Power conversion device

Abstract

To smooth heat generation of a plurality of switching elements included in a DAB converter.SOLUTION: In a DAB converter, when transmitting power from first DC sections (E1 and Ca) to second DC sections (E2 and Cb) while stepping down a voltage, a control circuit (13) performs control so as to include a first period in which the first DC sections (E1 and Ca) and a primary coil (n1) of an insulation transformer (TR1) are conducted and a second period in which both ends of the primary coil (n1) of the insulation transformer (TR1) are short-circuited in a first bridge circuit (11). The control circuit (13) generates a first pattern of shifting from the first period to the second period by turning off a fourth switching element (Q4), a second pattern of shifting to the second period by turning off a second switching element (Q2), a third pattern of shifting to the second period by turning off a first switching element (Q1), and a fourth pattern of shifting to the second period by turning off a third switching element (Q3).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a power conversion device that converts DC power into DC power of another voltage. [Background technology]

[0002] Power conditioners used in photovoltaic power generation systems and V2H (Vehicle to Home) systems are required to perform highly efficient power conversion. V2H systems can charge and discharge between storage batteries installed in electric vehicles (e.g., EVs and PHEVs) and commercial power grids or home loads. For example, the power generated by a home photovoltaic power generation system can be charged to the storage battery of an electric vehicle. The storage battery installed in an electric vehicle can also be used for peak shifting and backup purposes for home loads. DC / DC converters used in V2H systems are required to be highly efficient, insulated, and have a wide voltage range. This is because the voltage of storage batteries installed in electric vehicles varies greatly depending on the vehicle model. One DC / DC converter that meets these requirements is the DAB (Dual Active Bridge) converter.

[0003] In a typical DAB converter, when the potential difference between input and output is large, loss increases due to hard switching, and loss increases due to the flow of reactive current unrelated to power transmission.In response to this, for example, a DAB converter has been proposed that uses a phase shift method to step up or step down the voltage while turning off all PWM (Pulse Width Modulation) signals input to one of the legs of a bridge circuit in which four switching elements are bridge-connected (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] WO 16 / 125373 Summary of the Invention

Problems to be Solved by the Invention

[0005] When the proposed DAB converter is operated in a step-down mode, in the primary-side bridge circuit, the losses of the switching elements become non-uniform. Therefore, the amount of heat generated due to losses differs for each switching element, and thermal design is required for the amount of heat generated corresponding to the switching element with the maximum loss. This leads to an increase in the size of the cooling member (usually a heat sink) for cooling the switching element, an increase in the air volume of the fan for cooling the cooling member, an increase in the water volume of the water cooling mechanism for cooling the cooling member, and so on.

[0006] The present disclosure has been made in view of such a situation, and an object thereof is to provide a technique for equalizing the losses of a plurality of switching elements included in a DAB converter.

Means for Solving the Problems

[0007] To solve the above problems, a power conversion device according to an aspect of the present disclosure includes a first leg in which a first switching element and a second switching element are connected in series, and a second leg in which a third switching element and a fourth switching element are connected in series. The first leg and the second leg are connected in parallel to a first DC part, and a first bridge circuit; a third leg in which a fifth switching element and a sixth switching element are connected in series, and a fourth leg in which a seventh switching element and an eighth switching element are connected in series. The third leg and the fourth leg are connected in parallel to a second DC part, and a second bridge circuit; an isolation transformer connected between the first bridge circuit and the second bridge circuit; and a control circuit for controlling the first switching element to the eighth switching element. When stepping down the voltage from the first DC part to the second DC part and transmitting power, the control circuit controls to include a first period in which the first DC part and the primary winding of the isolation transformer are conducting, and a second period in which both ends of the primary winding of the isolation transformer are short-circuited within the first bridge circuit. From the first period, a first pattern in which the fourth switching element conducting in the forward direction turns off and transitions to the second period, a second pattern in which the second switching element conducting in the forward direction turns off and transitions to the second period, a third pattern in which the first switching element conducting in the forward direction turns off and transitions to the second period, and a fourth pattern in which the third switching element conducting in the forward direction turns off and transitions to the second period are generated.

Advantages of the Invention

[0008] According to the present disclosure, it is possible to equalize the losses of a plurality of switching elements included in the DAB converter.

Brief Description of the Drawings

[0009]

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Embodiments for Carrying Out the Invention

[0010] Fig. 1 is a diagram for explaining the configuration of a power conversion device 1 according to an embodiment. The power conversion device 1 is an isolated bidirectional DC / DC converter (DAB converter), which converts the DC power supplied from a first DC power source E1 and transmits it to a second DC power source E2. Further, the power conversion device 1 converts the DC power supplied from the second DC power source E2 and transmits it to the first DC power source E1. The power conversion device 1 can either step down and transmit power or step up and transmit power.

[0011] The first DC power supply E1 corresponds to, for example, a storage battery or an electric double layer capacitor mounted on an EV, or a stationary storage battery or an electric double layer capacitor. The second DC power supply E2 corresponds to, for example, a DC bus connected to a commercial power system via an inverter. Other storage batteries, solar cells, fuel cells, etc. may be connected to the DC bus via other DC / DC converters.

[0012] The power conversion device 1 includes a primary capacitor Ca, a first bridge circuit 11, a first inductance L1, an isolation transformer TR1, a second inductance L2, a second bridge circuit 12, a secondary capacitor Cb, and a control circuit 13.

[0013] The primary capacitor Ca is connected in parallel with the first DC power supply E1. The secondary capacitor Cb is connected in parallel with the second DC power supply E2. For example, electrolytic capacitors are used for the primary capacitor Ca and the secondary capacitor Cb. In this specification, the first DC power supply E1 and the primary capacitor Ca are collectively referred to as the first DC section, and the second DC power supply E2 and the secondary capacitor Cb are collectively referred to as the second DC section.

[0014] The first bridge circuit 11 is a full bridge circuit configured by connecting in parallel a first leg in which a first switching element Q1 and a second switching element Q2 are connected in series, and a second leg in which a third switching element Q3 and a fourth switching element Q4 are connected in series. The first bridge circuit 11 is connected in parallel with the first DC section, and the midpoints of the first leg and the second leg are respectively connected to both ends of the primary winding n1 of the isolation transformer TR1. The first bridge circuit 11 can convert the primary DC voltage supplied from the first DC section into an AC voltage and output it to the primary winding n1 of the isolation transformer TR1. Also, the first bridge circuit 11 can convert the AC voltage supplied from the primary winding n1 of the isolation transformer TR1 into a DC voltage and output it to the first DC section.

[0015] The second bridge circuit 12 is a full-bridge circuit formed by connecting in parallel a third leg in which a fifth switching element Q5 and a sixth switching element Q6 are connected in series and a fourth leg in which a seventh switching element Q7 and an eighth switching element Q8 are connected in series. The second bridge circuit 12 is connected in parallel with the second DC section, and the midpoints of the third leg and the fourth leg are respectively connected to both ends of the secondary winding n2 of the isolation transformer TR1. The second bridge circuit 12 can convert the DC voltage on the secondary side supplied from the second DC section into an AC voltage and output it to the secondary winding n2 of the isolation transformer TR1. Also, the second bridge circuit 12 can convert the AC voltage supplied from the secondary winding n2 of the isolation transformer TR1 into a DC voltage and output it to the second DC section.

[0016] A first diode D1 - an eighth diode D8 are respectively formed or connected in reverse parallel to the first switching element Q1 - the eighth switching element Q8. Also, a first capacitor C1 - an eighth capacitor C8 are respectively formed or connected in parallel to the first switching element Q1 - the eighth switching element Q8.

[0017] For example, MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) or IGBTs (Insulated Gate Bipolar Transistors) can be used for the first switching element Q1 - the eighth switching element Q8. When MOSFETs are used for the first switching element Q1 - the eighth switching element Q8, the parasitic diodes respectively formed between the drains and sources of the first switching element Q1 - the eighth switching element Q8 are used as the first diode D1 - the eighth diode D8, or external diode elements are respectively connected as the first diode D1 - the eighth diode D8. Also, the parasitic capacitances respectively formed between the drains and sources of the first switching element Q1 - the eighth switching element Q8 are used as the first capacitor C1 - the eighth capacitor C8, or external capacitors are respectively connected between the drains and sources of the first switching element Q1 - the eighth switching element Q8 as the first capacitor C1 - the eighth capacitor C8.

[0018] When IGBTs are used for the first switching element Q1 to the eighth switching element Q8, external diode elements are respectively connected as the first diode D1 to the eighth diode D8 between the collector and emitter of the first switching element Q1 to the eighth switching element Q8. Further, an external capacitor is respectively connected as the first capacitor C1 to the eighth capacitor C8 between the collector and emitter of the first switching element Q1 to the eighth switching element Q8, or the parasitic capacitances respectively formed between the collector and emitter of the first switching element Q1 to the eighth switching element Q8 are used as the first capacitor C1 to the eighth capacitor C8.

[0019] Since a tail current does not occur in the MOSFET as compared with the IGBT, the switching loss at turn-off can be reduced to reduce heat generation, so that the cooler (for example, heat sink) can be miniaturized. Further, since high-frequency driving is possible as compared with the IGBT, passive components (for example, transformers, capacitors) can be miniaturized. However, the MOSFET usually has a larger on-resistance than the IGBT.

[0020] In recent years, as a switching element with high breakdown voltage and low loss, switching elements using wide-gap semiconductors (for example, silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga2O3), diamond (C)) have been spreading. In the present embodiment, it is assumed that SiC-MOSFETs are used for the first switching element Q1 to the eighth switching element Q8. Since the SiC-MOSFET has a higher breakdown voltage than the Si-MOSFET, it can be miniaturized and the on-resistance per unit area can be reduced.

[0021] The isolation transformer TR1 is connected between the AC terminals of the first bridge circuit 11 and the AC terminals of the second bridge circuit 12. The isolation transformer TR1 converts the output voltage of the first bridge circuit 11 connected to the primary winding n1 according to the turns ratio of the primary winding n1 and the secondary winding n2, and outputs it to the second bridge circuit 12 connected to the secondary winding n2. Also, the isolation transformer TR1 converts the output voltage of the second bridge circuit 12 connected to the secondary winding n2 according to the turns ratio of the secondary winding n2 and the primary winding n1, and outputs it to the first bridge circuit 11 connected to the primary winding n1.

[0022] The first inductor L1 is connected in series or formed between the AC terminal of the first bridge circuit 11 and the primary winding n1 of the isolation transformer TR1. The second inductor L2 is connected in series or formed between the AC terminal of the second bridge circuit 12 and the secondary winding n2 of the isolation transformer TR1. In the example shown in FIG. 1, the first inductor L1 is composed of a reactive element connected between the midpoint of the first leg of the first bridge circuit 11 and the primary winding n1 of the isolation transformer TR1. The second inductor L2 is composed of a reactive element connected between the midpoint of the third leg of the second bridge circuit 12 and the secondary winding n2 of the isolation transformer TR1.

[0023] Note that the first inductor L1 may be composed of the leakage inductance of the primary winding n1 formed between the midpoint of the first leg of the first bridge circuit 11 and the primary winding n1 of the isolation transformer TR1. The second inductor L2 may be composed of the leakage inductance of the secondary winding n2 formed between the midpoint of the third leg of the second bridge circuit 12 and the secondary winding n2 of the isolation transformer TR1. Note that either one of the first inductor L1 and the second inductor L2 may be omitted.

[0024] The control circuit 13 executes the following control as basic control. When transmitting power from the first DC section to the second DC section (when discharging from the first DC power supply E1), the control circuit 13 controls the first switching element Q1 - the eighth switching element Q8 so that the current value detected by the secondary current sensor 23 maintains the current command value. Note that the voltage value detected by the first voltage sensor 21 on the primary side, the secondary voltage value detected by the second voltage sensor 22 on the secondary side, and the current value detected by a current sensor (not shown) on the primary side may be controlled using the target values.

[0025] Also, when transmitting power from the second DC section to the first DC section (when charging the first DC power supply E1), the control circuit 13 controls the first switching element Q1 - the eighth switching element Q8 so that the current value detected by a current sensor (not shown) on the primary side maintains the current command value. Note that the voltage value detected by the first voltage sensor 21 on the primary side, the secondary voltage value detected by the second voltage sensor 22 on the secondary side, and the current value detected by the secondary current sensor 23 may be controlled using the target values.

[0026] In this way, the DAB converter has a symmetric configuration on the primary and secondary sides and can transmit power bidirectionally. Hereinafter, the operation of the power conversion device 1 will be described.

[0027] (Comparative Example 1) Figs. 2(a) - (c) are diagrams for explaining the operating states according to Comparative Example 1 of the power conversion device 1. In the first state shown in Fig. 2(a), the control circuit 13 controls the first switching element Q1, the fourth switching element Q4, the sixth switching element Q6, and the seventh switching element Q7 to be in the on state, and the second switching element Q2, the third switching element Q3, the fifth switching element Q5, and the eighth switching element Q8 to be in the off state. In this state, power is charged from the first DC power supply E1 to the first inductance L1, and power is charged from the second DC power supply E2 to the second inductance L2.

[0028] In the second state shown in FIG. 2(b), the control circuit 13 controls the first switching element Q1, the fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 to be in the on state, and the second switching element Q2, the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 to be in the off state. In this state, the power of the first DC power supply E1, the power stored in the first inductor L1, and the power stored in the second inductor L2 are transmitted to the second DC power supply E2.

[0029] In the third state (not shown), the control circuit 13 controls the second switching element Q2, the third switching element Q3, the fifth switching element Q5, and the eighth switching element Q8 to be in the on state, and the first switching element Q1, the fourth switching element Q4, the sixth switching element Q6, and the seventh switching element Q7 to be in the off state. In this state, power is charged from the first DC power supply E1 to the first inductor L1, and power is charged from the second DC power supply E2 to the second inductor L2.

[0030] In the fourth state (not shown), the control circuit 13 controls the second switching element Q2, the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 to be in the on state, and the first switching element Q1, the fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 to be in the off state. In this state, the power of the first DC power supply E1, the power stored in the first inductor L1, and the power stored in the second inductor L2 are transmitted to the second DC power supply E2.

[0031] In the control according to the comparative example 1, in the first state (see FIG. 2(a)) and the third state (not shown), the power of the second DC power supply E2 is charged to the second inductor L2. In the subsequent second state (see FIG. 2(b)) and the fourth state (not shown), the power stored in the second inductor L2 is discharged to the second DC power supply E2. That is, an idle current that has nothing to do with power transmission flows on the secondary side. Due to the flow of this idle current, useless losses are generated.

[0032] Fig. 2(c) shows the current flow when the voltage of the first DC power supply E1 drops significantly with respect to the voltage of the second DC power supply E2 in the second state shown in Fig. 2(b). When the voltage of the second DC power supply E2 becomes higher than the voltage of the first DC power supply E1, the direction of the current is reversed, and the current flows backward from the second DC power supply E2 to the first DC power supply E1. In this state, when the first switching element Q1 and the fourth switching element Q4 are turned off and the second switching element Q2 and the third switching element Q3 are turned on to transition to the next state, the second switching element Q2 and the third switching element Q3 become hard switching, and the first diode D1 of the first switching element Q1 and the fourth diode D4 of the fourth switching element Q4 undergo a recovery operation, increasing the loss.

[0033] (Comparative Example 2) Figs. 3(a)-(b) are diagrams for explaining the current flow in the step-down operation according to Comparative Example 2 of the power conversion device 1. Fig. 4 is a diagram for explaining the switching pattern of the first switching element Q1 - the eighth switching element Q8 during the step-down operation according to Comparative Example 2 of the power conversion device 1. Figs. 5(a)-(b) are diagrams for explaining the current flow in the boost operation according to Comparative Example 2 of the power conversion device 1. Fig. 6 is a diagram for explaining the switching pattern of the first switching element Q1 - the eighth switching element Q8 during the boost operation according to Comparative Example 2 of the power conversion device 1.

[0034] In Comparative Example 2, a phase shift method is adopted. The duty ratios of the first switching element Q1 - the sixth switching element Q6 are fixed at 50%, and the seventh switching element Q7 and the eighth switching element Q8 maintain a fully off state.

[0035] Fig. 3(a) shows the state in which power is transmitted from the first DC power supply E1 to the second DC power supply E2 (hereinafter referred to as the transmission state). Fig. 3(b) shows the state in which the first DC power supply E1 and the isolation transformer TR1 are disconnected, and power is transmitted from the first inductor L1 and the second inductor L2 to the second DC power supply E2 (hereinafter referred to as the commutation state). In the step-down operation, the voltage or current of the transmitted power is controlled by the ratio of the transmission state and the commutation state. The higher the ratio of the commutation state, the lower the voltage or current of the transmitted power is controlled.

[0036] Specifically, as shown in Fig. 4, the phase of the first leg (the first switching element Q1 and the second switching element Q2) is fixed, the phase of the second leg (the third switching element Q3 and the fourth switching element Q4) is variable, and by controlling the phase of the second leg, the phase difference between the first leg and the second leg is controlled. The third leg (the fifth switching element Q5 and the sixth switching element Q6) is controlled in synchronization with the second leg. When the control circuit 13 increases the power transmitted from the primary side to the secondary side, it controls the phase difference to become smaller (shifts the phase of the second leg to the left), and when the power transmitted from the primary side to the secondary side is decreased, it controls the phase difference to become larger (shifts the phase of the second leg to the right).

[0037] Fig. 5(a) shows the state in which the isolation transformer TR1 and the second DC power supply E2 are disconnected, and power is accumulated from the first DC power supply E1 to the first inductor L1 and the second inductor L2 (hereinafter referred to as the accumulation state). Fig. 5(b) shows the transmission state in which power is transmitted from the first DC power supply E1, the first inductor L1, and the second inductor L2 to the second DC power supply E2. In the boost operation, the voltage or current of the transmitted power is controlled by the ratio of the transmission state and the accumulation state. The higher the ratio of the accumulation state, the higher the voltage or current of the transmitted power is controlled.

[0038] Specifically, as shown in FIG. 6, the phases of the first leg and the second leg are fixed, the phase of the third leg is variable, and by controlling the phase of the third leg, the phase difference between the first leg and the second leg and the third leg is controlled. When the control circuit 13 increases the power transmitted from the primary side to the secondary side, it controls so that the phase difference becomes large (shifts the phase of the third leg to the right), and when decreasing the power transmitted from the primary side to the secondary side, it controls so that the phase difference becomes small (shifts the phase of the third leg to the left).

[0039] In Comparative Example 2, since the seventh switching element Q7 and the eighth switching element Q8 maintain the all-off state, no current flows backward from the second DC power supply E2 to the second inductor L2, the first inductor L1, and the first DC power supply E1. That is, no reactive current is generated due to the backward flow of current from the second DC power supply E2 as shown in FIG. 2(a) of Comparative Example 1. Also, even when the voltage of the second DC power supply E2 becomes higher than the voltage of the first DC power supply E1 during the step-down operation, no current flows backward from the second DC power supply E2 to the first DC power supply E1, so it is possible to suppress the next on / off switching of the first switching element Q1 - the fourth switching element Q4 on the primary side from becoming hard switching.

[0040] As shown in FIG. 4, in the step-down operation of Comparative Example 2, the turn-off of the first switching element Q1 and the second switching element Q2 included in the first leg is a turn-off in the transmission state, and the turn-off of the third switching element Q3 and the fourth switching element Q4 included in the second leg is a turn-off in the commutation state. Since the turn-off in the transmission state is a turn-off in a state where more current flows, the turn-off losses of the first switching element Q1 and the second switching element Q2 included in the first leg are larger than the turn-off losses of the third switching element Q3 and the fourth switching element Q4 included in the second leg. As shown in FIG. 4, the turn-off of the first switching element Q1 and the second switching element Q2 is a turn-off that changes the absolute value of the current from increasing to decreasing, so the switching loss is large.

[0041] As shown in Fig. 6, in the boosting operation of Comparative Example 2, the fourth leg is completely off. Therefore, the switching losses of the fifth switching element Q5 and the sixth switching element Q6 included in the third leg are greater than the switching losses of the seventh switching element Q7 and the eighth switching element Q8 included in the fourth leg. As shown in Fig. 6, the turn-off of the fifth switching element Q5 and the sixth switching element Q6 is a turn-off that causes the absolute value of the current to change from increasing to decreasing, so the switching loss is large.

[0042] (Example (Step-down)) Fig. 7 is a diagram showing the switching patterns of the first switching element Q1 - the eighth switching element Q8 and the transition example of the transformer current IL during the step-down operation according to the embodiment of the power conversion device 1. The transformer current IL is the current flowing through the isolation transformer TR1. Figs. 8(a)-(d) are diagrams for explaining the first example of the switching pattern and the current flow in the step-down operation according to the embodiment of the power conversion device 1 (Part 1). Figs. 9(a)-(d) are diagrams for explaining the first example of the switching pattern and the current flow in the step-down operation according to the embodiment of the power conversion device 1 (Part 2). Figs. 10(a)-(d) are diagrams for explaining the second example of the switching pattern and the current flow in the step-down operation according to the embodiment of the power conversion device 1 (Part 1). Figs. 11(a)-(d) are diagrams for explaining the second example of the switching pattern and the current flow in the step-down operation according to the embodiment of the power conversion device 1 (Part 2).

[0043] This embodiment uses control that combines the PWM method and the phase shift method. When the control circuit 13 steps down the voltage from the first DC section to the second DC section to transmit power, it controls to include a first period T1 (step-down) and a second period T2 (step-down). The first period T1 (step-down) is a period in which the first bridge circuit 11 conducts the first DC section and the primary winding n1 of the isolation transformer TR1, and the second bridge circuit 12 conducts the secondary winding n2 of the isolation transformer TR1 and the second DC section. The second period T2 (step-down) is a period in which both ends of the primary winding n1 of the isolation transformer TR1 are short-circuited within the first bridge circuit 11, and the second bridge circuit 12 conducts the secondary winding n2 of the isolation transformer TR1 and the second DC section to enter a commutation state.

[0044] The control circuit 13 generates the first period T1 (step-down) and the second period T2 (step-down) in the first pattern (step-down) - the fourth pattern (step-down). The first example of the switching pattern and the current flow in the step-down operation shown in FIGS. 8(a)-(d) and FIGS. 9(a)-(d) is an example of synchronous rectification by controlling the switching element to be controlled in the rectification state in the second bridge circuit 12 to be in the on state. The second example of the switching pattern and the current flow in the step-down operation shown in FIGS. 10(a)-(d) and FIGS. 11(a)-(d) is an example of diode rectification by controlling the switching element to be controlled in the rectification state in the second bridge circuit 12 to be in the off state.

[0045] In the first period T1-1 (step-down) of the first pattern (step-down), the first switching element Q1 and the fourth switching element Q4 are in the on state, the fifth switching element Q5 and the eighth switching element Q8 are in the rectification state, and the second switching element Q2, the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 are in the off state (see FIGS. 8(a) and 10(a)).

[0046] In the first pattern (step-down), the second period T2-1 (step-down) is a state in which the first switching element Q1 is in the on state, the third switching element Q3, the fifth switching element Q5, and the eighth switching element Q8 are in the rectifying state, and the second switching element Q2, the fourth switching element Q4, the sixth switching element Q6, and the seventh switching element Q7 are in the off state (see FIGS. 8(b) and 10(b)).

[0047] In the first pattern (step-down), the control circuit 13 turns off the fourth switching element Q4 that is conducting in the forward direction from the first period T1-1 (step-down) and makes a transition to the second period T2-1 (step-down). Here, "conducting in the forward direction" refers to a state in which current flows from the drain to the source direction or from the collector to the emitter direction of the switching element.

[0048] In the second pattern (step-down), the first period T1-2 (step-down) is a state in which the second switching element Q2 and the third switching element Q3 are in the on state, the sixth switching element Q6 and the seventh switching element Q7 are in the rectifying state, and the first switching element Q1, the fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 are in the off state (see FIGS. 8(c) and 10(c)).

[0049] In the second pattern (step-down), the second period T2-2 (step-down) is a state in which the third switching element Q3 is in the on state, the first switching element Q1, the sixth switching element Q6, and the seventh switching element Q7 are in the rectifying state, and the second switching element Q2, the fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 are in the off state (see FIGS. 8(d) and 10(d)).

[0050] In the second pattern (step-down), the control circuit 13 turns off the second switching element Q2 that is conducting in the forward direction from the first period T1-2 (step-down) and makes a transition to the second period T2-2 (step-down).

[0051] In the third pattern (step - down), the first period T1 - 3 (step - down) is such that the first switching element Q1 and the fourth switching element Q4 are in the on - state, the fifth switching element Q5 and the eighth switching element Q8 are in the rectifying state, and the second switching element Q2, the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 are in the off - state (see FIGS. 9(a) and 11(a)).

[0052] In the third pattern (step - down), the second period T2 - 3 (step - down) is such that the fourth switching element Q4 is in the on - state, the second switching element Q2, the fifth switching element Q5, and the eighth switching element Q8 are in the rectifying state, and the first switching element Q1, the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 are in the off - state (see FIGS. 9(b) and 11(b)).

[0053] In the third pattern (step - down), the control circuit 13 turns off the first switching element Q1 that is conducting in the forward direction from the first period T1 - 3 (step - down) and makes a transition to the second period T2 - 3 (step - down).

[0054] In the fourth pattern (step - down), the first period T1 - 4 (step - down) is such that the second switching element Q2 and the third switching element Q3 are in the on - state, the sixth switching element Q6 and the seventh switching element Q7 are in the rectifying state, and the first switching element Q1, the fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 are in the off - state (see FIGS. 9(c) and 11(c)).

[0055] In the fourth pattern (step - down), the second period T2 - 4 (step - down) is such that the second switching element Q2 is in the on - state, the fourth switching element Q4, the sixth switching element Q6, and the seventh switching element Q7 are in the rectifying state, and the first switching element Q1, the third switching element Q3, the fifth switching element Q5, and the eighth switching element Q8 are in the off - state (see FIGS. 9(d) and 11(d)).

[0056] In the fourth pattern (step-down), the control circuit 13 turns off the third switching element Q3 that is conducting in the forward direction from the first period T1-4 (step-down), and makes a transition to the second period T2-4 (step-down).

[0057] The control circuit 13 supplies different drive signals to the first switching element Q1 - the fourth switching element Q4 included in the first bridge circuit 11. Specifically, the control circuit 13 fixes the duty ratio of the drive signals supplied to two diagonal switching elements (for example, the first switching element Q1 and the fourth switching element Q4) of the first switching element Q1 - the fourth switching element Q4 at 50% regardless of the power transmission amount except for the dead time. The control circuit 13 varies the first phase difference φ1 between the two drive signals respectively supplied to the two diagonal switching elements within the range of 0 to 180° according to the power transmission amount except for the dead time. When the turn-on timings of the two drive signals respectively supplied to the two diagonal switching elements are simultaneous, the first phase difference φ1 is 0°.

[0058] The control circuit 13 varies the duty ratio of the drive signals supplied to the other two diagonal switching elements (for example, the second switching element Q2 and the third switching element Q3) of the first switching element Q1 - the fourth switching element Q4 within the range of 0 to 100% according to the power transmission amount except for the dead time. Specifically, the control circuit 13 decreases the duty ratio of the drive signal supplied to one of the other two diagonal switching elements by an amount corresponding to the first phase difference φ1 from 50% except for the dead time. At the same time, the control circuit 13 increases the duty ratio of the drive signal supplied to the other of the other two diagonal switching elements by an amount corresponding to the first phase difference φ1 from 50% except for the dead time.

[0059] As described above, in the first bridge circuit 11, the duty ratio and the phase shift amount are controlled using the same control amount according to the power transmission amount. As a result, as shown in FIG. 7, the turn-off that reverses the increase in the absolute value of the current can be distributed to the first switching element Q1 - the fourth switching element Q4, and the loss and heat generation can be distributed to the first switching element Q1 - the fourth switching element Q4. In Comparative Example 2 shown in FIG. 4 above, the loss and heat generation concentrate on the first switching element Q1 - the second switching element Q2.

[0060] The control circuit 13 synchronizes the turn-on timings of the drive signals supplied to two diagonal switching elements of the fifth switching element Q5 - the eighth switching element Q8. The control circuit 13 fixes the duty ratio of the drive signal supplied to one of the two diagonal switching elements (for example, the fifth switching element Q5 and the eighth switching element Q8) (for example, the fifth switching element Q5) to 50% regardless of the power transmission amount except for the dead time. The control circuit 13 changes the duty ratio of the drive signal supplied to the other of the two diagonal switching elements (for example, the eighth switching element Q8) within a range of 50% or less according to the power transmission amount except for the dead time. The control circuit 13 switches the setting of the duty ratio of the drive signals supplied to the two switching elements at the start timing of each switching cycle.

[0061] As described above, in the second bridge circuit 12, by fixing the phase and controlling the duty ratio according to the power transmission amount, the switching between the step-down mode and the buck-boost mode described later can be made smooth.

[0062] The control circuit 13 generates the first pattern (step-down) - the fourth pattern (step-down) twice in units of four switching cycles as shown in FIG. 7 above. In this case, the number of occurrences of the first pattern (step-down) - the fourth pattern (step-down) is the same. As a result, the switching losses generated on the primary side in the step-down operation can be evenly distributed to the first switching element Q1 - the fourth switching element Q4, and the heat generation can be dispersed.

[0063] Note that the control circuit 13 may control so that the number of occurrences of the first pattern (step-down) - the fourth pattern (step-down) is different. For example, the control circuit 13 may generate the first pattern (step-down) and the second pattern (step-down) four times each in three switching periods, and generate the third pattern (step-down) and the fourth pattern (step-down) two times each. Even in this case, a certain degree of loss dispersion is possible, and the peak of heat generation on the primary side can be suppressed compared to the control shown in Comparative Example 2.

[0064] FIGS. 12(a)-(d) are diagrams (part 1) for explaining a third example of the switching pattern and current flow in the step-down operation according to the embodiment of the power conversion device 1. FIGS. 13(a)-(d) are diagrams (part 2) for explaining a third example of the switching pattern and current flow in the step-down operation according to the embodiment of the power conversion device 1.

[0065] In the third example (step-down), the control circuit 13 controls the switching element to be in the rectifying state included in the second bridge circuit 12 to be in the on state for synchronous rectification in the first period T1 (step-down). The control circuit 13 controls the switching element to be in the rectifying state included in the second bridge circuit 12 to be in the off state for diode rectification in the second period T2 (step-down). Since the conduction loss by synchronous rectification is smaller than the forward loss Vf by diode rectification, the third example (step-down) can reduce the loss in the first period T1 (step-down) compared to the second example (step-down).

[0066] The switching pattern shown in FIG. 7 above shows a control combining the first example (step-down) and the second example (step-down). In principle, the control circuit 13 controls two diagonal switching elements to be in the on state for synchronous rectification in the second period T2 (step-down). When the absolute value of the transformer current IL becomes equal to or less than a predetermined threshold Ith in the second period T2 (step-down), the control circuit 13 turns off at least one of the two diagonal switching elements to switch to diode rectification. Note that the efficiency may decrease, but both switching elements may be turned off.

[0067] The threshold Ith is set to 0 A if the transformer current IL can be ideally measured and the delay of the control system and the drive system is ideally 0. In reality, considering the voltage measurement error and the delay of the drive signal supplied to the switching element, the threshold Ith is set to (0 ± margin) A. The value of the margin is set based on experiments and simulations by the designer so that the positive and negative of the transformer current IL do not reverse within the half cycle Tc / 2 of the switching period, and the synchronous rectification period T2s of the second period T2 (step-down) is maximized.

[0068] When the absolute value of the transformer current IL does not decrease to the threshold Ith within a predetermined time, the control circuit 13 skips the turn-off of at least one of the two diagonal switching elements included in the second bridge circuit 12. The predetermined time is set to half cycle Tc / 2 - dead time. That is, when the absolute value of the transformer current IL does not decrease to the threshold Ith within a predetermined time, the two diagonal switching elements included in the second bridge circuit 12 enter the dead time while remaining in the on state, and by entering the dead time, the control circuit 13 controls all of the fifth switching element Q5 - the eighth switching element Q8 to the off state. When the decrease rate of the absolute value of the transformer current IL is slow and there is no risk of power backflow, there is no need to switch from synchronous rectification to diode rectification. On the other hand, as shown in FIG. 7, when there is a risk of power backflow, switch from synchronous rectification to diode rectification. When power backflow occurs, a reactive current unrelated to power transmission flows, leading to an increase in loss. By switching to diode rectification, an increase in loss due to the reactive current can be prevented.

[0069] FIG. 14 is a diagram showing the switching pattern during reverse transmission of the first switching element Q1 to the eighth switching element Q8 and the transition example of the transformer current IL during the step-down operation according to the embodiment of the power conversion device 1. In the switching pattern of the first switching element Q1 to the eighth switching element Q8 shown in FIG. 7, an example of stepping down the voltage from the first DC section to the second DC section and transmitting power was described. In this regard, it is also possible to step down the voltage from the second DC section to the first DC section and transmit power. In this case, as shown in FIG. 14, the control circuit 13 may simply swap the drive signals supplied to the first switching element Q1 to the fourth switching element Q4 and the drive signals supplied to the fifth switching element Q5 to the eighth switching element Q8.

[0070] (Embodiment (Step-up)) FIG. 15 is a diagram showing the switching pattern of the first switching element Q1 to the eighth switching element Q8 and the transition example of the transformer current IL during the step-up operation according to the embodiment of the power conversion device 1. FIGS. 16(a)-(d) are diagrams for explaining the first example of the switching pattern and the current flow in the step-up operation according to the embodiment of the power conversion device 1 (Part 1). FIGS. 17(a)-(d) are diagrams for explaining the first example of the switching pattern and the current flow in the step-up operation according to the embodiment of the power conversion device 1 (Part 2). FIGS. 18(a)-(d) are diagrams for explaining the second example of the switching pattern and the current flow in the step-up operation according to the embodiment of the power conversion device 1 (Part 1). FIGS. 19(a)-(d) are diagrams for explaining the second example of the switching pattern and the current flow in the step-up operation according to the embodiment of the power conversion device 1 (Part 2).

[0071] This embodiment uses control that combines the PWM method and the phase shift method. When the control circuit 13 boosts the voltage from the first DC section to the second DC section and transmits power, it controls to include a first period T1 (boost) and a second period T2 (boost). The first period T1 (boost) is a period in which the first bridge circuit 11 conducts the first DC section and the primary winding n1 of the isolation transformer TR1, and both ends of the secondary winding n2 of the isolation transformer TR1 are in a storage state of being short-circuited within the second bridge circuit 12. The second period T2 (boost) is a period in which the first bridge circuit 11 conducts the first DC section and the primary winding n1 of the isolation transformer TR1, and the second bridge circuit 12 conducts the secondary winding n2 of the isolation transformer TR1 and the second DC section, which is a transmission state.

[0072] The control circuit 13 generates the first period T1 (boost) and the second period T2 (boost) in the first pattern - the fourth pattern (boost). The first example of the switching pattern and the current flow in the boost operation shown in FIGS. 16(a)-(d) and FIGS. 17(a)-(d) is an example of synchronous rectification by controlling the switching element to be controlled in the rectification state in the second bridge circuit 12 to be in the on state. The second example of the switching pattern and the current flow in the boost operation shown in FIGS. 18(a)-(d) and FIGS. 19(a)-(d) is an example of diode rectification by controlling the switching element to be controlled in the rectification state in the second bridge circuit 12 to be in the off state.

[0073] In the first period T1-1 (boost) in the first pattern (boost), the first switching element Q1, the fourth switching element Q4, and the seventh switching element Q7 are in the on state, the fifth switching element Q5 is in the rectification state, and the second switching element Q2, the third switching element Q3, the sixth switching element Q6, and the eighth switching element Q8 are in the off state (see FIGS. 16(a) and 18(a)).

[0074] In the first pattern (boost), the second period T2-1 (boost) is a state where the first switching element Q1 and the fourth switching element Q4 are on, the fifth switching element Q5 and the eighth switching element Q8 are in the rectifying state, and the second switching element Q2, the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 are off (see FIGS. 16(b) and 18(b)).

[0075] In the first pattern (boost), the control circuit 13 turns off the seventh switching element Q7 that is conducting in the forward direction from the first period T1-1 (boost), and makes a transition to the second period T2-1 (boost).

[0076] In the second pattern (boost), the first period T1-2 (boost) is a state where the second switching element Q2, the third switching element Q3, and the fifth switching element Q5 are on, the seventh switching element Q7 is in the rectifying state, and the first switching element Q1, the fourth switching element Q4, the sixth switching element Q6, and the eighth switching element Q8 are off (see FIGS. 16(c) and 18(c)).

[0077] In the second pattern (boost), the second period T2-2 (boost) is a state where the second switching element Q2 and the third switching element Q3 are on, the sixth switching element Q6 and the seventh switching element Q7 are in the rectifying state, and the first switching element Q1, the fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 are off (see FIGS. 16(d) and 18(d)).

[0078] In the second pattern (boost), the control circuit 13 turns off the fifth switching element Q5 that is conducting in the forward direction from the first period T1-2 (boost), and makes a transition to the second period T2-2 (boost).

[0079] In the first period T1-3 (boosting) of the third pattern (boosting), the first switching element Q1, the fourth switching element Q4, and the sixth switching element Q6 are in the on state, the eighth switching element Q8 is in the rectifying state, and the second switching element Q2, the third switching element Q3, the fifth switching element Q5, and the seventh switching element Q7 are in the off state (see FIGS. 17(a) and 19(a)).

[0080] In the second period T2-3 (boosting) of the third pattern (boosting), the first switching element Q1 and the fourth switching element Q4 are in the on state, the fifth switching element Q5 and the eighth switching element Q8 are in the rectifying state, and the second switching element Q2, the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 are in the off state (see FIGS. 17(b) and 19(b)).

[0081] In the third pattern (boosting), the control circuit 13 turns off the sixth switching element Q6 that is conducting in the forward direction from the first period T1-3 (boosting) and makes a transition to the second period T2-3 (boosting).

[0082] In the first period T1-4 (boosting) of the fourth pattern (boosting), the second switching element Q2, the third switching element Q3, and the eighth switching element Q8 are in the on state, the sixth switching element Q6 is in the rectifying state, and the first switching element Q1, the fourth switching element Q4, the fifth switching element Q5, and the seventh switching element Q7 are in the off state (see FIGS. 17(c) and 19(c)).

[0083] In the second period T2-4 (boosting) of the fourth pattern (boosting), the second switching element Q2 and the third switching element Q3 are in the on state, the sixth switching element Q6 and the seventh switching element Q7 are in the rectifying state, and the first switching element Q1, the fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 are in the off state (see FIGS. 17(d) and 19(d)).

[0084] In the fourth pattern (boost), the control circuit 13 turns off the eighth switching element Q8 that is conducting in the forward direction from the first period T1-4 (boost), and makes a transition to the second period T2-4 (boost).

[0085] The control circuit 13 supplies the same drive signals with synchronized on / off timings to one pair of diagonal first switching element Q1 and fourth switching element Q4 included in the first bridge circuit 11, and supplies the same drive signals with synchronized on / off timings to the other second switching element Q2 and third switching element Q3.

[0086] The control circuit 13 fixes the duty ratio of the drive signals supplied to the first switching element Q1 and the fourth switching element Q4 at 50% regardless of the power transmission amount except for the dead time. The control circuit 13 fixes the phase difference between the phase of the drive signals supplied to the first switching element Q1 and the fourth switching element Q4 and the phase of the drive signals supplied to the second switching element Q2 and the third switching element Q3 at 180° regardless of the power transmission amount.

[0087] In this way, in the first bridge circuit 12, by fixing the duty ratio and phase of the drive signals regardless of the power transmission amount, the loss can be minimized and the power transmission efficiency can be maximized.

[0088] The control circuit 13 supplies different drive signals to the fifth switching element Q5 - eighth switching element Q8 included in the second bridge circuit 12. Specifically, the control circuit 13 varies the duty ratio of the drive signals supplied to two switching elements of the first diagonal of the fifth switching element Q5 - eighth switching element Q8 (for example, the fifth switching element Q5 and the eighth switching element Q8) in the range of 0 to 100% according to the power transmission amount except for the dead time.

[0089] The control circuit 13 varies the duty ratio of the drive signals supplied to two diagonal switching elements (e.g., the sixth switching element Q6 and the seventh switching element Q7) out of the fifth to eighth switching elements Q5 to Q8 in the range of 0 to 50% according to the power transmission amount except for the dead time. The control circuit 13 controls the second phase difference φ2 between the turn-on timing of the drive signals supplied to the two diagonal switching elements and the turn-on timing of the first to fourth switching elements Q1 to Q4 according to the power transmission amount except for the dead time. More precisely, among the two diagonal switching elements, the switching element with a fixed duty ratio of 50% is phase-shifted, and the switching element that varies in the range of 0 to 50% is not phase-shifted.

[0090] The control circuit 13 decreases the duty ratio of the drive signal supplied to one of the two switching elements on the first diagonal by an amount corresponding to the second phase difference φ2 from 50% except for the dead time, and increases the duty ratio of the drive signal supplied to the other of the two switching elements by an amount corresponding to the second phase difference φ2 from 50% except for the dead time. The control circuit 13 alternately switches the switching element that decreases from 50% and the switching element that increases from 50% among the two switching elements on the first diagonal. The control circuit 13 fixes the duty ratio of the drive signal supplied to one of the two switching elements on the second diagonal at 50% except for the dead time, and decreases the duty ratio of the drive signal supplied to one of the two switching elements by an amount corresponding to the second phase difference φ2 from 50% except for the dead time. The control circuit 13 alternately switches the switching element with a fixed duty ratio of 50% and the switching element that decreases from 50% among the two switching elements on the second diagonal.

[0091] Thus, in the second bridge circuit 12, the duty ratio and the phase shift amount are simultaneously controlled according to the power transmission amount. As a result, as shown in FIG. 15, the turn-off that reverses the increase in the absolute value of the current can be distributed to the fifth switching element Q5 - the eighth switching element Q8, and the loss and heat generation can be distributed to the fifth switching element Q5 - the eighth switching element Q8. In Comparative Example 2 shown in FIG. 6 above, the loss and heat generation concentrate on the fifth switching element Q5 - the sixth switching element Q6.

[0092] FIG. 20 is a diagram showing a specific example 1 in which the switching patterns of the first switching element Q1 - the eighth switching element Q8 during the boosting operation according to the embodiment of the power conversion device 1 are periodically generated. In the switching pattern shown in FIG. 20, the control circuit 13 generates the first pattern (boosting) - the fourth pattern (boosting) twice each in units of four switching cycles. In the example shown in FIG. 20, the first pattern (boosting) → the second pattern (boosting) → the third pattern (boosting) → the second pattern (boosting) → the third pattern (boosting) → the fourth pattern (boosting) → the first pattern (boosting) → the fourth pattern (boosting) are generated in this order. In Specific Example 1, the number of occurrences of the first pattern (boosting) - the fourth pattern (boosting) is the same. As a result, the switching losses generated on the secondary side during the boosting operation can be evenly allocated to the fifth switching element Q5 - the eighth switching element Q8, and heat generation can be dispersed.

[0093] FIG. 21 is a diagram showing a specific example 2 in which the switching patterns of the first switching element Q1 - the eighth switching element Q8 during the boosting operation according to the embodiment of the power conversion device 1 are irregularly generated. In the switching pattern shown in FIG. 21, the control circuit 13 controls the switching pattern so that the number of occurrences of the first pattern (step-down) - the fourth pattern (step-down) is different. In the example shown in FIG. 21, the first pattern (boosting) is generated twice, the second pattern (boosting) is generated three times, the third pattern (boosting) is generated twice, and the fourth pattern (boosting) is generated once in units of four switching cycles. Also in Specific Example 2, a certain degree of loss dispersion is possible, and the peak of heat generation on the secondary side can be suppressed compared to the control shown in Comparative Example 2.

[0094] Figures 22(a)-(d) are diagrams (Part 1) for explaining a third example of the switching pattern and current flow in the boosting operation according to an embodiment of the power conversion device 1. Figures 23(a)-(d) are diagrams (Part 2) for explaining a third example of the switching pattern and current flow in the boosting operation according to an embodiment of the power conversion device 1.

[0095] In the third example (boosting), the control circuit 13 controls the switching element to be in the rectifying state and in the on state for synchronous rectification in the first period T1 (boosting) in the second bridge circuit 12. The control circuit 13 controls the switching element to be in the rectifying state and in the off state for diode rectification in the second period T2 (boosting) in the second bridge circuit 12. Since the conduction loss due to synchronous rectification is smaller than the forward loss Vf due to diode rectification, the third example (boosting) can reduce the loss in the first period T1 (boosting) compared with the second example (boosting).

[0096] The switching pattern shown in FIG. 15 above shows the control combining the first example (boosting) and the second example (boosting). The control circuit 13 basically controls two diagonal switching elements to be in the rectifying state and in the on state for synchronous rectification in the second period T2 (boosting) in the second bridge circuit 12. When the absolute value of the transformer current IL becomes equal to or less than the threshold Ith in the second period T2 (boosting), the control circuit 13 turns off at least one of the two diagonal switching elements to switch to diode rectification. Note that the efficiency decreases, but both switching elements may be turned off.

[0097] When the absolute value of the transformer current IL does not decrease to the threshold Ith within a predetermined time, the control circuit 13 skips turning off at least one of the two diagonal switching elements included in the second bridge circuit 12. The predetermined time is set to half a cycle Tc / 2 - dead time. That is, when the absolute value of the transformer current IL does not decrease to the threshold Ith within a predetermined time, the two diagonal switching elements included in the second bridge circuit 12 enter the dead time while remaining in the on state, and by entering the dead time, the control circuit 13 controls all of the fifth switching element Q5 - the eighth switching element Q8 to the off state. When the rate of decrease in the absolute value of the transformer current IL is slow and there is no risk of power backflow, there is no need to switch from synchronous rectification to diode rectification. On the other hand, as shown in FIG. 15, when there is a risk of power backflow, switch from synchronous rectification to diode rectification. When power backflow occurs, reactive current that has nothing to do with power transmission flows, leading to an increase in losses. By switching to diode rectification, an increase in losses due to reactive current can be prevented.

[0098] FIG. 24 is a diagram showing a switching pattern during reverse transmission of the first switching element Q1 - the eighth switching element Q8 and an example of the transition of the transformer current IL during the step-up operation according to an embodiment of the power conversion device 1. In the switching pattern of the first switching element Q1 - the eighth switching element Q8 shown in FIG. 15, an example of stepping up from the first DC section to the second DC section and transmitting power was described. In this regard, it is also possible to step up from the second DC section to the first DC section and transmit power. In this case, as shown in FIG. 24, the control circuit 13 may simply swap the drive signals supplied to the first switching element Q1 - the fourth switching element Q4 and the drive signals supplied to the fifth switching element Q5 - the eighth switching element Q8.

[0099] (Embodiment (Step-up / down)) FIG. 25 is a diagram showing the switching patterns of the first switching element Q1 to the eighth switching element Q8 and the transition example of the transformer current IL during the step-up / step-down operation according to an embodiment of the power conversion device 1. The step-up / step-down operation is an operation inserted during the period when switching from the step-down operation to the step-up operation or from the step-up operation to the step-down operation. The control circuit 13 controls to include a first period T1 (step-up), a second period T2 (step-up), and a third period T3 when stepping up / down the power from the first DC section to the second DC section and transmitting the power during the period of switching between step-up and step-down. The third period T3 is a period in which both ends of the primary winding n1 of the isolation transformer TR1 are short-circuited within the first bridge circuit 11, and the second bridge circuit 12 is in a commutation state in which it conducts the secondary winding n2 of the isolation transformer TR1 and the second DC section.

[0100] In the step-up / step-down operation, as shown in FIG. 25, the control circuit 13 provides a phase difference between the drive signal supplied to the first switching element Q1 and the drive signal supplied to the fourth switching element Q4, and provides a phase difference between the drive signal supplied to the second switching element Q2 and the drive signal supplied to the third switching element Q3. The period corresponding to this phase difference becomes the third period T3. In the step-up / step-down operation, before the end of the second period T2 (the first DC section and the isolation transformer TR1 are in a conducting state), the third period T3 (the first DC section and the isolation transformer TR1 are in a cut-off state) is inserted, so that the energy supplied from the first DC section to the second DC section is suppressed.

[0101] FIG. 26(a) is a diagram schematically showing the relationship between the duty ratio and the transmission power when the buck-boost mode is not provided. FIG. 26(b) is a diagram schematically showing the relationship between the duty ratio and the transmission power when the buck-boost mode is provided. As shown in FIG. 26(a), when the buck-boost mode is not provided, at the time of switching between the buck mode and the boost mode, one switching point where the slope of the transmission power (which may be considered as the transmission current) changes occurs. As shown in FIG. 26(b), when the buck-boost mode is provided, two switching points occur at the time of switching between the buck mode and the buck-boost mode and at the time of switching between the buck-boost mode and the boost mode. When the buck-boost mode is provided, the number of switching points becomes two, but the change in the slope of each switching point becomes gentler than the slope of the switching point when the buck-boost mode is not provided. By sandwiching the buck-boost mode between the boost mode and the buck mode as shown in FIG. 26(b), the circuit gain can be smoothly changed and the current oscillation can be suppressed to a small level.

[0102] FIG. 27 is a diagram showing a configuration example of the control circuit 13. The control circuit 13 includes detectors 131a-c, a subtraction unit 132, a controller 133, a synchronous rectification period calculation unit 135, and a PWM generation unit 136. The detector 131a includes an A / D converter. The A / D converter converts the analog output current value Io detected by the current sensor 23 into a digital output current value at a predetermined sampling rate. The detector 131b includes an A / D converter. The A / D converter converts the analog input voltage value V1 detected by the first voltage sensor 21 into a digital input voltage value at a predetermined sampling rate. The detector 131c includes an A / D converter. The A / D converter converts the analog output voltage value V2 detected by the second voltage sensor 22 into a digital input voltage value at a predetermined sampling rate. The A / D converter may be time-division shared by the detectors 131a-c.

[0103] The subtraction unit 132 calculates a deviation err between the current command value Iref that should be the target value and the output current value input from the detector 131. The controller 133 PI-controls the deviation err using a predetermined control gain to calculate a control operation amount duty. The controller 133 outputs the calculated control operation amount duty to the synchronous rectification period calculation unit 135 and the PWM generation unit 136.

[0104] During the synchronous rectification period calculation unit 135 calculates the synchronous rectification period in the second bridge circuit 12 based on the differential voltage (V1 - V2) or (V2 - V1) etc. between the input voltage V1 detected by the first voltage sensor 21 and the output voltage V2 detected by the second voltage sensor 22.

[0105] Figures 28(a)-(b) are diagrams showing an example of variables and constants of the theoretical formula for calculating the synchronous rectification period in a graph. Hereinafter, with reference to Figure 28(a), the calculation method of the transformer current IL in the step-down mode and the boost mode, and the synchronous rectification period T2s of the second period T2 will be described.

[0106] The synchronous rectification period calculation unit 135 calculates the transformer current IL in the first period T1 (step-down) by the following (Equation 1). V1 - V2 = L·(IL / T1) IL = (V1 - V2)·T1 / L ···(Equation 1) L is the inductance between the first bridge circuit 11 and the second bridge circuit 12 (in this embodiment, L = L1 + L2). T1 is the time of the first period T1 (step-down) based on feedback control. In this embodiment, it is determined by the control operation amount duty calculated by the controller 133.

[0107] The synchronous rectification period calculation unit 135 calculates the transformer current (Ith - IL) of the synchronous rectification period T2s in the second period T2 (step-down) by the following (Equation 2). 0 - V2 = L·((Ith - IL) / T2s) Ith - IL = -V2·T2s / L ···(Equation 2)

[0108] The synchronous rectification period calculation unit 135 calculates the synchronous rectification period T2s in the second period T2 (step-down) by the following (Equation 3). T2s = ((V1 - V2)·T1 / V2) - ((L / V2)·Ith) ···(Equation 3) Since the inductance L and the threshold value Ith are constants, the synchronous rectification period T2s of the second period T2 (step - down) can be calculated by detecting the input voltage V1 and the output voltage V2.

[0109] The synchronous rectification period calculation unit 135 calculates the transformer current IL of the first period T1 (step - up) by the following (Equation 4). V1 = L·(IL / T1) IL = V1·T1 / L ···(Equation 4) L is the inductance between the first bridge circuit 11 and the second bridge circuit 12 (in this embodiment, L = L1 + L2). T1 is the time of the first period T1 (step - up) based on feedback control. In this embodiment, it is determined by the control operation amount duty calculated by the controller 133.

[0110] The synchronous rectification period calculation unit 135 calculates the transformer current (Ith - IL) of the synchronous rectification period T2s of the second period T2 (step - up) by the following (Equation 5). V1 - V2 = L·((Ith - IL) / T2s) Ith - IL=(V1 - V2)·T2s / L ···(Equation 5)

[0111] The synchronous rectification period calculation unit 135 calculates the synchronous rectification period T2s of the second period T2 (step - up) by the following (Equation 6). T2s=(V1·T1 / (V2 - V1))-(L / (V2 - V1)·Ith) ···(Equation 6) Since the inductance L and the threshold value Ith are constants, the synchronous rectification period T2s of the second period T2 (step - up) can be calculated by detecting the input voltage V1 and the output voltage V2.

[0112] In the step-up / step-down mode, when the absolute value of the transformer current IL becomes equal to or less than the threshold value Ith during the second period T2 (step-up), the synchronous rectification period T2s of the second period T2 (step-up) can be calculated according to the above (Equation 4)-(Equation 6). In the step-up / step-down mode, when the absolute value of the transformer current IL does not become equal to or less than the threshold value Ith during the second period T2 (step-up), but becomes equal to or less than the threshold value Ith during the third period T3, with reference to FIG. 28(b), the calculation method of the transformer current IL and the synchronous rectification period T3s of the third period T3 will be described.

[0113] The synchronous rectification period calculation unit 135 calculates the transformer current IL1 of the first period T1 (step-up) in the step-up / step-down mode according to the following (Equation 7). V1 = L·(IL1 / T1) IL1 = V1·T1 / L ···(Equation 7) L is the inductance between the first bridge circuit 11 and the second bridge circuit 12 (in this embodiment, L = L1 + L2). T1 is the time of the first period T1 (step-up) based on feedback control. In this embodiment, it is determined by the control operation amount duty calculated by the controller 133.

[0114] When the absolute value of the transformer current IL does not become equal to or less than the threshold value Ith during the second period T2 (step-up) in the step-up / step-down mode, the synchronous rectification period calculation unit 135 calculates the transformer current IL2 of the second period T2 (step-up) according to the following (Equation 8). V1 - V2 = L·((IL2 - IL1) / T2) IL2 - IL1 = (V1 - V2)·T2 / L ···(Equation 8)

[0115] When the absolute value of the transformer current IL becomes equal to or less than the threshold value Ith during the third period T3 in the step-up / step-down mode, the synchronous rectification period calculation unit 135 calculates the transformer current (Ith - IL2) of the synchronous rectification period T3s of the third period T3 according to the following (Equation 9). -V2 = L·((Ith - IL2) / T3s) Ith - IL2 = -V2·T2 / L ···(Equation 9)

[0116] During the synchronous rectification period calculation unit 135 calculates the synchronous rectification period T3s of the third period T3 when the absolute value of the transformer current IL becomes equal to or less than the threshold Ith in the step-down / step-up mode according to the following (Equation 10). T3s = ((V1·T1+(V1-V2)·T2) / V2)-(L / V2·Ith) ···(Equation 10) Since the inductance L and the threshold Ith are constants, the synchronous rectification period T3s of the third period T3 in the step-down / step-up mode can be calculated by detecting the input voltage V1 and the output voltage V2.

[0117] The PWM generation unit 136 generates a PWM signal for driving the first switching element Q1 - the eighth switching element Q8 based on the control operation amount duty calculated by the controller 133, the synchronous rectification period T2s of the second period T2 or the synchronous rectification period T3s of the third period T3 calculated by the synchronous rectification period calculation unit 135.

[0118] As described above, according to this embodiment, the heat generation of a plurality of switching elements included in the DAB converter can be equalized. In particular, in the step-down mode, the losses and heat generation of the first switching element Q1 - the fourth switching element Q4 can be made uniform, and in the step-up mode, the losses and heat generation of the fifth switching element Q5 - the eighth switching element Q8 can be made uniform. As a result, the peak of the losses and heat generation of the switching element where the maximum loss occurs can be reduced, and miniaturization and cost reduction of a member (for example, a heat sink) for cooling the switching element can be realized. In addition, the variation in the life of the switching elements can be reduced, and product management becomes easy.

[0119] Also, in the second period T2 (step-down), the second period T2 (step-up), and the third period T3, a highly efficient DAB converter can be realized by synchronously rectifying the second bridge circuit 12. At this time, when the transformer current IL drops to near 0 A, by switching the second bridge circuit 12 from synchronous rectification to diode rectification, it is possible to prevent a decrease in efficiency due to the generation of reactive current. In particular, this embodiment is effective for a DAB converter using a SiC-MOSFET with a large loss in parasitic diodes.

[0120] As described above, the present disclosure has been explained based on the embodiments. It is understood by those skilled in the art that the embodiments are examples, and various modifications are possible for each combination of their components and each processing process, and such modifications are also within the scope of the present disclosure.

[0121] In the above embodiment, an example has been described in which the transformer current IL is calculated using the above (Equation 1)-(Equation 10) without installing current sensors between the first bridge circuit 11 and the isolation transformer TR1 and between the isolation transformer TR1 and the second bridge circuit 12. In this regard, a current sensor may be installed between the first bridge circuit 11 and the isolation transformer TR1 or between the isolation transformer TR1 and the second bridge circuit 12, and the current measured by the current sensor may be used as the transformer current IL and compared with the threshold Ith.

[0122] Figures 29(a)-(d) are diagrams (Part 1) for explaining still another example of the switching pattern and current flow in the step-down operation according to the embodiment of the power conversion device 1. Figures 30(a)-(d) are diagrams (Part 2) for explaining still another example of the switching pattern and current flow in the step-down operation according to the embodiment of the power conversion device 1.

[0123] In this example, in the second period T2-1 (step-down) of the first pattern (step-down), the first switching element Q1, the third switching element Q3, the fifth switching element Q5, and the eighth switching element Q8 are in the on state, and the second switching element Q2, the fourth switching element Q4, the sixth switching element Q6, and the seventh switching element Q7 are in the off state (see Fig. 29(b)).

[0124] In the second period T2-2 (step-down) of the second pattern (step-down), the first switching element Q1, the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 are in the on state, and the second switching element Q2, the fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 are in the off state (see Fig. 29(d)).

[0125] In the second period T2-3 (step-down) of the third pattern (step-down), the second switching element Q2, the fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 are in the on state, and the first switching element Q1, the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 are in the off state (see Fig. 30(b)).

[0126] In the second period T2-4 (step-down) of the fourth pattern (step-down), the second switching element Q2, the fourth switching element Q4, the sixth switching element Q6, and the seventh switching element Q7 are in the on state, and the first switching element Q1, the third switching element Q3, the fifth switching element Q5, and the eighth switching element Q8 are in the off state (see Fig. 30(d)).

[0127] In the second period T2, the loss can be minimized by performing synchronous rectification on both the primary side and the secondary side.

[0128] Note that the embodiments may be specified by the following items.

[0129] [Item 1] A first leg in which a first switching element (Q1) and a second switching element (Q2) are connected in series, and a second leg in which a third switching element (Q3) and a fourth switching element (Q4) are connected in series, wherein the first leg and the second leg are connected in parallel to a first DC part (E1, Ca), a first bridge circuit (11); A third leg in which a fifth switching element (Q5) and a sixth switching element (Q6) are connected in series, and a fourth leg in which a seventh switching element (Q7) and an eighth switching element (Q8) are connected in series, wherein the third leg and the fourth leg are connected in parallel to a second DC part (E2, Cb), a second bridge circuit (12); An isolation transformer (TR1) connected between the first bridge circuit (11) and the second bridge circuit (12); A control circuit (13) for controlling the first switching element (Q1) - the eighth switching element (Q8). The control circuit (13) When stepping down the voltage from the first DC part (E1, Ca) to the second DC part (E2, Cb) to transmit power, it controls to include a first period in which the first DC part (E1, Ca) and the primary winding (n1) of the isolation transformer (TR1) are conducting, and a second period in which both ends of the primary winding (n1) of the isolation transformer (TR1) are short-circuited within the first bridge circuit (11). From the first period, A first pattern in which the fourth switching element (Q4) conducting in the forward direction turns off and transitions to the second period; A second pattern in which the second switching element (Q2) conducting in the forward direction turns off and transitions to the second period; A third pattern in which the first switching element (Q1) conducting in the forward direction turns off and transitions to the second period; Generates a fourth pattern in which the third switching element (Q3) conducting in the forward direction turns off and transitions to the second period. A power conversion device (1). According to this, the switching losses generated in the first bridge circuit (11) during the step-down operation can be equalized and allocated to the first switching element (Q1) - the fourth switching element (Q4), and heat generation can be dispersed. [Item 2] The control circuit (13) generates the first pattern - the fourth pattern two times each in units of four switching cycles. The power conversion device (1) according to Item 1. According to this, the switching losses generated in the first bridge circuit (11) can be evenly allocated to the first switching element (Q1) - the fourth switching element (Q4). [Item 3] The control circuit (13) controls so that the number of occurrences of the first pattern - the fourth pattern is different. The power conversion device (1) according to Item 1. According to this, the switching losses generated in the first bridge circuit (11) can be equalized and allocated to the first switching element (Q1) - the fourth switching element (Q4). [Item 4] In the first period in the first pattern, the first switching element (Q1) and the fourth switching element (Q4) are in the on state, and the fifth switching element (Q5) and the eighth switching element (Q8) are in the rectifying state. In the second period in the first pattern, the first switching element (Q1) is in the on state, and the third switching element (Q3), the fifth switching element (Q5), and the eighth switching element (Q8) are in the rectifying state. In the first period in the second pattern, the second switching element (Q2) and the third switching element (Q3) are in the on state, and the sixth switching element (Q6) and the seventh switching element (Q7) are in the rectifying state. In the second period in the second pattern, the third switching element (Q3) is in the on state, and the first switching element (Q1), the sixth switching element (Q6), and the seventh switching element (Q7) are in the rectifying state. In the first period in the third pattern, the first switching element (Q1) and the fourth switching element (Q4) are in the on state, and the fifth switching element (Q5) and the eighth switching element (Q8) are in the rectifying state. In the second period in the third pattern, the fourth switching element (Q4) is in the on state, and the second switching element (Q2), the fifth switching element (Q5), and the eighth switching element (Q8) are in the rectifying state. In the first period in the fourth pattern, the second switching element (Q2) and the third switching element (Q3) are in the on state, and the sixth switching element (Q6) and the seventh switching element (Q7) are in the rectifying state. In the second period in the fourth pattern, the second switching element (Q2) is in the on state, and the fourth switching element (Q4), the sixth switching element (Q6), and the seventh switching element (Q7) are in the rectifying state. The power conversion device (1) according to item 1. According to this, the switching losses generated in the first bridge circuit (11) can be equalized and allocated to the first switching element (Q1) - the fourth switching element (Q4). [Item 5] The control circuit (13) fixes the duty ratio of the drive signals supplied to two diagonal switching elements out of the first switching element (Q1) - the fourth switching element (Q4) at 50%, and varies the phase difference between the two drive signals supplied to the two switching elements according to the power transmission amount. varies the duty ratio of the drive signals supplied to the other two diagonal switching elements of the first switching element (Q1) - the fourth switching element (Q4) according to the power transmission amount. The power conversion device (1) according to item 4. According to this, according to the power transmission amount, by simultaneously controlling the duty ratio and the phase difference of the first switching element (Q1) to the fourth switching element (Q4), it is possible to achieve loss dispersion and heat dispersion of the first bridge circuit (11). [Item 6] The control circuit (13) decreases the duty ratio of the drive signal supplied to one of the two diagonal switching elements of the other diagonal of the first switching element (Q1) to the fourth switching element (Q4) from 50% by an amount corresponding to the phase difference, and increases the duty ratio of the drive signal supplied to the other of the two switching elements from 50% by an amount corresponding to the phase difference. The power conversion device (1) according to Item 5. According to this, according to the power transmission amount, by simultaneously controlling the duty ratio and the phase difference of the first switching element (Q1) to the fourth switching element (Q4) using the same control amount, it is possible to achieve loss dispersion and heat dispersion of the first bridge circuit (11). [Item 7] The control circuit (13) synchronizes the turn-on timings of the drive signals supplied to the two diagonal switching elements of the fifth switching element (Q5) to the eighth switching element (Q8), fixes the duty ratio of the drive signal supplied to one of the two diagonal switching elements of the fifth switching element (Q5) to the eighth switching element (Q8) at 50%, and varies the duty ratio of the drive signal supplied to the other of the two switching elements in the range of 50% or less according to the power transmission amount, and switches the setting of the duty ratio of the drive signal supplied to the two switching elements at the start timing of the switching period. The power conversion device (1) according to Item 4. According to this, while fixing the phases of the fifth switching element (Q5) to the eighth switching element (Q8), by controlling the duty ratio of the fifth switching element (Q5) to the eighth switching element (Q8) according to the power transmission amount, it is possible to achieve loss dispersion and heat dispersion of the second bridge circuit (12). [Item 8] The control circuit (13) In the first period, controls the switching element to be in the rectifying state included in the second bridge circuit (12) to be in the on state, The power conversion device (1) according to item 4. According to this, by increasing the synchronous rectification period, the forward loss due to the diode, which is larger than the conduction loss due to the synchronous rectification, can be reduced, and the overall loss can be reduced. [Item 9] The control circuit (13) In the second period, controls two diagonal switching elements to be in the rectifying state included in the second bridge circuit (12) to be in the on state, and when the absolute value of the current flowing through the isolation transformer (TR1) becomes equal to or less than a predetermined threshold value, turns off at least one of the two diagonal switching elements, The power conversion device (1) according to item 4. According to this, while suppressing the reactive current, the synchronous rectification period can be increased, and the overall loss can be reduced. [Item 10] The control circuit (13) calculates, using a derived formula, the period T2s during which two diagonal switching elements included in the second bridge circuit (12) are controlled to be in the on state in the second period, The derived formula is T2s = ((V1 - V2)·T1 / V2) - ((L / V2)·Ith) V1 is the voltage of the first DC section (E1, Ca), V2 is the voltage of the second DC section (E2, Cb), T1 is the time of the first period based on feedback control, L is the inductance between the first bridge circuit (11) and the second bridge circuit (12), Ith is the predetermined threshold value, is The power conversion device (1) according to item 9. According to this, the optimal timing for switching from synchronous rectification to diode rectification can be derived. [Item 11] The control circuit (13) When stepping down the voltage and transmitting power from the second DC section (E2, Cb) to the first DC section (E1, Ca), Swap the drive signals supplied to the first switching element (Q1) - the fourth switching element (Q4) and the drive signals supplied to the fifth switching element (Q5) - the eighth switching element (Q8). The power conversion device (1) according to item 1. According to this, bidirectional operation becomes possible.

Explanation of symbols

[0130] E1 First DC power supply, E2 Second DC power supply, 1 Power conversion device, 11 First bridge circuit, 12 Second bridge circuit, 13 Control circuit, 21 First voltage sensor, 22 Second voltage sensor, 23 Current sensor, 131 Detector, 132 Subtraction unit, 133 Controller, 135 Synchronous rectification period calculation unit, 136 PWM generation unit, Q1 - Q8 Switching elements, D1 - D8 Diodes, C1 - C8 Capacitors, L1 First inductance, L2 Second inductance, TR1 Isolation transformer, n1 Primary winding, n2 Secondary winding, Ca Primary side capacitor, Cb Secondary side capacitor.

Claims

1. A first leg in which a first switching element and a second switching element are connected in series, and a second leg in which a third switching element and a fourth switching element are connected in series, wherein the first leg and the second leg are connected in parallel to a first DC section; a first bridge circuit; A third leg in which a fifth switching element and a sixth switching element are connected in series, and a fourth leg in which a seventh switching element and an eighth switching element are connected in series, wherein the third leg and the fourth leg are connected in parallel to a second DC section; a second bridge circuit; An isolation transformer connected between the first bridge circuit and the second bridge circuit; A control circuit for controlling the first switching element - the eighth switching element; and The control circuit is configured to: When stepping down the voltage from the first DC section to the second DC section to transmit power, control to include a first period in which the first DC section and the primary winding of the isolation transformer are conducting, and a second period in which both ends of the primary winding of the isolation transformer are short-circuited within the first bridge circuit; From the first period, Generate a first pattern in which the fourth switching element that is conducting in the forward direction turns off and transitions to the second period; Generate a second pattern in which the second switching element that is conducting in the forward direction turns off and transitions to the second period; Generate a third pattern in which the first switching element that is conducting in the forward direction turns off and transitions to the second period; Generate a fourth pattern in which the third switching element that is conducting in the forward direction turns off and transitions to the second period; A power conversion device.

2. The control circuit is configured to: Generate each of the first pattern - the fourth pattern twice in units of four switching cycles; The power conversion device according to Claim 1.

3. The control circuit is configured to: Control such that the number of occurrences of the first pattern - the fourth pattern is different; The power conversion device according to Claim 1.

4. In the first pattern, in the first period, the first switching element and the fourth switching element are in the on state, and the fifth switching element and the eighth switching element are in the rectifying state; In the first pattern, in the second period, the first switching element is in the on state, and the third switching element, the fifth switching element, and the eighth switching element are in the rectifying state; In the first period of the second pattern, the second switching element and the third switching element are in the on state, and the sixth switching element and the seventh switching element are in the rectifying state. In the second period of the second pattern, the third switching element is in the on state, and the first switching element, the sixth switching element, and the seventh switching element are in the rectifying state. In the first period of the third pattern, the first switching element and the fourth switching element are in the on state, and the fifth switching element and the eighth switching element are in the rectifying state. In the second period of the third pattern, the fourth switching element is in the on state, and the second switching element, the fifth switching element, and the eighth switching element are in the rectifying state. In the first period of the fourth pattern, the second switching element and the third switching element are in the on state, and the sixth switching element and the seventh switching element are in the rectifying state. In the second period of the fourth pattern, the second switching element is in the on state, and the fourth switching element, the sixth switching element, and the seventh switching element are in the rectifying state. The power conversion device according to claim 1.

5. The control circuit fixes the duty ratio of the driving signals supplied to one pair of diagonal two switching elements of the first switching element - the fourth switching element at 50%, and changes the phase difference between the two driving signals respectively supplied to the two switching elements according to the power transmission amount; changes the duty ratio of the driving signals supplied to the other pair of diagonal two switching elements of the first switching element - the fourth switching element according to the power transmission amount. The power conversion device according to claim 4.

6. The control circuit decreases the duty ratio of the driving signal supplied to one of the other pair of diagonal two switching elements of the first switching element - the fourth switching element by an amount corresponding to the phase difference from 50%, and increases the duty ratio of the driving signal supplied to the other of the two switching elements by an amount corresponding to the phase difference from 50%. The power conversion device according to claim 5.

7. The control circuit synchronizes the turn-on timings of the driving signals supplied to the pair of diagonal two switching elements of the fifth switching element - the eighth switching element. Fix the duty ratio of the driving signal supplied to one of the two switching elements at one diagonal of the fifth to eighth switching elements at 50%, and vary the duty ratio of the driving signal supplied to the other of the two switching elements within the range of 50% or less according to the power transmission amount. Switch the setting of the duty ratio of the driving signal supplied to the two switching elements at the start timing of the switching period. The power conversion device according to claim 4.

8. The control circuit In the first period, control the switching element to be in the rectifying state included in the second bridge circuit to be in the on state. The power conversion device according to claim 4.

9. The control circuit In the second period, control the two diagonal switching elements to be in the rectifying state included in the second bridge circuit to be in the on state, and when the absolute value of the current flowing through the isolation transformer becomes equal to or less than a predetermined threshold value, turn off at least one of the two diagonal switching elements. The power conversion device according to claim 4.

10. The control circuit calculates the period T2s during which the two diagonal switching elements included in the second bridge circuit are controlled to be in the on state in the second period using a derived formula. The derived formula is T2s = ((V1 - V2) · T1 / V2) - ((L / V2) · Ith) V1 is the voltage of the first DC section, V2 is the voltage of the second DC section, T1 is the time of the first period based on feedback control, L is the inductance between the first bridge circuit and the second bridge circuit, Ith is the predetermined threshold value, where The power conversion device according to claim 9.

11. The control circuit When stepping down the voltage from the second DC section to the first DC section to transmit power Switch the driving signal supplied to the first to fourth switching elements and the driving signal supplied to the fifth to eighth switching elements. The power conversion device according to claim 1.

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