Patent Publication Number: US-2022216805-A1

Title: Dc/dc converter and power conversion device

Description:
TECHNICAL FIELD 
     The present invention relates to a DC/DC converter and a power conversion device. 
     BACKGROUND ART 
     A DC/DC converter that performs bidirectional power transmission between two DC power sources is described, for example, in WO2018/016106 (PTL 1). In the DC/DC converter in PTL 1, a first converter of a full bridge circuit is provided on the first DC power source side and a second converter of the full bridge circuit is provided on the second DC power source side with a transformer interposed therebetween. Furthermore, a first reactor is provided between a first winding of the transformer and the first converter, and a second reactor is provided between a second winding of the transformer and the second converter. 
     In PTL 1, step-up operation is performed using the first reactor or the second reactor when voltage of the first DC power source or the second DC power source is higher than voltage generated in the first winding or the second winding of the transformer, that is, when the step-up operation is necessary. On the other hand, the step-up operation is not performed when voltage of the first DC power source or the second DC power source is lower than voltage generated in the first winding or the second winding of the transformer, that is, when step-down operation is necessary. 
     In the DC/DC converter described in PTL 1, an operation mode of performing step-up operation (step-up charge) and an operation mode of performing step-down operation (step-down charge) in first power transmission (charge of the second DC power source) in which power is transmitted from the first DC power source to the second DC power source, and an operation mode of performing step-up operation (step-up discharge) and an operation mode of performing step-down operation (step-down discharge) in second power transmission (discharge of the second DC power source) in which power is transmitted from the second DC power source to the first DC power source, that is, in total, four operation modes can be switched according to the duty ratio representing power transmission. 
     CITATION LIST 
     Patent Literature 
     PTL 1: WO2018/016106 
     SUMMARY OF INVENTION 
     Technical Problem 
     Unfortunately, in the DC/DC converter described in PTL 1, as will be explained in detail later, a circulating current path including the transformer without passing through either the first DC power source or the second DC power source may be formed in the operation mode of performing step-down operation with both of the upper arm and the lower arm kept in the off state in one bridge circuit on the power receiving side. 
     As a result, conduction loss is caused by current passing through the transformer, the DC reactor, and the semiconductor element, in a period of time in which both of the first bridge circuit and the second bridge circuit output zero voltage. At the same time, switching loss occurs when the on/off of the upper and lower arms is switched in the other bridge circuit on the power receiving side in the step-down operation mode. 
     In particular, in the operation modes of step-down charge and step-down discharge, the power transmission amount is smaller than in the operation modes of step-up charge and step-up discharge, and the conduction loss in the circulating current path and the switching loss on the power receiving side have a larger impact, leading to reduction in power conversion efficiency. 
     The present invention is made in order to solve such a problem and an object of the present invention is to improve power conversion efficiency in step-down operation with a small power transmission amount while enabling step-up operation and step-down operation in a DC/DC converter that performs bidirectional power transmission between first and second DC power sources. 
     Solution to Problem 
     In an aspect of the present invention, a DC/DC converter that performs bidirectional power transmission between a first DC power source and a second DC power source includes a transformer, a first converter, a second converter, and a control circuit. The transformer has a first winding and a second winding magnetically coupled. The first converter is connected between the first DC power source and the first winding. The second converter is connected between the second DC power source and the second winding. The first converter includes a first bridge circuit and a second bridge circuit connected in parallel to each other to the first DC power source. Each of the first bridge circuit and the second bridge circuit has a positive electrode-side switching element and a negative electrode-side switching element connected in series between a positive electrode and a negative electrode of the first DC power source. The first winding is connected between a connection point of the positive electrode-side switching element and the negative electrode-side switching element of the first bridge circuit and a connection point of the positive electrode-side switching element and the negative electrode-side switching element of the second bridge circuit. The second converter includes a third bridge circuit and a fourth bridge circuit connected in parallel to each other to the second DC power source. Each of the third bridge circuit and the fourth bridge circuit has a positive electrode-side switching element and a negative electrode-side switching element connected in series between a positive electrode and a negative electrode of the second DC power source. The second winding is connected between a connection point of the positive electrode-side switching element and the negative electrode-side switching element of the third bridge circuit and a connection point of the positive electrode-side switching element and the negative electrode-side switching element of the fourth bridge circuit. The control circuit performs on/off drive control of the respective positive electrode-side switching elements and the respective negative electrode-side switching elements of the first converter and the second converter. In first power transmission in which power is transmitted from the first DC power source to the second DC power source, in the first converter, the control circuit performs DC/AC power conversion by performing on/off drive control of the positive electrode-side switching element and the negative electrode-side switching element in each of the first bridge circuit and the second bridge circuit. In the first power transmission, in the second converter, when a first power transmission amount by the first power transmission is greater than a predetermined first reference value, the control circuit stops on/off drive of the positive electrode-side switching element and the negative electrode-side switching element in the third bridge circuit and performs on/off drive control of the positive electrode-side switching element and the negative electrode-side switching element in the fourth bridge circuit, whereas when the first power transmission amount is smaller than the first reference value, the control circuit performs AC/DC power conversion by stopping on/off drive of the positive electrode-side switching element and the negative electrode-side switching element in both of the third bridge circuit and the fourth bridge circuit. 
     Advantageous Effects of Invention 
     According to the present invention, in a DC/DC converter that performs bidirectional power transmission between first and second DC power sources, while step-up operation and step-down operation is enabled, occurrence of circulating current between the first and second converters can be prevented by keeping the switching elements in the power receiving-side converter in the off state at the time of step-down operation. As a result, the power conversion efficiency in step-down operation with a small power transmission amount can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic circuit configuration diagram of a DC/DC converter according to a first embodiment. 
         FIG. 2  is a time chart illustrating waveforms of on/off drive signals of switching elements in step-down charge of the DC/DC converter according to the first embodiment. 
         FIG. 3  is a time chart illustrating waveforms of on/off drive signals of switching elements in step-down charge in PTL 1 illustrated as a comparative example. 
         FIG. 4  is a time chart illustrating waveforms of on/off drive signals of switching elements in step-up charge of the DC/DC converter according to the first embodiment. 
         FIG. 5  is a first circuit diagram for explaining a current path in step-up charge operation according to  FIG. 4 . 
         FIG. 6  is a second circuit diagram for explaining a current path in step-up charge operation according to  FIG. 4 . 
         FIG. 7  is a first circuit diagram for explaining a current path in step-down charge operation according to  FIG. 2 . 
         FIG. 8  is a second circuit diagram for explaining a current path in step-down charge operation according to  FIG. 2 . 
         FIG. 9  is a third circuit diagram for explaining a current path in step-down charge operation according to  FIG. 2 . 
         FIG. 10  is a fourth circuit diagram for explaining a current path in step-down charge operation according to  FIG. 2 . 
         FIG. 11  is a time chart illustrating waveforms of on/off drive signals of switching elements in step-down discharge of the DC/DC converter according to the first embodiment. 
         FIG. 12  is a time chart illustrating waveforms of on/off drive signals of switching elements in step-down discharge in PTL 1 illustrated as a comparative example. 
         FIG. 13  is a time chart illustrating waveforms of on/off drive signals of switching elements in step-up discharge of the DC/DC converter according to the first embodiment. 
         FIG. 14  is a graph for explaining control of a phase shift amount based on a power transmission amount in the DC/DC converter according to the first embodiment. 
         FIG. 15  is a first circuit diagram for explaining a circulating current path that may be produced in a zero voltage period when step-down charge is performed in accordance with  FIG. 3 . 
         FIG. 16  is a second circuit diagram for explaining a circulating current path that may be produced in a zero voltage period when step-down charge is performed in accordance with  FIG. 3 . 
         FIG. 17  is a first circuit diagram for explaining a current path in step-down discharge by the DC/DC converter according to the first embodiment. 
         FIG. 18  is a second circuit diagram for explaining a current path in step-down discharge by the DC/DC converter according to the first embodiment. 
         FIG. 19  is a time chart illustrating waveforms of on/off drive signals of switching elements when the phase difference between a first phase shift amount and a second phase shift amount is small in step-up charge by the DC/DC converter according to the first embodiment. 
         FIG. 20  is a time chart illustrating waveforms of on/off drive signals of switching elements when the phase difference between the first phase shift amount and the second phase shift amount is large in step-up charge by the DC/DC converter according to the first embodiment. 
         FIG. 21  is a time chart illustrating waveforms of on/off drive signals of switching elements in step-down discharge by the DC/DC converter according to the first embodiment. 
         FIG. 22  is a time chart illustrating waveforms of on/off drive signals of switching elements when the phase difference between the first phase shift amount and the second phase shift amount is small in step-up discharge by the DC/DC converter according to the first embodiment. 
         FIG. 23  is a time chart illustrating waveforms of on/off drive signals of switching elements when the phase difference between the first phase shift amount and the second phase shift amount is large in step-up discharge by the DC/DC converter according to the first embodiment. 
         FIG. 24  is a graph for explaining control of the phase shift amount based on the power transmission amount in a DC/DC converter according to a second embodiment. 
         FIG. 25  is a block diagram for explaining a first modification of calculation of an output DUTY ratio by a control circuit. 
         FIG. 26  is a block diagram for explaining a second modification of calculation of the output DUTY ratio by the control circuit. 
         FIG. 27  is a block diagram for explaining a first configuration example of a power conversion device according to a third embodiment. 
         FIG. 28  is a block diagram for explaining a second configuration example of the power conversion device according to the third embodiment. 
         FIG. 29  is a block diagram for explaining a third configuration example of the power conversion device according to the third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described in detail below with reference to the drawings. In the following, like or corresponding parts in the drawings are denoted by like reference signs and a description thereof is basically not repeated. 
     First Embodiment 
     (Circuit Configuration)  FIG. 1  is a schematic circuit configuration diagram of a DC/DC converter  100  according to a first embodiment. DC/DC converter  100  performs bidirectional power transmission between a first DC power source PS 1  and a second DC power source PS 2 . 
     In the present embodiment, a description is premised on that second DC power source PS 2  is configured with a battery. More specifically, DC/DC converter  100  operates as a battery charging/discharging device that charges and discharges the battery. In the following, first DC power source PS 1  may be simply referred to as DC power source PS 1 , and second DC power source PS 2  may be referred to as battery PS 2 . 
     As will be described below, the configuration of DC/DC converter  100  according to the present embodiment is similar to the DC/DC converter described in PTL 1. 
     DC/DC converter  100  includes a transformer  3 , a first converter  10 , a second converter  20 , a first reactor  14 , a second reactor  24 , and a control circuit  30 . Transformer  3  has a first winding  3   a  and a second winding  3   b  wound around a not-shown core. With electromagnetic induction between first winding  3   a  and second winding  3   b  magnetically coupled to each other, a circuit on the first winding  3   a  side connected to DC power source PS 1  and a circuit on the second winding  3   b  side connected to battery PS 2  can perform power transmission bidirectionally while being electrically insulated from each other. 
     First converter  10  is configured with a full bridge circuit including a first bridge circuit  41  and a second bridge circuit  42 . First bridge circuit  41  includes semiconductor switching elements (hereinafter simply referred to as switching elements) Q 4 A and Q 4 B connected in series between a first positive electrode wire  11  and a first negative electrode wire  12 . Second bridge circuit  42  includes switching elements Q 3 A and Q 3 B connected in series between first positive electrode wire  11  and first negative electrode wire  12 . 
     In other words, first bridge circuit  41  is a series connection circuit of first switching element Q 4 A on the positive electrode side and first switching element Q 4 B on the negative electrode side. Second bridge circuit  42  is a series connection circuit of second switching element Q 3 A on the positive electrode side and second switching element Q 3 B on the negative electrode side. 
     First positive electrode wire  11  and first negative electrode wire  12  are electrically connected to the positive electrode and the negative electrode of DC power source PS 1 . The midpoint of first bridge circuit  41  and the midpoint of second bridge circuit  42  are respectively electrically connected to both terminals of first winding  3   a . In each bridge circuit, the midpoint corresponds to a connection point between the positive electrode-side switching element and the negative electrode-side switching terminal. First converter  10  performs DC/AC bidirectional power conversion between 
     DC power source PS 1  and first winding  3   a  of transformer  3  through on/off control of switching elements Q 3 A, Q 3 B, Q 4 A, and Q 4 B. 
     Similarly, second converter  20  is configured with a full bridge circuit including a third bridge circuit  43  and a fourth bridge circuit  44 . Third bridge circuit  43  includes switching elements Q 1 A and Q 1 B connected in series between a second positive electrode wire  21  and a second negative electrode wire  22 . Fourth bridge circuit  44  includes switching elements Q 2 A and Q 2 B connected in series between second positive electrode wire  21  and second negative electrode wire  22 . Third bridge circuit  43  is a series connection circuit of third switching element Q 1 A on the positive electrode side and third switching element Q 1 B on the negative electrode side. Fourth bridge circuit  44  is a series connection circuit of fourth switching element Q 2 A on the positive electrode side and fourth switching element Q 2 B on the negative electrode side. 
     In each of first bridge circuit  41 , second bridge circuit  42 , third bridge circuit  43 , and fourth bridge circuit  44 , a plurality of switching elements may be arranged on each of the positive electrode side and the negative electrode side. Switching elements Q 1 A to Q 4 A and Q 1 B to Q 4 B may be any switching elements that can be on/off controlled by a control signal from control circuit  30 , such as insulated gate bipolar transistors (IGBTs) or metal oxide semiconductor field effect transistors (MOSFETs). 
     A diode  51  (hereinafter may be referred to as antiparallel diode  51 ) is connected in antiparallel with each of switching elements Q 1 A to Q 4 A and Q 1 B to Q 4 B. To turn on and off each of switching elements Q 1 A to Q 4 A and Q 1 B to Q 4 B, it is preferable to apply zero voltage switching in which terminal-to-terminal voltage of the switching element is almost zero at the time of switching. A capacitor  52  (hereinafter may be referred to as parallel capacitor  52 ) is connected to each of switching elements Q 1 A to Q 4 A and Q 1 B to Q 4 B, if necessary. 
     Second positive electrode wire  21  and second negative electrode wire  22  are electrically connected to the positive electrode and the negative electrode of battery PS 2 . The midpoint of third bridge circuit  43  and the midpoint of fourth bridge circuit  44  are respectively electrically connected to both terminals of second winding  3   b . Second converter  20  performs DC/AC bidirectional power conversion between battery PS 2  and second winding  3   b  of transformer  3  through on/off control of switching elements Q 1 A, Q 1 B, Q 2 A, and Q 2 B. 
     On the first converter  10  side, first reactor  14  is connected in series in a connection path between first converter  10  and first winding  3   a . In the present embodiment, first reactor  14  is connected in series in a connection path between the midpoint of first bridge circuit  41  and a first terminal of first winding  3   a . Furthermore, first converter  10  further includes a first smoothing capacitor  13  connected in parallel to DC power source PS 1  between first positive electrode wire  11  and first negative electrode wire  12 . 
     On the second converter  20  side, second reactor  24  is connected in series in a connection path between second converter  20  and second winding  3   b . In the present embodiment, second reactor  24  is connected in series in a connection path between the midpoint of third bridge circuit  43  and a first terminal of second winding  3   b . Furthermore, second converter  20  further includes a second smoothing capacitor  23  connected in parallel to battery PS 2  between second positive electrode wire  21  and second negative electrode wire  22 . With first reactor  14  and second reactor  24 , in DC/DC converter  100 , inductance elements for excitation described later can be provided on a path including first converter  10  and first winding  3   a  and on a path including second converter  20  and second winding  3   b . The arrangement of first reactor  14  and second reactor  24  is not essential, and the inductance element may be configured with leakage inductance of first winding  3   a  and second winding  3   b.    
     However, if a reactor element is configured only with leakage inductance, adjustment of an inductance value is difficult. Moreover, increasing leakage inductance for adjustment of the inductance value may reduce the conversion efficiency in transformer  3 . Therefore, external first reactor  14  and second reactor  24  may be arranged as necessary, so that the inductance value of the inductance element can be appropriately ensured without excessively increasing leakage inductance, thereby improving control stability and efficiency. Alternatively, an external reactor may be provided only on the primary side or the secondary side of transformer  3 , that is, only one of first reactor  14  and second reactor  24  may be arranged. 
     A reactor  25  is connected in series to second positive electrode wire  21  between second smoothing capacitor  23  and battery PS 2 . Reactor  25  is provided with a not-shown current sensor for detecting charge/discharge current i (hereinafter simply referred to as “current i”) of battery PS 2 . The current sensor may be provided on the side closer to second converter  20  than second smoothing capacitor  23 . Current i is positive in the direction of arrow in  FIG. 1 . Therefore, current i is positive (i&gt;0) at the time of discharge of battery PS 2  and conversely, current i is negative (i&lt;0) at the time of charge of battery PS 2 . 
     Furthermore, a voltage sensor (not shown) that detects terminal-to-terminal voltage of first smoothing capacitor  13  is provided in order to detect an output voltage v output from first converter  10  to DC power source PS 1 . Output signals of the current sensor and the voltage sensor are input to control circuit  30 . Control circuit  30  can detect current i of battery PS 2  and output voltage v of first converter  10 , based on output signals from the current sensor and the voltage sensor. 
     Control circuit  30  includes a processing circuit to perform on/off drive control of each switching element. The processing circuit may be configured with an arithmetic processing device and a digital electronic circuit such as a storage device, may be configured with an analog electronic circuit such as a comparator, an operational amplifier, and a differential amplifier circuit, or may be configured with both of a digital electronic circuit and an analog electronic circuit. 
     Control circuit  30  generates a drive signal  31   a  for on/off drive control of each switching element Q 3 A, Q 3 B, Q 4 A, and Q 4 B of first converter  10  and a drive signal  31   b  for on/off drive control of each switching element Q 1 A, Q 1 B, Q 2 A, Q 2 B of second converter  20 , based on the power transmission amount between DC power source PS 1  and battery PS 2 . 
     In control circuit  30 , an output DUTY ratio can be used as an intermediate variable representing the transmission power amount, in the same manner as in PTL 1. As will be described in detail later, control circuit  30  calculates the output DUTY ratio based on a command value for the transmission power amount and generates drive signals  31   a  and  31   b  for on/off drive control of each switching element in first converter  10  and second converter  20  based on the calculated output DUTY ratio. In doing so, control circuit  30  changes the output DUTY ratio that is an intermediate variable by feedback control described later such that the actual transmission power amount approaches the command value. 
     (Reference Element and Diagonal Element in DC/DC Converter) 
     Control circuit  30  sets one of the switching elements on the positive electrode side and the negative electrode side as a first reference element QB 1  in first bridge circuit  41  and sets the switching element on the electrode side opposite to the first reference element in second bridge circuit  42  as a first diagonal element QO 1  to control first converter  10 . In the present embodiment, first switching element Q 4 A on the positive electrode side of first bridge circuit  41  is set as first reference element QB 1 , and in second bridge circuit  42 , second switching element Q 3 B on the negative electrode side that is the opposite electrode to first reference element QB 1  (positive electrode side) is set as first diagonal element QO 1 . 
     Alternatively, conversely, the bridge circuit of first converter  10  in which first reference element QB 1  is set may be defined as first bridge circuit  41 , and the bridge circuit of first converter  10  in which first diagonal element QO 1  is set may be defined as second bridge circuit  42 . In other words, one of switching elements Q 3 A and Q 3 B may be set as first reference element QB 1 , and one of switching elements Q 4 A and 
     Q 4 B (the electrode side opposite to the first reference element) may be set as first diagonal element QO 1 . 
     Similarly, control circuit  30  sets one of the switching elements on the positive electrode side and the negative electrode side in third bridge circuit  43  as a second reference element QB 2  and sets the switching element on the electrode side opposite to the second reference element in fourth bridge circuit  44  as a second diagonal element QO 2  to control second converter  20 . In the present embodiment, in third bridge circuit  43 , third switching element Q 1 A on the same positive electrode side as in first bridge circuit  41  is set as second reference element QB 2 . In fourth bridge circuit  44 , fourth switching element Q 2 B on the negative electrode side that is the opposite electrode to second reference element QB 2  set as the positive electrode side is set as second diagonal element QO 2 . 
     Alternatively, also in second converter  20 , the bridge circuit in second converter  20  in which second reference element QB 2  is set may be defined as third bridge circuit  43 , and the bridge circuit of second converter  20  in which second diagonal element QO 2  is set may be defined as fourth bridge circuit  44 . In other words, one of switching elements Q 2 A and Q 2 B may be set as second reference element QB 2 , and one of switching elements Q 1 A and Q 1 B (the electrode side opposite to the second reference element) may be set as second diagonal element QO 2 . 
     (Basic Control Behavior of First Power Transmission) 
     In DC/DC converter  100 , first power transmission in which electric power is transmitted from DC power source PS 1  to battery PS 2 , that is, battery PS 2  is charged, and second power transmission in which electric power is transmitted from battery PS 2  to DC power source PS 1 , that is, battery PS 2  is discharged, are selectively performed. First, the circuit operation of first power transmission will be described. 
     The first power transmission includes charge of battery PS 2  not involving step-up operation of second reactor  24  (which hereinafter may be referred to as step-down charge) and charge of battery PS 2  involving step-up operation of second reactor  24  (which may be referred to as step-up charge). 
       FIG. 2  shows a time chart illustrating waveforms of on/off drive signals of the switching elements in step-down charge of DC/DC converter  100  according to the first embodiment. In comparison,  FIG. 3  is a time chart illustrating waveforms of on/off drive signals of the switching elements in step-down charge in PTL 1 illustrated as a comparative example. Furthermore,  FIG. 4  shows a time chart illustrating waveforms of on/off drive signals of the switching elements in step-up charge of DC/DC converter  100  according to the first embodiment. 
       FIG. 2  to  FIG. 4  show waveform examples for explaining the principle of step-down charge and step-up charge and strictly speaking do not agree with the control of a first phase shift amount θ 1  and a second phase shift amount θ 2  explained later with reference to  FIG. 14  and the like. More specifically, in  FIG. 2  to  FIG. 4 , for simplicity of explanation, a switching period Tsw of first bridge circuit  41  is divided into ten periods of periods A to J, and in each of periods A to J, a gate pattern that is a combination pattern of on or off drive signals of switching elements Q 1 A to Q 4 A and Q 1 B to Q 4 B is set. 
     Referring to  FIG. 2 , in the step-down charge operation that is the first power transmission not involving step-up operation, control circuit  30  alternatively turns on switching elements Q 3 A, Q 3 B, Q 4 A, and Q 4 B on the positive electrode side and the negative electrode side in first bridge circuit  41  and second bridge circuit  42 , once for each, at equal intervals in a preset switching period Tsw. On the other hand, in the step-down charge operation, control circuit  30  keeps off third switching elements Q 1 A and Q 1 B on the positive electrode side and the negative electrode side of third bridge circuit  43  and fourth switching elements Q 2 A and Q 2 B on the positive electrode side and the negative electrode side of fourth bridge circuit  44 . In the following, the above-noted operation in which the switching elements on the positive electrode side and the negative electrode side in both of third bridge circuit  43  and fourth bridge circuit  44  are kept in the off state may be referred to as “two-leg off operation”. 
     In the present embodiment, control circuit  30  is configured such that the switching elements on the positive electrode side and the negative electrode side are alternately turned on at equal intervals with a short-circuit prevention time td interposed. In other words, the switching elements on the positive electrode side and the negative electrode side are each controlled at a 50% on-time ratio excluding the short-circuit prevention time td. The short-circuit prevention time td is a time period (called dead time) set for preventing simultaneous turning-on of the switching elements on the positive electrode side and the negative electrode side, and both of the switching elements on the positive electrode side and the negative electrode side are brought to the off state during the short-circuit prevention time td. 
     Specifically, for first bridge circuit  41 , control circuit  30  turns on a drive signal to correspond to the ON period of first switching element Q 4 A on the positive electrode side and turns on a drive signal of first switching element Q 4 B on the negative electrode side after the lapse of the short-circuit prevention time td since turning-off of first switching element Q 4 A. The drive signal is turned on to correspond to the ON period of first switching element Q 4 B. After the elapse of the short-circuit prevention time td since turning-off of first switching element Q 4 B, a drive signal of first switching element Q 4 A on the positive electrode side is turned on again. 
     The short-circuit prevention time td is preset to correspond to the time required for the voltage at parallel capacitor  52  of each switching element to increase to the voltage at first smoothing capacitor  13  or the time required for the voltage at parallel capacitor  52  to decrease to the vicinity of zero voltage, when each switching element of first converter  10  is turned on. As a result, the ON time Ton of each switching element is denoted by Ton=(Tsw−2×td)/2 using the switching period Tsw and the short-circuit prevention time td. 
     In the case of the step-down charge operation in  FIG. 2 , control circuit  30  controls, as a first phase shift amount θ 1 , the phase shift amount of the on/off drive signal of first diagonal element QO 1  (second switching element Q 3 B on the negative electrode side) with respect to the on/off drive signal of first reference element QB 1  (first switching element Q 4 A on the positive electrode side). Control circuit  30  changes the first phase shift amount θ 1  based on the transmission power amount (in the present example, output DUTY ratio). 
     On the other hand, as described above, the two-leg off operation is applied in step-down charge. Therefore, a second phase shift amount θ 2  is not set, which is the phase shift amount of the on/off drive signal of second diagonal element QO 2  (fourth switching element Q 2 B on the negative electrode side) with respect to the on/off drive signal of first reference element QB 1  (first switching element Q 4 A on the positive electrode side). 
     In comparison, as shown in  FIG. 3 , second converter  20  can be controlled, if necessary, in accordance with the second phase shift amount θ 2  virtually set so as to make a change in the same amount as in the first phase shift amount θ 1 . The first phase shift amount θ 1  and the second phase shift amount θ 2  are phase shift amounts in the advance direction. In step-down discharge in PTL 1, in a DC/DC converter similar to that in  FIG. 1 , the on/off of switching elements Q 1 A to Q 4 A and Q 1 B to Q 4 B is controlled in accordance with the gate patterns in  FIG. 3 . 
     Referring to  FIG. 3 , in the step-down charge operation in PTL 1, for switching elements Q 3 A, Q 3 B, Q 4 A, and Q 4 B in first converter  10  turned on/off in the same manner as in  FIG. 2 , fourth switching elements Q 2 A and Q 2 B of fourth bridge circuit  44  in second converter  20  are turned on/off in synchronization with second switching elements Q 3 A and Q 3 B in first converter  10  in accordance with the virtually set second phase shift amount θ 2  (θ 2 =θ 1 ). 
     On the other hand, in second converter  20 , third switching elements Q 1 A and Q 1 B of third bridge circuit  43  are kept off in the same manner as in  FIG. 2 . In the following, the above-noted operation in which the switching elements on the positive electrode side and the negative electrode side are brought to the off state in only one of third bridge circuit  43  and fourth bridge circuit  44  may be referred to as “one-leg off operation”. 
     In this way, the present first embodiment and PTL 1 differ in control of the switching elements (more specifically, fourth switching elements Q 2 A and Q 2 B) of second converter  20  in step-down discharge. 
     In the step-up charge operation in  FIG. 4 , control circuit  30  sets the phase shift amount of the on/off drive signal of first diagonal element QO 1  (second switching element Q 3 B on the negative electrode side) with respect to the on/off drive signal of first reference element QB 1  (first switching element Q 4 A on the positive electrode side), as the first phase shift amount θ 1 , and sets the phase shift amount of the on/off drive signal of second diagonal element QO 2  (fourth switching element Q 2 B on the negative electrode side) with respect to the on/off drive signal of first reference element QB 1 , as the second phase shift amount θ 2 , to perform control. 
     Control circuit  30  changes the first phase shift amount θ 1  and the second phase shift amount θ 2 , based on the transmission power amount (in the present example, output DUTY ratio). In  FIG. 4  also, the first phase shift amount θ 1  and the second phase shift amount θ 2  are phase shift amounts in the advance direction. In the case of the step-up charge operation in  FIG. 4 , the second phase shift amount θ 2  is a value greater than the first phase shift amount θ 1 . 
     Here, referring to  FIG. 2  and  FIG. 4 , the operation at the time of charge of battery PS 2  will be described in detail. 
     When a period in which first reference element QB 1  (first switching element Q 4 A on the positive electrode side) and first diagonal element QO 1  (second switching element Q 3 B on the negative electrode side) are simultaneously on in step-down charge ( FIG. 2 ) and step-up charge ( FIG. 4 ) is defined as a first diagonal ON time t 1 , the first diagonal ON time t 1  changes in accordance with the first phase shift amount θ 1 . Furthermore, the period in which first switching element Q 4 B on the negative electrode side and second switching element Q 3 A on the positive electrode side are simultaneously on (which may be referred to as first diagonal ON time t 1   a ) is equal to the first diagonal ON time t 1 . 
     In the step-down discharge in  FIG. 2 , switching elements Q 1 A and Q 1 B (third bridge circuit  43 ) and switching elements Q 2 A and Q 2 B (fourth bridge circuit  44 ) kept in the off state in  FIG. 2  may be virtually turned on/off. In the present embodiment, “virtual on/off” means a state in which the on/off drive signal of the switching element (off state) that is actually not turned on/off is generated in the inside of control circuit  30  but actually not output to the gate of the switching element. 
     Specifically, the on/off drive signals of switching elements Q 4 A and Q 4 B of first bridge circuit  41  can be set as the virtual on/off drive signals of switching elements Q 1 A and Q 1 B in third bridge circuit  43 , if necessary. Similarly, the on/off drive signals of switching elements Q 3 A and Q 3 B of second bridge circuit  42  can be set as the virtual on/off drive signals of switching elements Q 2 A and Q 2 B of fourth bridge circuit  44 , if necessary. 
     In this case, when a period in which the virtual on/off drive signal of second reference element QB 2  (third switching element Q 1 A on the positive electrode side) and the virtual on/off drive signal of second diagonal element QO 2  (fourth switching element Q 2 B on the negative electrode side) are simultaneously on is set as a second virtual diagonal ON time t 2 , the second virtual diagonal ON time t 2  changes in accordance with the virtually set second phase shift amount θ 2 . Furthermore, the second virtual diagonal ON time t 2   a  in which the virtual on/off drive signal of third switching element Q 1 B on the negative electrode side and the virtual on/off drive signal of fourth switching element Q 2 A on the positive electrode side are simultaneously on is also equal to the second virtual diagonal ON time t 2 . 
     Furthermore, in the step-up charge in  FIG. 4 , the on/off drive signals of first switching elements Q 4 A and Q 4 B of first bridge circuit  41  can be set as the virtual on/off drive signals of third switching elements Q 1 A and Q 1 B of third bridge circuit  43 . 
     Then, when a period in which the virtual on/off drive signal of second reference element QB 2  (third switching element Q 1 A on the positive electrode side) and the on/off drive signal of second diagonal element QO 2  (fourth switching element Q 2 B on the negative electrode side) are simultaneously on is set as the second virtual diagonal ON time t 2 , the second virtual diagonal ON time t 2  changes in accordance with the second phase shift amount θ 2 . Furthermore, the second virtual diagonal ON time t 2   a  in which the virtual on/off drive signal of third switching element Q 1 B on the negative electrode side and the on/off drive signal of fourth switching element Q 2 A on the positive electrode side are simultaneously on is also equal to the second virtual diagonal ON time t 2 . 
     The circuit operation of DC/DC converter (battery charging/discharging device)  100  in step-up charge is similar to that of PTL 1 and the current path corresponding to each gate pattern shown in  FIG. 4  is described also in PTL 1 above. Here, charge (step-up charge) of battery PS 2  involving the step-up operation of second reactor  24  will be described by describing the current path in period B and period C in  FIG. 4 . 
       FIG. 5  shows a current path corresponding to the gate pattern in period B in  FIG. 4 . 
     Referring to  FIG. 5 , in period B in  FIG. 4 , first switching element Q 4 A on the positive electrode side (first reference element QB 1 ) and second switching element Q 3 B on the negative electrode side (first diagonal element QO 1 ) are simultaneously on in first converter  10 , and the diagonal two elements become electrically continuous. Therefore, current through first switching element Q 4 A on the positive electrode side and second switching element Q 3 B on the negative electrode side allows energy to be transmitted from the DC power source PS 1  to first reactor  14  to excite first reactor  14 . 
     In period B, fourth switching element Q 2 A on the positive electrode side is turned on in second converter  20 . Therefore, current circulates through fourth switching element Q 2 A on the positive electrode side and antiparallel diode  51  of third switching element Q 1 A on the positive electrode side to second reactor  24 . This current excites second reactor  24 . As a result, in period B, first reactor  14  and second reactor  24  are excited. In the present embodiment, this excitation operation is referred to as step-up. 
       FIG. 6  shows a current path corresponding to the gate pattern in period C in  FIG. 4 . 
     Referring to  FIG. 6 , in period C in  FIG. 4  in the same manner as period B, first switching element Q 4 A on the positive electrode side (first reference element QB 1 ) and second switching element Q 3 B on the negative electrode side (first diagonal element QO 1 ) are simultaneously turned on in first converter  10 , thereby exciting first reactor  14 . 
     On the other hand, in period C, fourth switching element Q 2 A on the positive electrode side is turned off in second converter  20 , and current flows toward battery PS 2  through antiparallel diode  51  of third switching element Q 1 A on the positive electrode side and antiparallel diode  51  of fourth switching element Q 2 B on the negative electrode side. 
     Accordingly, in period C, excitation energy of first reactor  14  and second reactor  24  is transmitted toward battery PS 2 . Charge of battery PS 2  (step-up charge) involving step-up operation of second reactor  24  is thus carried out. 
     The circuit operation of step-down charge will now be described in further detail. 
       FIG. 7  shows a current path corresponding to the gate pattern in period C in  FIG. 2 . 
     Referring to  FIG. 7 , in period C in  FIG. 2 , since first switching element Q 4 A on the positive electrode side (first reference element QB 1 ) and second switching element Q 3 B on the negative electrode side (first diagonal element QO 1 ) are simultaneously turned on in first converter  10 , energy is transmitted from DC power source PS 1  to first reactor  14  thereby exciting first reactor  14 . 
     On the other hand, in period C, in second converter  20  in which the two-leg off operation is applied, a current path for charging battery PS 2  is formed through antiparallel diode  51  of third switching element Q 1 A on the positive electrode side (off) and antiparallel diode  51  of fourth switching element Q 2 B on the negative electrode side (off). In  FIG. 7 , a current path similar to that in  FIG. 6  is formed but second reactor  24  is not excited in the preceding period as will be described later. 
     Since a gate pattern similar to that in period C is applied in period D in  FIG. 2 , a current path is also formed in the same manner as in  FIG. 6 . 
     Subsequently, in period E in  FIG. 2 , first switching element Q 4 A on the positive electrode side is turned off in first converter  10 . 
       FIG. 8  shows a current path corresponding to the gate pattern in period E in  FIG. 2 . 
     Referring to  FIG. 8 , with turning-off of switching element Q 4 A, current in first converter  10  passes through a current path, not via DC power source PS 1 , through antiparallel diode  51  of first switching element Q 4 B on the negative electrode side and second switching element Q 3 B on the negative electrode side. At this moment, output voltage from DC power source PS 1  is not applied to first winding  3   a  of transformer  3 . On the other hand, in second converter  20  in which the two-leg off operation is applied, a current path via the antiparallel diodes of switching element Q 1 A and switching element Q 2 B in the off state is formed. As a result, energy of the excited first reactor  14  is transmitted toward battery PS 2  via transformer  3 . 
     When the circuit state in  FIG. 8  continues, current flowing toward battery PS 2  gradually decreases through the current path including antiparallel diodes  51  of switching element Q 1 A (off) and switching element Q 2 B (off) in second converter  20 . 
     Then, as shown in  FIG. 9 , second converter  20  reaches a state in which current disappears. The circuit state in  FIG. 9  is kept in periods F and G until period H in which the diagonal two elements (here, switching element Q 3 A and switching element Q 4 B) in first converter  10  are turned on again. 
       FIG. 10  shows a current path corresponding to the gate pattern in period H in  FIG. 2 . 
     Referring to  FIG. 10 , in period H, since second switching element Q 3 A on the positive electrode side is turned on, first reactor  14  is excited again by current through a path including second switching element Q 3 A on the positive electrode side and first switching element Q 4 B on the negative electrode side. After period H, the operation with a current direction opposite to that in periods C to G is repeated in first converter  10  and second converter  20 , and the circuit operation will not be further elaborated. 
     In this way, it can be understood that in the step-down charge operation in  FIG. 2 , battery PS 2  is charged without involving the step-up operation of second reactor  24 . 
     (Basic Control Behavior of Second Power Transmission) 
     Next, the circuit operation of second power transmission in which power is transmitted from battery PS 2  to DC power source PS 1 , that is, battery PS 2  is discharged will be described. The second power transmission also includes discharge of battery PS 2  not involving step-up operation of first reactor  14  (which hereinafter may be referred to as step-down discharge) and charge of battery PS 2  involving step-up operation of first reactor  14  (which may be referred to as step-up discharge). 
       FIG. 11  shows a time chart illustrating waveforms of on/off drive signals of the switching elements in step-down discharge of DC/DC converter  100  according to the first embodiment. In comparison,  FIG. 12  shows a time chart illustrating waveforms of on/off drive signals of the switching elements in step-down discharge in PTL 1 illustrated as a comparative example.  FIG. 13  shows a time chart illustrating waveforms of on/off drive signals of the switching elements in step-up discharge of DC/DC converter  100  according to the first embodiment. 
       FIG. 11  to  FIG. 13  also show waveform examples for explaining the principle of step-down charge and step-up charge, which strictly speaking do not agree with the control of a third phase shift amount θ 3  and a fourth phase shift amount θ 4  explained later. More specifically, also in  FIG. 11  to  FIG. 13 , for simplicity of explanation, switching period Tsw of third bridge circuit  43  is divided into ten periods, namely, periods A to J, and in each of periods A to J, a gate pattern that is a combination pattern of the on or off drive signals of the switching elements is set. 
     Referring to  FIG. 11 , in the step-down discharge operation that is the second power transmission not involving step-up operation, control circuit  30  alternatively turns on switching elements Q 1 A, Q 1 B, Q 1 A, and Q 1 B on the positive electrode side and the negative electrode side in third bridge circuit  43  and fourth bridge circuit  44 , once for each, at equal intervals in a preset switching period Tsw. On the other hand, in the step-down discharge operation, control circuit  30  keeps off first switching elements Q 4 A and Q 4 B on the positive electrode side and the negative electrode side of first bridge circuit  41  and second switching elements Q 3 A and Q 3 B on the positive electrode side and the negative electrode side of second bridge circuit  42 . In other words, the two-leg off operation is applied to first converter  10 . 
     Even in the second power transmission, the short-circuit prevention time td is applied and the switching elements on the positive electrode side and the negative electrode side are alternately turned on at equal intervals in bridge circuits  41  to  44 , in the same manner as the first power transmission ( FIG. 2  to  FIG. 4 ). 
     In the case of the step-down discharge operation in  FIG. 11 , control circuit  30  controls, as a third phase shift amount θ 3 , the phase shift amount of the on/off drive signal of second diagonal element QO 2  (fourth switching element Q 2 B on the negative electrode side) with respect to the on/off drive signal of second reference element QB 2  (third switching element Q 1 A on the positive electrode side). Control circuit  30  changes the third phase shift amount θ 3  based on the transmission power amount (in the present example, output DUTY ratio). 
     When the two-leg off operation is applied, the fourth phase shift amount θ 2  is not set, which is the phase shift amount of the on/off drive signal of first diagonal element QO 1  (second switching element Q 3 B on the negative electrode side) with respect to the on/off drive signal of second reference element QB 2  (third switching element Q 1 A on the positive electrode side). 
     Referring to  FIG. 12 , in the step-down discharge operation in PTL 1, a virtual phase shift amount is set in the same manner as described with reference to  FIG. 3 . Thus, for switching elements Q 1 A, Q 1 B, Q 2 A, and Q 2 B of second converter  20  that are turned on/off in the same manner as in  FIG. 11 , in first converter  10 , second switching elements Q 3 A and Q 3 B of second bridge circuit  42  are turned on/off in synchronization with fourth switching elements Q 2 A and Q 2 B in second converter  20 , in accordance with the virtually set fourth phase shift amount θ 4  (θ 4 =θ 3 ). 
     In second converter  20 , switching elements Q 4 A and Q 4 B of first bridge circuit  41  are kept off in the same manner as in  FIG. 11 . In other words, the one-leg off operation is applied to second converter  20 , in the same manner as first converter  10  in  FIG. 3 . 
     Referring to  FIG. 13 , in the step-up discharge operation, control circuit  30  controls, as the third phase shift amount θ 3 , the phase shift amount of the on/off drive signal of second diagonal element QO 2  (fourth switching element Q 2 B on the negative electrode side) with respect to the on/off drive signal of second reference element QB 2  (third switching element Q 1 A on the positive electrode side) and controls, as the fourth phase shift amount θ 4 , the phase shift amount of the on/off drive signal of first diagonal element QO 1  (second switching element Q 3 B on the negative electrode side) with respect to the on/off drive signal of second reference element QB 2 . 
     Then, control circuit  30  changes the third phase shift amount θ 3  and the fourth phase shift amount θ 4 , based on the transmission power amount (in the present example, output DUTY ratio). In  FIG. 13 , the third phase shift amount θ 3  and the fourth phase shift amount θ 4  are also phase shift amounts in the advance direction. In the case of the step-up discharge in  FIG. 13 , the fourth phase shift amount θ 4  is a value greater than the third phase shift amount θ 3 . 
     Here, referring to  FIG. 11  and  FIG. 13 , the operation at the time of discharge of battery PS 2  will be described in detail. In the on/off drive signals of the switching elements at the time of step-down discharge shown in  FIG. 11 , the on/off drive signals of first switching elements Q 4 A and Q 4 B and the on/off drive signals of third switching elements Q 1 A and Q 1 B at the time of step-down charge shown in  FIG. 2  are replaced with each other, and the on/off drive signals of second switching element Q 3 A and Q 3 B and the on/off drive signals of fourth switching elements Q 2 A and Q 2 B are replaced with each other. 
     Similarly, in the on/off drive signals of the switching elements at the time of step-up discharge shown in  FIG. 13 , the on/off drive signals of first switching elements Q 4 A and Q 4 B and the on/off drive signals of third switching elements Q 1 A and Q 1 B at the time of step-up charge shown in  FIG. 4  are replaced with each other, and the on/off drive signals of second switching element Q 3 A and Q 3 B and the on/off drive signals of fourth switching elements Q 2 A and Q 2 B are replaced with each other. 
     As shown in  FIG. 11  and  FIG. 13 , when a period in which second reference element QB 2  (third switching element Q 1 A on the positive electrode side) and second diagonal element QO 2  (fourth switching element Q 2 B on the negative electrode side) are simultaneously on is defined as a third diagonal ON time t 3 , the third diagonal ON time t 3  changes in accordance with the third phase shift amount θ 3 . Furthermore, a period in which third switching element Q 1 B on the negative electrode side and fourth switching element Q 2 A on the positive electrode side are simultaneously on (which may be referred to as third diagonal ON time t 3   a ) is equal to the third diagonal ON time t 3 . 
     Even in the step-down discharge in  FIG. 11 , switching elements Q 4 A and Q 4 B (first bridge circuit  41 ) and switching elements Q 3 A and Q 3 B (second bridge circuit  42 ) that are kept in the off state in  FIG. 11  may be virtually turned on/off, if necessary. Specifically, the on/off drive signals of third switching elements Q 1 A and Q 1 B of third bridge circuit  43  can be set as virtual on/off drive signals of first switching elements Q 4 A and Q 4 B of first bridge circuit  41 , if necessary. Similarly, the on/off drive signals of switching elements Q 2 A and Q 2 B of fourth bridge circuit  44  can be set as virtual on/off drive signals of switching elements Q 3 A and Q 3 B of second bridge circuit  42 , if necessary. 
     In this case, when a period in which the virtual on/off drive signal of first reference element QB 1  (first switching element Q 4 A on the positive electrode side) and the virtual on/off drive signal of first diagonal element QO 1  (second switching element Q 3 B on the negative electrode side) are simultaneously on is set as a fourth virtual diagonal ON time t 4 , the fourth virtual diagonal ON time t 4  changes in accordance with the virtually set fourth phase shift amount θ 4 . Furthermore, the fourth virtual diagonal ON time t 4   a  in which the virtual on/off drive signal of first switching element Q 4 B on the negative electrode side and the virtual on/off drive signal of second switching element Q 3 A on the positive electrode side are simultaneously on is also equal to the fourth virtual diagonal ON time t 4 . 
     Furthermore, in the step-up discharge in  FIG. 13 , the on/off drive signals of third switching elements Q 1 A and Q 1 B of third bridge circuit  43  can be set as virtual on/off drive signals of first switching elements Q 4 A and Q 4 B of first bridge circuit  41 . 
     Then, when a period in which the virtual on/off drive signal of first reference element QB 1  (first switching element Q 4 A on the positive electrode side) and the on/off drive signal of first diagonal element QO 1  (second switching element Q 3 B on the negative electrode side) are simultaneously on is set as a fourth virtual diagonal ON time t 4 , the fourth virtual diagonal ON time t 4  changes in accordance with the fourth phase shift amount θ 4 . Furthermore, the fourth virtual diagonal ON time t 4   a  in which the virtual on/off drive signal of first switching element Q 4 B on the negative electrode side and the on/off drive signal of second switching element Q 3 A on the positive electrode side are simultaneously on is also equal to the fourth virtual diagonal ON time t 4 . 
     In the step-up discharge and the step-down discharge in which the gate patterns shown in  FIG. 12  and  FIG. 14  are applied, the circuit operations of first converter  10  and second converter  20  are replaced with each other for the step-up charge and the step-down charge illustrated in  FIG. 5  to  FIG. 10 . The circuit operation in the step-up discharge and the step-down discharge therefore will not be further elaborated. 
     (Control of Phase Shift Amount Based on Power Transmission Amount) 
       FIG. 14  is a graph for explaining control of a phase shift amount based on the power transmission amount in DC/DC converter  100  according to the first embodiment. The horizontal axes in three graphs in  FIG. 14  show a power transmission amount P 1  [W] from first DC power source PS 1  to second DC power source (battery) PS 2  and a power transmission amount P 2  [W] from second DC power source (battery) PS 2  to first DC power source PS 1 , in common. On the horizontal axes in  FIG. 14 , the power transmission amount P 1  increases toward the right side, and the power transmission amount P 2  increases toward the left side. 
     For example, as shown in the top graph in  FIG. 14 , control circuit  30  calculates the output DUTY ratio based on a power transmission command value Pref. In  FIG. 14 , when the first power transmission (charge of battery PS 2 ) is performed, Pref=P 1  (command value) is set. In comparison, when the second power transmission (discharge of battery PS 2 ) is performed, Pref=−P 2  (command value) is set. In this way, control circuit  30  can calculate the output DUTY ratio so that it is proportional to power transmission command value Pref. 
     (Change of Phase Shift Amount in First Power Transmission) First, the case of the first power transmission (charge of battery PS 2 ) will be described in detail. As shown in the right half of the middle graph in  FIG. 14 , control circuit  30  performs the step-down charge operation when the power transmission amount P 1  is between 0 to a first reference value Pr 1  (Pr 1 &gt;0), in other words, the output DUTY ratio is 0 to a first reference value Dr 1  (Dr 1 &gt;0). 
     In the step-down charge operation, control circuit  30  decreases the first phase shift amount θ 1  as the power transmission amount P 1 , that is, the output DUTY ratio increases. Furthermore, the second phase shift amount θ 2  can be virtually set, if necessary, such that a change is made in the same amount as in the first phase shift amount θ 1 . 
     When the power transmission amount P 1  is greater than the first reference value Pr 1 , that is, when the output DUTY ratio is greater than the first reference value Dr 1 , control circuit  30  performs the step-up charge operation. At a switching point between step-down charge and step-up charge where Pref=Pr 1  (output DUTY ratio=Dr 1 ), the first phase shift amount θ 1  and the second phase shift amount θ 2  are equivalent. Hereinafter the first phase shift amount θ 1  and the second phase shift amount θ 2  at the switching point of P 1 =Pr 1  may be referred to as reference phase shift amount θr. 
     In the step-up charge operation, control circuit  30  further decreases the first phase shift amount θ 1  as the power transmission amount P 1 , that is, the output DUTY ratio increases from the switching point. In other words, in the entire region of Pref&gt;0, the first phase shift amount θ 1  continuously decreases with increase of the power transmission amount P 1  (output DUTY ratio). 
     On the other hand, in the step-up charge operation, control circuit  30  increases the second phase shift amount θ 2  from the switching point, with increase of the power transmission amount P 1  (output DUTY ratio). In this way, in step-up charge, as the power transmission amount P 1  (output DUTY ratio) increases, the first phase shift amount θ 1  is decreased while the second phase shift amount θ 2  is increased. 
     For example, the reference phase shift amount θr can be preset to correspond to the power transmission amount P 1  (output DUTY ratio) at which the first phase shift amount θ 1  and the second phase shift amount θ 2  are 25% of the switching period Tsw. 
     When the power transmission amount P 1  is in the range of 0≤P 1 ≤Pr 1 , control circuit  30  decreases the first phase shift amount θ 1  from the maximum amount to the reference phase shift amount θr (a phase shift amount corresponding to a time length of Tsw×0.25) at a constant slope. The maximum value is preset to a value (for example, a phase shift amount corresponding to a time length of Tsw×0.45) equal to or smaller than 50% of the switching period Tsw and greater than the reference phase shift amount θr (a phase shift amount corresponding to a time length of Tsw×0.25). The unit of phase shift amount is strictly speaking [rad], but the phase shift amount may be hereinafter denoted similarly using a time length corresponding to a multiple of the switching period Tsw. 
     On the other hand, when the power transmission amount P 1  is in the range of Pr 1 ≤P 1 ≤2× Pr 1 , control circuit  30  decreases the first phase shift amount θ 1  from the reference phase shift amount θr (25% of Tsw) to the minimum value (for example, Tsw×0.05) at the same slope as above. Furthermore, the second phase shift amount θ 2  is increased from the reference phase shift amount θr (Tsw×0.25) to the maximum amount (for example, Tsw×0.45) at the same first slope. 
     The right half of the bottom graph in  FIG. 14  shows changes of the first diagonal ON time t 1 , t 1   a  and the second virtual diagonal ON time t 2 , t 2   a  with respect to such changes of the first phase shift amount θ 1  and the second phase shift amount θ 2 . 
     As described above, the first diagonal ON time t 1 , t 1   a  is a value obtained by subtracting the first phase shift amount θ 1  from the ON period of first reference element QB 1 . Similarly, the second virtual diagonal ON time t 2 , t 2   a  is a value obtained by subtracting the second phase shift amount θ 2  from the ON period of first reference element QB 1 . Therefore, in  FIG. 14 , the behavior of the first diagonal ON time t 1 , t 1   a  and the second virtual diagonal ON time t 2 , t 2   a  has an upside-down graph waveform of the behavior of the first phase shift amount θ 1  and the second phase shift amount θ 2 . 
     Here, in the first power transmission (charge of battery PS 2 ), an output voltage from DC power source PS 1  is applied to first winding  3   a  of transformer  3 , and power transmission from first winding  3   a  to second winding  3   b  brings about a period in which voltage is produced on second winding  3   b . This period is both of the first diagonal ON time t 1  in which first reference element QB 1  (first switching element Q 4 A on the positive electrode side) and first diagonal element QO 1  (second switching element Q 3 B on the negative electrode side) simultaneously turn on and the first diagonal ON time t 1   a  in which first switching element Q 4 B on the negative electrode side and second switching element Q 3 A on the positive electrode side simultaneously turn on. 
     At the time of step-down charge, the power transmission amount is controlled by adjusting the first phase shift amount θ 1  of first converter  10  to adjust the first diagonal ON time t 1 , t 1   a . Furthermore, second converter  20  operates as a diode bridge and performs rectifying operation through the two-leg off operation that brings third bridge circuit  43  and fourth bridge circuit  44  into the off state both on the positive electrode side and the negative electrode side. The range of change of the first phase shift amount θ 1  at the time of step-down charge is the range from the maximum value to the reference phase shift amount θr (25% of Tsw). 
     On the other hand, in the step-down charge in PTL 1, as shown in  FIG. 3 , a virtual shift amount is generated such that the second phase shift amount θ 2  is the same amount as the first phase shift amount θ 1 , and the one-leg off operation is performed. Thus, second converter  20  matches the second virtual diagonal ON time t 2 , t 2   a  with the first diagonal ON time t 1 , t 1   a  by on/off of switching elements Q 2 A and Q 2 B and performs synchronous rectifying operation. 
     Thus, in the step-down charge in PTL 1, there is concern that a circulating current path as described below is produced in second converter  20  in which the one-leg off operation is performed in a period in which power transmission actually does not occur and both first converter  10  and second converter  20  output zero voltage. 
     As for “a period in which zero voltage is output” described above, in first converter  10 , each of a period in which a current path including both of switching element Q 3 A or its antiparallel diode  51  and switching element Q 4 A or its antiparallel diode  51  is formed and a period in which a current path including both of switching element Q 3 B or its antiparallel diode  51  and switching element Q 4 B or its antiparallel diode  51  is formed may be hereinafter referred to as zero voltage period of first converter  10 . 
     Similarly, in second converter  20 , each of a period in which a current path including both of switching element Q 1 A or its antiparallel diode  51  and switching Q 2 A or its antiparallel diode  51  is formed and a period in which a current path including both of switching element Q 1 B or its antiparallel diode  51  and switching Q 2 B or its antiparallel diode  51  is formed may be referred to as zero voltage period of second converter  20 . 
       FIG. 15  and  FIG. 16  show a circulating current path which may be produced in a zero voltage period in step-down charge (that is, step-down charge in PTL 1) according to  FIG. 3 . 
     Referring to  FIG. 15 , in period A in  FIG. 3 , in first converter  10 , since switching element Q 4 A is in the on state while switching element Q 3 B is in the off state, a current path CP 1  including switching element Q 4 A and antiparallel diode  51  of switching element Q 3 A is formed (that is, zero voltage period). Furthermore, also in second converter  20 , since switching element Q 2 A is brought to the on state, a current path CP 2  including switching element Q 2 A and antiparallel diode  51  of switching element Q 1 A is formed (that is, zero voltage period). 
     As a result, a circulating current path including first converter  10  and second converter  20  may be produced through transformer  3  by current paths CP 1  and CP 2  in a period in which power transmission actually does not occur. 
     Similarly, referring to  FIG. 16 , in period F in  FIG. 3 , in first converter  10 , since switching element Q 4 B is in the on state while switching element Q 3 A is in the off state, a current path CP 1  including switching element Q 4 B and antiparallel diode  51  of switching element Q 3 B is formed (that is, zero voltage period). Furthermore, also in second converter  20 , since switching element Q 2 B is brought to the on state, a current path CP 2  including switching element Q 2 B and antiparallel diode  51  of switching element Q 1 B is formed (that is, zero voltage period). 
     As a result, also in  FIG. 16 , a circulating current path including first converter  10  and second converter  20  may be produced through transformer  3  by current paths CP 1  and CP 2  in a period in which power transmission actually does not occur. 
     In comparison, in step-down charge of DC/DC converter  100  according to the first embodiment, the first phase shift amount θ 1  is gradually decreased with increase of the output DUTY ratio as described above, whereby the first diagonal ON time t 1 , t 1   a  in first converter  10  is gradually increased while second converter  20  performs rectifying operation as a diode bridge through the two-leg off operation. That is, in second converter  20 , all of third switching elements Q 1 A and Q 1 B of third bridge circuit  43  and fourth switching elements Q 2 A and Q 2  of fourth bridge circuit  44  are in the off state. 
       FIG. 17  and  FIG. 18  show a current path in the zero voltage period of the first converter in step-down charge of DC/DC converter  100  according to the first embodiment. 
       FIG. 17  shows a current path in period A in  FIG. 2 . In first converter  10 , current path CP 1  similar to that in  FIG. 15  is formed, whereas in second converter  20 , switching element Q 2 A is in the off state due to the two-leg off operation and therefore current path CP 2  in  FIG. 15  is not formed. As a result, in DC/DC converter  100 , occurrence of the circulating current path as shown in  FIG. 15  can be avoided. 
     Similarly,  FIG. 18  shows a current path in period F in  FIG. 2 . In first converter  10 , current path CP 1  similar to that in  FIG. 16  is formed, whereas in second converter  20 , switching element Q 2 B is in the off state due to the two-leg off operation and therefore current path CP 2  in  FIG. 16  is not formed. As a result, in DC/DC converter  100 , occurrence of the circulating current path as shown in  FIG. 16  can be avoided. 
     In this way, in step-down charge of DC/DC converter  100  according to the first embodiment, conduction loss due to circulating current between first converter  10  and second converter  20  as in PTL 1 can be avoided. 
     Furthermore, in the gate pattern in  FIG. 3 , switching loss involved with on/off of switching elements Q 2 A and Q 2 B on the positive electrode side and the negative electrode side of fourth bridge circuit  44  occurs in second converter  20 . On the other hand, in DC/DC converter  100  according to the first embodiment, all of switching elements Q 3 A, Q 3 , Q 4 A, and Q 4 B are kept in the off state through the two-leg off operation of second converter  20 . Therefore, switching loss in second converter  20  does not occur. 
     In this way, in step-down charge of DC/DC converter  100  according to the first embodiment, compared with step-down charge in PTL 1, conduction loss and switching loss can be reduced. Thus, the power conversion efficiency can be improved in step-down charge with a small power transmission amount. 
     Furthermore, in DC/DC converter  100  according to the first embodiment, power can be quickly adjusted at the time of switching between step-down charge and step-up charge as will be described below. 
     Referring to  FIG. 2  again, in period C of step-down charge, which is immediately after second switching element Q 3 B on the negative electrode side (first diagonal element QO 1 ) of first converter  10  turns on, first switching element Q 4 A on the positive electrode side (first reference element QB 1 ) and second switching element Q 3 B on the negative electrode side (first diagonal element QO 1 ) are simultaneously on, and the diagonal two elements become electrically continuous. Therefore, as described with reference to  FIG. 7 , energy is transmitted from the DC power source PS 1  to first reactor  14  through first switching element Q 4 A on the positive electrode side and second switching element Q 3 B on the negative electrode side to excite first reactor  14 . 
     On the other hand, in period C in  FIG. 2 , switching elements Q 1 A, Q 1 B, Q 2 A, and Q 2 B of second converter  20  are in the off state. Therefore, as described with reference to  FIG. 7 , power is transmitted from first winding  3   a  to second winding  3   b  to generate voltage on second winding  3   b , whereby power is transmitted from DC power source PS 1  to battery PS 2  through a current path including antiparallel diode  51  of third switching element Q 1 A on the positive electrode side and antiparallel diode  51  of fourth switching element Q 2 B on the negative electrode side in second converter  20 . At this moment, second reactor  24  is not excited, and step-up operation does not occur. 
       FIG. 19  is a time chart illustrating waveforms of on/off drive signals of the switching elements when the phase difference between the first phase shift amount θ 1  and the second phase shift amount is small in DC/DC converter  100  according to the first embodiment. For example, the gate pattern in  FIG. 19  occurs at the time of switching from step-down charge to step-up charge. 
       FIG. 19  shows a gate pattern when the first phase shift amount θ 1  decreases from the reference phase shift amount θr (θr=Tsw×0.25) by Tsw×0.05 and conversely, the second phase shift amount θ 2  increases from the reference phase shift amount θr by Tsw×0.05 in a region in which power transmission amount P 1 &gt;Pr 1  and step-up charge is applied in the graph in  FIG. 14 . 
     Therefore, as shown in  FIG. 19 , the phase difference between the first phase shift amount θ 1  and the second phase shift amount θ 2  is thus 10% of the switching period Tsw and equal to the short-circuit prevention time td. 
     Referring to  FIG. 19 , in period C, which is immediately after second switching element Q 3 B on the negative electrode side (first diagonal element QO 1 ) of first converter  10  turns on, first switching element Q 4 A on the positive electrode side (first reference element QB 1 ) and second switching element Q 3 B on the negative electrode side (first diagonal element QO 1 ) simultaneously turn on, and the diagonal two elements become electrically continuous. Thus, energy is transmitted from DC power source PS 1  to first reactor  14  through first switching element Q 4 A on the positive electrode side and second switching element Q 3 B on the negative electrode side to excite first reactor  14 . 
     On the other hand, in  FIG. 19 , since the phase difference between the first phase shift amount θ 1  and the second phase shift amount θ 2  is equal to the short-circuit prevention time td, in second converter  20 , period C is set as the short-circuit prevention time td for fourth switching elements Q 2 A and Q 2 B and fourth switching element Q 2 A on the positive electrode side is not turned on. 
     Therefore, in period C in  FIG. 19 , since a current path including antiparallel diode  51  of third switching element Q 1 A on the positive electrode side and antiparallel diode  51  of fourth switching element Q 2 B on the negative electrode side is formed in the same manner as at the time of step-down operation as described with reference to  FIG. 7 , power is transmitted from DC power source PS 1  to battery PS 2  without involving excitation of second reactor  24 . 
     In this way, even when power transmission amount P 1  is greater than first reference value Pr 1  and thus step-up charge is applied, step-up operation actually does not occur when the phase difference Δθ between the first phase shift amount θ 1  and the second phase shift amount θ 2  is equal to or smaller than the short-circuit prevention time td. In the middle graph in  FIG. 14 , at the time of switching from step-down charge to step-up charge, the phase difference Δθ is small and the gate pattern in  FIG. 19  is applied. 
       FIG. 20  is a time chart illustrating waveforms of on/off drive signals of the switching elements when the phase difference between the first phase shift amount θ 1  and the second phase shift amount θ 2  is greater than the short-circuit prevention time td. 
       FIG. 20  shows a gate pattern when the power transmission amount P 1  is greater than that in  FIG. 19 , the first phase shift amount θ 1  decreases from the reference phase shift amount θr (θr=Tsw×0.25) by Tsw×0.15, and conversely the second phase shift amount θ 2  increases from the reference phase shift amount θr by Tsw×0.15. Therefore, the phase difference Δθ between the first phase shift amount θ 1  and the second phase shift amount θ 2  is 30% of the switching period Tsw, three times as large as the short-circuit prevention time td. 
     In period B in  FIG. 20 , first switching element Q 4 A on the positive electrode side (first reference element QB 1 ) and second switching element Q 3 B on the negative electrode side (first diagonal element QO 1 ) of first converter  10  are simultaneously on, and the diagonal two elements become electrically continuous. Thus, power is transmitted from DC power source PS 1  to first reactor  14  to excite first reactor  14 , in the same manner as described with reference to  FIG. 5 . 
     If the phase difference Δθ is large, fourth switching element Q 2 A on the positive electrode side of second converter  20  turns on in this period B. Therefore, current on a path including fourth switching element Q 2 A on the positive electrode side and antiparallel diode  51  of third switching element Q 1 A on the positive electrode side circulates to second reactor  24  to excite second reactor  24 , in the same manner as described with reference to  FIG. 5 . Therefore, in period B, first reactor  14  and second reactor  24  are excited whereby step-up operation of second reactor  24  occurs. 
     Since the state in period C in  FIG. 20  is the same as in period B, the excitation of first reactor  14  and second reactor  24  continues. In period D, since first converter  10  is in the same state as in periods B and C, the excitation of first reactor  14  continues. 
     On the other hand, in period D, since the short-circuit prevention time td applies in second converter  20 , fourth switching element Q 2 A on the positive electrode side turns off. Thus, current flows toward battery PS 2  through antiparallel diode  51  of third switching element Q 1 A on the positive electrode side and antiparallel diode  51  of fourth switching element Q 2 B on the negative electrode side, in the same manner as described with reference to  FIG. 5 . 
     As a result, in period D, excitation energy of first reactor  14  and second reactor  24  is transmitted toward battery PS 2 . Accordingly, in the gate pattern shown in  FIG. 20 , charge of battery PS 2  actually involving step-up operation of second reactor  24 , that is, step-up charge is performed. 
     In this way, the step-up operation of second reactor  24  is performed actually in a period obtained by subtracting the short-circuit prevention time td from the phase difference Δθ between the first phase shift amount θ 1  and the second phase shift amount θ 2 . That is, in the gate pattern in  FIG. 19 , since the power transmission amount P 1  is greater than first reference value Pr 1 , step-up charge is applied. However, the phase difference Δθ between the first phase shift amount θ 1  and the second phase shift amount θ 2  does not increase to an extent exceeding the short-circuit prevention time td, and therefore actually step-up operation does not occur. Therefore, the power transmission amount in  FIG. 19  is equal to the power transmission amount in step-down charge in  FIG. 2 . 
     In this case, it can be determined whether step-up operation is involved by comparison of the phase difference Δθ between the first phase shift amount θ 1  and the second phase shift amount θ 2  set according to  FIG. 14  with the short-circuit prevention time td. In this way, when step-up operation is not involved because Δθ≤td, the first phase shift amount θ 1  is operated to allow second converter  20  to perform two-leg off operation. When second converter  20  shifts to the one-leg off operation involving step-up operation, the phase difference Δθ is made equal to the short-circuit prevention time td at the switching point of P 1 =Pr 1 , and the first phase shift amount θ 1  and the second phase shift amount θ 2  can be set such that the phase difference Δθ increases as the power transmission amount P 1  increases. For example, the second phase shift amount θ 2  can be set such that the phase difference Δθ from the first phase shift amount θ 1  (that is, the reference phase shift amount θr) at the switching point is equivalent to the short-circuit prevention time td (corresponding to  FIG. 17 ). Furthermore, the first phase shift amount θ 1  and the second phase shift amount θ 2  can be decreased or increased from the respective values at the switching point such that the phase difference Δθ increases as the power transmission amount P 1  increases from the switching point. 
     If the two-leg off operation does not shift to the one-leg off operation but the mode of two-leg off operation is shifted to a mode of allowing all the legs to perform switching operation, the rectifying function of antiparallel diode  51  in two-leg off operation need to be simulated by active switching operation by control circuit  30  at the moment of switching. This is likely to cause a difference in transmission power amount. In comparison, when second converter  20  shifts from two-leg off operation to one-leg off operation as in DC/DC converter  100  according to the first embodiment, the rectifying function of antiparallel diode  51  can be used as it is. Therefore, as shown in the gate pattern in  FIG. 3 , allowing second converter  20  to perform one-leg off operation in step-up charge enables smooth switching from step-down charge to step-up charge and facilitates control of the transmission power amount. 
     However, the circuit operation according to  FIG. 2  (step-down charge) and the circuit operation according to  FIG. 19  (step-up charge) are different in whether a circuiting current path is produced. Therefore, in practice, there is a possibility that a difference occurs in power transmission amount between them because circulating current produced by the effect of excitation current or circuit parasitic capacitance influences the power transmission amount. 
     (Change of Phase Shift Amount in Second Power Transmission) 
     Next, the case of the second power transmission (discharge of battery PS 2 ) will be described in detail. As shown in  FIG. 1 , the circuit configuration of DC/DC converter  100  is symmetric with respect to transformer  3 . Because of this circuit symmetry, the control operation of DC/DC converter  100  is symmetric between the first power transmission and the second power transmission in  FIG. 14 . 
     As shown by the left half of the top graph in  FIG. 14 , in the case of the second power transmission, the output DUTY ratio increases in the negative direction as the power transmission amount P 2  increases. In other words, the power transmission amount P 2  and the output DUTY ratio are reverse in sign. 
     When the power transmission amount P 2  is in the range of 0 to a second reference value Pr 2  (Pr 2 &gt;0), in other words, when the output DUTY ratio is in the range of 0 to second reference value Dr 2  (Dr 2 &lt;0), control circuit  30  performs the step-down discharge operation. 
     In the step-down discharge operation, control circuit  30  decreases the third phase shift amount θ 3  as the power transmission amount P 2  increases, that is, the output DUTY ratio increases in the negative direction. Furthermore, the fourth phase shift amount θ 4  may be virtually set, if necessary, such that a change in the same amount as in the third phase shift amount θ 3  is made. 
     When the power transmission amount P 2  is greater than the second reference value Pr 2 , that is, when the output DUTY ratio is greater than the second reference value Dr 2  in the negative direction, control circuit  30  performs the step-up discharge operation. At a switching point between step-down charge and step-up charge where Pref=−P 2  (output DUTY ratio=Dr 2 ), the first phase shift amount θ 1  and the second phase shift amount θ 2  are equivalent. 
     In the step-up discharge operation, control circuit  30  further decreases the third phase shift amount θ 3  as the power transmission amount P 2  increases from the switching point, that is, as the output DUTY ratio increases in the negative direction. In other words, in the entire region of Pref&lt;0, the third phase shift amount θ 3  continuously decreases with increase of the power transmission amount P 2  (increase of the output DUTY ratio in the negative direction). 
     On the other hand, in the step-up discharge operation, control circuit  30  increases the fourth phase shift amount θ 4  with increase of the power transmission amount P 2  (increase of the output DUTY ratio in the negative direction) from the switching point. In this way, in step-up discharge, as the power transmission amount P 2  increases (increase of the output DUTY ratio in the negative direction), the third phase shift amount θ 3  is decreased while the fourth phase shift amount θ 4  is increased. 
     For example, the reference phase shift amount θr corresponding to the second reference value Pr 2  can be preset to correspond to the power transmission amount P 1  (output DUTY ratio) at which the first phase shift amount θ 1  and the second phase shift amount θ 2  are 25% of the switching period Tsw, in the same manner as in the first power transmission. 
     When the power transmission amount P 2  is in the range of 0≤P 2 ≤Pr 2 , control circuit  30  decreases the first phase shift amount θ 1  from the maximum amount to the reference phase shift amount θr (Tsw×0.25) at a constant slope common to the first power transmission. On the other hand, when the power transmission amount P 2  is in the range of Pr 2 ≤P 2 ≤2×Pr 2 , control circuit  30  decreases the third phase shift amount θ 1  from the reference phase shift amount θr (25% of Tsw) to the minimum value at the slope above and increases the second phase shift amount θ 2  from the reference phase shift amount θr (Tsw×0.25) to the maximum value at the same slope. The maximum value and the minimum value are set in common with the first power transmission. 
     The left half of the bottom graph in  FIG. 14  shows changes of the third diagonal ON time t 3 , t 3   a  and the fourth virtual diagonal ON time t 4 , t 4   a  with respect to such changes of the third phase shift amount θ 3  and the fourth phase shift amount θ 4 . 
     As described above, the third diagonal ON time t 3 , t 3   a  is a value obtained by subtracting the third phase shift amount θ 3  from the ON period of second reference element QB 2 . Similarly, the fourth virtual diagonal ON time t 4 , t 4   a  is a value obtained by subtracting the fourth phase shift amount θ 4  from the ON period of second reference element QB 2 . Therefore, in  FIG. 14 , the behavior of the third diagonal ON time t 3 , t 3   a  and the fourth virtual diagonal ON time t 4 , t 4   a  has an upside-down graph waveform of the behavior of the third phase shift amount θ 3  and the fourth phase shift amount θ 4 . 
     In  FIG. 14 , both of the first phase shift amount θ 1  at the time of charge and the fourth phase shift amount θ 4  at the time of discharge correspond to the phase shift amount of first diagonal element QO 1  (second switching element Q 3 B on the negative electrode side) and are depicted by similar solid lines. 
     Furthermore, both of the second phase shift amount θ 2  at the time of charge and the third phase shift amount θ 3  at the time of discharge correspond to the phase shift amount of second diagonal element QO 2  (fourth switching element Q 2 B on the negative electrode side) and are depicted by similar dotted lines. Similarly, the first diagonal ON time t 1  and the fourth virtual diagonal ON time t 4  are depicted by similar solid lines, and the second virtual diagonal ON time t 2  and the third diagonal ON time t 3  are depicted by similar dotted lines. 
       FIG. 21  is a time chart illustrating waveforms of on/off drive signals of the switching elements in step-down discharge of DC/DC converter  100  according to the first embodiment. 
     Referring to  FIG. 21 , in the step-down discharge operation, conversely to step-down charge ( FIG. 2 ), second converter  20  is the power transmitting side and first converter  10  is the power receiving side. Therefore, third switching elements Q 1 A and Q 1 B of second converter  20  are turned on/off in the same manner as first switching elements Q 4 A and Q 4 B of first converter  10  in  FIG. 2  (step-down discharge). Similarly, fourth switching elements Q 2 A and Q 2 B of second converter  20  are turned on/off in the same manner as second switching elements Q 3 A and Q 3 B of first converter  10  in  FIG. 2  (step-down discharge). 
     Furthermore, first converter  10  on the power-receiving side performs two-leg off operation, in the same manner as first converter  10  in  FIG. 2  (step-down discharge). That is, first switching elements Q 4 A and Q 4 B of first bridge circuit  41  and second switching elements Q 3 A and Q 3 B of second bridge circuit  42  are kept in the off state. 
     Thus, even in step-down discharge, occurrence of a current path in first converter  10  on the power-receiving side can be avoided, in the same manner as second converter  20  in  FIG. 17  and  FIG. 18 . Thus, even in step-down charge of DC/DC converter  100  according to the first embodiment, occurrence of a circulating current path between first converter  10  and second converter  20  through transformer  3  as in PTL 1 can be suppressed. As a result, conduction loss due to circulating current and switching loss in first converter  10  can be reduced, thereby improving the power conversion efficiency in step-down discharge with a small power transmission amount. 
     Next, switching between step-down discharge and step-up discharge in DC/DC converter  100  according to the first embodiment will be described. 
       FIG. 22  is a time chart illustrating waveforms of on/off drive signals of the switching elements at the time of step-up discharge, corresponding to  FIG. 19  at the time of step-up charge. 
       FIG. 22  shows a gate pattern when the third phase shift amount θ 3  decreases from the reference phase shift amount θr (θr=Tsw×0.25) by Tsw×0.05 and conversely, the fourth phase shift amount θ 4  increases from the reference phase shift amount θr by Tsw×0.05, in a region in which the power transmission amount P 2 &gt;Pr 2  and step-up discharge is applied. As a result, the phase difference between the third phase shift amount θ 3  and the fourth phase shift amount θ 4  is 10% of the switching period Tsw and equal to the short-circuit prevention time td. 
     In the on/off drive signals of the switching elements at the time of step-up discharge shown in  FIG. 22 , the on/off drive signals of first switching elements Q 4 A and Q 4 B and the on/off drive signals of third switching elements Q 1 A and Q 1 B at the time of step-up charge shown in  FIG. 19  are replaced with each other, and the on/off drive signals of second switching element Q 3 A and Q 3 B and the on/off drive signals of fourth switching elements Q 2 A and Q 2 B are replaced with each other. 
     Therefore, the circuit operation in the gate pattern in  FIG. 22  is similar to the circuit operation in the gate pattern in  FIG. 19  and has the power transmission direction reversed. That is,  FIG. 22  shows a gate pattern in step-up discharge in which the power transmission amount P 2  is greater than the second reference value Pr 2 . However, the phase difference Δθ between the third phase shift amount θ 3  and the fourth phase shift amount θ 4  is equal to or smaller than the short-circuit prevention time td, and therefore, actually step-up operation of first reactor  14  does not occur. 
       FIG. 23  is a time chart illustrating waveforms of the drive signals of the switching elements at the time of step-up discharge, corresponding to  FIG. 20  at the time of step-up charge. 
       FIG. 23  shows a gate pattern when the power transmission amount P 2  is greater than that in  FIG. 22 , the third phase shift amount θ 3  decreases from the reference phase shift amount θr (θr=Tsw×0.25) by Tsw×0.15, and conversely the fourth phase shift amount θ 4  increases from the reference phase shift amount θr by Tsw×0.15. At this moment, the phase difference Δθ between the third phase shift amount θ 3  and the fourth phase shift amount θ 4  is 30% of switching period Tsw, three times as large as the short-circuit prevention time td. 
     In the on/off drive signals of the switching elements at the time of step-up discharge shown in  FIG. 23 , the on/off drive signals of first switching elements Q 4 A and Q 4 B and the on/off drive signals of third switching elements Q 1 A and Q 1 B at the time of step-up charge shown in  FIG. 20  are replaced with each other, and the on/off drive signals of second switching elements Q 3 A and Q 3 B and the on/off drive signals of fourth switching elements Q 2 A and Q 2 B are replaced with each other. 
     Therefore, the circuit operation in the gate pattern in  FIG. 23  is similar to the circuit operation in the gate pattern in  FIG. 20  and has the power transmission direction reversed. That is, in  FIG. 23 , the phase difference Δθ between the third phase shift amount θ 3  and the fourth phase shift amount θ 4  increases to an extent exceeding the short-circuit prevention time td, and step-up operation of first reactor  14  occurs. In this way, in step-up discharge, the step-up operation of first reactor  14  is actually performed in a period obtained by subtracting the short-circuit prevention time td from the phase difference Δθ between the third phase shift amount θ 3  and the fourth phase shift amount θ 4 . 
     Therefore, setting the third phase shift amount θ 3  and the fourth phase shift amount θ 4  in consideration of the short-circuit prevention time td in the same manner as the step-down charge described above enables smooth switching from step-down discharge to step-up discharge and facilitates control of the transmission power amount. 
     Specifically, when step-up operation is not involved because Δθ≤td, the third phase shift amount θ 3  is operated to allow first converter  10  to perform two-leg off operation. In addition, when first converter  10  shifts to one-leg off operation involving step-up operation, the phase difference Δθ is made equal to the short-circuit prevention time td at the switching point of P 2 =Pr 2 . For example, the fourth phase shift amount θ 4  can be set such that the phase difference Δθ from the third phase shift amount θ 3  (that is, the reference phase shift amount θr) at the switching point is equivalent to the short-circuit prevention time td (corresponding to  FIG. 20 ). Furthermore, the third phase shift amount θ 3  and the fourth phase shift amount θ 4  can be decreased or increased from the respective values at the switching point such that the phase difference Δθ increases as the power transmission amount P 2  increases from the switching point. In this way, allowing first converter  10  to perform one-leg off operation in step-up discharge enables smooth switching operation even from step-down discharge to step-up discharge and facilitates control of the power transmission amount. 
     As described above, in DC/DC converter  100  according to the present first embodiment, the power receiving-side converters of first converter  10  and second converter  20  perform two-leg off operation in step-down operation (step-down charge and step-down discharge), whereby occurrence of circulating current in first converter  10  and second converter  20  described with reference to  FIG. 15  and  FIG. 16  can be avoided, and switching loss can be suppressed in the power receiving-side converters, thereby improving the power conversion efficiency. Furthermore, in step-up operation (step-up charge and step-up discharge), the power receiving-side converters of first converter  10  and second converter  20  perform one-leg off operation, whereby switching from step-down operation to step-up operation can be smoothened. 
     Second Embodiment 
     A DC/DC converter according to a second embodiment will now be described. The DC/DC converter according to the second embodiment is similar to that of the first embodiment in circuit configuration and basic control but differs from the first embodiment in control of the phase shift amount based on the power transmission amount. In the second embodiment, a description of parts similar to those in the first embodiment is basically not repeated. 
       FIG. 24  is a graph for explaining control of the phase shift amount based on the power transmission amount in the DC/DC converter according to the second embodiment. 
     Referring to  FIG. 24 , the top graph is the same as that of  FIG. 14 , whereas the middle graph differs from that of  FIG. 14 . 
     First, the case of the first power transmission (charge of battery PS 2 ) will be described in detail. As shown in the right half of the middle graph in  FIG. 22 , control circuit  30  decreases the first phase shift amount θ 1  as the power transmission amount P 1  (output DUTY ratio) increases when the power transmission amount P 1  is in the range of 0 to first reference value Pr 1  (Pr 1 &gt;0), in other words, when the output DUTY ratio is in the range of 0 to first reference value Dr 1  (Dr 1 &gt;0). Furthermore, the second phase shift amount θ 2  can be virtually set, if necessary, such that a change in the same amount as in the first phase shift amount θ 1  is made. As described above, at the switching point of P 1 =Pr 1 , a phase difference equivalent to the short-circuit prevention time td may be provided between the first phase shift amount θ 1  and the second phase shift amount θ 2 . 
     When the power transmission amount P 1  (output DUTY ratio) is between the first reference value Pr 1  and the third reference value Pr 3  (Pr 3 &gt;Pr 1 ), control circuit  30  decreases the first phase shift amount θ 1  and increases the second phase shift amount θ 2 , with respect to the first phase shift amount θ 1  and the second phase shift amount θ 2  (reference phase shift amount θr) where P 1 =Pr 1 , as the power transmission amount P 1  (output DUTY ratio) increases. 
     When the power transmission amount P 1  (output DUTY ratio) is greater than the third reference value Pr 3  (Pr 3 &gt;Pr 1 ), control circuit  30  increases the second phase shift amount θ 2  with respect to the second phase shift amount θ 2  when P 1 =Pr 3 , as the power transmission amount P 1  (output DUTY ratio) increases. On the other hand, in the range of P 1 &gt;Pr 3 , control circuit  30  keeps the first phase shift amount θ 1  when P 1 =Pr 3 . 
     Even in the DC/DC converter according to the second embodiment, in the same manner as the first embodiment, the range in which the power transmission amount P 1  is from 0 to the first reference value Pr 1  is a section in which step-down charge is performed, and the range in which the power transmission amount P 1  is greater than the first reference value Pr 1  is a section in which step-up charge is performed. 
     In the second embodiment, the reference phase shift amount θr corresponding to the first phase shift amount θ 1  when P 1 =Pr 1  is preset to a value smaller than that in the first embodiment (for example, 20% of switching period Tsw). Furthermore, the third reference value Pr 3  is preset to equivalent to the power transmission amount P 1  (output DUTY ratio) when the first phase shift amount θ 1  is 5% of the switching period Tsw. 
     When the power transmission amount P 1  (output DUTY ratio) is between 0 and the first reference value Pr 1 , control circuit  30  decreases the first phase shift amount θ 1  from the maximum value (for example, Tsw×0.45 in common to the first embodiment) to the reference phase shift amount θr (for example, Tsw×0.2) at a constant slope. Furthermore, the virtually set second phase shift amount θ 2  is decreased in the same amount as in the first phase shift amount θ 1 , if necessary. 
     When the power transmission amount P 1  (output DUTY ratio) is between the first reference value Pr 1  and the third reference value Pr 3 , control circuit  30  decreases the first phase shift amount θ 1  from the first phase shift amount θ 1  at P 1 =Pr 1  to the minimum value (for example, Tsw×0.05 in common to the first embodiment) at the same constant slope as above. On the other hand, the second phase shift amount θ 2  is increased from the second phase shift amount θ 2  at P 1 =Pr 1  at the same slope as above. When the power transmission amount P 1  (output DUTY ratio) is between the third reference value Pr 3  and the value twice the first reference value Pr 1 , control circuit  30  fixes the first phase shift amount θ 1  to the minimum value and continuously increases the second phase shift amount θ 2  up to the maximum value while keeping the same slope. 
     As shown in the right half of the bottom graph in  FIG. 24 , the first diagonal ON time t 1 , t 1   a  and the second virtual diagonal ON time t 2 , t 2   a  have an upside-down shape of the first phase shift amount θ 1  and the second phase shift amount θ 2 . 
     Next, the case of the second power transmission (discharge of battery PS 2 ) will be described in detail. As shown in the left half of the middle graph in  FIG. 24 , control circuit  30  decreases the third phase shift amount θ 3  as the power transmission amount P 2  increases (the output DUTY ratio increases in the negative direction) when the power transmission amount P 2  is between 0 and the second reference value Pr 2  (Pr 2 &gt;0), in other words, the output DUTY ratio is between 0 and the second reference value Dr 2  (Dr 2 &lt;0). Furthermore, the fourth phase shift amount θ 4  can be virtually set, if necessary, such that a change in the same amount as in the third phase shift amount θ 3  is made. As described above, at the switching point of P 2 =Pr 2 , a phase difference equivalent to the short-circuit prevention time td may be provided between the third phase shift amount θ 3  and the fourth phase shift amount θ 4 . 
     When the power transmission amount P 2  is between the second reference value Pr 2  and the fourth reference value Pr 4  (Pr 4 &gt;Pr 2 ), control circuit  30  decreases the third phase shift amount θ 3  and increases the fourth phase shift amount θ 4 , with respect to the third phase shift amount θ 3  and the fourth phase shift amount θ 4  where P 2 =Pr 2 , as the power transmission amount P 2  increases (the output DUTY ratio increases in the negative direction). 
     When the power transmission amount P 2  is greater than the fourth reference value Pr 4  (Pr 4 &gt;Pr 2 ), that is, when the output DUTY ratio is greater than the second reference value Dr 2  in the negative direction, control circuit  30  increases the fourth phase shift amount θ 4  with respect to the fourth phase shift amount θ 4  when P 2 =Pr 4 , with increase of the power transmission amount P 2  (increase of the output DUTY ratio in the negative direction). On the other hand, in the range of P 2 ≥Pr 4 , control circuit  30  keeps the third phase shift amount θ 3  when P 2 =Pr 4 . 
     Even in the DC/DC converter according to the second embodiment, in the same manner as the first embodiment, the range in which the power transmission amount P 2  is from 0 to the second reference value Pr 2  is a range in which step-down discharge is performed, and the range in which the power transmission amount P 2  is greater than the second reference value Pr 2  is a range in which step-up discharge is performed. 
     In the second embodiment, the reference phase shift amount θr corresponding to the third phase shift amount θ 3  when P 2 =Pr 2  is preset to a value common to charge operation. Furthermore, the fourth reference value Pr 4  is preset to equivalent to the power transmission amount P 2  (output DUTY ratio) when the first phase shift amount θ 3  is 5% of the switching period Tsw. 
     When the power transmission amount P 2  is between 0 and the second reference value Pr 2 , control circuit  30  decreases the third phase shift amount θ 3  from the maximum value to the reference phase shift amount θr (for example, Tsw×0.2) at a constant slope. 
     When the power transmission amount P 2  is between the second reference value Pr 2  and the fourth reference value Pr 4 , control circuit  30  decreases the third phase shift amount θ 3  from the reference phase shift amount θr (Tsw×0.2) to the minimum value at the same constant slope as above. On the other hand, the fourth phase shift amount θ 4  is increased from the fourth phase shift amount θ 4  at P 2 =Pr 1  at the same slope as above. When the power transmission amount P 1  (output DUTY ratio) is between the fourth reference value Pr 4  and the value twice the second reference value Pr 2 , control circuit  30  fixes the third phase shift amount θ 3  to the minimum value and continuously increases the fourth phase shift amount θ 4  up to the maximum value while keeping the same slope. 
     As shown in the left half of the bottom graph in  FIG. 24 , the third diagonal ON time t 3 , t 3   a  and the fourth virtual diagonal ON time t 4 , t 4   a  have an upside-down shape of the third phase shift amount θ 3  and the fourth phase shift amount θ 4 . 
     In the DC/DC converter according to the second embodiment, compared with the first embodiment, the range of step-down charge or step-down discharge (the range of the power transmission amount P 1 , P 2  or the output DUTY ratio) is expanded. Thus, the effect of improving the power conversion efficiency at the time of step-down operation described in the first embodiment can be enhanced. 
     In  FIG. 14  and  FIG. 24 , a simple example in which the output DUTY ratio is set in a proportional relation to the command values of the power transmission amounts P 1  and P 2  has been described. However, the output DUTY ratio may be calculated by feedback control of the detected values of current and voltage in the same manner as in PTL 1. 
       FIG. 25  is a block diagram for explaining a first modification of calculation of the output DUTY ratio by control circuit  30 . 
     Referring to  FIG. 25 , control circuit  30  includes a subtractor  31  and a control calculator  32 . Subtractor  31  subtracts a current detection value i of battery PS 2  from a current command value i* of battery PS 2  to calculate a current deviation Δi. Current command value i* can be set based on the power transmission amount P 1  or P 2  between first DC power source PS 1  and second DC power source PS 2 . Current command value i* is set to a negative value (i*&lt;0) at the time of charge of battery PS 2  (first power transmission) and is set to a positive value (i*&gt;0) at the time of discharge (second power transmission). 
     Control calculator  32  calculates an output DUTY ratio by proportional integral (PI) control calculation of current deviation Δi. By doing so, feedback control to change the output DUTY ratio can be performed such that charge/discharge current (current i) approaches the current command value i* in charge (first power transmission) or discharge (second power transmission) of battery PS 2 . 
       FIG. 26  is a block diagram for explaining a second modification of calculation of the output DUTY ratio by control circuit  30 . 
     Referring to  FIG. 26 , control circuit  30  includes subtractors  33  and  35  and control calculators  34  and  36 . Subtractor  33  subtracts a voltage detection value v of DC power source PS 1  from a voltage command value v* of DC power source PS 1  to calculate a voltage deviation Δv. The voltage command value v* can be set based on the power transmission amount P 1  or P 2 . 
     Control calculator  34  calculates a current command value i* of battery PS 2  by proportional integral (PI) control calculation of the voltage deviation Δv. Furthermore, subtractor  35  subtracts the current detection value i of battery PS 2  from the current command value i* from control calculator  34  to calculate a current deviation Δi. Control calculator  36  calculates an output DUTY ratio by proportional integral (PI) control calculation of the current deviation Δi. 
     Thus, feedback control to change the output DUTY ratio can be performed such that the output voltage v of DC power source PS 1  approaches the voltage command value v* set based on the power transmission amounts P 1  and P 2 . Alternatively, the output DUTY ratio may be directly calculated by proportional integral (PI) control calculation for the voltage deviation Δv. 
     In the present embodiment, the output DUTY ratio as an intermediate variable can be calculated by any calculation formula as long as the object of controlling the power transmission amount by the first power transmission or the second power transmission is met. 
     Third Embodiment 
     In a third embodiment, a configuration example of a power conversion device including a plurality of DC/DC converters in the first embodiment or the second embodiment will be described. 
       FIG. 27  is a block diagram illustrating a first configuration example of the power conversion device according to the third embodiment. 
     Referring to  FIG. 27 , a power conversion device  110  according to the first example of the third embodiment includes DC/DC converters  101  and  102  connected in parallel. In the third embodiment, each of DC/DC converters  101  and  102  is configured with DC/DC converter  100  according to the first or second embodiment. 
     In power conversion device  110 , in DC/DC converters  101  and  102  connected in parallel, first positive electrode wires  11  ( FIG. 1 ) are connected in common to a power supply terminal N 11 , and first negative electrode wires  12  ( FIG. 1 ) are connected in common to a power supply terminal N 12 . Power supply terminal N 11  is electrically connected to the positive electrode of first DC power source PS 1 , and power supply terminal N 12  is electrically connected to the negative electrode of first DC power source PS 1 . 
     Similarly, in DC/DC converters  101  and  102  connected in parallel, second positive electrode wires  21  ( FIG. 1 ) are connected in common to a power supply terminal N 21 , and second negative electrode wires  22  ( FIG. 1 ) are connected in common to a power supply terminal N 22 . Power supply terminal N 21  is electrically connected to the positive electrode of second DC power source PS 2 , and power supply terminal N 22  is electrically connected to the negative electrode of first DC power source PS 2 . 
     In power conversion device  110  in the first configuration example, power can be transmitted bidirectionally between first DC power source PS 1  and second DC power source PS 2  using DC/DC converters  101  and  102  ( 100 ) connected in parallel. This configuration facilitates application to large power transmission. 
       FIG. 28  is a block diagram illustrating a second configuration example of the power conversion device according to the third embodiment. 
     Referring to  FIG. 28 , a power conversion device  120  according to the second example of the third embodiment includes DC/DC converters  101  and  102  connected in series parallel. Power supply terminal N 21  is electrically connected to the positive electrode of second DC power source PS 2 , and power supply terminal N 22  is electrically connected to the negative electrode of first DC power source PS 2 . 
     First positive electrode wires  11  ( FIG. 1 ) of DC/DC converters  101  and  102  are connected in common to power supply terminal N 11 , and first negative electrode wires  12  ( FIG. 1 ) are connected in common to power supply terminal N 12 . That is, DC/DC converters  101  and  102  are connected in parallel on the first DC power source side. 
     On the other hand, second positive electrode wire  21  of DC/DC converter  101  is connected to power supply terminal N 21  electrically connected to the positive electrode of second DC power source PS 2 . Second negative electrode wire  22  of DC/DC converter  102  is connected to power supply terminal N 22  electrically connected to the positive electrode of second DC power source PS 2 . Furthermore, second positive electrode wire  21  of DC/DC converter  102  is connected to second negative electrode wire  22  of DC/DC converter  102 . That is, DC/DC converters  101  and  102  are connected in series on the second DC power source side. 
     In power conversion device  110  in the second configuration example, power can be transmitted bidirectionally between first DC power source PS 1  and second DC power source PS 2  using DC/DC converters  101  and  102  ( 100 ) connected in series parallel. This configuration facilitates application to power transmission between DC power sources with different voltages. In the configuration in  FIG. 28 , the connections may be replaced with each other such that the first DC power source sides are connected in series and the second DC power source sides are connected in parallel. 
       FIG. 29  is a block diagram illustrating a third configuration example of the power conversion device according to the third embodiment. 
     Referring to  FIG. 29 , a power conversion device  130  according to the third example of the third embodiment includes DC/DC converters  101  and  102 . 
     In power conversion device  130 , in DC/DC converter  101 , first positive electrode wire  11  ( FIG. 1 ) is connected to a power supply terminal N 11   a , and first negative electrode wire  12  ( FIG. 1 ) is connected to a power supply terminal N 12   a . In DC/DC converter  101 , first positive electrode wire  11  ( FIG. 1 ) is connected to a power supply terminal N 11   b , and first negative electrode wire  12  ( FIG. 1 ) is connected to a power supply terminal N 12   b . Separate first DC power sources PS 1  are connected to power supply terminals N 11   a  and N 11   b  and to power supply terminals N 12   a  and N 12   b.    
     On the other hand, second positive electrode wires  21  ( FIG. 1 ) of DC/DC converters  101  and  102  are connected to power supply terminal N 21  electrically connected to the positive electrode of second DC power source PS 2 . Similarly, second negative electrode wires  22  ( FIG. 1 ) of DC/DC converters  101  and  102  are connected to power supply terminal N 22  electrically connected to the negative electrode of second DC power source PS 2 . 
     In power conversion device  110  in the third configuration example, power can be transmitted bidirectionally between first DC power sources PS 1  and second DC power source PS 2  which are different in number. In the configuration of  FIG. 29 , the respective numbers of first DC power sources PS 1  and second DC power sources PS 2  between which power transmission is performed can be set as desired. 
     In the third embodiment, control circuit  30  of DC/DC converters  101  and  102  may be configured in common using one controller, or separate controllers may be arranged individually for DC/DC converters  100  and communication may be performed between the controllers to perform drive control. 
     In the power conversion device according to the third embodiment, a plurality of DC/DC converters  100  according to the first or second embodiment are arranged and connected in parallel or in series to one or more first DC power source(s) PS 1  and second DC power source(s) PS 2 . In particular, by taking advantage of improvement in power conversion efficiency in a region with a small power transmission amount in DC/DC converter  100 , steady power conversion efficiency can be improved in power conversion devices  110  to  130  as a whole by applying control such as adjusting burden of the power transmission amount among a plurality of DC/DC converters  100  or stopping power transmission operation in some of DC/DC converters  100  as appropriate. 
     OTHER EMBODIMENTS 
     Finally, other embodiments of the present invention will be described. The configurations of the embodiments described below are not necessarily applied singly and may be applied in combination with a configuration of another embodiment as long as there is no discrepancy. 
     (1) In the foregoing embodiments, first switching element Q 4 A on the positive electrode side of first bridge circuit  41  is defined as “first reference element QB 1 ”, second switching element Q 3 B on the negative electrode side of second bridge circuit  42  is defined as “first diagonal element QO 1 ”, third switching element Q 1 A on the positive electrode side of third bridge circuit  43  is defined as “second reference element QB 2 ”, and fourth switching element Q 2 B on the negative electrode side of fourth bridge circuit  44  is defined as “second diagonal element QO 2 ”, as a typical example. 
     However, the embodiments of the present invention are not limited thereto. For example, first switching element Q 4 B on the negative electrode side of first bridge circuit  41  may be defined as “first reference element QB 1 ”, second switching element Q 3 A on the positive electrode side of second bridge circuit  42  may be defined as “first diagonal element QO 1 ”, third switching element Q 1 B on the negative electrode side of third bridge circuit  43  may be defined as “second reference element QB 2 ”, and fourth switching element Q 2 A on the positive electrode side of fourth bridge circuit  44  may be defined as “second diagonal element QO 2 ”. 
     (2) In the foregoing embodiments, in first converter  10  in  FIG. 1 , the bridge circuit on the left side is first bridge circuit  41  in which first reference element QB 1  is set, and the bridge circuit on the right side is second bridge circuit  42  in which first diagonal element QO 1  is set, and in second converter  20  in  FIG. 1 , the bridge circuit on the right side is third bridge circuit  43  in which second reference element QB 2  is set, and the bridge circuit on the left side is fourth bridge circuit  44  in which second diagonal element QO 2  is set, as a typical example. 
     However, the embodiments of the present invention are not limited thereto. For example, in first converter  10  in  FIG. 1 , the bridge circuit on the right side may be first bridge circuit  41  in which first reference element QB 1  is set, and the bridge circuit on the left side may be second bridge circuit  42  in which first diagonal element QO 1  is set, and in second converter  20  in  FIG. 1 , the bridge circuit on the left side may be third bridge circuit  43  in which second reference element QB 2  is set, and the bridge circuit on the right side may be fourth bridge circuit  44  in which second diagonal element QO 2  is set. 
     (3) In the foregoing embodiments, second DC power source PS 2  is a battery, by way of example. However, the embodiments of the present invention are not limited thereto. That is, each of first DC power source PS 1  and second DC power source PS 2  may be configured with any DC power source. The DC power source may be configured with a battery as described above, or a power storage element such as a large-capacity capacitor, a power supply device that converts AC power from an AC power source such as a commercial system into DC power, a rotating machine (DC motor) having the functions of a power generator and an electric motor in combination, or a unit having the rotating machine (AC motor) and an inverter (AC/DC converter) in combination. 
     (4) In the foregoing embodiments, in the diagrams such as  FIG. 2  illustrating the temporal waveforms of drive signals of the switching elements, the switching period Tsw is divided into ten periods, namely, periods A to J, and a gate pattern that is a combination pattern of the on or off drive signals of the switching elements is set in each of periods A to J, by way of example. The short-circuit prevention time td is equivalent to one period that is each of ten equal parts of the switching period, as a typical example. 
     However, the embodiments of the present invention are not limited thereto, and the switching period Tsw may be divided into any number of parts. Alternatively, the switching period Tsw is not necessarily divided into a plurality of periods, and the phase shift amounts θ 1  to θ 4  may be continuously changed. The short-circuit prevention time td can be set to any time length in a range that can avoid a simultaneous on state of the positive electrode-side switching elements and the negative electrode-side switching elements. 
     (5) In the first embodiment, the first reference value Pr 1  is preset to correspond to the first power transmission amount P 1  when the first phase shift amount θ 1  and the second phase shift amount θ 2  are 25% of the switching period Tsw, and the second reference value Pr 2  is preset to correspond to the second power transmission amount P 2  when the third phase shift amount θ 3  and the fourth phase shift amount θ 4  are 25% of the switching period Tsw, by way of example. 
     In the second embodiment, the first reference value Pr 1  is preset to correspond to the first power transmission amount P 1  when the first phase shift amount θ 1  and the second phase shift amount θ 2  are a preset value smaller than 25% of the switching period Tsw, and the second reference value Pr 2  is preset to correspond to the second power transmission amount P 2  when the third phase shift amount θ 3  and the fourth phase shift amount θ 4  are a preset value smaller than 25% of the switching period Tsw, as a typical example. However, the embodiments of the present invention are not limited thereto. That is, the first reference value Pr 1  can be set to correspond to the first power transmission amount P 1  when the first phase shift amount θ 1  and the second phase shift amount θ 2  are any predetermined α (%) from 0% to 50% of the switching period Tsw. Similarly, the second reference value Pr 2  can be set to correspond to the second power transmission amount P 2  when the third phase shift amount θ 3  and the fourth phase shift amount θ 4  are any predetermined β (%) from 0% to 50% of the switching period Tsw. Furthermore, for the first reference value Pr 1  and the second reference value Pr 2 , α and β may be the same value or may be different values. 
     (6) In the foregoing embodiments, the first to fourth phase shift amounts θ 1  to θ 4  increase or decrease at the same slope, with respect to increase or decrease of the power transmission amount (output DUTY ratio), as a typical example. However, the embodiments of the present invention are not limited thereto. That is, the slope at which each of the first to fourth phase shift amounts θ 1  to θ 4  changes with respect to change of the power transmission amount (output DUTY ratio) may vary in accordance with a range of the power transmission amount (output DUTY ratio). In step-up charge, the first phase shift amount θ 1  and the second phase shift amount θ 2  may increase or decrease at different slopes. Similarly, in step-up discharge, the third phase shift amount θ 3  and the fourth phase shift amount θ 4  may increase or decrease at different slopes. 
     It should be noted that, for a plurality of embodiments described above, any combinations that are not referred to in the description as well as any appropriate combinations of the configurations described in the embodiments in a range that does not cause inconsistency or contradiction are initially intended at the time of filing. 
     Embodiments disclosed here should be understood as being illustrative rather than being limitative in all respects. The scope of the present invention is shown not in the foregoing description but in the claims, and it is intended that all modifications that come within the meaning and range of equivalence to the claims are embraced here. 
     REFERENCE SIGNS LIST 
       3  transformer,  3   a  first winding,  3   b  second winding,  10  first converter,  11  first positive electrode wire,  12  first negative electrode wire,  13  first smoothing capacitor,  14  first reactor,  20  second converter,  21  second positive electrode wire,  22  second negative electrode wire,  23  second smoothing capacitor,  24  second reactor,  25  reactor (current detection),  30  control circuit,  31 ,  33 ,  35  subtractor,  31   a ,  31   b  drive signal,  32 ,  34 ,  36  control calculator,  41  first bridge circuit,  42  second bridge circuit,  43  third bridge circuit,  44  fourth bridge circuit,  51  antiparallel diode,  52  parallel capacitor,  100  to  102  DC/DC converter,  110 ,  120 ,  130  power conversion device, Dr 1  first reference value (output DUTY ratio), Dr 2  second reference value (output DUTY ratio), N 11 , N 11   a , N 11   b , N 12 , N 12   a , N 12   b , N 21 , N 22  power supply terminal, P 1  first power transmission amount, P 2  second power transmission amount, PS 1  first DC power source, PS 2  second DC power source (battery), Pr 1  first reference value (power transmission amount), Pr 2  second reference value (power transmission amount), Pr 3  third reference value (power transmission amount), Pr 4  fourth reference value (power transmission amount), Pref power transmission command value, Q 1 A to Q 4 A, Q 1 B to Q 4 A semiconductor switching element, QB 1  first reference element, QB 2  second reference element, QO 1  first diagonal element, QO 2  second diagonal element, Tsw switching period, t 1   a , t 1  first diagonal ON time, t 2 , t 2   a  second virtual diagonal ON time, t 3   a , t 3  third diagonal ON time, t 4 , t 4   a  fourth virtual diagonal ON time, td short-circuit prevention time.