Patent Publication Number: US-9853557-B2

Title: Power conversion apparatus and power conversion method

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2013-107417 filed on May 21, 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 invention relates to an apparatus and a method for performing a power conversion between a primary side circuit and a secondary side circuit that is magnetically coupled to the primary side circuit via a transformer. 
     2. Description of Related Art 
     A conventional power conversion apparatus can adjust an amount of power transmitted between a primary side circuit and a secondary side circuit by changing a phase difference between switching of the primary side circuit and switching of the secondary side circuit (see Japanese Patent Application Publication No. 2011-193713 (JP 2011-193713 A), for example). 
     SUMMARY OF THE INVENTION 
     However, transmitted power transmitted between the primary side circuit and the secondary side circuit may conventionally fail to be accurately adjusted. An object of the invention is to provide a power conversion apparatus and a power conversion method which allow the transmitted power transmitted between the primary side circuit and the secondary side circuit to be accurately adjusted. 
     A first aspect of the invention is a power conversion apparatus including: a primary side circuit; a secondary side circuit magnetically coupled to the primary side circuit through a transformer; and a control unit that adjusts transmitted power transmitted between the primary side circuit and the secondary side circuit by changing a phase difference between switching of the primary side circuit and switching of the secondary side circuit. The control unit suppresses a fluctuation in the transmitted power by suppressing a change in a duty ratio of the switching of the primary side circuit to the switching of the secondary side circuit. 
     A second aspect of the invention is a power conversion method for adjusting transmitted power transmitted between a primary side circuit and a secondary side circuit magnetically coupled to the primary side circuit through a transformer, by changing a phase difference between switching of the primary side circuit and switching of the secondary side circuit. A fluctuation in the transmitted power is suppressed by suppressing a change in a duty ratio of the switching of the primary side circuit to the switching of the secondary side circuit. 
     According to the first and second aspects of the invention, the transmitted power transmitted between the primary side circuit and the secondary side circuit can be accurately adjusted. 
    
    
     
       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 block diagram showing an example of a configuration of a power supply apparatus serving as an embodiment of a power conversion apparatus according to the invention; 
         FIG. 2  is a block diagram showing an example of a configuration of a control unit according to this embodiment; 
         FIG. 3  is a timing chart showing an example of switching operations of a primary side circuit and a secondary side circuit according to this embodiment; 
         FIG. 4  is a graph showing relations between transmitted power P and a phase difference φ and a duty ratio D according to the embodiment; 
         FIG. 5  is a block diagram showing a configuration example of a control unit according to the embodiment; 
         FIG. 6  is a flowchart showing an example of a power conversion method according to the invention; 
         FIG. 7  is a timing chart of one update of the duty ratio D (on time δ) for every switching period; 
         FIG. 8  is a timing chart of one update of the duty ratio D (on time δ) for every two switching periods; 
         FIG. 9  is a block diagram showing a configuration example of a control unit according to the embodiment; and 
         FIG. 10  is a flowchart showing an example of a power conversion method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a block diagram showing an example of a configuration of a power supply apparatus  101  serving as an 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  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 power conversion between the two selected input/output ports. 
     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 . Note 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 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   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 detection value Yd corresponding to the detected input/output value Y to the control unit  50 . The detection 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 a transmitted power P transmitted between the primary side conversion circuit  20  and the secondary side conversion circuit  30  via the transformer  400  converges to a set target transmitted power Po. The transmitted power will also be referred to as a power transmission amount. For example, the target transmitted power Po is a command value set by the control unit  50  or a predetermined apparatus other than the control unit  50  on the basis of a deviation between the detected value Yd and the target value Yo in one of the ports. 
     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 L, 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 . 
     An operation of the power supply apparatus  101  having the above configuration will now be described using  FIGS. 1 and 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 kT, 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 δ11=secondary side ON time δ12=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, 
     
       
         
           
             
               
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     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 δ11 of the primary side first upper arm U 11  and the primary side second upper arm V 1  and the ON time δ12 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 likewise activated at 180 degrees (π). Moreover, by changing the phase difference φ between U 1  and U 2 , the power transmission amount P between the primary side conversion circuit  20  and the secondary side conversion circuit  30  can be adjusted such that when the phase difference φ&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, power can be transmitted from the secondary side conversion circuit  30  to the primary side conversion circuit  20 . 
     The phase difference φ is a deviation (a time lag) between the 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 . For example, the phase difference φ is a deviation between the switching timings of the primary side first arm circuit  207  and the secondary side first arm circuit  307 , and a deviation between the switching timings of the primary side second arm circuit  211  and the secondary side second arm circuit  311 . These deviations are controlled to be equal to each other. In other words, the phase difference φ between U 1  and U 2  and the phase difference φ between V 1  and V 2  are controlled to identical values. 
     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 in accordance with 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  in accordance with 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 transmitted power P is adjusted by the control unit  50  changing the phase difference φ but is affected by the duty ratio D as shown in Equation 1 and  FIG. 4 .  FIG. 4  is a graph showing relations between the transmitted power P and the phase difference φ and the duty ratio D. The transmitted power P increases consistently with phase difference φ(φ11&lt;φ12&lt;φ13&lt;φ14). However, even when the phase difference φ is fixed to the same value, the transmitted power P decreases with the increasing duty ratio D if the duty ratio D is larger than 0.5, and decreases consistently with the duty ratio D if the duty ratio D is smaller than 0.5. 
     Thus, even when changing a command value φo for the phase difference φ so that the transmitted power P converges to the target transmitted power Po, the control unit  50  may fail to accurately adjust the transmitted power P to the target transmitted power Po depending on a command value Do for the duty ratio D. Similarly, even when changing the command value φo for the phase difference φ so that the I/O value Y for a predetermined I/O port converges to the target value Yo, the control unit  50  may fail to accurately adjust the I/O value Y to the target value Yo depending on a command value Do for the duty ratio D. If the transmitted power P or the I/O value Y fails to be accurately adjusted, the control of the transmitted power P or the I/O value Y is likely to oscillate. 
     Such an oscillation phenomenon is likely to occur, for example, when a difference equal to or larger than a predetermined value is present between a port voltage and a target voltage if the power consumption of any of the loads connected to the I/O ports changes rapidly or if the power supply circuit  10  is activated. This is because, when a difference equal to or larger than the predetermined value is present between the port voltage and the target voltage, the control unit  50  changes the phase difference φ and the duty ratio D at the same time, causing a change in the phase difference φ to be hindered by a change in duty ratio D. 
     Thus, the control unit  50  has suppression means for suppressing a fluctuation in the transmitted power P by suppressing a change in the duty ratio D. Thus, the adjustment of the transmitted power P is unlikely to be affected by the duty ratio D, allowing the transmitted power P to be accurately adjusted. The control unit  50 , for example, suppresses the change in the duty ratio D when the phase difference φ is changed so that the change in the duty ratio D is unlikely to affect the change in the phase difference φ. The control unit  50  can thus smoothly change the phase difference φ. As a result, the transmitted power P can be adjusted to a desired value with a fluctuation in the transmitted power P being suppressed. This allows the port voltage to converge smoothly to the target voltage. 
     Thus,  FIG. 5  is a block diagram showing a first configuration example of the control unit  50 . The control unit  50  has a PID control unit  51 , a magnitude-of-change detecting unit  52 , and an update timing control unit  53 . 
     The PID control unit  51  has a phase difference command value generating unit that performs PID control to generate, for every switching period T, the command value φo for the phase difference φ intended to cause the port voltage of at least one of the primary and secondary side ports to converge to the target value. For example, the phase difference command value generating unit of the PID control unit  51  performs PID control based on a deviation between the target voltage for a port voltage Va and the detected voltage of the port voltage Va acquired by a sensor unit  70 , to generate the command value φo for causing the deviation to converge to zero, for every switching period T. 
     The control unit  50  performs switching control on the primary side conversion circuit  20  and the secondary side conversion circuit  30  in accordance with the command value φo generated by the PID control unit  51  to adjust the transmitted power P defined by Equation 1 so that the port voltage converges to the target voltage. 
     Furthermore, the PID control unit  51  has a duty ratio command value generating unit that performs PID control to generate, for every switching period T, the command value Do for the duty ratio D intended to cause the port voltage of at least one of the primary and secondary side ports to converge to the target value. For example, the duty ratio command value generating unit of the PID control unit  51  performs PID control based on a deviation between the target voltage for a port voltage Vc and the detected voltage of the port voltage Vc acquired by the sensor unit  70 , to generate the command value Do for causing the deviation to converge to zero, for every switching period T. 
     Note that the PID control unit  51  may include an ON time command value generation unit that generates a command value δo of the ON time δ instead of the command value Do of the duty ratio D. 
     The magnitude-of-change detecting unit  52  is means for detecting the magnitude of change (the amount of change) in the phase difference φ obtained when the phase difference φ is changed from an unchanged value to a changed value. The magnitude of change in the phase difference φ refers to a value representing the difference between the unchanged value of the phase difference φ and the changed value of the phase difference φ. The magnitude-of-change detecting unit  52 , for example, detects the magnitude of change in the command value φo for the phase difference φ generated for every switching period T by the PID control unit  51 , as the actual magnitude of change in the phase difference φ. 
     The update timing control unit  53  is means for delaying an update timing for the command value Do for the duty ratio D generated by the PID control unit  51  until the next or subsequent switching period T in accordance with the magnitude of change in the phase difference φ detected by the magnitude-of-change detecting unit  52 . The update timing control unit  53  can delay a change timing for the actual duty ratio D by delaying the update timing for the command value Do. This enables a change in the duty ratio D to be suppressed. 
     For example, when the magnitude of change in the phase difference φ is equal to or larger than a predetermined value Z, the update timing control unit  53  suppresses the change in the duty ratio D by delaying the timing at which the command value Do for the duty ratio D is updated, to thereby reduce the frequency of updates of the command value Do for the duty ratio D. The update timing control unit  53 , for example, sets the command value Do specifying the duty ratio D for the current switching period T to an old command value for the duty ratio D generated during a switching period T before the last switching period T rather than to the latest command value Do for the duty ratio D generated during the current switching period T. 
     On the other hand, for example, when the magnitude of change in the phase difference φ is smaller than the predetermined value Z, the update timing control unit  53  does not delay the update timing for the command value Do for the duty ratio D so as not to suppress the change the duty ratio D (when the change in the duty ratio D has been suppressed, the suppression is cancelled). The update timing control unit  53 , for example, sets the command value Do specifying the duty ratio D for the current switching period T to the latest command value for the duty ratio D generated during the current switching period T. This sets the frequency of updates of the command value Do for the duty ratio D to a normal value. 
     The control unit  50  performs switching control on the primary side conversion circuit  20  and the secondary side conversion circuit  30  in accordance with the command value Do set by the update timing control unit  53 , to adjust the step-up/step-down ratio of the primary side conversion circuit  20  to the secondary side conversion circuit  30  so that the port voltage converges to the target voltage. 
       FIG. 6  is a flowchart showing an example of a power conversion method. The power conversion method shown in  FIG. 6  is executed by the control unit  50 . 
     In step S 10 , the update timing control unit  53  determines whether or not the magnitude of change in the phase difference φ detected by the magnitude-of-change detecting unit  52  is equal to or larger than the predetermined value Z. Determining whether or not the magnitude of change in the phase difference φ is equal to or larger than the predetermined value Z enables determination of whether or not the command value φo is about to oscillate. The update timing control unit  53  performs processing in step S 30  when the magnitude of change in the phase difference φ is equal to or larger than the predetermined value Z, and performs processing in step S 20  when the magnitude of change in the phase difference φ is smaller than the predetermined value Z. 
     In step S 20 , the update timing control unit  53  sets one update timing for the command value Do for the duty ratio D (the command value δo for the on time δ), for one switching period (1T) (see  FIG. 7 ). 
       FIG. 7  is a timing chart showing that one update timing for the duty ratio D (=δ/T) is set for every switching period (1T). 
     The control unit  50  changes the phase difference φ for every switching period. The control unit  50  sets the phase difference φ for the ith, current switching period T to a phase difference φ1 generated during the ith, current switching period T, and sets the phase difference φ for the i+1th, next switching period T to a phase difference φ2 generated during the i+1th, next switching period T. Thus, the control unit  50  increases the phase difference φ in order for every switching period (φ1&lt;φ2). In this case, a natural number is denoted by i. 
     On the other hand, the control unit  50  also changes the duty ratio D (on time δ) for every switching period. The control unit  50  sets the duty ratio D (on time δ) for the ith, current switching period T to a duty ratio D1 (on time δ1) generated during the ith, current switching period T, and sets the duty ratio D (on time δ) for the i+1 th , next switching period T to a duty ratio D2 (on time δ2) generated during the i+1 th , next switching period T. 
     On the other hand, in step S 30  in  FIG. 6 , the update timing control unit  53  sets one update timing for the command value Do for the duty ratio D (the command value δo for the on time δ), for every N switching periods (N×T) (see  FIG. 8 ). In this case, a natural number is denoted by N. 
       FIG. 8  is a timing chart showing that one update timing for the duty ratio D (=δ/T) is set for two switching periods (2T). That is,  FIG. 8  shows a case of N=2. 
     As in the case of  FIG. 7 , the control unit  50  changes the phase difference φ for every switching period T. The control unit  50  increases the phase difference φ in order for every switching period (φ1&lt;φ2&lt;φ3&lt;φ4). 
     On the other hand, the control unit  50  changes the duty ratio D (on time δ) for every two switching periods. The control unit  50  sets the duty ratio D (on time δ) for the ith switching period T to the duty ratio D1 (on time δ1) generated during the ith switching period T, and sets the duty ratio D (on time δ) for the i+1 th  switching period T to the duty ratio D1 (on time δ1) generated during the ith switching period T. The control unit  50  sets the duty ratio D (on time δ) for the i+3 th  switching period T to the duty ratio D2 (on time δ2) generated during the i+3 th  switching period T, and sets the duty ratio D (on time δ) for the i+4 th  switching period T to the duty ratio D2 (on time δ2) generated during the i+3 th  switching period T. 
     Thus, the control unit  50  forcibly sets the duty ratio D (on time δ) to the same value over a plurality of consecutive switching periods T to suppress a change in the duty ratio D even in a situation where the duty ratio D changes. 
     In step S 40  in  FIG. 6 , the control unit  50  performs processing in step S 20  or S 30  for a predetermined time (at least a time equal to or longer than “N×T”) and then repeats the processing in step S 10  and the subsequent steps. 
       FIGS. 7 and 8  show an example in which the phase difference φ and the duty ratio D gradually increase. However, the above description also applies to a case where the phase difference φ and the duty ratio D gradually decrease. 
       FIG. 9  is a block diagram showing a second configuration example of the control unit  50 . The control unit  50  has the PID control unit  51 , the magnitude-of-change detecting unit  52 , and a gain control unit  54 . Description is omitted which relates to a part of the configuration of the control unit  50  which is similar to the corresponding part of the above-described configuration example. 
     The gain control unit  54  is means for adjusting an amplification factor (gain) for a predetermined input value input to the PID control unit  51  in accordance with the magnitude of change in the phase difference φ detected by the magnitude-of-change detecting unit  52 , enabling suppression of the rate of change R in the command value Do for the duty ratio D with respect to the input value. The gain control unit  54  can suppress the rate of change R in the command value Do to suppress the rate of change in the actual duty ratio D. The rate of change R is an indicator indicating at what rate the duty ratio D has changed with respect to the amount of change in the input value. 
     The predetermined input value input to the PID control unit  51  is, for example, a deviation between the port voltage of at least one of the primary and secondary side ports and the target voltage. As a specific example,  FIG. 9  shows a deviation between the target voltage for the port voltage Vc and the detected voltage of the port voltage Vc acquired by the sensor unit  70 . Furthermore,  FIG. 9  illustrates, as the predetermined amplification factor (gain), a proportional gain P2 that is a constant determining a proportional operation of the PID control unit  51 . 
     The control unit  50  performs switching control on the primary side conversion circuit  20  and the secondary side conversion circuit  30  in accordance with the command value Do set based on the proportional gain P2 suppressed by the gain control unit  54 , to adjust the step-up/step-down ratio of the primary side conversion circuit  20  to the secondary side conversion circuit  30  so that the port voltage converges to the target voltage. 
       FIG. 10  is a flowchart showing a second example of the power conversion method. The power conversion method in  FIG. 10  is executed by the control unit  50 . 
     In step S 50 , the gain control unit  54  determines whether or not the magnitude of change in the phase difference φ detected by the magnitude-of-change detecting unit  52  is equal to or larger than the predetermined value Z. Determining whether or not the magnitude of change in the command value φo for the phase difference φ is equal to or larger than the predetermined value Z enables determination of whether or not the command value φo is about to oscillate. The gain control unit  54  performs processing in step S 70  when the magnitude of change in the phase difference φ is equal to or larger than the predetermined value Z, and performs processing in step S 60  when the magnitude of change in the phase difference φ is smaller than the predetermined value Z. 
     In step S 60 , the gain control unit  54  sets the proportional gain P2 to a normal value (for example, an initial value). In this case, a change in the duty ratio D is not suppressed. 
     On the other hand, in step S 70 , the gain control unit  54  sets the proportional gain P2 to the normal value (for example, the initial value)×a constant β. The constant β is a value smaller than 1. Thus, the proportional gain P2 can be made smaller than the normal value, allowing a change in the duty ratio D to be suppressed. 
     In step  80 , the control unit  50  performs the processing in step S 60  or S 70  for a predetermined time and then repeats the processing in step S 50  and the subsequent steps. 
     An embodiment of the power conversion apparatus and power conversion method was described above, 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.    
     Furthermore, by suppressing a change in the command value φo for the phase difference φ, the control unit  50  may suppress a fluctuation in the transmitted power P, adjusted by changing the duty ratio D. For example, the control unit  50  delays the timing at which the command value φo for the phase difference φ is updated to reduce the frequency of updates of the command value φo, thus suppressing a change in the phase difference φ.