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), a control circuit (13) performs control in such a manner that a second bridge circuit (12) includes a first period in which both ends of a secondary coil (n2) of an insulation transformer (TR1) are short-circuited in the second bridge circuit (12) and a second period in which the secondary coil (n2) of the insulation transformer (TR1) and the second DC sections (E2 and Cb) are conducted. The control circuit (13) sets a second dead time, which is set to a 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 second bridge circuit (12) 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 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, losses increase due to hard switching at low output and due to reactive currents that are not related 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] In the secondary-side bridge circuit of the proposed DAB converter, the losses are non-uniform between the switching elements that are all-off and the switching elements used for phase-shift control. Therefore, the amount of heat generation also becomes non-uniform, leading to an increase in the size of the members (e.g., heat sinks) for cooling the switching elements. Also, the variation in the lifespan between the switching elements becomes large.

[0006] The present disclosure has been made in view of such circumstances, and its object is to provide a technique for equalizing the heat generation of the 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, a second leg in which a third switching element and a fourth switching element are connected in series, 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, a fourth leg in which a seventh switching element and an eighth switching element are connected in series, 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 for controlling the first switching element - the eighth switching element. When boosting the voltage from the first DC section to the second DC section and transmitting power, the control circuit controls the second bridge circuit to include a first period in which both ends of the secondary winding of the isolation transformer are short-circuited within the second bridge circuit and a second period in which the secondary winding of the isolation transformer and the second DC section are conducting, 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 second bridge circuit when transitioning from the first period to the second period.

Advantages of the Invention

[0008] According to the present disclosure, the heat generation of the switching elements included in the DAB converter can be made uniform.

Brief Description of the Drawings

[0009]

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MODE 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 transmit power by stepping down or stepping up.

[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] A primary-side capacitor Ca is connected in parallel with the first DC power supply E1. A secondary-side capacitor Cb is connected in parallel with the second DC power supply 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 supply E1 and the primary-side capacitor Ca are collectively referred to as the first DC section, and the second DC power supply 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 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-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 configured 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 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 to an eighth diode D8 are formed or connected in anti-parallel to the first switching element Q1 to the eighth switching element Q8, respectively. Also, a first capacitor C1 to an eighth capacitor C8 are formed or connected in parallel to the first switching element Q1 to the eighth switching element Q8, respectively.

[0017] For example, a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) or an IGBT (Insulated Gate Bipolar Transistor) can be used for the first switching element Q1 to the eighth switching element Q8. When a MOSFET is used for the first switching element Q1 to the eighth switching element Q8, the parasitic diode formed between the drain and source of the first switching element Q1 to the eighth switching element Q8 is used as the first diode D1 to the eighth diode D8, or an external diode element is connected as the first diode D1 to the eighth diode D8, respectively. Also, the parasitic capacitance formed between the drain and source of the first switching element Q1 to the eighth switching element Q8 is used as the first capacitor C1 to the eighth capacitor C8, or an external capacitor is connected as the first capacitor C1 to the eighth capacitor C8 between the drain and source of the first switching element Q1 to the eighth switching element Q8, respectively.

[0018] When an IGBT is used for the first switching element Q1 to the eighth switching element Q8, an external diode element is 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, respectively. Also, an external capacitor is 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, respectively, or the parasitic capacitance formed between the collector and emitter of the first switching element Q1 to the eighth switching element Q8 is used as the first capacitor C1 to the eighth capacitor C8.

[0019] Since a MOSFET does not generate a tail current as compared with an IGBT, the switching loss during turn-off can be reduced to reduce heat generation, so that a cooler (e.g., a heat sink) can be miniaturized. Also, since it is possible to drive at high frequencies as compared with an IGBT, passive components (e.g., a transformer, a capacitor) can be miniaturized. However, a MOSFET generally has a larger on-resistance than an IGBT.

[0020] In recent years, as a switching element with high breakdown voltage and low loss, switching elements using wide-gap semiconductors (e.g., 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 a SiC-MOSFET has a higher breakdown voltage than a 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] Note that the first inductance 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 inductance 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 the first inductance L1 or 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), it 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 primary current sensor (not shown) may be used as target values for control.

[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 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 voltage value on the secondary side detected by the second voltage sensor 22 on the secondary side, and the current value detected by the current sensor 23 on the secondary side may be controlled as target values.

[0026] In this way, the DAB converter has a symmetrical 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 the 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 the 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 the 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 the 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 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. 2(b) shows a 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.

[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 reducing 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 stored from the first DC power supply E1 in the first inductor L1 and the second inductor L2 (hereinafter referred to as the storage state). Fig. 4(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 step-up operation, the voltage or current of the transmitted power is controlled by the ratio of the transmission state and the storage state. The higher the ratio of the storage 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 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). When the power transmitted from the primary side to the secondary side is decreased, it controls so that the phase difference becomes small (shifts the phase of the third leg to the left).

[0033] In the comparative example, all dead times are set the same. The dead time is set to prevent through current from flowing due to two series-connected switching elements being simultaneously on. When current flows through the diode during the dead time, diode loss occurs 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 section to the second DC section 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 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.

[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 equal to or longer than the time when the simultaneous on of two series-connected switching elements is ensured to be avoided.

[0039] The second dead time Td2 (step - down) is set to be longer than the time when the current flowing through the secondary winding n2 of the isolation transformer TR1 reaches zero during 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 when the current flowing through the secondary winding n2 is ensured to reach zero, regardless of the change in at least one of the voltages applied to the primary winding n1 or the 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] During the first period T1 - 1 (step - down), 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. 7(a)). The primary winding n1 of the isolation transformer TR1 is connected to the first DC part through the first switching element Q1 and the fourth switching element Q4.

[0042] During the first dead time Td1 - 1 (step - down), the first switching element Q1, 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 fourth switching element Q4, the sixth switching element Q6, and the seventh switching element Q7 are in the off state (see Fig. 7(b)). Both ends of the primary winding n1 of the isolation transformer TR1 are short - circuited through the third diode D3 and the first switching element Q1.

[0043] In the second period T2-1 (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. 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] In the second dead time Td2-1 (step-down), the third switching element Q3, 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 fourth switching element Q4, the sixth switching element Q6, and the seventh switching element Q7 are in the off state (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 conducted with both ends of the first DC section via the first diode D1 and the fourth switching element Q4.

[0049] It is desirable that the first dead time Td1 (step-down) be set to the minimum time that ensures avoidance of simultaneous on of the first switching element Q1 and the second switching element Q2, or simultaneous on of the third switching element Q3 and the fourth switching element Q4. The conduction loss due to synchronous rectification (simultaneous on of the first switching element Q1 and the fourth switching element Q4, or simultaneous 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) that becomes 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 experiments or simulations 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, which is derived by experiments or simulations. 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), when the detected value of the current obtained from the current sensor 23 becomes zero continuously for a set time or more, the control circuit 13 ends the second dead time Td2 (step - down). 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 - the eighth switching element Q8 and the 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, during the second period T2 (step - down), 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, 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, in the second period T2-1 (step-down), the control circuit 13 turns off the eighth switching element Q8 to switch from synchronous rectification to diode rectification, and in the second period T2-2 (step-down), the control circuit 13 turns off the seventh switching element Q7 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 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 half of the switching period and the synchronous rectification period in the second period T2 (step-down) is maximized.

[0055] In the example shown in FIG. 9, when the control circuit 13 transitions from the second period T2 (step-down) to the first period T1 (step-down), the dead time set for the fourth leg of the second bridge circuit 12 is set to a variable value greater than or equal to 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 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 period - 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 - eighth switching element Q8 to the off state.

[0057] As shown in FIG. 6, when the decreasing 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 the step-down operation according to the 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 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. 10, the control circuit 13 may switch the drive signal supplied to the first switching element Q1 - the fourth switching element Q4 and the drive signal 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 - the eighth switching element Q8 and a first example of the transition of the transformer current IL during the step-up operation according to the embodiment of the power conversion device 1. FIGS. 12(a)-(d) are diagrams for explaining 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. 13(a)-(d) are diagrams for explaining the switching pattern and the current flow in the step-up operation according to the embodiment of the power conversion device 1 (Part 2).

[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 the 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 (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, which is a transmission state.

[0061] When the control circuit 13 transitions from the first period T1 (boosting) to the second period T2 (boosting), the control circuit 13 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), the control circuit 13 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 serially connected seventh switching element Q7 and eighth switching element Q8 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 serially connected fifth switching element Q5 and sixth switching element Q6 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 (boosting), 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 longer than the time when it is ensured that the simultaneous on of two series-connected switching elements is avoided.

[0065] 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 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 that is longer than the time when it is ensured that 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 the secondary winding n2 of the isolation transformer TR1.

[0066] In one switching cycle, 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).

[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] The first dead time Td1-1 (boost) is a state in which the first switching element Q1, the fourth switching element Q4, and the fifth switching element Q5 are on, 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 off (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] The second period T2-1 (boost) is a state in which 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. 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] The second dead time Td2-1 (boost) is a state in which all of the first switching element Q1 - the eighth switching element Q8 are off (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] The first period T1-2 (boost) is a state in which the second switching element Q2, the third switching element Q3, the sixth switching element Q6, and the eighth switching element Q8 are on, and the first switching element Q1, the fourth switching element Q4, the fifth switching element Q5, and the seventh switching element Q7 are off (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] The first dead time Td1-2 (boost) is a state in which the second switching element Q2, the third switching element Q3, and the sixth switching element Q6 are on, and 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 off (see Fig. 13(b)). The secondary winding n2 of the isolation transformer TR1 conducts with the second DC section via the seventh diode D7 and the sixth switching element Q6.

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

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

[0075] The first dead time Td1 (boost) is desirably set to the minimum time that ensures avoidance of simultaneous on of the seventh switching element Q7 and the eighth switching element Q8 (or simultaneous on of the fifth switching element Q5 and the sixth switching element Q6). 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 an experiment or simulation 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 an experiment or simulation. When a fixed value is used for the second dead time Td2 (boost), the switching frequency can be fixed, and the complication of control can be avoided.

[0077] When a variable value is used for 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 longer, the control circuit 13 ends the second dead time Td2 (boost). 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 a 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 boost operation according to the embodiment of the power conversion device 1. FIG. 15 is a diagram showing a switching pattern of the first switching element Q1 to the eighth switching element Q8 and a third transition example of the transformer current IL during the boost operation according to the embodiment of the power conversion device 1. FIG. 16 is a diagram showing a switching pattern of the first switching element Q1 to the eighth switching element Q8 and a fourth transition example of the transformer current IL during the boost operation according to the embodiment of the power conversion device 1.

[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. In the control circuit 13, the eighth switching element Q8 of the fourth leg maintains the on state at the second dead time Td2-1 (boost), and the seventh switching element Q7 of the fourth leg maintains the on state at the second dead time Td2-2 (boost). In the second example, at the second dead time Td2 (boost), by maintaining the fourth leg in the synchronous rectification state, the loss in the fourth leg can be reduced.

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

[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 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 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 of 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 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 the boost 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. 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 interchange the drive signal supplied to the first switching element Q1 - the fourth switching element Q4 and the drive signal supplied to the fifth switching element Q5 - the eighth switching element Q8.

[0084] (Embodiment (Buck - Boost)) FIG. 18 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 and transmitting power from the first DC section to the second DC section 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.

[0085] FIGS. 19(a)-(b) are diagrams for explaining the switching pattern and the current flow in the third period during the step-up / step-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 / step-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 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 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] When the control circuit 13 transitions from the first period T1 (step-up) to the second period T2 (step-up), the first dead time Td1 (step-up) is set for the second bridge circuit 12. When the control circuit 13 transitions from the third period T3 to the first period T1 (step-up), the second dead time Td2 (step-up) is set for the first bridge circuit 11.

[0089] The control circuit 13 sets the second dead time Td2 (step-up) to be longer than the first dead time Td1 (step-up). Specifically, the first dead time Td1 (step-up) is set to be longer than the time when it is ensured that simultaneous on of two serially connected switching elements is avoided. The second dead time Td2 (step-up) is set to be longer than the time when it is ensured that the current flowing through the secondary winding n2 of the isolation transformer TR1 reaches zero in the second period T2 (step-up).

[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 current oscillation can be suppressed to a small level.

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

[0092] FIG. 21 is a diagram showing an example of the switching pattern of the first switching element Q1 to the eighth switching element Q8 during the boost operation of the power conversion device 1 and the transition of the transformer current IL when the bias magnetization occurs. In the example shown in FIG. 21, the values of the first dead time Td1 and the second dead time Td2 are set to be equal. If the transformer current IL is not 0 A at the switching of the half switching period, due to variations in the circuit, etc., 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 this embodiment, it is ensured that the transformer current IL is 0 A when switching between half-switching cycles, and it can operate in the discontinuous current mode. Thereby, the bias magnetic flux can be suppressed. Also, by setting the first dead time Td1 at a timing different from the switching between half-switching cycles 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 illustrative, 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 - 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 - the eighth switching element Q8 can be equalized.

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

[0097] [Item 1] A first bridge circuit (11) having 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 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) - the eighth switching element (Q8). The control circuit (13) When boosting power from the first DC section (E1, Ca) to the second DC section (E2, Cb) and transmitting power, the second bridge circuit (12) controls to include a first period in which both ends of the secondary winding (n2) of the isolation transformer (TR1) are short-circuited within the second bridge circuit (12), and a second period in which the secondary winding (n2) of the isolation transformer (TR1) and the second DC section (E2, Cb) are conductive. A second dead time set for the first bridge circuit (11) when transitioning from the second period to the first period is set longer than a first dead time set for the second bridge circuit (12) when transitioning from the first period to the second period. A 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 equal to or longer than a time when it is ensured that simultaneous on of two series-connected switching elements is avoided. The second dead time is set to be equal to or longer than a 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 bias magnetic by the second dead time, it is possible to reduce the loss of the entire dead time 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 when the voltage condition changes, it is possible to suppress bias magnetization, 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 when the voltage condition changes, it is possible to suppress bias magnetization, and 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 switching period between step-up and step-down, when power is transmitted from the first DC section (E1, Ca) to the second DC section (E2, Cb), the first period, the second period, and 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 controlled to include a third period in which the secondary winding (n2) of the isolation transformer (TR1) is made conductive to the second DC section (E2, Cb). When transitioning from the third period to the first period, the second dead time set for the first bridge circuit (11) is set longer than the first dead time set for the second bridge circuit (12) when transitioning from the first period to the second period. The power conversion device (1) according to item 1. According to this, during the switching period between step-up and step-down, the circuit gain can be smoothly changed, and current oscillation can be suppressed to a small level. Also, it is possible to achieve both the maximization of the synchronous rectification period and the suppression of bias magnetization. [Item 6] The control circuit (13) When stepping up 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.

Description of symbols

[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 to the eighth switching element, and comprising: The control circuit is configured such that: When boosting power from the first DC section to the second DC section and transmitting power, the second bridge circuit is controlled to include a first period in which both ends of the secondary winding of the isolation transformer are short-circuited within the second bridge circuit, and a second period in which the secondary winding of the isolation transformer and the second DC section are conducting; A second dead time set for the first bridge circuit when transitioning from the second period to the first period is set longer than a first dead time set for the second bridge circuit when transitioning from the first period to the second period; A power conversion device.

2. The first dead time is set to be equal to or longer than a time that ensures that simultaneous turning on of two series-connected switching elements is avoided; The second dead time is set to be equal to or longer than a time that ensures that the current flowing through the secondary winding of the transformer reaches 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 ensures 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 ensures 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 configured such that: When transmitting power from the first DC section to the second DC section during the switching period between step-up and step-down, control is performed to include a first period, a second period, and a third period in which both ends of the primary winding of the isolation transformer are short-circuited within the first bridge circuit and the second bridge circuit conducts the secondary winding of the isolation transformer to the second DC section. When transitioning from the third period to the first period, a second dead time set for the first bridge circuit is set longer than the first dead time set for the second bridge circuit when transitioning from the first period to the second period. The power conversion device according to claim 1.

6. The control circuit is When stepping up and transmitting power from the second DC section to the first DC section interchange 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.

Images