Abstract:
An AC-DC converter includes two series circuits, an inductor, and a common snubber circuit. Each series circuit includes a switch and a current block device. The block device blocks a current from its high potential side to its low potential side when the first device is closed and allows the current from the low potential side to the high potential side when the first device is opened. The inductor is interposed between a first connection point between the switch and the block device of one series circuit and a second connection point between the switch and the block device of the other series circuit. The common snubber circuit is connected between each of the first connection point and the second connection point and at least one of both ends of the one series circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on Japanese Patent Application No. 2013-008080 filed on Jan. 21, 2013, the contents of which are incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to an AC-DC converter including a first device capable of opening and closing a current path, a second device connected in series to the first device, a third device capable of opening and closing a current path, a fourth device connected in series to the third device, and an inductor interposed between a first connection point between the first device and the second device and a second connection point between the third device and the fourth device. 
       BACKGROUND 
       [0003]    JP-2011-152017A discloses a bridgeless power factor correction (PFC) circuit including two series circuits, each of which has a diode and a semiconductor switching device (an N-channel MOSFET). A connection point between the diode and the switching device of one series circuit is connected to a connection point between the diode and the switching device of the other series circuit through an inductor and an AC power source. This configuration reduces loss in conversion from an AC voltage to a DC voltage. 
         [0004]    In the above circuit, the switching device switches between an ON state and an OFF state at a high speed. Therefore, when the switching device switches from one of the ON state and the OFF state to the other of the ON state and the OFF state, a surge voltage occurs. Specifically, when the switching device switches from the ON state to the OFF state, a surge voltage occurs, and when the switching device switches from the OFF state to the ON state, a surge voltage due to a recovery of the diode occurs. As a result, loss in an AC-DC converter may be increased, reliability of the switching device and the diode may be is degraded, and high-frequency noise may be produced. 
       SUMMARY 
       [0005]    Such disadvantages can be solved by connecting a snubber circuit to each of the switching device and the diode. However, in this case, there is a need to add the same number of snubber circuits as the number of the switching devices or the diodes to the AC-DC converter. Therefore, the number of components of the AC-DC converter is increased, so that the cost of the AC-DC converter will be increased. 
         [0006]    In view of the above, it is an object of the present disclosure to provide an AC-DC converter for reducing an increase in cost due to addition of a snubber circuit. 
         [0007]    According to an aspect of the present disclosure, an AC-DC converter includes a first device, a second device, a third device, a fourth device, a main inductor, a common snubber circuit, a fifth device, and a sixth device. The first device is capable of switching between an open state to open a current path and a closed state to close the current path. The second device is connected in series to the first device to form a first series circuit with a first end defined by the first device and a second end defined by the second device. The third device is capable of switching between an open state to open the current path and a closed state to close the current path. The fourth device is connected in series to the third device to form a second series circuit with a first end defined by the third device and a second end defined by the fourth device. The main inductor is interposed between a first connection point between the first device and the second device and a second connection point between the third device and the fourth device. The common snubber circuit is connected between each of the first connection point and the second connection point and at least one of the first end and the second end of the first series circuit. The fifth device is connected between the first connection point and the snubber circuit. The sixth device is connected between the second connection point and the snubber circuit. The first end of the first series circuit is connected to the first end of the second series circuit. The second end of the first series circuit is connected to the second end of the second series circuit. The second device blocks a current from a high potential side to a low potential side of the second device when the first device is in the closed state and allows the current from the low potential side to the high potential side of the second device when the first device is in the open state. The fourth device blocks the current from a high potential side to a low potential side of the fourth device when the third device is in the closed state and allows the current from the low potential side to the high potential side of the fourth device when the third device is in the open state. When the first device switches from the closed state to the open state to block the current through the first device, the current flows between the main inductor and the first connection point in a first direction. The fifth device blocks the current between the main inductor and the first connection point in a second direction opposite to the first direction from flowing through the fifth device. When the third device switches from the closed state to the open state to block the current through the third device, the current flows between the main inductor and the second connection point in a third direction. The sixth device blocks the current between the main inductor and the second connection point in a fourth direction opposite to the third direction from flowing through the sixth device. 
         [0008]    An advantage of the above aspect is that the snubber circuit is shared between the first device and the third device and also between the second device and the fourth device. Thus, an increase in the number of components of the AC-DC converter due to addition of the snubber circuit is reduced. Accordingly, an increase in cost of the AC-DC converter is reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings, in which the same reference characters represent the same elements. In the drawings: 
           [0010]      FIG. 1  is a diagram illustrating a system according to a first embodiment of the present disclosure; 
           [0011]      FIGS. 2A ,  2 B,  2 C, and  2 D are diagrams illustrating a current flow in a converter according to the first embodiment; 
           [0012]      FIGS. 3A ,  3 B,  3 C, and  3 D are diagrams illustrating a current flow in the converter according to the first embodiment; 
           [0013]      FIG. 4  is a diagram illustrating a system according to a second embodiment of the present disclosure; 
           [0014]      FIGS. 5A ,  5 B,  5 C, and  5 D are diagrams illustrating a current flow in a converter according to the second embodiment; 
           [0015]      FIG. 6  is a diagram illustrating a system according to a third embodiment of the present disclosure; 
           [0016]      FIGS. 7A ,  7 B,  7 C, and  7 D are diagrams illustrating a current flow in a converter according to the third embodiment; 
           [0017]      FIGS. 8A ,  8 B,  8 C, and  8 D are diagrams illustrating a current flow in a converter according to the third embodiment; 
           [0018]      FIGS. 9A ,  9 B,  9 C, and  9 D are diagrams illustrating a current flow in a converter according to the third embodiment; 
           [0019]      FIG. 10  is a diagram illustrating a system according to a fourth embodiment of the present disclosure; 
           [0020]      FIGS. 11A ,  11 B,  11 C, and  11 D are diagrams illustrating a current flow in a converter according to the fourth embodiment; 
           [0021]      FIGS. 12A ,  12 B,  12 C, and  12 D are diagrams illustrating a current flow in a converter according to the fourth embodiment; 
           [0022]      FIG. 13  is a diagram illustrating a system according to a fifth embodiment of the present disclosure; 
           [0023]      FIGS. 14A ,  14 B,  14 C, and  14 D are diagrams illustrating a current flow in a converter according to the fifth embodiment; 
           [0024]      FIG. 15  is a diagram illustrating a system according to a sixth embodiment of the present disclosure; 
           [0025]      FIGS. 16A ,  16 B,  16 C, and  16 D are diagrams illustrating a current flow in a converter according to the sixth embodiment; 
           [0026]      FIG. 17  is a diagram illustrating a system according to a seventh embodiment of the present disclosure; 
           [0027]      FIGS. 18A ,  18 B,  18 C, and  18 D are diagrams illustrating a current flow in a converter according to the seventh embodiment; 
           [0028]      FIGS. 19A ,  19 B,  19 C, and  19 D are diagrams illustrating a current flow in a converter according to the seventh embodiment; 
           [0029]      FIGS. 20A and 20B  are diagrams illustrating a current flow in a converter according to the seventh embodiment; 
           [0030]      FIG. 21  is a diagram illustrating a system according to an eighth embodiment of the present disclosure; 
           [0031]      FIGS. 22A ,  22 B,  22 C, and  22 D are diagrams illustrating a current flow in a converter according to the eighth embodiment; 
           [0032]      FIGS. 23A ,  23 B,  23 C, and  23 D are diagrams illustrating a current flow in a converter according to the eighth embodiment; and 
           [0033]      FIG. 24  is a diagram illustrating a system according to a modification of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
       [0034]    A first embodiment of the present disclosure is described below with reference to the drawings. 
         [0035]    As shown in  FIG. 1 , a converter  10  converts an AC voltage, which is supplied from an AC power source (a commercial power source), to a DC voltage and applies the DC voltage to a load  14 . According to the first embodiment, the converter  10  is configured as a bridgeless PFC circuit. The converter  10  has a main section and an auxiliary section for reducing switching loss. The main section of the converter  10  includes a first main switch Sm 1 , a first main diode Dm 1 , a second main switch Sm 2 , a second main diode Dm 2 , a main inductor  16 , and a smoothing capacitor  18 . The first main switch Sm 1  and the first main diode Dm 1  are connected in series to form a first series circuit. The second main switch Sm 2  and the second main diode Dm 2  are connected in series to form a second series circuit. The first series circuit and the second series circuit are connected in parallel to form a parallel circuit. According to the first embodiment, each of the first main switch Sm 1  and the second main switch Sm 2  is an N-channel MOSFET. The first main switch Sm 1  corresponds to a first device recited in claims, the first main diode Dm 1  corresponds to a second device recited in claims, the second main switch Sm 2  corresponds to a third device recited in claims, and the second main diode Dm 2  corresponds to a fourth device recited in claims. 
         [0036]    The main section of the converter  10  is described below in detail. An anode of the first main diode Dm 1  is connected to a drain of the first main switch Sm 1  An anode of the second main diode Dm 2  is connected to a drain of the second main switch Sm 2 . Cathodes of the first main diode Dm 1  and the second main diode Dm 2  are connected together, and sources of the first main switch Sm 1  and the second main switch Sm 2  are connected together. 
         [0037]    A first connection point P 1  between the first main diode Dm 1  and the first main switch Sm 1  is connected to a second connection point P 2  between the second main diode Dm 2  and the second main switch Sm 2  through the main inductor  16  and the AC power source  12 . Each of the smoothing capacitor  18  and the load  14  is connected in parallel to the parallel circuit of the first series circuit and the second series circuit. 
         [0038]    Next, the auxiliary section of the converter  10  is described. 
         [0039]    The auxiliary section of the converter  10  includes a first diode D 1 , a second diode D 2 , a third diode D 3 , a fourth diode D 4 , and a snubber circuit. The snubber circuit includes a snubber inductor  20 , a snubber capacitor  22 , and a resistor  24 . A series circuit of the first diode D 1  and the snubber inductor  20  is connected in parallel to the first main diode Dm 1 . A series circuit of the second diode D 2  and the snubber inductor  20  is connected in parallel to the second main diode Dm 2 . It is noted that cathodes of the first diode D 1  and the second diode D 2  are connected to the snubber inductor  20 . Each of the first diode D 1  and the third diode D 3  corresponds to a fifth device recited in claims, and each of the second diode D 2  and the fourth diode D 4  corresponds to a sixth device recited in claims. 
         [0040]    A series circuit of the third diode D 3  and the snubber capacitor  22  is connected to the first main switch Sm 1 . A series circuit of the fourth diode D 4  and the snubber capacitor  22  is connected to the second main switch Sm 2 . It is noted that cathodes of the third diode D 3  and the fourth diode D 4  are connected to an end of the snubber capacitor  22 . The end of the snubber capacitor  22  is connected to the cathode of the second main diode Dm 2  through the resistor  24 . 
         [0041]    A control unit  25  controls an output voltage of the converter  10 . The control unit  25  outputs control signals gm 1  and gm 2  to gates of the first main switch Sm 1  and the second main switch Sm 2 , thereby controlling (i.e., turning ON and OFF) the first main switch Sm 1  and the second main switch Sm 2 , respectively. Thus, the control unit  25  turns ON and OFF the first main switch Sm 1  and the second main switch Sm 2  so that AC power supplied to the main inductor  16  can be converted to DC power. 
         [0042]    Next, operations of the auxiliary section of the converter  10  are described with reference to  FIGS. 2A-2D  and  FIGS. 3A-3D . 
         [0043]      FIGS. 2A-2D  show a current flow in the converter  10  observed when an output voltage of the AC power source  12  has positive polarity.  FIGS. 3A-3D  show a current flow in the converter  10  observed when the output voltage of the AC power source  12  has negative polarity. The following description is basically based on a current continuous mode. 
         [0044]    When the output voltage of the AC power source  12  has positive polarity, the first main switch Sm 1  is controlled (i.e., turned ON and OFF) under a condition that the second main switch Sm 2  is kept ON (i.e., kept closed). In this state, since a current always flows through the second main switch Sm 2 , the second connection point P 2  becomes ground potential so that a reverse voltage can be applied across both of the second diode D 2  and the fourth diode D 4 . Therefore, when the output voltage of the AC power source  12  has positive polarity, the converter  10  has four states as shown in  FIGS. 2A-2D . 
         [0045]      FIG. 2A  shows a first state S 1  where the first main switch Sm 1  is ON under the condition that the second main switch Sm 2  is kept ON. In the first state S 1 , the current flows through a closed loop circuit constructed with the AC power source  12 , the main inductor  16 , the first main switch Sm 1 , and the second main switch Sm 2 . Thus, magnetic energy is stored in the main inductor  16 . 
         [0046]      FIG. 2B  shows a second state S 2  subsequent to the first state S 1  shown in  FIG. 2A . When the first main switch Sm 1  is turned OFF, there occurs a transition from the first state S 1  to the second state S 2 . In the second state S 2 , the current flows through a closed loop circuit constructed with the AC power source  12 , the main inductor  16 , the third diode D 3 , the snubber capacitor  22 , and the second main switch Sm 2 . In this case, when the first main switch Sm 1  is turned OFF, a voltage between a pair of terminals (i.e., the drain and source) of the first main switch Sm 1  is limited by a speed at which a voltage of the snubber capacitor  22  increases (i.e., at which the snubber capacitor  22  is charged). Therefore, a surge voltage occurring when the first main switch Sm 1  is turned OFF can be reduced. 
         [0047]      FIG. 2C  shows a third state S 3  subsequent to the second state S 2  shown in  FIG. 2B . When the voltage of the snubber capacitor  22  reaches the output voltage of the converter  10  (i.e., voltage of the smoothing capacitor  18 ) so that the current outputted from the main inductor  16  can flow through each of the first main diode Dm 1  and the snubber inductor  20 , there occurs a transition from the second state S 2  to the third state S 3 . 
         [0048]      FIG. 2D  shows a fourth state S 4  subsequent to the third state S 3  shown in  FIG. 2C . When the first main switch Sm 1  is turned ON, there occurs a transition from the third state S 3  to the fourth state S 4 . According to the first embodiment, even when the first main switch Sm 1  is turned ON, the current flow from the main inductor  16  to the snubber inductor  20  through the first diode D 1  is continued by the action of the snubber inductor  20 . Further, when a recovery current flows due to application of a reverse voltage to the first main diode Dm 1 , an inductance of a current path where the recovery current flows is increased by the action of the snubber inductor  20 . Therefore, the rate of change in the recovery current is reduced. Accordingly, a surge voltage due to the recovery current is reduced. 
         [0049]    It is noted that one cycle of the operation of the auxiliary section is defined as one cycle of an ON and OFF operation of the first main switch Sm 1  In the third state S 3  shown in  FIG. 2C  and the fourth state S 4  shown in  FIG. 2D , the snubber capacitor  22  is discharged through the resistor  24 , because the voltage of the snubber capacitor  22  increases above the voltage of the smoothing capacitor  18 . When the current flowing through the snubber inductor  20  becomes zero in the fourth state S 4 , there occurs a transition from the fourth state S 4  to the first state S 1  shown in  FIG. 2A . 
         [0050]    In contrast, when the output voltage of the AC power source  12  has negative polarity, the second main switch Sm 2  is turned ON and OFF under a condition that the first main switch Sm 1  is kept ON (i.e., kept closed). In this state, since the current always flows through the first main switch Sm 1 , the first connection point P 1  becomes ground potential so that a reverse voltage can be applied across both of the first diode D 1  and the third diode D 3 . Therefore, when the output voltage of the AC power source  12  has negative polarity, the converter  10  has four states as shown in  FIGS. 3A-3D . 
         [0051]      FIG. 3A  shows a first state S 1  where the second main switch Sm 2  is ON under the condition that the first main switch Sm 1  is kept ON. In the first state S 1 , the current flows through a closed loop circuit constructed with the AC power source  12 , the second main switch Sm 2 , the first main switch Sm 1 , and the main inductor  16 . Thus, magnetic energy is stored in the main inductor  16 . 
         [0052]      FIG. 3B  shows a second state S 2  subsequent to the first state S 1  shown in  FIG. 3A . When the second main switch Sm 2  is turned OFF, there occurs a transition from the first state S 1  to the second state S 2 . In the second state S 2 , the current flows through a closed loop circuit constructed with the AC power source  12 , the fourth diode D 4 , the snubber capacitor  22 , the first main switch Sm 1 , and the main inductor  16 . In this case, when the second main switch Sm 2  is turned OFF, a voltage between a pair of terminals of the second main switch Sm 2  is limited by the speed at which the voltage of the snubber capacitor  22  increases. Therefore, a surge voltage occurring when the second main switch Sm 2  is turned OFF can be reduced. 
         [0053]      FIG. 3C  shows a third state S 3  subsequent to the second state S 2  shown in  FIG. 3B . When the voltage of the snubber capacitor  22  reaches the voltage of the smoothing capacitor  18  so that the current outputted from the main inductor  16  can flow through each of the second main diode Dm 2  and the snubber inductor  20 , there occurs a transition from the second state S 2  to the third state S 3 . It is noted that when the second main switch Sm 2  is turned OFF, the second diode D 2  and the fourth diode D 4  allow the current flowing in a first direction from the second connection point P 2  toward the second diode D 2  and the fourth diode D 4  but prevent the current flowing in a second direction opposite to the first direction. 
         [0054]      FIG. 3D  shows a fourth state S 4  subsequent to the third state S 3  shown in  FIG. 3C . When the second main switch Sm 2  is turned ON, there occurs a transition from the third state S 3  to the fourth state S 4 . According to the first embodiment, as described for the state  4  shown in  FIG. 2D , a surge voltage occurring when the second main switch Sm 2  is turned ON can be reduced by the action of the snubber inductor  20 . 
         [0055]    In the third state S 3  shown in  FIG. 3C  and the fourth state S 4  shown in  FIG. 3D , the snubber capacitor  22  is discharged through the resistor  24 , because the voltage of the snubber capacitor  22  increases above the voltage of the smoothing capacitor  18 . When the current flowing through the snubber inductor  20  becomes zero in the fourth state S 4 , there occurs a transition from the fourth state S 4  to the first state S 1  shown in  FIG. 3A . 
         [0056]    As described above, according to the first embodiment, the snubber circuit is shared between the first main switch Sm 1  and the second main switch Sm 2  and also between the first main diode Dm 1  and the second main diode Dm 2 . In such an approach, an increase in the number of components of the converter  10  due to addition of the snubber circuit is reduced. Accordingly, an increase in cost of the converter  10  can be reduced. 
       Second Embodiment 
       [0057]    A second embodiment of the present disclosure is described below. 
         [0058]    The second embodiment differs from the first embodiment in the configuration of the auxiliary section of the converter  10 . 
         [0059]      FIG. 4  shows the converter  10  according to the second embodiment. 
         [0060]    As shown in  FIG. 4 , the auxiliary section includes the first diode D 1 , the second diode D 2 , and a snubber circuit. The snubber circuit includes a secondary diode Ds, a snubber capacitor  22   a , a secondary inductor  26  for storing energy, and a secondary switch Sb. According to the second embodiment, the secondary switch Sb is an N-channel MOSFET. Specifically, a series circuit of the first diode D 1  and the snubber capacitor  22   a  is connected to the first main switch Sm 1 , and a series circuit of the second diode D 2  and the snubber capacitor  22   a  is connected to the second main switch Sm 2 . It is noted that the cathodes of the first diode D 1  and the second diode D 2  are connected to the snubber capacitor  22   a.    
         [0061]    A series circuit of the secondary inductor  26  and the secondary switch Sb is connected in parallel to the snubber capacitor  22   a . Specifically, a drain of the secondary switch Sb is connected to the secondary inductor  26 . An anode of the secondary diode Ds is connected to a connection point between the secondary inductor  26  and the secondary switch Sb. A cathode of the secondary diode Ds is connected to the cathode of the second main diode Dm. The secondary switch Sb is controlled (i.e., turned ON and OFF) by a control signal gs outputted from the control unit  25 . 
         [0062]    Next, operations of the auxiliary section according to the second embodiment are described with reference to  FIGS. 5A-5D . 
         [0063]      FIGS. 5A-5D  show a current flow in the converter  10  observed when the output voltage of the AC power source  12  has positive polarity. In this state, since the second main switch Sm 2  is kept ON, the current always flows through the second main switch Sm 2  so that a reverse voltage can be applied across the second diode D 2 . Therefore, when the output voltage of the AC power source  12  has positive polarity, the converter  10  has four states as shown in  FIGS. 5A-5D . 
         [0064]      FIG. 5A  shows a first state S 1 . In the main section, the first state S 1  shown in  FIG. 5A  is the same as the first state S 1  shown in  FIG. 2A . In the auxiliary section, the secondary switch Sb is OFF. 
         [0065]      FIG. 5B  shows a second state S 2  subsequent to the first state S 1  shown in  FIG. 5A . When the first main switch Sm 1  is turned OFF, there occurs a transition from the first state S 1  to the second state S 2 . In the second state S 2 , the current flows through a closed loop circuit constructed with the AC power source  12 , the main inductor  16 , the first diode D 1 , the snubber capacitor  22   a , and the second main switch Sm 2 . In this case, when the first main switch Sm 1  is turned OFF, the voltage between the pair of terminals of the first main switch Sm 1  is limited by a speed at which a voltage of the snubber capacitor  22   a  increases. Therefore, a surge voltage occurring when the first main switch Sm 1  is turned OFF can be reduced. 
         [0066]    Then, when the voltage of the snubber capacitor  22   a  reaches the voltage of the smoothing capacitor  18 , the current outputted from the main inductor  16  flows through the first main diode Dm 1 . 
         [0067]      FIG. 5C  shows a third state S 3  subsequent to the second state S 2  shown in  FIG. 5B . When the first main switch Sm 1  is turned ON, and the secondary switch Sb is turned ON, there occurs a transition from the second state S 2  to the third state S 3 . In the third state S 3 , the snubber capacitor  22  is discharged so that the current can flow through a closed loop circuit constructed with the snubber capacitor  22   a , the secondary inductor  26 , and the secondary switch Sb. In this way, electrical energy stored in the snubber capacitor  22   a  is transferred to the secondary inductor  26 . 
         [0068]      FIG. 5D  shows a fourth state S 4  subsequent to the third state S 3  shown in  FIG. 5C . When the secondary switch Sb is turned OFF, there occurs a transition from the third state S 3  to the fourth state S 4 . In the fourth state S 4 , magnetic energy stored in the secondary inductor  26  produces the current flowing in a direction from the secondary inductor  26  to the secondary diode Ds. 
         [0069]    When the voltage of the snubber capacitor  22  gradually decreases to zero in the fourth state S 4 , there occurs a transition from the fourth state  64  to the first state S 1  shown in  FIG. 5A . Because of the transfer of the charge in the snubber capacitor  22   a  in the third state S 3  and the fourth state S 4 , the energy stored in the snubber capacitor  22   a  is outputted to the smoothing capacitor  18  side without any loss of energy in theory and used as output energy. That is, the snubber circuit according to the second embodiment is a lossless snubber circuit. 
         [0070]    By the way, when the output voltage of the AC power source  12  has negative polarity, the auxiliary section operates in the same manner as described above for when the output voltage of the AC power source  12  has positive polarity. Therefore, the description of the operations of the auxiliary section observed when the output voltage of the AC power source  12  has negative polarity is omitted. 
         [0071]    As described above, according to the second embodiment, the snubber circuit has a lot of components. However, since the snubber circuit is shared between the first main switch Sm 1  and the second main switch Sm 2 , an increase in the number of components of the converter  10  due to addition of the snubber circuit is reduced. Accordingly, an increase in cost of the converter  10  can be reduced. 
       Third Embodiment 
       [0072]    A third embodiment of the present disclosure is described below. 
         [0073]    The third embodiment differs from the first embodiment in the configuration of the auxiliary section of the converter  10 . 
         [0074]      FIG. 6  shows the converter  10  according to the third embodiment. 
         [0075]    As shown in  FIG. 6 , the auxiliary section includes the first diode D 1 , the second diode D 2 , the third diode D 3 , the fourth diode D 4 , and a snubber circuit. The snubber circuit includes the snubber capacitor  22 , a secondary diode Dsa, a secondary switch Sba, and a transformer  30 . According to the third embodiment, the secondary switch Sba is an N-channel MOSFET. Specifically, a series circuit of a primary coil  30   a  of the transformer  30  and the secondary switch Sba is connected to the snubber capacitor  22 . It is noted that a drain of the secondary switch Sba is connected to the primary coil  30   a . The secondary switch Sba is controlled (i.e., turned ON and OFF) by a control signal gsa outputted from the control unit  25 . 
         [0076]    An anode of the secondary diode Dsa is connected to a connection point between the primary coil  30   a  and the secondary switch Sba. A cathode of the secondary diode Dsa is connected to the cathode of the second main diode Dm 2 . Further, the cathode of the secondary diode Dsa is connected to the cathodes of the first diode D 1  and the second diode D 2  through a series circuit of a secondary coil  30   b  of the transformer  30  and a secondary inductor  32 . 
         [0077]    The number of turns of the primary coil  30   a  is sufficiently larger than the number of turns of the secondary coil  30   b . According to the third embodiment, the secondary inductor  32  is provided by a leakage inductance of the transformer  30 . 
         [0078]    By the way, according to the third embodiment, since the converter  10  is configured as a semi-bridgeless PFC circuit, the main inductor  16  is divided. Specifically, the first connection point P 1  is connected to the second connection point P 2  through a first main inductor  16   a , the AC power source  12 , and a second main inductor  18   b . Further, a connection point between the first main inductor  16   a  and the AC power source  12  is connected to a cathode of a first auxiliary diode Da, and a connection point between the AC power source  12  and the second main inductor  16   b  is connected to a cathode of a second auxiliary diode Db. Anodes of the first auxiliary diode Da and the second auxiliary diode Db are connected to the source of the first main switch Sm 1 . 
         [0079]    Next, operations of the auxiliary section according to the third embodiment are described with reference to  FIGS. 7A-7D  and  FIGS. 8A-8D . When the output voltage of the AC power source  12  has negative polarity, the auxiliary section operates in the same manner as when the output voltage of the AC power source  12  has positive polarity. Therefore, the description of the operations of the auxiliary section observed when the output voltage of the AC power source  12  has negative polarity is omitted. 
         [0080]      FIGS. 7A-7D  and  FIGS. 8A-6D  show a current flow in the converter  10  observed when the output voltage of the AC power source  12  has positive polarity. In this state, since the second main switch Sm 2  is kept OFF, the current flows through the second auxiliary diode Db in the forward direction so that a reverse voltage can be applied across each of the second diode D 2  and the fourth diode D 4 . Therefore, when the output voltage of the AC power source  12  has positive polarity, the converter  10  has eight states as shown in  FIGS. 7A-7D  and  FIGS. 8A-8D . 
         [0081]      FIG. 7A  shows a first state S 1  where the first main switch Sm 1  is ON. In the first state S 1 , the current flows through a closed loop circuit constructed with the AC power source  12 , the first main Inductor  16   a , the first main switch Sm 1 , and the second auxiliary diode Db. Thus, magnetic energy is stored in the first main inductor  16   a.    
         [0082]      FIG. 7B  shows a second state S 2  subsequent to the first state S 1  shown in  FIG. 7A . When the first main switch Sm 1  is turned OFF, there occurs a transition from the first state S 1  to the second state S 2 . In the second state S 2 , the current outputted from the first main inductor  16   a  flows to the snubber capacitor  22  through the third diode D 3 . At this time, the voltage between the pair of terminals of the first main switch Sm 1  is limited by the speed at which the voltage of the snubber capacitor  22  increases. Therefore, a surge voltage occurring when the first main switch Sm 1  is turned OFF can be reduced. 
         [0083]      FIG. 7C  shows a third state S 3  subsequent to the second state S 2  shown in  FIG. 7B . When the voltage of the snubber capacitor  22   a  reaches the voltage of the smoothing capacitor  18 , there occurs a transition from the second state S 2  to the third state S 3 . In the third state S 3 , the current outputted from the first main inductor  16   a  flows to the smoothing capacitor  18  side through the first main diode Dm 1 . 
         [0084]      FIG. 7D  shows a fourth state S 4  subsequent to the third state S 3  shown in  FIG. 7C . When the secondary switch Sba is turned ON before the first main switch Sm 1  is turned ON, there occurs a transition from the third state S 3  to the fourth state S 4 . In the fourth state S 4 , the current is commutated from the first main diode Dm 1  toward the secondary coil  30   b  of the transformer  30 . Therefore, while the current flowing through the first main diode Dm 1  gradually decreases, the current flowing through the secondary coil  30   b  gradually increases. That is, when the secondary switch Sba is turned ON, part of the current flowing through the first main inductor  16   a  flows to the secondary switch Sba through the third diode D 3  and the primary coil  30   a . As shown in the drawings, polarities of the coils  30   a  and  30   b  are set such that when a voltage which is positive on the third diode D 3  side is applied to the primary coil  30   a , a voltage which is positive on the secondary inductor  32  side is applied to the secondary coil  30   b . Therefore, when the current flows through the primary coil  30   a , the current also flows through the secondary coil  30   b . In this case, since the number of turns of the primary coil  30   a  is larger than the number of turns of the secondary coil  30   b , the current is sufficiently large compared to the current flowing through the secondary switch Sba. At this time, magnetic energy is stored in the transformer  30  and the secondary inductor  32 . 
         [0085]      FIG. 8A  shows a fifth state S 5  subsequent to the fourth state S 4  shown in  FIG. 7D . When the first main switch Sm 1  is turned ON after the current flowing through the first main diode Dm 1  becomes zero, there occurs a transition from the fourth state S 4  to the fifth state S 5 . When the first main switch Sm 1  is turned ON, the current flowing through the secondary inductor  32  gradually decreases, and the current flowing through the first main switch Sm 1  gradually increases accordingly. Since the gradual increase speed is limited by an inductance of the secondary inductor  32 , a surge voltage occurring when the first main switch Sm 1  is turned ON can be reduced. 
         [0086]    Further, when the first main switch Sm 1  is turned ON, the first connection point P 1  decreases close to ground potential. Therefore, the snubber capacitor  22  is discharged so that current can flow through the secondary switch Sba. 
         [0087]    As described above, the secondary inductor  32  has a function of limiting the gradual increase of the speed in the current flowing though the first main switch Sm 1  In contrast, if there is no secondary inductor  32 , the transformer  30  cannot be an ideal transformer and needs to have a leakage inductance for the following reasons. In the case of an ideal transformer, a relationship between the voltage of the primary coil  30   a  and the voltage of the secondary coil  30   b  depends on the turn ratio. Therefore, the voltage of the secondary coil  30   b  cannot be increased to the output voltage of the converter  10 . As a result, the turn on of the first main switch Sm 1  does not induce current flow through the secondary coil  30   b,    
         [0088]      FIG. 8B  shows a sixth state S 6  subsequent to the fifth state S 5  shown in  FIG. 8A . When the secondary switch Sba is turned OFF, there occurs a transition from the fifth state S 5  to the sixth state S 6 . In the sixth state S 6 , since the magnetic energy stored in the transformer  30  is released through the secondary diode Dsa, the snubber capacitor  22  continues to be discharged. 
         [0089]      FIG. 8C  shows a seventh state S 7  subsequent to the sixth state S 6  shown in  FIG. 8B . When the current flowing through the first main inductor  16   a  entirely flows through the first main switch Sm 1 , there occurs a transition from the sixth state S 6  to the seventh state S 7 . 
         [0090]      FIG. 8D  shows an eighth state S 8  subsequent to the seventh state S 7  shown in  FIG. 8C . When the snubber capacitor  22  is fully discharged, there occurs a transition from the seventh state S 7  to the eighth state S 8 . In the eighth state S 8 , because of the magnetic energy stored in the transformer  30 , part of the current flowing through the first main inductor  16   a  flows into the transformer  30 . Then, when the magnetic energy stored in the transformer  30  becomes zero in the eighth state S 8  so that the entire current flowing through the first main inductor  16   a  can flow through the first main switch Sm 1 , there occurs a transition from the eighth state S 8  to the first state S 1  shown in  FIG. 7A . 
         [0091]    It is noted that one cycle of the operation of the auxiliary section is defined as one cycle of an ON and OFF operation of the first main switch Sm 1 . The energy of the snubber capacitor  22  discharged in the fifth, sixth, and seventh state S 7 s shown in  FIGS. 8A-8C  is outputted to the smoothing capacitor  18  side without any loss of energy in theory and used as output energy. 
         [0092]    Next, operations of the auxiliary section in a discontinuous current mode where the current flowing through the first main inductor  16   a  remains zero for a finite time are described with reference to  FIGS. 9A-9D . In  FIGS. 9A-9D , a state number is assigned to correspond to the state number shown in  FIGS. 7A-7D  and  8 A- 8 D. 
         [0093]      FIG. 9A  shows a third state S 3  where the current flowing through the first main inductor  16   a  is zero. 
         [0094]      FIG. 9B  shows a fourth state S 4  subsequent to the third state S 3  shown in  FIG. 9A . When the secondary switch Sb is turned ON, there occurs a transition from the third state S 3  to the fourth state S 4 . In the fourth state S 4 , the snubber capacitor  22  is discharged through the primary coil  30   a  of the transformer  30  and the secondary switch Sba. However, at this time, no current flows through the secondary coil  30   b  of the transformer  30 . The reason for this is that since the number of turns of the secondary coil  30   b  is smaller than the number of turns of the primary coil  30   a , a voltage induced in the secondary coil  30   b  becomes smaller than the voltage of the smoothing capacitor  18 . Therefore, the primary coil  30   a  functions as an inductor and stores the energy discharged by the snubber capacitor  22  as magnetic energy. 
         [0095]      FIG. 9C  shows a fifth state S 5  subsequent to the fourth state S 4  shown in  FIG. 9B . When the first main switch Sm 1  is turned ON, there occurs a transition from the fourth state S 4  to the fifth state S 5 . In the fifth state S 5 , a speed at which the current flowing through the first main switch Sm 1  gradually increases is limited by an inductance of the first main inductor  16   a . Therefore, a surge voltage occurring when the first main switch Sm 1  is turned ON can be reduced. 
         [0096]      FIG. 9D  shows a sixth state S 6  subsequent to the fifth state S 5  shown in  FIG. 9C . When the secondary switch Sba is turned OFF, there occurs a transition from the fifth state S 5  to the sixth state S 6 . In the sixth state S 6 , the magnetic energy stored in the transformer  30  is outputted to the smoothing capacitor  18  side through the primary coil  30   a  and the secondary diode Dsa. In this way, the energy stored in the snubber capacitor  22  is used as output energy without any loss of energy in theory. Then, when the voltage of the snubber capacitor  22  becomes zero, there occurs a transition to the first state S 1  shown in  FIG. 7A . 
         [0097]    Like the second embodiment, the third embodiment described above can reduce the increase in cost of the converter  10 , 
       Fourth Embodiment 
       [0098]    A fourth embodiment of the present disclosure is described below, 
         [0099]    The fourth embodiment differs from the first embodiment in the configuration of the auxiliary section of the converter  10 . 
         [0100]      FIG. 10  shows the converter  10  according to the fourth embodiment. 
         [0101]    As shown in  FIG. 10 , the auxiliary section of the converter  10  has a snubber circuit including a fifth diode D 5 , a sixth diode D 6 , a snubber capacitor  22   b , a secondary diode Dab, a first secondary switch Sbb, and a second secondary switch Sbc. According to the fourth embodiment, each of the first secondary switch Sbb and the second secondary switch Sbc is an insulated gate bipolar transistor (IGBT) with an antiparallel diode. It is noted that the first secondary switch Sbb corresponds to a first secondary switch recited in claims, the second secondary switch Sbc corresponds to a second secondary switch recited in claims, the secondary diode Dsb corresponds to a first secondary block device recited in claims, the fifth diode D 5  corresponds to a second secondary block device recited in claims, and the sixth diode D 6  corresponds to a third secondary block device. 
         [0102]    The auxiliary section is described in detail below. The first main switch Sm 1  is connected in parallel to a series circuit of the first diode D 1 , the snubber capacitor  22   b , and the first secondary switch Sbb. Specifically, the cathode of the first diode D 1  is connected to the snubber capacitor  22   b , and a collector of the first secondary switch Sbb is connected to the snubber capacitor  22   b.    
         [0103]    The second main switch Sm 2  is connected in parallel to a series circuit of the second diode D 2 , the snubber capacitor  22   b , and the first secondary switch Sbb. Specifically, the cathode of the second diode D 2  is connected to the snubber capacitor  22   b.    
         [0104]    An anode of the fifth diode D 5  is connected to the first connection point P 1 , and an anode of the sixth diode D 6  is connected to the second connection point P 2 . Cathodes of the fifth diode D 5  and the sixth diode D 6  are connected through the second secondary switch Sbc to a connection point between the snubber capacitor  22   b  and the first secondary switch Sbb. Specifically, the cathodes of the fifth diode D 5  and the sixth diode D 6  are connected to a collector of the second secondary switch Sbc. 
         [0105]    An anode of the secondary diode Dab is connected to a connection point between the snubber capacitor  22   b  and each of the cathodes of the first diode D 1  and the second diode D 2 . A cathode of the secondary diode Dab is connected to the cathode of the second main diode Dm 2 . 
         [0106]    The first and second secondary switches Sbb and Sbc are controlled (i.e., turned ON and OFF) by control signals gab and gsc outputted from the control unit  25 , respectively. In the fourth embodiment, like in the third embodiment, the main inductor  16  is divided into the first main inductor  16   a  and the second main inductor  16   b.    
         [0107]    Next, operations of the auxiliary section according to the fourth embodiment are described with reference to  FIGS. 11A-11D  and  FIGS. 12A-12D . When the output voltage of the AC power source  12  has negative polarity, the auxiliary section operates in the same manner as when the output voltage of the AC power source  12  has positive polarity. Therefore, the description of the operations of the auxiliary section observed when the output voltage of the AC power source  12  has negative polarity is omitted, 
         [0108]      FIGS. 11A-11D  and  FIGS. 12A-12D  show a current flow in the converter  10  observed when the output voltage of the AC power source  12  has positive polarity. In this state, a reverse voltage is applied across each of the second diode D 2  and the sixth diode D 6 . Therefore, when the output voltage of the AC power source  12  has positive polarity, the converter  10  has eight states as shown in  11 A- 11 D and  FIGS. 12A-12D , 
         [0109]      FIG. 11A  shows a first state S 1 . In the main section, the first state S 1  shown in  FIG. 11A  is the same as the first state S 1  shown in  FIG. 2A . In the auxiliary section, the first secondary switch Sbb is OFF, and the second secondary switch Sbc is ON. 
         [0110]      FIG. 11B  shows a second state S 2  subsequent to the first state S 1  shown in  FIG. 11A . When the first secondary switch Sbb is turned ON, and the second secondary switch Sbc is turned OFF, there occurs a transition from the first state S 1  to the second state S 2 . In the second state S 2 , the first main switch Sm 1  is connected in parallel to the snubber capacitor  22   b . Since no charge is stored in the snubber capacitor  22   b , no loss of energy occurs when the first secondary switch Sbb is turned ON. 
         [0111]      FIG. 11C  shows a third state S 3  subsequent to the second state S 2  shown in  FIG. 11B . When the first main switch Sm 1  is turned OFF, there occurs a transition from the second state S 2  to the third state S 3 . At this time, the voltage between the pair of terminals of the first main switch Sm 1  is limited by a speed at which a voltage of the snubber capacitor  22   b  increases. Therefore, a surge voltage occurring when the first main switch Sm 1  is turned OFF can be reduced. 
         [0112]      FIG. 11D  shows a fourth state S 4  subsequent to the third state S 3  shown in  FIG. 11C . When the voltage of the snubber capacitor  22   b  reaches the voltage of the smoothing capacitor  18 , there occurs a transition from the third state S 3  to the fourth state S 4 . In the fourth state S 4 , the currant outputted from the first and second main inductors  16   a  and  16   b  is outputted through the first main diode Dm 1 . 
         [0113]      FIG. 12A  shows a fifth state S 5  subsequent to the fourth state S 4  shown in  FIG. 11D . When the first main switch Sm 1  is turned ON, there occurs a transition from the fourth state S 4  to the fifth state S 5 . It is noted that since a reverse voltage is applied across the first diode D 1 , the snubber capacitor  22   b  is disconnected from the first main switch Sm 1 . Therefore, when the first main switch Sm 1  is turned ON, the snubber capacitor  22   b  is not discharged. 
         [0114]      FIG. 12B  shows a sixth state S 6  subsequent to the fifth state S 5  shown in  FIG. 12A . When the first secondary switch Sbb is turned OFF, and the second secondary switch Sbc is turned ON, there occurs a transition from the fifth state S 5  to the sixth state S 6 . At this time, the voltage of the snubber capacitor  22   b  is equal to the voltage of the smoothing capacitor  18 . Therefore, no current flows through the second secondary switch Sbc, when the second secondary switch Sbc is turned ON. Thus, no loss of energy occurs when the second secondary switch Sbc is turned ON. 
         [0115]      FIG. 12C  shows a seventh state S 7  subsequent to the sixth state S 6  shown in  FIG. 12B . When the first main switch Sm 1  is turned OFF, there occurs a transition from the sixth state S 6  to the seventh state S 7 . In the seventh state S 7 , the current outputted from the first and second main inductors  16   a  and  16   b  is outputted to the smoothing capacitor  18  side through the second secondary switch Sbc, the snubber capacitor  22   b , and the secondary diode Dsb. At this time, a speed at which the voltage of the first main switch Sm 1  increases is limited by a speed at which the voltage of the snubber capacitor  22   b  decreases (at which the snubber capacitor  22   b  is discharged). Therefore, a surge voltage occurring when the first main switch Sm 1  is turned OFF can be reduced. 
         [0116]      FIG. 12D  shows an eighth state S 8  subsequent to the seventh state S 7  shown in  FIG. 12C . When the current flowing through the second secondary switch Sbc becomes zero, there occurs a transition from the seventh state S 7  to the eighth state S 8 . Then, when the first main switch Sm 1  is turned ON, there occurs a transition to the first state S 1  shown in  FIG. 11A . 
         [0117]    It is noted that one cycle of the operation of the auxiliary section according to the fourth embodiment is defined as two cycles of an ON and OFF operation of the first main switch Sm 1 . That is, the snubber capacitor  22   b  is charged by the ON and OFF operation of the first main switch Sm 1  in the first cycle and discharged by the ON and OFF operation of the first main switch Sm 1  in the next cycle. 
         [0118]    Like the second embodiment, the fourth embodiment described above can reduce the increase in cost of the converter  10 . Further, the auxiliary section according to the fourth embodiment has no magnetic component. Accordingly, the size of the auxiliary section can be reduced. 
       Fifth Embodiment 
       [0119]    A fifth embodiment of the present disclosure is described below. 
         [0120]    The fifth embodiment differs from the second embodiment in the configuration of the auxiliary section of the converter  10 , 
         [0121]      FIG. 13  shows the converter  10  according to the fifth embodiment. 
         [0122]    As shown in  FIG. 13 , in the fifth embodiment, like in the third embodiment, the converter  10  is configured as a semi-bridgeless PFC circuit. The auxiliary section has a snubber circuit including the snubber capacitor  22   a , the secondary inductor  26 , a first secondary switch Sbd, a second secondary switch Sbe, a first secondary diode Dsd, and a second secondary diode Dse. According to the fifth embodiment, each of the first secondary switch Sbd and the second secondary switch Sbe is an IGBT with an antiparallel diode. 
         [0123]    A first end of the secondary inductor  26  is connected to the snubber capacitor  22   a . A second end of the secondary inductor  26  is connected to the cathode of the second auxiliary diode Db through a series circuit of the first secondary switch Sbd and the first secondary diode Dsd. Specifically, an emitter of the first secondary switch Sbd is connected to an anode of the first secondary diode Dsd. The second end of the secondary inductor  26  is also connected to the cathode of the first auxiliary diode Da through a series circuit of the second secondary switch Sbe and the second secondary diode Dse. Specifically, an emitter of the second secondary switch Sbe is connected to an anode of the second secondary diode Dse. 
         [0124]    Next, operations of the auxiliary section according to the fifth embodiment are described with reference to  FIGS. 14A-14D . When the output voltage of the AC power source  12  has negative polarity, the auxiliary section operates in the same manner as when the output voltage of the AC power source  12  has positive polarity. Therefore, the description of the operations of the auxiliary section observed when the output voltage of the AC power source  12  has negative polarity is omitted. 
         [0125]      FIGS. 14A-14D  show a current flow in the converter  10  observed when the output voltage of the AC power source  12  has positive polarity. In this state, a reverse voltage is applied across the second diode D 2 . Therefore, when the output voltage of the AC power source  12  has positive polarity, the converter  10  has four states as shown in  FIGS. 14A-14D . 
         [0126]      FIG. 14A  shows a first state S 1  where the first main switch Sm 1  is ON, and the second secondary switch Sbe is ON. 
         [0127]      FIG. 14B  shows a second state S 2  subsequent to the first state S 1  shown in  FIG. 14A . When the first main switch Sm 1  is turned OFF, and the second secondary switch Sbe is OFF, there occurs a transition from the first state S 1  to the second state S 2 . At this time, the voltage between the pair of terminals of the first main switch Sm 1  is limited by the speed at which the voltage of the snubber capacitor  22   a  increases. Therefore, a surge voltage occurring when the first main switch Sm 1  is turned OFF can be reduced. 
         [0128]      FIG. 14C  shows a third state S 3  subsequent to the second state S 2  shown in  FIG. 148 . When the voltage of the snubber capacitor  22   a  reaches the voltage of the smoothing capacitor  18 , there occurs a transition from the second state S 2  to the third state S 3 . 
         [0129]      FIG. 14D  shows a fourth state S 4  subsequent to the third state S 3  shown in  FIG. 14C . When the second secondary switch Sbe is turned ON, there occurs a transition from the third state S 3  to the fourth state S 4 . Immediately after the second secondary switch Sbe is turned ON, part of the current flowing through the first main inductor  16   a  flows to the secondary inductor  26  through the first diode D 1 . Then, the entire current flowing through the first main inductor  16   a  flows to the secondary inductor  26  through the first diode D 1 . In this state, the snubber capacitor  22   a  is discharged. The reason for this is that since the current flows though the second auxiliary diode Db in the forward direction, the second auxiliary diode Db conducts so that a resonant current can flow through a closed loop circuit constructed with the snubber capacitor  22   a , the secondary inductor  26 , the second secondary switch Sbe, the second secondary diode Dse, the AC power source  12 , and the second auxiliary diode Db. Thus, the energy stored in the snubber capacitor  22   a  is recovered by the AC power source  12  without any loss of energy in theory and used as input energy. 
         [0130]    By the way, after that, the first main switch Sm 1  is turned ON. At this time, the current to the secondary inductor  26  still continues. Therefore, when the first main switch Sm 1  is turned ON, the speed at which the current flowing though the first main switch Sm 1  increases is limited. Therefore, a surge voltage occurring when the first main switch Sm 1  is turned ON can be reduced. Then, the current flowing through the first main diode Dm 1  gradually increases, and the current flowing through the secondary inductor  26  gradually decreases accordingly. Then, when the current flowing through the secondary inductor  26  becomes zero, there occurs a transition to the first state S 1  shown in  FIG. 14A . 
         [0131]    Like the second embodiment, the fifth embodiment described above can reduce the increase in cost of the converter  10 . 
       Sixth Embodiment 
       [0132]    A sixth embodiment of the present disclosure is described below. 
         [0133]    The sixth embodiment differs from the first embodiment in the configuration of the auxiliary section of the converter  10 . 
         [0134]      FIG. 15  shows the converter  10  according to the sixth embodiment. According to the sixth embodiment, the third diode D 3  and the fourth diode D 4  are not included. 
         [0135]    As shown in  FIG. 15 , the auxiliary section has a snubber circuit (recovery assist circuit) including a capacitor  34 , a resistor  36 , a power source  38 , and a secondary switch Sbf. According to the sixth embodiment, the secondary switch Sbf is an N-channel MOSFET. The cathodes of the first diode D 1  and the second diode D 2  are connected to the cathode of the second main diode Dm 2  through a series circuit of the capacitor  34  and the secondary switch Sbf. Specifically, a source of the secondary switch Sbf is connected to the capacitor  34 . 
         [0136]    The capacitor  34  is connected in parallel to a series circuit of the resistor  36  and the power source  38 . Specifically, a positive terminal of the power source  38  is connected to the resistor  36 , and a negative terminal of the power source  38  is connected to the capacitor  34 . A terminal voltage Vom of the power source  38  is set smaller than a reverse voltage Vmax (e.g., maximum value of the reverse voltage) which is expected to be applied across the first main diode Dm 1  or the second main diode Dm 2  when the first main switch Sm 1  or the second main switch Sm 2  is turned ON. Specifically, for example, the terminal voltage Vom of the power source  38  can be set smaller than a target value of the output voltage of the converter  10 . 
         [0137]    Next, operations of the auxiliary section according to the sixth embodiment are described with reference to  FIGS. 16A-18D . When the output voltage of the AC power source  12  has negative polarity, the auxiliary section operates in the same manner as when the output voltage of the AC power source  12  has positive polarity. Therefore, the description of the operations of the auxiliary section observed when the output voltage of the AC power source  12  has negative polarity is omitted. 
         [0138]      FIGS. 16A-16D  show a current flow in the converter  10  observed when the output voltage of the AC power source  12  has positive polarity. In this state, a reverse voltage is applied across the second diode D 2 . Therefore, when the output voltage of the AC power source  12  has positive polarity, the converter  10  has four states as shown in  FIGS. 16A-16D . 
         [0139]      FIG. 16A  shows a first state S 1  where the first main switch Sm 1  is ON, and the secondary switch Sbf is OFF. 
         [0140]      FIG. 16B  shows a second state S 2  subsequent to the first state S 1  shown in  FIG. 16A . When the first main switch Sm 1  is turned OFF, there occurs a transition from the first state S 1  to the second state S 2 . 
         [0141]      FIG. 16C  shows a third state S 3  subsequent to the second state S 2  shown in  FIG. 16B . When the secondary switch Sb 1  is turned ON before the first main switch Sm 1  is turned ON, there occurs a transition from the second state S 2  to the third state S 3 . In the third state S 3 , a reverse voltage is applied across the first main diode Dm 1  by the power source  38 , and the current is commutated from the first main diode Dm 1  to a current path that passes the secondary switch Sbf. Although the current tries to flow to the power source  38  through the first diode D 1  upon turn-ON of the secondary switch Sb 1 , the current mainly flows to the capacitor  34  due to the presence of the resistor  36 . Thus, an increase in a ripple current of the power source  38  can be reduced. 
         [0142]      FIG. 16D  shows a fourth state S 4  subsequent to the third state S 3  shown in  FIG. 16C . When the first main switch Sm 1  is turned ON, there occurs a transition from the third state S 3  to the fourth state S 4 . In the third state S 3 , the reverse voltage is applied across the first main diode Dm 1  by the power source  38 . Therefore, when the first main switch Sm 1  is turned ON, a recovery current flowing from the main diode Dm 1  through the first main switch Sm 1  can be reduced. 
         [0143]    Like the second embodiment, the sixth embodiment described above can reduce the increase in cost of the converter  10 . 
       Seventh Embodiment 
       [0144]    A seventh embodiment of the present disclosure is described below. 
         [0145]    The seventh embodiment differs from the third embodiment in the configuration of the main section of the converter  10 . Accordingly, the seventh embodiment also differs from the third embodiment in the configuration of the auxiliary section of the converter  10 . 
         [0146]      FIG. 17  shows the converter  10  according to the seventh embodiment. 
         [0147]    Firstly, the main section according to the seventh embodiment is described. 
         [0148]    As shown in  FIG. 17 , the cathode of the first main diode Dm 1  is connected to the source of the first main switch Sm 1 , and the cathode of the second main diode Dm 2  is connected to the source of the second main switch Sm 2 . The anodes of the first main diode Dm 1  and the second main diode Dm 2  are connected together, and the drains of the first main switch Sm 1  and the second main switch Sm 2  are connected together. The first connection point P 1  is connected to the second connection point P 2  through the first main inductor  16   a , the AC power source  12 , and the second main inductor  16   b.    
         [0149]    It is noted that according to the seventh embodiment, the converter  10  is configured as a bridgeless PFC circuit, not a semi-bridgeless PFC circuit. 
         [0150]    Next, the auxiliary section according to the seventh embodiment is described. 
         [0151]    The first main switch Sm 1  is connected in parallel to a series circuit of the first diode D 1  and the snubber capacitor  22 , and the second main switch Sm 2  is connected in parallel to a series circuit of the second diode D 2  and the snubber capacitor  22 . Specifically, the anodes of the first diode D 1  and the second diode D 2  are connected to the snubber capacitor  22 . 
         [0152]    The anodes of the first diode D 1  and the second diode D 2  are connected to the source of the secondary switch Sba through the primary coil  30   a . The drain of the secondary switch Sba is connected to the drain of the second main switch Sm 2 . The source of the secondary switch Sba is connected to the cathode of the secondary diode Dsa, and the anode of the secondary diode Dsa is connected to the anode of the second main diode Dm 2 . 
         [0153]    The cathode of the third diode D 3  is connected to the first connection point P 1 , and the cathode of the fourth diode D 4  is connected to the second connection point P 2 . The anodes of the third diode D 3  and the fourth diode D 4  are connected to the anode of the secondary diode Dsa through the secondary coil  30   b  and the secondary inductor  32 . 
         [0154]    Next, operations of the auxiliary section according to the seventh embodiment are described with reference to  FIGS. 18A-18D  and  FIGS. 19A-19D . 
         [0155]    When the output voltage of the AC power source  12  has positive polarity, the second main switch Sm 2  is controlled (i.e., turned ON and OFF) under a condition that the first main switch Sm 1  is kept ON (i.e., kept closed). In this state, since the first main switch Sm 1  is always ON, a positive polarity of the voltage of the AC power source  12  is applied to the cathodes of the first diode D 1  and the third diode D 3  so that a reverse voltage can be applied across both of the first diode D 1  and the third diode D 3 . Therefore, when the output voltage of the AC power source  12  has positive polarity, the converter  10  has eight states as shown in  FIGS. 18A-18D  and  FIGS. 19A-19D . In  FIGS. 18A-18D  and  FIGS. 19A-19D , a state number is assigned to correspond to the state number shown in  FIGS. 7A-7D  and  8 A- 8 D. 
         [0156]      FIG. 18A  shows a first state S 1  where the second main switch Sm 1  is ON. In the first state S 1 , the current flows through a closed loop circuit constructed with the AC power source  12 , the first main inductor  16   a , the first main switch Sm 1 , the second main switch Sm 2 , and the second main inductor  16   b.    
         [0157]      FIG. 18B  shows a second state S 2  subsequent to the first state S 1  shown in  FIG. 18A . When the second main switch Sm 2  is turned OFF, there occurs a transition from the first state S 1  to the second state S 2 . In the second state S 2 , the current outputted from the first and second main inductors  16   a  and  16   b  flows to the snubber capacitor  22 . Therefore, a surge voltage occurring when the second main switch Sm 2  is turned OFF can be reduced. 
         [0158]      FIG. 18C  shows a third state S 3  subsequent to the second state S 2  shown in  FIG. 18B . When the voltage of the snubber capacitor  22  reaches the voltage of the smoothing capacitor  18 , there occurs a transition from the second state S 2  to the third state S 3 . 
         [0159]      FIG. 18D  shows a fourth state S 4  subsequent to the third state S 3  shown in  FIG. 18C . When the secondary switch Sba is turned ON before the second main switch Sm 2  is turned ON, there occurs a transition from the third state S 3  to the fourth state S 4 . In the fourth state S 4 , part of the current outputted from the first and second main inductors  16   a  and  16   b  flows to the primary coil  30   a  through the secondary switch Sba, so that the current can be commutated from the second main diode Dm 2  toward the secondary coil  30   b . Therefore, after that, while the current flowing through the second main diode Dm 2  gradually decreases, the current flowing through the secondary coil  30   b  gradually increases. 
         [0160]      FIG. 19A  shows a fifth state S 5  subsequent to the fourth state S 4  shown in  FIG. 18D . When the second main switch Sm 2  is turned ON after the current flowing through the second main diode Dm 2  becomes zero, there occurs a transition from the fourth state S 4  to the fifth state S 5 . When the second main switch Sm 2  is turned ON, the current flowing through the secondary inductor  32  gradually decreases, and the current flowing through the second main switch Sm 2  gradually increases accordingly. Since the gradual increase speed is limited by the inductance of the secondary inductor  32 , a surge voltage occurring when the second main switch Sm 2  is turned ON can be reduced. 
         [0161]    Further, when the second main switch Sm 2  is turned ON, the potential of the source of the second main switch Sm 2  increases. Therefore, the snubber capacitor  22  is discharged so that the current can flow through the secondary switch Sba. 
         [0162]      FIG. 19B  shows a sixth state S 6  subsequent to the fifth state S 5  shown in  FIG. 19A . When the secondary switch Sba is turned OFF, there occurs a transition from the fifth state S 5  to the sixth state S 6 . In the sixth state S 6 , since the magnetic energy stored in the transformer  30  is released through the secondary diode Dsa, the snubber capacitor  22  continues to be discharged. 
         [0163]      FIG. 19C  shows a seventh state S 7  subsequent to the sixth state S 6  shown in  FIG. 19B . When the current flowing through the secondary coil  30   b  becomes zero, there occurs a transition from the sixth state S 6  to the seventh state S 7 . 
         [0164]      FIG. 19D  shows an eighth state S 8  subsequent to the seventh state S 7  shown in  FIG. 19C . When the snubber capacitor  22  is fully discharged, there occurs a transition from the seventh state S 7  to the eighth state S 8 . In the eighth state S 8 , because of the magnetic energy stored in the transformer  30 , the current flows through the primary coil  30   a . Then, when the magnetic energy stored in the transformer  30  becomes zero, there occurs a transition from the eighth state S 8  to the first state S 1  shown in  FIG. 18A . 
         [0165]    Next, operations of the main section according to the seventh embodiment when the output voltage of the AC power source  12  has negative polarity are described with reference to  FIGS. 20A and 20B . When the output voltage of the AC power source  12  has negative polarity, the first main switch Sm 1  is controlled (Le., turned ON and OFF) under a condition that the second main switch Sm 2  is kept ON (Le., kept closed). In this state, since the second main switch Sm 2  is always ON, a positive polarity of the voltage of the AC power source  12  is applied to the cathodes of the second diode D 2  and the fourth diode D 4  so that a reverse voltage can be applied across both of the second diode D 2  and the fourth diode D 4 . Therefore, when the output voltage of the AC power source  12  has negative polarity, the converter  10  has states as shown in  FIGS. 20A and 20B . In  FIGS. 20A and 20B , a state number is assigned to correspond to the state number shown in  FIGS. 18A-18D . 
         [0166]    By the way, when the output voltage of the AC power source  12  has negative polarity, the auxiliary section operates in the same manner as described above for when the output voltage of the AC power source  12  has positive polarity. Therefore, the description of the operations of the auxiliary section observed when the output voltage of the AC power source  12  has negative polarity is omitted. 
         [0167]    Like the second embodiment, the seventh embodiment described above can reduce the increase in cost of the converter  10 . 
       Eighth Embodiment 
       [0168]    An eighth embodiment of the present disclosure is described below. 
         [0169]    The eighth embodiment differs from the seventh embodiment in that the configuration of the auxiliary section described in the fourth embodiment is employed. 
         [0170]      FIG. 21  shows the converter  10  according to the eighth embodiment. 
         [0171]    As shown in  FIG. 21 , the first main switch Sm 1  is connected in parallel to a series circuit of the first secondary switch Sbb, the second secondary switch Sbc, and the fifth diode D 5 . Specifically, the emitter of the first secondary switch Sbb is connected to the collector of the second secondary switch Sbc. Further, the second main switch Sm 2  is connected in parallel to a series circuit of the first secondary switch Sbb, the second secondary switch Sbc, and the sixth diode D 6 . 
         [0172]    The anode of the second main diode Dm 2  is connected to a connection point between the first secondary switch Sbb and the second secondary switch Sbc through a series circuit of the snubber capacitor  22   b  and the secondary diode Dsb. Specifically, the snubber capacitor  22   b  is connected to the cathode of the secondary diode Dsb. Further, the anodes of the first diode D 1  and the second diode D 2  are connected to a connection point between the snubber capacitor  22   b  and the secondary diode Dsb. 
         [0173]    According to the eighth embodiment, when the output voltage of the AC power source  12  has negative polarity, the auxiliary section operates in the same manner as when the output voltage of the AC power source  12  has positive polarity. Therefore, the description of the operations of the auxiliary section observed when the output voltage of the AC power source  12  has negative polarity is omitted.  FIGS. 22A-22D  and  FIGS. 23A-23D  show the operations of the auxiliary section observed when the output voltage of the AC power source  12  has positive polarity. The main section according to the eighth embodiment operates in the same manner as that described in the seventh embodiment. Further, the auxiliary section according to the eighth embodiment operates in the same manner as that described in the fourth embodiment. Therefore, the detailed descriptions of the operations of the main section and the auxiliary section are omitted. Like the second embodiment, the eighth embodiment described above can reduce the increase in cost of the converter  10 . 
       Modifications 
       [0174]    The embodiments can be modified in various ways. 
         [0175]    The snubber circuit is not limited to those described in the embodiments. For example, in the first embodiment, one of the snubber inductor  20  and the snubber capacitor  22  can be removed. The recovery assist circuit described in the sixth embodiment can be employed in any one of the first, second, third, fourth, fifth, seventh, and the eighth embodiments. That is, the snubber circuit can be constructed with at least one of the snubber inductor, the snubber capacitor, and the recovery assist circuit. When the snubber circuit is constructed with the snubber capacitor, it is not always necessary that the snubber capacitor is located in a current path that bypasses both the primary main switch Sm 1  and the secondary main switch Sm 2  as shown in  FIG. 1  of the first embodiment. For example, the snubber capacitor can be located in a current path that bypasses both the first main diode Dm 1  and the second main diode Dm 2 . For example, the snubber capacitor can be located not only in a current path that bypasses both the primary main switch Sm 1  and the secondary main switch Sm 2  but also in a current path that bypasses both the first main diode Dm 1  and the second main diode Dm 2 . In this case, common snubber capacitors are shared not only by a series circuit of the first main switch Sm 1  and the first main diode Dm 1  but also by a series circuit of the second main switch Sm 2  and the second main diode Dm 2 , and the common snubber capacitors are connected in series. 
         [0176]    Two or more same or different snubber circuits can be included. Specifically, for example, two or more snubber circuit, each of which is configured as described in the third embodiment, can be included. In this case, to prevent the snubber circuits from operating at the same time, a switch or the like can be used to select one of the snubber circuits to be operated. 
         [0177]    A passive lossless snubber circuit with a snubber capacitor is not limited to those described in the second to seventh embodiments. For example, a LC snubber circuit can be used as a passive lossless snubber circuit. 
         [0178]    In the first embodiment, a saturable inductor can be used as a snubber inductor. 
         [0179]    A main diode as the second device and the fourth device is not limited to those described in the embodiments. For example, a zener diode or a charge storage diode (CSD) can be used as a main diode. Further, to implement synchronous rectification, a switching device such as a MOSFET can be used instead of a main diode. For example, as shown in  FIG. 24 , the first main diode Dm 1  can be replaced with a third main switch Sm 3 , and the second main diode Dm 2  can be replaced with a fourth main switch Sm 4 . 
         [0180]    The first device and the second device are not limited to those described in the embodiments. For example, an IGBT, a thyristor, or a photomos relay can be used as the first device and the second device. 
         [0181]    The fifth device and the sixth device are not limited to a diode. For example, a switching device such as a MOSFET can be used as the fifth device and the sixth device. In this case, the fifth device and the sixth device are controlled (i.e., turned ON and OFF) in accordance with the states of the first main switch Sm 1  and the second main switch Sm 2 . 
         [0182]    A reverse voltage applying device is not limited to those described in the embodiments. For example, the capacitor  34  and the resistor  36  can be removed. Even in this case, a reverse voltage can be applied to the first main diode Dm 1  and the second main diode Dm 2 . 
         [0183]    In the third and seventh embodiments, the secondary inductor  32  can be an inductor as a passive device. 
         [0184]    In the fourth and fifth embodiments, the secondary switch can be a reverse blocking IGBT. Alternatively, the secondary switch can be a field-effect transistor. In this case, a diode connected in antiparallel to a secondary switch can be a parasitic diode of the transistor. 
         [0185]    The semi-bridgeless PFC circuit described in the third and fifth embodiments can be used in the first, second, fourth, and sixth to eighth embodiments. Further, the bridgeless PFC circuit described in the first embodiment can be used in the third and fifth embodiments.