Three-level DC-DC converter for photovoltaic hydrogen production system and control method of three-level DC-DC converter

Abstract

The invention discloses a three-level DC-DC converter for a photovoltaic hydrogen production system and a control method of the three-level DC-DC converter, and belongs to the technical field of power electronics. The converter comprises a pre-stage three-level voltage reduction circuit and a post-stage double-Buck circuit. The front-stage three-level voltage reduction circuit comprises an input source Ui, a first switch tube S1, a second switch tube S2, a first independent inductor L1, a diode D1, a diode D2, a capacitor C1 and a capacitor C2; the rear-stage double-Buck circuit comprises a third switch tube S3, a fourth switch tube S4, a second independent inductor L2, a third independent inductor L3, a diode D3, a diode D4, a capacitor C3, a capacitor C4 and a capacitor C0. The invention has the advantages of simple topological structure, convenient switch control, strong voltage reduction capability, low power device voltage / current stress, high electric energy conversion efficiency, excellent dynamic performance and the like.

Description

A three-level DC-DC converter for photovoltaic hydrogen production systems and its control method Technical Field

[0001] This invention relates to the field of power electronics technology, and more specifically, to a three-level DC-DC converter and its control method for a photovoltaic hydrogen production system. Background Technology

[0002] Hydrogen energy, as a clean secondary energy source, has attracted widespread attention due to its high energy density, large-scale long-term storage capability, and pollution-free emissions. Compared to traditional fossil fuel-based hydrogen production methods (such as steam reforming) and bio-based hydrogen production, water electrolysis offers significant advantages in terms of environmental friendliness, sustainability, and safety. Photovoltaic hydrogen production systems can be categorized into direct coupling and indirect coupling based on the presence or absence of a power electronic converter between the photovoltaic cells and the electrolyzer. Both require addressing the characteristic matching issues (volt-ampere characteristics, maximum efficiency point, dynamic response, etc.) between the photovoltaic cells and the electrolyzer ports; otherwise, stable system operation and efficiency cannot be guaranteed. In comparison, indirect coupling, which includes a power electronic converter, allows for more precise system control and is therefore more advantageous.

[0003] As one of the two mainstream hydrogen production electrolyzers, alkaline electrolyzers require power supplies with low voltage and high current, wide output voltage range, low current ripple, and high-efficiency conversion capabilities. Traditional Buck circuits, the most common non-isolated converters in hydrogen production systems, are widely used due to their simple structure, convenient control, and low cost. However, due to inherent limitations, they have limited voltage reduction capabilities, large current ripple, low energy conversion efficiency, and high voltage stress, leading to strong nonlinear disturbances in the electrolyzer. Secondary Buck converters, consisting of two Buck circuits connected in series, offer stronger voltage reduction capabilities compared to traditional Buck converters. However, the voltage stress on the switching transistors is higher, and cascading affects efficiency. Cascaded three-level Buck converters have only half the voltage stress on the switching transistors of traditional Buck converters and offer some efficiency improvement. However, coupling effects may exist between different power stages, affecting circuit stability.

[0004] Compared to non-isolated converters, isolated converters can utilize transformers for voltage reduction and achieve electrical isolation, while also reducing circulating current in multi-machine parallel operation. Half-bridge resonant converters offer advantages such as high step-down ratios and full-range soft-switching. However, their fault tolerance is limited, resulting in lower reliability and higher transformer current stress, making them unsuitable for high-power hydrogen production applications. Some researchers have added two initial voltage-dividing capacitors to a half-bridge resonant converter to create a novel converter. This new converter maintains high efficiency over a wider power range, enabling efficient hydrogen production. However, this converter exhibits significant output current ripple, requiring the installation of a filter. Summary of the Invention

[0005] 1. Technical Problem to be Solved by the Invention: This invention addresses the issues of improving the voltage reduction capability and power conversion efficiency of power electronic converter topologies in photovoltaic hydrogen production systems, as well as resolving strong nonlinear disturbances in electrolyzers. It provides a three-level DC-DC converter and its control method for photovoltaic hydrogen production systems. This invention is a two-stage non-isolated three-level DC / DC converter. Based on a three-level Buck circuit, it adds a subsequent double-Buck circuit, improving the converter's voltage reduction capability while ensuring reduced voltage stress on the switching transistors. This meets the low-voltage, high-current requirements of the electrolyzer, and also improves efficiency. Furthermore, the converter has a simple topology, is easy to control, and has high operational reliability.

[0006] 2. Technical Solution To achieve the above objectives, the technical solution provided by this invention is as follows: A three-level DC-DC converter for a photovoltaic hydrogen production system, comprising a front-stage three-level step-down circuit and a rear-stage dual-Buck circuit; the front-stage three-level step-down circuit includes an input source U... i The circuit comprises a first switching transistor S1, a second switching transistor S2, a first independent inductor L1, diodes D1 and D2, capacitors C1 and C2; the subsequent dual-Buck circuit includes a third switching transistor S3, a fourth switching transistor S4, a second independent inductor L2, a third independent inductor L3, diodes D3 and D4, capacitors C3, C4, and C0; the front-stage three-level buck converter adopts a midpoint clamping structure, and the subsequent dual-Buck circuit adopts an embedded structure, wherein the front-stage three-level buck converter is connected to the subsequent dual-Buck circuit.

[0007] Furthermore, in the aforementioned front-end three-level buck circuit, the input source U iThe positive terminal of the first switch S1 is connected to the positive terminal of capacitor C1 and the drain of the first switch S1; the negative terminal of capacitor C1 is connected to the positive terminal of capacitor C2, the anode of diode D1, and the cathode of diode D2; the source of the first switch S1 is connected to the cathode of diode D1 and one end of the first independent inductor L1; the other end of the first independent inductor L1 is connected to the positive terminal of capacitor C3 and the drain of the third switch S3; the drain of the second switch S2, the anode of diode D2, and the low potential terminal of the load are connected; the source of the second switch S2 is connected to the power supply U. i The negative terminal of capacitor C2 is connected to the negative terminal of capacitor C2.

[0008] Furthermore, in the subsequent double-Buck circuit, the source of the third switch S3 is connected to the cathode of the diode D3 and one end of the second independent inductor L2; the anode of the diode D3 is connected to the source of the fourth switch S4 and the cathode of the capacitor C4; the anode of the capacitor C4 is connected to the cathode of the diode D4 and one end of the third independent inductor L3; the other end of the second independent inductor L2 is connected to the other end of the third independent inductor L3, the anode of the capacitor C0, and the high-potential end of the load; the drain of the fourth switch S4 is connected to the cathode of the capacitor C3, the anode of the diode D4, the cathode of the capacitor C0, and the low-potential end of the load.

[0009] Furthermore, it also includes a control unit, which is configured to, in continuous inductor current mode, input control signals of the first switch S1 and the second switch S2 with a phase difference of duty cycle D1·180°, and the control signals of the third switch S3 and the fourth switch S4 are the same, and are logically ORed with the control signals of the first switch S1 and the second switch S2.

[0010] Furthermore, the duty cycle of the first switch S1 and the second switch S2 is D1, and the duty cycle of the third switch S3 and the fourth switch S4 is D2, where D2 = 3D1 / 2, and the duty cycle D1 is less than 0.5.

[0011] Furthermore, the converter includes four operating modes within one switching cycle, with the first to fourth operating modes executed sequentially, wherein: the first operating mode controls the second switch S2, the third switch S3, and the fourth switch S4 to be turned on simultaneously, and the diode D1 is turned on; the second operating mode controls all switches to be turned on simultaneously, and all diodes to be turned off simultaneously; the third operating mode controls the first switch S1, the third switch S3, and the fourth switch S4 to be turned on simultaneously, and the diode D2 is turned on; the fourth operating mode controls all switches to be turned off simultaneously, and all diodes to be turned on simultaneously.

[0012] Furthermore, in continuous inductor current mode, when the duty cycle D1 of the first switch S1 and the second switch S2 is less than 0.5, the converter's buck ratio is:

[0013] In the formula, U i U is the input voltage, and U0 is the output voltage.

[0014] Furthermore, in continuous inductor current mode, when the duty cycle D1 of the first switch S1 and the second switch S2 is less than 0.5, the voltage stress on the switches and diodes of the converter is:

[0015] In the formula, U S1 U S2 U S3 U S4 The voltage stresses of switching transistors S1, S2, S3, and S4 are respectively, U D1 U D2 U D3 U D4 These are the voltage stresses of diodes D1, D2, D3, and D4, respectively, and D2 is the duty cycle of the third switch S3 and the fourth switch S4.

[0016] Furthermore, when the duty cycle D1 of the first switch S1 and the second switch S2 is 0.3764, the buck ratio of the converter is 0.12; at the input source U i When the voltage is 400V, the output voltage of the converter is 48V, and the voltage stress of the switching transistor is 200V.

[0017] This invention discloses a control method for a three-level DC-DC converter, wherein: in the first operating mode, at time t0, control switches S2, S3, and S4 are turned on, switch S1 is turned off, diode D1 is turned on under forward voltage, and diodes D2, D3, and D4 are turned off under reverse voltage; capacitors C2, C3, and C4 begin to discharge, and capacitor C0 and inductors begin to charge, with all inductor currents increasing linearly; in the second operating mode, at time t1, control all switches to be turned on, and all diodes are turned off under reverse voltage. At this time, capacitors C1 and C2 discharge in series, and other charging and discharging conditions are the same as in the first operating mode. The operating modes are consistent; the increase in input voltage leads to a widening of the voltage difference across inductor L1, and the rate of change of current is significantly increased compared to the first operating mode; in the third operating mode, at time t2, control switches S1, S3, and S4 are turned on, switch S2 is turned off, diode D2 is turned on, and D1, D3, and D4 are turned off; at this time, the input power supply is switched from capacitor C2 to C1, while other charging and discharging conditions remain unchanged; and due to the halving of the input voltage, the rate of change of current in inductor L1 returns to the corresponding value of the first operating mode; in the fourth operating mode, at time t3, all control switches are turned off, all diodes are turned on, and voltage source U... iCapacitors C1 and C2 are charged. Inductor L1 charges capacitor C3 through diodes D1 and D2. Inductor L2 charges capacitor C4 through diodes D3 and D4. Inductor L3 freewheels through diode D4. The current of all inductors decreases linearly.

[0018] 3. Beneficial Effects Compared with the prior art, the technical solution provided by this invention has the following beneficial effects: (1) The three-level DC-DC converter for photovoltaic hydrogen production system of this invention has a high step-down ratio. The converter can achieve wide-range and high-precision output voltage regulation through a single control parameter, duty cycle D1. Its step-down capability is directly determined by the duty cycle D1, and it has excellent linear control characteristics. When the duty cycle value is 0.3764, the step-down ratio of the converter is 0.12, which achieves a significant step-down effect.

[0019] (2) The present invention provides a three-level DC-DC converter for a photovoltaic hydrogen production system, which greatly reduces the voltage stress of the switching transistors. It uses devices with low on-resistance and low rated voltage to reduce the voltage stress on the switching transistors and diodes, which also minimizes the losses and net cost of the converter and improves efficiency.

[0020] (3) In the present invention, a three-level DC-DC converter for a photovoltaic hydrogen production system has the same control signals for the third switch S3 and the fourth switch S4, and is logically ORed with the control signals of the first switch S1 and the second switch S2, which greatly simplifies the design process of the control circuit.

[0021] (4) The three-level DC-DC converter for photovoltaic hydrogen production system of the present invention has wide applicability and can be applied to a variety of low-voltage high-current applications and switching power supplies and other applications that require high step-down ratio. Attached Figure Description

[0022] Figure 1 is an equivalent circuit diagram of a three-level DC-DC converter for a photovoltaic hydrogen production system according to the present invention; Figure 2 is a schematic diagram of the main operating waveforms of the converter in CCM mode according to the present invention; Figure 3 is a schematic diagram of the equivalent circuit of the converter in the first operating mode in CCM mode according to the present invention; Figure 4 is a schematic diagram of the equivalent circuit of the converter in the second operating mode in CCM mode according to the present invention; Figure 5 is a schematic diagram of the equivalent circuit of the converter in the third operating mode in CCM mode according to the present invention; Figure 6 is a schematic diagram of the equivalent circuit of the converter in the fourth operating mode in CCM mode according to the present invention; Figure 7 is a schematic diagram of the buck ratio of the converter according to the present invention as a function of duty cycle D1; Figure 8 is a schematic diagram of the simulated waveform of the independent inductor current of the converter according to the present invention; Figure 9 is a schematic diagram of the simulated waveform of the diode voltage of the converter according to the present invention; Figure 10 is a schematic diagram of the simulated waveform of the diode current of the converter according to the present invention; Figure 11 is a schematic diagram of the simulated waveform of the switching transistor voltage of the converter according to the present invention; Figure 12 is a schematic diagram of the simulated waveform of the switching transistor current of the converter according to the present invention; Figure 13 is a schematic diagram of the simulated waveforms of the input voltage, output voltage and output current of the converter according to the present invention. Detailed Implementation

[0023] To further understand the content of this invention, a detailed description of the invention will be provided in conjunction with the accompanying drawings and embodiments.

[0024] Example 1, as shown in Figure 1, describes a three-level DC-DC converter for a photovoltaic hydrogen production system, comprising: a front-end three-level buck converter (3L-NPC Buck) and a rear-end dual-buckle converter. The front-end three-level buck converter includes an input source U... i The circuit consists of a first switching transistor S1 and a second switching transistor S2, a first independent inductor L1, diodes D1 and D2, capacitors C1 and C2. The front-stage three-level buck converter is connected to the rear-stage dual-Buck circuit, which includes a third switching transistor S3 and a fourth switching transistor S4, a second independent inductor L2, a third independent inductor L3, diodes D3 and D4, capacitors C3, C4, and C0. The front-stage three-level buck converter uses a midpoint clamping structure; the rear-stage dual-Buck circuit is an embedded structure, sharing a single output terminal.

[0025] Specifically, in the front-end three-level buck circuit of the converter, the power supply U iThe positive terminals of the transistors are connected to the positive terminal of capacitor C1 and the drain of the first switching transistor S1, respectively; the negative terminal of capacitor C1 is connected to the positive terminal of capacitor C2, the anode of diode D1, and the cathode of diode D2; the source of the first switching transistor S1 is connected to the cathode of diode D1 and one end of the first independent inductor L1; the other end of the first independent inductor L1 is connected to the positive terminal of capacitor C3 and the drain of the third switching transistor S3; the drain of the second switching transistor S2, the anode of diode D2, and the low-potential terminal of the load are connected; the source of the second switching transistor S2 is connected to the power supply U. i The negative terminal of the first switch S3 is connected to the negative terminal of the second independent inductor L2; in the subsequent double-Buck circuit, the source of the third switch S3 is connected to the cathode of the diode D3 and one end of the second independent inductor L2; the anode of the diode D3 is connected to the source of the fourth switch S4 and the negative terminal of the capacitor C4; the positive terminal of the capacitor C4 is connected to the cathode of the diode D4 and one end of the third independent inductor L3; the other end of the second independent inductor L2 is connected to the other end of the third independent inductor L3, the positive terminal of the capacitor C0, and the high-potential terminal of the load. The drain of the fourth switch S4 is connected to the negative terminal of the capacitor C3, the anode of the diode D4, the negative terminal of the capacitor C0, and the low-potential terminal of the load; the high-potential terminal of the load is connected to the positive terminal of the capacitor C0; the low-potential terminal of the load is connected to the negative terminal of the capacitor C0.

[0026] The converter topology and control of this embodiment are simple, and it has the advantages of strong step-down capability, low device voltage and current stress, high power conversion efficiency, and excellent dynamic performance. It can be applied to a variety of low-voltage, high-current applications such as photovoltaic hydrogen production systems, as well as a variety of applications requiring high step-down ratios such as switching power supplies.

[0027] Example 2: In this example, the three-level DC-DC converter for a photovoltaic hydrogen production system, under continuous current mode (CCM), has a phase difference of D1·180° between the control signals of the first switch S1 and the second switch S2, while the control signals of the third switch S3 and the fourth switch S4 are the same and are logically ORed with the control signals of the first switch S1 and the second switch S2. Assuming the duty cycle of the first switch S1 and the second switch S2 is D1, and the duty cycle of the third switch S3 and the fourth switch S4 is D2, where D2 = 3D1 / 2, and the duty cycle D1 is less than 0.5, it includes a first operating mode, a second operating mode, a third operating mode, and a fourth operating mode within one switching cycle. The first to fourth operating modes are executed sequentially, wherein: the first operating mode controls the second switch S2, the third switch S3 and the fourth switch S4 to be turned on simultaneously, and the diode D1 is turned on; the second operating mode controls all switches to be turned on simultaneously, and all diodes to be turned off simultaneously; the third operating mode controls the first switch S1, the third switch S3 and the fourth switch S4 to be turned on simultaneously, and the diode D2 is turned on; the fourth operating mode controls all switches to be turned off simultaneously, and all diodes to be turned on simultaneously.

[0028] Figure 2 shows the theoretical operating waveforms of the equivalent circuit of the converter shown in Figure 1. Figures 3-6 are the operating mode diagrams of the equivalent circuit of the converter shown in Figure 1. The first to fourth operating modes correspond to operating modes 1-4. Specifically, referring to Figure 3, operating mode 1 [t0-t1]: At time t0, switches S2, S3, and S4 are turned on, and S1 is turned off. Diode D1 is forward-biased and turned on, while D2, D3, and D4 are reverse-biased and turned off. At this time, capacitors C2, C3, and C4 begin to discharge, and capacitor C0 and the inductor begin to charge, with all inductor currents increasing linearly. This mode ends when the drive signal arrives at the next moment. The inductor voltage in this mode is as follows:

[0029] In the formula, U0 is the output voltage, U L1 U L2 U L3 The voltages U of inductors L1, L2, and L3 are respectively. C2 U C3 U C4 These are the voltages of capacitors C2, C3, and C4, respectively.

[0030] Referring to Figure 4, in operating mode 2 [t1-t2]: at time t1, all switches are turned on, and all diodes are turned off under reverse voltage. At this time, capacitors C1 and C2 discharge in series, and other charging and discharging conditions are the same as in mode 1. The increase in input voltage leads to a wider voltage difference across inductor L1, and the rate of change of current is significantly higher than in mode 1. The inductor voltage in this mode is as follows:

[0031] Referring to Figure 5, in operating mode 3 [t2-t3]: at time t2, switches S1, S3, and S4 are turned on, and S2 is turned off. Diode D2 is turned on, and D1, D3, and D4 are turned off. Unlike mode 1, the input power supply is switched from capacitor C2 to C1, while other charging and discharging conditions remain unchanged. Furthermore, due to the halving of the input voltage, the rate of change of current in inductor L1 returns to the corresponding value in mode 1. The inductor voltage in this mode is as follows:

[0032] Referring to Figure 6, operating mode 4 [t3-t4]: At time t3, all switches are off and all diodes are on. At this time, the voltage source U... i Capacitors C1 and C2 are charged. Inductor L1 charges capacitor C3 through diodes D1 and D2. Inductor L2 charges capacitor C4 through diodes D3 and D4. Inductor L3 freewheels through diode D4. All inductor currents decrease linearly. The inductor voltages in this mode are as follows:

[0033] In this embodiment, the three-level DC-DC converter, in continuous inductor current mode, when the duty cycle D1 is less than 0.5, has the following buck ratio:

[0034] In the formula, U i U is the input voltage, U0 is the output voltage, and D1 is the duty cycle of the first switch S1 and the second switch S2.

[0035] Figure 7 shows the curve of the converter's buck ratio as a function of duty cycle D1 in this embodiment. Furthermore, the voltage stress on the switching transistors and diodes of this converter is as follows:

[0036] In the formula, D1 is the duty cycle of the first switch S1 and the second switch S2, and D2 is the duty cycle of the third switch S3 and the fourth switch S4. S1 U S2 U S3 U S4 The voltage stresses of switching transistors S1, S2, S3, and S4 are respectively, U D1 U D2 U D3 U D4 The voltage stresses of diodes D1, D2, D3, and D4 are respectively.

[0037] In this embodiment of the converter, if the duty cycle of the first switch S1 and the second switch S2 is 0.3764, the step-down ratio of the converter is 0.12; at the input source U i When the voltage is 400V, the output voltage of the converter is 48V, and the voltage stress of the switching transistor is 200V.

[0038] To verify the theoretical analysis of the three-level DC-DC converter in this embodiment, a simulation platform was built based on the converter simulation parameters in Table 1 below. Table 1 is the converter simulation parameter table.

[0039] Table 1

[0040] The simulated waveforms of the independent inductor current of the converter in this embodiment under the simulation parameters in Table 1 are shown in Figure 8. The simulated waveforms of the diode voltage and current are shown in Figures 9 and 10. The variation process is basically consistent with the theoretical analysis, and it can be seen that the diode voltage stress is very low.

[0041] The simulated waveforms of the voltage and current of the switching transistors are shown in Figures 11 and 12. From Figure 11, it can be observed that the voltage stress of the first and second switching transistors S1 and S2 is 200V, which is consistent with the calculation results of the voltage stress expression derived from theory.

[0042] The simulated waveforms of input voltage, output voltage, and output current are shown in Figure 13. It can be observed that when the input voltage is 400V, the output voltage is 48V, and the current is 208.33A, which is consistent with the calculation result of the theoretically derived voltage gain expression.

[0043] The simulation results of the simulation platform built based on the parameters listed in Table 1 verified the correctness of the theoretical analysis results, and further strongly proved that the three-level DC-DC converter has the advantages of strong step-down capability and low device voltage and current stress. Therefore, in this embodiment, the converter and control method proposed based on the three-level step-down circuit and the double Buck circuit have a high step-down ratio of 9D1. 3 / 4, and low device voltage stress, device voltage stress is U i / 2, where D1 is the duty cycle of the first switch S1 and the second switch S2. Furthermore, the converter in this embodiment has a simple topology and control, and boasts advantages such as strong voltage reduction capability, low device voltage and current stress, high power conversion efficiency, and excellent dynamic performance. It can be applied to various low-voltage, high-current applications and switching power supplies, as well as other applications requiring high voltage reduction ratios.

[0044] The present invention and its embodiments have been described above illustratively. This description is not restrictive, and the figures shown are only one embodiment of the present invention; the actual structure is not limited thereto. Therefore, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the present invention, such designs should fall within the protection scope of the present invention.

Claims

1. A three-level DC-DC converter for a photovoltaic hydrogen production system, characterized in that, It includes a pre-stage three-level buck converter circuit and a post-stage dual-Buck converter circuit; the pre-stage three-level buck converter circuit includes an input source U i The circuit comprises a first switching transistor S1, a second switching transistor S2, a first independent inductor L1, diodes D1 and D2, capacitors C1 and C2; the subsequent dual-Buck circuit includes a third switching transistor S3, a fourth switching transistor S4, a second independent inductor L2, a third independent inductor L3, diodes D3 and D4, capacitors C3, C4, and C0; the front-stage three-level buck converter adopts a midpoint clamping structure, and the subsequent dual-Buck circuit adopts an embedded structure, wherein the front-stage three-level buck converter is connected to the subsequent dual-Buck circuit.

2. The three-level DC-DC converter according to claim 1, characterized in that, In the aforementioned front-end three-level buck circuit, the input source U i The positive terminal of the first switch S1 is connected to the positive terminal of capacitor C1 and the drain of the first switch S1; the negative terminal of capacitor C1 is connected to the positive terminal of capacitor C2, the anode of diode D1, and the cathode of diode D2; the source of the first switch S1 is connected to the cathode of diode D1 and one end of the first independent inductor L1; the other end of the first independent inductor L1 is connected to the positive terminal of capacitor C3 and the drain of the third switch S3; the drain of the second switch S2, the anode of diode D2, and the low potential terminal of the load are connected; the source of the second switch S2 is connected to the power supply U. i The negative terminal of capacitor C2 is connected to the negative terminal of capacitor C2.

3. The three-level DC-DC converter according to claim 1, characterized in that, In the subsequent double-Buck circuit, the source of the third switch S3 is connected to the cathode of the diode D3 and one end of the second independent inductor L2; the anode of the diode D3 is connected to the source of the fourth switch S4 and the cathode of the capacitor C4; the anode of the capacitor C4 is connected to the cathode of the diode D4 and one end of the third independent inductor L3; the other end of the second independent inductor L2 is connected to the other end of the third independent inductor L3, the anode of the capacitor C0, and the high-potential end of the load; the drain of the fourth switch S4 is connected to the cathode of the capacitor C3, the anode of the diode D4, the cathode of the capacitor C0, and the low-potential end of the load.

4. The three-level DC-DC converter according to any one of claims 1-3, characterized in that, It also includes a control unit, which is configured to, in the continuous inductor current mode, input the control signals of the first switch S1 and the second switch S2 with a phase difference of duty cycle D1·180°, and the control signals of the third switch S3 and the fourth switch S4 are the same, and are logically ORed with the control signals of the first switch S1 and the second switch S2.

5. The three-level DC-DC converter according to claim 4, characterized in that, The duty cycle of the first switch S1 and the second switch S2 is D1, and the duty cycle of the third switch S3 and the fourth switch S4 is D2, where D2 = 3D1 / 2, and the duty cycle D1 is less than 0.

5.

6. The three-level DC-DC converter according to claim 4, characterized in that, The converter includes four operating modes within one switching cycle, with the first to fourth operating modes executed sequentially: First operating mode: controlling the second switch S2, the third switch S3, and the fourth switch S4 to conduct simultaneously, and diode D1 to conduct; Second operating mode: controlling all switches to conduct simultaneously, and all diodes to turn off simultaneously; Third operating mode: controlling the first switch S1, the third switch S3, and the fourth switch S4 to conduct simultaneously, and diode D2 to conduct; Fourth operating mode: controlling all switches to turn off simultaneously, and all diodes to conduct simultaneously.

7. The three-level DC-DC converter according to claim 4, characterized in that, In continuous inductor current mode, when the duty cycle D1 of the first switch S1 and the second switch S2 is less than 0.5, the converter's buck ratio is: In the formula, U i U is the input voltage, and U0 is the output voltage.

8. The three-level DC-DC converter according to claim 4, characterized in that, In continuous inductor current mode, when the duty cycle D1 of the first switch S1 and the second switch S2 is less than 0.5, the voltage stress on the switches and diodes of the converter is: In the formula, U S1 U S2 U S3 U S4 The voltage stresses of switching transistors S1, S2, S3, and S4 are respectively, U D1 U D2 U D3 U D4 These are the voltage stresses of diodes D1, D2, D3, and D4, respectively, and D2 is the duty cycle of the third switch S3 and the fourth switch S4.

9. The three-level DC-DC converter according to claim 5, characterized in that, When the duty cycle D1 of the first switch S1 and the second switch S2 is 0.3764, the buck ratio of the converter is 0.12; when the input source U i When the voltage is 400V, the output voltage of the converter is 48V, and the voltage stress of the switching transistor is 200V.

10. A control method for a three-level DC-DC converter, characterized in that, For controlling the three-level DC-DC converter as described in any one of claims 1-9, wherein: in the first operating mode, at time t0, control switches S2, S3, and S4 are turned on, switch S1 is turned off, diode D1 is turned on under forward voltage, and diodes D2, D3, and D4 are turned off under reverse voltage; capacitors C2, C3, and C4 begin to discharge, capacitor C0 and inductors begin to charge, and the current of all inductors increases linearly; in the second operating mode, at time t1, all control switches are turned on, all diodes are turned off under reverse voltage, at this time capacitors C1 and C2 discharge in series, and other charging and discharging conditions are the same as described in the previous description. The first operating mode is the same; the increase in input voltage leads to a widening of the voltage difference across inductor L1, and the rate of change of current is significantly increased compared to the first operating mode; in the third operating mode, at time t2, control switches S1, S3, and S4 are turned on, switch S2 is turned off, diode D2 is turned on, and D1, D3, and D4 are turned off; at this time, the input power supply is switched from capacitor C2 to C1, and other charging and discharging conditions remain unchanged; and due to the halving of the input voltage, the rate of change of current in inductor L1 returns to the corresponding value of the first operating mode; in the fourth operating mode, at time t3, control switches are all turned off, all diodes are turned on, and voltage source U i Capacitors C1 and C2 are charged. Inductor L1 charges capacitor C3 through diodes D1 and D2. Inductor L2 charges capacitor C4 through diodes D3 and D4. Inductor L3 freewheels through diode D4. The current of all inductors decreases linearly.

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