Patent Publication Number: US-9425698-B2

Title: Power conversion apparatus and method

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2013-265638 filed on Dec. 24, 2013 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a technique for performing a power conversion between a primary side full bridge circuit and a secondary side full bridge circuit. 
     2. Description of Related Art 
     A power conversion apparatus that converts power between a primary side full bridge circuit and a secondary side full bridge circuit is known (see, for example, Japanese Patent Application Publication No. 2011-193713 (JP 2011-193713)). 
     However, if characteristics of respective switching elements constituting the full bridge circuits (for example, electrical characteristics such as current characteristics) vary between the respective switching elements, there is a possibility that a power conversion efficiency between the primary side full bridge circuit and the secondary side full bridge circuit decreases. 
     SUMMARY OF THE INVENTION 
     Therefore, there is provided a power conversion apparatus and a power conversion method, which can improve a power conversion efficiency between a primary side full bridge circuit and a secondary side full bridge circuit. 
     According to a first aspect of the invention, there is provided a power conversion apparatus including: a primary side full bridge circuit having a first arm circuit and a second arm circuit in parallel, a secondary side full bridge circuit having a third arm circuit and a fourth arm circuit in parallel, and a control unit configured to adjust a first phase difference between switching of the first arm circuit and switching of the third arm circuit and a second phase difference between switching of the second arm circuit and switching of the fourth arm circuit to control transmitted power that is transmitted between the primary side full bridge circuit and the secondary side full bridge circuit, wherein the control unit configured to cause, among the first phase difference and the second phase difference, the phase difference between the arm circuits having a higher efficiency to be larger than the phase difference between the arm circuits having a lower efficiency. 
     According to the aspect, the power conversion efficiency between the primary side full bridge circuit and the secondary side full bridge circuit is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a diagram showing an example of a configuration of a power conversion apparatus; 
         FIG. 2  is a block diagram showing an example of a configuration of a control unit; 
         FIG. 3  is a timing chart showing an example of switching operations of a primary side circuit and a secondary side circuit; 
         FIG. 4  is a flowchart showing an example of a power conversion method; 
         FIG. 5  is an example of a map for calculating a standard phase difference φ; 
         FIG. 6  is a flowchart showing an example of a power conversion method; and 
         FIG. 7  is a diagram showing an example of the configuration of the power conversion apparatus. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     &lt;Configuration of Power Supply Apparatus  101 &gt; 
       FIG. 1  is a block diagram showing an example of a configuration of a power supply apparatus  101  serving as an first embodiment of a power conversion apparatus. For example, the power supply apparatus  101  is a power supply system that includes a power supply circuit  10 , a control unit  50 , and a sensor unit  70 . For example, the power supply apparatus  101  is a system that is mounted on a vehicle such as an automobile, and distributes power to various loads of the vehicle. A hybrid vehicle, a plug-in hybrid vehicle, an electric vehicle, and so on may be cited as specific examples of this vehicle. The power supply apparatus  101  may also be mounted on a vehicle using an engine as a driving source. 
     For example, the power supply apparatus  101  includes, as primary side ports, a first input/output port  60   a  to which a primary side high voltage system load  61   a  is connected and a second input/output port  60   c  to which a primary side low voltage system load  61   c  and a primary side low voltage system power supply  62   c  are connected. The primary side low voltage system power supply  62   c  supplies power to the primary side low voltage system load  61   c , which is operated by an identical voltage system (a 12 V system, for example) to the primary side low voltage system power supply  62   c . Further, the primary side low voltage system power supply  62   c  supplies power stepped up by a primary side conversion circuit  20  provided in the power supply circuit  10  to the primary side high voltage system load  61   a , which is operated by a different voltage system (a higher 48 V system than the 12 V system, for example) to the primary side low voltage system power supply  62   c . A secondary battery such as a lead battery may be cited as a specific example of the primary side low voltage system power supply  62   c.    
     For example, the power supply apparatus  101  includes, as secondary side ports, a third input/output port  60   b  to which a secondary side high voltage system load  61   b  and a secondary side high voltage system power supply  62   b  are connected and a fourth input/output port  60   d  to which a secondary side low voltage system load  61   d  is connected. The secondary side high voltage system power supply  62   b  supplies power to the secondary side high voltage system load  61   b , which is operated by an identical voltage system (a higher 288 V system than the 12 V system and the 48 V system, for example) to the secondary side high voltage system power supply  62   b . Further, the secondary side high voltage system power supply  62   b  supplies power stepped down by a secondary side conversion circuit  30  provided in the power supply circuit  10  to the secondary side low voltage system load  61   d , which is operated by a different voltage system (a lower 72 V system than the 288 V system, for example) to the secondary side high voltage system power supply  62   b . A secondary battery such as a lithium ion battery may be cited as a specific example of the secondary side high voltage system power supply  62   b.    
     The power supply circuit  10  is a power conversion circuit that includes the four input/output ports described above and has functions for selecting two desired input/output ports from the four input/output ports and performing a power conversion between the two selected input/output ports. Further, the power supply apparatus  101  including the power supply circuit  10  may be an apparatus that includes a plurality of, at least three, input/output ports, and is capable of converting power between any two input/output ports from the plurality of, at least three, input/output ports. For example, the power supply circuit  10  may also be a circuit that has three input/output ports without the fourth input/output port  60   d.    
     Port powers Pa, Pc, Pb, Pd are input/output powers (input powers or output powers) of the first input/output port  60   a , the second input/output port  60   c , the third input/output port  60   b , and the fourth input/output port  60   d , respectively. Port voltages Va, Vc, Vb, Vd are input/output voltages (input voltages or output voltages) of the first input/output port  60   a , the second input/output port  60   c , the third input/output port  60   b , and the fourth input/output port  60   d , respectively. Port currents Ia, Ic, Ib, Id are input/output currents (input currents or output currents) of the first input/output port  60   a , the second input/output port  60   c , the third input/output port  60   b , and the fourth input/output port  60   d , respectively. 
     The power supply circuit  10  includes a capacitor C 1  provided in the first input/output port  60   a , a capacitor C 3  provided in the second input/output port  60   c , a capacitor C 2  provided in the third input/output port  60   b , and a capacitor C 4  provided in the fourth input/output port  60   d . Film capacitors, aluminum electrolytic capacitors, ceramic capacitors, polymer electrolytic capacitors, and so on may be cited as specific examples of the capacitors C 1 , C 2 , C 3 , C 4 . 
     The capacitor C 1  is inserted between a high potential side terminal  613  of the first input/output port  60   a  and a low potential side terminal  614  of the first input/output port  60   a  and the second input/output port  60   c . The capacitor C 3  is inserted between a high potential side terminal  616  of the second input/output port  60   c  and the low potential side terminal  614  of the first input/output port  60   a  and the second input/output port  60   c . The capacitor C 2  is inserted between a high potential side terminal  618  of the third input/output port  60   b  and a low potential side terminal  620  of the third input/output port  60   b  and the fourth input/output port  60   d . The capacitor C 4  is inserted between a high potential side terminal  622  of the fourth input/output port  60   d  and the low potential side terminal  620  of the third input/output port  60   b  and the fourth input/output port  60   d.    
     The capacitors C 1 , C 2 , C 3 , C 4  may be provided either inside or outside the power supply circuit  10 . 
     The power supply circuit  10  is a power conversion circuit configured to include the primary side conversion circuit  20  and the secondary side conversion circuit  30 . It should be noted that the primary side conversion circuit  20  and the secondary side conversion circuit  30  are connected via a primary side magnetic coupling reactor  204  and a secondary side magnetic coupling reactor  304 , and magnetically coupled by a transformer  400  (a center tapped transformer). The primary side ports configured of the first input/output port  60   a  and the second input/output port  60   c  are connected to the secondary side ports configured of the third input/output port  60   b  and the fourth input/output port  60   d  via the transformer  400 . 
     The primary side conversion circuit  20  is a primary side circuit configured to include a primary side full bridge circuit  200 , the first input/output port  60   a , and the second input/output port  60   c . The primary side full bridge circuit  200  is a primary side power conversion unit configured to include a primary side coil  202  of the transformer  400 , the primary side magnetic coupling reactor  204 , a primary side first upper arm U 1 , a primary side first lower arm/U 1 , a primary side second upper arm V 1 , and a primary side second lower arm /V 1 . Here, the primary side first upper arm U 1 , the primary side first lower arm/U 1 , the primary side second upper arm V 1 , and the primary side second lower arm /V 1  are constituted by switching elements respectively configured to include, for example, an N channel type metal oxide semiconductor field effect transistor (MOSFET) and a body diode serving as a parasitic element of the MOSFET. Additional diodes may be connected to the MOSFET in parallel. 
     The primary side full bridge circuit  200  includes a primary side positive electrode bus line  298  connected to the high potential side terminal  613  of the first input/output port  60   a , and a primary side negative electrode bus line  299  connected to the low potential side terminal  614  of the first input/output port  60   a  and the second input/output port  60   c.    
     A primary side first arm circuit  207  connecting the primary side first upper arm U 1  and the primary side first lower arm/U 1  in series is attached between the primary side positive electrode bus line  298  and the primary side negative electrode bus line  299 . The primary side first arm circuit  207  is a primary side first power conversion circuit unit (a primary side U phase power conversion circuit unit) capable of performing a power conversion operation by switching the primary side first upper arm U 1  and the primary side first lower arm/U 1  ON and OFF. Further, a primary side second arm circuit  211  connecting the primary side second upper arm V 1  and the primary side second lower arm /V 1  in series is attached between the primary side positive electrode bus line  298  and the primary side negative electrode bus line  299  in parallel with the primary side first arm circuit  207 . The primary side second arm circuit  211  is a primary side second power conversion circuit unit (a primary side V phase power conversion circuit unit) capable of performing a power conversion operation by switching the primary side second upper arm V 1  and the primary side second lower arm /V 1  ON and OFF. 
     The primary side coil  202  and the primary side magnetic coupling reactor  204  are provided in a bridge part connecting a midpoint  207   m  of the primary side first arm circuit  207  to a midpoint  211   m  of the primary side second arm circuit  211 . To describe connection relationships to the bridge part in more detail, one end of a primary side first reactor  204   a  of the primary side magnetic coupling reactor  204  is connected to the midpoint  207   m  of the primary side first arm circuit  207 , and one end of the primary side coil  202  is connected to another end of the primary side first reactor  204   a . Further, one end of a primary side second reactor  204   b  of the primary side magnetic coupling reactor  204  is connected to another end of the primary side coil  202 , and another end of the primary side second reactor  204   b  is connected to the midpoint  211   m  of the primary side second arm circuit  211 . Note that the primary side magnetic coupling reactor  204  is configured to include the primary side first reactor  204   a  and the primary side second reactor  204   b , which is magnetically coupled to the primary side first reactor  204   a  by a coupling coefficient k 1 . 
     The midpoint  207   m  is a primary side first intermediate node between the primary side first upper arm U 1  and the primary side first lower arm/U 1 , and the midpoint  211   m  is a primary side second intermediate node between the primary side second upper arm V 1  and the primary side second lower arm /V 1 . 
     The first input/output port  60   a  is a port provided between the primary side positive electrode bus line  298  and the primary side negative electrode bus line  299 . The first input/output port  60   a  is configured to include the terminal  613  and the terminal  614 . The second input/output port  60   c  is a port provided between the primary side negative electrode bus line  299  and a center tap  202   m  of the primary side coil  202 . The second input/output port  60   c  is configured to include the terminal  614  and the terminal  616 . 
     The center tap  202   m  is connected to the high potential side terminal  616  of the second input/output port  60   c . The center tap  202   m  is an intermediate connection point between a primary side first winding  202   a  and a primary side second winding  202   b  constituting the primary side coil  202 . 
     The secondary side conversion circuit  30  is a secondary side circuit configured to include a secondary side full bridge circuit  300 , the third input/output port  60   b , and the fourth input/output port  60   d . The secondary side full bridge circuit  300  is a secondary side power conversion unit configured to include a secondary side coil  302  of the transformer  400 , the secondary side magnetic coupling reactor  304 , a secondary side first upper arm U 2 , a secondary side first lower arm/U 2 , a secondary side second upper arm V 2 , and a secondary side second lower arm /V 2 . Here, the secondary side first upper arm U 2 , the secondary side first lower arm/U 2 , the secondary side second upper arm V 2 , and the secondary side second lower arm /V 2  are constituted by switching elements respectively configured to include, for example, an N channel type MOSFET and a body diode serving as a parasitic element of the MOSFET. Additional diodes may be connected to the MOSFET in parallel. 
     The secondary side full bridge circuit  300  includes a secondary side positive electrode bus line  398  connected to the high potential side terminal  618  of the third input/output port  60   b , and a secondary side negative electrode bus line  399  connected to the low potential side terminal  620  of the third input/output port  60   b  and the fourth input/output port  60   d.    
     A secondary side first arm circuit  307  connecting the secondary side first upper arm U 2  and the secondary side first lower arm/U 2  in series is attached between the secondary side positive electrode bus line  398  and the secondary side negative electrode bus line  399 . The secondary side first arm circuit  307  is a secondary side first power conversion circuit unit (a secondary side U phase power conversion circuit unit) capable of performing a power conversion operation by switching the secondary side first upper arm U 2  and the secondary side first lower arm/U 2  ON and OFF. Further, a secondary side second arm circuit  311  connecting the secondary side second upper arm V 2  and the secondary side second lower arm /V 2  in series is attached between the secondary side positive electrode bus line  398  and the secondary side negative electrode bus line  399  in parallel with the secondary side first arm circuit  307 . The secondary side second arm circuit  311  is a secondary side second power conversion circuit unit (a secondary side V phase power conversion circuit unit) capable of performing a power conversion operation by switching the secondary side second upper arm V 2  and the secondary side second lower arm /V 2  ON and OFF. 
     The secondary side coil  302  and the secondary side magnetic coupling reactor  304  are provided in a bridge part connecting a midpoint  307   m  of the secondary side first arm circuit  307  to a midpoint  311   m  of the secondary side second arm circuit  311 . To describe connection relationships to the bridge part in more detail, one end of a secondary side first reactor  304   a  of the secondary side magnetic coupling reactor  304  is connected to the midpoint  307   m  of the secondary side first arm circuit  307 , and one end of the secondary side coil  302  is connected to another end of the secondary side first reactor  304   a . Further, one end of a secondary side second reactor  304   b  of the secondary side magnetic coupling reactor  304  is connected to another end of the secondary side coil  302 , and another end of the secondary side second reactor  304   b  is connected to the midpoint  311   m  of the secondary side second arm circuit  311 . Note that the secondary side magnetic coupling reactor  304  is configured to include the secondary side first reactor  304   a  and the secondary side second reactor  304   b , which is magnetically coupled to the secondary side first reactor  304   a  by a coupling coefficient k 2 . 
     The midpoint  307   m  is a secondary side first intermediate node between the secondary side first upper arm U 2  and the secondary side first lower arm/U 2 , and the midpoint  311   m  is a secondary side second intermediate node between the secondary side second upper arm V 2  and the secondary side second lower arm /V 2 . 
     The third input/output port  60   b  is a port provided between the secondary side positive electrode bus line  398  and the secondary side negative electrode bus line  399 . The third input/output port  60   b  is configured to include the terminal  618  and the terminal  620 . The fourth input/output port  60   d  is a port provided between the secondary side negative electrode bus line  399  and a center tap  302   m  of the secondary side coil  302 . The fourth input/output port  60   d  is configured to include the terminal  620  and the terminal  622 . 
     The center tap  302   m  is connected to the high potential side terminal  622  of the fourth input/output port  60   d . The center tap  302   m  is an intermediate connection point between a secondary side first winding  302   a  and a secondary side second winding  302   b  constituting the secondary side coil  302 . 
     In  FIG. 1 , the power supply apparatus  101  includes the sensor unit  70 . The sensor unit  70  serves as detecting means that detects an input/output value Y of at least one of the first to fourth input/output ports  60   a ,  60   c ,  60   b ,  60   d  at predetermined detection period intervals and outputs a detected value Yd corresponding to the detected input/output value Y to the control unit  50 . The detected value Yd may be a detected voltage obtained by detecting the input/output voltage, a detected current obtained by detecting the input/output current, or a detected power obtained by detecting the input/output power. The sensor unit  70  may be provided either inside or outside the power supply circuit  10 . 
     The sensor unit  70  includes, for example, a voltage detection unit that detects the input/output voltage generated in at least one of the first to fourth input/output ports  60   a ,  60   c ,  60   b ,  60   d . For example, the sensor unit  70  includes a primary side voltage detection unit that outputs at least one detected voltage from among an input/output voltage Va and an input/output voltage Vc as a primary side voltage detection value, and a secondary side voltage detection unit that outputs at least one detected voltage from among an input/output voltage Vb and an input/output voltage Vd as a secondary side voltage detection value. 
     The voltage detection unit of the sensor unit  70  includes, for example, a voltage sensor that monitors an input/output voltage value of at least one port, and a voltage detection circuit that, outputs a detected voltage corresponding to the input/output voltage value monitored by the voltage sensor to the control unit  50 . 
     The sensor unit  70  includes, for example, a current detection unit that detects the input/output current flowing through at least one of the first to fourth input/output ports  60   a ,  60   c ,  60   b ,  60   d . For example, the sensor unit  70  includes a primary side current detection unit that outputs at least one detected current from among an input/output current Ia and an input/output current Ic as a primary side current detection value, and a secondary side current detection unit that outputs at least one detected current from among an input/output current Ib and an input/output current Id as a secondary side current detection value. 
     The current detection unit of the sensor unit  70  includes, for example, a current sensor that monitors an input/output current value of at least one port, and a current detection circuit that outputs a detected current corresponding to the input/output current value monitored by the current sensor to the control unit  50 . 
     The power supply apparatus  101  includes the control unit  50 . For example, the control unit  50  is an electronic circuit that includes a microcomputer having an inbuilt central processing unit (CPU). The control unit  50  may be provided either inside or outside the power supply circuit  10 . 
     The control unit  50  feedback-controls a power conversion operation performed by the power supply circuit  10  such that the detected value Yd of the input/output value Y of at least one of the first to fourth input/output ports  60   a ,  60   c ,  60   b ,  60   d  converges to a target value Yo set in the port. For example, the target value Yo is a command value set by the control unit  50  or a predetermined apparatus other than the control unit  50  on the basis of driving conditions defined in relation to the respective loads (the primary side low voltage system load  61   c  and so on, for example) connected to the input/output ports. The target value Yo functions as an output target value when power is output from the port and an input target value when power is input into the port, and may be a target voltage value, a target current value, or a target power value. 
     Further, the control unit  50  feedback-controls the power conversion operation performed by the power supply circuit  10  such that transmitted power P that is transmitted between the primary side conversion circuit  20  and the secondary side conversion circuit  30  via the transformer  400  converges to set target transmitted power Po. The transmitted power will also be referred to as a power transmission amount. The target transmitted power will also be referred to as a command transmitted power. 
     The control unit  50  feedback-controls the power conversion operation performed by the power supply circuit  10  by varying a value of a predetermined control parameter X, and is thus capable of adjusting the respective input/output values Y of the first to fourth input/output ports  60   a ,  60   c ,  60   b ,  60   d  of the power supply circuit  10 . Two control variables, namely a phase difference φ and a duty ratio D (an ON time δ) are used as the main control parameters X. 
     The phase difference φ is a deviation (a time lag) between switching timings of identical-phase power conversion circuit units of the primary side full bridge circuit  200  and the secondary side full bridge circuit  300 . The duty ratio D (the ON time δ) is a duty ratio (an ON time) between switching waveforms of the respective power conversion circuit units constituting the primary side full bridge circuit  200  and the secondary side full bridge circuit  300 . 
     The two control parameters X can be controlled independently of each other. The control unit  50  varies the input/output values Y of the respective input/output ports of the power supply circuit  10  by performing duty ratio control and/or phase control on the primary side full bridge circuit  200  and the secondary side full bridge circuit  300  using the phase difference φ and the duty ratio D (the ON time δ). 
       FIG. 2  is a block diagram of the control unit  50 . The control unit  50  is a control unit having a function for performing switching control on the respective switching elements of the primary side conversion circuit  20 , such as the primary side first upper arm U 1 , and the respective switching elements of the secondary side conversion circuit  30 , such as the secondary side first upper arm U 2 . The control unit  50  is configured to include a power conversion mode determination processing unit  502 , a phase difference φ determination processing unit  504 , an ON time δ determination processing unit  506 , a primary side switching processing unit  508 , and a secondary side switching processing unit  510 . For example, the control unit  50  is an electronic circuit that includes a microcomputer having an inbuilt CPU. 
     For example, the power conversion mode determination processing unit  502  selects and sets an operating mode from among power conversion modes A to L of the power supply circuit  10 , to be described below, on the basis of a predetermined external signal (for example, a signal indicating the deviation between the detected value Yd and the target value Yo in one of the ports). As regards the power conversion modes, in mode A, power input from the first input/output port  60   a  is converted and output to the second input/output port  60   c . In mode B, power input from the first input/output port  60   a  is converted and output to the third input/output port  60   b . In mode C, power input from the first input/output port  60   a  is converted and output to the fourth input/output port  60   d.    
     In mode D, power input from the second input/output port  60   c  is converted and output to the first input/output port  60   a . In mode E, power input from the second input/output port  60   c  is converted and output to the third input/output port  60   b . In mode F, power input from the second input/output port  60   c  is converted and output to the fourth input/output port  60   d.    
     In mode G, power input from the third input/output port  60   b  is converted and output to the first input/output port  60   a . In mode H, power input from the third input/output port  60   b  is converted and output to the second input/output port  60   c . In mode I, power input from the third input/output port  60   b  is converted and output to the fourth input/output port  60   d.    
     In mode J, power input from the fourth input/output port  60   d  is converted and output to the first input/output port  60   a . In mode K, power input from the fourth input/output port  60   d  is converted and output to the second input/output port  60   c . In mode L, power input from the fourth input/output port  60   d  is converted and output to the third input/output port  60   b.    
     The phase difference φ determination processing unit  504  has a function for setting a phase difference φ between switching period motions of the switching elements between the primary side conversion circuit  20  and the secondary side conversion circuit  30  in order to cause the power supply circuit  10  to function as a direct current-direct current (DC-DC) converter circuit. 
     The ON time δ determination processing unit  506  has a function for setting an ON time δ of the switching elements of the primary side conversion circuit  20  and the secondary side conversion circuit  30  in order to cause the primary side conversion circuit  20  and the secondary side conversion circuit  30  to function respectively as step-up/step-down circuits. 
     The primary side switching processing unit  508  has a function for performing switching control on the respective switching elements constituted by the primary side first upper arm U 1 , the primary side first lower arm/U 1 , the primary side second upper arm V 1 , and the primary side second lower arm /V 1 , on the basis of outputs of the power conversion mode determination processing unit  502 , the phase difference φ determination processing unit  504 , and the ON time δ determination processing unit  506 . 
     The secondary side switching processing unit  510  has a function for performing switching control on the respective switching elements constituted by the secondary side first upper arm U 2 , the secondary side first lower arm/U 2 , the secondary side second upper arm V 2 , and the secondary side second lower arm /V 2 , on the basis of the outputs of the power conversion mode determination processing unit  502 , the phase difference φ determination processing unit  504 , and the ON time δ determination processing unit  506 . 
     &lt;Operation of Power Supply Apparatus  101 &gt; 
     An operation of the power supply apparatus  101  having the above configuration will now be described using  FIG. 1  and  FIG. 2 . When, for example, an external signal requesting an operation in which the power conversion mode of the power supply circuit  10  is set at mode F is input, the power conversion mode determination processing unit  502  of the control unit  50  sets the power conversion mode of the power supply circuit  10  to mode F. At this time, a voltage input into the second input/output port  60   c  is stepped up by a step-up function of the primary side conversion circuit  20 , whereupon power having the stepped-up voltage is transmitted to the third input/output port  60   b  side by a DC-DC converter circuit function of the power supply circuit  10 , stepped down by a step-down function of the secondary side conversion circuit  30 , and then output from the fourth input/output port  60   d.    
     Here, a step-up/step-down function of the primary side conversion circuit  20  will be described in detail. Focusing on the second input/output port  60   c  and the first input/output port  60   a , the terminal  616  of the second input/output port  60   c  is connected to the midpoint  207   m  of the primary side first arm circuit  207  via the primary side first winding  202   a  and the primary side first reactor  204   a  connected in series to the primary side first winding  202   a . Respective ends of the primary side first arm circuit  207  are connected to the first input/output port  60   a , and as a result, a step-up/step-down circuit is attached between the terminal  616  of the second input/output port  60   c  and the first input/output port  60   a.    
     The terminal  616  of the second input/output port  60   c  is also connected to the midpoint  211   m  of the primary side second arm circuit  211  via the primary side second winding  202   b  and the primary side second reactor  204   b  connected in series to the primary side second winding  202   b . Respective ends of the primary side second arm circuit  211  are connected to the first input/output port  60   a , and as a result, a step-up/step-down circuit is attached in parallel between the terminal  616  of the second input/output port  60   c  and the first input/output port  60   a . Note that since the secondary side conversion circuit  30  is a circuit having a substantially identical configuration to the primary side conversion circuit  20 , two step-up/step-down circuits are likewise connected in parallel between the terminal  622  of the fourth input/output port  60   d  and the third input/output port  60   b . Hence, the secondary side conversion circuit  30  has an identical step-up/step-down function to the primary side conversion circuit  20 . 
     Next, the function of the power supply circuit  10  as a DC-DC converter circuit will be described in detail. Focusing on the first input/output port  60   a  and the third input/output port  60   b , the primary side full bridge circuit  200  is connected to the first input/output port  60   a , and the secondary side full bridge circuit  300  is connected to the third input/output port  60   b . When the primary side coil  202  provided in the bridge part of the primary side full bridge circuit  200  and the secondary side coil  302  provided in the bridge part of the secondary side full bridge circuit  300  are magnetically coupled by a coupling coefficient k T , the transformer  400  functions as a center tapped transformer having a number of windings 1:N. Hence, by adjusting the phase difference φ between the switching period motions of the switching elements in the primary side full bridge circuit  200  and the secondary side full bridge circuit  300 , power input into the first input/output port  60   a  can be converted and transmitted to the third input/output port  60   b  or power input into the third input/output port  60   b  can be converted and transmitted to the first input/output port  60   a.    
       FIG. 3  is a view showing a timing chart of ON/OFF switching waveforms of the respective arms provided in the power supply circuit  10  resulting from control executed by the control unit  50 . In  FIG. 3 , U 1  is an ON/OFF waveform of the primary side first upper arm U 1 , V 1  is an ON/OFF waveform of the primary side second upper arm V 1 , U 2  is an ON/OFF waveform of the secondary side first upper arm U 2 , and V 2  is an ON/OFF waveform of the secondary side second upper arm V 2 . ON/OFF waveforms of the primary side first lower arm /U 1 , the primary side second lower arm /V 1 , the secondary side first lower arm /U 2 , and the secondary side second lower arm /V 2  are inverted waveforms (not shown) obtained by respectively inverting the ON/OFF waveforms of the primary side first-upper arm-U 1 , the primary side second upper arm V 1 , the secondary-side first upper arm U 2 , and the secondary side second upper arm V 2 . Note that dead time is preferably provided between the respective ON/OFF waveforms of the upper and lower arms to prevent a through current from flowing when both the upper and lower arms are switched ON. Further, in  FIG. 3 , a high level indicates an ON condition and a low level indicates an OFF condition. 
     Here, by modifying the respective ON times δ of U 1 , V 1 , U 2 , and V 2 , step-up/step-down ratios of the primary side conversion circuit  20  and the secondary side conversion circuit  30  can be modified. For example, by making the respective ON times δ of U 1 , V 1 , U 2 , and V 2  equal to each other, the step-up/step-down ratio of the primary side conversion circuit  20  can be made equal to the step-up/step-down ratio of the secondary side conversion circuit  30 . 
     The ON time δ determination processing unit  506  make the respective ON times δ of U 1 , V 1 , U 2 , and V 2  equal to each other (respective ON times δ=primary side ON time δ 11 =secondary side ON time δ 12 =time value α) so that the respective step-up/step-down ratios of the primary side conversion circuit  20  and the secondary side conversion circuit  30  are equal to each other. 
     The step-up/step-down ratio of the primary side conversion circuit  20  is determined by the duty ratio D, which is a proportion of a switching period T of the switching elements (arms) constituting the primary side full bridge circuit  200  occupied by the ON time δ. Similarly, the step-up/step-down ratio of the secondary side conversion circuit  30  is determined by the duty ratio D, which is a proportion of the switching period T of the switching elements (arms) constituting the secondary side full bridge circuit  300  occupied by the ON time δ. The step-up/step-down ratio of the primary side conversion circuit  20  is a transformation ratio between the first input/output port  60   a  and the second input/output port  60   c , while the step-up/step-down ratio of the secondary side conversion circuit  30  is a transformation ratio between the third input/output port  60   b  and the fourth input/output port  60   d.    
     Therefore, for example, the step-up/step-down ratio of the primary side conversion circuit  20 =the voltage of the second input/output port  60   c /the voltage of the first input/output port  60   a =δ 11 /T=α/T, and the step-up/step-down ratio of the secondary side conversion circuit  30 =the voltage of the fourth input/output port  60   d /the voltage of the third input/output port  60   b =δ 12 /T=α/T. In other words, the respective step-up/step-down ratios of the primary side conversion circuit  20  and the secondary side conversion circuit  30  take identical values (=α/T). 
     Note that the ON time δ in  FIG. 3  represents both the ON time δ 11  of the primary side first upper arm U 11  and the primary side second upper arm V 1  and the ON time δ 12  of the secondary side first upper arm U 2  and the secondary side second upper arm V 2 . Further, the switching period T of the arms constituting the primary side full bridge circuit  200  and the switching period T of the arms constituting the secondary side full bridge circuit  300  are equal times. 
     Furthermore, a phase difference between U 1  and V 1  is activated at 180 degrees (π), and a phase difference between U 2  and V 2  is also activated at 180 degrees (π). The phase difference between U 1  and V 1  is a time difference between timing t 2  and timing t 6 , and the phase difference between U 2  and V 2  is a time difference between timing t 1  and timing t 5 . 
     Moreover, by changing at least one of a phase difference φu between U 1  and U 2  and a phase difference φv between V 1  and V 2 , the transmitted power P between the primary side conversion circuit  20  and the secondary side conversion circuit  30  can be adjusted. The phase difference φu is a time difference between timing t 1  and timing t 2 , and the phase difference φv is a time difference between timing t 5  and timing t 6 . 
     When the phase difference φu&gt;0 or the phase difference φv&gt;0, power can be transmitted from the primary side conversion circuit  20  to the secondary side conversion circuit  30 , and when the phase difference φ&lt;0 or the phase difference φv&lt;0, power can be transmitted from the secondary side conversion circuit  30  to the primary side conversion circuit  20 . That is, between identical-phase power conversion circuit units of the primary side full bridge circuit  200  and the secondary side full bridge circuit  300 , the transmitted power P is transmitted from the full bridge circuit of the power conversion circuit unit including an upper arm that is turned on first to the full bridge circuit of the power conversion circuit unit including an upper arm that is turned on later. 
     For example, in  FIG. 3 , the turn-on timing t 1  of the secondary side first upper arm U 2  is earlier than the turn-on timing t 2  of the primary side first upper arm U 1 . Therefore, the transmitted power P is transmitted from the secondary side full bridge circuit  300  including the secondary side first arm circuit  307  having the secondary side first upper arm U 2  to the primary side full bridge circuit  200  including the primary side first arm circuit  207  having the primary side first upper arm U 1 . Similarly, the turn-on timing t 5  of the secondary side second upper arm V 2  is earlier than the turn-on timing t 6  of the primary side second upper arm V 1 . Therefore, the transmitted power P is transmitted from the secondary side full bridge circuit  300  including the secondary side second arm circuit  311  having the secondary side second upper arm V 2  to the primary side full bridge circuit  200  including the primary side second arm circuit  211  having the primary side second upper arm V 1 . 
     The phase difference φ is a deviation (a time lag) between the switching timings of the identical-phase power conversion circuit units of the primary side full bridge circuit  200  and the secondary side full bridge circuit  300 . For example, the phase difference φu is a deviation between the switching timings of the phases corresponding to the primary side first arm circuit  207  and the secondary side first arm circuit  307 , and the phase difference φv is a deviation between the switching timings of the phases corresponding to the primary side second arm circuit  211  and the secondary side second arm circuit  311 . 
     The control unit  50  typically performs a control in a state that the phase difference φu and the phase difference φv are equal to each other. However, the control unit  50  may also perform the control in a state that the phase difference φu and the phase difference φv are offset from one another within a range in which an accuracy required for the transmitted power P is satisfied. That is, the phase difference φu and the phase difference φv are typically controlled to be values equal to each other, whereas if the accuracy required for the transmitted power P is satisfied, the phase difference φu and the phase difference φv may be controlled to be values different from each other. 
     Hence, when, for example, an external signal requesting an operation in which the power conversion mode of the power supply circuit  10  is set at mode F is input, the power conversion mode determination processing unit  502  selects and sets mode F. The ON time δ determination processing unit  506  then sets the ON time δ to define a step-up ratio required when the primary side conversion circuit  20  is caused to function as a step-up circuit that steps up the voltage input into the second input/output port  60   c  and outputs the stepped-up voltage to the first input/output port  60   a . Note that the secondary side conversion circuit  30  functions as a step-down circuit that steps down the voltage input into the third input/output port  60   b  at a step-down ratio defined according to the ON time δ set by the ON time δ determination processing unit  506 , and outputs the stepped-down voltage to the fourth input/output port  60   d . Further, the phase difference φ determination processing unit  504  sets the phase difference φ such that the power input into the first input/output port  60   a  is transmitted to the third input/output port  60   b  in the desired power transmission amount P. 
     The primary side switching processing unit  508  performs switching control on the respective switching elements constituted by the primary side first upper arm U 1 , the primary side first lower arm /U 1 , the primary side second upper arm V 1 , and the primary side second lower arm /V 1  to cause the primary side conversion circuit  20  to function as a step-up circuit and to cause the primary side conversion circuit  20  to function as a part of a DC-DC converter circuit. 
     The secondary side switching processing unit  510  performs switching control on the respective switching elements constituted by the secondary side first upper arm U 2 , the secondary side first lower arm/U 2 , the secondary side second upper arm V 2 , and the secondary side second lower arm /V 2  to cause the secondary side conversion circuit  30  to function as a step-down circuit and to cause the secondary side conversion circuit  30  to function as a part of a DC-DC converter circuit. 
     As described above, the primary side conversion circuit  20  and the secondary side conversion circuit  30  can be caused to function as a step-up circuit or a step-down circuit, and the power supply circuit  10  can be caused to function as a bidirectional DC-DC converter circuit. Therefore, power conversion can be performed in all of the power conversion modes A to L, or in other words, power conversion can be performed between two input/output ports selected from the four input/output ports. 
     The transmitted power P (also referred to as the power transmission amount P) adjusted by the control unit  50  according to the phase difference φ is power transmitted from one of the primary side conversion circuit  20  and the secondary side conversion circuit  30  to the other via the transformer  400 , and is expressed as
 
 P =( N×Va×Vb )/(π×ω× L )× F ( D ,φ)  Equation 1
 
     Note that N is a winding ratio of the transformer  400 , Va is the input/output voltage of the first input/output port  60   a , Vb is the input/output voltage of the third input/output port  60   b , π is pi, ω (=2π×f=2π/T) is an angular frequency of the switching operations of the primary side conversion circuit  20  and the secondary side conversion circuit  30 , f is a switching frequency of the primary side conversion circuit  20  and the secondary side conversion circuit  30 , T is the switching period of the primary side conversion circuit  20  and the secondary side conversion circuit  30 , L is an equivalent inductance of the magnetic coupling reactors  204 ,  304  and the transformer  400  relating to power transmission, and F (D, φ) is a function having the duty ratio D and the phase difference φ as variables and a variable that increases monotonically as the phase difference φ increases, independently of the duty ratio D. The duty ratio D and the phase difference φ are control parameters designed to vary within a range sandwiched between predetermined upper and lower limit values. 
     The control unit  50  varies the phase difference φ such that a port voltage Vp of at least one predetermined port of the primary side ports and the secondary side ports converges to a target port voltage Vo, thereby to adjust the transmitted power P. Therefore, even if the current consumed by a load connected to the predetermined port increases, the control unit  50  can adjust the transmitted power P by changing the phase difference φ, thereby to prevent the port voltage Vp from decreasing with respect to the target port voltage Vo. 
     For example, the control unit  50  changes the phase difference φ such that a port voltage Vp of one port that is the transmission destination of the transmitted power P of the primary side ports and the secondary side ports converges to a target port voltage Vo, thereby to adjust the transmitted power P. Therefore, even if the current consumed by a load connected to the port that is the transmission destination of the transmitted power P increases, the control unit  50  may adjust the transmitted power P in an increase direction by changing the phase difference φ to increase, thereby to prevent the port voltage Vp from decreasing with respect to the target port voltage Vo. 
     &lt;Power Conversion Method to Improve Power Conversion Efficiency&gt; 
     The control unit  50  is an example of a control unit of controlling the transmitted power P that is transmitted between the primary side full bridge circuit  200  and the secondary side full bridge circuit  300  by adjusting the phase difference φu and the phase difference φv (see  FIG. 3 ). 
     The phase difference φu is a time difference between switching of the primary side first arm circuit  207  and switching of the secondary side first arm circuit  307 . For example, the phase difference φu is a difference between the turn-on timing t 2  of the primary side first upper arm U 1  and the turn-on timing t 1  of the secondary side first upper arm U 2 . The control unit  50  controls the switching of the primary side first arm circuit  207  and the switching of the secondary side first arm circuit  307  in an identical-phase with each other (that is, in the U phase). Similarly, the phase difference φv is a time difference between switching of the primary side second arm circuit  211  and switching of the secondary side second arm circuit  311 . For example, the phase difference φv is a difference between the turn-on timing t 6  of the primary side second upper arm V 1  and the turn-on timing t 5  of the secondary side second upper arm V 2 . The control unit  50  controls the switching of the primary side second arm circuit  211  and the switching of the secondary side second arm circuit  311  in an identical-phase with each other (that is, in the V phase). 
     The control unit  50  performs a power conversion control to, among the phase difference φu and the phase difference φv, cause the phase difference between the arm circuits having a higher efficiency to be larger than the phase difference between the arm circuits having a lower efficiency. For example, when a power conversion efficiency ηu of an identical-phase between the primary side first arm circuit  207  and the secondary side first arm circuit  307  is larger than a power conversion efficiency ηv of an identical-phase between the primary side second arm circuit  211  and the secondary side second arm circuit  311 , the control unit  50  causes the phase difference φu to be larger than the phase difference φv. To the contrary, when the power conversion efficiency ηu of an identical-phase between the primary side first arm circuit  207  and the secondary side first arm circuit  307  is lower than the power conversion efficiency ηv of an identical-phase between the primary side second arm circuit  211  and the secondary side second arm circuit  311 , the control unit  50  causes the phase difference φv to be larger than the phase difference φu. 
     The efficiency ηu represents a power conversion efficiency for generating the transmitted power P that is transmitted during a period t 2 -t 3  in which the ON time of the primary side first upper arm U 1  and the ON time of the secondary side first upper arm U 2  overlap. The efficiency ηv represents a power conversion efficiency for generating the transmitted power P that is transmitted during a period t 6 -t 7  in which the ON time of the primary side second upper arm V 1  and the ON time of the secondary side second upper arm V 2  overlap. 
     Therefore, by, among the phase difference φu and the phase difference φv, causing the phase difference between the arm circuits having a higher efficiency to be larger than the phase difference between the arm circuits having a lower efficiency, even if characteristics of respective arms constituting the arm circuits vary between the respective arms, a transmission efficiency of the transmitted power P is improved. Therefore, a power conversion efficiency η between the primary side full bridge circuit  200  and the secondary side full bridge circuit  300  is improved. 
     For example, it may be defined as
 
the efficiency η u =(a drain efficiency of the primary side first upper arm  U 1+a drain efficiency of the primary side second lower arm/ V 1+a drain efficiency of the secondary side first upper arm  U 2+a drain efficiency of the secondary side second lower arm/ V 2)÷4  Equation 2
 
the efficiency η v =(a drain efficiency of the primary side second upper arm  V 1+a drain efficiency of the primary side first lower arm/ U 1+a drain efficiency of the secondary side second upper arm  V 2+a drain efficiency of the secondary side first lower arm/ U 2)÷4  Equation 3
 
     For example, the drain efficiency is defined as a ratio of output power that is output from the input/output port to which the full bridge circuit is connected (=output voltage×output current) to power consumed by respective phase arms when the respective phases are ON (≈drain voltage×drain current). For example, in the case of the primary side first upper arm U 1 , the power consumed by the respective phase arms when the respective phases are ON is a product of the drain voltage and the drain current when the primary side first upper arm U 1  is ON. The output power that is output from the input/output port to which the full bridge circuit is connected is port power that is output from one port among the primary side ports to which the primary full bridge circuit  200  is connected and the secondary side ports to which the secondary side full bridge circuit  300  is connected. 
     The drain voltages of the respective arms are drain-source voltages of the respective arms, and the drain currents of the respective arms are drain-source currents flowing through the respective arms. The drain voltages and the drain currents of the respective arms and the port power that is output from the one port are detected by the sensor unit  70 . 
     Further, for example, it can be defined as
 
the efficiency η u =(a heat generation amount of the primary side first upper arm  U 1+a heat generation amount of the primary side second lower arm/ V 1+a heat generation amount of the secondary side first upper arm  U 2+a heat generation amount of the secondary side second lower arm/ V 2)÷(4×the maximum heat generation amount of all the 8 arms)  Equation 4
 
the efficiency η v =(a heat generation amount of the primary side-second-upper arm  V 1+a heat generation amount of the primary side first lower arm/ U 1+a heat generation amount of the secondary side second upper arm  V 2+a heat generation amount of the secondary side first lower arm/ U 2)÷(4×the maximum heat generation amount of all the 8 arms)  Equation 5
 
     The heat generation amount can be replaced with temperature. The maximum heat generation amount of all the 8 arms (or the maximum temperature) is the maximum heat generation amount (or the highest temperature) of all the 8 arms of which the primary side full bridge circuit  200  and the secondary side full bridge circuit  300  are configured. The heat generation amount or the temperature of each arm is detected by the sensor unit  70 . 
     Further, the efficiencies ηu and ηv are not limited to the values as defined above, as long as the efficiencies ηu and ηv may be equivalently represented. 
       FIG. 4  is a flowchart showing a first example of a power conversion method performed by the control unit  50 . 
     In step S 10 , the control unit  50  calculates efficiencies of the respective arms, for example, calculates the efficiency ηu and the efficiency ηv as defined above. 
     In step S 20 , the control unit  50  compares the magnitude relationship between the efficiency ηu and the efficiency ηv that are calculated in step S 10 . If it is determined that the efficiency ηu is larger than the efficiency ηv, the control unit  50  causes the phase difference φu to be larger than the phase difference φv by extending the phase difference φu and shortening the phase difference φv (step S 30 ). On the other hand, if it is determined that the efficiency ηu is smaller than the efficiency ηv, the control unit  50  causes the phase difference φv to be larger than the phase difference φu by shortening the phase difference φu and extending the phase difference φv (step S 40 ). 
     In step S 30 , the control unit  50  sets a value that is obtained by adding a change amount Δφ to a standard phase difference φ as the phase difference φu and sets a value obtained by subtracting the change amount Δφ from the standard phase difference φ as the phase difference φv. On the other hand, in step S 40 , the control unit  50  sets a value that is obtained by adding the change amount Δφ to the standard phase difference φ as the phase difference φv and sets a value obtained by subtracting the change amount Δφ from the standard phase difference φ as the phase difference φu. 
     For example, the change amount Δφ is defined as
 
Δφ=the standard phase difference φ×(η u−ηv )  Equation 6
 
The right side of Equation 6 may be multiplied by any proportional constant.
 
     The control unit  50  may calculate the standard phase difference φ according to Equation 1, or may perform the calculation according to a map defining the relationship of Equation 1. 
       FIG. 5  is a diagram showing an example of a map for calculating the standard phase difference φ. The map is stored in advance in a memory. The control unit  50  calculates the standard phase difference φ (for example, φ 1 , φ 2 , φ 3 , φ 4 , . . . ) based on the command transmitted power P* and a command duty ratio D* according to the map of  FIG. 5 . The command transmitted power P* is a target value of the transmitted power P (for example, target transmission powers P 1 *, P 2 *, P 3 *, . . . ), and the command duty ratio D* is a target value of the duty ratio D (for example, target duty ratios D 1 *, D 2 *, D 3 *, D 4 *, D 5 *, . . . ). 
     The control unit  50  can calculate optimum values of the phase difference φu and the phase difference φv by correcting the standard phase difference φ based on the addition and subtraction of the change amount Δφ as in steps S 30  and S 40 . Further, the control unit  50  can improve the power conversion efficiency η by, among the phase difference φu and the phase difference φv, extending the phase difference between the arm circuits having a higher efficiency and shortening the phase difference between the arm circuits having a lower efficiency such that the transmitted power P does not vary with respect to the command transmitted power P*. Since variation of the transmitted power P with respect to the command transmitted power P* is suppressed, variation of output power that is output to a port from one full bridge circuit can be suppressed. 
     Note that there is a possibility that the ripple in the output of the full bridge circuit increases if the phase difference φu and the phase difference φv are adjusted to values different from each other. Therefore, for example, the upper limit value of the change amount Δφ can be set to a value such that the ripple of the output voltage converges within a predetermined range. 
     Further, if the control unit  50  calculates the efficiencies ηu and ηv for the respective switching periods of the respective arms, its calculation load increases. In view of this, for example, the control unit  50  may repeat the calculation of efficiencies ηu and ηv and the adjustment of the phase differences φu and φv at periods that are sufficiently lagged as compared with the switching periods of the respective arms (for example, 2-3 seconds). 
     Further, for example, in order to accommodate aging of the respective arms, the control unit  50  may also perform the calculation of efficiencies ηu and ηv and the adjustment of the phase differences φu and φv a predetermined number of times (for example, once) at any timing from turn-on to turn-off of an ignition switch. Further, for example, in order to accommodate changes in the temperatures of the respective arms, the control unit  50  may also perform the calculation of efficiencies ηu and ηv and the adjustment of the phase differences φu and φv at timing when the ambient temperature rises to a predetermined threshold temperature. 
       FIG. 6  is a flowchart showing a second example of the power conversion method performed by the control unit  50 . 
     In step S 50 , the control unit  50  calculates efficiencies of the respective arms, for example, calculates the efficiency ηu and the efficiency ηv as defined above. 
     In step S 60 , the control unit  50  detects gate currents Ig of the respective arms when the respective arms are turned on, and stores the detected values in a memory. If the gate currents Ig are detected a plurality of times during the period from turn-on to turn-off of the ignition switch, the control unit  50  stores the average value or the last detected value of the gate currents Ig that are detected a plurality of times in the memory. 
     The gate current Ig is an example of a control current flowing through the gate electrode of the arm, and the gate electrode is an example of the control electrode of the arm. In  FIG. 1 , the gate current flowing through the gate electrode G of the primary side first upper arm U 1  may be cited as an example of the gate current Ig. Of course, however, the gate current may also flow through the gate electrodes of the arms other than the primary side first upper arm U 1 . 
     In step S 70 , the control unit  50  determines whether the respective arms have a failure based on the changes in the gate currents Ig of the respective arms. For example, the control unit  50  determines an arm of which the change amount of the detected value of the gate current Ig that is currently detected with respect to the stored value of the gate current Ig stored in the memory is above a predetermined change amount to be an arm in which a failure occurs. Alternatively, for example, the control unit  50  determines an arm of which the detected value of the gate current Ig that is currently detected exceeds a predetermined current threshold to be an arm in which a failure occurs. 
     If it is determined that a failure does not occur in any one of the respective arms, the control unit  50  performs the processing of step S 80 , and if it is determined that a failure does occur in one of the respective arms, the control unit  50  performs the processing of step S 110 . 
     In step S 80 , the control unit  50  compares the magnitude relationship between the efficiency ηu and the efficiency ηv that are calculated in step S 50 . If it is determined that the efficiency ηu is larger than the efficiency ηv, the control unit  50  causes the phase difference φu to be larger than the phase difference φv by extending the phase difference φu and shortening the phase difference φv (step S 90 ). On the other hand, if it is determined that the efficiency ηu is smaller than the efficiency ηv, the control unit  50  causes the phase difference φv to be larger than the phase difference φu by shortening the phase difference φu and extending the phase difference φv (step S 100 ). Step S 90  and step S 100  may be the same as step S 30  and step S 40  as described above, respectively. 
     On the other hand, if it is determined that a failure occurs in one of the respective arms in step S 70 , the control unit  50  changes to a control that does not use the arm in which the failure occurs (step S 110 ). In this way, progress of the failure of the arm can be suppressed. 
     In step S 110 , for example, the control unit  50  performs at least one of the fail-safe controls as described below. For example, the control unit  50  configured to stop the control of causing the phase difference between the arm circuits having a higher efficiency to be larger than the phase difference between the arm circuits having a lower efficiency. For example, the control unit  50  notifies outside of the failure of the arm. For example, the control unit  50  controls the transmitted power P that is transmitted between the full bridge circuits and the output power that is output from one full bridge circuit by operating the full bridge circuit with a half bridge that does not use the arm having a gate current Ig above a reference value flow through the gate electrode. In this case, the control unit  50  preferably causes the transmitted power P or the upper limit of the output circuit to decrease as compared with typical case. 
     Therefore, it is possible to continue transmission of the transmitted power P even if a failure occurs in the arms. 
     &lt;Configuration of Power Supply Apparatus  102 &gt; 
       FIG. 7  is a block diagram showing an example of a configuration of a power supply apparatus  102  serving as a second embodiment of the power conversion apparatus. Description of the same or similar structure and effect to the embodiment described above is omitted or simplified. For example, the power supply apparatus  102  is a power supply system that includes a first power supply circuit  11 , a second power supply circuit  12 , a control unit  50 , and a sensor unit  70 . 
     The power supply apparatus  102  is a DC-DC converter that includes two power supply circuits  11 ,  12  that are connected in parallel to each other between the first input/output port  60   a , the second input/output port  60   c  and the third input/output port  60   b , the fourth input/output port  60   d . By providing redundantly a plurality of power supply circuits in this way, the output power capable of being supplied to the respective loads  61   a ,  61   b ,  61   c ,  61   d  may increase, and the fail-safe performance when a failure occurs in a portion of the plurality of power supply circuits can be improved. 
     The power supply apparatus  102  has the power supply circuit  11  and the power supply circuit  12  in parallel. Each of the power supply circuit  11  and the power supply circuit  12  is the same as the power supply circuit  10  of  FIG. 1 . 
     Port powers Pa 1 , Pc 1 , Pb 1 , Pd 1  are powers that are input or output in the power supply circuit  11  respectively, and port powers Pa 2 , Pc 2 , Pb 2 , Pd 2  are powers that are input or output in the power supply circuit  12  respectively. Port voltages Va 1 , Vc 1 , Vb 1 , Vd 1  are voltages that are input or output in the power supply circuit  11  respectively, and port voltages Va 2 , Vc 2 , Vb 2 , Vd 2  are voltage that are input or output in the power supply circuit  12  respectively. Port currents Ia 1 , Ic 1 , Ib 1 , Id 1  are currents that are input or output in the power supply circuit  11  respectively, and port currents Ia 2 , Ic 2 , Ib 2 , Id 2  are currents that are input or output in the power supply circuit  12  respectively. 
     The control unit  50  varies the input/output powers of the respective input/output ports of the power supply circuits  11  and  12  by adjusting the phase differences φ and the duty ratios D of the respective power supply circuits  11  and  12 . Thereby, the input/output values Y of the respective input/output ports  60   a  to  60   d  of the power supply apparatus  102  are varied. For example, the control unit  50  varies the duty ratio D and the phase difference φ of at least one of the power supply circuits  11  and  12  such that a detected value Yd of the input/output value Y follows a target value Yo. 
     The control unit  50  extends, among the power supply circuits  11  and  12 , the phase difference φu and the phase difference φv in the power supply circuit having a higher efficiency, and shortens the phase difference φu and the phase difference φv in the power supply circuit having a lower efficiency. Thereby, the power conversion efficiency between the primary side ports and the secondary side ports of the power supply apparatus  102  can be improved in consideration of the power conversion efficiencies of the respective power supply circuits  11  and  12 . Further, the power conversion efficiency between the primary side ports and the secondary side ports of the power supply apparatus  102  can be improved such that the transmitted power P does not vary with respect to the command transmitted power P*. Since variation of the transmitted power P with respect to the command transmitted power P* is suppressed, variation of output power that is output to a port from a full bridge circuit can be suppressed. 
     For example, if an efficiency η 11  of the power supply circuit  11  is higher than an efficiency η 12  of the power supply circuit  12 , the control unit  50  extends both the phase difference φu and the phase difference φv in the power supply circuit  11  and shortens both the phase difference φu and the phase difference φv in the power supply circuit  12 . To the contrary, if the efficiency η 12  of the power supply circuit  12  is higher than the efficiency η 11  of the power supply circuit  11 , the control unit  50  extends both the phase difference φu and the phase difference φv in the power supply circuit  12  and shortens both the phase difference φu and the phase difference φv in the power supply circuit  11 . 
     The efficiency η 11  is the power conversion efficiency between the primary side ports and the secondary side ports in the power supply circuit  11 , and is represented by a ratio of output power to input power in the power supply circuit  11 , for example. The efficiency η 12  is the power conversion efficiency between the primary side ports and the secondary side ports in the power supply circuit  12 , and is represented by a ratio of the output power to the input power in the power supply circuit  12 , for example. 
     When input power that is input to one port among the primary side ports and the secondary side ports is defined as Pin, output power that is output from other port is defined as Pout, an input voltage that is input to the one port is defined as Vin, an output voltage that is output from the other port is defined as Vout, an input current that is input to the one port is defined as Iin, and an output current that is output from the other port is defined as Iout, the efficiency η 11  can be expressed as
 
η 11   =P out/ P in=( V out× I out)/( V in× I in)  Equation 7
 
The efficiency η 12  can be expressed similarly as Equation 7.
 
     For example, in the power supply circuit  11  of  FIG. 7 , when the port power Pb 1  that is input to the third input/output port is converted and the converted port power Pa 1  is output to the first input/output port, and the power Pa 1  of the first input/output port is converted and the converted port power Pct is output to the second input/output port, according to Equation 7, the efficiency η 11  of the power supply circuit  11  can be expressed as
 
η 11 =( Va 1× Ia 1+ Vc 1× Ic 1)/( Vb 1× Ib 1)  Equation 8
 
Note that Equation 8 is an equation in the case of not using the fourth input/output port, and the input/output power of the fourth input/output port is set to zero (for example, a configuration without the secondary side low voltage system load  61   d  and the capacitor C 4 ). The efficiency η 12  can be expressed similarly as Equation 8.
 
     The power conversion method performed by the control unit  50  of the power supply apparatus  102  can invoke the flowchart shown in  FIG. 4  or  FIG. 6  as described above. Hereinafter, according to the flowchart shown in  FIG. 6 , the power conversion method to be executed by the control unit  50  of the power supply apparatus  102  will be explained. In addition, the same steps as the steps described above will not be described. 
     In step S 50 , the control unit  50  calculates efficiencies of the respective arms, for example, calculates the efficiency ηu and the efficiency ηv as defined above for the power supply circuits  11  and  12  respectively. Further, in step S 50 , the control unit  50  calculates the efficiency η 11  of the power supply circuit  11  and the efficiency η 12  of the power supply circuit  12 . 
     For example, it can be defined as
 
η 11 =output power of the power supply circuit 11÷(total drain current×total drain voltage of the power supply circuit 11)  Equation 9
 
η 12 =output power of the power supply circuit 12÷(total drain current×total drain voltage of the power supply circuit 12)  Equation 10
 
     The output power of the power supply circuit  11  (or the power supply circuit  12 ) is port power output from one full bridge circuit of the power supply circuit  11  (or the power supply circuit  12 ). The total drain current is the sum of the drain currents of the 8 arms constituting the power supply circuit, and the total drain voltage is the sum of the drain voltages of the 8 arms constituting the power supply circuit. 
     In step S 80 , the control unit  50  compares the magnitude relationship between the efficiency ηu and the efficiency ηv of the power supply circuit  11  that are calculated in step S 50 , and compares the magnitude relationship between the efficiency ηu and the efficiency ηv of the power supply circuit  12  that are calculated in step S 50 . Further, in step S 80 , the control unit  50  compares the magnitude relationship between the efficiency η 11  and the efficiency η 12  that are calculated in step S 50 . 
     According to the comparison result in step  80 , as described below, the control unit  50  derives the phase differences φu and φv by correcting the standard phase difference φ with change amounts Δφ and ΔΦ. For example, the change amount Δφ may be defined by Equation 6 above. For example, the change amount ΔΦ may be defined as
 
ΔΦ=the standard phase difference φ×(η 11 −η 12 )  Equation 11
 
The right side of Equation 11 may be multiplied by any proportional constant.
 
     If it is determined in step S 80  that the efficiency ηu of the power supply circuit  11  is larger than the efficiency ηv of the power supply circuit  11  and the efficiency η 11  is larger than the efficiency η 12 , the control unit  50  derives the phase differences φu and φv of the power supply circuit  11  by correcting the standard phase difference φ specified to the power supply circuit  11  with the change amounts Δφ and ΔΦ according to
 
φ u  of the power supply circuit 11=(φ+Δφ)+ΔΦ,φ v  of the power supply circuit 11=(φ−Δφ)+ΔΦ  Equation 12
 
On the other hand, if it is determined in step S 80  that the efficiency ηu of the power supply circuit  12  is larger than the efficiency ηv of the power supply circuit  12  and the efficiency η 11  is larger than the efficiency η 12 , the control unit  50  derives the phase differences φu and φv of the power supply circuit  12  by correcting the standard phase difference φ specified to the power supply circuit  12  with the change amounts Δφ and ΔΦ according to
 
φ u  of the power supply circuit 12=(φ+Δφ)−ΔΦ,φ v  of the power supply circuit 12=(φ−Δφ)−ΔΦ  Equation 13
 
     Further, if it is determined in step S 80  that the efficiency ηu of the power supply circuit  11  is larger than the efficiency ηv of the power supply circuit  11  and the efficiency η 11  is smaller than the efficiency η 12 , the control unit  50  derives the phase differences φu and φv of the power supply circuit  11  by correcting the standard phase difference φ specified to the power supply circuit  11  with the change amounts Δφ and ΔΦ according to
 
φ u  of the power supply circuit 11=(φ+Δφ)−ΔΦ,φ v  of the power supply circuit 11=(φ−Δφ)−ΔΦ  Equation 14
 
On the other hand, if it is determined in step S 80  that the efficiency ηu of the power supply circuit  12  is larger than the efficiency ηv of the power supply circuit  12  and the efficiency η 11  is smaller than the efficiency η 12 , the control unit  50  derives the phase differences φu and φv of the power supply circuit  12  by correcting the standard phase difference φ specified to the power supply circuit  12  with the change amounts Δφ and ΔΦ according to
 
φ u  of the power supply circuit 12=(φ+Δφ)+ΔΦ,φ v  of the power supply circuit 12=(φ−Δφ)+ΔΦ  Equation 15
 
     Further, if the efficiency ηu is smaller than the efficiency ηv, the control unit  50  derives the respective phase differences φu and φv of the power supply circuits  11  and  12  according to equations that are obtained by replacing (φ+Δφ) and (φ−Δφ) in right side of Equations 12 to 15 with (φ−Δφ) and (φ+Δφ) respectively. 
     When it is determined that a failure occurs in one of the respective arms in step S 70 , the control unit  50  changes to a control that does not use the arm in which the failure occurs (step S 110 ). 
     In step S 110 , for example, the control unit  50  performs at least one of the fail-safe controls as described below. For example, the control unit  50  completely stops one power supply circuit having an arm that is determined to be faulty and use the other power supply circuit having a normal arm that is not determined to be faulty, to control the transmitted power P and the output power. For example, the control unit  50  does not use a half bridge of an arm that is determined to be faulty to operate the full bridge circuit of one power supply circuit and uses the other power supply circuit having a normal arm that is not determined to be faulty, to control the transmitted power P and the output power. The control unit  50  may operate the normal power supply circuit if the maximum output is required to support the power supply circuit that is operating with a half bridge, or operate the power supply circuit having a faulty arm with a half bridge temporarily to prevent a shortage of the output power. 
     For example, a scene where a direction indicator is issued before a curve, before an intersection at a low speed and so on may be cited as the case that the maximum output is required. Locations where the maximum output is required are stored in a memory, and the control unit  50  performs the fail-safe control as described above at the locations where the maximum output is required or when a vehicle travels before the locations. 
     The power conversion apparatus and the power conversion method were described above by means of an embodiment, but the invention is not limited to the above embodiment, and various amendments and improvements, such as combining or replacing the above embodiment either partially or wholly with another embodiment, may be implemented within the scope of the invention. 
     For example, in the above embodiment, a MOSFET, which is a semiconductor element subjected to an ON/OFF operation, was cited as an example of the switching element. However, the switching element may be a voltage control type power element using an insulating gate such as an insulated gate bipolar transistor (IGBT) or a MOSFET, or a bipolar transistor, for example. 
     Further, a power supply may be connected to the first input/output port  60   a , and a power supply may be connected to the fourth input/output port  60   d . Furthermore, a power supply need not be connected to the second input/output port  60   c , and a power supply need not be connected to the third input/output port  60   b.    
     Further, in  FIG. 1 , although the primary side low voltage system power supply  62   c  is connected to the second input/output port  60   c , the power supply may be connected to neither the first input/output port  60   a  nor the second input/output port  60   c.    
     Further, the present invention is suitable for a power conversion apparatus that has a plurality of, at least three or more, input/output ports and is capable of converting power between any two input/output ports of the plurality of, at least three or more, input/output ports. For example, the present invention is also suitable for the power supply apparatus configured to not include any one input/output port of the four input/output ports as illustrated in  FIG. 1 . 
     Further, in the above description, the primary side may be defined as the second side, and the second side may be defined as the primary side. In the above description, although a case that the transmitted power P is transmitted to the primary side ports from the secondary side ports has been illustrated as an example, the above description can also be applied to the case that the transmitted power P is transmitted to the secondary side ports from the primary side ports. 
     Further, one phase difference of the phase difference φu and the phase difference φv being larger than the other phase difference has the same meaning as that the other phase difference is smaller that the one phase difference. The control unit  50  may cause one phase difference to be larger than the other phase difference by shortening the other phase difference while fixing the one phase difference. In contrary, the control unit  50  may also cause one phase difference to be larger than the other phase difference by extending the one phase difference while fixing the other phase difference. 
     Further, if it is determined that the efficiency ηu and the efficiency ηv are equal to each other, the control unit  50  may set the phase difference φv and the phase difference φv to values equal to each other (typically, the standard phase difference φ).