Quasi-single-stage AC-DC converter based on three active bridge structures

Abstract

The invention relates to the technical field of switching power supplies, and discloses a quasi-single-stage AC-DC converter based on a three-active-bridge structure. The three-active bridge structure comprises a first port, a second port and a third port; the first port is connected with an alternating current power supply and serves as an alternating current voltage input end of the AC-DC converter; the second port is connected with an external load and serves as a DC voltage output end of the AC-DC converter; the third port is reversely connected in series with the first port and is used for outputting a constant direct-current voltage with constant bias to the first port so as to convert an alternating-current voltage input into the first port into a sinusoidal voltage with bias, or outputting a sinusoidal voltage with variable bias to the first port so as to change the amplitude of the alternating-current voltage input into the first port so as to convert the alternating-current voltage into the sinusoidal voltage with bias; therefore, the number of times of electric energy conversion can be reduced, the overall working efficiency of the power supply is improved, the zero-crossing distortion of the input current at the network side is eliminated, and the power factor and the quality of the electric energy at the network side are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of switching power supplies, and in particular to a quasi-single-stage AC-DC converter based on a three-active bridge structure. Background Art

[0002] AC-DC converters are required for power conversion in many emerging power systems, including electric vehicle charging infrastructure, distributed energy resources and storage systems, and specialty power supplies. These converters must provide essential functions such as electrical isolation, multi-voltage output levels, and AC-side power factor correction (PFC). In a two-stage AC-DC converter implementation, a diode-controlled bridge rectifier generates a pulsating DC voltage, which is filtered by a large-capacity DC bus capacitor and provides a stable DC input voltage for the subsequent isolated DC-DC converter. Among single-stage AC-DC converters, the Dual Active Bridge (DAB) converter, with its wide voltage range adaptability, bidirectional power flow control, advanced soft switching strategies, and flexible multi-degree-of-freedom control, has become a preferred isolated converter solution. The combination of a power-frequency bridge rectifier in series with a DAB stage offers improved efficiency and power density compared to traditional two-stage converters. If the four switches in the DAB primary full-bridge are replaced with four sets of bidirectional power switches, and the primary side is directly connected to the AC side, a DAB-based matrix AC-DC converter is formed. A bridgeless single-stage DAB-based AC-DC converter reuses the DAB primary power switches in a totem-pole rectification scheme.

[0003] In the aforementioned AC-DC converter implementations, the front- and rear-stage circuits of traditional two-stage converters rely on large-capacity electrolytic capacitors or active decoupling circuits for energy buffering and decoupling control, severely impacting the converter's power density and service life. Furthermore, the inherent conduction losses of the diode rectifier bridge further reduce the converter's energy consumption, leading to reduced circuit reliability and efficiency. Although the DAB architecture with a rectifier bridge in series eliminates the need for large busbar capacitors, and the small-capacity clamping capacitors between the rectifier bridge and the rear-stage circuits are not used to buffer secondary pulsating energy but only to filter high-frequency ripple, the conduction losses of the front-stage rectifier bridge still exist. In a DAB-based matrix AC-DC converter, there is no rectifier bridge in this topology, but the switches in this topology need to flow both high-frequency and power-frequency currents, resulting in significant conduction losses. While the DAB-based bridgeless single-stage AC-DC implementation reduces conduction losses, the grid-side inductance couples with the transformer leakage inductance, resulting in large current peaks that make soft switching difficult and negate the advantages of the DAB converter.

[0004] AC-DC converters not only need to achieve power conversion but also need to ensure grid-side power quality and the system's power factor. However, near the zero-crossing point of the AC input voltage, the inductor current's rate of change is very small, resulting in a decrease in the inductor current's ability to track the reference current. Especially under light-load conditions, the discontinuous current operation mode reduces loop gain and bandwidth, further exacerbating input current distortion and severely impacting AC-side power quality. The aforementioned isolated AC-DC converters simplify the power conversion process by changing the topology of the preceding AC side, but none consider the impact of zero-crossing input voltage on grid-side power quality. Existing approaches to address zero-crossing distortion primarily focus on optimizing inductor parameters or increasing the current loop control bandwidth to enhance dynamic response. However, this approach requires a simultaneous increase in switching frequency, which in turn increases core loss and intensifies the pressure on core material and high-frequency device selection, creating a conflict between system efficiency and performance indicators. Summary of the Invention

[0005] The present invention aims to provide a quasi-single-stage AC-DC converter based on a three-active bridge structure, which can solve the problem that existing AC-DC converters do not consider the impact of input voltage zero crossing on grid-side power quality, and solve the problem that zero-crossing distortion cannot simultaneously balance system efficiency and performance indicators.

[0006] To solve the above technical problems, an embodiment of the present invention provides a quasi-single-stage AC-DC converter based on a three-active bridge structure, wherein the three-active bridge structure includes a first port, a second port, and a third port; The first port is connected to an AC power source and serves as an AC voltage input terminal of the AC-DC converter; The second port is connected to an external load and serves as a DC voltage output terminal of the AC-DC converter; The third port is anti-series connected to the first port and is used to output a constant DC voltage with a fixed bias to the first port, so that the AC voltage input to the first port is converted into a biased sinusoidal voltage, or to output a variable bias sinusoidal voltage to the first port, so as to change the amplitude of the AC voltage input to the first port so that the input voltage of the first port is a sinusoidal voltage that does not cross zero.

[0007] The first port adopts a full-bridge structure or a half-bridge structure, the second port adopts a full-bridge structure or a half-bridge structure, and the third port adopts a full-bridge structure or a half-bridge structure.

[0008] Optionally, the first port, the second port and the third port are all full-bridge structures formed by connecting four switching tubes.

[0009] Optionally, in the first port, the second port, and the third port, two switch tubes that are diagonally opposite to each other are turned on and off by using the same trigger signal according to a driving signal generated in real time by a control circuit.

[0010] Optionally, equivalent inductors are connected to the bridge ports of the full-bridge structure of the first port, the second port, and the third port to regulate energy between the first port, the second port, and the third port.

[0011] Optionally, the positive and negative electrodes of the AC power supply are connected to the positive electrodes of the first port and the third port respectively, and the first port and the third port are grounded in common.

[0012] Optionally, the power conversion process of the AC-DC converter is as follows: S1. Sampling the AC voltage output by the AC power supply and obtaining the phase information of the AC voltage input to the AC-DC converter through a phase-locked loop; S2. Sample the load voltage of the external load and compare it with the preset load voltage. The comparison result is corrected by the voltage outer loop controller and then outputs the amplitude information of the current inner loop; S3: Multiply the amplitude information in S2 by the phase information in S1 to obtain a reference current for the inner current loop. After correction by the inner loop controller, the AC voltage is tracked to perform power factor correction. The phase shift angle between the first port and the second port is output to obtain the drive signal of each switch tube inside the first port. S4. Sampling the bias voltage output by the third port and comparing it with the preset bias voltage of the third port. The generated error is passed through a control loop formed by a PI controller to generate another phase shift angle for stabilizing the output voltage of the third port, thereby obtaining a driving signal for each switch tube in the third port. S5. Repeat S1 to S4 during the input cycle of the AC voltage to obtain drive signals for all switching tubes until the AC-DC converter operates in a steady state.

[0013] The quasi-single-stage AC-DC converter based on the three-active bridge structure provided by the present invention has at least the following beneficial effects: The first port of the three-active bridge structure serves as the AC voltage input, the second port as the DC voltage output, and the third port, connected in anti-series with the first port, outputs a bias voltage (either a constant DC voltage with a fixed bias or a sinusoidal voltage with a variable bias) to the first port. This converts the input voltage at the first port into a non-zero sinusoidal voltage (achieved by applying a bias or varying the amplitude). This changes the AC input voltage form of the converter, significantly improving the rate of change of the inductor current at the AC voltage zero-crossing point, eliminating current distortion near the voltage zero-crossing point, and ensuring grid-side power quality. Furthermore, compared to a two-stage AC-DC converter, the AC-DC converter based on the three-active bridge structure reduces the pre-stage rectification stage. A single-stage power conversion is performed from the input side to the output side. Through coupling via an isolation transformer, a small amount of circulating power is absorbed at the bias voltage port on the AC side. Therefore, the AC-to-DC conversion is considered a quasi-single-stage conversion, improving AC power quality without introducing additional power conversion cycles, thereby increasing the converter's power conversion efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] One or more embodiments are exemplarily described by the figures in the corresponding drawings, and these exemplified descriptions do not constitute limitations on the embodiments.

[0015] Figure 1 1 is a schematic diagram of a topology structure of a quasi-single-stage AC-DC converter based on a three-active bridge structure according to an embodiment of the present invention; Figure 2 This is a schematic diagram of an input side voltage conversion principle provided according to an embodiment of the present invention. Figure 1 ; Figure 3 This is a schematic diagram of an input side voltage conversion principle provided according to an embodiment of the present invention. Figure 2 ; Figure 4 1 is a schematic diagram of a two-port input-output equivalent model provided according to an embodiment of the present invention; Figure 5 is a schematic diagram of a power transmission equivalent mode provided according to an embodiment of the present invention; Figure 6 1 is a schematic diagram of operating waveforms of a DAB type AC-DC converter provided according to an embodiment of the present invention; Figure 7 1 is a schematic diagram of a control strategy for AC-DC power conversion according to an embodiment of the present invention; Figure 8 1 is a schematic diagram of an expanded structure of a quasi-single-stage AC-DC converter based on a three-active bridge structure according to an embodiment of the present invention; Figure 9The figure is a schematic diagram of an AC side half-bridge anti-series structure provided according to an embodiment of the present invention. DETAILED DESCRIPTION

[0016] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, it will be understood by those skilled in the art that in the embodiments of the present invention, many technical details are provided to enable the reader to better understand the present invention. However, even without these technical details and the various changes and modifications based on the following embodiments, the technical solutions claimed in the present invention can be implemented. The division of the following embodiments is for convenience of description and should not constitute any limitation on the specific implementation of the present invention. The various embodiments can be combined with each other and referenced to each other under the premise that there is no contradiction.

[0017] To improve the operating efficiency of AC-DC converters and optimize the power quality on the AC input side, the present invention proposes a quasi-single-stage AC-DC converter with extremely low input current total harmonic distortion (THD) based on a three-active bridge structure. The AC side of the proposed topology uses an anti-series structure of two full-bridge ports to change the AC input voltage form of the converter, significantly improving the rate of change of the inductor current at the AC voltage zero-crossing point and eliminating current distortion near the voltage zero-crossing point. At the same time, compared with a two-stage AC-DC converter, the proposed topology reduces the pre-stage rectification link and performs a single-stage power conversion from the input side to the output side. After coupling with the isolation transformer, a small amount of circulating power is absorbed at the bias voltage port on the AC side. Therefore, the conversion from the AC side to the DC side is considered to be a quasi-single-stage conversion. While improving the power quality of the AC side, no additional power conversion times are introduced, thereby improving the power conversion efficiency of the converter.

[0018] One embodiment of the present invention relates to a quasi-single-stage AC-DC converter based on a three-active bridge structure. The three-active bridge structure includes a first port, a second port, and a third port. The first port is connected to an AC power source and serves as the AC voltage input terminal of the AC-DC converter. The second port is connected to an external load and serves as the DC voltage output terminal of the AC-DC converter. The third port is connected in anti-series with the first port and is configured to output a constant DC voltage with a fixed bias to the first port, thereby converting the AC voltage input to the first port into a biased sinusoidal voltage, or to output a variable bias sinusoidal voltage to the first port, thereby changing the amplitude of the AC voltage input to the first port so that the input voltage to the first port is a sinusoidal voltage that does not cross zero. The positive and negative poles of the AC power source are connected to the positive poles of the first port and the third port, respectively, and the first and third ports are connected to a common ground.

[0019] In some embodiments, the first port adopts a full-bridge structure or a half-bridge structure, the second port adopts a full-bridge structure or a half-bridge structure, and the third port adopts a full-bridge structure or a half-bridge structure.

[0020] The following is an example in which the first port, the second port, and the third port all adopt a full-bridge structure to specifically describe the implementation details of the quasi-single-stage AC-DC converter based on the three-active bridge structure of the present invention. The following content is only for the convenience of understanding the implementation details and is not necessary for the implementation of this solution.

[0021] like Figure 1 In the topology of the AC-DC converter based on a three-active bridge structure shown in FIG, the first port, the second port, and the third port (i.e., port 1, port 2, and port 3) are all full-bridge structures composed of four switching tubes connected. The four switching tubes are all MOSFET high-frequency switching tubes. The two switching tubes diagonally opposite each other in each port use the same trigger signal and control the conduction and shutdown of the corresponding switching tube according to the drive signal generated in real time by the control circuit.

[0022] For port 1, S11 and S14 are a group of switch tubes using the same trigger signal; S12 and S13 are a group of switch tubes using trigger signals complementary to S11 and S14; for port 2, S21 and S24 are a group of switch tubes using the same trigger signal; S22 and S23 are a group of switch tubes using trigger signals complementary to S21 and S24; for port 2, S31 and S34 are a group of switch tubes using the same trigger signal; S32 and S33 are a group of switch tubes using trigger signals complementary to S31 and S34.

[0023] The working principle of this quasi-single-stage AC-DC converter topology based on the three-active bridge structure is as follows: Port 1 serves as the equivalent AC input terminal of the entire converter u in The DC side of port 2 serves as the output port of the entire converter and provides a stable DC voltage to the load. u o Port 3 DC side voltage U dc is the AC side support voltage, and u ac After superposition, the input voltage of port 1 is generated u in . L 1. L 2. L 3 represents the series equivalent inductance of ports 1, 2, and 3 at the bridge entrance, which is the carrier for energy regulation between ports. The positive and negative poles of the AC power supply are connected to the positive poles of ports 1 and 3, respectively, and ports 1 and 3 share a common ground.

[0024] In an example, if the third port on the AC side of a quasi-single-stage AC-DC converter based on a three-active bridge structure adopts a constant DC voltage fixed bias scheme, that is, port 3 (i.e., the third port) outputs a constant DC voltage with a fixed bias, and port 1 serves as the equivalent AC input port of the entire converter. u in After the three ports are coupled, the full-bridge DC side of port 3 outputs a constant DC voltage. U dc Provide voltage support for port 1. That is, the equivalent input voltage of the converter at port 1 is the sum of the grid-side AC voltage and the fixed bias voltage at port 3. ,definition is the voltage amplitude of the AC power supply on the grid side. At this time, the voltage stress borne by the switch tube of port 1 is When the peak voltage of the grid power supply is 311V and the bias voltage of port 3 is When the voltage is 711V, each switch on port 1 will be subjected to a voltage stress of 711V. In practical applications, considering the voltage margin of the switch, a switch with a higher withstand voltage will be selected, which will increase the pressure on device selection and the cost of use.

[0025] It can be assumed that the bias voltage is 400V. The result of this is that the 220V AC input voltage on the grid side is converted to 400V by using the 400V bias voltage provided by the DC side of port 3. u ac It becomes a sinusoidal voltage with a peak-to-valley value of 89-711V, such as Figure 2 As shown. That is, through the anti-series connection of port 1 and port 3, a non-zero sinusoidal pulsating DC voltage is provided to the equivalent AC input terminal (port 1) of the converter. u in With the help of the support voltage of port 2, the maximum inductor current rise rate that can be provided by the AC side is increased, thereby avoiding the problem of poor inductor current tracking ability near the zero point of the AC-DC converter input voltage.

[0026] In another example, Figure 3 A solution based on variable bias support voltage is given in the paper, that is, port 3 outputs a sinusoidal voltage with variable bias. Assuming that the grid-side AC input voltage is a sinusoidal voltage with an amplitude of 311V, port 3 takes the form of The variable bias voltage provides voltage support for port 1. is the DC side output voltage of port 3 under the variable bias scheme. , At this point, the DC-side output voltage at port 3 is a sinusoidal voltage with an amplitude of 200V-400V, 180 degrees out of phase with the grid-side AC input voltage. Under this variable bias support voltage scheme, the equivalent input voltage at port 1 becomes 89V-511V, reducing the switch voltage stress by 200V compared to the fixed bias scheme, effectively alleviating device selection pressure and operating costs.

[0027] Then, the input and output power of the quasi-single-stage AC-DC converter based on the three-active bridge structure is analyzed: During the power frequency cycle, under the action of the power frequency input current on the AC side, the supporting capacitor of port 3 achieves energy balance. Port 3 does not do any work to the outside and only provides supporting voltage to the grid side. Ideally, in order to analyze the power transmission relationship between the input AC side and the output DC side of the entire converter, it is assumed that the DC side of port 3 outputs a constant DC voltage without any harmonic components other than DC. Port 3 can be regarded as part of the AC side input and connected in series with the AC voltage source to provide input voltage to port 1. If the energy exchange relationship within ports 1 and 3 is not considered, ports 1 and 2 are the main power transmission channels from the input side to the output side. Ports 1 and 2 can be simplified into a DAB, such as Figure 4 As shown. The converter equivalent input voltage is the sum of the grid-side AC voltage and the bias voltage of port 3, so ,definition is the grid-side AC power supply voltage amplitude, and the DC-side bias voltage of port 3 is Under the PFC control strategy, the converter is considered to work in the unity power factor mode, and the grid-side input current can be expressed as , the grid-side input current is in phase with the AC voltage, is the converter input current amplitude. Figure 3 For the DAB shown in the figure, the DC input of the primary full-bridge is composed of an AC voltage and a bias DC voltage. The input is no longer a constant DC voltage as in a traditional DAB DC-DC converter, but a DC voltage that pulsates at the power frequency cycle. The input value of the converter varies in each switching cycle, so the design of the circuit parameters requires that the converter can adapt to a wide range of voltage gains. For pulsating DC input, the transformer bridge voltage of the primary full-bridge The square wave voltage with a constant amplitude is also changed to a pulsating square wave voltage under the sinusoidal envelope. The primary and secondary full bridges of the converter can be equivalent to two square wave power supplies. The inductor connected in series with the secondary side of the isolation transformer is equivalent to the primary side. This further obtains the equivalent power transmission model from the input to the output side of the converter as follows: Figure 5 As shown, the primary input side and the secondary output side transmit energy at each moment according to the state of the two square wave voltage sources through the equivalent inductance between the two. Figure 5The equivalent power transfer model of the converter shows that although the converter's input voltage and phase shift ratio are no longer constant within an AC input cycle, the converter's power switches operate in a high-frequency trigger-off mode within each sinusoidal cycle. Therefore, the converter input can be considered constant during any switching cycle. Ignoring the harmonic components in the three-port bias voltage, the average power transferred by the converter over a complete switching cycle under the single-phase shift control strategy is as follows:

[0028] ; Where, is the turns ratio of the isolation transformer 1-port and 2-port windings, is the converter switching frequency, is the equivalent inductance between port 1 and port 2 after conversion. When the converter is in steady state, the AC input port 1 and output port 2 are shifted by .

[0029] Next, the single phase shift control of the DAB type AC-DC converter is analyzed: For DAB type AC-DC converter, the AC side input voltage is no longer a constant value but a pulsating DC voltage, which causes the primary input voltage of the converter to be different in adjacent switching cycles, that is, each switching cycle corresponds to a different AC reference and phase shift ratio. However, since the switching frequency is much greater than the fundamental pulsating frequency of the input voltage, when analyzing the different operating modes under the converter switching cycle, it can be assumed that the AC voltage and phase shift ratio remain unchanged within the current switching cycle. Its operating waveform is as follows Figure 6 As shown in the figure, the converter operates in one switching cycle when the phase of the primary full-bridge switch lags behind the corresponding switch on the secondary side.

[0030] For the present invention, it is necessary to make full use of the two control degrees of freedom to achieve the control target. Since the AC side anti-series structure of the converter also requires port 3 to provide a stable DC support voltage, the entire converter has three control targets: port 1 AC side power factor correction, port 2 DC load side voltage regulation output, and port 3 DC side bias voltage output. To achieve the above three control targets, the following measures are taken: Figure 7 The control strategy shown, port 2 is used as the phase shift reference, D 12 is the phase difference between the switch tubes corresponding to port 1 and port 2, D 32 is the phase difference between the switching tubes corresponding to port 3 and port 2.

[0031] Finally, the power conversion implementation of the quasi-single-stage AC-DC converter based on the three-active bridge structure is explained: Step 1: Sample the AC power supply and obtain the real-time phase of the grid-side AC signal through a phase-locked loop; Step 2: Port 2 is used as the DC output port of the converter. After sampling the load voltage, it is compared with the output given value. The comparison result is corrected by the voltage outer loop controller to adjust the amplitude information of the output current inner loop. Step 3: The amplitude information obtained in step 2 is multiplied by the real-time phase information in step 1 to obtain the complete given reference of the current inner loop. After correction by the inner loop controller, the AC signal is tracked to achieve power factor correction. The inner loop controller outputs the phase shift angle between port 1 and port 2. D 12 , and then obtain the driving signals of each switch tube inside port 1; Step 4: The bias voltage on the DC side of port 3 is sampled in real time. The error generated by comparing the sampled information with the given reference of port 3 is generated by the control loop composed of the PI controller to generate another phase shift angle to stabilize the output voltage of the port. D 32 , and then obtain the driving signals of each switch tube inside port 3; Step 5: Repeat the above steps during the AC input cycle to obtain the switch drive signal of each power switch device until the converter operates in a steady state.

[0032] The present invention addresses the problem of input current distortion in AC-DC converters due to insufficient ability of the inductor current to track the reference current when the input voltage passes through zero, and innovatively proposes a quasi-single-stage isolated AC-DC converter based on a three-active bridge structure. Compared with a two-stage or quasi-single-stage AC-DC converter with a rectifier bridge link, this topology can make the grid-side input current operate in a lower iTHD state without introducing additional power conversion times, thereby improving the power quality on the AC side and having higher working efficiency. At the same time, the present invention reduces the requirements for optimizing the adaptability of the control loop and inductor parameters in the AC-DC converter to improve the inductor current tracking capability, so that the converter does not need to operate at a higher switching frequency, reduces the core loss, and reduces the pressure on the core material design and high-frequency device selection. In summary, the present invention is suitable for power conversion scenarios that require step-up and step-down, low harmonic distortion, high efficiency, and strict requirements on electrical isolation.

[0033] In some embodiments, based on the above ideas and working principles, a half-bridge structure can also be applied to this topology, that is, each port can be selected as a full-bridge structure or a half-bridge structure to form different forms of three-port converters, such as Figure 8 Specifically, the full-bridge structure ( Figure 1 ) into a half-bridge structure ( Figure 9 ), the DC side remains unchanged. In this way, the number of switches on the AC side is reduced by half compared to the topology using a full-bridge structure, further reducing the cost of the converter. At this time, the bridge voltages at ports 1 and 3 are 、 (corresponding to the full-bridge structure) becomes 、 (Corresponding to the half-bridge structure.) Alternatively, port 2 on the DC side of the converter can be changed to a half-bridge structure, while ports 1 and 3 on the AC side still adopt a full-bridge structure. This creates another hybrid bridge topology based on the aforementioned principle, and the rest are not listed here.

[0034] The present invention is Figure 1 Taking the converter as an example, the design idea and working principle of the proposed converter are elaborated in detail. Figure 1 In addition to the full-bridge structure shown, a half-bridge or full-bridge structure can also be selected according to different working conditions and cost requirements, and the remaining 7 structures can be combined and expanded. In summary, this embodiment proposes an extremely low iTHD quasi-single-stage AC-DC converter based on a three-active bridge architecture with full-bridge or half-bridge internal ports. A total of 8 topologies based on the above design ideas are all protection solutions of this embodiment, such as Figure 8 shown.

[0035] Those skilled in the art will appreciate that the above-described embodiments are specific embodiments for implementing the present invention, and that in actual applications, various changes in form and detail may be made thereto without departing from the spirit and scope of the embodiments of the present invention. Any person skilled in the art may make various changes and modifications without departing from the spirit and scope of the embodiments of the present invention. Therefore, the scope of protection of the embodiments of the present invention shall be subject to the scope defined in the claims.

Claims

1. A quasi-single-stage AC-DC converter based on a three-active bridge structure, characterized in that: The three-active bridge structure includes a first port, a second port and a third port; The first port is connected to an AC power source and serves as an AC voltage input terminal of the AC-DC converter; The second port is connected to an external load and serves as a DC voltage output terminal of the AC-DC converter; The third port is anti-series connected to the first port and is used to output a constant DC voltage with a fixed bias to the first port, so that the AC voltage input to the first port is converted into a biased sinusoidal voltage, or to output a variable bias sinusoidal voltage to the first port, so as to change the amplitude of the AC voltage input to the first port so that the input voltage of the first port is a sinusoidal voltage that does not cross zero.

2. The quasi-single-stage AC-DC converter based on a three-active-bridge structure according to claim 1, characterized in that: The first port adopts a full-bridge structure or a half-bridge structure, the second port adopts a full-bridge structure or a half-bridge structure, and the third port adopts a full-bridge structure or a half-bridge structure.

3. The quasi-single-stage AC-DC converter based on a three-active-bridge structure according to claim 1, characterized in that: The first port, the second port and the third port are all full-bridge structures formed by connecting four switching tubes.

4. The quasi-single-stage AC-DC converter based on a three-active-bridge structure according to claim 3, characterized in that: In the first port, the second port and the third port, two switch tubes that are diagonally opposite to each other are turned on and off by using the same trigger signal according to the driving signal generated in real time by the control circuit.

5. The quasi-single-stage AC-DC converter based on a three-active-bridge structure according to claim 4, characterized in that: Equivalent inductors are connected to the bridge ports of the full-bridge structure of the first port, the second port and the third port to regulate the energy between the first port, the second port and the third port.

6. The quasi-single-stage AC-DC converter based on a three-active-bridge structure according to claim 1, characterized in that: The positive and negative electrodes of the AC power supply are connected to the positive electrodes of the first port and the third port respectively, and the first port and the third port share a common ground.

7. The quasi-single-stage AC-DC converter based on a three-active-bridge structure according to claim 1, characterized in that: The power conversion process of the AC-DC converter is as follows: S1. Sampling the AC voltage output by the AC power supply and obtaining the phase information of the AC voltage input to the AC-DC converter through a phase-locked loop; S2. Sample the load voltage of the external load and compare it with the preset load voltage. The comparison result is corrected by the voltage outer loop controller and then outputs the amplitude information of the current inner loop; S3: Multiply the amplitude information in S2 by the phase information in S1 to obtain a reference current for the inner current loop. After correction by the inner loop controller, the AC voltage is tracked to perform power factor correction. The phase shift angle between the first port and the second port is output to obtain the drive signal of each switch tube inside the first port. S4. Sampling the bias voltage output by the third port and comparing it with the preset bias voltage of the third port. The generated error is passed through a control loop formed by a PI controller to generate another phase shift angle for stabilizing the output voltage of the third port, thereby obtaining a driving signal for each switch tube in the third port. S5. Repeat S1 to S4 during the input cycle of the AC voltage to obtain drive signals for all switching tubes until the AC-DC converter operates in a steady state.

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