A control method suitable for flyback micro-reverse mode switching

Abstract

The present application relates to the field of distributed photovoltaic power generation, and aims to provide a control method suitable for flyback micro-inverter mode switching. The control method uses peak current control mode to control the inverter, so that the inverter works in current discontinuous mode (DCM) or critical discontinuous mode (BCM); according to the instantaneous value of the peak current control reference value i ref , a PWM signal for controlling the on-off of the main switch Q M is generated; the duty cycle maximum value Dp in the DCM mode is used to generate the reference current in the two working modes, and then the duty cycle of the main switch QM is adjusted to realize the sine AC grid-connected current, and the maximum power point tracking is realized based on the adjustment of the duty cycle maximum value Dp. The present application can realize the maximum power point tracking function of the micro-inverter under the condition of realizing the sine of the grid-connected current; the control logic is simple and easy to calculate, and can be realized by a very simple circuit; the hardware product cost is low and the operation is stable.

Description

Technical Field

[0001] This invention relates to the field of distributed photovoltaic power generation, and more specifically to a control method suitable for flyback micro-inverter mode switching. Background Technology

[0002] With the expansion of photovoltaic system applications, distributed and residential photovoltaic products will further develop. Currently, the main control-related devices for distributed photovoltaic products include micro-inverters and power optimizers.

[0003] Among microinverters, the flyback microinverter is currently one of the most commercially successful and widely used. The flyback microinverter uses a flyback circuit to convert the direct current output from the photovoltaic modules into alternating current for injection into the grid. Generally, flyback microinverters typically operate in discontinuous current mode (DCM) or boundary conduction mode (BCM). To ensure the inverter operates as efficiently as possible, it is usually operated in DCM mode when the instantaneous power is low, i.e., near the zero-crossing point of the grid voltage, to prevent losses caused by excessive switching frequency; while in BCM mode when the instantaneous power is high, i.e., near the peak of the grid voltage, the inverter operates to prevent losses caused by excessive conduction current. For DCM mode, there are currently open-loop control methods that utilize the maximum duty cycle for maximum power point tracking (MPPT) control; for BCM mode, voltage control methods requiring closed-loop control are typically used. However, the control parameters for the different control methods used in the two operating modes are not uniform. Taking peak current control as an example, if the control parameters for the two modes are not uniform, two separate control systems need to be set up to generate their own reference currents, which greatly increases the complexity of the system and control. To solve this problem, the control parameters for the two operating modes need to be unified.

[0004] Therefore, providing a control method that is capable of open-loop control, has simple logic, and can unify the control parameters of the two working modes, suitable for switching between flyback micro-inverter modes, is in line with the current industry needs. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a control method suitable for mode switching of flyback microinverters.

[0006] To solve the technical problem, the solution of the present invention is:

[0007] A control method for mode switching in a flyback microinverter is provided, the flyback microinverter including a main switch Q located in the primary circuit and connected to the primary winding of transformer T. M ;

[0008] This control method employs peak current control to operate the inverter in either discontinuous current mode (DCM) or critical discontinuous current mode (BCM); based on the peak current control reference value i... ref The instantaneous value is used to generate the control of the main switch Q. M The PWM signal is switched on and off; the real-time mains voltage V is sampled. g and grid-connected current i g The instantaneous grid-connected power p is calculated. g And compare its value with the inverter's preset power threshold Pt h Comparison; when p g Greater than P th When the inverter is in BCM mode, p g Less than P th The inverter is controlled to operate in DCM mode. The maximum duty cycle Dp in DCM mode is used to generate reference currents for both operating modes, thereby adjusting the duty cycle of the main switch QM to achieve sinusoidal AC grid connection of the current, and achieving maximum power point tracking based on the adjustment of the maximum duty cycle Dp.

[0009] As a preferred embodiment of the present invention, the peak current control reference value i ref The instantaneous value is obtained according to the following different calculation methods in DCM and BCM modes respectively;

[0010]

[0011] in:

[0012]

[0013] The maximum duty cycle Dp satisfies the following relationship:

[0014]

[0015] In the above formulas, i ref (t) is the reference value i ref The instantaneous value of D; p The maximum duty cycle value is given by I in DCM mode, where N is the turns ratio of the secondary to the primary winding in transformer T; pv V is the output current of the photovoltaic module. pv ω is the output voltage of the photovoltaic module. g The angular velocity of the grid voltage is given by d(t), where t represents time; and d(t) represents the angular velocity of the main switch Q.M The instantaneous value of the duty cycle, L m V is the magnetizing inductance of transformer T. g f is the effective value of the grid voltage. sw-DCM This indicates the operating frequency of the flyback microinverter in DCM mode.

[0016] As a preferred embodiment of the present invention, in order to achieve maximum power point tracking, the maximum duty cycle Dp in DCM mode is continuously adjusted during the control process; specifically, the following steps are included:

[0017] (1) Assume the initial value of the maximum duty cycle Dp is 0;

[0018] (2) The output current I of the photovoltaic module pv and output voltage V pv Perform multiplication to obtain the photovoltaic module output power P for this cycle. pv Compare this value with the output power of the previous cycle to determine the direction of change in output power.

[0019] (3) Adjust the maximum duty cycle value Dp and compare its value with the value of the previous cycle to obtain the direction of change of the maximum duty cycle value Dp;

[0020] (4) If the output power P in this cycle pv If the duty cycle maximum value Dp increases or decreases simultaneously, the value of the duty cycle maximum value Dp will increase in the next cycle; if the two change in opposite directions within the current cycle, the value of the duty cycle maximum value Dp will decrease in the next cycle.

[0021] The present invention further provides an inverter controller for implementing the aforementioned control method applicable to flyback micro-inverter mode switching. The controller includes a sampling module, an MPPT control module, a reference value calculation module, and a PWM generation module that are electrically connected in sequence. The sampling module is also electrically connected to the reference value calculation module and the PWM generation module respectively.

[0022] The sampling module is used to collect inverter and grid operating parameters, and output the photovoltaic module's output current I to the MPPT control module. pv and output voltage V pv The output voltage V of the photovoltaic module is output to the reference value calculation module. pv and the effective value of grid voltage V g Output transformer primary current i to the PWM generation module p and transformer secondary current i s and grid voltage v g and grid current i g The real-time value;

[0023] The MPPT control module is used to generate the maximum duty cycle Dp in DCM mode and output it to the reference value calculation module.

[0024] The reference value calculation module is used to calculate the reference value i for peak current control. ref The real-time value is output to the PWM generation module;

[0025] The PWM generation module is used to generate PWM signals and output them to the main switch Q. M To control its on / off state.

[0026] The present invention also provides a flyback microinverter for implementing the aforementioned maximum power point tracking control method, comprising a primary circuit, a transformer T, and a secondary circuit, wherein the primary circuit includes a main switch Q connected to the primary winding of the transformer T. M The inverter's input is connected to the photovoltaic module's output, and the inverter's output is connected to the power grid. g The flyback microinverter also includes the inverter controller, in which the PWM generation module is electrically connected to the main switch Q. M .

[0027] As a preferred embodiment of the present invention, the primary-side circuit further includes an input electrolytic capacitor C. in Input electrolytic capacitor C in C is connected in parallel across the output terminals of the photovoltaic module. in The positive terminal is connected to the positive terminal of the photovoltaic module, and this terminal is also connected to one end of the primary winding of the transformer; the other end of the primary winding of the transformer is connected to the main switch Q. M Drain connected, Q M Source connected to C in The negative terminal is connected to the ground terminal.

[0028] As a preferred embodiment of the present invention, the secondary circuit includes a rectifier diode D. M Secondary-side decoupling capacitor C o Bridge-type expansion circuit and CL filter circuit; secondary winding of transformer T and rectifier diode D M After being connected in series, it is connected to a bridge-type expansion circuit. A CL filter circuit is connected in parallel between the two bridge arms of the bridge-type expansion circuit. The output of the CL filter circuit is connected to the mains input. The secondary decoupling capacitor C o The secondary decoupling capacitor C is connected in parallel across the two ends of the bridge circuit. o The end connected to the secondary ground is not connected to the rectifier diode D in the secondary winding of the transformer. M The connected ends are connected.

[0029] As a preferred embodiment of the present invention, the bridge-type expansion circuit includes four MOS switches Q1, Q2, Q3 and Q4, wherein switches Q1 and Q4 are turned on when the mains voltage is in the positive half-cycle and turned off when the mains voltage is in the negative half-cycle; switches Q2 and Q3 are turned on when the mains voltage is in the negative half-cycle and turned off when the mains voltage is in the negative half-cycle.

[0030] As a preferred embodiment of the present invention, the CL filter circuit includes a filter capacitor C. f and filter inductor L f Among them, the filter capacitor C f The filter inductor L is connected between the source of switching transistor Q1 and the drain of switching transistor Q3. f Connect the filter capacitor C f On one side of the mains input terminal, and on the other side of the mains input terminal, the filter capacitor C. f Not with filter inductor L f The other end of the connection is connected.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] 1. The inverter control method provided by this invention can realize the maximum power point tracking function of a micro-inverter under the condition of achieving sinusoidal grid-connected current;

[0033] 2. The control logic of this invention is simple and easy to calculate, and can be implemented with a very simple circuit; therefore, its hardware product has low cost and stable operation. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the circuit structure of the flyback micro-inverter described in this invention.

[0035] Figure 2 This is a schematic diagram of the circuit structure of the converter controller described in this invention.

[0036] Figure 3 This is a flowchart of the maximum power point tracking control method described in this invention.

[0037] Figure 4 These are the primary and secondary currents of the transformer under mode switching in the flyback micro-inverter of this invention.

[0038] Figure 5 This is a derivation diagram of the maximum power point tracking implementation principle of the flyback micro-inverter described in this invention.

[0039] Figure 6 This is a flowchart of the flyback micro-inverter mode selection and judgment process described in this invention. Detailed Implementation

[0040] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0041] Figure 1 This is a schematic diagram of the flyback micro-inverter described in this invention. The flyback inverter includes a primary circuit, a high-frequency transformer T, and a secondary circuit. The input terminal of the inverter is connected to the photovoltaic module, and the output terminal is connected to the power grid. g Connected.

[0042] The primary circuit includes an input electrolytic capacitor C. in and main power MOSFET switch Q M (hereinafter referred to as the main switch transistor Q) M Its connection method is as follows: input electrolytic capacitor C in C is connected in parallel across the output terminals of the photovoltaic module. in The positive terminal is connected to the positive terminal of the photovoltaic module, and this terminal is also connected to one end of the primary winding of the transformer. The other end of the primary winding of the transformer is connected to the main switch Q. M Drain connected, Q M Source connected to C in The negative terminal is connected to the ground terminal.

[0043] The secondary circuit includes a rectifier diode D. M A secondary decoupling capacitor C o A bridge circuit consisting of MOS switches Q1, Q2, Q3, and Q4, and a filter capacitor C. f and filter inductor L f The circuit consists of a CL filter. Its connection method is as follows: the secondary side of the transformer is connected to the primary side and C... in The opposite terminal of one end is connected to the rectifier diode D. M The anode of the rectifier diode D is connected to the cathode. M Cathode and C o One end of the capacitor is connected to the source of Q1, and this end is also connected to the drain of Q1 and Q4; the source of Q1 is connected to the drain of Q2, and the source of Q4 is connected to the drain of Q3; the sources of Q2 and Q3 are connected to capacitor C. o The other end is connected, and this end is not connected to the secondary winding of D. M One end is connected. Filter capacitor C f Follow Q I Between the source and the drain of Q3, the filter inductor L f One end is connected to the source of Q1, and the other end is connected to the power grid v. g One end is connected, the power grid v gThe other end is connected to the drain of Q3. For Q1, Q2, Q3, and Q4, Q1 and Q3 are turned on when the grid voltage is in the positive half-cycle, and Q2 and Q4 are turned on when the grid voltage is in the negative half-cycle.

[0044] The main circuit structure of the flyback inverter is a well-known technology. Since its circuit structure and functional implementation principle have been described in many documents and books, this invention will not elaborate on them.

[0045] Figure 2 This is a schematic diagram of the controller of the flyback microinverter described in this invention.

[0046] The inverter controller includes a sampling module, an MPPT control module, a reference value calculation module, and a PWM generation module that are electrically connected in sequence. The sampling module is also electrically connected to the reference value calculation module and the PWM generation module.

[0047] The sampling module is used to collect inverter and grid operating parameters, and obtains the output voltage V of the photovoltaic module through sampling. pv Photovoltaic module output current I pv Transformer primary current i p The secondary current i of the transformer s Grid voltage v g The sampling value of the signal. The sampling module will output current I. pv and output voltage V p The output is sent to the MPPT module, which generates the maximum duty cycle Dp in DCM mode. Dp can be used to adjust the main switch Q. M The duty cycle is adjusted to achieve sinusoidal AC grid connection of the current.

[0048] In DCM mode, the maximum duty cycle Dp satisfies the following relationship:

[0049]

[0050] In the formula, I pv V is the output current of the photovoltaic module. pv L is the output voltage of the photovoltaic module. m f is the magnetizing inductance of transformer T. sw-DCM This refers to the switching frequency of the inverter when it operates at a fixed frequency in DCM mode.

[0051] The maximum duty cycle Dp in DCM mode and the output voltage V of the photovoltaic module pv and the effective value of grid voltage V g Together, they are input into the reference value calculation module, which calculates the peak current control reference value i for both DCM and BCM modes according to the following formula. ref Real-time values:

[0052]

[0053] in:

[0054]

[0055] In the formula, i ref (t) represents the current reference value i ref The instantaneous value; k is the duty cycle control factor, N is the duty cycle between the secondary and primary sides of transformer T, ω g The angular velocity of the grid voltage is given by d(t), where t represents time; and d(t) represents the angular velocity of the main switch Q. M The instantaneous value of the duty cycle.

[0056] Then the instantaneous value of the reference value i ref With the transformer primary current i p and transformer secondary current i s The common input is sent to the PWM generation module, and the voltage v output by the converter is also input. g The current ig is simultaneously input into the PWM generation module to calculate the instantaneous power p output by the inverter. g The PWM generation module determines the instantaneous value based on its magnitude and the power threshold P. th The comparison results are used to select the appropriate operating mode (DCM or BCM) to obtain the control Q. M The PWM signal is switched on and off. The implementation of the PWM generation module is a well-known technology, and since many documents and books have described how to generate PWM signals, this invention will not elaborate on it further.

[0057] Figure 3 This is a flowchart of the maximum power point tracking control method described in this invention.

[0058] To achieve maximum power point tracking, this invention requires continuous adjustment of the maximum duty cycle Dp in DCM mode during the control process; specifically, it includes the following steps:

[0059] First, let the initial value of Dp be 0; by adjusting V... pv and I pv Perform multiplication to obtain the real-time output power P of the photovoltaic module. pv This value is then compared with the power value obtained in the previous cycle to determine the direction of power change. If it increases, it is recorded as 1; otherwise, it is recorded as 0. Similarly, the direction of Dp change is determined. If, within this cycle, P... pv If P and Dp increase or decrease simultaneously, then the value of Dp will increase in the next period; if P increases or decreases simultaneously in the current period... pv If the direction of change of Dp is inconsistent with that of Dp, then the value of Dp will decrease in the next cycle. Clearly, there is an XOR relationship between them.

[0060] Specific examples are shown in the table below:

[0061] 1 (Added) 1 (Added) 1 (Added) 1 (Added) 0 (decrease) 0 (decrease) 0 (decrease) 0 (decrease) 1 (Added) 0 (decrease) 1 (Added) 0 (decrease)

[0062] By continuously adjusting the maximum duty cycle Dp in DCM mode during the control process, Dp can track the changes in operating parameters of the photovoltaic modules and the grid in real time, and its value changes affect the adjustment of the PWM control signal. Ultimately, by adjusting the operation of the flyback microinverter, the photovoltaic modules can operate at maximum power output.

[0063] It should be noted that although Dp is named the maximum duty cycle value in DCM mode in this invention, the peak current control reference value i in both DCM and BCM modes is actually used for the peak current control. ref This parameter is used in real-time numerical calculations; and in the control process of DCM and BCM modes, the specific value of this parameter can be adaptively adjusted by referring to the above process.

[0064] Figure 4 This refers to the primary and secondary currents of the transformer in the mode switching state of the flyback micro inverter of this invention.

[0065] The following is combined with Figures 1-5 The implementation principle of the control method described in this invention will be explained in more detail below:

[0066] First, let me briefly introduce the basic principle of controlling MPPT using the maximum duty cycle Dp in DCM mode.

[0067] Due to the main switching transistor Q M The switching frequency is much higher than the power grid frequency. Therefore, to simplify the analysis, it is assumed that the grid voltage remains constant during one switching cycle. Combining the operating characteristics of the flyback microinverter in DCM mode, it can be concluded that Q... M The conduction time is:

[0068]

[0069] In the formula, i ref-DCM (t) represents the current reference value i ref Real-time values ​​in DCM mode;

[0070] At the same time, it is not difficult to deduce the time t for the excitation inductor current to decrease from its peak value to 0. off The following relationship must be satisfied:

[0071]

[0072] Since the bridge circuit used on the secondary side only serves to convert the sine wave into a sine wave, the average value of the secondary current of transformer T can be considered as the grid-connected current.

[0073]

[0074] Among them, i g (t) represents the grid-connected current i g The instantaneous value of I g V is the effective value of the grid-connected current. g The voltage is the effective value of the grid voltage, and t represents time.

[0075] Combining the above equations, to ensure that the grid-connected current is sinusoidal, the conduction time t can be easily calculated. on The duty cycle needs to change according to a sinusoidal law. Since the switch operates at a fixed frequency in DCM mode, it can be assumed that the duty cycle needs to change according to a sinusoidal law, therefore t... on Satisfy the following formula:

[0076]

[0077] Dp represents the maximum duty cycle value in DCM mode, which is generated by the MPPT control circuit.

[0078] If Q M Given the above relationship, the average value of the secondary current at this time can be obtained as follows:

[0079]

[0080] Meanwhile, ignoring the converter's own losses, we assume that the input power equals the effective value of the output power:

[0081] P pv =V g I g =V pv I pv =P gactive

[0082] P gactive This represents the active power output of the inverter. Furthermore, the following relationship can be derived:

[0083]

[0084] It is not difficult to see from this relationship that at this time I pv It is V pv The direct proportional function is obtained by plotting this direct proportional function on the same graph as the IV and PV curves of the photovoltaic module. Figure 5 .from Figure 5 It is easy to see that as the Dp value increases, the slope of the proportional function also increases. At the same time, the proportional function has a unique intersection point with the IV characteristic curve of the photovoltaic module, which means that each Dp value corresponds to an operating point.

[0085] If MPPT is controlled by controlling the Dp value, the relevant logic can be described as follows: If the Dp value increases in this cycle, and the photovoltaic module output power P... pv If Dp also increases, then the value of Dp will continue to increase in the next control cycle; conversely, if Dp increases in the current cycle, then P... pv If the value of Dp decreases, then the value of Dp needs to be decreased in the next control cycle; if the value of Dp decreases in this cycle, and P also decreases... pv If the value of Dp decreases, the next control cycle needs to decrease the value of Dp; conversely, if the value of Dp increases, the value of Dp needs to increase. If we compare the value of Dp with P... pv An increase in is denoted as logic 1, and a decrease as logic 0. Based on the above description, we can obtain the logic relationship table shown above. It is easy to see that the direction of change of the Dp value in the next control cycle is related to the Dp value and P value in the current cycle. pv The direction of change is XOR-OR, which is also consistent with... Figure 3 The flowchart described matches. As long as the Dp value is controlled according to the above process, there will definitely be a definite value D. mpp This ensures that the operating point of the photovoltaic module in DCM mode is at the voltage V corresponding to the maximum power point. mpp and power P mpp Place.

[0086] The following section explains how control is performed in BCM mode. Similarly, if peak current control is used, the current reference value in BCM mode is:

[0087]

[0088] In the formula, i ref-BM (t) represents the current reference value i ref Real-time values ​​in BCM mode;

[0089] Since the bridge circuit used on the secondary side only serves to convert the sine wave into a sine wave, the average value of the secondary current of transformer T can be considered as the grid-connected current.

[0090]

[0091] At the same time, it is concluded that:

[0092]

[0093] Where d(t) represents the main switch Q in BCM mode. M The instantaneous value of the duty cycle.

[0094] Combining the above formulas, to ensure the grid-connected current is sinusoidal, and to use the maximum duty cycle Dp in DCM mode for MPPT control, the parameter adjustment can be used to determine Q. M The conduction time in BCM mode must satisfy the following relationship:

[0095]

[0096] The correctness of this formula is explained in detail below:

[0097] In BCM mode, Q M When the conduction time satisfies the above formula, it is not difficult to deduce that the average value of the secondary current is:

[0098]

[0099] Clearly, the average secondary current at this point is equal to the average current obtained in DCM mode. This means that if BCM mode is controlled, controlling Q... M If the conduction time meets the conduction time described in this patent, the grid-connected current obtained in both modes will always be constant, meaning that the output power in both modes will be equal. This also indicates that the MPPT function in BCM mode can be realized at this time.

[0100] Thus, the reference current value i of the flyback micro-inverter in BCM mode can be obtained at this time. ref-BCM The instantaneous value is:

[0101]

[0102] In summary, by controlling only the variable Dp and generating the reference current in the corresponding mode, the MPPT function and grid connection function of the flyback micro-inverter can be simultaneously satisfied in both DCM and BCM modes. Further derivation shows that, using Dp as the sole control variable, the current reference values ​​generated in both modes are as follows:

[0103]

[0104] Figure 6 A basic flowchart for selecting the operating mode of an inverter is shown.

[0105] In actual operation, the controller samples the real-time grid voltage v within half a power frequency cycle. g and grid-connected current i g Then the instantaneous grid-connected power p is calculated. g The power threshold P is set by technicians based on the actual operating conditions of the inverter. th When p g Greater than P thWhen the inverter is in BCM mode, p g Less than P th At this time, the inverter is controlled to operate in DCM mode.

[0106] In summary, under the control of the control circuit, the flyback microinverter can ultimately achieve maximum power point tracking while ensuring the sinusoidal nature of the grid-connected current, and can unify the control parameters in both DCM and BCM modes. Furthermore, the above control method is simple to implement and can also be achieved through analog control, which will not be elaborated upon here.

[0107] The specific embodiments described above illustrate the technical solution and beneficial effects of the present invention in detail. It should be understood that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A control method for mode switching in a flyback microinverter, the flyback microinverter including a main switch Q located in the primary circuit and connected to the primary winding of a transformer T. M Its characteristics are, This control method employs peak current control to operate the inverter in either discontinuous current mode (DCM) or critical discontinuous current mode (BCM); based on the peak current control reference value i... ref The instantaneous value is used to generate the control of the main switch Q. M The PWM signal is switched on and off; the real-time mains voltage V is sampled. g and grid-connected current i g The instantaneous grid-connected power p is calculated. g And compare its value with the inverter's preset power threshold P. th Comparison; when p g Greater than P th When the inverter is in BCM mode, p g Less than P th The inverter is controlled to operate in DCM mode; the maximum duty cycle Dp in DCM mode is used to generate reference currents for both operating modes, thereby adjusting the main switch Q. M The duty cycle is adjusted to achieve sinusoidal AC grid connection of the current, and maximum power point tracking is achieved based on the adjustment of the maximum duty cycle Dp. The peak current control reference value i ref The instantaneous value is obtained according to the following different calculation methods in DCM and BCM modes respectively; ； in: ； The maximum duty cycle Dp satisfies the following relationship: ； In the above formulas, For reference value i ref The instantaneous value of D; p The maximum duty cycle value is given by I in DCM mode, where N is the turns ratio of the secondary to the primary winding in transformer T; pv V is the output current of the photovoltaic module. pv ω is the output voltage of the photovoltaic module. g The angular velocity of the grid voltage is given by d(t), where t represents time; and d(t) represents the angular velocity of the main switch Q. M The instantaneous value of the duty cycle, L m V is the magnetizing inductance of transformer T. g f is the effective value of the grid voltage. sw-DCM This indicates the operating frequency of the flyback microinverter in DCM mode.

2. The method according to claim 1, characterized in that, To achieve maximum power point tracking, the maximum duty cycle Dp in DCM mode is continuously adjusted during the control process; Specifically, the following steps are included: (1) Assume the initial value of the maximum duty cycle Dp is 0; (2) The output current I of the photovoltaic module pv and output voltage V pv Perform multiplication to obtain the photovoltaic module output power P for this cycle. pv Compare this value with the output power of the previous cycle to determine the direction of change in output power. (3) Adjust the maximum duty cycle value Dp and compare its value with the value of the previous cycle to obtain the direction of change of the maximum duty cycle value Dp; (4) If the output power P in this cycle pv If the duty cycle maximum value Dp increases or decreases simultaneously, the value of the duty cycle maximum value Dp will increase in the next cycle; if the two change in opposite directions within the current cycle, the value of the duty cycle maximum value Dp will decrease in the next cycle.

3. An inverter controller for implementing the control method for mode switching of a flyback microinverter as described in claim 1, characterized in that, The controller includes a sampling module, an MPPT control module, a reference value calculation module, and a PWM generation module that are electrically connected in sequence. The sampling module is also electrically connected to the reference value calculation module and the PWM generation module respectively. The sampling module is used to collect inverter and grid operating parameters, and output the photovoltaic module's output current I to the MPPT control module. pv and output voltage V pv The output voltage V of the photovoltaic module is output to the reference value calculation module. pv and the effective value of grid voltage V g Output transformer primary current i to the PWM generation module p and transformer secondary current i s and grid voltage v g and grid current i g The real-time value; The MPPT control module is used to generate the maximum duty cycle Dp in DCM mode and output it to the reference value calculation module. The reference value calculation module is used to calculate the reference value i for peak current control. ref The real-time value is output to the PWM generation module; The PWM generation module is used to calculate the PWM signal and output it to the main switch Q. M To control its on / off state; This module will calculate the instantaneous grid-connected power p. g The size of the instantaneous power p when connected to the grid g Power P less than the threshold th At that time, the PWM generation module generates a PWM signal that enables the inverter to operate in DCM mode based on the current reference value in DCM mode; when the instantaneous power p of the grid connection is... g Power greater than threshold P th At this time, the PWM generation module generates a PWM signal that enables the inverter to operate in BCM mode based on the current reference value in BCM mode.

4. A flyback microinverter for implementing the control method for mode switching of a flyback microinverter as described in claim 1, comprising a primary circuit, a transformer T, and a secondary circuit, wherein the primary circuit includes a main switch Q connected to the primary winding of the transformer T. M The inverter's input is connected to the photovoltaic module's output, and the inverter's output is connected to the power grid. g Connected; characterized in that, The flyback micro inverter also includes the inverter controller as described in claim 3, wherein the PWM generation module in the controller is electrically connected to the main switching transistor Q. M .

5. The flyback microinverter according to claim 4, characterized in that, The primary circuit also includes an input electrolytic capacitor C. in Input electrolytic capacitor C in C is connected in parallel across the output terminals of the photovoltaic module. in The positive terminal is connected to the positive terminal of the photovoltaic module, and this terminal is also connected to one end of the primary winding of the transformer; the other end of the primary winding of the transformer is connected to the main switch Q. M Drain connected, Q M Source connected to C in The negative terminal is connected to the ground terminal.

6. The flyback microinverter according to claim 4, characterized in that, The secondary circuit includes a rectifier diode D. M Secondary-side decoupling capacitor C o Bridge-type expansion circuit and CL filter circuit; secondary winding of transformer T and rectifier diode D M After being connected in series, it is connected to a bridge-type expansion circuit. A CL filter circuit is connected in parallel between the two bridge arms of the bridge-type expansion circuit. The output of the CL filter circuit is connected to the mains input. The secondary decoupling capacitor C o The secondary decoupling capacitor C is connected in parallel across the two ends of the bridge circuit. o The end connected to the secondary ground is not connected to the rectifier diode D in the secondary winding of the transformer. M The connected ends are connected.

7. The flyback microinverter according to claim 6, characterized in that, The bridge-type expansion circuit includes four MOS switches Q1, Q2, Q3 and Q4, wherein MOS switches Q1 and Q4 are turned on when the mains voltage is in the positive half-cycle and turned off when the mains voltage is in the negative half-cycle. MOS switches Q2 and Q3 are turned on when the mains voltage is in the negative half-cycle and turned off when the mains voltage is in the positive half-cycle.

8. The flyback microinverter according to claim 7, characterized in that, The CL filter circuit includes a filter capacitor C. f and filter inductor L f Among them, the filter capacitor C f The filter inductor L is connected between the source of switching transistor Q1 and the drain of switching transistor Q3. f Connect the filter capacitor C f On one side of the mains input terminal, and on the other side of the mains input terminal, the filter capacitor C. f Not with filter inductor L f The other end of the connection is connected.

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