Power conversion device

Abstract

To achieve both biased magnetization suppression and efficiency improvement 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 secondary coil (n2) of an insulation transformer (TR1) are conducted and a second period in which both ends of a primary coil (n1) of the insulation transformer (TR1) are short-circuited in a first bridge circuit (11). The control circuit (13) sets a second dead time, which is set to the first bridge circuit (11) when shifting from the second period to the first period, longer than a first dead time which is set to the first bridge circuit (11) when shifting from the first period to the second period.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 Art

[0002] Power conditioners used in photovoltaic power generation systems and V2H (Vehicle to Home) systems are required to perform highly efficient power conversion. The V2H system can charge and discharge between a storage battery mounted on an electric vehicle (e.g., EV, PHEV) and a commercial power grid or a load in a house. For example, power generated by a household photovoltaic power generation system can be used to charge the storage battery of an electric vehicle. Also, the storage battery mounted on an electric vehicle can be used for peak shifting or backup of loads in a house. The DC / DC converter used in the V2H system is required to be highly efficient, isolated, and have a wide voltage range. This is because the voltage of the storage battery mounted on an electric vehicle varies greatly depending on the vehicle type. One of the DC / DC converters that satisfy these requirements is a DAB (Dual Active Bridge) converter (see, for example, Patent Document 1).

[0003] In a DAB converter, when operating in the current continuous mode, the voltage periods applied to the transformer do not match in the positive and negative half-cycles. As a result, a phenomenon called "bias magnetization" occurs in which a DC current flows through the exciting current and the exciting inductance rapidly decreases due to magnetic saturation. Due to this bias magnetization phenomenon, the exciting current flowing through the transformer increases, leading to an increase in losses and protection stops or equipment failures exceeding the rated current of the circuit. Therefore, it is necessary to operate the circuit so that the bias magnetization phenomenon does not occur.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In order to suppress the bias magnetic field, to operate the DAB converter in the discontinuous current mode, a long dead time until the current flowing through the transformer becomes zero is required. Usually, a fixed value is used for the dead time of the switching elements in the full-bridge circuit. When the dead time is set long, the bias magnetic field can be suppressed, but the loss increases. On the contrary, when the dead time is set short, the loss can be reduced, but the bias magnetic field is likely to occur.

[0006] The present disclosure has been made in view of such a situation, and an object thereof is to provide a technique for achieving both bias magnetic field suppression and high efficiency 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, and a first bridge circuit in which the first leg and the second leg are connected in parallel to a first DC section, 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, and a second bridge circuit in which the third leg and the fourth leg are connected in parallel to a second DC section, an isolation transformer connected between the first bridge circuit and the second bridge circuit, and a control circuit that controls the first switching element to the eighth switching element. When stepping down and transmitting power from the first DC section to the second DC section, the control circuit controls to include a first period in which the first DC section and the secondary winding of the isolation transformer are conductive, and a second period in which both ends of the primary winding of the isolation transformer are short-circuited within the first bridge circuit, and sets a second dead time set for the first bridge circuit when transitioning from the second period to the first period to be longer than a first dead time set for the first bridge circuit when transitioning from the first period to the second period.

[0008] According to the present disclosure, in a DAB converter, it is possible to achieve both bias magnetic suppression and high efficiency.

Brief Description of the Drawings

[0009]

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Modes 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 DC power supplied from a first DC power source E1 and transmits it to a second DC power source E2. The power conversion device 1 also converts 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 the voltage for power transmission or step up the voltage for power transmission.

[0011] The first DC power source E1 corresponds to, for example, a battery or an electric double layer capacitor mounted on an EV, or a stationary battery or an electric double layer capacitor. The second DC power source E2 corresponds to, for example, a DC bus connected to a commercial power system via an inverter. Other 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-side 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-side capacitor Cb, and a control circuit 13.

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

[0014] The first bridge circuit 11 is a full-bridge circuit formed 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 a 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-side 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 a 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 secondary-side DC voltage 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 anti-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 the first switching element Q1 to the eighth switching element Q8, for example, MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) or IGBT (Insulated Gate Bipolar Transistor) can be used. When MOSFET is used for the first switching element Q1 to the eighth switching element Q8, the parasitic diodes respectively formed between the drain and source of the first switching element Q1 to the eighth switching element Q8 are used as the first diode D1 to the eighth diode D8, or external diode elements are respectively connected as the first diode D1 to the eighth diode D8. Also, whether to use the parasitic capacitances respectively formed between the drain and source of the first switching element Q1 to the eighth switching element Q8 as the first capacitance C1 to the eighth capacitance C8, or external capacitors are respectively connected between the drain and source of the first switching element Q1 to the eighth switching element Q8 as the first capacitance C1 to the eighth capacitance C8.

[0018] When IGBT is used for the first switching element Q1 to the eighth switching element Q8, external diode elements are respectively connected between the collector and emitter of the first switching element Q1 to the eighth switching element Q8 as the first diode D1 to the eighth diode D8. Also, whether to respectively connect external capacitors between the collector and emitter of the first switching element Q1 to the eighth switching element Q8 as the first capacitance C1 to the eighth capacitance C8, or use the parasitic capacitances respectively formed between the collector and emitter of the first switching element Q1 to the eighth switching element Q8 as the first capacitance C1 to the eighth capacitance C8.

[0019] Since MOSFET does not generate tail current compared with IGBT, the switching loss during turn-off can be reduced to reduce heat generation, so the cooler (for example, heat sink) can be miniaturized. Also, since it can be driven at high frequency compared with IGBT, passive components (for example, transformers, capacitors) can be miniaturized. However, MOSFET usually has a larger on-resistance than IGBT.

[0020] In recent years, as a high-voltage and low-loss switching element, a switching element using a wide-bandgap semiconductor (for example, silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga2O3), diamond (C)) has been becoming popular. 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 inductance 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 inductance 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 inductance L1 is composed of a reactor 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 inductance L2 is composed of a reactor 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] Incidentally, the first inductance L1 may be constituted by 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 inductance L2 may be constituted by 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. Incidentally, either one of the first inductance L1 and the second inductance L2 may be omitted.

[0024] The control circuit 13 executes the following control as basic control. When the control circuit 13 transmits 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. Incidentally, 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 primary current sensor (not shown) may be controlled with target values.

[0025] Also, when the control circuit 13 transmits 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 primary current sensor (not shown) maintains the current command value. Incidentally, 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 with target values.

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

[0027] (Comparative Example) Figs. 2(a)-(b) are diagrams for explaining the current flow in the step-down operation according to a comparative example of the power conversion device 1. Fig. 3 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 a comparative example of the power conversion device 1. Figs. 4(a)-(b) are diagrams for explaining the current flow in the boost operation according to a comparative example of the power conversion device 1. Fig. 5 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 a comparative example of the power conversion device 1.

[0028] In the comparative example, the 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 the fully-off state.

[0029] Fig. 2(a) shows a state where 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. 2(b) shows a state where 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 to the commutation state. The higher the ratio of the commutation state, the lower the voltage or current of the transmitted power is controlled.

[0030] Specifically, as shown in FIG. 3, 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 so that the phase difference becomes smaller (shifts the phase of the second leg to the left), and when it decreases the power transmitted from the primary side to the secondary side, it controls so that the phase difference becomes larger (shifts the phase of the second leg to the right).

[0031] FIG. 4(a) shows a 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. 4(b) shows a 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 step-up operation, the voltage or current of the transmitted power is controlled by the ratio of the transmission state to the accumulation state. The higher the ratio of the accumulation state, the higher the voltage or current of the transmitted power is controlled.

[0032] Specifically, as shown in FIG. 5, 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 differences between the first leg and the second leg and the third leg are 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 larger (shifts the phase of the third leg to the right), and when it decreases the power transmitted from the primary side to the secondary side, it controls so that the phase difference becomes smaller (shifts the phase of the third leg to the left).

[0033] In the comparative example, all dead times are set to be the same. The dead time is set to prevent through current from flowing due to two series-connected switching elements being turned on simultaneously. When current flows through the diode during the dead time, diode losses occur during the dead time. This diode loss increases as the dead time becomes longer.

[0034] (Example (Step - down)) FIG. 6 is a diagram showing the switching pattern of the first switching element Q1 - the eighth switching element Q8 and the first transition example of the transformer current IL during the step - down operation according to an embodiment of the power conversion device 1. The transformer current IL is the current flowing through the isolation transformer TR1. FIGS. 7(a) - (d) are diagrams for explaining the switching pattern and the current flow in the step - down operation according to an embodiment of the power conversion device 1 (Part 1). FIGS. 8(a) - (d) are diagrams for explaining the switching pattern and the current flow in the step - down operation according to an embodiment of the power conversion device 1 (Part 2).

[0035] When the control circuit 13 steps down the voltage from the first DC part to the second DC part and transmits 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 part 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 part. 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 part to enter a commutation state.

[0036] When the control circuit 13 transitions from the first period T1 (step - down) to the second period T2 (step - down), it sets a first dead time Td1 (step - down) for the first bridge circuit 11. When the control circuit 13 transitions from the second period T2 (step - down) to the first period T1 (step - down), it sets a second dead time Td2 (step - down) for the first bridge circuit 11.

[0037] In the first dead time Td1 (step - down) and the second dead time Td2 (step - down), when the on / off states of the series - connected first switching element Q1 and second switching element Q2 are switched, the control circuit 13 controls both the first switching element Q1 and the second switching element Q2 to be in the off state. When the on / off states of the series - connected third switching element Q3 and fourth switching element Q4 are switched, the control circuit 13 controls both the third switching element Q3 and the fourth switching element Q4 to be in the off state.

[0038] In this embodiment, the control circuit 13 sets the second dead time Td2 (step - down) to be longer than the first dead time Td1 (step - down). Specifically, the first dead time Td1 (step - down) is set to be longer than the time guaranteed to avoid simultaneous on of two series - connected switching elements.

[0039] The second dead time Td2 (step - down) is set to be longer than the time guaranteed for the current flowing through the secondary winding n2 of the isolation transformer TR1 to reach zero in the second period T2 (step - down). More specifically, the second dead time Td2 (step - down) is set to a fixed value or a variable value that is longer than the time guaranteed for the current flowing through the secondary winding n2 to reach zero regardless of the change in at least one of the voltages applied to the primary winding n1 or secondary winding n2 of the isolation transformer TR1.

[0040] The control circuit 13 switches the switching state in the order of the first period T1 - 1 (step - down) → the first dead time Td1 - 1 (step - down) → the second period T2 - 1 (step - down) → the second dead time Td2 - 1 (step - down) → the first period T1 - 2 (step - down) → the first dead time Td1 - 2 (step - down) → the second period T2 - 2 (step - down) → the second dead time Td2 - 2 (step - down) in one switching cycle.

[0041] The first period T1-1 (step-down) is a state where the first switching element Q1, the fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 are on, 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 Fig. 7(a)). The primary winding n1 of the isolation transformer TR1 is connected to the first DC section via the first switching element Q1 and the fourth switching element Q4.

[0042] The first dead time Td1-1 (step-down) is a state where the first switching element Q1, the fifth switching element Q5, and the eighth switching element Q8 are on, and the second switching element Q2, the third switching element Q3, the fourth switching element Q4, the sixth switching element Q6, and the seventh switching element Q7 are off (see Fig. 7(b)). Both ends of the primary winding n1 of the isolation transformer TR1 are short-circuited via the third diode D3 and the first switching element Q1.

[0043] The second period T2-1 (step-down) is a state where the first switching element Q1, the third switching element Q3, the fifth switching element Q5, and the eighth switching element Q8 are on, and the second switching element Q2, the fourth switching element Q4, the sixth switching element Q6, and the seventh switching element Q7 are off (see Fig. 7(c)). Both ends of the primary winding n1 of the isolation transformer TR1 are short-circuited via the third switching element Q3 and the first switching element Q1.

[0044] The second dead time Td2-1 (step-down) is a state where the third switching element Q3, the fifth switching element Q5, and the eighth switching element Q8 are on, and the first switching element Q1, the second switching element Q2, the fourth switching element Q4, the sixth switching element Q6, and the seventh switching element Q7 are off (see Fig. 7(d)). The primary winding n1 of the isolation transformer TR1 is connected to the first DC section via the second diode D2 and the third switching element Q3.

[0045] In the first period T1-2 (step-down), the second switching element Q2, the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 are 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 are in the off state (see Fig. 8(a)). The primary winding n1 of the isolation transformer TR1 is connected to the first DC section via the second switching element Q2 and the third switching element Q3.

[0046] In the first dead time Td1-2 (step-down), the second switching element Q2, 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 fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 are in the off state (see Fig. 8(b)). Both ends of the primary winding n1 of the isolation transformer TR1 are short-circuited via the second switching element Q2 and the fourth diode D4.

[0047] In the second period T2-2 (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. 8(c)). Both ends of the primary winding n1 of the isolation transformer TR1 are short-circuited via the second switching element Q2 and the fourth switching element Q4.

[0048] In the second dead time Td2-2 (step-down), 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 second switching element Q2, the third switching element Q3, the fifth switching element Q5, and the eighth switching element Q8 are in the off state (see Fig. 8(d)). The primary winding n1 of the isolation transformer TR1 is connected to both ends of the first DC section via the first diode D1 and the fourth switching element Q4.

[0049] The first dead time Td1 (step - down) is desirably set to the minimum time that ensures the avoidance of simultaneous turn - on of the first switching element Q1 and the second switching element Q2, or the simultaneous turn - on of the third switching element Q3 and the fourth switching element Q4. The conduction loss due to synchronous rectification (simultaneous turn - on of the first switching element Q1 and the fourth switching element Q4, or simultaneous turn - on of the second switching element Q2 and the third switching element Q3) is smaller than the forward loss Vf due to diode rectification. By minimizing the first dead time Td1 (step - down) which is the diode rectification period, the synchronous rectification period can be maximized, and the loss in the first bridge circuit 11 can be reduced.

[0050] The second dead time Td2 (step - down) is set to be longer than the time when it is ensured that the current flowing through the secondary winding n2 reaches zero in order to suppress the bias magnetization of the isolation transformer TR1. When a fixed value is used for the second dead time Td2 (step - down), the second dead time Td2 (step - down) is set to a value derived in advance by an experiment or simulation based on the assumed voltage fluctuations of the first DC part and the assumed voltage fluctuations of the second DC part. For example, it may be set to a value obtained by adding a margin to the minimum value that can avoid the current continuous mode derived by an experiment or simulation. When a fixed value is used for the second dead time Td2 (step - down), the switching frequency can be fixed, and the complexity of control can be avoided.

[0051] When a variable value is used for the second dead time Td2 (step - down), the control circuit 13 terminates the second dead time Td2 (step - down) when the detected value of the current obtained from the current sensor 23 becomes continuously zero for a set time or more. When a variable value is used for the second dead time Td2 (step - down), the effective value of the current can be made smaller, and the loss can be further reduced.

[0052] FIG. 9 is a diagram showing the switching pattern of the first switching element Q1 to the eighth switching element Q8 and a second transition example of the transformer current IL during the step-down operation according to the embodiment of the power conversion device 1. In principle, the control circuit 13 controls two diagonal switching elements included in the second bridge circuit 12 to be in the on state for synchronous rectification during 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 during the second period T2 (step-down), the control circuit 13 turns off at least one of the two diagonal switching elements and switches to diode rectification. Although the efficiency decreases, both switching elements may be turned off.

[0053] In the example shown in FIG. 9, the control circuit 13 turns off the eighth switching element Q8 in the second period T2-1 (step-down) to switch from synchronous rectification to diode rectification, and turns off the seventh switching element Q7 in the second period T2-2 (step-down) to switch from synchronous rectification to diode rectification.

[0054] The threshold Ith is set to 0 A if the transformer current IL can be ideally measured and the delay of the control system / drive system is ideally 0. In practice, 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 half a switching period and the synchronous rectification period of the second period T2 (step-down) is maximized.

[0055] In the example shown in FIG. 9, when transitioning from the second period T2 (step-down) to the first period T1 (step-down), the control circuit 13 sets the dead time set for the fourth leg of the second bridge circuit 12 to a variable value equal to or greater than the first dead time Td1. The earlier the switching to diode rectification occurs, the longer the dead time set for the fourth leg becomes.

[0056] 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 the half-cycle - 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.

[0057] As shown in FIG. 6, 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. 9, when there is a risk of power backflow, switch from synchronous rectification to diode rectification. When power backflow occurs, reactive current unrelated to 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.

[0058] FIG. 10 is a diagram showing the 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 step-down 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. 6, an example of stepping down the voltage from the first DC part to the second DC part and transmitting power was described. In this regard, it is also possible to step down the voltage from the second DC part to the first DC part and transmit power. In this case, as shown in FIG. 10, the control circuit 13 may 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.

[0059] (Embodiment (Step-up)) FIG. 11 is a diagram showing the switching pattern of the first switching element Q1 to the eighth switching element Q8 and the first transition example of the transformer current IL during the boosting operation according to the embodiment of the power conversion device 1. FIGS. 12(a)-(d) are diagrams (part 1) for explaining the switching pattern and the current flow in the boosting operation according to the embodiment of the power conversion device 1. FIGS. 13(a)-(d) are diagrams (part 2) for explaining the switching pattern and the current flow in the boosting operation according to the embodiment of the power conversion device 1.

[0060] 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 (boosting) and a second period T2 (boosting). The first period T1 (boosting) 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 short-circuited within the second bridge circuit 12. The second period T2 (boosting) 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.

[0061] When the control circuit 13 transitions from the first period T1 (boosting) to the second period T2 (boosting), it sets a first dead time Td1 (boosting) in the second bridge circuit 12. When the control circuit 13 transitions from the second period T2 (boosting) to the first period T1 (boosting), it sets a second dead time Td2 (boosting) in the first bridge circuit 11.

[0062] In the first dead time Td1 (boosting), when the on / off states of the seventh switching element Q7 and the eighth switching element Q8 connected in series are switched, the control circuit 13 controls both the seventh switching element Q7 and the eighth switching element Q8 to be in the off state. When the on / off states of the fifth switching element Q5 and the sixth switching element Q6 connected in series are switched, the control circuit 13 controls both the fifth switching element Q5 and the sixth switching element Q6 to be in the off state.

[0063] In the second dead time Td2 (boost), when the on / off states of the serially connected first switching element Q1 and second switching element Q2 are switched, the control circuit 13 controls both the first switching element Q1 and the second switching element Q2 to be in the off state. When the on / off states of the serially connected third switching element Q3 and fourth switching element Q4 are switched, the control circuit 13 controls both the third switching element Q3 and the fourth switching element Q4 to be in the off state.

[0064] In this embodiment, the control circuit 13 sets the second dead time Td2 (boost) to be longer than the first dead time Td1 (boost). Specifically, the first dead time Td1 (boost) is set to be equal to or longer than the time when simultaneous on of two serially connected switching elements is ensured to be avoided.

[0065] The second dead time Td2 (boost) is set to be equal to or longer than the time when the current flowing through the secondary winding n2 of the isolation transformer TR1 reaches zero in the second period T2 (boost). More specifically, the second dead time Td2 (boost) is set to a fixed value or a variable value equal to or longer than the time when the current flowing through the secondary winding n2 reaches zero regardless of the change in at least one of the voltages applied to the primary winding n1 or secondary winding n2 of the isolation transformer TR1.

[0066] The control circuit 13 switches the switching state in the order of the first period T1-1 (boost) → the first dead time Td1-1 (boost) → the second period T2-1 (boost) → the second dead time Td2-1 (boost) → the first period T1-2 (boost) → the first dead time Td1-2 (boost) → the second period T2-2 (boost) → the second dead time Td2-2 (boost) in one switching cycle.

[0067] In the first period T1-1 (boost), the first switching element Q1, the fourth switching element Q4, the fifth switching element Q5, and the seventh switching element Q7 are in the on 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 Fig. 12(a)). Both ends of the secondary winding n2 of the isolation transformer TR1 are short-circuited via the fifth switching element Q5 and the seventh switching element Q7.

[0068] In the first dead time Td1-1 (boost), the first switching element Q1, the fourth switching element Q4, and the fifth switching element Q5 are in the on state, and the second switching element Q2, the third switching element Q3, the sixth switching element Q6, the seventh switching element Q7, and the eighth switching element Q8 are in the off state (see Fig. 12(b)). The secondary winding n2 of the isolation transformer TR1 conducts with the second DC section via the fifth switching element Q5 and the eighth diode D8.

[0069] In the second period T2-1 (boost), the first switching element Q1, the fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 are 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 are in the off state (see Fig. 12(c)). The secondary winding n2 of the isolation transformer TR1 conducts with the second DC section via the fifth switching element Q5 and the eighth switching element Q8.

[0070] In the second dead time Td2-1 (boost), all of the first switching element Q1 - the eighth switching element Q8 are in the off state (see Fig. 12(d)). The secondary winding n2 of the isolation transformer TR1 conducts with the second DC section via the fifth diode D5 and the eighth switching element Q8.

[0071] During the first period T1-2 (boost), the second switching element Q2, the third switching element Q3, the sixth switching element Q6, and the eighth switching element Q8 are in the on state, while 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 Fig. 13(a)). Both ends of the secondary winding n2 of the isolation transformer TR1 are short-circuited via the eighth switching element Q8 and the sixth switching element Q6.

[0072] During the first dead time Td1-2 (boost), the second switching element Q2, the third switching element Q3, and the sixth switching element Q6 are in the on state, while the first switching element Q1, the fourth switching element Q4, the fifth switching element Q5, the seventh switching element Q7, and the eighth switching element Q8 are in the off state (see Fig. 13(b)). The secondary winding n2 of the isolation transformer TR1 is connected to the second DC section via the seventh diode D7 and the sixth switching element Q6.

[0073] During the second period T2-2 (boost), the second switching element Q2, the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 are in the on state, while 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 Fig. 13(c)). The secondary winding n2 of the isolation transformer TR1 is connected to the second DC section via the seventh switching element Q7 and the sixth switching element Q6.

[0074] During the second dead time Td2-2 (boost), all of the first switching element Q1 to the eighth switching element Q8 are in the off state (see Fig. 13(d)). The secondary winding n2 of the isolation transformer TR1 is connected to the second DC section via the seventh diode D7 and the sixth diode D6.

[0075] The first dead time Td1 (boost) is preferably set to the minimum time that ensures that the simultaneous on of the seventh switching element Q7 and the eighth switching element Q8 (or the simultaneous on of the fifth switching element Q5 and the sixth switching element Q6) is avoided. The conduction loss due to synchronous rectification (simultaneous on of the fifth switching element Q5 and the eighth switching element Q8, or simultaneous on of the sixth switching element Q6 and the seventh switching element Q7) is smaller than the forward loss Vf due to diode rectification. By minimizing the first dead time Td1 (boost) that becomes the diode rectification period, the synchronous rectification period can be maximized, and the loss in the second bridge circuit 12 can be reduced.

[0076] The second dead time Td2 (boost) is set to be longer than the time when it is ensured that the current flowing through the secondary winding n2 reaches zero in order to suppress the bias magnetization of the isolation transformer TR1. When a fixed value is used for the second dead time Td2 (boost), the second dead time Td2 (boost) is set to a value derived in advance by experiments or simulations based on the assumed voltage fluctuations of the first DC section and the assumed voltage fluctuations of the second DC section. For example, it may be set to a value obtained by adding a margin to the minimum value that can avoid the current continuous mode derived by experiments or simulations. When a fixed value is used for the second dead time Td2 (boost), the switching frequency can be fixed, and the complexity of control can be avoided.

[0077] When a variable value is used for the second dead time Td2 (boost), the control circuit 13 ends the second dead time Td2 (boost) when the detected value of the current acquired from the current sensor 23 becomes zero continuously for a set time or more. When a variable value is used for the second dead time Td2 (boost), the effective value of the current can be made smaller, and the loss can be further reduced.

[0078] FIG. 14 is a diagram showing the switching pattern of the first switching element Q1 to the eighth switching element Q8 during the boost operation according to an embodiment of the power conversion device 1 and a second transition example of the transformer current IL. FIG. 15 is a diagram showing the switching pattern of the first switching element Q1 to the eighth switching element Q8 during the boost operation according to an embodiment of the power conversion device 1 and a third transition example of the transformer current IL. FIG. 16 is a diagram showing the switching pattern of the first switching element Q1 to the eighth switching element Q8 during the boost operation according to an embodiment of the power conversion device 1 and a fourth transition example of the transformer current IL.

[0079] In the first example shown in FIG. 10, the control circuit 13 fixes the duty ratios of the first leg, the second leg, and the third leg to 50% except for the first dead time Td1 (boost) and the second dead time Td2 (boost), and fixes the phase difference between the first leg, the second leg, and the third leg to 0°. The control circuit 13 makes the second dead time Td2 (boost) coincide between the first leg and the fourth leg.

[0080] The second example shown in FIG. 14 is different from the first example shown in FIG. 10 in the following points. The control circuit 13 maintains the on state of the eighth switching element Q8 of the fourth leg at the second dead time Td2-1 (boost), and maintains the on state of the seventh switching element Q7 of the fourth leg at the second dead time Td2-2 (boost). In the second example, by maintaining the synchronous rectification state of the fourth leg at the second dead time Td2 (boost), the loss in the fourth leg can be reduced.

[0081] The third example shown in Fig. 15 differs from the second example shown in Fig. 14 in the following points. The control circuit 13 varies the second dead time Td2 (boost) in the third leg. Specifically, the control circuit 13 controls the fifth switching element Q5 to be in the on state for as long as possible while ensuring the minimum dead time that avoids the simultaneous on of the fifth switching element Q5 and the sixth switching element Q6 at the second dead time Td2-1 (boost). The control circuit 13 controls the sixth switching element Q6 to be in the on state for as long as possible while ensuring the minimum dead time that avoids the simultaneous on of the fifth switching element Q5 and the sixth switching element Q6 at the second dead time Td2-2 (boost). In the third example, at the second dead time Td2 (boost), the loss in the third leg can be reduced by lengthening the synchronous rectification period in the third leg.

[0082] The fourth example shown in Fig. 16 differs from the third example shown in Fig. 15 in the following points. The control circuit 13 fixes the second dead time Td2 (boost) in the first leg to the minimum time. Specifically, the control circuit 13 maximizes the on time of the second switching element Q2 while ensuring the minimum dead time that avoids the simultaneous on of the first switching element Q1 and the second switching element Q2 at the second dead time Td2-1 (boost). The control circuit 13 maximizes the on time of the first switching element Q1 while ensuring the minimum dead time that avoids the simultaneous on of the first switching element Q1 and the second switching element Q2 at the second dead time Td2-2 (boost). In the fourth example, at the second dead time Td2 (boost), the loss in the first leg can be reduced by lengthening the synchronous rectification period in the first leg. Note that the control of the first leg and the second leg in the fourth example may be interchanged.

[0083] FIG. 17 is a diagram showing a switching pattern during reverse transmission of the first switching element Q1 to the eighth switching element Q8 and an example of the transition of the transformer current IL during the boosting 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. 11, an example of boosting from the first DC section to the second DC section and transmitting power was described. In this regard, it is also possible to boost from the second DC section to the first DC section and transmit power. In this case, as shown in FIG. 17, the control circuit 13 may switch the drive signal supplied to the first switching element Q1 to the fourth switching element Q4 and the drive signal supplied to the fifth switching element Q5 to the eighth switching element Q8.

[0084] (Embodiment (Step-up / down)) FIG. 18 is a diagram showing a switching pattern of the first switching element Q1 to the eighth switching element Q8 and an example of the transition of the transformer current IL during the step-up / down operation according to the embodiment of the power conversion device 1. The step-up / 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. When the control circuit 13 steps up / down from the first DC section to the second DC section and transmits power during the period of switching between step-up and step-down, it controls to include a first period T1 (step-up), a second period T2 (step-up), and a third period T3. 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.

[0085] FIG. 19(a)-(b) is a diagram for explaining the switching pattern and current flow in the third period in the step-up / down operation according to an embodiment of the power conversion device 1. In the third period T3-1, 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 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 FIG. 19(a)). Both ends of the primary winding n1 of the isolation transformer TR1 are short-circuited via the fourth switching element Q4 and the second diode D2.

[0086] In the third period T3-2, the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 are in the on state, and the first switching element Q1, 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. 19(b)). Both ends of the primary winding n1 of the isolation transformer TR1 are short-circuited via the first diode D1 and the third switching element Q3.

[0087] In the step-up / down operation, as shown in FIG. 18, 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 is the third period T3. In the step-up / down operation, before the end of the second period T2 (the first DC section and the isolation transformer TR1 are in the conducting state), the third period T3 (the first DC section and the isolation transformer TR1 are in the cut-off state) is inserted, so that the energy supplied from the first DC section to the second DC section is suppressed.

[0088] The control circuit 13 sets a first dead time Td1 (step-up) for the second bridge circuit 12 when transitioning from the first period T1 (step-up) to the second period T2 (step-up). The control circuit 13 sets a second dead time Td2 (step-up) for the first bridge circuit 11 when transitioning from the third period T3 to the first period T1 (step-up).

[0089] The control circuit 13 sets the second dead time Td2 (boost) to be longer than the first dead time Td1 (boost). Specifically, the first dead time Td1 (boost) is set to be longer than the time guaranteed to avoid simultaneous on of two serially connected switching elements. The second dead time Td2 (boost) is set to be longer than the time guaranteed for the current flowing through the secondary winding n2 of the isolation transformer TR1 to reach zero during the second period T2 (boost).

[0090] FIG. 20(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. 20(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. 20(a), when the buck-boost mode is not provided, one switching point where the slope of the transmission power (which may be considered as the transmission current) changes occurs at the switching between the buck mode and the boost mode. As shown in FIG. 20(b), when the buck-boost mode is provided, two switching points occur at the switching between the buck mode and the buck-boost mode and at the 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. 20(b), the circuit gain can be smoothly changed and the current oscillation can be suppressed to a small level.

[0091] As described above, according to this embodiment, by setting the second dead time Td2 to be longer than the first dead time Td1, it is possible to suppress the bias magnetization while achieving high efficiency.

[0092] FIG. 21 is a diagram showing the switching patterns of the first switching element Q1 to the eighth switching element Q8 during the boosting operation of the power conversion device 1 and an example of the transition of the transformer current IL when flux imbalance occurs. In the example shown in FIG. 21, the values of the first dead time Td1 and the second dead time Td2 are set equal. If the transformer current IL is not 0 A at the switching of the half-switching period, due to circuit variations or the like, the current time integral A in the positive period and the current time integral B in the negative period become unbalanced, and a DC component is generated.

[0093] In this regard, in the present embodiment, it is ensured that the transformer current IL is 0 A at the switching of the half-switching period, and the operation can be performed in the current discontinuous mode. Thereby, flux imbalance can be suppressed. Also, by setting the first dead time Td1 at a timing different from the switching of the half-switching period to be short, the period during which the diode conducts can be shortened, and the loss can be reduced. Further, within the range where reverse current or through current can be avoided, by setting the synchronous rectification period as long as possible, the loss can be further reduced.

[0094] As described above, the present disclosure has been described based on the embodiments. The embodiments are examples, and it is understood by those skilled in the art that various modifications are possible for each of the components and combinations of each processing process, and such modifications are also within the scope of the present disclosure.

[0095] In the buck mode, the switching patterns of the first leg and the second leg may be switched every predetermined switching period. In this case, the heat generation of the first switching element Q1 to the fourth switching element Q4 can be equalized. In the boost mode, the switching patterns of the third leg and the fourth leg may be switched every predetermined switching period. In this case, the heat generation of the fifth switching element Q5 to the eighth switching element Q8 can be equalized.

[0096] Note that the embodiment may be specified by the following items.

[0097] [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, and a first bridge circuit (11) in which the first leg and the second leg are connected in parallel to a first DC section (E1, Ca); 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, and a second bridge circuit (12) in which the third leg and the fourth leg are connected in parallel to a second DC section (E2, Cb); 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) to the eighth switching element (Q8). The control circuit (13) is When stepping down the voltage from the first DC section (E1, Ca) to the second DC section (E2, Cb) to transmit power, it is controlled to include a first period in which the first DC section (E1, Ca) and the secondary winding (n2) 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). The second dead time set for the first bridge circuit (11) when transitioning from the second period to the first period is set longer than the first dead time set for the first bridge circuit (11) when transitioning from the first period to the second period. Power conversion device (1). According to this, it is possible to achieve both bias magnetic suppression and high efficiency. [Item 2] The first dead time is set to be longer than the time when it is ensured that simultaneous on of two series-connected switching elements is avoided. The second dead time is set to be longer than the time when it is ensured that the current flowing through the secondary winding (n2) of the transformer reaches zero during the second period. The power conversion device (1) according to item 1. According to this, while suppressing the bias magnetization by the second dead time, the loss of the entire dead time can be reduced by the first dead time. [Item 3] The second dead time is set to a fixed value that ensures that the current flowing through the secondary winding (n2) of the transformer reaches zero during the second period. The power conversion device (1) according to item 2. According to this, even if the voltage condition changes, bias magnetization can be suppressed, and since the switching frequency can be fixed, the control can be simplified. [Item 4] The second dead time is set to a variable value that ensures that the current flowing through the secondary winding (n2) of the transformer reaches zero during the second period. The power conversion device (1) according to item 2. According to this, even if the voltage condition changes, bias magnetization can be suppressed, the effective value of the current can be further reduced, and the loss can be further decreased. [Item 5] The control circuit (13) During the second period, two diagonal switching elements included in the second bridge circuit (12) are controlled 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, at least one of the two diagonal switching elements is turned off. When transitioning from the second period to the first period, the dead time set in one leg of the second bridge circuit (12) is set to a variable value equal to or greater than the first dead time. The power conversion device (1) according to item 1. According to this, generation of the reactive current can be suppressed while reducing the loss in the second bridge circuit (12). [Item 6] The control circuit (13) When stepping down the voltage from the second DC section (E2, Cb) to the first DC section (E1, Ca) and transmitting power 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 Signs

[0098] 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, 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 When stepping down the voltage from the first DC section to the second DC section to transmit power, the control circuit controls to include a first period in which the first DC section and the secondary 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. When transitioning from the first period to the second period, the second dead time set for the first bridge circuit is set longer than the first dead time set for the first bridge circuit when transitioning from the second period to the first period. A power conversion device.

2. The first dead time is set to be equal to or longer than the time guaranteed to avoid simultaneous on of two series-connected switching elements. The second dead time is set to be equal to or longer than the time guaranteed for the current flowing through the secondary winding of the transformer to reach zero during the second period. The power conversion device according to Claim 1.

3. The second dead time is set to a fixed value that guarantees that the current flowing through the secondary winding of the transformer reaches zero during the second period. The power conversion device according to Claim 2.

4. The second dead time is set to a variable value that guarantees that the current flowing through the secondary winding of the transformer reaches zero during the second period. The power conversion device according to Claim 2.

5. The control circuit is During the second period, the control circuit controls two diagonal switching elements included in the second bridge circuit to be in an on state, and when the absolute value of the current flowing through the isolation transformer becomes equal to or less than a predetermined threshold, at least one of the two diagonal switching elements is turned off. When transitioning from the second period to the first period, set the dead time set in one leg of the second bridge circuit to a variable value equal to or greater than the first dead time. The power conversion device according to claim 1.

6. The control circuit When stepping down the voltage from the second DC section to the first DC section and transmitting power Swap the drive signals supplied to the first switching element - the fourth switching element and the drive signals supplied to the fifth switching element - the eighth switching element. The power conversion device according to claim 1.

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