Power Conversion Equipment

Abstract

To provide a power conversion device capable of insulating input and output by fewer active components compared with a case of using a full-bridge circuit and converting AC voltage into DC voltage.SOLUTION: A power conversion device 1 converts AC voltage into DC voltage, and has a primary side circuit 10 having a transformer 18 with a primary winding W1 and a secondary winding W2, a first capacitor 17, a pair of input ends t21 and t22, a first inductor 11, an upper arm switching element 15, a lower arm switching element 16, an upper diode 12, a lower arm diode 13, and a buffer capacitor Cc, and a rectification smoothing circuit 20 having a second inductor 21, a secondary side diode 22, an output capacitor 23, a first output end t91, and a second output end t92. The primary side circuit 10 is connected to the primary winding W1 of the transformer 18 through the first capacitor 17.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a power conversion device, and more particularly to a circuit that converts AC voltage to DC voltage and isolates the input from the output. [Background technology]

[0002] Conventionally, as a circuit for insulating input and output and converting AC voltage to DC voltage, a power conversion device is known that includes a transformer, a rectifier circuit and an inverter circuit provided on the primary side of the transformer, and a rectifier circuit provided on the secondary side of the transformer (for example, Patent Document 1). In the power conversion device described in Patent Document 1, both the rectifier circuits and the inverter circuit are each configured as a full-bridge circuit. In this circuit, the input AC voltage is converted to a DC voltage by the rectifier circuit, then converted to a high-frequency AC voltage by the inverter circuit and applied to the transformer, transmitted in an insulated manner by the transformer, and converted back to DC by the rectifier circuit. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-41428 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in conventional technology, it was difficult to reduce the number of active elements because a total of three bridge circuits, consisting of two rectifier circuits and an inverter circuit, were used. Here, active elements are a general term for switching elements and rectifier elements. [Means for solving the problem]

[0005] A power conversion device that achieves the above object is a power conversion device that converts an AC voltage input from an AC power supply into a DC voltage, and includes: a transformer having a primary winding and a secondary winding; a first capacitor; a pair of input terminals to which the AC power supply is connected; a primary side circuit having a first inductor, an upper arm switching element, a lower arm switching element, an upper arm rectifying element, a lower arm rectifying element, and a buffer capacitor; and a rectifying and smoothing circuit having a second inductor, a secondary side rectifying element, an output capacitor, a first output terminal, and a second output terminal. The series connection of the upper arm switching element and the lower arm switching element and the series connection of the upper arm rectifier element and the lower arm rectifier element form a bridge circuit, and a connection point between the upper arm switching element and the lower arm switching element and a connection point between the upper arm rectifier element and the lower arm rectifier element are connected to the pair of input terminals via the first inductor, the primary side circuit is connected to the primary winding of the transformer via the first capacitor, and the rectifier smoothing circuit is connected to the secondary winding of the transformer.

[0006] According to this configuration, by switching the upper arm switching element and the lower arm switching element, the transformer insulates the input and output and converts AC voltage to DC voltage. This makes it possible to reduce the number of active elements compared to conventional circuits. In addition, by using a buffer capacitor, it is possible to match the current in the circuit and suppress surge voltage.

[0007] In the above power conversion device, the buffer capacitor may be connected in parallel to the bridge circuit. In the above power conversion device, the buffer capacitor may be connected in parallel with a primary winding of the transformer via the upper arm switching element.

[0008] According to this configuration, the power conversion device can suppress surge voltage and can lower the voltage applied to the buffer capacitor. The above power conversion device may further include a second capacitor, and the rectifying and smoothing circuit may be configured such that the first output terminal is connected to one end of the output capacitor and the second output terminal is connected to the other end of the output capacitor, one end of the second inductor is connected to one end of the output capacitor via the secondary side rectifying element and the other end of the second inductor is connected to the other end of the output capacitor, the rectifying and smoothing circuit is connected to a secondary winding of the transformer via the second capacitor, and the primary winding and the secondary winding are magnetically coupled to have a polarity such that the secondary side rectifying element is conductive when the lower arm switching element is in a non-conductive state.

[0009] The above power conversion device may further include a second capacitor, and the rectifying and smoothing circuit may be configured such that the first output terminal and one end of the output capacitor are connected, the second output terminal and the other end of the output capacitor are connected, one end of the secondary side rectifying element is connected to one end of the output capacitor via the second inductor, and the other end of the secondary side rectifying element is connected to the other end of the output capacitor, the rectifying and smoothing circuit is connected to a secondary winding of the transformer via the second capacitor, and the primary winding and the secondary winding are magnetically coupled to have a polarity such that the secondary side rectifying element is conductive when the lower arm switching element is in a non-conductive state.

[0010] In the above power conversion device, the rectifier smoothing circuit may have a first secondary side rectifier element and a second secondary side rectifier element as the secondary side rectifier elements, the first output terminal and one end of the output capacitor are connected, the second output terminal and the other end of the output capacitor are connected, one end of the first secondary side rectifier element is connected to one end of the output capacitor via the second inductor, the other end of the first secondary side rectifier element is connected to one end of the secondary winding, one end of the second secondary side rectifier element is connected to a connection point between the second inductor and the first secondary side rectifier element, the other end of the secondary side rectifier element is connected to the other end of the secondary winding, and the other end of the output capacitor is connected to a midpoint of the secondary winding.

[0011] The above power conversion device may further include a control unit that controls switching of the switching elements when any or all of the upper arm switching element, the lower arm switching element, the upper arm rectifier element, the lower arm rectifier element, and the secondary side rectifier element are composed of switching elements, and the control unit may control the switching so as to improve the power factor of the input power based on the AC voltage and the current flowing through the first inductor.

[0012] According to this configuration, the power factor of the power supplied from the AC power supply can be controlled. In the above power conversion device, when any or all of the upper arm switching element, the lower arm switching element, the upper arm rectifier element, the lower arm rectifier element, and the secondary side rectifier element are composed of switching elements, the power conversion device may further include a control unit that controls switching of the switching elements, and the control unit may control the switching so that a predetermined DC voltage is output based on the voltage between the first output terminal and the second output terminal.

[0013] With this configuration, the output voltage can be controlled to a desired value. Effect of the Invention

[0014] According to the present invention, it is possible to realize a power conversion device that insulates the input from the output and converts AC voltage to DC voltage using fewer active elements than conventional circuits. [Brief description of the drawings]

[0015] [Figure 1] FIG. 2 is a diagram showing an example of the operation of the power conversion device 1 according to the first embodiment. [Diagram 2] FIG. 2 is a diagram showing an example of the operation of the power conversion device 1 according to the first embodiment. [Diagram 3] FIG. 2 is a diagram showing an example of the operation of the power conversion device 1 according to the first embodiment. [Figure 4]FIG. 2 is a diagram showing an example of the operation of the power conversion device 1 according to the first embodiment. [Diagram 5] FIG. 2 is a diagram showing an example of the operation of the power conversion device 1 according to the first embodiment. [Figure 6] FIG. 2 is a diagram showing an example of the operation of the power conversion device 1 according to the first embodiment. [Figure 7] FIG. 2 is a diagram showing an example of the operation of the power conversion device 1 according to the first embodiment. [Figure 8] FIG. 2 is a diagram showing an example of the operation of the power conversion device 1 according to the first embodiment. [Figure 9] FIG. 2 is a diagram showing an example of the operation of the power conversion device 1 according to the first embodiment. [Figure 10] FIG. 2 is a diagram showing an example of the operation of the power conversion device 1 according to the first embodiment. [Figure 11] FIG. 2 is a diagram showing an example of the operation of the power conversion device 1 according to the first embodiment. [Figure 12] FIG. 2 is a diagram showing an example of the operation of the power conversion device 1 according to the first embodiment. [Figure 13] FIG. 11 is a diagram showing an example of the operation of the power conversion device 2 according to the second embodiment. [Figure 14] FIG. 11 is a diagram showing an example of the operation of the power conversion device 2 according to the second embodiment. [Figure 15] FIG. 11 is a diagram showing an example of the operation of the power conversion device 2 according to the second embodiment. [Figure 16] FIG. 11 is a diagram showing an example of the operation of the power conversion device 2 according to the second embodiment. [Figure 17] FIG. 11 is a diagram showing an example of the operation of the power conversion device 2 according to the second embodiment. [Figure 18] FIG. 11 is a diagram showing an example of the operation of the power conversion device 2 according to the second embodiment. [Figure 19] FIG. 11 is a diagram showing an example of the operation of the power conversion device 2 according to the second embodiment. [Figure 20] FIG. 11 is a diagram showing an example of the operation of the power conversion device 2 according to the second embodiment. [Figure 21] FIG. 11 is a diagram showing an example of the operation of the power conversion device 2 according to the second embodiment. [Figure 22] FIG. 11 is a diagram showing an example of the operation of the power conversion device 2 according to the second embodiment. [Diagram 23] FIG. 11 is a diagram showing an example of the operation of the power conversion device 2 according to the second embodiment. [Figure 24] FIG. 11 is a diagram showing an example of the operation of the power conversion device 2 according to the second embodiment. [Diagram 25] FIG. 4 is a diagram showing an example of the operation of the power conversion device 3 according to the first modification. [Figure 26] FIG. 4 is a diagram showing an example of the operation of the power conversion device 3 according to the first modification. [Figure 27] FIG. 4 is a diagram showing an example of the operation of the power conversion device 3 according to the first modification. [Figure 28] FIG. 4 is a diagram showing an example of the operation of the power conversion device 3 according to the first modification. [Figure 29] FIG. 4 is a diagram showing an example of the configuration of a power conversion device 4 according to a first modified example. [Diagram 30] FIG. 11 is a diagram showing an example of the operation of the power conversion device 5 according to the second modification. [Diagram 31] FIG. 11 is a diagram showing an example of the operation of the power conversion device 5 according to the second modification. [Diagram 32] FIG. 11 is a diagram showing an example of the operation of the power conversion device 5 according to the second modification. [Diagram 33] FIG. 11 is a diagram showing an example of the operation of the power conversion device 5 according to the second modification. [Diagram 34] FIG. 11 is a diagram showing an example of the configuration of a power conversion device 6 according to a second modification. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] First Embodiment An embodiment of the present invention will be described below. The power conversion device 1 is a device that converts an AC voltage supplied from an AC power source V1 into a DC voltage and supplies the DC voltage to a load connected to the power conversion device 1.

[0017] The power conversion device 1 includes, for example, a primary side circuit 10, a first capacitor 17, a transformer 18, a second capacitor 19, a rectifying and smoothing circuit 20, and a control unit 50. [Regarding the primary circuit 10] The primary side circuit 10 includes, for example, a first inductor 11, an upper arm diode 12, a lower arm diode 13, an upper arm switching element 15, a lower arm switching element 16, a first connecting line CL1, a second connecting line CL2, a first intermediate line ML1, a second intermediate line ML2, a first input terminal t21, a second input terminal t22, and a buffer capacitor Cc. The upper arm diode 12 is an example of an "upper arm rectifier element", and the lower arm diode 13 is an example of a "lower arm rectifier element".

[0018] The AC power supply V1 and the primary side circuit 10 are electrically connected. Specifically, a first terminal t11 of the AC power supply V1 and a first input terminal t21 of the primary side circuit 10 are connected by a first input line L1. A second terminal t12 of the AC power supply V1 and a second input terminal t22 of the primary side circuit 10 are connected by a second input line L2. As a result, an AC voltage is input to the input terminals t21 and t22.

[0019] The upper arm diode 12 and the lower arm diode 13 are connected in series by a second connection line CL2. Specifically, the second connection line CL2 connects the anode of the upper arm diode 12 and the cathode of the lower arm diode 13.

[0020] The upper arm switching element 15 has a first end t31 and a second end t32, and the lower arm switching element 16 has a first end t41 and a second end t42. The first connection line CL1 connects the second end t32 of the upper arm switching element 15 and the first end t41 of the lower arm switching element 16. The upper arm switching element 15 and the lower arm switching element 16 are, for example, power switching elements, and in this embodiment, are n-type power MOSFETs. Both switching elements 15, 16 have body diodes. However, the upper arm switching element 15 and the lower arm switching element 16 are not limited to MOSFETs, and may be any other elements such as IGBTs having anti-parallel diodes.

[0021] The first intermediate line ML1 connects the first connection line CL1 and the first input terminal t21. A first inductor 11 is provided on the first intermediate line ML1. The second intermediate line ML2 connects the second connection line CL2 and the second input terminal t22. Therefore, the connection point between the upper arm switching element 15 and the lower arm switching element 16 and the connection point between the upper arm diode 12 and the lower arm diode 13 are connected to the pair of input terminals t21, t22 via the first inductor 11.

[0022] The cathode of the upper arm diode 12 is connected to a first end t31 of the upper arm switching element 15, and the anode of the lower arm diode 13 is connected to a second end t42 of the lower arm switching element 16. Therefore, the series connection of the upper arm switching element 15 and the lower arm switching element 16 and the series connection of the upper arm diode 12 and the lower arm diode 13 form a bridge circuit 14.

[0023] The transformer 18 is an insulating transformer having a primary winding W1 and a secondary winding W2. The starting end of the primary winding W1 is connected to the first connection line CL1 via the first capacitor 17. Specifically, the first capacitor 17 has a first end t51 and a second end t52, the starting end of the primary winding W1 is connected to the second end t52 of the first capacitor 17, and the first end t51 of the first capacitor 17 is connected to the first connection line CL1. The end of the primary winding W1 is connected to the anode of the lower arm diode 13 and the second end t42 of the lower arm switching element 16. Therefore, the primary side circuit 10 is connected to the primary winding W1 of the transformer 18 via the first capacitor 17. The starting end of the primary winding W1 is the side marked with a black circle in FIG. 1, and the end of the primary winding W1 is the side not marked with a black circle in FIG. 1.

[0024] [Regarding the rectifying and smoothing circuit 20] The rectifying smoothing circuit 20 includes a second inductor 21, a secondary side diode 22, an output capacitor 23, a first output line OL1, a second output line OL2, a first output end t91, and a second output end t92. The second inductor 21 has a first end t71 and a second end t72, and the output capacitor 23 has a first end t81 and a second end t82. The secondary side diode 22 is an example of a "secondary side rectifying element." The first end t81 is an example of a "one end of the output capacitor," and the second end t82 is an example of a "other end of the output capacitor."

[0025] The start end of the secondary winding W2 is connected to the first end t71 of the second inductor 21 and the anode of the secondary side diode 22 via the second capacitor 19. Specifically, the second capacitor 19 has a first end t61 and a second end t62, and the start end of the secondary winding W2 is connected to the first end t61 of the second capacitor 19, and the second end t62 of the second capacitor 19 is connected to the first end t71 of the second inductor 21 and the anode of the secondary side diode 22. The end end of the secondary winding W2 is connected to the second end t72 of the second inductor 21. Therefore, the rectifying smoothing circuit 20 is connected to the secondary winding W2 of the transformer 18 via the second capacitor 19. The start end of the secondary winding W2 is the side marked with a black circle in FIG. 1, and the end end of the secondary winding W2 is the side not marked with a black circle in FIG. 1.

[0026] The output capacitor 23 is connected to both the first output line OL1 and the second output line OL2. Specifically, a first end t81 of the output capacitor 23 is connected to the first output line OL1. And a second end t82 of the output capacitor 23 is connected to the second output line OL2.

[0027] The second inductor 21 is connected to both the secondary side diode 22 and the second output line OL2. Specifically, a first end t71 of the second inductor 21 is connected to the anode of the secondary side diode 22, and a second end t72 of the second inductor 21 is connected to the second output line OL2. The first end t71 is an example of "one end of the second inductor", and the second end t72 is an example of "the other end of the second inductor".

[0028] The secondary side diode 22 is provided in a portion between the second inductor 21 and the output capacitor 23. As described above, the anode of the secondary side diode 22 and the first end t71 of the second inductor 21 are connected. In addition, the cathode of the secondary side diode 22 and the first end t81 of the output capacitor 23 are connected.

[0029] [About the buffer capacitor Cc] The buffer capacitor Cc has a terminal tc1 and a terminal tc2. The buffer capacitor Cc is connected in parallel with the bridge circuit 14. In detail, the terminal tc1 of the buffer capacitor Cc is connected to a connection point between the cathode of the upper-arm diode 12 and the first end t31 of the upper-arm switching element 15. In addition, the terminal tc2 of the buffer capacitor Cc is connected to a connection point between the anode of the lower-arm diode 13 and the second end t42 of the lower-arm switching element 16.

[0030] [Regarding the control unit 50] 1, the power conversion device 1 includes a current sensor C1 that detects a current flowing through the first inductor 11. The power conversion device 1 also includes a voltage sensor C2 that detects an input voltage, and a voltage sensor C3 that detects an output voltage. The current sensor C1, the voltage sensor C2, and the voltage sensor C3 output the detection results to the control unit 50.

[0031] The control unit 50 turns on / off the upper arm switching element 15 and the lower arm switching element 16 based on signals from the current sensor C1, the voltage sensor C2, and the voltage sensor C3. The power conversion device 1 converts the AC voltage supplied by the AC power source V1 into a DC voltage by switching the upper arm switching element 15 and the lower arm switching element 16 between a conductive state and a non-conductive state, and outputs the DC voltage from both output terminals t91, t92.

[0032] The operation of the power conversion device 1 will be described in detail below with reference to Fig. 1 to Fig. 12. The control method differs between a state in which the potential of the first terminal t11 is higher than the potential of the second terminal t12 in the AC power supply V1 and a state in which the potential of the first terminal t11 is lower than the potential of the second terminal t12. In the following description, the state in which the AC power supply V1 has the potential of the first terminal t11 higher than the potential of the second terminal t12 will also be referred to as a "state in which the AC potential is positive," and the state in which the AC power supply V1 has the potential of the first terminal t11 lower than the potential of the second terminal t12 will also be referred to as a "state in which the AC potential is negative."

[0033] For ease of explanation, it is assumed that the capacitors 17, 19, and 23 are charged when the power conversion device 1 starts operating. It is also assumed that currents flow through the first inductor 11, the primary winding W1 and secondary winding W2 of the transformer 18, and the second inductor 21.

[0034] [Operation when AC potential is positive] First, switching control in a state in which the AC potential is positive will be described with reference to FIGS.

[0035] In this state, the control unit 50 switches the upper arm switching element 15 and the lower arm switching element 16 between a conductive state and a non-conductive state. This causes the state of the power conversion device 1 to transition from the first state to the sixth state. Each state will be described in detail below.

[0036] [Positive AC potential: 1st state] 1 is a state in which the control unit 50 controls the upper arm switching element 15 to a non-conductive state and controls the lower arm switching element 16 to a conductive state. As shown in FIG 1, in the first state, a current flows in the primary side circuit 10 through a current path ra1 associated with the supply of power from the AC power source V1 and a current path ra2 associated with the discharge of the first capacitor 17. In addition, in the first state, a current flows in the rectifying smoothing circuit 20 through a current path rb1 associated with the transfer of power from the primary winding W1 to the secondary winding W2 by electromagnetic induction and the supply of power from the secondary winding W2 of the transformer 18, and a current path rb2 associated with the discharge of the output capacitor 23.

[0037] The current path ra1 is a path from a first end t11 of the AC power supply V1 to a second end t12 of the AC power supply V1 via the first inductor 11, the lower arm switching element 16, and the lower arm diode 13. In the first state, the current flowing through the current path ra1 increases over time in the direction from the AC power supply V1 to the first inductor 11.

[0038] The current path ra2 is a path from the first end t51 of the first capacitor 17, via the lower arm switching element 16 and the end to the starting end of the primary winding W1, to the second end t52 of the first capacitor 17. The voltage across the first capacitor 17 is applied to the primary winding W1 of the transformer 18, and the potential at the end of the primary winding W1 becomes higher than the potential at the starting end. As a result, a voltage is generated across the secondary winding W2 of the transformer 18 such that the potential at the end becomes higher than the potential at the starting end.

[0039] The current path rb1 is a path from the end of the secondary winding W2 to the beginning of the secondary winding W2, via the second inductor 21 and the second capacitor 19. In the first state, the current flowing through the current path rb1 increases over time in the direction from the second end t72 to the first end t71 of the second inductor 21, and the magnetic flux of the second inductor 21 also increases with the increase in the current.

[0040] The current path rb2 is a path from the first end t81 of the output capacitor 23 to the second end t82 of the output capacitor 23 via the first output end t91, a load (not shown) connected to the power conversion device 1, and the second output end t92. A DC voltage, which is the voltage across the output capacitor 23, is generated between the first output end t91 and the second output end t92. A current is supplied to the load by discharging the output capacitor 23. In the first state, the secondary side diode 22 is not in a conductive state.

[0041] [Positive AC potential: second state] The second state shown in FIG. 2 is a state in which the control unit 50 controls the lower arm switching element 16 to be in a non-conducting state while keeping the upper arm switching element 15 in a non-conducting state after a predetermined time has elapsed since the transition to the first state. As shown in FIG. 2, in the second state, a current flows in the primary side circuit 10 through a current path ra3 due to the supply of power from the AC power source V1 and the continuity of the current in the first inductor 11, and a current path ra4 due to the continuity of the current in the leakage inductance Rw1 of the primary winding W1. In addition, in the second state, a current flows in the rectifying and smoothing circuit 20 through a current path rb2 from the output capacitor 23 to the load, and a current path rb3 due to the continuity of the current in the second inductor 21 and the leakage inductance Rw2 of the secondary winding W2. The current path rb2 has already been described, so a detailed description thereof will be omitted.

[0042] The current path ra3 is a path from the first end t11 of the AC power supply V1 to the second end t12 of the AC power supply V1 via the first inductor 11, the body diode of the upper-arm switching element 15, the buffer capacitor Cc, and the lower-arm diode 13. In the second state, the current flowing through the first inductor 11 flows through the body diode of the upper-arm switching element 15 to the buffer capacitor Cc, charging the buffer capacitor Cc. As a result, the direction of the voltage applied to the first inductor 11 changes, and the current flowing through the first inductor 11 decreases.

[0043] The current path ra4 is a path from the leakage inductance Rw1 of the primary winding W1 through the first capacitor 17, the body diode of the upper arm switching element 15, and the buffer capacitor Cc to the leakage inductance Rw1. In the second state, the current flowing through the primary winding W1 flows through the body diode of the upper arm switching element 15 to the buffer capacitor Cc, charging the buffer capacitor Cc. Before and after the control to the second state, the direction of the voltage applied to the primary winding W1 is reversed. Therefore, the current flowing through the primary winding W1 decreases rapidly.

[0044] The current path rb3 is a path that runs from the leakage inductance Rw2 of the secondary winding W2, via the second inductor 21 and the second capacitor 19, to the secondary winding W2 and the leakage inductance Rw2.

[0045] [Positive AC potential: third state] The third state shown in Fig. 3 is a state in which, after a predetermined time has elapsed since transition to the second state, the control unit 50 controls the upper arm switching element 15 to a conductive state while maintaining the lower arm switching element 16 in a non-conductive state. As shown in Fig. 3, in the third state, currents flow through current paths ra3' and ra4' in the primary side circuit 10, and currents flow through current paths rb2 and rb3 in the rectifying smoothing circuit 20. As the current paths rb2 and rb3 have already been described, detailed description thereof will be omitted.

[0046] Current path ra3' and current path ra4' are obtained by changing current path ra3 and current path ra4, which in the second state passed through the body diode of upper arm switching element 15, to a path between the drain and source of upper arm switching element 15 in the third state. This makes it possible to reduce the resistance of current path ra3' and current path ra4' compared to current path ra3 and current path ra4. As in the second state, the currents in current path ra3' and current path ra4' also decrease in the third state.

[0047] Here, the period during which the primary side circuit 10 is in the second state may be short as long as the time required to control the lower arm switching element 16 to a completely non-conductive state is ensured. In other words, the period during which the primary side circuit 10 is in the second state may be short as long as all of the switching elements 15, 16 are in a conductive state and the buffer capacitor Cc is not short-circuited during the transition period from the first state to the third state.

[0048] [Positive AC potential: fourth state] The fourth state shown in Fig. 4 is a state in which a predetermined time has elapsed since transition to the third state. As shown in Fig. 4, in the fourth state, in the primary side circuit 10, a current flows through a current path ra5 due to the supply of power from the AC power source V1 and the continuity of the current in the first inductor 11, and a current path ra6 through which a current difference between the current flowing in the first inductor 11 and the current flowing in the primary winding W1 flows. In addition, in the fourth state, in the secondary side circuit 20, a current flows through a current path rb4.

[0049] The current path ra5 is a path from the first end t11 of the AC power supply V1 to the second end t12 of the AC power supply V1 via the first inductor 11, the first capacitor 17, the primary winding W1, and the lower arm diode 13. When the current flowing through the current path ra4' in the third state decreases and disappears, a current flows from the AC power supply V1 through the current path ra5 via the first inductor 11 and the first capacitor 17 to the primary winding W1. At this time, the current flowing through the primary winding W1 is a current in the opposite direction to the current flowing through the primary winding W1 in the third state. In addition, the current flowing through the current path ra5 charges the first capacitor 17, and the current flowing through the first inductor 11 decreases. In addition, the potential at the start end of the primary winding W1 of the transformer 18 becomes higher than the potential at the end. Therefore, the potential at the start end of the secondary winding W2 also becomes higher than the potential at the end.

[0050] The current path ra6 is a path from the connection point of the first inductor 11, the second terminal t32, and the first terminal t51 to the connection point of the terminal tc2, the anode of the lower-arm diode 13, and the second terminal t42, via the upper-arm switching element 15 and the buffer capacitor Cc. The current path ra6 is a path of the difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1. Specifically, when the current flowing through the first inductor 11 is larger than the current flowing through the primary winding W1, the current flows through the buffer capacitor Cc via the upper-arm switching element 15, and the buffer capacitor Cc is charged. When the current flowing through the first inductor 11 is smaller than the current flowing through the primary winding W1, the current discharged from the buffer capacitor Cc flows through the upper-arm switching element 15 to the primary winding W1.

[0051] The current path rb4 includes two paths. One is a path from the first end t71 of the second inductor 21 to the second end t72 of the second inductor 21 via the secondary side diode 22, the first output end t91, the load connected to the power conversion device 1, and the second output end t92. This current path is due to the continuity of the current in the second inductor 21. The other is a path from the start end of the secondary winding W2 to the end end of the secondary winding W2 via the second capacitor 19, the secondary side diode 22, the first output end t91, the load connected to the power conversion device 1, and the second output end t92. This is a current path formed because the potential of the start end of the secondary winding W2 is higher than the potential of the end end. In the fourth state, the secondary side diode 22 is conductive. In other words, when the upper arm switching element 15 is in a conductive state and the lower arm switching element 16 is in a non-conductive state, the primary winding W1 and secondary winding W2 of the transformer 18 are magnetically coupled with each other with a polarity such that the secondary side diode 22 is conductive.

[0052] [Positive AC potential: 5th state] The fifth state shown in Fig. 5 is a state in which, after a predetermined time has elapsed since transition to the fourth state, the control unit 50 controls the upper arm switching element 15 to be in a non-conductive state while keeping the lower arm switching element 16 in a non-conductive state. As shown in Fig. 5, in the fifth state, in the primary side circuit 10, a current that is a difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1 flows through a current path ra7. In addition, in the fifth state, a current flows through a current path rb4 in the rectifying smoothing circuit 20. The current paths ra5 and rb4 have already been described, and therefore detailed description thereof will be omitted.

[0053] The current path ra7 is a path from the connection point of the terminal tc2, the anode of the lower-arm diode 13, and the second end t42 to the first connection line CL1 via the body diode of the lower-arm switching element 16. In the fifth state, the upper-arm switching element 15 is controlled to a non-conductive state, so that the difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1 flows through the current path ra7. Specifically, the current flowing from the buffer capacitor Cc to the primary winding W1 immediately before the transition from the fourth state to the fifth state flows through the current path ra7 and back to the primary winding W1 after the transition to the fifth state. When the body diode of the lower-arm switching element 16 becomes conductive, the polarity of the voltage applied to the primary winding W1 is reversed, so that the current flowing through the current path ra5 starts to increase in the direction from the AC power source V1 to the first inductor 11. When the current flowing through the first inductor 11 and the current flowing through the primary winding W1 match, the body diode of the lower arm switching element 16 becomes non-conductive.

[0054] Here, the period during which the primary side circuit 10 is in the fifth state may be short as long as the time required to control the upper arm switching element 15 to a completely non-conductive state is ensured. In other words, the period during which the primary side circuit 10 is in the fifth state may be short as long as the buffer capacitor Cc is not short-circuited due to all of the switching elements 15, 16 being in a conductive state during the transition from the fourth state to the sixth state described next.

[0055] [Positive AC potential: 6th state] The sixth state shown in Fig. 6 is a state in which, after a predetermined time has elapsed since transition to the fifth state, the control unit 50 controls the lower arm switching element 16 to be in a conductive state while keeping the upper arm switching element 15 in a non-conductive state. As shown in Fig. 6, in the sixth state, in the primary side circuit 10, a current flows through a current path ra1 and a current path ra8 due to the continuity of the current through the leakage inductance Rw1 of the primary winding W1. In addition, in the sixth state, in the rectifying smoothing circuit 20, a current flows through a current path rb5 due to the continuity of the current through the leakage inductance Rw2 of the secondary winding W2. As the current path ra1 has already been described, a detailed description thereof will be omitted.

[0056] The current path ra8 is a path from the leakage inductance Rw1 of the primary winding W1 through the primary winding W1, the lower arm switching element 16, and the first capacitor 17 to the leakage inductance Rw1. In the sixth state, the current flows through the current path ra1, and the current flowing through the first inductor 11 increases. As a result, the current flowing through the first inductor 11 stops in the primary winding W1, but the leakage inductance Rw1 tries to continue to pass the current, so that a current is generated through the current path ra8. However, the voltage of the first capacitor 17 is applied to the primary winding W1 in a direction that resists the current flowing through the current path ra8. As a result, the current flowing through the current path ra8 decreases rapidly.

[0057] The current path rb5 includes two paths. One is a path from the first end t71 of the second inductor 21 to the second end t72 of the second inductor 21 via the secondary side diode 22, the first output end t91, the load connected to the power conversion device 1, and the second output end t92. The other is a path from the leakage inductance Rw2 of the secondary winding W2 to the secondary winding W2 and the leakage inductance Rw2 via the second capacitor 19, the secondary side diode 22, the first output end t91, the load connected to the power conversion device 1, and the second output end t92. While the voltage applied to the secondary winding W2 is reversed, the second capacitor 19 is discharged and the second inductor 21 continues to flow a current due to the stored energy, so that a current continues to flow through the current path rb5. As a result, the output capacitor 23 is charged, and the magnetic flux of the second inductor 21 decreases.

[0058] After that, when the current flowing through the primary winding W1 becomes zero, the direction of the current flow is reversed. The direction of the current flowing through the secondary winding W2 is also reversed, and the secondary-side diode 22 becomes non-conductive. This causes the state of the power conversion device 1 to return to the first state of FIG.

[0059] [Operation when AC potential is negative] Next, switching control in a state in which the AC potential is negative will be described with reference to FIGS.

[0060] In this state, the control unit 50 switches between the conductive state and the non-conductive state of the upper arm switching element 15 and the lower arm switching element 16. This causes the state of the power conversion device 1 to transition from the first state to the sixth state.

[0061] [AC potential is negative: 1st state] The first state shown in Fig. 7 is a state in which the control unit 50 controls the upper arm switching element 15 to a conductive state and controls the lower arm switching element 16 to a non-conductive state. As shown in Fig. 7, in the first state, a current flows in the primary side circuit 10 through a current path ra9 associated with the supply of power from the AC power source V1 and a current path ra10 associated with the voltage difference between the buffer capacitor Cc and the first capacitor 17. Also, in the first state, a current flows in the rectifying smoothing circuit 20 through a current path rb4. As the current path rb4 has already been described, a detailed description thereof will be omitted.

[0062] The current path ra9 is a path from the second terminal t12 of the AC power supply V1 to the first terminal t11 of the AC power supply V1 via the upper arm diode 12, the upper arm switching element 15, and the first inductor 11. In the first state, the current flowing through the current path ra9 increases over time in the direction from the first inductor 11 to the AC power supply V1, and the magnetic flux of the first inductor 11 also increases with the increase in the current.

[0063] A current path ra10 is a path from a terminal tc1 of the buffer capacitor Cc to a terminal tc2 of the buffer capacitor Cc via the upper arm switching element 15, the first capacitor 17, and the primary winding W1. In the first state, the difference in voltage between the buffer capacitor Cc and the first capacitor 17 is applied to the primary winding W1. The voltage across the buffer capacitor Cc is higher than the voltage across the first capacitor 17. Therefore, the potential at the start end of the primary winding W1 becomes higher than the potential at the end, and a voltage is generated in the secondary winding W2 such that the potential at the start end becomes higher than the potential at the end.

[0064] [AC potential is negative: second state] The second state shown in FIG. 8 is a state in which, after a predetermined time has elapsed since the transition to the first state, the control unit 50 controls the upper arm switching element 15 to be in a non-conducting state while keeping the lower arm switching element 16 in a non-conducting state. As shown in FIG. 8, in the second state, in the primary side circuit 10, a current flows through a current path ra11 due to the supply of power from the AC power source V1 and the continuity of the current in the first inductor 11, and a current path ra8'. In addition, in the second state, a current flows through a current path rb5 in the rectifying smoothing circuit 20. The current path rb5 has already been described, so a detailed description will be omitted. The current path ra8' is similar to the current path ra8, except that in the lower arm switching element 16, the current flows through a body diode instead of between the drain and source.

[0065] The current path ra11 is a path from the second end t12 of the AC power supply V1 to the first end t11 of the AC power supply V1 via the upper arm diode 12, the buffer capacitor Cc, the body diode of the lower arm switching element 16, and the first inductor 11. In the second state, the current flowing through the first inductor 11 flows to the buffer capacitor Cc and charges the buffer capacitor Cc. Accordingly, the current in the first inductor 11 decreases.

[0066] [AC potential is negative: third state] The third state shown in FIG. 9 is a state in which, after a predetermined time has elapsed since the transition to the second state, the control unit 50 controls the lower arm switching element 16 to be in a conductive state while keeping the upper arm switching element 15 in a non-conductive state. As shown in FIG. 9, in the third state, a current flows through a current path ra8 and a current path ra11' in the primary side circuit 10. In addition, in the fourth state, a current flows through a current path rb5 in the rectifying smoothing circuit 20. The current path ra8 and the current path rb5 have already been described, and therefore a detailed description thereof will be omitted. The current path ra11' is similar to the current path ra11, except that the current flows between the drain and source of the lower arm switching element 16, rather than through the body diode.

[0067] Here, the period during which the primary side circuit 10 is in the second state may be short as long as the time required to control the upper arm switching element 15 to a completely non-conductive state is ensured. In other words, the period during which the primary side circuit 10 is in the second state may be short as long as all of the switching elements 15, 16 are in a conductive state and the buffer capacitor Cc is not short-circuited during the transition period from the first state to the third state.

[0068] [AC potential is negative: fourth state] The fourth state shown in Fig. 10 is a state in which a predetermined time has elapsed since transition to the third state. As shown in Fig. 10, in the fourth state, in the primary side circuit 10, a current flows through a current path ra12 due to the supply of power from the AC power source V1 and the continuity of the current in the first inductor 11, and a current path ra13 through which a current difference between the current flowing in the first inductor 11 and the current flowing in the primary winding W1 flows. In addition, in the fourth state, a current flows through a current path rb1 and a current path rb2 in the rectifying and smoothing circuit 20. The current path rb1 and the current path rb2 have already been described, and therefore detailed description thereof will be omitted.

[0069] The current path ra12 is a path from the second end t12 of the AC power supply V1 to the first end t11 of the AC power supply V1 via the upper arm diode 12, the buffer capacitor Cc, the primary winding W1, the first capacitor 17, and the first inductor 11. When the current flowing through the current path ra8 decreases and disappears in the third state, as shown by the current path ra12, a current flows from the AC power supply V1 to the primary winding W1 via the upper arm diode 12 and the buffer capacitor Cc. At this time, the current flowing through the primary winding W1 is a current in the opposite direction to the current flowing through the primary winding W1 in the third state. In addition, the current flowing through the current path ra12 charges the first capacitor 17, and the current flowing through the first inductor 11 decreases. A voltage is applied to the primary winding W1 of the transformer 18 so that the potential at the starting end is lower than the potential at the end, and a voltage is generated in the secondary winding W2 so that the potential at the starting end is lower than the potential at the end.

[0070] The current path ra13 is a path from the connection point of the first inductor 11, the second terminal t32, and the first terminal t51 to the connection point of the terminal tc2, the anode of the lower-arm diode 13, and the second terminal t42, via the lower-arm switching element 16. The current path ra13 is a path of the difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1. Specifically, when the current flowing through the first inductor 11 is larger than the current flowing through the primary winding W1, the current flows from the second terminal t42 to the first terminal t41 of the lower-arm switching element 16. When the current flowing through the first inductor 11 is smaller than the current flowing through the primary winding W1, the current flows from the first terminal t41 to the second terminal t42 of the lower-arm switching element 16.

[0071] [AC potential is negative: 5th state] The fifth state shown in FIG. 11 is a state in which the control unit 50 controls the lower arm switching element 16 to be in a non-conductive state while keeping the upper arm switching element 15 in a non-conductive state after a predetermined time has elapsed since the transition to the fourth state. As shown in FIG. 11, in the fifth state, in the primary side circuit 10, a current flows through a current path ra12 associated with the supply of power from the AC power source V1, the continuity of the current in the first inductor 11, and the leakage inductance Rw1 of the primary winding W1, and a current path ra15 through which a current difference between the current flowing in the first inductor 11 and the current flowing in the primary winding W1 flows. In addition, in the fifth state, a current flows through a current path rb2 and a current path rb3 in the rectifying and smoothing circuit 20. The current path ra12, the current path rb2, and the current path rb3 have already been described, and therefore detailed description thereof will be omitted.

[0072] The current path ra15 is a path from the connection point of the first inductor 11, the second terminal t32, and the first terminal t51 to the connection point of the cathode of the upper arm diode 12, the first terminal t31, and the terminal tc1 via the body diode of the upper arm switching element 15. In the fifth state, the lower arm switching element 16 is controlled to a non-conductive state, so that the difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1 flows through the current path ra15. Specifically, the current discharged from the first capacitor 17 and flowing through the lower arm switching element 16 immediately before the transition from the fourth state to the fifth state flows through the current path ra15, the buffer capacitor Cc, the primary winding W1, and returns to the first capacitor 17 after the transition to the fifth state. When the body diode of the upper arm switching element 15 becomes conductive, the polarity of the voltage applied to the primary winding W1 is reversed, and the current flowing through the current path ra15 rapidly decreases.When the current flowing through the first inductor 11 and the current flowing through the primary winding W1 match, the body diode of the upper arm switching element 15 becomes non-conductive.

[0073] Here, the period during which the primary side circuit 10 is in the fifth state may be short as long as the time required to control the lower arm switching element 16 to a completely non-conductive state is ensured. In other words, the period during which the primary side circuit 10 is in the fifth state may be short as long as the buffer capacitor Cc is not short-circuited due to all of the switching elements 15, 16 being in a conductive state during the transition from the fourth state to the sixth state described next.

[0074] [AC potential is negative: 6th state] The sixth state shown in Fig. 12 is a state in which, after a predetermined time has elapsed since the transition to the fifth state, the control unit 50 controls the upper arm switching element 15 to a conductive state while maintaining the lower arm switching element 16 in a non-conductive state. As shown in Fig. 12, in the sixth state, currents flow through current paths ra9 and ra16 in the primary side circuit 10. Also, in the sixth state, currents flow through current paths rb2 and rb3 in the rectifying smoothing circuit 20. As the current paths ra9, rb2, and rb3 have already been described, detailed description thereof will be omitted.

[0075] The current path ra16 is a path from the leakage inductance Rw1 of the primary winding W1 through the first capacitor 17, the upper arm switching element 15, and the buffer capacitor Cc to the leakage inductance Rw1. In the sixth state, the current flows through the current path ra9, and the current flowing through the first inductor 11 increases. As a result, the current flowing through the first inductor 11 stops in the primary winding W1, but the leakage inductance Rw1 tries to continue to pass the current, so that a current flows through the current path ra16. However, the voltage of the first capacitor 17 is applied to the primary winding W1 in a direction that resists the current flowing through the current path ra16. As a result, the current flowing through the current path ra16 decreases rapidly.

[0076] After that, when the current flowing through the primary winding W1 becomes zero, the direction of the current flow is reversed. The direction of the current flowing through the secondary winding W2 is also reversed, and the secondary-side diode 22 becomes non-conductive. This causes the state of the power conversion device 1 to return to the first state in FIG.

[0077] As described above, whether the AC potential is positive or negative, the time from the start of the first state to the end of the sixth state may or may not be constant. However, in either case, it is desirable that the time from the start of the first state to the end of the sixth state be sufficiently shorter than the period of the input voltage.

[0078] [Another example of state transitions] In the above description, the state of the power conversion device 1 transitions from the first state to the sixth state in a state where the AC potential is positive and in a state where the AC potential is negative, but the present invention is not limited to this. For example, in the first state, when the current in the current path ra2 is small or the leakage inductance Rw1 of the primary winding W1 is small, the current in the current path ra4 or the current path ra8' may become zero while the second state continues. In this case, during the second state, the current flowing through the first inductor 11 may be divided into a current flowing through the body diode of the upper arm switching element 15 to charge the buffer capacitor Cc and a current flowing through the primary winding W1 through the first capacitor 17, and then the upper arm switching element 15 may be turned on, transitioning to the fourth state. In the fourth state, if the current flowing through the primary winding W1 is small or the leakage inductance Rw1 of the primary winding W1 is small, the currents in the current paths ra7 and ra15 may become zero while the fifth state continues. In this case, during the fifth state, all of the current in the first inductor 11 passes through the first capacitor 17 and flows to the primary winding W1, and then the lower arm switching element 16 becomes conductive when the AC potential is positive, and the upper arm switching element 15 becomes conductive when the AC potential is negative, so that a transition to the sixth state may occur.

[0079] [Operation of the power conversion device 1 of this embodiment] As described above, when the AC potential is positive, the control unit 50 switches the upper arm switching element 15 and the lower arm switching element 16 between a conductive state and a non-conductive state, thereby repeatedly changing the state of the power conversion device 1 from the first state to the sixth state. Also, when the AC potential is negative, the control unit 50 switches the upper arm switching element 15 and the lower arm switching element 16 between a conductive state and a non-conductive state, thereby repeatedly changing the state of the power conversion device 1 from the first state to the sixth state. In this way, power conversion is performed in the power conversion device 1.

[0080] In detail, the power conversion device 1 repeats transitions from the first state to the sixth state depending on the polarity of the AC power supply V1. As a result, the potential of the first end t31 of the upper arm switching element 15 is always higher than the potential of the second end t42 of the lower arm switching element 16. That is, the AC voltage of the AC power supply V1 is converted into a DC voltage by the bridge circuit 14 including the upper arm switching element 15, the lower arm switching element 16, the upper arm diode 12, and the lower arm diode 13.

[0081] In the fourth to sixth states in which the AC potential is positive and the first to third states in which the AC potential is negative, the start end of the primary winding W1 has a higher potential than the end end of the primary winding W1. At this time, the first capacitor 17 is charged. Furthermore, at this time, the secondary side diode 22 is conductive, and a current flows from the secondary winding W2 of the transformer 18 through the second capacitor 19 and the secondary side diode 22 to the load. Therefore, energy is supplied from the primary side to the load via the transformer 18.

[0082] In the first to third states in which the AC potential is positive and in the fourth to sixth states in which the AC potential is negative, the potential at the end of the primary winding W1 is higher than the potential at the beginning of the primary winding W1. At this time, a current flows from the secondary winding W2 of the transformer 18 to the second inductor 21 and the second capacitor 19, and is stored as electromagnetic energy and electrostatic energy, respectively.

[0083] The energy stored in the second inductor 21 and the second capacitor 19 when the AC potential is in the first to third states where it is positive and the fourth to sixth states where it is negative is supplied to the load when the AC potential is in the fourth to sixth states where it is positive and the first to third states where it is negative. Regardless of the polarity of the voltage applied to the primary winding W1 of the transformer 18, energy is transferred from the primary side to the secondary side of the transformer 18, and therefore the utilization efficiency of the transformer 18 is high.

[0084] In the second to fourth states, a current that is the difference between the current through the first inductor 11 and the current through the primary winding W1 of the transformer 18 flows through the upper arm switching element 15 to the buffer capacitor Cc. By charging and discharging the buffer capacitor Cc, the currents in the circuit are matched, and the occurrence of a surge voltage is suppressed.

[0085] [Regarding control by the control unit 50] In this embodiment, the control unit 50 can control switching based on the current flowing through the first inductor 11 and the input voltage so that the power factor of the input power is improved. For example, the control unit 50 controls the time of the first state of the upper arm switching element 15 and the lower arm switching element 16 to operate the upper arm diode 12, the lower arm diode 13, and the bridge circuit 14 as a power factor correction (PFC) circuit. In the first state, the AC power source V1 and the first inductor 11 form a closed circuit, and the current from the AC power source V1 is limited by the first inductor 11. Since the increase in the current flowing through the first inductor 11 is proportional to the time the first state continues, it is possible to control the current of the first inductor 11 by controlling the time the first state continues. The control unit 50 controls the duration of the first state based on signals from the current sensor C1 and the voltage sensor C2 so that the waveform of the current flowing through the first inductor 11 becomes approximately sinusoidal and the frequency and phase of the sinusoidal wave match the frequency and phase of the AC power source V1.

[0086] The control unit 50 can also perform switching control so that a predetermined DC voltage is output based on the detection result of the voltage sensor C3. In detail, the control unit 50 performs duty ratio control of both switching elements 15, 16 based on, for example, the difference between the output voltage detected by the voltage sensor C3 and a predetermined DC voltage (target voltage). Furthermore, the control unit 50 can simultaneously improve the power factor of the input power and control the output voltage.

[0087] [Effects of the power conversion device 1 of this embodiment] According to the above embodiment, the following effects can be obtained. (1-1) The power conversion device 1 transitions between the first state to the sixth state by the control unit 50 controlling the switching elements 15, 16 in accordance with the polarity of the AC power supply V1. This causes the bridge circuit 14 to convert the AC voltage of the AC power supply V1 into a DC voltage.

[0088] At the same time, a high-frequency AC voltage is applied to the primary winding W1 of the transformer 18 by the switching operation of the upper arm switching element 15 and the lower arm switching element 16. As a result, power is transmitted to the secondary side of the transformer 18 while being insulated.

[0089] In the conventional circuit, the conversion from AC voltage to DC voltage and the conversion from the converted DC voltage to high frequency AC voltage are performed by separate bridge circuits. On the other hand, in the power conversion device 1 of the present embodiment, the conversion from AC to DC and the conversion from the converted DC to high frequency AC are performed by a single bridge circuit 14.

[0090] This allows the power conversion device 1 to have a smaller number of active elements such as switching elements and rectifying elements, compared to a case where a full-bridge circuit is used as in a conventional circuit. Also, the power conversion device 1 of the present embodiment has a smaller number of active elements through which current passes, compared to a case where a full-bridge circuit is used as in a conventional circuit, so that the conduction loss in the active elements can be reduced.

[0091] (1-2) In the power conversion device 1 in the second to fourth states, a current that is the difference between the current through the first inductor 11 and the current through the primary winding W1 of the transformer 18 flows through the upper arm switching element 15 to the buffer capacitor Cc.

[0092] According to this configuration, the power conversion device 1 can match the currents in the circuit by charging and discharging the buffer capacitor Cc, thereby suppressing the occurrence of surge voltages. (1-3) The transformer 18 transmits power from the primary side to the secondary side in all of the first to sixth states, regardless of the direction of the voltage applied to the start and end of the primary winding W1 of the transformer 18. Therefore, the power conversion device 1 can increase the utilization efficiency of the transformer 18.

[0093] (1-4) The power conversion device 1 further includes a control unit 50 that controls switching of the upper arm switching element 15 and the lower arm switching element 16. The control unit 50 controls the switching based on the current flowing through the first inductor 11 and the input voltage from the AC power source V1 so as to improve the power factor.

[0094] With this configuration, the power conversion device 1 can improve the power factor of the power supplied from the AC power supply V1. (1-5) The control unit 50 controls the switching of the upper arm switching element 15 and the lower arm switching element 16 based on the voltage between the first output terminal t91 and the second output terminal t92 so that a predetermined DC voltage is output.

[0095] According to this configuration, the power conversion device 1 performs feedback control based on the voltage between the first output terminal t91 and the second output terminal t92, and can bring the DC voltage output by the power conversion device 1 closer to a predetermined DC voltage (target voltage).

[0096] <Second embodiment> Hereinafter, the second embodiment will be described with reference to the drawings. In the second embodiment, the connection between the primary side circuit 10 and the buffer capacitor Cc in the power conversion device 2 is different from that in the first embodiment. Note that the same reference numerals are used for the same configurations as in the first embodiment, and the description thereof will be omitted.

[0097] 13, the power conversion device 2 has a similar configuration to that of the power conversion device 1, but the connections of the primary side circuit 10, the first capacitor 17, the transformer 18, and the buffer capacitor Cc are different from those of the power conversion device 1. Specifically, in the power conversion device 2, the starting end of the primary winding W1 is electrically connected to the second end t32 of the upper arm switching element 15 and the first end t41 of the lower arm switching element 16. In addition, the ending end of the primary winding W1 is connected to the first end t51 of the first capacitor 17, and the second end t52 of the first capacitor 17 is connected to the cathode of the lower arm diode 13 and the first end t41 of the lower arm switching element 16.

[0098] Furthermore, a terminal tc1 of the buffer capacitor Cc is connected to the bridge circuit 14 (more specifically, the cathode and first end t31 of the upper arm diode 12). Furthermore, a terminal tc2 of the buffer capacitor Cc is connected to an end of the primary winding W1 and a first end t51 of the first capacitor 17. As a result, the buffer capacitor Cc is connected in parallel to the primary winding W1 of the transformer 18 via the upper arm switching element 15. Furthermore, the buffer capacitor Cc is connected in series to the first capacitor 17.

[0099] Hereinafter, the operation of the power conversion device 2 will be described in detail with reference to Figs. 13 to 24. For ease of explanation, it is assumed that the capacitors 17, 19, 23, and Cc are charged at the timing when the power conversion device 2 starts to operate. It is also assumed that current flows through the first inductor 11, the primary winding W1 of the transformer 18, and the second inductor 21. Furthermore, in the following explanation, the transition timing of each state is the same as in the above-mentioned embodiment, and the conductive state or non-conductive state of each switching element 15, 16 controlled by the control unit 50 is the same as the conductive state or non-conductive state of each state in the first embodiment, so the explanation will be omitted.

[0100] [Operation when AC potential is positive] First, switching control in a state in which the AC potential of the first terminal t11 is higher than the potential of the second terminal t12 in the AC power supply V1 and the AC potential is positive will be described with reference to FIGS.

[0101] [Positive AC potential: 1st state] 13, in the first state, in the primary side circuit 10, a current flows through a current path ra1 and a current path ra21 associated with the discharge of the first capacitor 17. In addition, in the first state, a current flows through a current path rb1 and a current path rb2 in the rectifying smoothing circuit 20. The current path ra1, the current path rb1, and the current path rb2 have already been described, and therefore detailed description thereof will be omitted.

[0102] The current path ra21 is a path from a first end t51 of the first capacitor 17 to a second end t52 of the first capacitor 17 via the primary winding W1 and the lower arm switching element 16. The voltage across the first capacitor 17 is applied to the primary winding W1 of the transformer 18, and the potential at the end of the primary winding W1 becomes higher than the potential at the starting end. As a result, a voltage is generated across the secondary winding W2 of the transformer 18 such that the potential at the end becomes higher than the potential at the starting end.

[0103] [Positive AC potential: second state] 14, in the second state, in the primary side circuit 10, a current flows through a current path ra22 associated with the supply of power from the AC power source V1, and a current path ra23 due to the continuation of the current through the leakage inductance Rw1 of the primary winding W1. In addition, in the second state, a current flows through a current path rb2 and a current path rb3 in the rectifying smoothing circuit 20. The current path rb2 and the current path rb3 have already been described, and therefore detailed description thereof will be omitted.

[0104] The current path ra22 is a path from the first end t11 of the AC power supply V1 to the second end t12 of the AC power supply V1 via the first inductor 11, the body diode of the upper arm switching element 15, the buffer capacitor Cc, the first capacitor 17, and the lower arm diode 13. In the second state, the current flowing through the first inductor 11 flows through the body diode of the upper arm switching element 15 to the buffer capacitor Cc and the first capacitor 17. Therefore, the current flowing through the current path ra22 charges the buffer capacitor Cc and the first capacitor 17. The current flowing through the first inductor 11 decreases.

[0105] The current path ra23 is a path from the primary winding W1 through the body diode of the upper arm switching element 15 and the buffer capacitor Cc to the primary winding W1. In the second state, the current flowing through the primary winding W1 flows through the body diode of the upper arm switching element 15 to the buffer capacitor Cc. Therefore, the current flowing through the current path ra23 charges the buffer capacitor Cc. Before and after the control to the second state, the direction of the voltage applied to the primary winding W1 is reversed. Therefore, the current flowing through the primary winding W1 rapidly decreases.

[0106] [Positive AC potential: third state] 15, in the third state, current flows through a current path ra22' and a current path ra23' in the primary side circuit 10. In addition, in the third state, current flows through a current path rb2 and a current path rb3 in the rectifying smoothing circuit 20. As the current path rb2 and the current path rb3 have already been described, detailed description thereof will be omitted.

[0107] Current path ra22' and current path ra23' are obtained by changing current path ra22 and current path ra23, which in the second state passed through the body diode of upper arm switching element 15, to a path between the drain and source of upper arm switching element 15 in the third state. This makes it possible to reduce the resistance of current path ra22' and current path ra23' compared to current path ra22 and current path ra23. As in the second state, in the third state, the currents in current path ra22' and current path ra23' decrease and the current in current path rb3 also decreases.

[0108] Here, the period during which the primary side circuit 10 is in the second state may be short as long as the time required to control the lower arm switching element 16 to a completely non-conductive state is ensured. In other words, the period during which the primary side circuit 10 is in the second state may be short as long as all of the switching elements 15, 16 are in a conductive state during the transition period from the first state to the third state and the series connection circuit of the buffer capacitor Cc and the first capacitor 17 is not short-circuited.

[0109] [Positive AC potential: fourth state] 16, in the fourth state, in the primary side circuit 10, a current flows through a current path ra24 associated with the supply of power from the AC power source V1 and a current path ra25 associated with the difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1. In addition, in the fourth state, a current flows through a current path rb4 in the rectifying smoothing circuit 20. As the current path rb4 has already been described, a detailed description thereof will be omitted.

[0110] The current path ra24 is a path from the first end t11 of the AC power supply V1 to the second end t12 of the AC power supply V1 via the first inductor 11, the primary winding W1, the first capacitor 17, and the lower arm diode 13. When the current flowing through the current path ra23' in the third state decreases and disappears, a current flows from the AC power supply V1 to the primary winding W1 via the current path ra24 via the first inductor 11. At this time, the current flowing through the primary winding W1 is a current in the opposite direction to the current flowing through the primary winding W1 in the third state. Furthermore, the current flowing through the current path ra24 charges the first capacitor 17, and the current flowing through the first inductor 11 decreases.

[0111] The current path ra25 is a path from the connection point of the first inductor 11, the second end t32, and the start end of the primary winding W1 to the connection point of the terminal tc2, the end end of the primary winding W1, and the first end t51, via the upper arm switching element 15 and the buffer capacitor Cc. The current path ra25 is a path of the difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1. Specifically, when the current flowing through the first inductor 11 is larger than the current flowing through the primary winding W1, a current flows from the primary winding W1 to the buffer capacitor Cc, and the buffer capacitor Cc is charged. On the other hand, when the current flowing through the first inductor 11 is smaller than the current flowing through the primary winding W1, a current discharged from the buffer capacitor Cc flows through the upper arm switching element 15 to the primary winding W1.

[0112] [Positive AC potential: 5th state] 17, in the fifth state, in the primary side circuit 10, a current flows through a current path ra24 and a current path ra26 through which a current equal to the difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1 flows. In addition, in the fifth state, a current flows through a current path rb4 in the rectifying smoothing circuit 20. The current path ra24 and the current path rb4 have already been described, and therefore detailed description thereof will be omitted.

[0113] In the fifth state, the upper arm switching element 15 is controlled to be in a non-conductive state, so that the difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1 flows through the current path ra26. Specifically, the current discharged from the buffer capacitor Cc and flowing through the primary winding W1 in the fourth state returns to the primary winding W1 from the current path ra26 in the fifth state. When the body diode of the lower arm switching element 16 becomes conductive, the polarity of the voltage applied to the primary winding W1 is reversed, so that the current flowing through the current path ra24 is rapidly reduced. When the current flowing through the first inductor 11 and the current flowing through the primary winding W1 match, the body diode of the lower arm switching element 16 becomes in a non-conductive state.

[0114] Here, the period during which the primary side circuit 10 is in the fifth state may be short as long as the time required to control the upper arm switching element 15 to a completely non-conductive state is ensured. In other words, the period during which the primary side circuit 10 is in the fifth state may be short as long as the series connection circuit of the buffer capacitor Cc and the first capacitor 17 is not short-circuited due to all of the switching elements 15 and 16 being in a conductive state during the transition from the fourth state to the sixth state described next.

[0115] [Positive AC potential: 6th state] 18, in the sixth state, a current flows through a current path ra1 and a current path ra27 due to a continuation of a current through a leakage inductance Rw1 of the primary winding W1 in the primary side circuit 10. In addition, in the sixth state, a current flows through a current path rb5 in the rectifying smoothing circuit 20. The current paths ra1 and rb5 have already been described, and therefore detailed description thereof will be omitted.

[0116] The current path ra27 is a path from the primary winding W1 through the first capacitor 17 and the lower arm switching element 16 to the primary winding W1. In the sixth state, the current flowing through the first inductor 11 flows through the current path ra1. As a result, the current flowing through the first inductor 11 stops in the primary winding W1, but the leakage inductance Rw1 tries to continue to pass the current, so that a current flows through the current path ra27. However, the voltage of the first capacitor 17 is applied to the primary winding W1 in a direction that resists the current flowing through the current path ra27. As a result, the current flowing through the current path ra27 decreases rapidly.

[0117] After that, when the current flowing through the primary winding W1 becomes zero, the direction of the current flow is reversed. The direction of the current flowing through the secondary winding W2 is also reversed, and the secondary-side diode 22 becomes non-conductive. This causes the state of the power conversion device 2 to return to the first state in FIG.

[0118] [Operation when AC potential is negative] Next, switching control in a state in which the AC potential of the AC power supply V1 is negative, that is, the potential of the first terminal t11 is lower than the potential of the second terminal t12, will be described with reference to FIGS.

[0119] [AC potential is negative: 1st state] 19, in the first state, a current flows through current path ra9 and current path ra28 in the primary side circuit 10. In addition, in the second state, a current flows through current path rb4 in the rectifier smoothing circuit 20. As current path ra9, current path ra28, and current path rb4 have already been described, detailed description thereof will be omitted. As described above, in the first state, the current flowing through current path ra9 increases over time.

[0120] A current path ra28 is a path from a terminal tc1 of the buffer capacitor Cc to a terminal tc2 via the upper arm switching element 15 and the primary winding W1. The voltage across the buffer capacitor Cc is applied to the primary winding W1 of the transformer 18, and the potential at the start end of the primary winding W1 becomes higher than the potential at the end. As a result, a voltage is generated across the secondary winding W2 of the transformer 18 such that the potential at the start end is higher than the potential at the end.

[0121] [AC potential is negative: second state] As shown in Fig. 20, in the second state, in the primary side circuit 10, a current flows through a current path ra27' and a current path ra29 associated with the supply of power from the AC power supply V1. In addition, in the second state, in the rectifying smoothing circuit 20, a current flows through a current path rb2 and a current path rb3. As the current path rb2 and the current path rb3 have already been described, detailed description will be omitted. The current path ra27' is similar to the current path ra27, except that in the lower arm switching element 16, a current flows through the body diode instead of between the drain and source.

[0122] The current path ra29 is a path from the second end t12 of the AC power supply V1 to the first end t11 of the AC power supply V1 via the upper arm diode 12, the buffer capacitor Cc, the first capacitor 17, the body diode of the lower arm switching element 16, and the first inductor 11. In the second state, the current flowing through the first inductor 11 flows through the body diode of the lower arm switching element 16 to the buffer capacitor Cc, charging the buffer capacitor Cc and the first capacitor 17. Accordingly, the current flowing through the first inductor 11 decreases.

[0123] [AC potential is negative: third state] As shown in Fig. 21, in the third state, a current flows through a current path ra27 and a current path ra29' in the primary side circuit 10. Also, in the third state, a current flows through a current path rb5 in the rectifying smoothing circuit 20. As the current path ra27, the current path rb2, and the current path rb3 have already been described, detailed description thereof will be omitted. The current path ra29' is similar to the current path ra29, except that a current flows between the drain and source of the lower arm switching element 16, rather than through the body diode.

[0124] Here, the period during which the primary side circuit 10 is in the second state may be short as long as the time required to control the upper arm switching element 15 to a completely non-conductive state is ensured. In other words, the period during which the primary side circuit 10 is in the second state may be short as long as all of the switching elements 15, 16 are in a conductive state during the transition period from the first state to the third state and the series connection circuit of the buffer capacitor Cc and the first capacitor 17 is not short-circuited.

[0125] [AC potential is negative: fourth state] 22, in the fourth state, in the primary side circuit 10, a current flows through a current path ra30 associated with the supply of power from the AC power source V1, and a current path ra31 associated with the difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1. In addition, in the fourth state, a current flows through a current path rb1 and a current path rb2 in the rectifying smoothing circuit 20. The current path rb1 and the current path rb2 have already been described, and therefore detailed description thereof will be omitted.

[0126] The current path ra30 is a path from the second end t12 of the AC power supply V1 to the first end t11 of the AC power supply V1 via the upper arm diode 12, the buffer capacitor Cc, the primary winding W1, and the first inductor 11. When the current flowing through the current path ra27' in the third state decreases and disappears, a current flows from the AC power supply V1 to the primary winding W1 through the current path ra30 via the upper arm diode 12. At this time, the current flowing through the primary winding W1 is a current in the opposite direction to the current flowing through the primary winding W1 in the third state. Furthermore, the current flowing through the current path ra30 charges the buffer capacitor Cc, and the current flowing through the first inductor 11 decreases.

[0127] The current path ra31 is a path from the connection point of the first inductor 11, the second end t32, and the start end of the primary winding W1 to the connection point of the terminal tc2, the end end of the primary winding W1, and the first end t51, via the lower arm switching element 16 and the first capacitor 17. The current path ra31 is a path of the difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1. Specifically, when the current flowing through the first inductor 11 is larger than the current flowing through the primary winding W1, a current flows from the primary winding W1 to the first capacitor 17, and the first capacitor 17 is charged. When the current flowing through the first inductor 11 is smaller than the current flowing through the primary winding W1, a current discharged from the first capacitor 17 flows through the lower arm switching element 16 to the primary winding W1.

[0128] [AC potential is negative: 5th state] 23, in the fifth state, in the primary side circuit 10, a current flows through a current path ra30 and a current path ra32 through which a current equal to the difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1 flows. In addition, in the fifth state, a current flows through a current path rb2 and a current path rb3 in the rectifying smoothing circuit 20. The current path ra30, the current path rb2, and the current path rb3 have already been described, and therefore detailed description thereof will be omitted.

[0129] The current path ra32 is a path from the connection point of the first inductor 11, the second terminal t32, and the start terminal of the primary winding W1 to the connection point of the cathode of the upper arm diode 12, the first terminal t31, and the terminal tc1 via the body diode of the upper arm switching element 15. In the fifth state, the lower arm switching element 16 is controlled to a non-conductive state, so that the difference between the current flowing through the first inductor 11 and the current flowing through the primary winding W1 flows through the current path ra32. Specifically, in the fourth state, the current discharged from the first capacitor 17 and flowing through the primary winding W1 flows through the current path ra32 and returns to the primary winding W1 in the fifth state. When the body diode of the upper arm switching element 15 becomes conductive, the polarity of the voltage applied to the primary winding W1 is reversed, so that the current flowing through the current path ra30 rapidly decreases. When the current flowing through the first inductor 11 and the current flowing through the primary winding W1 match, the body diode of the upper arm switching element 15 becomes non-conductive.

[0130] Here, the period during which the primary side circuit 10 is in the fifth state may be short as long as the time required to control the lower arm switching element 16 to a completely non-conductive state is ensured. In other words, the period during which the primary side circuit 10 is in the fifth state may be short as long as the series connection circuit of the buffer capacitor Cc and the first capacitor 17 is not short-circuited due to all of the switching elements 15, 16 being in a conductive state during the transition from the fourth state to the sixth state described next.

[0131] [Positive AC potential: 6th state] 24, in the sixth state, current flows through the current path ra9 and the current path ra23' in the primary side circuit 10. In addition, in the sixth state, current flows through the current path rb2 and the current path rb3 in the rectifying smoothing circuit 20. As the current path ra9, the current path ra23', the current path rb2, and the current path rb3 have already been described, detailed description thereof will be omitted.

[0132] [Effects of the power conversion device 2 of this embodiment] (2-1) In the power conversion device 2, the buffer capacitor Cc is connected in series with the first capacitor 17. In addition, in the second state to the fourth state, the buffer capacitor Cc and the first capacitor 17 have a current that is a difference between the current of the first inductor 11 and the current flowing through the primary winding W1 of the transformer 18 flow through the upper arm switching element 15 to the buffer capacitor Cc and the first capacitor 17. That is, the currents in the circuit are matched by charging and discharging the buffer capacitor Cc and the first capacitor 17, and the surge voltage is suppressed. In addition, according to the power conversion device 2, the voltage applied to both ends of the buffer capacitor Cc in the power conversion device 1 is divided into a voltage applied to the first capacitor 17 and a voltage applied to the buffer capacitor Cc in this embodiment. As a result, the withstand voltage of the buffer capacitor Cc can be made smaller than that of the buffer capacitor Cc used in the power conversion device 1.

[0133] <Variation 1> Hereinafter, a first modified example of the first embodiment described above will be described with reference to the drawings. In the above embodiment, the power conversion device 1 is described as including the rectifying smoothing circuit 20. In the first modified example, the power conversion device 3 is described as including a rectifying smoothing circuit 30 instead of the rectifying smoothing circuit 20. Note that the same reference numerals are used to designate the same components as those in the above embodiment, and the description thereof will be omitted.

[0134] 25, the power conversion device 3 includes a primary side circuit 10, a first capacitor 17, a transformer 18, a second capacitor 19, a rectifying smoothing circuit 30, a buffer capacitor Cc, and a control unit 50. The rectifying smoothing circuit 30 includes a second inductor 21, a secondary side diode 22, an output capacitor 23, a first output line OL1, a second output line OL2, a first output terminal t91, and a second output terminal t92.

[0135] An end of the secondary winding W2 is electrically connected to a first end t71 of the second inductor 21 and a cathode of the secondary side diode 22 via the second capacitor 19. Specifically, an end of the secondary winding W2 is connected to a first end t61 of the second capacitor 19, and a second end t62 of the second capacitor 19 is connected to the first end t71 of the second inductor 21 and the cathode of the secondary side diode 22. A start of the secondary winding W2 is connected to an anode of the secondary side diode 22 and a second output line OL2. In this embodiment, the cathode of the secondary side diode 22 is an example of "one end of the secondary side rectifier", and the anode of the secondary side diode 22 is an example of "the other end of the secondary side rectifier".

[0136] The output capacitor 23 is connected to both the first output line OL1 and the second output line OL2. Specifically, a first end t81 of the output capacitor 23 is connected to the first output line OL1. And a second end t82 of the output capacitor 23 is connected to the second output line OL2.

[0137] The second end t72 of the second inductor 21 is connected to the first output line OL1. As described above, the first end t71 of the second inductor 21 and the cathode of the secondary side diode 22 are connected to each other. In addition, the second end t72 of the second inductor 21 and the first end t81 of the output capacitor 23 are connected to each other. Therefore, the rectifying smoothing circuit 30 is connected to the secondary winding W2 of the transformer 18 via the second capacitor 19.

[0138] Hereinafter, the operation of the power conversion device 3 will be described in detail with reference to Fig. 25 to Fig. 28. Note that the operation of the primary side circuit 10 is the same as that of the first embodiment described above, and therefore the description thereof will be omitted.

[0139] [Operation in the first state for positive condition, and the fourth state for negative condition] Fig. 25 is a diagram showing current paths when the AC potential is in a first state where it is positive and in a fourth state where it is negative. As shown in Fig. 25, in the first state where the AC potential is positive and in the fourth state where the AC potential is negative, currents flow through current paths ra2 and ra12 from the end to the start in the primary winding W1 of the primary side circuit 10. In addition, in the first state where the AC potential is positive and in the fourth state where it is negative, current flows through a current path rb6 from the secondary winding W2 in the rectifying smoothing circuit 30. The current paths ra2 and ra12 have already been described, so a detailed description will be omitted.

[0140] The current path rb6 is a path from the end of the secondary winding W2 to the beginning of the secondary winding W2 via the second capacitor 19, the second inductor 21, the first output terminal t91, the load connected to the power conversion device 3, and the second output terminal t92. In the first state in which the AC potential is positive and the fourth state in which the AC potential is negative, the current flowing through the second inductor 21 increases with time. In addition, the current flowing through the current path rb6 discharges the second capacitor 19 and charges the output capacitor 23. A voltage that is the sum of the voltages across the secondary winding W2 and the voltages across the second capacitor 19 minus the DC voltages output from the first output terminal t91 and the second output terminal t92 is applied to the second inductor 21. Accordingly, the current flowing through the second inductor 21 increases. In the first state in which the AC potential is positive and the fourth state in which the AC potential is negative, the secondary diode 22 is not in a conductive state.

[0141] [Operation in the second to third states in positive state, and the fifth to sixth states in negative state] Fig. 26 is a diagram showing current paths in the second and third states where the AC potential is positive and in the fifth and sixth states where the AC potential is negative. As shown in Fig. 26, in the second and third states where the AC potential is positive and in the fifth and sixth states where the AC potential is negative, a current flows from the end to the start in the primary winding W1 of the primary side circuit 10 through current paths ra4, ra4', ra14, and ra16 due to the continuity of the current through the leakage inductance Rw1 of the primary winding W1. The current paths ra4, ra4', ra14, and ra16 have already been described, and therefore will not be described in detail.

[0142] The current path rb7 is a path from the secondary winding W2 to the start end of the secondary winding W2 via the second capacitor 19, the second inductor 21, the first output end t91, the load connected to the power conversion device 3, and the second output end t92. In the second and third states where the AC potential is positive and in the fifth and sixth states where the AC potential is negative, the current flowing through the primary winding W1 decreases, and therefore the current flowing through the current path rb7 also decreases.

[0143] [Operation in states 4 to 5 in positive state, and state 1 in negative state] FIG. 27 is a diagram showing current paths in the fourth to fifth states in which the AC potential is positive and in the first state in which the AC potential is negative. As shown in FIG. 27, in the fourth to fifth states in which the AC potential is positive and in the first state in which the AC potential is negative, currents flow through current paths ra5 and ra10 from the starting end to the ending end in the primary winding W1 of the primary side circuit 10. In addition, in the fourth to fifth states in which the AC potential is positive and in the first state in which the AC potential is negative, the rectifying smoothing circuit 30 has a current path rb8 due to the current from the secondary winding W2 and a current path rb9 due to the continuity of the current in the second inductor 21. The current paths ra5 and ra10 have already been described, so detailed description will be omitted.

[0144] The current path rb8 is a path from the start of the secondary winding W2 to the end of the secondary winding W2 via the secondary diode 22 and the second capacitor 19. The second capacitor 19 is charged by the current flowing through the current path rb8. In the fourth and fifth states in which the AC potential is positive and in the first state in which the AC potential is negative, the secondary diode 22 is conductive. That is, in the first modification, when the upper arm switching element 15 is conductive and the lower arm switching element 16 is non-conductive, the primary winding W1 and the secondary winding W2 of the transformer 18 are magnetically coupled with each other with a polarity such that the secondary diode 22 is conductive. As the current flowing through the current paths ra5 and ra10 decreases, the current flowing through the current path rb8 also decreases.

[0145] The current path rb9 is a path from the second end t72 of the second inductor 21 to the first end t71 via the first output end t91, the load connected to the power conversion device 3, the second output end t92, and the secondary side diode 22. As described above, in the fourth state to the fifth state in which the AC potential is positive and in the first state in which the AC potential is negative, the secondary side diode 22 is conductive, so that a current continues to flow through the current path rb9 due to the energy stored in the second inductor 21. The output capacitor 23 outputs a DC voltage from the first output end t91 and the second output end t92. In addition, the current flowing through the current path rb9 charges the output capacitor 23, and the current flowing through the second inductor 21 decreases.

[0146] [Operation in the sixth state in positive state, and the second to third states in negative state] FIG. 28 is a diagram showing current paths in the sixth state in which the AC potential is positive and the second to third states in which the AC potential is negative. As shown in FIG. 28, in the sixth state in which the AC potential is positive and the second to third states in which the AC potential is negative, a current flows from the start end to the end end in the primary winding W1 of the primary side circuit 10 through current paths ra8 and ra8' due to the continuity of the current through the leakage inductance Rw1 of the primary winding W1. In addition, in the sixth state in which the AC potential is positive and the second to third states in which the AC potential is negative, a current flows through the current path rb9 and a current path rb10 due to the continuity of the current through the leakage inductance Rw2 of the secondary winding W2 in the rectifying smoothing circuit 30. The current paths ra8 and ra8' and the current path rb9 have already been described, and therefore detailed description thereof will be omitted.

[0147] The current path rb10 is a path from the secondary winding W2 to the secondary winding W2 via the secondary side diode 22 and the second capacitor 19. When a current flows through the current path rb10, the second capacitor 19 is charged.

[0148] [Effects of power conversion device 3 of modification 1] (3-1) According to the power conversion device 3 of the first modification, a current is supplied to the load of the power conversion device 3 by the second inductor 21 regardless of the polarity of the AC power supply V1 and the conductive / non-conductive state of the upper arm switching element 15 and the lower arm switching element 16.

[0149] [Another example of the power conversion device 3] In addition, in the power conversion device 3, as in the second embodiment, the connection between the primary side circuit 10 and the buffer capacitor Cc may be different from the above-mentioned example. As shown in FIG. 29, the power conversion device 4 has, for example, the same configuration as the power conversion device 3, while the connection between the primary side circuit 10, the first capacitor 17, the transformer 18, and the buffer capacitor Cc is different from that of the power conversion device 3. Specifically, in the power conversion device 4, the start end of the primary winding W1 is electrically connected to the second end t32 of the upper arm switching element 15 and the first end t41 of the lower arm switching element 16. In addition, the end end of the primary winding W1 is connected to the first end t51 of the first capacitor 17, and the second end t52 of the first capacitor 17 is connected to the cathode of the lower arm diode 13 and the first end t41 of the lower arm switching element 16.

[0150] Furthermore, a terminal tc1 of the buffer capacitor Cc is connected to the bridge circuit 14 (more specifically, the cathode and first end t31 of the upper arm diode 12). Furthermore, a terminal tc2 of the buffer capacitor Cc is connected to an end of the primary winding W1 and a first end t51 of the first capacitor 17. As a result, the buffer capacitor Cc is connected in parallel to the primary winding W1 of the transformer 18 via the upper arm switching element 15. Furthermore, the buffer capacitor Cc is connected in series to the first capacitor 17.

[0151] The operation of the primary side circuit 10 in the power conversion device 4 is similar to that of the primary side circuit 10 described in the above embodiment, and therefore a description thereof will be omitted. Also, the operation of the rectifying smoothing circuit 30 corresponding to the primary side circuit 10 in the power conversion device 4 is similar to that of the above modification 1, and therefore a description thereof will be omitted.

[0152] According to this configuration, the power conversion device 4 can reduce the withstand voltage of the buffer capacitor Cc, similar to the effect of the above-described second embodiment. Furthermore, similar to the effect of the above-described first modification, the power conversion device 4 supplies a current to the load of the power conversion device 4 by the second inductor 21 regardless of the polarity of the AC power supply V1 and the conductive / non-conductive state of the upper arm switching element 15 and the lower arm switching element 16.

[0153] <Variation 2> Hereinafter, a second modification of the first embodiment described above will be described with reference to the drawings. In the above-described embodiment and modifications, the power conversion devices 1 and 3 are provided with the rectifying smoothing circuit 20 and the rectifying smoothing circuit 30. In the second modification, a case will be described in which the power conversion device 4 is provided with a rectifying smoothing circuit 60 instead of the rectifying smoothing circuit 20 and the rectifying smoothing circuit 30. Note that the same components as those in the above-described embodiment are denoted by the same reference numerals and description thereof will be omitted.

[0154] 30, the power conversion device 5 includes a primary side circuit 10, a first capacitor 17, a transformer 18a, a buffer capacitor Cc, a control unit 50, and a rectifying smoothing circuit 60. The transformer 18 includes a first secondary winding W21 and a second secondary winding W22 as a secondary winding W2. The rectifying smoothing circuit 60 includes a second inductor 21, an output capacitor 23, a first secondary side diode 24, a second secondary side diode 25, a first output line OL1, a second output line OL2, a first output terminal t91, and a second output terminal t92.

[0155] The starting end of the first secondary winding W21 is electrically connected to the anode of the first secondary side diode 24. The ending end of the second secondary winding W22 is electrically connected to the anode of the second secondary side diode 25. The cathode of the first secondary side diode 24 is connected to one end of the output capacitor 23 via the second inductor 21. Specifically, the cathode of the first secondary side diode 24 is connected to the first end t71 of the second inductor 21. The cathode of the second secondary side diode 25 is connected to the connection point between the cathode of the first secondary side diode 24 and the first end t71. The ending end of the first secondary winding W21 and the starting end of the second secondary winding W22 are connected.

[0156] The first secondary side diode 24 and the second secondary side diode 25 are connected as described above, so that each allows one of the current flowing through the first secondary winding W21 and the current flowing through the second secondary winding W22 and restricts the other. Specifically, when the potential at the start end of the primary winding W1 is higher than the potential at the end end, the first secondary side diode 24 allows current to flow from the first secondary winding W21 of the secondary winding W2 to the second inductor 21 and beyond. Also, the second secondary side diode 25 prohibits current from flowing from the second secondary winding W22 of the secondary winding W2 to the second inductor 21 and beyond. Conversely, when the potential at the end of the primary winding W1 is higher than the potential at the starting end, the second secondary side diode 25 allows current to flow from the second secondary winding W22 of the secondary winding W2 to the second inductor 21 and beyond, and the first secondary side diode 24 prohibits current from flowing from the first secondary winding W21 of the secondary winding W2 to the second inductor 21 and beyond.

[0157] In the second modification, the first secondary side diode 24 is an example of a "first secondary side rectifier element", the cathode of the first secondary side diode 24 is an example of a "one end of the first secondary side rectifier element", and the anode of the first secondary side diode 24 is an example of a "other end of the first secondary side rectifier element". The second secondary side diode 25 is an example of a "second secondary side rectifier element", the cathode of the second secondary side diode 25 is an example of a "one end of the second secondary side rectifier element", and the anode of the second secondary side diode 25 is an example of a "other end of the second secondary side rectifier element".

[0158] The output capacitor 23 is connected to both the first output line OL1 and the second output line OL2. Specifically, a first end t81 of the output capacitor 23 is connected to the first output line OL1. And a second end t82 of the output capacitor 23 is connected to the second output line OL2.

[0159] The second inductor 21 is provided in a portion on the first output line OL1 between the first secondary side diode 24 and the output capacitor 23. Specifically, the second inductor 21 is provided in a portion on the first output line OL1 between the connection point of the first secondary side diode 24 and the connection point of the output capacitor 23. As described above, the cathode of the first secondary side diode 24 and the cathode of the second secondary side diode 25 are connected to the first end t71 of the second inductor 21. In addition, the second end t72 of the second inductor 21 and the first end t81 of the output capacitor 23 are connected to each other.

[0160] The connection point between the first secondary winding W21 and the second secondary winding W22 is connected to the second end t82 of the output capacitor 23 and the second output line OL2. The connection point between the first secondary winding W21 and the second secondary winding W22 is, for example, the midpoint of the secondary winding W2. Therefore, the second end t82 of the output capacitor 23 is connected to the midpoint of the secondary winding W2.

[0161] Hereinafter, the operation of the power conversion device 5 will be described in detail with reference to Fig. 30 to Fig. 33. Note that the operation of the primary side circuit 10 is similar to that of the first embodiment described above, and therefore the description thereof will be omitted.

[0162] [Operation in the first state for positive condition, and the fourth state for negative condition] Fig. 30 is a diagram showing current paths in the first state where the AC potential is positive and negative. As shown in Fig. 30, in the first state where the AC potential is positive and in the fourth state where the AC potential is negative, currents flow through current paths ra2 and ra12 from the end to the start in the primary winding W1 of the primary side circuit 10. In addition, in the first state where the AC potential is positive and in the fourth state where the AC potential is negative, a current flows through rb11 from the second secondary winding W22 in the rectifying smoothing circuit 60. The current paths ra2 and ra12 have already been described, so a detailed description will be omitted.

[0163] The current path rb11 is a path from the end of the second secondary winding W22 to the start of the second secondary winding W22 via the second secondary side diode 25, the second inductor 21, the first output terminal t91, the load connected to the power conversion device 5, and the second output terminal t92. A voltage is generated in the secondary winding W2 in a direction that makes the first secondary side diode 24 non-conductive and the second secondary side diode 25 conductive. In a first state in which the AC potential is positive and a fourth state in which the AC potential is negative, a voltage that is the difference between the voltage applied to the second secondary winding W22 and the voltage across the output capacitor 23 (i.e., the output voltage) is applied to the second inductor 21.

[0164] [Operation in the second to third states in positive state, and the fifth to sixth states in negative state] Fig. 31 is a diagram showing current paths in the second and third states where the AC potential is positive and in the fifth and sixth states where the AC potential is negative. As shown in Fig. 31, in the second and third states where the AC potential is positive and in the fifth and sixth states where the AC potential is negative, a current flows from the end to the start in the primary winding W1 of the primary side circuit 10 through current paths ra4, ra4', ra14, and ra16 due to the continuity of the current through the leakage inductance Rw1 of the primary winding W1. The current paths ra4, ra4', ra14, and ra16 have already been described, and therefore will not be described in detail.

[0165] The current path rb12 is due to the leakage inductance Rw22 of the second secondary winding W22 and the continuity of the current in the second inductor 21. The current path rb12 is a path from the second secondary winding W22 to the second secondary winding W22 via the second secondary side diode 25, the second inductor 21, the first output terminal t91, the load connected to the power conversion device 5, and the second output terminal t92. In the second and third states in which the AC potential is positive and the fifth and sixth states in which the AC potential is negative, a voltage is generated in the secondary winding W2 in a direction that makes the first secondary side diode 24 non-conductive and the second secondary side diode 25 conductive. In the second and third states in which the AC potential is positive and the fifth and sixth states in which the AC potential is negative, the current flowing through the primary winding W1 decreases, and the current flowing through the current path rb12 also decreases. The output capacitor 23 discharges to compensate for the decrease in the current flowing through the current path rb12.

[0166] [Operation in states 4 to 5 in positive state, and state 1 in negative state] Fig. 32 is a diagram showing current paths in the fourth and fifth states in which the AC potential is positive and in the first state in which the AC potential is negative. As shown in Fig. 32, in the fourth and fifth states in which the AC potential is positive and in the first state in which the AC potential is negative, currents flow through current paths ra5 and ra10 from the starting end to the ending end in the primary winding W1 of the primary side circuit 10. In addition, in the fourth and fifth states, a current flows through a current path rb13 associated with the first secondary winding W21 in the rectifying smoothing circuit 60. The current paths ra5 and ra10 have already been described, and therefore detailed description thereof will be omitted.

[0167] The current path rb13 is a path from the start of the first secondary winding W21 to the end of the first secondary winding W21 via the first secondary side diode 24, the second inductor 21, the first output terminal t91, the load connected to the power conversion device 5, and the second output terminal t92. In the fourth state to the fifth state in which the AC potential is positive and in the first state in which the AC potential is negative, a voltage is generated in the secondary winding W2 in a direction that makes the first secondary side diode 24 conductive and the second secondary side diode 25 non-conductive. A voltage that is the difference between the voltage applied to the second secondary winding W22 and the voltage across the output capacitor 23 (i.e., the output voltage) is applied to the second inductor 21.

[0168] [Operation in the sixth state in positive state, and the second to third states in negative state] FIG. 33 is a diagram showing current paths in the sixth state in which the AC potential is positive and the second to third states in which the AC potential is negative. As shown in FIG. 33, in the sixth state in which the AC potential is positive and the second to third states in which the AC potential is negative, a current flows from the start end to the end end of the primary winding W1 of the primary side circuit 10 through current paths ra8 and ra8' due to the continuity of the current through the leakage inductance Rw1 of the primary winding W1. In addition, in the sixth state in which the AC potential is positive and the second to third states in which the AC potential is negative, a current flows through a current path rb14 due to the continuity of the current through the leakage inductance Rw21 of the first secondary winding W21 in the rectifying smoothing circuit 60. The current paths ra8 and ra8' have already been described, and therefore will not be described in detail.

[0169] The current path rb14 is due to the leakage inductance Rw21 of the first secondary winding W21 and the continuity of the current in the second inductor 21. The current path rb14 is a path from the first secondary winding W21 to the first secondary winding W21 via the first secondary side diode 24, the second inductor 21, the first output terminal t91, the load connected to the power conversion device 5, and the second output terminal t92. In the sixth state in which the AC potential is positive and the second to third states in which the AC potential is negative, a voltage is generated in the secondary winding W2 in a direction that makes the first secondary side diode 24 conductive and the second secondary side diode 25 non-conductive. In the sixth state in which the AC potential is positive and the second to third states in which the AC potential is negative, the current flowing through the primary winding W1 decreases, and the current flowing through the current path rb14 also decreases. The output capacitor 23 discharges to compensate for the decrease in the current flowing through the current path rb14.

[0170] As described above, in the first to sixth states, in the rectifying smoothing circuit 60, either the first secondary side diode 24 or the second secondary side diode 25 is conductive. [Effects of power conversion device 5 of modification 2] (4-1) According to the power conversion device 5 of the second modification, a current is supplied to the load of the power conversion device 5 by the second inductor 21 regardless of the polarity of the AC power supply V1 and the conductive / non-conductive state of the upper arm switching element 15 and the lower arm switching element 16. Furthermore, the power conversion device 5 commutates the current flowing through the first secondary winding W21 and the second secondary winding W22 according to the direction of the current flowing through the primary winding W1 by the first secondary side diode 24 and the second secondary side diode 25, and can cause the current to flow through the second inductor 21 without using the second capacitor 19. Therefore, according to the power conversion device 5 of the second modification, the second capacitor 19 can be omitted.

[0171] [Another example of the power conversion device 5] In the power conversion device 5, as in the second embodiment, the connection between the primary side circuit 10 and the buffer capacitor Cc may be different from the above-mentioned example. As shown in FIG. 34, the power conversion device 6 has, for example, the same configuration as the power conversion device 5, while the connection between the primary side circuit 10, the first capacitor 17, the transformer 18, and the buffer capacitor Cc is different from that of the power conversion device 5. Specifically, in the power conversion device 6, the start end of the primary winding W1 is electrically connected to the cathode of the upper arm diode 12 and the first end t31 of the upper arm switching element 15. In addition, the end end of the primary winding W1 is connected to the first end t51 of the first capacitor 17, and the second end t52 of the first capacitor 17 is connected to the cathode of the upper arm diode 12 and the first end t31 of the upper arm switching element 15.

[0172] Furthermore, a terminal tc1 of the buffer capacitor Cc is connected to the bridge circuit 14 (more specifically, the cathode and first end t31 of the upper arm diode 12). Furthermore, a terminal tc2 of the buffer capacitor Cc is connected to an end of the primary winding W1 and a first end t51 of the first capacitor 17. As a result, the buffer capacitor Cc is connected in parallel to the primary winding W1 of the transformer 18 via the upper arm switching element 15. Furthermore, the buffer capacitor Cc is connected in series to the first capacitor 17.

[0173] The operation of the primary side circuit 10 in the power conversion device 6 is similar to that of the primary side circuit 10 described in the above embodiment, and therefore a description thereof will be omitted. Also, the operation of the rectifying smoothing circuit 60 corresponding to the primary side circuit 10 in the power conversion device 6 is similar to that of the above modification 2, and therefore a description thereof will be omitted.

[0174] According to this configuration, the power conversion device 6 can reduce the withstand voltage of the buffer capacitor Cc, similar to the effect of the second embodiment described above. Also, the power conversion device 6 can omit the second capacitor 19, similar to the effect of the second modification described above.

[0175] The above-described embodiments may be modified as follows: The above-described embodiments and the following examples may be combined with each other as long as they are not technically inconsistent. In the above-described embodiments, diodes are used as the upper arm rectifier elements and the lower arm rectifier elements, but this is not limiting and switching elements may be used instead of the diodes. In this case, so-called synchronous rectification control is performed in which the switching elements are turned on at the timing when the upper arm diode 12 and the lower arm diode 13 are conductive. By using switching elements instead of diodes, the conduction loss can be reduced.

[0176] In the above description, the rectifier smoothing circuit 20 and the rectifier smoothing circuit 30 include the secondary side diode 22 as the secondary side rectifier element, and the rectifier smoothing circuit 60 includes the first secondary side diode 24 and the second secondary side diode 25 as the secondary side rectifier element. However, this is not limiting. The rectifier smoothing circuit 20 and the rectifier smoothing circuit 30 may include a switching element instead of the secondary side diode 22. The rectifier smoothing circuit 60 may include a switching element instead of the first secondary side diode 24 and the second secondary side diode 25. In this case, the control unit 50 performs so-called synchronous rectification control, in which the switching element is turned on in a switching pattern state in which the secondary side diode 22, the first secondary side diode 24, and the second secondary side diode 25 are conductive. As a result, the rectifier smoothing circuit 20, the rectifier smoothing circuit 30, and the rectifier smoothing circuit 60 can reduce conduction loss by using switching elements instead of diodes.

[0177] In the above-described embodiments and modifications, the first inductor 11 is connected between the first input terminal t21 and the first connecting line CL1, but the position of the first inductor 11 is not limited thereto and may be connected between the second input terminal t22 and the second connecting line CL2. Furthermore, the first inductor 11 may be two inductors, and may be connected both between the first input terminal t21 and the first connecting line CL1 and between the second input terminal t22 and the second connecting line CL2.

[0178] In the first embodiment described above, the first capacitor 17 is connected between the start end of the primary winding W1 and the second end t32 of the upper arm switching element 15, but the position of the first capacitor 17 is not limited thereto, and the first capacitor 17 may be connected between the end end of the primary winding W1 and the second end t42 of the lower arm switching element 16. Also, the first capacitor 17 may be composed of two capacitors, and in this case, the first capacitor 17 is connected both between the start end of the primary winding W1 and the second end t32 of the upper arm switching element 15 and between the end end of the primary winding W1 and the second end t42 of the lower arm switching element 16.

[0179] In the first and second embodiments described above, the second capacitor 19 is connected between the start of the secondary winding W2 and the anode of the secondary diode 22, but the position of the second capacitor 19 is not limited to this, and the second capacitor 19 may be connected between the end of the secondary winding W2 and the second end t72 of the second inductor 21. The second capacitor 19 may also be composed of two capacitors, and in this case, the second capacitor 19 is connected both between the start of the secondary winding W2 and the anode of the secondary diode 22 and between the end of the secondary winding W2 and the second end t72 of the second inductor 21.

[0180] In the above-mentioned modified example 1, the second capacitor 19 is connected between the start of the secondary winding W2 and the cathode of the secondary side diode 22, but the position of the second capacitor 19 is not limited to this, and the second capacitor 19 may be connected between the end of the secondary winding W2 and the anode of the secondary side diode 22. Also, the second capacitor 19 may be composed of two capacitors, in which case it is connected both between the start of the secondary winding W2 and the cathode of the secondary side diode 22 and between the end of the secondary winding W2 and the cathode of the secondary side diode 22.

[0181] The current sensor C1 may be provided on the second intermediate line ML2 instead of the first intermediate line ML1. The buffer capacitor Cc may be provided separately from the primary circuit 10. [Explanation of symbols]

[0182] Reference Signs List 1, 2, 3, 4, 5, 6...power conversion device, 10...primary side circuit, 11...first inductor, 12...upper arm diode, 13...lower arm diode, 14...bridge circuit, 15...upper arm switching element, 16...lower arm switching element, 17...first capacitor, 18, 18a...transformer, 19...second capacitor, 20, 30, 60...rectifying and smoothing circuit, 21...second inductor, 22...secondary side diode, 23...output capacitor, 24...first secondary side diode node, 25...second secondary side diode, 50...control section, C1...current sensor, C2, C3...voltage sensor, Cc...buffer capacitor, CL1...first connecting line, CL2...second connecting line, L1...first input line, L2...second input line, ML1...first intermediate line, ML2...second intermediate line, OL1...first output line, OL2...second output line, t91...first output terminal, t92...second output terminal, V1...AC power supply, W1...primary winding, W2...secondary winding, W21...first secondary winding, W22...second secondary winding.

Claims

1. A power conversion device that converts an AC voltage input from an AC power source into a DC voltage, a transformer having a primary winding and a secondary winding; A first capacitor; a primary side circuit including a pair of input terminals to which the AC power supply is connected, a first inductor, an upper arm switching element, a lower arm switching element, an upper arm rectifying element, a lower arm rectifying element, and a buffer capacitor that suppresses generation of a surge voltage; a rectifying and smoothing circuit having a secondary side rectifying element, a first output terminal, and a second output terminal; The primary side circuit includes: a series connection of the upper arm switching element and the lower arm switching element and a series connection of the upper arm rectifier element and the lower arm rectifier element form a bridge circuit, and the first inductor is connected between a first connection point of the upper arm switching element and the lower arm switching element and one of the pair of input terminals, and a second connection point of the upper arm rectifier element and the lower arm rectifier element is connected to the other of the pair of input terminals, or the first inductor is connected between the second connection point and the other end, and the first connection point is connected to the one end, or there are two first inductors, and one of the first inductors is connected between the first connection point and the one end, and the other of the first inductors is connected between the second connection point and the other end, the first connection point and a start end of a primary winding of the transformer are connected; the first capacitor is connected between an end of a primary winding of the transformer and a connection point of the lower arm switching element and the lower arm rectifying element; the buffer capacitor and the upper arm switching element are connected in parallel to a primary winding of the transformer, one end of the buffer capacitor is connected to a connection point between the upper arm rectifier element and the upper arm switching element, and the other end of the buffer capacitor is connected to a connection point between an end of a primary winding of the transformer and the first capacitor, The rectifying and smoothing circuit and the secondary winding of the transformer are connected in parallel. A power conversion device comprising:

2. A power conversion device that converts an AC voltage input from an AC power source into a DC voltage, a transformer having a primary winding and a secondary winding; A first capacitor; a primary side circuit including a pair of input terminals to which the AC power supply is connected, a first inductor, an upper arm switching element, a lower arm switching element, an upper arm rectifying element, a lower arm rectifying element, and a buffer capacitor that suppresses generation of a surge voltage; a rectifying and smoothing circuit having a secondary side rectifying element, a first output terminal and a second output terminal; a second capacitor; The primary side circuit includes: a series connection of the upper arm switching element and the lower arm switching element and a series connection of the upper arm rectifier element and the lower arm rectifier element form a bridge circuit, and the first inductor is connected between a first connection point of the upper arm switching element and the lower arm switching element and one of the pair of input terminals, and a second connection point of the upper arm rectifier element and the lower arm rectifier element is connected to the other of the pair of input terminals, or the first inductor is connected between the second connection point and the other end, and the first connection point is connected to the one end, or there are two first inductors, and one of the first inductors is connected between the first connection point and the one end, and the other of the first inductors is connected between the second connection point and the other end, the first capacitor is connected between the first connection point and a start end of the primary winding of the transformer, and an end end of the primary winding of the transformer is connected to a connection point of the lower arm switching element and the lower arm rectifier element, or the first capacitor is connected between an end end of the primary winding of the transformer and a connection point of the lower arm switching element and the lower arm rectifier element, and the first connection point is connected to a start end of the primary winding of the transformer, or there are two first capacitors, one of which is connected between the first connection point and a start end of the primary winding of the transformer, and the other first capacitor is connected between an end end of the primary winding of the transformer and a connection point of the lower arm switching element and the lower arm rectifier element, The rectifying and smoothing circuit includes: a second inductor, a secondary side rectifier, an output capacitor, a first output terminal, and a second output terminal; the first output terminal is connected to one terminal of the output capacitor; the second output terminal is connected to the other terminal of the output capacitor; the secondary side rectifier element is connected between one end of the second inductor and one end of the output capacitor, and the other end of the second inductor and the other end of the output capacitor are connected, or the secondary side rectifier element is connected between the other end of the second inductor and the other end of the output capacitor, and the one end of the second inductor and the one end of the output capacitor are connected, the second capacitor is connected between a start end of the secondary winding of the transformer and one end of the second inductor, and an end end of the secondary winding of the transformer is connected to the other end of the second inductor, or the second capacitor is connected between an end end of the secondary winding of the transformer and the other end of the second inductor, and a start end of the secondary winding of the transformer is connected to one end of the second inductor, or there are two second capacitors, one of the second capacitors is connected between a start end of the secondary winding of the transformer and one end of the second inductor, and the other second capacitor is connected between an end end of the secondary winding of the transformer and the other end of the second inductor, the secondary-side rectifier element is connected to conduct a current flowing from one end of the second inductor to one end of the output capacitor, or a current flowing from the other end of the output capacitor to the other end of the second inductor; one end of the buffer capacitor is connected to a connection point between the upper arm rectifier element and the upper arm switching element, and the other end of the buffer capacitor is connected to a connection point between the lower arm switching element and the lower arm rectifier element; Power conversion equipment.

3. A power conversion device that converts an AC voltage input from an AC power source into a DC voltage, a transformer having a primary winding and a secondary winding; A first capacitor; a primary side circuit including a pair of input terminals to which the AC power supply is connected, a first inductor, an upper arm switching element, a lower arm switching element, an upper arm rectifying element, a lower arm rectifying element, and a buffer capacitor that suppresses generation of a surge voltage; a rectifying and smoothing circuit having a secondary side rectifying element, a first output terminal and a second output terminal; a second capacitor; The primary side circuit includes: a series connection of the upper arm switching element and the lower arm switching element and a series connection of the upper arm rectifier element and the lower arm rectifier element form a bridge circuit, and the first inductor is connected between a first connection point of the upper arm switching element and the lower arm switching element and one of the pair of input terminals, and a second connection point of the upper arm rectifier element and the lower arm rectifier element is connected to the other of the pair of input terminals, or the first inductor is connected between the second connection point and the other end, and the first connection point is connected to the one end, or there are two first inductors, and one of the first inductors is connected between the first connection point and the one end, and the other of the first inductors is connected between the second connection point and the other end, the first capacitor is connected between the first connection point and a start end of the primary winding of the transformer, and an end end of the primary winding of the transformer is connected to a connection point of the lower arm switching element and the lower arm rectifier element, or the first capacitor is connected between an end end of the primary winding of the transformer and a connection point of the lower arm switching element and the lower arm rectifier element, and the first connection point is connected to a start end of the primary winding of the transformer, or there are two first capacitors, one of which is connected between the first connection point and a start end of the primary winding of the transformer, and the other first capacitor is connected between an end end of the primary winding of the transformer and a connection point of the lower arm switching element and the lower arm rectifier element, The rectifying and smoothing circuit includes: a second inductor, a secondary side rectifier, an output capacitor, a first output terminal, and a second output terminal; the first output terminal is connected to one terminal of the output capacitor; the second output terminal is connected to the other terminal of the output capacitor; the second inductor is connected between one end of the secondary side rectifier element and one end of the output capacitor, and the other end of the secondary side rectifier element and the other end of the output capacitor are connected, or the second inductor is connected between the other end of the secondary side rectifier element and the other end of the output capacitor, and the one end of the secondary side rectifier element and the one end of the output capacitor are connected, the second capacitor is connected between a starting end of the secondary winding of the transformer and the other end of the secondary side rectifier element, and an end of the secondary winding of the transformer is connected to one end of the secondary side rectifier element, or the second capacitor is connected between an end of the secondary winding of the transformer and one end of the secondary side rectifier element, and a starting end of the secondary winding of the transformer is connected to the other end of the secondary side rectifier element, or there are two second capacitors, one of the second capacitors is connected between a starting end of the secondary winding of the transformer and the other end of the secondary side rectifier element, and the other second capacitor is connected between an end of the secondary winding of the transformer and one end of the secondary side rectifier element, the secondary side rectifier element is connected to conduct a current flowing from the other end of the secondary side rectifier element to one end of the secondary side rectifier element, one end of the buffer capacitor is connected to a connection point between the upper arm rectifier element and the upper arm switching element, and the other end of the buffer capacitor is connected to a connection point between the lower arm switching element and the lower arm rectifier element; Power conversion equipment.

4. a control unit that controls switching of the switching elements when any or all of the upper arm switching element, the lower arm switching element, the upper arm rectifier element, the lower arm rectifier element, and the secondary side rectifier element are configured with switching elements, the control unit controls switching based on the AC voltage and the current flowing through the first inductor so as to improve a power factor of the input power. The power conversion device according to claim 1 .

5. a control unit that controls switching of the switching elements when any or all of the upper arm switching element, the lower arm switching element, the upper arm rectifier element, the lower arm rectifier element, and the secondary side rectifier element are configured with switching elements, The control unit controls switching based on a voltage between the first output terminal and the second output terminal so that a predetermined DC voltage is output. The power conversion device according to claim 1 .

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