Power conversion device and control method for power conversion device

Abstract

The present invention provides a power conversion device capable of achieving both overcharging prevention and loss reduction. The power conversion device comprises: a first DC / AC circuit that has a primary side bridge circuit 101 which is disposed on an input side of a transformer Tr1; a secondary side bridge circuit 102 which is disposed on an output side of the transformer Tr1 and to which a first load 109 is connected; a second DC / AC circuit 104 which is disposed on the output side of the transformer Tr1, which has a tertiary side bridge circuit 103 and a capacitor C3 that is connected in parallel to the tertiary side bridge circuit 103, and to which a second load 110 is connected; and a control unit 106 which, when the second load 110 is a prescribed power consumption value or lower with respect to the second DC / AC circuit 104, controls the voltage of the capacitor C3 with the tertiary side bridge circuit 103 and applies to the secondary side bridge circuit 102 the current of an output side winding of the transformer Tr1 connected to the tertiary side bridge circuit 103.

Description

Power conversion device and control method for power conversion device

[0001] The present invention relates to a configuration of a power conversion device and a control method thereof, and more particularly to a technique that is effective when applied to an isolated DC / DC converter that integrates a plurality of power conversion circuits using a transformer.

[0002] As one variation of on-board power conversion equipment, development is underway on an isolated DC / DC converter that combines a DC / DC converter for a low-voltage battery (LVBAT) and a DC / AC converter for a 100V AC outlet (V2L) using a transformer. Integrating multiple power conversion circuits via a transformer makes it possible to achieve both a compact and lightweight power conversion equipment.

[0003] In Patent Document 1, an AC power source 1 such as a commercial AC power source or a private power generator, a first DC voltage source 2 such as a high-voltage battery for driving a vehicle, a DC / DC converter 8, 28 for an LVBAT that supplies a second DC voltage source 3 such as a lead battery that is an LVBAT, and a DC / AC converter 4 applicable to a V2L are coupled by a composite winding transformer 10 (see, for example, FIG. 1 and paragraph

[0011] of Patent Document 1).

[0004] JP 2016-146681 A

[0005] The power conversion circuit coupled by the transformer described above is effective in reducing the size and weight of the circuit by reducing the number of common circuit components.

[0006] However, when the above-mentioned LVBAT DC / DC converter and V2L DC / AC converter are integrated, the LVBAT DC / DC converter operates continuously while the vehicle is running, whereas the V2L DC / AC converter operates intermittently depending on the load connected to the outlet.

[0007] Therefore, even when the V2L DC / AC converter is at rest with no load, voltage is applied via the transformer, the body diode of the switch conducts, and the V2L side DC link capacitor is charged.

[0008] Furthermore, since the circuit contains parasitic inductance components such as transformer leakage inductance, a surge is superimposed on the applied voltage, causing the V2L side DC link capacitor to become overcharged during pauses.

[0009] In Patent Document 1, only the LVBAT DC / DC converter is driven, and the bridge of the V2L DC / AC converter is turned off when there is no load on the V2L side. In this case, there is a risk that the V2L side DC link capacitor may be overcharged due to the influence of driving the LVBAT DC / DC converter, so a separate discharge device such as a discharge resistor is required.

[0010] Therefore, an object of the present invention is to provide a power conversion device and a control method for a power conversion device in which two or more DC / DC circuits and two or more DC / AC circuits are coupled by a transformer, which can prevent overcharging of a DC link capacitor that is expected in a sleep mode and reduce loss without adding a discharge device.

[0011] In order to solve the above problems, the present invention is configured as follows.

[0012] The power conversion device is a power conversion device in which the AC sides of at least two DC / AC circuits are coupled by a transformer having at least three windings, and includes: a first DC / AC circuit having a primary bridge circuit arranged on the input side of the transformer; a secondary bridge circuit arranged on the output side of the transformer and connected to a first load; a second DC / AC circuit arranged on the output side of the transformer, having a tertiary bridge circuit and a capacitor connected in parallel with the tertiary bridge circuit, and connected to a second load; and a control unit that controls the voltage of the capacitor in the tertiary bridge circuit in the second DC / AC circuit when the second load is equal to or less than a predetermined power consumption value, thereby causing the current of the output winding of the transformer connected to the tertiary bridge circuit to flow back to the secondary bridge circuit.

[0013] Furthermore, in a control method for a power conversion device including a primary bridge circuit arranged on the input side of at least two DC / AC circuits connected to a transformer having at least three windings, each of the AC sides of the DC / AC circuits; a secondary bridge circuit arranged on the output side of the transformer and connected to a first load; and a second DC / AC circuit having a capacitor connected in parallel with the tertiary bridge circuit and connected to a second load, the second DC / AC circuit controls the voltage of the capacitor using the tertiary bridge circuit when the second load is equal to or less than a predetermined power consumption value, and causes the current of the output winding of the transformer connected to the tertiary bridge circuit to flow back to the secondary bridge circuit.

[0014] According to the present invention, it is possible to provide a power conversion device and a control method for a power conversion device that can prevent overcharging and reduce losses in a power conversion device in which two or more DC / DC circuits and two or more DC / AC circuits are coupled by a transformer.

[0015] Problems, configurations, and effects other than those described above will become apparent from the following description of the embodiments.

[0016] FIG. 1 is a diagram showing a main circuit of a power conversion device according to a first embodiment of the present invention. FIG. 2 is a diagram showing an operation control unit of a power conversion device according to a first embodiment of the present invention. FIG. 3 is a state transition diagram showing a control method for a power conversion device according to a first embodiment of the present invention. FIG. 4 is a state transition diagram showing a control method for a power conversion device in an example different from the present invention. FIG. 5 is a state transition diagram showing a control method for a power conversion device according to a first embodiment of the present invention. FIG. 6 is a graph showing operation waveforms of control in the first embodiment of the present invention. FIG. 7 is a diagram showing a charging mechanism of a capacitor. FIG. 8 is a diagram showing a discharging mechanism of a capacitor. FIG. 9 is a circuit diagram showing a power conversion device according to a second embodiment of the present invention. FIG. 10 is a graph showing operation waveforms of the second embodiment of the present invention. FIG. 11 is a graph explaining a control method according to a third embodiment of the present invention. FIG. 12 is a graph explaining a control method according to a fourth embodiment of the present invention. FIG. 13 is a graph explaining a control technique according to a fifth embodiment of the present invention.

[0017] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same components are designated by the same reference numerals, and detailed descriptions of overlapping parts will be omitted.

[0018] First Embodiment A power conversion device and a method for controlling the power conversion device according to a first embodiment of the present invention will be described with reference to FIGS. 1 to 7. FIG.

[0019] FIG. 1 is a circuit diagram showing a main circuit 100 of a power conversion device according to the first embodiment.

[0020] As shown in Fig. 1, the power conversion device of the first embodiment is a power conversion device in which two or more DC / DC circuits and a DC / AC circuit are coupled by a transformer. The power conversion device of the first embodiment is an isolated DC / DC converter, and is not limited to a phase shift converter, but may also be an LLC converter or the like. Reference numeral 101 denotes a primary-side bridge circuit, which is composed of semiconductor switches Q1-1 to Q1-4. A first DC / AC circuit includes the primary-side bridge circuit 101 and a smoothing capacitor C1.

[0021] Reference numeral 102 denotes a secondary bridge circuit, which is composed of semiconductor switches Q2-1 and Q2-2. A first load 109 is connected in parallel to a smoothing capacitor C2 of the secondary bridge circuit 102. The secondary bridge circuit 102 is not limited to a center tap configuration, and may be a full bridge configuration, etc.

[0022] Reference numeral 103 denotes a tertiary bridge circuit, which is composed of semiconductor switches Q3-1 to Q3-4. Reference numeral 104 denotes a DC / AC bridge circuit, which is composed of semiconductor switches Q4-1 to Q4-4. A second load 110 is connected to the DC / AC bridge circuit 104.

[0023] The semiconductor switches Q1-1 to Q1-4, Q2-1, Q2-2, Q3-1 to Q3-4, and Q4-1 to Q4-4 are not limited to MOSFETs, and may be IGBTs, etc. In Fig. 1, an example of the semiconductor switches Q1-1 to Q1-4, Q2-1, Q2-2, Q3-1 to Q3-4, and Q4-1 to Q4-4 is shown in which diodes are connected in antiparallel.

[0024] Tr1 is a transformer having a leakage inductance L1. Smoothing capacitors C1, C2, and C3 are connected to the primary, secondary, and tertiary sides, respectively.

[0025] FIG. 2 is a diagram illustrating the operation control unit 108 of the power conversion device according to the first embodiment of the present invention.

[0026] 2, the operation control unit 108 includes a main circuit 100, a voltage / current detection circuit 105, a control circuit (control unit) 106, and a signal generation circuit 107. The voltage / current detection circuit 105 reads the voltage and current of the main circuit 100, converts them into digital signals, and transmits them to the control circuit 106. The voltage / current detection circuit 105 detects the input voltage V1 input to the primary-side bridge circuit 101 in FIG. 1, the output voltage V2 of the secondary-side bridge circuit 102, the DC link voltage (voltage of capacitor C3) V3, the output voltage V4 of the secondary-side bridge circuit 102, the output current, etc.

[0027] The control circuit 106 generates the phase shift amount by controlling the voltages of the output voltages V2 and V3. When the second load 110 consumes less than a predetermined power, the third DC / AC circuit 104 controls the voltage of the capacitor C3 in the tertiary bridge circuit 103 to return the current of the output winding of the transformer TR1 connected to the tertiary bridge circuit 103 to the secondary bridge circuit 102.

[0028] PI control or the like can be used as a control method for the control circuit 106. The signal generation circuit 107 generates gate signals that take into account the amount of phase shift output from the control circuit 106, and drives the semiconductor switches Q1-1 to Q1-4, Q2-1, Q2-2, Q3-1 to Q3-4, and Q4-1 to Q4-4 of the main circuit 100. Fig. 3 is a state transition diagram showing a control method for the power conversion device according to the first embodiment of the present invention.

[0029] As shown in Fig. 3 , in the initial state, the first load 109 and the second load 110 are disconnected from both the secondary-side bridge circuit 102 and the tertiary-side bridge circuit 103, and the circuits are in the OFF state. This state is expressed as EV:OFF. From this state, the secondary-side bridge circuit 102 transitions to a state in which the first load 109 is connected (one of the two states shown in the lower part of Fig. 3 ). This state is expressed as EV:ON. However, since the second load 110 of the tertiary-side bridge circuit 103 may be in either a connected state or a disconnected state, the circuit transitions to the respective state.

[0030] In EV:ON, the state transitions according to the load condition of the tertiary bridge circuit 103. However, the secondary bridge circuit 102 is basically designed to be driven at all times, and enters a state (EV:OFF) in which the first load 109 is disconnected only when the circuit is stopped.

[0031] FIG. 4A is a state transition diagram of a control different from that of the present invention, and FIG. 4B is a state transition diagram of the first embodiment of the present invention.

[0032] As shown in FIG. 4A , in a control different from the present invention, control is performed by switching between driving and pausing both the tertiary-side bridge circuit 103 and the DC / AC bridge circuit 104 depending on the state of the second load 110 of the tertiary-side bridge circuit 103.

[0033] On the other hand, as shown in FIG. 4B , in the control according to the first embodiment of the present invention, only the DC / AC bridge circuit 104 is controlled by switching between drive and pause depending on the state of the second load 110 of the tertiary-side bridge circuit 103, and the tertiary-side bridge circuit 103 is driven at all times.

[0034] FIG. 5 is a graph showing an example of an operational waveform of the control in the first embodiment of the present invention.

[0035] 5, an example of control for load disconnection of the tertiary bridge circuit 103 will be described. When the secondary bridge circuit 102 and the tertiary bridge are operating, the semiconductor switches Q1-1 to Q1-4 of the primary bridge circuit 101 control the secondary voltage V2, the semiconductor switches Q3-1 to Q3-4 of the tertiary bridge circuit 103 control the tertiary voltage V3, and the semiconductor switches Q4-1 to Q4-4 of the DC / AC bridge circuit 104 control the output voltage V4.

[0036] By the above control, the voltage V3 applied to the smoothing capacitor C3 becomes constant, and overcharging does not occur.

[0037] When the second load 110 of the tertiary bridge circuit 103 is disconnected at time t1, under control different from that of the present invention, as indicated by the dashed lines, the tertiary bridges Q3-1 to Q3-4 and the DC / AC bridges Q4-1 to Q4-4 enter a resting state and are fixed in the OFF state. At this time, as shown in Figure 6, a surge is superimposed on the voltage Vtr applied to the transformer Tr1 due to the leakage inductance L1 included in the transformer Tr1 and the parasitic inductance of the circuit. Because the semiconductor switch has a reverse-connected diode, when Vtr > V3, the diode conducts, and the current shown by the dashed line flows, charging the capacitor C3.

[0038] This may cause the capacitor C3 to be charged, increasing V3 and resulting in overcharging.

[0039] Therefore, in the control according to the first embodiment of the present invention, the semiconductor switches Q3-1 to Q3-4 and Q4-1 to Q4-4 of the tertiary bridge circuit 103 are continuously driven. By controlling the tertiary bridge circuit 103, it is possible to provide a period during which the capacitor C3 is discharged to the transformer Tr side, as shown in FIG.

[0040] 6 and 7, the voltage V3 of the capacitor C3 is controlled by repeating the charging and discharging of the capacitor C3 using the phase shift amount θ between the primary bridge circuit 101 and the tertiary bridge circuit 103.

[0041] By applying the control according to the first embodiment of the present invention as shown by the solid line in Fig. 5, it is possible to prevent overcharging of the voltage V3 of the capacitor C3 even after the second load 110 on the tertiary side is disconnected. Furthermore, by supplying the energy stored in the capacitor C3 to the secondary bridge circuit 102 and the first load 109, which is the secondary load, via the transformer Tr1 as shown in Fig. 6, it is expected that loss will be reduced.

[0042] According to the first embodiment of the present invention, it is possible to provide a power conversion device and a control method for the power conversion device that can achieve both overcharging prevention and loss reduction in a power conversion device in which two or more DC / DC circuits and two or more DC / AC circuits are coupled by a transformer.

[0043] Second Embodiment A power conversion device and a method for controlling the power conversion device according to a second embodiment of the present invention will be described with reference to FIGS. 8 and 9. FIG.

[0044] FIG. 8 is a diagram showing a main circuit 100 of a power conversion device according to the second embodiment.

[0045] The power conversion device of Fig. 8 differs from the power conversion device of Fig. 1 in that it has a smoothing inductor L2 in the secondary bridge circuit 102. The primary bridge circuit 101 and the secondary bridge circuit 102 are driven as a phase shift converter. Other configurations of the power conversion device shown in Fig. 8 are the same as those of the first embodiment. Fig. 9 shows the operating waveforms of the primary bridge circuit 101 and the secondary bridge circuit 102, which are phase shift converters in the second embodiment.

[0046] In the phase shift converter, when the diagonal semiconductor switches (combinations of Q1-1 and Q1-4, Q1-2 and Q1-3, etc.) in the primary side bridge circuit 101 are ON, an input voltage V1 is applied to the transformer Tr1.

[0047] On the other hand, when the upper semiconductor switches (Q1-1 and Q1-3) or the lower semiconductor switches (Q1-2 and Q1-4) are ON, a short circuit occurs within the bridge, the input voltage V1 is disconnected, and the transformer voltage Vtr becomes 0V.

[0048] Therefore, the waveform of the transformer voltage Vtr becomes as shown in Fig. 9. At this time, the output voltage V2 of the secondary side bridge circuit 102 is the average value of the transformer voltage Vtr output by the smoothing inductor L2 and the smoothing capacitor C2, so the period during which the voltage is applied to the transformer Tr1, i.e., the average value, can be controlled by the phase shift amount θ1.

[0049] When the second load 110 of the tertiary-side bridge circuit 103 is disconnected at time t1, the control described in the first embodiment is applied. In the control described in the first embodiment, the voltage of the capacitor C3 is controlled by the phase shift amount θ3 between the primary-side bridge circuit 101 and the tertiary-side bridge circuit 103. At this time, if the diagonal semiconductor switches in the primary-side bridge circuit 101 are ON, the input voltage is applied to the transformer Tr1 as described above. In addition, if the diagonal semiconductor switches (Q3-1 and Q3-4, Q3-2 and Q3-3) in the tertiary-side bridge circuit 103 are ON, the voltage of the capacitor C3 is applied to the transformer Tr1.

[0050] Therefore, the period during which a voltage is applied to the transformer Tr1 increases, resulting in a waveform like that shown in FIG. 9, and an increase in the average value of the transformer voltage Vtr.

[0051] 8 and 9, the power conversion device and the control method for the power conversion device according to the second embodiment can obtain the same effects as those of the first embodiment, and also obtain the effect of expanding the output voltage range of the secondary-side voltage V2. By expanding the output voltage range of the secondary-side voltage V2, it is possible to set a voltage that results in low loss.

[0052] Third Embodiment A power conversion device and a method for controlling the power conversion device according to a third embodiment of the present invention will be described with reference to FIGS. 8 and 10. FIG.

[0053] FIG. 10 is a graph illustrating the control method of the third embodiment.

[0054] 8, when the first load 109 of the secondary-side bridge circuit 102 fluctuates, the output voltage V2 of the secondary-side bridge circuit 102 fluctuates due to the influence of a voltage drop in the power conversion device. In particular, when the load factor of the first load 109 of the secondary-side bridge circuit 102 increases and becomes heavy, the output voltage drops as shown by the dashed line V2' compared to during steady operation.

[0055] At this time, as shown in FIG. 10, the control circuit (control unit) 106 increases the voltage V3 applied to the capacitor C3, thereby increasing the voltage applied to the transformer Tr1 and increasing the output voltage V2.

[0056] From the above results, as shown in FIG. 10, by applying the control according to the third embodiment, it is possible to obtain the effect of expanding the output voltage range of the output voltage V2 of the secondary-side bridge circuit 102.

[0057] 10 is an example, in which a threshold value Vth is set for the load factor (secondary load factor) of the secondary bridge circuit 102, and when the secondary load factor falls below the threshold value Vth, the command value of the voltage V3 applied to the capacitor C3 is increased, thereby increasing the output voltage V2. Furthermore, when the secondary load factor increases and exceeds the threshold value Vth, the command value of the voltage V3 applied to the capacitor C3 is decreased.

[0058] In the example shown in Figure 10, the voltage V3 applied to the capacitor C3 is controlled using two values, but it is also possible to consider a method of continuously varying the threshold value of the secondary load factor or the voltage V3 applied to the capacitor C3.

[0059] In the power conversion device and the control method for the power conversion device according to the third embodiment, the output voltage range of the secondary side voltage V2 can be expanded, and the output voltage can be adjusted to accommodate heavy loads.

[0060] Fourth Embodiment A power conversion device and a control method for the power conversion device according to a fourth embodiment of the present invention will be described with reference to Fig. 8 and Fig. 11. Fig. 11 is a graph illustrating the control method according to the fourth embodiment.

[0061] In the power conversion device shown in Fig. 8, when the input voltage V1 fluctuates, the voltage applied to the transformer Tr1 also fluctuates, causing the output voltage V2 of the secondary-side bridge circuit 102 to fluctuate. In particular, when the input voltage V1 drops, the output voltage V2 drops compared to steady-state operation, as indicated by the dashed line V2' in Fig. 11. In this case, as shown in Fig. 11, by increasing the voltage V3 applied to the capacitor C3, the voltage applied to the transformer Tr1 is increased, and the output voltage is increased.

[0062] From the above results, as shown in FIG. 11 , by applying the control according to the fourth embodiment, it is possible to expand the output voltage range of the output voltage V2 of the secondary-side bridge circuit 102, and to obtain the effect that the output voltage V2 can be adjusted when the input voltage V1 drops.

[0063] The example shown in FIG. 11 is an example of control, in which a threshold value Vth is set for the input voltage V1. When the input voltage V1 falls below the threshold value Vth, the command value for the voltage V3 of the capacitor C3 is increased, thereby increasing the output voltage. Furthermore, when the input voltage V1 increases and exceeds the threshold value Vth, the command value for the voltage V3 applied to the capacitor C3 is decreased. In the example shown in FIG. 11, the voltage V3 applied to the capacitor C3 is controlled using two values, but other methods are also possible, such as continuously varying the threshold value for the input voltage V1 or continuously varying the voltage V3 applied to the capacitor C3.

[0064] The power conversion device and the control method for the power conversion device according to the fourth embodiment have the effect of expanding the output voltage range of the secondary side voltage V2 and appropriately adjusting the output voltage in response to fluctuations in the input voltage.

[0065] Fifth Embodiment A power conversion device and a method for controlling the power conversion device according to a fifth embodiment of the present invention will be described with reference to FIGS.

[0066] FIG. 12 is a graph illustrating the control method of the fifth embodiment.

[0067] In the power conversion device shown in Fig. 1, when the tertiary bridge circuit 103 is inactive while there is no load on the tertiary bridge circuit 103, the capacitor C3 is overcharged. In this case, as shown in Fig. 12, a threshold value Vth is set for the voltage V3 of the capacitor C3, and the tertiary bridge circuit 103 is inactive when the voltage V3 of the capacitor C3 is equal to or lower than the threshold value Vth.

[0068] When the voltage V3 of the capacitor C3 exceeds the threshold Vth, the tertiary bridge circuit 103 is driven in the same manner as in the first embodiment.

[0069] The above control is repeated to intermittently operate the tertiary bridge circuit 103. As shown in Fig. 12, by applying the control according to the fifth embodiment, it is possible to reduce the switching loss in the tertiary bridge circuit 103, thereby obtaining the effect of reducing the loss.

[0070] Note that the control shown in FIG. 12 is an example, and other methods may be considered, such as how to set the threshold value of the input voltage V1 or a method of controlling the discharge current when the tertiary bridge circuit 103 is driven.

[0071] According to the fifth embodiment, in addition to the same effects as those of the first embodiment, the fifth embodiment also has the effect of reducing the switching loss in the tertiary bridge circuit 103, thereby achieving the effect of reducing the loss.

[0072] The present invention has been described above with reference to a power conversion device having two DC / AC circuits and a control method for the power conversion device. However, the present invention can also be applied to a power conversion device having two or more DC / AC circuits and a control method for the power conversion device.

[0073] 100: Main circuit, 101: Primary side bridge circuit, 102: Secondary side bridge circuit, 103: Tertiary side bridge circuit, 104: DC / AC bridge, 105: Voltage / current detection circuit, 106: Control circuit (control unit), 107: Signal generation circuit, 108: Operation control unit, 109: First load, 110: Second load, C1, C2, C3: Smoothing capacitor, Tr1: Transformer, L1: Transformer leakage inductance, L2: Smoothing inductor, Q1-1 to Q1-4, Q2-1 to Q2-2, Q3-1 to Q3-4, Q4-1 to Q4-4: Semiconductor switches, V1: Input voltage, V2, V4: Output voltage, V3: DC link voltage (voltage of capacitor C3)

Claims

1. A power conversion device in which the AC sides of at least two DC / AC circuits are coupled by a transformer having at least three windings, comprising: a first DC / AC circuit having a primary bridge circuit arranged on the input side of the transformer; a secondary bridge circuit arranged on the output side of the transformer and connected to a first load; a second DC / AC circuit arranged on the output side of the transformer and having a tertiary bridge circuit and a capacitor connected in parallel with the tertiary bridge circuit, to which a second load is connected; and a control unit for controlling the voltage of the capacitor in the tertiary bridge circuit in the second DC / AC circuit when the second load is equal to or less than a predetermined power consumption value, thereby causing the current of the output winding of the transformer connected to the tertiary bridge circuit to flow back to the secondary bridge circuit.

2. A power conversion device according to claim 1, wherein the secondary bridge circuit has a smoothing inductor.

3. A power conversion device according to claim 2, wherein the control unit controls the voltage applied to the capacitor in accordance with the load connected to the secondary bridge circuit.

4. A power conversion device according to claim 2, wherein the control unit controls the voltage applied to the capacitor in accordance with the input voltage of the primary bridge circuit.

5. A power conversion device according to claim 1, wherein the control unit stops the tertiary bridge circuit when the voltage of the capacitor is equal to or lower than a certain threshold value.

6. A control method for a power conversion device comprising: a primary bridge circuit arranged on the input side of at least two DC / AC circuits connected to a transformer having at least three windings on each AC side of the transformer; a secondary bridge circuit arranged on the output side of the transformer and connected to a first load; and a second DC / AC circuit having a capacitor connected in parallel with the tertiary bridge circuit and connected to a second load, wherein the second DC / AC circuit controls the voltage of the capacitor using the tertiary bridge circuit when the second load is equal to or less than a predetermined power consumption value, and causes the current of the output winding of the transformer connected to the tertiary bridge circuit to flow back to the secondary bridge circuit.

7. A control method for a power conversion device according to claim 6, wherein the secondary bridge circuit has a smoothing inductor.

8. A control method for a power conversion device according to claim 7, characterized in that the voltage applied to the capacitor is controlled in accordance with the load connected to the secondary bridge circuit.

9. A control method for a power conversion device according to claim 7, characterized in that the voltage applied to the capacitor is controlled in accordance with the input voltage of the primary side bridge circuit.

10. A control method for a power conversion device according to claim 6, characterized in that when the voltage of the capacitor is equal to or lower than a certain threshold, the tertiary bridge circuit is stopped.

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