Single-phase three-phase input compatible bidirectional ac-dc converter without dc link electrolytic capacitor

Abstract

The application discloses a DC-link-free electrolytic capacitor single-phase three-phase input compatible bidirectional AC-DC converter, and belongs to the field of AC-DC converters in the field of power electronics. The topology of the application is composed of an input filter inductor, a three-phase full-bridge circuit and a full-bridge DAB circuit. When the three-phase input, the grid voltage is rectified through the three-phase full-bridge circuit, and a six times power frequency pulsating DC voltage is output as the input of the DAB converter, and only a small capacity film capacitor is needed for filtering at the input side of the DAB converter. When the single-phase input, the front two-phase bridge arms of the three-phase full-bridge are connected in parallel and then connected to the live line of the grid, the middle point of the third-phase bridge arm is connected to the zero line and works at a low frequency, and a two times power frequency steamed bun wave voltage is output after rectification as the input of the DAB converter. The control of the input AC current is realized by directly controlling the DAB input current, and the stable DC voltage of the DAB output is realized. The application realizes electrical isolation through a high-frequency transformer, has a simple structure, no DC bus electrolytic capacitor, and significantly improves the power density and service life.

Description

Technical Field

[0001] This invention relates to a bidirectional AC-DC converter compatible with single-phase and three-phase inputs without DC-link electrolytic capacitors, belonging to the field of bidirectional AC-DC converters that can be used for electric vehicle charging in the field of power electronics. Background Technology

[0002] With the development of new energy power generation technologies and the widespread use of electric vehicles, isolated bidirectional AC-DC converters are widely used in electric vehicle charging piles, energy storage systems, and hybrid microgrids. Isolated AC-DC converters are divided into single-stage and two-stage structures. Traditional two-stage structures typically use a non-isolated AC-DC converter in the front stage for power factor correction, followed by an isolated DC-DC converter in the rear stage for output regulation and electrical isolation. Large-capacity DC-link electrolytic capacitors are usually required between the front and rear stages, resulting in low power density and short lifespan. Furthermore, most converters can only operate in one input mode and are not compatible with single-phase or three-phase inputs, limiting their application scenarios. Therefore, research on high-power-density, long-life converters that support multiple input modes is particularly important.

[0003] The 2025 paper "Single-Stage Isolated Three-Phase SWISS AC-DC Converter Based on Dual Active Bridge Converters," published in *IEEE Transactions on Power Electronics*, modifies the SWISS converter, proposing a single-stage SWISS-type AC-DC converter based on a dual active bridge (DAB). The paper connects the low-frequency voltage selector output port of the SWISS converter in parallel to two DAB converters. Based on the analysis of the input current relationship between the SWISS converter and the two parallel DAB converters, the optimal triple phase shift (TPS) control of the DAB converter is derived, achieving zero-voltage switching (ZVS) and minimum current stress. However, this parallel structure results in topology redundancy, with a large number of switching devices and energy storage components, leading to low power density. Furthermore, the large real-time power fluctuations handled by the two DAB converters result in a large design margin for the transformer, requiring over-design of the switching transistors and transformer windings, and ultimately, low utilization.

[0004] The international conference paper "Ultra-Efficient Three-Phase Integrated-Active-Filter Isolated Rectifier for AI Data Center Applications," published in May 2025 at the 2025 IEEE Energy Conversion Congress & Exposition Asia (ECCE-Asia), proposes an AC-DC converter based on an integrated active filter rectifier. Three-phase sinusoidal input current control is achieved by injecting a third harmonic current into the bridge arm at the voltage selector output. A triangular wave modulation mode for the injection arm is introduced to ensure zero-voltage switching throughout the grid cycle. Simultaneously, a multi-objective Pareto optimization method is used to optimize circuit parameters, determining the limiting performance of the front-stage integrated active filter rectifier paired with the back-stage isolated DC-DC converter, meeting the power requirements of high-efficiency AI data center power supplies. However, due to the presence of the "horizontal tube" in the voltage selector, this topology still requires a relatively large number of switching devices, and the power density still needs to be improved. Furthermore, the above circuit topologies can only operate in three-phase input mode and are not compatible with single-phase input mode. Summary of the Invention

[0005] To address the problems of existing electric vehicle chargers, such as large-capacity DC-link electrolytic capacitors, numerous and redundant switching devices in complex topologies, and incompatibility with single-phase and three-phase input modes, this invention proposes a DAB-based bidirectional AC-DC converter with no DC-link electrolytic capacitors and compatibility with both single-phase and three-phase input modes. By connecting different bridge arms, it can meet the requirements for both single-phase and three-phase input modes, thus broadening the converter's application scenarios. Corresponding control strategies are proposed for different single-phase and three-phase input modes to achieve power factor correction and stable output voltage control, achieving high-efficiency power conversion. This invention has advantages such as simple structure, fewer switching devices, and no DC-link electrolytic capacitors.

[0006] The objective of this invention is achieved through the following technical solution.

[0007] This invention discloses a bidirectional AC-DC converter compatible with both single-phase and three-phase inputs, without DC-link electrolytic capacitors. It consists of an input filter inductor, a three-phase full-bridge circuit, and a full-bridge DAB circuit. Only a thin-film capacitor exists between the three-phase full-bridge circuit and the full-bridge DAB circuit; no electrolytic capacitors are required. In three-phase input mode, the grid voltage is connected to the three-phase full-bridge circuit, rectified, and output as a pulsating DC voltage six times the power frequency, i.e., the envelope of the line voltage, which serves as the input to the DAB converter. In single-phase input mode, the first two phase arms of the three-phase full-bridge circuit are connected in parallel to the live wire of the single-phase grid voltage. The third phase bridge arm is connected to the neutral line at its midpoint and operates at a low frequency. After rectification, it outputs a pulsating DC voltage at twice the power frequency, i.e., a ripple voltage at twice the power frequency, which serves as the input to the DAB converter. In the DAB converter, the primary and secondary sides of the transformer are two full-bridge circuits, electrically isolated by a high-frequency transformer, and the output is controlled as DC. The DAB converter directly controls the input current of the DAB through a three-phase-shift modulation strategy, thereby controlling the AC input current. Power factor correction is achieved through coordinated control of the front and rear stages, ensuring good AC power quality and a stable output voltage. The circuit topology disclosed in this invention is simple in structure, compatible with single-phase and three-phase grid voltage inputs, has fewer switching devices, eliminates the need for DC link electrolytic capacitors, and improves the power density and lifespan of the converter.

[0008] The bidirectional AC-DC converter disclosed in this invention, which is compatible with both single-phase and three-phase inputs and has no DC-link electrolytic capacitor, is available in two modes: three-phase input mode control and single-phase input mode control.

[0009] The bidirectional AC-DC converter, which is compatible with both single-phase and three-phase inputs and has no DC-link electrolytic capacitor, operates in three-phase input mode by the following steps:

[0010] Step 1: Connect relays R1 and R2, and disconnect R3 and R4; the converter operates in three-phase input mode.

[0011] Step 2: Divide the circuit into sectors based on the grid input voltage. For a three-phase full-bridge circuit, the upper switch of the corresponding bridge arm with the maximum instantaneous phase voltage is turned on, and the lower switch of the corresponding bridge arm with the minimum instantaneous phase voltage is turned on. The switch of the corresponding bridge arm with the intermediate instantaneous phase voltage is controlled by high-frequency switching for third harmonic injection current. Therefore, the output voltage of the front-end pn node is a pulsating DC voltage six times the power frequency, i.e., the envelope of the line voltage. After adjustment by the full-bridge DAB converter, the output load-side DC voltage v o .

[0012] Step 3: Set the output voltage reference v oref The output voltage v at the sampled load terminal o The difference is calculated, and the voltage controller outputs the control quantity y. vThe per-unit value representing the input current amplitude ranges from [-1, 1]. The sampled three-phase current undergoes an abc-dq transformation, and the per-unit value I of the input current amplitude is obtained after normalization calculation. m * The range is [-1, 1]; then y v with I m * The difference is calculated, and the difference is passed through the current controller. The output is the input current amplitude control quantity y of the converter, which ranges from [-1, 1].

[0013] Step 4: Sample the grid input voltage v a After passing through a phase-locked loop, the phase angle θ of phase a voltage is obtained, and the magnitude of θ is limited to 0 to 2π. Adding and subtracting 2π / 3 to the phase angle θ yields the phase angles of phases c and b, thus obtaining the cosine values ​​of the three-phase phase angles. Multiplying the input current amplitude control quantity y by the median value of the cosine values ​​gives yc. 3rd * As a reference for the per-unit value of the third harmonic injection current; its expression is:

[0014]

[0015] The per-unit value y of the injected current is obtained by selecting the median value of the three-phase current in each sector and performing per-unit calculation. 3rd ; will y 3rd * With y 3rd The difference is calculated, and the difference is passed through the injection current controller to output the actual duty cycle of the injection current bridge arm switch, which is used to control the third harmonic injection current; the high-frequency drive signals HDS1 and HDS2, or HDS3 and HDS4, or HDS5 and HDS6 output by the injection current controller are distributed to each phase bridge arm according to the sector.

[0016] The input current amplitude control value y is obtained by multiplying the maximum value of the cosine value. p Its expression is

[0017]

[0018] Based on the volt-second balance of the input filter inductor of each phase during each switching cycle, the theoretical steady-state duty cycle of the high-frequency switching transistor in the injected current bridge arm is obtained as follows:

[0019]

[0020] Among them, D h To control the theoretical duty cycle of the switch on the injected current bridge arm, D l The duty cycle for the next switch; v max ,v mid and v minThese represent the maximum phase voltage value, the intermediate phase voltage value, and the minimum phase voltage value within each sector;

[0021] y 3rd * With D h Multiplying them together gives y h , then y p With y h Adding them together gives y pp = y p +y h As a given value for the per-unit value of the DAB input current.

[0022] Step 5: Directly control the input current y using three-phase shift modulation of the DAB converter. pp This enables control of the AC input current, achieving power factor correction; power factor correction is achieved through coordinated control of the pre- and post-stages, eliminating the need for DC-link electrolytic capacitors; when the power is positive, y pp >0, phase shift angle φ s Take positive; when the power is reversed, y pp <0, phase shift angle φ s Inverting the value results in a negative value; this enables bidirectional power control.

[0023] The bidirectional AC-DC converter, which is compatible with both single-phase and three-phase inputs and has no DC-link electrolytic capacitor, has the following two control strategies and operating principles in single-phase input mode:

[0024] The bidirectional AC-DC converter, compatible with both single-phase and three-phase inputs and without DC-link electrolytic capacitors, operates in the first case of single-phase input mode, which is the power frequency rectification mode. This mode is commonly used in scenarios with low output voltage and low power, and includes the following steps:

[0025] Step 1: Disconnect relays R1 and R2, and connect R3 and R4; the converter operates in single-phase input mode.

[0026] Step 2: Sample the grid input voltage v a When the voltage is positive, i.e., v a When the voltage is greater than 0, switches S1, S3, and S6 are turned on, while switches S2, S4, and S5 are turned off; when the voltage is negative, i.e., v... a When the voltage is less than 0, switches S2, S4, and S5 are turned on, while switches S1, S3, and S6 are turned off; S1-S6 operate at low frequency; therefore, the output voltage of the front-end pn node is a pulsating DC voltage at twice the power frequency, i.e., a wave voltage at twice the power frequency, with an amplitude equal to the phase voltage amplitude; after regulation by the full-bridge DAB converter, the output load terminal DC voltage v o .

[0027] Step 3: Set the output voltage reference v orefThe output voltage v at the sampled load terminal o The difference is calculated, and the voltage controller outputs the control quantity y. v It represents the per-unit value of the input current amplitude, ranging from [-1, 1]; the per-unit value I of the input current amplitude is obtained by normalizing the sampled input phase current. m * The range is [-1, 1]; then y v with I m * The difference is calculated, and the difference is passed through the current controller. The output is the input current amplitude control quantity y of the converter, which ranges from [-1, 1].

[0028] Step 4: Sample the grid input voltage v a After passing through a phase-locked loop, the phase angle θ of phase a voltage is obtained, and the magnitude of θ is limited to 0 to 2π. The input current amplitude control quantity y is multiplied by the absolute value of the sine of the phase angle θ to obtain y. pp =y |sin(θ)|, which serves as the given value for the per-unit input current of the DAB.

[0029] Step 5: Directly control the input current y using three-phase shift modulation of the DAB converter. pp This enables control of the AC input current, achieving power factor correction; power factor correction is achieved through coordinated control of the pre- and post-stages, eliminating the need for DC-link electrolytic capacitors; when the power is positive, y pp >0, phase shift angle φ s Take positive; when the power is reversed, y pp <0, phase shift angle φ s Inverting the value results in a negative value; this enables bidirectional power control.

[0030] The bidirectional AC-DC converter with single-phase and three-phase input compatibility and no DC-link electrolytic capacitor described above, in its second case under single-phase input mode, is a boost mode, commonly used in scenarios requiring high output voltage and high power, and includes the following steps:

[0031] Step 1: Disconnect relays R1 and R2, and connect R3 and R4; the converter operates in single-phase input mode.

[0032] Step 2: Sample the grid input voltage v a When the voltage is positive, i.e., v a When the voltage is greater than 0, switch S6 is turned on and S5 is turned off; when the voltage is negative, i.e., v a When V < 0, switch S5 is turned on and S6 is turned off; switches S1, S2, S3, and S4 operate at high frequency, with S1 and S3 having the same drive signal, and S2 and S4 having the same drive signal; S1 and S2 are complementary switches, and S3 and S4 are complementary switches; when V aWhen the duty cycle is greater than 0, the duty cycles of switching transistors S2 and S4 are set to D. n The duty cycles of switching transistors S1 and S3 are set to ( When v a When <0, the duty cycle of switching transistors S1 and S3 is set to D. n The duty cycles of switching transistors S2 and S4 are set to ( Then the output voltage of the preceding pn node is a pulsating DC voltage at twice the power frequency, that is, a wavy voltage at twice the power frequency, with an amplitude of 1 / ( The phase voltage amplitude is doubled; the DC voltage at the output load terminal after regulation by the full-bridge DAB converter is v. o Preferably, when the duty cycles of switches S1, S2, S3, and S4 are set to 0.5, the output voltage of the preceding pn node is a pulsating DC voltage at twice the power frequency, i.e., a wavy voltage at twice the power frequency, with an amplitude twice the phase voltage amplitude. Preferably, when v a When v > 0, the duty cycle of switches S2 and S4 is set to 2 / 3, and the duty cycle of switches S1 and S3 is set to 1 / 3; when v a When <0, the duty cycle of switches S1 and S3 is set to 2 / 3, and the duty cycle of switches S2 and S4 is set to 1 / 3; then the output voltage of the front-stage pn node is a pulsating DC voltage twice the power frequency, that is, a wavy voltage twice the power frequency, and its amplitude is three times the phase voltage amplitude.

[0033] Step 3: Set the output voltage reference v oref The output voltage v at the sampled load terminal o The difference is calculated, and the voltage controller outputs the control quantity y. v It represents the per-unit value of the input current amplitude, ranging from [-1, 1]; the per-unit value I of the input current amplitude is obtained by normalizing the sampled input phase current. m * The range is [-1, 1]; then y v with I m * The difference is calculated, and the difference is passed through the current controller. The output is the input current amplitude control quantity y of the converter, which ranges from [-1, 1].

[0034] Step 4: Sample the grid input voltage v a After passing through a phase-locked loop, the phase angle θ of phase a voltage is obtained, and the magnitude of θ is limited to 0 to 2π. The input current amplitude control quantity y is multiplied by the absolute value of the sine of the phase angle θ to obtain y. pp =y |sin(θ)|, which serves as the given value for the per-unit input current of the DAB.

[0035] Step 5: Directly control the input current y using three-phase shift modulation of the DAB converter. ppThis enables control of the AC input current, achieving power factor correction; power factor correction is achieved through coordinated control of the pre- and post-stages, eliminating the need for DC-link electrolytic capacitors; when the power is positive, y pp >0, phase shift angle φ s Take positive; when the power is reversed, y pp <0, phase shift angle φ s Inverting the value results in a negative value; this enables bidirectional power control.

[0036] Beneficial effects:

[0037] 1. This invention discloses a bidirectional AC-DC converter compatible with both single-phase and three-phase inputs without a DC-link electrolytic capacitor. It features a simple structure and few switching devices. Power factor correction is achieved through coordinated control of the preceding and following stages, eliminating the need for a DC-link electrolytic capacitor and improving the converter's power density and lifespan. The proposed topology can satisfy single-phase and three-phase input mode compatibility by connecting different bridge arms, broadening the converter's application scenarios.

[0038] 2. The bidirectional AC-DC converter disclosed in this invention, which is compatible with both single-phase and three-phase inputs and has no DC-link electrolytic capacitor, achieves electrical isolation and wide-range voltage adaptation through a high-frequency isolation transformer. Leveraging the inherent wide operating range of DABs, this invention maintains stable power delivery and excellent dynamic response performance across a wide input and output voltage range.

[0039] 3. The bidirectional AC-DC converter with single-phase and three-phase input compatibility and no DC-link electrolytic capacitor disclosed in this invention uses the input current amplitude y as a control quantity, and solves for y through the working principle of the front-end circuit. pp Given the per-unit value of the DAB input current, the analytical relationship between the DAB input current and the AC input current is solved. Then, using the three-phase-shift modulation strategy of the DAB, the relationship between the DAB input current and various control quantities is solved to obtain the analytical expressions for the duty cycle and phase shift angle. Finally, the input current is directly controlled through three-phase-shift modulation. This control strategy enables power factor correction, resulting in better power quality for the input AC current.

[0040] 4. The bidirectional AC-DC converter with single-phase and three-phase input compatibility and no DC-link electrolytic capacitor disclosed in this invention has two operating modes in single-phase input mode. One is the power frequency rectification mode, which is commonly used in low-voltage and low-power output scenarios. S1-S6 operate at low frequency, forming a single-stage topology, performing only one power conversion, avoiding repeated energy conversion in multi-stage structures, reducing losses, and improving efficiency. The other is the boost mode, which is commonly used in high-voltage and high-power output scenarios. S1-S4 operate at high frequency, and S5-S6 operate at low frequency. S1 and S3 have the same drive signal, and S2 and S4 have the same drive signal; S1 and S2 are complementary in conduction, and S3 and S4 are complementary in conduction; boosting is achieved by setting the duty cycle of the high-frequency switches of S1-S4. When the duty cycle of the high-frequency switches is set to 0.5, the DAB input voltage is a pulsating DC voltage twice the power frequency, i.e., a ripple voltage twice the power frequency, with an amplitude twice the phase voltage amplitude. In high-power applications, boost mode can reduce the current stress on the downstream DAB switching transistors.

[0041] 5. The bidirectional AC-DC converter with single-phase and three-phase input compatibility and no DC-link electrolytic capacitor disclosed in this invention achieves ZVS / ZCS soft switching across the entire operating range of all switching transistors in the DAB through a three-phase-shift modulation strategy. This eliminates the need for external auxiliary circuitry, reducing hardware costs, minimizing switching losses, and improving system reliability and stability. Attached Figure Description

[0042] Figure 1 This diagram illustrates the topology of a bidirectional AC-DC converter compatible with single-phase and three-phase inputs without DC-link electrolytic capacitors, as shown in this embodiment.

[0043] Figure 2 A schematic diagram of the converter of the present invention operating in three-phase input mode is shown.

[0044] Figure 3 This diagram shows the topology of the converter of the present invention in the power frequency rectification mode when operating in single-phase input mode;

[0045] Figure 4 This diagram illustrates the boost mode topology of the converter of the present invention operating in single-phase input mode.

[0046] Figure 5 The diagram shows the power factor correction closed-loop control block diagram of the converter of the present invention operating in three-phase input mode;

[0047] Figure 6 The diagram shows the power factor correction closed-loop control block diagram of the converter of the present invention in the power frequency rectification mode when the converter is operating in the single-phase input mode.

[0048] Figure 7The diagram shows the power factor correction closed-loop control block diagram of the converter of the present invention in boost mode when operating in single-phase input mode. Detailed Implementation

[0049] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. The technical problems solved by the present invention and its beneficial effects will also be described. It should be noted that the described embodiments are only intended to facilitate the understanding of the present invention and do not limit it in any way.

[0050] Figure 1 The diagram shows the bidirectional AC-DC converter topology of this embodiment, which is compatible with both single-phase and three-phase inputs and has no DC-link electrolytic capacitor. It consists of four relays, an input filter inductor, a three-phase full-bridge circuit, and a full-bridge DAB circuit. When relays R1 and R2 are connected and R3 and R4 are disconnected, the converter operates in three-phase input mode, as shown below. Figure 2 As shown. When relays R3 and R4 are connected and R1 and R2 are disconnected, the converter operates in single-phase input mode, as shown. Figure 3 , Figure 4 As shown. v a , v b and v c For the three-phase power grid voltage, i a i b and i c This represents three-phase alternating current. In the full-bridge DAB circuit, nodes A and B are the midpoints of the two arms of the primary side full bridge, respectively, and nodes C and D are the midpoints of the two arms of the secondary side full bridge, respectively. r i represents the current in the primary winding of the transformer. pp This represents the input current of the DAB converter. pn and v o These represent the input and output voltages on the primary and secondary sides of the DAB converter, respectively. AB V is the voltage between node A and node B on the primary side of the DAB converter. CD This represents the voltage between node C and node D on the secondary side of DAB.

[0051] like Figure 1As shown, the three-phase voltages a, b, and c on the grid side correspond to three bridge arms, with S1, S3, and S5 forming the upper bridge arm, and S2, S4, and S6 forming the lower bridge arm. Phase a bridge arm consists of switches S1 and S2, with the source of S1 connected to the drain of S2; phase b bridge arm consists of switches S3 and S4, with the source of S3 connected to the drain of S4; phase c bridge arm consists of switches S5 and S6, with the source of S5 connected to the drain of S6. The drains of switches S1, S3, and S5 are connected together as the positive terminal of the DAB converter input connected to the pn node, defined as node p; the sources of switches S2, S4, and S6 are connected together as the negative terminal of the DAB converter input connected to the pn node, defined as node n; a small thin-film capacitor C is connected between the pn nodes. in .

[0052] The primary side full-bridge DAB converter consists of four transistors Q1-Q4. Transistors Q1 and Q2 form one bridge arm, with the source of Q1 connected to the drain of Q2. Transistors Q3 and Q4 form another bridge arm, with the source of Q3 connected to the drain of Q4. The drains of transistors Q1 and Q3 are connected together and then connected to node p. The sources of transistors Q2 and Q4 are connected together and then connected to node n. The source of transistor Q1 is connected to a series inductor L. r One end of the inductor is connected to the same-name terminal of the primary transformer winding, and the opposite-name terminal of the primary transformer winding is connected to the source of Q3. The secondary full-bridge consists of four switching transistors Q5-Q8. Switches Q5 and Q6 form one bridge arm, with the source of Q5 connected to the drain of Q6. Switches Q7 and Q8 form another bridge arm, with the source of Q7 connected to the drain of Q8. The drains of switches Q5 and Q7 are connected together to the positive terminal of the output capacitor, serving as the positive terminal of the output voltage. The sources of switches Q6 and Q8 are connected together to the negative terminal of the output capacitor, serving as the negative terminal of the output voltage. The same-name terminal of the transformer secondary winding is connected to the source of switch Q5, and the opposite-name terminal of the transformer secondary winding is connected to the source of switch Q7.

[0053] First Example

[0054] Figure 2 The diagram shown is a schematic topology of the bidirectional AC-DC converter compatible with single-phase and three-phase inputs without a DC link electrolytic capacitor in this embodiment, in three-phase input mode. Its corresponding power factor correction closed-loop control block diagram is shown below. Figure 5 As shown. The specific steps are as follows:

[0055] Step 1: Connect relays R1 and R2, and disconnect R3 and R4; the converter operates in three-phase input mode.

[0056] Step 2: Divide the sector according to the grid input voltage. When va > v b >v c When v is in the range, it is defined as sector 1; when v is in the range, it is defined as sector 1. b > v a >v c When v is in the range of sector 2, it is defined as sector 2; when v is in the range of sector 2, it b > v c >v a When v is defined as sector 3; when v c > v b >v a When v is defined as sector 4; when v c > v a >v b When v is defined as sector 5; when v a > v c >v b At this time, sector 6 is defined. For a three-phase full-bridge circuit, the upper switch of the corresponding bridge arm is turned on when the instantaneous phase voltage is at its maximum, and the lower switch of the corresponding bridge arm is turned on when the instantaneous phase voltage is at its minimum. The switch of the corresponding bridge arm with the intermediate instantaneous phase voltage is controlled by high-frequency switching for third harmonic injection current. Therefore, the output voltage of the front-end pn node is a pulsating DC voltage six times the power frequency, i.e., the envelope of the line voltage. After regulation by the full-bridge DAB converter, the DC voltage at the output load terminal is v. o ;

[0057] Step 3: Set the output voltage reference v oref The output voltage v at the sampled load terminal o The difference is calculated, and the voltage controller outputs the control quantity y. v The per-unit value representing the input current amplitude ranges from [-1, 1]. The sampled three-phase current undergoes an abc-dq transformation, and the per-unit value I of the input current amplitude is obtained after normalization calculation. m * The range is [-1, 1]; then y v with I m * The difference is calculated, and the difference value passes through the current controller. The output is the input current amplitude control quantity y of the converter, which has a range of [-1, 1].

[0058] Step 4: Sample the grid input voltage v a After passing through a phase-locked loop, the phase angle θ of phase a voltage is obtained, and the magnitude of θ is limited to 0 to 2π. Adding and subtracting 2π / 3 to the phase angle θ yields the phase angles of phases c and b, thus obtaining the cosine values ​​of the three-phase phase angles. Multiplying the input current amplitude control quantity y by the median value of the cosine values ​​gives yc. 3rd * As a reference for the per-unit value of the third harmonic injection current; its expression is:

[0059]

[0060] The per-unit value y of the injected current is obtained by selecting the median value of the three-phase current in each sector and performing per-unit calculation. 3rd ; will y 3rd * With y 3rd The difference is calculated, and the difference value is passed through the injection current controller to output the actual duty cycle of the injection current bridge arm switch, which is used to control the third harmonic injection current; the high-frequency drive signals HDS1 and HDS2, or HDS3 and HDS4, or HDS5 and HDS6 output by the injection current controller are distributed to each phase bridge arm according to the sector.

[0061] v in sector 1 a > v b > v c Then, the upper transistor S1 corresponding to phase a bridge arm is turned on, the lower transistor S6 corresponding to phase c bridge arm is turned on, and the switches S3 and S4 of phase b bridge arm are alternately turned on and operate at high frequency to control the third harmonic injection current. That is, in sector 1, the high-frequency drive signal output by the injection current controller is distributed to phase b bridge arm, namely HDS3 and HDS4. Thus, the drive signals DS1-DS6 of all switches of the front-stage three-phase full bridge are obtained.

[0062] v in sector 2 b > v a > v c Then, the upper transistor S3 corresponding to phase b bridge arm conducts, and the lower transistor S6 corresponding to phase c bridge arm conducts. Phase a bridge arm switches S1 and S2 alternately conduct at high frequency to control the third harmonic injection current. That is, in sector 1, the high-frequency drive signal output by the injection current controller is distributed to phase a bridge arm, namely HDS1 and HDS2. This yields the drive signals DS1-DS6 for all switches in the preceding three-phase full-bridge circuit.

[0063] v in sector 3 b > v c > v a Then, the upper transistor S3 corresponding to phase b bridge arm conducts, the lower transistor S2 corresponding to phase a bridge arm conducts, and the switches S5 and S6 of phase c bridge arm alternately conduct at high frequency to control the third harmonic injection current. That is, in sector 1, the high-frequency drive signal output by the injection current controller is distributed to phase c bridge arm, namely HDS5 and HDS6. Thus, the drive signals DS1-DS6 of all switches of the front-stage three-phase full bridge are obtained.

[0064] v in sector 4 c > v b > v aThen, the upper transistor S5 corresponding to phase c bridge arm conducts, the lower transistor S2 corresponding to phase a bridge arm conducts, and the switches S3 and S4 of phase b bridge arm alternately conduct at high frequency to control the third harmonic injection current. That is, in sector 1, the high-frequency drive signal output by the injection current controller is distributed to phase b bridge arm, namely HDS3 and HDS4. This yields the drive signals DS1-DS6 for all switches in the preceding three-phase full-bridge circuit.

[0065] In sector 5, v c > v a > v b Then, the upper transistor S5 corresponding to phase c bridge arm conducts, the lower transistor S4 corresponding to phase b bridge arm conducts, and the switches S1 and S2 of phase a bridge arm alternately conduct at high frequency to control the third harmonic injection current. That is, in sector 1, the high-frequency drive signal output by the injection current controller is distributed to phase a bridge arm, namely HDS1 and HDS2. This yields the drive signals DS1-DS6 for all switches in the preceding three-phase full-bridge circuit.

[0066] In sector 6, v a > v c > v b Then, the upper transistor S1 corresponding to phase a bridge arm is turned on, the lower transistor S4 corresponding to phase b bridge arm is turned on, and the switches S5 and S6 of phase c bridge arm are alternately turned on and operate at high frequency to control the third harmonic injection current. That is, in sector 1, the high-frequency drive signal output by the injection current controller is distributed to phase c bridge arm, namely HDS5 and HDS6. Thus, the drive signals DS1-DS6 of all switches of the front-stage three-phase full bridge are obtained.

[0067] The input current amplitude control value y is obtained by multiplying the maximum value of the cosine value. p Its expression is

[0068]

[0069] Based on the volt-second balance of the input filter inductor of each phase during each switching cycle, the theoretical steady-state duty cycle of the high-frequency switching transistor in the injected current bridge arm is obtained as follows:

[0070]

[0071] Among them, D h To control the theoretical duty cycle of the switch on the injected current bridge arm, D l The duty cycle for the next switch; v max ,v mid and v min These represent the maximum phase voltage value, the intermediate phase voltage value, and the minimum phase voltage value within each sector;

[0072] y 3rd * With Dh Multiplying them together gives y h , then y p With y h Adding them together gives y pp = y p +y h As the given value for the per-unit value of the DAB input current;

[0073] Step 5: Sample the DAB input voltage v pn With output voltage v o The corresponding modulation parameter, phase shift angle φ, is calculated using a three-phase-shift modulation strategy. s Duty cycles D1 and D2. D1 and D2 are used to adjust the duty cycles of the primary and secondary sides of DAB, φ s The phase difference between the primary and secondary voltage waveforms is determined to achieve power transfer control. This generates drive signals for the two full-bridge switches in the DAB converter, which drive the eight switches Q1-Q8. The drive signals for the eight switches Q1-Q8 in the DAB converter are all 50% square wave signals, with DQ1 and DQ2 being complementary, DQ3 and DQ4 complementary, DQ5 and DQ6 complementary, and DQ7 and DQ8 complementary, with a dead time between the complementary drive signals. The input current y is directly controlled by the three-phase-shift modulation of the DAB converter. pp This allows for control of the AC input current, achieving power factor correction. In summary, power factor correction is achieved through coordinated control of the front and rear stages, eliminating the need for DC-link electrolytic capacitors. When the power is positive, y pp >0, phase shift angle φ s Take positive; when the power is reversed, y pp <0, phase shift angle φ s Inverting the value results in a negative value; this enables bidirectional power control.

[0074] Second example

[0075] Figure 3 The diagram shown is a schematic of the power frequency rectification mode topology of the single-phase input mode bidirectional AC-DC converter compatible with single-phase and three-phase inputs without DC link electrolytic capacitors in this embodiment. The corresponding power factor correction closed-loop control block diagram is shown below. Figure 6 As shown. The specific steps are as follows:

[0076] Step 1: Disconnect relays R1 and R2, and connect R3 and R4; the converter operates in single-phase input mode.

[0077] Step 2: Sample the grid input voltage v a When the voltage is positive, i.e., v a When the voltage is greater than 0, switches S1, S3, and S6 are turned on, while switches S2, S4, and S5 are turned off; when the voltage is negative, i.e., v...a When the voltage is less than 0, switches S2, S4, and S5 are turned on, while switches S1, S3, and S6 are turned off; S1-S6 operate at low frequency; therefore, the output voltage of the front-end pn node is a pulsating DC voltage at twice the power frequency, i.e., a wave voltage at twice the power frequency, with an amplitude equal to the phase voltage amplitude; after regulation by the full-bridge DAB converter, the output load terminal DC voltage v o ;

[0078] Step 3: Set the output voltage reference v oref The output voltage v at the sampled load terminal o The difference is calculated, and the voltage controller outputs the control quantity y. v It represents the per-unit value of the input current amplitude, ranging from [-1, 1]; the per-unit value I of the input current amplitude is obtained by normalizing the sampled input phase current. m * The range is [-1, 1]; then y v with I m * The difference is calculated, and the difference value passes through the current controller. The output is the input current amplitude control quantity y of the converter, which has a range of [-1, 1].

[0079] Step 4: Sample the grid input voltage v a After passing through a phase-locked loop, the phase angle θ of phase a voltage is obtained, and the magnitude of θ is limited to 0 to 2π. The input current amplitude control quantity y is multiplied by the absolute value of the sine of the phase angle θ to obtain ya. pp =y |sin(θ)|, which serves as the given value for the per-unit input current of the DAB.

[0080] Step 5: Sample the DAB input voltage v pn With output voltage v o The corresponding modulation parameter, phase shift angle φ, is calculated using a three-phase-shift modulation strategy. s Duty cycles D1 and D2. D1 and D2 are used to adjust the duty cycles of the primary and secondary sides of DAB, φ s The phase difference between the primary and secondary voltage waveforms is determined to achieve power transfer control. This generates drive signals for the two full-bridge switches in the DAB converter, which drive the eight switches Q1-Q8. The drive signals for the eight switches Q1-Q8 in the DAB converter are all 50% square wave signals, with DQ1 and DQ2 being complementary, DQ3 and DQ4 complementary, DQ5 and DQ6 complementary, and DQ7 and DQ8 complementary, with a dead time between the complementary drive signals. The input current y is directly controlled by the three-phase-shift modulation of the DAB converter. pp This allows for control of the AC input current, achieving power factor correction. In summary, power factor correction is achieved through coordinated control of the front and rear stages, eliminating the need for DC-link electrolytic capacitors. When the power is positive, ypp >0, phase shift angle φ s Take positive; when the power is reversed, y pp <0, phase shift angle φ s Inverting the value results in a negative value; this enables bidirectional power control.

[0081] Third Example

[0082] Figure 4 The diagram shown is a schematic of the boost mode topology of the single-phase input bidirectional AC-DC converter compatible with single-phase and three-phase inputs without a DC link electrolytic capacitor in this embodiment. The corresponding power factor correction closed-loop control block diagram is shown below. Figure 7 As shown. The specific steps are as follows:

[0083] Step 1: Disconnect relays R1 and R2, and connect R3 and R4; the converter operates in single-phase input mode.

[0084] Step 2: Sample the grid input voltage v a When the voltage is positive, i.e., v a When the voltage is greater than 0, switch S6 is turned on and S5 is turned off; when the voltage is negative, i.e., v a When V < 0, switch S5 is turned on and S6 is turned off; switches S1, S2, S3, and S4 operate at high frequency, with S1 and S3 having the same drive signal, and S2 and S4 having the same drive signal; S1 and S2 are complementary switches, and S3 and S4 are complementary switches; when V a When the duty cycle is greater than 0, the duty cycles of switching transistors S2 and S4 are set to D. n The duty cycles of switching transistors S1 and S3 are set to ( When v a When <0, the duty cycle of switching transistors S1 and S3 is set to D. n The duty cycles of switching transistors S2 and S4 are set to ( Then the output voltage of the preceding pn node is a pulsating DC voltage at twice the power frequency, that is, a wavy voltage at twice the power frequency, with an amplitude of 1 / ( The phase voltage amplitude is doubled; the DC voltage at the output load terminal after regulation by the full-bridge DAB converter is v. o Preferably, when the duty cycles of switches S1, S2, S3, and S4 are set to 0.5, the output voltage of the preceding pn node is a pulsating DC voltage at twice the power frequency, i.e., a wavy voltage at twice the power frequency, with an amplitude twice the phase voltage amplitude. Preferably, when v a When v > 0, the duty cycle of switches S2 and S4 is set to 2 / 3, and the duty cycle of switches S1 and S3 is set to 1 / 3; when v aWhen <0, the duty cycle of switches S1 and S3 is set to 2 / 3, and the duty cycle of switches S2 and S4 is set to 1 / 3; then the output voltage of the front-stage pn node is a pulsating DC voltage twice the power frequency, that is, a wavy voltage twice the power frequency, and its amplitude is three times the phase voltage amplitude.

[0085] Step 3: Set the output voltage reference v oref The output voltage v at the sampled load terminal o The difference is calculated, and the voltage controller outputs the control quantity y. v It represents the per-unit value of the input current amplitude, ranging from [-1, 1]; the per-unit value I of the input current amplitude is obtained by normalizing the sampled input phase current. m * The range is [-1, 1]; then y v with I m * The difference is calculated, and the difference value passes through the current controller. The output is the input current amplitude control quantity y of the converter, which has a range of [-1, 1].

[0086] Step 4: Sample the grid input voltage v a After passing through a phase-locked loop, the phase angle θ of phase a voltage is obtained, and the magnitude of θ is limited to 0 to 2π. The input current amplitude control quantity y is multiplied by the absolute value of the sine of the phase angle θ to obtain ya. pp =y |sin(θ)|, which serves as the given value for the per-unit input current of the DAB.

[0087] Step 5: Sample the DAB input voltage v pn With output voltage v o The corresponding modulation parameter, phase shift angle φ, is calculated using a three-phase-shift modulation strategy. s Duty cycles D1 and D2. D1 and D2 are used to adjust the duty cycles of the primary and secondary sides of DAB, φ s The phase difference between the primary and secondary voltage waveforms is determined to achieve power transfer control. This generates drive signals for the two full-bridge switches in the DAB converter, which drive the eight switches Q1-Q8. The drive signals for the eight switches Q1-Q8 in the DAB converter are all 50% square wave signals, with DQ1 and DQ2 being complementary, DQ3 and DQ4 complementary, DQ5 and DQ6 complementary, and DQ7 and DQ8 complementary, with a dead time between the complementary drive signals. The input current y is directly controlled by the three-phase-shift modulation of the DAB converter. pp This allows for control of the AC input current, achieving power factor correction. In summary, power factor correction is achieved through coordinated control of the front and rear stages, eliminating the need for DC-link electrolytic capacitors. When the power is positive, y pp >0, phase shift angle φ s Take positive; when the power is reversed, y pp<0, phase shift angle φ s Inverting the value results in a negative value; this enables bidirectional power control.

[0088] In summary, the bidirectional AC-DC converter disclosed in this invention, which is compatible with both single-phase and three-phase inputs without a DC-link electrolytic capacitor, can meet the requirements of single-phase and three-phase input modes by connecting different bridge arms, thus broadening the converter's application scenarios. In single-phase mode, it offers two functions: power frequency rectification mode and boost mode. The power frequency rectification mode is commonly used in low-voltage and low-power output scenarios, performing only one power conversion, avoiding repeated energy conversion in multi-stage structures, reducing losses, and improving efficiency. The boost mode is commonly used in high-voltage and high-power output scenarios. In high-power applications, the boost mode can reduce the current stress on the downstream DAB switching transistors. This multifunctional AC-DC converter provides an excellent solution for electric vehicle chargers with a wide output voltage range.

[0089] The above detailed description further illustrates the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A bidirectional AC-DC converter compatible with single-phase and three-phase inputs without DC-link electrolytic capacitors, characterized in that: The topology consists of an input filter inductor, a three-phase full-bridge circuit, and a full-bridge DAB circuit. Only a thin-film capacitor connects the three-phase full-bridge circuit and the full-bridge DAB circuit. In three-phase input mode, the grid voltage is connected to the three-phase full-bridge circuit, rectified, and output as a pulsating DC voltage six times the power frequency (i.e., the envelope of the line voltage), which serves as the input to the DAB converter. In single-phase input mode, the first two phase arms of the three-phase full-bridge circuit are connected to the live wire of the single-phase grid voltage, and the midpoint of the third phase arm is connected to the neutral wire and operates at a low frequency. After rectification, it outputs a pulsating DC voltage twice the power frequency (i.e., a two-times-power-frequency ripple voltage), which serves as the input to the DAB converter. The primary and secondary sides of the transformer in the DAB converter are two full-bridge circuits, electrically isolated by a high-frequency transformer, with DC output control. The DAB converter directly controls the input current of the DAB circuit through a three-phase-shift modulation strategy, thereby controlling the AC input current. Power factor correction is achieved through coordinated control of the preceding and following stages, ensuring good AC power quality and a stable output voltage.

2. The bidirectional AC-DC converter with single-phase and three-phase input compatibility without DC-link electrolytic capacitors as described in claim 1, characterized in that: The operating method in three-phase input mode includes the following steps. Step 1: Connect relays R1 and R2, and disconnect R3 and R4; the converter operates in three-phase input mode. Step 2: Divide the circuit into sectors based on the grid input voltage. For a three-phase full-bridge circuit, the upper switch of the corresponding bridge arm with the maximum instantaneous phase voltage is turned on, and the lower switch of the corresponding bridge arm with the minimum instantaneous phase voltage is turned on. The switch of the corresponding bridge arm with the intermediate instantaneous phase voltage is controlled by high-frequency switching for third harmonic injection current. Therefore, the output voltage of the front-end pn node is a pulsating DC voltage six times the power frequency, i.e., the envelope of the line voltage. After adjustment by the full-bridge DAB converter, the output load-side DC voltage v o ; Step 3: Set the output voltage reference v oref The output voltage v at the sampled load terminal o The difference is calculated, and the voltage controller outputs the control quantity y. v The per-unit value representing the input current amplitude ranges from [-1, 1]. The sampled three-phase current undergoes an abc-dq transformation, and the per-unit value I of the input current amplitude is obtained after normalization calculation. m * The range is [-1, 1]; then y v with I m * The difference is calculated, and the difference value passes through the current controller. The output is the input current amplitude control quantity y of the converter, which has a range of [-1, 1]. Step 4: Sample the grid input voltage v a After passing through a phase-locked loop, the phase angle θ of phase a voltage is obtained, and the magnitude of θ is limited to 0 to 2π. Adding and subtracting 2π / 3 to the phase angle θ yields the phase angles of phases c and b, thus obtaining the cosine values ​​of the three-phase phase angles. Multiplying the input current amplitude control quantity y by the median value of the cosine values ​​gives yc. 3rd * As a reference for the per-unit value of the third harmonic injection current; its expression is: The per-unit value y of the injected current is obtained by selecting the median value of the three-phase current in each sector and performing per-unit calculation. 3rd ; will y 3rd * With y 3rd The difference is calculated, and the difference value is passed through the injection current controller to output the actual duty cycle of the injection current bridge arm switch, which is used to control the third harmonic injection current. The high-frequency drive signals HDS1 and HDS2, or HDS3 and HDS4, or HDS5 and HDS6 output by the injection current controller are distributed to each phase bridge arm according to the sector. The input current amplitude control value y is obtained by multiplying the maximum value of the cosine value. p Its expression is Based on the volt-second balance of the input filter inductor of each phase during each switching cycle, the theoretical steady-state duty cycle of the high-frequency switching transistor in the injected current bridge arm is obtained as follows: Among them, D h To control the theoretical duty cycle of the switch on the injected current bridge arm, D l The duty cycle for the next switch; v max , v mid and v min These represent the maximum phase voltage value, the intermediate phase voltage value, and the minimum phase voltage value within each sector; y 3rd * With D h Multiplying them together gives y h , then y p With y h Adding them together gives y pp = y p +y h As the given value for the per-unit value of the DAB input current; Step 5: Directly control the input current y using three-phase shift modulation of the DAB converter. pp This enables control of the AC input current, achieving power factor correction; power factor correction is achieved through coordinated control of the pre- and post-stages, eliminating the need for DC-link electrolytic capacitors; when the power is positive, y pp >0, phase shift angle φ s Take positive; when the power is reversed, y pp <0, phase shift angle φ s Inverting the value results in a negative value; this enables bidirectional power control.

3. The bidirectional AC-DC converter with single-phase and three-phase input compatibility without DC-link electrolytic capacitors as described in claim 1, characterized in that: The operating method of the power frequency rectification mode in single-phase input mode includes the following steps: Step 1: Disconnect relays R1 and R2, and connect R3 and R4; the converter operates in single-phase input mode. Step 2: Sample the grid input voltage v a When the voltage is positive, i.e., v a When the voltage is greater than 0, switches S1, S3, and S6 are turned on, while switches S2, S4, and S5 are turned off; when the voltage is negative, i.e., v... a When the voltage is less than 0, switches S2, S4, and S5 are turned on, while switches S1, S3, and S6 are turned off; S1-S6 operate at low frequency; therefore, the output voltage of the front-end pn node is a pulsating DC voltage at twice the power frequency, i.e., a wave voltage at twice the power frequency, with an amplitude equal to the phase voltage amplitude; after regulation by the full-bridge DAB converter, the output load terminal DC voltage v o ; Step 3: Set the output voltage reference v oref The output voltage v at the sampled load terminal o The difference is calculated, and the voltage controller outputs the control quantity y. v It represents the per-unit value of the input current amplitude, ranging from [-1, 1]; the per-unit value I of the input current amplitude is obtained by normalizing the sampled input phase current. m * The range is [-1, 1]; then y v with I m * The difference is calculated, and the difference value passes through the current controller. The output is the input current amplitude control quantity y of the converter, which has a range of [-1, 1]. Step 4: Sample the grid input voltage v a After passing through a phase-locked loop, the phase angle θ of phase a voltage is obtained, and the magnitude of θ is limited to 0 to 2π. The input current amplitude control quantity y is multiplied by the absolute value of the sine of the phase angle θ to obtain y. pp =y |sin(θ)|, which serves as the given value for the per-unit value of the DAB input current; Step 5: Directly control the input current y using three-phase shift modulation of the DAB converter. pp This enables control of the AC input current, achieving power factor correction; power factor correction is achieved through coordinated control of the pre- and post-stages, eliminating the need for DC-link electrolytic capacitors; when the power is positive, y pp >0, phase shift angle φ s Take positive; when the power is reversed, y pp <0, phase shift angle φ s Inverting the value results in a negative value; this enables bidirectional power control.

4. The bidirectional AC-DC converter with single-phase and three-phase input compatibility without DC-link electrolytic capacitor as described in claim 1, characterized in that: The operation method of the boost mode in the single-phase input mode includes the following steps: Step 1: Disconnect relays R1 and R2, and connect R3 and R4; the converter operates in single-phase input mode. Step 2: Sample the grid input voltage v a When the voltage is positive, i.e., v a When the voltage is greater than 0, switch S6 is turned on and S5 is turned off; when the voltage is negative, i.e., v a When V < 0, switch S5 is turned on and S6 is turned off; switches S1, S2, S3, and S4 operate at high frequency, with S1 and S3 having the same drive signal, and S2 and S4 having the same drive signal; S1 and S2 are complementary switches, and S3 and S4 are complementary switches; when V a When the duty cycle is greater than 0, the duty cycles of switching transistors S2 and S4 are set to D. n The duty cycles of switching transistors S1 and S3 are set to ( When v a When <0, the duty cycle of switching transistors S1 and S3 is set to D. n The duty cycles of switching transistors S2 and S4 are set to ( Then the output voltage of the preceding pn node is a pulsating DC voltage at twice the power frequency, that is, a wavy voltage at twice the power frequency, with an amplitude of 1 / ( The phase voltage amplitude is doubled; the DC voltage at the output load terminal after regulation by the full-bridge DAB converter is v. o ; Step 3: Set the output voltage reference v oref The output voltage v at the sampled load terminal o The difference is calculated, and the voltage controller outputs the control quantity y. v It represents the per-unit value of the input current amplitude, ranging from [-1, 1]; the per-unit value I of the input current amplitude is obtained by normalizing the sampled input phase current. m * The range is [-1, 1]; then y v with I m * The difference is calculated, and the difference value passes through the current controller. The output is the input current amplitude control quantity y of the converter, which has a range of [-1, 1]. Step 4: Sample the grid input voltage v a After passing through a phase-locked loop, the phase angle θ of phase a voltage is obtained, and the magnitude of θ is limited to 0 to 2π. The input current amplitude control quantity y is multiplied by the absolute value of the sine of the phase angle θ to obtain y. pp =y |sin(θ)|, which serves as the given value for the per-unit value of the DAB input current; Step 5: Directly control the input current y using three-phase shift modulation of the DAB converter. pp This enables control of the AC input current, achieving power factor correction; power factor correction is achieved through coordinated control of the pre- and post-stages, eliminating the need for DC-link electrolytic capacitors; when the power is positive, y pp >0, phase shift angle φ s Take positive; when the power is reversed, y pp <0, phase shift angle φ s Inverting the value results in a negative value; this enables bidirectional power control.

5. The bidirectional AC-DC converter with single-phase and three-phase input compatibility without DC-link electrolytic capacitor as described in claim 4, characterized in that: In step two, the duty cycle of switches S1, S2, S3 and S4 is set to 0.5; then the output voltage of the front-stage pn node is a pulsating DC voltage twice the power frequency, that is, a wavy voltage twice the power frequency, and its amplitude is twice the phase voltage amplitude.

6. The bidirectional AC-DC converter with single-phase and three-phase input compatibility without DC-link electrolytic capacitor as described in claim 4, characterized in that: In step two, when v a When v > 0, the duty cycle of switches S2 and S4 is set to 2 / 3, and the duty cycle of switches S1 and S3 is set to 1 / 3; when v a When <0, the duty cycle of switches S1 and S3 is set to 2 / 3, and the duty cycle of switches S2 and S4 is set to 1 / 3; then the output voltage of the front-stage pn node is a pulsating DC voltage twice the power frequency, that is, a wavy voltage twice the power frequency, and its amplitude is three times the phase voltage amplitude.

7. The bidirectional AC-DC converter with single-phase and three-phase input compatibility without DC-link electrolytic capacitor as described in claim 1, characterized in that: One end of relay R1 is connected to the b-phase mains voltage, and the other end is connected to the b-phase input filter inductor L. b One end of relay R2 is connected to the c-phase mains voltage, and the other end is connected to the c-phase input filter inductor L. c Relay R3 is connected at one end to the neutral line of the mains voltage and at the other end to the midpoint of phase c bridge arm; relay R4 is connected at one end to phase b mains voltage and at the other end to phase a mains voltage; when operating in three-phase input mode, R1 and R2 are connected, and R3 and R4 are disconnected; when operating in single-phase input mode, R1 and R2 are connected... Disconnect R3 and R4; the structure of the three-phase full-bridge circuit is as follows: the grid-side three-phase voltages a, b, and c correspond to three bridge arms respectively; S1, S3, and S5 form the upper bridge arm, and S2, S4, and S6 form the lower bridge arm; the a-phase bridge arm consists of switching transistors S1 and S2, with the source of S1 connected to the drain of S2; the b-phase bridge arm consists of switching transistors S3 and S4, with the source of S3 connected to the drain of S4; the c-phase bridge arm consists of switching transistors S5 and S6, with the source of S5 connected to the drain of S6; the drains of switching transistors S1, S3, and S5 are connected together as the positive terminal of the DAB converter input connected to the pn node, defined as node p; the sources of switching transistors S2, S4, and S6 are connected together as the negative terminal of the DAB converter input connected to the pn node, defined as node n; a small thin-film capacitor C is connected between the pn nodes. in Finally, the DAB converter is connected at the pn node.

8. The bidirectional AC-DC converter with single-phase and three-phase input compatibility without DC-link electrolytic capacitor as described in claim 1, characterized in that: The capacitance of the thin-film capacitor between the three-phase full-bridge circuit and the full-bridge DAB circuit does not exceed 20 microfarads.

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