Neutral-point-clamped dual-active bridge converter and modulation method thereof

Abstract

The invention provides a neutral-point-clamped dual-active bridge converter and a modulation method thereof.The neutral-point-clamped dual-active bridge converter comprises a first switch group, a transformer, a second switch group and a working mode selection unit, the first switch group is connected with the main side of the transformer, the second switch group is connected with the secondary side of the transformer, and the working mode selection unit is connected with the main side of the transformer. And the working mode selection unit calculates and obtains the working state of each switch in the switch group I and the switch group II according to the normalized power P * and the voltage gain k, so that voltage matching is realized on two sides. By adjusting the working mode of the switch, the circuit works in the ZVS boundary diagram, so that the converter can work near a voltage matching point through the switching mode when the input voltage and the output voltage are not matched, and the efficiency of the converter is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of converters, and particularly relates to a neutral point clamped dual active bridge converter and its modulation method. Background Art

[0002] The switching mode of the neutral point clamped dual active bridge (NPC DAB) converter is flexible and is often applied to high-voltage power electronic converters. The traditional phase-shift modulation method is simple, but it has good soft-switching performance and high efficiency only when the input and output voltages match. When the voltages do not match, the loss is high. Summary of the Invention

[0003] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a neutral point clamped dual active bridge converter and its modulation method. The solution of the present invention can solve the problems existing in the above prior art.

[0004] The technical solution of the present invention:

[0005] According to the first aspect, a neutral point clamped dual active bridge converter is provided, including a first switch group, a transformer, a second switch group, and a working mode selection unit. The first switch group is connected to the primary side of the transformer, and the second switch group is connected to the secondary side of the transformer. The working mode selection unit calculates the working states of each switch in the first switch group and the second switch group according to the normalized power P* and the voltage gain k, so as to achieve voltage matching on both sides. The first switch group includes switches Q1, Q2, Q3, and Q4 connected in series in sequence, and switches Q5, Q6, Q7, and Q8 connected in series in sequence. The two sets of software switches after series connection are connected in parallel. One end of the primary side of the transformer is connected to the circuit between switches Q2 and Q3, and the other end of the primary side of the transformer is connected between switches Q6 and Q7. The second switch group includes switches S1, S2, S3, and S4 connected in series in sequence, and switches S5, S6, S7, and S8 connected in series in sequence. The two sets of software switches after series connection are connected in parallel. One end of the secondary side of the transformer is connected to the circuit between switches S2 and S3, and the other end of the secondary side of the transformer is connected between switches S6 and S7.

[0006] Further, the neutral point clamped dual active bridge converter further includes an inductor and an isolation capacitor. One end of the inductor is connected to switches Q2 and Q3, and the other end is connected to the positive pole of the primary side of the transformer. There are two isolation capacitors. One end of isolation capacitor C b1 is connected to the negative pole of the primary side of the transformer, and the other end is connected to switches Q6 and Q7. One end of isolation capacitor C b2 is connected to the negative pole of the secondary side of the transformer, and the other end is connected to switches S6 and S7.

[0007] Furthermore, the working mode selection unit includes a normalized power calculation component, a switching condition calculation unit, a ZVS boundary diagram drawing unit, and a mode selection unit. The switching condition calculation unit calculates the switching conditions and sends them to the ZVS boundary diagram drawing unit. The normalized power calculation component calculates the normalized power based on the transmission power, reference power, and voltage gain, and transmits the result to the ZVS boundary diagram drawing unit. The ZVS boundary diagram drawing unit draws the ZVS boundary diagram based on the obtained data, and the mode selection unit selects the working states of each switch to make the circuit operate within the ZVS boundary diagram.

[0008] Preferably, the switch is a MOSFET.

[0009] Furthermore, the working states of each switch include:

[0010] The mode MN state refers to the state where the primary side is in mode M and the secondary side is in mode N, where M = A, B, C, D; N = A, B, C, D;

[0011] Among them, mode A means that for two arms of the primary side or secondary side, in the first half cycle, the left arm is in the P state and the right arm is in the N state, and in the second half cycle, the left arm is in the N state and the right arm is in the P state;

[0012] Mode B means that for two arms of the primary side or secondary side, in the first half cycle, the left arm is in the P state and the right side is in the N state; in the second half cycle, the left side is in the N state and the right side is in the O state;

[0013] Mode C means that for two arms of the primary side or secondary side, in the first half cycle, the left arm is in the P state and the right side is in the O state; in the second half cycle, the left side is in the N state and the right side is in the O state;

[0014] Mode D means that for two arms of the primary side or secondary side, in the first half cycle, the left arm is in the P state and the right side is in the O state; in the second half cycle, the left side is in the O state and the right side is in the O state;

[0015] Taking the primary side as an example, when Q1 and Q2 are turned on and Q3 and Q4 are turned off, the arm voltage U AO1 = 0.5V i , this switching state is called the P state. When Q3 and Q4, Q1 and Q2 are turned off, UA O1 = -0.5V i , this switching mode is called the N state; when Q2 and Q3 are turned on and Q1 and Q4 are turned off, UA O1 = 0, this switching state is called the O state.

[0016] Further, when several operating states of the switch all satisfy that the circuit operates within the ZVS boundary diagram, the priority order of the selection modes is: Mode A > Mode B > Mode C > Mode D.

[0017] Further, if the operating states of the switch do not satisfy that the circuit operates within the ZVS boundary diagram, the operating mode is selected according to the normalized power and voltage gain in the vicinity.

[0018] Further, the expression of the normalized power is:

[0019] where k1 = U AE / V i , k2 = U CF / V o , k = nV o / V i , U AE is the primary voltage of the transformer, V i is the input voltage of the converter, U CF is the secondary voltage of the transformer, V o is the output voltage of the converter.

[0020] According to the second aspect, a modulation method for a neutral-point clamped dual-active bridge converter is provided, including the following steps:

[0021] Detect the input voltage and current, output voltage and current, and transmission power parameters of the converter, and calculate the normalized power P* and voltage gain k;

[0022] Calculate the ZVS boundary diagram based on the normalized power P* and voltage gain k;

[0023] Determine the operating state of the switch that satisfies the boundary condition according to the obtained ZVS boundary diagram;

[0024] Select the best operating state according to the selection method of the switch operating state to complete the modulation of the converter.

[0025] Further, the selection method of the switch operating state is: when several operating states of the switch all satisfy that the circuit operates within the ZVS boundary diagram, the priority order of the selection modes is: Mode A > Mode B > Mode C > Mode D; when only one operating state of the switch satisfies that the circuit operates within the ZVS boundary diagram, this operating state is adopted; when the operating states of the switch do not satisfy that the circuit operates within the ZVS boundary diagram, the operating mode is selected according to the normalized power and voltage gain in the vicinity.

[0026] The beneficial effects of the present invention compared with the prior art:

[0027] By adjusting the working mode of the switch, the present invention enables the circuit to operate within the ZVS boundary diagram, so that the converter can also operate near the voltage matching point by switching modes when the input and output voltages do not match, thereby improving the efficiency of the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The accompanying drawings included are used to provide a further understanding of the embodiments of the present invention, which form a part of the specification, illustrate the embodiments of the present invention, and together with the written description, explain the principles of the present invention. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0029] Figure 1 Shows the neutral-point clamped dual-active-bridge converter topology and control block diagram provided according to an embodiment of the present invention;

[0030] Figure 2 Shows the switching-mode voltage diagram provided according to an embodiment of the present invention;

[0031] Figure 3 Shows the main waveform diagram of the converter operation provided according to an embodiment of the present invention;

[0032] Figure 4 Shows the ZVS region where the converter operates in the AA mode provided according to an embodiment of the present invention.

[0033] Figure 5 Shows the ZVS boundary condition diagram of all working modes of the converter provided according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. The following description of at least one exemplary embodiment is actually only illustrative and in no way limits the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0035] It should be noted that the terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular forms are also intended to include the plural forms. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they specify the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0036] Unless otherwise specifically stated, the relative arrangements of components and steps, numerical expressions, and values set forth in these embodiments do not limit the scope of the present invention. At the same time, it should be understood that, for the sake of convenience of description, the dimensions of the various parts shown in the drawings are not drawn in actual proportional relationships. Technologies, methods, and devices known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, the said technologies, methods, and devices should be regarded as part of the authorized specification. In all the examples shown and discussed herein, any specific values should be construed as merely exemplary and not as a limitation. Therefore, other examples of the exemplary embodiments may have different values. It should be noted that like reference numerals and letters denote like items in the following drawings, and thus, once an item is defined in one drawing, it does not need to be further discussed in subsequent drawings.

[0037] As Figure 1 shown, according to an embodiment of the first aspect of the present invention, a midpoint-clamped dual-active-bridge converter is provided, which includes a first switch group, a transformer, a second switch group, and a working mode selection unit. The first switch group is connected to the primary side of the transformer, the second switch group is connected to the secondary side of the transformer, and the working mode selection unit calculates the working states of each switch in the first switch group and the second switch group according to the normalized power P* and the voltage gain k, so as to achieve voltage matching on both sides of the transformer; the first switch group includes switches Q1, Q2, Q3, and Q4 connected in series in sequence, switches Q5, Q6, Q7, and Q8 connected in series in sequence, and the two groups of software switches after being connected in series are connected in parallel. One end of the primary side of the transformer is connected to the circuit between switches Q2 and Q3, and the other end of the primary side of the transformer is connected between switches Q6 and Q7; the second switch group includes switches S1, S2, S3, and S4 connected in series in sequence, switches S5, S6, S7, and S8 connected in series in sequence, and the two groups of software switches after being connected in series are connected in parallel. One end of the secondary side of the transformer is connected to the circuit between switches S2 and S3, and the other end of the secondary side of the transformer is connected between switches S6 and S7.

[0038] By adjusting the working mode of the switches, the circuit works within the ZVS boundary diagram, so that the converter can also work near the voltage matching point by switching modes when the input and output voltages are mismatched, thereby improving the efficiency of the converter.

[0039] Preferably, in one embodiment, the switch is a MOSFET.

[0040] Further, in one embodiment, a midpoint-clamped dual-active-bridge converter further includes an inductor and an isolation capacitor. One end of the inductor is connected to switches Q2 and Q3, and the other end is connected to the positive pole of the primary side of the transformer. There are two isolation capacitors. One end of isolation capacitor C b1 is connected to the negative pole of the primary side of the transformer, and the other end is connected to switches Q6 and Q7. One end of isolation capacitor C b2 is connected to the negative pole of the secondary side of the transformer, and the other end is connected to switches S6 and S7.

[0041] Further, in one embodiment, the operating mode selection unit includes a normalized power calculation component, a switching condition calculation unit, a ZVS boundary diagram drawing unit, and a mode selection unit. The switching conditions are calculated by the switching condition calculation unit and sent to the ZVS boundary diagram drawing unit. The normalized power calculation component calculates the normalized power according to the transmitted power, the reference power, and the voltage gain, and transmits the result to the ZVS boundary diagram drawing unit. The ZVS boundary diagram drawing unit draws the ZVS boundary diagram based on the obtained data, and the mode selection unit selects the operating states of each switch to make the circuit operate within the ZVS boundary diagram. Through this setting, the operating mode of the switch is determined according to the normalized power and the voltage gain.

[0042] Further, in one embodiment, the operating states of each switch include:

[0043] Mode MN state means that the primary side is in mode M and the secondary side is in mode N, where M = A, B, C, D; N = A, B, C, D;

[0044] Among them, mode A means that for two arms of the primary side or the secondary side, in the first half cycle, the left arm is in the P state and the right arm is in the N state, and in the second half cycle, the left arm is in the N state and the right arm is in the P state;

[0045] Mode B means that for two arms of the primary side or the secondary side, in the first half cycle, the left arm is in the P state and the right side is in the N state; in the second half cycle, the left side is in the N state and the right side is in the O state;

[0046] Mode C means that for two arms of the primary side or the secondary side, in the first half cycle, the left arm is in the P state and the right side is in the O state; in the second half cycle, the left side is in the N state and the right side is in the O state;

[0047] Mode D means that for two arms of the primary side or the secondary side, in the first half cycle, the left arm is in the P state and the right side is in the O state; in the second half cycle, the left side is in the O state and the right side is in the O state;

[0048] Taking the original side as an example, when Q1 and Q2 are turned on and Q3 and Q4 are turned off, the arm voltage U AO1 = 0.5V i , this switching state is called the P state. When Q3 and Q4, Q1 and Q2 are turned off, UA O1 = -0.5V i , this switching mode is called the N state; when Q2 and Q3 are turned on and Q1 and Q4 are turned off, UA O1 = 0, this switching state is called the O state.

[0049] Further, in an embodiment, when several working states of the switch all satisfy that the circuit works within the ZVS boundary diagram, the priority order of the selection modes is: Mode A > Mode B > Mode C > Mode D.

[0050] Further, in an embodiment, if the working states of the switch do not all satisfy that the circuit works within the ZVS boundary diagram, the working mode is selected nearby according to the normalized power and voltage gain.

[0051]

[0052] where k1 = U AE / V i , k2 = U CF / V o , k = nV o / V i , U AE is the primary voltage of the transformer, V i is the input voltage of the converter, U CF is the secondary voltage of the transformer, V o is the output voltage of the converter.

[0053] According to the embodiments of the second aspect, a modulation method for a neutral point clamped dual active bridge converter is provided, including the following steps:

[0054] Detect the input voltage and current, output voltage and current, and transmission power parameters of the converter, and calculate the normalized power P* and voltage gain k;

[0055] Calculate the ZVS boundary diagram based on the normalized power P* and voltage gain k;

[0056] Determine the working state of the switch that satisfies the boundary condition according to the obtained ZVS boundary diagram;

[0057] Select the best working state according to the switch working state selection method to complete the modulation of the converter.

[0058] Further, in one embodiment, the method for selecting the switching operating state is as follows: when several operating states of the switch all satisfy that the circuit operates within the ZVS boundary diagram, the priority order of selection modes is: Mode A > Mode B > Mode C > Mode D; when only one operating state of the switch satisfies that the circuit operates within the ZVS boundary diagram, then this operating state is adopted; when none of the operating states of the switch satisfy that the circuit operates within the ZVS boundary diagram, the operating mode is selected according to the normalized power and voltage gain proximately.

[0059] In order to further elaborate on a neutral-point clamped dual-active-bridge converter and its modulation method according to the present invention, the present invention will be described in detail below with reference to the accompanying drawings.

[0060] The circuit topology is as Figure 1 shown. The topology of the NPC DAB is as Figure 1 shown, where L is the sum of the externally added inductor of the converter and the leakage inductance of the transformer, C b1 is the isolation capacitor on the primary side, C b2 is the isolation capacitor on the secondary side; n is the turns ratio of the transformer; V i is the input voltage of the DC-DC converter, and V o is the output voltage of the converter.

[0061] The neutral-point clamped bridge arm can output different levels through different switch combinations. There are two bridge arms on the primary side and two bridge arms on the secondary side. The arm voltage refers to the voltage difference between the output points A, B, C, D of the bridge arm and the voltage midpoint O1 or O2, that is, U AO1 , U BO1 , U CO2 , U DO2 . Here, U AO1 is used as an example. For example, when Q1 and Q2 are turned on and Q3 and Q4 are turned off, the arm voltage U AO1 = 0.5V i , and this switching state is called state P. When Q3 and Q4, Q1 and Q2 are turned off, UA O1 = -0.5V i , and this switching mode is called state N. When Q2 and Q3 are turned on and Q1 and Q4 are turned off, UA O1 = 0, and this switching state is called state O. U AO1 and U BO1 can each have three different levels. Then the voltage U AB between the two bridge arms will have multiple levels. By selecting a suitable combination of switching states, voltages of different amplitudes can be generated. The isolation capacitor will eliminate the DC component in the voltage, and the voltage amplitudes of U AE and U CF can be 0.25V i , 0.5V i, 0.75V i and V i , similarly for the secondary side. Voltage matching means that the voltages U AE and nU CF on both sides of the transformer are equal. At this time, the converter works in a state with higher efficiency and good switching conditions. For the traditional phase-shifted control method, the working mode will not be switched. Then, only when V i = nV o can voltage matching be achieved. By using mode switching, when the input and output voltages deviate from the traditional phase-shifted matching point, the converter can be made to work near the voltage matching point again by switching the working mode. The voltage waveforms of different switching modes are as shown in Figure 2 .

[0062] The main working waveforms of the NPC DAB converter are as shown in Figure 3 . i L(1) and i L(2) are the transformer current waveforms. When U AE > nU CF , the current shows the i L(1) state. When U AE < nU CF , the current shows the i L(2) state. T h represents half of the switching period. k1 = U AE / V i , k2 = U CF / V o , k = nV o / V i . k1 and k2 are used to represent the relationship between U AE , U CF and the input and output voltages. The voltage gain k represents the relationship between the input voltage and the output voltage.

[0063] According to the ZVS conditions of this topology, through mathematical derivation, the soft-switching conditions can be obtained. And using the soft-switching conditions, a ZVS boundary diagram related to the normalized power P* and the voltage gain k is drawn, as shown in Figure 4 . Similarly, the ZVS boundary for working in any mode is as shown in Figure 5 . The multiple working modes of the NPC DAB converter can almost cover all working states. By selecting the appropriate working mode for the NPC DAB and designing the converter parameters reasonably, this modulation strategy will greatly broaden the ZVS range of the converter, thereby promoting the improvement of efficiency. In one embodiment, the solution process of the normalized power P* is as follows:

[0064] The transformer current is

[0065]

[0066]

[0067] The transmitted power can be expressed as

[0068]

[0069] The reference power is set as

[0070]

[0071] The normalized power is expressed as

[0072]

[0073] If the transformer current is negative at time t0, the energy stored in the transformer will charge or discharge the parasitic capacitance of the MOSFET in the converter, and zero-voltage turn-on can be achieved in this case. This situation can be expressed as

[0074]

[0075] C p is the parasitic capacitance of each MOSFET, and a i is a coefficient. Combining formula (3), (4) and formula (9), the boundary condition for zero-voltage turn-on on the primary side can be expressed as

[0076]

[0077] The condition for zero-voltage turn-on on the secondary side can be obtained in the same way

[0078]

[0079]

[0080] Substituting formula (8) and formula (9) into formula (5), the zero-voltage turn-on condition can be expressed in another form:

[0081]

[0082]

[0083] The parasitic capacitance of each MOSFET is different, and for the same MOSFET operating under different voltage and current conditions, the parasitic capacitance is also different. To simplify the analysis and reduce the computational load of the chip, the parasitic capacitance is ignored.

[0084]

[0085]

[0086] According to Equation (12) and Equation (13), the ZVS boundary conditions of the NPC DAB converter operating in the AA mode are as follows Figure 4 shown.

[0087] The converter samples and detects the input voltage, current, output voltage, current, and transmission power parameters, and determines the operating mode that can achieve ZVS based on the power and voltage. If there is only one operating mode of the converter that can achieve bilateral switching, then this mode is selected. If, under the same operating condition, multiple operating modes can achieve ZVS, the mode priority from high to low is ABCD. For example, under a certain operating condition, both mode AA and mode AB can achieve ZVS of the primary and secondary MOSFETs, then the converter preferentially selects AA. If there are extremely rare cases where the converter operates within a range where multiple operating modes cannot achieve ZVS, the operating mode is selected nearby according to P* and k.

[0088] The beneficial effects of the present invention compared with the prior art are as follows

[0089] By adjusting the operating mode of the switch, the present invention enables the circuit to operate within the ZVS boundary diagram, so that the converter can also operate near the voltage matching point by switching modes when the input and output voltages do not match, thereby improving the efficiency of the converter.

[0090] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A midpoint-clamped dual-active-bridge converter, characterized in that, It includes Switch Group 1, a transformer, Switch Group 2 and a working mode selection unit. Switch Group 1 is connected to the primary side of the transformer, and Switch Group 2 is connected to the secondary side of the transformer. The working mode selection unit calculates the working states of each switch in Switch Group 1 and Switch Group 2 based on the normalized power P* and the voltage gain k to achieve voltage matching on both sides. Switch Group 1 includes switches Q1, Q2, Q3, and Q4 connected in series in sequence, and switches Q5, Q6, Q7, and Q8 connected in series in sequence. The two groups of software switches after series connection are in parallel. One end of the primary side of the transformer is connected to the circuit between switches Q2 and Q3, and the other end of the primary side of the transformer is connected between switches Q6 and Q7. Switch Group 2 includes switches S1, S2, S3, and S4 connected in series in sequence, and switches S5, S6, S7, and S8 connected in series in sequence. The two groups of software switches after series connection are in parallel. One end of the secondary side of the transformer is connected to the circuit between switches S2 and S3, and the other end of the secondary side of the transformer is connected between switches S6 and S7.

2. The dual-active-bridge converter with midpoint clamping according to claim 1, wherein The described midpoint-clamped dual-active-bridge converter further includes an inductor and isolation capacitors. One end of the inductor is connected to switches Q2 and Q3, and the other end is connected to the positive pole of the primary side of the transformer. There are two isolation capacitors. One end of isolation capacitor C b1 is connected to the negative pole of the primary side of the transformer, and the other end is connected to switches Q6 and Q7. One end of isolation capacitor C b2 is connected to the negative pole of the secondary side of the transformer, and the other end is connected to switches S6 and S7.

3. The dual-active-bridge converter with midpoint clamping according to claim 2, wherein, The working mode selection unit includes a normalized power calculation component, a switch condition calculation unit, a ZVS boundary diagram drawing unit, and a mode selection unit. The switch condition calculation unit calculates the switch conditions and sends them to the ZVS boundary diagram drawing unit. The normalized power calculation component calculates the normalized power based on the transmission power, the reference power, and the voltage gain and transmits the result to the ZVS boundary diagram drawing unit. The ZVS boundary diagram drawing unit draws the ZVS boundary diagram based on the obtained data. The mode selection unit selects the working states of each switch to make the circuit work within the ZVS boundary diagram.

4. A midpoint-clamped dual-active-bridge converter according to claim 3, characterized in that, The switch is a MOSFET.

5. A neutral-point clamped dual-active-bridge converter according to claim 4, characterized in that, The working states of each switch include: The mode MN state refers to the state where the primary side is in mode M and the secondary side is in mode N, where M = A, B, C, D; N = A, B, C, D; Among them, mode A means that for two bridge arms on the primary side or the secondary side, in the first half cycle, the left bridge arm is in the P state and the right bridge arm is in the N state, and in the second half cycle, the left bridge arm is in the N state and the right bridge arm is in the P state; Mode B means that for two bridge arms on the primary side or the secondary side, in the first half cycle, the left bridge arm is in the P state and the right side is in the N state; in the second half cycle, the left side is in the N state and the right side is in the O state; Mode C means that for two bridge arms on the primary side or the secondary side, in the first half cycle, the left bridge arm is in the P state and the right side is in the O state; in the second half cycle, the left side is in the N state and the right side is in the O state; Mode D means that for two bridge arms on the primary side or the secondary side, in the first half cycle, the left bridge arm is in the P state and the right side is in the O state; in the second half cycle, the left side is in the O state and the right side is in the O state; Taking the original side as an example, when Q1 and Q2 are turned on and Q3 and Q4 are turned off, the arm voltage UAO1 = 0.5Vi. This switching state is called the P state. When Q3 and Q4, Q1 and Q2 are turned off, UAO1 = -0.5Vi. This switching mode is called the N state; when Q2 and Q3 are turned on and Q1 and Q4 are turned off, UAO1 = 0. This switching state is called the O state.

6. The dual-active-bridge converter with midpoint clamping according to claim 5, wherein When several working states of the switch all meet the condition that the circuit operates within the ZVS boundary diagram, the priority order of the selection modes is: Mode A > Mode B > Mode C > Mode D.

7. A midpoint-clamped dual-active-bridge converter according to claim 6, wherein If the working states of the switch do not all meet the condition that the circuit operates within the ZVS boundary diagram, select the working mode according to the normalized power and voltage gain in the vicinity.

8. A neutral-point clamped dual-active-bridge converter according to claim 7, wherein, The expression for the normalized power is as follows: Among them, k1 = UAE / Vi, k2 = UCF / Vo, k = nVo / Vi. UAE is the primary voltage of the transformer, Vi is the input voltage of the converter, UCF is the secondary voltage of the transformer, and Vo is the output voltage of the converter.

9. A modulation method for a midpoint-clamped dual-active-bridge converter using the midpoint-clamped dual-active-bridge converter according to any one of claims 1 to 8, characterized in that, It includes the following steps: Detect the input voltage and current, output voltage and current, and transmission power parameters of the converter, and calculate the normalized power P* and the voltage gain k; Calculate the ZVS boundary diagram based on the normalized power P* and the voltage gain k; Determine the working state of the switch that meets the boundary condition according to the obtained ZVS boundary diagram; Select the best working state according to the selection method of the switch working state to complete the modulation of the converter.

10. The modulation method of a midpoint-clamped dual-active-bridge converter according to claim 9, characterized in that, The selection method of the switch working state is as follows: when several working states of the switch all meet the condition that the circuit operates within the ZVS boundary diagram, the priority order of the selection modes is: Mode A > Mode B > Mode C > Mode D; when only one working state of the switch meets the condition that the circuit operates within the ZVS boundary diagram, then adopt this working state; when the working states of the switch do not all meet the condition that the circuit operates within the ZVS boundary diagram, select the working mode according to the normalized power and voltage gain in the vicinity.

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