A quasi-single-stage ac-dc converter based on three-active-bridge structure

Abstract

The application 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 direct-current voltage output end of the AC-DC converter; and the third port is connected with the first port in anti-series connection and is used for outputting a constant direct-current voltage with a fixed bias to the first port, so that the alternating-current voltage input into the first port is converted into a sine voltage with a bias, or a sine voltage with a variable bias is output to the first port, the amplitude of the alternating-current voltage input into the first port is changed, the input voltage of the first port is a sine voltage without zero, the number of times of electric energy conversion can be reduced, the overall working efficiency of the power supply can be improved, the zero-crossing distortion of the input current at the grid side can be eliminated, and therefore the power factor and the electric energy quality at the grid side can be improved.

Description

Technical Field

[0001] This invention relates to the field of switching power supply technology, and in particular to a quasi-single-stage AC-DC converter based on a three-active-bridge structure. Background Technology

[0002] In many emerging power system fields such as electric vehicle charging facilities, distributed energy and energy storage systems, and special power supplies, AC-DC converters are required for power conversion. These AC-DC converters should have basic functions such as electrical isolation, multi-voltage output, and AC-side power factor correction (PFC). In a two-stage implementation of an AC-DC converter, the front stage uses a diode-controlled uncontrolled rectifier bridge to generate a pulsating DC voltage, which is then filtered by a large-capacity DC bus capacitor to provide a stable DC input voltage for the subsequent isolated DC-DC converter. In a single-stage implementation of an AC-DC converter, the dual active bridge (DAB) converter, with its wide-range voltage adaptation capability, bidirectional power flow control, advanced soft-switching strategy, and multi-degree-of-freedom flexible control, has become the preferred solution among isolated converters. Among these, the scheme of connecting the front-stage power frequency rectifier bridge in series with the subsequent DAB improves efficiency and power density compared to the traditional two-stage converter. If the four switches on the primary side of the DAB full-bridge are replaced with four sets of bidirectional power switches, and the primary side is directly connected to the AC side, a matrix AC-DC converter based on DAB is formed. A bridgeless single-stage AC-DC converter based on DAB uses a totem-pole rectification-like approach to multiplex the DAB primary-side power switches.

[0003] In the aforementioned AC-DC converter implementation schemes, 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 lifespan. Furthermore, the inherent conduction losses of the diode rectifier bridge further reduce the converter's energy consumption, leading to decreased circuit reliability and efficiency. Although the rectifier bridge-DAB series architecture eliminates the need for large-capacity bus capacitors, and the small clamping capacitor between the rectifier bridge and the subsequent stage is used only to filter high-frequency ripple and not to buffer secondary pulsating energy, the conduction losses of the front-stage rectifier bridge still exist. For DAB-based matrix AC-DC converters, there is no rectifier bridge in this topology, but the switching transistors in this topology need to carry both high-frequency and power-frequency currents, resulting in significant conduction losses. While the bridgeless single-stage AC-DC implementation based on DAB reduces conduction losses, the coupling between the grid-side inductor and transformer leakage inductance creates large current peaks, making soft-switching difficult and negating the advantages of DAB converters.

[0004] For AC-DC converters, in addition to achieving power conversion, it is also necessary to ensure grid-side power quality and system power factor. However, the inductor current change rate is very small near the zero-crossing point of the AC input voltage, which reduces 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 aggravating input current distortion and severely affecting AC-side power quality. The aforementioned isolated AC-DC converters simplify the power conversion process by changing the topology of the AC side in the preceding stage, but none of them consider the impact of the zero-crossing input voltage on grid-side power quality. Existing approaches to solving zero-crossing distortion mainly focus on optimizing inductor parameters or increasing the current loop control bandwidth to enhance dynamic response. However, this method requires a simultaneous increase in switching frequency, which leads to increased core losses and intensifies the selection pressure of core materials and high-frequency components, creating a contradiction between system efficiency and performance indicators. Summary of the Invention

[0005] The purpose of this invention is to provide a quasi-single-stage AC-DC converter based on a three-active-bridge structure, which can solve the problems of existing AC-DC converters not considering the impact on grid-side power quality when the input voltage crosses zero, and the inability to simultaneously balance system efficiency and performance indicators when zero-crossing distortion occurs.

[0006] To address the aforementioned technical problems, embodiments of the present invention provide 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;

[0007] The first port is connected to the AC power supply and serves as the AC voltage input terminal of the AC-DC converter.

[0008] The second port is connected to an external load and serves as the DC voltage output terminal of the AC-DC converter.

[0009] The third port is connected in anti-series with the first port and is used to output a constant bias DC voltage 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 that the amplitude of the AC voltage input to the first port is a sinusoidal voltage that does not cross zero.

[0010] 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.

[0011] Optionally, the first port, the second port, and the third port are all full-bridge structures consisting of four connected switching transistors.

[0012] Optionally, in the first port, the second port, and the third port, two switching transistors that are diagonally opposite each other are turned on and off by using the same trigger signal according to the drive signal generated in real time by the control circuit.

[0013] Optionally, an equivalent inductor is connected to the bridge port 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.

[0014] Optionally, the positive and negative terminals of the AC power supply are connected to the positive terminals of the first port and the third port, respectively, and the first port and the third port share a common ground.

[0015] Optionally, the power conversion process of the AC-DC converter is as follows:

[0016] S1. Sample the AC voltage output from the AC power supply, and obtain the phase information of the AC voltage input to the AC-DC converter after passing through a phase-locked loop;

[0017] S2. Sample the load voltage of the external load and compare it with the preset load voltage. After the comparison result is corrected by the voltage outer loop controller, output the amplitude information of the current inner loop.

[0018] S3. Multiply the amplitude information in S2 with the phase information in S1 to obtain the reference current of 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 in the first port.

[0019] S4. The bias voltage output from the third port is sampled and compared with the preset bias voltage of the third port. The resulting error is used by the control loop formed by the PI controller to generate another phase shift angle to stabilize the output voltage of the third port, so as to obtain the drive signal of each switch inside the third port.

[0020] S5. Repeat S1 to S4 within the AC voltage input cycle to obtain drive signals for all switching transistors until the AC-DC converter is in steady-state operation.

[0021] The quasi-single-stage AC-DC converter based on a three-active-bridge structure provided by this invention has at least the following beneficial effects:

[0022] Using a three-active-bridge structure, the first port is the AC voltage input, the second port is the DC voltage output, and the third port is anti-connected to the first port, outputting a bias voltage (a constant bias DC voltage or a variable bias sinusoidal voltage) to the first port. This converts the input voltage at the first port into a sinusoidal voltage that does not cross zero (achieved by biasing or changing the amplitude), thereby changing the form of the AC input voltage of the converter, significantly improving the rate of change of inductor current at the zero-crossing point of the AC voltage, eliminating current distortion near the zero-crossing point, and ensuring the power quality on the grid side. Simultaneously, compared to a two-stage AC-DC converter, the three-active-bridge AC-DC converter reduces the number of front-end rectification stages, performing a single-stage power conversion from the input to the output side. Due to coupling via an isolation transformer, a small amount of circulating current 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 the AC power quality without introducing additional power conversion steps, thus improving the converter's power conversion efficiency. Attached Figure Description

[0023] One or more embodiments are illustrated by way of example with reference to the accompanying drawings, and these illustrative descriptions do not constitute a limitation on the embodiments.

[0024] Figure 1 This is a schematic diagram of the topology of a quasi-single-stage AC-DC converter based on a three-active-bridge structure according to an embodiment of the present invention;

[0025] Figure 2 This is a schematic diagram of an input-side voltage transformation principle according to an embodiment of the present invention. Figure 1 ;

[0026] Figure 3 This is a schematic diagram of an input-side voltage transformation principle according to an embodiment of the present invention. Figure 2 ;

[0027] Figure 4 This is a schematic diagram of a two-port input / output equivalent model according to an embodiment of the present invention;

[0028] Figure 5 This is a schematic diagram of an equivalent power transmission mode according to an embodiment of the present invention;

[0029] Figure 6 This is a schematic diagram of the working waveform of a DAB-type AC-DC converter according to an embodiment of the present invention;

[0030] Figure 7 This is a schematic diagram of a control strategy for AC-DC power conversion according to an embodiment of the present invention;

[0031] Figure 8This is a schematic diagram illustrating an extended structure of a quasi-single-stage AC-DC converter based on a three-active-bridge structure according to an embodiment of the present invention.

[0032] Figure 9 This is a schematic diagram of an AC-side half-bridge anti-series structure according to an embodiment of the present invention. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the various embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, those skilled in the art will understand that many technical details are presented in the embodiments of the present invention to facilitate a better understanding of the invention. However, the technical solutions claimed in the present invention can be implemented even without these technical details and various variations and modifications based on the following embodiments. The division of the following embodiments is for ease of description and should not constitute any limitation on the specific implementation of the present invention. The various embodiments can be combined with and referenced by each other without contradiction.

[0034] To improve the efficiency of AC-DC converters and optimize the power quality on the AC input side, this 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 proposed topology utilizes a two-full-bridge anti-series structure on the AC side to alter the form of the AC input voltage, significantly increasing the rate of change of inductor current at the AC voltage zero-crossing point and eliminating current distortion near the zero-crossing point. Simultaneously, compared to a two-stage AC-DC converter, the proposed topology reduces the number of preceding rectification stages, performing a single-stage power conversion from the input to the output side. Due to coupling via an isolation transformer, a small amount of circulating current is absorbed at the AC side bias voltage port; therefore, the AC-to-DC conversion is considered a quasi-single-stage conversion, improving AC power quality without introducing additional power conversion steps, thus enhancing the converter's power conversion efficiency.

[0035] 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 supply 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 used to output a constant biased DC voltage to the first port, thereby converting the AC voltage input to the first port into a biased sinusoidal voltage, or to output a variable biased sinusoidal voltage to the first port, thereby changing the amplitude of the AC voltage input to the first port so that the input voltage at the first port is a sinusoidal voltage that does not cross zero. The positive and negative terminals of the AC power supply are connected to the positive terminals of the first and third ports, respectively, and the first and third ports share a common ground.

[0036] 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.

[0037] The following describes the implementation details of the quasi-single-stage AC-DC converter based on the three active bridge structure of the present invention, taking the first, second, and third ports all adopting a full-bridge structure as an example. The following content is only for the convenience of understanding and is not necessary for implementing this solution.

[0038] like Figure 1 The AC-DC converter topology shown is based on a three-active-bridge structure. 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 transistors. All four switching transistors are high-frequency MOSFET switching transistors. The two switching transistors on each port that are diagonally opposite each other use the same trigger signal, and the corresponding switching transistors are turned on and off according to the drive signal generated in real time by the control circuit.

[0039] For port 1, S11 and S14 are a pair of switches using the same trigger signal; S12 and S13 are a pair of switches using a complementary trigger signal to S11 and S14. For port 2, S21 and S24 are a pair of switches using the same trigger signal; S22 and S23 are a pair of switches using a complementary trigger signal to S21 and S24. For port 2, S31 and S34 are a pair of switches using the same trigger signal; S32 and S33 are a pair of switches using a complementary trigger signal to S31 and S34.

[0040] The working principle of this quasi-single-stage AC-DC converter topology based on a three-active-bridge structure is as follows:

[0041] Port 1 serves as the equivalent AC input of the entire converter. u inThe DC side of port 2 serves as the output port of the entire converter, providing a stable DC voltage to the load. u o DC side voltage at port 3 U dc For AC side support voltage, and u ac The superposition generates the input voltage of port 1. u in . L 1. L 2. L 3 represents the equivalent series inductance of ports 1, 2, and 3 at the bridge port, serving as the carrier for energy regulation between the ports. The positive and negative terminals of the AC power supply are connected to the positive terminals of ports 1 and 3, respectively, and ports 1 and 3 share a common ground.

[0042] In one 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 bias scheme, that is, port 3 (i.e., the third port) outputs a constant bias DC voltage, and port 1 serves as the equivalent AC input terminal of the entire converter. u in After being coupled through three ports, port 3 outputs a constant DC voltage on its full-bridge DC side. U dc Provide voltage support for port 1. That is, the equivalent input voltage of the converter at port 1 is the superposition of the grid-side AC voltage and the constant bias voltage at port 3. ,definition This represents the amplitude of the AC power supply voltage on the grid side. At this time, the voltage stress borne by the switch transistor at port 1 is... When the peak voltage of the grid-side power supply is 311V and the constant bias voltage at port 3 is... At that time, each switch at 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, resulting in greater pressure on device selection and an increase in usage costs.

[0043] We can assume the bias voltage is 400V. The result is that the 400V bias voltage provided by the DC side of port 3 will be used to bias the 220V AC input voltage from the grid side. u ac Transformed into a sinusoidal voltage with peak-to-valley values ​​of 89-711V, such as Figure 2 As shown, the anti-zero sinusoidal pulsating DC voltage is provided to the equivalent AC input terminal (port 1) of the converter through the anti-series connection of port 1 and port 3. u inBy leveraging the support voltage at port 2, the maximum inductor current rise rate that the AC side can provide is increased, thereby avoiding the problem of poor inductor current tracking capability near the zero-crossing point of the AC-DC converter input voltage.

[0044] In another example, Figure 3 The paper presents a scheme based on variable bias support voltage, where port 3 outputs a variable bias sinusoidal voltage. Assuming the grid-side AC input voltage is a sinusoidal voltage with an amplitude of 311V, port 3 adopts the form of... The variable bias voltage provides voltage support for port 1. For the DC-side output voltage of port 3 under the variable bias scheme, at this time , At this time, the DC output voltage of port 3 is a sinusoidal voltage with an amplitude of 200V-400V, which is 180 degrees out of phase with the AC input voltage on the grid side. Under the above variable bias support voltage scheme, the equivalent input voltage of port 1 becomes 89V-511V, and the voltage stress on the switching transistor is reduced by 200V compared with the fixed bias scheme, effectively alleviating the pressure of device selection and usage costs.

[0045] Then, the input and output power of the quasi-single-stage AC-DC converter based on the three active bridge structure is analyzed:

[0046] During the power frequency cycle, under the influence of the AC power frequency input current, the supporting capacitor at port 3 achieves energy balance. Port 3 does no external work, only providing supporting voltage to the grid side. Ideally, to analyze the power transfer relationship between the AC input side and the DC output side of the entire converter, it is assumed that the DC output voltage at port 3 is constant and contains no harmonic components other than DC. Port 3 can be considered as part of the AC input, connected in series with the AC voltage source to provide the input voltage for port 1. If the energy exchange relationship within ports 1 and 3 is not considered, ports 1 and 2 are the main power transfer channels from the input side to the output side. Ports 1 and 2 can be simplified to a DAB, such as... Figure 4 As shown. The equivalent input voltage of the converter is the superposition of the grid-side AC voltage and the bias voltage at port 3, therefore... ,definition The AC power supply voltage amplitude is on the grid side, and the DC bias voltage at port 3 is... Under the PFC control strategy, assuming the converter operates in unity power factor mode, the grid-side input current can be expressed as: The grid-side input current is in phase with the AC voltage. This refers to the input current amplitude of the converter. Figure 3In the case of the DAB type, the DC input of the primary-side full-bridge converter is composed of the superposition of AC voltage and bias DC voltage. The input is no longer the constant DC voltage found in traditional DAB-type DC-DC converters, but rather a DC voltage that pulsates with the power frequency cycle. The input value differs in each switching cycle, therefore the circuit design requires the converter to adapt to a wide range of voltage gains. For the pulsating DC input, the transformer bridge port voltage of the primary-side full-bridge... The voltage also changes from a constant amplitude square wave voltage to a pulsating square wave voltage under a sinusoidal envelope. The full-bridge converter can be equivalently represented by two square wave power supplies. The series inductor on the secondary side of the isolation transformer is equivalently represented on the primary side. From this, the equivalent power transfer model from the converter input to the output side is further obtained as follows: Figure 5 As shown, energy is transferred between the primary input side and the secondary output side via the equivalent inductance between them at each moment, depending on the state of the two square wave voltage sources. Figure 5 From the equivalent model of the power transfer of the converter, it can be seen that although the input voltage and phase shift of the converter are no longer constant within one AC input cycle, the power switch of the converter is in a high-frequency triggered turn-off mode within each sinusoidal cycle. Therefore, the converter input can be considered constant in any given switching cycle. Ignoring the harmonic components in the 3-port bias voltage, the average power transferred by the converter in a complete switching cycle under the single-phase-shift control strategy is as follows:

[0047] ;

[0048] In the formula, The turns ratio of the isolation transformer's port 1 and port 2 windings. For the converter switching frequency, The equivalent inductance between port 1 and port 2 is the equivalent inductance between the AC input port 1 and output port 2 in steady state of the converter. .

[0049] Next, the single-phase shift control of the DAB-type AC-DC converter is analyzed:

[0050] For DAB-type AC-DC converters, the AC input voltage is no longer a constant value but a pulsating DC voltage. This results in unequal primary-side input voltages in adjacent switching cycles, meaning each switching cycle corresponds to a different AC input voltage and shift ratio. However, since the switching frequency is much higher than the fundamental pulsation frequency of the input voltage, when analyzing different operating modes of the converter during the switching cycle, the AC voltage and shift ratio can be considered constant within the current switching cycle, and its operating waveform is as follows: Figure 6 As shown in the figure, the waveform of the converter during one switching cycle is displayed when the phase of the primary-side full-bridge switch lags behind the corresponding secondary-side switch.

[0051] For this invention, it is necessary to fully utilize two degrees of control freedom to achieve the control objectives. 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 objectives: AC side power factor correction at port 1, DC load side regulated output at port 2, and DC side bias voltage output at port 3. To achieve these three control objectives, the following measures were taken: Figure 7 The control strategy shown uses port 2 as the phase shift reference. D 12 The phase difference between the corresponding switches at port 1 and port 2. D 32 This represents the phase difference between the corresponding switches at ports 3 and 2.

[0052] Finally, the power conversion implementation method of the quasi-single-stage AC-DC converter based on the three active bridge structure is explained:

[0053] Step 1: Sample the AC power supply and obtain the real-time phase of the grid-side AC signal after passing through a phase-locked loop;

[0054] Step 2: Port 2 serves as the DC-side output port of the converter. After sampling the load voltage, it is compared with the output setpoint. The comparison result is used by the voltage outer loop controller to correct the amplitude information of the output current inner loop.

[0055] Step 3: Multiply the amplitude information obtained in Step 2 with the real-time phase information in Step 1 to obtain the complete reference for the inner current loop. After correction by the inner loop controller, it achieves tracking of the AC signal and power factor correction. The phase shift angles of the inner loop controller output ports 1 and 2 are then calculated. D 12 This allows us to obtain the drive signals for each switch transistor inside port 1.

[0056] Step 4: The bias voltage on the DC side of port 3 is sampled in real time. The sampled information is compared with the given reference at port 3, and the resulting error is used by a control loop composed of a PI controller to generate another phase shift angle to stabilize the output voltage of this port. D 32 This allows us to obtain the drive signals for each switch transistor inside port 3.

[0057] Step 5: Repeat the above steps within the AC input cycle to obtain the switching drive signals of each power switching device until the converter is in steady-state operation.

[0058] This invention addresses the problem of input current distortion in AC-DC converters caused by insufficient inductor current tracking of the reference current when the input voltage crosses zero. It innovatively proposes a quasi-single-stage isolated AC-DC converter based on a three-active-bridge structure. Compared to two-stage or quasi-single-stage AC-DC converters with rectifier bridges, this topology allows the grid-side input current to operate at a lower iTHD without introducing additional power conversion cycles, improving AC-side power quality and achieving higher efficiency. Simultaneously, this invention reduces the requirements for control loop and inductor parameter optimization in AC-DC converters to improve inductor current tracking capability, eliminating the need for high switching frequencies, reducing core losses, and lessening the pressure on core material design and high-frequency component selection. In summary, this invention is suitable for power conversion scenarios requiring step-up / step-down voltage, low harmonic distortion, high efficiency, and strict electrical isolation.

[0059] 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 As shown. Specifically, the full-bridge structure in port 1 and port 3 of the converter's AC side ( Figure 1 ) becomes a half-bridge structure ( Figure 9 The DC side remains unchanged. This reduces the number of AC side switches by half compared to a full-bridge topology, further lowering converter costs. At this point, the bridge port voltages at ports 1 and 3 are... , (The structure corresponding to the full bridge) 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 produces another hybrid bridge topology based on the aforementioned principle, and the rest will not be listed.

[0060] This invention is based on Figure 1 Taking this as an example, the design concept and working principle of the proposed converter are explained in detail. The internal ports of the converter, in addition to adopting... Figure 1 Besides the full-bridge structure shown, a half-bridge or full-bridge configuration can be selected based on different operating conditions and cost requirements, resulting in the remaining seven structures. In summary, this embodiment proposes an ultra-low iTHD quasi-single-stage AC-DC converter based on a three-active-bridge architecture, where each port uses either a full-bridge or half-bridge configuration. All eight topologies based on the aforementioned design concept serve as protection schemes for this embodiment, such as... Figure 8 As shown.

[0061] Those skilled in the art will understand that the above embodiments are specific embodiments for implementing the present invention, and in practical applications, various changes in form and detail can be made without departing from the spirit and scope of the embodiments of the present invention. Any person skilled in the art can make various modifications and alterations 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 should be determined by 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 comprises a first port, a second port and a third port; The first port and the third port are commonly connected with an alternating current power supply as an alternating voltage input end of an AC-DC converter; The second port is connected with an external load as a direct current voltage output end of the AC-DC converter; The third port is reversely connected in series with the first port, and a constant direct current voltage with a fixed bias is outputted by the third port to convert the input voltage of the first port into a sine voltage with a bias, or a sine voltage with a variable bias is outputted by the third port to change the amplitude of the alternating voltage inputted into the first port, so that the input voltage of the first port is a sine voltage without zero; The isolation transformer of the AC-DC converter is located at the center of the three active bridge structure, and the bridge structures of the first port, the second port and the third port are connected to the isolation transformer through respective inductors, so as to convert the alternating voltage inputted into the first port and the third port into a direct current voltage through the isolation transformer and output the direct current voltage from the second port.

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 composed of 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 switching tubes which are diagonal to each other are turned on and turned off by using the same trigger signal according to the driving signal generated by the control circuit in real time.

5. 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 alternating voltage outputted by the alternating current power supply, and obtaining the phase information of the alternating voltage inputted into the AC-DC converter after phase-locked loop; S2, sampling the load voltage of the external load, comparing the load voltage with the preset load voltage, and outputting the amplitude information of the current inner loop after the comparison result is corrected by the voltage outer loop controller; S3, multiplying the amplitude information in S2 with the phase information in S1 to obtain the reference current of the current inner loop, correcting the reference current by the inner loop controller, tracking the alternating voltage to perform power factor correction, outputting the phase shift angle between the first port and the second port, and obtaining the driving signal of each switching tube inside the first port; S4, sampling the bias voltage outputted by the third port, comparing the bias voltage with the preset bias voltage of the third port, and generating another phase shift angle of the third port output voltage by the error generated by the PI controller to obtain the driving signal of each switching tube inside the third port; S5, repeating S1 to S4 in the input period of the alternating voltage to obtain the driving signal of all the switching tubes until the AC-DC converter operates stably.

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