Power converter, control method, control program, and control device

Abstract

To provide a power conversion device, a control method, a control program, and a control apparatus, capable of preventing an inrush current from increasing.SOLUTION: A power conversion device comprises: a transformer having first and second coils; a first bridge circuit; a second bridge circuit; and a control circuit that controls the first and second bridge circuits to perform power conversion. The control circuit comprises: a first control part that controls to increase a period when a voltage is applied to the first and second coils to a first period, at activation of the power conversion device; a second control part that holds the period when the voltage is applied to the second coil to the first period after the termination of the control of the first control part, and performs a control so that the period when the voltage is applied to the first coils is changed to the second period shorter than the first period and a voltage waveform applied to the first coil is changed until it becomes a first waveform; and a third control part that performs a control so that a phase difference of the voltage waveforms applied to the first and second coils is held at a predetermined value after the termination of the control of the second control part.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This disclosure relates to a power conversion device, a control method, a control program, and a control device.

Background Art

[0002] As a power conversion device, what is called the so-called DAB (Dual Active Bridge) method is known. The power conversion device of the DAB method can perform power conversion bidirectionally. In the power conversion device of the DAB method, furthermore, so-called soft switching is possible, and if the voltage conversion ratio is low, power conversion can be performed efficiently.

[0003] However, in the power conversion device of the DAB method, it is known that when the voltage conversion ratio becomes large, hard switching occurs and the switching loss becomes large.

[0004] A proposal as a countermeasure is disclosed in Patent Document 1. Patent Document 1 discloses a DAB isolation converter capable of outputting a five-level voltage. The five levels refer to voltages of 0, ±V / 2, and ±V. Since there are three absolute values of these voltages, it is also called a three-level isolation converter. In this specification, such an isolation converter is called a three-level isolation converter. On the other hand, a conventional DAB isolation converter can output a voltage of ±V. In this case, since there are two levels of voltage, in this specification, such an isolation converter is called a two-level isolation converter.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The technology disclosed in Patent Document 1 has the effect of suppressing switching losses over a wide range of transformation ratios. However, it has been found that it is difficult to prevent the generation of inrush current when the power is turned on in the case of a 3-level isolation converter. In the case of a 2-level isolation converter, the problem of inrush current when the power is turned on has already been solved. However, as will be described later, it is difficult to directly apply the solution for a 2-level isolation converter to a 3-level isolation converter.

[0007] This disclosure aims to provide a power conversion device, a control method, a control program, and a control device that can prevent the inrush current from becoming excessive. [Means for solving the problem]

[0008] A power converter according to the first aspect of this disclosure includes a transformer having a first coil and a second coil, a first bridge circuit connected to the first coil, a second bridge circuit connected to the second coil, and a control circuit that, in steady state, controls the first bridge circuit and the second bridge circuit to periodically apply to the first coil and the second coil a voltage controlled to have a first waveform with 3 levels and a second waveform with 2 levels, respectively, at a predetermined period, thereby performing power conversion between the first coil and the second coil via the transformer, wherein, when the power converter is started, the control circuit maintains the voltage waveform applied to the first coil and the second coil at the 2 levels, and increases the period during which voltage is applied to the first coil and the second coil until the first period is reached, by controlling the first bridge circuit and the second The system includes: a first control unit for controlling a two-bridge circuit; a second control unit for controlling the first and second bridge circuits after the first control unit has finished controlling the system, such that the period during which voltage is applied to the second coil by the second bridge circuit is maintained as the first period, and that the period during which voltage is applied to the first coil changes to a second period shorter than the first period, and that the voltage waveform applied to the first coil changes to the first waveform; and a third control unit for controlling the first and second bridge circuits after the second control unit has finished controlling the system, such that the period during which voltage is applied to the second coil is shifted until the phase difference between the voltage waveforms applied to the first and second coils reaches a predetermined value, and that the phase difference between the voltage waveform applied to the first coil and the voltage waveform applied to the second coil is maintained at the predetermined value on a steady basis.

[0009] This disclosure can be implemented not only as a power converter and control device equipped with such characteristic processing, but also as a power conversion method and control method in which such characteristic processing is performed in steps, or as a program for causing a computer to execute such steps. Furthermore, this disclosure can be implemented as a semiconductor integrated circuit that implements part or all of the power converter and control device, or as a power conversion system including the power converter or control device.

[0010] The above and other purposes, features, aspects and advantages of this disclosure will become apparent from the following detailed description of this disclosure, which will be understood in conjunction with the attached drawings. [Effects of the Invention]

[0011] As described above, this disclosure provides a power conversion device, a control method, a control program, and a control device that can prevent the inrush current from becoming large. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a circuit block diagram of an isolation converter, which is a power conversion circuit according to the first embodiment of this disclosure. [Figure 2] Figure 2 is a waveform diagram showing the primary and secondary voltages that drive the transformer of the isolation converter shown in Figure 1 in a steady state. [Figure 3] Figure 3 schematically illustrates how the soft-start technology used in conventional two-level isolation converters can be directly applied to a three-level isolation converter. [Figure 4] Figure 4 is a waveform diagram illustrating the power conversion when the technology shown in Figure 3 is applied to a three-level isolation converter. [Figure 5] Figure 5 is a schematic waveform diagram showing the first step of the soft-start technology according to the first embodiment. [Figure 6] Figure 6 is a schematic waveform diagram showing the final state of the first step of the soft-start technology according to the first embodiment. [Figure 7] FIG. 7 is a schematic waveform diagram showing the second step of the soft start technique according to the first embodiment. [Figure 8] FIG. 8 is a schematic waveform diagram showing the third step of the soft start technique according to the first embodiment. [Figure 9] FIG. 9 is a schematic waveform diagram showing the details of the second step shown in FIG. 7. [Figure 10] FIG. 10 is a functional block diagram of a control circuit for controlling each switching element of the isolation converter according to the first embodiment. [Figure 11] FIG. 11 is a block diagram showing the hardware configuration of an MCU (Micro Controller Unit) that realizes the control circuit. [Figure 12] [ FIG. 12 is a flowchart showing the control structure of the main routine of a program that the control circuit executes to realize the soft start of the isolation converter. [Figure 13] FIG. 13 is a flowchart showing the control structure of a program that realizes the soft start mode 0 shown in FIG. 12. [Figure 14] FIG. 14 is a flowchart showing the control structure of a program that realizes the soft start mode 1 shown in FIG. 12. [Figure 15] FIG. 15 is a flowchart showing the control structure of a program that realizes the soft start mode 2 shown in FIG. 12. [Figure 16] FIG. 16 is a diagram showing the simulation settings of the isolation converter according to the first embodiment in a tabular form. [Figure 17] FIG. 17 is a graph showing the simulation result of power conversion when starting a three-level isolation converter without soft start. [Figure 18] FIG. 18 is a graph showing the simulation result when the soft start technique in a two-level isolation converter is directly applied to a three-level isolation converter. [Figure 19]FIG. 19 is a graph at the time of starting a three-level insulated converter to which the soft start technology according to the first embodiment is applied. [Figure 20] FIG. 20 is a schematic waveform diagram showing a waveform deformation of the second step in the first modification of the first embodiment. [Figure 21] FIG. 21 is a schematic waveform diagram showing a waveform deformation method of the third step in the second modification of the first embodiment. [Figure 22] FIG. 22 is a circuit block diagram of an insulated converter which is a power conversion circuit according to the second embodiment. [Figure 23] FIG. 23 is a detailed view of the midpoint switch shown in FIG. 22.

Mode for Carrying Out the Invention

[0013] [Description of Embodiments of the Present Disclosure] In the following description and drawings, the same components are denoted by the same reference numerals. Therefore, detailed descriptions thereof will not be repeated. Note that at least a part of the embodiments described below may be arbitrarily combined.

[0014] (1) A power converter according to the first aspect of this disclosure includes a transformer having a first coil and a second coil, a first bridge circuit connected to the first coil, a second bridge circuit connected to the second coil, and a control circuit that, in steady state, controls the first bridge circuit and the second bridge circuit to periodically apply to the first coil and the second coil a voltage controlled to have a first waveform with 3 levels and a second waveform with 2 levels, respectively, at a predetermined period, thereby performing power conversion between the first coil and the second coil via the transformer, wherein the control circuit maintains the voltage waveform applied to the first coil and the second coil at the 2 levels when the power converter is started, and increases the period during which voltage is applied to the first coil and the second coil until the first period is reached, by controlling the first bridge circuit and the The power converter includes a first control unit that controls a second bridge circuit; a second control unit that, after the control by the first control unit has finished, maintains the period during which voltage is applied to the second coil by the second bridge circuit as the first period, and changes the period during which voltage is applied to the first coil to a second period shorter than the first period, and changes the voltage waveform applied to the first coil to the first waveform; and a third control unit that, after the control by the second control unit has finished, moves the period during which voltage is applied to the second coil until the phase difference between the voltage waveforms applied to the first and second coils reaches a predetermined value, and then controls the first and second bridge circuits so that the phase difference between the voltage waveform applied to the first coil and the voltage waveform applied to the second coil is steadily maintained at the predetermined value. This configuration prevents inrush current from occurring when a power converter that applies a first waveform with three levels to the first coil is powered on.

[0015] (2) In (1) above, the voltage waveform applied to the first coil has a first level period in which the voltage is substantially zero, a second level period in which the absolute value of the voltage is substantially the first voltage, and a third level period in which the absolute value of the voltage is substantially higher than the first voltage. This configuration prevents the occurrence of inrush current when the power is turned on of the power converter that applies a first waveform having a first level period, a second level period, and a third level period to the first coil.

[0016] (3) In (2) above, the period before and after the third level period is the second level period. This configuration prevents inrush current from occurring when the power is turned on to the power converter that applies the first waveform having three levels to the first coil.

[0017] (4) In (3) above, the first period is half the period of the predetermined period, and the centers of the first period, the second level period and the third level period may coincide with each other. This configuration prevents the occurrence of inrush current when the power is turned on of a power converter that applies a time-symmetrical first waveform with three levels to the first coil.

[0018] (5) In any one of (2) to (4) above, the second control unit extends the first level period of the voltage waveform applied to the first coil so that the third level period becomes the second period, and then extends the second level period of the voltage waveform applied to the first coil until the third level period becomes a third period shorter than the second period. With this configuration, when the power is turned on to the power converter that applies a first waveform with three levels to the first coil, the change in current flowing through the power converter becomes smoother, and inrush current can be prevented from occurring.

[0019] (6) In any one of (2) to (4) above, the second control unit changes the voltage waveform applied to the first coil while keeping the ratio of the first level period to the sum of the first level period and the second level period constant, until the sum of the second level period and the third level period equals the second period. With this configuration, when the power is turned on to the power converter that applies a first waveform with three levels to the first coil, the change in current flowing through the power converter becomes smoother, and inrush current can be prevented from occurring.

[0020] (7) In any one of (2) to (4) above, the second control unit expands the second level period of the voltage waveform applied to the first coil so that the third level period becomes a third period shorter than the second period, and then changes the voltage waveform applied to the first coil so that the second level period becomes the second period while maintaining the third level period. With this configuration, when the power is turned on to the power converter that applies a first waveform with three levels to the first coil, the change in current flowing through the power converter becomes smoother and inrush current can be prevented.

[0021] (8) A control method for a power converter according to a second aspect of this disclosure, comprising a transformer having a first coil and a second coil, a first bridge circuit connected to the first coil, and a second bridge circuit connected to the second coil, wherein the power converter performs power conversion between the first coil and the second coil via the transformer by controlling the first bridge circuit and the second bridge circuit during steady-state operation to periodically apply voltages to the first coil and the second coil at a predetermined period, which are controlled to be a first waveform having 3 levels and a second waveform having 2 levels, respectively, to the first bridge circuit, wherein the method includes a computer that, in response to the power converter being started, maintains the voltage waveform applied to the first coil and the second coil at the 2 levels and increases the period during which voltage is applied to the first coil and the second coil until it becomes a first period. The configuration includes: a first step of controlling the bridge circuit and the second bridge circuit; a second step of controlling the first bridge circuit and the second bridge circuit after the completion of the first step, such that the period during which voltage is applied to the second coil by the second bridge circuit is maintained as the first period, and then changes until the period during which voltage is applied to the first coil becomes a second period shorter than the first period, and the voltage waveform applied to the first coil changes until it becomes the first waveform; and a third step of controlling the first bridge circuit and the second bridge circuit after the completion of the second step, such that the period during which voltage is applied to the second coil is shifted until the phase difference between the voltage waveforms applied to the first coil and the voltage waveform applied to the second coil becomes a predetermined value, and then the phase difference between the voltage waveform applied to the first coil and the voltage waveform applied to the second coil is maintained at the predetermined value on a steady basis. This configuration prevents the occurrence of inrush current when a power converter that applies a first waveform with three levels to the first coil is powered on.

[0022] (9) A control program for a power converter relating to a third aspect of this disclosure is a control program for a power converter including a transformer having a first coil and a second coil, a first bridge circuit connected to the first coil, and a second bridge circuit connected to the second coil, wherein the power converter performs power conversion between the first coil and the second coil via the transformer by controlling the first bridge circuit and the second bridge circuit in a steady state to periodically apply voltages to the first coil and the second coil at a predetermined period, which are controlled to be a first waveform having 3 levels and a second waveform having 2 levels, respectively, and the program, in response to the power converter being started, maintains the voltage waveform applied to the first coil and the second coil at the 2 levels and increases the period during which voltage is applied to the first coil and the second coil until it becomes a first period. The computer is configured to perform the following steps: a first step of controlling the bridge circuit and the second bridge circuit; a second step of controlling the first bridge circuit and the second bridge circuit so that, after the completion of the first step, the period during which voltage is applied to the second coil by the second bridge circuit is maintained as the first period, and then changing until the period during which voltage is applied to the first coil becomes a second period shorter than the first period, and so that the voltage waveform applied to the first coil changes until it becomes the first waveform; and a third step of controlling the first bridge circuit and the second bridge circuit so that, after the completion of the second step, the period during which voltage is applied to the second coil is shifted until the phase difference between the voltage waveforms applied to the first coil and the voltage waveform applied to the second coil becomes a predetermined value, and then maintaining the phase difference between the voltage waveform applied to the first coil and the voltage waveform applied to the second coil at a constant predetermined value. This configuration prevents inrush current from occurring when a power converter that applies a first waveform with three levels to the first coil is powered on.

[0023] (10) A control device relating to a fourth aspect of this disclosure is a power converter including a transformer having a first coil and a second coil, a first bridge circuit connected to the first coil, and a second bridge circuit connected to the second coil, wherein in a steady state, the control device controls the first bridge circuit and the second bridge circuit to periodically apply to the first coil and the second coil, at a predetermined period, voltages controlled to have a first waveform with 3 levels and a second waveform with 2 levels, respectively, to the first bridge circuit and the second bridge circuit, wherein when the power converter is started, the voltage waveforms applied to the first coil and the second coil are kept at the 2 levels, and the period during which voltage is applied to the first coil and the second coil increases until the first period is reached, by controlling the first bridge circuit and the second bridge circuit. The power converter includes a first control unit for controlling a ridge circuit, a second control unit for controlling the first and second bridge circuits so that, after the control by the first control unit ends, the period during which voltage is applied to the second coil by the second bridge circuit is maintained as the first period, and so that the period during which voltage is applied to the first coil changes to a second period shorter than the first period, and so that the voltage waveform applied to the first coil changes until it becomes the first waveform, and so that after the control by the second control unit ends, the period during which voltage is applied to the second coil is shifted until the phase difference between the voltage waveforms applied to the first and second coils becomes a predetermined value, and so that the phase difference between the voltage waveform applied to the first coil and the voltage waveform applied to the second coil is maintained at the predetermined value on a steady basis. This configuration prevents the occurrence of inrush current when a power converter that applies a first waveform with three levels to the first coil is powered on.

[0024] [Details of the embodiments of this disclosure] Specific examples of power converters, control methods, control programs, and control devices according to embodiments of this disclosure will be described below with reference to the drawings. However, this disclosure is not limited to these examples and is intended to include all modifications within the meaning and scope of the claims as indicated by the claims.

[0025] 1. First Embodiment A.Configuration Figure 1 shows a circuit diagram of an isolation converter 50, which is a power conversion circuit according to the first embodiment of this disclosure. Referring to Figure 1, the isolation converter 50 includes a transformer 64 having a leakage inductance 62, a three-level primary full-bridge circuit 60, or first bridge circuit, connected to the primary input terminals 68 and 70, and the primary coil, which is the first coil of the transformer 64, and which applies a voltage to the primary coil that changes with a predetermined period, and a two-level secondary full-bridge circuit 66, or second bridge circuit, connected to the output terminals 72 and 74, and the secondary terminal, which is the second coil of the transformer 64.

[0026] The primary-side full-bridge circuit 60 includes capacitors 80 and 82 connected in series between input terminals 68 and 70 via node 84, switching elements S1 and S4 connected in series between input terminals 68 and 70 via node 88, and similarly, switching elements S5 and S8 connected in series between input terminals 68 and 70 via node 90. Node 88 is connected to the first terminal of the primary-side terminals of transformer 64. Node 90 is connected to the second terminal of the primary-side terminals of transformer 64. Switching elements S1 and S4 form the first leg of the primary-side full-bridge circuit 60. Switching elements S5 and S8 constitute the second leg of the primary-side full-bridge circuit 60.

[0027] The primary full-bridge circuit 60 further includes a midpoint switch 86 connected to nodes 84, 88, and 90, which is necessary for the three-level control in the primary full-bridge circuit 60. The midpoint switch 86 includes switching elements S2 and S3 connected in series on the wiring connecting nodes 88 and 84, and switching elements S6 and S7 connected in series on the wiring connecting nodes 90 and 84.

[0028] The secondary full-bridge circuit 66 includes switching elements S9 and S10 connected in series between output terminals 72 and 74 via node 102, and switching elements S11 and S12 connected in series between output terminals 72 and 74 via node 104. Switching elements S9 and S10 form a first leg. Switching elements S11 and S12 form a second leg.

[0029] Figure 2 shows the primary voltage waveform 150 of the transformer 64 generated by the primary full-bridge circuit 60 shown in Figure 1, and the secondary voltage waveform 152 of the transformer 64 generated by the secondary full-bridge circuit 66. The primary voltage waveform 150 and the secondary voltage waveform 152 are periodic waveforms whose phase changes from 0 to 2π with a predetermined period. The waveform 160 in the first half of one period and the waveform 162 in the second half are symmetrical above and below the line where the voltage is 0. The secondary voltage waveform 152 is phase lag behind waveform 160 by φ. The waveform 164 in the first half of the secondary voltage waveform 152 and the waveform 166 in the second half are symmetrical above and below the line where the voltage is 0.

[0030] If the phase at the beginning of one period of the primary voltage waveform 150 is 0, then the voltage of waveform 160 is 0 volts (V) during the first level period 170 from phase 0 to phase α, V1 / 2 (V) during the second level period 172 from phase α to phase β, V1 (V) during the third level period 174 from phase β to phase π-β, V1 / 2 (V) during the second level period 176 from phase π-β to phase π-α, and 0 (V) during the first level period 178 from phase π-α to phase π. The latter half of waveform 162 is the same shape as waveform 160 except that π is added to the phase of waveform 160 and the sign of the voltage is reversed. The primary voltage waveform 150 then repeats this waveform 160 and waveform 162 with the same period.

[0031] If the phase at the beginning of one period of the primary voltage waveform 150 is set to 0, then the voltage of the first half of waveform 164 of the secondary voltage waveform 152 is V2(V) from phase φ to phase π+φ, and -V2(V) from phase π+φ to phase 2π+φ. The secondary voltage waveform 152 repeats waveforms 164 and 166 with a phase delay of φ compared to the primary voltage waveform 150, and with the same period as the primary voltage waveform 150. It is already known that by driving each switching element of the isolation converter 50 basically according to this waveform, it is possible to make the isolation converter 50 perform the desired voltage conversion operation, and how to drive each switching element to obtain such a waveform is already known, so the details will not be repeated here.

[0032] Here, we will explain a method for achieving soft start in a conventional two-level isolation converter. Conventionally, pulses with a narrowed waveform 164, as shown in Figure 2, are generated on both the primary and secondary sides, and the transformer 64 is driven while gradually widening the width of these pulses. After the pulse widths on both sides reach half a cycle, the transformer 64 is driven while gradually delaying the phase of the secondary waveform relative to the primary waveform. After the phase shift reaches a desired value (for example, φ), steady-state operation begins.

[0033] A simple application of these techniques to the isolation converter 50 having the configuration according to this first embodiment will be explained with reference to Figure 3. Note that the conventional method can be applied directly to the lower part of Figure 2. Furthermore, the same approach can be applied to the first and second halves of each period of the primary voltage waveform 150 shown in Figure 2. Therefore, in the following explanation, only the first half-period of the primary voltage waveform 150 will be described.

[0034] Referring to Figure 3, when the power is turned on, a waveform 200 is first generated by reducing the width of waveform 160 shown in Figure 2. Next, the transformer 64 is driven while gradually widening the width of this waveform 200, and this process is continued until the same waveform as waveform 160 shown in Figure 2 is obtained. Waveform 200 can be thought of as consisting of a bottom 210 of voltage V1 / 2 and an upper part 212 of voltage V1 / 2 stacked in the center. If the conventional method is applied as is, the width of the bottom 210 and the width of the upper part 212 will be kept the same as the ratio of the bottom to the top in waveform 160 shown in Figure 2, while widening the left and right sides. After obtaining the same waveform as waveform 160 in this way, the steady-state power conversion process is started.

[0035] However, this method has the following shortcomings: it assumes that no power is transmitted during soft start. However, if the conventional method described above is used as is, power will be transmitted in the isolation converter 50 even during soft start, as explained below. The reason for this is as follows. For the sake of simplicity, in the following explanation, the primary voltage V1 and the secondary voltage V2 will be assumed to be equal.

[0036] The power transfer equation for a conventional 2-level-2-level power converter is as follows. In the following equation, P is power [W], V1 is the primary voltage [V], V2 is the secondary voltage [V], n is the transformer turns ratio, ω is the switching angular velocity of the switching element [rad], Llk is the leakage inductance [H], and φ is the phase difference between the primary and secondary sides [rad].

number

[0037] From this equation, in a 2-level to 2-level power converter, power transfer is 0W when the phase difference φ=0, and no current is transferred.

[0038] On the other hand, power transfer in a 3-level to 2-level power converter is shown by the following equation. In this equation, α and β represent the phase [rad] shown in Figure 2.

number

[0039] As can be seen from this equation, in the case of a 3-level to 2-level power converter, even if φ is set to 0, the terms α and β remain. Therefore, the power P transmitted from the primary side to the secondary side is not 0, and power is transmitted.

[0040] Let's explain the above circumstances in detail. Referring to Figure 4, we compare the primary waveform 252 and the secondary waveform 250 during soft start. In the first half of one cycle, we focus on the period 260 from phase 0 to α, the period 262 from phase α to β, the period 264 from phase β to phase π-β, the period 266 from phase π-β to phase π-α, and the period 268 from phase π-α to phase π. In period 264, the voltages of the primary waveform 252 and the secondary waveform 250 are the same. However, in periods 260 and 268, the absolute value of the difference is V2, and in periods 262 and 266, the absolute value of the difference is V2-V1 / 2. In other words, there are periods in which a voltage difference occurs between the primary coil and the secondary coil of the transformer 64.

[0041] This is also true for the latter half of the period. Specifically, we focus on period 270 from phase π to π+α, period 272 from phase π+α to π+β, period 274 from phase π+β to phase 2π-β, period 276 from phase 2π-β to phase 2π-α, and period 278 from phase 2π-α to phase 2π. In period 274, the voltages of the primary waveform 256 and the secondary waveform 254 are the same. However, in periods 270 and 278, the absolute value of the difference is V2, and in periods 272 and 276, the absolute value of the difference is V2-V1 / 2. In other words, there are periods in which a voltage difference occurs between the primary and secondary coils of the transformer 64.

[0042] This voltage difference can cause power transmission during the soft start process.

[0043] In the isolation converter 50 according to the first embodiment of this disclosure, this condition is avoided by the following configuration. That is, referring to Figure 5, unlike in the case of Figure 3, when the isolation converter 50 is started up, in the first half of one cycle, simple rectangular waveforms 300 and 302 with the same width for voltage V1 and voltage V2, respectively, are generated at the center of the half cycle, and in the second half, simple rectangular waveforms 304 and 306 with the same width for voltage -V1 and voltage -V2, respectively, are generated at the center of the half cycle. Then, the width of these waveforms is sequentially widened as a function of time until the width of each waveform reaches half a cycle. In this specification, "startup" includes not only when operation starts when the power of the device is turned on, but also when operation is resumed after being temporarily stopped.

[0044] Figure 6 shows the waveforms 350 for the first half of the primary side and 354 for the second half of the primary side, and the waveforms 352 for the first half of the secondary side and 356 for the second half of the secondary side, respectively, when the width reaches half a rotation. The operation of the isolation converter 50 up to this point is the same as the soft start of a conventional two-level isolation converter.

[0045] Next, referring to Figure 7, while keeping the secondary waveforms 352 and 354 the same as those shown in Figure 6, the primary voltage is gradually changed as a function of time so that the waveform of the first half of the primary period becomes the same waveform 400 as the waveform 160 shown in Figure 2, and the waveform of the second half of the primary period becomes the same waveform 402 as the waveform 162 shown in Figure 2, that is, so that the time of the portion in which these waveforms change within one period becomes a predetermined value. Such waveform changes can be achieved by various methods. The method in this embodiment is as follows. The following explanation concerns the first half waveform 400.

[0046] Referring to Figures 6 and 7, in this embodiment, the leading portion of the waveform 350 shown in Figure 6 is gradually shifted as a function of time from phase 0 to phase α, and the trailing portion is shifted from phase π to phase π-α. That is, by expanding the region where the voltage is 0 from both ends of the half-period towards the center, the width of the portion of the waveform 350 where the voltage is V1 is reduced. As a result, periods in which the primary voltage is 0 occur, as shown in periods 410 and 418 in Figure 7. This is shown in the upper part of Figure 9. Figure 9 shows the waveform 450 during the process of width reduction.

[0047] Next, by adjusting the switching timing of the switching elements of the primary full-bridge circuit 60, the portion where the voltage is V1 is shifted as a function of time from phase α to phase β, and from phase π-α to phase π-β at the trailing end, while maintaining the voltage at V1 / 2. That is, the width of the upper half of the waveform 400 is gradually reduced as a function of time so that the beginning of the period 414 where the voltage is V1 shifts from phase α to phase β, and from phase π-α to phase π-β. This is shown in the lower part of Figure 9. Waveform 452 shown in the lower part of Figure 9 shows this process in progress. As shown in Figure 9, while maintaining the positions of both ends of the waveform 450, the region where the voltage is V1 / 2 is expanded from both ends of the waveform 452 towards the center. The width of the bottom of the waveform 452 is kept constant, while the width of the upper part gradually narrows as a function of time. That is, periods 412 and 416 shown in Figure 7 widen and period 414 shortens. As a result, the final waveform 400 shown in Figure 8 is obtained.

[0048] The same processing is applied to the waveform 402 in the latter half of the cycle shown in Figure 8. Once the waveform obtained in Figure 8 is acquired, the soft start of the isolation converter 50 is completed and steady-state operation begins.

[0049] In this embodiment, the primary side of the isolation converter 50 is expected to be connected to, for example, a storage battery, and the secondary side is expected to be connected to a load. The secondary side is also connected to the grid power. The operation of the isolation converter 50 includes operation when the secondary side of the isolation converter 50 is connected to the grid (interconnected operation) and operation when the isolation converter 50 is disconnected from the grid (standalone operation).

[0050] Figure 10 shows the functional configuration of the control circuit 500 that generates control signals to drive each switching element of the isolation converter 50 shown in Figure 1. Referring to Figure 10, the control circuit 500 includes a timing control unit 510 for generating parameters α and β by feedback to determine the phase of the primary side so that the secondary voltage becomes a desired value V2 during steady operation of the isolation converter 50, and a phase difference control unit 512 for feedback control of the phase difference φ between the primary side waveform and the secondary side waveform during steady operation of the isolation converter 50. The control circuit 500 further includes an initial startup calculation unit 514 for calculating the parameters α, β and phase difference φ during the soft start described above, and a selector 516 for selecting the parameters α, β and phase difference φ output by the initial startup calculation unit 514 during initial startup, and for selecting and outputting the parameters α, β and phase difference φ output by the timing control unit 510 after the soft start is completed. The control circuit 500 further includes a PWM (Pulse Width Modulation) control unit 518 that receives the parameters α, β, and phase difference φ output from the selector 516 and outputs a PWM control signal to drive the switching element S12 from the switching element S shown in Figure 1. Given the parameters α, β, and phase difference φ, the PWM control unit 518 has the function of outputting a PWM control signal that defines the on and off timing for the primary and secondary switching elements S1 and S12, respectively, so that these values ​​are realized. This function of the PMW control unit 518 is already an established technology.

[0051] The timing control unit 510 includes a calculation unit 540 that takes the measured primary voltage VM1 and the measured secondary voltage VM2 as inputs, performs calculations on parameters α and β by feedback so that the secondary voltage becomes a desired value V2, and inputs these calculations to the selector 516.

[0052] The phase difference control unit 512 includes a grid connection control unit 560, an independent operation control unit 562, and a selector 564 that selects the grid connection control unit 560 to control the isolation converter 50 when the isolation converter 50 is connected to the grid, and selects the independent operation control unit 562 to control the isolation converter 50 when it is in independent operation.

[0053] The interconnection control unit 560 performs primary-side constant current control with respect to the phase φ. That is, the interconnection control unit 560 receives a normal current target value I* and, with respect to the output current I1, performs feedback control of the phase φ by PI control with respect to the current target value I*.

[0054] The autonomous operation control unit 562 performs a constant voltage control of the secondary voltage with respect to the phase φ. That is, the autonomous operation control unit 562 uses the secondary side measured voltage value VM2 of the isolation converter 50 to obtain a current target value I* by PI control with respect to the voltage target value V*. The autonomous operation control unit 562 further performs feedback control of the phase φ by a minor loop of current feedback control using PI control with respect to this current target value I*.

[0055] Figure 11 shows the hardware configuration of the MCU 580 that implements the control circuit 500. Referring to Figure 11, the MCU 580 includes a processor, an MPU (Micro Processing Unit) 602, a high-speed bus 600 to which the MPU 602 is connected, an SRAM (Static Random Access Memory) 604 connected to the high-speed bus 600, a flash memory 606 connected to the high-speed bus 600, and a ROM (Read-Only Memory) 608 connected to the high-speed bus 600. The SRAM 604 holds data necessary for program execution. The flash memory 606 stores a program 626 for implementing the functions realized by the control circuit 500. The ROM 608 stores the boot-up program for the MPU 602, etc.

[0056] The MCU further includes a low-speed bus 610 connected to the high-speed bus 600 via a bridge 612, and a serial interface 614, an analog-to-digital converter 616, a timer / counter 618, a clock generator 620, a power supply control unit 622, and a general-purpose interface 624, all connected to the low-speed bus 610. The clock generator 620 has the function of outputting a clock signal of a constant frequency that defines the operation of the MCU 580. The timer / counter 618 has the function of generating PMW control signals according to the timing instructed by the program, outputting signals indicating the timing of acquiring the output of various sensors, and generating various timings necessary for the MPU 602 to execute the program and control the isolation converter 50.

[0057] The outputs of voltage sensors and current sensors (not shown) located at various points in the isolation converter 50 are input to the MCU 580 via the ADC 616. Each PWM control signal is output from the MCU 580 via the general-purpose I / F 624.

[0058] Since the operation of the MCU is well known, and the relevant aspects of the embodiment are the functions of the program it executes, the operation of the MCU itself will not be described in the following explanation.

[0059] B. Program Structure Figure 12 shows the control structure of the main routine of the program that controls the operation of the isolation converter 50 during soft-start, which is executed by the control circuit 500 (MCU 580 shown in Figure 11) according to this embodiment. In this program, it is assumed that whether or not to perform a soft start can be specified in advance by a hardware switch or a variable value stored in SRAM 604. As described above, the soft start according to this embodiment includes three steps: a step of expanding the rectangular waveform on both the primary and secondary sides to half a cycle of the operation cycle; a step of shaping the rectangular waveform expanded to half a cycle on the primary side into a 3-level waveform; and a step of shifting the waveform on the secondary side relative to the waveform on the primary side until the phase difference between the 3-level waveform on the primary side and the 2-level waveform on the secondary side reaches a desired value. In the following description, a variable called "soft-start mode" (hereinafter referred to as "mode" for simplicity) is provided to indicate which of the three steps the control of the isolation converter 50 is in. In other words, step 1, which performs control as the first control unit, is represented by mode=0; step 2, which performs control as the second control unit, is represented by mode=1; and step 3, which performs control as the third control unit, is represented by mode=2. The operating mode of mode=0 is simply called mode 0. The same applies to modes 1 and 2.

[0060] The program described below is for controlling each switch on the primary side. For the secondary side, you can simply apply the same program as the primary side, except that you do not make any changes to the settings in mode 1 and maintain the last state of mode 0.

[0061] Referring to Figure 12, the program includes step 650, which starts in response to the power being turned on to the isolation converter 50 and initiates the soft-start process; step 652, which assigns 0 to the mode; and step 654, which branches the control flow depending on whether or not it is specified to perform a soft start.

[0062] The program further includes, if the determination in step 654 is negative, step 656 which calculates parameter α and parameter β values ​​according to whether it is linked operation or autonomous operation; step 658 which performs feedback control of the phase difference φ using the parameter α and parameter β values ​​obtained in step 656; and step 660 which sets the timer counter 618 to perform PWM control for switching element S1 to switching element S12 according to the parameter α, parameter β and phase difference φ obtained in the above steps and returns control to step 654.

[0063] The program further includes step 662, which determines whether the mode is 0, 1, or 2 when the determination in step 654 is negative, and branches the control flow according to the determination result; and steps 664, 666, and 668, which execute the soft start process for mode 0, mode 1, and mode 2, respectively, according to the determination result in step 662. After steps 664, 666, and 668, the control proceeds to step 660.

[0064] Referring to Figure 13, step 664 includes step 700, which sets a variable representing the phase difference φ (this variable is also represented by φ) to 0, and step 702, which branches the control flow according to whether the repeating control variable i, which defines the following repetition, is 90 or greater. In this embodiment, the value of variable i must be initialized to 0, for example, in step 650 shown in Figure 12. This initialization is similarly necessary for other variables, although it will not be described in detail here.

[0065] Step 664 further includes, if the determination in step 702 is negative, step 704 setting both parameter α and parameter β to 90-i, and step 706 adding 1 to i to terminate the execution of step 664.

[0066] Step 664 further includes, if the determination in step 702 is positive, step 708, which involves substituting 0 for parameters α and β; step 710, which involves substituting 0 for variable i; and step 712, which involves setting the mode to 1 and ending the execution of step 664.

[0067] Referring to Figure 14, the soft-start mode 1 process performed in step 666 of Figure 12 includes step 750, which assigns 0 to the variable φ, and step 752, which calculates target values ​​for parameters α and β to maintain the secondary voltage at a desired value based on the measured primary voltage VM1 and the measured secondary voltage VM2. Step 666 further includes step 754, which sets parameters α and β to the values ​​of variable j, step 756, which branches the control flow depending on whether the value of variable j is greater than or equal to the target value of parameter α, and step 758, which, if the determination in step 756 is negative, adds 1 to the value of variable j and terminates the execution of step 666. Note that the value of variable j is initialized to 0, for example, in step 650 of Figure 12.

[0068] Step 666 further includes step 760, which sets parameter α to the target value of parameter α if the determination in step 756 is positive, and step 762, which branches the control flow depending on whether the value of variable j is greater than or equal to the target value of β. If the determination in step 762 is negative, the control proceeds to step 758.

[0069] Step 666 further includes, if the determination in step 762 is positive, step 764 setting parameter β to the target value of parameter β, step 766 setting the value of variable j to 0, and step 768 setting the mode to 2 and ending the execution of step 666.

[0070] Referring to Figure 15, the program executed in step 668 of Figure 12 includes step 800, which performs calculations for parameters α and β; step 802, which calculates the phase difference φ using feedback; step 804, which performs a variation limiter process to limit the feedback calculation value φ to a predetermined variation amount and increase it; and step 804, which branches the control flow depending on whether the command value of the phase difference φ matches the feedback calculation value of the phase difference φ. If the determination in step 806 is negative, the execution of step 668 is terminated.

[0071] Step 668 further includes, if the determination in step 806 is positive, step 808 setting the mode to 0 and step 810 starting steady-state operation by performing the soft-start termination process and returning control to step 654 in Figure 12. Specifically, step 810 sets the value of the variable related to whether or not to start the soft-start to a value representing "do not start".

[0072] C.Operation The operation of the control circuit 500, i.e., the MCU 580, will be explained below, primarily with reference to Figures 12 to 15. In the following explanation, it will be assumed that the variable related to whether or not to perform a soft start has a value pre-stored indicating that a soft start will be performed. Furthermore, the following explanation primarily concerns the control of each switching element on the primary side. For the secondary side, the MCU 580 operates in the same manner as the primary side control, except that no specific settings related to operation are changed in mode 1.

[0073] Referring to Figure 12, the MCU 580 starts up in response to the power being turned on to the isolation converter 50 and starts the soft-start process in step 650. At this time, for example, the repeating control variable i and the repeating control variable j are set to 0. In step 652, the MCU 580 assigns 0 to the variable representing the mode. The MCU 580 further branches the control flow in step 654, as shown in Figure 12, depending on whether or not it is specified to perform a soft start. Since it is specified to perform a soft start according to the current conditions, the control proceeds to step 662.

[0074] In step 662, the MCU580 further determines whether the mode is 0, 1, or 2, and branches the control flow according to the determination result. The mode is set to 0. Therefore, the control proceeds to step 664.

[0075] Referring to Figures 13 to 15, the MCU580 operates as follows in step 664. Note that in the following explanation, only the operation of the MCU580 on the primary side will be described. The operation of the MCU580 on the secondary side is the same as on the primary side, except that the operation of each switching element in mode 1 is maintained in the same state as in the final state of mode 0.

[0076] In step 700, the MCU580 sets the variable φ to 0. In the following step 702, the MCU580 determines whether the value of the iterative control variable i is 90 or greater. Here, the value of variable i is 0. Therefore, the determination in step 702 is negative, and the control proceeds to step 704. In step 704, the MCU580 sets both parameter α and parameter β to 90-i. Since the value of variable i is 0, both parameter α and parameter β are set to 90. In the following step 706, the MCU580 adds 1 to variable i and terminates the execution of step 664. As a result, the value of variable i becomes 1. Therefore, both parameter α and parameter β are set to 90. The control proceeds to step 660, where parameter α and parameter β = 0 and phase difference φ = 0, and PWM control is performed in step 660, supplying a PWM control signal from switching element S1 to switching element S12 as shown in Figure 1. Note that step 660 is always executed in each iteration of the initial startup calculation unit 514. Therefore, in order to make the explanation easier to understand, the explanation regarding the execution of step 660 will not be repeated below.

[0077] When both parameter α and parameter β are 90, it means that both waveform 300 and waveform 304 shown in Figure 5 are located in the center of each half-cycle and have a width of 0. In other words, in this state, waveforms 300 and 304 in Figure 5 do not essentially exist.

[0078] The following steps 702 to 706, and step 660 shown in Figure 12, are repeatedly executed, increasing the value of variable i by 1 until the value of variable i reaches 90. As a result, as shown in Figure 5, the waveform 300 for the first half of one cycle and the waveform 304 for the second half of one cycle on the primary side extend to 180°, with the width increasing by 1° in phase from the center of each half-cycle. The judgment in step 702 becomes positive when the value of variable i reaches 90. At this time, the voltage waveform on the primary side is a waveform that covers the entire half-cycle, as shown by waveforms 350 and 352 in Figure 6. In step 708, the MCU580 assigns 0 to parameters α and β, and in step 710, it assigns 0 to variable i. That is, parameters α, β, and variable i are cleared. After this, in step 712, the MCU580 sets the mode to 1 and ends the repetition of the execution of step 664. In other words, when step 712 is completed, the operation of the MCU580 transitions from mode 0 to mode 1. Control returns to step 654 as shown in Figure 12, and thereafter the determination in step 654 is affirmative, and the determination in step 662 is mode=1, and the mode 1 processing in step 666 is repeatedly executed.

[0079] Referring to Figure 14, when processing in mode 1 begins, the MCU 580 assigns 0 to the variable φ in step 750. In step 752, the MCU 580 calculates target values ​​for parameters α and β to maintain the secondary voltage at the desired value, based on the measured primary voltage VM1 and the measured secondary voltage VM2. The MCU 580 further sets parameters α and β to the values ​​of variable j in step 754. At the first execution of step 666, the value of variable j is 0. Therefore, both parameters α and β are set to 0. The primary voltage waveform at this time is shown by waveforms 350 and 354 in Figure 6.

[0080] The MCU580 further branches the control flow in step 756 depending on whether the value of variable j is greater than or equal to the target value of parameter α. In the first execution of step 664, the value of variable j is 0, and the determination in step 756 is negative. In step 758, the MCU580 adds 1 to the value of variable j and terminates the execution of step 666. As a result, the value of variable j becomes 1. After this, the control returns from step 660 in Figure 12, through steps 654 and 662, to step 666, that is, to step 750 in Figure 14.

[0081] The MCU580 repeats this process, incrementing the value of variable j by 1 until the value of variable j exceeds the target value of parameter α. As a result, the value of parameter α increases from 0 by 1. The primary voltage waveform, as shown in the upper part of Figure 9, gradually narrows as both ends move towards the center, starting from a width spanning half a period. When the value of variable j exceeds the target value of parameter α, the judgment in step 756 becomes positive. In step 760, the MCU580 sets parameter α to its target value.

[0082] In step 762, the MCU580 further determines whether the value of variable j is greater than or equal to the target value of parameter β. Since parameter β > parameter α, the determination in step 762 is negative when step 666 is executed for the first time. Control proceeds to step 758, where the MCU580 adds 1 to the value of variable j and terminates the execution of step 666. After this, control returns to step 666 via steps 654 and 662 as shown in Figure 12, and the processing from step 750 onwards is executed with the value of variable j set to 1. Thereafter, the MCU580 repeatedly executes step 666, adding 1 to the value of variable j until the value of variable j becomes greater than or equal to the target value of parameter β. The value of parameter α is fixed at its target value. As a result, as shown in the lower part of Figure 9, the bottom of the primary voltage waveform is maintained in the shape shown in the upper part of Figure 9, while the width of the upper part gradually narrows from both ends towards the center.

[0083] Thus, when the value of variable j becomes greater than or equal to the target value of parameter β, the judgment in step 762 becomes positive. In step 764, MCU580 sets parameter β to its target value. Furthermore, in step 766, MCU580 sets the value of variable j to 0, and in step 768, sets the mode to mode 2. When mode 2 is activated, the execution of step 668 begins based on the judgments in step 654 and step 662 in Figure 12.

[0084] Referring to Figure 15, in step 800 of step 668, the MCU 580 performs calculations for parameters α and β using the primary side measured voltage value VM1 and the secondary side measured voltage value VM2, as shown by the timing control unit 510 in Figure 10. In step 802, the MCU 580 performs feedback calculations for the phase difference φ, as shown by the grid connection control unit 560 in Figure 10 when the isolation converter 50 is connected to the grid, and as shown by the standalone operation control unit 562 during standalone operation. In step 804, the MCU 580 further increases the amount of variation of the feedback calculation value φ, limiting it to a predetermined range. In step 804, the MCU 580 branches the control flow depending on whether the command value of the phase difference φ matches the feedback calculation value of the phase difference φ.

[0085] When step 666 is executed for the first time, the decision in step 806 is negative. Therefore, the MCU 580 terminates the processing of step 668. Control returns to step 654 in Figure 12.

[0086] Subsequently, the determination in step 654 in Figure 12 becomes positive, and as a result of the determination in step 662, the MCU 580 selects the process in step 668. That is, step 668 is executed repeatedly. During this time, the phase difference φ between the primary voltage waveform and the secondary voltage waveform increases. That is, the capacitor 80 between the primary waveforms 400 and 402 and the secondary waveforms 352 and 354 shown in Figure 8 gradually expands. When the determination in step 806 becomes positive, the MCU 580 sets the mode to 0 in step 808 and performs the soft start termination process in step 810, returning control to step 654 in Figure 12. Specifically in step 810, the MCU 580 sets the value of the variable related to whether or not to start the soft start to a value that represents "do not start".

[0087] After this, the control returns to step 654 in Figure 12. Since the setting is to not initiate soft start, the determination in subsequent steps 654 will always be negative, and steps 656, 658, and 660 are repeatedly executed using the specified values ​​for parameter α, parameter β, and phase difference φ, and power conversion from the primary side to the secondary side is performed.

[0088] C. Simulation Results To verify the effects of the above embodiment, the following simulation was performed. The circuit used was as shown in Figure 1. The simulation conditions are shown in Figure 16. Referring to Figure 16, the primary voltage V1 was set to 350V and the secondary voltage to 350V. The leakage inductance 62, i.e., Llk, of the transformer 64 was set to 40uH, and the turns ratio of the transformer was set to 1:1. Although not shown in Figure 1, the capacitance of the snubber circuit added to each switching element was set to 0.1uH, the snubber resistance to 200Ω, the switching frequency of each switching element to 40kHz, the dead time of the primary full-bridge circuit 60 and the secondary full-bridge circuit 66 shown in Figure 1 to 500nsec, the phase α to 5°, the phase β to 40°, and the phase difference φ between the primary and secondary sides to 43°.

[0089] In the following simulation, the duration from the start to the end of the soft start was set to 2 msec. However, in actual use, a duration of approximately 1 msec for the soft start should be sufficient.

[0090] Figure 17 shows the simulation results when soft start is disabled, that is, when the determination in step 654 of Figure 12 is negative when the isolation converter 50 is started. The graphs in Figure 17 show the transformer voltage, transformer current, and power supply current, respectively, from top to bottom. The graphs in Figure 17 are a little difficult to understand because the primary and secondary sides are superimposed, but it can be seen that both the transformer current and power supply current are large. In other words, it can be seen that an inrush current occurs when soft start is disabled.

[0091] Figure 18 shows the simulation results when a soft start in a 2-level-2-level power converter is applied to an isolated converter 50 using a primary voltage such as waveform 160 shown in Figure 2. The graphs shown in Figure 2, from top to bottom, are the transformer voltage, transformer current, and power supply current, and fourthly, the graphs of the temporal changes in phase α, phase β, phase difference φ, and the duty cycle D of the primary waveform.

[0092] In this example, no obvious inrush current occurs as shown in Figure 17. However, as shown by the 850 region of the transformer current graph and the 860 region of the power supply current graph, a phenomenon occurred where the current temporarily increased, then decreased, and then increased again before and after the phase difference φ began to increase during the soft start. Such instability in current values ​​is undesirable both during grid connection and standalone operation.

[0093] Figure 19 shows the simulation results when the primary voltage is controlled according to the above embodiment. Each graph in Figure 19 corresponds to each graph in Figure 18. As is clear from Figure 19, this simulation shows that no inrush current occurs. Furthermore, it can be seen that the isolation converter 50 can be started with a smooth increase in current, without the unstable current behavior shown in regions 850 and 860 of Figure 18.

[0094] As described above, according to this embodiment, in a 3-level to 2-level power conversion circuit such as the isolation converter 50, by devising a way to control the waveform change on the 3-level side at startup, it is possible to prevent the occurrence of inrush current and enable smooth startup.

[0095] D. First variation In the first embodiment described above, the primary voltage waveform is first expanded to cover the entire half-period while remaining rectangular, then the width of the voltage waveform is narrowed by an amount corresponding to parameter α, and then the width of the upper part of the voltage waveform is narrowed to a position corresponding to parameter beta, thereby generating the waveform shown in Figure 2, waveform 160 (and waveform 162). However, this disclosure is not limited to such embodiments.

[0096] In this first modified example, as shown in Figure 20, when the voltage waveform, which has been expanded over the entire half-period, is reduced, the primary voltage waveform 900 is deformed by simultaneously reducing the bottom and top portions corresponding to parameters α and β. As a result, in this modified example, when the waveform is reduced, a convex portion is formed from the beginning, and it gradually approaches the final waveform.

[0097] To achieve this, only a slight modification of the program shown in Figure 14 is needed. For example, in step 754, instead of setting the value of parameter β to the value of variable j, the value of variable j is multiplied by a constant value, (target value of parameter β) / (target value of parameter α). That is, the value of parameter β is set to (target value of parameter β) / (target value of parameter α). Then, if the judgment in step 756 is positive, in step 760 not only the value of parameter α but also the value of parameter β is set to the target value of parameter β, and steps 766 and 768 are executed. At this time, while the ratio of the value of parameter α to parameter β is kept constant, the first level period and the second level period are expanded, and the waveform is reduced until the values ​​of parameter α and parameter β become equal to the final target values. The width of the upper part of the final waveform where the voltage is V1 is π-2β in phase.

[0098] It is clear that the same effects as those of the first embodiment can be obtained by such modifications.

[0099] E. Second variation Unlike the first embodiment and the first modification described above, it is also possible to generate the waveform 160 (and waveform 162) shown in Figure 2 by first introducing a deformation related to phase β into the primary waveform, and then applying a deformation related to phase α to the primary waveform. This second modification is an example of such a modification.

[0100] Specifically, referring to Figure 21, for the primary waveform expanded over the entire half-period, as shown in the upper panel, the width of the upper part of the waveform is sequentially reduced by an amount corresponding to the phase difference β to obtain waveform 910. That is, the portion of waveform 900 where the voltage is V1 / 2 is gradually expanded from both sides towards the center as a function of time until the target value of parameter β is reached. Then, the lower part of the waveform, i.e., the portion where the voltage is V1 / 2, is sequentially reduced as a function of time until the parameter α also reaches the target value to obtain the final waveform 912.

[0101] To achieve this, steps 754, 756, and 758 on the left side of Figure 14 should first be performed for the phase difference β, and then the same process should be performed for the phase difference α.

[0102] It is clear that the same effects as those of the first embodiment can be obtained by such modifications.

[0103] F. Other Variations The method for modifying the waveform to obtain a three-level waveform after expanding the primary waveform over the entire half-period is flexible, as long as it allows for a smooth increase in transformer current without generating inrush current. In other words, any modification method can be used as long as it allows for the final three-level waveform to be obtained through intermediate modifications different from those shown in the first embodiment, first modification, and second modification.

[0104] 2. Second Embodiment In the first embodiment described above, a primary full-bridge circuit 60 having a center switch 86 as shown in Figure 1 is used to perform three-level control of the primary full-bridge circuit. However, this embodiment is not limited to that. As a full-bridge circuit that performs three-level control, a primary full-bridge circuit 960 including a center switch 974 including a diode may be used, as shown in Figure 22. That is, the isolation converter 950, which is a power conversion circuit according to the second embodiment, includes a leakage inductance 62, a transformer 64 and a secondary full-bridge circuit 66 similar to the first embodiment, and a primary full-bridge circuit 960 having a center switch 974.

[0105] Referring to Figures 22 and 23, in the midpoint switch 974, switching element S2 is connected between switching element S1 and node 88 via node 990. Switching element S3 is connected between node 88 and switching element S4 via node 992. Switching element S6 is connected between switching element S5 and node 90 via node 994. Switching element S7 is connected between node 90 and switching element S8 via node 996.

[0106] The midpoint switch 974 further includes diodes 1002 and 1000 connected in series in this direction via node 980 between node 992 and node 990, and diode 1006 and capacitor 1004 connected in series in this direction via node 982 between node 996 and node 994. Node 980 is connected to the connection point of capacitors 80 and 82 at node 970. Node 982 is connected to the connection point of capacitors 80 and 82 at node 972.

[0107] It is known that an isolated converter 950 with such a configuration can perform the same 3-level power conversion as the isolated converter 50 (see Figure 1) according to the first embodiment by driving each switching element in exactly the same way as the isolated converter 50. Therefore, this isolated converter 950 can also operate in the same way as the first embodiment using the same program as the first embodiment, and thus obtain the same effects.

[0108] Furthermore, as mentioned above, the DAB type power converter is capable of bidirectional power conversion. In the above embodiment, the secondary side can also be configured with three voltage waveforms. In this case, the soft start described above can be implemented in exactly the same way as in the above embodiment, except that the phase difference φ is reversed.

[0109] Each process (each function) of the above-described embodiment is implemented by a processing circuit (Circuitry) including one or more processors. The processing circuit may consist of one or more memories, various analog circuits, various digital circuits, and other integrated circuits in addition to the one or more processors. The one or more memories store programs (instructions) that cause the one or more processors to execute each of the above processes. The one or more processors may execute each of the above processes according to the programs read from the one or more memories, or they may execute each of the above processes according to logic circuits that have been pre-designed to execute each of the above processes. The processors may be various processors suitable for computer control, such as a CPU (Central Processing Unit), GPU (Graphics Processing Unit), DSP (Digital Signal Processor), FPGA (Field-Programmable Gate Array), and ASIC (Application Specific Integrated Circuit). The physically separated multiple processors may cooperate with each other to execute each of the above processes. For example, the processors installed in each of several physically separated computers may cooperate with each other via a network such as a LAN (Local Area Network), WAN (Wide Area Network), or the Internet to perform the above processes. The program may be installed in the memory via the network from an external server device, or it may be distributed on a recording medium such as a CD-ROM (Compact Disc Read-Only Memory), DVD-ROM (Digital Versatile Disc Read-Only Memory), or semiconductor memory, and then installed in the memory from the recording medium.

[0110] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this disclosure is not defined by the description in the detailed disclosure but by the claims, and all modifications within the meaning and scope equivalent to the wording of the claims are intended. [Explanation of symbols]

[0111] S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, S11, S12 Switching elements 50,950 Isolation Converter 60, 960 Primary side full bridge circuit 62 Leakage Inductance 64 transformers 66 Secondary side full bridge circuit 68, 70 Input terminals 72, 74 Output terminals 80, 82, 100 capacitors Nodes 84, 88, 90, 102, 104, 970, 972, 980, 982, 990, 992, 994, 996 86, 974 Center switch 150 Primary voltage waveform 152 Secondary voltage waveform 160, 162, 164, 166, 200, 300, 302, 304, 306, 350, 352, 354, 356, 400, 402, 450, 452, 910, 912 waveform 170, 172, 174, 176, 178, 260, 262, 264, 266, 268, 270, 272, 274, 276, 278, 410, 412, 414, 416, 418 period 210 Bottom 212 Top 250, 254 Secondary waveform 252, 256 Primary waveform 500 control circuit 510 Timing Control Unit 512 Phase difference control section 514 Initial startup calculation section 516, 564 selector 518 PMW Control Unit 540 Arithmetic section 560 Interconnection Control Unit 562 Autonomous Operation Control Unit 580 MCU 600 Express bus 602 MPU 604 SRAM 606 Flash Memory 608 ROM 610 Slow Bus 612 Bridge 614 Serial I / F 616 ADC 618 Timer Counter 620 Clock Generator 622 Power Control Unit 624 General Purpose Interface 626 Programs 850, 860 area 900 Voltage waveform 1000, 1002, 1004, 1006 diodes

Claims

1. A transformer having a first coil and a second coil, A first bridge circuit connected to the first coil, A second bridge circuit connected to the second coil, A power conversion device including a control circuit that, in a steady state, controls the first bridge circuit and the second bridge circuit to periodically apply a voltage controlled to the first coil and the second coil, respectively, to the first coil and the second coil at a predetermined period, such that the voltages are a first waveform with three levels and a second waveform with two levels, thereby performing power conversion between the first coil and the second coil via the transformer, The aforementioned control circuit is A first control unit controls the first and second bridge circuits to maintain the voltage waveforms applied to the first and second coils at the startup of the power converter, and to increase the period during which voltage is applied to the first and second coils until the first period is reached. After the control by the first control unit is completed, the second control unit controls the first bridge circuit and the second bridge circuit to maintain the period during which voltage is applied to the second coil by the second bridge circuit as the first period, and to change the period during which voltage is applied to the first coil to a second period which is shorter than the first period, and to change the voltage waveform applied to the first coil until it becomes the first waveform. A power conversion device including a third control unit that, after the completion of control by the second control unit, moves the period during which voltage is applied to the second coil until the phase difference between the voltage waveforms applied to the first coil and the second coil becomes a predetermined value, and then controls the first bridge circuit and the second bridge circuit so that the phase difference between the voltage waveform applied to the first coil and the previous voltage waveform applied to the second coil is maintained at the predetermined value on a steady basis.

2. The power conversion device according to claim 1, wherein the voltage waveform applied to the first coil has a first level period in which the voltage is substantially zero, a second level period in which the absolute value of the voltage is substantially the first voltage, and a third level period in which the absolute value of the voltage is substantially the second voltage which is higher than the first voltage.

3. The power conversion device according to claim 2, wherein the period before and after the third level period is the second level period.

4. The power conversion device according to claim 3, wherein the first period is half the period of the predetermined cycle, and the centers of the first period, the second level period, and the third level period coincide with each other.

5. The power conversion device according to any one of claims 2 to 4, wherein the second control unit extends the first level period of the voltage waveform applied to the first coil so that the third level period becomes the second period, and then extends the second level period of the voltage waveform applied to the first coil until the third level period becomes a third period shorter than the second period.

6. The power conversion device according to any one of claims 2 to 4, wherein the second control unit changes the voltage waveform applied to the first coil while keeping the ratio of the first level period to the sum of the first level period and the second level period constant, until the sum of the second level period and the third level period equals the second period.

7. The power conversion device according to any one of claims 2 to 4, wherein the second control unit expands the second level period of the voltage waveform applied to the first coil so that the third level period becomes a third period shorter than the second period, and then changes the voltage waveform applied to the first coil so that the second level period becomes the second period while maintaining the third level period.

8. A transformer having a first coil and a second coil, A first bridge circuit connected to the first coil, A control method for a power converter including a second bridge circuit connected to the second coil, The power conversion device, in a steady state, controls the first bridge circuit and the second bridge circuit to periodically apply voltages controlled to the first coil and the second coil at a predetermined period, such that the voltages are a first waveform with three levels and a second waveform with two levels, respectively, thereby performing power conversion between the first coil and the second coil via the transformer. The aforementioned method, A first step in which the computer controls the first bridge circuit and the second bridge circuit in response to the activation of the power converter, such that the voltage waveforms applied to the first coil and the second coil are maintained at the two levels, and the period during which voltage is applied to the first coil and the second coil increases until it becomes a first period. A second step in which the computer controls the first bridge circuit and the second bridge circuit so that, after the completion of the first step, the period during which voltage is applied to the second coil by the second bridge circuit is maintained as the first period, and the period during which voltage is applied to the first coil changes to a second period which is shorter than the first period, and the voltage waveform applied to the first coil changes until it becomes the first waveform. A method for controlling a power converter, comprising: a third step in which, after the completion of the second step, the computer moves the period during which voltage is applied to the second coil until the phase difference between the voltage waveforms applied to the first coil and the second coil becomes a predetermined value, and then controls the first bridge circuit and the second bridge circuit so that the phase difference between the voltage waveform applied to the first coil and the voltage waveform applied to the second coil is maintained at the predetermined value on a steady basis.

9. A transformer having a first coil and a second coil, A first bridge circuit connected to the first coil, A control program for a power converter including a second bridge circuit connected to the second coil, The power conversion device, in a steady state, controls the first bridge circuit and the second bridge circuit to periodically apply voltages controlled to the first coil and the second coil at a predetermined period, such that the voltages are a first waveform with three levels and a second waveform with two levels, respectively, thereby performing power conversion between the first coil and the second coil via the transformer. The aforementioned program, A first step is to control the first bridge circuit and the second bridge circuit in response to the activation of the power converter, such that the voltage waveforms applied to the first coil and the second coil are maintained at the two levels, and the period during which voltage is applied to the first coil and the second coil increases until the first period is reached. A second step involves controlling the first and second bridge circuits, after the completion of the first step, such that the period during which voltage is applied to the second coil by the second bridge circuit is maintained as the first period, and that the period during which voltage is applied to the first coil changes to a second period shorter than the first period, and that the voltage waveform applied to the first coil changes until it becomes the first waveform. A control program for a power converter, which causes a computer to perform a third step of controlling the first bridge circuit and the second bridge circuit so that, after the completion of the second step, the period during which voltage is applied to the second coil is shifted until the phase difference between the voltage waveforms applied to the first coil and the voltage waveform applied to the second coil becomes a predetermined value, and then the phase difference between the voltage waveform applied to the first coil and the voltage waveform applied to the second coil is kept at the predetermined value on a regular basis.

10. A transformer having a first coil and a second coil, A first bridge circuit connected to the first coil, In a power conversion device including a second bridge circuit connected to the second coil, A control device that performs power conversion between the first coil and the second coil via the transformer by controlling the first bridge circuit and the second bridge circuit during steady-state operation and periodically applying voltages controlled to the first coil and the second coil at a predetermined period so that they have a first waveform with three levels and a second waveform with two levels, respectively, to the first coil and the second coil, respectively, A first control unit controls the first and second bridge circuits to maintain the voltage waveforms applied to the first and second coils at the startup of the power converter, and to increase the period during which voltage is applied to the first and second coils until the first period is reached. After the control by the first control unit is completed, the second control unit controls the first bridge circuit and the second bridge circuit to maintain the period during which voltage is applied to the second coil by the second bridge circuit as the first period, and to change the period during which voltage is applied to the first coil to a second period which is shorter than the first period, and to change the voltage waveform applied to the first coil until it becomes the first waveform. A control device comprising: a third control unit that, after the completion of control by the second control unit, moves the period during which voltage is applied to the second coil until the phase difference between the voltage waveforms applied to the first coil and the second coil becomes a predetermined value, and then controls the first bridge circuit and the second bridge circuit so that the phase difference between the voltage waveform applied to the first coil and the voltage waveform applied to the second coil is maintained at the predetermined value on a steady basis.