Power conversion device

Abstract

In the present invention, a step-down mode includes a step-down delay phase mode in which, after a primary-side voltage is raised from a low level to a middle level, a secondary-side voltage is raised from the low level to a high level and, then, the primary-side voltage is raised from the middle level to the high level. The difference between a first time, at which the primary-side voltage is raised from the low level to the middle level, and a second time, at which the secondary-side voltage is raised from the low level to the high level, is a first phase difference. The difference between the second time and a third time, at which the primary-side voltage is raised from the middle level to the high level, is a second phase difference. In the step-down delay phase mode, a control unit (50) derives a combination which is a combination of the first phase difference and the second phase difference, with which a required power can be output, with which a reactor current flowing through the reactor (L) becomes greater than or equal to a reactor current threshold at the second time, and with which the reactor current becomes less than or equal to the reactor current threshold at the third time.

Description

Power Conversion Device

[0001] The present disclosure relates to a power conversion device.

[0002] The power conversion device disclosed in Patent Document 1 includes a transformer, a primary-side full bridge circuit, a secondary-side full bridge circuit, and a control unit. The transformer includes a primary winding and a secondary winding. The primary-side full bridge circuit includes a plurality of primary-side switching elements. The secondary-side full bridge circuit includes a plurality of secondary-side switching elements. The control unit controls the primary-side full bridge circuit so that three levels of primary-side voltages are applied to the primary winding. The control unit controls the secondary-side full bridge circuit so that three levels of secondary-side voltages are applied to the secondary winding. The control unit realizes soft switching of the primary-side switching elements and the secondary-side switching elements by controlling the phase difference between the primary-side voltage and the secondary-side voltage.

[0003] International Publication No. 2020 / 003717

[0004] The control unit may control the power conversion device so that two levels of voltage are applied to one of the primary winding and the secondary winding. Even in this case, soft switching of the primary switching element and the secondary switching element is required.

[0005] A power conversion device according to one aspect of the present disclosure includes a transformer having a primary winding with a number of turns N1 and a secondary winding with a number of turns N2, a reactor connected to the primary winding, the primary winding and the reactor forming a series connection, a primary full bridge circuit having a plurality of primary switching elements connected to the reactor, a secondary full bridge circuit having a plurality of secondary switching elements connected to the secondary winding, and a secondary full bridge circuit having a plurality of primary switching elements and a secondary full bridge circuit having a plurality of secondary switching elements connected to the plurality of primary switching elements and the plurality of secondary switching elements. an input voltage input to the primary-side full bridge circuit is Vin, an output voltage output from the secondary-side full bridge circuit is Vout, and an equivalent voltage ratio which is an equivalent voltage ratio when a winding ratio of the transformer is converted to 1:1 is (Vout×N1) / (Vin×N2), and the control unit has, as control modes for controlling the plurality of primary-side switching elements and the plurality of secondary-side switching elements, a step-up mode when the equivalent voltage ratio is greater than 1 and a step-down mode when the equivalent voltage ratio is less than 1, When the control mode is the step-down mode, the control unit controls the primary-side full bridge circuit so that a three-level primary-side voltage is applied to the series-connected body, and controls the secondary-side full bridge circuit so that a two-level secondary-side voltage is applied to the secondary-side winding. The step-down mode includes a step-down delay phase mode in which the primary-side voltage is raised from a low level to a middle level, and then the secondary-side voltage is raised from a low level to a high level, and then the primary-side voltage is raised from the middle level to a high level. a first phase difference is a difference between a first time when the primary-side voltage rises to a low level and a second time when the secondary-side voltage rises to a high level from a low level, and a second phase difference is a difference between the second time when the primary-side voltage rises to a high level from a middle level, and a second phase difference is a difference between the second time when the primary-side voltage rises to a high level from a middle level, and a condition for performing soft switching of the plurality of primary-side switching elements and the plurality of secondary-side switching elements is that a reactor current flowing through the reactor at the second time is equal to or greater than a reactor current threshold, and the reactor current is equal to or less than the reactor current threshold at the third time, and the control unitIn the step-down delay phase mode, a combination of the first phase difference and the second phase difference that can output the required power and that satisfies the condition is derived, and the primary side full bridge circuit and the secondary side full bridge circuit are controlled to realize the first phase difference and the second phase difference in the derived combination, thereby outputting the required power.

[0006] In the step-down delay phase mode, low power can be output according to the required power. The control unit derives a combination of the first phase difference and the second phase difference so as to satisfy the conditions for performing soft switching of the primary-side switching element and the secondary-side switching element in the step-down delay phase mode. The control unit controls the primary-side full bridge circuit and the secondary-side full bridge circuit so as to achieve the first phase difference and the second phase difference, thereby enabling soft switching while outputting low power according to the required power.

[0007] FIG. 1 is a circuit diagram of a power conversion device. FIG. 2 is a diagram showing the relationship between the equivalent voltage ratio and the control mode. FIG. 3 is a diagram showing the primary side voltage, secondary side voltage, and reactor current in the step-down delay phase mode. FIG. 4 is a flowchart showing the switching control performed by the control unit. FIG. 5 is a diagram showing the primary side voltage, secondary side voltage, and reactor current in the step-down inverse phase mode. FIG. 6 is a diagram showing the primary side voltage, secondary side voltage, and reactor current in the step-down advance phase mode. FIG. 7 is a diagram showing the primary side voltage, secondary side voltage, and reactor current in the step-up advance phase mode. FIG. 8 is a diagram showing the primary side voltage, secondary side voltage, and reactor current in the step-up inverse phase mode. FIG. 9 is a diagram showing the primary side voltage, secondary side voltage, and reactor current in the step-up delay phase mode.

[0008] An embodiment of a power conversion device will be described. As shown in Fig. 1, a power supply system 100 includes a DC power supply 110, a load 120, and a power conversion device 10. The DC power supply 110 inputs DC power to the power conversion device 10. The DC power supply 110 is, for example, a battery or a power supply circuit. The power supply circuit is, for example, an AC / DC converter that converts AC power into DC power and outputs it. The load 120 is, for example, a secondary battery that can charge and discharge DC power. The secondary battery is, for example, a lithium-ion battery or a lead-acid battery.

[0009] <Power Conversion Device> The power conversion device 10 is a dual active bridge DC / DC converter. The power conversion device 10 is provided between a DC power supply 110 and a load 120. The power conversion device 10 is capable of converting DC power input from the DC power supply 110 and outputting the converted power to the load 120. The power conversion device 10 is capable of converting DC power input from the load 120 and outputting the converted power to the DC power supply 110. In the following description, the primary side is treated as the input and the secondary side as the output. That is, the power conversion device 10 converts a DC voltage input from the DC power supply 110 and outputs the converted power to the load 120.

[0010] The power conversion device 10 includes two primary terminals 11 and 12 and two secondary terminals 13 and 14. A DC power supply 110 is electrically connected to the primary terminals 11 and 12. A load 120 is electrically connected to the secondary terminals 13 and 14.

[0011] The power conversion device 10 includes a transformer 20. The transformer 20 is an insulated type. The transformer 20 includes a magnetic core 21, a primary winding 22, and a secondary winding 23. The primary winding 22 and the secondary winding 23 are wound around the core 21. The transformer 20 has a reactor L. The reactor L may be an element such as a choke coil, or may be leakage inductance of the primary winding 22 and the secondary winding 23. The reactor L is connected to the primary winding 22. The primary winding 22 and the reactor L form a series connection 24.

[0012] The power conversion device 10 includes a primary-side full-bridge circuit 30. The primary-side full-bridge circuit 30 includes a first leg 31 and a second leg 32. The first leg 31 and the second leg 32 are connected to primary-side terminals 11 and 12 so as to be connected in parallel with each other. As a result, the primary-side full-bridge circuit 30 is electrically connected to a DC power supply 110 via the primary-side terminals 11 and 12. The first leg 31 includes a first switching element Q1, a second switching element Q2, diodes D1 and D2, and capacitors C1 and C2. The first switching element Q1 and the second switching element Q2 are connected in series with each other. The second leg 32 includes a third switching element Q3, a fourth switching element Q4, diodes D3 and D4, and capacitors C3 and C4. The third switching element Q3 and the fourth switching element Q4 are connected in series with each other. The first switching element Q1 and the third switching element Q3 form an upper arm, and the second switching element Q2 and the fourth switching element Q4 form a lower arm.

[0013] The first switching element Q1, the second switching element Q2, the third switching element Q3, and the fourth switching element Q4 constitute a plurality of primary-side switching elements Q1 to Q4. The primary-side switching elements Q1 to Q4 are, for example, n-type metal oxide semiconductor field effect transistors (MOSFETs). The primary-side switching elements Q1 to Q4 may also be p-type MOSFETs, IGBTs (insulated gate bipolar transistors), or GaN-HEMTs.

[0014] The diodes D1 to D4 and the capacitors C1 to C4 are connected in parallel to the primary-side switching elements Q1 to Q4, respectively. The diodes D1 to D4 may be parasitic diodes or elements. The capacitors C1 to C4 may be parasitic capacitances, elements, or a combination of parasitic capacitances and elements.

[0015] The connection point between the first switching element Q1 and the second switching element Q2, and the connection point between the third switching element Q3 and the fourth switching element Q4 are each connected to the primary winding 22. As a result, the primary full-bridge circuit 30 is connected to the primary winding 22. The primary full-bridge circuit 30 is also connected to the reactor L.

[0016] The power conversion device 10 includes a primary-side voltage sensor 37. The primary-side voltage sensor 37 detects an input voltage Vin input from a DC power supply 110 to the primary-side full-bridge circuit 30. The power conversion device 10 includes a secondary-side full-bridge circuit 40. The secondary-side full-bridge circuit 40 includes a third leg 41 and a fourth leg 42. The third leg 41 and the fourth leg 42 are connected to secondary-side terminals 13 and 14 so as to be connected in parallel with each other. As a result, the secondary-side full-bridge circuit 40 is electrically connected to the load 120. The third leg 41 includes a fifth switching element Q5, a sixth switching element Q6, diodes D5 and D6, and capacitors C5 and C6. The fifth switching element Q5 and the sixth switching element Q6 are connected in series with each other. The fourth leg 42 includes a seventh switching element Q7, an eighth switching element Q8, diodes D7 and D8, and capacitors C7 and C8. The seventh switching element Q7 and the eighth switching element Q8 are connected in series. The fifth switching element Q5 and the seventh switching element Q7 form an upper arm. The sixth switching element Q6 and the eighth switching element Q8 form a lower arm.

[0017] The fifth switching element Q5, the sixth switching element Q6, the seventh switching element Q7, and the eighth switching element Q8 constitute a plurality of secondary-side switching elements Q5 to Q8. The secondary-side switching elements Q5 to Q8 are, for example, n-type MOSFETs. The secondary-side switching elements Q5 to Q8 may also be p-type MOSFETs, IGBTs, or GaN-HEMTs.

[0018] Diodes D5 to D8 and capacitors C5 to C8 are connected in parallel to secondary-side switching elements Q5 to Q8, respectively. Diodes D5 to D8 may be parasitic diodes or elements. Capacitors C5 to C8 may be parasitic capacitances, elements, or a combination of parasitic capacitances and elements.

[0019] The connection point between the fifth switching element Q5 and the sixth switching element Q6, and the connection point between the seventh switching element Q7 and the eighth switching element Q8 are each connected to the secondary winding 23. As a result, the secondary full bridge circuit 40 is connected to the secondary winding 23.

[0020] The output power of the secondary-side full-bridge circuit 40 is supplied to a load 120. The power conversion device 10 includes a secondary-side sensor 47. The secondary-side sensor 47 detects the output voltage Vout of the power conversion device 10.

[0021] The power conversion device 10 includes a control unit 50. The control unit 50 includes a processor and a memory unit. The processor may be, for example, a central processing unit (CPU), a graphics processing unit (GPU), or a digital signal processor (DSP). The memory unit includes a random access memory (RAM) and a read-only memory (ROM). The memory unit stores program code or instructions configured to cause the processor to execute processing. The memory unit, i.e., a computer-readable medium, includes any available medium accessible by a general-purpose or special-purpose computer. The control unit 50 may be configured with a hardware circuit such as an application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA). The control unit 50, which is a processing circuit, may include one or more processors operating according to a computer program, one or more hardware circuits such as an ASIC or an FPGA, or a combination thereof.

[0022] The control unit 50 converts the input voltage Vin into the output voltage Vout by controlling the primary switching elements Q1 to Q4 and the secondary switching elements Q5 to Q8. The control unit 50 has a step-up mode and a step-down mode as control modes for controlling the primary switching elements Q1 to Q4 and the secondary switching elements Q5 to Q8. Here, the number of turns of the primary winding 22 of the transformer 20 is N1, and the number of turns of the secondary winding 23 is N2. The ratio between the input voltage Vin input to the primary full-bridge circuit 30 and the output voltage Vout output from the secondary full-bridge circuit 40 is the voltage ratio. The equivalent voltage ratio, which is the voltage ratio when the winding ratio of the transformer 20 is converted to 1:1, is (Vout × N1) / (Vin × N2). Vin × N2 is the equivalent input voltage when the winding ratio of the transformer is converted to 1:1. Vout×N1 is the equivalent output voltage when the winding ratio of the transformer is converted to 1:1. The step-up mode is the control mode when the equivalent voltage ratio is greater than 1. The step-down mode is the control mode when the equivalent voltage ratio is less than 1.

[0023] In the step-up mode, the secondary-side full-bridge circuit 40 is controlled in three levels. In the step-down mode, the primary-side full-bridge circuit 30 is controlled in three levels. In the step-up mode, the primary-side full-bridge circuit 30 is controlled in two levels. In the step-down mode, the secondary-side full-bridge circuit 40 is controlled in two levels. In the step-down mode, the three-level control is a control in which the voltage applied to the series connection 24 of the primary-side winding 22 and the reactor L is switched between three levels: positive, negative, or zero. In the step-down mode, the two-level control is a control in which the voltage applied to the secondary-side winding 23 is switched between two levels: positive or negative. In the step-up mode, the three-level control is a control in which the voltage applied to the secondary-side winding 23 is switched between three levels: positive, negative, or zero. In the step-up mode, two-level control is control in which the voltage applied to the series connection 24 of the primary winding 22 and the reactor L is switched between two levels: positive and negative. In the three-level control, a positive voltage applied to the series connection 24 or the secondary winding 23 is referred to as a high level, a zero voltage applied to the series connection 24 or the secondary winding 23 is referred to as a middle level, and a negative voltage applied to the series connection 24 or the secondary winding 23 is referred to as a low level. In the two-level control, a positive voltage applied to the series connection 24 or the secondary winding 23 is referred to as a high level, and a negative voltage applied to the series connection 24 or the secondary winding 23 is referred to as a low level. For simplicity of explanation, in the following description and in FIG. 2 and subsequent figures, N1 = N2. When N1=N2, the mode is step-up when Vin<Vout, and step-down when Vin>Vout. The voltage applied to the series connection 24 of the primary winding 22 and the reactor L is referred to as the primary voltage V1, and the voltage applied to the secondary winding 23 is referred to as the secondary voltage V2.

[0024] <Step-Down Mode> The control performed by the control unit 50 when the control mode is the step-down mode will be described. When the control mode is the step-down mode, the primary-side full-bridge circuit 30 is controlled in three levels, and the secondary-side full-bridge circuit 40 is controlled in two levels.

[0025] When performing three-level control of the primary-side full-bridge circuit 30, the control unit 50 independently controls the first leg 31 and the second leg 32. The switching patterns of the primary-side full-bridge circuit 30 include first to fourth patterns.

[0026] The first pattern is a switching pattern in which the first switching element Q1 is turned on, the second switching element Q2 is turned off, the third switching element Q3 is turned off, and the fourth switching element Q4 is turned on.

[0027] The second pattern is a switching pattern in which the first switching element Q1 is turned on, the second switching element Q2 is turned off, the third switching element Q3 is turned on, and the fourth switching element Q4 is turned off.

[0028] The third pattern is a switching pattern in which the first switching element Q1 is turned off, the second switching element Q2 is turned on, the third switching element Q3 is turned on, and the fourth switching element Q4 is turned off.

[0029] The fourth pattern is a switching pattern in which the first switching element Q1 is turned off, the second switching element Q2 is turned on, the third switching element Q3 is turned off, and the fourth switching element Q4 is turned on.

[0030] When performing two-level control of the secondary-side full-bridge circuit 40, the control unit 50 controls the third leg 41 and the fourth leg 42 in conjunction with each other. The control unit 50 simultaneously turns on the fifth switching element Q5 and the eighth switching element Q8. The control unit 50 simultaneously turns on the sixth switching element Q6 and the seventh switching element Q7. The switching patterns of the secondary-side full-bridge circuit 40 include a fifth pattern and a sixth pattern.

[0031] The fifth pattern is a switching pattern in which the fifth switching element Q5 is turned on, the sixth switching element Q6 is turned off, the seventh switching element Q7 is turned off, and the eighth switching element Q8 is turned on.

[0032] The sixth pattern is a switching pattern in which the fifth switching element Q5 is turned off, the sixth switching element Q6 is turned on, the seventh switching element Q7 is turned on, and the eighth switching element Q8 is turned off.

[0033] The control unit 50 outputs an output voltage Vout from the secondary full bridge circuit 40 by combining any one of the first to fourth patterns of the primary full bridge circuit 30 with any one of the fifth and sixth patterns of the secondary full bridge circuit 40.

[0034] 2, the step-down mode includes a step-down delay phase mode. The step-down delay phase mode is a step-down mode in which a small amount of power is output in response to the power requested by the load 120.

[0035] As shown in FIG. 3, the step-down delay phase mode is a step-down mode in which the primary side voltage V1 is raised from a low level to an intermediate level, the secondary side voltage V2 is raised from a low level to a high level, and then the primary side voltage V1 is raised from the intermediate level to a high level.

[0036] The difference between the first time T1 when the primary voltage V1 rises from a low level to a middle level and the second time T2 when the secondary voltage V2 rises from a low level to a high level is the first phase difference θ1. The difference between the second time T2 and the third time T3 when the primary voltage V1 rises from a middle level to a high level is the second phase difference θ2.

[0037] The switching control performed by the control unit 50 in the step-down delay phase mode will be described. The switching control is a control for outputting the output voltage Vout by switching the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8. In the step-down mode, the primary-side full-bridge circuit 30 is PWM-controlled to control the first phase difference θ1 and the second phase difference θ2. Then, power according to the first phase difference θ1 and the second phase difference θ2 is output.

[0038] As shown in FIGS. 3 and 4 , in step S1, the control unit 50 derives a soft switching region. The soft switching region is a region set for the reactor current IL flowing through the reactor L. The soft switching region at the second time T2 is a region equal to or greater than the reactor current threshold I1. The soft switching region at the third time T3 is a region equal to or less than the reactor current threshold I1. The soft switching region is a region in which soft switching is achieved by switching the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8 while the reactor current IL is within the soft switching region. Therefore, the condition for soft switching of the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8 is that the reactor current IL is equal to or greater than the reactor current threshold I1 at the second time T2 and that the reactor current IL is equal to or less than the reactor current threshold I1 at the third time T3. Note that FIGS. 3 , 5 , and 6 show the reactor current threshold I1 at the second time T2.

[0039] In this embodiment, at second time T2, the switching pattern of the secondary-side full-bridge circuit 40 is switched from pattern 6 to pattern 5. At this time, if the reactor current IL is equal to or greater than the reactor current threshold I1, soft switching of the secondary-side switching elements Q5 to Q8 is established. At third time T3, the switching pattern of the primary-side full-bridge circuit 30 is switched from pattern 3 to pattern 4. At this time, if the reactor current IL is equal to or less than the reactor current threshold I1, soft switching of the primary-side switching elements Q1 to Q4 is established.

[0040] As described above, the soft switching region is determined by the reactor current threshold I1. Therefore, the soft switching region can be determined by deriving the reactor current IL. The reactor current threshold I1 is determined by the following equation (1):

[0041] I1: reactor current threshold, k1: coefficient, V1: primary side voltage, L1: inductance of reactor L, C01: capacitance corresponding to one switching element with respect to the combined capacitance of all capacitors C1 to C4 arranged in parallel with switching elements Q1 to Q4 of primary side full bridge circuit 30. If the capacitance of capacitor C1 is C11, the capacitance of capacitor C2 is C12, the capacitance of capacitor C3 is C13, and the capacitance of capacitor C4 is C14, C01 = (C11 + C12 + C13 + C14) / 4 may be used. If Cx is the largest capacitance among C11 to C14, C01 = Cx may be used.

[0042] A condition for soft switching of the secondary-side switching elements Q5 to Q8 is that the secondary-side current IS flowing through the secondary-side winding 23 exceeds the secondary-side threshold when the secondary-side switching elements Q5 to Q8 perform switching. In this embodiment, the inductance of the secondary-side winding 23 is set so that the secondary-side current IS flowing through the secondary-side winding 23 due to the excitation current generated in the secondary-side winding 23 exceeds the secondary-side threshold. As a result, soft switching is achieved when the reactor current IL flowing through the reactor L at the second time T2 is equal to or greater than the reactor current threshold I1, and when the reactor current IL flowing through the reactor L at the third time T3 is equal to or less than the reactor current threshold I1.

[0043] Next, in step S2, the control unit 50 calculates a combination of the target current at the second time T2 and the target current at the third time T3. The target current is a target value of the reactor current IL. The control unit 50 calculates the combination of the target current at the second time T2 and the target current at the third time T3 so that the current is within the soft switching region and the power required by the load 120 can be output.

[0044] Next, the control unit 50 derives a combination of the first phase difference θ1 and the second phase difference θ2 from the combination of the target current at the second time T2 and the target current at the third time T3. This combination of the first phase difference θ1 and the second phase difference θ2 is derived so that the reactor current IL follows the target current derived in step S2. That is, the combination of the first phase difference θ1 and the second phase difference θ2 is a combination that can output the power required by the load 120, that makes the reactor current IL equal to or greater than the reactor current threshold I1 at the second time T2, and that makes the reactor current IL equal to or less than the reactor current threshold I1 at the third time T3.

[0045] In step S4, the control unit 50 controls the primary-side full-bridge circuit 30 and the secondary-side full-bridge circuit 40 so as to achieve the first phase difference θ1 and the second phase difference θ2 derived in step S3.

[0046] 2, the step-down mode includes a step-down in-phase mode. The step-down in-phase mode is a step-down mode in which medium power is output in response to the power required by the load 120. The medium power is greater than the small power.

[0047] As shown in FIG. 5, the step-down in-phase mode is a step-down mode in which, after the primary side voltage V1 is raised from a low level to an intermediate level, the secondary side voltage V2 is raised from a low level to a high level and the primary side voltage V1 is raised from an intermediate level to a high level simultaneously.

[0048] The difference between the first time T1 when the primary voltage V1 rises from a low level to a middle level and the second time T2 when the secondary voltage V2 rises from a low level to a high level is the first phase difference θ1. The difference between the second time T2 and the third time T3 when the primary voltage V1 rises from a middle level to a high level is the second phase difference θ2. In the same phase step-down mode, the second time T2 and the third time T3 are the same time. Therefore, the second phase difference θ2 is zero.

[0049] The switching control performed by the control unit 50 in the step-down in-phase mode will now be described. In step S1, the control unit 50 derives a soft-switching region in the same manner as in the step-down delayed-phase mode. The condition for soft switching in the step-down in-phase mode is that the reactor current IL is equal to or greater than the reactor current threshold I1 at the second time T2 and that the reactor current IL is equal to or less than the reactor current threshold I1 at the third time T3. In the step-down in-phase mode, the second time T2 and the third time T3 are the same time, so the condition for soft switching of the primary-side switching elements Q1 to Q4 and the multiple secondary-side switching elements Q5 to Q8 is that the reactor current IL matches the reactor current threshold I1 at the second time T2. That is, the soft-switching region in the step-down in-phase mode is the reactor current threshold I1.

[0050] In step S2, the control unit 50 calculates a combination of the target current at the second time T2 and the target current at the third time T3. In the case of the in-phase step-down mode, the second time T2 and the third time T3 are the same time, so the target current is calculated to match the reactor current threshold value I1 at the second time T2.

[0051] In step S3, the control unit 50 derives a combination of the first phase difference θ1 and the second phase difference θ2 that can output the required power and that causes the reactor current IL at the second time T2 to match the reactor current threshold I1. In this manner, in the step-down in-phase mode, a combination of the first phase difference θ1 and the second phase difference θ2 is derived so that the reactor current IL at the second time T2 matches the reactor current threshold I1. Note that in the step-down in-phase mode, the second phase difference θ2 is zero, so a combination of the first phase difference θ1 and zero, which is different from zero, is derived. Even in this case, the soft switching conditions for the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8 are met. This is because k1 is set as a margin in equation (1) that defines the reactor current threshold I1.

[0052] 2, the buck mode includes a buck-advance phase mode. The buck-advance phase mode is a buck mode in which high power is output in response to the power required by the load 120. The high power is power greater than the medium power.

[0053] As shown in FIG. 6, the step-down leading phase mode is a step-down mode in which the primary side voltage V1 is raised from a low level to a middle level, and then the primary side voltage V1 is raised from the middle level to a high level, and then the secondary side voltage V2 is raised from a low level to a high level.

[0054] The difference between the first time T1 when the primary voltage V1 rises from a low level to a middle level and the second time T2 when the secondary voltage V2 rises from a low level to a high level is the first phase difference θ1. The difference between the second time T2 and the third time T3 when the primary voltage V1 rises from a middle level to a high level is the second phase difference θ2.

[0055] The switching control performed by the control unit 50 in the buck lead phase mode is the same as that in the buck lag phase mode. That is, in step S1, the control unit 50 derives a soft switching region. In step S2, the control unit 50 calculates a combination of a target current at a second time T2 and a target current at a third time T3. In step S3, the control unit 50 derives a combination of the first phase difference θ1 and the second phase difference θ2 that can output the required power, where the reactor current IL is equal to or greater than the reactor current threshold I1 at the second time T2 and equal to or less than the reactor current threshold I1 at the third time T3. In step S4, the control unit 50 outputs the required power by controlling the primary-side full bridge circuit 30 and the secondary-side full bridge circuit 40 to achieve the derived first phase difference θ1 and second phase difference θ2. The conditions for achieving soft switching in the buck lead phase mode are the same as those in the buck lag phase mode.

[0056] <Boost Mode> The control performed by the control unit 50 when the control mode is the boost mode will be described. When the control mode is the boost mode, the primary side full bridge circuit 30 is controlled in two levels, and the secondary side full bridge circuit 40 is controlled in three levels.

[0057] When performing two-level control of the primary-side full-bridge circuit 30, the control unit 50 controls the first leg 31 and the second leg 32 in conjunction with each other. The control unit 50 simultaneously turns on the first switching element Q1 and the fourth switching element Q4. The control unit 50 simultaneously turns on the second switching element Q2 and the third switching element Q3. The switching patterns of the primary-side full-bridge circuit 30 include a seventh pattern and an eighth pattern.

[0058] The seventh pattern is a switching pattern in which the first switching element Q1 is turned on, the second switching element Q2 is turned off, the third switching element Q3 is turned off, and the fourth switching element Q4 is turned on.

[0059] The eighth pattern is a switching pattern in which the first switching element Q1 is turned off, the second switching element Q2 is turned on, the third switching element Q3 is turned on, and the fourth switching element Q4 is turned off.

[0060] When performing three-level control of the secondary-side full-bridge circuit 40, the control unit 50 independently controls the third leg 41 and the fourth leg 42. The switching patterns of the secondary-side full-bridge circuit 40 include ninth to twelfth patterns.

[0061] The ninth pattern is a switching pattern in which the fifth switching element Q5 is turned on, the sixth switching element Q6 is turned off, the seventh switching element Q7 is turned off, and the eighth switching element Q8 is turned on.

[0062] The tenth pattern is a switching pattern in which the fifth switching element Q5 is turned on, the sixth switching element Q6 is turned off, the seventh switching element Q7 is turned on, and the eighth switching element Q8 is turned off.

[0063] The eleventh pattern is a switching pattern in which the fifth switching element Q5 is turned off, the sixth switching element Q6 is turned on, the seventh switching element Q7 is turned on, and the eighth switching element Q8 is turned off.

[0064] The twelfth pattern is a switching pattern in which the fifth switching element Q5 is turned off, the sixth switching element Q6 is turned on, the seventh switching element Q7 is turned off, and the eighth switching element Q8 is turned on.

[0065] The control unit 50 outputs an output voltage Vout from the secondary full bridge circuit 40 by combining either the seventh or eighth pattern of the primary full bridge circuit 30 with either the ninth to twelfth patterns of the secondary full bridge circuit 40.

[0066] 2, the boost mode includes a boost phase mode, which is a boost mode in which a small amount of power is output in response to the power requested by the load 120.

[0067] As shown in FIG. 7, the boost leading phase mode is a boost mode in which the secondary side voltage V2 is raised from a low level to a middle level, the primary side voltage V1 is raised from a low level to a high level, and then the secondary side voltage V2 is raised from the middle level to a high level.

[0068] The difference between the first time T11 when the secondary voltage V2 rises from the low level to the middle level and the second time T12 when the primary voltage V1 rises from the low level to the high level is the first phase difference θ11. The difference between the second time T12 and the third time T13 when the secondary voltage V2 rises from the middle level to the high level is the second phase difference θ12.

[0069] The conditions for performing soft switching of the primary side switching elements Q1 to Q4 and the secondary side switching elements Q5 to Q8 are that the reactor current IL is greater than or equal to the reactor current threshold I1 at the first time T11 and that the reactor current IL is less than or equal to the reactor current threshold I1 at the second time T12.

[0070] In the boost mode, the control unit 50 controls the first phase difference θ11 and the second phase difference θ12 by PWM controlling the secondary-side full-bridge circuit 40. Then, power according to the first phase difference θ11 and the second phase difference θ12 is output.

[0071] The control performed by the control unit 50 in the boost lead phase mode will be described with reference to Fig. 4. In step S1, the control unit 50 derives a soft switching region. The soft switching region at a first time T11 is a region where the reactor current is equal to or greater than the reactor current threshold I1. The soft switching region at a second time T12 is a region where the reactor current is equal to or less than the reactor current threshold I1.

[0072] In step S2, the control unit 50 calculates a combination of the target current at the first time T11 and the target current at the second time T12. In step S3, the control unit 50 derives a combination of the first phase difference θ11 and the second phase difference θ12 that can output the required power, where the reactor current IL is equal to or greater than the reactor current threshold I1 at the first time T11 and the reactor current IL is equal to or less than the reactor current threshold I1 at the second time T12. In step S4, the control unit 50 outputs the required power by controlling the primary-side full-bridge circuit 30 and the secondary-side full-bridge circuit 40 to achieve the derived first phase difference θ11 and second phase difference θ12. Note that FIGS. 7 to 9 show the reactor current threshold I1 at the first time T11.

[0073] 2, the boost mode includes a boost in-phase mode. The boost in-phase mode is a boost mode in which medium power is output in response to the power required by the load 120. The medium power is greater than the small power.

[0074] As shown in FIG. 8, the boost in-phase mode is a boost mode in which the primary side voltage V1 is raised from a low level to a high level and the secondary side voltage V2 is raised from a low level to an intermediate level simultaneously, and then the secondary side voltage V2 is raised from the intermediate level to a high level.

[0075] The difference between the first time T11 when the secondary voltage V2 rises from a low level to a middle level and the second time T12 when the primary voltage V1 rises from a low level to a high level is defined as the first phase difference θ11. The difference between the second time T12 and the third time T13 when the secondary voltage V2 rises from a middle level to a high level is defined as the second phase difference θ12. In the boost in-phase mode, the first time T11 and the second time T12 are the same time. Therefore, the first phase difference θ11 is zero.

[0076] The condition for performing soft switching of the primary side switching elements Q1 to Q4 and the secondary side switching elements Q5 to Q8 is that the reactor current IL is equal to or greater than the reactor current threshold I1 at the first time T11 and that the reactor current IL is equal to or less than the reactor current threshold I1 at the second time T12. In the boost in-phase mode, the first time T11 and the second time T12 are the same time, and therefore the condition for performing soft switching is that the reactor current IL at the first time T11 matches the reactor current threshold I1.

[0077] In step S1, the control unit 50 derives a soft switching region. In step S2, the control unit 50 calculates a combination of a target current at a first time T11 and a target current at a second time T12. In step S3, the control unit 50 derives a combination of the first phase difference θ11 and the second phase difference θ12 that can output the required power and that causes the reactor current IL to match the reactor current threshold I1 at the first time T11. Note that in the boost in-phase mode, the first phase difference θ11 is 0, and therefore a combination of the second phase difference θ12, which is different from 0, and 0 is derived. In step S4, the control unit 50 outputs the required power by controlling the primary-side full bridge circuit 30 and the secondary-side full bridge circuit 40 to achieve the derived first phase difference θ11 and second phase difference θ12.

[0078] 2, the boost mode includes a boost delay phase mode. The boost delay phase mode is a boost mode in which high power is output in response to the power requested by the load 120. The high power is power greater than the medium power.

[0079] As shown in FIG. 9, the boost delay phase mode is a boost mode in which the primary side voltage V1 is raised from a low level to a high level, the secondary side voltage V2 is raised from a low level to a middle level, and then the secondary side voltage V2 is raised from the middle level to a high level.

[0080] The difference between the first time T11 when the secondary voltage V2 rises from the low level to the middle level and the second time T12 when the primary voltage V1 rises from the low level to the high level is the first phase difference θ11. The difference between the second time T12 and the third time T13 when the secondary voltage V2 rises from the middle level to the high level is the second phase difference θ12.

[0081] The conditions for performing soft switching of the primary side switching elements Q1 to Q4 and the secondary side switching elements Q5 to Q8 are that the reactor current IL is greater than or equal to the reactor current threshold I1 at the first time T11 and that the reactor current IL is less than or equal to the reactor current threshold I1 at the second time T12.

[0082] In step S1, the control unit 50 derives a soft switching region. In step S2, the control unit 50 calculates a combination of a target current at a first time T11 and a target current at a second time T12. In step S3, the control unit 50 derives a combination of the first phase difference θ11 and the second phase difference θ12 that can output the required power, where the reactor current IL is equal to or greater than the reactor current threshold I1 at the first time T11 and the reactor current IL is equal to or less than the reactor current threshold I1 at the second time T12. In step S4, the control unit 50 outputs the required power by controlling the primary-side full-bridge circuit 30 and the secondary-side full-bridge circuit 40 to achieve the derived first phase difference θ11 and second phase difference θ12.

[0083] [Operation of the Present Embodiment] When the control mode is the step-down mode, the primary-side full-bridge circuit 30 is controlled in three levels. The secondary-side full-bridge circuit 40 is controlled in two levels. The control unit 50 controls the first phase difference θ1 and the second phase difference θ2 to realize soft switching of the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8, while outputting power according to the required power from the secondary-side full-bridge circuit 40. At this time, the required power differs depending on the state of the load 120. The control unit 50 then derives the first phase difference θ1 and the second phase difference θ2 according to the required power.

[0084] When the control mode is the step-down mode and the required power is low, the timing at which the primary-side voltage V1 rises from a middle level to a high level is later than the timing at which the secondary-side voltage V2 rises from a low level to a high level. This state is the step-down delayed phase mode. When the control mode is the step-down mode and the required power is medium, the timing at which the primary-side voltage V1 rises from a middle level to a high level is the same as the timing at which the secondary-side voltage V2 rises from a low level to a high level. This state is the step-down in-phase mode. When the control mode is the step-down mode and the required power is high, the timing at which the primary-side voltage V1 rises from a middle level to a high level is earlier than the timing at which the secondary-side voltage V2 rises from a low level to a high level. This state is the step-down advanced phase mode.

[0085] The timing at which the primary-side voltage V1 rises corresponds to the timing at which the switching pattern of the primary-side switching elements Q1 to Q4 switches. When the primary-side voltage V1 rises from a middle level to a high level, soft switching is achieved by switching the primary-side switching elements Q1 to Q4 when the reactor current IL is equal to or less than the reactor current threshold I1. When the secondary-side voltage V2 rises from a low level to a high level, soft switching is achieved by switching the secondary-side switching elements Q5 to Q8 when the reactor current IL is equal to or greater than the reactor current threshold I1. In this embodiment, the first phase difference θ1 and the second phase difference θ2 are derived so as to satisfy the above-described conditions for soft switching, and the control unit 50 performs control to realize the derived first phase difference θ1 and second phase difference θ2. This enables soft switching to be achieved.

[0086] When the control mode is the boost mode, the primary-side full-bridge circuit 30 is controlled in two levels, and the secondary-side full-bridge circuit 40 is controlled in three levels. The control unit 50 controls the first phase difference θ1 and the second phase difference θ2 to achieve soft switching of the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8, and outputs power according to the required power from the secondary-side full-bridge circuit 40.

[0087] When the control mode is the boost mode and the required power is low, the timing at which the secondary-side voltage V2 rises from a low level to a middle level is earlier than the timing at which the primary-side voltage V1 rises from a low level to a high level. This state is the boost advanced phase mode. When the control mode is the boost mode and the required power is medium, the timing at which the secondary-side voltage V2 rises from a low level to a middle level is the same as the timing at which the primary-side voltage V1 rises from a low level to a high level. This state is the boost in-phase mode. When the control mode is the boost mode and the required power is high, the timing at which the secondary-side voltage V2 rises from a low level to a middle level is later than the timing at which the primary-side voltage V1 rises from a low level to a high level. This state is the boost delayed phase mode.

[0088] The timing at which the primary-side voltage V1 rises corresponds to the timing at which the switching pattern of the primary-side switching elements Q1 to Q4 switches. When the primary-side voltage V1 rises from low level to high level, soft switching is achieved by switching the primary-side switching elements Q1 to Q4 when the reactor current IL is equal to or less than the reactor current threshold I1. When the secondary-side voltage V2 rises from low level to middle level, soft switching is achieved by switching the secondary-side switching elements Q5 to Q8 when the reactor current IL is equal to or greater than the reactor current threshold I1. In this embodiment, the first phase difference θ1 and the second phase difference θ2 are derived so as to satisfy the above-described conditions for soft switching, and the control unit 50 performs control to realize the derived first phase difference θ1 and second phase difference θ2. This enables soft switching to be achieved.

[0089] [Effects of the Present Embodiment] (1) In the step-down delay phase mode, the control unit 50 derives a combination of the first phase difference θ1 and the second phase difference θ2 that can output the required power, where the reactor current IL is equal to or greater than the reactor current threshold I1 at the second time T2 and equal to or less than the reactor current threshold I1 at the third time T3. In the step-down delay phase mode, the reactor current IL satisfying the above condition is a condition for performing soft switching of the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8. The control unit 50 derives the first phase difference θ1 and the second phase difference θ2 so as to satisfy this condition, and performs control so that the first phase difference θ1 and the second phase difference θ2 are achieved, thereby enabling soft switching while outputting a small amount of power according to the required power.

[0090] (2) In the step-down in-phase mode, the control unit 50 derives a combination of the first phase difference θ1 and the second phase difference θ2 that allows the required power to be output and that causes the reactor current IL to match the reactor current threshold I1 at the second time T2. In the step-down in-phase mode, the reactor current IL satisfying the above condition is a condition for performing soft switching of the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8. The control unit 50 derives the first phase difference θ1 and the second phase difference θ2 so as to satisfy this condition, and performs control so that the first phase difference θ1 and the second phase difference θ2 are achieved, thereby enabling soft switching while outputting medium power according to the required power.

[0091] In particular, in the step-down in-phase mode, the condition for achieving soft switching becomes stricter when the second phase difference θ2 becomes zero. When the second phase difference θ2 is zero, the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8 must be switched simultaneously, and therefore the reactor current IL must be made to match the reactor current threshold I1 at the second time T2. As in this embodiment, by controlling the first phase difference θ1 and the second phase difference θ2, the reactor current IL is made to match the reactor current threshold I1 at the second time T2, and soft switching can be performed.

[0092] (3) In the step-down lead phase mode, the control unit 50 derives a combination of the first phase difference θ1 and the second phase difference θ2 that can output the required power, where the reactor current IL is equal to or greater than the reactor current threshold I1 at the second time T2 and equal to or less than the reactor current threshold I1 at the third time T3. In the step-down lead phase mode, the reactor current IL satisfying the above condition is a condition for performing soft switching of the primary side switching elements Q1 to Q4 and the secondary side switching elements Q5 to Q8. The control unit 50 derives the first phase difference θ1 and the second phase difference θ2 so as to satisfy this condition, and performs control so that the first phase difference θ1 and the second phase difference θ2 are achieved, thereby enabling soft switching while outputting a large amount of power according to the required power.

[0093] (4) In any of the step-down delayed phase mode, step-down same phase mode, and step-down advanced phase mode, the control unit 50 can perform soft switching of the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8 by controlling the first phase difference θ1 and the second phase difference θ2. Soft switching can be performed while outputting low, medium, or high power according to the required power.

[0094] (5) In the boost lead phase mode, the control unit 50 derives a combination of the first phase difference θ11 and the second phase difference θ12 that can output the required power, where the reactor current IL is equal to or greater than the reactor current threshold I1 at the first time T11 and equal to or less than the reactor current threshold I1 at the second time T12. In the boost lead phase mode, the reactor current IL satisfying the above condition is a condition for performing soft switching of the primary side switching elements Q1 to Q4 and the secondary side switching elements Q5 to Q8. The control unit 50 derives the first phase difference θ11 and the second phase difference θ12 so as to satisfy this condition, and performs control so that the first phase difference θ11 and the second phase difference θ12 are obtained, thereby enabling soft switching while outputting a small amount of power according to the required power.

[0095] (6) In the boost in-phase mode, the control unit 50 derives a combination of the first phase difference θ11 and the second phase difference θ12 that allows the required power to be output and that causes the reactor current IL to match the reactor current threshold I1 at the first time T11. In the boost in-phase mode, the reactor current IL satisfying the above condition is a condition for performing soft switching of the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8. The control unit 50 derives the first phase difference θ11 and the second phase difference θ12 so as to satisfy this condition, and performs control to achieve the first phase difference θ11 and the second phase difference θ12, thereby enabling soft switching while outputting medium power according to the required power.

[0096] In particular, in the boost in-phase mode, the condition for achieving soft switching becomes stricter when the second phase difference θ12 becomes 0. When the second phase difference θ12 is 0, the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8 must be switched simultaneously, and therefore the reactor current IL must be made to match the reactor current threshold I1 at the second time T12. As in this embodiment, by controlling the first phase difference θ11 and the second phase difference θ12, the reactor current IL is made to match the reactor current threshold I1 at the first time T11, thereby enabling soft switching.

[0097] (7) In the boost delay phase mode, the control unit 50 derives a combination of the first phase difference θ11 and the second phase difference θ12 that can output the required power, where the reactor current IL is equal to or greater than the reactor current threshold I1 at the first time T11 and equal to or less than the reactor current threshold I1 at the second time T12. In the boost delay phase mode, the reactor current IL satisfying the above condition is a condition for performing soft switching of the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8. The control unit 50 derives the first phase difference θ11 and the second phase difference θ12 so as to satisfy this condition, and performs control so that the first phase difference θ11 and the second phase difference θ12 are obtained, thereby enabling soft switching while outputting a large amount of power according to the required power.

[0098] (8) The control unit 50 can perform soft switching of the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8 by controlling the first phase difference θ11 and the second phase difference θ12 in any of the boost lead phase mode, boost inline phase mode, and boost lag phase mode. Soft switching can be performed while outputting low power, medium power, or high power according to the required power.

[0099] (9) The inductance of the secondary winding 23 is set so that the secondary current IS flowing through the secondary winding 23 due to the excitation current generated in the secondary winding 23 exceeds the secondary threshold value. Therefore, if the reactor current IL satisfies the conditions for soft switching, soft switching of the primary switching elements Q1 to Q4 and the secondary switching elements Q5 to Q8 can be performed.

[0100] (10) When performing three-level control on both the primary-side full bridge circuit 30 and the secondary-side full bridge circuit 40, the control performed by the control unit 50 becomes complicated because three phase differences must be controlled. Furthermore, when performing three-level control, the number of switching patterns increases compared to when performing two-level control, and the number of control signals for switching the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8 also increases. In contrast, when performing two-level control on one of the primary-side full bridge circuit 30 and the secondary-side full bridge circuit 40, it is sufficient to control two phase differences. This simplifies the control performed by the control unit 50. Furthermore, the number of control signals can be reduced.

[0101] [Modifications] The embodiment can be modified as follows: The embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

[0102] The control unit 50 may perform the switching control of the embodiment in at least one of the buck delay phase mode, the buck in-phase mode, the buck advance phase mode, the boost advance phase mode, the boost in-phase mode, and the boost delay phase mode. In other words, it is sufficient to perform soft switching of the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8 in at least one of the buck delay phase mode, the buck in-phase mode, the buck advance phase mode, the boost advance phase mode, the boost in-phase mode, and the boost delay phase mode. The expression "at least one" used in this specification means "one or more" of the desired options. For example, when the number of options is two, the expression "at least one" used in this specification means "only one option" or "both of two options." For another example, when the number of options is three or more, the expression "at least one" used in this specification means "only one option" or "any combination of two or more options."

[0103] L...reactor, Q1 to Q4...primary side switching elements, Q5 to Q8...secondary side switching elements, 20...transformer, 22...primary side winding, 23...secondary side winding, 30...primary side full bridge circuit, 40...secondary side full bridge circuit, 50...control unit.

Claims

1. A transformer having a primary winding with N1 turns and a secondary winding with N2 turns; a reactor connected to the primary winding, the primary winding and the reactor forming a series connection; a primary full bridge circuit having a plurality of primary switching elements, the secondary full bridge circuit being a circuit connected to the secondary winding, the secondary full bridge circuit having a plurality of secondary switching elements; and a control unit configured to control the plurality of primary switching elements and the plurality of secondary switching elements, wherein an input voltage input to the primary full bridge circuit is Vin, an output voltage output from the secondary full bridge circuit is Vout, and an equivalent voltage ratio which is an equivalent voltage ratio when the winding ratio of the transformer is converted to 1:1 is (Vout x N1) / (Vin x N2), and the control unit controls the plurality of primary switching elements and the plurality of secondary switching elements in the following control modes: a boost mode when the equivalent voltage ratio is greater than 1; and a step-down mode when the equivalent voltage ratio is smaller than 1, and when the control mode is the step-down mode, the control unit controls the primary-side full bridge circuit so that three levels of primary-side voltages are applied to the series-connected body, and controls the secondary-side full bridge circuit so that two levels of secondary-side voltages are applied to the secondary winding, and the step-down mode includes a step-down delay phase mode in which the primary-side voltage is raised from a low level to a middle level, and then the secondary-side voltage is raised from a low level to a high level, and then the primary-side voltage is raised from the middle level to the high level, a difference between a first time when the primary-side voltage rises from a low level to a middle level and a second time when the secondary-side voltage rises from a low level to a high level is a first phase difference, and a difference between the second time and a third time when the primary-side voltage rises from a middle level to a high level is a second phase difference, a condition for performing soft switching of the plurality of primary side switching elements and the plurality of secondary side switching elements is that a reactor current flowing through the reactor at the second time is equal to or greater than a reactor current threshold, and that the reactor current is equal to or less than the reactor current threshold at the third time;the control unit is configured to derive a combination of the first phase difference and the second phase difference that can output a required power and satisfy the condition in the step-down delay phase mode, and to control the primary side full bridge circuit and the secondary side full bridge circuit to realize the first phase difference and the second phase difference in the derived combination, thereby outputting the required power.

2. A transformer having a primary winding with a number of turns N1 and a secondary winding with a number of turns N2, a reactor connected to the primary winding, the primary winding and the reactor forming a series connection, a primary full bridge circuit having a plurality of primary switching elements, a secondary full bridge circuit having a plurality of secondary switching elements, a circuit connected to the reactor, the primary full bridge circuit having a plurality of secondary switching elements, and a control unit configured to control the primary switching elements and the secondary switching elements, an input voltage input to the primary full bridge circuit is Vin, an output voltage output from the secondary full bridge circuit is Vout, an equivalent voltage ratio which is an equivalent voltage ratio when the winding ratio of the transformer is converted to 1:1 is (Vout x N1) / (Vin x N2), and the control unit controls the primary switching elements and the secondary switching elements in the following control modes: a boost mode when the equivalent voltage ratio is greater than 1; and a step-down mode when the equivalent voltage ratio is smaller than 1, and when the control mode is the step-down mode, the control unit controls the primary-side full bridge circuit so that three levels of primary-side voltages are applied to the series-connected body, and controls the secondary-side full bridge circuit so that two levels of secondary-side voltages are applied to the secondary-side winding, and the step-down mode includes a step-down in-phase mode in which, after the primary-side voltage is raised from a low level to a middle level, the secondary-side voltage is raised from a low level to a high level and the primary-side voltage is raised from the middle level to a high level simultaneously, a difference between a first time when the primary-side voltage rises from a low level to a middle level and a second time when the secondary-side voltage rises from a low level to a high level is a first phase difference, and a difference between the second time and a third time when the primary-side voltage rises from a middle level to a high level is a second phase difference,a condition for performing soft switching of the plurality of primary side switching elements and the plurality of secondary side switching elements is that a reactor current flowing through the reactor at the second time is equal to or greater than a reactor current threshold and that the reactor current is equal to or less than the reactor current threshold at the third time; and the control unit is configured to derive, in the step-down same phase mode, a combination of the first phase difference and the second phase difference that can output a required power, where the reactor current matches the reactor current threshold at the second time, and to control the primary side full bridge circuit and the secondary side full bridge circuit to realize the first phase difference and the second phase difference in the derived combination, thereby outputting the required power.

3. A transformer having a primary winding with N1 turns and a secondary winding with N2 turns; a reactor connected to the primary winding, the primary winding and the reactor forming a series connection; a primary full bridge circuit having a plurality of primary switching elements, the secondary full bridge circuit being a circuit connected to the secondary winding, the secondary full bridge circuit having a plurality of secondary switching elements; and a control unit configured to control the plurality of primary switching elements and the plurality of secondary switching elements, wherein an input voltage input to the primary full bridge circuit is Vin, an output voltage output from the secondary full bridge circuit is Vout, and an equivalent voltage ratio which is an equivalent voltage ratio when the winding ratio of the transformer is converted to 1:1 is (Vout x N1) / (Vin x N2), and the control unit controls the plurality of primary switching elements and the plurality of secondary switching elements in the following control modes: a boost mode when the equivalent voltage ratio is greater than 1; and a step-down mode when the equivalent voltage ratio is smaller than 1, and when the control mode is the step-down mode, the control unit controls the primary-side full bridge circuit so that three levels of primary-side voltages are applied to the series-connected body, and controls the secondary-side full bridge circuit so that two levels of secondary-side voltages are applied to the secondary winding, and the step-down mode includes a step-down leading phase mode in which the primary-side voltage is raised from a low level to a middle level, and then the primary-side voltage is raised from a middle level to a high level, and then the secondary-side voltage is raised from a low level to a high level, a difference between a first time when the primary-side voltage rises from a low level to a middle level and a second time when the secondary-side voltage rises from a low level to a high level is a first phase difference, and a difference between the second time and a third time when the primary-side voltage rises from a middle level to a high level is a second phase difference, a condition for performing soft switching of the plurality of primary side switching elements and the plurality of secondary side switching elements is that a reactor current flowing through the reactor at the second time is equal to or greater than a reactor current threshold, and that the reactor current is equal to or less than the reactor current threshold at the third time;the control unit is configured to derive a combination of the first phase difference and the second phase difference that can output a required power and satisfy the condition in the step-down leading phase mode, and to control the primary side full bridge circuit and the secondary side full bridge circuit to realize the first phase difference and the second phase difference in the derived combination, thereby outputting the required power.

4. A transformer having a primary winding with a number of turns N1 and a secondary winding with a number of turns N2, a reactor connected to the primary winding, the primary winding and the reactor forming a series connection, a primary full bridge circuit having a plurality of primary switching elements, a secondary full bridge circuit having a plurality of secondary switching elements, a circuit connected to the reactor, the primary full bridge circuit having a plurality of secondary switching elements, and a control unit configured to control the primary switching elements and the secondary switching elements, wherein an input voltage input to the primary full bridge circuit is Vin, an output voltage output from the secondary full bridge circuit is Vout, and an equivalent voltage ratio which is an equivalent voltage ratio when the winding ratio of the transformer is converted to 1:1 is (Vout x N1) / (Vin x N2), and the control unit controls the primary switching elements and the secondary switching elements in the following control modes: a boost mode when the equivalent voltage ratio is greater than 1; a step-down mode when the equivalent voltage ratio is smaller than 1, and when the control mode is the step-down mode, the control unit controls the primary-side full bridge circuit so that three levels of primary-side voltages are applied to the series-connected body, and controls the secondary-side full bridge circuit so that two levels of secondary-side voltages are applied to the secondary-side winding, and the step-down mode comprises: a step-down delayed phase mode in which the primary-side voltage is raised from a low level to a middle level, and then the secondary-side voltage is raised from a low level to a high level, and then the primary-side voltage is raised from the middle level to the high level; a step-down same phase mode in which the primary-side voltage is raised from a low level to a middle level, and then the secondary-side voltage is raised from a low level to a high level and the primary-side voltage is raised from the middle level to a high level simultaneously; and a step-down advanced phase mode in which the primary-side voltage is raised from a low level to a middle level, and then the primary-side voltage is raised from the middle level to a high level, and then the secondary-side voltage is raised from the low level to the high level,a first phase difference is a difference between a first time when the primary side voltage rises from a low level to an intermediate level and a second time when the secondary side voltage rises from a low level to a high level, and a second phase difference is a difference between the second time and a third time when the primary side voltage rises from an intermediate level to a high level; a condition for performing soft switching of the plurality of primary side switching elements and the plurality of secondary side switching elements is that a reactor current flowing through the reactor at the second time is equal to or greater than a reactor current threshold and that the reactor current is equal to or less than the reactor current threshold at the third time; the control unit derives a combination of the first phase difference and the second phase difference that can output required power and satisfies the condition in the step-down delay phase mode, and controls the primary side full bridge circuit and the secondary side full bridge circuit to realize the first phase difference and the second phase difference in the derived combination, thereby outputting the required power; the primary side full bridge circuit and the secondary side full bridge circuit are controlled to realize the first phase difference and the second phase difference in the derived combination, thereby outputting the required power; and the step-down lead phase mode, the step-down lead phase mode, the combination of the first phase difference and the second phase difference in which the required power can be output and which satisfies the condition, and the primary side full bridge circuit and the secondary side full bridge circuit are controlled to realize the first phase difference and the second phase difference in the derived combination, thereby outputting the required power.

5. A transformer having a primary winding with a number of turns N1 and a secondary winding with a number of turns N2, a reactor connected to the primary winding, the primary winding and the reactor forming a series connection, a primary full bridge circuit having a plurality of primary switching elements, a secondary full bridge circuit having a plurality of secondary switching elements, a circuit connected to the reactor, the primary full bridge circuit having a plurality of secondary switching elements, and a control unit configured to control the primary switching elements and the secondary switching elements, wherein an input voltage input to the primary full bridge circuit is Vin, an output voltage output from the secondary full bridge circuit is Vout, and an equivalent voltage ratio which is an equivalent voltage ratio when the winding ratio of the transformer is converted to 1:1 is (Vout x N1) / (Vin x N2), and the control unit controls the primary switching elements and the secondary switching elements in the following control modes: a boost mode when the equivalent voltage ratio is greater than 1; and a step-down mode when the equivalent voltage ratio is smaller than 1, and when the control mode is the step-up mode, the control unit controls the primary-side full bridge circuit so that two levels of primary-side voltage are applied to the series-connected body, and controls the secondary-side full bridge circuit so that three levels of secondary-side voltage are applied to the secondary winding, and the step-up mode includes a step-up leading phase mode in which the secondary-side voltage is raised from a low level to a middle level and then the primary-side voltage is raised from a low level to a high level, and then the secondary-side voltage is raised from the middle level to the high level, a difference between a first time when the secondary-side voltage rises from a low level to a middle level and a second time when the primary-side voltage rises from a low level to a high level is a first phase difference, and a difference between the second time and a third time when the secondary-side voltage rises from a middle level to a high level is a second phase difference, a condition for performing soft switching of the plurality of primary side switching elements and the plurality of secondary side switching elements is that a reactor current flowing through the reactor at the first time is equal to or greater than a reactor current threshold, and the reactor current is equal to or less than the reactor current threshold at the second time;the control unit is configured to derive a combination of the first phase difference and the second phase difference that can output a required power and satisfy the condition in the boost leading phase mode, and to control the primary side full bridge circuit and the secondary side full bridge circuit to realize the derived first phase difference and second phase difference, thereby outputting the required power.

6. A transformer having a primary winding with a number of turns N1 and a secondary winding with a number of turns N2, a reactor connected to the primary winding, the primary winding and the reactor forming a series connection, a primary full bridge circuit having a plurality of primary switching elements, a secondary full bridge circuit having a plurality of secondary switching elements, a circuit connected to the reactor, the primary full bridge circuit having a plurality of secondary switching elements, and a control unit configured to control the primary switching elements and the secondary switching elements, wherein an input voltage input to the primary full bridge circuit is Vin, an output voltage output from the secondary full bridge circuit is Vout, and an equivalent voltage ratio which is an equivalent voltage ratio when the winding ratio of the transformer is converted to 1:1 is (Vout x N1) / (Vin x N2), and the control unit controls the primary switching elements and the secondary switching elements in the following control modes: a boost mode when the equivalent voltage ratio is greater than 1; and a step-down mode when the equivalent voltage ratio is smaller than 1, and when the control mode is the step-up mode, the control unit controls the primary-side full bridge circuit so that two levels of primary-side voltage are applied to the series-connected body, and controls the secondary-side full bridge circuit so that three levels of secondary-side voltage are applied to the secondary winding, and the step-up mode includes a step-up in-phase mode in which the primary-side voltage is raised from a low level to a high level and the secondary-side voltage is raised from a low level to a middle level simultaneously, and then the secondary-side voltage is raised from a middle level to a high level, a difference between a first time when the secondary-side voltage rises from a low level to a middle level and a second time when the primary-side voltage rises from a low level to a high level is a first phase difference, and a difference between the second time and a third time when the secondary-side voltage rises from a middle level to a high level is a second phase difference,a condition for performing soft switching of the plurality of primary side switching elements and the plurality of secondary side switching elements being a reactor current flowing through the reactor at the first time being equal to or greater than a reactor current threshold and at the second time being equal to or less than the reactor current threshold, and the control unit is configured to derive, in the boost same phase mode, a combination of the first phase difference and the second phase difference that can output a required power, where the reactor current matches the reactor current threshold at the first time, and to control the primary side full bridge circuit and the secondary side full bridge circuit to realize the derived first phase difference and second phase difference, thereby outputting the required power.

7. A transformer having a primary winding with N1 turns and a secondary winding with N2 turns, a reactor connected to the primary winding, the primary winding and the reactor forming a series connection, a primary full bridge circuit having a plurality of primary switching elements, a secondary full bridge circuit having a plurality of secondary switching elements, a circuit connected to the reactor, the primary full bridge circuit having a plurality of secondary switching elements, and a control unit configured to control the primary switching elements and the secondary switching elements, wherein an input voltage input to the primary full bridge circuit is Vin, an output voltage output from the secondary full bridge circuit is Vout, and an equivalent voltage ratio which is an equivalent voltage ratio when the winding ratio of the transformer is converted to 1:1 is (Vout x N1) / (Vin x N2), and the control unit controls the primary switching elements and the secondary switching elements in the following control modes: a boost mode when the equivalent voltage ratio is greater than 1; and a step-down mode when the equivalent voltage ratio is smaller than 1, and when the control mode is the step-up mode, the control unit controls the primary-side full bridge circuit so that two levels of primary-side voltage are applied to the series-connected body, and controls the secondary-side full bridge circuit so that three levels of secondary-side voltage are applied to the secondary winding, and the step-up mode includes a step-up delay phase mode in which the primary-side voltage is raised from a low level to a high level, and then the secondary-side voltage is raised from a low level to a middle level, and then the secondary-side voltage is raised from the middle level to the high level, a difference between a first time when the secondary-side voltage rises from a low level to a middle level and a second time when the primary-side voltage rises from a low level to a high level is a first phase difference, and a difference between the second time and a third time when the secondary-side voltage rises from a middle level to a high level is a second phase difference, a condition for performing soft switching of the plurality of primary side switching elements and the plurality of secondary side switching elements is that a reactor current flowing through the reactor at the first time is equal to or greater than a reactor current threshold, and the reactor current is equal to or less than the reactor current threshold at the second time;the control unit is configured to derive a combination of the first phase difference and the second phase difference that can output a required power and satisfy the condition in the boost delay phase mode, and to control the primary side full bridge circuit and the secondary side full bridge circuit to realize the derived first phase difference and second phase difference, thereby outputting the required power.

8. A transformer having a primary winding with a number of turns N1 and a secondary winding with a number of turns N2, a reactor connected to the primary winding, the primary winding and the reactor forming a series connection, a primary full bridge circuit having a plurality of primary switching elements, a secondary full bridge circuit having a plurality of secondary switching elements, a circuit connected to the reactor, the primary full bridge circuit having a plurality of secondary switching elements, and a control unit configured to control the primary switching elements and the secondary switching elements, wherein an input voltage input to the primary full bridge circuit is Vin, an output voltage output from the secondary full bridge circuit is Vout, and an equivalent voltage ratio which is an equivalent voltage ratio when the winding ratio of the transformer is converted to 1:1 is (Vout x N1) / (Vin x N2), and the control unit controls the primary switching elements and the secondary switching elements in the following control modes: a boost mode when the equivalent voltage ratio is greater than 1; and a step-down mode when the equivalent voltage ratio is smaller than 1, and when the control mode is the step-up mode, the control unit controls the primary-side full bridge circuit so that two levels of primary-side voltage are applied to the series-connected body, and controls the secondary-side full bridge circuit so that three levels of secondary-side voltage are applied to the secondary winding, and the step-up mode comprises: a step-up advance phase mode in which the secondary-side voltage is raised from a low level to a middle level, and then the primary-side voltage is raised from a low level to a high level, and then the secondary-side voltage is raised from the middle level to the high level; a step-up same phase mode in which the primary-side voltage is raised from a low level to a high level and the secondary-side voltage is raised from a low level to a middle level simultaneously, and then the secondary-side voltage is raised from the middle level to a high level; and a step-up delay phase mode in which the primary-side voltage is raised from a low level to a high level, and then the secondary-side voltage is raised from a low level to a middle level, and then the secondary-side voltage is raised from the middle level to the high level,a first phase difference is a difference between a first time when the secondary side voltage rises from a low level to an intermediate level and a second time when the primary side voltage rises from a low level to a high level, and a second phase difference is a difference between the second time and a third time when the secondary side voltage rises from a intermediate level to a high level; a condition for performing soft switching of the plurality of primary side switching elements and the plurality of secondary side switching elements is that a reactor current flowing through the reactor at the first time is equal to or greater than a reactor current threshold and that the reactor current is equal to or less than the reactor current threshold at the second time; the control unit derives a combination of the first phase difference and the second phase difference that can output required power and satisfies the condition in the boost leading phase mode, and controls the primary side full bridge circuit and the secondary side full bridge circuit to realize the derived first phase difference and second phase difference, thereby outputting the required power; the power conversion device configured to: in the boost in-phase mode, derive a combination of the first phase difference and the second phase difference which can output the required power, the combination causing the reactor current to match the reactor current threshold value at the first time, and control the primary-side full bridge circuit and the secondary-side full bridge circuit to realize the derived first phase difference and second phase difference, thereby outputting the required power; and in the boost delayed phase mode, derive a combination of the first phase difference and the second phase difference which can output the required power and satisfies the condition, and control the primary-side full bridge circuit and the secondary-side full bridge circuit to realize the derived first phase difference and second phase difference, thereby outputting the required power.

9. A power conversion device according to any one of claims 1 to 8, wherein the reactor current threshold is defined by the following formula (1). I1: reactor current threshold, k1: coefficient, V1: primary side voltage, L1: reactor inductance, C01: capacitance corresponding to one switching element with respect to the combined capacitance of all capacitors arranged in parallel to the switching elements of the primary side full bridge circuit.

10. A power conversion device as described in any one of claims 1 to 8, wherein the inductance of the secondary winding is set so that the secondary current flowing in the secondary winding due to the excitation current generated in the secondary winding exceeds a secondary threshold value.

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