Control circuit and power conversion device

Abstract

To provide a control circuit and a power conversion device capable of performing soft switching by three-level voltage control with practical calculation load.SOLUTION: A control circuit, for controlling a power conversion device including a transformer, a primary side three-level bridge circuit, and a secondary side bridge circuit, includes: a feedback circuit for performing feedback control of a phase difference φ based on a given current command value and a state value of the transformer; a timing calculation circuit for calculating two phases α and β determining switching timing for each switching element of the primary side three-level bridge circuit, where 0<α<β<π, for the current command value according to each relational expression; and a PWM signal generation circuit for generating a PWM signal for controlling switching elements of the primary side three-level bridge circuit and the secondary side bridge circuit using the phase difference φ, and the phases α and β.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present disclosure relates to a control circuit and a power conversion device. [Background technology]

[0002] A known power conversion device is a dual active bridge (DAB) isolated converter, which uses a full bridge circuit on the primary side of a transformer. In such converters, when the step-up ratio is large or the transmitted power is small, switching becomes so-called hard switching, which is known to result in large losses.

[0003] One proposal for solving these problems is disclosed in Non-Patent Document 1, which will be listed later. The technology disclosed in Non-Patent Document 1 relates to a DAB converter in which the voltage applied to the transformer is three levels instead of two. According to Non-Patent Document 1, by using three levels of voltage and appropriately defining parameters, it is possible to avoid hard switching and achieve soft switching. In the following explanation, for the sake of simplicity, when explaining the configuration of the converter, "three levels" may be represented as "3L" and "two levels" as "2L." [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] A. Filba-Martinez, S. Busquets-Monge, J. Nicolas-Apruzzese and J. Bordonau, "Operating Principle and Performance Optimization of a Three-Level NPC Dual-Active-Bridge DC-DC Converter," in IEEE Transactions on Industrial Electronics, vol. 63, no. 2, pp. 678-690, Feb. 2016. Summary of the Invention [Problem to be solved by the invention]

[0005] However, the technology disclosed in Non-Patent Document 1 generates three-level voltages to be applied to a transformer. This increases the number of semiconductor switching elements in the primary circuit to be controlled, and the number of parameters for controlling them also increases. The technology disclosed in Non-Patent Document 1 proposes feedback control of these parameters. This proposal performs feedback control under various conditions, but the large number of parameters to be controlled also poses a problem of high computational load. As a result, the required functions cannot be realized due to the processing capabilities and computation speed of microprocessors used in ordinary power conversion devices, making it impractical.

[0006] An object of this disclosure is to provide a control circuit and a power conversion device that are capable of soft switching using three-level voltage control with a practical calculation load. [Means for solving the problem]

[0007] A control circuit according to a first aspect of the present disclosure is a control circuit for controlling a power conversion device that includes a transformer, a primary-side three-level bridge circuit connected to the primary side of the transformer, and a secondary-side bridge circuit connected to the secondary side of the transformer, and performs power conversion by changing a phase difference φ between the phase of a voltage waveform that the primary-side three-level bridge circuit applies to the transformer and the phase of a voltage waveform that the secondary-side bridge circuit applies to the transformer. The control circuit includes: a feedback circuit that feedback-controls the phase difference φ based on a given current command value and a state value of the transformer; a timing calculation circuit that calculates two phases α and β, where 0<α<β<π, for the current command value according to respective relational expressions, that determine the switching timing of each switching element of the primary-side three-level bridge circuit; and a PWM signal generation circuit that generates PWM (Pulse Width Modulation) signals that control the switching elements of the primary-side three-level bridge circuit and the secondary-side bridge circuit using the phase difference φ and the phases α and β.

[0008] A power conversion device according to a second aspect of this disclosure includes the above-mentioned control circuit, a transformer, a primary-side three-level bridge circuit connected to the primary side of the transformer, and a secondary-side circuit connected to the secondary side of the transformer, the primary-side three-level bridge circuit including a semiconductor switching element whose switching is controlled by the control circuit.

[0009] A power conversion device according to a third aspect of this disclosure includes the above-mentioned control circuit, a transformer, a primary-side three-level bridge circuit connected to the primary side of the transformer, the primary-side three-level bridge circuit including semiconductor switching elements whose switching is controlled by a primary-side PWM signal generating circuit of the control circuit, and a secondary-side three-level bridge circuit connected to the secondary side of the transformer, the secondary-side three-level bridge circuit including semiconductor switching elements whose switching is controlled by a secondary-side PWM signal generating circuit of the control circuit. [Effects of the Invention]

[0010] As described above, this disclosure provides a control circuit and a power conversion device that are capable of soft switching using three-level voltage control with a practical calculation load. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram for explaining loss due to hard switching. [Figure 2] FIG. 2 is a circuit block diagram of a DAB converter according to the first embodiment of the present disclosure. [Figure 3] FIG. 3 is a graph for explaining parameters related to the three-level control in the first embodiment. [Figure 4] FIG. 4 is a graph showing changes in the primary voltage, secondary voltage, and transformer current of the transformer in the first embodiment. [Figure 5] FIG. 5 is a block diagram showing the configuration of a control circuit in the first embodiment. [Figure 6] FIG. 6 is a graph showing the range of combinations of phase difference φ and transformation ratio m that allow soft switching to be achieved for various phases β. [Figure 7] FIG. 7 is a diagram plotting transformation ratios at which soft switching can be achieved for combinations of phase difference φ and phase β based on the relationship shown in FIG. [Figure 8] FIG. 8 is a diagram plotting the maximum current value at which soft switching is possible for each combination of phase difference φ and phase β. [Figure 9] FIG. 9 is a diagram that combines the region in FIG. 7 where soft switching can be achieved at a desired transformation ratio with the graph in FIG. 8, and shows a state in which lines are drawn to indicate combinations of phase difference φ and phase β that allow a desired range of current to flow at a transformation ratio of 2.5. [Figure 10] FIG. 10 is a graph obtained by plotting the line shown in FIG. 9 as a relationship between the current command value I and the phase β, and further approximating this graph with a cubic curve. [Figure 11] FIG. 11 is a block diagram showing the configuration of a control circuit according to the second embodiment. [Figure 12] FIG. 12 is a diagram showing a method of determining the phase β when the transformation ratio is changed based on the graph shown in FIG. [Figure 13] FIG. 13 is a block diagram showing the configuration of a control circuit according to the third embodiment. [Figure 14] FIG. 14 is a block diagram showing the configuration of a control circuit using constant voltage control. [Figure 15] FIG. 15 is a diagram showing the results of a simulation comparing the range in which soft switching can be achieved by the 3L-2L converter according to the present disclosure with that of a conventional 2L-2L converter. [Figure 16] FIG. 16 is a diagram showing transformer voltage waveforms and transformer current waveforms in a simulation of a 2L-2L converter. [Figure 17] FIG. 17 is a diagram showing gate-source voltage waveforms and current waveforms of switching elements in a simulation of a 2L-2L converter. [Figure 18] FIG. 18 is a diagram showing transformer voltage waveforms and transformer current waveforms in a simulation of a 3L-2L converter according to the present disclosure. [Figure 19] FIG. 19 shows gate-source voltage waveforms and current waveforms of switching elements in a simulation of a 3L-2L converter according to the present disclosure. [Figure 20] FIG. 20 is a circuit block diagram of a 3L-3L converter according to a modified example of this disclosure. [Figure 21] FIG. 21 is a block diagram showing the configuration of the control circuit shown in FIG. [Figure 22] FIG. 22 is a circuit block diagram of a 3L-2L converter according to another modification of this disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] [Description of the embodiments of the present disclosure] In the following description and drawings, the same parts are denoted by the same reference numerals. Therefore, detailed description thereof will not be repeated. Note that any features of one or more of the following embodiments may be combined.

[0013] (1) A control circuit according to a first aspect of this disclosure is a control circuit for controlling a power conversion device that includes a transformer, a primary-side three-level bridge circuit connected to the primary side of the transformer, and a secondary-side bridge circuit connected to the secondary side of the transformer, and performs power conversion by changing the phase difference φ between the phase of a voltage waveform applied to the transformer by the primary-side three-level bridge circuit and the phase of a voltage waveform applied to the transformer by the secondary-side bridge circuit. The control circuit includes: a feedback circuit that feedback-controls the phase difference φ based on a given current command value and a state value of the transformer; a timing calculation circuit that calculates two phases α and β, where 0<α<β<π, for the current command value according to respective relational expressions, that determine the switching timing of each switching element of the primary-side three-level bridge circuit; and a PWM signal generation circuit that generates PWM signals that control the switching elements of the primary-side three-level bridge circuit and the secondary-side bridge circuit using the phase difference φ and the phases α and β.

[0014] Since the phases α and β are calculated using a relational expression with the current command value, the load for control is reduced. As a result, a control circuit capable of soft switching by three-level voltage control can be provided with a practical calculation load.

[0015] (2) In (1) above, the calculation period of the two phases α and β by the timing calculation circuit is longer than the feedback period by the feedback circuit.

[0016] Since the calculation period of the phases α and β can be lengthened, the load for control is further reduced.

[0017] (3) In the above (1) or (2), the relational expression may include a first relational expression relating to the phase α and the current command value, and a second relational expression relating to the phase β and the current command value.

[0018] The phases α and β are calculated using separate relational expressions related to the current command values, which allows these phases to be calculated simply and independently of each other, thereby reducing the load on the control.

[0019] (4) In the above (3), the first relational expression may be a function of the current command value that uniquely determines the phase α for a certain range of the current command value.

[0020] Since the phase α is uniquely determined, control becomes simple.

[0021] (5) In the above (4), the first relational expression may be a monotonically decreasing function in a strong sense of the current command value within a certain range that determines the phase α.

[0022] The relationship between the phase α and the current command value is determined one-to-one by a monotonically decreasing function in the strong sense. As a result, the phase α can be uniquely determined by a simple calculation, which simplifies control.

[0023] (6) In any one of the above (3) to (5), the second relational expression may be a function of the current command value that uniquely determines the phase β for a certain range of the current command value.

[0024] Since the phase β is uniquely determined, control becomes simple.

[0025] (7) In the above (6), the second relational expression may be a monotonically decreasing function in a strong sense of the current command value within a certain range that determines the phase β.

[0026] By doing so, the relationship between the current command value and the phase β is determined in one-to-one correspondence, which results in the effect that the phase β can be uniquely determined by a simple calculation, simplifying control.

[0027] (8) In any one of (1) to (7) above, each switching element of the primary-side three-level bridge circuit may have a first terminal, a second terminal, and a control terminal that controls conduction between the first terminal and the second terminal, and the relational expression may be selected so that switching by each switching element is a soft switch.

[0028] By performing soft switching by the switching elements, it is possible to prevent power loss caused by the switching elements.

[0029] (9) In the above (8), the relational expressions may be selected to determine the phases α and β so that the switching elements are conductive while the direction of the current flowing through the body diode of each switching element is opposite to the direction of the voltage applied to the first terminal and the second terminal when each switching element is not conductive.

[0030] When the switching element is turned on, the direction of the current flowing through the body diode of the switching element is opposite to the direction of the voltage, so no power loss occurs when the switching element is turned on, and the efficiency of the power conversion circuit can be improved.

[0031] (10) In any one of (1) to (9) above, the primary-side three-level bridge circuit may function to apply 0, ±V1 / 2, and ±V1 to the primary side of the transformer, and phase α may determine the timing at which the voltage applied to the primary side of the transformer changes from 0 to ±V1 / 2, and the phase obtained by subtracting phase α from phase π may determine the timing at which the voltage applied to the primary side of the transformer changes from ±V1 / 2 to 0.

[0032] By defining the phase α in this way, the voltage applied to the primary side of the transformer can be switched between three levels. This reduces the voltage change caused by the switching of the switching element. This allows for smaller switching elements to be used. As a result, soft switching can be achieved over a wide range of transformation ratios while keeping costs down. This allows for low losses in the power conversion device while keeping costs down.

[0033] (11) In (10) above, phase β may determine the timing at which the voltage applied to the primary side of the transformer is changed from ±V1 / 2 to ±V1, and the phase obtained by subtracting phase β from phase π may determine the timing at which the voltage applied to the primary side of the transformer is changed from ±V1 to ±V1 / 2.

[0034] By defining the phase β in this way, the voltage applied to the primary side of the transformer can be switched between three levels. Furthermore, the voltage change due to switching of each switching element is reduced. This allows for smaller switching elements to be used. As a result, soft switching can be achieved for a wide range of transformation ratios while keeping costs down. As a result, losses in the power conversion device can be kept low while keeping costs down.

[0035] (12) In any one of (1) to (11) above, the relational expression may be a polynomial of the current command value.

[0036] By adopting a polynomial for the current command value as the relational expression, the phases α and β can be calculated with a low amount of calculation, which enables soft switching by three-level voltage control with a practical calculation load.

[0037] (13) In the above (12), the relational expression may be a polynomial of the current command value of degree three or less.

[0038] By using a third-order or lower polynomial of the current command value as the relational expression, phases α and β can be calculated with a very low amount of calculation. As a result, soft switching by three-level voltage control is possible with a practical calculation load.

[0039] (14) In any one of (1) to (13) above, the control circuit may further include a current command value generation circuit that generates a current command value by feedback based on the voltage command value and the voltage applied to the primary side of the transformer.

[0040] This configuration enables soft switching with three-level voltage control with a practical computation load by utilizing the configuration of constant current control even under constant voltage control.

[0041] (15) In any one of the above (1) to (14), the secondary bridge circuit may include a secondary three-level bridge circuit, and the control circuit may further include a secondary timing calculation circuit that calculates two phases γ and δ, where 0<γ<δ<π, that determine the switching timing of each switching element of the secondary three-level bridge circuit according to each relational expression for the current command value, and the PWM signal generation circuit is a 3-level-3-level PWM signal generation circuit that controls the switching elements of the primary three-level bridge circuit using the phase difference φ and the phases α and β, and controls the switching elements of the secondary three-level bridge circuit using the phase difference φ and the phases γ and δ. The road It may include.

[0042] By employing a three-level bridge circuit not only on the primary side but also on the secondary side, and by determining the phases γ and δ required for its control from a relational expression with the current command value, it is possible to provide a control circuit that can achieve stable soft switching over a wide range of transformation ratios, even with large voltage fluctuations, and that can operate with a practical calculation load.

[0043] (16) A power conversion device according to a second aspect of this disclosure includes a control circuit according to any one of (1) to (15) above, a transformer, a primary-side three-level bridge circuit connected to the primary side of the transformer, the primary-side three-level bridge circuit including semiconductor switching elements whose switching is controlled by the control circuit, and a secondary-side circuit connected to the secondary side of the transformer.

[0044] Since the phases α and β are calculated using a relational expression with the current command value, the load for control is reduced. As a result, a power conversion device can be provided that enables soft switching using three-level voltage control with a practical calculation load and reduces losses.

[0045] (17) A power conversion device according to a third aspect of the present disclosure includes the control circuit of (15), a transformer, and 3 level - for 3 levels A primary-side three-level bridge circuit connected to the primary side of a transformer, including a semiconductor switching element whose switching is controlled by a PWM signal generating circuit, and a control circuit 3 level - for 3 levels It includes a semiconductor switching element whose switching is controlled by a PWM signal generation circuit. fruit, The inverter includes a secondary-side three-level bridge circuit connected to the secondary side of the transformer.

[0046] The phases α, β, γ, and δ are calculated using a relational expression with the current command value. This reduces the load for control. As a result, a power conversion device can be provided that enables soft switching using three-level voltage control with a practical calculation load and reduces losses.

[0047] [Details of the embodiments of the present disclosure] Specific examples of converters according to embodiments of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0048] 1. First embodiment 1A. Background The occurrence of losses due to hard switching will be explained with reference to Figure 1. As shown in voltage waveform 50 in Figure 1, assume a state in which there is a voltage difference across a semiconductor switch when no current flows through it. When the semiconductor switch is turned on in this state, current begins to flow through the semiconductor switch, as shown by current waveform 52, before the voltage difference across the semiconductor disappears due to the capacitance component of the semiconductor switch. As a result, power loss 58 occurs in region 54 where both the current and voltage are non-zero. The same is true when the semiconductor switch is turned off, and power loss 60 occurs in region 56 where both the current and voltage are non-zero.

[0049] This embodiment is intended to prevent such power loss from occurring, and to that end, as described below, a 3L full-bridge circuit is employed on the primary side, and the switching timing of the semiconductor switches is controlled using a simple configuration, thereby preventing power loss over a wide range of transformation ratios and current values.

[0050] 1B.Configuration 2 shows a schematic configuration of a 3L-2L converter 100 according to this embodiment. Referring to Fig. 2, the 3L-2L converter 100 includes a transformer 114 having a leakage inductance 116, and a primary-side 3L full-bridge circuit 110 connected between the storage battery 102 and a primary-side terminal of the transformer 114. The 3L-2L converter 100 further includes a secondary-side full-bridge circuit 112 connected between a secondary-side terminal of the transformer 114 and the DC bus 104.

[0051] The primary-side 3L full-bridge circuit 110 includes switching elements S1, S2, S3, S4, S5, S6, S7, and S8, all of which are MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors). Of these, switching elements S1, S4, and S5, and S8 form a full-bridge circuit. Switching elements S2, S3, S6, and S7 form a switching element group 124 required for three-level control. A node 128 connecting switching elements S1 and S4 is connected to a first terminal on the primary side of a transformer 114. A node 130 connecting switching elements S5 and S8 is connected to a second terminal on the primary side of the transformer 114.

[0052] Capacitors 120 and 122 are connected in series via a node 126 between two terminals of the primary-side 3L full-bridge circuit 110 that are connected to the storage battery 102 side. The node 126 is connected to nodes 128 and 130 via separate connection lines. Switching elements S2 and S3 are connected in this order between the node 126 and the node 128. Switching elements S6 and S7 are connected in this order between the node 126 and the node 130.

[0053] The secondary-side full-bridge circuit 112 includes switching elements S9, S10, S11, and S12. Switching elements S9 and S10 are connected via a node 142. Switching elements S11 and S12 are connected via a node 144. Node 142 is connected to a first terminal on the secondary side of a transformer 114. Node 144 is connected to a second terminal on the secondary side of the transformer 114.

[0054] The secondary full bridge circuit 112 further includes a capacitor 140 connected between the two DC buses 104 .

[0055] The 3L-2L converter 100 further includes a control circuit 250 that generates control signals for the switching elements included in the primary-side 3L full-bridge circuit 110 and the secondary-side full-bridge circuit 112 based on the current IL of the transformer 114 and the voltages applied to the primary and secondary sides of the transformer 114, and outputs the control signals to the primary-side 3L full-bridge circuit 110 and the secondary-side full-bridge circuit 112. The configuration of the control circuit 250 will be described later with reference to FIG. 5.

[0056] Parameters for controlling the primary-side 3L full-bridge circuit 110 will be described with reference to Fig. 3. The upper part of Fig. 3 shows a voltage waveform 170 that the primary-side 3L full-bridge circuit 110 applies to the transformer 114. The lower part of Fig. 3 shows a voltage waveform 172 that the secondary-side full-bridge circuit 112 applies to the secondary side of the transformer 114.

[0057] As shown in the lower part of Figure 3, in this embodiment, the voltage waveform 172 is a normal square wave, alternating between voltages of ±V2 at a fixed cycle. In the following description, one cycle is represented by 2π. Meanwhile, as shown in the upper part of Figure 3, the voltage waveform 170 generated by the primary-side 3L full-bridge circuit 110 takes voltages of ±V1, 1 / 2, and 0.

[0058] If the phase at the beginning of one cycle is 0, then the voltage waveform 170 is zero from phase 0 to phase α (where 0<α<π / 2). At phase α, the voltage waveform 170 becomes V1 / 2. At phase β (where α<β<π / 2), the voltage waveform 170 becomes V1. From phase β to phase π-β, the voltage waveform 170 maintains V1, and at phase π-β, it becomes V1 / 2. Furthermore, at phase π-α, the voltage waveform 170 changes to 0.

[0059] In the half cycle after the phase π, the voltage waveform 170 has a waveform in which the polarity is reversed from that in the half cycle from phase 0 to π. After that, the voltage waveform 170 repeats the waveform described above.

[0060] Here, voltage waveform 172 lags behind voltage waveform 170 by a phase difference φ. The energy generated by the voltage difference between the primary and secondary sides during this delay is stored in leakage inductance 116 of transformer 114 and then released, so that 3L-2L converter 100 also functions as a buck-boost converter.

[0061] Fig. 4 shows an example of a primary voltage 200, a secondary voltage 202, and a current waveform 204 of the transformer 114 when the 3L-2L converter 100 is operated in the manner shown in Fig. 3. In this example, the primary voltage V1 is 200V, and the secondary voltage is 350V.

[0062] 5 shows the configuration of the control circuit 250. Referring to FIG. 5, the control circuit 250 is for performing constant current control on the 3L-2L converter 100. The control circuit 250 controls a current command value I * and the current I of the transformer 114, a subtractor 260 for feedback-controlling the phase difference φ, a PI control unit 262, and a conversion unit 264 for converting the PI control unit 262 to a phase difference φ; * and the current command value I *and a relational equation converter 268 that converts the phase difference φ from the converter 264 and the phases α and β from the relational equation converter 268 into the phases α and β according to a predetermined relational equation. The control circuit 250 further includes a PWM signal generator 266 that receives the phase difference φ from the converter 264 and the phases α and β from the relational equation converter 268 and outputs PWM signals that control the switching elements S1 to S12.

[0063] Note that the phase difference φ is generated by feedback in the same way as in conventional 2L-2L converters. The mechanism for generating PWM signals to control switching elements S to S12 based on the phase difference φ and phases α and β is a well-known technique, and is clear from the description in the aforementioned Non-Patent Document 1, so the details will not be repeated here.

[0064] 1C. How to determine the relationship Hereinafter, the current command value I * This section explains how to determine the relational expression for converting the current command value I into phases α and β. The method for determining this relational expression is basically the same for both phases α and β, and the concept is also common to any of the switching elements in the primary-side 3L full-bridge circuit 110. Therefore, only the phase β in the control of the switching element S1 will be explained here. The relational expression explained below is based on the current command value I * The purpose is to uniquely determine the value of the phase β from

[0065] The power transfer equation in the 3L-2L converter 100 shown in FIG. 2 is expressed as follows, with the phase difference φ, and the phases α and β as parameters.

[0066]

number

[0067] This formula can be further elaborated as follows:

[0068]

number

[0069]

number

[0070] This equation, viewed as the relationship between φ and the transformation ratio m, is plotted in Figure 6 for various values ​​of β. While Figure 6 is a little difficult to read, it shows the range in which the above equation holds when β = 80 degrees, for example, between curve 308 and line 318. Similarly, the ranges between curve 306 and line 316, curve 304 and line 314, curve 302 and line 312, and curve 300 and line 310 show the ranges in which the above equation holds when β = 60 degrees, 40 degrees, 20 degrees, and 0 degrees, respectively. Note that in the graph shown in Figure 6, curves 300, 302, 304, 306, and 308 continue to infinity, with lines 310, 312, 314, 316, and 318 as asymptote, respectively. Therefore, only the range of transformation ratios below 2.4 is shown in the graph.

[0071] Figure 7 shows the relationship shown in Figure 6 plotted to show the transformation ratio values ​​for combinations of phase difference φ and phase β. Although Figure 7 shows simple shading, during design, the screen is drawn in different colors depending on the transformation ratio value. The colored areas are the areas that satisfy the above relationship. Therefore, it can be seen that soft switching is possible in the colored areas. Furthermore, by looking at the color of a point, it is possible to see what the possible transformation ratio is for the combination of phase difference φ and phase β corresponding to that point.

[0072] On the other hand, Fig. 8 shows a diagram in which the horizontal axis represents the phase difference φ and the vertical axis represents the phase β, and the maximum possible transformer current I for the 3L-2L converter 100 is plotted for each combination. The current values ​​indicated in the legend in Fig. 8 are in amperes. The maximum current I is calculated by assuming α = 0 in the power transfer equation above.

[0073] From these two diagrams, the relationship between the transformer current and the phase β is identified as follows. In FIG. 7, a region where a predetermined transformation ratio, for example, a transformation ratio m=2.5, can be achieved is specified. This specification can be determined by looking at the drawing color of the region, as described above. Next, in FIG. 8, the region corresponding to the specified region in the diagram shown in FIG. 7 is cut out and displayed on a new screen. This region is shown as region 350 in FIG. 9. In FIG. 7 and FIG. 8, the horizontal and vertical axes are the same, making this operation easy to perform. In FIG. 9, the horizontal axis also represents the phase difference φ, and the vertical axis represents the phase β.

[0074] Referring to FIG. 9, this figure shows a region 350 where a desired transformation ratio can be achieved by combining the phase difference φ and the phase β that allows soft switching, along with the distribution of the maximum current value that can be achieved in that region.

[0075] 9 is drawn inside this region 350. Then, line 352 gives a combination (relationship) of phase difference φ and phase β that allows a transformation ratio of 2.5 and a current in a predetermined range (for example, 0 amperes to 40 amperes) to flow.

[0076] Therefore, the line 352 depicted in FIG. 9 can be re-plotted on a graph, as shown in FIG. 10, with the horizontal axis representing the current and the vertical axis representing the phase β. The resulting graph is curve 380. As described above, curve 380 can be considered to represent the combination of phase difference φ and phase β that allows a predetermined range of current to flow at a predetermined transformation ratio, as expressed in the form of a function of phase β versus phase difference φ. This graph is approximated by a predetermined function, for example, a polynomial. Curve 382 shown in FIG. 10 represents curve 380 using a cubic polynomial of φ. This cubic polynomial can determine the phase β relative to the current command value I. This function is a simple polynomial, and the phase β can be easily calculated given the current command value I. Furthermore, for example, the calculation of phase β can be performed independently of the calculation of phase α. Therefore, unlike the technology described in Non-Patent Document 1, this method can be sufficiently processed even by a low-power microprocessor.

[0077] However, due to the nature of control, the phase β must be uniquely determined for a certain value of the current command value I. Therefore, the curve 380 shown in FIG. 10 must be a monotonic function in the strong sense (a monotonically decreasing function in the case of FIG. 10). Otherwise, the phase β cannot be determined. In the design stage of the 3L-2L converter 100, if the curve drawn in FIG. 10 after drawing the line 352 shown in FIG. 9 is not a monotonic function, the line 352 must be redrawn.

[0078] When approximating a curve using a polynomial, there is no particular limit to the degree of the polynomial. However, since it is necessary to keep the amount of calculation low, a certain degree, for example, third degree, is desirable. In the example shown in FIG. 10, there are what appear to be inflection points on the curve 380, so approximation is performed using a third degree polynomial. However, if a large error range can be ensured, approximation using a second degree polynomial or a first degree polynomial is also possible. Conversely, the curve may also be approximated using a fourth degree polynomial or higher.

[0079] In this embodiment, the fluctuation of the current command value I is small. Therefore, the control period of the phases α and β can be made longer than the control period of the phase difference φ. As a result, the load on the microprocessor that performs the control is reduced, and three-level control of the full-bridge circuit can be achieved with a simple configuration.

[0080] If the microprocessor has ample storage capacity, it is also possible to prepare a table of combinations of current command values ​​I and phases β that make up curve 380, rather than using an approximation of curve 380. In this case, when a current command value I is given, the corresponding phase β can be identified by table lookup. If the specified current command value I does not exist in the table, the phase β can be interpolated using nearby data in the table.

[0081] Furthermore, in the above embodiment, FIG. 9 is created from FIGS. 7 and 8, line 352 is drawn on the map of FIG. 9, and line 352 is then converted into the map of FIG. 10. However, this disclosure is not limited to such an embodiment. Conversely, a desired curve (a monotonically decreasing function in a strong sense) showing the relationship between the current command value I and the phase β may be drawn on the map of FIG. 10 and mapped to FIG. 9. In this case, if the line obtained from the map (a line corresponding to line 352 in FIG. 9) falls within region 350, the initially drawn curve can be used to show the relationship between the current command value I and the phase β. Thereafter, as in the above embodiment, an approximation formula or table lookup may be used. If the line obtained from the map does not fall within region 350, simply return to FIG. 10 and redraw the curve.

[0082] Furthermore, in the above-described embodiment, for example, in FIG. 7, the value of the transformation ratio m is represented by a color on the map. However, this disclosure is not limited to such an embodiment. Because the transformation ratio is a function of the phase difference φ and the phase β, it can be represented by a three-dimensional graph with the phase difference φ on the x-axis, the phase β on the y-axis, and the transformation ratio on the z-axis. If the transformation ratio is represented by a three-dimensional graph, when this three-dimensional graph is cut by two planes perpendicular to the Z-axis, which indicate the range of the desired transformation ratio, a figure corresponding to region 350 in FIG. 9 can be obtained by projecting the region enclosed by the cut lines on the three-dimensional graph onto the xy plane.

[0083] 2. Second embodiment In the first embodiment, the transformation ratio m is limited to a certain region, and when a current command value I is given, the phases α and β are calculated as functions of the current command value I. However, this disclosure is not limited to such an embodiment. In this second embodiment, it is assumed that each point in region 350 in FIG. 9 represents the transformation ratio shown in FIG. 7 and the current value shown in FIG. 8. The resulting graph can then be converted into a three-dimensional graph (three-dimensional surface) in which each point in region 350 is plotted, with the current command value on the x-axis, the transformation ratio m on the y-axis, and the phase β on the z-axis. This three-dimensional graph has the same meaning as the curve 380 in FIG. 10. Therefore, by approximating this surface with a function having two variables, the current command value I and the transformation ratio m, or by storing it as a two-dimensional table, it is possible to obtain the same effect as in the first embodiment.

[0084] FIG. 11 shows a conversion unit 410 that calculates the phases α and β in the control circuit 400 according to the second embodiment. Referring to FIG. 11, the conversion unit 410 receives a current command value and the primary and secondary voltages of the transformer 114 as inputs. The conversion unit 410 calculates the phases α and β by inputting the current command value and a transformation ratio (=secondary voltage / (n*primary voltage)) to functions that calculate the phases α and β, respectively, and provides the phases α and β to a PWM signal generator 412. By replacing the relational equation conversion unit 268 in FIG. 5 with the conversion unit 410 shown in FIG. 11, the switching elements of the 3L-2L converter 100 can be controlled with a simple configuration, as in the first embodiment. The time fluctuations of the current command value, primary voltage, and secondary voltage are small. Therefore, in this embodiment, too, the calculation of the phases α and β can be performed at longer intervals than in the feedback control of the phase difference φ. As a result, the load on the microprocessor for control can be reduced, and three-level control can be performed in a practical manner.

[0085] 3. Third embodiment In the first embodiment, the case where the transformation ratio m=2.5 has been described as an example. When the transformation ratio is different, the relational expressions for determining the current command value and the phases α and β are obtained by the procedure described in the first embodiment based on the different transformation ratio. However, if the relational expressions for determining the phases α and β for a predetermined transformation ratio have been obtained, by following this third embodiment, the values ​​of the phases α and β can be obtained for any transformation ratio by slightly modifying the control according to the first embodiment.

[0086] Referring again to Figure 3, when the primary side is a three-level full-bridge circuit, the transmitted power drops by the amount indicated by phases α and β. On the other hand, for example, when the transformation ratio approaches 1, there is no need to increase either phase α or β. Therefore, by reducing phases α and β, the transmitted power can be increased. In other words, when the transformation ratio approaches 1 from 2.5, both phases α and β can be reduced.

[0087] This is shown in FIG. 12. Curve 380 shown in FIG. 12 represents the relationship between the current command value and phase β when the transformation ratio is 2.5, as determined in the first embodiment. Curve 382 is obtained by approximating curve 380 using a third-order polynomial. As described above, when the transformation ratio approaches 1, the phase β can be reduced. For example, as shown in FIG. 12, when the current command value I is approximately 12.5 amperes, the phase β determined from curve 382 is approximately 0.31. However, when the transformation ratio approaches 1 (when the transformation ratio decreases), the phase β can be reduced. To utilize this result, the phase β corresponding to point 450, which is obtained by compressing the entire curve 382 downward along the vertical axis, can be used. For example, curve 382 can be multiplied by a coefficient k determined by the following equation, where m is the transformation ratio.

[0088]

number

[0089] According to the third embodiment, when a relational expression between the current command value I and the phases α and β for a specific transformation ratio is obtained, the phases α and β can be calculated by simple control using that relational expression even for a transformation ratio different from that transformation ratio. As a result, there is an advantage that the three-level full-bridge circuit can be controlled by a control circuit with a simple configuration even for different transformation ratio values. In addition, there is an advantage that the control circuit itself can be designed simply by slightly changing the control circuit for a specific transformation ratio.

[0090] 4. Fourth embodiment The above first, second, and third embodiments all relate to a control circuit that performs constant current control on the primary-side 3L full-bridge circuit 110 of a 3L-2L converter. However, this circuit is not limited to such embodiments. This disclosure can also be applied to cases where the primary-side 3L full-bridge circuit is subjected to constant voltage control.

[0091] 14 is a block diagram showing the configuration of a control circuit 500 for constant voltage control of the primary-side 3L full-bridge circuit 110 according to the fourth embodiment. Referring to FIG. 14, the control circuit 500 includes a control circuit 514 as a minor loop, which is similar to the control circuit 250 shown in FIG. 5, and a control circuit 514 for controlling a voltage command value V * a subtraction circuit 510 that subtracts the primary voltage V of the transformer 114 from the output of the subtraction circuit 510; and a current command value I * and inputs it to the plus terminal of the subtractor 260.

[0092] The control circuit 514 serving as a minor loop has the same configuration as the control circuit 250 shown in Fig. 5. Therefore, similar to the first embodiment, the relational equation conversion unit 268 can calculate the phases α and β required for the PWM signal generation unit 266 to generate a switching signal that controls the primary-side 3L full-bridge circuit 110.

[0093] In this fourth embodiment, too, the phases α and β can be calculated using simple relational expressions. This requires a small amount of calculation. Furthermore, the period for calculating the phases α and β can be made longer than the control period for the phase difference φ. As a result, the primary-side 3L full-bridge circuit 110 can be practically controlled using a simple circuit with a small amount of calculation.

[0094] 5. Simulation Results The voltage / current ranges in which soft switching can be achieved were investigated by simulation for a conventional 2L-2L converter and the 3L-2L converter 100 according to the first embodiment. The results are shown in FIG. 15. In the two tables shown in FIG. 15, the upper row shows the results for the 2L-2L converter. The lower row shows the results for the 3L-2L converter 100. In the two tables shown in FIG. 15, the value shown on the far left is the primary-side voltage. In this simulation, the secondary-side voltage was fixed at 350 V. In FIG. 15, "OK" indicates that soft switching was achieved, and "NG" indicates that soft switching was not achieved.

[0095] As can be seen from Figure 15, in a conventional 2L-2L converter, soft switching cannot be achieved when the absolute value of the current becomes small. It can also be seen that soft switching cannot be achieved as the transformation ratio increases.

[0096] In contrast, such a result does not occur in the case of the 3L-2L converter 100 according to the first embodiment. It can be seen that soft switching can be achieved even if the absolute value of the current is small and the transformation ratio is large.

[0097] Fig. 16 shows a voltage waveform 550 and a current waveform 552 on the primary side of the transformer in a conventional 2L-2L converter in a simulation. Fig. 17 shows a voltage waveform 560 and a current waveform 562 of the switching element S1 shown in Fig. 2. The phase difference φ at this time was 24.6 degrees.

[0098] 17, it can be seen that at time 500 microseconds, when switching element S1 turns on, the current is almost zero, and the current begins to increase almost simultaneously with the rise in the voltage of switching element S1. Therefore, in an actual device, loss occurs in this part.

[0099] Fig. 18 shows a transformer voltage waveform 570 and a current waveform 572 on the primary side when a simulation is performed under similar conditions for the 3L-2L converter 100 in the first embodiment. Fig. 19 shows a gate-source voltage waveform 590 and a current waveform 592 of the switching element S1 before and after the switching element S1 is switched from off to on. At this time, the phase difference φ was 31.4 degrees, the phase α was 16.1 degrees, and the phase β was 32.2 degrees.

[0100] Fig. 18 shows that the primary-side transformer voltage is controlled to three levels. Fig. 19 also shows that the current flowing through switching element S1 is negative when switching element S1 switches from off to on. In other words, the switching of switching element S1 is soft switching, and no loss occurs.

[0101] As can be seen from the above simulations, the above control circuit is effective.

[0102] 6. Variations In the above embodiments, a three-level full-bridge circuit is used on the primary side and a two-level full-bridge circuit is used on the secondary side. However, this disclosure is not limited to such embodiments. A three-level full-bridge circuit may be used on the secondary side as well as the primary side. Figure 20 shows an example of the configuration of such a 3L-3L converter.

[0103] 20 , this 3L-3L converter 600 includes the same primary 3L full bridge circuit 110 as in the first embodiment, connected between the storage battery 102 and the transformer 114; a secondary 3L full bridge circuit 610 connected to the DC bus 104; and a control circuit 620 for controlling the primary 3L full bridge circuit 110 and the secondary 3L full bridge circuit 610. The combination of the primary 3L full bridge circuit 110, the transformer 114, and the secondary 3L full bridge circuit 610 is publicly known, and a method for driving them in three levels is also publicly known, for example, from the description in Non-Patent Document 1. However, unlike the prior art, the control circuit 620 proposed in this disclosure not only determines the phases α and β for controlling the primary 3L full bridge circuit 110 described in each of the above embodiments using a relationship equation with a current command value, but also determines the phases γ and delta for driving the secondary 3L full bridge circuit 610 in three levels using a relationship equation with a current command value. The phase γ corresponds to the phase α in the primary side 3L full bridge circuit 110. The phase delta corresponds to the phase β in the primary side 3L full bridge circuit 110.

[0104] 21, control circuit 620 has the same configuration as control circuit 250 shown in FIG. 5. However, control circuit 620 uses a current command value I instead of relational expression conversion unit 268 shown in FIG. * 5. The control circuit 620 differs from the control circuit 250 in that it includes a relational equation conversion unit 630 that calculates and outputs the phases α, β, γ, and δ by substituting the values ​​of α, β, γ, and δ into the corresponding functions. The control circuit 620 also differs from the control circuit 250 in that it includes a 3L-3L PWM signal generation unit 632, instead of the PWM signal generation unit 266 shown in FIG. 5, that receives the phase difference φ output by the conversion unit 264 and the phases α, β, γ, and δ output by the control circuit 620 and outputs PWM signals that control the switching elements of the primary-side 3L full bridge circuit 110 and the switching elements of the secondary-side 3L full bridge circuit 610.

[0105] The method for determining the relational expressions for calculating the phases γ and δ as functions of the current command value is the same as the method for determining the relational expression for calculating the phase β described in the first embodiment.

[0106] The basic operation of the control circuit 620 is also basically the same as that of the control circuit 250 of the first embodiment. However, unlike the first embodiment, the control circuit 620 differs from the control circuit 250 in that a relational equation conversion unit 630 calculates values ​​of not only the phases α and β but also the phases γ and δ as functions of the current command value, and that the number of PWM signals output by a 3L-3L PWM signal generation unit 632 to control each switching element of the secondary-side 3L full-bridge circuit 610 is greater than that of the PWM signal generation unit 266 of the first embodiment.

[0107] This modification also provides the same effect as the above embodiment in that the 3L-3L converter 600 can be controlled with a simple configuration and a small amount of calculation.

[0108] The above-described embodiments all relate to an NPC-type DAB-type isolated converter as shown in FIG. 2. However, this disclosure is not limited to such embodiments. For example, a diode-clamped primary-side 3L full-bridge circuit 660 may be used instead of the NPC-type primary-side 3L full-bridge circuit 110 shown in FIG. 2. FIG. 22 shows a circuit block diagram of a 3L-2L converter 650 according to such a modification.

[0109] 22, a 3L-2L converter 650 is obtained by replacing the primary 3L full bridge circuit 110 shown in FIG. 2 with a diode-clamped primary 3L full bridge circuit 660. Other than this, the components of the 3L-2L converter 650 are similar to those of the primary 3L full bridge circuit 110. The circuit configuration of the control circuit 250 is also similar to that of the first embodiment. The method of determining the relational expressions (functions) for calculating the phases α and β from the current command values ​​in the control circuit 250 is also similar to the method in the first embodiment.

[0110] As with the above-described embodiments, this 3L-2L converter 650 also has the effect of being able to practically control the 3L-2L converter 650 with a simple configuration and a small amount of calculation.

[0111] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present disclosure is not defined by the detailed description of the disclosure, but by the claims of the appended claims, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0112] S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, S11, S12 switching elements 50, 170, 172, 550, 560 Voltage waveform 52, 204, 552, 562, 572, 592 current waveform 54, 56, 350 areas 58, 60 Power loss 100, 650 3L-2L converter 102 Storage battery 104 DC bus 110, 660 Primary side 3L full bridge circuit 112 Secondary side full bridge circuit 114 Trans 116 Leakage Inductance 120, 122, 140 capacitors 124 Switching elements 126, 128, 130, 142, 144 nodes 200 Primary voltage 202 Secondary voltage 250, 400, 460, 500, 514, 620 control circuit 260 Subtractor 262, 512 PI control unit 264, 410 conversion unit 266, 412 PWM signal generation section 268, 630 Relational equation conversion section 300, 302, 304, 306, 308, 380, 382 curve 310, 312, 314, 316, 318 straight line 352 lines 470 Multiplication Circuit 510 Subtraction Circuit 570 Transformer voltage waveform 590 Gate-Source Voltage Waveform 600 3L-3L Converter 610 Secondary side 3L full bridge circuit 632 PWM signal generator for 3L-3L

Claims

1. A control circuit for controlling a power conversion device including a transformer, a primary-side three-level bridge circuit connected to a primary side of the transformer, and a secondary-side bridge circuit connected to a secondary side of the transformer, the control circuit performing power conversion by changing a phase difference φ between a phase of a voltage waveform applied to the transformer by the primary-side three-level bridge circuit and a phase of a voltage waveform applied to the transformer by the secondary-side bridge circuit, a feedback circuit that feedback-controls the phase difference φ based on a given current command value and a state value of the transformer; a timing calculation circuit that calculates two phases α and β, where 0<α<β<π, that determine the switching timing of each switching element of the primary side three-level bridge circuit in accordance with each relational expression for the current command value; a PWM signal generating circuit that generates a PWM signal for controlling switching elements of the primary-side three-level bridge circuit and the secondary-side bridge circuit using the phase difference φ and the phases α and β, Each switching element of the primary-side three-level bridge circuit has a first terminal, a second terminal, and a control terminal that controls conduction between the first terminal and the second terminal, The control circuit, wherein the relational expression is selected so that switching by each of the switching elements is soft switching.

2. 2. The control circuit according to claim 1, wherein a calculation period of the two phases α and β by the timing calculation circuit is longer than a feedback period by the feedback circuit.

3. 3. The control circuit according to claim 1, wherein the relational expressions include a first relational expression relating to the phase α and the current command value, and a second relational expression relating to the phase β and the current command value.

4. 4. The control circuit according to claim 3, wherein the first relational expression is a function of the current command value that uniquely determines the phase α for a certain range of the current command value.

5. 5. The control circuit according to claim 4, wherein the first relational expression is a strong monotonically decreasing function of the current command value within the certain range that determines the phase α.

6. 4. The control circuit according to claim 3, wherein the second relational expression is a function of the current command value that uniquely determines the phase β for a certain range of the current command value.

7. 7. The control circuit according to claim 6, wherein the second relational expression is a strong monotonically decreasing function of the current command value within the certain range, which determines the phase β.

8. 3. The control circuit according to claim 1, wherein the relational expression is selected to determine the phase α and the phase β so that the switching element is conductive while a direction of a current flowing through a body diode of each of the switching elements is opposite to a direction of a voltage applied to the first terminal and the second terminal when each of the switching elements is not conductive.

9. the primary side three-level bridge circuit functions to apply 0, ±V1 / 2, and ±V1 to the primary side of the transformer; the phase α determines the timing at which the voltage applied to the primary side of the transformer is changed from 0 to ±V1 / 2; 3. The control circuit according to claim 1, wherein a phase obtained by subtracting the phase α from the phase π determines a timing at which the voltage applied to the primary side of the transformer is changed from ±V1 / 2 to 0.

10. The phase β determines the timing at which the voltage applied to the primary side of the transformer is changed from ±V1 / 2 to ±V1, 10. The control circuit according to claim 9, wherein a phase obtained by subtracting the phase β from the phase π determines a timing for changing the voltage applied to the primary side of the transformer from the ±V1 to the ±V1 / 2.

11. 3. The control circuit according to claim 1, wherein the relational expression is a polynomial of the current command value.

12. The control circuit according to claim 11 , wherein the relational expression is a polynomial of the current command value of degree three or less.

13. 3. The control circuit according to claim 1, further comprising a current command value generating circuit that generates the current command value by feedback based on a voltage command value and a voltage applied to a primary side of the transformer.

14. the secondary-side bridge circuit includes a secondary-side three-level bridge circuit, The control circuit further comprises: a secondary-side timing calculation circuit that calculates two phases γ and δ, where 0<γ<δ<π, that determine the switching timing of each switching element of the secondary-side three-level bridge circuit in accordance with each relational expression for the current command value; The PWM signal generating circuit 3. The control circuit according to claim 1, further comprising a 3-level-3-level PWM signal generation circuit that controls switching elements of the primary-side three-level bridge circuit using the phase difference φ and the phases α and β, and controls switching elements of the secondary-side three-level bridge circuit using the phase difference φ and the phases γ and δ.

15. a control circuit according to claim 1 or claim 2; The transformer; a primary-side three-level bridge circuit connected to a primary side of the transformer, the primary-side three-level bridge circuit including a semiconductor switching element whose switching is controlled by the control circuit; a secondary-side circuit connected to the secondary side of the transformer.

16. a control circuit according to claim 14; The transformer; the primary-side three-level bridge circuit connected to the primary side of the transformer, the primary-side three-level bridge circuit including semiconductor switching elements whose switching is controlled by the three-level-three-level PWM signal generating circuit of the control circuit; a secondary-side three-level bridge circuit connected to the secondary side of the transformer, the secondary-side three-level bridge circuit including a semiconductor switching element whose switching is controlled by the three-level-three-level PWM signal generation circuit of the control circuit.

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