Control method and device of micro-inverter, and micro-inverter

Abstract

The invention discloses a control method and device of a micro-inverter and the micro-inverter, and belongs to the technical field of inverters. The method comprises the following steps: in a first working mode, for any two branches in a secondary bridge arm, if an absolute value of a resonance current between a transformer and the secondary bridge arm is smaller than a current threshold value when a pulse control signal generates voltage jump, controlling each switch tube in the two branches to keep an on-off state; until the absolute value of the resonance current reaches a current threshold value, each switching tube in the two branches is controlled to change the on-off state; if the absolute value of the resonance current is greater than or equal to the current threshold value when the voltage jumps, each switching tube in the two branches is controlled to change the on-off state; the action sequence of each switch tube is controlled according to the direction of the resonance current in the switching process; in the first working mode, at least two switching tubes with opposite directions in the two branches are turned off at the same moment, and resonant current flows through one branch at any moment. According to the invention, the reliability of the micro-inverter can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of inverters, and in particular, to a control method and device for a micro-inverter, and a micro-inverter. Background Art

[0002] A micro photovoltaic grid-connected inverter, referred to as a micro-inverter, is a power conversion unit based on an independent photovoltaic module, which can modularize the power conversion process and achieve maximum power point tracking for a single module. Since each single module is connected by a DC-AC converter that performs maximum power point tracking and system integration, the system composed of micro-inverters can exhibit higher reliability, and the failure of any inverter module can be treated as an individual failure without affecting the overall system. The secondary arm of the micro-inverter includes at least two groups of bidirectional switches, which are respectively connected to different output terminals of the micro-inverter. The switching of the bidirectional switches needs to be judged according to the polarity of the voltage at the output terminal of the micro-inverter. However, both software and hardware switching have a certain delay. When the grid voltage is at the zero-crossing stage or fluctuates and the polarity reverses at a relatively high voltage, due to sampling delay or inaccurate polarity judgment, the bidirectional switch tubes may not have time to make real-time adjustments, resulting in a direct connection phenomenon between the arms between the two output terminals, causing a sharp rise in the arm current and burning out the bidirectional switch tubes. Summary of the Invention

[0003] The present invention provides a control method and device for a micro-inverter, and a micro-inverter, so as to avoid the direct connection phenomenon of the secondary arm and ensure that there is always a freewheeling path for the resonant current, thereby improving the reliability of the micro-inverter.

[0004] In a first aspect, an embodiment of the present invention provides a control method for a micro-inverter. The micro-inverter includes: a transformer and a secondary arm; the secondary arm includes: at least two branches, all connected to the same end of the secondary winding of the transformer and respectively corresponding to different output terminals of the micro-inverter; each of the branches includes two switch tubes connected in series with opposite directions.

[0005] The control method of the micro-inverter includes: a first working mode.

[0006] In the first working mode, for any two of the branches in the secondary arm:

[0007] When the absolute value of the resonant current between the transformer and the secondary side bridge arm is less than the current threshold when the pulse control signal undergoes a voltage jump, control each of the switching tubes in the two branches to maintain their on / off states unchanged until the absolute value of the resonant current reaches the current threshold, and then control each of the switching tubes in the two branches to change their on / off states; wherein, before the voltage jump of the pulse control signal, the two switching tubes in one of the branches are both conducting, and the two switching tubes in the other branch are both off; the voltage jump of the pulse control signal is used to control the switching of the branch through which the resonant current flows.

[0008] When the absolute value of the resonant current is greater than or equal to the current threshold when the pulse control signal undergoes a voltage jump, control each of the switching tubes in the two branches to change their on / off states.

[0009] Wherein, during the process of controlling each of the switching tubes in the two branches to change their on / off states, control the action sequence of each of the switching tubes according to the direction of the resonant current; in the first mode, at least two switching tubes with opposite directions in the two branches are off at the same time, and at any moment the resonant current flows through one of the branches.

[0010] Optionally, controlling each of the switching tubes in the two branches to change their on / off states includes:

[0011] Sequentially control the first target switching tube to turn off, the second target switching tube to turn on, the third target switching tube to turn off, and the fourth target switching tube to turn on;

[0012] Wherein, the first target switching tube is the switching tube in the first target branch whose freewheeling direction of the body diode is the same as the direction of the resonant current, the second target switching tube is the switching tube in the second target branch whose freewheeling direction of the body diode is opposite to the direction of the resonant current, the third target switching tube is the switching tube in the first target branch whose freewheeling direction of the body diode is opposite to the direction of the resonant current, and the fourth target switching tube is the switching tube in the second target branch whose freewheeling direction of the body diode is the same as the direction of the resonant current; in the two branches, the first target branch is the branch through which the resonant current flows before the voltage jump of the pulse control signal, and the second target branch is the other branch.

[0013] Optionally, the two branches include: a first branch and a second branch, which are respectively connected to the first output terminal and the second output terminal of the micro-inverter; the first branch includes a first switching tube and a second switching tube, and the second branch includes a third switching tube and a fourth switching tube; the first switching tube, the second switching tube, the third switching tube, and the fourth switching tube are sequentially connected in series between the first output terminal and the second output terminal, and the directions of adjacent two switching tubes are opposite, and the connection node between the second switching tube and the third switching tube is connected to the secondary winding of the transformer;

[0014] When the voltage difference between the first output terminal and the second output terminal is positive:

[0015] If the resonant current flows from the transformer to the secondary bridge arm, the first target branch is the second branch, the second target branch is the first branch, the first target switching tube is the fourth switching tube, the second target switching tube is the second switching tube, the third target switching tube is the third switching tube, and the fourth target switching tube is the first switching tube;

[0016] If the resonant current flows from the secondary bridge arm to the transformer, the first target branch is the first branch, the second target branch is the second branch, the first target switching tube is the second switching tube, the second target switching tube is the fourth switching tube, the third target switching tube is the first switching tube, and the fourth target switching tube is the third switching tube;

[0017] When the voltage difference between the first output terminal and the second output terminal is negative:

[0018] If the resonant current flows from the transformer to the secondary bridge arm, the first target branch is the first branch, the second target branch is the second branch, the first target switching tube is the first switching tube, the second target switching tube is the third switching tube, the third target switching tube is the second switching tube, and the fourth target switching tube is the fourth switching tube;

[0019] If the resonant current flows from the secondary bridge arm to the transformer, the first target branch is the second branch, the second target branch is the first branch, the first target switching tube is the third switching tube, the second target switching tube is the first switching tube, the third target switching tube is the fourth switching tube, and the fourth target switching tube is the second switching tube.

[0020] Optionally, the control method of the micro-inverter further includes:

[0021] When the voltage difference between the output terminals connected by the two branches is within the voltage range between the negative voltage threshold and the positive voltage threshold, the first working mode is adopted;

[0022] When the voltage difference between the output terminals connected by the two branches is outside the voltage range between the negative voltage threshold and the positive voltage threshold, the second working mode is adopted; wherein, in the second working mode, according to the positive and negative of the voltage difference between the output terminals connected by the two branches and the pulse control signal, the on-off states of the switching tubes in the two branches are controlled, and at most three switching tubes in the two branches are conducting at the same time.

[0023] Optionally, the two branches include: a first branch and a second branch, which are respectively connected to the first output terminal and the second output terminal of the micro-inverter; the first branch includes a first switching tube and a second switching tube; the second branch includes: a third switching tube and a fourth switching tube;

[0024] The second working mode includes:

[0025] When the voltage difference between the first output terminal and the second output terminal is positive, control the second switching tube and the fourth switching tube to conduct, and control the first switching tube and the third switching tube to conduct alternately according to the pulse control signal;

[0026] When the voltage difference between the first output terminal and the second output terminal is negative, control the first switching tube and the third switching tube to conduct, and control the second switching tube and the fourth switching tube to conduct alternately according to the pulse control signal.

[0027] Optionally, the current threshold is positively correlated with the absolute value of the voltage difference amplitude between the output terminals connected by the two branches.

[0028] In a second aspect, an embodiment of the present invention further provides a control device for a micro-inverter, including: a first control module for controlling the micro-inverter in the first working mode; wherein, in the first working mode, for any two branches in the secondary bridge arm of the micro-inverter, the first control module is used for:

[0029] When the absolute value of the resonant current between the transformer and the secondary bridge arm in the micro-inverter is less than the current threshold when the pulse control signal has a voltage jump, control each switch tube in the two branches to maintain its on / off state unchanged until the absolute value of the resonant current reaches the current threshold, and then control each switch tube in the two branches to change its on / off state; wherein, before the voltage jump of the pulse control signal, two switch tubes in one of the branches are both conducting, and two switch tubes in the other branch are both off; the voltage jump of the pulse control signal is used to control the switching of the branch through which the resonant current flows.

[0030] When the absolute value of the resonant current is greater than or equal to the current threshold when the pulse control signal has a voltage jump, control each switch tube in the two branches to change its on / off state.

[0031] Wherein, during the process of controlling each switch tube in the two branches to change its on / off state, control the action sequence of each switch tube according to the direction of the resonant current; in the first mode, at least two switch tubes with opposite directions in the two branches are off at the same time, and at any moment the resonant current flows through one of the branches.

[0032] Optionally, the first control module is used to control the micro-inverter in the first working mode when the voltage difference between the output terminals connected by the two branches is within the voltage range between the negative voltage threshold and the positive voltage threshold.

[0033] The control device of the micro-inverter further includes: a second control module, configured to control the micro-inverter in a second working mode when the voltage difference between the output terminals connected by the two branches is outside the voltage range between the negative voltage threshold and the positive voltage threshold; wherein, in the second working mode, the second control module is configured to control the on / off states of each switch tube in the two branches according to the positive and negative of the voltage difference between the output terminals connected by the two branches and the pulse control signal, and at most three switch tubes in the two branches are conducting at the same time.

[0034] In a third aspect, an embodiment of the present invention further provides a micro-inverter, including: a primary bridge arm, a transformer, a secondary bridge arm, and a controller.

[0035] The primary side bridge arms are respectively connected to the input end of the micro-inverter and the primary side winding of the transformer, and the secondary side bridge arms are respectively connected to the secondary side winding of the transformer and the output ends of the micro-inverter; wherein, the secondary side bridge arm includes: at least two branches, all connected to the same end of the secondary side winding of the transformer, and respectively corresponding to different output ends of the micro-inverter; each of the branches includes two switching tubes connected in series with opposite directions; the controller is connected to the control poles of the primary side bridge arm and each of the switching tubes; the controller is used to execute the control method of the micro-inverter provided in any embodiment of the present invention.

[0036] Optionally, the micro-inverter includes one such secondary side bridge arm and one capacitor bridge arm;

[0037] Each of the branches in the secondary side bridge arm is connected to the first end of the secondary side winding of the transformer;

[0038] The capacitor bridge arm includes: capacitors with the same number as the branches, the first ends of each of the capacitors are all connected to the second end of the secondary side winding of the transformer, and the second ends of each of the capacitors are respectively corresponding to different output ends of the micro-inverter;

[0039] Or,

[0040] The micro-inverter includes two such secondary side bridge arms; each of the branches in one secondary side bridge arm is connected to the first end of the secondary side winding of the transformer, and each of the branches in the other secondary side bridge arm is connected to the second end of the secondary side winding of the transformer.

[0041] In the control method of the micro-inverter provided by the embodiment of the present invention, by setting the first working mode, when the absolute value of the resonant current is less than the current threshold, each switching tube is controlled to maintain its state and the switching tube is not allowed to act, which is equivalent to setting an absolute safety area to avoid incorrect polarity judgment when the absolute value of the resonant current is small, resulting in the switching tube acting in the wrong order; after the resonant current passes through zero and the absolute value reaches the set current threshold, then according to the direction of the resonant current, each switching tube is controlled to perform correct switching according to the corresponding action sequence. Such a setting is equivalent to controlling each switching tube to act in the correct order on the basis of ensuring the correct direction of the determined resonant current, so as to ensure that the resonant current can still flow through one of the branches during the branch switching process, thereby ensuring that there is a freewheeling path for the resonant current at any time in the first working mode and avoiding overvoltage of the switching tube. And, in the first mode, at least two switching tubes with opposite directions in the two branches are turned off at the same time, and at most two switching tubes are turned on, which can effectively avoid the phenomenon of bridge arm direct connection and avoid overcurrent burning of the switching tube. Therefore, the embodiment of the present invention can effectively improve the reliability of the micro-inverter.

[0042] It should be understood that the content described in this section is not intended to identify the key or important features of the embodiments of the present invention, nor is it used to limit the scope of the present invention. Other features of the present invention will become readily apparent from the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the accompanying drawings required for the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0044] Figure 1 is a schematic structural diagram of a micro-inverter provided by an embodiment of the present invention;

[0045] Figure 2 is a schematic flowchart of a control method of a micro-inverter provided by an embodiment of the present invention;

[0046] Figure 3 is a schematic signal timing diagram in the first working mode provided by an embodiment of the present invention;

[0047] Figure 4 is another schematic signal timing diagram in the first working mode provided by an embodiment of the present invention;

[0048] Figure 5 is a schematic diagram of the action sequence of switching tubes in the first working mode provided by an embodiment of the present invention;

[0049] Figure 6 is another schematic diagram of the action sequence of switching tubes in the first working mode provided by an embodiment of the present invention;

[0050] Figure 7 is yet another schematic diagram of the action sequence of switching tubes in the first working mode provided by an embodiment of the present invention;

[0051] Figure 8 is still another schematic diagram of the action sequence of switching tubes in the first working mode provided by an embodiment of the present invention;

[0052] Figure 9 is a schematic signal timing diagram in the second working mode provided by an embodiment of the present invention;

[0053] Figure 10 is another schematic signal timing diagram in the second working mode provided by an embodiment of the present invention;

[0054] Figure 11 is a schematic diagram of working mode switching provided by an embodiment of the present invention;

[0055] Figure 12 It is a schematic structural diagram of a secondary side bridge arm provided by an embodiment of the present invention;

[0056] Figure 13 It is a simulation topology structure diagram of a micro-inverter provided by an embodiment of the present invention;

[0057] Figure 14 It is a simulation result diagram of grid-connected voltage and grid-connected current provided by an embodiment of the present invention;

[0058] Figure 15 It is a simulation result diagram when the output voltage polarity is positive in the first working mode provided by an embodiment of the present invention;

[0059] Figure 16 It is Figure 15 an enlarged view of area AA in

[0060] Figure 17 It is Figure 15 an enlarged view of area BB in

[0061] Figure 18 It is a simulation result diagram when the output voltage polarity is positive in the second working mode provided by an embodiment of the present invention;

[0062] Figure 19 It is a simulation result diagram when the output voltage polarity is negative in the first working mode provided by an embodiment of the present invention;

[0063] Figure 20 It is Figure 19 an enlarged view of area CC in

[0064] Figure 21 It is Figure 19 an enlarged view of area DD in

[0065] Figure 22 It is a simulation result diagram when the output voltage polarity is negative in the second working mode provided by an embodiment of the present invention;

[0066] Figure 23 It is a schematic structural diagram of another secondary side bridge arm provided by an embodiment of the present invention;

[0067] Figure 24 It is a schematic structural diagram of yet another secondary side bridge arm provided by an embodiment of the present invention. Detailed implementation manners

[0068] To enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0069] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and do not necessarily need to describe a specific order or sequence. It should be understood that such data can be interchanged under appropriate circumstances so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusion.

[0070] The embodiment of the present invention provides a control method for a micro-inverter to improve the reliability of the micro-inverter. To facilitate the explanation of this control method, the topological structure of the micro-inverter will be briefly described below. Figure 1 is a schematic structural diagram of a micro-inverter provided by an embodiment of the present invention. Refer to Figure 1 , the micro-inverter 100 may be a single-stage micro-inverter, and its topological structure includes a primary bridge arm 10, a transformer 20 (such as a high-frequency transformer), and a secondary bridge arm 30. Among them, the primary bridge arm 10 is respectively connected to the photovoltaic module PV and the primary winding of the transformer 20, and the secondary bridge arm 30 is respectively connected to the secondary winding of the transformer 20 and each output terminal of the micro-inverter 100. Each output terminal of the micro-inverter 100 is connected to the power grid Vg, for example. Multiple switching tubes may be included in both the primary bridge arm 10 and the secondary bridge arm 30. Figure 1 According to the characteristics of the transformer 20, the leakage inductance L of the transformer 20 is also equivalently drawn in k and the exciting inductance L m ; among them, the leakage inductance L of the transformer k can be used as a resonant inductance. During the operation of the micro-inverter 100, a resonant current i will appear on the inductor (such as the leakage inductance L of the transformer k ) between the transformer 20 and the secondary bridge arm 30 s . The direction (or polarity) of the resonant current i s can be specified as follows: the direction of the resonant current i s refers to its transmission direction between the transformer 20 and the secondary bridge arm 30. Taking the resonant current i sThe flow from the transformer 20 to the secondary bridge arm 30 is positive, and the flow from the secondary bridge arm 30 to the transformer 20 is negative. The micro-inverter 100 may further include a controller for controlling the on / off states of the switching tubes in the primary bridge arm 10 and the secondary bridge arm 30. For example, the controller may generate control signals for the switching tubes in the secondary bridge arm 30 according to the pulse control signals output by the modulation module; the pulse control signals are used to control the phase shift angles of the secondary switching tubes, and the pulse control signals may specifically be pulse signals with a duty cycle of 50%.

[0071] During the operation of the inverter, due to factors such as sampling delay or inaccurate judgment of the output voltage polarity / resonant current direction, the situation of direct conduction of the secondary bridge arm 30 may occur, causing a short circuit at the output end of the micro-inverter and burning out the switching tubes, or there may be no freewheeling path between the transformer 20 and the secondary bridge arm 30, resulting in the resonant current i s not being able to flow and causing overvoltage damage to the switching tubes.

[0072] To solve the above problems, an embodiment of the present invention proposes a control method for a micro-inverter, which can ensure that the secondary bridge arm 30 does not have a direct conduction situation at any time, and there is a freewheeling path for the resonant current i s at any time. This control method mainly involves the on / off control strategy of the switching tubes in the secondary bridge arm 30, and the following is a specific description of this control method.

[0073] Specifically, the secondary bridge arm 30 includes: at least two branches, all connected to the same end of the secondary winding of the transformer 20 and respectively corresponding to different output ends of the micro-inverter 100. For example, the first ends of the branches are all connected to the same end of the secondary winding of the transformer 20, and the second ends of the branches are respectively corresponding to different output ends of the micro-inverter 100. Figure 1 Two branches are exemplarily given, namely the first branch 31 and the second branch 32, and the two branches are respectively connected to the first output end L1 and the second output end L2 of the micro-inverter 100. Each branch includes two switching tubes connected in series with opposite directions, for example, connected in series between the first end and the second end of the branch. In other words, any branch is a bidirectional switch branch composed of a pair of top-to-bottom switching tubes, and the directions of the body diodes of the two switching tubes in the same branch are opposite, for example, the anodes of the two body diodes are connected to each other. The two switching tubes connecting the secondary winding of the transformer 20 in any two branches are also two switching tubes with opposite directions.

[0074] The control method of the micro-inverter includes: a first operating mode. In the first operating mode, for any two branches in the secondary bridge arm 30, the on / off control of the switching tubes can be performed using Figure 2 the shown process. Specifically, referring to Figure 2 , for any two branches in the secondary bridge arm 30, the first operating mode includes:

[0075] S110. If, when a voltage jump occurs in the pulse control signal, the absolute value of the resonant current between the transformer and the secondary bridge arm is less than the current threshold, control each switch in the two branches to maintain its on / off state unchanged until the absolute value of the resonant current reaches the current threshold, and then control each switch in the two branches to change its on / off state.

[0076] Among them, the voltage jump of the pulse control signal is used to control the branch for switching the flowing resonant current. For example, for the first branch 31 and the second branch 32, if the resonant current i s flows through the first branch 31 before the voltage jump of the pulse control signal, then after the voltage jump of the pulse control signal, the resonant current i s needs to be switched to flow through the second branch 32, and vice versa. In the first operating mode, before the voltage jump occurs in the pulse control signal, or rather, during most of the time intervals when the level of the pulse control signal remains constant, two switches in one of the branches are both turned on to allow the resonant current i s to flow, and two switches in the other branch are both turned off. Among them, a switch maintaining its on / off state unchanged means that the level of the control signal transmitted to the control electrode of the switch remains unchanged, so that the switch maintains the on / off state when the voltage jump occurs in the pulse control signal. Controlling a switch to change its on / off state means controlling the currently conducting switch to switch to the off state and controlling the currently off switch to switch to the on state.

[0077] Taking the control of the first branch 31 and the second branch 32 as an example, the control timing of the control signals of each switch in these two branches can be seen in Figure 3 . Specifically, the first branch 31 may include a first switch Q1 and a second switch Q2. The control electrode of the first switch Q1 is connected to the first control signal S 1H , and the control electrode of the second switch Q2 is connected to the second control signal S 1L ; the second branch 32 may include a third switch Q3 and a fourth switch Q4. The control electrode of the third switch Q3 is connected to the third control signal S 2L , and the control electrode of the fourth switch Q4 is connected to the fourth control signal S 2H . Figure 3 shows the waveform of the resonant current i s changing with time, as well as the waveforms of the control signals of the four switches changing with time t. The two red lines respectively correspond to the current magnitudes when the absolute values of the positive and negative currents are equal to the current threshold. Among them, the positive current threshold is represented by i + , and the negative current threshold is represented by i - ; each control signal is shown with the high level as the conducting level and the low level as the cut-off level as an example. See Figure 3, the dotted line indicates the voltage jump moment of the pulse control signal. It can be seen that at times t1, t2, and t3, the absolute value of the resonant current i s is less than the current threshold. For example, when the resonant current i s is positive, it is less than the positive current threshold i + or when it is negative, it is greater than the negative current threshold i - . Therefore, at the voltage jump moment of the above pulse control signal, all four control signals are controlled to maintain the level before the voltage jump of the pulse control signal, so that all four switching tubes remain in their current on / off states without acting (specifically, see the gray shaded part), until the absolute value of the resonant current i s reaches the current threshold for the first time after this voltage jump moment, and then the four switching tubes are controlled to act according to the established action sequence, and the resonant current i s is orderly switched to flow on the other branch. It can be understood that when switching branches, there is an established action sequence for the actions of the four switching tubes. Therefore, when controlling the actions of each switching tube, it is necessary to ensure that the action sequence of each switching tube remains unchanged and their action times are uniformly delayed. Exemplarily, the action sequence of the four switching tubes is related to the direction of the resonant current and can also be related to the positive and negative of the voltage between the two output terminals connected by the two branches.

[0078] S120. If the absolute value of the resonant current is greater than or equal to the current threshold when the voltage of the pulse control signal jumps, then control the on / off states of the switching tubes in both branches to change.

[0079] Still taking the control of the first branch 31 and the second branch 32 as an example, in this case, the control timing of the control signals of the switching tubes in both branches can be seen in Figure 4 . See Figure 4 , the dotted line indicates the voltage jump moment of the pulse control signal. It can be seen that at times t4, t5, and t6, the absolute value of the resonant current is greater than the current threshold. For example, when the resonant current i s is positive, it is greater than the positive current threshold i + or when it is negative, it is less than the negative current threshold i - . Therefore, at the voltage jump moment of the above pulse control signal, the four switching tubes can be immediately controlled to act according to the established action sequence, and the resonant current i s is orderly switched to flow on the other branch.

[0080] Among them, in the first working mode, whether it is for the situation shown in Figure 3 or the situation shown in Figure 4 , at least two switching tubes with opposite directions in the two branches are turned off at the same time, that is, at most two switching tubes are turned on at the same time; and at any moment, the resonant current i sFlow through one of the two branches. Taking the first branch 31 and the second branch 32 as an example, the two switching tubes with opposite directions can be: the first switching tube Q1 and the second switching tube Q2, the second switching tube Q2 and the third switching tube Q3, the third switching tube Q3 and the fourth switching tube Q4, and the fourth switching tube Q4 and the first switching tube Q1. Specifically, in the process of controlling the on-off states of each of the switching tubes in the two branches to change, the action sequence of each switching tube can be controlled according to the direction of the resonant current i s so as to ensure that during the switching process, at any moment, the resonant current i s can flow through a conducting switching tube and the body diode of another switching tube in one of the branches; before and after the on-off state change, the resonant current i s can flow through two conducting switching tubes in one of the branches.

[0081] In the control method of the micro-inverter provided by the embodiment of the present invention, by setting the first working mode, when the absolute value of the resonant current is less than the current threshold, each switching tube is controlled to maintain its state and is not allowed to act, which is equivalent to setting an absolute safety area to avoid the situation of incorrect polarity judgment when the absolute value of the resonant current is small, resulting in the switching tubes acting in the wrong order; after the resonant current passes through zero and its absolute value reaches the set current threshold, then according to the direction of the resonant current, each switching tube is controlled to perform correct switching according to the corresponding action sequence. Such a setting is equivalent to controlling each switching tube to act in the correct order on the basis of ensuring the correct direction of the determined resonant current, so as to ensure that the resonant current can still flow through one of the branches during the branch switching process, thereby ensuring that there is a freewheeling path for the resonant current at any moment in the first working mode and avoiding overvoltage of the switching tubes. And, in the first mode, at least two switching tubes with opposite directions in the two branches are turned off at the same time, and at most two switching tubes are turned on, which can effectively avoid the phenomenon of bridge arm direct connection and avoid overcurrent burning of the switching tubes. Therefore, the embodiment of the present invention can effectively improve the reliability of the micro-inverter.

[0082] On the basis of the above embodiments, optionally, controlling the on-off states of each of the switching tubes in the two branches to change may specifically include: sequentially controlling the first target switching tube to turn off, the second target switching tube to turn on, the third target switching tube to turn off, and the fourth target switching tube to turn on.

[0083] Among them, the first target switch is the switch in the first target branch where the freewheeling direction of the body diode is the same as the direction of the resonant current; the second target switch is the switch in the second target branch where the freewheeling direction of the body diode is opposite to the direction of the resonant current; the third target switch is the switch in the first target branch where the freewheeling direction of the body diode is opposite to the direction of the resonant current; the fourth target switch is the switch in the second target branch where the freewheeling direction of the body diode is the same as the direction of the resonant current. In the two branches, the first target branch is the branch where the resonant current flows before the pulse control signal undergoes a voltage jump; the second target branch is the other branch, that is, the branch where the resonant current flows after the switching.

[0084] With such a setting in this embodiment, it is equivalent to determining the action sequence of each switch according to the direction of the resonant current, and controlling one switch to act each time, which can ensure that there is always a freewheeling path for the resonant current during the branch switching process. Specifically, when the first target switch is turned off and the second target switch is turned on, the resonant current still flows through the first target branch, specifically through the body diode of the third target switch that is turned on and the first target switch that is turned off in the first target branch; when the third target switch is turned off, the resonant current switches to flow through the second target branch, specifically through the body diode of the second target switch that is turned on and the fourth target switch that is turned off in the second target branch; when the fourth target switch is turned on, both switches in the second target branch are turned on, and the resonant current flows through the fully turned-on second target branch. It can be understood that during the branch switching process, the direction of the resonant current remains unchanged, for example, maintaining the direction of flowing out of the transformer or flowing into the transformer, only the flowing branch is switched.

[0085] On the basis of the above embodiments, optionally, during the branch switching process, in order to ensure that the next switch is controlled to act after the previous switch's action is completed, a dead time can be set. The dead time can be set according to the action characteristics of the switch, for example, set to 100 ns. Refer to Figure 3 and Figure 4 , after the voltage of the control signal for controlling any switch undergoes a jump, after a dead time td, the control signal of the next switch can be controlled to undergo a voltage jump. Therefore, the branch switching process in the first working mode requires a duration of 3td, that is, from the moment when the control of the first target switch is turned off until the moment when the control of the fourth target switch is turned on, a total of three dead times td pass.

[0086] On the basis of the above embodiments, optionally, the value of the current threshold is related to the slope of the resonant current. For example, it is necessary to ensure that within the 3td duration of the branch switching, the resonant current does not undergo a polarity inversion (i.e., direction switching) under the maximum slope of the resonant current, to avoid the loss of the resonant current freewheeling loop and avoid overvoltage of the switch.

[0087] Based on the above embodiments, optionally, the current threshold is positively correlated with the absolute value of the voltage difference amplitude between the output terminals connected to the two branches. When the absolute value of the voltage difference amplitude between the output terminals connected to the two branches is relatively large, that is, when the output voltage is relatively high, the current threshold can be set relatively high; when the absolute value of the voltage difference amplitude between the output terminals connected to the two branches is relatively small, that is, when the output voltage is relatively low, the current threshold can be set relatively low.

[0088] Based on the above embodiments, optionally, the two branches include: a first branch 31 and a second branch 32, which are respectively connected to the first output terminal L1 and the second output terminal L2 of the micro-inverter 100; the first branch 31 includes a first switching tube Q1 and a second switching tube Q2, and the second branch 32 includes a third switching tube Q3 and a fourth switching tube Q4; the first switching tube Q1, the second switching tube Q2, the third switching tube Q3, and the fourth switching tube Q4 are sequentially connected in series between the first output terminal L1 and the second output terminal L2, and the directions of adjacent two switching tubes are opposite, and the connection node between the second switching tube Q2 and the third switching tube Q3 is connected to one end of the secondary winding of the transformer 20. The following combines Figures 5 - 8 Taking the switching tube action sequence when switching the first branch 31 and the second branch 32 as an example, the control strategies in different cases are described. The control strategies of any two branches can refer to the control strategies of the first branch 31 and the second branch 32. Among them, Figures 5 - 8 in, the dotted-line switching tubes represent the turned-off switching tubes, the solid-line switching tubes represent the turned-on switching tubes, and the voltage provided by the secondary winding of the transformer 20 is denoted as the secondary voltage V p , the first branch 31 and the second branch 32 are, for example, connected to the positive end of the secondary winding. Exemplarily, according to the positive and negative of the voltage difference between the first output terminal L1 and the second output terminal L2 and the direction of the resonant current i s , the action sequence of each switching tube can be determined, and the following is a specific description.

[0089] First, in the case where the voltage difference between the first output terminal L1 and the second output terminal L2 is positive, for example, when the first output terminal L1 outputs a positive voltage and the second output terminal L2 outputs a negative voltage:

[0090] 1) Refer to Figure 5 , if the resonant current i s flows from the transformer to the secondary bridge arm, that is, the resonant current i s is positive, then the first target branch is the second branch 32, the second target branch is the first branch 31, the first target switching tube is the fourth switching tube Q4, the second target switching tube is the second switching tube Q2, the third target switching tube is the third switching tube Q3, and the fourth target switching tube is the first switching tube Q1.

[0091] Among them, when the voltage difference between the first output terminal L1 and the second output terminal L2 is positive and the resonant current i s flows to the secondary side bridge arm, it can be correspondingly referred to Figure 3 and Figure 4 the waveform above the abscissa in. If when the signal for turning off the second branch 32 arrives (for example, when the rising edge of the pulse control signal appears), the resonant current i s is less than the positive current threshold i + , then the third switch tube Q3 and the fourth switch tube Q4 are still kept conducting without any action until the resonant current i s reaches the positive current threshold i + or when the signal for turning off the second branch 32 arrives and the resonant current i s is greater than or equal to the positive current threshold i + , in the order from left to right in Figure 5 , the fourth switch tube Q4 is controlled to turn off, the second switch tube Q2 is controlled to turn on, the third switch tube Q3 is controlled to turn off, and the first switch tube Q1 is controlled to turn on.

[0092] 2) Refer to Figure 6 , if the resonant current i s flows from the secondary side bridge arm to the transformer, that is, the resonant current i s is negative, then the first target branch is the first branch 31, the second target branch is the second branch 32, the first target switch tube is the second switch tube Q2, the second target switch tube is the fourth switch tube Q4, the third target switch tube is the first switch tube Q1, and the fourth target switch tube is the third switch tube Q3.

[0093] Among them, when the voltage difference between the first output terminal L1 and the second output terminal L2 is positive and the resonant current i s flows to the transformer, it can be correspondingly referred to Figure 3 and Figure 4 the waveform below the abscissa in. If when the signal for turning off the first branch 31 arrives (for example, when the falling edge of the pulse control signal appears), the resonant current i s is greater than the negative current threshold i - , then the first switch tube Q1 and the second switch tube Q2 are still kept conducting without any action until the resonant current i s reaches the negative current threshold i - or when the signal for turning off the first branch 31 arrives and the resonant current i s is less than or equal to the negative current threshold i - , in the order from right to left in Figure 6 , the second switch tube Q2 is controlled to turn off, the fourth switch tube Q4 is controlled to turn on, the first switch tube Q1 is controlled to turn off, and the third switch tube Q3 is controlled to turn on.

[0094] Second, when the voltage difference between the first output terminal L1 and the second output terminal L2 is negative, for example, when the first output terminal L1 outputs a negative voltage and the second output terminal L2 outputs a positive voltage:

[0095] 1) Refer to Figure 7 , if the resonant current i s flows from the transformer to the secondary side bridge arm, that is, the resonant current i s is positive, then the first target branch is the first branch 31, the second target branch is the second branch 32, the first target switch is the first switch Q1, the second target switch is the third switch Q3, the third target switch is the second switch Q2, and the fourth target switch is the fourth switch Q4.

[0096] Specifically, when the signal for turning off the first branch 31 arrives (for example, when the rising edge of the pulse control signal appears), if the resonant current i s is less than the positive current threshold i + , then the first switch Q1 and the second switch Q2 still remain conducting and do not act until the resonant current i s reaches the positive current threshold i + or when the signal for turning off the first branch 31 arrives and the resonant current i s is greater than or equal to the positive current threshold i + , in the order from right to left in Figure 7 , control the first switch Q1 to turn off, the third switch Q3 to turn on, the second switch Q2 to turn off, and the fourth switch Q4 to turn on in sequence.

[0097] 2) Refer to Figure 8 , if the resonant current i s flows from the secondary side bridge arm to the transformer, that is, the resonant current i s is negative, then the first target branch is the second branch 32, the second target branch is the first branch 31, the first target switch is the third switch Q3, the second target switch is the first switch Q1, the third target switch is the fourth switch Q4, and the fourth target switch is the second switch Q2.

[0098] Specifically, when the signal for turning off the second branch 32 arrives (for example, when the falling edge of the pulse control signal appears), if the resonant current i s is greater than the negative current threshold i - , then the third switch Q3 and the fourth switch Q4 still remain conducting and do not act until the resonant current i s reaches the negative current threshold i - or when the signal for turning off the second branch 32 arrives and the resonant current i s is less than or equal to the negative current threshold i - , in the order from right to left in Figure 8In the order from left to right, the third switching transistor Q3 is controlled to turn off, the first switching transistor Q1 is controlled to turn on, the fourth switching transistor Q4 is controlled to turn off, and the second switching transistor Q2 is controlled to turn on.

[0099] In summary, in the first operating mode, while maintaining a freewheeling path for the resonant current between the transformer and the secondary bridge arm at all times, at least two opposing switching transistors (or switching transistors with opposite directions) in the secondary bridge arm are kept off, thereby avoiding the situation of the secondary bridge arm being short-circuited directly and the micro-inverter circuit being short-circuited, and improving the reliability of the inverter operation. This solution is particularly applicable to the topological structure in which the switching transistors of the secondary bridge arm can achieve zero-voltage turn-on, avoiding the delay of the secondary switch and ensuring control stability. And according to the action sequence provided in the above embodiments, the resonant current i s always has a path to flow through, and there will be no sudden interruption of the current, so there will be no high voltage stress. The specific analysis of the soft switching of the secondary bridge arm is as follows: Taking the case where the polarity of the voltage difference between the first output terminal and the second output terminal is positive as an example, the condition for the first switching transistor Q1 to achieve soft switching is that the resonant current flows in the direction of the secondary bridge arm during the dead time, and the current is greater than the specified value (the junction capacitance of the first switching transistor Q1 is discharged completely, and the body diode is forced to conduct), which is consistent with the requirement that the resonant current i s is greater than or equal to the positive current threshold i + in the above solution, and the action requirements of the switching transistor are the same. The condition for the third switching transistor Q3 to achieve soft switching is that the resonant current flows in the direction of the transformer during the dead time, and the current is less than or equal to the specified value (the junction capacitance of the third switching transistor Q3 is discharged completely, and the body diode is forced to conduct), which is consistent with the requirement that the resonant current i s is less than or equal to the negative current threshold i - in the above solution, and the action requirements of the switching transistor are the same. Therefore, when adopting this solution, it is suitable for the control strategy in which the switching transistors of the secondary bridge arm can achieve zero-voltage turn-on.

[0100] The above embodiments have detailed the control strategy in the first operating mode, which can avoid the direct short-circuit of the secondary bridge arm and always have a freewheeling loop, and the inverter has high operating reliability. However, the first operating mode needs to be controlled strictly according to the given switching transistor action sequence, the operation is relatively complex, and the driving loss of the switching transistor is relatively high, resulting in a reduction in conversion efficiency. In response to this, the embodiment of the present invention also proposes a second operating mode to provide a simpler control strategy.

[0101] In the second operating mode, for any two branches of the secondary bridge arm, the on-off states of the switching transistors in the two branches can be controlled according to the positive and negative of the voltage difference between the output terminals connected by the two branches and the level of the pulse control signal, and at most three switching transistors in the two branches are turned on at the same time.

[0102] Specifically, still taking the control of the first branch 31 and the second branch 32 as an example, the second working mode includes:

[0103] When the voltage difference between the first output terminal L1 and the second output terminal L2 is positive, control the second switching transistor Q2 and the fourth switching transistor Q4 to conduct, and control the first switching transistor Q1 and the third switching transistor Q3 to conduct alternately according to the pulse control signal. The waveform of the control signals of the four switching transistors can be specifically referred to Figure 9 , where each control signal takes the high level as the conduction level and the low level as the cut-off level for display. Then the second control signal S 1L and the fourth control signal S 2H can maintain the high level to keep the second switching transistor Q2 and the fourth switching transistor Q4 constantly conducting; the first control signal S 1H and the third control signal S 2L can both be pulse signals with alternating high and low levels. The high levels of the first control signal S 1H and the third control signal S 2L appear at different times to control the first switching transistor Q1 and the third switching transistor Q3 to conduct complementarily in turn. For example, they conduct alternately with a 50% duty cycle. Exemplarily, when the voltage difference between the first output terminal L1 and the second output terminal L2 is positive, when the pulse control signal is at the high level, control the first switching transistor Q1 to conduct, and when the pulse control signal is at the low level, control the third switching transistor Q3 to conduct.

[0104] When the voltage difference between the first output terminal L1 and the second output terminal L2 is negative, control the first switching transistor Q1 and the third switching transistor Q3 to conduct, and control the second switching transistor Q2 and the fourth switching transistor Q4 to conduct alternately according to the pulse control signal. The waveform of the control signals of the four switching transistors can be specifically referred to Figure 10 , where the first control signal S 1H and the third control signal S 2L can maintain the high level to keep the first switching transistor Q1 and the third switching transistor Q3 constantly conducting; the second control signal S 1L and the fourth control signal S 2H can both be pulse signals with alternating high and low levels. The high levels of the second control signal S 1L and the fourth control signal S 2H appear at different times to control the second switching transistor Q2 and the fourth switching transistor Q4 to conduct complementarily in turn. For example, they conduct alternately with a 50% duty cycle. Exemplarily, when the voltage difference between the first output terminal L1 and the second output terminal L2 is negative, when the pulse control signal is at the high level, control the fourth switching transistor Q4 to conduct, and when the pulse control signal is at the low level, control the second switching transistor Q2 to conduct.

[0105] In summary, in the second operating mode, when the voltage difference between the first output terminal L1 and the second output terminal L2 is positive and negative, the two switching transistors are respectively controlled to be always on, which can effectively reduce the switching loss. The second operating mode has the advantages of simple and easy-to-operate control timing, low switching loss of the switching transistors, and high conversion efficiency. However, when the controller cannot accurately determine the voltage polarity between the first output terminal L1 and the second output terminal L2, the second operating mode may cause the secondary side bridge arm 30 to conduct directly, resulting in overcurrent of the micro-inverter and low operating reliability. Therefore, when the controller cannot accurately determine the voltage polarity between the first output terminal L1 and the second output terminal L2, it can switch to the first operating mode, so that there is always a freewheeling loop for the resonant current between the secondary side bridge arms, and the secondary side bridge arm will not conduct directly, avoiding overvoltage and overcurrent of the micro-inverter and improving the operating reliability of the micro-inverter.

[0106] Specifically, when the first operating mode + the second operating mode are used to cooperate in controlling the operation of the micro-inverter, a positive voltage threshold and a negative voltage threshold can be set. For details, please refer to Figure 11 , when the voltage difference Vout between the output terminals connected by the two branches is within the voltage range between the negative voltage threshold V - and the positive voltage threshold V + (that is, V - ≤ Vout ≤ V + ), the first operating mode is adopted; when the voltage difference Vout between the output terminals connected by the two branches is outside the voltage range between the negative voltage threshold V - and the positive voltage threshold V + (that is, Vout < V - or Vout > V + ), the second operating mode is adopted. Among them, the absolute values of the positive and negative voltage thresholds can be the same or different, and can be specifically set according to actual requirements.

[0107] Exemplarily, the optional ranges of the positive voltage threshold and the negative voltage threshold are related to the amplitude of the voltage difference (or called output voltage) between the two output terminals. The larger the absolute values of the positive and negative voltage thresholds are taken, the smaller the probability of the secondary side bridge arm conducting directly, that is, the safer the circuit is. However, the larger the absolute values of the positive and negative voltage thresholds are taken, the longer the time of the first operating mode will be, and the time required for the branch switching in the first operating mode is 3td, which is longer than the delay of the second operating mode. Therefore, when determining the positive and negative voltage thresholds, the above two points can be comprehensively considered for trade-offs.

[0108] It can be understood that in practical applications, the micro-inverter can be controlled to operate in a single first operating mode as needed, or the first operating mode and the second operating mode can be used in combination. Regardless of which operating mode, the controller can control the on-off of each switch tube in the secondary side bridge arm according to the pulse control signal provided by the modulation module and in combination with the polarity of the voltage between the two output terminals. Among them, the secondary side bridge arm can be connected to the power frequency grid, and the pulse control signal can be a high-frequency pulse signal with a duty cycle of 50%. This micro-inverter can be applied to a photovoltaic power generation system.

[0109] In the above embodiments, the case where the secondary side bridge arm 30 includes two branches is exemplarily given, but it is not a limitation to the present invention. The above control method is also applicable to a circuit structure with multiple branches in a multi-way bidirectional switch combination. For details, please refer to Figure 12 , the secondary side bridge arm 30 may include n branches, and the control signals applied to the two switch tubes in the k-th branch are S kL and S kH , and the k-th branch is connected to the k-th output terminal L k , 1 ≤ k ≤ n.

[0110] Then, the switch tube control strategy between the x-th branch and the y-th branch is as follows, 1 ≤ x ≤ n, 1 ≤ y ≤ n, and x ≠ y:

[0111] In the first operating mode:

[0112] When the polarity of the voltage at the output terminal of the secondary side bridge arm (the voltage VLx of the x-th output terminal L x minus the voltage VLy of the y-th output terminal L y ) is positive:

[0113] If the resonant current i s flows in the direction of the secondary side bridge arm, and the resonant current i s is less than the positive current threshold, the pulse control signal undergoes a voltage jump, and the control signals S yL and S yH remain unchanged in level; if the resonant current i s flows in the direction of the secondary side bridge arm, and the resonant current i s is greater than or equal to the positive current threshold, and when the pulse control signal undergoes a voltage jump, S yH is sequentially controlled to jump to the cut-off level, S xL jumps to the conduction level, S yL jumps to the cut-off level, and S xH jumps to the conduction level.

[0114] If the resonant current i s flows in the direction of the transformer, and the resonant current i s is greater than the negative current threshold, the pulse control signal undergoes a voltage jump, and the control signal SxL and S xH The level remains unchanged; if the resonant current i s flows in the direction of the transformer, and the resonant current i s is less than or equal to the negative current threshold, and when the pulse control signal has a voltage jump, sequentially control S xL to jump to the cut-off level, S yH to jump to the conducting level, S xH to jump to the cut-off level, S yL to jump to the conducting level.

[0115] When the polarity of the output terminal voltage (VLx - VLy) is negative:

[0116] If the resonant current i s flows in the direction of the secondary side bridge arm, the resonant current i s is less than the positive current threshold, and when the pulse control signal has a voltage jump, still keep the control signals S xL and S xH at the unchanged level; if the resonant current i s flows in the direction of the secondary side bridge arm, and the resonant current i s is greater than or equal to the positive current threshold, and when the pulse control signal has a voltage jump, sequentially control S xH to jump to the cut-off level, S yL to jump to the conducting level, S xL to jump to the cut-off level, S yH to jump to the conducting level.

[0117] If the resonant current i s flows in the direction of the transformer, the resonant current i s is greater than the negative current threshold, and when the pulse control signal has a voltage jump, still keep the control signals S yL and S yH at the unchanged level; if the resonant current i s flows in the direction of the transformer, and the resonant current i s is less than or equal to the negative current threshold, and when the pulse control signal has a voltage jump, sequentially control S yL to jump to the cut-off level, S xH to jump to the conducting level, S yH to jump to the cut-off level, S xL to jump to the conducting level.

[0118] In the second working mode:

[0119] When the polarity of the output terminal voltage (VLx - VLy) is positive:

[0120] Make the control signals S xL and S yH remain at the conducting level, and make the control signal SxH and S yL are both pulse signals, and their conduction levels appear complementarily in sequence.

[0121] When the polarity of the output terminal voltage (VLx - VLy) is negative:

[0122] make the control signal S xH and S yL maintain their conduction levels, and make the control signal S xL and S yH are both pulse signals, and their conduction levels appear complementarily in sequence.

[0123] To verify the correctness and feasibility of each working mode in the proposed control method, the inventor uses Figure 13 the shown topological structure for simulation verification. See Figure 13 , in this circuit, a capacitor Cbus is also connected in parallel on both sides of the photovoltaic module PV. The control poles of the four switching tubes in the primary bridge arm are respectively connected to the first primary control signal S1, the second primary control signal S2, the third primary control signal S3, and the fourth primary control signal S4; the primary current of the transformer is denoted as i p ; one end of the secondary winding of the transformer is connected to the secondary bridge arm, and the other end is connected to the capacitor bridge arm. The capacitor bridge arm includes a first capacitor C1 and a second capacitor C2; the secondary winding of the transformer is also connected to a resonant inductor L r and a resonant capacitor C r ; the secondary bridge arm is connected to the power grid Vg through an EMI filter. The circuit parameters are shown in Table 1.

[0124] Table 1

[0125]

[0126]

[0127] The simulation results can be seen in Figures 14 - 22 . Among them, as Figure 14 shown, the control method provided by the embodiment of the present invention can realize the grid-connected operation of the dual-active-bridge micro-inverter. Figure 14 In it, for uniformly using the left vertical coordinate scale, the reduced grid-connected voltage, that is, 0.02v g 's waveform is provided, and the waveform of the grid-connected current i g is provided. See Figure 14 . It can be known that the grid-connected power factor is greater than 0.99, and the THD of the grid-connected current is 3.24%, meeting the harmonic distortion rate requirement standard.

[0128] Set the positive voltage threshold to 155V, the negative voltage threshold to -155V, set the positive current threshold to 0.5A in the first working mode, and the negative current threshold to -0.5A.

[0129] Figures 15 - 18 The waveform diagram of the control signal of the secondary side bridge arm switch tube is given when the polarity of the output voltage (i.e., the voltage difference obtained by subtracting the voltage of the second output terminal from the voltage of the first output terminal) is positive. Refer to Figures 15 - 17 , when the output voltage is between 0 and 155V, the system operates in the first operating mode. After judging the direction of the resonant current and comparing the resonant current with the positive current threshold, the on / off of the secondary side bridge arm switch tube is controlled in sequence; to clearly show the action sequence of the switch tube during the branch switching, Figure 16 and Figure 17 respectively give Figure 15 the enlarged views of region AA and region BB in Figure 18 . When the output voltage is between 155 and 311V, the system operates in the second operating mode, and the waveform of the control signal of the secondary side bridge arm switch tube is as shown in

[0130] Figures 19 - 22 The waveform diagram of the control signal of the secondary side bridge arm switch tube is given when the polarity of the output voltage (i.e., the voltage difference obtained by subtracting the voltage of the second output terminal from the voltage of the first output terminal) is negative. Refer to Figures 19 - 21 , when the output voltage is between -155 and 0V, the system operates in the first operating mode. After judging the direction of the resonant current and comparing the resonant current with the negative current threshold, the on / off of the secondary side bridge arm switch tube is controlled in sequence; to clearly show the action sequence of the switch tube during the branch switching, Figure 20 and Figure 21 respectively give Figure 19 the enlarged views of region CC and region DD in Figure 21 . When the output voltage is between -311 and -155V, the system operates in the second operating mode, and the waveform of the control signal of the secondary side bridge arm switch tube is as shown in

[0131] The embodiment of the present invention also provides a control device for a micro-inverter, which is used to execute the control method of the micro-inverter provided in any embodiment of the present invention, and has corresponding beneficial effects.

[0132] The control device of the micro-inverter includes: a first control module; the first control module is used to control the micro-inverter in a first operating mode. Among them, in the first operating mode, for any two branches in the secondary bridge arm of the micro-inverter, the first control module is used to: if the absolute value of the resonant current between the transformer and the secondary bridge arm is less than the current threshold when the pulse control signal has a voltage jump, control each switch tube in the two branches to maintain its on / off state unchanged until the absolute value of the resonant current reaches the current threshold, and then control each switch tube in the two branches to change its on / off state; among them, before the voltage jump of the pulse control signal, two switch tubes in one branch are both turned on, and two switch tubes in the other branch are both turned off; the voltage jump of the pulse control signal is used to control the switching of the branch through which the resonant current flows; if the absolute value of the resonant current is greater than or equal to the current threshold when the pulse control signal has a voltage jump, control each switch tube in the two branches to change its on / off state; among them, during the process of controlling each switch tube in the two branches to change its on / off state, control the action sequence of each switch tube according to the direction of the resonant current; in the first mode, at least two switch tubes with opposite directions in the two branches are turned off at the same time, and at any moment, the resonant current flows through one of the branches.

[0133] Specifically, the first control module controls each switch tube in the two branches to change its on / off state, which specifically includes: sequentially controlling the first target switch tube to turn off, the second target switch tube to turn on, the third target switch tube to turn off, and the fourth target switch tube to turn on. Among them, the first target switch tube is the switch tube in the first target branch whose freewheeling direction of the body diode is the same as the direction of the resonant current, the second target switch tube is the switch tube in the second target branch whose freewheeling direction of the body diode is opposite to the direction of the resonant current, the third target switch tube is the switch tube in the first target branch whose freewheeling direction of the body diode is opposite to the direction of the resonant current, and the fourth target switch tube is the switch tube in the second target branch whose freewheeling direction of the body diode is the same as the direction of the resonant current; in the two branches, the first target branch is the branch through which the resonant current flows before the voltage jump of the pulse control signal, and the second target branch is the other branch.

[0134] On the basis of the above embodiments, optionally, the first control module is used to control the micro-inverter in the first operating mode when the voltage difference between the output terminals connected by the two branches is within the voltage range between the negative voltage threshold and the positive voltage threshold.

[0135] The control device of the micro-inverter further includes: a second control module; the second control module is used to control the micro-inverter in a second operating mode when the voltage difference between the output terminals connected by the two branches is outside the voltage range between the negative voltage threshold and the positive voltage threshold. Wherein, in the second operating mode, the second control module is used to control the on-off states of the switching tubes in the two branches according to the positive and negative of the voltage difference between the output terminals connected by the two branches and the pulse control signal, and at most three switching tubes in the two branches are turned on at the same time.

[0136] Specifically, in the second operating mode, the second control module is specifically used to: when the voltage difference between the first output terminal and the second output terminal is positive, control the second switching tube and the fourth switching tube to turn on, and control the first switching tube and the third switching tube to alternately turn on according to the pulse control signal; when the voltage difference between the first output terminal and the second output terminal is negative, control the first switching tube and the third switching tube to turn on, and control the second switching tube and the fourth switching tube to alternately turn on according to the pulse control signal.

[0137] The embodiment of the present invention also provides a micro-inverter, which can be controlled by using the control method of the micro-inverter provided in any embodiment of the present invention, and has corresponding beneficial effects. See Figure 1 , the micro-inverter 100 may include: a primary bridge arm 10, a transformer 20, a secondary bridge arm 30, and a controller (not shown in the figure).

[0138] Wherein, the primary bridge arm 10 is respectively connected to the input terminal of the micro-inverter 100 (which can be connected to the photovoltaic module PV) and the primary winding of the transformer 20, and the secondary bridge arm 30 is respectively connected to the secondary winding of the transformer 20 and the output terminals of the micro-inverter 100 (which can be connected to the power grid Vg). Wherein, the secondary bridge arm 30 includes: at least two branches, both of which are connected to the same end of the secondary winding of the transformer 20 and are respectively connected to different output terminals of the micro-inverter 100; each branch includes two switching tubes connected in series with opposite directions; the controller is connected to the primary bridge arm 10 (for example, connected to the control poles of the switching tubes in the primary bridge arm 10) and the control poles of each switching tube; the controller is used to execute the control method of the micro-inverter provided in any embodiment of the present invention, and the control device of the micro-inverter provided in any embodiment of the present invention can be integrated in the controller.

[0139] On the basis of the above embodiments, optionally, the secondary side of the micro-inverter 100 may be a half-bridge structure. See Figure 1 and Figure 23, the micro-inverter 100 may include a secondary side bridge arm 30 and a capacitor bridge arm 40. Among them, each branch in the secondary side bridge arm is connected to the first end (e.g., the positive end) of the secondary winding of the transformer 20. The capacitor bridge arm 40 may include: capacitors with the same number of branches, the first end of each capacitor is connected to the second end (e.g., the negative end) of the secondary winding of the transformer 20, and the second end of each capacitor is respectively connected to different output terminals of the micro-inverter 100. For example Figure 23 shows a three-phase structure. The secondary side bridge arm 30 has three branches, the capacitor bridge arm 40 includes three capacitors (C1, C2, and C3 respectively), and the three output terminals L1, L2, and L3 of the micro-inverter 100 respectively correspond to the three-phase outputs of abc.

[0140] Alternatively, the micro-inverter 100 may include two secondary side bridge arms to form a full-bridge secondary side structure. Each branch in one secondary side bridge arm is connected to the first end of the secondary winding of the transformer, and each branch in the other secondary side bridge arm is connected to the second end of the secondary winding of the transformer. Among them, the control strategy of each switching tube in any secondary side bridge arm can refer to the control strategies in the above first working mode and second working mode. Specifically, refer to Figure 24 , the two secondary side bridge arms include a first secondary side bridge arm 301 and a second secondary side bridge arm 302. The control signals of the j-th branch in the first secondary side bridge arm 301 are respectively denoted as S pjL and S pjH , 1≤j≤3. The control signals of the j-th branch in the second secondary side bridge arm 302 are respectively denoted as S njL and S njH .

[0141] In summary, the control method provided by the embodiments of the present invention can be applied to circuit structures of secondary side half-bridge bidirectional switches, full-bridge bidirectional switches, or multi-way bidirectional switches. The transformer 20 can be non-isolated or isolated, which is not limited here.

[0142] It should be understood that various forms of the processes shown above can be used, reordering, adding, or deleting steps. For example, the steps described in the present invention can be executed in parallel, sequentially, or in a different order, as long as the desired results of the technical solutions of the present invention can be achieved, which is not limited herein.

[0143] The above specific embodiments do not constitute a limitation to the protection scope of the present invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principle of the present invention should be included within the protection scope of the present invention.

Claims

1. A control method for a micro-inverter, characterized in that, The micro-inverter includes: a transformer and a secondary side bridge arm; the secondary side bridge arm includes: at least two branches, both connected to the same end of the secondary winding of the transformer and respectively connected to different output terminals of the micro-inverter; each of the branches includes two switching tubes connected in series with opposite directions. The control method of the micro-inverter includes: a first operating mode. In the first operating mode, for any two of the branches in the secondary side bridge arm: If the absolute value of the resonant current between the transformer and the secondary side bridge arm is less than the current threshold when the pulse control signal has a voltage jump, control each of the switching tubes in the two branches to maintain their on / off states unchanged until the absolute value of the resonant current reaches the current threshold, and then control each of the switching tubes in the two branches to change their on / off states; where, before the voltage jump of the pulse control signal, the two switching tubes in one of the branches are both on, and the two switching tubes in the other branch are both off; the voltage jump of the pulse control signal is used to control the switching of the branch through which the resonant current flows. If the absolute value of the resonant current is greater than or equal to the current threshold when the pulse control signal has a voltage jump, control each of the switching tubes in the two branches to change their on / off states. Wherein, during the process of controlling each of the switching tubes in the two branches to change their on / off states, control the action sequence of each of the switching tubes according to the direction of the resonant current; in the first mode, at least two switching tubes with opposite directions in the two branches are off at the same time, and at any moment the resonant current flows through one of the branches.

2. The control method of the micro-inverter according to claim 1, wherein Controlling each of the switching tubes in the two branches to change their on / off states includes: Controlling the first target switching tube to turn off, the second target switching tube to turn on, the third target switching tube to turn off, and the fourth target switching tube to turn on in sequence. Wherein, the first target switching tube is the switching tube in the first target branch whose body diode's freewheeling direction is the same as the direction of the resonant current, the second target switching tube is the switching tube in the second target branch whose body diode's freewheeling direction is opposite to the direction of the resonant current, the third target switching tube is the switching tube in the first target branch whose body diode's freewheeling direction is opposite to the direction of the resonant current, the fourth target switching tube is the switching tube in the second target branch whose body diode's freewheeling direction is the same as the direction of the resonant current; in the two branches, the first target branch is the branch through which the resonant current flows before the voltage jump of the pulse control signal, and the second target branch is the other branch.

3. The control method of the micro-inverter according to claim 2, wherein The two branches include: a first branch and a second branch, which are respectively connected to the first output terminal and the second output terminal of the micro-inverter; the first branch includes a first switching tube and a second switching tube, and the second branch includes a third switching tube and a fourth switching tube; the first switching tube, the second switching tube, the third switching tube, and the fourth switching tube are sequentially connected in series between the first output terminal and the second output terminal, and the directions of adjacent two switching tubes are opposite, and the connection node between the second switching tube and the third switching tube is connected to the secondary winding of the transformer; When the voltage difference between the first output terminal and the second output terminal is positive: If the resonant current flows from the transformer to the secondary bridge arm, the first target branch is the second branch, the second target branch is the first branch, the first target switching tube is the fourth switching tube, the second target switching tube is the second switching tube, the third target switching tube is the third switching tube, and the fourth target switching tube is the first switching tube; If the resonant current flows from the secondary bridge arm to the transformer, the first target branch is the first branch, the second target branch is the second branch, the first target switching tube is the second switching tube, the second target switching tube is the fourth switching tube, the third target switching tube is the first switching tube, and the fourth target switching tube is the third switching tube; When the voltage difference between the first output terminal and the second output terminal is negative: If the resonant current flows from the transformer to the secondary bridge arm, the first target branch is the first branch, the second target branch is the second branch, the first target switching tube is the first switching tube, the second target switching tube is the third switching tube, the third target switching tube is the second switching tube, and the fourth target switching tube is the fourth switching tube; If the resonant current flows from the secondary bridge arm to the transformer, the first target branch is the second branch, the second target branch is the first branch, the first target switching tube is the third switching tube, the second target switching tube is the first switching tube, the third target switching tube is the fourth switching tube, and the fourth target switching tube is the second switching tube.

4. The control method of the micro-inverter according to any one of claims 1-3, characterized in that It further includes: When the voltage difference between the output terminals connected by the two branches is within the voltage range between the negative voltage threshold and the positive voltage threshold, the first working mode is adopted; When the voltage difference between the output terminals connected by the two branches is outside the voltage range between the negative voltage threshold and the positive voltage threshold, the second working mode is adopted; wherein, in the second working mode, according to the positive and negative of the voltage difference between the output terminals connected by the two branches and the pulse control signal, the on-off states of the switching tubes in the two branches are controlled, and at most three switching tubes in the two branches are conducting at the same time.

5. The control method of the micro-inverter according to claim 4, characterized in that, The two branches include: a first branch and a second branch, which are respectively connected to the first output terminal and the second output terminal of the micro-inverter; the first branch includes a first switching tube and a second switching tube; the second branch includes: a third switching tube and a fourth switching tube; The second operating mode includes: When the voltage difference between the first output terminal and the second output terminal is positive, controlling the second switching tube and the fourth switching tube to conduct, and controlling the first switching tube and the third switching tube to conduct alternately according to the pulse control signal; When the voltage difference between the first output terminal and the second output terminal is negative, controlling the first switching tube and the third switching tube to conduct, and controlling the second switching tube and the fourth switching tube to conduct alternately according to the pulse control signal.

6. The control method of the micro-inverter according to claim 1, wherein, The current threshold is positively correlated with the absolute value of the voltage difference amplitude between the output terminals connected by the two branches.

7. A control device for a micro-inverter, characterized in that, Including: A first control module for controlling the micro-inverter in a first operating mode; wherein, in the first operating mode, for any two branches in the secondary bridge arm of the micro-inverter, the first control module is configured to: If, when the voltage of the pulse control signal jumps, the absolute value of the resonance current between the transformer and the secondary bridge arm in the micro-inverter is less than the current threshold, controlling each switching tube in the two branches to maintain its on-off state unchanged until the absolute value of the resonance current reaches the current threshold, and then controlling each switching tube in the two branches to change its on-off state; wherein, before the voltage of the pulse control signal jumps, two switching tubes in one of the branches are both conducting, and two switching tubes in the other branch are both off; the voltage jump of the pulse control signal is used to control the switching of the branch through which the resonance current flows; If, when the voltage of the pulse control signal jumps, the absolute value of the resonance current is greater than or equal to the current threshold, controlling each switching tube in the two branches to change its on-off state; Wherein, in the process of controlling each switching tube in the two branches to change its on-off state, the action sequence of each switching tube is controlled according to the direction of the resonance current; in the first mode, at least two switching tubes with opposite directions in the two branches are off at the same time, and at any moment the resonance current flows through one of the branches.

8. The control device of the micro-inverter according to claim 7, characterized in that, The first control module is configured to control the micro-inverter in the first operating mode when the voltage difference between the output terminals connected by the two branches is within the voltage range between the negative voltage threshold and the positive voltage threshold; The control device of the micro-inverter further includes: a second control module, configured to control the micro-inverter in a second operating mode when the voltage difference between the output terminals connected by the two branches is outside the voltage range between the negative voltage threshold and the positive voltage threshold; wherein, in the second operating mode, the second control module is configured to control the on-off states of the switching tubes in the two branches according to the positive and negative of the voltage difference between the output terminals connected by the two branches and the pulse control signal, and at most three of the switching tubes in the two branches are turned on at the same time.

9. A micro-inverter, characterized in that, including: a primary bridge arm, a transformer, a secondary bridge arm, and a controller; The primary bridge arm is respectively connected to the input terminal of the micro-inverter and the primary winding of the transformer, and the secondary bridge arm is respectively connected to the secondary winding of the transformer and the output terminals of the micro-inverter; wherein, the secondary bridge arm includes: at least two branches, both connected to the same end of the secondary winding of the transformer, and respectively corresponding to different output terminals of the micro-inverter; each of the branches includes two switching tubes connected in series in opposite directions; the controller is connected to the control poles of the primary bridge arm and each of the switching tubes; the controller is configured to execute the control method of the micro-inverter according to any one of claims 1-6.

10. The micro-inverter according to claim 9, wherein One secondary bridge arm and one capacitor bridge arm are included in the micro-inverter; Each of the branches in the secondary bridge arm is connected to the first end of the secondary winding of the transformer; The capacitor bridge arm includes: capacitors having the same number as the branches, the first end of each capacitor is connected to the second end of the secondary winding of the transformer, and the second end of each capacitor is respectively corresponding to different output terminals of the micro-inverter; Or, Two secondary bridge arms are included in the micro-inverter; each of the branches in one secondary bridge arm is connected to the first end of the secondary winding of the transformer, and each of the branches in the other secondary bridge arm is connected to the second end of the secondary winding of the transformer.

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