Global soft switching buck-boost rectifier based on device multiplexing and modulation method

Abstract

The invention discloses a global soft switching buck-boost rectifier based on device multiplexing and a modulation method. The rectifier comprises a DC-AC servo driver, a low-voltage DC energy storage module, an auxiliary unit, a bus capacitor and a DC load. The DC-AC servo driver comprises three bridge arms; the low-voltage direct-current energy storage module comprises an energy storage battery, an energy storage switch tube S7 and a direct-current energy storage inductor; the auxiliary unit comprises an auxiliary switch tube Sa, an auxiliary resonant inductor Lr and an auxiliary capacitor Cr; the DC load is connected in parallel with the bus capacitor. According to the invention, a single-stage resonant soft switching architecture is innovatively adopted, zero-voltage switching operation of all power devices in a rectification link is realized, the loss problem of a traditional hard switch is thoroughly eliminated, and the power density and the conversion efficiency of the system are remarkably improved; in addition, switching tube multiplexing and dual-port output are achieved, and through the switching tube multiplexing technology, the topological structure is simplified, and meanwhile high-voltage and low-voltage dual-direct-current-port output is achieved.

Description

Technical Field

[0001] This invention relates to the field of rectifiers, and more particularly to a global soft-switching buck-boost rectifier and modulation method based on device multiplexing. Background Technology

[0002] Currently, step-up and step-down rectifiers based on traditional hard-switching technology are widely used in new energy power generation, smart microgrids, and industrial frequency conversion. Due to the large range of grid voltage fluctuations and frequent load changes, traditional solutions often adopt multi-stage conversion structures, resulting in large system size, limited power density, and the complex topology significantly increases manufacturing costs and maintenance difficulty.

[0003] The current main solution in the industry to address this problem is: traditional buck-boost rectifier topologies such as... Figure 1 As shown, this system can achieve rectification and voltage reduction. The front stage obtains high-voltage DC through a three-phase rectifier AC-DC converter, while the rear stage uses a Buck DC-DC converter. The output voltage is precisely controlled by adjusting the PWM duty cycle in real time to meet the DC bus voltage requirements under different operating conditions. However, traditional hard-switching rectifiers face severe challenges under high-frequency conditions. The switching transistors experience significant voltage and current overlap losses during switching, leading to a decrease in overall efficiency. The voltage spikes generated during turn-off and the current surges during turn-on not only threaten device reliability but also cause serious electromagnetic interference problems. These inherent defects severely restrict the improvement of system power density and significantly increase the difficulty of heat dissipation design and electromagnetic compatibility design. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a global soft-switching buck-boost rectifier and modulation method based on device reuse.

[0005] The objective of this invention is achieved through the following technical solution: A first aspect of the present invention provides a global soft-switching buck-boost rectifier based on device reuse, comprising a DC-AC servo driver, a low-voltage DC energy storage module, an auxiliary unit, a bus capacitor, and a DC load; The DC-AC servo driver includes three bridge arms, each of which has an upper bridge arm tube and a lower bridge arm tube. The midpoints of the three bridge arms are respectively connected to the three-phase input of the three-phase power grid. The upper bridge arm tubes of the three bridge arms are connected to one end of the bus capacitor through an auxiliary unit, and the lower bridge arm tubes of the three bridge arms are connected to the other end of the bus capacitor. The low-voltage DC energy storage module includes an energy storage battery, an energy storage switch S7, and a DC energy storage inductor. The energy storage switch S7 is located on the lower arm of the third bridge arm and between the lower arm tube of the third bridge arm and the other end of the bus capacitor. The positive terminal of the energy storage battery is connected to the common connection point between the energy storage switch S7 and the lower arm tube of the third bridge arm through the DC energy storage inductor. The negative terminal of the energy storage battery is connected to the other end of the bus capacitor. The auxiliary unit includes an auxiliary switch Sa, an auxiliary resonant inductor Lr, and an auxiliary capacitor Cr. The auxiliary switch Sa and the auxiliary capacitor Cr are connected in series and then connected in parallel with the auxiliary resonant inductor Lr. One end of the auxiliary resonant inductor Lr is connected to the upper transistor of the three bridge arms, and the other end of the auxiliary resonant inductor Lr is connected to one end of the bus capacitor. The DC load is connected in parallel to the bus capacitor.

[0006] A second aspect of the present invention provides a modulation method for a global soft-switching buck-boost rectifier based on device multiplexing as described in the first aspect. The method includes a single-port output three-phase switching step, wherein the three phases respectively include a sustaining phase, a phase to be turned on, and a phase to be turned off, and an energy storage switch S7 serves as the lower arm switch of the corresponding phase arm. The single-port output three-phase switching step includes the following sub-steps: In the first mode, the upper tube of the bridge arm of the two phase arms of the maintaining phase and the phase to be turned on is closed and the lower tube of the bridge arm is turned on; the upper tube of the bridge arm of the phase to be turned off is turned on and the lower tube of the bridge arm is closed; the auxiliary switch tube Sa is turned on. In the second mode, the auxiliary switch Sa is turned off; In the third mode, the upper transistor of the bridge arm of the phase to be turned on with zero voltage, the lower transistor of the bridge arm of the phase to be turned on, the upper transistor of the bridge arm of the phase to be turned off, and the lower transistor of the bridge arm of the phase to be turned off are turned on. In the fourth mode, the auxiliary switch Sa is turned on.

[0007] A third aspect of the present invention provides a modulation method for a global soft-switching buck-boost rectifier based on device multiplexing as described in the first aspect. The method includes a two-port output three-phase switching step, wherein the three phases respectively include a sustaining phase, a phase to be turned on, and a phase to be turned off, and the lower transistor of the third bridge arm is always on and never turned off. The two-port output three-phase switching step includes the following sub-steps: In the first mode, the upper tube of the bridge arm of the two phase arms of the maintenance phase and the phase to be turned on is closed and the lower tube of the bridge arm is turned on; the upper tube of the bridge arm of the phase to be turned off is turned on and the lower tube of the bridge arm is closed; the auxiliary switch tube Sa is turned on and the energy storage switch tube S7 is turned off. In the second mode, the auxiliary switch Sa is turned off; In the third mode, the upper tube of the bridge arm of the phase to be turned on and the energy storage switch tube S7 are turned on at zero voltage, the lower tube of the bridge arm of the phase to be turned on and the upper tube of the bridge arm of the phase to be turned off are turned off, and the lower tube of the bridge arm of the phase to be turned off is turned on. In the fourth mode, the auxiliary switch Sa is turned on.

[0008] The beneficial effects of this invention are: (1) High-efficiency operation of full soft switching: This exemplary embodiment innovatively adopts a single-stage resonant soft-switching architecture to achieve zero-voltage switching operation of all power devices in the rectification stage, completely eliminating the loss problem of traditional hard switching and significantly improving the system power density and conversion efficiency.

[0009] (2) Dual DC Port Output: Switching transistor multiplexing and dual-port output. By using switching transistor multiplexing technology, high and low voltage dual DC port output is achieved while simplifying the topology. This design abandons the traditional multi-stage conversion architecture and completes high and low voltage output with only a single-stage conversion, which greatly reduces system complexity and cost. Attached Figure Description

[0010] Figure 1 A schematic diagram of a conventional hard-switching buck-boost rectifier; Figure 2 This is a schematic diagram of a global soft-switching buck-boost rectifier based on device multiplexing provided in an exemplary embodiment of the present invention; Figure 3 This is a schematic diagram of a modulation method for a global soft-switching buck-boost rectifier based on device multiplexing provided in an exemplary embodiment of the present invention; Figure 4 This is a schematic diagram of a modulation method for a global soft-switching buck-boost rectifier based on device multiplexing, provided in another exemplary embodiment of the present invention. Detailed Implementation

[0011] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0012] In the description of this invention, it should be noted that the directions or positional relationships indicated by terms such as "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer" are based on the directions or positional relationships shown in the accompanying drawings and are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0013] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0014] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0015] See Figure 2 , Figure 2 A schematic diagram of a global soft-switching buck-boost rectifier based on device multiplexing provided in an exemplary embodiment of the present invention is shown, including a DC-AC servo driver, a low-voltage DC energy storage module, an auxiliary unit, a bus capacitor, and a DC load; The DC-AC servo driver includes three bridge arms, each with an upper bridge arm tube (S1, S2, S3) and a lower bridge arm tube (S4, S5, S6). The midpoints of the three bridge arms are respectively connected to the three-phase input of the three-phase power grid. The upper bridge arm tubes (S1, S2, S3) of the three bridge arms are connected to one end of the bus capacitor Cbus through an auxiliary unit, and the lower bridge arm tubes (S4, S5, S6) of the three bridge arms are connected to the other end of the bus capacitor Cbus. The low-voltage DC energy storage module includes an energy storage battery Vdc, an energy storage switch S7, and a DC energy storage inductor Ldc. The energy storage switch S7 is located on the lower arm of the third bridge arm and between the lower arm tube S6 of the third bridge arm and the other end of the bus capacitor Cbus. The positive terminal of the energy storage battery Vdc is connected to the common connection point between the energy storage switch S7 and the lower arm tube S6 of the third bridge arm through the DC energy storage inductor Ldc. The negative terminal of the energy storage battery Vdc is connected to the other end of the bus capacitor Cbus. The auxiliary unit includes an auxiliary switch Sa, an auxiliary resonant inductor Lr, and an auxiliary capacitor Cr. The auxiliary switch Sa and the auxiliary capacitor Cr are connected in series and then connected in parallel with the auxiliary resonant inductor Lr. One end of the auxiliary resonant inductor Lr is connected to the upper transistors (S1, S2, S3) of the three bridge arms, and the other end of the auxiliary resonant inductor Lr is connected to one end of the bus capacitor Cbus. The DC load Load is connected in parallel to the bus capacitor Cbus.

[0016] Specifically, in an exemplary embodiment of the present invention, Figure 2 The improved step-down soft-switching rectifier has seven switching transistors (S1, S2, S3, S4, S5, S6, S7) in total across the three bridge arms. An auxiliary unit consisting of an auxiliary switching transistor Sa, an auxiliary capacitor Cr, and an auxiliary resonant inductor Lr is connected in series on the DC bus. It also includes two DC ports: a DC energy storage module and a DC load Load.

[0017] This converter features a compact structure and a small number of components. It employs soft-switching control technology combined with variable duty cycle adjustment for stable bus voltage. Under various conditions, it can achieve efficient input voltage to bus conversion, bus voltage regulation and grid-side current optimization, flexible energy interaction between the energy storage unit and the grid, as well as bus voltage balancing and characteristic harmonic compensation. This ensures efficient and stable system operation over a wide input voltage range and under dynamic load conditions. This invention reduces switching losses and improves power device reuse through soft-switching technology, while simultaneously optimizing grid-side power quality through adaptive compensation control strategies, ensuring that current harmonic distortion is controlled within a reasonable range.

[0018] Specifically, regarding soft-switching control technology: when the upper transistors (S1, S2, S3) of the bridge arm are turned off and switched to the lower transistors (S4, S5, S6) of the same bridge arm, the freewheeling current of the auxiliary resonant inductor Lr forms a loop through the other bridge arm (the bridge arm to be turned on) within the dead time, automatically discharging the junction capacitance of the auxiliary switch transistor Sa. This significantly reduces the voltage stress on the lower transistor and even achieves zero-voltage turn-off. During this process, the voltage of the upper transistor smoothly rises from zero to the DC bus voltage. The entire process achieves spontaneous soft switching without additional control. For cases where spontaneous soft switching cannot be achieved when switching from the lower transistor to the upper transistor, this exemplary embodiment innovatively introduces an auxiliary resonant unit, i.e., the auxiliary unit. By precisely controlling the timing of the operation of the auxiliary switch transistor Sa, the oscillating current generated by the resonant circuit charges and discharges the junction capacitance of the auxiliary switch transistor Sa, thereby ensuring zero-voltage conduction when the upper transistor is turned on. This hybrid soft-switching solution retains the simplicity of traditional spontaneous soft-switching while expanding the applicability of soft-switching through auxiliary units, greatly simplifying control complexity while ensuring high efficiency.

[0019] Meanwhile, this exemplary embodiment selects different control strategies based on usage conditions, and achieves multi-mode buck-boost rectifier operation by optimizing switch combinations. Combined with characteristic subharmonic compensation control, harmonic components of the grid-side current are extracted in real time and injected into the modulation wave, effectively reducing total harmonic distortion and ensuring stable grid-connected current quality. For example: Figure 3 The diagram shown is the modal diagram of a buck-boost soft-switching rectifier with only a DC load; while Figure 4 The figure shows the modal diagram of the DC two ports of the buck-boost soft-switching rectifier operating simultaneously.

[0020] In conclusion: (1) High-efficiency operation of full soft switching: This exemplary embodiment innovatively adopts a single-stage resonant soft-switching architecture to achieve zero-voltage switching operation of all power devices in the rectification stage, completely eliminating the loss problem of traditional hard switching and significantly improving the system power density and conversion efficiency.

[0021] (2) Dual DC Port Output: Switching transistor multiplexing and dual-port output. By using switching transistor multiplexing technology, high and low voltage dual DC port output is achieved while simplifying the topology. This design abandons the traditional multi-stage conversion architecture and completes high and low voltage output with only a single-stage conversion, which greatly reduces system complexity and cost.

[0022] See Figure 3 , Figure 3 This illustrates another exemplary embodiment of the invention, as shown below. Figure 2 The modulation method for a global soft-switching buck-boost rectifier based on device multiplexing includes a single-port output three-phase switching step. The three phases include a sustaining phase, a phase to be turned on, and a phase to be turned off. The energy storage switch S7 serves as the lower arm transistor of the corresponding phase arm. The single-port output three-phase switching step includes the following sub-steps: In the first mode, the upper tube of the bridge arm of the two phase arms of the maintaining phase and the phase to be turned on is closed and the lower tube of the bridge arm is turned on; the upper tube of the bridge arm of the phase to be turned off is turned on and the lower tube of the bridge arm is closed; the auxiliary switch tube Sa is turned on. In the second mode, the auxiliary switch Sa is turned off; In the third mode, the upper transistor of the bridge arm of the phase to be turned on with zero voltage, the lower transistor of the bridge arm of the phase to be turned on, the upper transistor of the bridge arm of the phase to be turned off, and the lower transistor of the bridge arm of the phase to be turned off are turned on. In the fourth mode, the auxiliary switch Sa is turned on.

[0023] Specifically, in this exemplary embodiment, as Figure 3As shown, taking phase B as the maintenance phase, phase A as the phase to be turned on, and phase C as the phase to be turned off as an example, it is necessary to turn off the upper tube S3 of the C phase bridge arm, turn on the lower tube S6 of the C phase bridge arm, and turn on the upper tube S1 of the A phase bridge arm and turn off the upper tube S4 of the A phase bridge arm. At this time, the buck-boost soft-switching rectifier only carries a DC load. The specific process is as follows: In the first mode, the energy storage switch S7 acts as the lower switch of the corresponding phase bridge arm. At this time, the switching pulses of the lower switches S6 and S7 of the C phase bridge arm are the same, and the low-voltage DC energy storage module does not work. The upper switch S3 of the C phase bridge arm is turned on, the lower switch S4 of the A phase bridge arm and the lower switch S5 of the B phase bridge arm are turned on, and the C phase power grid releases energy to the bus capacitor Cbus through the upper switch S3 of the C phase bridge arm. The bus capacitor Cbus then provides energy to the DC load Load. At the next moment, the A phase is turned on. However, the switching process from the lower switch S4 of the A phase bridge arm to the upper switch S1 of the A phase bridge arm is a hard switch, which causes switching losses and brings voltage spikes that endanger the safe operation of the system. In the second mode, commutation does not occur initially. At this time, the auxiliary switch Sa is turned off, and the auxiliary resonant inductor Lr resonates with the resonant capacitance of the upper transistor S1 of the A-phase bridge arm. The direction of the resonant inductor current is as follows: Figure 3 As shown in mode 2, the soft switching of the upper tube S1 of the bridge arm of phase A is finally realized. In the third mode, after the freewheeling of the auxiliary resonant inductor Lr ends, the direction of the resonant inductor current changes due to the effect of the bus capacitor Cbus. The bus capacitor Cbus charges the auxiliary resonant inductor Lr, which discharges the parallel equivalent capacitor of the auxiliary switch Sa, realizing the zero-voltage turn-on of the auxiliary switch Sa, and finally transitioning to the fourth mode, where the auxiliary switch Sa turns on.

[0024] See Figure 4 , Figure 4 This illustrates another exemplary embodiment of the invention, as shown below. Figure 2 The modulation method for a global soft-switching buck-boost rectifier based on device multiplexing includes a dual-port output three-phase switching step. The three phases include a sustain phase, a phase to be turned on, and a phase to be turned off. The lower transistor of the third bridge arm is always on and never off. The dual-port output three-phase switching step includes the following sub-steps: In the first mode, the upper tube of the bridge arm of the two phase arms of the maintenance phase and the phase to be turned on is closed and the lower tube of the bridge arm is turned on; the upper tube of the bridge arm of the phase to be turned off is turned on and the lower tube of the bridge arm is closed; the auxiliary switch tube Sa is turned on and the energy storage switch tube S7 is turned off. In the second mode, the auxiliary switch Sa is turned off; In the third mode, the upper tube of the bridge arm of the phase to be turned on and the energy storage switch tube S7 are turned on at zero voltage, the lower tube of the bridge arm of the phase to be turned on and the upper tube of the bridge arm of the phase to be turned off are turned off, and the lower tube of the bridge arm of the phase to be turned off is turned on. In the fourth mode, the auxiliary switch Sa is turned on.

[0025] Specifically, in this exemplary embodiment, as Figure 4 As shown, taking phase B as the maintenance phase, phase A as the phase to be turned on, and phase C as the phase to be turned off as an example, it is necessary to turn off the upper tube S3 of the C phase bridge arm and turn on the lower tube S6 of the C phase bridge arm, and turn on the upper tube S1 of the A phase bridge arm and turn off the upper tube S4 of the A phase bridge arm. At this time, the buck-boost soft-switching rectifier operates simultaneously on both DC ports (DC load and low-voltage DC energy storage module). The specific process is as follows: In the first mode, the upper transistor S3 of the C-phase bridge arm is turned on, and the lower transistors S4 and S5 of the A-phase bridge arm are turned on. At this time, the C-phase power grid releases energy to the bus capacitor Cbus through the upper transistor S3 of the C-phase bridge arm. At the same time, the switching pulses of the lower transistors S6 and S7 of the C-phase bridge arm are different, and the low-voltage DC energy storage module works. The energy storage battery Vdc releases energy to the bus capacitor Cbus through the upper transistors S3 and S6 of the C-phase bridge arm, and the bus capacitor Cbus then provides energy to the DC load Load. In the next moment, the A-phase is turned on. However, the switching process from the lower transistor S4 to the upper transistor S1 of the A-phase bridge arm is a hard switch, which causes switching losses and brings voltage spikes that endanger the safe operation of the system. The next state switches from phase C to phase A. In the second mode, the auxiliary switch Sa is turned off. At this time, the direction of the resonant current on the auxiliary resonant inductor Lr is opposite to the direction of the current on the bus capacitor Cbus. The resonant current discharges the equivalent parallel capacitor of the upper arm transistor S1 of the A phase bridge arm, realizing the zero-voltage turn-on of the upper arm transistor S1 and the energy storage switch S7 of the A phase bridge arm, and the main circuit mode switching.

[0026] In the third mode, after the freewheeling of the auxiliary resonant inductor Lr ends, the bus capacitor Cbus charges the auxiliary resonant inductor Lr, providing conditions for the soft switching of the auxiliary switch Sa. The resonant current discharges the equivalent parallel capacitor of the auxiliary switch Sa, ultimately realizing the soft switching of the auxiliary switch Sa, and finally transitioning to the fourth mode, where the auxiliary switch Sa is turned on.

[0027] It should be noted that at this time, only the lower tube S6 of the C-phase bridge arm and the energy storage switch tube S7 need to be controlled separately. The analysis process is similar to that of only having a DC load.

[0028] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A global soft-switching buck-boost rectifier based on device multiplexing, characterized in that: Includes DC-AC servo drive, low-voltage DC energy storage module, auxiliary unit, bus capacitor and DC load; The DC-AC servo driver includes three bridge arms, each of which has an upper bridge arm tube and a lower bridge arm tube. The midpoints of the three bridge arms are respectively connected to the three-phase input of the three-phase power grid. The upper bridge arm tubes of the three bridge arms are connected to one end of the bus capacitor through an auxiliary unit, and the lower bridge arm tubes of the three bridge arms are connected to the other end of the bus capacitor. The low-voltage DC energy storage module includes an energy storage battery, an energy storage switch S7, and a DC energy storage inductor. The energy storage switch S7 is located on the lower arm of the third bridge arm and between the lower arm tube of the third bridge arm and the other end of the bus capacitor. The positive terminal of the energy storage battery is connected to the common connection point between the energy storage switch S7 and the lower arm tube of the third bridge arm through the DC energy storage inductor. The negative terminal of the energy storage battery is connected to the other end of the bus capacitor. The auxiliary unit includes an auxiliary switch Sa, an auxiliary resonant inductor Lr, and an auxiliary capacitor Cr. The auxiliary switch Sa and the auxiliary capacitor Cr are connected in series and then connected in parallel with the auxiliary resonant inductor Lr. One end of the auxiliary resonant inductor Lr is connected to the upper transistor of the three bridge arms, and the other end of the auxiliary resonant inductor Lr is connected to one end of the bus capacitor. The DC load is connected in parallel to the bus capacitor.

2. The modulation method for a global soft-switching buck-boost rectifier based on device multiplexing as described in claim 1, characterized in that: The method includes a single-port output three-phase switching step, wherein the three phases include a sustaining phase, a phase to be turned on, and a phase to be turned off, and the energy storage switch tube S7 serves as the lower tube of the corresponding phase bridge arm. The single-port output three-phase switching step includes the following sub-steps: In the first mode, the upper tube of the bridge arm of the two phase arms of the maintaining phase and the phase to be turned on is closed and the lower tube of the bridge arm is turned on; the upper tube of the bridge arm of the phase to be turned off is turned on and the lower tube of the bridge arm is closed; the auxiliary switch tube Sa is turned on. In the second mode, the auxiliary switch Sa is turned off; In the third mode, the upper transistor of the bridge arm of the phase to be turned on with zero voltage, the lower transistor of the bridge arm of the phase to be turned on, the upper transistor of the bridge arm of the phase to be turned off, and the lower transistor of the bridge arm of the phase to be turned off are turned on. In the fourth mode, the auxiliary switch Sa is turned on.

3. The modulation method for a global soft-switching buck-boost rectifier based on device multiplexing as described in claim 1, characterized in that: The method includes a dual-port output three-phase switching step, wherein the three phases include a sustaining phase, a phase to be turned on, and a phase to be turned off, and the lower tube of the third bridge arm is always on and never turned off. The dual-port output three-phase switching step includes the following sub-steps: In the first mode, the upper tube of the bridge arm of the two phase arms of the maintenance phase and the phase to be turned on is closed and the lower tube of the bridge arm is turned on; the upper tube of the bridge arm of the phase to be turned off is turned on and the lower tube of the bridge arm is closed; the auxiliary switch tube Sa is turned on and the energy storage switch tube S7 is turned off. In the second mode, the auxiliary switch Sa is turned off; In the third mode, the upper tube of the bridge arm of the phase to be turned on and the energy storage switch tube S7 are turned on at zero voltage, the lower tube of the bridge arm of the phase to be turned on and the upper tube of the bridge arm of the phase to be turned off are turned off, and the lower tube of the bridge arm of the phase to be turned off is turned on. In the fourth mode, the auxiliary switch Sa is turned on.

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