Method and system for controlling power device of string photovoltaic grid-connected inverter

Abstract

The invention belongs to the field of power supply systems, and provides a control method and system for a string type photovoltaic grid-connected inverter power device, the system comprises a control circuit and an isolation circuit, the control circuit is connected with a photovoltaic grid-connected inverter, the output current and the input DC voltage of the photovoltaic grid-connected inverter are collected, and the driving parameter and the duty ratio of the power device are calculated; the isolation circuit is respectively connected with the control circuit and the control chip, the control chip is connected with the push-pull amplification circuit, the push-pull amplification circuit is connected with the driving circuit, the driving circuit comprises a first driving branch and a second driving branch which are connected in parallel, and the first driving branch comprises a first control switch and a first driving resistor which are connected in series; the second driving branch comprises a second control switch and a second driving resistor which are connected in series; the driving circuit is connected with the power device and drives and adjusts the switching speed of the power device. Based on the output current and the input DC voltage, the drive circuit is dynamically adjusted to control the on-off of the switch, so that the switching loss is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power supply systems, and particularly relates to a control method and system for power devices of a string-type photovoltaic grid-connected inverter. BACKGROUND

[0002] The statements in this section merely provide background information related to the application and do not necessarily constitute prior art.

[0003] Overexploitation and use of fossil fuels have become an important inducement for global climate warming and environmental pollution. In this context, new energy power generation technologies represented by photovoltaics have achieved large-scale application and rapid development in recent years. As the core equipment of a photovoltaic power generation system, a photovoltaic inverter undertakes the key task of converting direct current generated by a photovoltaic module into alternating current, and its performance is directly related to the power generation efficiency, operation safety and long-term reliability of the entire system. Therefore, research and development of photovoltaic inverters with high efficiency, high safety and high reliability have become an important direction for promoting the sustainable and healthy development of the photovoltaic industry.

[0004] The core components of a photovoltaic inverter are power devices and their driving circuits. The current mainstream power devices include IGBTs and SiC-MOSFETs. With the continuous improvement of the capacity and power level of photovoltaic systems, higher requirements are put forward for the efficiency, power density and operation reliability of inverters. However, there is a close coupling relationship between the switching characteristics of power devices and the output power of inverters, which brings significant challenges to the design of driving circuits. Specifically, in the light load or low power operating range, to suppress electromagnetic interference (EMC), it is necessary to reduce the switching speed, and at this time, larger turn-on and turn-off resistors should be configured; while in the high power output range, to reduce switching loss and improve system efficiency, smaller switching resistors should be used to speed up the switching process.

[0005] The widely used driving scheme currently configures driving resistors with fixed resistance values, which cannot flexibly adjust the switching characteristics according to the actual operating power. This "one-size-fits-all" design approach results in the inability of the inverter to simultaneously meet the performance requirements of low electromagnetic interference and low switching loss in a wide power range, thereby limiting the optimization of the overall performance of the system. SUMMARY

[0006] To address the aforementioned issues, this invention proposes a control method and system for power devices in a string photovoltaic grid-connected inverter. This invention aims to resolve the core contradiction that traditional fixed drive resistors cannot simultaneously balance efficiency, electromagnetic compatibility (EMC), and device safety across the entire power range. By acquiring the inverter's output current and DC bus voltage in real time, the control circuit analyzes and makes decisions, and dynamically adjusts the drive resistor combination of power devices (such as IGBTs and SiC-MOSFETs) using a digital logic control chip and drive circuit. This method achieves online optimized control of the voltage change rate (du / dt) and current change rate (di / dt) during the switching process of the power devices, thereby realizing intelligent regulation at the system level.

[0007] According to some embodiments, the first aspect of the present invention provides a control system for the power devices of a string photovoltaic grid-connected inverter, employing the following technical solution: A control system for power devices in a string photovoltaic grid-connected inverter includes a control circuit and an isolation circuit. The control circuit is connected to the photovoltaic grid-connected inverter, collects the output current and input DC voltage of the photovoltaic grid-connected inverter, and calculates the driving parameters and duty cycle of the power devices. The isolation circuit is connected to the control circuit and the control chip respectively, and is used to transmit the driving parameters and duty cycle of the power device calculated by the control circuit to the control chip. The control chip is connected to a push-pull amplifier circuit, which is connected to a drive circuit. The drive circuit includes a first drive branch and a second drive branch connected in parallel. The first drive branch includes a first control switch and a first drive resistor connected in series. The second drive branch includes a second control switch and a second drive resistor connected in series. The drive circuit is connected to the power device and adjusts the switching speed of the power device based on the drive signal amplified by the push-pull amplifier circuit.

[0008] Furthermore, it also includes an isolated power supply, the primary side of which is connected to the control circuit for power supply; the secondary side of which is connected to the control chip and the push-pull amplifier circuit for power supply.

[0009] Furthermore, the push-pull amplifier circuit includes two transistors of different types.

[0010] Furthermore, the isolation circuit employs a digital isolator, which transmits the drive parameters and duty cycle of the power devices calculated by the control circuit to the control chip via isolated CAN communication.

[0011] Furthermore, the resistance value of the first driving resistor is greater than the resistance value of the second driving resistor.

[0012] Furthermore, the control circuit employs a digital signal processor circuit.

[0013] According to some embodiments, the second aspect of the present invention provides a control method for power devices in a string photovoltaic grid-connected inverter, employing the following technical solution: A control method for power devices in a string photovoltaic grid-connected inverter, based on the control system for power devices in a string photovoltaic grid-connected inverter described in the first scheme, includes: When the power devices are turned on, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted according to the relationship between the output current and the rated current of the photovoltaic grid-connected inverter. When the power devices are turned off, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted according to the relationship between the output current and the rated current of the photovoltaic grid-connected inverter, and the relationship between the input DC voltage and the maximum operating voltage.

[0014] Furthermore, when the power devices are turned on, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted based on the relationship between the output current and the rated current of the photovoltaic grid-connected inverter, including: When the output current of the photovoltaic grid-connected inverter is lower than the first percentage of the rated current, the first control switch is closed and the second control switch is opened. When the output current of the photovoltaic grid-connected inverter is greater than the first percentage of the rated current and less than the second percentage of the rated current, the first control switch is disconnected and the second control switch is closed. When the output current of the photovoltaic grid-connected inverter is greater than the second percentage of the rated current, the first control switch and the second control switch are closed simultaneously.

[0015] Furthermore, when the power devices are turned off, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted based on the relationship between the input DC voltage and the maximum operating voltage of the photovoltaic grid-connected inverter, and the relationship between the output current and the rated current. This includes: When the input DC voltage of the photovoltaic grid-connected inverter is less than the set percentage of the maximum operating voltage, the first control switch and the second control switch are closed simultaneously. When the input DC voltage of the photovoltaic grid-connected inverter is greater than the set percentage of the maximum operating voltage, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted according to the relationship between the output current and the rated current of the photovoltaic grid-connected inverter.

[0016] Furthermore, based on the relationship between the output current and rated current of the photovoltaic grid-connected inverter, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted, including: When the output current of the photovoltaic grid-connected inverter is lower than the first percentage of the rated current, the first control switch and the second control switch are closed simultaneously. When the output current of the photovoltaic grid-connected inverter is greater than the first percentage of the rated current and less than the second percentage of the rated current, the first control switch is disconnected and the second control switch is closed. When the output current of the photovoltaic grid-connected inverter is greater than the second percentage of the rated current, the first control switch is closed and the second control switch is opened.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention significantly improves system efficiency by dynamically reducing the drive resistance under high power conditions, effectively accelerating the switching process, and greatly reducing the switching losses of power devices, thereby improving the energy conversion efficiency of the inverter across the entire load range.

[0018] This invention optimizes electromagnetic compatibility (EMC) performance. Under low power or light load conditions, the system automatically configures a larger drive resistor to slow down the switching speed, suppress the generation of high-frequency electromagnetic interference (EMI) from the source, reduce the level of conducted and radiated emissions, and improve the system's EMC margin.

[0019] This invention enhances system reliability and safety by precisely controlling the du / dt ratio during the turn-off process, effectively suppressing voltage spikes generated when power devices are turned off and reducing the voltage stress they bear. This not only improves the protection of the power devices themselves but also enhances the long-term operational reliability of the entire system under complex power grid conditions.

[0020] This invention achieves intelligent adaptive control, breaking through the limitations of fixed parameter drive, enabling the drive system to adaptively adjust according to the actual operating state (power, voltage) of the inverter, and realizing the optimal dynamic balance between switching losses, EMC performance and device stress. Attached Figure Description

[0021] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0022] Figure 1 This is a control system framework diagram of a string photovoltaic grid-connected inverter power device according to an embodiment of the present invention; Figure 2 This is a push-pull amplifier circuit diagram in an embodiment of the present invention; Figure 3 This is a flowchart of the power device turn-on state control in an embodiment of the present invention; Figure 4 This is a flowchart of the power device shutdown state control in an embodiment of the present invention. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0025] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0026] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0027] Example 1 This embodiment provides a control system for the power devices of a string photovoltaic grid-connected inverter, including a control circuit and an isolation circuit. The control circuit is connected to the photovoltaic grid-connected inverter, collects the output current and input DC voltage of the photovoltaic grid-connected inverter, and calculates the driving parameters and duty cycle of the power devices. The isolation circuit is connected to the control circuit and the control chip respectively, and is used to transmit the driving parameters and duty cycle of the power device calculated by the control circuit to the control chip. The control chip is connected to a push-pull amplifier circuit, which is connected to a drive circuit. The drive circuit includes a first drive branch and a second drive branch connected in parallel. The first drive branch includes a first control switch and a first drive resistor connected in series. The second drive branch includes a second control switch and a second drive resistor connected in series. The drive circuit is connected to the power device and adjusts the switching speed of the power device based on the drive signal amplified by the push-pull amplifier circuit.

[0028] like Figure 1 As shown, this embodiment mainly includes a control circuit, an isolation circuit, a sampling line, a control chip, a drive circuit, an isolation power supply, and power devices, etc. The drive circuit consists of a first control switch Q1, a second control switch Q2, a first drive resistor R1, and a second drive resistor R2. The combination of drive resistors is adjusted according to the output current lout and input DC voltage Vdc of the photovoltaic grid-connected inverter to achieve the function of adjusting the du / dt and di / dt of the power device, thereby improving the efficiency of the photovoltaic grid-connected inverter, reducing the EMC interference of the photovoltaic grid-connected inverter, and reducing the turn-off voltage stress of the power device S1.

[0029] The control circuit uses a digital signal processor (DSP control circuit) to collect the output current lout and input DC voltage Vdc of the photovoltaic grid-connected inverter. Based on the collected values ​​of lout and Vdc, the control circuit calculates the driving parameters and duty cycle of the power device S1. The information is transmitted to the control chip (FPGA) through isolated CAN communication. The control chip adjusts the switching speed of the power device by controlling the switching states of Q1 and Q2 and sends a drive signal to drive S1 to work. A push-pull amplifier circuit is also connected between the control chip and the drive circuit. The control chip outputs a PWM drive signal, which is amplified by the push-pull amplifier circuit to increase the drive voltage. The increased drive voltage drives the drive circuit to control the power switch S1 to turn on or off.

[0030] The push-pull amplifier circuit includes two transistors of different types. The working principle of the push-pull amplifier circuit is to amplify the voltage and current of the drive signal sent by the FPGA. It is generally composed of an NPN transistor and a PNP transistor, such as... Figure 2 As shown, V+ is the positive driving voltage, typically 15Vdc, and V- is the negative driving voltage, typically -10Vdc.

[0031] The isolated power supply uses a DC power supply. The primary side of the isolated power supply is connected to the control circuit for power supply; the secondary side of the isolated power supply is connected to the control chip and the push-pull amplifier circuit for power supply; the DC power supply - primary side mainly provides power to the DSP control circuit, generally 5V and 3.3Vdc; the DC power supply - secondary side mainly provides power to the FPGA and the push-pull amplifier circuit, generally 3.3V, +15Vdc and -10Vdc are the positive and negative driving voltages of the power device S1.

[0032] The isolation circuit includes a CAN controller, a digital isolator, and a CAN transceiver. The digital isolator is positioned between the CAN controller and the CAN transceiver to establish electrical isolation between the control circuit (low-voltage side) and the external CAN bus (high-voltage side), thereby preventing common-mode noise and high-voltage surges from the bus side from entering the core controller and enhancing the system's communication reliability and anti-interference capability. The low-voltage side control circuit outputs RXD and TXD, which are converted to CANH and CANL by the CAN controller in the isolation circuit. These are then transmitted to the high-voltage side CAN transceiver via the digital isolator. The CAN transceiver converts CANH and CANL back to RXD and TXD, which are then transmitted to the high-voltage side FPGA.

[0033] A key advantage of this embodiment is that it replaces the traditional isolation optocoupler solution with an isolation + push-pull amplifier circuit. Q1 and Q2 are the first and second control switches, primarily using low-voltage MOSFETs. R1 and R2 are the first and second drive resistors, respectively. Q1, Q2, and R1 and R2 together form the drive circuit. The drive circuit parameters can be adjusted as needed, while simultaneously decoupling the turn-on and turn-off circuits for independent control. The resistance value of the first drive resistor R1 is greater than the resistance value of the second drive resistor R2.

[0034] The isolation circuit uses a digital isolator to transmit the drive parameters and duty cycle of the power devices calculated by the control circuit to the control chip through isolated CAN communication.

[0035] Example 2 This embodiment provides a control method for power devices in a string photovoltaic grid-connected inverter, based on the control system for power devices in a string photovoltaic grid-connected inverter described in Embodiment 1, including: When the power devices are turned on, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted according to the relationship between the output current and the rated current of the photovoltaic grid-connected inverter. When the power devices are turned off, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted according to the relationship between the output current and the rated current of the photovoltaic grid-connected inverter, and the relationship between the input DC voltage and the maximum operating voltage.

[0036] Furthermore, when the power devices are turned on, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted based on the relationship between the output current and the rated current of the photovoltaic grid-connected inverter, including: When the output current of the photovoltaic grid-connected inverter is lower than the first percentage of the rated current, the first control switch is closed and the second control switch is opened. When the output current of the photovoltaic grid-connected inverter is greater than the first percentage of the rated current and less than the second percentage of the rated current, the first control switch is disconnected and the second control switch is closed. When the output current of the photovoltaic grid-connected inverter is greater than the second percentage of the rated current, the first control switch and the second control switch are closed simultaneously.

[0037] Furthermore, when the power devices are turned off, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted based on the relationship between the input DC voltage and the maximum operating voltage of the photovoltaic grid-connected inverter, and the relationship between the output current and the rated current. This includes: When the input DC voltage of the photovoltaic grid-connected inverter is less than the set percentage of the maximum operating voltage, the first control switch and the second control switch are closed simultaneously. When the input DC voltage of the PV grid-connected inverter is greater than the set ratio of the maximum operating voltage, the on / off states of the first control switch and the second control switch of the drive circuit are dynamically adjusted according to the relationship between the output current of the PV grid-connected inverter and the rated current.

[0038] Further, dynamically adjusting the on / off states of the first control switch and the second control switch of the drive circuit according to the relationship between the output current of the PV grid-connected inverter and the rated current includes: When the output current of the PV grid-connected inverter is lower than the first ratio of the rated current, the first control switch and the second control switch are closed simultaneously; When the output current of the PV grid-connected inverter is greater than the first ratio of the rated current and less than the second ratio of the rated current, the first control switch is turned off and the second control switch is turned on; When the output current of the PV grid-connected inverter is greater than the second ratio of the rated current, the first control switch is turned on and the second control switch is turned off.

[0039] The control logic of this embodiment is as follows: [[ID=1从ID=14]]As Figure 3 shown, when the power device is turned on, the output current lout of the PV grid-connected inverter is detected; in order to reduce losses and EMC; When lout is lower than 30% of the rated current, the current is small and the switching loss is not the main factor. The first control switch Q1 is closed and the second control switch Q2 is turned off, and the power device is driven through the first drive resistor R1 to reduce the on-state di / dt and du / dt, thereby reducing EMC interference; When 30% of the rated current < lout < 60% of the rated current, the first control switch Q1 is turned off and the second control switch Q2 is turned on, and the power device is driven through the second drive resistor R2. R2 is smaller than R1, which can accelerate the turn-on speed of the power device, reduce losses. At the same time, according to the turn-on characteristics of the power device, as the current increases, the resistance is reduced, keeping di / dt and du / dt unchanged, and reducing EMC interference; When lout > 60% of the rated current, the first control switch Q1 and the second control switch Q2 are closed simultaneously. The first drive resistor R1 and the second drive resistor R2 are in parallel, further reducing the resistance value in the on state, accelerating the turn-on speed of the power device, reducing the power device loss. At the same time, keeping di / dt and du / dt unchanged, reducing EMC interference; By adjusting the drive resistor at different currents, the power device is in the best turn-on state within the full current range, which not only reduces the turn-on loss, improves the efficiency, but also reduces the EMC interference.

[0040] 从ID=25]]As Figure 4As shown, when the power device is turned off, the output current lout and the input DC voltage Vdc of the PV grid-connected inverter are detected. By adjusting the resistance value in the off state, the off-state voltage stress of the power device is reduced, the off loss is reduced, and the EMC interference is reduced; When Vdc < 80% of the maximum operating voltage, since Vdc is relatively low and can withstand a relatively high voltage spike in the off state, a relatively small resistance value in the off state can be selected. At the same time, the first control switch Q1 and the second control switch Q2 are closed, and the first drive resistor R1 and the second drive resistor R2 are in parallel. The resistance value in the off state is the smallest, which improves the off speed of the power device, reduces the loss, and improves the efficiency; When Vdc > 80% of the maximum operating voltage, the output current lout of the PV grid-connected inverter is detected simultaneously. When lout < 30% of the rated current, the first control switch Q1 and the second control switch Q2 are closed simultaneously, and the first drive resistor R1 and the second drive resistor R2 are in parallel. The resistance value in the off state is the smallest, which improves the off speed of the power device, reduces the loss, and improves the efficiency; When 30% of the rated current < lout < 60% of the rated current, the first control switch Q1 is opened, and the second control switch Q2 is closed. The resistance value of the second drive resistor R2 is at an intermediate value, which can not only ensure the lowest off loss but also control the off-state voltage spike voltage not to exceed the dangerous value; When lout ≥ 60% of the rated current, the first control switch Q1 is closed, and the second control switch Q2 is opened. The first drive resistor R1 makes the resistance in the off state the largest, and controls the off-state voltage spike not to exceed the dangerous value even under the maximum current, protecting the power device from overvoltage breakdown.

[0041] This embodiment improves the efficiency of the PV grid-connected inverter, reduces the EMC interference of the PV grid-connected inverter, and reduces the off-state electrical stress of the power device. CAN communication is used to reduce the drive signal interference and reduce the delay. At the same time, the drive parameters are adjusted by the control chip according to the DC input voltage and the AC output current of the PV grid-connected inverter, improving the inverter efficiency and reducing the EMC interference.

[0042] Although the specific implementation manners of the present invention are described above in conjunction with the drawings, it is not a limitation to the protection scope of the present invention. Those skilled in the art should understand that based on the technical solutions of the present invention, various modifications or deformations that can be made without creative efforts by those skilled in the art are still within the protection scope of the present invention.

Claims

1. A control system for power devices in a string photovoltaic grid-connected inverter, characterized in that, It includes a control circuit and an isolation circuit. The control circuit is connected to the photovoltaic grid-connected inverter, collects the output current and input DC voltage of the photovoltaic grid-connected inverter, and calculates the driving parameters and duty cycle of the power devices. The isolation circuit is connected to the control circuit and the control chip respectively, and is used to transmit the driving parameters and duty cycle of the power device calculated by the control circuit to the control chip. The control chip is connected to a push-pull amplifier circuit, which is connected to a drive circuit. The drive circuit includes a first drive branch and a second drive branch connected in parallel. The first drive branch includes a first control switch and a first drive resistor connected in series. The second drive branch includes a second control switch and a second drive resistor connected in series. The drive circuit is connected to the power device and adjusts the switching speed of the power device based on the drive signal amplified by the push-pull amplifier circuit.

2. The control system for the power devices of a string photovoltaic grid-connected inverter as described in claim 1, characterized in that, It also includes an isolation power supply, the primary side of which is connected to the control circuit for power supply; the secondary side of which is connected to the control chip and the push-pull amplifier circuit for power supply.

3. The control system for the power devices of a string photovoltaic grid-connected inverter as described in claim 1, characterized in that, The push-pull amplifier circuit includes two transistors of different types.

4. The control system for the power devices of a string photovoltaic grid-connected inverter as described in claim 1, characterized in that, The isolation circuit uses a digital isolator to transmit the drive parameters and duty cycle of the power devices calculated by the control circuit to the control chip through isolated CAN communication.

5. The control system for the power devices of a string photovoltaic grid-connected inverter as described in claim 1, characterized in that, The resistance of the first driving resistor is greater than the resistance of the second driving resistor.

6. The control system for the power devices of a string photovoltaic grid-connected inverter as described in claim 1, characterized in that, The control circuit uses a digital signal processor circuit.

7. A control method for power devices in a string photovoltaic grid-connected inverter, characterized in that, The control system implementation based on any one of claims 1-6 for a string photovoltaic grid-connected inverter power device includes: When the power devices are turned on, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted according to the relationship between the output current and the rated current of the photovoltaic grid-connected inverter. When the power devices are turned off, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted according to the relationship between the output current and the rated current of the photovoltaic grid-connected inverter, and the relationship between the input DC voltage and the maximum operating voltage.

8. The control method for power devices in a string photovoltaic grid-connected inverter as described in claim 7, characterized in that, When the power devices are turned on, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted according to the relationship between the output current and the rated current of the photovoltaic grid-connected inverter, including: When the output current of the photovoltaic grid-connected inverter is lower than the first percentage of the rated current, the first control switch is closed and the second control switch is opened. When the output current of the photovoltaic grid-connected inverter is greater than the first percentage of the rated current and less than the second percentage of the rated current, the first control switch is disconnected and the second control switch is closed. When the output current of the photovoltaic grid-connected inverter is greater than the second percentage of the rated current, the first control switch and the second control switch are closed simultaneously.

9. The control method for power devices in a string photovoltaic grid-connected inverter as described in claim 7, characterized in that, When the power devices are turned off, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted based on the relationship between the input DC voltage and the maximum operating voltage of the photovoltaic grid-connected inverter, and the relationship between the output current and the rated current. This includes: When the input DC voltage of the photovoltaic grid-connected inverter is less than the set percentage of the maximum operating voltage, the first control switch and the second control switch are closed simultaneously. When the input DC voltage of the photovoltaic grid-connected inverter is greater than the set percentage of the maximum operating voltage, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted according to the relationship between the output current and the rated current of the photovoltaic grid-connected inverter.

10. The control method for power devices in a string photovoltaic grid-connected inverter as described in claim 9, characterized in that, Based on the relationship between the output current and rated current of the photovoltaic grid-connected inverter, the on / off state of the first and second control switches of the drive circuit is dynamically adjusted, including: When the output current of the photovoltaic grid-connected inverter is lower than the first percentage of the rated current, the first control switch and the second control switch are closed simultaneously. When the output current of the photovoltaic grid-connected inverter is greater than the first percentage of the rated current and less than the second percentage of the rated current, the first control switch is disconnected and the second control switch is closed. When the output current of the photovoltaic grid-connected inverter is greater than the second percentage of the rated current, the first control switch is closed and the second control switch is opened.

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