Power conversion device

Abstract

To equalize heat generation of switching elements included in a DAB converter.SOLUTION: In the DAB converter, when transmitting power after lowering the voltage, a control circuit (13) generates a first period in which a first bridge circuit (11) electrically connects a first DC part (E1, Ca) and a primary coil (n1) of an insulating transformer (TR1) and a second bridge circuit (12) electrically connects a secondary coil (n2) of the insulating transformer (TR1) and a second DC part (E2, Cb), and a second period in which both ends of the primary coil (n1) of the insulating transformer (TR1) are shorted together in the first bridge circuit (11) and the second bridge circuit (12) electrically connects the secondary coil (n2) of the insulating transformer (TR1) and the second DC part (E2, Cb). The control circuit (13) swaps drive signals supplied to four switching elements (Q5-Q8) included in the second bridge circuit (12) between a third leg and a fourth leg.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 [Problem to be solved by the invention]

[0005] In the bridge circuit on the secondary side of the proposed DAB converter, the losses are not uniform between the switching elements that are all turned off and the switching elements that are used for phase shift control. This results in non-uniform heat generation, which leads to the need for larger components (e.g., heat sinks) to cool the switching elements. In addition, the life span of the switching elements varies greatly.

[0006] The present disclosure has been made in consideration of these circumstances, and has an object to provide a technique for uniformly dissipating heat from switching elements included in a DAB converter. [Means for solving the problem]

[0007] In order to solve the above problems, a power conversion device according to an embodiment of the present disclosure includes a first bridge circuit having a first leg in which a first switching element and a second switching element are connected in series and a second leg in which a third switching element and a fourth switching element are connected in series, the first leg and the second leg being connected in parallel to a first DC section, a second bridge circuit having a third leg in which a fifth switching element and a sixth switching element are connected in series and a fourth leg in which a seventh switching element and an eighth switching element are connected in series, the third leg and the fourth leg being 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-the eighth switching element. When transmitting power by stepping down the voltage from the first DC section to the second DC section, the control circuit generates a first period during which the first bridge circuit conducts the first DC section and the primary winding of the isolation transformer and the second bridge circuit conducts the secondary winding of the isolation transformer with the second DC section, and a second period during 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 with the second DC section, and switches drive signals supplied to four switching elements included in the second bridge circuit between the third leg and the fourth leg. Effect of the Invention

[0008] According to the present disclosure, it is possible to uniformly distribute heat among switching elements included in a DAB converter. [Brief description of the drawings]

[0009] [Figure 1] 1 is a diagram for explaining a configuration of a power conversion device according to an embodiment; [Diagram 2] 2(a) to 2(c) are diagrams for explaining the operating state of the power conversion device according to the first comparative example. [Diagram 3] 3(a) and 3(b) are diagrams for explaining the current flow in the voltage step-down operation of the power conversion device according to Comparative Example 2. In FIG. [Figure 4]FIG. 11 is a diagram for explaining a switching pattern of a first switching element-an eighth switching element during a step-down operation of a power conversion device according to Comparative Example 2. [Diagram 5] 5(a) and 5(b) are diagrams for explaining the current flow in the boost operation of the power conversion device according to the second comparative example. [Figure 6] FIG. 11 is a diagram for explaining a switching pattern of a first switching element to an eighth switching element during a boost operation of the power conversion device according to Comparative Example 2. [Figure 7] 11 is a diagram for explaining a switching pattern of a first switching element-an eighth switching element during a step-down operation of the power conversion device according to the embodiment, and a first example of a transformer current. FIG. [Figure 8] 13 is a diagram for explaining a switching pattern of the first switching element-the eighth switching element during a step-down operation of the power conversion device according to the embodiment, and a second example of a transformer current. FIG. [Figure 9] 9(a)-(c) are diagrams for explaining a current flow in a step-down operation according to an embodiment of the power conversion device (part 1). [Figure 10] 10(a)-(c) are diagrams for explaining a current flow in a step-down operation according to an embodiment of the power conversion device (part 2). [Figure 11] FIG. 13 is a diagram for explaining an example of a switching pattern during reverse transmission between a first switching element and an eighth switching element during a step-down operation in the embodiment of the power conversion device. [Figure 12] FIG. 13 is a diagram for explaining a first modification of a switching pattern of a first switching element-an eighth switching element during a step-down operation in the embodiment of the power conversion device. [Figure 13] FIG. 13 is a diagram for explaining a second modified example of a switching pattern of a first switching element-an eighth switching element during a step-down operation in the embodiment of the power conversion device. [Figure 14]11A and 11B are diagrams for explaining a switching pattern of a first switching element-an eighth switching element and a first example of a transformer current during a boost operation in the embodiment of the power conversion device. [Figure 15] 13 is a diagram for explaining a switching pattern of a first switching element-an eighth switching element during a boost operation of the power conversion device according to the embodiment, and a second example of a transformer current. FIG. [Figure 16] 16(a)-(c) are diagrams for explaining a current flow in a boost operation according to an embodiment of the power conversion device (part 1). [Figure 17] 17(a)-(c) are diagrams for explaining a current flow in a boost operation according to an embodiment of the power conversion device (part 2). [Figure 18] 13A and 13B are diagrams for explaining an example of a switching pattern during reverse transmission of a first switching element-an eighth switching element and a transformer current during a boost operation in the embodiment of the power conversion device. [Figure 19] 11A and 11B are diagrams for explaining a switching pattern of a first switching element-an eighth switching element during a step-up / step-down operation in the embodiment of the power conversion device, and a first example of a transformer current. [Figure 20] 13A and 13B are diagrams for explaining a switching pattern of the first switching element-the eighth switching element during step-up / step-down operation in the embodiment of the power conversion device, and a second example of the transformer current. [Figure 21] 21(a)-(b) are diagrams for explaining a current flow in a voltage step-up / step-down operation according to an embodiment of the power conversion device (part 1). [Figure 22] 22(a)-(b) are diagrams for explaining a current flow in a voltage step-up / step-down operation according to an embodiment of the power conversion device (part 2). [Figure 23] FIG. 13 is a diagram for explaining an example of a switching pattern during reverse transmission between a first switching element and an eighth switching element during voltage step-up / step-down operation in the embodiment of the power conversion device. [Figure 24]FIG. 13 is a diagram for explaining a first modification of a switching pattern of a first switching element-an eighth switching element during a step-up / step-down operation in the embodiment of the power conversion device. [Diagram 25] FIG. 13 is a diagram for explaining a second modified example of a switching pattern of a first switching element-an eighth switching element during a step-up / step-down operation in the embodiment of the power conversion device. [Figure 26] 5A to 5C are diagrams for explaining switching between a step-down operation, a step-up operation, and a step-up / step-down operation according to an embodiment. [Figure 27] FIG. 2 is a diagram illustrating an example of the configuration of a control circuit. [Figure 28] 28(a)-(b) are graphs showing an example of variables and constants in a theoretical formula for calculating a synchronous rectification period. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 insulated 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 the power.

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

[0012] The power conversion device 1 includes a primary side capacitor Ca, a first bridge circuit 11, a first inductor L1, an insulating transformer TR1, a second inductor 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 to the first DC power source E1. A secondary side capacitor Cb is connected in parallel to the second DC power source E2. For example, an electrolytic capacitor is 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 a first DC section, and the second DC power source E2 and the secondary side capacitor Cb are collectively referred to as a 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 midpoint of the first leg and the midpoint of the second leg are connected to both ends of the primary winding n1 of the isolation transformer TR1, respectively. 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. The first bridge circuit 11 can also 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 midpoint of the third leg and the midpoint of the fourth leg are connected to both ends of the secondary winding n2 of the isolation transformer TR1, respectively. 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. The second bridge circuit 12 can also 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 capacitance C1 to an eighth capacitance 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, a 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, respectively, or an external diode element is connected as the first diode D1 to the eighth diode D8, respectively. In addition, a 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 capacitance C1 to the eighth capacitance C8, respectively, or an external capacitor is connected between the drain and source of the first switching element Q1 to the eighth switching element Q8, respectively.

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

[0019] MOSFETs do not generate tail currents compared to IGBTs, so they can reduce switching losses at turn-off and reduce heat generation, allowing coolers (e.g., heat sinks) to be made smaller. In addition, MOSFETs can be driven at higher frequencies than IGBTs, allowing passive components (e.g., transformers, capacitors) to be made smaller. However, MOSFETs usually have a higher on-resistance than IGBTs.

[0020] In recent years, switching elements using wide-gap semiconductors (e.g., silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga2O3), diamond (C)) have become popular as switching elements with high withstand voltage and low loss. In this embodiment, it is assumed that SiC-MOSFETs are used for the first switching element Q1 to the eighth switching element Q8. SiC-MOSFETs have a higher withstand voltage than Si-MOSFETs, so they can be made smaller and their on-resistance per unit area can be reduced.

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

[0022] The first inductance L1 is connected or formed in series 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 or formed in series 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] The first inductance L1 may be formed of a 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 formed of a 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. 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 transmitting power from the first DC section to the second DC section (when discharging from the first DC power source E1), the control circuit 13 controls the first switching element Q1 to the eighth switching element Q8 so that the current value detected by the secondary side 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 side voltage value detected by the second voltage sensor 22 on the secondary side, and the current value detected by the primary side current sensor (not shown) may be controlled as target values.

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

[0026] In this way, the DAB converter has a symmetrical configuration between the primary side and the secondary side, and can transmit power in both directions. The operation of the power conversion device 1 will now be described.

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

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

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

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

[0031] In the control according to the comparative example 1, in the first state (see FIG. 2(a)) and the third state (not shown), the power of the second DC power source E2 is charged to the second inductance L2. In the subsequent second state (see FIG. 2(b)) and the fourth state (not shown), the power stored in the second inductance L2 is discharged to the second DC power source E2. That is, a reactive current unrelated to power transmission flows on the secondary side. The flow of this reactive current causes unnecessary loss.

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

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

[0034] A phase shift method is adopted in Comparative Example 2. The duty ratio of the first switching element Q1 to the sixth switching element Q6 is fixed at 50%, and the seventh switching element Q7 and the eighth switching element Q8 are maintained in an all-off state.

[0035] Fig. 3(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. 3(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 inductance L1 and the second inductance 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.

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

[0037] Fig. 5(a) shows a state in which the isolation transformer TR1 and the second DC power source E2 are disconnected and power is stored in the first inductance L1 and the second inductance L2 from the first DC power source E1 (hereinafter referred to as the storage state). Fig. 5(b) shows a transmission state in which power is transmitted from the first DC power source E1 to the second DC power source E2. In the boost operation, the voltage or current of the transmitted power is controlled according to the ratio between 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 to be.

[0038] 6, the phases of the first and second legs are fixed, the phase of the third leg is variable, and the phase difference between the first and second legs and the third leg is controlled by controlling the phase of the third leg. When increasing the power transmitted from the primary side to the secondary side, the control circuit 13 controls the phase difference to be larger (shifts the phase of the third leg to the right), and when decreasing the power transmitted from the primary side to the secondary side, the control circuit 13 controls the phase difference to be smaller (shifts the phase of the third leg to the left).

[0039] In Comparative Example 2, the seventh switching element Q7 and the eighth switching element Q8 maintain the all-off state, so that the current does not flow back from the second DC power source E2 to the second inductance L2, the first inductance L1, and the first DC power source E1. That is, no reactive current is generated due to the current flowing back from the second DC power source E2 as shown in FIG. 2(a) of Comparative Example 1. In addition, even if the voltage of the second DC power source E2 becomes higher than the voltage of the first DC power source E1 during the step-down operation, the current does not flow back from the second DC power source E2 to the first DC power source E1, so that the next on / off switching of the first switching element Q1-fourth switching element Q4 on the primary side can be prevented from becoming hard switching.

[0040] 4, in the step-down operation of Comparative Example 2, the fourth leg is fully turned off. Therefore, the switching loss of the fifth switching element Q5 and the sixth switching element Q6 included in the third leg is larger than the switching loss of the seventh switching element Q7 and the eighth switching element Q8 included in the fourth leg.

[0041] Also, the first switching element Q1 and the second switching element Q2 included in the first leg are turned off in the transmission state, and the third switching element Q3 and the fourth switching element Q4 included in the second leg are turned off in the commutation state. Since more current flows when turned off in the transmission state, the turn-off loss of the first switching element Q1 and the second switching element Q2 included in the first leg is larger than the turn-off loss of the third switching element Q3 and the fourth switching element Q4 included in the second leg.

[0042] 6, in the boost operation of Comparative Example 2, the fourth leg is fully turned off. Therefore, the switching loss of the fifth switching element Q5 and the sixth switching element Q6 included in the third leg is larger than the switching loss of the seventh switching element Q7 and the eighth switching element Q8 included in the fourth leg.

[0043] (Example (hypertensive)) Fig. 7 is a diagram for explaining a switching pattern of the first switching element Q1 to the eighth switching element Q8 during a step-down operation according to the embodiment of the power conversion device 1, and a first example of a transformer current IL. Fig. 8 is a diagram for explaining a switching pattern of the first switching element Q1 to the eighth switching element Q8 during a step-down operation according to the embodiment of the power conversion device 1, and a second example of a transformer current IL. Figs. 9(a)-(c) are diagrams for explaining a current flow during a step-down operation according to the embodiment of the power conversion device 1 (part 1). Figs. 10(a)-(c) are diagrams for explaining a current flow during a step-down operation according to the embodiment of the power conversion device 1 (part 2).

[0044] This embodiment employs a PWM method, and the control circuit 13 controls the voltage or current of the power transmitted from the first DC section to the second DC section by the on-time and off-time of the drive signal supplied to each of the first switching element Q1 to the eighth switching element Q8. In addition, in order to reduce the current flowing through the fifth diode D5 to the eighth diode D8 of the fifth switching element Q5 to the eighth switching element Q8 included in the second bridge circuit 12, the leg of the second bridge circuit 12 that is not PWM controlled is synchronously rectified in principle. This makes it possible to reduce the diode loss in the second bridge circuit 12.

[0045] When transferring power by stepping down the voltage from the first DC section to the second DC section, the control circuit 13 generates a first period T1 and a second period T2 and alternately switches between the first period T1 and the second period T2. When switching from the second period T2 to the first period T1, the control circuit 13 inserts a dead time Td that controls the first switching element Q1 to the eighth switching element Q8 to an all-off state.

[0046] In the first period T1, 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, resulting in a transmission state. In the first period T1, the control circuit 13 controls two diagonal switching elements (the first switching element Q1 and the fourth switching element Q4, or the second switching element Q2 and the third switching element Q3) included in the first bridge circuit 11 to the ON state, and controls two diagonal switching elements (the fifth switching element Q5 and the eighth switching element Q8, or the sixth switching element Q6 and the seventh switching element Q7) included in the second bridge circuit 12 to the ON state.

[0047] At the start of the second period T2, a dead time of the first leg (the first switching element Q1 and the second switching element Q2 are simultaneously turned off) is inserted. When the second period T2 starts, both ends of the primary winding n1 of the isolation transformer TR1 are short-circuited in the first bridge circuit 11, and the second bridge circuit 12 enters a commutation state in which the secondary winding n2 of the isolation transformer TR1 is conductive with the second DC section.

[0048] In the second period T2, the control circuit 13 controls the two lower switching elements (the second switching element Q2 and the fourth switching element Q4) or the two upper switching elements (the first switching element Q1 and the third switching element Q3) included in the first bridge circuit 11 to the on state, and controls the two diagonal switching elements (the fifth switching element Q5 and the eighth switching element Q8, or the sixth switching element Q6 and the seventh switching element Q7) included in the second bridge circuit 12 to the on state.

[0049] In the second period T2, when the absolute value of the current IL (hereinafter referred to as the transformer current IL) flowing through the insulating transformer TR1 becomes equal to or less than a predetermined threshold value Ith, the control circuit 13 turns off at least one of the two diagonal switching elements included in the second bridge circuit 12. Although efficiency decreases, both switching elements may be turned off. The threshold value Ith is set to 0 A if the transformer current IL can be ideally measured and the delay of the control system and the driving system is ideally 0. In practice, the threshold value Ith is set to (0 ± margin) A, taking into consideration voltage measurement errors and delays of the driving signals supplied to the switching elements. The margin value is set based on experiments and simulations by the designer so that the positive and negative of the current flowing through the insulating transformer TR1 is not inverted within the half cycle Tc / 2 and the synchronous rectification period T2s of the second period T2 is maximized.

[0050] When the absolute value of the transformer current IL does not fall 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. As shown in FIG. 8, the predetermined time is set to a half cycle Tc / 2-dead time Td. That is, when the absolute value of the transformer current IL does not fall to the threshold Ith within a predetermined time (Tc / 2-Td), the two diagonal switching elements included in the second bridge circuit 12 enter the dead time Td while remaining in the on state, and entering the dead time Td causes the control circuit 13 to control all of the fifth switching element Q5 to the eighth switching element Q8 to be in the off state. When there is no risk of a reverse flow of power as shown in FIG. 8, there is no need to switch from synchronous rectification to diode rectification. On the other hand, when there is a risk of a reverse flow of power as shown in FIG. 7, the synchronous rectification is switched to diode rectification.

[0051] The first period T1 in the step-down period includes a first pattern and a second pattern. In the first pattern, the first switching element Q1 and the fourth switching element Q4 are in the ON state, the second switching element Q2 and the third switching element Q3 are in the OFF state, and the second bridge circuit 12 is in the rectifying state (see FIG. 9(a)). In the second pattern, the second switching element Q2 and the third switching element Q3 are in the ON state, the first switching element Q1 and the fourth switching element Q4 are in the OFF state, and the second bridge circuit 12 is in the rectifying state (see FIG. 10(a)). In this embodiment, the second bridge circuit 12 in the first period T1 is controlled to a synchronous rectifying state in which two diagonal switching elements are in the ON state.

[0052] The second period T2 includes a third pattern and a fourth pattern. In the third pattern, the first switching element Q1 and the third switching element Q3 are in an on state and the second switching element Q2 and the fourth switching element Q4 are in an off state, or the second switching element Q2 and the fourth switching element Q4 are in an on state and the first switching element Q1 and the third switching element Q3 are in an off state, and the second bridge circuit 12 is in a rectification state. As shown in FIG. 9(b), when the absolute value of the transformer current IL is greater than the threshold value Ith, the fifth switching element Q5 and the eighth switching element Q8 are in an on state, which is a synchronous rectification state T2-1s. As shown in FIG. 9(c), when the absolute value of the transformer current IL is less than the threshold value Ith, the fifth switching element Q5 or the eighth switching element Q8 is in an off state, which is a diode rectification state T2-1d. In FIG. 9(c), the fifth diode D5 or the eighth diode D8 prevents the reverse flow of power from the second DC power source E2 to the second inductance L2.

[0053] In the fourth pattern, the second bridge circuit 12 is in a rectifying state with the second switching element Q2 and the fourth switching element Q4 in an on state and the first switching element Q1 and the third switching element Q3 in an off state, or the first switching element Q1 and the third switching element Q3 in an on state and the second switching element Q2 and the fourth switching element Q4 in an off state. As shown in FIG. 10(b), when the absolute value of the transformer current IL is greater than the threshold value Ith, the sixth switching element Q6 and the seventh switching element Q7 are in an on state, which is a synchronous rectifying state T2-2s. As shown in FIG. 10(c), when the absolute value of the transformer current IL is less than the threshold value Ith, the sixth switching element Q6 or the seventh switching element Q7 is in an off state, which is a diode rectifying state T2-2d.

[0054] The control circuit 13 synchronizes the period of the first pattern with the period of the second pattern. That is, the control circuit 13 controls the period of the first pattern with the period of the second pattern to be substantially the same time. The control circuit 13 also synchronizes the period of the third pattern with the period of the fourth pattern. That is, the control circuit 13 controls the period of the third pattern with the period of the fourth pattern to be substantially the same time. These controls result in symmetrical positive and negative operations, making it possible to suppress the occurrence of DC bias magnetism in the insulating transformer TR1.

[0055] The control circuit 13 switches the drive signals supplied to the first switching element Q1 to the fourth switching element Q4 included in the first bridge circuit 11 between the first leg and the second leg at each predetermined period. The predetermined period is basically set to one period. That is, the roles of the first leg and the second leg are switched at each period. The predetermined period may be set to multiple periods (for example, 100 periods). However, if the switching period is too long, the peak heat generation of each switching element becomes high, making it difficult to miniaturize the cooler.

[0056] Furthermore, the control circuit 13 switches the drive signals supplied to the fifth switching element Q5 to the eighth switching element Q8 included in the second bridge circuit 12 between the third leg and the fourth leg every predetermined period. The predetermined period is basically set to one period. That is, the roles of the third leg and the fourth leg are switched every period. Note that the predetermined period may be set to multiple periods.

[0057] In the step-down operation, the duty ratio of one of the first and second legs is fixed to 100% (duty ratio=1), and the duty ratio of the other leg is variable. In this embodiment, the duty ratio=100% (duty ratio=1) is set to (half period (Tc / 2)-dead time Td). In the first period Tc1, the duty ratio of the second leg is fixed to 100%, and the duty ratio of the first leg is variable. In the second period Tc2, the duty ratio of the first leg is fixed to 100%, and the duty ratio of the second leg is variable. As the duty ratio of the leg with a variable duty ratio increases, the transmission period becomes longer than the commutation period, and the transmitted power increases. In this way, in the step-down operation, the voltage or current of the transmitted power is controlled by the primary side PWM control.

[0058] In the first period Tc1 of the step-down operation, the control circuit 13 turns on the fourth switching element Q4, the fifth switching element Q5, and the eighth switching element Q8 in synchronization with the turn-on of the first switching element Q1. Next, the control circuit 13 turns off the first switching element Q1. After the dead time of the first leg, the control circuit 13 turns on the second switching element Q2. If the absolute value of the transformer current IL falls to the threshold value Ith within a predetermined time (Tc / 2-Td), the control circuit 13 turns off the second switching element Q2 and the fifth switching element Q5. If the absolute value of the transformer current IL does not fall to the threshold value Ith within the predetermined time (Tc / 2-Td), the turn-off of the second switching element Q2 and the fifth switching element Q5 is skipped.

[0059] Next, the control circuit 13 turns off the eighth switching element Q8 in synchronization with the turning off of the fourth switching element Q4. When the fifth switching element Q5 is in the on state, the control circuit 13 also turns off the fifth switching element Q5. When the second switching element Q2 is in the on state, the control circuit 13 also turns off the second switching element Q2.

[0060] After a dead time Td, the control circuit 13 turns on the third switching element Q3, the sixth switching element Q6, and the seventh switching element Q7 in synchronization with the turn-on of the second switching element Q2. Next, the control circuit 13 turns off the second switching element Q2. After a dead time of the first leg, the control circuit 13 turns on the first switching element Q1. If the absolute value of the transformer current IL falls to the threshold value Ith within a predetermined time (Tc / 2-Td), the control circuit 13 turns off the first switching element Q1 and the sixth switching element Q6. If the absolute value of the transformer current IL does not fall to the threshold value Ith within the predetermined time (Tc / 2-Td), the turn-off of the first switching element Q1 and the sixth switching element Q6 is skipped.

[0061] Next, the control circuit 13 turns off the seventh switching element Q7 in synchronization with the turning off of the third switching element Q3. If the sixth switching element Q6 is in the on state, the control circuit 13 also turns off the sixth switching element Q6. If the first switching element Q1 is in the on state, the control circuit 13 also turns off the first switching element Q1. This completes the first period Tc1.

[0062] In the second period Tc2, the control of the first switching element Q1 and the fourth switching element Q4 is switched, the control of the second switching element Q2 and the third switching element Q3 is switched, the control of the fifth switching element Q5 and the eighth switching element Q8 is switched, and the control of the sixth switching element Q6 and the seventh switching element Q7 is switched.

[0063] FIG. 11 is a diagram for explaining an example of a switching pattern during reverse transmission of the first switching element Q1-the eighth switching element Q8 during 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. 7 and FIG. 8, an example in which power is stepped down from the first DC section to the second DC section and transmitted is explained. In this regard, it is also possible to step down and transmit power from the second DC section to the first DC section. In this case, as shown in FIG. 11, the control circuit 13 may simply replace the drive signal supplied to the first switching element Q1-the fourth switching element Q4 with the drive signal supplied to the fifth switching element Q5-the eighth switching element Q8.

[0064] Fig. 12 is a diagram for explaining a first modification of the switching pattern of the first switching element Q1 to the eighth switching element Q8 during step-down operation in the embodiment of the power conversion device 1. Fig. 13 is a diagram for explaining a second modification of the switching pattern of the first switching element Q1 to the eighth switching element Q8 during step-down operation in the embodiment of the power conversion device 1.

[0065] In a first modification shown in Fig. 12, the control circuit 13 switches the drive signals supplied to the first switching element Q1-fourth switching element Q4 included in the first bridge circuit 11 between the first leg and the second leg at every predetermined period. No switching is performed between the third leg and the fourth leg. In a second modification shown in Fig. 13, the control circuit 13 switches the drive signals supplied to the fifth switching element Q5-eighth switching element Q8 included in the second bridge circuit 12 between the third leg and the fourth leg at every predetermined period. No switching is performed between the first leg and the second leg.

[0066] (Example (Boost)) Fig. 14 is a diagram for explaining a switching pattern of the first switching element Q1 to the eighth switching element Q8 during boost operation in the embodiment of the power conversion device 1 and a first example of the transformer current IL. Fig. 15 is a diagram for explaining a switching pattern of the first switching element Q1 to the eighth switching element Q8 during boost operation in the embodiment of the power conversion device 1 and a second example of the transformer current IL. Figs. 16(a)-(c) are diagrams for explaining a current flow in boost operation in the embodiment of the power conversion device 1 (part 1). Figs. 17(a)-(c) are diagrams for explaining a current flow in boost operation in the embodiment of the power conversion device 1 (part 2).

[0067] When transmitting power by boosting from the first DC section to the second DC section, the control circuit 13 generates a first period T1 and a second period T2 and alternately switches between the first period T1 and the second period T2. When switching from the second period T2 to the first period T1, the control circuit 13 inserts a dead time Td that controls the first switching element Q1 to the eighth switching element Q8 to an all-off state.

[0068] In the first period T1, the first bridge circuit 11 is in a storage state in which the first DC section and the primary winding n1 of the isolation transformer TR1 are conductive and both ends of the secondary winding n2 of the isolation transformer TR1 are short-circuited within the second bridge circuit 12. In the first period T1, the control circuit 13 controls two diagonal switching elements (the first switching element Q1 and the fourth switching element Q4, or the second switching element Q2 and the third switching element Q3) included in the first bridge circuit 11 to an ON state, and controls two lower switching elements (the sixth switching element Q6 and the eighth switching element Q8) or two upper switching elements (the fifth switching element Q5 and the seventh switching element Q7) included in the second bridge circuit 12 to an ON state.

[0069] At the start of the second period T2, a dead time of the third leg (the fifth switching element Q5 and the sixth switching element Q6 are simultaneously turned off) is inserted. When the second period T2 starts, a transmission state is established 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.

[0070] In the second period T2, the control circuit 13 controls two diagonal switching elements (the first switching element Q1 and the fourth switching element Q4, or the second switching element Q2 and the third switching element Q3) included in the first bridge circuit 11 to the on state, and controls two diagonal switching elements (the fifth switching element Q5 and the eighth switching element Q8, or the sixth switching element Q6 and the seventh switching element Q7) included in the second bridge circuit 12 to the on state.

[0071] In the second period T2, when the absolute value of the transformer current IL becomes equal to or smaller than a predetermined threshold value Ith, the control circuit 13 turns off at least one of the two diagonal switching elements included in the second bridge circuit 12. Note that, although efficiency decreases, both switching elements may be turned off.

[0072] When the absolute value of the transformer current IL does not fall 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. As shown in FIG. 15, the predetermined time is set to a half cycle Tc / 2-dead time Td. That is, when the absolute value of the transformer current IL does not fall to the threshold Ith within a predetermined time (Tc / 2-Td), the two diagonal switching elements included in the second bridge circuit 12 enter the dead time Td while remaining in the on state, and by entering the dead time Td, the control circuit 13 controls all of the fifth switching element Q5 to the eighth switching element Q8 to be in the off state. When there is no risk of a reverse flow of power as shown in FIG. 15, there is no need to switch from synchronous rectification to diode rectification. On the other hand, when there is a risk of a reverse flow of power as shown in FIG. 14, the synchronous rectification is switched to diode rectification.

[0073] The first period T1 in the boost period includes a first pattern and a second pattern. In the first pattern, the first switching element Q1 and the fourth switching element Q4 are in an on state, the second switching element Q2 and the third switching element Q3 are in an off state, and the second bridge circuit 12 shorts both ends of the secondary winding n2 of the isolation transformer TR1 (see FIG. 16(a)). In the second pattern, the second switching element Q2 and the third switching element Q3 are in an on state, the first switching element Q1 and the fourth switching element Q4 are in an off state, and the second bridge circuit 12 shorts both ends of the secondary winding n2 of the isolation transformer TR1 (see FIG. 17(a)). In this embodiment, the second bridge circuit 12 in the first period T1 is controlled so that the upper or lower two switching elements are in an on state (one switching element is in a synchronous rectification state).

[0074] The second period T2 includes a third pattern and a fourth pattern. In the third pattern, the first switching element Q1 and the fourth switching element Q4 are in an ON state, the second switching element Q2 and the third switching element Q3 are in an OFF state, and the second bridge circuit 12 is in a rectifying state. As shown in FIG. 16(b), when the absolute value of the transformer current IL is greater than the threshold value Ith, the fifth switching element Q5 and the eighth switching element Q8 are in an ON state, which is a synchronous rectifying state T2-1s. As shown in FIG. 16(c), when the absolute value of the transformer current IL is less than the threshold value Ith, the fifth switching element Q5 or the eighth switching element Q8 is in an OFF state, which is a diode rectifying state T2-1d. In FIG. 16(c), the fifth diode D5 or the eighth diode D8 prevents the reverse flow of power from the second DC power source E2 to the second inductance L2.

[0075] In the fourth pattern, the second switching element Q2 and the third switching element Q3 are in the ON state, the first switching element Q1 and the fourth switching element Q4 are in the OFF state, and the second bridge circuit 12 is in the rectification state. As shown in Fig. 17(b), when the absolute value of the transformer current IL is greater than the threshold value Ith, the sixth switching element Q6 and the seventh switching element Q7 are in the ON state, which is a synchronous rectification state T2-2s. As shown in Fig. 17(c), when the absolute value of the transformer current IL is less than the threshold value Ith, the sixth switching element Q6 or the seventh switching element Q7 is in the OFF state, which is a diode rectification state T2-2d.

[0076] The control circuit 13 synchronizes the period of the first pattern with the period of the second pattern. That is, the control circuit 13 controls the period of the first pattern with the period of the second pattern to be substantially the same time. The control circuit 13 also synchronizes the period of the third pattern with the period of the fourth pattern. That is, the control circuit 13 controls the period of the third pattern with the period of the fourth pattern to be substantially the same time. These controls result in symmetrical positive and negative operations, making it possible to suppress the occurrence of DC bias magnetism in the insulating transformer TR1.

[0077] The control circuit 13 switches the drive signals supplied to the fifth switching element Q5 to the eighth switching element Q8 included in the second bridge circuit 12 between the third leg and the fourth leg at each predetermined period. The predetermined period is basically set to one period. That is, the roles of the third leg and the fourth leg are switched at each period. The predetermined period may be set to multiple periods.

[0078] In boost operation, the duty ratios of both the first and second legs are fixed at 100%, the duty ratio of one of the third and fourth legs is fixed at 100%, and the duty ratio of the other leg is variable. In the first period Tc1, the duty ratio of the fourth leg is fixed at 100%, and the duty ratio of the third leg is variable. In the second period Tc2, the duty ratio of the third leg is fixed at 100%, and the duty ratio of the fourth leg is variable. As the duty ratio of the leg with a variable duty ratio increases, the accumulation period becomes longer than the transmission period, and the transmitted power increases. In this way, in boost operation, the voltage or current of the transmitted power is controlled by PWM control on the secondary side.

[0079] In the first period Tc1 of the boost operation, the control circuit 13 turns on the fourth switching element Q4, the sixth switching element Q6, and the eighth switching element Q8 in synchronization with the turn-on of the first switching element Q1. Next, the control circuit 13 turns off the sixth switching element Q6. After the dead time of the third leg, the control circuit 13 turns on the fifth switching element Q5. If the absolute value of the transformer current IL falls to the threshold value Ith within a predetermined time (Tc / 2-Td), the control circuit 13 turns off the fifth switching element Q5. If the absolute value of the transformer current IL does not fall to the threshold value Ith within the predetermined time (Tc / 2-Td), the turn-off of the fifth switching element Q5 is skipped.

[0080] Next, the control circuit 13 turns off the fourth switching element Q4 and the eighth switching element Q8 in synchronization with the turning off of the first switching element Q1. If the fifth switching element Q5 is in the on state, the control circuit 13 also turns off the fifth switching element Q5.

[0081] After a dead time Td, the control circuit 13 turns on the third switching element Q3, the fifth switching element Q5, and the seventh switching element Q7 in synchronization with the turn-on of the second switching element Q2. Next, the control circuit 13 turns off the fifth switching element Q5. After a dead time of the third leg, the control circuit 13 turns on the sixth switching element Q6. If the absolute value of the transformer current IL falls to the threshold value Ith within a predetermined time (Tc / 2-Td), the control circuit 13 turns off the sixth switching element Q6. If the absolute value of the transformer current IL does not fall to the threshold value Ith within the predetermined time (Tc / 2-Td), the turn-off of the sixth switching element Q6 is skipped.

[0082] Next, the control circuit 13 turns off the third switching element Q3 and the seventh switching element Q7 in synchronization with the turning off of the second switching element Q2. If the sixth switching element Q6 is in the on state, the control circuit 13 also turns off the sixth switching element Q6. This completes the first period Tc1.

[0083] In the second period Tc2, the control of the sixth switching element Q6 and the seventh switching element Q7 is switched, and the control of the fifth switching element Q5 and the eighth switching element Q8 is switched.

[0084] FIG. 18 is a diagram for explaining an example of a switching pattern of the first switching element Q1-the eighth switching element Q8 during reverse transmission during boost operation according to the embodiment of the power conversion device 1, and a transformer current IL. In the switching pattern of the first switching element Q1-the eighth switching element Q8 shown in FIG. 14 and FIG. 15, an example in which power is boosted from the first DC section to the second DC section and then transmitted is explained. In this regard, it is also possible to boost power from the second DC section to the first DC section and then transmitted. In this case, as shown in FIG. 18, the control circuit 13 may simply replace the drive signal supplied to the first switching element Q1-the fourth switching element Q4 with the drive signal supplied to the fifth switching element Q5-the eighth switching element Q8.

[0085] (Example (Step-up / step-down)) Fig. 19 is a diagram for explaining a switching pattern of the first switching element Q1 to the eighth switching element Q8 during step-up and step-down operation according to the embodiment of the power conversion device 1, and a first example of a transformer current IL. Fig. 20 is a diagram for explaining a switching pattern of the first switching element Q1 to the eighth switching element Q8 during step-up and step-down operation according to the embodiment of the power conversion device 1, and a second example of a transformer current IL. Figs. 21(a)-(b) are diagrams for explaining a current flow during step-up and step-down operation according to the embodiment of the power conversion device 1 (part 1). Figs. 22(a)-(b) are diagrams for explaining a current flow during step-up and step-down operation according to the embodiment of the power conversion device 1 (part 2).

[0086] The step-up / step-down operation is an operation inserted in a period between switching from step-down operation to step-up operation or switching from step-up operation to step-down operation. The step-up / step-down operation is initiated when the duty ratio of the first leg reaches a threshold value α in step-down operation (see FIG. 26 described later). The threshold value α is set to a value obtained by subtracting a duty ratio corresponding to a dead time from 100%. The threshold value α can be set to an arbitrary value as appropriate. The step-up / step-down operation is initiated when the duty ratio of the third leg reaches a threshold value β in step-up operation. The threshold value β is set to a value obtained by adding a duty ratio corresponding to a dead time to 0%. The threshold value β can be set to an arbitrary value as appropriate. In the step-up / step-down operation, power control by PWM control on the primary side in step-down operation and power control by PWM control on the secondary side in step-up operation coexist.

[0087] When transferring power from the first DC section to the second DC section by stepping up or down during a period when switching between step-up and step-down, the control circuit 13 generates a first period T1, a second period T2, and a third period T3, and switches between the first period T1, the second period T2, and the third period T3 in that order. When switching from the third period T3 to the first period T1, the control circuit 13 inserts a dead time Td that controls the first switching element Q1 to the eighth switching element Q8 to be all in an off state.

[0088] In the first period T1, the first bridge circuit 11 is in a storage state in which the first DC section and the primary winding n1 of the isolation transformer TR1 are conductive and both ends of the secondary winding n2 of the isolation transformer TR1 are short-circuited within the second bridge circuit 12. In the first period T1, the control circuit 13 controls two diagonal switching elements (the first switching element Q1 and the fourth switching element Q4, or the second switching element Q2 and the third switching element Q3) included in the first bridge circuit 11 to an ON state, and controls two lower switching elements (the sixth switching element Q6 and the eighth switching element Q8) or two upper switching elements (the fifth switching element Q5 and the seventh switching element Q7) included in the second bridge circuit 12 to an ON state.

[0089] At the start of the second period T2, a dead time of the third leg (the fifth switching element Q5 and the sixth switching element Q6 are simultaneously turned off) is inserted. When the second period T2 starts, a transmission state is established 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.

[0090] In the second period T2, the control circuit 13 controls two diagonal switching elements (the first switching element Q1 and the fourth switching element Q4, or the second switching element Q2 and the third switching element Q3) included in the first bridge circuit 11 to the on state, and controls two diagonal switching elements (the fifth switching element Q5 and the eighth switching element Q8, or the sixth switching element Q6 and the seventh switching element Q7) included in the second bridge circuit 12 to the on state.

[0091] In the third period T3, both ends of the primary winding n1 of the isolation transformer TR1 are short-circuited in the first bridge circuit 11, and the second bridge circuit 12 is in a commutation state in which the secondary winding n2 of the isolation transformer TR1 is conductive with the second DC section. In the third period T3, the control circuit 13 controls at least one lower switching element (at least one of the second switching element Q2 and the fourth switching element Q4) or at least one upper switching element (at least one of the first switching element Q1 and the third switching element Q3) included in the first bridge circuit 11 to an ON state, and controls two diagonal switching elements (the fifth switching element Q5 and the eighth switching element Q8, or the sixth switching element Q6 and the seventh switching element Q7) included in the second bridge circuit 12 to an ON state.

[0092] In the second period T2 and the third period T3, 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 included in the second bridge circuit 12. Note that, although the efficiency decreases, both switching elements may be turned off.

[0093] When the absolute value of the transformer current IL does not fall 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. As shown in FIG. 20, the predetermined time is set to a half cycle Tc / 2-dead time Td. That is, when the absolute value of the transformer current IL does not fall to the threshold Ith within a predetermined time (Tc / 2-Td), the two diagonal switching elements included in the second bridge circuit 12 enter the dead time Td while remaining in the on state, and by entering the dead time Td, the control circuit 13 controls all of the fifth switching element Q5 to the eighth switching element Q8 to be in the off state. When there is no risk of a reverse flow of power as shown in FIG. 20, there is no need to switch from synchronous rectification to diode rectification. On the other hand, when there is a risk of a reverse flow of power as shown in FIG. 19, the synchronous rectification is switched to diode rectification.

[0094] The first period T1 in the voltage boost / buck period includes a first pattern and a second pattern. The first pattern and the second pattern are the same as the first pattern and the second pattern of the first period T1 in the voltage boost period described above. The second period T2 in the voltage boost / buck period includes a third pattern and a fourth pattern. The third pattern and the fourth pattern are the same as the third pattern and the fourth pattern of the second period T2 in the voltage boost period described above.

[0095] The third period T3 in the step-up / step-down period includes a fifth pattern and a sixth pattern. In the fifth pattern, the first switching element Q1 or the fourth switching element Q4 and the third switching element Q3 or the second switching element Q2 are in an on state, the fourth switching element Q4 or the first switching element Q1 and the second switching element Q2 or the third switching element Q3 are in an off state, and the second bridge circuit 12 is in a rectifying state. As shown in FIG. 21(a), when the absolute value of the transformer current IL is greater than the threshold value Ith, the fifth switching element Q5 and the eighth switching element Q8 are in an on state, which is a synchronous rectifying state T3-1s. As shown in FIG. 21(b), when the absolute value of the transformer current IL is less than the threshold value Ith, the fifth switching element Q5 or the eighth switching element Q8 is in an off state, which is a diode rectifying state T3-1d. In FIG. 21(b), the fifth diode D5 or the eighth diode D8 prevents the reverse flow of power from the second DC power source E2 to the second inductance L2.

[0096] In the sixth pattern, the second bridge circuit 12 is in a rectifying state, with the second switching element Q2 or the third switching element Q3 and the fourth switching element Q4 or the first switching element Q1 being in an on state, and the first switching element Q1 or the fourth switching element Q4 and the third switching element Q3 or the second switching element Q2 being in an off state. As shown in FIG. 22(a), when the absolute value of the transformer current IL is greater than the threshold value Ith, the sixth switching element Q6 and the seventh switching element Q7 are in an on state, in a synchronous rectifying state T3-2s. As shown in FIG. 22(b), when the absolute value of the transformer current IL is less than the threshold value Ith, the sixth switching element Q6 or the seventh switching element Q7 is in an off state, in a diode rectifying state T3-2d.

[0097] The control circuit 13 synchronizes the period of the first pattern with the period of the second pattern. That is, the control circuit 13 controls the period of the first pattern with the period of the second pattern to be substantially the same time. The control circuit 13 also synchronizes the period of the third pattern with the period of the fourth pattern. That is, the control circuit 13 controls the period of the third pattern with the period of the fourth pattern to be substantially the same time. The control circuit 13 also synchronizes the period of the fifth pattern with the period of the sixth pattern. That is, the control circuit 13 controls the period of the fifth pattern with the period of the sixth pattern to be substantially the same time. These controls result in a positive-negative symmetrical operation, making it possible to suppress the occurrence of DC bias magnetism in the insulating transformer TR1.

[0098] The control circuit 13 switches the drive signals supplied to the first switching element Q1 to the fourth switching element Q4 included in the first bridge circuit 11 between the first leg and the second leg every predetermined period. The predetermined period is basically set to one period. That is, the roles of the first leg and the second leg are switched every period. Note that the predetermined period may be set to multiple periods.

[0099] Furthermore, the control circuit 13 switches the drive signals supplied to the fifth switching element Q5 to the eighth switching element Q8 included in the second bridge circuit 12 between the third leg and the fourth leg every predetermined period. The predetermined period is basically set to one period. That is, the roles of the third leg and the fourth leg are switched every period. Note that the predetermined period may be set to multiple periods.

[0100] In the first period Tc of the step-up / step-down operation, the control circuit 13 turns on the fourth switching element Q4, the sixth switching element Q6, and the eighth switching element Q8 in synchronization with the turn-on of the first switching element Q1. Next, the control circuit 13 turns off the sixth switching element Q6. After the dead time of the third leg, the control circuit 13 turns on the fifth switching element Q5. Next, the control circuit 13 turns off the first switching element Q1. After turning on the fifth switching element Q5, if the absolute value of the transformer current IL falls to the threshold value Ith within a predetermined time (Tc / 2-Td), the control circuit 13 turns off the fifth switching element Q5. If the absolute value of the transformer current IL does not fall to the threshold value Ith within the predetermined time (Tc / 2-Td), the turn-off of the fifth switching element Q5 is skipped.

[0101] Next, the control circuit 13 turns off the eighth switching element Q8 in synchronization with the turning off of the fourth switching element Q4. When the fifth switching element Q5 is in the on state, the control circuit 13 also turns off the fifth switching element Q5. When the second switching element Q2 is in the on state, the control circuit 13 also turns off the second switching element Q2.

[0102] After the dead time Td, the control circuit 13 turns on the third switching element Q3, the fifth switching element Q5, and the seventh switching element Q7 in synchronization with the turn-on of the second switching element Q2. Next, the control circuit 13 turns off the fifth switching element Q5. After the dead time of the third leg, the control circuit 13 turns on the sixth switching element Q6. Next, the control circuit 13 turns off the second switching element Q2. After turning on the sixth switching element Q6, if the absolute value of the transformer current IL falls to the threshold value Ith within a predetermined time (Tc / 2-Td), the control circuit 13 turns off the sixth switching element Q6. If the absolute value of the transformer current IL does not fall to the threshold value Ith within the predetermined time (Tc / 2-Td), the turn-off of the sixth switching element Q6 is skipped.

[0103] Next, the control circuit 13 turns off the seventh switching element Q7 in synchronization with the turning off of the third switching element Q3. If the sixth switching element Q6 is in the on state, the control circuit 13 also turns off the sixth switching element Q6. If the first switching element Q1 is in the on state, the control circuit 13 also turns off the first switching element Q1. This ends the first period Tc1.

[0104] In the second period Tc, the control of the first switching element Q1 and the fourth switching element Q4 is switched, the control of the second switching element Q2 and the third switching element Q3 is switched, the control of the fifth switching element Q5 and the eighth switching element Q8 is switched, and the control of the sixth switching element Q6 and the seventh switching element Q7 is switched.

[0105] FIG. 23 is a diagram for explaining an example of a switching pattern of the first switching element Q1-the eighth switching element Q8 during reverse transmission during the step-up / 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. 19 and FIG. 20, an example of transmitting power by stepping up / down from the first DC section to the second DC section has been described. In this regard, it is also possible to transmit power by stepping up / down from the second DC section to the first DC section. In this case, as shown in FIG. 23, the control circuit 13 may simply replace the drive signal supplied to the first switching element Q1-the fourth switching element Q4 with the drive signal supplied to the fifth switching element Q5-the eighth switching element Q8.

[0106] Fig. 24 is a diagram for explaining a first modification of the switching pattern of the first switching element Q1 to the eighth switching element Q8 during step-up and step-down operation in the embodiment of the power conversion device 1. Fig. 25 is a diagram for explaining a second modification of the switching pattern of the first switching element Q1 to the eighth switching element Q8 during step-up and step-down operation in the embodiment of the power conversion device 1.

[0107] In a first modification shown in FIG. 24, the control circuit 13 switches the drive signals supplied to the fifth switching element Q5-eighth switching element Q8 included in the second bridge circuit 12 between the third leg and the fourth leg at every predetermined period. No switching is performed between the first leg and the second leg. In a second modification shown in FIG. 25, the control circuit 13 switches the drive signals supplied to the first switching element Q1-fourth switching element Q4 included in the first bridge circuit 11 between the third leg and the fourth leg at every predetermined period. No switching is performed between the third leg and the fourth leg.

[0108] 26 is a diagram for explaining switching between a step-down operation, a step-up operation, and a step-up / step-down operation according to an embodiment of the present invention. In this embodiment, a step-up / step-down operation is interposed between a step-down operation and a step-up operation.

[0109] The control circuit 13 calculates a control operation amount based on the deviation between a target value of the current or voltage of the controlled object and an actual detected value. For example, the control circuit 13 calculates the control operation amount by performing PI compensation on the deviation. The method of calculating the control operation amount will be described in detail later. The control circuit 13 switches the operation of the DAB converter based on the calculated control operation amount.

[0110] In the step-down operation, the control circuit 13 PWM controls the first leg, fixes the second leg and the fourth leg to a duty ratio of 100% (maximum PWM width), and synchronously rectifies the third leg. The PWM width of the third leg varies depending on the conditions. In the step-up operation, the control circuit 13 PWM controls the third leg, and fixes the first leg, the second leg, and the fourth leg to a duty ratio of 100% (maximum PWM width). In the step-up / step-down operation, the control circuit 13 PWM controls the first leg and the third leg, and fixes the second leg and the fourth leg to a duty ratio of 100% (maximum PWM width).

[0111] The control circuit 13 switches to the buck-boost operation when the PWM width of the first leg rises to a threshold value α during the buck operation. The control circuit 13 switches to the buck-boost operation before the PWM width of the first leg reaches a maximum during the buck operation. The control circuit 13 also switches to the buck-boost operation when the PWM width of the third leg falls to a threshold value β during the boost operation. The control circuit 13 switches to the buck-boost operation before the PWM width of the third leg reaches 0 during the boost operation. This allows smooth switching between the buck operation and the boost operation. It is possible to prevent a dead period from occurring between the buck operation and the boost operation, and to suppress distortion in the output current.

[0112] 27 is a diagram showing an example of the configuration of the control circuit 13. The control circuit 13 includes a detector 131, a subtraction unit 132, a controller 133, a synchronous rectification period calculation unit 135, and a PWM generation unit 136. The detector 131 includes an A / D converter. The A / D converter converts the analog output current value Io detected by the current sensor 23 into a digital output current value at a predetermined sampling rate.

[0113] The subtraction unit 132 calculates a deviation err between the current command value Iref to be a target value and the output current value input from the detector 131. The controller 133 uses a predetermined control gain to perform PI control of the deviation err and calculates a control operation amount duty. The controller 133 outputs the calculated control operation amount duty to the synchronous rectification period calculation unit 135 and the PWM generation unit 136.

[0114] The synchronous rectification period calculation unit 135 calculates the synchronous rectification period in the second bridge circuit 12 by using the differential voltage between the input voltage V1 detected by the first voltage sensor 21 and the output voltage V2 detected by the second voltage sensor 22.

[0115] 28(a)-(b) are graphs showing an example of variables and constants in a theoretical formula for calculating the synchronous rectification period. Hereinafter, a method for calculating the transformer current IL in the step-down operation and step-up operation and the synchronous rectification period T2s of the second period T2 will be described with reference to FIG. 28(a).

[0116] The synchronous rectification period calculation unit 135 calculates the transformer current IL in the first period T1 in the step-down operation by the following formula (1). V1-V2=L (IL / T1) IL=(V1-V2)·T1 / L (Equation 1) L is the inductance between the first bridge circuit 11 and the second bridge circuit 12 (in this embodiment, L=L1+L2). T1 is the duration of the first period T1 based on the feedback control, and is determined by the control operation amount duty calculated by the controller 133 in this embodiment.

[0117] The synchronous rectification period calculation unit 135 calculates the transformer current (Ith-IL) in the synchronous rectification period T2s of the second period T2 in the step-down operation by the following (Equation 2). 0-V2=L·((Ith-IL) / T2s) Ith-IL=-V2·T2s / L (Formula 2)

[0118] The synchronous rectification period calculation unit 135 calculates the synchronous rectification period T2s of the second period T2 in the step-down operation by the following (Equation 3). T2s=((V1-V2)·T1 / V2)-((L / V2)·Ith) (Formula 3) Since the inductance L and the threshold value Ith are constants, the synchronous rectification period T2s of the second period T2 in the step-down operation can be calculated by detecting the input voltage V1 and the output voltage V2.

[0119] The synchronous rectification period calculation unit 135 calculates the transformer current IL in the first period T1 in the voltage step-up operation by the following (Equation 4). V1 = L (IL / T1) IL=V1 T1 / L (Equation 4) L is the inductance between the first bridge circuit 11 and the second bridge circuit 12 (in this embodiment, L=L1+L2). T1 is the duration of the first period T1 based on the feedback control, and is determined by the control operation amount duty calculated by the controller 133 in this embodiment.

[0120] The synchronous rectification period calculation unit 135 calculates the transformer current (Ith-IL) in the synchronous rectification period T2s of the second period T2 in the boost operation by the following (Equation 5). V1-V2=L ((Ith-IL) / T2s) Ith-IL=(V1-V2)·T2s / L···(Formula 5)

[0121] The synchronous rectification period calculation unit 135 calculates the synchronous rectification period T2s of the second period T2 in the boost operation by the following (Equation 6). T2s=(V1·T1 / (V2-V1))-(L / (V2-V1)·Ith) (Formula 6) Since the inductance L and the threshold value Ith are constants, the synchronous rectification period T2s of the second period T2 in the boost operation can be calculated by detecting the input voltage V1 and the output voltage V2.

[0122] In the step-up / step-down operation, if the absolute value of the current flowing through the isolation transformer TR1 in the second period T2 is equal to or less than the threshold Ith, the synchronous rectification period T2s in the second period T2 can be calculated by the above (Equation 4) to (Equation 6). In the step-up / step-down operation, if the absolute value of the current flowing through the isolation transformer TR1 does not become equal to or less than the threshold Ith in the second period T2, but becomes equal to or less than the threshold Ith in the third period T3, a method for calculating the transformer current IL and the synchronous rectification period T3s in the third period T3 will be described with reference to Fig. 28(b).

[0123] The synchronous rectification period calculation unit 135 calculates the transformer current IL1 in the first period T1 in the voltage step-up / step-down operation by the following (Equation 7). V1 = L (IL1 / T1) IL1=V1·T1 / L (Formula 7) L is the inductance between the first bridge circuit 11 and the second bridge circuit 12 (in this embodiment, L=L1+L2). T1 is the duration of the first period T1 based on the feedback control, and is determined by the control operation amount duty calculated by the controller 133 in this embodiment.

[0124] In the step-up / step-down operation, the synchronous rectification period calculation unit 135 calculates the transformer current IL2 in the second period T2 by the following (Equation 8) when the absolute value of the current flowing through the insulating transformer TR1 in the second period T2 does not become equal to or lower than the threshold value Ith. V1-V2=L ((IL2-IL1) / T2) IL2-IL1=(V1-V2)·T2 / L···(Formula 8)

[0125] In the step-up / step-down operation, the synchronous rectification period calculation unit 135 calculates the transformer current (Ith-IL2) of the synchronous rectification period T3s of the third period T3 when the absolute value of the current flowing through the insulating transformer TR1 in the third period T3 is equal to or less than the threshold value Ith, using the following (Equation 9). -V2=L·((Ith-IL2) / T3s) Ith-IL2=-V2·T2 / L (Formula 9)

[0126] In the step-up / step-down operation, the synchronous rectification period calculation unit 135 calculates the synchronous rectification period T3s of the third period T3 when the absolute value of the current flowing through the insulating transformer TR1 in the third period T3 is equal to or less than the threshold Ith, using the following (Equation 10). T3s=((V1·T1+(V1-V2)·T2) / V2)-(L / V2·Ith) ···(Formula 10) Since the inductance L and the threshold value Ith are constants, the synchronous rectification period T3s of the third period T3 in the step-up / step-down operation can be calculated by detecting the input voltage V1 and the output voltage V2.

[0127] The PWM generating unit 136 generates a PWM signal for driving the first switching element Q1 to the eighth switching element Q8 based on the control operation amount duty calculated by the controller 133 and the synchronous rectification period T2s of the second period T2 or the synchronous rectification period T3s of the third period T3 calculated by the synchronous rectification period calculating unit 135.

[0128] As described above, according to this embodiment, the drive signal of the first leg and the drive signal of the second leg are switched every predetermined period, and / or the drive signal of the third leg and the drive signal of the fourth leg are switched every predetermined period. This makes it possible to equalize at least one of the losses of the first switching element Q1-fourth switching element Q4 included in the first bridge circuit 11 and the losses of the fifth switching element Q5-eighth switching element Q8 included in the second bridge circuit 12. This makes it possible to equalize the heat generation amount of the first switching element Q1-fourth switching element Q4 or the heat generation amount of the fifth switching element Q5-eighth switching element Q8, and to reduce the peak of the heat generation amount of the switching element that generates the maximum loss. Therefore, it is possible to realize the miniaturization and cost reduction of the member (for example, heat sink) for cooling the switching elements. In addition, it is possible to reduce the variation in life span between the switching elements, and product management becomes easier.

[0129] Moreover, by performing synchronous rectification in the second bridge circuit 12 in the second period T2 in the step-down operation, the second period T2 in the step-up operation, and the second period T2 or the third period T3 in the step-up / step-down operation, a highly efficient DAB converter can be realized. In this case, when the transformer current IL drops to about 0 A, the second bridge circuit 12 is switched from synchronous rectification to diode rectification, thereby preventing a decrease in efficiency due to the generation of reactive current. This embodiment is particularly effective for a DAB converter using a SiC-MOSFET in which the loss of the parasitic diode is large.

[0130] Furthermore, according to this embodiment, since control is performed by the PWM method rather than the phase shift method, it is possible to easily generate a dead time during which the first switching element Q1 through the eighth switching element Q8 are all in an off state, thereby suppressing the occurrence of recovery loss in the diodes.

[0131] The present disclosure has been described above based on the embodiments. The embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of each component and each processing step, and that such modifications are also within the scope of the present disclosure.

[0132] In the above embodiment, an operation example in which the DAB converter is controlled by the PWM method has been described. In this regard, the present disclosure can also be applied to a case in which the DAB converter is controlled by the phase shift method as shown in Fig. 4 and Fig. 6, in which the drive signal of the first leg and the drive signal of the second leg are interchanged every predetermined period, and / or the drive signal of the third leg and the drive signal of the fourth leg are interchanged every predetermined period.

[0133] In the above embodiment, in order to reduce the current flowing through the fifth diode D5-eighth diode D8 of the fifth switching element Q5-eighth switching element Q8 included in the second bridge circuit 12, the leg of the second bridge circuit 12 that is not PWM controlled is synchronously rectified in principle. In this regard, the leg of the second bridge circuit 12 that is not PWM controlled may be controlled to be always off. Even in this case, the present disclosure can be applied in which the drive signal of the first leg and the drive signal of the second leg are interchanged every predetermined period, and / or the drive signal of the third leg and the drive signal of the fourth leg are interchanged every predetermined period. For example, in the first period Tc1 during the step-down operation shown in FIG. 7, the fourth leg (the seventh switching element Q7 and the eighth switching element Q8) is controlled to be always off, and in the second period Tc2, the third leg (the fifth switching element Q5 and the sixth switching element Q6) is controlled to be always off.

[0134] In the above embodiment, an example has been described in which the transformer current IL is calculated using the above (Equation 1) to (Equation 10) without installing a current sensor between the first bridge circuit 11 and the isolation transformer TR1 and between the isolation transformer TR1 and the second bridge circuit 12. In this regard, a current sensor may be installed between the first bridge circuit 11 and the isolation transformer TR1 or between the isolation transformer TR1 and the second bridge circuit 12, and the current measured by the current sensor may be set as the transformer current IL and compared with the threshold value Ith.

[0135] The embodiment may be specified by the following items.

[0136] [Item 1] a first bridge circuit (11) including 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 leg and the second leg being connected in parallel to a first DC unit (E1, Ca); a second bridge circuit (12) including 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 third leg and the fourth leg being connected in parallel to a second DC unit (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) through the eighth switching element (Q8); The control circuit (13) When power is stepped down from the first DC section (E1, Ca) to the second DC section (E2, Cb) and transmitted, a first period during which the first bridge circuit (11) conducts the first DC section (E1, Ca) 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 (E2, Cb), and a second period during 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 (E2, Cb), switching the drive signals supplied to the four switching elements (Q5-Q8) included in the second bridge circuit (12) between the third leg and the fourth leg; Power conversion device (1). The control circuit (13) may alternate the drive signals supplied to the four switching elements (Q5-Q8) included in the second bridge circuit (12) between the third leg and the fourth leg at predetermined intervals or irregularly. This allows the loss occurring on the secondary side during voltage step-down operation to be evenly allocated to the fifth switching element (Q5) to the eighth switching element (Q8). [Item 2] the first period includes a first pattern and a second pattern, and the second period includes a third pattern and a fourth pattern; the first pattern is a state in which the first switching element (Q1) and the fourth switching element (Q4) are in an on state, the second switching element (Q2) and the third switching element (Q3) are in an off state, and the second bridge circuit (12) is in a rectifying state; the second pattern is a state in which the second switching element (Q2) and the third switching element (Q3) are in an on state, the first switching element (Q1) and the fourth switching element (Q4) are in an off state, and the second bridge circuit (12) is in a rectifying state; the third pattern is a state in which the first switching element (Q1) and the third switching element (Q3) are in an on state and the second switching element (Q2) and the fourth switching element (Q4) are in an off state, or the second switching element (Q2) and the fourth switching element (Q4) are in an on state and the first switching element (Q1) and the third switching element (Q3) are in an off state, and the second bridge circuit (12) is in a rectifying state; the fourth pattern is a state in which the second switching element (Q2) and the fourth switching element (Q4) are in an on state and the first switching element (Q1) and the third switching element (Q3) are in an off state, or a state in which the first switching element (Q1) and the third switching element (Q3) are in an on state and the second switching element (Q2) and the fourth switching element (Q4) are in an off state, and the second bridge circuit (12) is in a rectifying state; The power conversion device (1) according to item 1. This makes it possible to suppress reactive current during voltage step-down operation and improve efficiency. [Item 3] The control circuit (13) Synchronizing a period of the first pattern with a period of the second pattern; synchronizing a period of the third pattern with a period of the fourth pattern; The power conversion device (1) according to item 2. This results in a positive / negative symmetrical operation, making it possible to suppress the occurrence of DC bias magnetism. [Item 4] The control circuit (13) The voltage or current of the power supplied from the first DC unit (E1, Ca) to the second DC unit (E2, Cb) is controlled by the on-time and off-time of the drive signal supplied to each switching element (Q1-Q8). The power conversion device (1) according to any one of items 1 to 3. According to this, by controlling using the PWM method instead of the phase shift method, a total off period can be provided, and recovery loss can be reduced. [Item 5] The control circuit (13) When power is stepped down and transmitted from the second DC unit (E2, Cb) to the first DC unit (E1, Ca), The drive signals supplied to the first switching element (Q1) through the fourth switching element (Q4) are switched with the drive signals supplied to the fifth switching element (Q5) through the eighth switching element (Q8). The power conversion device (1) according to any one of items 1 to 3. This allows for bidirectional operation. [Explanation of symbols]

[0137] E1 first DC power supply, E2 second DC power supply, 1 power conversion device, 11 first bridge circuit, 12 second bridge circuit, 13 control circuit, 21 first voltage sensor, 22 second voltage sensor, 23 current sensor, 131 detector, 132 subtraction unit, 133 controller, 135 synchronous rectification period calculation unit, 136 PWM generation unit, Q1-Q8 switching elements, D1-D8 diodes, C1-C8 capacitance, 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 bridge circuit including a first leg in which a first switching element and a second switching element are connected in series, and a second leg in which a third switching element and a fourth switching element are connected in series, the first leg and the second leg being connected in parallel to a first DC section; a second bridge circuit including a third leg in which a fifth switching element and a sixth switching element are connected in series, and a fourth leg in which a seventh switching element and an eighth switching element are connected in series, the third leg and the fourth leg being connected in parallel to a second DC section; an isolation transformer connected between the first bridge circuit and the second bridge circuit; A control circuit that controls the first switching element to the eighth switching element, The control circuit includes: When power is stepped down from the first DC section to the second DC section and transmitted, a first period during which the first bridge circuit conducts the first DC section and a primary winding of the isolation transformer and the second bridge circuit conducts the secondary winding of the isolation transformer with the second DC section, and a second period during 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 with the second DC section, switching drive signals supplied to four switching elements included in the second bridge circuit between the third leg and the fourth leg; Power conversion equipment.

2. the first period includes a first pattern and a second pattern, and the second period includes a third pattern and a fourth pattern; the first pattern is a state in which the first switching element and the fourth switching element are in an on state, the second switching element and the third switching element are in an off state, and the second bridge circuit is in a rectifying state; the second pattern is a state in which the second switching element and the third switching element are in an on state, the first switching element and the fourth switching element are in an off state, and the second bridge circuit is in a rectifying state; the third pattern is a state in which the first switching element and the third switching element are in an on state and the second switching element and the fourth switching element are in an off state, or the second switching element and the fourth switching element are in an on state and the first switching element and the third switching element are in an off state, and the second bridge circuit is in a rectifying state; The fourth pattern is a state in which the second switching element and the fourth switching element are in an on state and the first switching element and the third switching element are in an off state, or a state in which the first switching element and the third switching element are in an on state and the second switching element and the fourth switching element are in an off state, and the second bridge circuit is in a rectifying state. The power conversion device according to claim 1 .

3. The control circuit includes: Synchronizing a period of the first pattern with a period of the second pattern; synchronizing a period of the third pattern with a period of the fourth pattern; The power conversion device according to claim 2 .

4. The control circuit includes: a voltage or a current of the power supplied from the first DC unit to the second DC unit is controlled by an on-time and an off-time of a drive signal supplied to each switching element; The power conversion device according to claim 1 .

5. The control circuit includes: When power is stepped down from the second DC section to the first DC section and transmitted, The drive signals supplied to the first switching element to the fourth switching element are interchanged with the drive signals supplied to the fifth switching element to the eighth switching element. The power conversion device according to claim 1 .

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