Power Conversion Device

Abstract

To provide a power conversion device with a mounted capacitor of small capacity and that can be controlled with ease.SOLUTION: A power conversion device comprises: a primary coil 15 provided between a connection point between a fifth switching element S5 and a sixth switching element S6, and a connection point between a seventh switching element S7 and an eighth switching element S8; a reactor L1 provided between a tap of the primary coil 15 and a connection point between a first capacitor C1 and a second capacitor C2; and an input reactor L2 connected with a connection point between a first switching element S1 and a second switching element S2. One terminal 17p is connected with the input reactor L2, and the other terminal 17n is connected with a connection point between a third switching element S3 and a fourth switching element S4.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a power conversion device that transmits power by switching. [Background technology]

[0002] Patent Document 1 discloses a boost-type PFC (Power Factor Correction) control device that boosts AC voltage using a boost coil and a full-bridge circuit to generate DC voltage that is higher than the peak value of the input AC voltage.

[0003] Non-Patent Document 1 discloses a matrix converter circuit using a DAB (Dual Active Bridge) system having full-bridge circuits on both sides of an isolation transformer.

[0004] The mechanism by which a power conversion device using a boost coil and a full-bridge circuit can boost an AC voltage to generate a DC voltage, and the reason why a capacitor with a large capacity is required will be described below with reference to FIGS. 11 to 14.

[0005] Fig. 11 is a diagram showing the configuration of a power conversion device 200 using a boost coil and a full-bridge circuit. As shown in Fig. 11, the power conversion device 200 includes a first switching arm SA1, a second switching arm SA2, a capacitor arm C, an input reactor L, and a filter capacitor Cf. The power conversion device 200 also includes a positive terminal T1 and a negative terminal T2 on the input side, and a positive terminal T3 and a negative terminal T4 on the output side.

[0006] The first switching arm SA1 includes a first switching element S1 and a second switching element S2 connected in series. The second switching arm SA2 includes a third switching element S3 and a fourth switching element S4 connected in series. The first switching arm SA1 and the second switching arm SA2 are connected in parallel. The first switching arm SA1 and the second switching arm SA2 are connected in parallel in this manner to form a full-bridge inverter FBI1.

[0007] The capacitor arm C includes a first capacitor C1 and a second capacitor C2 connected in series. The capacitor arm C is connected in parallel with the full-bridge inverter FBI1. That is, the capacitor arm C is connected between two parallel connection points of the first switching arm SA1 and the second switching arm SA2. The upper and lower parallel connection points of the first switching arm SA1, the second switching arm SA2, and the capacitor arm C are connected to the positive terminal T3 and the negative terminal T4, respectively. An output load, such as a primary winding of a transformer, is connected between the positive terminal T3 and the negative terminal T4.

[0008] One end of an input reactor L is connected to the connection point between the first switching element S1 and the second switching element S2. A filter capacitor Cf and a positive terminal T1 are connected to the other end of the input reactor L. Furthermore, a filter capacitor Cf and a negative terminal T2 are connected to the connection point between the third switching element S3 and the fourth switching element S4.

[0009] An AC voltage Vin is applied between the positive terminal T1 and the negative terminal T2. In this state where the AC voltage Vin is applied, the operation of each switching element of the power conversion device 200 and the current I flowing through the input reactor L are LThe fluctuation of is shown in Figure 12. In Figure 12, the on / off operation of each switching element is represented by a high state for on and a low state for off. When the AC voltage Vin is positive, as shown in Figure 12, the third switching element S3 is fixed off and the fourth switching element S4 is fixed on, and the first switching element S1 and the second switching element S2 are switched on and off alternately. When the first switching element S1 is on, the second switching element S2 is always off, and when the first switching element S1 is off, the second switching element S2 is always on. However, when the AC voltage Vin is negative, the first switching element S1 is fixed off and the second switching element S2 is fixed on, and the third switching element S3 and the fourth switching element S4 are switched on and off alternately. Although the AC voltage Vin is strictly a sine wave, the value of the AC voltage Vin in FIG. 12 is almost constant because the switching period of the first switching element S1 and the second switching element S2 is sufficiently shorter than the period of the AC voltage Vin.

[0010] During the period (1) shown in FIG. 12, the second switching element S2 and the fourth switching element S4 are on, and the first switching element S1 and the third switching element S3 are off, so that current flows through the path indicated by the dashed arrow in FIG. 13, charging the input reactor L.

[0011] During the period (2) shown in FIG. 12, the first switching element S1 and the fourth switching element S4 are on, and the second switching element S2 and the third switching element S3 are off, so that current flows through the path shown by the dashed arrow in FIG. 14, the charged input reactor L is discharged, and the first capacitor C1 and the second capacitor C2 are charged.

[0012] In this manner, by repeating a cycle in which the input reactor L is charged during the period (1) shown in FIG. 12 and then discharged during the period (2) shown in FIG. 12 to charge the first capacitor C1 and the second capacitor C2, the input capacitor voltage Vc, which is the sum of the voltage Vc1 of the first capacitor C1 and the voltage Vc2 of the second capacitor C2, can be boosted. This input capacitor voltage Vc becomes the output voltage of the power conversion device 200. FIG. 15 shows the results of a simulation performed under the conditions in which the full-bridge inverter FBI1 is operated in this manner. FIG. 15 shows the simulated waveforms of the AC input current Iin, the pre-capacitor current Iac, and the input capacitor voltage Vc flowing between the positive terminal T1 and the negative terminal T2. The pre-capacitor current Iac is a current that has been full-wave rectified by the full-bridge inverter FBI1. As shown in FIG. 15, the power conversion device 200 can boost the input capacitor voltage Vc. However, the first capacitor C1 and the second capacitor C2 for converting the pre-capacitor current Iac, which has been full-wave rectified by the full-bridge inverter FBI1, into a direct current are required to have very large capacitances. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-99946 [Non-patent literature]

[0014] [Non-Patent Document 1] Shunsuke Takuma and Junichi Ito: "Surge Voltage Reduction Method for DAB Matrix Converters Using Return Current" <URL:http: / / itohserver01.nagaokaut.ac.jp / itohlab / paper / 2021 / 20210801_%E9%9B%BB%E5%AD%A6%E8%AB%96D / takuma.pdf> Summary of the Invention [Problem to be solved by the invention]

[0015] The boost PFC control device disclosed in Patent Document 1 can vary the output DC voltage using a boost coil and a full-bridge circuit, but requires a large-capacity smoothing capacitor, which increases costs.

[0016] A power conversion device that can prevent the capacitance of the capacitor to be mounted from increasing in this way is the matrix converter circuit disclosed in Non-Patent Document 1. However, although the matrix converter circuit disclosed in Non-Patent Document 1 can reduce the capacitance of the capacitor to be mounted, the control is complicated because the matrix converter controls the current (voltage) and output voltage to perform ZVS (Zero Voltage Switching), and the inverter controls the output voltage.

[0017] Therefore, an object of the present invention is to provide a power conversion device that has a small capacity capacitor and is easy to control. [Means for solving the problem]

[0018] The power conversion device according to the present invention includes a first switching arm including a first switching element and a second switching element connected in series, a second switching arm including a third switching element and a fourth switching element connected in series, a capacitor arm including a first capacitor and a second capacitor connected in series, a third switching arm including a fifth switching element and a sixth switching element connected in series, a fourth switching arm including a seventh switching element and an eighth switching element connected in series, and a primary winding magnetically coupled to a secondary winding, the primary winding being provided between a connection point of the fifth switching element and the sixth switching element and a connection point of the seventh switching element and the eighth switching element; a secondary winding coupled to the primary winding and constituting a transformer together with the primary winding;the first switching arm, the second switching arm, the capacitor arm, the third switching arm, the first capacitor side, the fifth switching element side, and the fourth switching arm, the seventh switching element side, are connected in common; and the first switching arm, the second switching arm, the capacitor arm, the third switching arm, the fourth switching arm, the second capacitor side, the sixth switching element side, and the fourth switching arm, the eighth switching element side, are connected in common; the first switching arm, the second switching arm, the capacitor arm, the third switching arm, and the fourth switching arm are connected in parallel; and one of the input terminals is connected to the input reactor, and the other input terminal is connected to the connection point of the third switching element and the fourth switching element.

[0019] Power conversion device according to the present invention teeth,a first state in which the second switching element, the fourth switching element, the sixth switching element, and the eighth switching element are on, and the first switching element, the third switching element, the fifth switching element, and the seventh switching element are off; a second state in which only one of the first switching element and the second switching element is on, and the fourth switching element, the fifth switching element, and the eighth switching element are on, and the third switching element, the sixth switching element, and the seventh switching element are off; a third state in which the second switching element, the third switching element, the sixth switching element, and the eighth switching element are turned off; a fourth state in which the second switching element, the fourth switching element, the fifth switching element, and the seventh switching element are turned on, and the first switching element, the third switching element, the sixth switching element, and the eighth switching element are turned off; and a fifth state in which only one of the first switching element and the second switching element is turned on, the fourth switching element, the sixth switching element, and the seventh switching element are turned on, and the third switching element, the fifth switching element, and the eighth switching element are turned off. When it is desired to increase the power supply to the transformer, the frequency of the second state and the fifth state is increased, and when it is desired to decrease the power supply to the transformer, the frequency of the second state and the fifth state is decreased.

[0020] In one aspect of the power conversion device of the present invention, in the first state, a current may circulate between the second capacitor and the reactor, and in the fourth state, a current may circulate between the first capacitor and the reactor.

[0021] In one aspect of the power conversion device of the present invention, in the first state, a voltage may be formed in the second capacitor by circulating a current between the second capacitor and the reactor, and in the fourth state, a voltage may be formed in the first capacitor by circulating a current between the first capacitor and the reactor.

[0022] In one aspect of the power conversion device according to the present invention, a voltage obtained by rectifying and boosting an AC voltage applied between the input terminals may be applied to the primary winding. [Effects of the Invention]

[0023] The present invention can provide a power conversion device that includes a capacitor with a small capacity and that is simple to control. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a circuit diagram of a power conversion system including a power conversion device according to an embodiment of the present invention. [Figure 2] 3 is a diagram showing a current path in a first state of the power conversion device of the present embodiment. FIG. [Figure 3] FIG. 4 is a diagram showing a current path in a second state of the power conversion device of the present embodiment. [Figure 4] FIG. 10 is a diagram showing a current path in a third state of the power conversion device of the present embodiment. [Figure 5] FIG. 10 is a diagram showing current paths in a fourth state of the power conversion device of the present embodiment. [Figure 6] FIG. 10 is a diagram showing a current path in the fifth state of the power conversion device of the present embodiment. [Figure 7] 10 is a diagram showing the results of a simulation performed on the power conversion device of the present embodiment under conditions in which the order of the first state, the second state, the third state, the fourth state, and the fifth state is changed. FIG. [Figure 8] FIG. 7 is a diagram illustrating the definitions of current and voltage, showing the results of the simulation. [Figure 9]FIG. 10 is a diagram showing the results of a simulation of the power conversion device of the present embodiment under low load conditions. [Figure 10] 10A and 10B are diagrams illustrating the results of a simulation of the power conversion device of the present embodiment under high load conditions. [Figure 11] FIG. 1 is a circuit diagram showing the configuration of a power conversion device using a boost coil and a full-bridge circuit. [Figure 12] 1 is a diagram illustrating the operation of a power conversion device using a boost coil and a full-bridge circuit. [Figure 13] FIG. 13 is a diagram showing the flow of current in the period (1) shown in FIG. [Figure 14] FIG. 13 is a diagram showing the flow of current in the period (2) shown in FIG. [Figure 15] FIG. 10 is a diagram showing the results of a simulation of a power conversion device using a boost coil and a full-bridge circuit. DETAILED DESCRIPTION OF THE INVENTION

[0025] A power conversion device 10 according to an embodiment of the present invention will be described below with reference to Fig. 1 to Fig. 10. A circuit diagram of a power conversion system 100 including the power conversion device 10 of this embodiment is shown in Fig. 1. As shown in Fig. 1, the power conversion system 100 includes the power conversion device 10 and a power conversion device 20.

[0026] The power conversion device 10 includes a first switching arm 11, a second switching arm 12, a capacitor arm C, a third switching arm 13, a fourth switching arm 14, a primary winding 15, a reactor L1, an input reactor L2, a filter capacitor 16, a positive terminal 17p, and a negative terminal 17n.

[0027] The first switching arm 11 includes a first switching element S1 and a second switching element S2 connected in series. The second switching arm 12 includes a third switching element S3 and a fourth switching element S4 connected in series. The third switching arm 13 includes a fifth switching element S5 and a sixth switching element S6 connected in series. The fourth switching arm 14 includes a seventh switching element S7 and an eighth switching element S8 connected in series. Each switching element may be an insulated gate bipolar transistor (IGBT) or a metal oxide semiconductor field effect transistor (MOSFET). When an IGBT is used as a switching element, connecting two IGBTs in series means connecting the emitter terminal of one IGBT to the collector terminal of the other IGBT. When a MOSFET is used as a switching element, connecting two MOSFETs in series means connecting the source terminal of one MOSFET to the drain terminal of the other MOSFET. Each switching element includes a diode. When an IGBT is used as the switching element, the anode terminal is connected to the emitter terminal and the cathode terminal is connected to the collector terminal.When a MOSFET is used as the switching element, the anode terminal is connected to the source terminal and the cathode terminal is connected to the drain terminal.

[0028] The first switching arm 11 and the second switching arm 12 are connected in parallel. That is, a terminal of the first switching element S1 opposite to the second switching element S2 (upper side in FIG. 1) is connected to a terminal of the third switching element S3 opposite to the fourth switching element S4 (upper side in FIG. 1), and a terminal of the second switching element S2 opposite to the first switching element S1 (lower side in FIG. 1) is connected to a terminal of the fourth switching element S4 opposite to the third switching element S3 (lower side in FIG. 1). The first switching arm 11 and the second switching arm 12 are connected in parallel in this manner, thereby constituting a first full-bridge inverter FBI1.

[0029] The third switching arm 13 and the fourth switching arm 14 are connected in parallel. That is, the terminal (upper terminal) of the fifth switching element S5 opposite to the sixth switching element S6 is connected to the terminal (upper terminal) of the seventh switching element S7 opposite to the eighth switching element S8, and the terminal (lower terminal) of the sixth switching element S6 opposite to the fifth switching element S5 is connected to the terminal (lower terminal) of the eighth switching element S8 opposite to the seventh switching element S7. The third switching arm 13 and the fourth switching arm 14 are connected in parallel in this manner, thereby constituting a second full-bridge inverter FBI2.

[0030] The capacitor arm C includes a first capacitor C1 and a second capacitor C2 connected in series. The capacitor arm C is connected in parallel with the first full-bridge inverter FBI1 and the second full-bridge inverter FBI2. That is, the capacitor arm C is connected between two parallel connection points of the first switching arm 11 and the second switching arm 12, and is also connected between two parallel connection points of the third switching arm 13 and the fourth switching arm 14.

[0031] One end of an input reactor L2 is connected to the connection point between the first switching element S1 and the second switching element S2. The other end of the input reactor L2 is connected to a filter capacitor 16 and a positive terminal 17p. The connection point between the third switching element S3 and the fourth switching element S4 is connected to a filter capacitor 16 and a negative terminal 17n.

[0032] A primary winding 15 is connected between the connection point of the fifth switching element S5 and the sixth switching element S6 and the connection point of the seventh switching element S7 and the eighth switching element S8. A reactor L1 is connected between a tap provided at a midpoint of the conductor constituting the primary winding 15 and the connection point of the first capacitor C1 and the second capacitor C2. The tap may be a center tap located at the midpoint of the conductor constituting the primary winding 15.

[0033] The same applies to the switching elements described below, in that an IGBT, a MOSFET, or the like is used for each switching element, that each switching element includes a diode, and that the series connection of switching elements is defined.

[0034] The power conversion device 20 includes a switching arm 21, a switching arm 22, a capacitor C3, a secondary winding 23, a positive terminal 24p, and a negative terminal 24n.

[0035] The switching arm 21 includes a ninth switching element S9 and a tenth switching element S10 connected in series. The switching arm 22 includes an eleventh switching element S11 and a twelfth switching element S12 connected in series. The switching arm 21 and the switching arm 22 are connected in parallel. That is, a terminal of the ninth switching element S9 opposite the tenth switching element S10 (upper side in FIG. 1) is connected to a terminal of the eleventh switching element S11 opposite the twelfth switching element S12 (upper side in FIG. 1), and a terminal of the tenth switching element S10 opposite the ninth switching element S9 (lower side in FIG. 1) is connected to a terminal of the twelfth switching element S12 opposite the eleventh switching element S11 (lower side in FIG. 1). The parallel connection of the switching arm 21 and the switching arm 22 in this manner constitutes a third full-bridge inverter FBI3.

[0036] A secondary winding 23 is connected between the connection point of the ninth switching element S9 and the tenth switching element S10 and the connection point of the eleventh switching element S11 and the twelfth switching element S12. The secondary winding 23 is coupled to the primary winding 15 and together with the primary winding 15 constitutes a transformer.

[0037] The capacitor C3 is connected in parallel with the third full-bridge inverter FBI3. That is, the capacitor C3 is connected between two parallel connection points of the switching arms 21 and 22. The parallel connection point on one side (upper side in FIG. 1) of the switching arms 21, 22, and capacitor C3 is connected to the positive terminal 24p, and the parallel connection point on the other side (lower side in FIG. 1) of the switching arms 21, 22, and capacitor C3 is connected to the negative terminal 24n.

[0038] An AC voltage Vac is applied between a positive terminal 17p and a negative terminal 17n of the power conversion device 10. A load is connected between a positive terminal 24p and a negative terminal 24n of the power conversion device 20. The configuration of the power conversion device 20 is not particularly limited, and it may be any circuit that includes a secondary winding 23 magnetically coupled to the primary winding 15 of the power conversion device 10 and supplies power to the load.

[0039] In power conversion device 10, the state changes over time in the following order: first state, second state, third state, fourth state, and fifth state. In power conversion device 10, the process of switching from first state to fifth state in this order is repeated. Figures 2 to 6 show the states of current flowing through power conversion device 10 in the first state, second state, third state, fourth state, and fifth state, respectively.

[0040] FIG. 7 shows the results of a simulation of the capacitor current Iac, input capacitor voltage Vc, transformer current Itr, and transformer voltage Vtr when the power conversion device 10 is switched in the order of state 1, state 2, state 3, state 4, and state 5. The first state is entered at the timing (1) shown in FIG. 7, the period (2) shown in FIG. 7 is the second state, the period (3) shown in FIG. 7 is the third state, the period (4) shown in FIG. 7 is the fourth state, and the period (5) shown in FIG. 7 is the fifth state. FIG. 7 also shows the on and off states of eight switching elements, from the first switching element S1 to the eighth switching element S8. In FIG. 7, each switching element is on when its on / off signal is high and off when its on / off signal is low.

[0041] The definitions of the pre-capacitor current Iac, input capacitor voltage Vc, transformer current Itr, and transformer voltage Vtr, whose simulated waveforms are shown in Figure 7, are explained below with reference to Figure 8. The pre-capacitor current Iac is the current flowing from the full-bridge inverter FBI1 to the capacitor arm C and the second full-bridge inverter FBI2. The input capacitor voltage Vc is the sum of the voltage Vc1 of the first capacitor C1 and the voltage Vc2 of the second capacitor C2. The transformer current Itr is the current flowing through the primary winding 15 from one end to the other, and the transformer voltage Vtr is the voltage applied across the primary winding 15.

[0042] The first state is a state in which the second switching element S2, the fourth switching element S4, the sixth switching element S6, and the eighth switching element S8 are on, and the first switching element S1, the third switching element S3, the fifth switching element S5, and the seventh switching element S7 are off.

[0043] The current flow in the power conversion device 10 in the first state is indicated by dashed arrows in FIG. 2. In the first state, current flows through the second switching element S2 and the fourth switching element S4 of the first full-bridge inverter FBI1, charging the input reactor L2. In the second full-bridge inverter FBI2, the sixth switching element S6 and the eighth switching element S8 are on, and both ends of the primary winding 15 are shorted, causing the transformer voltage Vtr to be zero. Therefore, current flows from the second capacitor C2 through the reactor L1, branches off at a tap on the primary winding 15, and returns to the second capacitor C2 via the sixth switching element S6 or the eighth switching element S8. That is, current circulates between the second capacitor C2, the reactor L1, and the primary winding 15, applying a voltage to the second capacitor C2.

[0044] The second state is a state in which only one of the first switching element S1 and the second switching element S2 is on, the fourth switching element S4, the fifth switching element S5 and the eighth switching element S8 are on, and the third switching element S3, the sixth switching element S6 and the seventh switching element S7 are off.

[0045] The dashed arrows in Figure 3 indicate the current flow in the power conversion device 10 in the second state, in which the first switching element S1 is turned on and the second switching element S2 is turned off. In the second state, current flows through the first switching element S1 and the fourth switching element S4 in the first full-bridge inverter FBI1, discharging the input reactor L2 and charging the first capacitor C1 and the second capacitor C2. Then, in the second full-bridge inverter FBI2, a transformer voltage Vtr is applied across the primary winding 15, causing a transformer current Itr to flow through the primary winding 15. In this case, in the capacitor arm C, the charging current flowing from the input reactor L2 and the discharging current flowing to the primary winding 15 cancel each other out, and current flows directly from the input reactor L2 to the primary winding 15. Therefore, in the second state, as shown in Figure 7, the input capacitor voltage Vc rises, power is supplied to the load, and the transformer current Itr increases.

[0046] In the second state, the first switching element S1 may be turned off and the second switching element S2 may be turned on. In this case, as in the fifth state described below, the first capacitor C1 and the second capacitor C2 apply the transformer voltage Vtr across the primary winding 15, causing the transformer current Itr to flow through the primary winding 15.

[0047] The third state is a state in which the first switching element S1, the fourth switching element S4, the fifth switching element S5, and the seventh switching element S7 are turned on, and the second switching element S2, the third switching element S3, the sixth switching element S6, and the eighth switching element S8 are turned off.

[0048] The current flow in the power conversion device 10 in the third state is indicated by dashed arrows in Figure 4. In the third state, current flows through the first switching element S1 and the fourth switching element S4 of the first full-bridge inverter FBI1, discharging the input reactor L2 and charging the first capacitor C1 and the second capacitor C2. In the second full-bridge inverter FBI2, both ends of the primary winding 15 are shorted, so the transformer voltage Vtr is zero. Therefore, current flows from the first capacitor C1 through the sixth switching element S6 or the eighth switching element S8, joins at a tap of the primary winding 15, passes through the reactor L1, and returns to the first capacitor C1. That is, current circulates between the first capacitor C1, the reactor L1, and the primary winding 15. In the third state, the discharge current of the input reactor L2 flows into the first capacitor C1 and the second capacitor C2, causing the input capacitor voltage Vc to rise, as shown in Figure 7.

[0049] The fourth state is a state in which the second switching element S2, the fourth switching element S4, the fifth switching element S5, and the seventh switching element S7 are on, and the first switching element S1, the third switching element S3, the sixth switching element S6, and the eighth switching element S8 are off.

[0050] The current flow in the power conversion device 10 in the fourth state is indicated by dashed arrows in Fig. 5. In the fourth state, as shown in Fig. 5, in the first full-bridge inverter FBI1, current flows through the first switching element S1 and the fourth switching element S4, thereby charging the input reactor L2. In the second full-bridge inverter FBI2, the fifth switching element S5 and the seventh switching element S7 are on, and as in the third state, current circulates between the first capacitor C1, the reactor L1, and the primary winding 15, forming a voltage across the first capacitor C1.

[0051] The fifth state is a state in which only one of the first switching element S1 and the second switching element S2 is on, the fourth switching element S4, the sixth switching element S6, and the seventh switching element S7 are on, and the third switching element S3, the fifth switching element S5, and the eighth switching element S8 are off.

[0052] The dashed arrows in Figure 6 indicate the current flow in the power conversion device 10 in the fifth state, in which the first switching element S1 is turned off and the second switching element S2 is turned on. In the fifth state, current flows through the second switching element S2 and the fourth switching element S4 in the first full-bridge inverter FBI1, thereby charging the input reactor L2. In the second full-bridge inverter FBI2, a current path is formed through the sixth switching element S6 and the seventh switching element S7. This applies a transformer voltage Vtr from the first capacitor C1 and the second capacitor C2 to both ends of the primary winding 15, causing a transformer current Itr to flow through the primary winding 15. As a result, the input capacitor voltage Vc drops, as shown in Figure 7.

[0053] In the fifth state, the first switching element S1 may be on and the second switching element S2 may be off. In this case, similar to the second state in which the first switching element S1 is on and the second switching element S2 is off, in the capacitor arm C, the charging current flowing in from the input reactor L2 and the discharging current flowing out to the primary winding 15 cancel each other out, and a current flows directly from the input reactor L2 to the primary winding 15.

[0054] The five states from state 1 to state 5 can be classified into three periods: a power transmission period (states 2 and 5), a voltage control period (state 3), and a circulation period (states 1 and 4). The power transmission period is a period during which a transformer voltage Vtr is applied to the primary winding 15, a transformer current Itr flows, and power is transmitted between the primary winding 15 and the power conversion device 20. During the voltage control period, the input reactor L2 is discharged to charge the capacitor arm C, thereby controlling the input capacitor voltage Vc. During the circulation period, a current is circulated between the first capacitor C1 or the second capacitor C2 and the primary winding 15, maintaining the input capacitor voltage Vc.

[0055] In this way, in the power conversion device 10, when it is desired to increase the power supply to the transformer, the frequency of the power transmission period is increased, and when it is desired to decrease the power supply to the transformer, the frequency of the power transmission period is decreased. For example, when the load is low, the power supply to the transformer can be small, so the operation frequency of the power transmission period is decreased. When the load is high, it is necessary to increase the power supply to the transformer, so the operation frequency of the power transmission period is increased.

[0056] Figure 9 shows the results of a simulation under low load conditions for the capacitor current Iac, input capacitor voltage Vc, transformer current Itr, and transformer voltage Vtr, while Figure 10 shows the results of a simulation under high load conditions. As shown in Figure 9, under low load conditions, the operation frequency during the power transfer period is low, and the voltages of the first capacitor C1 and the second capacitor C2 are controlled by the voltage control period and circulation period. Then, as shown in Figure 10, under high load conditions, the operation frequency during the power transfer period is increased, thereby increasing the power supply to the transformer.

[0057] During the power transmission period, the power conversion device 10 can pass current directly from the input reactor L2 to the primary winding 15 rather than supplying power from the first capacitor C1 and the second capacitor C2 to the transformer, thereby reducing the charge / discharge amounts of the first capacitor C1 and the second capacitor C2. Furthermore, the power conversion device 10 can control the input capacitor voltage Vc by controlling the pre-capacitor current Iac by increasing or decreasing the operation frequency during the power transmission period, thereby reducing the charge / discharge amounts of the first capacitor C1 and the second capacitor C2. Therefore, the power conversion device 10 can reduce the capacitance of the capacitors it incorporates. Furthermore, with this simple control, the power conversion device 10 can rectify and boost the AC voltage applied between the positive terminal 17p and the negative terminal 17n and apply the resulting voltage to the primary winding 15. [Explanation of symbols]

[0058] 10 power conversion device, 11 first switching arm, 12 second switching arm, 13 third switching arm, 14 fourth switching arm, 15 primary winding, 16 filter capacitor, 17p positive terminal, 17n negative terminal, 20 power conversion device, 21, 22 switching arms, 23 secondary winding, 24p positive terminal, 24n negative terminal, 100 power conversion system, 200 power conversion device, C capacitor arm, C1 first capacitor, C2 second capacitor, C3 capacitor, FBI1 first full-bridge inverter, FBI2 second full-bridge inverter, FBI3 third full-bridge inverter, L1 reactor, L2 input reactor, S1 first switching element, S2 second switching element, S3 third switching element, S4 fourth switching element, S5 fifth switching element, S6 sixth switching element, S7 seventh switching element, S8 eighth switching element, S9 9th switching element, S10 10th switching element, S11 11th switching element, S12 12th switching element.

Claims

1. a first switching arm including a first switching element and a second switching element connected in series; a second switching arm including a third switching element and a fourth switching element connected in series; a capacitor arm including a first capacitor and a second capacitor connected in series; a third switching arm including a fifth switching element and a sixth switching element connected in series; a fourth switching arm including a seventh switching element and an eighth switching element connected in series; a primary winding magnetically coupled to the secondary winding, the primary winding being provided between a connection point between the fifth switching element and the sixth switching element and a connection point between the seventh switching element and the eighth switching element; a secondary winding coupled to the primary winding and constituting a transformer together with the primary winding; a reactor provided between a tap of the primary winding and a connection point of the first capacitor and the second capacitor; an input reactor connected to a connection point of the first switching element and the second switching element, one end of the first switching arm on the first switching element side, one end of the second switching arm on the third switching element side, one end of the capacitor arm on the first capacitor side, one end of the third switching arm on the fifth switching element side, and one end of the fourth switching arm on the seventh switching element side are commonly connected, and one end of the first switching arm on the second switching element side, one end of the second switching arm on the fourth switching element side, one end of the capacitor arm on the second capacitor side, one end of the third switching arm on the sixth switching element side, and one end of the fourth switching arm on the eighth switching element side are commonly connected, thereby the first switching arm, the second switching arm, the capacitor arm, the third switching arm, and the fourth switching arm are connected in parallel, one of the input terminals is connected to the input reactor, and the other of the input terminals is connected to a connection point between the third switching element and the fourth switching element; a first state in which the second switching element, the fourth switching element, the sixth switching element, and the eighth switching element are turned on, and the first switching element, the third switching element, the fifth switching element, and the seventh switching element are turned off; a second state in which only one of the first switching element and the second switching element is turned on, the fourth switching element, the fifth switching element, and the eighth switching element are turned on, and the third switching element, the sixth switching element, and the seventh switching element are turned off; a third state in which the first switching element, the fourth switching element, the fifth switching element, and the seventh switching element are turned on, and the second switching element, the third switching element, the sixth switching element, and the eighth switching element are turned off; a fourth state in which the second switching element, the fourth switching element, the fifth switching element, and the seventh switching element are turned on, and the first switching element, the third switching element, the sixth switching element, and the eighth switching element are turned off; a fifth state is sequentially realized in which only one of the first switching element and the second switching element is turned on, the fourth switching element, the sixth switching element, and the seventh switching element are turned on, and the third switching element, the fifth switching element, and the eighth switching element are turned off; a power conversion device characterized in that when it is desired to increase the power supply to the transformer, the frequency of the second state and the fifth state is increased, and when it is desired to decrease the power supply to the transformer, the frequency of the second state and the fifth state is decreased.

2. The power conversion device according to claim 1, the power conversion device, wherein, in the second state or the fifth state, when the first switching element is turned on and the second switching element is turned off, a charging current flowing from the input reactor to the capacitor arm and a discharging current flowing from the capacitor arm to the primary winding cancel each other out, and a current flows directly from the input reactor to the primary winding.

3. The power conversion device according to claim 1 or 2, In the first state, a current circulates between the second capacitor and the reactor, In the fourth state, a current circulates between the first capacitor and the reactor.

4. The power conversion device according to any one of claims 1 to 3, A power conversion device capable of applying a voltage obtained by rectifying and boosting an AC voltage applied between the input terminals to the primary winding.

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