Patent Publication Number: US-2023133167-A1

Title: Bidirectional dc-dc converter, traffic system, control method, and non-transitory computer-readable storage medium

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure, relates to a bidirectional DC-DC converter, a traffic system, a control method, and a non-transitory computer-readable storage medium. 
     Description of Related Art 
     As electric vehicles in traffic systems such as automated guideway transit (AGT) and an automated people mover (APM), there is an electric vehicle that travels using electric power supplied from an overhead line. 
     Patent Document 1 discloses, as a related technology, a technology related to voltage switching control of an electric vehicle.
     [Patent Documents]   

     [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2011-130579 
     SUMMARY OF THE INVENTION 
     Electric vehicles that travel using electric power supplied from overhead lines may use electric components for railway vehicles that travel using electric power supplied from the overhead lines (for example, an electric vehicle shown in  FIG.  21    to be described below). These electric components for railroad vehicles generally have a high withstand voltage (for example, 1700 volts) compared to the withstand voltage of electric components for an electric vehicle (EV) and general-purpose electric components (for example, 1200 volts). Electric components for railroad vehicles are generally more expensive than electric components for an electric vehicle (EV) and general-purpose electric components. Therefore, there is a demand for a technology that can realize the same function as when a high withstand voltage electric component is used even if an electric component including a high withstand voltage electronic component is replaced with an electric component including an electronic component with a lower withstand voltage than the electric component. 
     The present disclosure has been made in order to solve the problems described above, and an object thereof is to provide a bidirectional DC-DC converter, a traffic system, a control method, and a non-transitory computer-readable storage medium that can realize the sale function as when a high withstand voltage electric component is used even if an electric component including a high withstand voltage electronic component is replaced with an electric component including an electronic component with a lower withstand voltage than the electric component 
     To solve the above problems, a bidirectional DC-DC converter according to the present disclosure includes a first circuit that is configured to process a first voltage being a DC voltage and that includes a first electronic component including a first switching element; second circuit that is configured to process a second voltage or a third voltage, the second voltage being a DC voltage supplied to an electric vehicle, the third voltage being a DC voltage generated in an electric vehicle, and that includes a second electric component with a lower withstand voltage than the first electronic component, the second electric component including a second switching element; and a control circuit configured to control switching of at least one of the first switching element and the second switching element wherein the bidirectional DC-DC converter is configured to convert the first voltage into the second voltage or convert the third voltage into the first voltage. 
     The traffic system according to the present disclosure includes the bidirectional DC-DC converter described above, and a host system that transmits a torque command to the bidirectional DC-DC converter. 
     A control method according to the present disclosure is a control method to be executed by a bidirectional DC-DC converter that includes a first circuit that is configured to process a first voltage being a DC voltage and that includes a first electronic component including a first switching element, and a second circuit that is configured to process a second voltage or a third voltage, the second voltage being a DC voltage supplied to an electric vehicle, the third voltage being a DC voltage generated in an electric vehicle, and that includes a second electric component with a lower withstand voltage than the first electronic component, the second electric component including a second switching element, the method comprising: controlling switching of at least one of the first switching element and the second switching element; and converting the first voltage into the second voltage or converting the third voltage into the first voltage. 
     A non-transitory computer-readable storage medium storing a program according to the present disclosure causes a computer of a bidirectional DC-DC converter that includes a first circuit that is configured to process a first voltage being a DC voltage and that includes a first electronic component including a first switching element, and a second circuit that is configured to process a second voltage or a third voltage, the second voltage being a DC voltage supplied to an electric vehicle, the third voltage being a DC voltage generated in an electric vehicle, and that includes a second electric component with a lower withstand voltage than the first electronic component, the second electric component including a second switching element to execute controlling at least one of the first switching element and the second switching element, and converting the first voltage into the second voltage or converting the third voltage into the first voltage. 
     According to the bidirectional DC-DC converter, traffic system, control method, and non-transitory computer-readable storage medium according to the present disclosure, even if an electric component including a high withstand voltage electronic component is replaced with an electric component including an electronic component with a lower withstand voltage than the electric component, it is possible to provide a bidirectional DC-DC converter that can realize the same function as when the high withstand voltage electric component is used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram which shows a configuration of a traffic system according to a first embodiment of the present disclosure. 
         FIG.  2    is a diagram which shows an example of a configuration of a control circuit according to the first embodiment of the present disclosure. 
         FIG.  3    is a diagram which shows an example of a configuration of a traffic system according to a first modified example of the first embodiment of the present disclosure. 
         FIG.  4    is a diagram which shows an example of a configuration of a control circuit according to the first modified example of the first embodiment of the present disclosure. 
         FIG.  5    is a diagram which shows an example of a processing flow of the traffic system according to the first modified example of the first embodiment. 
         FIG.  6    is a diagram which shows an example of a configuration of a DAB circuit according to a second modified example of the first embodiment of the present disclosure. 
         FIG.  7 A  is a first diagram for describing problems that occur when a DC-DC converter of a general DAB type is controlled. 
         FIG.  7 B  is a second diagram for describing problems that occur when a DC-DC converter of a general DAB type is controlled. 
         FIG.  7 C  is a third diagram for describing problems that occur when a DC-DC converter of a general DAB type is controlled. 
         FIG.  8    is a fourth diagram for describing the problems that occur when the DC-DC converter of the general DAB type is controlled. 
         FIG.  9    is a fifth diagram for describing the problems that occur when the DC-DC converter of the general DAB type is controlled. 
         FIG.  10    is a first diagram for describing control content of a control unit according to a third modified example of the first embodiment. 
         FIG.  11    is a second diagram for describing the control content of the control unit according to the third modified example of the first embodiment. 
         FIG.  12    is a third diagram for describing the control content of the control unit according to the third modified example of the first embodiment. 
         FIG.  13    is a fourth diagram for describing the control content of the control unit according to the third modified example of the first embodiment. 
         FIG.  14    is a fifth diagram for describing the control content of the control unit according to the third modified example of the first embodiment. 
         FIG.  15    is a first diagram which shows an example of wavefonns of simulation of charging a capacitor according to the third modified example of the first embodiment. 
         FIG.  16    is a second diagram which shows an example of waveforms of a simulation of charging the capacitor according to the third modified example of the first embodiment. 
         FIG.  17    is a third diagram which shows an example of waveforms of a simulation of charging the capacitor according to the third modified example of the first embodiment. 
         FIG.  18    is a diagram which shows an example of a configuration of a traffic system according to a second embodiment of the present disclosure. 
         FIG.  19    is a diagram which shows an example of a configuration of a DC-DC converter according to the second embodiment of the present disclosure. 
         FIG.  20    is a schematic block diagram which shows a configuration of a computer according to at least one embodiment. 
         FIG.  21    is a diagram which shows an example of a configuration of a traffic system to be compared with the traffic systems according to each embodiment of the present disclosure. 
         FIG.  22    is a diagram which shows an example of a configuration of a VFD inverter used in the traffic system to be compared with the traffic systems according to each embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First, a traffic system la to be compared with a traffic system  1  according to each embodiment will be described. 
     (Configuration of Traffic System to be Compared) 
       FIG.  21    is a diagram which shows an example of a configuration of a traffic system  1   a  to be compared with the traffic system  1  according to each embodiment of the present disclosure. The traffic system  1   a  includes an overhead line  10 , an electric vehicle  20   a , and a host system  30 , as shown in  FIG.  21   . 
     The overhead line  10  supplies electric power to the electric vehicle  20   a . For example, the overhead line  10  supplies electric power to the electric vehicle  20   a  at a predetermined DC voltage (for example, a DC voltage (nominal voltage) of 750 volts). This predetermined DC voltage can vary largely. For example, in “Section 5.2.1 of Japanese Industrial Standards (RS),” Table 0A standard voltage and frequency for train lines and their fluctuation ranges in a supply of electric power from train lines, and a maximum voltage of 900 V and a minimum voltage of 500 V with a standard voltage of 750 V DC in Type 1 are described. In other words, the overhead line voltage with the standard voltage of 750 volts DC may fluctuate from 500 volts to 900 volts. The host system  30  outputs a torque command to the electric vehicle  20   a.    
     The electric vehicle  20   a  operates using electric power supplied from an overhead line  10  via a pantograph  203   a . The electric vehicle  20   a  includes, as shown in  FIG.  21   , a vehicle main body  201   a , wheels  202   a   1  and  202   a   2 , a pantograph  203   a , motors  204   a   1  and  204   a   2 , a variable-frequency drive or variable voltage variable frequency (VDF) inverter  205   a , an auxiliary power unit (APL)  206   a , transformers  207   a   1 ,  207   a   2 , and  207   a   3 , motors  208   a   1 ,  208   a   2 , and  208   a   3 , a heating, ventilation, and air conditioning (HVAC) compressor  209 , a brake air compressor  210 , a cooling fan  211 , an accessory  212 , a rectifier circuit  213   a , a control power supply  214   a , and a control circuit  215   a , The vehicle main body  201   a  includes the motors  204   a   1  and  204   a   2 , the VDF inverter  205   a , the APU  206   a , the transformers  207   a   1 ,  207   a   2 , and  207   a   3 , the motors  208   a   1 ,  208   a   2 , and  208   a   3 , an HVAC compressor  209   a , a brake air compressor  210   a , a cooling fan  211   a , an accessory  212   a , the rectifier circuit  213   a , the control power supply  214   a , and the control circuit  215   a.    
     The wheels  202   a   1  are, for example, two front wheels. In addition, the wheels  202   a   2  are, for example, two rear wheels. Each of the wheels  202   a   1  and  202   a   2  has, for example, a rubber tire. 
     The pantograph  203   a  receives electric power supplied from the overhead line  10 . The electric power received by the pantograph  203   a  is supplied to the VDF inverter  205   a  and the APU  206   a.    
     The motors  204   a   1  and  204   a   2  drive the wheels  202   a   1  and  202   a   2 . For example, the rotor  204   a   1  rotates the wheels  202   a   1  according to an AC voltage output from the VDF inverter  205   a . In addition, for example, the motor  204   a   2  rotates the wheels  202   a   2  according to the AC voltage output from the VDF inverter  205   a.    
     The VDF inverter  205   a  converts a DC voltage into an AC voltage. The VDF inverter  205   a  is a VDF inverter for railroad vehicles, and is configured from electric components having a higher withstand voltage (for example, 1700 volts) than a withstand voltage of electronic components used in inverters for electric vehicles (EV) and general-purpose inverters for example, 1200 volts). Examples of the electronic components include resistors, capacitors, inductors, and semiconductor elements such as diodes and transistors. 
       FIG.  22    is a diagram which shows an example of a configuration of the VDF inverter  205   a  used in the traffic system  1   a  to be compared with the traffic system  1  according to each embodiment of the present disclosure. The VDF inverter  205   a  includes a switching circuit  2051   a  and a control circuit  2052   a  as show in  FIG.  22   . 
     The switching circuit  2051   a  converts a DC voltage supplied from the overhead line  10  into an AC voltage to be supplied to the motors  204   a   1  and  204   a   2  by performing a switching operation under control of the control circuit  2052   a.    
     The control circuit  2052   a  includes a detection unit  2052   a   1  and a control unit  2052   a   2 , as shown in  FIG.  22   . The detection unit  2052   a   1  detects the amount of change in rotation speed (acceleration) of each of the motors  204   a   1  and  204   a   2 . For example, the detection unit  2052   a   1  detects these amounts of change in the rotation speed as the amounts of change in a frequency of a motor current flowing through the motors  204   a   1  and  204   a   2 . The detection unit  2052   a   1  may detect the amount of change in the rotation speed (acceleration) of each of the motors  204   a   1  and  204   a   2  using a rotation number sensor capable of detecting the number of rotations of the motors. 
     The control unit  2052   a   2  causes the switching circuit  2051   a  to switch so that the torque corresponds to a torque command from the host system  30 . The VDF inverter  205  has a function of improving the idling when it is detected that the wheels  202   a   1  and  202   a   2  have idled. When the control unit  2052   a   2  detects the idling of wheels (that is, when it is determined that any one of the amounts of change in the frequency of the motor current flowing through the motors  204   a   1  and  204   a   2  exceeds an allowable value, or when it is determined that a difference in the number of rotations between a plurality of motors (here, the motor  204   a   1  and motor  204   a   2 ) has exceeded the allowable value)), the function of improving the idling of wheels involves performing control to narrow down a current value of the motor  204   a   1  or the motor  204   a   2  whose amount of change in the frequency of the motor current is determined to have exceeded the allowable value (that is, to reduce the torque of the motor  204   a   1  or motor  204   a   2 ). Control for improving the idling of wheels by reducing the member of rotations of a motor when the idling of wheels is detected is called “idling re-adhesion control” or “idling detection/re-adhesion control.” 
     The APU  206   a  converts a DC voltage into an AC voltage. The converted AC voltage output by the APU  206   a  has fixed amplitude and frequency. The APU  206   a  is used as an auxiliary power supply device. The APU  206   a  is also configured from electronic components with a higher withstand voltage (for example, 1700 volts) than a withstand voltage of electronic components used in an inverter for EV or a general-purpose inverter (for example, 1200 volts), like the VDF inverter  205   a . Since the electronic component of a high withstand voltage (for example, 1700 volts) is expensive, auxiliary power supply devices using the high withstand voltage electronic component used in the electric vehicle  20   a  are combined into one to be the APU  206   a  shown in  FIG.  21   , and thereby a manufacturing cost of the APU  206   a  is reduced. 
     The transformer  207   a   1  converts the AC voltage output by the APU  206   a  into an AC voltage that can drive the motors  208   a   1 ,  208   a   2 , and  208   a   3 . The transformer  207   a   2  converts the AC voltage output by the APU  206   a  into an AC voltage that can be supplied to the accessory  212   a . The transformer  207   a   3  converts the AC voltage output by the 
     APU  206   a  into an AC voltage such that a DC voltage rectified by the rectifier circuit  213   a  is large enough to be supplied to the control power supply  214   a.    
     The motor  208   a   1  drives the FIVAC compressor  209   a . The motor  208   a   1  rotates according to an AC voltage with fixed amplitude and frequency; output by the transformer  207   a   1 . That is, the motor  208   a   1  rotates with the fixed number of rotations. 
     The motor  208   a   2  drives the brake air compressor  210 . The motor  208   a   2 , like the motor  208   a   1 , rotates with the fixed number of rotations. 
     A motor  208   a   3  drives the cooling fan  211 . The motor  208   a   3 , like the motors  208   a   1  and  208   a   2 , rotates with the fixed number of rotations, The HVAC compressor  209   a  sucks in the refrigerant that has evaporated in an air conditioner of the electric vehicle  20   a  and changes it into a liquid. The brake air compressor  210   a  generates compressed air that operates pneumatic equipment for applying a brake to the electric vehicle  20   a . The cooling fan  211   a  cools a heat-generating portion within the vehicle main body  201   a  by circulating air in the heat-generating portion. 
     The accessory  212   a  operates using an AC voltage with fixed amplitude and frequency, output by the transformer  207   a   1 , as a power supply. Examples of the accessory  212   a  include communication equipment, defrosters, and other equipment that operates when electric power is supplied through an outlet. The rectifier circuit  213   a  rectifies the AC voltage with fixed amplitude and frequency, output by the transformer  207   a   3 , and outputs the rectified DC voltage. Examples of the rectifier circuit  213   a  include a bridge circuit of diodes and the like. 
     The control power supply  214   a  generates a voltage (for example, 110 volts) for the control circuit  215   a  from the DC voltage output by the rectifier circuit  213   a , and supplies the generated voltage to the control circuit  215   a.    
     The control circuit  215   a  controls the electric vehicle  20   a . The control circuit  215   a  includes a control unit  215   a   1 , as shown in  FIG.  21   . For example, the control unit  215   a   1  controls the VDI inverter  205   a  and the APU  206   a . Specifically, the control unit  215   a   1  controls an AC voltage output by the VDF inverter  205   a  and an AC voltage output by the APU  206   a  by generating a pulse width modulation (PWM) signal for each of the 
     VDF inverter  205   a  and the APU  206   a  and controlling the VDF inverter  205   a  and the APU  206   a  with the generated PWM signal. In addition, the control unit  215   a   1  controls communication with the host system  30  in the electric vehicle  20   a.    
     The host system  30  controls traveling of the electric vehicle  20   a  in the traffic system  1   a . For example, the host system  30  outputs a torque command to the electric vehicle  20   a , and the electric vehicle  20   a  travels according to the torque command, 
     The traffic system la to be compared with the traffic system  1  according to each embodiment of the present disclosure has been described above. The electric vehicle  20   a  described above is configured from electronic components with a high withstand voltage (for example, 1700 volts) such as the VDF inverter  205   a  and the APU  206   a . In addition, the VDF inverter  205   a  has a function of improving the idling of wheels, which is usually not provided in an inverter. For this reason, the VDF inverter  205   a  becomes expensive, and as a result, the electric vehicle  20   a  also becomes expensive. Moreover, as described above, since the auxiliary power supply devices using the high withstand voltage electronic component used in the electric vehicle  20   a  are combined into one to be the APU  206   a , the motors  208   a   1 ,  208   a   2 , and  208   a   3  rotate with the fixed number of rotations. For this reason, drive efficiency of the motors  208   a   1 ,  208   a   2  and  208   a   3  is lower than the drive efficiency when the motors  208   a   1 ,  208   a   2  and  208   a   3  are driven by an inverter with a varying frequency. 
     (First Embodiment) 
     Hereinafter, an embodiment will be described in detail with reference to the drawings. First, the traffic system  1  according to a first embodiment of the present disclosure will be described. 
     (Configuration of Traffic System) 
       FIG.  1    is a diagram which shows a configuration of the traffic system  1  according to the first embodiment of the present disclosure. The traffic system  1  includes, as shown in  FIG.  1   , an overhead line  10 , an electric vehicle  20 , and a host system  30 . The traffic system  1  is a system in which the number of high withstand voltage electronic components is reduced compared to the number of high withstand voltage electronic components used in the electric vehicle  20   a  to be compared by providing an insulated DC-DC converter between the overhead line  10  and the electric vehicle  20  and using electronic components with a high withstand voltage (for example 1700 volts) only on the overhead line  10  side in the DC-DC converter. 
     The overhead line  10  supplies electric power to the electric vehicle  20 . For example, the overhead line  10  supplies electric power to the electric vehicle  20  at a predetermined DC voltage (for example, a DC voltage (nominal voltage) of 750 volts). This predetermined DC voltage may vary largely. For example, in an overhead line voltage with a standard voltage of 750 volts, the predetermined DC voltage may vary from 500 volts to 900 volts. The electric vehicle  20  operates using electric power supplied from the overhead line  10  via the pantograph  203   a . The electric vehicle  20  includes, as shown in  FIG.  1   , the vehicle main body  201   a , the wheels  202   a   1  and  202   a   2 , the pantograph  203   a , the motors  204   a   1  and  204   a   2 , the motors  208   a   1 ,  208   a   2 , and  208   a   3 , the HVAC compressor  209   a , the brake air compressor  210   a , the cooling fan  211   a , the accessory  212   a , the control power supply  214   a , a control circuit  215 , a DC-DC converter  216   a , inverters  217   a   1 ,  217   a   2 ,  217   a   3 ,  217   a   4 ,  217   a   5 , and  217   a   6 , and a DC-DC converter  218   a.    
     The vehicle main body  201   a  stores each of the motors  204   a   1  and  204   a   2 , the motors  208   a   1 ,  208   a   2 , and  208   a   3 , the HVAC compressor  209   a , the brake air compressor  210   a , the cooling fan  211   a , the accessory  212   a , the control power supply  214   a , the DC-DC converter  216   a , the inverter  217   a   1 ,  217   a   2 ,  217   a   3 ,  217   a   4 ,  217   a   5 , and  217   a   6 , and the DC-DC converter  218   a.    
     The wheels  202   a   1  are, for example, two front wheels. The wheels  202   a   2  are, for example, two rear wheels. Each of the wheels  202   a   1  and  202   a   2  has, for example, a rubber tire. 
     The pantograph  203   a  receives electric power supplied from the overhead line  10 . The electric power received by the pantograph  203   a  is supplied to the DC-DC converter  216   a , 
     The motors  204   a   1  and  204   a   2  drive the wheels  202   a   1  and  202   a   2 . For example, the motor  204   a   1  rotates the wheels  202   a   1  according to an AC voltage output from the inverter  217   a   1 . In addition, for example, the motor  204   a   2  rotates the wheels  202   a   2  according to an AC voltage output from the inverter  217   a   2 . 
     The motor  208   a   1  drives the HVAC compressor  209   a . The motor  208   a   1  rotates according to an AC voltage output by the inverter  217   a   3 . 
     The motor  208   a   2  drives the brake air compressor  210   a . The motor  208   a   2  rotates according to an AC voltage output by the inverter  217   a   4 . 
     The motor  208   a   3  drives the cooling fan  211   a . The motor  208   a   3  rotates according to an AC voltage output by the inverter  217   a   5 . 
     The HVAC compressor  209   a  sucks in a refrigerant that has evaporated in an air conditioner of the electric vehicle  20  and changes it into liquid. The brake air compressor  210   a  generates compressed air that operates pneumatic equipment for applying a brake to the electric vehicle  20 . The cooling fan  211  cools a heat-generating portion within the vehicle main body  201   a  by circulating air in the heat-generating portion. 
     The accessory  212   a  operates using an AC voltage output from the inverter  217   a   6  as a power source. Examples of the accessory  212   a  include communication equipment, defrosters, and other equipment that operates when electric power is supplied through an outlet. 
     The control power supply  214   a  generates a voltage for the control circuit  215  from a DC voltage output by the DC-DC converter  218   a , and supplies the generated voltage to the control circuit  215 . 
     The control circuit  215  controls the electric vehicle  20 .  FIG.  2    is a diagram which shows an example of a configuration of the control circuit  215  according to the first embodiment of the present disclosure. The control circuit  215  has a control unit  2151  as shown in  FIG.  2   . 
     The control unit  2151  controls the DC-DC converter  216   a , the inverters  217   a   1 ,  217   a   2 ,  217   a   3 ,  217   a   4 ,  217   a   5 , and  217   a   6 , and the DC-DC converter  218   a . Specifically, the control unit  2151  controls AC voltages output from each of the DC-DC converter  216   a  and the inverters  217   a    1 ,  217   a   2 ,  217   a   3 ,  217   a   4 ,  217   a   5  and  217   a   6  by generating a PWM signal for each of the DC-DC converter  216   a  and the inverters  217   a   1 ,  217   a   2 ,  217   a   3 ,  217   a   4 ,  217   a   5 , and  217   a   6 , and controlling the DC-DC converter  216   a  and the inverters  217   a   1 ,  217   a   2 ,  217   a   3 ,  217   a   4 ,  217   a   5  and  217   a   6  using the generated PWM signal. 
     In addition, specifically, the control unit  2151  controls the DC voltage output from the DC-DC converter  218   a  by performing chopper control for a switching element of the DC-DC converter  218   a.    
     Also, the control unit  2151  controls communication with the host system  30  in the electric vehicle  20 . Control by the control unit  2151  is performed via, for example, a controller area network (CAN) communication. 
     The DC-DC converter  216   a  is an insulated bidirectional DC-DC converter capable of transmitting a voltage in both of a direction from the overhead line  10  side to the electric vehicle  20  side and a direction from the electric vehicle  20  side to the overhead line  10  side. 
     For example, the DC-DC converter  216   a  includes an inverter  216   a   1 , a converter  216   a   2 , and a transformer  216   a   3  when electric power is transmitted from the overhead line  10  side to the electric vehicle  20  side. The inverter  216   a   1  converts a DC voltage of the overhead line,  10  into an AC voltage. The inverter  216   a   1  is configured from electronic components with a high withstand voltage (for example, 1700 volts). 
     The converter  216   a   2  converts an AC voltage transmitted from a primary side (that is, the overhead line  10  side) of the transformer  216   a   3  to a secondary side (the electric vehicle  20  side) into a DC voltage. The converter  216   a   2  is configured from electronic components with a lower withstand voltage (for example, 1200 volts) than the inverter  216   a   1 . 
     The transformer  216   a   3  converts an AC voltage output by the inverter  216   a   1  into an AC voltage in accordance with a winding ratio between the primary side and the secondary side such that a DC voltage output by the converter  216   a   2  is a DC voltage of magnitude suitable for each of the inverters  217   a   1 ,  217   a   2 ,  217   a   3 ,  217   a   4 ,  217   a   5 , and  217   a   6 , and the DC-DC converter  218   a.    
     In addition, when electric power is transmitted from the electric vehicle  20  side to the overhead line  10  side (for example, when the electric vehicle  20  applies an electric brake and regenerative electric power is generated in the electric vehicle  20 ), the DC-DC converter  216   a  includes the inverter  216   a   2 , the converter  216   a   1 , and the transformer  216   a   3  after swapping the inverter and the converter each other. The operation of the DC-DC converter  216   a  when electric power is transmitted from the electric vehicle  20  side to the overhead line  10  side can be considered similar to the operation of the DC-DC converter  216   a  when electric power is transmitted from the overhead line  10  side to the electric vehicle  20  side by replacing the inverter  216   a   1  with the converter  216   a   2 , and the converter  216   a   2  with the converter  216   a   1 , respectively, and furthermore swapping the primary side and the secondary side of the transformer  216   a   3  each other in the operation described above of the DC-DC converter  216   a  when electric power is transmitted from the overhead line  10  side to the electric vehicle  20  side. The inverter  217   a   1  converts the DC voltage output by the DC-DC converter  216   a  into an AC voltage that drives the motor  204   a   1 . The inverter  217   a   1  is configured from electronic components with a low withstand voltage (for example, 1200 volts). An amplitude and a frequency of the AC voltage output by the inverter  217   a   1  are not fixed and can be changed depending on a situation, The inverter  217   a   2  converts the DC voltage output by the DC-DC converter  216   a  into an AC voltage that drives the motor  204   a   2 . The inverter  217   a   2  is configured from the electronic components with a low withstand voltage (for example, 1200 volts). An amplitude and a frequency of the AC voltage output by the inverter  217   a   2  are not fixed and can be changed depending on a situation, The inverter  217   a   3  converts the DC voltage output from the DC-DC converter  216   a  into an AC voltage that drives the motor  208   a   1 . The inverter  217   a   3  is configured from the electronic components with a low withstand voltage (for example, 1200 volts). An amplitude and a frequency of the AC voltage output by the inverter  217   a   3  are not fixed and can be changed depending on a situation. The inverter  217   a   4  converts the DC voltage output by the DC-DC converter  216   a  into an AC voltage that drives the motor  208   a   2 . The inverter  217   a   4  is configured from the electronic components with a low withstand voltage (for example, 1200 volts). An amplitude and a frequency of the AC voltage output by the inverter  217   a   4  are not fixed and can be changed depending on a situation. The inverter  217   a   5  converts the DC voltage output by the DC-DC converter  216   a  into an AC voltage that drives the motor  208   a   3 . The inverter  217   a   5  is configured from the electronic components with a low withstand voltage (for example, 1200 volts). An amplitude and a frequency of the AC voltage output by the inverter  217   a   5  are not fixed and can be changed according to a situation. The inverter  217   a   6  converts the DC voltage output by the DC-DC converter  216   a  into an AC voltage that can be supplied to the accessory  212   a . The inverter  217   a   6  is configured from the electronic components with a low withstand voltage (for example, 1200 volts). An amplitude and a frequency of the AC voltage output by the inverter  217   a   6  are not fixed and can be changed depending on a situation. 
     The DC-DC converter  218   a  converts the DC voltage output by the DC-DC converter  216   a  into a DC voltage that can be supplied to the control power supply  214   a.    
     The host system  30  controls traveling of the electric vehicle  20  in the traffic system  1 . For example, the host system  30  outputs a torque command to the electric vehicle  20 , and the electric vehicle  20  travels according to the torque command 
     (Advantage) 
     The traffic system  1  according to the first embodiment of the present disclosure has been described above. In the DC-DC converter  216   a  of the traffic system  1 , the inverter  216   a   1  is configured from electronic components with a high withstand voltage (fir example, 1700 volts), and the converter  216   a   2  is configured from electronic components with a low withstand voltage (for example, 1200 volts). By configuring the DC-DC converter  216   a  of the traffic system  1  in this manner, even if an electric component including a high withstand voltage electronic component is replaced with an electric component including an electronic component with a lower withstand voltage than the electric component, it is possible to provide a bidirectional DC-DC converter that can realize the same function as when the high withstand voltage electric component is used. 
     (First Modified Example of the First Embodiment) 
     Next, the traffic system  1  according to a first modified example of the first embodiment of the present disclosure will be described. The traffic system  1  according to the first modified example of the first embodiment is a system having a function of improving the idling of wheels, like the VDF inverter  205   a  of the electric vehicle  20   a  to be compared. 
     In the first modified example of the first embodiment, the function of improving the idling of wheels is realized by the control circuit  215 , the host system  30 , and wheel rotation sensors  40   a   1  and  40   a   2  to be described below. A difference between the traffic system  1  according to the first modified example of the first embodiment and the traffic system  1  according to the first embodiment will be described below. 
     (Configuration of Traffic System) 
       FIG.  3    is a diagram which shows an example of the configuration of the traffic system  1  according to the first modified example of the first embodiment of the present disclosure. The traffic system  1  according to the first modified example of the first embodiment includes, similar to the traffic system  1  of the first embodiment shown in  FIG.  1   , the vehicle main body  201   a , the wheels  202   a   1  and  202   a   2 , the pantograph  203   a , the motors  204   a   1  and  204   a   2 , the motors  208   a   1 ,  208   a   2 , and  208   a   3 , the HVAC compressor  209   a , the brake air compressor  210   a , the cooling fan  211   a , the accessory  212   a , the control power supply  214   a , the control circuit  215 , the DC-DC converter  216   a , the inverters  217   a   1 ,  217   a   2 ,  217   a   3 ,  217   a   4 ,  217   a   5 , and  217   a   6 , and the DC-DC converter  218   a . However, the control circuit  215  according to the first modified example of the first embodiment differs from the control circuit  215  according to the first embodiment. In addition, the traffic system  1  according to the first modified example of the first embodiment further includes rotation sensors  40   a   1  and  40   a   2 . 
     The rotation sensor  40   a   1  detects the number of rotations of the wheels  202   a   1  per unit time. The rotation sensor  40   a   2  detects the number of rotations of the wheels  202   a   2  per unit time. 
     The control circuit  215  controls the electric vehicle  20 .  FIG.  4    is a diagram which shows an example of the configuration of the control circuit  215  according to the first modified example of the first embodiment of the present disclosure. The control circuit  215  includes, as shown in  FIG.  4   , a control unit  2151 , an acquisition unit  2152 , and a determination unit  2153 . 
     The acquisition unit  2152  acquires results of detecting the number of rotations from each of the rotation sensors  40   a   1  and  40   a   2 . 
     The determination unit  2153  calculates the rate of change in the number of rotations of the wheels  202   a   1  per unit time and the rate of change in the number of rotations of the wheels  202   a   2  per unit time from each result of detection acquired by the acquisition unit  2152  (that is, the number of rotations of the wheels  202   a   1  per unit time and the number of rotations of the wheels  202   a   2  per unit time), compares between these rates, and determines the idling of the axles (that is, the front and rear axle) on the basis of a result of the comparison. For example, when the rate of change in the number of rotations of the wheels  202   a   2  per unit time with respect to the rate of change in the number of rotations of the —heels  202   a   1  deviates from a range of the rate of change in the number of rotations of the wheels  202   a   2  per unit time determined in advance based on the rate of change in the number of rotations of the wheels  202   a   1  per unit time, that is, an allowable value of the rate of change in the number of rotations, the determination unit  2153  determines that an axle to which wheels with the higher rate of change in the number of rotations per unit time are connected is idling. In addition, the determination unit  2153  determines that the axles are not idling when it does not deviate from the range of the rate of change in the number of rotations, that is, the allowable value of the rate of change in the number of rotations. The determination unit  2153  may determine whether the idling of the axles occurs by using “the number of rotations of wheels  202   a   1  per unit time and the number of rotations of wheels  202   a   2  per unit time” instead of “the rate of change in the number of rotations of the wheels  202   a   1  per unit time and the rate of change in the number of rotations of the wheels  202   a   2  per unit time.” 
     The control unit  2151  controls the DC-DC converter  216   a , the inverters  217   a   1 ,  217   a   2 ,  217   a   3 ,  217   a   4 ,  217   a   5 , and  217   a   6 , and the DC-DC converter  218   a , like the control unit  2151  according to the first embodiment. Specifically, the control unit  2151  generates a PWM signal for each of the DC-DC converter  216   a , and the inverters  217   a   1 ,  217   a   2 ,  217   a   3 ,  217   a   4 ,  217   a   5 , and  217   a   6 , and controls the DC-L)C converter  216   a , and the inverters  217   a   1 ,  217   a   2 ,  217   a   3 ,  217   a   4 ,  217   a   5  and  217   a   6  using the generated PWM thereby controlling AC voltages output from each of the DC-DC converter  216   a , and the inverters  217   a   1 ,  217   a   2 ,  217   a   3 ,  217   a   4 ,  217   a   5  and  217   a   6 . In addition, specifically, the control unit  2151  controls a DC voltage output from the DC-DC converter  218   a  by performing chopper control for switching elements of the DC-DC converter  218   a.    
     Moreover, the control unit  2151  controls communication with the host system  30  in the electric vehicle  20 , like the control unit  2151  according to the first embodiment. However, in the first modified example of the first embodiment, the control unit  2151  controls each of the inverters  217   a   1  and  217   a   2  on the basis of a result of determination by the determination unit  2053 . Specifically, when the determination unit  2053  determines that an axle is idling (that is, at least one of the front and rear axles), the control unit  2151  controls an inverter (that is the inverter  217   a   1  or the inverter  217   a   2 ) corresponding to each individual axle to reduce the torque of wheels (that is, the wheels  202   a   1  or the wheels  202   a   2 ) that are connected to the axle determined to be idling. A degree of torque reduction is determined by a combination of the control content of the control unit  2151  for the inverter and the rate of change in the number of rotations of the wheels per unit time. In this manner, the control unit  2151  controls each of the inverters  217   a   1  and  217   a   2  on the basis of a result of the determination by the determination unit  2053 , and thereby it is possible to immediately improve the idling of axles even when at least one of the from and rear axles idles. 
     In addition to the functions possessed by the host system  30  according to the first embodiment, the host system  30  has a function of detecting the amount of change in rotation speed (acceleration) of each of the motors  204   a   1  and  204   a   2 , like a detection unit  2051   a   1  and a control unit  2052   a   2  provided in the VDF inverter  205   a  of the electric vehicle  20   a  to be compared. In addition, when the host system  30  detects the idling of both axles (front and rear axles) (in the example shown here, when it is determined that the amount of change in the frequency of both motor currents flowing through the motors  204   a   1  and  204   a   2  has exceeded the allowable value), it has a function of transmitting a torque command that narrows down the current values of the motors  204   a   1  and  204   a   2  (that is, reduces the torque of the motor  204   a   1  and the motor  204   a   2 ) determined that the amount of change in the frequency of the motor current has exceeded the allowable value to the electric vehicle  20 . 
     Specifically; the host system  30  controls the traveling of the electric vehicle  20  in the traffic system  1 . For example, the host system  30  outputs a torque command to the electric vehicle  20 , and the electric vehicle  20  travels according to the torque command. However, the host system  30  detects the amount of change in the rotation speed (acceleration) of each of the motors  204   a   1  and  204   a   2 . For example, the host system  30  detects the amount of change in the rotation speeds as the amount of change in the frequency of the motor current flowing through the motors  204   a   1  and  204   a   2 . When the host system  30  determines whether the amount of change in the frequency of both motor currents flowing through the motors  204   a   1  and  204   a   2  exceeds the allowable value, and determines that the amount of change in the frequency of both motor currents has exceeded the allowable value, it determines that both axles are idling. In addition, when the host system  30  determines that the amount of change in the frequency of at least one of the motor currents does not exceed the allowable value, it determines that the idling of both axles does not occur. 
     Then, when the host system  30  detects the idling of both axles (in the example shown here, when it determines that the amount of change in the frequency of both motor currents flowing through the motors  204   a   1  and  204   a   2  has exceeded the allowable value), it transmits a torque command that narrows down the current values of the motors  204   a   1  and  204   a   2  (that is, reduces the torque of the motor  204   a   1  and the motor  204   a   2 ) determined that the amount of change in the frequency of the motor current has exceeded the allowable value to the electric vehicle  20 . In this case, the control unit  2151  of the electric vehicle  20  controls each of the inverters  217   a   1  and  217   a   2  according to the torque command transmitted from the host system  30 . 
     (Processing Performed by Traffic System) 
       FIG.  5    is a diagram which shows an example of a processing flow of the traffic system according to the first modified example of the first embodiment. Next, processing for improving the idling of wheels by the traffic system  1  according to the first modified example of the first embodiment will be described with reference to  FIG.  5   . 
     The traffic system  1  sets an idling allowable value (step S 1 ). Specifically, for example, the determination unit  2153  sets and holds a range of the rate of change in the number of rotations of the wheels  202   a   2  per unit time determined in advance by based on the rate of change in the number of rotations of the wheels  202   a   1  per unit time, that is, an allowable value for the rate of change in the number of rotations. In addition, the host system  30  sets and holds an allowable value for the amount of change in the rotation speed (acceleration) of each of the motors  204   a   1  and  204   a   2  (for example, an allowable value for the amount of change in the frequency of motor current flowing through the motors  204   a   1  and  204   a   2 ). 
     The traffic system  1  detects idling of the axle (step S 2 ). Specifically, in the electric vehicle  20 , the acquisition unit  2152  acquires results of detecting the number of rotations from the rotation sensors  40   a   1  and  40   a   2 . The determination unit  2153  calculates the rate of change in the number of rotations of the wheels  202   a    1  per unit time and the rate of change in the number of rotations of the wheels  202   a   2  per unit time from each result of detection acquired by the acquisition unit  2152  (that is, the rate of change in the number of rotations of the wheels  202   a   1  per unit time and the rate of change in the number of rotations of the wheels  202   a   2  per unit time), compares between these, and determines the idling of the axle on the basis of a result of the comparison. For example, when the rate of change in the number of rotations of the wheels  202   a   2  per unit time with respect to the rate of change in the number of rotations of the wheels  202   a   1  per unit time deviates from a range of the rate of change in the number of rotations of the wheels  202   a   2  per unit time determined in advance based on the rate of change in the number of rotations of the wheels  202   a   1  per unit time, that is, the allowable value of the rate of change in the number of rotations, the determination unit  2153  determines that an axle to which the wheels with the higher rate of change in the number of rotations per unit time are connected is idling. In addition, the determination unit  2153  determines that the axle is not idling when it does not deviate from the range of the rate of change in the number of rotations, that is, the allowable value of the rate of change in the number of rotations. 
     In addition, specifically, the host system  30  determines whether the amount of change in the frequency of both motor currents flowing through the motors  204   a   1  and  204   a   2  exceeds the allowable value, and determines that both axles (front and rear axles) are idling when it is determined that the amount of change in the frequency of both motor currents exceeds the allowable value. Moreover, when the host system  30  determines that the amount of change in the frequency of at least one of the motor currents does not exceed the allowable value, it determines that the idling of both axles has not occurred. in processing of step S 2 , when the traffic system  1  determines that at least one of the front and rear axles is idling (“control for each individual axle” in step S 2 ), an inverter corresponding to each individual axle is controlled (step S 3 ) to reduce the torque of the wheels connected to the axle determined to be idling. Specifically, when the determination limit  2053  determines that the axle is idling, the control unit  2151  controls the inverter corresponding to each individual axle (that is, inverter  217   a   1  or inverter  217   a   2 ) to reduce the torque of the wheels (that is, the wheels  202   a   1  or  202   a   2 ) connected to the axle determined to be idling. 
     In addition, in the processing of step S 2 , when the traffic system  1  determines that it has detected the idling of both the front and rear axles, the inverter corresponding  113  to each individual axle is controlled to reduce the torque of the wheels connected to both axles. Specifically, when the host system  30  detects the idling of both axles (in the example shown here, when it is determined that the amount of change in the frequency of both motor currents flowing through the motors  204   a   1  and  204   a   2  exceeds the allowable value)), the host system  30  transmits a torque command value that narrows down current values of the motors  204   a   1  and  204   a   2  (that is, reduces torque of the motor  204   a   1  and the motor  204   a   2 ) determined that the amount of change in the frequency of the motor current has exceeded the allowable value to the electric vehicle  20 . In this case, the control unit  2151  of the electric vehicle  20  controls each of the inverters  217   a   1  and  217   a   2  according to the torque command transmitted from the host system  30 . Also, in the processing of step  52 , if the traffic system  1  determines that both axles are not idling (“within the idling allowable value” in step S 2 ), it returns to the determination of step S 2 . 
     In the traffic system  1 , when the determination unit  2053  determines that the axle is idling and the host system  30  transmits the torque command value to the electric vehicle  20 , the control unit  2151  gives priority to the torque command value transmitted by the host system  30  to the electric vehicle  20  and controls the inverter corresponding to each of both axles. In other words, the traffic system  1  executes processing of step S 4  with priority over processing of step S 3 . 
     (Advantage) The traffic system  1  according to the first modified example of the first embodiment has been described above. In the traffic system  1  according to the first modified example of the first embodiment, processing of steps S 2  and S 4  described above makes it possible to improve the idling of wheels equivalent to the VDF inverter  205   a  of the electric vehicle  20   a  to be compared. In addition, since the improvement of wheel idling equivalent to this VDF inverter  205   a  can be realized by the host system  30  present in advance in the traffic system  1 , an inverter of the electric vehicle  20  does not need the function of improving the idling of the wheels, such as the VDF inverter  205   a , and can be a low-performance inverter. As a result, a cost reduction in the manufacturing of the electric vehicle  20  can be expected. 
     Moreover, in the traffic system  1  according to the first modified example of the first embodiment, it is possible to determine the idling of the front and rear axles and improve the idling of the wheels connected to the idling axle on the basis of a result of comparing rates of change in the number of rotations between the front wheel and the rear wheel, which are not provided in the electric vehicle  20   a  to be compared, according to the processing of steps S 2  and S 3  described above, 
     (Second Modified Example of the First Embodiment) 
     Next, the traffic system  1  according to a second modified example of the first embodiment of the present disclosure will be described. The traffic system  1  according to the second modified example of the first embodiment is a traffic system that includes a DC-DC converter in a Dual Active Bridge (DAB) method (hereinafter, referred to as a DAB circuit  216   a ) as a specific example of the DC-DC converter  216   a , which is an insulated bidirectional DC-DC converter included in the traffic system  1  of the first embodiment. The traffic system  1  according to the second modified example of the first embodiment is a traffic system in which problems occurring at the time of controlling a. general DC-DC converter in a DAB method are improved. 
     (Configuration of DAB Circuit) 
       FIG.  6    is a diagram which shows an example of a configuration of the DAB circuit  216   a  according to the second modified example of the first embodiment of the present disclosure. The DAB circuit  216   a  includes, as shown in  FIG.  6   , a primary side (overhead line  10  side) circuit  216   a   1 , a secondary side (electric vehicle  20  side) circuit  216   a   2 , and the transformer  216   a   3 . For convenience of description, a circuit on the overhead line  10  side is called the primary circuit  216   a   1 , and a circuit on the electric vehicle  20  side is called the secondary circuit  216   a   2 . However, since the DAB circuit  216   a  is an insulated bidirectional DC-DC converter, there is no need to mention that the electric vehicle  20  side becomes the primary side and the overhead line  10  side becomes the secondary side in an actual operation, and it is also possible to transmit electric power from the electric vehicle  20  to the overhead line  10 . 
     The primary circuit  216   a   1  includes, as shown in  FIG.  6   , four switching elements  709 ,  710 ,  711 , and  712 , four diodes  717 ,  718 ,  719 , and  720 , five capacitors  725 ,  726 ,  727 ,  728 , and  733 , and a reactor  735 . The switching elements  709 ,  710 ,  711 , and  712  are, for example, power semiconductors. Examples of the power semiconductors include metal oxide semiconductor (MOS) transistors, insulated gate bipolar transistors (IGBT), silicon carbide (SiC) transistors, and the like. 
     For example, if the switching elements  709 ,  710 ,  711 , and  712  are MOS transistors, the diode  717  is a body diode of the switching element  709  (that is, a parasitic diode between source and drain). Moreover, the diodes  718 ,  719  and  720  are body diodes of the switching elements  710 ,  711  and  712 , respectively. The capacitors  725 ,  726 ,  727  and  728  are snubber capacitors. 
     In addition, the secondary circuit  216   a   2  includes, as shown in  FIG.  6   , four switching elements  713 ,  714 ,  715 , and  716 , four diodes  721 ,  722 ,  723 , and  724 , and five capacitors  729 ,  730 ,  731 ,  732 , and  734 . The switching elements  713 ,  714 ,  715 , and  716  are, for example, power semiconductors. The switching elements  713 ,  714 ,  715 , and  716  may be different types of power semiconductors from the switching elements  709 ,  710 ,  711 , and  712 . For example, the switching elements  709 ,  710 ,  711 ,  712  may be IGBTs and the switching elements  713 ,  714 ,  715 ,  716  may be MOS transistors. 
     For example, if the switching elements  713 ,  714 ,  715 , and  716  are MOS transistors, the diode  721  is a body diode of the switching element  713 . The diodes  722 ,  723  and  724  are body diodes of the switching elements  714 ,  715  and  716 , respectively. The capacitors  729 ,  730 ,  731  and  732  are snubber capacitors. The transformer  216   a   3  includes, as shown in  FIG.  6   , a primary coil  736  and a secondary coil  737 . 
     Connection between elements of the DAB circuit  216   a  shown in  FIG.  6    is the same as connection of a general DAB type DC-DC converter. In the traffic system  1  according to the second modified example of the first embodiment, the switching elements  709 ,  710 ,  711 ,  712 ,  713 ,  714 ,  715 , and  716  are controlled such that they are in the ON state or the OFF state by the control unit  2151  of the control circuit  215 . In the traffic system  1  according to the second modified example of the first embodiment, control of the switching elements  709 ,  710 ,  711 ,  712 ,  713 ,  714 ,  715 , and  716  by the control unit  2151  is different from general control performed on the DAB type DC-DC converter. 
     Here, problems that occur at the time of controlling a general DAB type DC-DC converter will be described.  FIG.  7 A  is a first diagram for describing problems that occur at the time of controlling the general DAB type DC-DC converter.  FIG.  7 B  is a second diagram for describing problems that occur at the time of controlling the general DAB type DC-DC converter.  FIG.  7 C  is a third diagram for describing problems that occur at the time of controlling the general DAB type DC-DC converter.  FIG.  8    is a fourth diagram for describing problems that occur at the time of controlling the general DAB type DC-DC converter.  FIG.  9    is a fifth diagram for describing problems that occur at the time of controlling the general DAB type DC-DC converter.  FIG.  8    is an enlarged view of  FIG.  7 A . Moreover,  FIG.  9    is an enlarged view of a part including one cycle in which an operation is performed in  FIG.  7 B . 
     The  FIG.  7 A  is a diagram which shows a result of simulation when the control unit  2151  switches the switching elements  709 ,  710 ,  711  and  712  of the DAB circuit  216   a  using a signal of a constant cycle (in the example shown here, a constant cycle of 20 kHz)) with a duty ratio of 50%, and transmits 5 kW of electric power from the primary circuit  216   a   1  to the secondary circuit  216   a   2 . When electric power is transmitted from the primary circuit  216   a   1  to the secondary circuit  216   a   2 , the switching elements  713 ,  714 ,  715 , and  716  of the secondary circuit  216   a   2  are in the OFF state, and the bridge circuit configured from the diodes  721  to  724  rectifies a voltage transmitted from the primary circuit  216   a   1  to the secondary circuit  216   a   2  via the transformer  216   a   3 . In addition, an input voltage of the primary circuit  216   a   1  is 800 volts, an output voltage of the secondary circuit  216   a   2  is 500 volts, and a dead time is provided for the switching of the switching elements to prevent the switching element  709  and the switching element  710  connected in series up and down from being in the ON state at the same time, in addition, to prevent the switching element  711  and the switching element  712  from being in the ON state at the same time (that is, to prevent the through current from flowing). 
     Control that switches the switching elements using a signal of the constant cycle with a duty ratio of 50% is one of control commonly performed. When the control unit  2151  performs control of switching the switching elements using a signal of the constant cycle with a duty ratio of 50% on a general DAB type DC-DC converter, since a dead time is provided even when, for example, it is controlled that the switching elements  709  and  712  are in the ON state and the switching elements  710  and  711  are in the OFF state, there is a state in which both the switching elements  709  and  710  are in the OFF state. However, even if the switching element  710  is in the OFF state, current flows through a path of the diode  718 , the reactor  735 , the primary coil  736 , and the switching element  712  via the diode  718 . In this state, when dead time ends and the switching element  709  is turned on, a phenomenon called recovery occurs in which a voltage substantially equal to the input voltage of the primary circuit  216   a   1  due to the DC voltage of the overhead line  10  is applied to the diode  718 , and current flows in the diode  718  in an opposite direction. In this case, the switching element  710  is also turned on, and a problem occurs that a through current flows from the switching element  709  to the switching element  710  (that is, hard switching). When the control unit  2151  performs the control of switching the switching elements using a signal of the constant cycle with a duty ratio of 50%, a phenomenon called recovery occurs every half cycle. A cause of the phenomenon called recovery occurring when the control unit  2151  performs the control of switching the switching elements using a signal of the constant cycle with a duty ratio of 50% is because, as shown in  FIG.  8   , the transformer current changes from a positive current to a negative current and crosses zero in a period in which the transformer voltage is kept at the high level, and the transformer current changes from a negative current to a positive current and crosses zero in a period in which the transformer voltage is kept at the low level. When the input voltage of the primary circuit  216   a   1  and the output voltage of the secondary circuit  216   a   2  are equal, there is almost no change in the transformer current and no zero crossing, so the phenomenon called recovery does not occur. 
     A driving method called an intermittent operation has been proposed to prevent the phenomenon called recovery described above. This intermittent operation is control of switching the switching elements before the transformer current described above changes and crosses zero. In  FIG.  7 B , the high level and the low level are switched before the transformer current crosses zero by changing the period in which the transformer voltage is kept at the high level and the period in which the transformer voltage is kept at the low level without changing the constant cycle (in this case, 20 kHz). However, since electric power transmitted from the primary circuit  216   a   1  to the secondary circuit  216   a   2  needs to be the same as that before the change (in the example shown here, it needs to be 5 kW), the electric power to be transmitted is adjusted by providing a period in which the transformer voltage is zero. However, when the intermittent operation is performed without changing the constant cycle (20 kHz in this case) hard switching may occur as shown in  FIG.  9   , that is, the phenomenon called recovery cannot be completely eliminated. 
     Therefore, in the traffic system  1  according to the second modified example of the first embodiment, the constant cycle is shortened (for example, 40 kHz corresponding to half the cycle), and a period is provided to make the transformer voltage zero such that the electric power transmitted from the primary circuit  216   a   1  to the secondary circuit  216   a   2  becomes the same as before the change. The control unit  2151  controls the switching elements  709 ,  710 ,  711 , and  712  of the DAB circuit  216   a  with such signals, thereby preventing hard switching, that is, eliminating the phenomenon called recovery.  FIG.  7 C  shows an example of signals used when the control unit  2151  controls the switching elements  709 ,  710 ,  711  and  712  of the DAB circuit  216   a . As shown in  FIG.  7 C , as described above, the control unit  2151  controls the switching elements  709 ,  710 ,  711 ,  712  of the DAB circuit  216   a  by shortening the constant cycle (for example, 40 kHz corresponding to half the period), and using a signal with a period provided to make the transformer voltage zero such that the electric power transmitted from the primary circuit  216   a   1  to the secondary circuit  216   a   2  becomes the same as before the change. As a result, it is possible to switch between a high level and a lo level before the transformer current crosses zero, and it is possible to prevent hard switching, that is, to eliminate the phenomenon called recovery. In general, when the constant cycle is shortened, a frequency is increased, that is, the number of switching times increases, so an efficiency tends to decrease due to heat generation caused by switching. However, in the traffic system  1  according to the second modified example of the first embodiment, even if the constant cycle is shortened, the pause period is set accordingly, and the number of switching times per cycle can be made to be equal to or less than before the constant cycle is shortened. For this reason, in the traffic system  1  according to the second modified example of the first embodiment, it is possible to improve efficiency. 
     When the load of the DC-DC converter  216   a  is light, such as when the air conditioner is stopped in the electric vehicle  20 , the transformer current is decreased. When the transformer current is small, if the transformer current changes due to something such as noise, a ratio of the change to the transformer current is increased, and negligible change in the transformer current cannot be neglected when the transformer current is large. That is, when the transformer current is small, a phenomenon called recovery may occur in the same manner as when the transformer current changes and the input voltage of the primary circuit  216   a   1  is different from the output voltage of the secondary circuit  216   a   2 . Therefore, when the transformer current is small, in the same manner as when the input voltage of the primary circuit  216   a   1  is different from the output voltage of the secondary circuit  216   a   2 , the control unit  2151  may shorten the constant cycle, set a pause period such that the electric power transmitted per unit time becomes desired electric power, and control the switching elements  709 ,  710 ,  711 , and  712  of the DAB circuit  216   a.    
     When the input voltage of the primary circuit  216   a   1  and the output voltage of the secondary circuit  216   a   2  are considered to be the same, or when the load is considered to be heavy; the control unit  2151  may perform control such that the switching elements  709 ,  710 ,  711  and  712  are switched by a signal having a constant cycle (in the example shown here, a constant cycle of 20 kHz) with a duty ratio of 50% shown in  FIG.  7 A , 
     In addition, when transmitting electric power from the electric vehicle  20  to the overhead line  10  (for example, when the electric vehicle  20  applies an electric brake and regenerative electric power is generated in the electric vehicle  20 ), when the voltage of the electric power generated on the electric vehicle  20  side (an example of a third voltage) is different from a voltage of the overhead line  10  (an example of the first voltage), or when the load of the overhead line  10  is light, the control unit  2151  may turn off the switching elements  709 ,  710 ,  711 , and  712 , and control the switching elements  713 ,  714 ,  715 , and  716  in the same mariner as the switching elements  709 ,  710 ,  711 , and  712  described above, 
     (Advantage) 
     The traffic system  1  according to the second modified example of the first embodiment has been described above. The control unit  2151  shortens a cycle of switching the switching elements  709 ,  710 ,  711  and  712  and provides a period in which the switching is not performed when an input voltage (an example of a first voltage) of the primary circuit  216   a   1  is different from an output voltage (an example of a second voltage) of the secondary circuit  216   a   2 , or when a load of the DC-DC converter  216   a  in the electric vehicle  20  is light. In addition, when a voltage of electric power generated on the electric vehicle  20  side (an example of the third voltage) is different from the voltage of the overhead line  10  (an example of the first voltage), or when the load of the overhead line  10  is light, the control unit  2151  shortens a cycle of switching the switching elements  713 ,  714 ,  715  and  716  and provides the period in which the switching is not performed. By configuring the DC-DC converter  216   a  of the traffic system  1  in this manner, it is possible to suppress occurrence of hard switching. 
     (Third Modified Example of First Embodiment) 
     Next, a traffic system  1  according to a third modified example of the first embodiment of the present disclosure will be described. The traffic system  1  according to the third modified example of the first embodiment is a traffic system that reduces a possibility that malfunction occurs in electronic components of the DAB circuit  216   a  when the DAB circuit  216   a  included in the traffic system  1  according to the second modified example of the first embodiment is started, and performs charging of the capacitor  734  provided in the secondary circuit  216   a   2 . 
     (Configuration of Traffic System) 
     The configuration of the DAB circuit  216   a  according to the third modified example of the first embodiment is the same as the configuration of the DAB circuit  216   a  according to the second modified example of the first embodiment shown in  FIG.  6   . The traffic system  1  according to the third modified example of the first embodiment differs from the traffic system  1  according to the second modified example of the first embodiment in the content of control over the switching elements  709 ,  710 ,  711 , and  712  by the control unit  2151 . 
       FIG.  10    is the first diagram for describing control content of the control unit  2151  according to the third modified example of the first embodiment.  FIG.  11    is a second diagram for describing the control content of the control unit  2151  according to the third modified example of the first embodiment.  FIG.  12    is a third diagram for describing the control content of the control unit  2151  according to the third modified example of the first embodiment.  FIG.  13    is a fourth diagram for describing the control content of the control unit  2151  according to the third modified example of the first embodiment.  FIG.  14    is a fifth diagram for describing the control content of the control unit  2151  according to the third modified example of the first embodiment. 
       FIG.  10    is a diagram which shows an example of control signals used when the control unit  2151  switches the switching elements  709 ,  710 ,  711 , and  712 .  FIG.  11    shows a current path in the DAB circuit  216   a  in a period indicated as  1  in  FIG.  10   . In the period indicated as  1  in  FIG.  10   , the control unit  2151  controls the switching elements  709  and  712  such that they are in the ON state and controls the switching elements  710  and  711  such that they are in the OFF state. In the period indicated as  1  in  FIG.  10   , current flows through a path of the overhead line  10 , the switching element  709 , the reactor  735 , the primary coil  736 , the switching element  712 , and the overhead line  10 , as shown in  FIG.  11   . 
       FIG.  12    shows the current path in the DAB circuit  216   a  in a period indicated as  2  in  FIG.  10   . In the period indicated as  2  in  FIG.  10   , the control unit  2151  controls the switching element  712  such that it is in the ON state and controls the switching elements  709 ,  710 , and  711  such that they are in the OFF state. In the period indicated as  2  in  FIG.  10   , current flows through a path of the reactor  735 , the primary coil  736 , the switching element  712 , and the diode  718 , as shown in  FIG.  12   . 
       FIG.  13    shows the current path in the DAB circuit  216   a  in a period indicated as  3  in  FIG.  10   . In the period indicated as  3  in  FIG.  10   , the control unit  2151  controls the switching elements  710  and  711  such that they are in the ON state and controls the switching elements  709  and  712  such that they are in the OFF state. In the period indicated as  3  in  FIG.  10   , current flows through a path of the overhead line  10 , the switching element  711 , the primary coil  736 , the reactor  735 , the switching element  710 , and the overhead line  10 , as shown in  FIG.  13   . 
       FIG.  14    shows the current path in the DAB circuit  216   a  in the period indicated as  4  in  FIG.  10   . In the period indicated as  4  in  FIG.  10   , the control unit  2151  controls the switching element  711  such that it is in the ON state and controls the switching elements  709 ,  710 , and  712  such that they are in the OFF state. In the period indicated as  4  in  FIG.  10   , current flows through a path of the switching element  711 , the primary coil  736 , the reactor  735 , and the diode  717 , as shown in  FIG.  14   . 
     When the DAB circuit  216   a  is started, the periods indicated as  1  to  4  in  FIG.  10    are repeated, and the current flows through the paths shown in  FIGS.  11  to  14    repeatedly. 
     In this case, current flowing through the reactor  735  changes the direction, that is, alternating current flows through the reactor  735 . This alternating current is transmitted to the second circuit  216   a   2  via the transformer  216   a   3  and rectified by a bridge circuit of the diodes  721  to  724 . As a result, the capacitor  734  is charged. As shown in  FIG.  10   , the period in which the switching element  709  and the switching element  710  are in the ON state is shorter than the period in which the switching element  711  and the switching element  712  are in the ON state. For this reason, the capacitor  734  repeats being slightly charged in a short time. That is, a possibility that the capacitor  734  is charged with excessive current is reduced, 
     Note that the control unit  2151  may lengthen the period in which the switching element  709  and the switching element  710  are in the ON state similar to the period in which the switching element  711  and the switching element  712  are in the ON state when the charge of the capacitor  734  reaches a predetermined state (for example, when it is determined that the charge reaches 80% of the full charge). 
       FIG.  15    is a first diagram which shows an example of waveforms of simulation for charging the capacitor  734 .  FIG.  16    is a second diagram which shows an example of the waveforms of the simulation fir charging the capacitor  734 .  FIG.  17    is a third diagram which shows an example of the waveforms of the simulation for charging the capacitor  734 . 
       FIG.  15    shows an example of the waveforms of the simulation when charging of the capacitor  734  starts. Based on the waveforms shown in  FIG.  15   , it can be seen that a voltage of the capacitor  734  rises slowly. 
       FIG.  16    shows an example of the waveforms of the simulation in the middle of charging the capacitor  734 . Based on the waveforms shown in  FIG.  16   , it can be seen that a change in voltage of the capacitor  734  rises more slowly than when the charging starts. 
       FIG.  17    shows an example of waveforms of simulation when the capacitor  734  is charged to a predetermined state (in this example, a state in which it is charged to 80% of the full charge) and the control unit  2151  lengthens a period in which the switching elements  709  and  710  are in the ON state similarly to a period in which the switching element  711  and the switching element  712  are in the ON state. Based on the waveforms shown in  FIG.  17   , it can be seen that a voltage of the voltage of the, capacitor  734  rises quickly from a timing when the control unit  2151  lengthens the period in which the switching element  709  and the switching element  710  are in the ON state similar to the period in which the switching element  711  and the switching element  712  are in the ON state. 
     When electric power is transmitted from the electric vehicle  20  to the overhead line  10  (for example, when the electric vehicle  20  applies an electric brake and regenerative electric power is generated in the electric vehicle  20 ), the control unit  2151  may turn off the switching elements  709 ,  710 ,  711  and  712 , and control the switching elements  713 ,  714 ,  715  and  716  in the same manner as the switching elements  709 ,  710 ,  711  and  712  described above. 
     (Advantage) 
     The traffic system  1  according to the third modified example of the first embodiment has been described above. In the traffic system  1  according to the third modified example of the first embodiment, the control unit  2151  can get the capacitor  734  to be charged without overcurrent flowing through the capacitor  734  by making a period in which some switching elements (for example, switching element  709  and switching element  710 ) are in the ON state shorter than a period in which other switching elements (for example, the switching element  711  and the switching element  712 ) are in the ON state and switching these switching elements. 
     (Second Embodiment) 
     Next, a traffic system  1  according to a second embodiment of the present disclosure will be described.  FIG.  18    is a diagram which shows an example of a configuration of the traffic system  1  according to the second embodiment of the present disclosure. Unlike the traffic system  1  according to the first embodiment, the DC-DC converter  216   a  in the traffic system  1  according to the second embodiment of the present disclosure is a bidirectional DC-DC converter under chopper control. In the DC-DC converter  216   a , a circuit handling the voltage of the overhead line  10  is configured from the electronic components with a high withstand voltage (for example, 1700 volts). In addition, in the DC-DC converter  216   a , a circuit that handles a voltage supplied to the electric vehicle  20  or a voltage generated in the electric vehicle  20  (for example, a voltage indicated by the regenerative electric power) is configured from electronic components with a low withstand voltage (for example, 1200 volts). Chopper control for the switching element in the DC-DC converter  216   a  may be performed by the control circuit  215 . 
     The DC-DC converter  216   a  according to the second embodiment is, for example, a bidirectional chopper circuit.  FIG.  19    is a diagram which shows an example of a configuration of the DC-DC converter  216   a  according to the second embodiment of the present disclosure. The DC-DC converter  216   a  includes, as shown in  FIG.  19   , the switching elements  709  and  710 , the capacitors  733  and  734 , and the reactor  735 . Note that the DC-DC converter  216   a  shown in  FIG.  19    has the same configuration as a bidirectional chopper circuit with a general configuration shown in  FIG.  4    of Japanese Unexamined Patent Application, First Publication No. 2006-006061. 
     The switching elements  709  and  710  are, for example, power semiconductors. 
     For example, if the switching elements  709  and  710  are MOS transistors, the diode  717  is the body diode of the switching element  709 . In addition, diode  718  is the body diode of the switching element  710 . The DC-DC converter  216   a  shown in  FIG.  19    can step down a voltage between a ground GND terminal on the left and a terminal a 1  and output it as a voltage tween a ground GND terminal on the right and a terminal a 2 . In addition, the DC-DC converter  216   a  shown in  FIG.  19    can step up the voltage between the ground GND terminal on the right side and the terminal a 2  and output it as the voltage between the ground GND terminal on the left side and the terminal a 1 . In the case of the DC-DC converter  216   a  shown in  FIG.  19   , the switching elements  709  and  710 , the capacitor  733 , and the reactor  735  are electronic components with a high withstand voltage (for example, 1700 volts) that constitute the circuit handling the voltage of the overhead line  10 . In addition, in the case of the DC-DC converter  216   a  shown in  FIG.  19   , the capacitor  734  is an electronic component with a low withstand voltage (for example, 1200 volts) that constitutes a circuit handling the voltage generated. in the electric vehicle  20 . 
     In addition, the DC-DC converter  216   a  is not limited to a bidirectional chopper circuit shown in  FIG.  19   , and may be any kind of DC-DC converter as long as it can convert a voltage on the overhead line  10  side into a voltage on the electric vehicle  20  side when electric power is supplied from the overhead line  10  to the electric vehicle  20 , and convert and transmit regenerative electric power generated in the electric vehicle  20  from the voltage on the electric vehicle  20  side to the voltage on the overhead line  10  side, for example, when an electric brake is applied. 
     (Advantage) 
     The traffic system  1  according to the second embodiment has been described above. In the DC-DC converter  216   a  of the traffic system  1  according to the second embodiment, a circuit handling the voltage of the overhead line  10  is configured from electronic components with a high withstand voltage (for example, 1700 volts). In addition, the circuit that handles a voltage supplied to the electric vehicle  20  or a voltage generated in the electric vehicle  20  (for example, a voltage indicated by regenerative electric power) is configured from electronic components with a low withstand voltage (for example, 1200 volts). By configuring the DC-DC converter  216   a  of the traffic system  1  in this manner, even if an electric component including a high withstand voltage electronic component is replaced with an electric component including an electronic component with a lower withstand voltage than the electric component, it is possible to realize the same function as when the high withstand voltage electric component is used. 
     The order of processing in the embodiments of the present disclosure may be changed as long as appropriate processing is performed. 
     Each of the storage units and storage devices (including registers and latches) in the embodiments of the present disclosure may be provided anywhere in a range in which appropriate information transmission and reception are performed. Further, each of the storage units and storage devices may exist in a plurality and store data in a distributed manner in the range in which appropriate information transmission and reception are performed. 
     Although the embodiments of the present disclosure have been described, the electric vehicle  20 , the control circuit  215 , the control unit  2151 , the host system  30 , and other control devices may have a computer system inside. Then, processes of the processing described above are stored in a computer-readable recording medium in the form of a program, and the processing is performed by reading and executing this program by a computer. A concrete example of the computer is shown below.  FIG.  20    is a schematic block diagram which shows a configuration of a computer according to at least one embodiment. A computer  5  includes, as shown in  FIG.  20   , a CPU  6 , a main memory  7 , a storage  8 , and an interface  9 . For example, each of the electric vehicle  20 , the control circuit  215 , the control unit  2151 , the host system  30  and other control devices described above are mounted in the computer  5 . Then, the operation of each processing unit described above is stored in the storage  8  in the form of a program. The CPU  6  reads the program from the storage  8 , develops it in the main memory  7 , and executes the above processing according to the program. In addition, the CPU  6  secures storage areas corresponding to the storage units described above in the main memory  7  according to the program. Examples of the storage  8  include a hard disk drive (HDD), a solid-state drive (SSD), magnetic disk, a magneto-optical disk, a compact disc read only memory (CD-ROM), a digital versatile disc read only memory (DVD-ROM), a semiconductor memory, and the like. The storage  8  may be an internal medium directly connected to a bus of the computer  5 , or an external medium connected to the computer  5  via an interface  9  or communication line. Moreover, when this program is delivered to the computer  5  via a communication line, the computer  5  that receives the delivery may develop the program in the main memory  7  and execute the processing described above. In at least one embodiment, the storage  8  is a non-transitory tangible storage medium. 
     In addition, the program described above may realize some of the functions described above. Furthermore, the program described above may be a file capable of realizing the functions described above in combination with a program already recorded in a computer system, that is, a so-called difference file (difference program). 
     Although several embodiments of the present disclosure have been described, these embodiments are examples and do not limit the scope of the disclosure. Various additions, omissions, replacements, and modifications may be made to these embodiments within a range not departing from the gist of the disclosure. 
     (Appendix) 
     The electric vehicle ( 20 ), the traffic system (I), the control method, and the program described in each embodiment of the present disclosure are ascertained, for example, as follows. 
     (1) A bidirectional DC-DC converter ( 216   a ) according to a first aspect includes a first circuit ( 216   a   1 ) that is configured to process a first voltage beings a DC voltage and that includes a first electronic component including a first switching element, a second circuit ( 216   a   2 ) that is configured to process a second voltage or a third voltage, the second voltage being a DC voltage supplied to the electric vehicle ( 20 ), the third voltage being a DC voltage generated in an electric vehicle ( 20 ), and that includes a second. electric component with a lower withstand voltage than the first electronic component, the second electric component including a second switching element, and a control circuit ( 215 ) configured to control switching of at least one of the first switching element and the second switching element, wherein the bidirectional DC-DC converter is configured to convert the first voltage into the second voltage or convert the third voltage into the first voltage. 
     As a result, even if an electric component including a high withstand voltage electronic component is replaced with an electric component including an electronic component with a lower withstand voltage than the electric component, the bidirectional DC-DC converter ( 216   a ) can realize the same function as when the high withstand voltage electric component is used. 
     (2) The bidirectional DC-DC converter ( 216   a ) according to a second aspect is a bidirectional DC-DC converter of (1), and may include a transformer with a primary side and a secondary side, the primary side being connected to the first circuit ( 216   a   1 ), and the secondary side being connected to the second circuit ( 216   a   2 ). 
     As a result, the bidirectional DC-DC converter ( 216   a ) can insulate the first circuit ( 216   a   1 )) and the second circuit ( 216   a   2 ). 
     (3) The bidirectional DC-DC converter ( 216   a ) according to a third aspect is a bidirectional DC-DC converter ( 216   a ) of (1), wherein the control circuit ( 215 ) may also perform chopper control for at least one of the first switching element and the second switching element. 
     As a result, the bidirectional DC-DC converter ( 216   a ) can separate a voltage on the overhead line  10  side and a voltage of the electric vehicle ( 20 ) without using a transformer. 
     (4) The bidirectional DC-DC converter ( 216   a ) according to a fourth aspect is the bidirectional DC-DC converter ( 216   a ) of any one of (1) to (3), wherein when the first voltage is different from the second voltage or when the first voltage is different from the third voltage, the control circuit ( 215 ) may shorten a cycle of switching at least one of the first switching element and the second switching element and provides a period during which the switching is not performed. 
     As a result, the bidirectional DC-DC converter ( 216   a ) can suppress hard switching. 
     (5) The bidirectional DC-DC converter ( 216   a ) according to a fifth aspect is the bidirectional DC-DC converter ( 216   a ) of any one of (1) to (4), wherein the control circuit ( 215 ) may control switching of the first switching element and the second switching element so as to shorten a first period than a second period, wherein one switching element between the first switching element and the second switching element is in an ON state in the first period, and the other switching element between the first switching element and the second switching element is in the ON state in the second period when the bidirectional DC-DC converter ( 216   a ) is started. 
     As a result, the bidirectional DC-DC converter ( 216   a ) can suppress overcurrent at the start and safely charge the capacitor to which electric power is transmitted. 
     ( 6 ) The traffic system ( 1 ) according to a sixth aspect includes the bidirectional DC-DC converter ( 216   a ) according to any one of first to fifth aspects, and a host system configured to transmit a torque command to the bidirectional DC-DC converter ( 216   a ). 
     As a result, even if an electric component including a high withstand voltage electronic component is replaced with an electric component including an electronic component with a lower withstand voltage than the electric component, it is possible to realize the same function as when the high withstand voltage electric component is used in the traffic system ( 1 ). 
     (7) A control method according to a seventh aspect is a control method to be executed by a bidirectional DC-DC converter that includes a first circuit ( 216   a   1 ) that is configured to process a first voltage being a DC voltage and that includes a first electronic component including a first switching element, and a second circuit ( 216   a   2 ) that is configured to process a second voltage or a third voltage, the second voltage being a DC voltage, supplied to an electric vehicle ( 20 ), the third voltage being a DC voltage generated in an electric vehicle ( 20 ), and that includes a second electric component with a lower withstand voltage than the first electronic component, the second electric component including a second switching element, the method including controlling switching of at least one of the first switching element and the second switching element, and converting the first voltage into the second voltage or converting the third voltage into the first voltage. 
     As a result, even if an electric component including a high withstand voltage electronic component is replaced with an electric component including an electric component with a lower withstand voltage than the electric component, it is possible to realize the same function as when the high withstand voltage electric component is used in the control method. 
     (8) A non-transitory computer-readable storage medium storing a program according to an eighth aspect causes a computer of a bidirectional DC-DC converter that includes a first circuit ( 216   a   1 ) that is configured to process a first voltage being a DC voltage and that includes a first electronic component including a first switching element, and a second circuit ( 216   a   2 ) that is configured to process a second voltage or a third voltage, the second voltage being a DC voltage supplied to an electric vehicle ( 20 ), the third voltage being a DC voltage generated in an electric vehicle ( 20 ), and that includes a second electric component with a lower withstand voltage than the first electronic component, the second electric component including a second switching element to execute controlling at least one of the first switching element and the second switching element, and converting the first voltage into the second voltage or converting the third voltage into the first voltage, 
     As a result, even if an electric component including a high withstand voltage electronic component is replaced with an electric component including an electric component with a lower withstand voltage than the electric component, it is possible to realize the same function as when the high withstand voltage electric component is used in the program.