Flying capacitor three-level dc-dc converter, photovoltaic system and control method

Abstract

The application discloses a flying capacitor three-level DCDC converter, a photovoltaic system and a control method. The flying capacitor three-level DCDC converter comprises an inductor, a first switch tube, a second switch tube, a first diode, a second diode, a third diode, a flying capacitor and a controller. The difference between the DC bus voltage and the voltage of the flying capacitor is greater than or equal to the withstand voltage of the second diode, the controller controls the DCDC converter to be inoperative, and the DC bus voltage is reduced. When the DC bus voltage is too high or the flying capacitor pre-charge voltage is too low, the DCDC converter works at this time, and the second diode will bear too high voltage, which is easy to damage the second diode. Therefore, by reducing the DC bus voltage, the flying capacitor Cf will be charged and lifted synchronously until the difference between the DC bus voltage and the voltage of the flying capacitor is less than the withstand voltage of the second diode, at which time the DCDC converter works, and the safety of the second diode can be ensured.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, specifically to a flying capacitor three-level DC-DC converter, a photovoltaic system, and a control method. Background Technology

[0002] To improve the power generation efficiency of photovoltaic (PV) strings, a DC-DC converter is typically added between the PV strings and the inverter. This usually includes multiple DC-DC converters, with their outputs connected in parallel to the inverter's input. High-voltage systems commonly use three-level boost converters. Compared to two-level boost circuits, three-level boost circuits halve the voltage stress on power devices, significantly reduce input current ripple, and lower inductor size and cost.

[0003] See Figure 1 The figure shows a schematic diagram of a three-level DC-DC converter with a flying capacitor and a clamping diode.

[0004] A three-level Boost circuit with a flying capacitor includes: an inductor L, a first switching transistor Q1, a second switching transistor Q2, a first diode D1, a second diode D2, a third diode D3, and a flying capacitor Cf; it also includes an input capacitor Cin, with an input voltage of Vin. The Boost circuit also includes two output capacitors Co1 and Co2 connected in series, with their common point being the midpoint of the DC bus. The output voltage is the DC bus voltage Vbus. To synchronously pre-charge the flying capacitor and ensure the voltage stress requirements of each power device during Boost circuit startup, a clamping diode D3 is also included. The second terminal of the flying capacitor Cf is connected to the midpoint of the DC bus through the third diode D3. The first terminal of the flying capacitor Cf is connected to the common terminal of D1 and D2.

[0005] Because the outputs of multiple Boost circuits in a photovoltaic system are connected in parallel, some Boost circuits may have high input voltages while others have low input voltages. When multiple Boost outputs are connected in parallel, their parallel output DC bus voltage is determined by the Boost circuit with the highest input voltage. This will cause the flying capacitor voltage of the Boost circuit with the lower input voltage to only charge to a level far less than half the bus voltage, or even fail to achieve pre-charging. If the switching transistor of the Boost circuit with the lower input voltage is activated at this time, its diode D2 will experience excessively high reverse voltage, leading to the risk of overvoltage failure. Summary of the Invention

[0006] To address the aforementioned issues, this application provides a flying capacitor three-level DC-DC converter, a photovoltaic system, and a control method, which can protect the safety of each power device in the flying capacitor three-level DC-DC converter.

[0007] This application provides a flying capacitor three-level DC-DC converter, including: an inductor, a first switching transistor, a second switching transistor, a first diode, a second diode, a third diode, a flying capacitor, and a controller;

[0008] The first end of the inductor is connected to the positive input terminal of the DC-DC converter, and the second end of the inductor, the anode of the first diode, and the first end of the first switching transistor are all connected to the first node;

[0009] The cathode of the first diode, the anode of the second diode, and the first terminal of the flying capacitor are all connected to the second node;

[0010] The second terminal of the first switching transistor is connected to the negative input terminal of the DC-DC converter through the second switching transistor. The second terminal of the flying capacitor is connected to the midpoint of the DC bus through the third diode. The cathode of the second diode is connected to the positive output terminal of the DC-DC converter. The negative output terminal and the negative input terminal of the DC-DC converter are connected together.

[0011] The controller is used to prevent the DC-DC converter from operating and reduce the DC bus voltage when the difference between the DC bus voltage and the voltage of the flying capacitor is greater than or equal to the withstand voltage of the second diode.

[0012] Preferably, the controller is specifically used to control the operation of the DCAC circuit to reduce the DC bus voltage, and the input terminal of the DCAC circuit is used to connect to the DC bus;

[0013] or,

[0014] Control the operation of the load connected to the DC bus to reduce the DC bus voltage.

[0015] Preferably, the controller is also used to control the operation of the DC-DC converter when the difference between the DC bus voltage and the voltage of the flying capacitor is less than the withstand voltage of the second diode.

[0016] Preferably, the controller is specifically used to control the duty cycle of the second switch to be greater than the duty cycle of the first switch when the voltage of the flying capacitor is less than a preset voltage value; and to control the duty cycle of the second switch to be less than the duty cycle of the first switch when the voltage of the flying capacitor is greater than the preset voltage value.

[0017] Preferably, the controller is also used to ensure that the voltage across the capacitor is equal to a preset voltage value, and to control the duty cycle of the second switch to be equal to the duty cycle of the first switch.

[0018] This application also provides a photovoltaic system, including at least two of the described flying capacitor three-level DC-DC converters; and also includes: a DCAC circuit;

[0019] The outputs of at least two flying capacitor three-level DC-DC converters are connected in parallel to the input of the DCAC circuit.

[0020] The input of each flying capacitor three-level DC-DC converter is used to connect to the corresponding photovoltaic string.

[0021] This application also provides a control method for a three-level DC-DC converter with a flying capacitor. The DC-DC converter includes: an inductor, a first switching transistor, a second switching transistor, a first diode, a second diode, a third diode, and a flying capacitor.

[0022] The method includes:

[0023] Obtain the DC bus voltage and the voltage of the flying capacitor;

[0024] If the difference between the DC bus voltage and the voltage of the flying capacitor is greater than or equal to the withstand voltage of the second diode, the DC-DC converter is controlled to stop working, thereby reducing the DC bus voltage.

[0025] Preferably, reducing the DC bus voltage specifically includes:

[0026] The DCAC circuit is controlled to reduce the DC bus voltage. The input terminal of the DCAC circuit is used to connect to the DC bus.

[0027] or,

[0028] Controlling the operation of loads connected to the DC bus to reduce DC bus voltage

[0029] Preferably, it further includes: the difference between the DC bus voltage and the voltage of the flying capacitor is less than the withstand voltage of the second diode, thereby controlling the operation of the DC-DC converter.

[0030] Preferably, controlling the operation of the DC-DC converter specifically includes:

[0031] When the voltage across the flying capacitor is less than the preset voltage value, the duty cycle of the second switch is controlled to be greater than that of the first switch.

[0032] When the voltage across the flying capacitor is greater than the preset voltage value, the duty cycle of the second switch is controlled to be less than that of the first switch.

[0033] Preferably, it further includes: the voltage of the flying capacitor is equal to a preset voltage value, and the duty cycle of the second switching transistor is controlled to be equal to the duty cycle of the first switching transistor.

[0034] Therefore, this application has the following beneficial effects:

[0035] The flying capacitor three-level DC-DC converter provided in this application selects whether to start the three-level DC-DC converter by judging whether the difference between the DC bus voltage and the voltage of the flying capacitor is greater than the withstand voltage of the second diode. When the difference between the DC bus voltage and the voltage of the flying capacitor is greater than or equal to the withstand voltage of the second diode, it indicates that the DC bus voltage is too high or the pre-charge voltage of the flying capacitor is too low. At this time, if the DC-DC converter operates, the second diode will be subjected to too high a voltage, which may easily damage the second diode. Therefore, by reducing the DC bus voltage, the flying capacitor Cf will be charged and raised synchronously until the difference between the DC bus voltage and the voltage of the flying capacitor is less than the withstand voltage of the second diode. At this time, the DC-DC converter operates, which can ensure the safety of the second diode. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of a three-level Boost circuit with a flying capacitor;

[0037] Figure 2 A schematic diagram of a photovoltaic system provided in this application;

[0038] Figure 3 This is a schematic diagram illustrating the voltage withstand capability of the second diode provided in this application.

[0039] Figure 4 A schematic diagram of a flying capacitor three-level DC-DC converter provided in an embodiment of this application;

[0040] Figure 5 A schematic diagram of a photovoltaic system provided in an embodiment of this application;

[0041] Figure 6 A flowchart illustrating a control method for a flying capacitor three-level DC-DC converter provided in an embodiment of this application. Detailed Implementation

[0042] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the embodiments of this application will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0043] The specific application scenarios of the embodiments in this application are not limited to the flying capacitor three-level DC-DC converter. As long as the output terminals of multiple DC-DC converters are connected in parallel, it can be used in photovoltaic system scenarios, where the input terminal of each DC-DC converter is connected to the corresponding photovoltaic string.

[0044] For ease of description, the flying capacitor three-level DC-DC converter will be referred to as a DC-DC converter in the following text.

[0045] The following section uses the application of DC-DC converters in photovoltaic systems as an example.

[0046] See Figure 2 The figure shows a schematic diagram of a photovoltaic system.

[0047] A photovoltaic system consists of two stages: a DC-DC converter and a DC-AC circuit. This explanation will focus on the DC-DC converter, which includes a boost circuit.

[0048] For ease of description, we will take two Boost circuits connected in parallel as an example.

[0049] The input terminal of the first Boost circuit 101 is connected to the photovoltaic string PV1, and the input terminal of the second Boost circuit 102 is connected to the photovoltaic string PV2. The output terminals of the first Boost circuit 101 and the second Boost circuit 102 are connected in parallel and both are connected to the input terminal of the DCAC circuit 103. The output terminal of the DCAC circuit 103 can be connected to the power grid or to an AC load. The DCAC circuit 103 can be three-phase or single-phase.

[0050] The following description uses a three-level Boost circuit with a flying capacitor as an example.

[0051] See also Figure 1 .

[0052] A three-level Boost circuit with a flying capacitor includes: an inductor L, a first switch Q1, a second switch Q2, a first diode D1, a second diode D2, a third diode D3, and a flying capacitor Cf;

[0053] The first end of the inductor L is connected to the positive input terminal of the DC-DC converter, and the second end of the inductor L, the anode of the first diode D1, and the first end of the first switch Q1 are all connected to the first node.

[0054] The cathode of the first diode D1, the anode of the second diode D2, and the first terminal of the flying capacitor Cf are all connected to the second node;

[0055] The second terminal of the first switching transistor Q1 is connected to the negative input terminal of the DC-DC converter through the second switching transistor Q2. The second terminal of the flying capacitor Cf is connected to the midpoint of the DC bus through the third diode D3. The cathode of the second diode D2 is connected to the positive output terminal of the DC-DC converter. The negative output terminal and the negative input terminal of the DC-DC converter are connected together.

[0056] D3 can perform clamping, simultaneously pre-charging the flying capacitor Cf and clamping the voltage stress of the switching transistor when the input is powered on. It pre-charges the voltage of the flying capacitor Cf to nearly half of the bus voltage, thus eliminating the need for an additional pre-charging circuit and ensuring the voltage stress requirements of each power device when the Boost circuit starts up.

[0057] In addition, the Boost circuit includes an input capacitor Cin, which is connected in parallel between the positive and negative input terminals of the Boost circuit. The input voltage is Vin. The Boost circuit also includes two output capacitors Co1 and Co2 connected in series. The common point of Co1 and Co2 is the midpoint of the DC bus. In principle, the capacitance values ​​of Co1 and Co2 are equal. The voltage at the midpoint of the DC bus is half of the DC bus voltage Vbus. The output voltage is the DC bus voltage Vbus.

[0058] It should be understood that since the outputs of multiple Boost circuits are connected in parallel to the DC bus, the DC bus voltage Vbus will affect the output voltage of a single Boost circuit. In steady state, the output voltage of each Boost circuit will be consistent with the DC bus voltage Vbus.

[0059] In photovoltaic system applications, the input voltage of each Boost circuit may differ. For example, due to the influence of power plant site terrain, solar irradiance, and external environmental factors (such as the installation tilt angle of photovoltaic modules, module obstruction by clouds or vegetation, snow / dust / sandstorm coverage, etc.), the input voltage of each Boost circuit may vary significantly.

[0060] Because the outputs of multiple Boost circuits are connected in parallel, the DC bus voltage Vbus is determined by the Boost circuit with the highest input voltage. This results in the flying capacitor voltage of a Boost circuit with a lower input voltage only charging to a level far less than half of the DC bus voltage Vbus, i.e., far less than half the bus voltage, or even failing to pre-charge. Furthermore, due to the low voltage of the flying capacitor Cf, if the switching transistor is activated (e.g., Q2 conducts), diode D2 is at risk of overvoltage (Vbus - Vcf) failure. A detailed analysis is provided below with reference to the attached diagram.

[0061] See Figure 3 The figure is a schematic diagram of the voltage withstand capability of the second diode provided in this application.

[0062] Taking a 1500V photovoltaic system as an example, which includes two Boost circuits connected in parallel, assuming that the input voltage Vin of one Boost circuit is 800V and the input voltage of the other Boost circuit is 1400V, the output bus voltage will reach 1400V.

[0063] For a Boost circuit with an input voltage Vin of 800V, since the half-bus voltage, i.e. the voltages on Co1 and Co2 are equal, both are Vco2 = 700V, the input voltage Vin can only precharge the voltage of the flying capacitor Cf to 100V. If the switching transistor Q2 is directly turned on at this time, the diode D2 will be subject to the risk of high voltage failure (VD2 = Vbus - Vcf = 1400V - 100V = 1300V), where Vcf represents the voltage of the flying capacitor Cf.

[0064] To address the above-mentioned technical problems, this application provides a flying capacitor three-level DC-DC converter, which will be described in detail below with reference to the accompanying drawings.

[0065] See Figure 4 The figure is a schematic diagram of a flying capacitor three-level DC-DC converter provided in an embodiment of this application.

[0066] The connection relationships of the various components in the flying capacitor three-level DC-DC converter provided in this embodiment can be found in [reference]. Figure 2 The description will not be repeated here.

[0067] The controller 400 is used to control the DC-DC converter to stop working and reduce the DC bus voltage Vbus when the difference between the DC bus voltage Vbus and the voltage of the flying capacitor Cf is greater than or equal to the withstand voltage of the second diode D2.

[0068] The embodiments of this application do not specifically limit the withstand voltage of the second diode D2. The second diode can be selected according to the DC bus voltage level of the application scenario of the flying capacitor three-level DC-DC converter. For example, the DC bus voltage level can be around 1500V, or it can be a higher voltage level or a lower voltage level.

[0069] As the DC bus voltage decreases, the flying capacitor Cf is gradually charged, and the voltage of the flying capacitor Cf gradually increases.

[0070] The embodiments of this application do not specifically limit the specific method of reducing the DC bus voltage Vbus. For example, a controller is specifically used to control the operation of the DCAC circuit to reduce the DC bus voltage Vbus. The input terminal of the DCAC circuit is used to connect to the DC bus. The DCAC circuit can reduce the DC bus voltage Vbus by controlling the change of electrical parameters.

[0071] or,

[0072] The DC bus voltage Vbus is reduced by controlling the operation of the load connected to the DC bus, that is, by consuming energy to reduce the DC bus voltage Vbus.

[0073] For example, the load can be a switching power supply device or a discharge circuit device, etc.

[0074] The controller 400 is also used to control the DC-DC converter to operate when the difference between the DC bus voltage Vbus and the voltage of the flying capacitor Cf is less than the withstand voltage of the second diode D2. That is, when the DC-DC converter operates, the second diode D2 will not be subjected to excessively high withstand voltage and will not be damaged.

[0075] The controller 400 controls the operation of the DC-DC converter by sending drive signals to Q1 and Q2, thus controlling their switching states. When the controller 400 stops the DC-DC converter from operating, it stops sending drive signals, effectively blocking the waveform; Q1 and Q2 remain disconnected and do not operate.

[0076] The flying capacitor three-level DC-DC converter provided in this application selects whether to start the three-level DC-DC converter by judging whether the difference between the DC bus voltage and the voltage of the flying capacitor is greater than the withstand voltage of the second diode. When the difference between the DC bus voltage and the voltage of the flying capacitor is greater than or equal to the withstand voltage of the second diode, it indicates that the DC bus voltage is too high or the pre-charge voltage of the flying capacitor is too low. At this time, if the DC-DC converter is working, the second diode will be subjected to too high a voltage, which may easily damage the second diode. Therefore, by reducing the DC bus voltage, the flying capacitor Cf will be charged and raised synchronously until the difference between the DC bus voltage and the voltage of the flying capacitor is less than the withstand voltage of the second diode. At this time, the DC-DC converter can operate to ensure the safety of the second diode.

[0077] The flying capacitor three-level DC-DC converter provided in this application embodiment can directly start the DC-DC converter even under conditions of high voltage on the DC bus and low voltage input of a certain DC-DC converter. Therefore, it can broaden the MPPT operating range of the DC-DC converter and thus increase the system power generation. At the same time, this solution does not require additional costs. By adopting a software control scheme, it can solve the voltage stress risk of the diodes and the pre-charging problem of the floating capacitor voltage when starting the DC-DC converter corresponding to the low voltage input path under high voltage conditions.

[0078] The controller controls the DC-DC converter to operate, which can specifically include the following three situations:

[0079] First:

[0080] The controller is specifically used to control the duty cycle of the second switch to be greater than that of the first switch when the voltage of the flying capacitor is less than a preset voltage value; that is, in order to charge the flying capacitor, the voltage of the flying capacitor is increased.

[0081] second:

[0082] When the voltage across the flying capacitor exceeds a preset voltage value, the duty cycle of the second switch is reduced to be less than that of the first switch. In other words, to discharge the flying capacitor, its voltage is lowered.

[0083] third:

[0084] The controller is also used to ensure that the voltage across the capacitor is equal to a preset voltage value, and to control the duty cycle of the second switch to be equal to the duty cycle of the first switch.

[0085] The flying capacitor three-level DC-DC converter provided in this application embodiment can not only ensure the safety of the second diode, but also stabilize the voltage of the flying capacitor at a preset voltage value while ensuring the safe operation of the second diode.

[0086] Furthermore, the flying capacitor three-level DC-DC converter provided in this application embodiment can also detect the input voltage of the flying capacitor three-level DC-DC converter in real time, i.e., the voltage of the photovoltaic string. Since the voltage of the flying capacitor three-level DC-DC converter is too low, even if the converter is not working, the voltage of the flying capacitor cannot be increased by charging. Even if the DC bus voltage is reduced using the method provided above, it still cannot satisfy the requirement that the difference between the DC bus voltage and the voltage of the flying capacitor be less than the withstand voltage of the second diode. Therefore, the control mode described above can be determined by judging the input voltage of the flying capacitor three-level DC-DC converter, i.e., controlling the DC-DC converter to not work and reducing the DC bus voltage.

[0087] Based on the flying capacitor three-level DC-DC converter provided in the above embodiments, this application also provides a photovoltaic system, which will be described in detail below with reference to the accompanying drawings.

[0088] See Figure 5 The figure is a schematic diagram of a photovoltaic system provided in an embodiment of this application.

[0089] The photovoltaic system provided in this application includes at least the flying capacitor three-level DC-DC converter described above. This application does not specifically limit the number of flying capacitor three-level DC-DC converters connected in parallel; for ease of description, the specific number is not limited. Figure 5 This paper uses two flying capacitor three-level DC-DC converters as an example. Specifically, there is a first flying capacitor three-level DC-DC converter 101 and a second flying capacitor three-level DC-DC converter 102. The input terminal of each flying capacitor three-level DC-DC converter is used to connect to the corresponding photovoltaic string. That is, the first flying capacitor three-level DC-DC converter 101 is connected to PV1, and the input terminal of the second flying capacitor three-level DC-DC converter 102 is connected to PV2.

[0090] The photovoltaic system also includes: DCAC circuit 103;

[0091] The outputs of at least two flying capacitor three-level DC-DC converters are connected in parallel to the input of DCAC circuit 103; that is, the outputs of the first flying capacitor three-level DC-DC converter 101 and the second flying capacitor three-level DC-DC converter 102 are both connected to the input of DCAC circuit 103.

[0092] For example, the input voltage Vin of the first flying capacitor three-level DC-DC converter 101 is 800V, and the input voltage of the second flying capacitor three-level DC-DC converter 102 is 1400V. At this time, the DC bus voltage Vbus will reach 1400V. If Q2 in the first flying capacitor three-level DC-DC converter 101 is turned on, D2 in the first flying capacitor three-level DC-DC converter 101 will be at risk of failure. Therefore, in order to protect D2, the controller controls both the first flying capacitor three-level DC-DC converter 101 and the second flying capacitor three-level DC-DC converter 102 to be inactive, reducing the DC bus voltage Vbus so that the difference between the DC bus voltage and the voltage of the flying capacitor is less than the withstand voltage of D2, before controlling the first flying capacitor three-level DC-DC converter 101 and the second flying capacitor three-level DC-DC converter 102 to operate.

[0093] The photovoltaic system provided in this application embodiment can ensure the safety of the second diode in each flying capacitor three-level DC-DC converter. Therefore, it can ensure the safety of each flying capacitor three-level DC-DC converter, thereby enabling the photovoltaic system to work normally and improving the power generation efficiency of the photovoltaic system.

[0094] Based on the above embodiments, a flying capacitor three-level DC-DC converter and photovoltaic system are provided. This application also provides a control method for the flying capacitor three-level DC-DC converter, which will be described in detail below with reference to the accompanying drawings.

[0095] See Figure 6 The figure is a flowchart of a control method for a flying capacitor three-level DC-DC converter provided in an embodiment of this application.

[0096] The control method for the flying capacitor three-level DC-DC converter provided in this embodiment is applied to the flying capacitor three-level DC-DC converter described in the above embodiment. The flying capacitor three-level DC-DC converter includes: an inductor, a first switching transistor, a second switching transistor, a first diode, a second diode, a third diode, and a flying capacitor.

[0097] The method includes:

[0098] S601: Obtain the DC bus voltage and the voltage of the flying capacitor;

[0099] S601: If the difference between the DC bus voltage and the voltage of the flying capacitor is greater than or equal to the withstand voltage of the second diode, the DC-DC converter will be deactivated to reduce the DC bus voltage.

[0100] Reducing the DC bus voltage specifically includes:

[0101] The DCAC circuit is controlled to reduce the DC bus voltage. The input terminal of the DCAC circuit is used to connect to the DC bus.

[0102] or,

[0103] Control the operation of the load connected to the DC bus to reduce the DC bus voltage.

[0104] The control method provided in this embodiment further includes: the difference between the DC bus voltage and the voltage of the flying capacitor is less than the withstand voltage of the second diode, thereby controlling the DC-DC converter to work.

[0105] The control of the DC-DC converter specifically includes:

[0106] When the voltage across the flying capacitor is less than the preset voltage value, the duty cycle of the second switch is controlled to be greater than that of the first switch.

[0107] When the voltage across the flying capacitor is greater than the preset voltage value, the duty cycle of the second switch is controlled to be less than that of the first switch.

[0108] The method further includes: setting the voltage of the flying capacitor to a preset voltage value, and controlling the duty cycle of the second switching transistor to be equal to the duty cycle of the first switching transistor. This ensures that the charging and discharging of the flying capacitor remains balanced.

[0109] The control method for the flying capacitor three-level DC-DC converter provided in this application selects whether to enable the flying capacitor three-level DC-DC converter by judging whether the difference between the DC bus voltage and the voltage of the flying capacitor is greater than the withstand voltage of the second diode. When the difference between the DC bus voltage and the voltage of the flying capacitor is greater than or equal to the withstand voltage of the second diode, it indicates that the DC bus voltage is too high or the pre-charge voltage of the floating capacitor is too low. At this time, if the DC-DC converter is working, the second diode will be subjected to too high a voltage, which may easily damage the second diode. Therefore, by reducing the DC bus voltage, the flying capacitor Cf will be charged and raised synchronously until the difference between the DC bus voltage and the voltage of the flying capacitor is less than the withstand voltage of the second diode. At this time, the DC-DC converter can operate, which can ensure the safety of the second diode.

[0110] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A flying capacitor three-level DCDC converter, characterized by, The flyback capacitor three-level DCDC converter comprises: an inductor, a first switch tube, a second switch tube, a first diode, a second diode, a third diode, a flyback capacitor and a controller; a first end of the inductor is connected to a positive input end of the DCDC converter, a second end of the inductor, an anode of the first diode and a first end of the first switch tube are all connected to a first node; a cathode of the first diode, an anode of the second diode and a first end of the flyback capacitor are all connected to a second node; a second end of the first switch tube is connected to a negative input end of the DCDC converter through the second switch tube, a second end of the flyback capacitor is connected to a DC bus midpoint through the third diode, a cathode of the second diode is connected to a positive output end of the DCDC converter, and a negative output end of the DCDC converter is connected to the negative input end together; the controller is configured to control the DCDC converter to be inactivated when a difference between a DC bus voltage and a voltage of the flyback capacitor is greater than or equal to a withstand voltage of the second diode, so as to reduce the DC bus voltage.

2. The DCDC converter of claim 1, wherein, the controller is specifically configured to control a DCAC circuit to be activated to reduce the DC bus voltage, and an input end of the DCAC circuit is configured to be connected to the DC bus; alternatively, a load connected to the DC bus is controlled to be activated to reduce the DC bus voltage.

3. The DCDC converter of claim 1 or 2, wherein, the controller is further configured to control the DCDC converter to be activated when the difference between the DC bus voltage and the voltage of the flyback capacitor is less than the withstand voltage of the second diode.

4. The DCDC converter of claim 3, wherein, the controller is specifically configured to control a duty cycle of the second switch tube to be greater than a duty cycle of the first switch tube when the voltage of the flyback capacitor is less than a preset voltage value, and control the duty cycle of the second switch tube to be less than the duty cycle of the first switch tube when the voltage of the flyback capacitor is greater than the preset voltage value.

5. The DCDC converter of claim 4, wherein, the controller is further configured to control the duty cycle of the second switch tube to be equal to the duty cycle of the first switch tube when the voltage of the flyback capacitor is equal to the preset voltage value.

6. A photovoltaic system characterized by, The flyback capacitor three-level DCDC converter comprises at least two flyback capacitor three-level DCDC converters according to any one of claims 1-5, and further comprises a DCAC circuit. output ends of the at least two flyback capacitor three-level DCDC converters are connected together to an input end of the DCAC circuit. an input end of each of the flyback capacitor three-level DCDC converters is configured to be connected to a corresponding photovoltaic string.

7. A control method of a flying capacitor three-level DCDC converter, characterized by, The DCDC converter comprises an inductor, a first switch tube, a second switch tube, a first diode, a second diode, a third diode and a flying capacitor; a first end of the inductor is connected to a positive input end of the DCDC converter, a second end of the inductor, an anode of the first diode and a first end of the first switch tube are all connected to a first node; a cathode of the first diode, an anode of the second diode and a first end of the flying capacitor are all connected to a second node; a second end of the first switch tube is connected to a negative input end of the DCDC converter through the second switch tube, a second end of the flying capacitor is connected to a DC bus midpoint through the third diode, a cathode of the second diode is connected to a positive output end of the DCDC converter, and a negative output end and the negative input end of the DCDC converter are connected together. The method comprises: obtaining a DC bus voltage and a voltage of the flying capacitor; determining that a difference between the DC bus voltage and the voltage of the flying capacitor is greater than or equal to a withstand voltage of the second diode, and controlling the DCDC converter to be inoperative to reduce the DC bus voltage.

8. The control method according to claim 7, characterized by The reduction of the DC bus voltage specifically comprises: controlling a DCAC circuit to be operative to reduce the DC bus voltage, an input end of the DCAC circuit being used to be connected to the DC bus; or, controlling a load connected to the DC bus to be operative to reduce the DC bus voltage.

9. The control method according to claim 7 or 8, characterized by, Further comprising: the difference between the DC bus voltage and the voltage of the flying capacitor is less than the withstand voltage of the second diode, and the DCDC converter is controlled to be operative.

10. The control method according to claim 9, characterized by, The control of the DCDC converter to be operative specifically comprises: the voltage of the flying capacitor is less than a preset voltage value, and a duty cycle of the second switch tube is controlled to be greater than a duty cycle of the first switch tube; the voltage of the flying capacitor is greater than the preset voltage value, and the duty cycle of the second switch tube is controlled to be less than the duty cycle of the first switch tube.

11. The control method according to claim 10, characterized by Further comprising: the voltage of the flying capacitor is equal to the preset voltage value, and the duty cycle of the second switch tube is controlled to be equal to the duty cycle of the first switch tube.

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