Electric power conversion apparatus

Abstract

A control unit (50) executes first control to control a primary-side full-bridge circuit (30) so that a primary-side voltage of three levels is applied to a primary-side winding (22) and also control a secondary-side full-bridge circuit (40) so that a secondary-side voltage of two levels is applied to a secondary-side winding (23) and a series connection body (24) of a reactor (L1). In the first control, the control unit (50) controls the primary-side full-bridge circuit (30) and the secondary-side full-bridge circuit (40) to achieve a combination of a first phase difference and a second phase difference that can output required electric power and that satisfy condition 1 and condition 2, and thereby outputs the required electric power.

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] If the equivalent voltage ratio, which corresponds to the ratio of the input voltage to the output voltage when the transformer winding ratio is converted to 1:1, is close to 1, soft switching of the primary-side switching element and the secondary-side switching element may not be possible.

[0005] A power conversion device according to one aspect of the present disclosure includes a transformer having a primary winding and a secondary winding, 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 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 a condition for performing soft switching of the plurality of primary switching elements is condition 1, and a condition for performing soft switching of the plurality of secondary switching elements is condition 2, and condition 1 is a current that flows in the primary winding when at least one of the primary switching elements switches between ON and OFF, the value of a current that flows in a forward direction of a diode connected in parallel to the primary switching element that switches from OFF to ON among the plurality of primary switching elements is greater than or equal to a value of a current that flows in a forward direction of the primary winding. and condition 2 is that the value of a current flowing in the secondary winding when at least one of the secondary switching elements is switched between ON and OFF, the value of a current flowing in a forward direction of a diode connected in parallel to the secondary switching element that is switched from OFF to ON among the plurality of secondary switching elements, is equal to or greater than the absolute value of a secondary winding current threshold, and the control unit controls the primary full bridge circuit so that two levels of primary voltage are applied to the series connection, and a first control for controlling the secondary-side full bridge circuit so that a three-level secondary-side voltage is applied to a primary-side winding, the primary-side voltage and the secondary-side voltage have waveforms of the same frequency that are inverted in phase every 180 degrees, 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 first time and a third time when the secondary-side voltage rises from the middle level to a high level is a second phase difference, and the control unit, in the first control,The primary-side full-bridge circuit and the secondary-side full-bridge circuit are controlled to realize a combination of the first phase difference and the second phase difference that can output the required power and that satisfies the conditions 1 and 2, thereby outputting the required power.

[0006] To soft-switch the primary-side switching element, condition 1 must be satisfied. To soft-switch the secondary-side switching element, condition 2 must be satisfied. When the equivalent input voltage and equivalent output voltage when the transformer turns ratio is converted to 1:1, the equivalent output voltage / equivalent input voltage is close to 1, the voltage applied to the reactor is low. When the voltage applied to the reactor is low, even if the primary-side full-bridge circuit is controlled so that three levels of primary-side voltage are applied to the series-connected element, the amount of change in current is small, and soft switching may not be possible. On the other hand, when the voltage applied to the reactor is low, the amount of change in current can be increased by controlling the secondary-side full-bridge circuit so that three levels of secondary-side voltage are applied to the secondary winding. This enables soft-switching of the primary-side switching element and the secondary-side switching element.

[0007] A power conversion device according to one aspect of the present disclosure includes a transformer having a primary winding and a secondary winding, a reactor connected to the secondary winding, wherein the secondary winding and the reactor form a series connection, a primary full bridge circuit connected to the primary winding, the primary full bridge circuit having a plurality of primary switching elements, a secondary full bridge circuit connected to the reactor, 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 a condition for performing soft switching of the plurality of primary switching elements is condition 1, a condition for performing soft switching of the plurality of secondary switching elements is condition 2, and condition 1 is a current that flows in the primary winding when at least one of the primary switching elements switches between ON and OFF, the value of a current that flows in a forward direction of a diode connected in parallel to the primary switching element that switches from OFF to ON among the plurality of primary switching elements is greater than or equal to a value of a current that flows in a forward direction of the primary winding. and condition 2 is that the value of a current flowing in the secondary winding when at least one of the secondary switching elements switches between ON and OFF, the value of a current flowing in a forward direction of a diode connected in parallel to the secondary switching element that switches from OFF to ON among the plurality of secondary switching elements, is equal to or greater than the absolute value of a secondary winding current threshold, and the control unit controls the primary full bridge circuit so that three levels of primary voltage are applied to the primary winding, and The control unit is configured to execute a first control for controlling the secondary-side full bridge circuit so that two levels of secondary-side voltages are applied to the column-connected bodies, the primary-side voltage and the secondary-side voltage have waveforms of the same frequency that are inverted in phase every 180 degrees, 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 first time and a third time when the primary-side voltage rises from the middle level to a high level is a second phase difference, and in the first control, the control unit:The primary-side full-bridge circuit and the secondary-side full-bridge circuit are controlled to realize a combination of the first phase difference and the second phase difference that can output the required power and that satisfies the conditions 1 and 2, thereby outputting the required power.

[0008] To soft-switch the primary-side switching element, condition 1 must be satisfied. To soft-switch the secondary-side switching element, condition 2 must be satisfied. When the equivalent input voltage and equivalent output voltage when the transformer turns ratio is converted to 1:1, the equivalent output voltage / equivalent input voltage is close to 1, the voltage applied to the reactor is low. When the voltage applied to the reactor is low, even if the secondary-side full-bridge circuit is controlled so that three levels of secondary-side voltage are applied to the series-connected element, the amount of change in current is small, and soft switching may not be possible. On the other hand, when the voltage applied to the reactor is low, the amount of change in current can be increased by controlling the primary-side full-bridge circuit so that three levels of primary-side voltage are applied to the primary winding. This enables soft-switching of the primary-side switching element and the secondary-side switching element.

[0009] FIG. 1 is a circuit diagram of a power conversion device according to a first embodiment. FIG. 2 is a diagram illustrating the relationship between an equivalent voltage ratio and a control mode. FIG. 3 is a diagram illustrating a primary-side voltage and a secondary-side voltage in a first buck delay phase mode according to the first embodiment. FIG. 4 is a diagram illustrating a primary-side voltage and a secondary-side voltage in a first buck inline phase mode according to the first embodiment. FIG. 5 is a diagram illustrating a primary-side voltage and a secondary-side voltage in a first buck lead phase mode according to the first embodiment. FIG. 6 is a flowchart illustrating switching control performed by a control unit. FIG. 7 is a diagram illustrating a primary-side current and a secondary-side current in a first buck delay phase mode according to the first embodiment. FIG. 8 is a diagram illustrating a primary-side voltage and a secondary-side voltage in a first boost delay phase mode according to the first embodiment. FIG. 9 is a diagram illustrating a primary-side voltage and a secondary-side voltage in a first boost inline phase mode according to the first embodiment. FIG. 10 is a diagram illustrating a primary-side voltage and a secondary-side voltage in a first boost lead phase mode according to the first embodiment. FIG. 11 is a diagram illustrating a primary-side voltage and a secondary-side voltage in a second buck delay phase mode according to the first embodiment. FIG. 12 is a diagram showing the primary-side voltage and the secondary-side voltage in the second buck-inline phase mode of the first embodiment. FIG. 13 is a diagram showing the primary-side voltage and the secondary-side voltage in the second buck-inline phase mode of the first embodiment. FIG. 14 is a diagram showing the relationship between the equivalent voltage ratio and the output current of the power conversion device of the first embodiment. FIG. 15 is a circuit diagram of the power conversion device of the second embodiment. FIG. 16 is a diagram showing the primary-side voltage and the secondary-side voltage in the first buck-inline phase mode of the second embodiment. FIG. 17 is a diagram showing the primary-side voltage and the secondary-side voltage in the first buck-inline phase mode of the second embodiment. FIG. 18 is a diagram showing the primary-side voltage and the secondary-side voltage in the first buck-inline phase mode of the second embodiment. FIG. 19 is a diagram showing the primary-side voltage and the secondary-side voltage in the first boost-inline phase mode of the second embodiment. FIG. 20 is a diagram showing the primary-side voltage and the secondary-side voltage in the first boost-inline phase mode of the second embodiment. Fig. 21 is a diagram showing the primary-side voltage and the secondary-side voltage in the first boost-leading phase mode of the second embodiment. Fig. 22 is a diagram showing the primary-side voltage and the secondary-side voltage in the second boost-delaying phase mode of the second embodiment. Fig. 23 is a diagram showing the primary-side voltage and the secondary-side voltage in the second boost-independent phase mode of the second embodiment.Fig. 24 is a diagram showing the primary side voltage and the secondary side voltage in the second boost lead phase mode of the second embodiment. Fig. 25 is a diagram showing the relationship between the equivalent voltage ratio and the output current of the power conversion device of the second embodiment.

[0010] [First Embodiment] A first 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.

[0011] <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.

[0012] 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.

[0013] 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 L1. The reactor L1 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 L1 is connected to the primary winding 22.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] The connection point between the first switching element Q1 and the second switching element Q2 is connected to the primary winding 22 via the reactor L1. The connection point between the third switching element Q3 and the fourth switching element Q4 is connected to the primary winding 22. As a result, the primary full-bridge circuit 30 is connected to the primary winding 22. The primary winding 22 and the reactor L1 form a series connection 24.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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 voltage sensor 47. The secondary-side voltage sensor 47 detects the output voltage Vout of the power conversion device 10.

[0023] 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.

[0024] The control unit 50 converts the input voltage Vin into the output voltage Vout by controlling the plurality of primary side switching elements Q1 to Q4 and the plurality of secondary side switching elements Q5 to Q8.

[0025] <First Control> The control unit 50 performs first control in which one of the primary side full bridge circuit 30 and the secondary side full bridge circuit 40, through which the excitation current flows, is controlled in three levels, and the other of the secondary side full bridge circuit 40, through which the excitation current does not flow, is controlled in two levels. The one of the primary side full bridge circuit 30 and the secondary side full bridge circuit 40, through which the excitation current flows, is a circuit to which the reactor L1 is not connected. The one of the primary side full bridge circuit 30 and the secondary side full bridge circuit 40, through which the excitation current does not flow, is a circuit to which the reactor L1 is connected. In this embodiment, 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.

[0026] Three-level control is control in which the voltage applied to the secondary winding 23 is switched between three levels: positive, negative, or zero. Two-level control is control in which the voltage applied to the series connection 24 of the primary winding 22 and the reactor L1 is switched between two levels: positive and negative. In three-level control, a positive voltage applied to the secondary winding 23 is referred to as a high level, a zero voltage applied to the secondary winding 23 is referred to as a middle level, and a negative voltage applied to the secondary winding 23 is referred to as a low level. In two-level control, a positive voltage applied to the series connection 24 is referred to as a high level, and a negative voltage applied to the series connection 24 is referred to as a low level. The voltage applied to the series connection 24 of the primary winding 22 and the reactor L1 is referred to as a primary voltage V1, and the voltage applied to the secondary winding 23 is referred to as a secondary voltage V2. The primary-side voltage V1 is the potential difference between 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. The secondary-side voltage V2 is the potential difference between 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. The arrows in FIG. 1 indicate the positive directions of the primary-side voltage V1 and the secondary-side voltage V2. During normal operation where the output is not changed, the primary-side voltage V1 and the secondary-side voltage V2 have waveforms of the same frequency that are inverted in phase every 180 degrees.

[0027] 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 first pattern and a second pattern.

[0028] 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.

[0029] The second 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.

[0030] 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 a third pattern to a sixth pattern.

[0031] The third 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 fourth 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.

[0033] The fifth 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.

[0034] 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 off, and the eighth switching element Q8 is turned on.

[0035] The control unit 50 outputs an output voltage Vout from the secondary full bridge circuit 40 by combining either the first or second pattern of the primary full bridge circuit 30 with any of the third to sixth patterns of the secondary full bridge circuit 40.

[0036] 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 of the input voltage Vin input to the primary full bridge circuit 30 to the output voltage Vout output from the secondary full bridge circuit 40 is the voltage ratio. The equivalent voltage ratio, which is the equivalent 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 control unit 50 has a first step-down mode and a first step-up mode as control modes of the first control. The first step-down mode is a control mode in which the equivalent voltage ratio is less than 1. The first step-up mode is a control mode in which the equivalent voltage ratio is greater than 1.

[0037] For example, if the turns ratio of transformer 20 is 1:2, the input voltage Vin=200V, and the output voltage Vout=400V, converting the turns ratio to 1:1 results in an equivalent voltage ratio of (Vout×1) / (Vin×2) being 1. Therefore, if the turns ratio of transformer 20 is 1:2 and the input voltage Vin=200V, the mode is step-down when the output voltage Vout is less than 400V, and the mode is step-up when the output voltage Vout is greater than 400V. Note that the following explanation will be given assuming a turns ratio of 1:1 unless otherwise specified.

[0038] <First Step-Down Mode> As shown in FIG. 2, the first step-down mode includes a first step-down delay phase mode, a first step-down in-phase mode, and a first step-down lead phase mode.

[0039] 3, the first step-down delay phase mode is a first step-down mode in which the secondary-side voltage V2 is raised from a low level to an intermediate 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 intermediate level to a high level. The first step-down delay phase mode is a first step-down mode in which a small amount of power is output in response to the power required by the load 120.

[0040] In the first step-down mode, the difference between a first time T1 when the secondary-side voltage V2 rises from a low level to an intermediate level and a second time T2 when the primary-side voltage V1 rises from a low level to a high level is a first phase difference θ1. The difference between the first time T1 and a third time T3 when the secondary-side voltage V2 rises from an intermediate level to a high level is a second phase difference θ2.

[0041] As described above, the primary-side voltage V1 and the secondary-side voltage V2 have waveforms of the same frequency that are inverted in phase every 180 degrees during normal operation with no change in output. Therefore, the time when the secondary-side voltage V2 falls from high to middle level is also the first time T1, the time when the primary-side voltage V1 falls from high to low level is also the second time T2, and the time when the secondary-side voltage V2 falls from middle to low level is also the third time T3.

[0042] As shown in FIG. 4 , the first in-phase step-down mode is a first step-down mode in which the secondary-side voltage V2 is raised from a low level to a middle level and the primary-side voltage V1 is raised from a low level to a high level simultaneously, and then the secondary-side voltage V2 is raised from a middle level to a high level. In the first in-phase step-down mode, the first time T1 and the second time T2 are the same time. Therefore, the first phase difference θ1 is 0. The first in-phase step-down mode is a first step-down mode in which medium power is output in response to the power requested by the load 120. Medium power is power greater than small power.

[0043] 5, the first step-down lead phase mode is a first step-down 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. The first step-down lead phase mode is a first step-down mode in which high power is output in response to the power required by the load 120. The high power is power greater than medium power.

[0044] The first buck delay phase mode is an example of a light load mode, the first buck advance phase mode is an example of a heavy load mode, and the first buck in-phase mode is a mode used when switching between the light load mode and the heavy load mode.

[0045] The following describes the switching control performed by the control unit 50 in the first step-down mode. 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.

[0046] 6 and 7, in step S1, the control unit 50 derives a soft switching region. The soft switching region is set for each of the primary current I1 and the secondary current I2. The primary current I1 is a current flowing through the primary winding 22. The secondary current I2 is a current flowing through the secondary winding 23. In FIG. 1, the arrows indicate the positive directions of the primary current I1 and the secondary current I2.

[0047] A condition for soft switching of the multiple primary-side switching elements Q1 to Q4 is that, when at least one of the primary-side switching elements Q1 to Q4 switches between ON and OFF, the value of the primary-side current I1, which is a current flowing in the forward direction of a diode connected in parallel to one of the primary-side switching elements Q1 to Q4 that changes from OFF to ON, is equal to or greater than the absolute value of a primary-side winding current threshold value TI1. Hereinafter, this condition will be referred to as condition 1. The region that satisfies condition 1 is the soft-switching region of the primary-side current I1. The time when at least one of the primary-side switching elements Q1 to Q4 switches between ON and OFF is the time when at least one of the primary-side switching elements Q1 to Q4 changes from ON to OFF or from OFF to ON. In the first control of this embodiment, the time when at least one of the primary side switching elements Q1 to Q4 switches between ON and OFF is the same as the second time T2 when the primary side voltage V1 rises or falls.

[0048] The condition for soft switching of the multiple secondary-side switching elements Q5 to Q8 is that, when at least one of the secondary-side switching elements Q5 to Q8 switches between ON and OFF, the value of the secondary-side current I2, which flows in the forward direction of a diode connected in parallel to one of the secondary-side switching elements Q5 to Q8 that switches from OFF to ON, is equal to or greater than the absolute value of a secondary-side winding current threshold value TI2. Hereinafter, this condition will be referred to as condition 2. The region that satisfies condition 2 is the soft-switching region of the secondary-side current I2. The time when at least one of the secondary-side switching elements Q5 to Q8 switches between ON and OFF is the time when at least one of the secondary-side switching elements Q5 to Q8 switches from ON to OFF or from OFF to ON. In the first control of this embodiment, the time when at least one of the secondary-side switching elements Q5 to Q8 switches between ON and OFF is the first time T1 or the third time T3. The first time T1 is the time when the secondary-side voltage V2 rises from a low level to a middle level, or the time when the secondary-side voltage V2 falls from a high level to a middle level. The third time is the time when the secondary-side voltage V2 rises from a middle level to a high level, or the time when the secondary-side voltage V2 falls from a middle level to a low level.

[0049] As described above, the primary voltage V1 and the secondary voltage V2 have waveforms of the same frequency that are inverted in phase every 180 degrees, and therefore, since the positive and negative signs are simply reversed at the rising and falling edges, it is sufficient to consider only one of them. The following explanation will focus on the rising edge only.

[0050] FIG. 7 shows, as an example, the primary-side current I1 and the secondary-side current I2 in the first step-down delay phase mode. As can be seen from a comparison with the voltage waveforms in FIG. 3 , in the example shown in FIG. 7 , at the first time T1, the switching pattern of the secondary-side full bridge circuit 40 is switched from the fifth pattern to the fourth pattern. At the third time T3, the switching pattern of the secondary-side full bridge circuit 40 is switched from the fourth pattern to the third pattern. If condition 2 is satisfied at the first time T1 and the third time T3, soft switching of the secondary-side switching elements Q5 to Q8 is achieved. At the second time T2, the switching pattern of the primary-side full bridge circuit 30 is switched from the second pattern to the first pattern. At this time, if condition 1 is satisfied, soft switching of the primary-side switching elements Q1 to Q4 is achieved.

[0051] The primary winding current threshold TI1 is defined by the following equation (1).

[0052] TI1: primary winding current threshold, k1: coefficient, V1: primary voltage, L: inductance of reactor L1, and C01: capacitance corresponding to one switching element relative to the combined capacitance of all capacitors C1 to C4 arranged in parallel with switching elements Q1 to Q4 of primary 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, then C01 = (C11 + C12 + C13 + C14) / 4 may be used. If Cx is the largest capacitance among C11 to C14, then C01 = Cx may be used. The secondary winding current threshold TI2 is similarly defined by replacing each value with a secondary value. In this way, the soft switching region is derived.

[0053] Next, in step S2, the control unit 50 calculates a target current. The target current is a current value that can output the power required by the load 120 and that satisfies conditions 1 and 2. In this control mode, of the primary-side switching elements Q1 to Q4, the first switching element Q1 and the fourth switching element Q4 change from OFF to ON at the second time T2. The first switching element Q1 and the fourth switching element Q4 are primary-side switching elements that change from OFF to ON when the switching pattern changes from the second pattern to the first pattern.

[0054] To satisfy Condition 1, the primary current I1 at the second time T2, which flows forward through the diodes D1 and D4 connected in parallel to the first switching element Q1 and the fourth switching element Q4, respectively, and which change from OFF to ON at this time, needs to be equal to or greater than the absolute value "|TI1|" of the primary winding current threshold value TI1. This current value is the value "-I1" in FIG. 1. As can be seen from FIG. 7, the primary current I1 is a negative value at the second time T2, so "-I1" becomes a positive value.

[0055] In this control mode, to satisfy Condition 2, the secondary-side current I2 at the first time T1 and the third time T3 is taken into consideration. The time at which the absolute value of the secondary-side current is smaller between the first time T1 and the third time T3 in this control mode is the first time. Therefore, it is sufficient to consider the secondary-side current I2 at the first time T1. Therefore, to satisfy Condition 2, the secondary-side current I2 at the first time T1 must be such that the value of the forward current flowing through the diode D5 connected in parallel with the fifth switching element Q5, which changes from OFF to ON at this time, is equal to or greater than the absolute value "|TI2|" of the secondary-side winding current threshold TI2. This current value is the value "+I2" in FIG. 1. The fifth switching element Q5 is the secondary-side switching element that changes from OFF to ON when the switching pattern changes from the fifth pattern to the fourth pattern. As can be seen from FIG. 7, the secondary-side current I2 is a positive value at the first time T1, so "+I2" is also a positive value. Moreover, the time at which the absolute value of the secondary current becomes small, either the first time T1 or the third time T3, varies depending on the control mode.

[0056] In practice, for example, condition 2 may be satisfied by comparing TI2', which is the secondary winding current threshold value TI2 converted to the primary side, with the primary current I1 at the first time T1, or vice versa. In this case, it is sufficient to consider either the primary current I1 or the secondary current I2. Furthermore, the inductance L of the reactor L1 may be set in advance so that if one of condition 1 or condition 2 is satisfied, the other will also be satisfied.

[0057] In this way, it is possible to calculate a combination of target currents that can output the required power from the load 120 and that satisfies conditions 1 and 2. If there is no combination of target currents that can output the required power and that satisfies conditions 1 and 2, this means that soft switching is not possible for that required power and control mode.

[0058] Next, in step S3, the control unit 50 derives a combination of the first phase difference θ1 and the second phase difference θ2 from the calculated target current. Here, the control unit 50 derives the combination of the first phase difference θ1 and the second phase difference θ2 so as to follow the calculated target current at the first time T1 and the calculated target current at the second time T2. In other words, the combination of the first phase difference θ1 and the second phase difference θ2 is a combination that can output the required power of the load 120 and satisfies conditions 1 and 2.

[0059] 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 realize the first phase difference θ1 and the second phase difference θ2 derived in step S3.

[0060] The required power, the first phase difference θ1, and the second phase difference θ2 may be calculated each time, or may be calculated in advance and stored in a map, etc. <First Boost Mode> As shown in Fig. 2, the first boost mode includes a first boost delay phase mode, a first boost in-phase mode, and a first boost lead phase mode.

[0061] As shown in Fig. 8, the first boost delay phase mode is a first 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. The first boost delay phase mode is a first boost mode in which a small amount of power is output in response to the power requested by the load 120. The first boost delay phase mode differs from the first buck delay phase mode in that the high level of the secondary-side voltage V2 is higher in voltage than the high level of the primary-side voltage V1. The first boost delay phase mode also differs from the first buck delay phase mode in that the low level of the secondary-side voltage V2 is lower in voltage than the low level of the primary-side voltage V1.

[0062] In the first step-up mode, the difference between a first time T11 when the secondary-side voltage V2 rises from a low level to a middle level and a second time T12 when the primary-side voltage V1 rises from a low level to a high level is the first phase difference θ11. The difference between the first time T11 and a third time T13 when the secondary-side voltage V2 rises from a middle level to a high level is the second phase difference θ12.

[0063] As described above, during normal operation with no change in output, the primary-side voltage V1 and the secondary-side voltage V2 have waveforms of the same frequency that are inverted in phase every 180 degrees. Therefore, the time when the secondary-side voltage V2 falls from high to middle level is also the first time T11, the time when the primary-side voltage V1 falls from high to low level is also the second time T12, and the time when the secondary-side voltage V2 falls from middle to low level is also the third time T13.

[0064] As shown in FIG. 9 , the first in-phase boost mode is a first boost mode in which the secondary-side voltage V2 is raised from a low level to a middle level and the primary-side voltage V1 is raised from a low level to a high level simultaneously, and then the secondary-side voltage V2 is raised from a middle level to a high level. In the first in-phase boost mode, the first time T11 and the second time T12 are the same time. Therefore, the first phase difference θ11 is zero. The first in-phase boost mode is a first boost mode in which medium power is output in response to the power requested by the load 120. Medium power is greater than low power. The first in-phase boost mode differs from the first in-phase buck mode in that the high level of the secondary-side voltage V2 is higher in voltage than the high level of the primary-side voltage V1. The first in-phase boost mode also differs from the first in-phase buck mode in that the low level of the secondary-side voltage V2 is lower in voltage than the low level of the primary-side voltage V1.

[0065] As shown in Fig. 10 , the first boost-lead phase mode is a first 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. The first boost-lead phase mode is a first boost mode in which a large amount of power is output in response to the power requested by the load 120. The first boost-lead phase mode differs from the first buck-lead phase mode in that the high level of the secondary-side voltage V2 is higher in voltage than the high level of the primary-side voltage V1. The first boost-lead phase mode also differs from the first buck-lead phase mode in that the low level of the secondary-side voltage V2 is lower in voltage than the low level of the primary-side voltage V1.

[0066] The first boost delay phase mode is an example of a light load mode, the first boost advance phase mode is an example of a heavy load mode, and the first boost in-phase mode is a mode used when switching between the light load mode and the heavy load mode.

[0067] The switching control performed by the control unit 50 when the control mode is the first voltage step-up mode is the same as that performed in the first voltage step-down mode. That is, 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. This combination satisfies conditions 1 and 2. The control unit 50 then 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.

[0068] <Second Control> The control unit 50 may execute a second control. In the second control, the control unit 50 controls the primary-side full-bridge circuit 30 so that a three-level primary-side voltage V1 is applied to the series-connected body 24, and controls the secondary-side full-bridge circuit 40 so that a two-level secondary-side voltage V2 is applied to the secondary-side winding 23.

[0069] In the first embodiment, when the second control is executed, the control unit 50 executes the second control when the equivalent voltage ratio is less than a predetermined threshold value that is less than 1. The control unit 50 executes the first control when the equivalent voltage ratio is equal to or greater than the predetermined threshold value.

[0070] 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 seventh to tenth patterns.

[0071] 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.

[0072] The eighth 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.

[0073] The ninth 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.

[0074] The tenth 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.

[0075] 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 an eleventh pattern and a twelfth pattern.

[0076] The eleventh 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.

[0077] 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 on, and the eighth switching element Q8 is turned off.

[0078] The control unit 50 outputs an output voltage Vout from the secondary full bridge circuit 40 by combining any one of the seventh to tenth patterns of the primary full bridge circuit 30 with any one of the eleventh and twelfth patterns of the secondary full bridge circuit 40.

[0079] <Second Step-Down Mode> The control unit 50 has a second step-down mode as a control mode of the second control. The second step-down mode is a control mode when the equivalent voltage ratio is smaller than 1. The second step-down mode has a second step-down delay phase mode, a second step-down in-phase mode, and a second step-down lead phase mode.

[0080] 11 , the second step-down delay phase mode is a second 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. The second step-down delay phase mode is a second step-down mode in which a small amount of power is output in response to the power required by the load 120.

[0081] In the second control, the difference between a first time T21 when the primary voltage V1 rises from a low level to a middle level and a second time T22 when the secondary voltage V2 rises from a low level to a high level is a first phase difference θ21. The difference between the first time T21 and a third time T23 when the primary voltage V1 rises from a middle level to a high level is a second phase difference θ22.

[0082] As described above, during normal operation with no change in output, the primary-side voltage V1 and the secondary-side voltage V2 have waveforms of the same frequency that are inverted in phase every 180 degrees. Therefore, the time when the primary-side voltage V1 falls from high to middle level is also the first time T21, the time when the secondary-side voltage V2 falls from high to low level is also the second time T22, and the time when the primary-side voltage V1 falls from middle to low level is also the third time T23.

[0083] 12 , the second in-phase step-down mode is a second step-down mode in which, after the primary voltage V1 is raised from a low level to a middle level, the secondary voltage V2 is raised from a low level to a high level and the primary voltage V1 is raised from a middle level to a high level simultaneously. In the second in-phase step-down mode, the first time T21 and the second time T22 are the same time. Therefore, the first phase difference θ21 and the second phase difference θ22 have the same value. The second in-phase step-down mode is a second step-down mode in which medium power is output in response to the power requested by the load 120.

[0084] 13 , the second step-down lead phase mode is a second step-down mode in which the primary side voltage V1 is raised from a low level to a middle level, 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. The second step-down lead phase mode is a second step-down 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 middle power.

[0085] The second buck delay phase mode is an example of a light load mode, the second buck advance phase mode is an example of a heavy load mode, and the second buck in-phase mode is a mode used when switching between the light load mode and the heavy load mode.

[0086] In the second step-down mode, the control unit 50 performs switching control by controlling the first phase difference θ21 and the second phase difference θ22. The control unit 50 derives a combination of the first phase difference θ21 and the second phase difference θ22 that allows the required power to be output. This combination is a condition for performing soft switching of the primary-side switching elements Q1 to Q4, and satisfies the same condition as condition 1 of the first control, and is also a condition for performing soft switching of the secondary-side switching elements Q5 to Q8, and satisfies the same condition as condition 2 of the first control.

[0087] In the second control, when at least one of the primary-side switching elements Q1 to Q4 switches between ON and OFF, it is the time when at least one of the primary-side switching elements Q1 to Q4 switches from ON to OFF or from OFF to ON. This time is the same as the first time T21 or the third time T23. The first time T21 is the time when the primary-side voltage V1 rises from a low level to a middle level or the time when the primary-side voltage V1 falls from a high level to a middle level. The third time T23 is the time when the primary-side voltage V1 rises from a middle level to a high level or the time when the primary-side voltage V1 falls from a middle level to a low level. Furthermore, in the second control, when at least one of the secondary-side switching elements Q5 to Q8 switches between ON and OFF, it is the time when at least one of the secondary-side switching elements Q5 to Q8 switches from ON to OFF or from OFF to ON. This point in time is the same as the second time T22, which is the time when the secondary voltage V2 rises from a low level to a high level or the time when the secondary voltage V2 falls from a high level to a low level.

[0088] 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 so as to realize the derived first phase difference θ21 and second phase difference θ22.

[0089] [Operation of First Embodiment] Figure 14 shows the relationship between the equivalent voltage ratio and the output current. Line L12 is the boundary line where switching occurs between the heavy load mode and the light load mode when the second control is performed. Region A2 indicates the operating region where soft switching is impossible when the second control is performed. As can be seen from Figure 14, region A2 includes an equivalent voltage ratio of 1. Thus, with the second control, soft switching cannot be performed even in the heavy load mode in region A2 where the equivalent voltage ratio includes 1.

[0090] In the second step-down mode, the primary-side full-bridge circuit 30 is controlled so that a three-level primary voltage V1 is applied to the series-connected body 24. The voltage applied to the reactor L1 is the primary-side voltage V1 minus the voltage applied to the primary winding 22. When the equivalent voltage ratio is close to 1, the voltage applied to the reactor L1 is low. When the voltage applied to the reactor L1 is low, even if the primary-side full-bridge circuit 30 is controlled so that a three-level primary voltage V1 is applied to the series-connected body 24, the amount of change in current is small, and soft switching may not be possible. In other words, the primary-side current I1 and the secondary-side current I2 may not be changed to values ​​within the soft switching region.

[0091] When the voltage applied to the reactor L1 is low, the amount of change in current can be increased by controlling the secondary full-bridge circuit 40 so that a three-level secondary voltage V2 is applied to the secondary winding 23. This enables soft switching of the primary switching elements Q1 to Q4 and the secondary switching elements Q5 to Q8.

[0092] Line L11 in Figure 14 is the boundary line where switching occurs between heavy load mode and light load mode when the first control is performed. Region A1 indicates the operating region where soft switching is impossible when the first control is performed. In this way, the region where soft switching is impossible shifts from region A2 under the second control to region A1 under the first control. Region A1 where soft switching is impossible under the first control does not overlap with region A2 where soft switching is impossible under the second control. Therefore, even in region A2 where soft switching is impossible under the second control, soft switching can be performed by performing the first control.

[0093] As described above, the control unit 50 executes the second control when the equivalent voltage ratio is less than a predetermined threshold value that is less than 1. This threshold value may be, for example, a value that is smaller than the lower limit value of the equivalent voltage ratio represented by the region A2 and greater than the upper limit value of the equivalent voltage ratio represented by the region A1. That is, the threshold value may be set so that the second control is executed in the region A1 and the first control is executed in the region A2.

[0094] [Effects of the First Embodiment] (1-1) When the equivalent voltage ratio is close to 1, the voltage applied to the reactor L1 is low. When the voltage applied to the reactor L1 is low, even if the primary-side full-bridge circuit 30 is controlled so that a three-level primary-side voltage V1 is applied to the series-connected body 24, the amount of change in current is small, and soft switching may not be possible (area A2). When the voltage applied to the reactor L1 is low, the amount of change in current can be increased by controlling the secondary-side full-bridge circuit 40 so that a three-level secondary-side voltage V2 is applied to the secondary-side winding 23. As a result, even when the equivalent voltage ratio is close to 1, soft switching of the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8 is possible.

[0095] (1-2) The control unit 50 executes the second control when the equivalent voltage ratio is less than a threshold value, and executes the first control when the equivalent voltage ratio is equal to or greater than the threshold value. When the equivalent voltage ratio is less than the threshold value, soft switching may not be possible with the first control (area A1). By executing the second control in this case, soft switching can be performed even when soft switching is not possible with the first control. When the equivalent voltage ratio is equal to or greater than the threshold value, soft switching may not be possible with the second control (area A2). By executing the first control in this case, soft switching can be performed even when soft switching is not possible with the second control.

[0096] [Second Embodiment] A second embodiment of the power conversion device will be described. As shown in Fig. 15 , the second embodiment differs from the first embodiment in that a reactor L2 is connected to a secondary winding 23 instead of the primary winding 22, and the three-level control and two-level control are reversed between the primary and secondary sides. The secondary winding 23 and the reactor L2 form a series connection 25.

[0097] <First Control> The first control of the second embodiment is a control in which 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.

[0098] Three-level control is control in which the voltage applied to the primary winding 22 is switched between three levels: positive, negative, or zero. Two-level control is control in which the voltage applied to the series connection 25 of the reactor L2 and the secondary winding 23 is switched between two levels: positive and negative. In three-level control, a positive voltage applied to the primary winding 22 is referred to as a high level, a zero voltage applied to the primary winding 22 is referred to as a middle level, and a negative voltage applied to the primary winding 22 is referred to as a low level. In two-level control, a positive voltage applied to the series connection 25 is referred to as a high level, and a negative voltage applied to the series connection 25 is referred to as a low level.

[0099] The control unit 50 outputs an output voltage Vout from the primary side full bridge circuit 30 by combining any one of the seventh to tenth patterns of the primary side full bridge circuit 30 with any one of the eleventh and twelfth patterns of the secondary side full bridge circuit 40.

[0100] In the second embodiment, the control unit 50 has a first step-down mode and a first step-up mode as control modes of the first control. <First step-down mode> The first step-down mode has a first step-down delay phase mode, a first step-down in-phase mode, and a first step-down delay phase mode.

[0101] 16, the first step-down delay phase mode is a first 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. The first step-down delay phase mode is a first step-down mode in which a small amount of power is output in response to the power required by the load 120.

[0102] The difference between the first time T31 when the primary voltage V1 rises from the low level to the middle level and the second time T32 when the secondary voltage V2 rises from the low level to the high level is the first phase difference θ31. The difference between the first time T31 and the third time T33 when the primary voltage V1 rises from the middle level to the high level is the second phase difference θ32.

[0103] During normal operation with no change in output, the primary voltage V1 and the secondary voltage V2 are controlled to invert their phases every 180 degrees and have the same frequency. Therefore, the time when the primary voltage V1 falls from high to middle level is also the first time T31, the time when the secondary voltage V2 falls from high to low level is also the second time T32, and the time when the primary voltage V1 falls from middle to low level is also the third time T33.

[0104] 17 , the first in-phase step-down mode is a first step-down mode in which the primary voltage V1 is raised from a low level to a middle level, and then the primary voltage V1 is raised from a middle level to a high level and the secondary voltage V2 is raised from a low level to a high level simultaneously. In the first in-phase step-down mode, the second time T32 and the third time T33 are the same time. Therefore, the first phase difference θ31 and the second phase difference θ32 have the same value. The first in-phase step-down mode is a first step-down mode in which medium power is output in response to the power requested by the load 120.

[0105] 18 , the first step-down lead phase mode is a first 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. The first step-down lead phase mode is a first step-down mode in which a large amount of power is output in response to the power required by the load 120.

[0106] In the first step-down mode, the control unit 50 performs switching control by controlling the first phase difference θ31 and the second phase difference θ32, as in the first embodiment. The control unit 50 derives a combination of the first phase difference θ31 and the second phase difference θ32 that allows the required power to be output. This combination satisfies the conditions for performing soft switching on the primary-side switching elements Q1 to Q4, which are the same as condition 1 of the first control, and the conditions for performing soft switching on the secondary-side switching elements Q5 to Q8, which are the same as condition 2 of the first control.

[0107] In the first control of this embodiment, the time when at least one of the primary-side switching elements Q1 to Q4 switches between ON and OFF is the time when at least one of the primary-side switching elements Q1 to Q4 switches from ON to OFF or from OFF to ON. This time is the same as the first time T31 or the third time T33. The first time T31 is the time when the primary-side voltage V1 rises from a low level to a middle level, or the time when the primary-side voltage V1 falls from a high level to a middle level. The third time T33 is the time when the primary-side voltage V1 rises from a middle level to a high level, or the time when the primary-side voltage V1 falls from a middle level to a low level. In the first control of this embodiment, the time when at least one of the secondary-side switching elements Q5 to Q8 switches between ON and OFF is the time when at least one of the secondary-side switching elements Q5 to Q8 switches from ON to OFF or from OFF to ON. This time is the same as the second time T32 when the secondary-side voltage V2 rises from a low level to a high level or when the secondary-side voltage V2 falls from a high level to a low level.

[0108] 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 so as to realize the derived first phase difference θ31 and second phase difference θ32.

[0109] <First Boost Mode> The first boost mode of the second embodiment includes a first boost delay phase mode, a first boost in-phase mode, and a first boost delay phase mode.

[0110] As shown in Fig. 19, the first boost delay phase mode is a first boost mode in which the primary-side voltage V1 is raised from a low level to a middle 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 middle level to a high level. The first boost delay phase mode is a first boost mode in which a small amount of power is output in response to the power requested by the load 120. The first boost delay phase mode differs from the first buck delay phase mode in that the high level of the secondary-side voltage V2 is higher in voltage than the high level of the primary-side voltage V1. The first boost delay phase mode also differs from the first buck delay phase mode in that the low level of the secondary-side voltage V2 is lower in voltage than the low level of the primary-side voltage V1.

[0111] The difference between a first time T41 when the primary voltage V1 rises from a low level to a middle level and a second time T42 when the secondary voltage V2 rises from a low level to a high level is a first phase difference θ41. The difference between the first time T41 and a third time T43 when the primary voltage V1 rises from a middle level to a high level is a second phase difference θ42.

[0112] As described above, during normal operation with no change in output, the primary voltage V1 and the secondary voltage V2 have waveforms of the same frequency that are inverted in phase every 180 degrees. Therefore, the time when the primary voltage V1 falls from high to middle level is also the first time T41, the time when the secondary voltage V2 falls from high to low level is also the second time T42, and the time when the primary voltage V1 falls from middle to low level is also the third time T43.

[0113] As shown in FIG. 20 , the first in-phase boost mode is a first boost mode in which the primary voltage V1 is raised from a low level to a middle level, and then the primary voltage V1 is raised from a middle level to a high level and the secondary voltage V2 is raised from a low level to a high level simultaneously. In the first in-phase boost mode, the second time T42 and the third time T43 are the same time. Therefore, the first phase difference θ41 and the second phase difference θ42 have the same value. The first in-phase boost mode is a first boost mode in which medium power is output in response to the power requested by the load 120. The first in-phase boost mode differs from the first in-phase buck mode in that the high level of the secondary voltage V2 is higher than the high level of the primary voltage V1. The first in-phase buck mode also differs from the first in-phase buck mode in that the low level of the secondary voltage V2 is lower than the low level of the primary voltage V1.

[0114] As shown in Fig. 21 , the first boost-advance phase mode is a first boost mode in which the primary-side voltage V1 is raised from a low level to a middle level, then the primary-side voltage V1 is raised from a middle level to a high level, and then the secondary-side voltage V2 is raised from a low level to a high level. The first boost-advance phase mode is a first boost mode in which a large amount of power is output in response to the power requested by the load 120. The first boost-advance phase mode differs from the first buck-advance phase mode in that the high level of the secondary-side voltage V2 is higher in voltage than the high level of the primary-side voltage V1. The first boost-advance phase mode also differs from the first buck-advance phase mode in that the low level of the secondary-side voltage V2 is lower in voltage than the low level of the primary-side voltage V1.

[0115] The switching control performed by the control unit 50 when the control mode is the first voltage step-up mode is the same as that performed in the first voltage step-down mode. That is, the control unit 50 derives a combination of the first phase difference θ41 and the second phase difference θ42 that allows the required power to be output. This combination satisfies conditions 1 and 2. The control unit 50 then 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 θ41 and second phase difference θ42.

[0116] <Second Control> The control unit 50 may execute the second control. The second control in the second embodiment controls the primary-side full bridge circuit 30 so that a two-level primary-side voltage V1 is applied to the primary-side winding 22, and controls the secondary-side full bridge circuit 40 so that a three-level secondary-side voltage V2 is applied to the series-connected body 25. In the second embodiment, when the second control is executed, the control unit 50 executes the second control when the equivalent voltage ratio is greater than a predetermined threshold value greater than 1. The control unit 50 executes the first control when the equivalent voltage ratio is equal to or less than the predetermined threshold value.

[0117] The control unit 50 outputs an output voltage Vout from the secondary full bridge circuit 40 by combining either the first or second pattern of the primary full bridge circuit 30 with any of the third to sixth patterns of the secondary full bridge circuit 40.

[0118] <Second Boost Mode> The control unit 50 has a second boost mode as a control mode of the second control. The second boost mode is a control mode when the equivalent voltage ratio is greater than 1. The second boost mode has a second boost lead phase mode, a second boost in-phase mode, and a second boost lag phase mode.

[0119] 22 , the second boost delay phase mode is a second boost mode in which the secondary-side voltage V2 is raised from a low level to an intermediate 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 intermediate level to a high level. The second boost delay phase mode is a second boost mode in which a small amount of power is output in response to the power required by the load 120.

[0120] In the second control, the difference between a first time T51 when the secondary-side voltage V2 rises from a low level to a middle level and a second time T52 when the primary-side voltage V1 rises from a low level to a high level is a first phase difference θ51. The difference between the first time T51 and a third time T53 when the secondary-side voltage V2 rises from a middle level to a high level is a second phase difference θ52.

[0121] As described above, during normal operation with no change in output, the primary-side voltage V1 and the secondary-side voltage V2 have waveforms of the same frequency that are inverted in phase every 180 degrees. Therefore, the time when the secondary-side voltage V2 falls from high to middle level is also the first time T51, the time when the primary-side voltage V1 falls from high to low level is also the second time T52, and the time when the secondary-side voltage V2 falls from middle to low level is also the third time T53.

[0122] 23 , the second in-phase boost mode is a second boost mode in which the secondary-side voltage V2 is raised from a low level to a middle level and the primary-side voltage V1 is raised from a low level to a high level simultaneously, and then the secondary-side voltage V2 is raised from a middle level to a high level. In the second in-phase boost mode, the first time T51 and the second time T52 are the same time. Therefore, the first phase difference θ51 is 0. The second in-phase boost mode is a second boost mode in which medium power is output in response to the power requested by the load 120.

[0123] 24, the second boost mode is a second boost mode in which the primary voltage V1 is raised from a low level to a high level, the secondary voltage V2 is raised from a low level to a middle level, and then the secondary voltage V2 is raised from the middle level to a high level. The second boost mode is a second boost mode in which a large amount of power is output in response to the power required by the load 120.

[0124] The second boost delay phase mode is an example of a light load mode, the second boost advance phase mode is an example of a heavy load mode, and the second boost same phase mode is a mode used when switching between the light load mode and the heavy load mode.

[0125] In the second step-up mode, the control unit 50 performs switching control by controlling the first phase difference θ51 and the second phase difference θ52. The control unit 50 derives a combination of the first phase difference θ51 and the second phase difference θ52 that allows the required power to be output. This combination is a condition for performing soft switching of the primary-side switching elements Q1 to Q4, which satisfies the same condition as condition 1 of the first control, and is also a condition for performing soft switching of the secondary-side switching elements Q5 to Q8, which satisfies condition 2 of the first control.

[0126] In the second control, when at least one of the primary-side switching elements Q1 to Q4 switches between ON and OFF, it is the time when at least one of the primary-side switching elements Q1 to Q4 switches from ON to OFF or from OFF to ON. This time is the same as second time T52, which is the time when the primary-side voltage V1 rises from low to high or the time when the primary-side voltage V1 falls from high to low. Also, in the second control, when at least one of the secondary-side switching elements Q5 to Q8 switches between ON and OFF, it is the time when at least one of the secondary-side switching elements Q5 to Q8 switches from ON to OFF or from OFF to ON. This time is the same as first time T51 or third time T53. The first time T51 is the time when the secondary-side voltage V2 rises from a low level to a middle level or falls from a high level to a middle level. The third time T53 is the time when the secondary-side voltage V2 rises from a middle level to a high level or falls from a middle level to a low level.

[0127] 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 so as to realize the derived first phase difference θ51 and second phase difference θ52.

[0128] [Operation of the Second Embodiment] Figure 25 shows the relationship between the equivalent voltage ratio and the output current. Line L22 is the boundary line where switching occurs between the heavy load mode and the light load mode when the second control is performed. Region A12 indicates the region where soft switching is impossible when the second control is performed. As can be seen from Figure 25, region A12 includes an equivalent voltage ratio of 1. Thus, with the second control, soft switching cannot be performed even in the heavy load mode in region A12 where the equivalent voltage ratio includes 1.

[0129] In the second step-up mode, the secondary-side full bridge circuit 40 is controlled so that a three-level secondary-side voltage V2 is applied to the series-connected body 25. The voltage applied to the reactor L2 is a value obtained by subtracting the voltage applied to the secondary-side winding 23 from the secondary-side voltage V2. When the equivalent voltage ratio is close to 1, the voltage applied to the reactor L2 is low. When the voltage applied to the reactor L2 is low, even if the secondary-side full bridge circuit 40 is controlled so that a three-level secondary-side voltage V2 is applied to the series-connected body 25, soft switching may not be possible because the amount of change in current is small.

[0130] When the voltage applied to the reactor L2 is low, the amount of change in current can be increased by controlling the primary full-bridge circuit 30 so that a three-level primary voltage V1 is applied to the primary winding 22. This enables soft switching of the primary switching elements Q1 to Q4 and the secondary switching elements Q5 to Q8.

[0131] Line L21 in Figure 25 is the boundary line where switching occurs between heavy load mode and light load mode when the first control is performed. Area A11 indicates the area where soft switching is impossible when the first control is performed. In this way, the area where soft switching is impossible shifts from area A12 under the second control to area A11 under the first control. Area A11 where soft switching is impossible under the first control and area A12 where soft switching is impossible under the second control do not overlap. Therefore, even in area A11 where soft switching is impossible under the first control, soft switching can be performed by performing the second control.

[0132] As described above, when the second control is executed, the control unit 50 executes the second control when the equivalent voltage ratio is greater than a predetermined threshold value greater than 1. This threshold value may be, for example, a value greater than the upper limit value of the equivalent voltage ratio represented by the region A12 and less than the lower limit value of the equivalent voltage ratio represented by the region A11. That is, the threshold value may be set so that the second control is executed in the region A11 and the first control is executed in the region A12.

[0133] [Effects of the Second Embodiment] (2-1) When the voltage applied to the reactor L2 is low, even if the secondary-side full-bridge circuit 40 is controlled so that a three-level secondary-side voltage V2 is applied to the series-connected body 25, the amount of change in current is small, and soft switching may not be possible (area A12). In contrast, when the voltage applied to the reactor L2 is low, the amount of change in current can be increased by controlling the primary-side full-bridge circuit 30 so that a three-level primary-side voltage V1 is applied to the primary-side winding 22. As a result, soft switching of the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8 can be performed even when the equivalent voltage ratio is close to 1.

[0134] (2-2) The control unit 50 executes the second control when the equivalent voltage ratio is greater than a threshold value, and executes the first control when the equivalent voltage ratio is equal to or less than the threshold value. When the equivalent voltage ratio is greater than the threshold value, soft switching may not be possible with the first control (area A11). By executing the second control in this case, soft switching can be performed even when soft switching is not possible with the first control. When the equivalent voltage ratio is equal to or less than the threshold value, soft switching may not be possible with the second control (area A12). By executing the first control in this case, soft switching can be performed even when soft switching is not possible with the second control.

[0135] L1, L2...reactor, Q1 to Q4...primary side switching elements, Q5 to Q8...secondary side switching elements, 10...power conversion device, 20...transformer, 22...primary side winding, 23...secondary side winding, 24, 25...series-connected body, 30...primary side full bridge circuit, 40...secondary side full bridge circuit, 50...control unit.

Claims

1. A transformer having a primary winding and a secondary winding, a reactor connected to the primary winding, the primary winding and the reactor forming a series connection, a circuit connected to the reactor, the primary side full bridge circuit having a plurality of primary side switching elements, a secondary side full bridge circuit connected to the secondary winding, the secondary side full bridge circuit having a plurality of secondary side switching elements, and a control unit configured to control the plurality of primary side switching elements and the plurality of secondary side switching elements, wherein a condition for performing soft switching of the plurality of primary side switching elements is condition 1 and a condition for performing soft switching of the plurality of secondary side switching elements is condition 2, the condition 1 is a current flowing through the primary winding when at least one of the primary switching elements switches between ON and OFF, the value of a current flowing in a forward direction through a diode connected in parallel to the primary switching element of the multiple primary switching elements that changes from OFF to ON being equal to or greater than an absolute value of a primary winding current threshold; the condition 2 is a current flowing through the secondary winding when at least one of the secondary switching elements switches between ON and OFF, the value of a current flowing in a forward direction through a diode connected in parallel to the secondary switching element of the multiple secondary switching elements that changes from OFF to ON being equal to or greater than an absolute value of a secondary winding current threshold; and the control unit is configured to execute a first control that controls the primary full bridge circuit so that two levels of primary voltage are applied to the series connection, and controls the secondary full bridge circuit so that three levels of secondary voltage are applied to the secondary winding, the primary side voltage and the secondary side voltage have waveforms of the same frequency that are inverted in phase every 180 degrees, 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 first time and a third time when the secondary side voltage rises from a middle level to a high level is a second phase difference,the control unit controls the primary side full bridge circuit and the secondary side full bridge circuit in the first control so as to realize a combination of the first phase difference and the second phase difference that can output the required power and that satisfies the condition 1 and the condition 2, thereby outputting the required power.

2. The power conversion device according to claim 1, wherein the number of turns of the primary winding of the transformer is N1, the number of turns of the secondary winding is N2, 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 turns ratio of the transformer is converted to 1:1 is (Vout x N1) / (Vin x N2), and the control unit is configured to: when the equivalent voltage ratio is less than a predetermined threshold which is less than 1, control the primary full bridge circuit so that three levels of primary voltages are applied to the series connection, and to execute a second control to control the secondary full bridge circuit so that two levels of secondary voltages are applied to the secondary winding, and to execute the first control when the equivalent voltage ratio is equal to or greater than the predetermined threshold.

3. A transformer having a primary winding and a secondary winding, a reactor connected to the secondary winding, the secondary winding and the reactor forming a series connection, a circuit connected to the primary winding, the primary side full bridge circuit having a plurality of primary side switching elements, a secondary side full bridge circuit connected to the reactor, the circuit having a plurality of secondary side switching elements, and a control unit configured to control the plurality of primary side switching elements and the plurality of secondary side switching elements, wherein a condition for performing soft switching of the plurality of primary side switching elements is condition 1 and a condition for performing soft switching of the plurality of secondary side switching elements is condition 2, the condition 1 is a current flowing through the primary winding when at least one of the primary switching elements switches between ON and OFF, the value of a current flowing in a forward direction through a diode connected in parallel to the primary switching element of the multiple primary switching elements that changes from OFF to ON being equal to or greater than an absolute value of a primary winding current threshold; the condition 2 is a current flowing through the secondary winding when at least one of the secondary switching elements switches between ON and OFF, the value of a current flowing in a forward direction through a diode connected in parallel to the secondary switching element of the multiple secondary switching elements that changes from OFF to ON being equal to or greater than an absolute value of a secondary winding current threshold; and the control unit is configured to execute a first control that controls the primary full bridge circuit so that three levels of primary voltages are applied to the primary winding, and controls the secondary full bridge circuit so that two levels of secondary voltages are applied to the series connection; the primary side voltage and the secondary side voltage have waveforms of the same frequency that are inverted in phase every 180 degrees, 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 first time and a third time when the primary side voltage rises from the middle level to the high level is a second phase difference,the control unit controls the primary side full bridge circuit and the secondary side full bridge circuit in the first control so as to realize a combination of the first phase difference and the second phase difference that can output the required power and that satisfies the condition 1 and the condition 2, thereby outputting the required power.

4. The power conversion device according to claim 3, wherein the number of turns of the primary winding of the transformer is N1, the number of turns of the secondary winding is N2, 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 turns ratio of the transformer is converted to 1:1 is (Vout x N1) / (Vin x N2), and the control unit is configured to: when the equivalent voltage ratio is greater than a predetermined threshold value which is greater than 1, control the primary full bridge circuit so that two levels of primary voltages are applied to the primary winding, and to control the secondary full bridge circuit so that three levels of secondary voltages are applied to the series-connected body, execute a second control; and when the equivalent voltage ratio is equal to or less than the predetermined threshold value, execute the first control.

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