Patent Publication Number: US-9893617-B2

Title: Electric power conversion system

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2015-086867 filed on Apr. 21, 2015 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     The disclosure relates to an electric power conversion system that is able to step up or step down voltage for two direct-current power supplies in parallel with each other. 
     2. Description of Related Art 
     In a hybrid vehicle or an electric vehicle, which uses a rotary electric machine as a driving source, the rotary electric machine is driven by alternating-current power that is converted by an inverter from the direct-current power of a battery. In addition, a step-up/step-down converter is provided between the battery and the inverter. The step-up/step-down converter steps up a battery voltage or steps down electric power regenerated by the rotary electric machine. 
     A voltage converter is, for example, described in Japanese Patent Application Publication No. 2014-193090 (JP 2014-193090 A) as the one that has the extended function of the step-up/step-down converter. The voltage converter includes four switching elements, and is connected to two batteries. The voltage converter is able to switch the two batteries between series connection and parallel connection. 
     The above-described voltage converter steps up or steps down voltage for the two batteries in parallel with each other at the time of the parallel connection (parallel mode). Step-up/step-down operation is controlled via a PWM signal indicating a duty ratio to each of step-up/step-down circuits. The duty ratio is the ratio of an on time to a single PWM control period. The voltage converter described in JP 2014-193090 A has such a circuit configuration that the switching elements are shared between two step-up/step-down circuits. The on/off operation of each switching element is controlled in accordance with the logical addition of both PWM signals based on so-called principle of superposition. For example, when a predetermined one of the switching elements is controlled based on the PWM signal PWM 1  for one of the step-up/step-down circuits and the PWM signal PWM 2  for the other one of the step-up/step-down circuits, the on/off operation of the predetermined one of the switching elements is controlled by a composite signal of the PWM 1  and the PWM 2 . 
     A power loss (hereinafter, also simply referred to as loss) arises with the on/off operation of each switching element. Specifically, examples of the loss include a switching loss (turn-on loss) that arises at the time when each switching element switches from an off state (interruption of current) to an on state (conduction of current) and a switching loss (turn-off loss) that arises at the time when each switching element switches from the on state to the off state, as shown in the upper timing chart of  FIG. 17 . Another example of the loss includes a steady loss that arises due to an on voltage (collector-to-emitter saturation voltage) at the time when each switching element is in the on state and a current flowing at this time. 
     The steady loss is classified into an overlap loss and an on-state loss. The overlap loss is caused when currents from the two step-up/step-down circuits are overlappingly supplied to each switching element. The on-state loss is caused when only current of one of the two step-up/step-down circuits is supplied to each switching element. Because of the magnitude relation in current, the overlap loss is larger than the on-state loss. 
     Each switching element is heated by the loss that arises in the switching element. In order to prevent overheating of each switching element, the phases of the PWM signals for the two step-up/step-down circuits are shifted from each other (phase shift control) in JP 2014-193090 A. 
     In phase shift control, as shown in the lower timing chart of  FIG. 17 , one or both of the phases of the PWM signals PWM 1 , PWM 2  are shifted such that the leading edge of an on duty (OnDuty 1 ) of the PWM signal PWM 1  is brought into coincidence (connection) with the trailing edge of an on duty (OnDuty 2 ) of the PWM signal PWM 2 . Thus, the number of times of switching is reduced as compared to the PWM signals shown in the upper timing chart of  FIG. 17 , with the result that the switching loss is reduced. In addition, the duration of the overlap loss is also shortened. 
     SUMMARY 
     Incidentally, a further reduction of the loss is desired in order to prevent overheating of switching elements. The disclosure provides an electric power conversion system that is able to reduce a power loss in each switching element to which current is supplied from two step-up circuits as compared to the existing power conversion system, particularly, at the time of parallel step-up operation. 
     The disclosure relates to an electric power conversion system. The system includes a first battery, a second battery, and a voltage converter. The voltage converter includes a plurality of switching elements. The voltage converter bidirectionally steps up or steps down voltage between each of the first and second batteries and an output line by turning on or off the plurality of switching elements in accordance with PWM signals. At the time of parallel step-up operation in which voltages of the first and second batteries are stepped up in parallel with each other, the voltage converter is configured to step up the voltage of the first battery by using a first step-up circuit and output the stepped-up voltage to the output line, and is configured to step up the voltage of the second battery by using a second step-up circuit and output the stepped-up voltage to the output line. The electric power conversion system further includes an electronic control unit configured to control the first and second step-up circuits by generating a first PWM signal for executing step-up control over the first step-up circuit and a second PWM signal for executing step-up control over the second step-up circuit. The plurality of switching elements include a common switching element that is supplied with current from both the first and second step-up circuits at the time of the parallel step-up operation. The electronic control unit is configured to, at the time of the parallel step-up operation and when a temperature of the common switching element exceeds a threshold temperature, execute on time change control for changing an on time of at least one of the first PWM signal and the second PWM signal such that a trailing edge of one of the first PWM signal and the second PWM signal and a leading edge of the other one of the first PWM signal and the second PWM signal connect with each other and the sum of the on time of the first PWM signal and the on time of the second PWM signal in a single PWM control period falls within a range from the single PWM control period to an allowable period obtained by adding a predetermined time to the single PWM control period. 
     In the above system, the electronic control unit may be configured to execute the on time change control such that the sum of the on time of the first PWM signal and the on time of the second PWM signal in the single PWM control period coincides with the single PWM control period. 
     In the above system, the electronic control unit may be configured to, when the sum of the on time of the first PWM signal and the on time of the second PWM signal in the single PWM control period before execution of the on time change control exceeds the single PWM control period, set the allowable period such that the allowable period is shorter than the sum of the on time of the first PWM signal and the on time of the second PWM signal. 
     In the above system, the electronic control unit may be configured to execute the on time change control when the sum of the on time of the first PWM signal and the on time of the second PWM signal in the single PWM control period before execution of the on time change control is shorter than the single PWM control period and when a power loss that arises in the common switching element based on the first PWM signal and the second PWM signal after execution of the on time change control is smaller than a power loss that arises in the common switching element based on the first PWM signal and the second PWM signal before execution of the on time change control. 
     In the above system, the electronic control unit may be configured to execute the on time change control when a power loss that arises in the common switching element based on the first PWM signal and the second PWM signal after execution of the on time change control is smaller than a power loss that arises in the common switching element at the time when phase shift control for shifting the trailing edge of at least one of the first PWM signal and the second PWM signal to the leading edge of the other one of the first PWM signal and the second PWM signal without extending or shortening the on time of the first PWM signal before execution of the on time change control or the on time of the second PWM signal before execution of the on time change control. 
     In the above system, the electric power conversion system may further include an inverter configured to convert direct-current power, output from the first and second step-up circuits, to alternating-current power, and the electronic control unit may be configured to change a conduction ratio in the inverter in response to a change between output voltages of the first step-up circuit and second step-up circuit before execution of the on time change control and output voltages of the first step-up circuit and second step-up circuit after execution of the on time change control. 
     In the above system, the electric power conversion system may further include a rotary electric machine configured to be supplied with alternating-current power converted by the inverter, and the electronic control unit may be configured to change the conduction ratio in the inverter in response to a change in efficiency of the rotary electric machine commensurate with a change between the output voltages of the first step-up circuit and second step-up circuit before execution of the on time change control and the output voltages of the first step-up circuit and second step-up circuit after execution of the on time change control. 
     According to the system, it is possible to reduce the power loss in the switching element that is supplied with current from two step-up circuits as compared to the existing system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a view that illustrates an electric power conversion system according to an embodiment; 
         FIG. 2  is a view that illustrates the operation of a voltage converter according to the embodiment at the time of parallel step-up operation when first and second step-up circuits are in a charge process; 
         FIG. 3  is a view that illustrates the operation of the voltage converter according to the embodiment at the time of parallel step-up operation when the first and second step-up circuits are in a discharge process; 
         FIG. 4  is a view that illustrates a first example of on time change control according to the embodiment; 
         FIG. 5  is a view that illustrates a second example of on time change control according to the embodiment; 
         FIG. 6  is a view that illustrates a third example of on time change control according to the embodiment; 
         FIG. 7  is a view that illustrates a fourth example of on time change control according to the embodiment; 
         FIG. 8  is a view that illustrates a fifth example of on time change control according to the embodiment; 
         FIG. 9  is a view that illustrates a sixth example of on time change control according to the embodiment; 
         FIG. 10  is the first half of a table that illustrates calculation of a loss in a common switching element; 
         FIG. 11  is the second half of the table that illustrates calculation of a loss in the common switching element; 
         FIG. 12  is a view that illustrates a flowchart of overheating protection control (on time change control and phase shift control) over the common switching element in the electric power conversion system according to the embodiment; 
         FIG. 13  is a graph that shows the relationship between a step-up voltage command value and each of the duty ratio of a PWM signal PWM 1  and the duty ratio of a PWM signal PWM 2  at the time of parallel step-up operation; 
         FIG. 14  is a view that illustrates a VH* map for obtaining a step-up voltage command value VH* intended for the voltage converter; 
         FIG. 15  is a view that shows another example of the voltage converter according to the embodiment; 
         FIG. 16  is a view that shows further another example of the voltage converter according to the embodiment; and 
         FIG. 17  is a view that illustrates phase shift control at the time when the sum of the on time of PWM 1  and the on time of PWM 2  exceeds a single PWM control period. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the disclosure will be described with reference to the accompanying drawings.  FIG. 1  illustrates a configuration view of an electrical system of a vehicle, including an electric power conversion system  10  according to the present embodiment. The alternate long and short dashes lines in  FIG. 1  represent signal lines. In  FIG. 1 , for the sake of easy understanding, part of components not associated with electric power conversion are not shown in the drawing. 
     The electric power conversion system  10  includes a first battery B 1 , a second battery B 2 , a voltage converter  11 , an inverter  18  and a controller  22 . The electric power conversion system  10  is mounted on a vehicle, such as a hybrid vehicle and an electric vehicle. A rotary electric machine  20  that serves as a drive source is mounted on the vehicle. The controller  22  may be a computer called electronic control unit (ECU). The controller  22  includes, for example, a CPU that is an arithmetic circuit, a storage unit, such as a memory, and a device and sensor interface, which are connected with each other via an internal bus. Hereinafter, the controller  22  is referred to as ECU  22 . 
     As shown in  FIG. 1 , each of the first battery B 1  and the second battery B 2  is separately connected to the voltage converter  11 . The voltage converter  11  steps up direct-current voltages VB 1 , VB 2  from the first battery B 1  and the second battery B 2 , and outputs the stepped-up direct-current voltages VB 1 , VB 2  to the inverter  18 . 
     The inverter  18  is a three-phase inverter. The output side of the inverter  18  is connected to the rotary electric machine  20 . The inverter  18  converts direct-current power, stepped up by the voltage converter  11 , to three-phase alternating-current power, and outputs the three-phase alternating-current power to the rotary electric machine  20 . Thus, the rotary electric machine  20  is driven to rotate. The driving force of the rotary electric machine  20  is transmitted to drive wheels (not shown). 
     During braking of the vehicle, regenerative braking is carried out by the rotary electric machine  20 . Regenerated electric power obtained at this time is converted by the inverter  18  from alternating-current power to direct-current power, the direct-current power is stepped down by the voltage converter  11 , and the stepped-down direct-current power is supplied to the first battery B 1  and the second battery B 2 . 
     The ECU  22  includes a CNV ECU  13  that controls the on/off states of switching elements S 1  to S 4  of the voltage converter  11 . By controlling the on/off states of the switching elements S 1  to S 4 , step-up/step-down (voltage conversion) operation and series-parallel switching operation over the voltage converter  11  are controlled. 
     The ECU  22  further includes an INV ECU  15  that controls the on/off states of switching elements (not shown) of the inverter  18 . By controlling the on/off states of the switching elements of the inverter  18 , DC-AC conversion or AC-DC conversion of the inverter  18  is controlled. 
     In this way, the ECU  22  controls the driving of the rotary electric machine  20  by controlling the voltage converter  11  and the inverter  18  via the CNV ECU  13  and the INV ECU  15 . 
     The ECU  22  is able to execute overheating protection control over a specific switching element at the time of parallel step-up operation. The parallel step-up operation means voltage conversion for stepping up the voltage of the first battery B 1  and the voltage of the second battery B 2  in parallel with each other. The specific switching element means a switching element that is supplied with current from both a first step-up circuit and a second step-up circuit at the time of parallel step-up operation. The first step-up circuit steps up the voltage of the first battery B 1 . The second step-up circuit steps up the voltage of the second battery B 2 . In the present embodiment, such a switching element is referred to as common switching element. As will be described later, in the example shown in  FIG. 1 , the switching element S 3  is the common switching element. 
     When the temperature of the common switching element S 3  exceeds a predetermined threshold temperature, the CNV ECU  13  changes the on time of at least one of a first PWM signal PWM 1  and a second PWM signal PWM 2  as overheating protection control. The first PWM signal PWM 1  is used to control the step-up operation of the first step-up circuit. The second PWM signal PWM 2  is used to control the step-up operation of the second step-up circuit. Specifically, the CNV ECU  13  changes the on time of at least one of the PWM 1  and the PWM 2  such that the trailing edge of one of the PWM 1  and the PWM 2  connects with the leading edge of the other one of the PWM 1  and the PWM 2  and the sum of the on time of the PWM 1  and the on time of the PWM 2  in a single PWM control period falls within a range from the single PWM control period to an allowable period obtained by adding a predetermined time to the single PWM control period. 
     By executing the above-described overheating protection control, the common switching element S 3  is kept in the on state over the single PWM control period. Thus, a switching loss in the common switching element S 3  is avoided. In addition, the on time of the PWM 1  and the on time of the PWM 2  alternately appear without any overlap between the on time of the PWM 1  and the on time of the PWM 2  or the on time of the PWM 1  and the on time of the PWM 2  appear in a state where an overlap period is shorter than that before on time change control if there is an overlap between the on time of the PWM 1  and the on time of the PWM 2 . Thus, an overlap loss in the common switching element S 3  is avoided or at least reduced. By such loss reduction, it is possible to prevent overheating of the common switching element S 3 . 
     Each of the first battery B 1  and the second battery B 2  is a direct-current power supply formed of a secondary battery, and is formed of, for example, a lithium-ion storage battery or a nickel-metal hydride storage battery. At least one of the first battery B 1  and the second battery B 2  may be an electrical storage element, such as an electric double layer capacitor, instead of a secondary battery. 
     The voltage converter  11  includes the switching elements S 1  to S 4 . Step-up/step-down (voltage conversion) operation is bidirectionally performed between each of the first battery B 1  and the second battery B 2  and an output line (high-voltage line  26 ) by controlling the on/off states of the switching elements S 1  to S 4  in response to the PWM signals that are generated by the CNV ECU  13 . In addition, the voltage converter  11  switches connection of the first battery B 1  and the second battery B 2  with the high-voltage line  26  between series connection and parallel connection. 
     Each of the switching elements S 1  to S 4  of the voltage converter  11  is, for example, a transistor element, such as an IGBT. The switching elements S 1  to S 4  are connected in series with each other such that the direction from the high-voltage line  26  toward a reference line  28  is a forward direction. The high-voltage line  26  is the output line of the voltage converter  11 . In addition, diodes Dd 1  to Dd 4  are respectively connected in antiparallel with the switching elements S 1  to S 4 . 
     The voltage converter  11  includes a first reactor L 1  and a first capacitor C 1 . The first reactor L 1  is connected in series with the first battery B 1 . The first capacitor C 1  is connected in parallel with the first battery B 1 . The voltage converter  11  includes a second reactor L 2  and a second capacitor C 2 . The second reactor L 2  is connected in series with the second battery B 2 . The second capacitor C 2  is connected in parallel with the second battery B 2 . 
     The first battery B 1  is connected between a connection point (node)  40  and the reference line  28 . The connection point (node)  40  is provided between the second switching element S 2  and the third switching element S 3  from the high-voltage line  26  side. In addition, the second battery B 2  is connected between a connection point  42  and a connection point  44 . The connection point  42  is provided between the first switching element S 1  and the second switching element S 2  from the high-voltage line  26  side. The connection point  44  is provided between the third switching element S 3  and the fourth switching element S 4  from the high-voltage line  26  side. 
     The inverter  18  converts direct-current power, stepped up by the voltage converter  11 , to three-phase alternating-current power by turning on or off the switching elements (not shown), and supplies the three-phase alternating-current power to the rotary electric machine  20 . The inverter  18  also converts regenerated electric power (three-phase alternating-current power), regenerated by the rotary electric machine  20 , to direct-current power by turning on or off the switching elements (not shown), and supplies the direct-current power to the first battery B 1  and the second battery B 2  via the voltage converter  11 . 
     The controller  22 , as will be described later, executes various operation controls over the vehicle, including voltage conversion and switching of power supply connection over the voltage converter  11 . 
     The storage unit of the ECU  22  stores a control program for the switching elements S 1  to S 4 , a VH* map (described later), a phase shift program (described later), an on time change program (described later), and the like. 
     The ECU  22  receives signals from various sensors via the device and sensor interface. Specifically, the ECU  22  receives detected values from battery voltage sensors  46 A,  46 B and battery current sensors  48 A,  48 B as the signals associated with the first battery B 1  and the second battery B 2 . The battery voltage sensors  46 A,  46 B respectively measure battery voltage values VB 1 ,VB 2 . The battery current sensors  48 A,  48 B respectively measure battery current values IL 1 , IL 2 . The ECU  22  receives a detected value from an output voltage sensor  50  as the signal associated with the output voltage of the voltage converter  11 . The output voltage sensor  50  is connected in parallel with a smoothing capacitor CH, and measures a potential difference VH (output voltage) between the high-voltage line  26  and the reference line  28 . 
     The ECU  22  receives an actual rotation angle of the rotary electric machine  20  and detected signals of three-phase alternating currents from a rotation speed sensor  52  and current sensors  54 A  54 B as the signals associated with the rotary electric machine  20 . The ECU  22  receives pedal depression amounts from an accelerator pedal depression amount sensor and a brake pedal depression amount sensor (not shown) as other vehicle information. 
     In addition, the ECU  22  receives a temperature signal from a temperature sensor  17 . The temperature sensor  17  detects the temperature of the switching element S 3  that is the common switching element. 
     The ECU  22  includes the CNV ECU  13  and the INV ECU  15 . The ECU  22 , the CNV ECU  13  and the INV ECU  15  may be incorporated in a single computer. Part of resources, such as the CPU and the memory, are allocated to the CNV ECU  13  and the INV ECU  15 , so the CNV ECU  13  and the INV ECU  15  each are able to operate independently of the ECU  22 . The ECU  22 , the CNV ECU  13  and the INV ECU  15  may be respectively formed of separate computers. 
     The ECU  22  transmits control commands to the CNV ECU  13  and the INV ECU  15 . For example, the ECU  22  transmits a step-up voltage command value VH* to the CNV ECU  13  based on the VH* map (described later). The ECU  22  transmits a command frequency of alternating-current power to the INV ECU  15  based on an actual rotation speed of the rotary electric machine  20 , a torque command value, and the like. The CNV ECU  13  and the INV ECU  15  may communicate with each other. As will be described later, a step-up voltage command value VH*′ changed as a result of on time change control over the PWM signals may be transmitted from the CNV ECU  13  to the INV ECU  15 . 
     The INV ECU  15  controls the inverter  18  by executing a control program for the switching elements (not shown), stored in the storage unit of the computer, current compensation control (described later), and loss compensation control (described later). 
     The CNV ECU  13  controls the voltage converter  11  by executing the control program for the switching elements S 1  to S 4 , an overheating protection control program (described later), and the like, stored in the storage unit of the computer. As will be described later, at the time of parallel step-up operation, the CNV ECU  13  generates the PWM signals PWM 1 , PWM 2  for step-up control, and outputs the PWM signals PWM 1 , PWM 2  respectively to the first step-up circuit and the second step-up circuit. 
     The detailed operation of the voltage converter  11  is already known from the above-described JP 2014-193090 A, or the like, so only a parallel step-up mode associated with overheating protection control over the common switching element (described later) will be simply described. 
     The step-up operation mainly includes two processes, that is, a charge process and a discharge process. In the charge process, electric charge of the battery is accumulated in the reactor. In the discharge process, electric charge accumulated in the reactor and electric charge of the battery are superimposed and discharged to a load. 
     In the parallel step-up mode, the above-described step-up operations are performed in parallel with each other. That is, as shown in  FIG. 2  and  FIG. 3 , a first step-up circuit BCNV 1  and a second step-up circuit BCNV 2  are provided in the electric power conversion system  10 . In the first step-up circuit BCNV 1 , the voltage of the first battery B 1  is stepped up by the voltage converter  11 , and the stepped-up voltage is output to the high-voltage line  26  (output line). In the second step-up circuit BCNV 2 , the voltage of the second battery B 2  is stepped up by the voltage converter  11 , and the stepped-up voltage is output to the high-voltage line  26  (output line). In addition, a charge and a discharge are performed in each of these circuits. 
       FIG. 2  illustrates the charge process at the time of parallel step-up operation. In the first step-up circuit BCNV 1 , the switching elements S 3 , S 4  turn on, with the result that a loop path from the first battery B 1  via the reactor L 1 , the switching element S 3  and the switching element S 4  back to the first battery B 1  is established, as indicated by the current IL 1 . 
     In the second step-up circuit BCNV 2 , the switching elements S 2 , S 3  turn on, with the result that a loop path from the second battery B 2  via the reactor L 2 , the switching element S 2  and the switching element S 3  back to the second battery B 2  is established, as indicated by the current IL 2 . 
     As shown in the drawing, the switching element S 3  is the common switching element that is supplied with current (IL 1  and IL 2 ) from both the first step-up circuit BCNV 1  and the second step-up circuit BCNV 2 . 
       FIG. 3  shows the operation of the discharge process at the time of parallel step-up operation in the first step-up circuit BCNV 1  and the second step-up circuit BCNV 2 . In the first step-up circuit BCNV 1 , the switching elements S 3 , S 4  turn off, with the result that the current IL 1  flows through a path from the first battery B 1  via the reactor L 1 , the diode Dd 2 , the diode Dd 1  and the load (rotary electric machine  20 ) back to the first battery B 1 . 
     In the second step-up circuit BCNV 2 , the switching elements S 2 , S 3  turn off, with the result that the current IL 2  flows through a path from the second battery B 2  via the reactor L 2 , the diode Dd 1 , the load (rotary electric machine  20 ) and the diode Dd 4  back to the second battery B 2 . 
     The CNV ECU  13  generates and outputs the PWM signals in order to cause the switching elements to perform the operations shown in  FIG. 2  and  FIG. 3  at the time of parallel step-up operation. Specifically, the CNV ECU  13  generates the PWM signal PWM 1  for causing the switching elements S 3 , S 4  to perform a charge (S 3  is on, S 4  is on) and discharge (S 3  is off, S 4  is off) of the first step-up circuit BCNV 1 . The CNV ECU  13  generates the PWM signal PWM 2  for causing the switching elements S 2 , S 3  to perform a charge (S 2  is on, S 3  is on) and discharge (S 2  is off, S 3  is off) of the second step-up circuit BCNV 2 . 
     The PWM signals may be generated and output to the switching element S 1  such that the switching element S 1  is in the off state (fixed to the off state) over the charge process and the discharge process or the inversion signal (/PWM 1  or /PWM 2 ) of the PWM 1  or PWM 2  may be generated and output to the switching element S 1 , in order to prevent the switching elements S 1  to S 4  enter the on state at the same time. 
     Overheating protection control over the common switching element S 3  will be described based on the first and second step-up circuits BCNV 1 , BCNV 2  shown in  FIG. 2  and  FIG. 3 . As described above, the common switching element S 3  is supplied with current from the first step-up circuit BCNV 1  and the second step-up circuit BCNV 2 . At this time, as shown in the upper timing chart of  FIG. 17 , when current of the first step-up circuit BCNV 1  and current of the second step-up circuit BCNV 2  are superimposed and supplied to the common switching element S 3 , an overlap loss may arise. As shown in the upper timing chart of  FIG. 4 , when the on/off timing of the PWM signal PWM 1  for the first step-up circuit BCNV 1  and the on/off timing of the PWM signal PWM 2  for the second step-up circuit BCNV 2  deviate from each other, a switching loss may arise. 
     The common switching element S 3  is heated by these losses (power losses). If the loss is excessive, there is a concern about overheating of the common switching element S 3 . The CNV ECU  13  executes the following overheating protection control when the temperature of the common switching element S 3  acquired from the temperature sensor  17  exceeds a predetermined threshold temperature. 
     In this embodiment, when the sum of the on time of the PWM 1  and the on time of the PWM 2  per single PWM control period differs from the single PWM control period, the sum of the on time of the PWM 1  and the on time of the PWM 2  per single PWM control period is brought into coincidence with the single PWM control period by changing the on time of at least one of the PWM 1  and the PWM 2 . In addition, the trailing edge of one of the PWM 1  and the PWM 2  is connected to the leading edge of the other one of the PWM 1  and the PWM 2 . 
     The leading edge of each of the PWM 1  and the PWM 2  means the timing of switching from the off time to the on time. The trailing edge of each of the PWM 1  and the PWM 2  means the timing of switching from the on time to the off time. 
     The ratio of the on time to the single PWM control period is referred to as duty ratio or simply referred to as duty. On the assumption that the single PWM control period is fixed, when the above-described on time change control translates, by using the duty ratio, to, when the sum of the duty ratio of the PWM 1  and the duty ratio of the PWM 2  is different from 100%, bring the sum of the duty ratio of the PWM 1  and the duty ratio of the PWM 2  100% by changing the on time (on duty) of at least one of the PWM 1  and the PWM 2 . In addition, the trailing edge of one of the PWM 1  and the PWM 2  is connected to the leading edge of the other one of the PWM 1  and the PWM 2 . 
       FIG. 4  shows an example of on time change control in the case where the sum of the on time of the PWM 1  and the on time of the PWM 2  in the single PWM control period is shorter than the single PWM control period, in other words, in the case where the sum of the duty ratio of the PWM 1  and the duty ratio of the PWM 2  is smaller than 100% (D 1 +D 2 &lt;100%). The upper timing chart of  FIG. 4  shows the PWM 1  and the PWM 2  before on time change control. 
     The lower timing chart of  FIG. 4  shows the waveforms at the time when on time change control is executed over the upper waveforms. In this example, the trailing edge of the PWM 2  is delayed to the leading edge of the PWM 1  (OnDuty 2 →OnDuty 2 ′), and the trailing edge of the PWM 1  is delayed to the leading edge of the PWM 2  (OnDuty 1 →OnDuty 1 ′). In this way, the trailing edge of one of the PWM 1  and the PWM 2  is connected to the leading edge of the other one of the PWM 1  and the PWM 2 , and the sum of the on time of the PWM 1  and the on time of the PWM 2  in the single PWM control period coincides with the single PWM control period. In other words, the sum of the duty ratio D 1 ′ of the PWM 1  and the duty ratio D 2 ′ of the PWM 2  after on time change control becomes 100%. 
     After on time change control, the common switching element S 3  is constantly in the on state over the single PWM control period, so the switching loss becomes zero. In addition to this, current of the first step-up circuit BCNV 1  and current of the second step-up circuit BCNV 2  are alternately supplied to the common switching element S 3  without any overlap. As a result, as is apparent from comparison between the upper timing chart and lower timing chart of  FIG. 4 , the switching loss becomes zero. 
       FIG. 5  shows an example of on time change control in the case where the sum of the on time of the PWM 1  and the on time of the PWM 2  in the single PWM control period exceeds the single PWM control period, in other words, in the case where the sum of the duty ratio of the PWM 1  and the duty ratio of the PWM 2  exceeds 100% (D 1 +D 2 &gt;100%). The upper timing chart of  FIG. 5  shows the PWM 1  and the PWM 2  before on time change control. 
     The lower timing chart of  FIG. 5  shows the waveforms at the time when on time change control is executed over the upper waveforms. In this example, the trailing edge of the PWM 2  is advanced (moved forward) to the leading edge of the PWM 1  (OnDuty 2 →OnDuty 2 ′), and the trailing edge of the PWM 1  is delayed to the leading edge of the PWM 2  (OnDuty 1 →OnDuty 1 ′). In this way, the trailing edge of one of the PWM 1  and the PWM 2  is connected to the leading edge of the other one of the PWM 1  and the PWM 2 , and the sum of the on time of the PWM 1  and the on time of the PWM 2  in the single PWM control period coincides with the single PWM control period. By executing on time change control, the overlap loss and the switching loss become zero as shown in the lower timing chart of  FIG. 5 . 
     In the on time change control described above, the trailing edge of each of the PWM 1  and the PWM 2  is aligned with the leading edge of the other one of the PWM 1  and the PWM 2 ; however, the disclosure is not limited to this mode. For example, as shown in  FIG. 6 , the on time change control is executed over the PWM 1  and the PWM 2  such that the leading edge of each of the PWM 1  and the PWM 2  is aligned with the trailing edge of the other one of the PWM 1  and the PWM 2 . With this example as well, as shown in the lower timing chart of  FIG. 6 , the trailing edge of one of the PWM 1  and the PWM 2  is connected to the leading edge of the other one of the PWM 1  and the PWM 2 , and the sum of the on time of the PWM 1  and the on time of the PWM 2  in the single PWM control period coincides with the single PWM control period. In this timing chart, both the on time of the PWM 1  and the on time of the PWM 2  are changed; however, depending on waveforms, only the on time of one of the PWM 1  and the PWM 2  may be changed. 
     In  FIG. 4  to  FIG. 6 , the process of shortening or extending the on time and the process of connecting the trailing edge and the leading edge to each other are performed at the same time; however, the system is not limited to this mode. For example, each of the processes may be performed step by step. 
       FIG. 7  shows an example in which the on time is shortened or extended and then phase shift control is executed. In order to distinguish the on time change control in the present application from the phase shift control in the existing technique, hereinafter, the phase shift control is defined as control for shifting (connecting) the leading edge of at least one of the PWM 1  and the PWM 2  to the trailing edge of the other one of the PWM 1  and the PWM 2  without shortening or extending the on time. 
     Initially, as shown in the middle timing chart of  FIG. 7 , the sum of the on time of the PWM 1  and the on time of the PWM 2  in the single PWM control period is brought into coincidence with the single PWM control period (OnDuty 1 →OnDuty 1 ′, OnDuty 2 →OnDuty 2 ′). At this time, the rate of change (the rate of reduction in  FIG. 7 ) in the PWM 1  and the rate of change (the rate of reduction in  FIG. 7 ) in the PWM 2  may be equal to each other. 
     Subsequently, as shown in the lower timing chart of  FIG. 7 , phase shift control is executed in order to connect the trailing edge of one of the PWM 1  and the PWM 2  to the leading edge of the other one of the PWM 1  and the PWM 2 . Through the above-described two processes, the trailing edge of one of the PWM 1  and the PWM 2  is connected to the leading edge of the other one of the PWM 1  and the PWM 2 , and the sum of the on time of the PWM 1  and the on time of the PWM 2  in the single PWM control period coincides with the single PWM control period. 
     In the example of  FIG. 7 , as compared to  FIG. 4  to  FIG. 6 , the rate of change in the PWM 1  and the rate of change in the PWM 2  are made equal to each other, so the example of  FIG. 7  has the advantage of not significantly changing the balance between electric power supplied from the first battery B 1  and electric power supplied from the second battery B 2  before and after on time change control. 
     In the examples shown in  FIG. 4  to  FIG. 7 , on time change control is executed such that the sum of the on time of the PWM 1  and the on time of the PWM 2  coincides with the single PWM control period; however, the system is not limited to this mode. In short, the power loss of the common switching element S 3  after on time change control just needs to be smaller than the power loss before on time change control, so, for example, the sum of the on time of the PWM 1  and the on time of the PWM 2  may slightly exceed the single PWM control period. In other words, the on time of at least one of the PWM 1  and the PWM 2  is changed such that the sum of the on time of the PWM 1  and the on time of the PWM 2  falls within the range from the single PWM control period to the allowable period obtained by adding the predetermined time to the single PWM control period. 
     The on time change control will be described by using the duty ratio. The duty ratios D 1 ′, D 2 ′ are set such that the sum of the duty ratio D 1 ′ of the PWM 1  and the duty ratio D 2 ′ of the PWM 2  after on time change control is larger than or equal to 100%, that is, 100%+α((100+α)%≧D 1 ′+D 2 ′≧100%). α is any positive number. 
       FIG. 8  shows an example in which on time change control is executed over such waveforms that the sum of the on time of the PWM 1  and the on time of the PWM 2  is shorter than the single PWM control period (D 1 +D 2 &lt;100%) and is executed such that the sum of the on time of the PWM 1  and the on time of the PWM 2  exceeds the single PWM control period. In this example, the on time of the PWM 1  and the on time of the PWM 2  are changed such that the trailing edge of the PWM 1  is delayed and connected to the leading edge of the PWM 2  and the trailing edge of the PWM 2  is delayed to timing slightly (temporally) after the leading edge of the PWM 1 . 
     When the sum of the on time of the PWM 1  and the on time of the PWM 2  is longer than or equal to the single PWM control period, the switching loss becomes zero in theory. For example, when the switching loss occupies the majority of the power loss that arises in the common switching element S 3 , it is possible to effectively reduce the power loss by executing the above-described on time change control. 
     In this example, as a result of the on time change control, the on-state loss increases, and a new overlap loss arises. Therefore, a power loss before on time change control and a power loss after on time change control may be predicted, and then the waveforms that cause a smaller one of the power losses may be used. That is, when the power loss that arises in the common switching element S 3  based on the PWM 1  and the PWM 2  after on time change control is smaller than the power loss that arises in the common switching element S 3  based on the PWM 1  and the PWM 2  before on time change control, the on time change control may be allowed to be executed. 
       FIG. 9  shows an example in which, for such waveforms that the sum of the on time of the PWM 1  and the on time of the PWM 2  exceeds the single PWM control period (D 1 +D 2 &gt;100%), the on time of the PWM 1  and the on time of the PWM 2  are changed such that the sum of the on time of the PWM 1  and the on time of the PWM 2  falls within the range from the single PWM control period to the allowable period. In this example, the on time of the PWM 1  and the on time of the PWM 2  are changed such that the trailing edge of the PWM 1  is delayed and connected to the leading edge of the PWM 2  and the trailing edge of the PWM 2  is advanced to timing slightly (temporally) after the leading edge of the PWM 1 . 
     Through the on time change control, the switching loss becomes zero in theory, and the overlap loss is also reduced. In executing such on time change control, the allowable period should be set so as to be longer than or equal to the single PWM control period and shorter than the sum of the on time of the PWM 1  and the on time of the PWM 2  before on time change control. In other words, on time change control should be executed such that the difference between the sum of the on time of the PWM 1  and the on time of the PWM 2  and the single PWM control period is shorter than the difference before on time change control. 
     In any of the above-described embodiments of on time change control, on time change control over the PWM 1  and the PWM 2  is not limited to the modes shown in the timing charts. For example, depending on waveforms, any mode, such as a delay or advance of the on time of only the PWM 1 , a delay or advance of the on time of only the PWM 2 , an advance of the on time of each of the PWM 1  and the PWM 2 , a delay of the on time of each of the PWM 1  and the PWM 2 , and a delay of the on time of one of the PWM 1  and the PWM 2  and an advance of the on time of the other one of the PWM 1  and the PWM 2 , may be applied. 
     As described above, depending on the waveforms of the PWM 1  and PWM 2 , the period of the on-state loss extends for a reduction in the switching loss to zero as a result of on time change control. In some cases, a new overlap loss arises. Therefore, when the amount of increase in the on-state loss or the overlap loss is larger than the amount of reduction in the switching loss, there is a concern that on time change control contrarily increases the loss in the common switching element S 3 . 
     Therefore, the CNV ECU  13  may calculate losses in the common switching element S 3  before and after on time change control in advance and then control the on/off state of the common switching element S 3  based on the PWM signals that cause a smaller loss. 
       FIG. 10  and  FIG. 11  show an example of estimating a loss in the common switching element S 3 . In this example, on the assumption of the example of  FIG. 7 , that is, the example in which the process of shortening or extending the on time and the phase shift process are executed step by step and the embodiment in which the sum of the on time of the PWM 1  and the on time of the PWM 2  after change is brought into coincidence with the single PWM control period, the loss in the common switching element S 3  is estimated. 
     In this estimation, the voltage VB 1  of the first battery B 1  is set to 300 [V], and the voltage VB 2  of the second battery B 2  is set to 200 [V]. In addition, the switching frequency (carrier frequency) fsw is set to 10 [kHz], and the on voltage Vice (collector-to-emitter saturation voltage) of the common switching element S 3  is set to 2 [V]. Furthermore, the turn-on time Ton of the common switching element S 3  is set to 110 [ns], and the turn-off time Toff of the common switching element S 3  is set to 170 [ns]. 
     Initially, the duty ratio D 1  [rate] of the PWM 1  is shown at the left end of the table of  FIG. 10 . In this table, the minimum value of D 1  is set to 5%, and the duty ratio (the ratio of on duty) is increased in units of 5% below. 
     The duty ratio D 2  of the PWM 2  and the stepped-up voltage VH are calculated by using the duty ratio D 1  and the battery voltages VB 1 , VB 2 . The operation mode of the voltage converter  11  in the present embodiment assumes the parallel step-up mode, so the stepped-up voltage of the first step-up circuit BCNV 1  and the stepped-up voltage of the second step-up circuit BCNV 2  are equal to each other in theory. In consideration of this precondition, simultaneous equations of the following mathematical expressions (1) and (2) are obtained. 
     
       
         
           
               
             
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     By solving the mathematical expressions (1) and (2), as shown in the table of  FIG. 10 , the duty ratio D 2  and the stepped-up voltage VH for any duty ratio D 1  are obtained. In the above-described mathematical expressions (1) and (2) and the mathematical expressions (3) to (7), each of the duty ratios D 1 , D 2  indicates not a percentage but an absolute value (for example, 100%→1.00). 
     The sum D 1 +D 2  [rate] of the duty ratios D 1 , D 2  is shown on the right side of the columns of the duty ratios D 1 , D 2 . A cell in which the value of D 1 +D 2  is 100% is shown near the middle of the table. 
     In addition, the current IL 1  [A] that is supplied from the first step-up circuit BCNV 1  to the common switching element S 3  is obtained based on the voltage VB 1  of the first battery B 1  and the output electric power Wout [W]. Similarly, the current IL 2  [A] that is supplied from the second step-up circuit BCNV 2  to the common switching element S 3  is obtained based on the voltage VB 2  of the second battery B 2  and the output electric power Wout [W]. In the example of  FIG. 10 , the output electric power Wout is set to 50 [kW]. 
     By using the thus obtained D 1 , D 2 , VH, IL 1  and IL 2 , the power losses [mJ] in the common switching element S 3  as shown at the right side of the sheet in  FIG. 10  are obtained. The power losses, that is, a turn-on loss Eon [mJ], a turn-off loss Eoff [mJ] and a steady loss (which indicates an on-state loss) Esat [mJ], are obtained for each of the PWM 1  and the PWM 2 . That is, six losses in total, that is, Eon 1  and Eon 2  as the turn-on loss, Eoff 1  and Eoff 2  as the turn-off loss and Esat 1  and Esat 2  as the steady loss, are obtained. 
     The turn-on losses, the turn-off losses and the steady losses are respectively allowed to be obtained from the following mathematical expressions (3), (4), and (5). 
                     Eon   k     =       1   2     ×   VH   ×     IL   k     ×   Ton             (   3   )                 Eoff   k     =       1   2     ×   VH   ×     IL   k     ×   Toff             (   4   )                 Esat   k     =     Vce   ×     IL   K     ×       D   k     fsw               (   5   )               
where k=1 or 2
 
     As for the loss obtained as described above, the total Etotal [mJ] of the six losses obtained in  FIG. 10  is shown in the fourth column from the left side of the sheet of  FIG. 11  via the arrow. In addition, in the columns to the right side of the above column, the total Etotal′ of the losses at the time when phase shift control is executed and the difference delta [mJ] between Etotal′ and Etotal are shown. 
     In the example shown in  FIG. 11 , in calculating the loss Etotal′ at the time of execution of phase shift control, a calculation method is changed between when D 1 +D 2 &lt;100% and when D 1 +D 2 ≧100%. 
     That is, when D 1 +D 2 &lt;100%, on the assumption of control for connecting the trailing edge of the PWM 2  to the leading edge of the PWM 1 , Etotal′ is obtained by subtracting the turn-on loss Eon 1  of the PWM 1 (D 1 ) and the turn-off loss Eoff 2  of the PWM 2 (D 2 ) from the total loss Etotal before phase shift control. 
     When D 1 +D 2 ≧100%, the entire switching loss disappears as a result of phase shift control, so Etotal′ is obtained by subtracting Eon 1 , Eon 2 , Eoff 1  and Eoff 2  from the total loss Etotal before the phase shift control. 
     As is apparent when the total loss Etotal before phase shift control and the total loss Etotal′ after phase shift control are compared with each other, that is, when delta next on the right side of the Etotal′ on the sheet of  FIG. 11  is referenced, the loss is reduced through phase shift control in any of the duty ratios D 1 , D 2  (in any of the rows). 
     The columns further next on the right side of  FIG. 11  show the total loss Etotal″ [mJ] in the common switching element S 3  and the difference delta [mJ] between the total loss Etotal″ and the total loss Etotal′ at the time of phase shift control in the case where on time change control is executed after phase shift control. 
     In this column, the total loss Etotal′ (=43.3 [mJ]) after phase shift control at the time when the sum of the duty ratios D 1  and D 2  is 100% is applied to all the cells in the column as the total loss Etotal″ after on time change control. 
     As is apparent when the difference delta next on the right side of the total loss Etotal″ after on time change control is referenced, the loss is reduced for all the cells in the column in the region in which the sum of the duty ratios D 1  and D 2  exceeds 100%. On the other hand, in the region in which the sum of the duty ratios D 1  and D 2  is smaller than 100%, there are a case where the total loss increases and a case where the total loss is reduced. 
     In consideration of the above-described calculation results, the CNV ECU  13  may determine whether on time change control is allowed to be executed based on comparison between the loss in the common switching element S 3  before on time change control and the loss in the common switching element S 3  after on time change control. 
       FIG. 12  illustrates a flowchart of overheating protection control over the common switching element S 3 , which is executed by the CNV ECU  13 , in consideration of the above-described calculation results. This control flowchart is based on the embodiment shown in  FIG. 7 . That is,  FIG. 12  shows the control flowchart in the case where the process of shortening or extending the on time and the phase shift process are executed step by step and on time change control that brings the sum of the on time of the PWM 1  and the on time of the PWM 2  into coincidence with the single PWM control period is executed. 
     The CNV ECU  13  initially determines whether the voltage converter  11  is executing the parallel step-up operation (S 10 ). When the parallel step-up operation is not being executed, the process proceeds to Return at the end of the flowchart. 
     When the parallel step-up operation is being executed, the CNV ECU  13  acquires the temperature of the common switching element S 3  from the temperature sensor  17  (S 12 ), and determines whether the acquired temperature exceeds the predetermined threshold temperature (S 14 ). When the temperature of the common switching element S 3  does not exceed the predetermined threshold temperature, the process proceeds to Return at the end of the flowchart. 
     When the temperature of the common switching element S 3  exceeds the threshold temperature, the CNV ECU  13  acquires the duty ratio D 1  of the PWM 1  and the duty ratio D 2  of the PWM 2  (S 16 ), and determines whether the value of D 1 +D 2  is smaller than 100% (S 18 ). 
     When the value of D 1 +D 2  is not smaller than 100%, that is, when the value of D 1 +D 2  is larger than or equal to 100%, the CNV ECU  13  determines whether the value of D 1 +D 2  exceeds 100% (S 20 ). 
     When the value of D 1 +D 2  does not exceed 100% in step S 20 , the value of D 1 +D 2  is equal to 100% (D 1 +D 2 =100%). The CNV ECU  13  keeps the duty ratios at D 1  and D 2  (S 22 ), and executes phase shift control (S 24 ). 
     When it is determined in step S 20  that the value of D 1 +D 2  exceeds 100%, the CNV ECU  13  acquires the voltage VB 1  of the first battery B 1  and the voltage VB 2  of the second battery B 2  (S 26 ). In addition, the CNV ECU  13  calculates the duty ratios D 1 ′ and D 2 ′ after on time change control (D 1 ′+D 2 ′=100%). 
     In calculating the duty ratios D 1 ′ and D 2 ′, the CNV ECU  13  solves the simultaneous equations of the following mathematical expression (6), from which VH is removed from the above-described mathematical expressions (1) and (2), and the mathematical expression (7) for the duty ratios D 1 ′ and D 2 ′ (S 28 ). 
     
       
         
           
               
             
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     Furthermore, the CNV ECU  13  calculates a step-up voltage command value VH*′ after on time change control based on the obtained duty ratio D 1 ′ and the first battery voltage VB 1  or the obtained duty ratio D 2 ′ and the second battery voltage VB 2 . 
     Calculation of the duty ratios D 1 ′, D 2 ′ by using the mathematical expressions (6) and (7) and calculation of the step-up voltage command value VH*′ after on time change control are hereinafter referred to as first process. 
     The CNV ECU  13  updates (reduces) the duty ratio from D 1  to D 1 ′ and the duty ratio from D 2  to D 2 ′ (S 30 ), and updates the step-up voltage command value from VH* to VH*′ (S 32 ). In addition, the CNV ECU  13  executes phase shift control over the updated duty ratios D 1 ′, D 2 ′ (S 24 ). 
     Back to step S 18 , when the sum of the duty ratios D 1 , D 2  is smaller than 100%, the CNV ECU  13  compares the total loss in the common switching element S 3  before on time change control with the total loss in the common switching element S 3  after on time change control, and then uses the duty ratio, which causes a lower total loss, in controlling the on/off state of the common switching element S 3 . 
     The CNV ECU  13  acquires the voltage VB 1  of the first battery B 1  from the battery voltage sensor  46 A and the voltage VB 2  of the second battery B 2  from the battery voltage sensor  46 B. The CNV ECU  13  also acquires the current IL 1  of the first battery B 1  from the battery current sensor  48 A and the current IL 2  of the second battery B 2  from the battery current sensor  48 B. The potential difference VH (output voltage) between the high-voltage line  26  and the reference line  28  is acquired from the output voltage sensor  50  (S 34 ). 
     Subsequently, the CNV ECU  13  calculates the duty ratios D 1 ′, D 2 ′ after on time change control and the step-up voltage command value VH*′ by executing the above-described first process (S 36 ). The CNV ECU  13  obtains a switching loss Esw and a steady loss Esat that arise in the common switching element S 3  in the case where phase shift control is executed over the duty ratios D 1 , D 2  before on time change control (S 38 ). It is noted that Esw=Eon+Eoff 
     As for the switching loss Esw, in consideration of the amount of decrease in loss resulting from phase shift control, the turn-on loss Eon 1  of the PWM 1 (D 1 ) and the turn-off loss Eoff 2  of the PWM 2  do not need to be incorporated, as in the case of  FIG. 11  described above. That is, the sum of the turn-off loss Eoff 1  of the PWM 1 (D 1 ) and the turn-on loss Eon 2  of the PWM 2  may be regarded as the switching loss Esw. 
     The CNV ECU  13  obtains a steady loss Esat′ that arises in the common switching element S 3  after on time change control and phase shift control based on the duty ratios D 1 ′, D 2 ′ after on time change control (S 40 ). 
     Subsequently, the CNV ECU  13  obtains the total Esw+Esat of the switching loss and steady loss based on the switching loss Esw and steady loss Esat obtained in step S 38 . The total Esw+Esat of the switching loss and steady loss arise at the time of controlling the on/off state of the common switching element S 3  based on the PWM 1  and the PWM 2  before on time change control and after phase shift control. The CNV ECU  13  determines whether the loss Esw+Esat before on time change control and after phase shift control exceeds the steady loss Esat′ after on time change control and after phase shift control (S 42 ). 
     When the loss Esw+Esat before on time change control and after phase shift control does not exceed the loss Esat′ after on time change control and after phase shift control, the loss in the common switching element S 3  is low when no on time change control is executed, so the CNV ECU  13  keeps the duty ratios at D 1  and D 2  (S 22 ), and executes phase shift control (S 24 ). 
     When the loss Esw+Esat before on time change control and after phase shift control exceeds the loss Esat′ after on time change control and after phase shift control, the CNV ECU  13  updates (increases) the duty ratio from D 1  to D 1 ′ and the duty ratio from D 2  to D 2 ′ (S 44 ), and updates the step-up voltage command value from VH* to VH*′ (S 46 ). The CNV ECU  13  executes phase shift control over the updated duty ratios D 1 ′, D 2 ′ (S 24 ). The on/off state of the common switching element S 3  is controlled based on the logical addition of D 1 ′ and D 2 ′ after phase shift control. 
     As shown in step S 32  and step S 46  in  FIG. 12 , as the on times (duty ratios) are changed, the step-up voltage command value is changed (VH*→VH*′).  FIG. 13  shows a view that illustrates these changes. The ordinate axis represents the duty ratio of the PWM signal PWM 1  for the first step-up circuit BCNV 1 , and the abscissa axis represents the duty ratio of the PWM signal PWM 2  for the second step-up circuit BCNV 2 . 
     In this graph, the upward-sloping line (parallel step-up operation line) shows the step-up voltage command value VH at the time of parallel step-up operation. That is, when a selected point is plotted on the parallel step-up operation line and then a perpendicular is drawn from the plot to the abscissa axis, the intersection of the perpendicular with the abscissa axis is the duty ratio of the second step-up circuit BCNV 2 . Similarly, when a perpendicular is drawn from the plot on the parallel step-up operation line to the ordinate axis, the intersection of the perpendicular with the ordinate axis is the duty ratio of the first step-up circuit BCNV 1 . 
     The downward-sloping dashed line in  FIG. 13  indicates a line (100% line) on which the sum of the duty ratio D 1  of the first step-up circuit BCNV 1  and the duty ratio D 2  of the second step-up circuit BCNV 2  is 100%. As a result of on time change control, the step-up voltage command value VH* takes the value VH*′ at the intersection of the parallel step-up operation line with the 100% line. The duty ratio D 1  of the first step-up circuit BCNV 1  and the duty ratio D 2  of the second step-up circuit BCNV 2  both are changed to the values D 1 ′, D 2 ′ corresponding to the intersection VH*′ of the parallel step-up operation line with the 100% line. 
     The step-up voltage command value VH* before change may be obtained by using the VH* map shown in  FIG. 14 . The VH* map is a map for obtaining the step-up voltage command value VH* for the voltage converter  11 . The step-up voltage command value VH* (VH* 1 , VH* 2 , VH* 3 , or the like) is stored in correspondence with an actual rotation speed (abscissa axis) of the rotary electric machine  20  and a torque command value (ordinate axis). The step-up voltage command value VH* is obtained by substituting the torque command value based on the depression amount of the accelerator pedal (not shown) and the actual rotation speed of the rotary electric machine  20 , acquired from the rotation speed sensor  52 , into the VH* map. 
     When the step-up voltage command value VH* obtained based on the torque command value and the actual rotation speed of the rotary electric machine  20  is changed to VH*′ through on time change control, there is a concern that a desired torque or rotation speed is not obtained. 
     Therefore, even when the step-up voltage command value is changed through on time change control, compensation control (current compensation) for obtaining a desired torque and rotation speed from the rotary electric machine  20  may be executed. For example, the current compensation is executed by the cooperation of the CNV ECU  13  and the INV ECU  15 . 
     When on time change control is executed by the CNV ECU  13  and, as a result, the step-up voltage command value is changed from VH* to VH*′, information about the change is transmitted from the CNV ECU  13  to the INV ECU  15 . The INV ECU  15  sets the conduction ratio (that is, duty ratio) of current in the inverter  18  based on the changed step-up voltage command value VH*′ and a predetermined electric power command value. 
     More specifically, the electric power command value is transmitted from the ECU  22  to the INV ECU  15  in advance. The electric power command value is a required electric power value from the load (rotary electric machine  20 ) to which alternating-current power converted by the inverter  18  from direct-current power is supplied. The electric power command value is, for example, obtained by multiplying the step-up voltage command value VH*, obtained from the VH* map, by a predetermined proportional control gain or an integral control gain. 
     As information about the change in the step-up voltage command value (VH*→VH*′) is received from the CNV ECU  13 , the INV ECU  15  obtains a current value based on the changed step-up voltage command value VH*′ and the electric power command value, and then controls the on/off states of the switching elements in the inverter  18  based on the duty ratio corresponding to the current value. 
     The above-described VH* map may be created based on so-called maximum torque control for maximizing the efficiency of the rotary electric machine  20 . That is, as the step-up voltage command values VH* (VH* 1 , VH* 2 , VH* 3 , and the like) plotted on the VH* map, voltage values corresponding to the optimal operating points of the rotary electric machine  20  (maximum efficiency voltage values) may be stored. 
     If the step-up voltage command value is changed from the maximum efficiency voltage value VH* to VH*′ as a result of the above-described on time change control, there is a concern that, even when the amount of change (VH*→VH*′) in the voltage value is compensated by current with the use of the inverter  18 , the efficiency of the rotary electric machine  20  decreases and, as a result, a desired rotation speed or torque is not obtained. 
     The INV ECU  15  may determine the conduction ratio (duty ratio) in the inverter  18  such that the amount of decrease in the efficiency of the rotary electric machine  20  resulting from the change in the step-up voltage command value is compensated in addition to the change in the step-up voltage command value or solely. For example, the INV ECU  15  may determine the duty ratio of the switching elements in the inverter  18  based on a value obtained by multiplying a current value, which is used to compensate the difference (VH*−VH*′) between the voltage value VH* before the change in the step-up voltage command value and the voltage value VH*′ after the change in the step-up voltage command value, by a coefficient (loss compensation coefficient) proportional to the difference. 
     In the above-described embodiment, a so-called series-parallel converter that includes four switching elements and that is able to switch between series connection and parallel connection is provided as the voltage converter  11 ; however, the system is not limited to this mode. In short, as long as a voltage converter is able to perform parallel step-up operation and includes a common switching element to which current is commonly supplied from two step-up circuits, overheating protection control according to the present embodiment is applicable to the voltage converter. 
       FIG. 15  shows another example of the voltage converter  11 . In this voltage converter  11 , the three switching elements S 1 , S 2 , S 3  are connected in series in the direction from the high-voltage line  26  toward the reference line  28  as the forward direction. In addition, the diodes Dd 1  to Dd 3  are respectively connected in antiparallel with the switching elements S 1  to S 3 . 
     The first battery B 1  is connected between the connection point  40  and the reference line  28 . The connection point  40  is provided between the second switching element S 2  and the third switching element S 3  from the high-voltage line  26  side. In addition, the first reactor L 1  is provided in series with the first battery B 1 , and the first capacitor C 1  is provided in parallel with the first battery B 1 . 
     The second battery B 2  is connected between the connection point  42  and the reference line  28 . The connection point  42  is provided between the first switching element S 1  and the second switching element S 2  from the high-voltage line  26  side. In addition, the second reactor L 2  is provided in series with the second battery B 2 , and the second capacitor C 2  is provided in parallel with the second battery B 2 . 
     The first step-up circuit BCNV 1  and the second step-up circuit BCNV 2  are provided at the time of parallel step-up operation. In the first step-up circuit BCNV 1 , the voltage of the first battery B 1  is stepped up by the voltage converter  11 , and the stepped-up voltage is output to the high-voltage line  26  (output line). In the second step-up circuit BCNV 2 , the voltage of the second battery B 2  is stepped up by the voltage converter  11 , and the stepped-up voltage is output to the high-voltage line  26  (output line). 
     The switching element S 3  establishes or opens the loop path (the loop including the first battery B 1  and the first reactor L 1 ) of the first step-up circuit BCNV 1 . The switching elements S 2 , S 3  establish or open the loop path (the loop including the second battery B 2  and the second reactor L 2 ) of the second step-up circuit BCNV 2 . From the configurations of both step-up circuits, the common switching element is the switching element S 3 . 
     When the loop path is established or opened based on the PWM signal from the CNV ECU  13 , the on/off operation of the switching element S 3  is controlled by using the PWM 1  for the first step-up circuit BCNV 1 . The on/off operation of each of the switching elements S 2 , S 3  is controlled by using the PWM 2  for the second step-up circuit BCNV 2 . 
     The CNV ECU  13  monitors the temperature of the common switching element S 3 . When the temperature exceeds the threshold temperature, the CNV ECU  13  executes the above-described overheating protection control via phase shift control and on time change control over the PWM 1  and the PWM 2 . 
       FIG. 16  shows further another example of the voltage converter  11 . As in the case of  FIG. 15 , in the voltage converter  11 , the three switching elements S 1 , S 2 , S 3  are connected in series in the direction from the high-voltage line  26  toward the reference line  28  as the forward direction. In addition, the diodes Dd 1  to Dd 3  are respectively connected in antiparallel with the switching elements S 1  to S 3 . 
     The voltage converter  11  shown in  FIG. 16  differs from the voltage converter shown in  FIG. 15  in the arrangement of the first step-up circuit BCNV 1 . That is, the first battery B 1 , the first reactor L 1  and the first capacitor C 1  are connected between the connection point  42  and the connection point  40 . The connection point  42  is provided between the first switching element S 1  and the second switching element S 2  from the high-voltage line  26  side. The connection point  40  is provided between the second switching element S 2  and the third switching element S 3  from the high-voltage line  26  side. 
     The first step-up circuit BCNV 1  and the second step-up circuit BCNV 2  are provided at the time of parallel step-up operation. In the first step-up circuit BCNV 1 , the voltage of the first battery B 1  is stepped up by the voltage converter  11 , and the stepped-up voltage is output to the high-voltage line  26  (output line). In the second step-up circuit BCNV 2 , the voltage of the second battery B 2  is stepped up by the voltage converter  11 , and the stepped-up voltage is output to the high-voltage line  26  (output line). 
     The switching element S 2  establishes or opens the loop path (the loop including the first battery B 1  and the first reactor L 1 ) of the first step-up circuit BCNV 1 . The switching elements S 2 , S 3  establish or open the loop path (the loop including the second battery B 2  and the second reactor L 2 ) of the second step-up circuit BCNV 2 . From the configurations of both step-up circuits, the common switching element is the switching element S 2 . 
     When the loop path is established or opened based on the PWM signal from the CNV ECU  13 , the on/off operation of the switching element S 2  is controlled by using the PWM 1  for the first step-up circuit BCNV 1 . The on/off operation of each of the switching elements S 2 , S 3  is controlled by using the PWM 2  for the second step-up circuit BCNV 2 . 
     The CNV ECU  13  monitors the temperature of the common switching element S 2 . When the temperature exceeds the threshold temperature, the CNV ECU  13  executes the above-described overheating protection control via phase shift control and on time change control over the PWM 1  and the PWM 2 . 
     The embodiment according to the disclosure will be summarized below. The electric power conversion system  10  establishes the first step-up circuit BCNV 1  and the second step-up circuit BCNV 2  at the time of parallel step-up operation. In the parallel step-up operation, the voltages of the first and second batteries B 1 , B 2  are stepped up in parallel with each other. The first step-up circuit BCNV 1  steps up the voltage of the first battery B 1  by using the voltage converter  11 , and outputs the stepped-up voltage to the output line  26 . The second step-up circuit BCNV 2  steps up the voltage of the second battery B 2  by using the voltage converter  11 , and outputs the stepped-up voltage to the output line  26 . The plurality of switching elements S 1  to S 4  of the voltage converter  11  include the common switching element S 3  to which current is supplied from both the first and second step-up circuits BCNV 1 , BCNV 2  at the time of the parallel step-up operation. At the time of the parallel step-up operation and when the temperature of the common switching element S 3  exceeds the threshold temperature, the ECU  22  of the electric power conversion system  10  connects the trailing edge of one of the first PWM signal PWM 1  and the second PWM signal PWM 2  to the leading edge of the other one of the first PWM signal PWM 1  and the second PWM signal PWM 2 , and changes the on time of at least one of the first PWM signal PWM 1  and the second PWM signal PWM 2  such that the sum of the on time of the first PWM signal PWM 1  and the on time of the second PWM signal PWM 2  in the single PWM control period falls within the range from the single PWM control period to the allowable period obtained by adding the predetermined time to the single PWM control period.