Abstract:
An electric power converter facilitates performing soft switching in the two-way electric-power-conversion operation thereof, and reducing the manufacturing costs thereof and the losses caused therein, The electric power converter includes a first switching device; a second switching device; a first series circuit including capacitor, a diode, the primary winding of transformer, and a third switching device; a second series circuit including a capacitor, a fourth switching device, the primary winding of transformer, and a diode; a third series circuit including a diode and the secondary winding of transformer; and a voltage clamping element connected in parallel to the primary winding of transformer. The first series circuit is connected in parallel to the first switching device, and the second series circuit is connected in parallel to second switching device. The third series circuit is connected between the DC output terminals.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of U.S. patent application Ser. No. 12/379,392 filed on Feb. 20, 2009 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to an electric power converter that generates a DC output from a DC power supply or from an AC power supply. Specifically, the present invention relates to the soft switching function of an electric power converter capable of conducting two-way operations. 
     The circuit of a conventional electric power converter capable of conducting two-way operations is disclosed in the following Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-147475. The conventional circuit disclosed in the Patent Document 1 is shown in  FIG. 3A . 
     The conventional circuit shown in  FIG. 3A  is described in connection with a single-phase AC power supply. The conventional circuit consists of a rectifier circuit including a diode bridge circuit having diodes  2  through  5 , and a chopper circuit including reactor  21 , diode  6 , and switching device  15 . 
     As switching device  15  is turned on, AC power supply  1  is short-circuited via the diode bridge circuit and reactor  21 , energy is stored in reactor  21 , and an AC input current increases. 
     Then, as switching device  15  is turned off, the energy stored in reactor  21  is fed via diode  6  to capacitor  33  and load  34 , which constitute a DC output. 
     By controlling the ON and OFF of switching device  15 , a rectified AC voltage (DC voltage) is converted to an arbitrary DC voltage. A soft switching circuit for the chopper circuit is configured by capacitor  31 , diodes  7 ,  9 ,  10 , voltage clamping element  30 , transformer  22  and switching device  17 . 
       FIG. 3B  is a wave chart describing the operations of the circuit shown in  FIG. 3A . 
     As switching device  17  is turned on, the current that circulates, during a period t 1 , from reactor  21  to reactor  21  via diode  6 , capacitor  33 , diode bridge circuit  40 , and AC power supply  1  gradually changes the current path so as to circulate, due to the influence of the leakage inductance of transformer  22 , from reactor  21  to reactor  21  via diode  7 , primary winding  22   a  of transformer  22 , switching device  17 , diode bridge circuit  40 , and AC power supply  1 . Since the current that flows through switching device  17  increases gradually from zero during the commutation described above, switching device  17  performs soft switching at the turn-ON thereof. 
     Then, a period t 2  starts. During the period t 2 , the current flowing through switching device  17  becomes equal to the current flowing through reactor  21  and diode  6  becomes OFF. Since the current flowing through diode  6  decreases gradually to zero, the surge voltage and the reverse recovery losses caused by the reverse recovery are reduced. At the same time, the electric charge stored in capacitor  31  (or in the parasitic capacitance of switching device  15 ) is discharged via a path connecting capacitor  31 , diode  7 , primary winding  22   a  of transformer  22 , switching device  17 , and capacitor  31 . The electric charge stored in capacitor  31  is regenerated to the output side via secondary winding  22   b  of transformer  22  and diode  10 . 
     By turning on switching device  15  after the voltage thereof lowers to zero in a period t 3 , a difference current, which is the difference between the current flowing through primary winding  22   a  of transformer  22  and the current flowing through reactor  21 , flows through switching device  15 . Since the difference current that flows through switching device  15  initially flows through parasitic diode  12 , the current that flows through switching device  15  increases gradually from a negative value. Therefore, switching device  15  performs soft switching at the state of the turn-ON thereof. 
     Then, the current that has been circulating from reactor  21  to reactor  21  via diode  7 , primary winding  22   a  of transformer  22 , switching device  17 , diode bridge circuit  40 , and AC power supply  1  gradually changes so as to circulate from reactor  21  to reactor  21  via switching device  15 , diode bridge circuit  40 , and AC power supply  1 . At the same time, the energy stored in the leakage inductance of transformer  22  is fed to the output side via secondary winding  22   b  of transformer  22  and diode  10 . The current that flows through switching device  17  decreases gradually to zero. Since switching device  17  is brought into the OFF-state thereof after the current that flows through switching device  17  reaches zero, switching device  17  performs soft switching at the state of the turn-OFF thereof. 
     When switching device  15  is turned off, the voltage of switching device  15  rises gradually due to the current flowing through capacitor  31 . Therefore, the turn-OFF losses are reduced. Thus, switching devices  15  and  17  perform soft switching. 
     In a period t 4 , a reset voltage equal to the voltage clamped by voltage clamping element  30  is caused across primary winding  22   a  of transformer  22 . A voltage, which is as high as the product of the reset voltage and the winding ratio of transformer  22 , is generated across secondary winding  22   b  of transformer  22 . The sum of the DC output voltage and the voltage across secondary winding  22   b  of transformer  22  is applied to diode  10 . By setting the clamping voltage of voltage clamping element  30  to be low, the voltage applied to diode  10  is reduced. 
       FIG. 4A  is a circuit diagram of another conventional electric power converter disclosed in the Patent Document 1. 
     In  FIG. 4A , a rectifier circuit is configured by reactor  21 , diodes  2  through  5 , and switching devices  15  and  16 . Switching device  15  and capacitor  31  are connected in parallel to diode  3 . Switching device  16  and capacitor  32  are connected in parallel to diode  5 . AC power supply  1  is connected between the series connection point of diodes  2  and  3  and the series connection point of diodes  4  and  5  via reactor  21 . Capacitor  33  and load  34  are connected between the DC terminals of the diode bridge circuit. 
     The parasitic diode of switching device  15  may be used in substitution for diode  3 . The parasitic diode of switching device  16  may be used in substitution for diode  5 . The soft switching circuit for the rectifier circuit is configured by diodes  7  through  10 , switching device  17 , transformer  20 , and voltage clamping element  30 . 
       FIG. 4B  is a wave chart describing the operations of the circuit shown in  FIG. 4A . 
     As switching device  15  is turned on when the AC power supply voltage is positive, the AC input current, circulating from AC power supply  1  to AC power supply  1  via reactor  21 , switching device  15 , and diode  5 , increases while storing energy in reactor  21 . Then, as switching device  15  is turned off, the energy stored in reactor  21  is fed to the DC output side via a path connecting reactor  21 , diode  2 , capacitor  33 , diode  5 , AC power supply  1  and reactor  21 . Therefore, it is possible to convert an AC power supply voltage to an arbitrary DC voltage by controlling the ON and OFF of switching device  15  when the AC power supply voltage is positive. In the same manner, it is possible to convert an AC power supply voltage to an arbitrary DC voltage by controlling the ON and OFF of switching device  16  when the AC power supply voltage is negative. 
     In  FIG. 4A , diodes  7  and  8  are disposed in substitution for diode  7  in  FIG. 3A . In  FIG. 4A , diode  8  works for diode  7  in  FIG. 3A , when the AC power supply voltage is positive. Diode  7  works for diode  7  in  FIG. 3A , when the AC power supply voltage is negative. Since switching device  15  is turned on and off when the AC power supply voltage is positive, the electric charge stored in capacitor  31  is regenerated to the DC output side through the operations similar to the operations conducted in the circuit shown in  FIG. 3A . Since a current always flows through diode  5  when the AC power supply voltage is positive, capacitor  32  stores no electric charge. 
     When the AC power supply voltage is negative, the electric charge stored in capacitor  32  is regenerated to the load side through the operations similar to the operations conducted in the circuit shown in  FIG. 3A . Therefore, the circuit shown in  FIG. 4A  conducts operations similar to the operations conducted by the circuit shown in  FIG. 3A . Switching devices  15 ,  16 , and  17  and diodes  2  and  4  conduct soft switching. Since the sum of the DC output voltage and the secondary winding voltage of transformer  22  is applied to diode  10  in the circuit shown in  FIG. 4A  in the same manner as in  FIG. 3A , the voltage applied to diode  10  is reduced by setting the clamping voltage of voltage clamping element  30  to be low. 
     For performing two-way electric power conversion, Patent Document 2: Japanese Unexamined Patent Application Publication No. Sho 64 (1989)-064557 discloses a combination of a buck chopper and a boost chopper. For the boost chopper, a boost chopper including an auxiliary chopper and disclosed in Patent Document 3: Japanese Unexamined Patent Application Publication No. Hei 05 (1993)-328714 may be used. However, the boost chopper including an auxiliary chopper and disclosed in the Patent Document 3 includes many circuit component parts. Moreover, the boost chopper including an auxiliary chopper and disclosed in the Patent Document 3 is large in size and expensive. 
     For realizing two-way electric power conversion in the conventional circuit shown in  FIG. 3A , it is necessary to replace diode  6  by a switching device. For realizing two-way electric power conversion in the conventional circuit shown in  FIG. 4A , it is necessary to replace diodes  2  and  4  by switching devices. The replacing switching device or the replacing switching devices can not perform soft switching. 
     In view of the foregoing, it would be desirable to obviate the problems described above, and to provide a two-way electric power converter that facilitates conducting soft switching operations inexpensively with low conversion losses. 
     Further objects and advantages of the invention will be apparent from the following description of the invention. 
     SUMMARY OF THE INVENTION 
     According to the subject matter of a first aspect of the invention, there is provided an electric power converter including: 
     a first series circuit including a reactor and a first switching device, the first series circuit being connected between DC input terminals; 
     a second series circuit including a second switching device and an output capacitor including a terminal working for a DC output terminal, the second series circuit being connected in parallel to the first switching device; 
     a load connected in parallel to the output capacitor; 
     a third series circuit including a first capacitor, a first diode, a primary winding of a transformer, and a third switching device, the third series circuit being connected in parallel to the first switching device; 
     a fourth series circuit including a second capacitor, a fourth switching device, the primary winding of the transformer, and a second diode, the fourth series circuit being connected in parallel to the second switching device; 
     a fifth series circuit including a third diode and the secondary winding of the transformer, the fifth series circuit being connected between the DC output terminals; and 
     a voltage clamping means connected in parallel to the primary winding of the transformer. 
     According to the subject matter of a second aspect of the invention, there is provided an electric power converter including: 
     an AC power supply; 
     a first series circuit including a first switching device and a second switching device connected in series to each other via an internal connection point, N pieces of the first series circuits being connected in parallel to each other, said N being a nonnegative integer equal to or more than 2; 
     a reactor connected between the AC power supply and the internal connection point in the first one of the first series circuits; 
     an output capacitor including a DC output terminal, the DC output terminals being connected between the parallel connection points of the N pieces of the first series circuits; 
     a load connected between the DC output terminals of the output capacitor; 
     the first series circuit including a first capacitor and a second capacitor connected in parallel to the first switching device and the second switching device, respectively; 
     a first diode including an anode terminal connected to the internal connection point of the first series circuit and a cathode terminal, the cathode terminals of the first diodes being connected collectively; 
     a second series circuit including the primary winding of a transformer and a third switching device; the second series circuit being connected between the cathode terminals of the first diodes and the DC output terminal; 
     a second diode including a cathode terminal connected to the internal connection point of the first series circuit and an anode terminal, the anode terminals of the second diodes being connected collectively; 
     a third series circuit including the primary winding of the transformer and a fourth switching device, the third series circuit being connected between the anode terminals of the second diodes and the DC output terminal; 
     a fourth series circuit including a third diode and the secondary winding of the transformer, the fourth series circuit being connected between the DC output terminals; and 
     a voltage clamping means connected in parallel to the primary winding of the transformer. 
     The electric power converter according to the invention that conducts two-way electric power conversion facilitates performing soft switching with a minimal circuit added thereto and reducing the losses caused thereby. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a circuit diagram showing the circuit configuration of an electric power converter according to a first embodiment of the invention. 
         FIG. 1B  is a wave chart describing the operations of the circuit shown in  FIG. 1A . 
         FIG. 2A  is a circuit diagram showing the circuit configuration of an electric power converter according to a second embodiment of the invention. 
         FIG. 2B  is a wave chart describing the operations of the circuit shown in  FIG. 2A . 
         FIG. 2C  is a circuit diagram showing the circuit configuration of an electric power converter according to a third embodiment of the invention. 
         FIG. 2D  is a circuit diagram showing the circuit configuration of an electric power converter according to a fourth embodiment of the invention. 
         FIG. 3A  is a circuit diagram of a conventional electric power converter. 
         FIG. 3B  is a wave chart describing the operations of the circuit shown in  FIG. 3A . 
         FIG. 4A  is a circuit diagram of another conventional electric power converter. 
         FIG. 4B  is a wave chart describing the operations of the circuit shown in  FIG. 4A . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Now, the invention will be described in detail hereinafter with reference to the accompanied drawings which illustrate the preferred embodiments of the invention. 
       FIG. 1A  is a circuit diagram showing the circuit configuration of an electric power converter according to a first embodiment of the invention. 
     In the circuit shown in  FIG. 1A , DC power supply  51  is employed in substitution for AC power supply  1  and rectifier circuit  40 . Switching device  18  is connected in parallel to diode  6 . DC power supply  51 , reactor  21 , diodes  6  and  12 , and switching devices  15  and  18  constitute a chopper circuit. 
     By turning on and off switching device  15  in the chopper circuit described above, electric power is fed from the DC power supply side to the load side. By turning on and off switching device  18  in the chopper circuit described above, electric power is regenerated from the load side to the DC power supply side. A soft switching circuit is configured by diodes  7 ,  9 ,  10 ,  41 , and  42 ; switching devices  17  and  20 ; transformer  22 ; and voltage clamping element  30 . 
     For feeding electric power from the DC power supply side to the load side, switching devices  15  and  17  and diode  6  are made to conduct soft switching in the same manner as in the circuit shown in  FIG. 3A . The electric power converter according to the first embodiment is different from the conventional electric power converters in that the electric power converter according to the first embodiment makes it possible to conduct soft switching in regenerating electric power from the load side to the DC power supply side by adding a few circuit component parts. Now the regeneration operation conducted by the electric power converter according to the first embodiment will be described in detail below. 
     By turning on switching device  18  in  FIG. 1A , the energy stored in capacitor  33  is transferred to reactor  21  via switching device  18  and regenerated to DC power supply  51 . Then, as switching device  18  is turned off, the energy transferred to reactor  21  is regenerated to DC power supply  51  through a path connecting reactor  21 , DC power supply  51 , and diode  12 . Thus, the energy stored in the capacitor on the load side is regenerated to the DC power supply side by controlling the ON and OFF of switching device  18 . 
     Capacitor  71 ; diodes  9 ,  10 ,  41 , and  42 ; voltage clamping element  30 , transformer  22 ; and switching device  20  form a soft switching circuit for the regeneration operation mode that regenerates electric power from the load side to the DC power supply side. In the same manner as in  FIG. 3A , diodes  9  and  10 ; voltage clamping element  30 ; and transformer  22  are employed also for configuring a soft switching circuit for the operation mode that feeds electric power from the DC power supply side to the load side. 
       FIG. 1B  is a wave chart describing the operations of the circuit shown in  FIG. 1A . 
     As switching device  20  is turned on, the electric charge stored in capacitor  71  (or in the parasitic capacitance of switching device  18 ) is discharged in a period t 1  through a path connecting capacitor  71 , switching device  20 , the primary winding of transformer  22 , and diode  42 . At the same time, the electric charge stored in capacitor  71  is regenerated to the output side via the secondary winding of transformer  22  and diode  10 . Since the current flowing through switching device  20  gradually increases due to the leakage inductance of transformer  22 , switching device  20  performs soft switching during the state of the turn-ON thereof. 
     As soon as the current value flowing through switching device  20  becomes equal to the current value flowing through reactor  21 , a period t 2  starts and diode  12  becomes OFF. Since the current flowing through diode  12  decreases gradually to zero, the surge voltage caused by the reverse recovery and the reverse recovery losses are reduced. As switching device  18  is turned on in a period t 3  after the voltage of switching device  18  becomes zero, a difference current, equal to the difference between the current flowing through the primary winding of transformer  22  and the current flowing through reactor  21 , flows through switching device  18 . Since the difference current that flows through switching device  18  initially flows through diode  6 , the current that flows through switching device  18  gradually increases from a negative value. Therefore, switching device  18  performs soft switching during the state of the turn-ON thereof. 
     When switching device  15  is turned off, the voltage of switching device  15  rises gradually due to the current flowing through capacitor  31 . Therefore, the turn-OFF losses are reduced. Thus, switching devices  15  and  18  perform soft switching at the turn-OFF thereof. In a period t 4 , a reset voltage equal to the voltage clamped by voltage clamping element  30  is caused across the primary winding of transformer  22 . A voltage, which is as high as the product of the reset voltage and the winding ratio of transformer  22 , is generated across the secondary winding of transformer  22 . The sum of the DC output voltage and the voltage across the secondary winding of transformer  22  is applied to diode  10 . By setting the clamping voltage of voltage clamping element  30  to be low, the voltage applied to diode  10  is reduced. 
       FIG. 2A  is a circuit diagram showing the circuit configuration of an electric power converter according to a second embodiment of the invention. 
     As shown in  FIG. 2A , a rectifier circuit is configured by reactor  21 , diodes  2  through  5 , and switching devices  15 ,  16 ,  18  and  19 . Switching device  18  and capacitor  71  are connected in parallel to diode  2  in a diode bridge circuit configured by diodes  2  through  5 . Switching device  15  and capacitor  31  are connected in parallel to diode  3  in the diode bridge circuit. Switching device  19  and capacitor  72  are connected in parallel to diode  4  in the diode bridge circuit. Switching device  16  and capacitor  32  are connected in parallel to diode  5  in the diode bridge circuit. AC power supply  1  is connected between the series connection point of diodes  2  and  3  and the series connection point of diodes  4  and  5  via reactor  21 . Diodes  2  through  5  may be replaced by the parasitic diodes of switching devices  15 ,  16 ,  18 , and  19 , respectively. 
     Diodes  7  through  10 ,  13 ,  41  through  43 ; switching devices  17  and  20 ; transformer  22 ; and voltage clamping element  30  form a soft switching circuit. In detail, the soft switching circuit is configured in the following manner. The anode of diode  8  is connected to the series connection point of diodes  2  and  3 . The anode of diode  7  is connected to the series connection point of diodes  4  and  5 . The cathode of diode  42  is connected to the series connection point of diodes  2  and  3 . The cathode of diode  43  is connected to the series connection point of diodes  4  and  5 . The cathodes of diodes  7  and  8  and the source terminal of switching device  20 , to which diode  41  is connected in parallel, are connected to the first terminal of the primary winding in transformer  22 . The anodes of diodes  42  and  43  and the drain terminal of switching device  17 , to which diode  13  is connected in parallel, are connected to the second terminal of the primary winding in transformer  22 . The drain terminal of switching device  20  is connected to the positive terminal of the DC output. The source terminal of switching device  17  is connected to the negative terminal of the DC output. A series circuit of diode  9  and voltage clamping element  30  is connected in parallel to the primary winding of transformer  22 . A series circuit of diode  10  and the secondary winding of transformer  22  is connected in parallel to capacitor  33 , that is the DC output. The parasitic diodes of switching devices  17  and  20  may be employed in substitution for diodes  13  and  41  with no problem. 
     In feeding electric power from the AC power supply side to the load side in the circuit shown in  FIG. 2A , soft switching is performed by switching devices  15  through  17  and diodes  2  and  4  in the same manner as in the conventional circuit shown in  FIG. 4A . The circuit shown in  FIG. 2A  is different from the conventional circuit shown in  FIG. 4A  in that the circuit shown in  FIG. 2A  facilitates performing soft switching even in regenerating electric power from the load side to the AC power supply side with a few circuit component parts added thereto. 
     As switching devices  16  and  18  are turned on when the AC power supply voltage is positive in the circuit configuration shown in  FIG. 2A , the energy stored in capacitor  33  is transferred to reactor  21  through a path connecting capacitor  33 , switching device  18 , reactor  21 , AC power supply  1 , and switching device  16  and regenerated to AC power supply  1 . Then, by turning off switching device  18 , the energy transferred to reactor  21  is regenerated to AC power supply  1  through a path connecting reactor  21 , AC power supply  1 , switching device  16  and diode  3 . 
     As switching devices  19  and  15  are turned on when the AC power supply voltage is negative in the circuit configuration shown in  FIG. 2A , the energy stored in capacitor  33  is transferred to reactor  21  through a path connecting capacitor  33 , switching device  19 , AC power supply  1 , reactor  21 , and switching device  15  and regenerated to AC power supply  1 . Then, by turning off switching device  19 , the energy transferred to reactor  21  is regenerated to AC power supply  1  through a path connecting reactor  21 , AC power supply  1 , switching device  15  and diode  5 . Thus, by controlling the ON and OFF of switching device  18  or  19 , the energy stored on the load side is regenerated to the AC power supply side. 
     Capacitors  71  and  72 , diodes  9 ,  10 ,  41  through  43 , voltage clamping element  30 , transformer  22 , and switching device  20  form a soft switching circuit for the regeneration operation mode that regenerates electric power from the load side to the AC power supply side. In the same manner as described with reference to  FIG. 4A , diodes  9  and  10 , voltage clamping element  30 , and transformer  22  are employed also for configuring a soft switching circuit for the operation mode that feeds electric power from the AC power supply side to the load side. 
       FIG. 2C  is a circuit diagram showing the circuit configuration of an electric power converter according to a third embodiment of the invention. 
     As shown in  FIG. 2C , reactors  211  through  213  are connected to each phase of a three-phase AC power supplies. A rectifier circuit is configured by diodes  111  through  116  and switching devices  101  through  106 , and a rectifier diode bridge circuit configured by diodes  111  through  116 . Switching device  101  and capacitor  121  are connected in parallel to diode  111 . Switching devices  102  through  106  and capacitors  122  through  126  are connected in parallel to diodes  112  through  116 , respectively. AC power supplies are connected between the series connection point of diodes  111  and  112  via reactor  211 . AC power supplies are connected between the series connection point of diodes  113  and  114  and the series connection point of diodes  115  and  116  via reactors  212  and  213 , respectively. Diodes  111  through  116  may be replaced by the parasitic diodes of switching devices  101  through  106 , respectively. 
     Diodes  9 ,  10 ,  131  through  133 ,  173 ,  174 , and  141  through  143 , switching devices  171  and  172 , transformer  22 , and voltage clamping element  30  form a soft switching circuit. 
     In detail, the soft switching circuit is configured in the following manner. The anode of diode  131  is connected to the series connection point of diodes  111  and  112 . The anode of diode  132  is connected to the series connection point of diodes  113  and  114 . The anode of diode  133  is connected to the series connection point of diodes  115  and  116 . The cathode of diode  141  is connected to the series connection point of diodes  111  and  112 . The cathode of diode  142  is connected to the series connection point of diodes  113  and  114 . The cathode of diode  143  is connected to the series connection point of diodes  115  and  116 . The cathodes of diodes  131  through  133  and the source terminal of switching device  171 , to which diode  173  is connected in parallel, are connected to the first terminal of the primary winding in transformer  22 . The anodes of diodes  141  through  143  and the drain terminal of switching device  172 , to which diode  174  is connected in parallel, are connected to the second terminal of the primary winding in transformer  22 . The drain terminal of switching device  171  is connected to the positive terminal of the DC output. The source terminal of switching device  172  is connected to the negative terminal of the DC output. A series circuit of diode  9  and voltage clamping element  30  is connected in parallel to the primary winding of transformer  22 . A series circuit of diode  10  and the secondary winding of transformer  22  is connected in parallel to capacitor  33 , that is the DC output. The parasitic diodes of switching devices  171  and  172  may be employed in substitution for diodes  173  and  174  with no problem. 
     In feeding electric power from the AC power supply side to the load side or from the load side to the AC power supply side shown in  FIG. 2C , soft switching is performed by switching devices  101  through  106 ,  171 , and  172 , and diodes  111  through  116 . The operation is described as follows. 
     As switching devices  101  and  104  are turned on when the U-phase of AC power supply voltage is positive in the circuit configuration shown in  FIG. 2C , the energy stored in capacitor  33  is transferred to reactors  211  and  212  through a path connecting capacitor  33 , switching device  101 , reactor  211 , AC power supply, reactor  212 , and switching device  104  and regenerated to AC power supply. Then, by turning off switching device  101 , the energy transferred to reactor  211  is regenerated to AC power supply through a path connecting reactor  211 , AC power supply, reactor  212 , switching device  104 , and diode  112 . 
     As switching devices  113  and  112  are turned on when the U-phase of AC power supply voltage is negative in the circuit configuration shown in  FIG. 2C , the energy stored in capacitor  33  is transferred to reactors  211  and  212  through a path connecting capacitor  33 , switching device  113 , reactor  212 , AC power supply, reactor  211 , and switching device  102  and regenerated to AC power supply. Then, by turning off switching device  113 , the energy transferred to reactors  211  and  212  are regenerated to AC power supply through a path connecting reactor  212 , AC power supply, reactor  211 , switching device  112 , and diode  113 . Thus, by controlling the ON and OFF of switching devices  101  through  106 , the energy stored on the load side is regenerated to the AC power supply side. 
     Capacitors  121  through  124 , diodes  9 ,  10 ,  173 ,  174 ,  131 ,  132 ,  141 , and  142 , voltage clamping element  30 , transformer  22 , and switching devices  171  and  172  form a soft switching circuit for the regeneration operation mode that regenerates electric power from the load side to the AC power supply side. In the same manner as described with reference to  FIG. 4A , diodes  9  and  10 , voltage clamping element  30 , and transformer  22  are employed also for configuring a soft switching circuit for the operation mode that feeds electric power from the AC power supply side to the load side. 
       FIG. 2D  is a circuit diagram showing the circuit configuration of an electric power converter according to a fourth embodiment of the invention. 
     As shown in  FIG. 2D , reactors  211  through  214  are connected to each phase of a four-phase AC power supplies. A rectifier circuit is configured by diodes  111  through  118  and switching devices  101  through  108 , and a rectifier diode bridge circuit configured by diodes  111  through  118 . Switching device  101  and capacitor  121  are connected in parallel to diode  111 . Switching devices  102  through  108  and capacitors  122  through  128  are connected in parallel to diodes  112  through  118 , respectively. AC power supplies are connected between the series connection point of diodes  111  and  112  via reactor  211 . AC power supplies are connected between the series connection point of diodes  113  and  114 , the series connection point of diodes  115  and  116 , and the series connection point of diodes  117  and  118  via reactors  212 ,  213 , and  214 , respectively. Diodes  111  through  118  may be replaced by the parasitic diodes of switching devices  101  through  108 , respectively. 
     Diodes  9 ,  10 ,  131  through  134 ,  173 ,  174 , and  141  through  144 , switching devices  171  and  172 , transformer  22 , and voltage clamping element  30  form a soft switching circuit. 
     In detail, the soft switching circuit is configured in the following manner. The anode of diode  131  is connected to the series connection point of diodes  111  and  112 . The anode of diode  132  is connected to the series connection point of diodes  113  and  114 . The anode of diode  133  is connected to the series connection point of diodes  115  and  116 . The anode of diode  134  is connected to the series connection point of diodes  117  and  118 . The cathode of diode  141  is connected to the series connection point of diodes  111  and  112 . The cathode of diode  142  is connected to the series connection point of diodes  113  and  114 . The cathode of diode  143  is connected to the series connection point of diodes  115  and  116 . The cathode of diode  144  is connected to the series connection point of diodes  117  and  118 . The cathodes of diodes  131  through  134  and the source terminal of switching device  171 , to which diode  173  is connected in parallel, are connected to the first terminal of the primary winding in transformer  22 . The anodes of diodes  141  through  144  and the drain terminal of switching device  172 , to which diode  174  is connected in parallel, are connected to the second terminal of the primary winding in transformer  22 . The drain terminal of switching device  171  is connected to the positive terminal of the DC output. The source terminal of switching device  172  is connected to the negative terminal of the DC output. A series circuit of diode  9  and voltage clamping element  30  is connected in parallel to the primary winding of transformer  22 . A series circuit of diode  10  and the secondary winding of transformer  22  are connected in parallel to capacitor  33 , that is the DC output. The parasitic diodes of switching devices  171  and  172  may be employed in substitution for diodes  173  and  174  with no problem. 
     In feeding electric power from the AC power supply side to the load side or from the load side to the AC power supply side shown in  FIG. 2D , soft switching is performed by switching devices  101  through  108 ,  171 , and  172 , and diodes  111  through  118 . The operation is described as follows. 
     In  FIG. 2D , in the same manner as described in  FIG. 2C , as switching devices are turned on based on the voltage of each phase of the AC power supplies, the energy stored in capacitor  33  is transferred to reactors  211  through  214  through a path connecting capacitor  33 , switching device (such as  101 ), reactor (such as  211 ), AC power supply, reactor (such as  212 ), and switching device ( 104 ) and regenerated to AC power supply. Then, by turning off switching device (such as  101 ), the energy transferred to reactor is regenerated to AC power supply through a path connecting reactor ( 211 ), AC power supply, reactor ( 212 ), switching device ( 104 ), and diode ( 112 ). 
     Thus, by controlling the ON and OFF of switching devices  101  through  108 , the energy stored on the load side is regenerated to the AC power supply side. 
     Capacitors  121  through  124 , diodes  9 ,  10 ,  173 ,  174 ,  131 ,  132 ,  141 , and  142 , voltage clamping element  30 , transformer  22 , and switching devices  171  and  172  form a soft switching circuit for the regeneration operation mode that regenerates electric power from the load side to the AC power supply side. In the same manner as described with reference to  FIG. 4A , diodes  9  and  10 , voltage clamping element  30 , and transformer  22  are employed also for configuring a soft switching circuit for the operation mode that feeds electric power from the AC power supply side to the load side. 
       FIG. 2B  is a wave chart describing the operations of the circuit shown in  FIG. 2A . 
     By turning on switching device  20  when the AC power supply voltage is positive, the electric charge stored in capacitor  71  (or in the parasitic capacitance of switching device  18 ) is discharged in a period t 1  through a path connecting capacitor  71 , switching device  20 , the primary winding of transformer  22 , and diode  42 . At the same time, the electric charge stored in capacitor  71  is regenerated to the output side via the secondary winding of transformer  22  and diode  10 . Since the current flowing through switching device  20  increases gradually from zero due to the leakage inductance of transformer  22 , switching device  20  performs soft switching at the turn-ON thereof. 
     As soon as the current value flowing through switching device  20  becomes equal to the current value flowing through reactor  21 , a period t 2  starts and diode  3  becomes OFF. Since the current flowing through diode  3  decreases gradually to zero, the surge voltage caused by the reverse recovery and the reverse recovery losses are reduced. As switching device  18  is turned on in a period t 3  after the voltage of switching device  18  becomes zero, a difference current, equal to the difference between the current flowing through the primary winding of transformer  22  and the current flowing through reactor  21 , flows through switching device  18 . Since the difference current that flows through switching device  18  initially flows through diode  2 , the current flowing through switching device  18  increases gradually from a negative value. Therefore, switching device  18  performs soft switching at the turn-ON thereof. 
     When switching device  18  is turned off, the voltage of switching device  18  rises gradually due to the current flowing through capacitor  71 . Therefore, the turn-OFF losses are reduced. Thus, switching devices  18  and  20  perform soft switching. 
     In a period t 4 , a reset voltage equal to the voltage clamped by voltage clamping element  30  is caused across the primary winding of transformer  22 . A voltage, which is as high as the product of the reset voltage and the winding ratio of transformer  22 , is generated across the secondary winding of transformer  22 . The sum of the DC output voltage and the voltage across the secondary winding of transformer  22  is applied to diode  10 . By setting the clamping voltage of voltage clamping element  30  to be low, the voltage applied to diode  10  is reduced. 
     When the AC power supply voltage is negative, the electric charges stored in capacitor  72  are regenerated to the load side in the same manner as described above. Therefore, the rectifier circuit in  FIG. 2A  works in the same manner as the rectifier circuit in  FIG. 1A . Switching devices  15  through  20  and diodes  2  through  5  perform soft switching. 
     The Disclosure of Japanese Patent Application No. 2008-047706 filed on Feb. 28, 2008 is incorporated in the application. 
     While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.