating current flowing through the three-phase coil  330  of the motor generator  360 . 
         [0029]    The motor generator  320  generates a rotation magnetic field by a three-phase alternating current flowing through the three-phase coil  290  wound around the stator and accelerates or decelerates a rotor  300  by the generated rotation magnetic field to generate a torque of the motor generator  320 . The generated torque of the motor generator  320  is transmitted to a transmission  380  via a motor generator shaft  370 . 
         [0030]    The motor generator  360 , similarly to the motor generator  320 , generates a rotation magnetic field by a three-phase alternating current flowing through the three-phase coil  330  wound around the stator and accelerates or decelerates a rotor  340  by the generated rotation magnetic field to generate a torque of the motor generator  360 . The generated torque of the motor generator  360  is transmitted to the transmission  380  via a motor generator shaft  375 . 
         [0031]    Although the motor generator  320  and the motor generator  360  are desirably small-sized, highly efficient and high output permanent magnet motors, they can be induction motors. 
         [0032]    An engine  385  controls intake, compression, combustion and exhaust of fuel according to a torque command of the engine  385  received from the vehicle control device (not shown) to generate a torque of the engine  385 . The generated torque of the engine  385  is transmitted to the transmission  380  via a crank shaft  390 . 
         [0033]    The transmission  380  transmits a torque as the addition of the transmitted torques of the motor generators  320  and  360  and the transmitted torque of the engine  385  to a differential gear  410  via a propeller shaft  400 . The differential gear  410  converts the torque transmitted from the transmission  380  into a drive shaft torque and transmits the obtained torque to a drive shaft  420 . 
         [0034]    The drive shaft  420  accelerates or decelerates a vehicle (not shown) by accelerating or decelerating rotation of a drive shaft  430  of the vehicle by the transmitted drive shaft torque. The motor generator  320  is capable of converting the torque of the engine  385  transmitted to the motor generator shaft  370  via the transmission  380  into power and charging the battery  1  with the converted power via the inverter  190  and the step-up converter  100 , or alternatively supplying the power to the motor generator  360  via the inverter  190  and the inverter  260 . Further, the motor generator  320  is also capable of converting the rotation energy of the drive shaft  430  transmitted to the motor generator shaft  370  sequentially via the drive shaft  420 , the differential gear  410 , the propeller shaft  400  and the transmission  380  into power and charging the battery  1  with the converted power via the inverter  190  and the step-up converter  100 . 
         [0035]    The motor generator  360 , similarly to the motor generator  320 , is capable of converting the torque of the engine  385  transmitted to the motor generator shaft  375  via the transmission  380  into power and charging the battery  1  with the converted power via the inverter  260  and the step-up converter  100 , or alternatively supplying the power to the motor generator  320  via the inverter  260  and the inverter  190 . 
         [0036]    Further, the motor generator  360  is also capable of converting the rotation energy of the drive shaft  430  transmitted to the motor generator shaft  375  sequentially via the drive shaft  420 , the differential gear  410 , the propeller shaft  100  and the transmission  380  into power and charging the battery  1  with the converted power via the inverter  260  and the step-up converter  100 . 
         [0037]    Here, a mode in which power is supplied from the battery  1  to the motor generators  320  and  360  via the step-up converter  100  and the inverters  190  and  260  is defined as a power running mode and a mode in which power generated by the motor generators  320  and  360  is charged in the battery  1  via the inverters  190  and  260  and the step-up converter inn defined as a regenerative mode. For reducing fuel consumption, a hybrid vehicle makes the most of the power running mode at the time of starting or accelerating the vehicle and makes the most of the regenerative mode at the time of decelerating the vehicle. 
         [0038]    The step-up converter  100  has a Y capacitor  20  and a Y capacitor  2030 , a chassis ground  10 , a filter capacitor  40 , a voltage sensor  50 , a current sensor  60 , a reactor  70 , the switching element  80  of the upper arm which is configured with an IGBT and diode, and the switching element  90  of the lower arm which is configured with an IGBT and a diode. The Y capacitor  20  has its high potential side connected to a high potential side of the battery  1 , a high potential side of the filter capacitor  40 , one end of the voltage sensor  50 , and one end of the reactor  70  via the current sensor  60 , and has its low potential side connected to the chassis ground  10  and a high potential side of a Y capacitor  30 . 
         [0039]    The Y capacitor  30  has its low potential side connected to a low potential side of the battery  1 , a low potential side of the filter capacitor  40 , the other end of the voltage sensor  50 , and an emitter side of the IGBT and an anode side of the diode configuring the switching element  90  of the lower arm of the step-up converter  100 . 
         [0040]    The filter capacitor  40  smooths the output of the battery  1  and has the same voltage as that of the battery  1  when the system relay  2  is closed. The reactor  70  has a thermistor  71  for measuring a coil temperature of the reactor  70 . In other words, the thermistor  71  functions as a temperature detection circuit unit. 
         [0041]    A collector side of the IGBT and a cathode side of the diode configuring the switching element  90  of the lower arm of the step-up converter  100  are connected to an emitter side of the IGBT and an anode side of the diode configuring the switching element  80  of the upper arm of the step-up converter  100 , and a middle point connecting them and the other end of the reactor  70  are connected. 
         [0042]    A collector side of the IGBT and a cathode side of the diode configuring the switching element  80  of the upper arm of the step-up converter  100  are connected to a high potential side of the smoothing capacitor  110 . 
         [0043]    The emitter side of the IGBT and the anode side of the diode configuring the switching element of  90  of the lower arm of the step-up converter  100  are connected to a low potential side of the smoothing capacitor  110 . 
         [0044]    The inverter  190  has the switching element  130  of the upper arm and the switching element  140  of the lower arm which are of a U-phase, the switching element  150  of the upper arm and the switching element  160  of the lower arm which are of a V-phase, and the switching element  170  of the upper arm and the switching element  180  of the lower arm which are of a U-phase. 
         [0045]    An emitter side of the IGBT and an anode side of the diode configuring the U-phase switching element  130  of the upper arm of the inverter  190  are connected to a collector side of the IGBT and a cathode side of the diode configuring the U-phase switching element  140  of the lower arm of the inverter  190 , and a middle point connecting them is connected to a U-phase coil of the three-phase coil  290  of the motor generator  320  via the current sensor  270 . 
         [0046]    An emitter side of the IGBT and an anode side of the diode configuring the V-phase switching element  150  of the upper arm of the inverter  190  are connected to a collector side of the IGBT and a cathode side of the diode configuring the V-phase switching element  160  of the lower arm of the inverter  190 , and a middle point connecting them is connected to a V-phase coil of the three-phase coil  290  of the motor generator  320  via the current sensor  270 . 
         [0047]    An emitter side of the IGBT and an anode side of the diode configuring the W-phase switching element  170  of the upper arm of the inverter  190  are connected to a collector side of the IGBT and cathode side of the diode configuring the W-phase switching element  180  of the lower arm of the inverter  190 , and a middle point connecting them is connected to a W-phase coil of the three-phase coil  290  of the motor generator  320  via the current sensor  270 . 
         [0048]    A collector side of the IGBT and a cathode side of the diode of each of the UVW phase switching elements  130 ,  150  and  170  of the upper arm of the inverter  190  are connected to the high potential side of the smoothing capacitor  110 , and an emitter side of the IGBT and an anode side of the diode configuring the UVW phase switching elements  140 ,  160  and  180  of the lower arm of the inverter  190  are connected to the low potential side of the smoothing capacitor  110 . 
         [0049]    The inverter  260  has the U-phase switching elements  200  and  210  of the upper and lower arms of the inverter  260 , the V-phase switching elements  220  and  230  of the upper and lower arms of the inverter  260 , and the W-phase switching elements  240  and  250  of the upper and lower arms of the inverter  260 . 
         [0050]    An emitter side of the IGBT and an anode side of the diode configuring the U-phase switching element  200  of the upper arm of the inverter  260  are connected to a collector side of the IGBT and a cathode side of the diode configuring the U-phase switching element  210  of the lower arm of the inverter  260 , and a middle point connecting them is connected to a U-phase coil of the three-phase coil  330  of the motor generator  360  via the current sensor  280 . 
         [0051]    An emitter side of the IGBT and an anode side of the diode configuring the V-phase switching element  220  of the upper arm of the inverter  260  are connected to a collector side of the IGBT and a cathode side of the diode configuring the V-phase switching element  230  of the lower arm of the inverter  260 , and a middle point connecting them is connected to a V-phase coil of the three-phase coil  330  of the motor generator  360  via the current sensor  280 . 
         [0052]    An emitter side of the IGBT and an anode side of the diode configuring the W-phase switching element  240  of the upper arm of the inverter  260  are connected to a collector side of the IGBT and a cathode of the diode configuring the W-phase switching element  250  of the lower arm of the inverter  260 , and a middle point connecting them is connected to a W-phase coil of the three-phase coil  330  of the motor generator  360  via the current sensor  280 . The switching elements  80  and  90  of the step-up converter  100 , the switching elements  130 ,  140 ,  150 ,  160 ,  170 ,  180  of the inverter  190 , and the switching elements  200 ,  210 ,  220 ,  230 ,  240 ,  250  of the inverter  260  may be each configured with a MOSFET or the like. 
         [0053]    The control device  530  controls switching of the switching elements  80  and  90  of the step-up converter  100  based on the output voltage command vcs* of the step-up converter  100  received from the vehicle control device (not shown), a current value it flowing through the reactor  70  detected by the current sensor  60 , a voltage value vcin at both ends of the filter capacitor  40  detected by the voltage sensor  50 , and voltage values vcs at both ends of the smoothing capacitor  110  detected by the voltage sensor  120 . 
         [0054]    Further, the control device  530  controls switching of the switching elements  130 ,  140 ,  150 ,  160 ,  170 ,  180  of the inverter  190  and the switching elements  200 ,  210 ,  220 ,  230 ,  240 ,  250  of the inverter  260  based on the torque command Trq 1 * of the motor generator  320  received from the vehicle control device (not shown) and a magnetic pole position θ 1  of the rotor  300  of the motor generator  320  detected by an angle detector  310 . 
         [0055]    The gate drive circuit  540  generates a voltage which enables turning-on or turning-off of the IGBT configuring each of the switching elements  130 ,  140 ,  150 ,  160 ,  170 ,  180  of the inverter  190  based on switching signals Sup 1 , Sun 1 , Svp 1 , Svn 1 , Swp 1  and Swn 1  for the switching elements  130 ,  140 ,  150 ,  160 ,  170 ,  180  of the inverter  190  which are generated by a switching signal generation unit  460  (see  FIG. 2 ) provided in the control device  530  and applies the generated voltage to between a gate and the emitter of the IGBT configuring each of the switching elements  130 ,  140 ,  150 ,  160 ,  170 ,  180  of the inverter  190 . 
         [0056]    Further, the gate drive circuit  540  generates a voltage which enables turning-on or turning-off of the IGBT configuring each of the switching elements  200 ,  210 ,  220 ,  230 ,  240 ,  250  of the inverter  260  based on switching signals Sup 2 , Sun 2 , Svp 2 , Svn 2 , Swp 2  and Swn 2  for the switching elements  200 ,  210 ,  220 ,  230 ,  240 ,  250  of the inverter  260  which are generated by the switching signal generation unit  460  (see  FIG. 2 ) provided in the control device  530 , and applies the generated voltage to between a gate and an emitter of the IGBT configuring each of the switching elements  200 ,  210 ,  220 ,  230 ,  240 ,  250  of the inverter  260 . 
         [0057]    Further, the gate drive circuit  540  generates a voltage which enables turning-on or turning-off of the IGBT configuring each of the switching elements  80  and  90  of the step-up converter  100  based on switching signals Sbp and Sbn for the switching elements  80  and  90  of the step-up converter  100  which are generated by the switching signal generation unit  460  provided in the control device  530 , and applies the generated voltage to between gate of the switching elements  80  and  90  and the emitter. 
         [0058]      FIG. 2  is a diagram for explaining a configuration of the control device  530  illustrated in  FIG. 1 . 
         [0059]    The control device  530  has a UVW phase/dq-axis conversion unit  490 , a rotation speed calculation unit  510 , a dg-axis current command generation unit  500 , a UVW phase voltage command generation unit  480  for the inverter  190  and the inverter  260 , a P-Q voltage command generation unit  440  for the step-up converter  100 , a carrier generation unit  520 , a UVW phase duty command generation unit  470  for the inverters  190  and  260 , a duty command generation unit  450  for the step-up converter  100 , and the switching signal generation unit  460 . 
         [0060]    The UVW phase/dq-axis conversion unit  490  calculates dg-axis current values id 1  and iq 1  of the motor generator  320  based on UVW phase current values iud 1 , ivd 1 , iwd 1  flowing through the three-phase coil  290  of the motor generator  320  which are detected by current sensor  270 , and the magnetic pole position  91  of the rotor  300  of the motor generator  320  which is detected by the angle detector  310 , as well as calculating dq-axis current values id 2  and ig 2  of the motor generator  360  based on UVW phase current values iud 2 , ivd 2 , iwd 2  flowing through the three-phase coil  330  of the motor generator  360  which are detected by the current sensor  280 , and a magnetic pole position  92  of the rotor  340  of the motor generator  360  which is detected by the angle detector  310 . The calculated dq-axis current values id 1 , iq 1  of motor generator  320  and the calculated dq-axis current values id 2 , ig 2  of the motor generator  360  are input to the UVW phase voltage command generation unit  480  for the inverters  190  and  260  and to the carrier generation unit  520 . For angle detectors  310 ,  350 , a resolver or an encoder is used. 
         [0061]    The rotation speed calculation unit  510  calculates a rotation speed ω 1  of the rotor  300  of the motor generator  320  based on the magnetic pole position θ 1  of the rotor  300  of the motor generator  320  detected by the angle detector  310 , as well as calculating a rotation speed ω 2  of the rotor  340  of the motor generator  360  based on the magnetic pole position θ 2  of the rotor  340  of the motor generator  360  detected by the angle detector  350 . The rotation speed ω 1  and the rotation speed ω 2  are input to the dq-axis current command generation unit  500 . 
         [0062]    The dq-axis current command generation unit  500  generates dq-axis current command values id 1 *, iq 1 * of the motor generator  320  based on the torque command Trq 1 * of the motor generator  320  received from the vehicle control device (not shown) and the rotation speed ω 1  calculated by the rotation speed calculation unit  510 , as well as generating dq-axis current command values id 2 *, iq 2 * of the motor generator  360  based on a torque command Trq 2 * of the motor generator  360  received from the vehicle control device (not shown) and the rotation speed ω 2  calculated by the rotation speed calculation unit  510 . The generated dq-axis current command values id 1 *, iq 1 * and dq-axis current command values id 2 *, iq 2 * are input to the UVW phase voltage command generation unit  480  for the inverters  190  and  260 . 
         [0063]    The UVW phase voltage command generation unit  480  for the inverters  190  and  260  generates UVW phase voltage command values vu 1 *, vv 1 *, vw 1 * of the inverter  190  such that the dq-axis current values id 1 , iq 1  of the motor generator  320  coincide with the dq-axis current command values id 1 *, iq 1 * of the motor generator  320  based on the magnetic pole position θ 1  of the rotor  300 , the dq-axis current values id 1 , iq 1  calculated by the UVW phase/dq-axis conversion unit  490  and the dq-axis current command values id 1 *, iq 1 * generated by the dq-axis current command generation unit  500 . 
         [0064]    Further, the UVW phase voltage command generation unit  480  for the inverters  190  and  260  generates UVW phase voltage command values vu 2 *, vv 2 *, vw 2 * of the inverter  260  such that the dq-axis current values id 2 , ig 2  of the motor generator  360  coincide with the dq-axis current values id 2 *, iq 2 * of the motor generator  360  based on the magnetic pole position θ 2  of the rotor  340 , the dq-axis current values id 2 , calculated by the UVW phase/dg-axis conversion unit  490  and the dq-axis current command values id 2 *, iq 2  generated by the dq-axis current command generation unit  500 . 
         [0065]    The generated UVW phase voltage command values vu 1 *, vw 1 *, vv 1 *, of the inverter  190  and UVW phase voltage command values vu 2 *, vv 2 *, vw 2 * of the inverter  260  are input to the UVW phase duty command generation unit  470  for the inverters  190  and  260 . 
         [0066]    The P-Q voltage command generation unit  440  of the step-up converter  100  generates a command value vpg* (hereinafter, referred to as a P-Q voltage command value vpg*) of a voltage to be applied to between a P point (see  FIG. 1 ) which connects the other end of the reactor  70  and the switching elements  80  and  90  of the upper and lower arms of the step-up converter  100  and a Q point (see  FIG. 1 ) to which the low potential side of the battery  1 , and the emitter side of the IGFT and the anode side of the diode configuring the switching element  90  of the lower arm of the step-up converter  100  are connected such that the voltage values vcs at both ends of the smoothing capacitor  110  coincide with the output voltage command vcs* of the step-up converter  100  based on the output voltage command vcs* of the step-up converter  100  received from the vehicle control device (not shown), the current value it flowing through the reactor  70  which is detected by the current sensor  60 , the voltage values vcin at both ends of the filter capacitor  40  which are detected by the voltage sensor  50 , and the voltage values vcs at both ends of the smoothing capacitor  110  which are detected by the voltage sensor  120 . The generated P-Q voltage command value vpq* is input to the duty command generation unit  450  of the step-up converter  100 . 
         [0067]    The carrier generation unit  520  generates a carrier frequency fcarrier 1  for the step-up converter  100 , a triangular wave carrier carrier 1  for the step-up converter  100 , a carrier frequency fcarrier 2  for the inverters  190  and  260 , and a triangular wave carrier carrier 2  for the inverters  190  and  260  based on the voltage values vcs at both ends of the smoothing capacitor  110 , the current value it flowing through the reactor  70 , the dq-axis current values id 1 , iq 1  of the motor generator  320  calculated by the UVW phase/dq-axis conversion unit  490 , and the dq-axis current values id 2 , iq 2  of the motor generator  360 . 
         [0068]    The triangular wave carrier carrier 1  is input to the duty command generation unit  450  and the switching signal generation unit  460  for the step-up converter  100 . Additionally, the generated triangular wave carrier carrier 2  for the inverters  190  and  260  is input to the UVW phase duty command generation unit  470  and the switching signal generation unit  460  for the inverters  190  and  260 , and the generated carrier frequency fcarrier 1  for the step-up converter  100  and the carrier frequency fcarrier 2  for the inverters  190  and  260  are input to the switching signal generation unit  460 . 
         [0069]    The UVW phase duty command generation unit  470  generates UVW phase duty command values Du 1 *, Dv 1 *, Dw 1 * of the inverter  190  based on the UVW phase voltage command values vu 1 *, vv 1 *, vw 1 *, the voltage values vcs at both ends of the smoothing capacitor  110 , and the triangular wave carrier carrier 2 . Further, the UVW phase duty command generation unit  470  generates UVW phase duty command values Du 2 *, Dv 2 *, Dw 2 * of the inverter  260  based on the UVW phase voltage command values vu 2 *, vv 2 *, vw 2 *, the voltage values vcs at both ends of the smoothing capacitor  110 , and the triangular wave carrier carrier 2 . The generated UVW phase duty command values Du 1 *, Dv 1 *, Dw 1 * of the inverter  190  and UVW phase duty command values Du 2 *, Dv 2 *, Dw 2 * of the inverter  260  are input to the switching signal generation unit  460 . The duty command generation unit  450  of the step-up converter  100  generates a duty command value Db* of the step-up converter  100  based on the voltage values vcs at both ends of the smoothing capacitor  110 , the P-Q voltage command value vpq* and the triangular wave carrier carrier 1 . The generated duty command value Db* of the step-up converter  100  is input to the switching signal generation unit  460 . The switching signal generation unit  460  generates the switching signal Svp 1  for the U-phase switching element  130  of the upper arm, the switching signal Sun 1  for the U-phase switching element  140  of the lower arm, the switching signal Svp 1  for the V-phase switching element  150  of the upper arm, the switching signal Svn 1  for the switching element  160  of the lower arm, the switching signal Swp 1  for the U-phase switching element  170  of the upper arm, and the switching signal Swn 1  for the W-phase switching element  180  of the lower arm based on the UVW phase duty command values Du 1 *, Dv 1 *, Dw 1 *, the triangular wave carrier carrier 2  for the inverters  190  and  260 , the carrier frequency fcarrier 2  for the inverters  190  and  260 , and the carrier frequency fcarrier 1  for the step-up converter  100 . 
         [0070]    Further, the switching signal generation unit  460  generates the switching signal Sup 2  for the U-phase switching element  200  of the upper arm, the switching signal Sun 2  for the U-phase switching element  210  of the lower arm, the switching signal Svp 2  for the V-phase switching element  220  of the upper arm, the switching signal Svn 2  for the V-phase switching element  230  of the lower arm, the switching signal Swp 2  for the W-phase switching element  240  of the upper arm, and the switching signal Swn 2  for the W-phase switching element  250  of the lower arm based on the UVW phase duty command values Du 2 *, Dv 2 *, Dw 2 *, the triangular wave carrier carrier 2  for the inverters  190  and  260 , the carrier frequency fcarrier 2  for the inverters  190  and  260 , and the carrier frequency fcarrier 1  for the step-up converter  100 . 
         [0071]    Further, the switching signal generation unit  460  generates the switching signal Sbp for the switching element  80  of the upper arm of the step-up converter  100 , and the switching signal Sbn for the switching element  90  of the lower arm of the step-up converter  100  based on the duty command value Db* of the step-up converter  100  generated by the duty command generation unit  450  for the step-up converter  100 , the triangular wave carrier carrier 1  for the step-up converter  100 , the carrier frequency fcarrier 1  for the step-up converter  100 , and the carrier frequency fcarrier 2  for the inverters  190  and  260 . 
         [0072]    The generated switching signals Sup 1 , Sun 1 , Svp 1  Svn 1 , Swp 1 , Swn 1  for the switching elements  130 ,  140 ,  150 ,  160 ,  170 ,  180  of the inverter  190 , switching signals Sup 2 , Sun 2 , Svp 2 , Svn 2 , Swp 2 , Swn 2  for the switching elements  200 ,  210 ,  220 ,  230 ,  240 ,  250  of the inverter  260  and switching signals Sbp and Sbn for the switching elements  80  and  90  of the step-up converter  100  are input to the gate drive circuit  540 . 
         [0073]    Thus configuring the power conversion system enables such control as to make the torque of the motor generator  320  coincide with the torque command Trq 1 * of the motor generator  320  received from the vehicle, control device (not shown). It is further possible to control the torque of the motor generator  360  to coincide with the torque command Trq 2 * of the motor generator  360 . It is further possible to control the output voltage vcs (voltage values vcs at both ends of the smoothing capacitor  110 ) of the step-up converter  100  to coincide with the output voltage command vcs* of the step-up converter  100 .  FIG. 3  is a flow chart for explaining a flow from start to stop of the control device  530 . 
         [0074]    At Step A 10 , the control device  530  starts to transit to Step A 20 . 
         [0075]    At Step A 20 , the system relay  2  is closed. As a result, the battery  1  and the step-up converter  100  are connected to apply a voltage to the filter capacitor  40 . At this time, since the switching element  80  of the upper arm of the step-up converter  100  is set to be on and the switching element  90  of the lower arm is set to be off, the voltage of the battery  1  is applied also to the smoothing capacitor  110 . When the determination is made that the voltages of the filter capacitor  40  and the smoothing capacitor  110  become generally the same as the voltage of the battery  1 , and preparation for controlling the motor generator  320  and the motor generator  360  is completed, the step transits to Step A 30 . 
         [0076]    At Step A 30 , the step-up operation of the step-up converter  100  and the drive control of the motor generator  320  and the motor generator  360  are performed at the control device  530  to transit to Step A 40 . 
         [0077]    At Step A 40 , determination is made whether a stop request is received from the system or not. When a stop request to the control device  530  such as an ignition off signal of an external ECU is confirmed, the step transits to Step A 50 . Without a stop request to the control device  530 , the step again transits to Step A 30  to continue the control of the step-up converter  100 , the motor generator  320  and the motor generator  360 . 
         [0078]    At Step A 50 , switching of the inverter  190  and the inverter  260  is stopped. 
         [0079]    At Step A 60 , the system relay  2  closed to electrically cut off the battery  1  and the step-up converter  100 , so that the step transits to Step A 60 . 
         [0080]    At Step A 70 , discharging processing is performed of discharging electric charges remaining in the filter capacitor  40  and the smoothing capacitor  110  to transit to Step A 70 . The discharging processing is realized by switching operation of the step-up converter  100 . A discharge control mode is selected in the duty ratio command generation unit  450  for the step-up converter  100  to select and output a duty ratio Ddischg* set in advance. The switching operation is performed based on the output duty ratio Ddischg* to consume the residual charges of the smoothing capacitor  110 . 
         [0081]    At Step A 80 , determination is made whether the discharging processing is completed or not. When the voltages of the filter capacitor  40  and the smoothing capacitor  110  fall below a predetermined value due to the discharging processing, completion of the discharging processing is confirmed to transit to Step A 80 . When the voltages of the filter capacitor  40  and the smoothing capacitor  110  fail to exceed the predetermined value, the step again transits to Step A 60  to continue the discharging processing. 
         [0082]    At Step A 90 , the switching of the step-up converter  100  is stopped to transit to Step A 90 . At Step A 100 , the control device  530  stops. 
         [0083]      FIG. 4  illustrates a waveform, varying with time, of an on/off signal of the system relay  2 , the duty command value Db* of the step-up converter  100 , the voltage values vcs at both ends of the smoothing capacitor  110  and the voltage values vcin at both ends of the filter capacitor  40  when processing from start to stop of the control device  530  illustrated in  FIG. 3  is executed. 
         [0084]    In a period E 1 , the control device  530  executes the processing of Step A 10  in the flow chart illustrated in  FIG. 3 , so that the device is in a system start request waiting state. At this time, a system start request signal is off. Additionally, at this time, the system relay  2  is off and the battery  1  and the step-up converter  100  are electrically cut off. Additionally, at this time, since the inverter  190  and the inverter  260  stop switching, the step-up converter, the inverter  190  and the inverter  260  are electrically cut off. 
         [0085]    Additionally, in the period E 1 , since the duty command value Db* of the step-up converter  100  is set to 1, the switching element  80  of the upper arm of the step-up converter  100  is set to be on and the switching element  90  of the lower arm is set to be off, and the step-up converter  100 , the inverter  190  and the inverter  260  are being electrically connected. 
         [0086]    Additionally, in the period E 1 , the voltage values vcin at both ends of the filter capacitor  40  are 0 because the battery  1  and the step-up converter  100  are electrically cut off. At this time, the voltage values vcs at both ends of the smoothing capacitor  110  are 0 because the battery  1  and the step-up converter  100  are electrically cut off. 
         [0087]    At time T 1 , the control device  530  receives the system start request signal, for example, an ignition on signal, from the external ECU to transit to the processing of Step A 20  in the flow chart illustrated in  FIG. 3 . 
         [0088]    In a period P 2 , the control device  530  is performing Step A 20  of the flow chart illustrated in  FIG. 3  to send an on signal to the system relay  2 . At this time, the system start request signal is on. Additionally, at this time, the system relay  2  is off, and the battery  1  and the step-up converter  100  are being electrically cut off. Additionally, at this time, since the inverter  190  and the inverter  260  stop switching, the step-up converter, the inverter  190  and the inverter  260  are electrically cut off. 
         [0089]    Additionally, in the period P 2 , the duty command value Db of the step-up converter  100  is set to 0 and remains unchanged from the state in the period E 1 . At this time, the voltage values vcin at both ends of the filter capacitor  40  are 0 and remain unchanged from the state in the period E 1 . At this time, the voltage values vcs at both ends of the smoothing capacitor  110  are 0 and remain unchanged from the state in the period E 1 . 
         [0090]    At time T 2 , the system relay  2  receives the on signal to be turned on. 
         [0091]    In a period E 3 , the control device  530  is performing Step A 20  of the flow chart illustrated in  FIG. 3 . At this time, the system start request signal is one Additionally, at this time, the system relay  2  is on, and the battery  1  and the step-up converter  100  are being electrically connected. Additionally, at this time, the inverter  190  and the inverter  260  stop switching, so that the step-up converter, the inverter  190  and the inverter  260  are electrically cut off. 
         [0092]    Additionally, in the period E 3 , the duty command value Db* of the step-up converter  100  remains unchanged from the state in the period E 2 . At this time, since the battery  1  and the step-up converter  100  are electrically connected, as the voltage values vein at both ends of the filter capacitor  40 , the voltage of the battery  1  is applied. As a result, charges are stored in the filter capacitor  40  to increase the voltage value vcin from 0. 
         [0093]    Additionally, in the period E 3 , because the battery  1 , the step-up converter  100 , the inverter  190  and the inverter  260  are electrically connected, as the voltage values vcs at both ends of the smoothing capacitor  110 , the voltage of the battery  1  is applied. As a result, charges are stored in the smoothing capacitor  110  to increase the voltage value vcs from 0. 
         [0094]    At time T 3 , the voltage values vcs at both ends of the smoothing capacitor  110  and the voltage values vcin at both ends of the filter capacitor  40  become generally equal to the voltage of the battery  1 , so that the control device  530  determines that preparation for controlling the motor generator  320  and the motor generator  360  is completed to transit to Step A 30 . 
         [0095]    In a period E 4 , the control device  530  is repeatedly performing Steps A 30  and A 40  in the flow chart illustrated in  FIG. 3 , in which at Step A 30 , the step-up converter  100 , the motor generator  320  and the motor generator  360  perform control and at Step A 40 , the device monitors presence/non-presence of a system stop request. At this time, the system start request signal is on. Additionally, at this time, the system relay  2  is on and the battery  1  and the step-up converter  100  are electrically connected. 
         [0096]    Additionally, at this time, the inverter  190  and the inverter  260  perform the switching operation based on the torque command Trq 1 * of the motor generator  320  and the torque command Trq 2 * of the motor generator  360  which are received from the vehicle control device (not shown). 
         [0097]    Additionally, in the period E 4 , as the duty command value Db* of the step-up converter  100 , a duty ratio which is generated by the duty command generation unit  450  of the step-up converter  100  is output such that the voltage values vcs at both ends of the smoothing capacitor  110  coincide with the output voltage command vcs* of the step-up converter  100 . At this time, the duty command value Db* varies with a change of the output voltage command vcs* of the step-up converter  100 . At this time, being connected to the battery  1 , the voltage values vcin at both ends of the filter capacitor  40  are generally the same as the voltage of the battery  1 . Then, the voltage values vcs at both ends of the smoothing capacitor  110  change so as to coincide with the output voltage command vcs of the step-up converter  100  due to the step-up operation according to the duty command value Db* of the step-up converter  100 . 
         [0098]    At time T 4 , the control device  530  receives the system start request signal, for example, the ignition off signal, from the external ECU to transit to the processing of Step A 50  in the flow chart illustrated in  FIG. 3 , thereby stopping the switching of the inverter  190  and the inverter  260 . 
         [0099]    In a period E 5 , the control device  530  is performing Steps A 50  and A 60  in the flow chart illustrated in  FIG. 3 . After stopping the switching of the inverter  190  and the inverter  260  at Step A 50 , the step transits to Step A 60  where the off signal is sent to the system relay  2 . At this time, the system start request signal is off. Additionally, at this time, the system relay  2  is off and the battery  1  and the step-up converter  100  are being electrically connected. Additionally, at this time, the inverter  190  and the inverter  260  stop switching, so that the step-up converter, the inverter  190  and the inverter  260  are electrically cut off. 
         [0100]    Additionally, in the period E 5 , the duty command value Db* of the step-up converter  100  maintains the duty ratio output at time T 4 . At this time, the voltage values vcin at both ends of the filter capacitor  40  similarly maintain the capacitor voltage output at time T 4  because the duty command value Db* of the step-up converter  100  maintains the duty ratio output at time T 4 . At this time, the voltage values vcs at both ends of the smoothing capacitor  110  similarly maintain the capacitor voltage which is output at time T 4  because the duty command value Db* of the step-up converter  100  maintains the duty ratio output at time T 4 . 
         [0101]    In such a state, execution of the switching operation of the step-up converter  100  causes electric charges to move between the filter capacitor  40  and the smoothing capacitor  110 . When the changes move, the charges pass through the reactor  70 , and the switching element  80  of the upper arm and the switching element  90  of the lower arm of the step-up converter, so that the charges are consumed by resistances held by the switching elements and the reactor to realize the discharge operation. 
         [0102]    As a result, the step-up converter  100 , the inverter  190  and the inverter  260  are electrically cut off to prevent erroneous operation of the motor due to the discharge operation. 
         [0103]    At time T 5 , the system relay  2  receives the off signal to be turned off, so that the step transits to the processing of Step A 60  in the flow chart illustrated in  FIG. 3 . 
         [0104]    In a period E 6 , the control device  530  sequentially executing Steps A 60  and A 70  in the flow chart illustrated in  FIG. 3 , in which at Step A 60 , the discharge control mode is selected by the duty ratio command generation unit  450  of the step-up converter  100  to select and output the duty ratio Ddischg* set in advance. Based on the output duty ratio Ddischg*, the switching operation is performed to consume residual charges of the smoothing capacitor  110 . Further, at Step A 70 , determination is made whether the discharging processing is completed or not. At this time, the system start request signal is off. Additionally, at this time, the system relay  2  is off and the battery  1  and the step-up converter  100  are being electrically cut off. Additionally, at this time, since the inverter  190  and the inverter  260  stop switching, the step-up converter, the inverter  190  and the inverter  260  are electrically cut off. At this time, the duty command value Db* of the step-up converter  100  maintains the duty ratio Ddischg*. 
         [0105]    Additionally, in the period E 6 , the voltage values vcin at both ends of the filter capacitor  40  are decreasing while maintaining a fixed ratio to the voltage values vcs at both ends of the smoothing capacitor  110  because as the duty command value Db* of the step-up converter  100 , the duty ratio Ddischg* is output. At this time, the voltage values vcs at both ends of the smoothing capacitor  110  are decreasing while maintaining a fixed ratio to the voltage values vcin at both ends of the filter capacitor  40  because as the duty command value Db*of the step-up converter  100 , the duty ratio Ddischg* is output. 
         [0106]    At Time T 6 , the voltage values vcin at both ends of the filter capacitor  40  and the voltage values vcs at both ends of the smoothing capacitor  110  fall below the predetermined values, so that the control device  530  determines that the discharging processing is completed at Step A 70  to transit to the processing of Step A 80  in the flow chart illustrated in  FIG. 3  and after stopping the switching of the step-up converter  100 , transit to Step A 90 . 
         [0107]    In a period E 7 , the control device  530  is in the state of Step A 90  in the flow chart illustrated in  FIG. 3  and the control device  530  stops. At this time, the system start request signal is off. Additionally, at this time, the system relay  2  is off and the battery  1  and the step-up converter  100  are being electrically cut off. Additionally, at this time, the inverter  190  and the inverter  260  stop switching, so that the step-up converter, the inverter  190  and the inverter  260  are electrically cut off. 
         [0108]    Additionally, in the period E 7 , the step-up converter  100  has the duty command value Db* of 1 to stop switching. The switching element  80  of the upper arm of the step-up converter  100  is set to be on, the switching element  90  of the lower arm is set to be off, and the step-up converter  100 , the inverter  190  and the inverter  260  are being electrically connected. At this time, the voltage values vcin at both ends of the filter capacitor  40  are generally 0 or a value not more than the predetermined voltage value set at Step A 70  in the flow chart illustrated in  FIG. 3  which is used for determining completion of the discharging processing, and residual charges of the filter capacitor  40  are consumed through the discharge resistance  115 . At this time, the voltage values vcs at both ends of the smoothing capacitor  110  are generally 0 or a value not more than the predetermined voltage value set at Step A 70  in the flow chart illustrated in  FIG. 3  which is used for determining completion of the discharging processing, and residual charges of the smoothing capacitor  110  are consumed through the discharge resistance  115 . 
         [0109]      FIG. 5  is a diagram for explaining a configuration of the carrier generation unit  520  according to the first embodiment of the present Example. 
         [0110]    The carrier generation unit  520  has a carrier frequency setting unit  590  for the step-up converter  100 , a carrier generation unit  580  for the step-up converter  100 , a phase current maximum value calculation unit  550 , a carrier frequency setting unit  560  for the inverters  190  and  260 , and a carrier generation unit  570  for the inverters  190  and  260 . 
         [0111]    The carrier frequency setting unit  590  outputs a value of the carrier frequency fcarrier 1  for the step-up converter  100  which is stored in advance. The output value of carrier frequency fcarrier 1  for the step-up converter  100  is input to the carrier generation unit  580  and the switching signal generation unit  460  for the step-up converter  100 . 
         [0112]    The carrier generation unit  580  generates the triangular wave carrier carrier 1  based on the carrier frequency fcarrier 1  input from the carrier frequency setting unit  590 . The generated triangular wave carrier carrier 1  is input to the carrier frequency setting unit  560  for the inverters  190  and  260 , the carrier generation unit  570  and the switching signal generation unit  460  for the inverters  190  and  260 , and the duty command generation unit  450  for the step-up converter  100 . 
         [0113]    The phase current maximum value calculation unit  550  calculates a phase current maximum value iphmax 1  of the motor generator  320  by using the dq-axis current values id 1 , iq 1  of the motor generator  320 . Further, using the dq-axis current values id 2 , iq 2  of the motor generator  360 , a phase current maximum value iphmax 2  of the motor generator  360  is calculated. 
         [0114]    The carrier frequency setting unit  560  sets the carrier frequency fcarrier 2  for the inverters  190  and  260  based on the current value it flowing through the reactor  70 , the voltage values vcs at both ends of the smoothing capacitor  110 , the carrier frequency fcarrier 1  for the step-up converter  100  set by the carrier frequency setting unit  590  for the step-up converter  100 , the triangular wave carrier carrier 1  for the step-up converter  100  which is generated by the carrier generation unit  580  for the step-up converter  100 , the phase current maximum value iphmax 1  of the motor generator  320  which is calculated by the phase current maximum value calculation unit  550 , and the phase current maximum value iphmax 2  of the motor generator  360 . 
         [0115]    The carrier generation unit  570  generates the triangular wave carrier carrier 2  for the inverters  190  and  260  based on the carrier frequency fcarrier 2  for the inverters  190  and  260  which is set by the carrier frequency setting unit  560  for the inverters  190  and  260 , and the triangular wave carrier carrier 1  for the step-up converter  100  which is generated by the carrier generation unit  580  for the step-up converter  100 . 
         [0116]      FIG. 6  is a diagram for explaining a processing procedure of the carrier frequency setting unit  590  for the step-up converter  100  according to the first embodiment of the present invention. 
         [0117]    At Step B 10 , determination is made whether a stop request received from the system or not. When confirming the stop request to the control device  530  such as the ignition off signal of the external ECU, the step transits to Step B 30 . When there is no stop request to the control device  530 , the step transits to Step B 20 . 
         [0118]    At Step B 20 , an already stored first control mode carrier frequency fcarrier 1 - 1  is obtained to transit to B 40 . 
         [0119]    A Step B 30 , an already stored discharge control mode carrier frequency fcarrier 1 - 2  is obtained to transit to B 40 . 
         [0120]    At Step B 40 , the carrier frequency selected at Step B 20  or B 30  is output as the fcarrier 1 , and the output fcarrier 1  is input to the carrier generation unit for the step-up converter  100 . At this time, the discharge control mode carrier frequency fcarrier 1 - 2  desirably has a smaller value than that of the first control mode carrier frequency fcarrier 1 - 1 . This enables control with a fixed duty and enables a carrier frequency at the time of discharge control to be set lower than that of normal operation, thereby further increasing a discharge speed. 
         [0121]      FIG. 7  is a diagram for explaining a processing procedure of the duty command generation unit  450  for the step-up converter  100  according to the first embodiment of the present invention. 
         [0122]    At Step C 10 , determination is made whether a stop request received from the system or not. When confirming the stop request to the control device  530  such as the ignition off signal of the external ECU, the step transits to Step C 30  to execute the operation in the discharge operation mode. When there is no stop request to the control device  530 , the step transits to Step C 20  to perform operation in the first control mode. 
         [0123]    At Step C 20 , operation in the first control mode is performed. The obtained P-Q voltage command value vpq* of the step-up converter  100  and the voltage values vcs at both ends of the smoothing capacitor  110  are substituted into the following formula to calculate the duty command value Db* of the step-up converter  100  and then the step again transits to Step C 10 . 
         [0124]    At Step C 30 , determination is made whether a thermistor temperature TL of the reactor obtained by a thermistor  71  exceeds a predetermined value α stored in advance or not. When the thermistor temperature TL exceeds the predetermined value α, the step transits to Step C 40  and when the same falls below the predetermined value α, the step transits to Step C 50 . 
         [0125]    At Step C 40 , after outputting the fixed duty ratio Ddischg in the discharge operation mode which is stored in advance as the duty command value Db* of the step-up converter  100 , the step again transits to Step C 10 . 
         [0126]    At Step C 50 , after outputting a fixed duty ratio 1 as the duty command value Db* of the step-up converter  100 , the step transits to Step C 60 . 
         [0127]    At Step C 60 , determination is made whether the thermistor temperature TL of the reactor obtained by the thermistor  71  exceeds the predetermined value  3  stored in advance or not. When the thermistor temperature TL exceeds the predetermined value β, the step transits to Step C 50  and when the same falls below the predetermined value β, the step transits to Step C 10 . 
         [0128]    This enables discharge operation prohibiting processing to be performed at Step C 50  when the thermistor temperature TL exceeds the predetermined value a even during the execution of the discharge operation mode at Step C 40 , thereby preventing the reactor temperature from further increasing to protect the reactor. 
         [0129]    Here, the predetermined value a is preferably selected within a range of an upper limit temperature at which the reactor is operable according to temperature characteristic. 
         [0130]    Additionally, although at Step C 50 , as a switching operation stop means in the discharge control mode, the duty command value 1 is applied, and the switching element  80  of the upper arm of the step-up converter  100  is set to be on and the switching element  90  of the lower arm is set to be off, the switching elements  80  and  90  may be simultaneously set to be off to serve as a switching operation stop means. 
         [0131]    Additionally, at Step C 50 , even when the discharge operation prohibiting processing is in execution, if the thermistor temperature TL falls below the predetermined value β, the processing in the discharge operation mode at Step C 40  is again executed, so that the discharge operation can be completed while protecting the reactor. 
         [0132]    Here, the predetermined value β is desirably set to be a smaller value than the predetermined value α. 
         [0133]    Additionally, even when determination is made whether or not a stop request is received from the system at Step C 10  while the coil temperature of the reactor is increased due to operation in the first control mode at Step C 20 , if the thermistor temperature TL exceeds the predetermined value a at Step C 30 , the discharge operation prohibiting processing at Step C 50  is executed to prevent the reactor temperature from further increasing to protect the reactor. 
         [0134]      FIG. 8  illustrates temperature characteristic of a coil and a core of the reactor  70  when the step-up converter is driven. 
         [0135]    Illustrated is a time change of the reactor coil and core temperatures when duty control is performed according a ratio of the input voltage Vin to the output voltage Vout of the step-up converter  100 . According to  FIG. 3 , as a reactor temperature change, a temperature rise the largest when Vin/Vout=0.5, that is, when the output voltage is double the input, voltage. 
         [0136]    A large temperature increase means that a damage due to flowing of a current through the reactor  70  is large, and when Vin/Vout=0.5, the reactor damage will be the largest. The reason will be described as follows. 
         [0137]    Formula (1) shows an alternating current copper loss. The alternating current copper loss is proportional to the square of an effective value of a reactor current ripple. Therefore, the lower a carrier frequency is, the larger the alternating current copper loss becomes, thereby increasing the coil temperature. 
         [0000]      P coil   _   ac =I LRipple   2 R ac    Formula (1)
 
         [0138]    Pcoil_ac [W]: alternating current copper loss, 
         [0139]    ILRipple [Arms]: reactor current ripple (effective value), 
         [0140]    Rac [Ω]: alternating current resistance. 
         [0141]    Formula (2) shows a reactor current ripple. 
         [0000]    
       
         
           
             
               
                 
                   
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                        
                       Time 
                     
                     = 
                     
                       
                         
                           
                             V 
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                           L 
                         
                         · 
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                         · 
                         
                           1 
                           
                             f 
                             c 
                           
                         
                       
                       = 
                       
                         
                           
                             V 
                             L 
                           
                           L 
                         
                         · 
                         
                           ( 
                           
                             1 
                             - 
                             
                               
                                 V 
                                 in 
                               
                               
                                 V 
                                 out 
                               
                             
                           
                           ) 
                         
                         · 
                         
                           1 
                           
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                   Formula 
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         [0142]    ILRipple [Ap-p]: reactor current ripple (peak-to-peak value), 
         [0143]    Vt [V]: voltage applied to reactor, 
         [0144]    L [H]: inductance of reactor, 
         [0145]    Time [s]: voltage application time, 
         [0146]    fc [Hz]: carrier frequency, 
         [0147]    Vin [V]: input voltage, 
         [0148]    Vout [V]: output voltage. 
         [0149]    Here, assuming that when the switch of the lower arm of the step-up converter is on, Vt equals Vin, Formula (2) will be the same as Formula (3). 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     LRipple 
                   
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                             V 
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                   Formula 
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         [0150]    The ILRipple takes the maximum value when Formula (4) results in 0, that is, when Formula (5) establishes. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                        
                       
                         I 
                         LRipple 
                       
                     
                     
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                           V 
                           in 
                         
                         L 
                       
                       · 
                       
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                               V 
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                     V 
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                       out 
                     
                   
                 
               
               
                 
                   Formula 
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         [0151]    From Formula (5), when an output voltage is twice as much as the input voltage, the reactor current ripple becomes the largest. Since from Formula (1), an alternating current copper loss of the coil is proportional to an effective value of the reactor current ripple, when the output voltage is double the input voltage, the alternating current copper loss becomes larger than that in other voltage conditions, so that the coil temperature is increased. Also with respect to the core temperature, the core loss becomes the largest with Vin/Vout=0.5 when the reactor current ripple is the largest, so that the core temperature becomes higher than with other input voltage. 
         [0152]    With this arrangement, switching the control mode so as to select a predetermined duty ratio such that a loss becomes the largest during the discharge operation enables speed-up of capacitor discharging in the discharge operation of the step-up converter  100 . 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  battery 
           10  chassis ground 
           20  capacitor 
           30  Y capacitor 
           40  filter capacitor 
           110  smoothing capacitor 
           70  reactor 
           50  voltage sensor 
           120  voltage sensor 
           60  current sensor 
           270  current sensor 
           280  current sensor 
           310  angle detector 
           350  angle detector 
           100  step-up converter 
           80  switching element of upper arm of step-up converter  100   
           90  switching element of lower arm of step-up converter  100   
           440  P-Q voltage command generation unit of step-up converter  100   
           450  duty command generation unit of step-up converter  100   
           580  carrier generation unit for step-up converter  100   
           590  carrier frequency setting unit for step-up converter  100   
           620  duty command calculation unit of step-up converter  100   
           620  duty command updating unit of step-up converter  100   
           650  comparison unit for step-up converter  100   
           670  switching prohibiting processing unit for step-up converter  100   
           190  inverter 
           260  inverter 
           130  U-phase switching element of upper arm of inverter  190   
           140  U-phase switching element of lower arm of inverter  190   
           150  V-phase switching element of upper arm of inverter  190   
           160  V-phase switching element of lower arm of inverter  190   
           170  W-phase switching element of upper arm of inverter  190   
           180  U-phase switching element of lower arm of inverter  190   
           200  U-phase switching element of upper arm of inverter  260   
           210  U-phase switching element of lower arm inverter  260   
           220  V-phase switching element of upper arm of inverter  260   
           230  V-phase switching element of lower arm of inverter  260   
           240  W-phase switching element of upper arm of inverter  260   
           250  U-phase switching element of lower arm of inverter  260   
           470  UVW phase duty command generation unit for inverters  190  and  260   
           480  UVW phase voltage command generation unit for inverters  190  and  260   
           560  carrier frequency setting unit for inverters  190  and  260   
           570  carrier generation unit for inverters  190  and  260   
           600  UVW phase duty command calculation unit for inverters  190  and  260   
           610  UVW phase duty command updating unit for inverters  190  and  260   
           640  comparison unit for inverters  190  and  260   
           660  switching prohibiting processing unit for inverters  190  and  260   
           290  three-Phase coil wound around stator of motor generator  320   
           330  three-phase coil wound around a stator of the motor generator  360   
           300  rotor of motor generator  320   
           340  rotor of motor generator  360   
           320  motor generator 
           360  motor generator 
           370  motor generator shaft of motor generator  320   
           375  motor generator shaft of motor generator  360   
           460  switching signal generation unit 
           500  dc-axis current command generation unit 
           520  carrier generation unit 
           490  UVW phase/dg-axis conversion unit 
           510  rotation speed calculation unit 
           530  control device 
           540  gate drive circuit 
           550  phase current maximum value calculation unit 
           430  drive shaft of vehicle 
           385  engine 
           390  crank shaft 
           400  propeller shaft 
           420  drive shaft 
           380  transmission 
           410  differential gear 
         iL current value flowing through reactor  70   
         θ 1  magnetic pole position of rotor  300  of motor generator  320   
         θ 2  magnetic pole position of rotor  340  of motor generator  360   
         ω 1  rotation speed of rotor  300  of motor generator  320   
         ω 2  rotation speed of rotor  340  of motor generator  360   
         vcs voltage values at both ends of smoothing capacitor  110   
         Db* duty command value of step-up converter  100   
         Sbp switching signal of upper arm of step-up converter  100   
         Sbn switching signal of lower arm of step-up converter  100   
         id 1  d-axis current value of motor generator  320   
         iq 1  c-axis current value of motor generator  320   
         id 2  d-axis current value of motor generator  360   
         iq 2  q-axis current value of motor generator  360   
         vcin voltage values at both ends of filter capacitor  40   
         Sup 1  U-phase switching signal of upper arm of inverter  190   
         Sun 1  U-phase switching signal of lower arm of inverter  190   
         Svp 1  V-phase switching signal of upper arm of inverter  190   
         Svn 1  V-phase switching signal of lower arm of inverter  190   
         Swp 1  U-phase switching signal of upper arm of inverter  190   
         Swn 1  W-phase switching signal of lower arm of inverter  190   
         Sup 2  U-phase switching signal of upper arm of inverter  260   
         Sun 2  U-phase switching signal of lower arm of inverter  260   
         Svp 2  V-phase switching signal of upper arm of inverter  260   
         Svn 2  V-chase switching signal of lower arm of inverter  260   
         Swp 2  W-phase switching signal of upper arm of inverter  260   
         Swn 2  W-phase switching signal of lower arm of inverter  260   
         iud 1  current value flowing through U-phase coil of motor generator  320   
         ivd 1  current value flowing through V-phase coil of motor generator  320   
         iwd 1  current value flowing through U-phase coil of motor generator  320   
         iud 2  current value flowing through U-phase coil of motor generator  360   
         ivd 2  current value flowing through V-phase coil of motor generator  360   
         iwd 2  current value flowing through W-phase coil of motor generator  360   
         id 1 * d-axis current command value of motor generator  320   
         ig 1 * g-axis current command value of motor generator  320   
         id 2 * d-axis current command value of motor generator  360   
         ig 2 * g-axis current command value of motor generator  360   
         vcs* output voltage command of step-up converter  100   
         vpa* P-Q voltage command value 
         vu 1 * U-phase voltage command value of inverter  190   
         vv 1 * v-phase voltage command value or inverter  190   
         vw 1 * W-phase voltage command value of inverter  190   
         vu 2 * U-phase voltage command value of inverter  260   
         vv 2 * V-phase voltage command value of inverter  260   
         vw 2 * U-phase voltage command value of inverter  260   
         Du 1 * U-phase duty command value of inverter  190   
         Dv 1 * V-chase duty command value of inverter  190   
         Dw 1 * U-phase duty command value of inverter  190   
         Du 2 * U-phase duty command value of inverter  260   
         Dv 2 * V-phase duty command value of inverter  260   
         Dw 2 * W-phase duty command value of inverter  260   
         carrier 1  triangular wave carrier for step-up converter  100   
         carrier 2  triangular wave carrier for inverters  190  and  260   
         fcarrier 1  carrier frequency for step-up converter  100   
         fcarrier 2  carrier frequency for inverters  190  and  260   
         Tcarrier 1  cycle of triangular wave carrier carrier 1   
         Tcarrier 2  cycle of triangular wave carrier carrier 2