Patent Publication Number: US-RE41303-E

Title: Load driver and control method for safely driving DC load and computer-readable recording medium with program recorded thereon for allowing computer to execute the control

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a load driver for driving a DC load connected to a DC power supply. The present invention further relates to a control method for driving the DC load connected to the DC power supply. Moreover, the present invention relates to a computer-readable recording medium on which a program is recorded that allows a computer to execute the control for driving the DC load. 
     2. Description of the Background Art 
     Hybrid vehicles and electric vehicles are now attracting considerable attention as they help the environment. Some hybrid vehicles are now commercially available. 
     The hybrid vehicle includes, as its power source, a DC power supply, an inverter and a motor driven by the inverter in addition to a conventional engine. Specifically, the engine is driven to generate power while a DC voltage from the DC power supply is converted into AC voltage by the inverter to rotate the motor by the AC voltage and accordingly generate power. The power source of the electric vehicle is a DC power supply, an inverter and a motor driven by the inverter. 
     Such a hybrid vehicle or electric vehicle is designed for example to include a motor driver as shown in FIG.  16 . Referring to  FIG. 16 , motor driver  600  includes a DC power supply B, system relays SR 1  and SR 2 , a capacitor C, a bidirectional voltage converter  410 , and an inverter  420 . Bidirectional voltage converter  410  includes a reactor L, NPN transistors Q 10  and Q 11 , and diodes D 10  and D 11 . Reactor L has one end connected to a power supply line of DC power supply B and the other end connected to an intermediate point between NPN transistors Q 10  and Q 11 , i.e., between the emitter of NPN transistor Q 10  and the collector of NPN transistor Q 11 . NPN transistors Q 10  and Q 11  are connected in series between a power supply line and a ground line. NPN transistor Q 10  has its collector connected to the power supply line of inverter  420  while NPN transistor Q 11  has its emitter connected to the ground line. Between the emitter and collector of NPN transistors Q 10  and Q 11  each, corresponding one of diodes D 10  and D 11  is provided to flow current from the emitter to the collector. 
     DC power supply B supplies a DC voltage to capacitor C when system relays SR 1  and SR 2  are made on. Capacitor C smoothes the DC voltage from DC power supply B to supply the smoothed DC voltage to bidirectional voltage converter  410 . Bidirectional voltage converter  410  is controlled by a control unit (not shown) to boost the DC voltage from capacitor C in response to a period during which NPN transistor Q 11  is kept on. Converter  410  then supplies the boosted DC voltage to inverter  420 . Bidirectional voltage converter  410  is also controlled by the control unit to down-convert a DC voltage converted by inverter  420  to charge DC power supply B in regenerative power generation by a motor M. 
     Inverter  420  receives the DC voltage from bidirectional voltage converter  410  via a smoothing capacitor (not shown) and converts the DC voltage into an AC voltage under control by a control unit (not shown) to drive motor M. Further, in regenerative power generation mode by motor M, inverter  420  receives an AC voltage from motor M and converts the AC voltage into a DC voltage under control by the control unit to supply the DC voltage to bidirectional voltage converter  410 . Motor M is driven by inverter  420  to generate predetermined torque. In addition, motor M serves as a regenerative generator to supply the generated AC voltage to inverter  420 . 
     DC/DC converter  430  is located between bidirectional voltage converter  410  and DC power supply B to be connected to DC power supply B and receives the DC voltage from DC power supply B. DC/DC converter  430  is used for auxiliary equipment of the vehicle and down-converts the DC voltage from DC power supply B and supplies the down-converted DC voltage to an inverter (not shown) driving an air conditioner (not shown) provided in the hybrid or electric vehicle. 
     In motor driver  600 , DC power supply B supplies the DC voltage to capacitor C when system relays SR 1  and SR 2  are made on, and then capacitor C smoothes the DC voltage to supply the smoothed voltage to bidirectional voltage converter  410  and DC/DC converter  430 . Bidirectional voltage converter  410  boosts the DC voltage in response to a period during which NPN transistor Q 11  is kept on and supplies the boosted DC voltage to inverter  420  via the smoothing capacitor (not shown). Inverter  420  converts the DC voltage into the AC voltage to drive motor M. Motor M generates predetermined torque. On the other hand, DC/DC converter  430  down-converts the DC voltage from capacitor C to supply the down-converted voltage to the inverter which drives the air conditioner. 
     In regenerative braking of the hybrid or electric vehicle, motor M generates the AC voltage to be supplied to inverter  420 . Inverter  420  converts the AC voltage from motor M into the DC voltage to be supplied to bidirectional voltage converter  410 . Bidirectional voltage converter  410  down-converts the DC voltage from inverter  420  to charge DC power supply B. In this way, motor driver  600  boosts the DC voltage from DC power supply B to drive motor M, and motor driver  600  also charges DC power supply B with the voltage generated by motor M in regenerative braking. 
     Alternatively, a hybrid or electric vehicle is designed to include a motor driver as shown in FIG.  17 . Referring to  FIG. 17 , motor driver  700  has the same configuration as that of motor driver  600  except that a DC/DC converter  440  of motor driver  700  is connected to the output of bidirectional voltage converter  410 . 
     DC/DC converter  440  receives a voltage which is boosted by bidirectional voltage converter  410  and down-converts the boosted voltage to charge an auxiliary buttery  450  (with output voltage of 12 V for example) which supplies electric power to such a control circuit as an ECU (Electrical Control Unit). Regarding the configuration as shown in  FIG. 17 , even if any abnormal event of DC power supply B, fuse blowing or any abnormal event of system relays SR 1  and SR 2  for example occurs, DC/DC converter  440  is supplied with a DC voltage generated by motor M 1  and converted by inverter  420 . In other words, even if any abnormal event occurs in the circuitry between bidirectional voltage converter  410  and DC power supply B, auxiliary battery  450  for driving such a control circuit as ECU never becomes empty and thus the vehicle is prevented from being unable to move. 
     As for the conventional motor driver  600  in regenerative power generation, if DC power supply B is separated due to malfunction of system relays SR 1  and SR 2  or break, a voltage Vb appearing on the DC power supply B side of bidirectional voltage converter  410  increases resulting in a problem that an overvoltage is applied to DC/DC converter  430  which is a DC load. 
     In order to protect DC load system from the overvoltage, the withstand voltage of the DC load system should be enhanced which requires components with a high withstand voltage. Then, the overall cost cannot be reduced. Therefore, it is necessary to prevent the overvoltage from being applied to the DC load system in regenerative power generation if the DC power supply B is separated due to any reason. 
     As for the conventional motor driver  700 , DC/DC converter  440  is connected to the output of bidirectional voltage converter  410 . Then, a high withstand voltage is required and accordingly, the requirements of the specification of components are considerably severe. A resultant problem is that the configuration of the circuitry becomes complicated which leads to difficulty in reduction of the cost and size. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is thus to provide a load driver that safely drives a DC load connected to a DC power supply. 
     Another object of the present invention is to provide a control method for safely driving a DC load connected to a DC power supply. 
     Still another object of the present invention is to provide a computer-readable recording medium on which a program is recorded for allowing a computer to execute control for safely driving a DC load connected to a DC power supply. 
     According to the present invention, a load driver includes a DC power supply, a voltage converter, a DC load, and a control unit. 
     The DC power supply outputs a DC voltage. The voltage converter is connected to the DC power supply to provide, toward the DC power supply, a DC voltage based on electric power generated by a power-generating unit. The DC load is connected in parallel with the voltage converter to the DC power supply. The control unit executes at least one of first control and second control when a malfunction is detected in an electrical system between the DC power supply and the voltage converter, the first control being executed to protect an electrical system of the DC load and the second control being executed to continue operation of the DC load. 
     Preferably, the control unit executes the first control to protect the electrical system of the DC load when the malfunction is detected in the electrical system between the DC power supply and the voltage converter. 
     Preferably, when the malfunction is detected in the electrical system between the DC power supply and the voltage converter, the control unit controls the voltage converter to apply a voltage smaller than a predetermined value to the electrical system of the DC load. 
     Preferably, the load driver further includes a voltage sensor detecting a voltage applied to the DC load. The control unit stops operation of the voltage converter when the voltage detected by the voltage sensor reaches at least the predetermined value. 
     Preferably, the load driver further includes a voltage sensor. The voltage sensor detects a DC voltage on an input of the voltage converter when a DC current is supplied from the DC power supply to the voltage converter. The control unit stops operation of the voltage converter when the DC voltage detected by the voltage sensor reaches at least the predetermined value. 
     Preferably, the power-generating unit is formed of at least one generator. 
     Preferably, each of at least one generators is an AC generator, and the load driver further includes at least one inverters provided correspondingly to that at least one generators and each converting an AC voltage supplied from the corresponding AC generator into a DC voltage. The control unit controls each of at least one inverters in a normal operation to convert the AC voltage into the DC voltage and supply the DC voltage converted from the AC voltage to the voltage converter. 
     Preferably, the load driver further includes a first voltage sensor and a second voltage sensor. The first voltage sensor detects a first DC voltage output from the DC power supply. The second voltage sensor detects a second DC voltage on an input of the voltage converter when a DC current is supplied from the DC power supply to the voltage converter. The control unit stops operation of the voltage converter when the first voltage detected by the first voltage sensor differs from the second voltage detected by the second voltage sensor. 
     Preferably, the power-generating unit is formed of at least one generator. 
     Preferably, the generator is an AC generator, and the load driver further includes an inverter converting an AC voltage supplied from the AC generator into a DC voltage. The control unit controls the inverter in a normal operation to convert the AC voltage into the DC voltage and supply the DC voltage converted from the AC voltage to the voltage converter. 
     Preferably, the power-generating unit is formed of a plurality of generators. 
     Preferably, the control unit further controls a plurality of drivers corresponding respectively to those generators to keep a balance between supply and consumption of electric energy with respect to those generators, and controls the electrical system of the DC load to drive the DC load by electric power supplied from the DC power supply. 
     Preferably, those generators are each an AC generator. The load driver further includes a plurality of inverters provided correspondingly to those generators and each converting an AC voltage supplied from a corresponding AC generator into a DC voltage. The control unit controls each of the inverters in a normal operation to convert the AC voltage into the DC voltage and supply the DC voltage converted from the AC voltage to the voltage converter. 
     Preferably, the generator is a drive motor generating drive power for a vehicle. 
     Preferably, the load driver further includes first and second voltage sensors. The first voltage sensor detects a first DC voltage output from the DC power supply and the second voltage sensor detects a second DC voltage on an input of the voltage converter when a DC current is supplied from the DC power supply to the voltage converter. The control unit executes the second control when the first voltage detected by the first voltage sensor differs from the second voltage detected by the second voltage sensor. 
     Preferably, the control unit executes control for supplying a DC voltage based on electric power generated by the power-generating unit to the DC load. 
     Preferably, the control unit controls the voltage converter for directly supplying to the DC load a DC voltage based on the electric power generated by the power-generating unit and having a voltage level lower than a predetermined value. 
     Preferably, the voltage converter includes first and second switching elements and a reactor. The first and second switching elements are connected in series between terminals receiving the DC voltage, switching of at least one of the switching elements being controlled in voltage-up-converting operation and voltage-down-converting operation. The reactor has one end connected to a point of connection between the first switching element and the second switching element. The reactor and the second switching element are connected in series between terminals of the DC power supply. The control unit keeps the first switching element continuously in a conducting state and keeps the second switching element continuously in a disconnected state. 
     Preferably, the load driver further includes a supply unit and a switching unit. The supply unit directly supplies to the DC load a DC voltage based on the electric power generated by the power-generating unit and having a voltage level lower than a predetermined value. The switching unit switches supply of the DC voltage between the voltage converter and the supply unit. The control unit controls the switching unit to supply the DC voltage to the supply unit. 
     According to the present invention, a control method for safely driving a DC load connected to a DC power supply includes a first step of detecting a malfunction in an electrical system between the DC power supply and a voltage converter converting voltage, and a second step of executing at least one of first control and second control when the malfunction is detected, the first control being executed to protect an electrical system of the DC load connected in parallel with the voltage converter to the DC power supply and the second control being executed to continue operation of the DC load. 
     Preferably, the first control is executed in the second step to protect the electrical system of the DC load connected in parallel with the voltage converter to the DC power supply. 
     Preferably, the first step includes a first sub step of detecting a voltage applied to the DC load and a second sub step of detecting whether or not the detected voltage is equal to or more than a predetermined value. In the second step, operation of the voltage converter is stopped when the detected voltage is equal to or more than the predetermined value. 
     Preferably, the first step includes a first sub step of detecting a DC voltage on an input of the voltage converter when a DC current is supplied from the DC power supply to the voltage converter and a second sub step of detecting whether or not the detected voltage is equal to more than a predetermined value. In the second step, operation of the voltage converter is stopped when the detected voltage is equal to or more than the predetermined value. 
     Preferably, the first step includes a first sub step of detecting a first voltage output from the DC power supply, a second sub step of detecting a second DC voltage on an input of the voltage converter when a DC current is supplied from the DC power supply to the voltage converter, and third sub step of detecting whether or not the first voltage detected in the first sub step matches the second voltage detected in the second sub step. In the second step, operation of the voltage converter is stopped when the first voltage does not match the second voltage. 
     Preferably, the voltage converter is connected to a plurality of inverters provided correspondingly to a plurality of power-generating units. The control method further includes a third step of controlling those inverters to maintain a balance between supply and consumption of electric energy with respect to those power-generating units, and a fourth step of controlling the electrical system of the DC load to drive the DC load by electric power supplied from the DC power supply. 
     Preferably, the first step includes a first sub step of detecting a first voltage output from the DC power supply, a second sub step of detecting a second DC voltage on an input of the voltage converter when a DC current is supplied from the DC power supply to the voltage converter and a third sub step of detecting whether or not the first voltage detected in the first sub step matches the second voltage detected in the second sub step. In the second step, the second control is executed when the first voltage does not match the second voltage. 
     Preferably, in the second step, control is executed to supply, to the DC load, DC power based on electric power generated by a power-generating unit. 
     Preferably, in the second step, the voltage converter is controlled to directly supply, to the DC load, a DC voltage based on the electric power generated by the power-generating unit and having a voltage level lower than a predetermined value. 
     Preferably, the voltage converter includes first and second switching elements and a reactor. The first and second switching elements are connected in series between terminals receiving the DC voltage, switching of at least one of the switching elements being controlled in voltage-up-converting operation and voltage-down-converting operation. The reactor has one end connected to a point of connection between the first switching element and the second switching element. The reactor and the second switching element are connected in series between terminals of the DC power supply. 
     Here, the second step of the control method includes a fourth sub step of keeping the first switching element continuously in a conducting state and a fifth sub step of keeping the second switching element continuously in a disconnected state. 
     Preferably, the DC load is connected to a supply unit and the voltage converter, the supply unit supplying, toward the DC power supply, a DC voltage based on the electric power generated by the power-generating unit. The supply unit and the voltage converter are connected to a switching unit switching supply of the DC voltage between the supply unit and the voltage converter. In the second step of the control method, the switching unit is controlled to supply, to the supply unit, a DC voltage based on the electric power generated by the power-generating unit and having a voltage level lower than a predetermined value. 
     According to the present invention, a computer-readable recording medium has a program recorded thereon to allow a computer to execute control for safely driving a DC load connected to a DC power supply. The computer executes a first step of detecting a malfunction in an electrical system between the DC power supply and a voltage converter converting voltage, and a second step of executing at least one of first control and second control when the malfunction is detected, the first control being executed to protect an electrical system of the DC load connected in parallel with the voltage converter to the DC power supply and the second control being executed to continue operation of the DC load. 
     Preferably, the first control is executed in the second step to protect the electrical system of the DC load connected in parallel with the voltage converter to the DC power supply. 
     Preferably, the first step includes a first sub step of detecting a voltage applied to the DC load and a second sub step of detecting whether or not the detected voltage is euqal to or more than a predetermined value. In the second step, operation of the voltage converter is stopped when the detected voltage is equal to or more than the predetermined value. 
     Preferably, the first step includes a first sub step of detecting a DC voltage on an input of the voltage converter when a DC current is supplied from the DC power supply to the voltage converter and a second subs step of detecting whether or not the detected voltage is equal to or more than a predetermined value. In the second step, operation of the voltage converter is stopped when the detected voltage is equal to or more than the predetermined value. 
     Preferably, the first step includes a first sub step of detecting a first voltage output from the DC power supply, a second sub step of detecting a second DC voltage on an input of the voltage converter when a DC current is supplied from the DC power supply to the voltage converter, and a third sub step of detecting whether or not the first voltage detected in the first sub step matches the second voltage detected in the second sub step. In the second step, operation of the voltage converter is stopped when the first voltage does not match the second voltage. 
     Preferably, the voltage converter is connected to a plurality of inverters provided correspondingly to a plurality of power-generating units. The program allows the computer to further execute a third step of controlling those inverters to maintain a balance between supply and consumption of electric energy with respect to those power-generating units, and a fourth step of controlling the electrical system of the DC load to drive the DC load by electric power supplied from the DC power supply. 
     Preferably, the first step includes a first sub step of detecting a first voltage output from the DC power supply, a second sub step of detecting a second DC voltage on an input of the voltage converter when a DC current is supplied from the DC power supply to the voltage converter, and a third sub step of detecting whether or not the first voltage detected in the first sub step matches the second voltage detected in the second sub step. In the second step, the second control is executed when the first voltage does not match the second voltage. 
     Preferably, in the second step, control is executed to supply, to the DC load, DC power based on electric power generated by a power-generating unit. 
     Preferably, in the second step, the voltage converter is controlled to directly supply, to the DC load, a DC voltage based on the electric power generated by the power-generating unit and having a voltage level lower than a predetermined value. 
     Preferably, the voltage converter includes first and second switching elements and a reactor. The first and second switching elements are connected in series between terminals receiving the DC voltage, switching of at least one of the switching elements being controlled in voltage-up-converting operation and voltage-down-converting operation. The reactor has one end connected to a point of connection between the first switching element and the second switching element. The reactor and the second switching element are connected in series between terminals of the DC power supply. 
     Here, the second step of the program includes a fourth sub step of keeping the first switching element continuously in a conducting state and a fifth sub step of keeping the second switching element continuously in a disconnected state. 
     Preferably, the DC load is connected to a supply unit and the voltage converter, the supply unit supplying, toward the DC power supply, a DC voltage based on the electric power generated by the power-generating unit. The supply unit and the voltage converter are connected to a switching unit switching supply of the DC voltage between the supply unit and the voltage converter. 
     Here, in the second step of the program, the switching unit is controlled to supply, to the supply unit, a DC voltage based on the electric power generated by the power-generating unit and having a voltage level lower than a predetermined value. 
     In this way, according to the present invention, the DC load connected between the DC power supply and the voltage converter is safely driven. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram schematically showing a motor driver according to a first embodiment. 
         FIG. 2  is a functional block diagram of a control unit shown in FIG.  1 . 
         FIG. 3  is a functional block diagram illustrating the function of motor torque control means shown in FIG.  2 . 
         FIG. 4  is a flowchart illustrating an operation of the motor driver shown in FIG.  1 . 
         FIG. 5  is a flowchart illustrating another operation of the motor driver shown in FIG.  1 . 
         FIG. 6  is a flowchart illustrating still another operation of the motor driver shown in FIG.  1 . 
         FIG. 7  is a block diagram schematically showing a motor driver according to a second embodiment of the present invention. 
         FIG. 8  is a functional block diagram of a control unit shown in FIG.  7 . 
         FIG. 9  is a flowchart illustrating an operation of the motor driver shown in FIG.  7 . 
         FIG. 10  is a block diagram schematically showing a motor driver according to a third embodiment of the present invention. 
         FIG. 11  is a flowchart illustrating an operation of the motor driver shown in FIG.  10 . 
         FIG. 12  is a flowchart illustrating another operation of the motor driver shown in FIG.  10 . 
         FIG. 13  is a block diagram schematically showing a motor driver according to a fourth embodiment. 
         FIG. 14  is a flowchart illustrating an operation of the motor driver shown in FIG.  13 . 
         FIG. 15  is a flowchart illustrating another operation of the motor driver shown in FIG.  13 . 
         FIG. 16  is a block diagram schematically showing a conventional motor driver. 
         FIG. 17  is a block digram schematically showing another conventional motor driver. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are hereinafter described in detail in conjunction with the drawings. It is noted that the same or corresponding components in the drawings are denoted by the same reference character and description thereof is not repeated here. 
     First Embodiment 
     Referring to  FIG. 1 , a motor driver  100  having a load driver according to a first embodiment of the present invention includes a DC power supply B, voltage sensors  10 ,  11 ,  13  and  18 , system relays SR 1  and SR 2 , capacitors C 1  and C 2 , an up-converter  12 , an inverter  14 , a current sensor  24 , and a control unit  30 . Motor driver  100  drives one motor M 1 . Motor M 1  is a drive motor generating torque for driving drive wheels of a hybrid or electric vehicle. This motor serves as a power generator driven by an engine as well as an electric motor for the engine. Alternatively, the motor may be capable of starting the engine and incorporated as such into a hybrid vehicle. 
     Up-converter  12  includes a reactor L 1 , NPN transistors Q 1  and Q 2  and diodes D 1  and D 2 . Reactor L 1  has one end connected to a power supply line of DC power supply B and the other end connected to the intermediate point between NPN transistors Q 1  and Q 2 , i.e., between the emitter of NPN transistor Q 1  and the collector of NPN transistor Q 2 . 
     NPN transistors Q 1  and Q 2  are connected in series between a power supply line of inverter  14  and a ground line. The collector of NPN transistor Q 1  is connected to the power supply line and the emitter of NPN transistor Q 2  is connected to the ground line. Between the collector and emitter of NPN transistors Q 1  and Q 2  each, corresponding one of diodes D 1  and D 2  is connected for flowing current from the emitter to the collector. 
     Inverter  14  is constituted of U-phase arm  15 , a V-phase arm  16  and a W-phase arm  17 . U-phase arm  15 , V-phase arm  16  and W-phase arm  17  are connected in parallel between the power supply line and the ground. 
     U-phase arm  15  is constituted of series-connected NPN transistors Q 3  and Q 4 , V-phase arm  16  is constituted of series-connected NPN transistors Q 5  and Q 6 , and W-phase arm  17  is constituted of series-connected NPN transistors Q 7  and Q 8 . Diodes D 3  to D 8  are each connected between the collector and emitter of a corresponding one of NPN transistors Q 3 -Q 8  for allowing current to flow from the emitter to the collector. 
     The U, V and W-phase arms have respective intermediate points connected to respective ends of phase coils of motor M 1 . Motor M 1  is a three-phase permanent-magnet motor with respective three coils of U, V and W phases each having one end connected commonly to the center. The other end of the U-phase coil is connected to the intermediate point between NPN transistors Q 3  and Q 4 , the other end of the V-phase coil is connected to the intermediate point between NPN transistors Q 5  and Q 6 , and the other end of the W-phase coil is connected to the intermediate point between NPN transistors Q 7  and Q 8 . 
     DC power supply B is formed of a nickel-hydrogen or lithium-ion secondary battery. DC power supply B outputs a DC voltage of 200-300 V for example. Voltage sensor  10  detects a voltage V 1  from DC power supply B to output the detected voltage V 1  to control unit  30 . System relays SR 1  and SR 2  are made on by a signal SE from control unit  30 . 
     Capacitor C 1  smoothes a DC voltage supplied from DC power supply B to provide the smoothed DC voltage to up-converter  12  and a DC/DC converter  19 . Voltage sensor  11  detects a voltage V 2  on the input side of up-converter  12  to output the detected voltage V 2  to control unit  30 . 
     Up-converter  12  boosts the DC voltage from capacitor C 1  to supply the boosted voltage to capacitor C 2 . More specifically, up-converter  12  receives a signal PWU from control unit  30  to boost and supply the DC voltage to capacitor C 2  in response to a period in which NPN transistor Q 2  is made on by signal PWU. In this case, NPN transistor Q 1  is turned off by signal PWU. Further, up-converter  12  receives a signal PWD from control unit  30  to down-convert a DC voltage supplied from inverter  14  via capacitor C 2  and accordingly charge DC power supply B. In addition, up-converter  12  receives a signal STP from control unit  30  to stop operating. 
     Capacitor C 2  smoothes the DC voltage from up-converter  12  to supply the smoothed DC voltage to inverter  14 . Voltage sensor  13  detects the voltage on both ends of capacitor C 2 , i.e., voltage IVV to be supplied to inverter  14  and outputs the detected input voltage IVV to control unit  30 . 
     Inverter  14  receives the DC voltage from capacitor C 2  to convert, according to a signal PWM 1  from control unit  30 , the DC voltage into an AC voltage and accordingly drive motor M 1 . Then, motor M 1  is driven to generate torque designated by a torque control value TR. In regenerative braking of a hybrid or electric vehicle including motor driver  100 , inverter  14  converts an AC voltage generated by motor M 1  into a DC voltage according to a signal PWMC from control unit  30  and supplies the converted voltage to up-converter  12  via capacitor C 2 . 
     Here, “regenerative braking” includes braking which is caused when a driver (operator) of a hybrid or electric vehicle manages the foot brake and which is accompanied by regenerative power generation as well as deceleration (or stopping of acceleration) of the vehicle by releasing the accelerator (pedal) in driving without managing the foot brake, which is also accompanied by regenerative power generation. 
     Voltage sensor  18  detects a voltage Vf applied from DC power supply B to DC/DC converter  19  to output the detected voltage Vf to control unit  30 . 
     Current sensor  24  detects a motor current MCRT flowing to motor M 1  to output the detected motor current MCRT to control unit  30 . 
     Control unit  30  generates, based on torque control value TR and motor rotation number MRN supplied from an externally placed ECU (electrical control unit), voltage V 1  from voltage sensor  10 , input voltage IVV from voltage sensor  13  and motor current MCRT from current sensor  24 , signal PWU for driving up-converter  12  and signal PWM 1  for driving inverter  14  following a method as described hereinbelow, and provides the signals PWU and PWM 1  to up-converter  12  and inverter  14  respectively. 
     Control unit  30  receives from the external ECU a signal indicating that the hybrid or electric vehicle enters a regenerative braking mode to generate signal PWMC for converting an AC voltage generated by motor M 1  into a DC voltage and output the signal to inverter  14 . In this case, switching of NPN transistors Q 4 , Q 6  and Q 8  of inverter  14  is controlled by signal PWMC. Specifically, NPN transistors Q 6  and Q 8  are turned on when electric power is generated by the U phase of motor M 1 , NPN transistors Q 4  and Q 8  are turned on when the electric power is generated by the V phase thereof, and NPN transistors Q 4  and Q 6  are turned on when the electric power is generated by the W phase thereof. In this way, inverter  14  converts the AC voltage generated by motor M 1  into the DC voltage to supply the DC voltage to up-converter  12 . 
     Moreover, control unit  30  receives voltage V 2  from voltage sensor  11  (or voltage Vf from voltage sensor  18 ) to determine whether or not the received voltage V 2  (or Vf) is higher than a predetermined value. When control unit  30  determines that voltage V 2  (or Vf) is higher than the predetermined value, control unit  30  accordingly determines that an overvoltage is applied to the input of up-converter  12  to generate signal STP for stopping up-converter  12  and supply signal STP to up-converter  12 . 
     In this case, control unit  30  may determine whether or not voltage V 1  from voltage sensor  10  matches voltage V 2  from voltage sensor  11  and generates signal STP to output the signal STP to up-converter  12  when voltage V 1  does not match voltage V 2 . The fact that voltage V 1  does not match voltage V 2  means that DC power supply B is separated from capacitor C 1 , up-converter  12  and DC/DC converter  19  due to any malfunction of system relays SR 1  and SR 2  or break. 
     It is seen from the above that the first embodiment is characterized in that up-converter  12  is stopped when voltage V 2  applied to the input to up-converter  12  (or voltage Vf applied to DC/DC converter  19 ) is an overvoltage or DC power supply B is separated due to any reason. 
     Moreover, control unit  30  generates signal SE for allowing system relays SR 1  and SR 2  to be made on to supply signal SE to relays SR 1  and SR 2 . 
     DC/DC converter  19  down-converts the DC voltage from DC power supply B to provide the down-converted voltage to an inverter  20 . Inverter  20  converts the DC voltage from DC/DC converter  19  into an AC voltage for driving a motor  21  used for an air conditioner. Air-conditioner motor  21  drives the compressor of the air conditioner. 
     DC/DC converter  19 , inverter  20  and air-conditioner motor  21  constitute auxiliary equipment provided to the hybrid or electric vehicle. In addition, DC/DC converter  19  constitutes a DC load provided to the auxiliary equipment. 
     As for motor driver  100 , capacitor C 2  is driven with approximately 500 V at the maximum, and accordingly the electrical system of capacitor C 2  and inverter  14  provided on the output side of up-converter  12  is constituted of components having an withstand voltage in the range from 750 V to 900 V. 
     On the other hand, the auxiliary equipment-related circuitry including DC/DC converter  19 , inverter  20  and air-conditioner motor  21  is constituted of components having a withstand voltage of approximately 400 V. 
       FIG. 2  is a functional block diagram of control unit  30 . Referring to  FIG. 2 , control unit  30  includes motor-torque control means  301  and voltage-conversion control means  302 . Motor-torque control means  301  generates, based on torque control value TR, output voltage V 1  of DC power supply B, motor current MCRT, motor rotation number MRN and inverter input voltage IVV, signal PWU for turning on/off NPN transistors Q 1  and Q 2  of up-converter  12  and signal PWM 1  for turning on/off NPN transistors Q 3 -Q 8  of inverter  14 , when motor M 1  is driven, following a method as described hereinbelow, and provides the generated signals PWU and PWM 1  respectively to up-converter  12  and inverter  14 . 
     Voltage-conversion control means  302  receives voltage V 2  from voltage sensor  11  (or voltage Vf from voltage sensor  18 ) to generate signal STP for stopping up-converter  12  if voltage V 2  (or Vf) is higher than a predetermined value and provide the signal STP to up-converter  12 . Further voltage-conversion control means  302  receives voltage V 1  from voltage sensor  10  to generate signal STP if voltage V 1  differs from voltage V 2  and provide the signal STP to up-converter  12 . Moreover, in regenerative braking, voltage-conversion control means  302  generates a signal PWD for down-converting the DC voltage supplied from inverter  14  to output the signal PWD to up-converter  12 . Up-converter  12  thus serves as a bidirectional converter since converter  12  can also down-convert or reduce the voltage by signal PWD for down-converting the DC voltage. In addition, voltage-conversion control means  302  generates signal PWMC for converting the AC voltage generated by motor M 1  into DC voltage to supply the signal PWMC to inverter  14 . 
       FIG. 3  is a functional block diagram of motor-torque control means  301 . Referring to  FIG. 3 , motor-torque control means  301  includes a phase voltage calculating unit  40  for controlling the motor, a PWM signal converting unit  42  for the inverter, an inverter-input-voltage calculating unit  50 , a duty ratio calculating unit  52  for the converter, and a PWM signal converting unit  54  for the converter. 
     Phase voltage calculating unit  40  receives, from voltage sensor  13 , input voltage IVV to inverter  14 , receives, from current sensor  24 , motor current MCRT flowing to each phase of motor M 1 , and receives torque control value TR from the external ECU. Based on the supplied signal, current and voltage, phase voltage calculating unit  40  calculates a voltage to be applied to the coil of each phase of motor M 1  and supplies the calculated voltage to PWM signal converting unit  42 . Then, based on the calculated voltage supplied from phase voltage calculating unit  40 , PWM signal converting unit  42  generates signal PWM 1  for actually turning on/off each of NPN transistors Q 3 -Q 8  of inverter  14  and supplies the generated signal PWM 1  to each of NPN transistors Q 3 -Q 8  of inverter  14 . 
     Switching of NPN transistors Q 3 -Q 8  each is thus controlled and NPN transistors Q 3 -Q 8  accordingly control the current to be supplied to each phase of motor M 1  so that motor M 1  generates any designated torque. The motor drive current is controlled in this way to output the motor torque according to torque control value TR. 
     On the other hand, inverter-input-voltage calculating unit  50  calculates an optimum value (target value) of an inverter input voltage based on torque control value TR and motor rotation number MRN and provides the calculated optimum value to duty ratio calculating unit  52 . Duty ratio calculating unit  52  calculates, based on the optimum value of the inverter input voltage from inverter-input-voltage calculating unit  50 , inverter input voltage IVV from voltage sensor  13  and voltage V 1  from voltage sensor  10 , a duty ratio for setting inverter input voltage IVV from voltage sensor  13  at the optimum value of the inverter input voltage supplied from inverter-input-voltage calculating unit  50 , and provides the calculated duty ratio to PWM signal converting unit  54 . Based on the duty ratio supplied from duty ratio calculating unit  52 , PWM signal converting unit  54  generates signal PWU for turning on/off each of NPN transistors Q 1  and Q 2  of up-converter  12  and provides the generated signal PWU to NPN transistors Q 1  and Q 2  of up-converter  12 . 
     A greater amount of electric power is accumulated by reactor L 1  by increasing on-duty of NPN transistor Q 2  which is the lower transistor of up-converter  12 , and accordingly a higher-voltage output is obtained. The voltage on the power supply line of inverter  14  is decreased by increasing the on-duty of the upper transistor, i.e., NPN transistor Q 1 . The duty ratio of NPN transistors Q 1  and Q 2  can thus be controlled to control the voltage on the power supply line such that the voltage on the power supply line is an arbitrary voltage of at least the output voltage of DC power supply B. 
     Motor-torque control means  301  of control unit  30  thus controls up-converter  12  and inverter  14  for allowing motor M 1  to generate torque according to torque control value TR supplied from the external ECU. Motor M 1  accordingly generates the torque designated by torque control value TR. 
     Referring to  FIG. 4 , motor driver  100  operates as described below. The operation is started and voltage sensor  11  detects input voltage V 2  to up-converter  12  (step S 1 ) to output the detected voltage V 2  to control unit  30 . Voltage-conversion control means  302  of control unit  30  receives voltage V 2  from voltage sensor  11  to determine whether or not the received voltage V 2  is higher than a predetermined value (step S 2 ). 
     This predetermined value is determined according to a formula: predetermined value=V 0 +α, where V 0  represents a voltage which is output from DC power supply B, and α is determined in such a way that the sum of V 0  and α is a voltage which is impossible to be output from DC power supply B. In other words, the predetermined value is set at a certain voltage which is never output from DC power supply B. Then, if the voltage output from DC power supply B varies, to the maximum value of the varying output voltage, α is added to determine the predetermined value. 
     In step S 2 , if it is determined that voltage V 2  is higher than the predetermined value, voltage-conversion control means  302  generates signal STP for stopping up-converter  12  and provides that signal to NPN transistors Q 1  and Q 2  of up-converter  12 . Accordingly, NPN transistors Q 1  and Q 2  are stopped by signal STP and thus up-converter  12  is stopped (step S 3 ). This is because voltage-conversion control means  302  judges, from the fact that voltage V 2  is higher than the predetermined voltage, that an overvoltage is applied to the input of up-converter  12 , and then stops up-converter  12  in order to prevent the overvoltage equal to or higher than the withstand voltage from being applied to capacitor C 1  and DC/DC converter  19 . 
     Up-converter  12  is thus stopped and then a DC voltage is supplied from DC power supply B to DC/DC converter  19  via capacitor C 1  (step S 4 ). DC/DC converter  19  down-converts the supplied DC voltage and provides the resultant voltage to inverter  20  which converts the DC voltage into an AC voltage for driving motor  21  for the air conditioner. 
     As described above, when it is determined that an overvoltage is applied to the input of up-converter  12 , up-converter  12  is stopped from operating to eliminate the cause for the overvoltage and accordingly continue driving of the auxiliary equipment constituted of DC/DC converter  19 , inverter  20  and air-conditioner motor  21 . The series of steps of the operation are completed in this way (step S 5 ). 
     In step S 2 , if voltage V 2  is equal to or lower than the predetermined value, voltage-conversion control means  302  receives, from the external ECU, a signal KR indicating whether or not the hybrid or electric vehicle is in the regenerative braking mode. Based on this signal KR, it is determined that whether the vehicle is in the regenerative braking mode (step S 6 ). If voltage-conversion control means  302  determines that the vehicle is in the regenerative braking mode, control means  302  generates signal PWMC for converting an AC voltage from motor M 1  into a DC voltage, provides signal PWMC to inverter  14 , and accordingly controls inverter  14  such that inverter  14  converts the AC voltage from motor M 1  into the DC voltage (step S 7 ). Accordingly, switching of NPN transistors Q 4 , Q 6  and Q 8  of inverter  14  is controlled as discussed above by signal PWMC and inverter  14  converts the AC voltage from motor M 1  into the DC voltage which is then supplied to up-converter  12 . 
     Further, voltage-conversion control means  302  generates signal PWD and provides this signal to up-converter  12  in order to control up-converter  12  such that up-converter  12  down-converts the DC voltage from inverter  14  to charge DC power supply B (step S 8 ). Then, in up-converter  12 , NPN transistor Q 1  is turned on while NPN transistor Q 2  is turned off to down-convert the DC voltage from inverter  14  and accordingly charge DC power supply B (step S 9 ). After this, this operation returns to step S 2 . 
     In step S 6 , if it is determined that the vehicle is not in the regenerative braking mode, motor-torque control means  301  generates signals PWU and PWM 1  as described above based on torque control value TR and motor rotation number MRN from the external ECU, output voltage V 1  of DC power supply B that is provided from voltage sensor  10 , input voltage IVV from voltage sensor  13 , and motor current MCRT from current sensor  24 . The generated signals PWU and PWM 1  are supplied respectively to up-converter  12  and inverter  14 . Inverter  14  driving motor M 1  is then controlled such that motor M 1  outputs the torque which is designated by torque control value TR (step S 10 ). The operation thereafter returns to step S 2  and the steps discussed above are carried out. 
     In the flowchart shown in  FIG. 4 , the operation of steps S 3  and S 4  is performed, when the overvoltage is applied to the input of up-converter  12 , by eliminating the cause for the overvoltage to continuously drive the auxiliary equipment. The operation of steps S 7 -S 9  is performed, in the regenerative braking mode, by converting the AC voltage generated by motor M 1  into the DC voltage to charge DC power supply B. The operation of step S 10  is performed to allow motor M 1  to generate torque. 
     Further, in the flowchart shown in  FIG. 4 , the determination as to whether voltage V 2  on the input of up-converter  12  is higher than the predetermined value (step S 2 ) precedes the determination as to whether the vehicle is in the regenerative braking mode (step S 6 ) as described above, the determination regarding the regenerative braking mode may precede the determination as to whether voltage V 2  is higher than the predetermined voltage. In this case, regardless of whether it is determined that the vehicle is in the regenerative braking mode or it is determined that the vehicle is not in the regenerative braking mode, the determination as to if voltage V 2  is higher than the predetermined value is made. 
     Instead of the operation of motor driver  100  shown in the flowchart in  FIG. 4 , an operation shown in the flowchart in  FIG. 5  may be employed. The flowchart in  FIG. 5  is the same as that in  FIG. 4  except that steps S 1  and S 2  in  FIG. 4  are replaced with steps S 20  and S 21  respectively. 
     Referring to  FIG. 5 , the operation is started and voltage sensor  18  detects voltage Vf applied to the DC load (DC/DC converter  19 ) (step S 20 ) and then outputs the detected voltage Vf to control unit  30 . Voltage-conversion control means  302  of control unit  30  determines whether voltage Vf from voltage sensor  18  is higher than a predetermined value (step S 21 ). If it is determined that voltage Vf is higher than the predetermined value, the operation proceeds to step S 3 . If it is determined that voltage Vf is equal to or lower than the predetermined value, the operation proceeds to step S 6 . Subsequent steps are the same as those described above in connection with the flowchart shown in FIG.  4 . 
     In the flowchart shown in  FIG. 5 , the determination as to whether or not voltage Vf applied to the DC load is higher than the predetermined value (step S 21 ) precedes the determination as to whether the vehicle is in the regenerative braking mode (step S 6 ) as described above. Instead of this, the determination regarding the regenerative braking mode may precede the determination as to whether voltage Vf is higher than the predetermined value. In this case, regardless of whether it is determined that the vehicle is in the regenerative braking mode or it is determined that the vehicle is not in the regenerative braking mode, the determination as to if voltage Vf is higher than the predetermined value is made. 
     According to the flowchart shown in  FIG. 5 , when voltage Vf applied to the DC load (DC/DC converter  19 ) is higher than the predetermined value, it is determined that the overvoltage is applied to the DC load to stop up-converter  12  for eliminating the cause for the overvoltage. Therefore, the predetermined value used in step S 21  is determined by the above-described method based on the withstand voltage of the DC load-related circuitry. 
     In addition, the operation of motor driver  100  may follow the flowchart shown in FIG.  6 . The flowchart in  FIG. 6  is the same as that in  FIG. 4  except that steps S 1  and S 2  in  FIG. 4  are replaced with steps S 30 -S 32 . 
     Referring to  FIG. 6 , the operation is started and voltage sensor  10  detects voltage V 1  output from DC power supply B (step S 30 ) and provides the detected voltage V 1  to control unit  30 . Voltage sensor  11  detects voltage V 2  on the input of up-converter  12  (step S 31 ) and provides the detected voltage V 2  to control unit  30 . 
     Voltage-conversion control means  302  of control unit  30  then determines whether voltage V 1  from voltage sensor  10  matches voltage V 2  from voltage sensor  11  (step S 32 ). If voltage V 1  does not match voltage V 2 , the operation proceeds to step S 3 . If voltage V 1  matches voltage V 2 , the operation proceeds to step S 6 . The subsequent steps are the same as those described above in connection with FIG.  4 . 
     Here, it is indicated in step S 4  that the voltage is supplied to the DC load, which means that the voltage is supplied from capacitor C 1  to DC/DC converter  19 . If voltage V 1  does not match voltage V 2 , DC power supply B is separated from capacitor C 1  and accordingly, the power accumulated in capacitor C 1  is supplied to DC/DC converter  19 . 
     In the flowchart shown in  FIG. 6 , the determination as to whether or not voltage V 1  matches voltage V 2  (step S 32 ) precedes the determination as to whether the vehicle is in the regenerative braking mode (step S 6 ) as described above. Instead of this, the determination regarding the regenerative braking mode may precede the determination as to whether voltage V 1  matches voltage V 2 . In this case, regardless of whether it is determined that the vehicle is in the regenerative braking mode or it is determined that the vehicle is not in the regenerative braking mode, the determination as to if voltage V 1  matches voltage V 2  is made. 
     According to the flowchart shown in  FIG. 6 , it is determined whether or not voltage V 1  output from DC power supply B matches voltage V 2  on the input of up-converter  12  and, up-converter  12  is stopped if the voltages do not match. The fact that voltage V 1  does not match voltage V 2  indicates that DC power supply B is separated from capacitor C 1 , up-converter  12  and DC/DC converter  19  due to any malfunction of system relays SR 1  and SR 2  or brake. In this case, if regenerative braking occurs with DC power supply B separated, an overvoltage is applied to the input of up-converter  12 . In order to avoid this, up-converter  12  is stopped for eliminating the cause for the overvoltage when it is found that DC power supply B is separated. Here, control unit  30  does not particularly control inverter  14 . 
     According to the present invention as discussed above, voltage-conversion control means  302  judges whether or not an overvoltage is applied to the input of up-converter  12  according to whether input voltage V 2  of up-converter  12  is higher than a predetermined value or not, or whether voltage Vf applied to the DC load is higher than a predetermined value or not. When control means  302  judges that the overvoltage is applied thereto, control means  302  stops up-converter  12 . Moreover, according to the present invention, voltage-conversion control means  302  detects whether or not DC power supply B is separated according to the determination as to whether or not output voltage V 1  of DC power supply B matches input voltage V 2  of up-converter  12 . Then, if DC power supply B is separated, up-converter  12  is stopped. 
     The present invention is thus characterized in that up-converter  12  is stopped when the overvoltage is applied to the input of up-converter  12  or DC power supply B is separated. Specifically, the fact that the overvoltage is applied to the input of up-converter  12  or the fact that DC power supply B is separated means any malfunction occurs in the electrical system between the DC power supply and a voltage converter (up-converter  12 ). Here, the operation of stopping up-converter  12  corresponds to control of the voltage converter (up-converter  12 ) for protecting the electrical system of the DC load. 
     Moreover, the present invention is characterized in that, in the motor driver having one motor M 1 , up-converter  12  is stopped when any malfunction occurs in the electrical system between the DC power supply and the voltage converter (up-converter  12 ). 
     When voltage V 2  from voltage sensor  11  is used for detecting an overvoltage on the input of up-converter  12 , voltage sensor  11 , up-converter  12 , inverter  14 , DC/DC converter  19 , and control unit  30  constitute “load driver.” 
     In addition, when voltage Vf from voltage sensor  18  is used for detecting the overvoltage on the DC load, up-converter  12 , inverter  14 , voltage sensor  18 , DC/DC converter  19 , and control unit  30  constitute “load driver.” 
     Further, when voltage V 1  from voltage sensor  10  and voltage V 2  from voltage sensor  11  are used for detecting whether DC power supply B is separated or not, voltage sensors  10  and  11 , up-converter  12 , inverter  14 , DC/DC converter  19 , and control unit  30  constitute “load driver.” 
     According to the description above, it is detected that DC power supply B is separated if voltage V 1  from voltage sensor  10  does not match voltage V 2  from voltage sensor  11 . Instead of this, the ECU external to the voltage converter may detect whether DC power supply B is separated or not according to the present invention. In this case, control unit  30  receives a detection signal indicative of separation of DC power supply B from the external ECU and, according to the detection signal, control unit  30  generates signal STP for stopping up-converter  12  and provides the signal STP to up-converter  12 . 
     Further, according to the description above, motor M 1  generates electric power. The device with the power-generating function may generally be an AC power generator according to the present invention. 
     In addition, according to the description above, the electrical system of the DC load connected to the DC power supply B is constituted of DC/DC converter  19 , inverter  20  and air-conditioner motor  21 . Here, the electrical system may be any auxiliary equipment or circuitry mounted on a hybrid or electric vehicle. 
     According to the present invention, a control method for safely driving the DC load follows any of the flowcharts shown in  FIGS. 4-6  respectively. 
     Moreover, the control by control unit  30  for safely driving the DC load is actually carried out by a CPU (Central Processing Unit). CPU reads, from a ROM (Read-Only Memory), a program including the steps shown in any of the flowcharts in  FIGS. 4-6 , and then executes the program read from the ROM to control driving of the DC load according to any of the flowcharts shown in  FIGS. 4-6 . The ROM thus corresponds to a computer (CPU)-readable recording medium on which a program is recorded that includes the steps of any of the flowcharts shown respectively in  FIGS. 4-6 . 
     According to the first embodiment, the load driver has the control unit which controls the up-converter in such a way that the up-converter is stopped from operating if any malfunction occurs in the electrical system between the DC power supply and the up-converter. Accordingly, an overvoltage is prevented from being applied to the input of the up-converter. 
     Second Embodiment 
     Referring to  FIG. 7 , a motor driver  200  having a load driver according to a second embodiment includes a DC power supply B, voltage sensors  10 ,  11 ,  13  and  18 , system relays SR 1  and SR 2 , capacitors C 1  and C 2 , an up-converter  12 , inverters  14  and  31 , current sensors  24  and  28 , and a control unit  300 . Motor driver  200  drives two motors M 1  and M 2 . Of the motors M 1  and M 2 , one motor M 1  generates torque for driving drive wheels of a hybrid or electric vehicle and the other motor M 2  is used for a power generator or for auxiliary equipment if the vehicle is the hybrid vehicle and is used for auxiliary equipment if the vehicle is the electric vehicle. 
     DC power supply B, voltage sensors  10 ,  11 ,  13  and  18 , system relays SR 1  and SR 2 , capacitors C 1  and C 2 , up-converter  12 , inverter  14 , and current sensor  24  are as those described above in connection with the first embodiment. Here, capacitor C 2  receives a DC voltage from up-converter  12  via nodes N 1  and N 2  to smooth the received DC voltage and supplies the smoothed voltage to inverter  31  as well as inverter  14 . 
     Current sensor  24  detects a motor current MCRT 1  which is then provided to control unit  300 . Inverter  14  converts, according to a signal PWMI 1  from control unit  330 , the DC voltage from capacitor C 2  into an AC voltage to drive motor M 1  and, according to a signal PWMC 1 , converts an AC voltage generated by motor M 1  into a DC voltage. 
     Inverter  31  has the same configuration as that of inverter  14 . Inverter  31  converts, according to a signal PWMI 2  from control unit  300 , the DC voltage from capacitor C 2  into an AC voltage to drive motor M 2  and, according to a signal PWMC 2 , converts an AC voltage generated by motor M 2  into a DC voltage. Current sensor  28  detects a motor current MCRT 2  flowing to each phase of motor M 2  and outputs the detected current to control unit  300 . 
     Control unit  300  receives output voltage V 1  of DC power supply B from voltage sensor  10 , receives voltage V 2  on the input of up-converter  12  from voltage sensor  11 , receives motor currents MCRT 1  and MCRT 2  from respective current sensors  24  and  28 , receives input voltage IVV to inverters  14  and  31  from voltage sensor  13 , and receives torque control values TR 1  and TR 2  and motor rotation number MRN 1  and MRN 2  from an external ECU. Based on voltage V 1 , input voltage IVV, motor current MCRT 1 , torque control value TR 1  and motor rotation number MRN 1 , control unit  300  generates signal PWMI 1  for controlling switching of NPN transistors Q 3 -Q 8  of inverter  14  when inverter  14  drives motor M 1  following the above-described method, and provides the generated signal PWMI 1  to inverter  14 . 
     Further, based on voltage V 1 , input voltage IVV, motor current MCRT  2 , torque control value TR 2  and motor rotation number MRN 2 , control unit  300  generates signal PWMI 2  for controlling switching of NPN transistors Q 3 -Q 8  of inverter  31  when inverter  31  drives motor M 2  following the above-described method, and provides the generated signal PWMI 2  to inverter  31 . 
     When inverter  14  (or  31 ) drives motor M 1  (or M 2 ), control unit  300  generates signal PWU for controlling switching of NPN transistors Q 1  and Q 2  of up-converter  12  following the above-described method, based on voltage V 1 , input voltage IVV, motor current MCRT 1  (or MCRT 2 ), torque control value TR 1  (or TR 2 ) and motor rotation number MRN 1  (or MRN 2 ) and provides the generated signal PWU to up-converter  12 . 
     Further, control unit  300  determines, based on voltage V 2  from voltage sensor  11  or voltage Vf from voltage sensor  18 , whether or not the overvoltage is applied to the input of up-converter  12 , following the above-described method. If the overvoltage is applied thereto, control unit  300  generates signal STP for stopping up-converter  12  and provides the signal STP to up-converter  12 . Alternatively, control unit  300  may determine, based on voltages V 1  and V 2 , whether or not DC power supply B is separated, following the above-described method to generate signal STP for stopping up-converter  12  if DC power supply B is separated and provide the generated signal STP to up-converter  12 . 
     In regenerative braking mode, control unit  300  generates signal PWMC 1  for converting the AC voltage generated by motor M 1  into the DC voltage or generates signal PWMC 2  for converting the AC voltage generated by motor M 2  into the DC voltage, and supplies the generated signal PWMC 1  or PWMC 2  to inverter  14  or inverter  31 , respectively. At this time, control unit  300  generates signal PWD for controlling up-converter  12  such that up-converter  12  down-converts the DC voltage from inverter  14  or  31  to charge DC power supply B, and provides the generated signal PWD to up-converter  12 . 
     In addition, control unit  300  generates signal SE for making system relays SR 1  and SR 2  on to provide the signal SE to system relays SR 1  and SR 2 . 
       FIG. 8  is a functional block diagram of control unit  300 . Control unit  300  includes voltage-conversion control means  302  and motor-torque control means  303 . Voltage-conversion control means  302  performs, in addition to the functions discussed in connection with the first embodiment, a function of outputting signal STP not only to up-converter  12  but also motor-torque control means  303 . This signal STP is generated by control means  302  when it is detected that the overvoltage is applied to the input of up-converter  12  or that DC power supply B is separated. Voltage-conversion control means  302  generates, in regenerative braking mode, two signals PWMC 1  and PWMC 2  to be supplied to inverters  14  and  31  respectively. 
     Further, motor-torque control means  303  performs a function in addition to the functions discussed above in connection with the first embodiment. Specifically, when motor-torque control means  303  receives signal STP from voltage-conversion control means  302 , motor-torque control means  303  generates signals for driving motors M 1  and M 2  to maintain a balance between supply and consumption of the electric energy held in the circuitry on the output side of up-converter  12 , that is, the balance between the supply and consumption of the electric energy is kept with respect to motors M 1  and M 2 . The signals thus generated by motor-torque control means  303  are output respectively to inverters  14  and  31 . Here again, switching of NPN transistors Q 3 -Q 8  in inverters  14  and  31  is controlled and accordingly, motor-torque control means  303  provides, to inverters  14  and  31 , respective signals PWMI 1  and PWMI 2  for driving motors M 1  and M 2  to maintain a balance between supply and consumption (of electric energy) with respect to motor M 1  and motor M 2 . Then, inverter  14  drives motor M 1  according to signal PWMI 1  while inverter  31  drives motor M 2  according to signal PWMI 2  in such a way that the balance between supply and consumption of the electric energy with respect to motors M 1  and M 2  is maintained. 
     Motor driver  200  operates as discussed below. Motor driver  200  stops up-converter  12  when the overvoltage is detected on the input of up-converter  12 . The operation here of motor driver  200  thus follows the flowchart shown in  FIG. 4  or FIG.  5 . 
     Motor driver  200  may alternatively operate following the flowchart shown in FIG.  9 . The flowchart in  FIG. 9  is the same as that in  FIG. 6  except that the flowchart in  FIG. 9  includes an additional step S 33 . 
     Referring to  FIG. 9 , up-converter  12  is stopped (step S 3 ), and then motors M 1  and M 2  are operated to keep the balance between supply and consumption of the electric energy with respect to motors M 1  and M 2  (step S 33 ). The operation here then proceeds to step S 4  described above. 
     Motors M 1  and M 2  may be operated in various manners in step S 33 . Motors M 1  and M 2  may typically be operated as follows: 
     (1) when up-converter  12  is stopped, motors M 1  and M 2  are operated with the electric power accumulated in capacitor C 2 ; or 
     (2) one of motors M 1  and M 2  serves as a regenerative power generator to generate power which is used for charging capacitor C 2  and accordingly operating the other motor. 
     When the motors are operated in manner (1), motor torque control means  303  generates signals PWMI 1  and PWMI 2  by the above-discussed method to output the signals to inverters  14  and  31  respectively. According to signal PWMI 1 , inverter  14  converts the DC voltage from capacitor C 2  into an AC voltage for driving motor M 1 . According to signal PWMI 2 , inverter  31  converts the DC voltage from capacitor C 2  into an AC voltage for driving motor M 2 . Motors M 1  and M 2  are stopped from operating when the electric power accumulated in capacitor C 2  becomes zero. 
     When the motors are operated in manner (2), motor torque control means  303  generates, by the above-described method, signals PWMI 1  and PWMC 2  or signals PWMC 1  and PWMI 2  to provide the signals to inverters  14  and  31 . If motor torque control means  303  outputs signals PWMI 1  and PWMC 2 , inverter  31  converts an AC voltage generated by motor M 2  into a DC voltage in response to signal PWMC 2  to charge capacitor C 2  while inverter  14  converts the DC voltage from capacitor C 2  into an AC voltage in response to signal PWMI 1  to drive motor M 1 . 
     If motor torque control means  303  outputs signals PWMC 1  and PWMI 2 , inverter  14  converts an AC voltage generated by motor M 1  into a DC voltage in response to signal PWMC 1  to charge capacitor C 2  while inverter  31  converts the DC voltage from capacitor C 2  into an AC voltage in response to signal PWMI 2  to drive motor M 2 . 
     In this way, when up-converter  12  is stopped for the reason that DC power supply B is separated, motors M 1  and M 2  are operated to maintain the balance between supply and consumption of the electric energy with respect to motors M 1  and M 2 . 
     Other details are the same as those of the first embodiment. 
     According to the description above, two motors are employed. Instead, three or more motors may be used according to the present invention. In this case, depending on the number of additional motors, one or any number of combinations each consisting of a motor and an inverter for driving the motor are connected to nodes N 1  and N 2  shown in FIG.  7 . Specifically, one or any number of combinations each of a motor and an inverter are connected in parallel to nodes N 1  and N 2 . 
     The second embodiment applied to the motor driver driving at least two motors is characterized in that, as the first embodiment, the up-converter is stopped when the overvoltage is detected on the input of the up-converter. 
     Moreover, the second embodiment applied to the motor driver driving at least two motors is characterized in that the up-converter is stopped when the DC power supply is separated while the motors are operated to keep the balance between supply and consumption of the electric energy with respect to these motors. 
     Here, when voltage V 2  from voltage sensor  11  is used to detect the overvoltage on the input of up-converter  12 , voltage sensor  11 , up-converter  12 , inverter  14 , DC/DC converter  19  and control unit  300  constitute “load driver.” 
     When voltage Vf from voltage sensor  18  is used to detect the overvoltage to the DC load, up-converter  12 , inverter  14 , voltage sensor  18 , DC/DC converter  19  and control unit  300  constitute “load driver.” 
     When voltage V 1  from voltage sensor  10  and voltage V 2  from voltage sensor  11  are used to detect that DC power supply B is separated, voltage sensors  10  and  11 , up-converter  12 , inverter  14 , DC/DC converter  19  and control unit  300  constitute “load driver.” 
     According to the present invention, a control method for safely driving the DC load follows any of the flowcharts shown respectively in  FIGS. 4 ,  5  and  9 . 
     The control by control unit  300  for safely driving the DC load is actually carried out by a CPU (Central Processing Unit). CPU reads, from a ROM (Read-Only Memory), a program including the steps shown in any of the flowcharts in  FIGS. 4 ,  5  and  9 , and then executes the program read from the ROM to control driving of the DC load according to any of the flowcharts shown in  FIGS. 4 ,  5  and  9 . The ROM thus corresponds to a computer (CPU)-readable recording medium on which a program is recorded that includes the steps of any of the flowcharts shown respectively in  FIGS. 4 ,  5  and  9 . 
     According to the second embodiment, the load driver has the control unit which controls the up-converter in such a way that the up-converter is stopped from operating if any malfunction occurs in the electrical system between the DC power supply and the up-converter. Accordingly, the overvoltage is prevented from being applied to the input of the up-converter. 
     In addition, when any malfunction occurs in the electrical system between the DC power supply and the up-converter, the control unit of the load driver stops the up-converter from operating and then controls a plurality of inverters respectively driving a plurality of motors in such a way that the balance between supply and consumption of electric energy with respect to these motors is maintained. Accordingly, the electrical system of the DC load connected between the DC power supply and the up-converter is protected and the energy is effectively used. 
     Third Embodiment 
     Referring to  FIG. 10  a motor driver  400  having a load driver according to a third embodiment is the same as motor driver  200  except that motor driver  400  includes a control unit  300 A instead of control unit  300  of motor driver  200 . In addition, an engine  35  is connected to motor M 2 . Motor M 2  thus serves to electromagnetically transmit torque from the output shaft of engine  35  to the vehicle-driving-shaft and also serves as a power generator converting a part or the whole of engine torque into electric energy. In addition, an auxiliary battery  60  is connected to the DC/DC converter  19 . 
     In motor driver  400 , DC/DC converter  19  is connected between DC power supply B and up-converter  12  to down-convert a DC voltage from DC power supply B and accordingly charge auxiliary battery  60  (e.g. output voltage 12 V). As DC/DC converter  19  is placed between DC power supply B and up-converter  12 , a required withstand voltage of DC/DC converter  19  is determined according to an output voltage of DC power supply B. Then, the withstand voltage of DC/DC converter  19  placed between DC power supply B and up-converter  12  is smaller than that of DC/DC converter  19  placed between up-converter  12  and inverters  14  and  31 . 
     Moreover, requirements of the specification of the components of DC/DC converter  19  are made less severe, which means the circuit configuration of DC/DC converter  19  may be simplified. Consequently, reduction in the cost and size of DC/DC converter  19  is achieved. 
     Auxiliary battery  60  is used as a power supply of such a control circuit as control unit  300 A. 
     Control unit  300 A performs, in addition to the functions of control unit  300 , a function as specifically described below. 
     When voltage V 1  from voltage sensor  10  does not match voltage V 2  from voltage sensor  11 , control unit  300 A decreases the output of engine  35  while controlling inverters  14  and  31  in such a way that the DC voltage obtained by converting, by inverter  31 , the voltage generated by motor M 2  is lower than the withstand voltage of DC/DC converter  19 . 
     More specifically, control unit  300 A generates signal PWMC 2  for converting the AC voltage generated by motor M 2  from torque of engine  35  into the DC voltage to supply the generated signal to inverter  31 , and generates signal PWMI 1  for converting the DC voltage from capacitor C 2  into the AC voltage to drive motor M 1  or a signal STP 1  for stopping inverter  14  to provide the resultant signal to inverter  14 . 
     When the DC voltage obtained by converting by inverter  31  the voltage generated by motor M 2  based on the torque from engine  35  is equal to or higher than the withstand voltage of DC/DC converter  19 , control unit  300 A drives inverter  14  such that voltage IVV from voltage sensor  13  is lower than the withstand voltage of DC/DC converter  19 . Then, control unit  300 A generates signal PWMI 1  and provides this signal to inverter  14  for converting the DC voltage from capacitor C 2  into an AC voltage so as to cause a part of the DC power accumulated in capacitor C 2  to be consumed by motor M 1 . 
     On the other hand, signal STP 1  is generated and provided to inverter  14  for stopping inverter  14  when the DC voltage obtained by converting by inverter  31  the voltage generated by motor M 2  based on torque from engine  35  is lower than the withstand voltage of DC/DC converter  19 . 
     In this way, control unit  300 A controls inverters  14  and  31  such that the AC voltage generated based on the torque of engine  35  is converted into a DC voltage which is lower than the withstand voltage of DC/DC converter  19 . 
     Moreover, control unit  300 A generates signal PWH for keeping NPN transistor Q 2  continuously in OFF state and keeping NPN transistor Q 1  continuously in ON state and provides signal PWH to up-converter  12 . Then the configuration of up-converter  12  is changed to allow up-converter  12  to directly output a DC voltage supplied from nodes N 1  and N 2  to DC power supply B. Then, the voltage generated by motor M 2  and converted by inverters  31  into a DC voltage is supplied to DC/DC converter  19 . 
     Preferably, when voltage V 1  does not match voltage V 2 , control unit  300 A determines whether voltage V 2  from voltage sensor  11  (or voltage Vf from voltage sensor  18 ) is equal to or more than a predetermined value. Then, if voltage V 2  (or voltage Vf) is smaller than the predetermined value, control unit  300 A controls engine  35  and inverters  14  and  31  to supply the voltage generated by motor M 2  and converted by inverter  31  into the DC voltage directly by DC/DC converter  19 . 
     As discussed above, when voltage V 1  does not match voltage V 2 , that is, when it is detected that DC power supply B is separated from capacitor C 1 , up-converter  12  and DC/DC converter  19 , control unit  300 A controls engine  35  and inverters  14  and  31  to generate a DC voltage lower than the withstand voltage of DC/DC converter  19  and supply the generated DC voltage directly to DC/DC converter  19 . Preferably, control unit  300 A confirms that no overvoltage is applied to DC/DC converter  19  to directly supply the voltage generated by motor M 2  and converted by inverter  31  into the DC voltage to DC/DC converter  19 . 
     Referring to  FIG. 11 , motor driver  400  operates as detailed below. The flowchart in  FIG. 11  is the same as that in  FIG. 6  except that steps S 3  and S 4  in the flowchart shown in  FIG. 6  are replaced with steps S 34  and S 35  in the flowchart shown in FIG.  11 . 
     Referring to  FIG. 11 , it is determined in step S 32  that voltage V 1  does not match voltage V 2 . Then, control unit  300 A controls engine  35  to decrease the output therefrom and generates signal PWMC 2  for converting an AC voltage generated by motor M 2  based on torque of engine  35  into a DC voltage to provide the generated signal to inverter  31 . Further, control unit  300 A generates signal PWMI 1  or signal STP 1  to provide the generated signals to inverter  14  as described above (step S 34 ). 
     Control unit  300 A generates signal PWH which is output to up-converter  12 . The AC voltage generated based on the torque of engine  35  is converted into the DC voltage which is lower than the withstand voltage of DC/DC converter  19 . The resultant DC voltage is directly supplied via up-converter  12  to DC/DC converter  19  (step S 35 ). A series of steps of this operation is completed accordingly. Other details are as those described in connection with the first embodiment. 
     Motor driver  400  may operate following the flowchart shown in FIG.  12 . The flowchart in  FIG. 12  is the same as that in  FIG. 11  except that steps S 33 , S 3  and S 4  are added to the flowchart of FIG.  11 . The operation of steps S 3  and S 4  is as described above in connection with the first embodiment. 
     Referring to  FIG. 12 , it is determined in step S 32  that voltage V 1  does not match voltage V 2 . Then, control unit  300 A determines whether or not voltage V 2  from voltage sensor  11  (or voltage Vf from voltage sensor  18 ) is equal to or higher than a predetermined value Vstd (step S 33 ). In step S 33  if it is determined that voltage V 2  (or voltage Vf) is equal to or more than predetermined value Vstd, the operation proceeds to step S 3 . If it is determined that voltage V 2  (or voltage Vf) is smaller than predetermined value Vstd, the operation proceeds to step S 34 . After this, steps S 3  and S 4  or steps S 34  and S 35  are carried out. 
     In other words, when voltage V 2  (or voltage Vf) is equal to or more than predetermined value Vstd, up-converter  12  is stopped to supply electric power accumulated in capacitor C 1  to DC/DC converter  19 . When it is determined that voltage V 2  (or voltage Vf) is smaller than predetermined value Vstd, voltage generated by motor M 2  based on the torque of engine  35  and converted into DC voltage by inverter  31  is supplied directly to DC/DC converter  19 . 
     According to the third embodiment as described above, when any failure in system relays SR 1  and SR 2  or brake causes DC power supply B to be separated from capacitor C 1 , up-converter  12  and DC/DC converter  19 , the voltage generated according to the torque of engine  35  and converted into the DC voltage lower than the withstand voltage of DC/DC converter  19  is directly supplied to DC/DC converter  19 . 
     Preferably, the DC voltage generated according to the torque of engine  35  and converted into the DC voltage lower than the withstand voltage of DC/DC converter  19  is supplied directly to DC/DC converter  19  after it is confirmed that no overvoltage is applied to DC/DC converter  19 . 
     In this way, even if DC power supply B is separated from capacitor C 1 , up-converter  12  and DC/DC converter  19 , DC/DC converter  19  can continue its operation. Then, the vehicle with motor driver  400  mounted thereon surely keeps moving. 
     Voltage sensors  10 ,  11  and  18 , up-converter  12 , inverters  14  and  31 , DC/DC converter  19  and control unit  300 A constitute “load driver.” 
     A control method according to the present invention for safely driving a DC load follows the flowchart shown in  FIG. 11  or  12 . 
     Moreover, the control by control unit  300 A for safely driving the DC load is actually carried out by a CPU (Central Processing Unit). CPU reads, from a ROM (Read-Only Memory), a program including the steps shown in the flowchart in  FIG. 11  or  12 , and then executes the program read from the ROM to control driving of the DC load according to the flowchart shown in  FIG. 11  or  12 . The ROM thus corresponds to a computer (CPU)-readable recording medium on which a program is recorded that includes the steps of the flowchart shown in  FIG. 11  or  12 . 
     Other details are the same as those of the second embodiment. 
     According to the third embodiment, the load driver has the control unit and, under the control by the control unit, the power generated based on the torque of the engine is converted into the DC voltage lower than the withstand voltage of the DC load and the resultant DC voltage is supplied directly to the DC load, when the DC power supply is separated. The DC load can thus be kept driven while the DC load connected between the DC power supply and the up-converter is protected. 
     Fourth Embodiment 
     Referring to  FIG. 13 , a motor driver  500  having a load driver according to a fourth embodiment is the same as motor driver  400  except that control unit  300 A of motor driver  400  is replaced with a control unit  300 B and that motor driver  500  includes diodes D 9  and D 10  and a system relay SR 3  in addition to the components of motor driver  400 . 
     System relay SR 3  and diode D 10  are connected in series between node N 3  and node N 4 . Diode D 10  is connected in the direction in which a DC current flows from system relay SR 3  to node N 4 . System relay SR 3  is made on/off in response to a signal CHG from control unit  300 B. 
     Diode D 9  is connected between a power supply line of DC power supply B and node N 4 . Diode D 9  is connected in the direction in which a DC current from DC power supply B flows to node N 4 . 
     Even when diodes D 9  and D 10  cause system relay SR 3  to be made on, short circuit is prevented that occurs between DC power supply B and inverters  14  and  31  through the path of system relay SR 3  and diode D 10 . 
     In addition to the functions of control unit  300 A, control unit  300 B performs a function of generating signal CHG to be output to system relay SR 3  for turning on/off system relay SR 3 . 
     As for motor driver  500 , control unit  300 B controls inverters  14  and  31  and engine  35  in such a way that, when voltage V 1  does not match voltage V 2 , a DC voltage lower than the withstand voltage of DC/DC converter  19  is generated and supplied to up-converter  12  via nodes N 1  and N 2 . Control unit  300 B further generates signal STP for stopping up-converter  12  as well as signal CHG for making system relay SR 3  on and provides the signals respectively to up-converter  12  and system relay SR 3 . 
     Accordingly, the voltage generated based on the torque of engine  35  and converted into a DC voltage is supplied via nodes N 1  and N 2  to up-converter  12  and supplied directly to DC/DC converter  19  via system relay SR 3  and diode D 10 . 
     In this way, the operation of DC/DC converter  19  can be continued while overvoltage is prevented from being applied. 
     Referring to  FIG. 14 , motor driver  500  operates as described below. The flowchart in  FIG. 14  is the same as the flowchart in  FIG. 11  except that step S 35  in  FIG. 11  is replaced with step S 36 . 
     After step S 34 , control unit  300 B generates signal STP for stopping up-converter  12  and signal CHG for making system relay SR 3  on and provides the generated signals respectively to up-converter  12  and system relay SR 3 . 
     The voltage generated based on the torque of engine  35  and converted into the DC voltage is thus supplied via nodes N 1  and N 2  to up-converter  12  and directly to DC/DC converter  19  via system relay SR 3  and diode D 10  (step S 36 ). A series of the steps of the operation is then completed. Other details of the operation are described above. 
     Motor driver  500  may operate following the flowchart shown in FIG.  15 . The flowchart in  FIG. 15  is the same as that in  FIG. 12  except that step S 35  in the flowchart of  FIG. 12  is replaced with step S 36  in FIG.  15 . 
     Referring to  FIG. 15 , after step S 34 , step S 36  as described above is carried out. Other details are described above. 
     According to the fourth embodiment as described above, when voltage V 1  does not match voltage V 2 , which means that DC power supply B is separated, the voltage generated based on the torque of engine  35  and converted into the DC voltage is supplied directly to DC/DC converter  19  via system relay SR 3 . 
     The voltage generated by motor M 2  and converted by inverter  31  into the DC voltage is supplied to DC/DC converter  19  via system relay SR 3 , so that the power generated by motor M 2  can be supplied to DC/DC converter  19  even if NPN transistor Q 1  of up-converter  12  fails. 
     Voltage sensors  10 ,  11  and  18 , up-converter  12 , inverters  14  and  31 , DC/DC converter  19 , system relay SR 3 , diodes D 9  and D 10  and control unit  300 B constitute “load driver.” 
     Diode D 10  constitutes “supply unit” which directly supplies, to the DC load (DC/DC converter  19 ), a DC voltage produced from the power generated by motor M 2  and having a voltage level lower than a predetermined value (voltage of at least the withstand voltage of DC/DC converter  19 ). 
     System relay SR 3  constitutes “switching unit” switching supply of a DC voltage between a voltage converter (up-converter  12 ) and a supply unit (diode D 10 ). 
     A control method according to the present invention for safely driving a DC load follows the flowchart shown in  FIG. 14  or  15 . 
     Moreover, the control by control unit  300 B for safely driving the DC load is actually carried out by a CPU (Central Processing Unit). CPU reads, from a ROM (Read-Only Memory), a program including the steps shown in the flowchart in  FIG. 14  or  15 , and then executes the program read from the ROM to control driving of the DC load according to the flowchart shown in  FIG. 14  or  15 . The ROM thus corresponds to a computer (CPU)-readable recording medium on which a program is recorded that includes the steps of the flowchart shown in  FIG. 14  or  15 . 
     Other details are the same as those of the second and third embodiments. 
     According to the fourth embodiment, the load driver has the control unit and, under the control of the control unit, the power generated based on the torque of the engine is converted into the DC voltage lower than the withstand voltage of the DC load and the resultant DC voltage is supplied directly to the DC load, when the DC power supply is separated. The load driver further has the switching unit for switching supply of voltage generated by engine and converted into a DC voltage. Accordingly, even if the up-converter fails, the electric power generated according to torque of the engine can surely be supplied to the DC load connected between the DC power supply and the up-converter. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.