Control method of power conversion circuit, power conversion equipment and power supply system

Abstract

The invention provides a control method of a power conversion circuit, power conversion equipment and a power supply system. The control method comprises the following steps: acquiring a real-time alternating-current voltage on a second end of the secondary bridge circuit, and when the real-time alternating-current voltage is in a preset transition region of a voltage zero crossing point, controlling each switching tube in the secondary bridge circuit to perform on-off action within secondary switching time so as to enable the secondary bridge circuit to perform power conversion; and when the real-time AC voltage is in the preset transition region of the voltage zero crossing point, controlling a part of switch tubes in the secondary side bridge circuit to be switched on within the secondary side dead zone time, so that the secondary side bridge circuit and the secondary side of the transformer form a follow current loop. By means of the method, the power conversion circuit is not prone to generating current oscillation near the voltage zero crossing point, and therefore overstress damage of a secondary side switch tube can be avoided, and the current waveform can be optimized.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to a control method for a power conversion circuit, a power conversion device, and a power supply system. Background Art

[0002] In the field of new energy, dual active bridge micro inverters are increasingly being used due to their small size, low cost, high efficiency, etc. However, when the dual active bridge micro inverter uses a conventional drive scheme, the error in voltage sampling may cause inaccurate switching of the drive signal of the dual active bridge micro inverter, or the presence of the filter may cause a delay in voltage polarity switching, causing the secondary switch tube to not switch at the voltage zero point, and the current then oscillates near the voltage zero point, which is not conducive to device safety and normal operation of the circuit. Summary of the invention

[0003] In view of the shortcomings of the prior art, the present application provides a control method, a power conversion device and a power supply system for a power conversion circuit, which can solve the problem of current oscillation at the voltage zero point caused by the secondary side switch tube not switching at the voltage zero point.

[0004] The first aspect of the present application provides a control method for a power conversion circuit. The power conversion circuit includes a primary bridge circuit, a transformer and a secondary bridge circuit, wherein the first end of the primary bridge circuit is connected to the primary side of the transformer, the second end of the primary bridge circuit is used to connect a DC power supply or a DC load, the first end of the secondary bridge circuit is connected to the secondary side of the transformer, and the second end of the secondary bridge circuit is used to connect an AC load or an AC power supply. Correspondingly, the control method includes: obtaining a real-time AC voltage on the second end of the secondary bridge circuit; when the real-time AC voltage is in a preset transition region of a voltage zero crossing, controlling each switch tube in the secondary bridge circuit to perform an on-off action within the secondary switching time, so that the secondary bridge circuit performs power conversion; wherein there is a secondary dead time between two adjacent secondary switching times; when the real-time AC voltage is in a preset transition region of a voltage zero crossing, controlling part of the switch tubes in the secondary bridge circuit to conduct within the secondary dead time, so that the secondary bridge circuit and the secondary side of the transformer form a freewheeling loop.

[0005] In one embodiment, the secondary bridge circuit includes a first secondary bridge arm and a second secondary bridge arm connected in series, the first secondary bridge arm includes an upper half bridge arm and a lower half bridge arm connected in series, the upper half bridge arm includes a first secondary switch tube and a second secondary switch tube connected in reverse series, the lower half bridge arm includes a third secondary switch tube and a fourth secondary switch tube connected in reverse series, and the second secondary bridge arm includes a first capacitor and a second capacitor connected in series. Accordingly, when the real-time AC voltage is in a preset transition region of a voltage zero crossing, the process of controlling some of the switches in the secondary bridge circuit to be turned on within the secondary dead time includes: when the real-time AC voltage is in a preset transition region of a voltage zero crossing, within the secondary dead time, controlling both the first secondary switch tube and the third secondary switch tube to be turned on, or controlling both the second secondary switch tube and the fourth secondary switch tube to be turned on.

[0006] In one embodiment, when the real-time AC voltage is in a preset transition region of the voltage zero crossing point, within the secondary side dead time, the process of controlling the first secondary side switch tube and the third secondary side switch tube to be turned on, or controlling the second secondary side switch tube and the fourth secondary side switch tube to be turned on includes: if the real-time AC voltage is in the preset transition region and is positive polarity, then within the secondary side dead time, the second secondary side switch tube and the fourth secondary side switch tube are controlled to be turned on, and the first secondary side switch tube and the third secondary side switch tube are controlled to be turned off; if the real-time AC voltage is in the preset transition region and is negative polarity, then within the secondary side dead time, the first secondary side switch tube and the third secondary side switch tube are controlled to be turned on, and the second secondary side switch tube and the fourth secondary side switch tube are controlled to be turned off.

[0007] In one embodiment, the secondary bridge circuit includes a first secondary bridge arm and a second secondary bridge arm connected in series, the first secondary bridge arm includes an upper half bridge arm and a lower half bridge arm connected in series, the upper half bridge arm includes a first secondary switch tube and a second secondary switch tube connected in series in reverse order, the lower half bridge arm includes a third secondary switch tube and a fourth secondary switch tube connected in series in reverse order, and the second secondary bridge arm includes a first capacitor and a second capacitor connected in series. Accordingly, when the real-time AC voltage is in a preset transition region of a voltage zero crossing, the process of controlling each switch tube in the secondary bridge circuit to perform on-off actions within the secondary switching time so that the secondary bridge circuit performs power conversion includes: when the real-time AC voltage is in a preset transition region of a voltage zero crossing, within the secondary switching time, controlling the first secondary switch tube and the third secondary switch tube to be alternately turned on, and the second secondary switch tube and the fourth secondary switch tube to be alternately turned on, wherein the first secondary switch tube is synchronously turned on with the second secondary switch tube, and the third secondary switch tube is synchronously turned on with the fourth secondary switch tube.

[0008] In one embodiment, the control method further includes: when the real-time AC voltage is not in a preset transition region of the voltage zero point, controlling each switch tube in the secondary bridge circuit to perform on-off action within the secondary switching time so that the secondary bridge circuit performs power conversion.

[0009] In one embodiment, the secondary bridge circuit includes a first secondary bridge arm and a second secondary bridge arm connected in series, the first secondary bridge arm includes an upper half bridge arm and a lower half bridge arm connected in series, the upper half bridge arm includes a first secondary switch tube and a second secondary switch tube connected in reverse series, the lower half bridge arm includes a third secondary switch tube and a fourth secondary switch tube connected in reverse series, and the second secondary bridge arm includes a first capacitor and a second capacitor connected in series. Correspondingly, when the real-time AC voltage is not in the preset transition zone of the voltage zero crossing point, each switch tube in the secondary bridge circuit is controlled to perform on-off action within the secondary switching time so that the secondary bridge circuit performs power conversion. The process includes: if the real-time AC voltage is not in the preset transition zone and is positive polarity, then within the secondary switching time, the first secondary switch tube and the third secondary switch tube are controlled to be turned on alternately, and the second secondary switch tube and the fourth secondary switch tube are both kept turned on; if the real-time AC voltage is not in the preset transition zone and is negative polarity, then within the secondary switching time, the first secondary switch tube and the third secondary switch tube are controlled to be turned on alternately, and the second secondary switch tube and the fourth secondary switch tube are turned on alternately.

[0010] In one embodiment, the control method further includes: controlling each switch tube in the primary bridge circuit to perform on-off action during the primary switching time, so that the primary bridge circuit performs power conversion.

[0011] In one embodiment, the primary bridge circuit includes a first primary bridge arm and a second primary bridge arm connected in series, the first primary bridge arm includes a first primary switch tube and a second primary switch tube connected in series, and the second primary bridge arm includes a third primary switch tube and a fourth primary switch tube connected in series. Accordingly, during the primary switching time, the process of controlling each switch tube in the primary bridge circuit to perform on-off actions so that the primary bridge circuit performs power conversion includes: during the primary switching time, controlling the first primary switch tube and the second primary switch tube to be alternately turned on, and controlling the third primary switch tube and the fourth primary switch tube to be alternately turned on.

[0012] The second aspect of the present application provides a power conversion device. The power conversion device includes a power conversion circuit and a controller, the power conversion circuit includes a primary bridge circuit, a transformer and a secondary bridge circuit, the first end of the primary bridge circuit is connected to the primary side of the transformer, the second end of the primary bridge circuit is used to connect a DC power supply or a DC load, the first end of the secondary bridge circuit is connected to the secondary side of the transformer, and the second end of the secondary bridge circuit is used to connect an AC load or an AC power supply. The controller is connected to the power conversion circuit, and the controller is used to: when the real-time AC voltage on the second end of the secondary bridge circuit is in a preset transition area of ​​the voltage zero crossing point, control each switch tube in the secondary bridge circuit to perform an on-off action within the secondary switching time, so that the secondary bridge circuit performs power conversion; wherein there is a secondary dead time between two adjacent secondary switching times; when the real-time AC voltage is in a preset transition area of ​​the voltage zero crossing point, control some of the switch tubes in the secondary bridge circuit to conduct within the secondary dead time, so that the secondary bridge circuit and the secondary side of the transformer form a freewheeling loop.

[0013] In one embodiment, the secondary bridge circuit includes a first secondary bridge arm and a second secondary bridge arm connected in series, the first secondary bridge arm includes an upper half bridge arm and a lower half bridge arm connected in series, the upper half bridge arm includes a first secondary switch tube and a second secondary switch tube connected in reverse series, the lower half bridge arm includes a third secondary switch tube and a fourth secondary switch tube connected in reverse series, and the second secondary bridge arm includes a first capacitor and a second capacitor connected in series. The controller is used to: when the real-time AC voltage is in a preset transition region of the voltage zero crossing point, within the secondary dead time, control the first secondary switch tube and the third secondary switch tube to be turned on, or control the second secondary switch tube and the fourth secondary switch tube to be turned on, so that the upper half bridge arm and the secondary side of the transformer and the first capacitor form a freewheeling loop, and the lower half bridge arm and the secondary side of the transformer and the second capacitor form another freewheeling loop.

[0014] In one embodiment, the controller is used to generate a control signal, and the control signal is used to control the on-off state of each switch tube in the primary bridge circuit and the secondary bridge circuit; wherein, when the real-time AC voltage is in a preset transition area of ​​the voltage zero point, the control signal waveform of the second secondary switch tube and the control signal waveform of the fourth secondary switch tube overlap within the secondary dead time, or the control signal waveform of the first secondary switch tube and the control signal waveform of the third secondary switch tube overlap within the secondary dead time.

[0015] In one embodiment, the waveform of the real-time AC current outputted by the second end of the secondary bridge circuit is a substantially smooth sine wave.

[0016] The third aspect of the present application provides a power supply system. The power supply system includes: a DC power supply and a power conversion device as described in the second aspect or any one of the embodiments of the second aspect, the DC power supply is connected to the second end of the primary bridge circuit, the second end of the secondary bridge circuit is connected to a common coupling point, the DC power supply includes at least one of an energy storage device and a photovoltaic power generation device, and the common coupling point is used to connect at least one of a power grid and an AC load.

[0017] Compared with the prior art, this application has at least the following advantages: 1. The control method of the power conversion circuit of the present application can control some switching tubes in the secondary bridge circuit to be turned on within the secondary dead time when the real-time AC voltage is in a preset transition area of ​​the voltage zero point, so that the secondary bridge circuit and the secondary side of the transformer form a freewheeling loop, thereby reducing the current oscillation of the power conversion circuit near the voltage zero point, avoiding overstress damage of the secondary switch tube and reducing the power loss of the device, and optimizing the sinusoidality of the AC current, so that the quality of the power output is higher, which is beneficial to the safety of the device and the normal operation of the circuit.

[0018] 2. The control method of the power conversion circuit of the present application does not require the addition of additional hardware circuit structure. The soft switching of the switch tube can be realized near the voltage zero point through the on-off logic control of the switch tube. Therefore, the method of the present application is simple, easy to implement and low in cost.

[0019] 3. The control method of the power conversion circuit of the present application performs corresponding on-off control on the secondary side switch tube during the secondary side switching time and the secondary side dead time corresponding to the preset transition area. Therefore, it can more comprehensively and effectively reduce the current oscillation of the power conversion circuit near the voltage zero point, so that the current stress risk of the secondary side switch tube can be reduced to a lower level. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a structural schematic diagram of a power supply system provided in an embodiment of the present application.

[0021] Figure 2 It is a circuit diagram of a power conversion circuit provided in an embodiment of the present application.

[0022] Figure 3 yes Figure 2 The power conversion circuit shown is a working waveform diagram when a conventional driving scheme is adopted.

[0023] Figure 4 yes Figure 3 A local enlarged view of the working waveform at a voltage zero crossing point.

[0024] Figure 5 yes Figure 2A schematic diagram of the AC voltage and AC current generated when the power conversion circuit adopts a conventional driving scheme.

[0025] Figure 6 It is a flow chart of a control method of a power conversion circuit provided in an embodiment of the present application.

[0026] Figure 7 yes Figure 2 The power conversion circuit shown is a working waveform diagram when all secondary side switch tubes are disconnected during the dead time control.

[0027] Figure 8 It is the dead time control Figure 2 A flow diagram of AC current when all secondary side switches of the power conversion circuit shown are disconnected.

[0028] Fig. 9 It is the dead time control Figure 2 Another flow diagram of AC current when all secondary side switches of the power conversion circuit shown are disconnected.

[0029] Fig.10 yes Figure 2 The power conversion circuit shown is a working waveform diagram when using the method of the embodiment of the present application.

[0030] Fig.11 yes Figure 2 The shown power conversion circuit is a flow diagram of the alternating current generated when the method of the embodiment of the present application is adopted.

[0031] Fig.12 yes Figure 2 Another flow diagram of the alternating current generated when the power conversion circuit shown adopts the method of the embodiment of the present application.

[0032] Fig.13 yes Figure 2 The power conversion circuit shown is a waveform diagram of the AC voltage and AC current generated when the method of the embodiment of the present application is adopted.

[0033] Main component symbols 100-power supply system, 10-DC power supply, 11-energy storage equipment, 12-photovoltaic power generation equipment, 20-power conversion equipment, 21-power conversion circuit, 211-primary bridge circuit, 2111-first primary bridge arm, 2112-second primary bridge arm, 212-transformer, 213-resonant inductor, 214-secondary bridge circuit, 2141-first secondary bridge arm, 2142-second secondary bridge arm, 22-controller, 30-common coupling point, 40-grid, 50-AC load. DETAILED DESCRIPTION

[0034] In order to enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only embodiments of a part of the present application, not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work should fall within the scope of protection of the present application. In the absence of conflict, the following different embodiments and features in the embodiments can be combined with each other.

[0035] See also Figure 1 , showing a topological diagram of a power supply system 100 provided in an embodiment of the present application.

[0036] like Figure 1 As shown, the power supply system 100 includes a DC power supply 10 and a power conversion device 20. For the convenience of description, the DC power supply 10 is an energy storage device 11 and a photovoltaic power generation device 12. At this time, the power supply system 100 can also be called a photovoltaic storage and charging system.

[0037] It should be understood that in other scenarios, only the photovoltaic power generation device 12 or the energy storage device 11 may be used as the DC power source 10, or other devices that can provide DC power may be used as the DC power source 10. In the scenario where only the photovoltaic power generation device 12 is used as the DC power source 10, the power supply system 100 may also be called a photovoltaic system, and in the scenario where only the energy storage device 11 is used as the DC power source 10, the power supply system 100 may also be called an energy storage system or a storage and charging system.

[0038] The power conversion device 20 is a single-stage micro inverter. The power conversion device 20 includes a power conversion circuit 21 and a controller 22. One end of the power conversion circuit 21 is connected to the energy storage device 11 and the photovoltaic power generation device 12. The energy storage device 11 includes a rechargeable energy storage battery. The other end of the power conversion circuit 21 is connected to the common coupling point 30 (abbreviated as PCC). The common coupling point 30 can be used to connect the power grid 40 and the AC load 50. The power conversion circuit 21 is also connected to the controller 22, and its working process is controlled by the controller 22.

[0039] In the embodiments of this application, please refer to Figure 2 The power conversion circuit 21 includes a primary bridge circuit 211, a transformer 212, and a secondary bridge circuit 214, wherein the primary bridge circuit 211 may adopt a full-bridge structure or a half-bridge structure, and the secondary bridge circuit 214 adopts a half-bridge structure. Therefore, the power conversion circuit 21 may also be called a resonant dual active bridge (DAB) conversion circuit.

[0040] For the convenience of description, the present application embodiment is Figure 2 The primary bridge circuit 211 shown is a full-bridge structure, and the secondary bridge circuit 214 is a half-bridge structure for exemplary description.

[0041] like Figure 2 As shown, the primary bridge circuit 211 includes a first primary bridge arm 2111 and a second primary bridge arm 2112 connected in parallel, the first primary bridge arm 2111 includes a first primary switch tube Q1 and a second primary switch tube Q2 connected in series, and the second primary bridge arm 2112 includes a third primary switch tube Q3 and a fourth primary switch tube Q4 connected in series. The midpoint of the first primary bridge arm 2111 and the midpoint of the second primary bridge arm 2112 together constitute the first end of the primary bridge circuit 211, and the first end of the primary bridge circuit 211 is connected to the primary side of the transformer 212. The two ends of the first primary bridge arm 2111 and the two ends of the second primary bridge arm 2112 constitute the second end of the primary bridge circuit 211. The second end of the primary bridge circuit 211 can be connected to the capacitor Cdc, and can be used to connect the DC power supply 10 or the DC load.

[0042] The secondary bridge circuit 214 includes a first secondary bridge arm 2141 and a second secondary bridge arm 2142 connected in parallel, wherein the first secondary bridge arm 2141 includes a first secondary switch tube Q5, a second secondary switch tube Q6, a third secondary switch tube Q7 and a fourth secondary switch tube Q8 connected in series in sequence, wherein the first secondary switch tube Q5 and the second secondary switch tube Q6 are connected back to back (i.e., connected in reverse series) to form a bidirectional switch, and the third secondary switch tube Q7 and the fourth secondary switch tube Q8 are connected back to back (i.e., connected in reverse series) to form another bidirectional switch. The second secondary bridge arm 2142 includes a first capacitor Cr1 and a second capacitor Cr2 connected in series. The midpoint of the first secondary bridge arm 2141 and the midpoint of the second secondary bridge arm 2142 together form a first end of the secondary bridge circuit 214, and the first end of the secondary bridge circuit 214 is connected to the secondary side of the transformer 212.

[0043] Among them, the switch tubes Q1~Q8 can use corresponding types of semiconductor switch devices according to actual needs. For the convenience of description, Figure 2 In the example, the switch tubes Q1 to Q8 are all MOSFET. The turns ratio between the secondary side and the primary side of the transformer 212 is n:1, n>1. The secondary side leakage inductance of the transformer 212 can be used as a resonant inductor. For ease of description, Figure 2 The resonant inductor 213 (corresponding to Figure 2The resonant inductor 213 is located between the midpoint of the first secondary bridge arm 2141 and the secondary side of the transformer 212. In other embodiments, the resonant inductor Lr may also be an independently arranged inductor element. Alternatively, the resonant inductor Lr may also be composed of an independently arranged inductor element and a secondary leakage inductor. In some embodiments, the resonant inductor Lr may constitute a resonant circuit alone. In some other embodiments, the resonant inductor Lr, the first capacitor Cr1 and the second capacitor Cr2 may constitute a resonant circuit together.

[0044] The two ends of the first secondary bridge arm 2141 and the two ends of the second secondary bridge arm 2142 constitute the second end of the secondary bridge circuit 214. The second end of the secondary bridge circuit 214 can be used to connect to the common coupling point 30, thereby connecting to the AC power supply or the AC load 50. A filter or filtering circuit can be provided between the second end of the secondary bridge circuit 214 and the common coupling point 30, for example Figure 2 The filter inductor Lf in.

[0045] Based on such a design, when the power supply system 100 is working, the energy storage device 11 can be used as a DC power source 10, and the power conversion circuit 21 can transmit the power provided by the energy storage device 11 to the AC load 50 and / or the power grid 40. When the power conversion circuit 21 feeds power to the power grid 40, the power grid 40 can be regarded as an AC load 50. Conversely, the energy storage device 11 can also be used as a DC load, and the power grid 40 can be used as an AC power source. The power conversion circuit 21 can transmit the power provided by the power grid 40 to the energy storage device 11 to charge the energy storage device 11.

[0046] In the embodiment of the present application, the process of transmitting power from the primary bridge circuit 211 to the secondary bridge circuit 214 can be referred to as the forward operation process of the power conversion circuit 21, and the process of transmitting power from the secondary bridge circuit 214 to the primary bridge circuit 211 can be referred to as the reverse operation process of the power conversion circuit 21. In other embodiments, the forward and reverse directions can also be reversed, which is not specifically limited here.

[0047] The primary bridge circuit 211 constitutes a DC / AC conversion circuit, and the secondary bridge circuit 214 constitutes an AC / AC conversion circuit. The voltage at the second end of the primary bridge circuit 211 is a DC voltage Vdc, the voltage at the second end of the secondary bridge circuit 214 is an AC voltage Vac, and the current at the second end of the secondary bridge circuit 214 is an AC current iac.

[0048] The control ends of the switch tubes Q1~Q8 are respectively connected to the controller 22. The controller 22 can be a microcontroller unit (MCU), a digital signal processor (DSP) or other control elements or circuits. The controller 22 can be used to sample the voltage of the first end of the primary bridge circuit 211 and the first end of the secondary bridge circuit 214, and generate corresponding control signals based on the sampling results and send them to the switch tubes Q1~Q8. The control signal is used to control the on-off state of the switch tubes Q1~Q8.

[0049] Furthermore, the control signal of the first primary switch tube Q1 is S1, the control signal of the second primary switch tube Q2 is S2, the control signal of the third primary switch tube Q3 is S2, and the control signal of the fourth primary switch tube Q4 is S4. The control signal of the first secondary switch tube Q5 is S5, the control signal of the second secondary switch tube Q6 is S6, the control signal of the third secondary switch tube Q7 is S7, and the control signal of the fourth secondary switch tube Q8 is S8. For the convenience of description, the embodiment of the present application is described by taking the control signal being high level (for example, 1) to control the switch tube to be turned on, and the control signal being low level (for example, 0) to control the switch tube to be turned off as an example.

[0050] The transformer 212 is used to isolate and couple the primary bridge circuit 211 and the secondary bridge circuit 214. The resonant circuit is used to store and transmit energy. The voltage on the primary side of the transformer 212 can be called the primary voltage Vp. The primary voltage Vp is the voltage generated on the first end of the primary bridge circuit 211, or the voltage between the midpoint of the first primary bridge arm 2111 and the midpoint of the second primary bridge arm 2112 of the primary bridge circuit 211. The current on the secondary side of the transformer 212 can be called the secondary current is. The secondary current is is also the inductor current flowing through the resonant inductor Lr. The voltage generated on the secondary side of the transformer 212 can be called the secondary voltage Vs. The secondary voltage Vs is the voltage on the first end of the secondary bridge circuit 214, or the voltage between the midpoint of the first secondary bridge arm 2141 and the midpoint of the second secondary bridge arm 2142 of the secondary bridge circuit 214.

[0051] In a conventional driving scheme, for the primary bridge circuit 211, during the primary switching time, the controller 22 controls the first primary switch tube Q1 and the second primary switch tube Q2 to be turned on alternately at high frequency, and the third primary switch tube Q3 and the fourth primary switch tube Q4 to be turned on alternately at high frequency. The primary switch tubes Q1 to Q4 can all be referred to as primary high-frequency tubes.

[0052] For the secondary bridge circuit 214, please refer to Figure 3 and Figure 4 ,in Figure 3This is the waveform diagram of the control signals S5~S8 and the AC voltage Vac under the conventional driving scheme. Figure 4 yes Figure 3 Vac in the 10ms time is near the zero point (that is, Figure 3 A local enlarged view of the area within the elliptical dotted box in FIG.

[0053] Combination Figure 3 and Figure 4 It can be seen that when the real-time AC voltage Vac is positive, during the secondary switching time, the controller 22 controls the first secondary switch tube Q5 and the third secondary switch tube Q7 to be alternately turned on at high frequency, and the second secondary switch tube Q6 and the fourth secondary switch tube Q8 to be normally turned on. Among them, the secondary switch tubes Q5 and Q7 can both be called secondary high-frequency tubes, and the secondary switch tubes Q6 and Q8 can both be called secondary normally-on tubes.

[0054] When the real-time AC voltage Vac is negative, during the secondary switching time, the controller 22 controls the second secondary switch tube Q6 and the fourth secondary switch tube Q8 to be alternately turned on at high frequency, and the first secondary switch tube Q5 and the third secondary switch tube Q7 to be normally turned on. The secondary switch tubes Q6 and Q8 can both be called secondary high-frequency tubes, and the secondary switch tubes Q5 and Q7 can both be called secondary normally-on tubes.

[0055] In the embodiment of the present application, there is a secondary dead time between two adjacent secondary switching times. The secondary dead time can reserve time for the secondary high-frequency tube to switch state, ensuring that the secondary high-frequency tube has switched state when entering the next secondary switching time, thereby avoiding the first secondary bridge arm from being directly turned on.

[0056] Specific, combined Figure 3 and Figure 4 It can be seen that when the real-time AC voltage Vac is positive, during the secondary dead time, the controller 22 controls the first secondary switch tube Q5 and the third secondary switch tube Q7 to be turned off, and the second secondary switch tube Q6 and the fourth secondary switch tube Q8 to be always on. Overall, the control signal waveform of the first secondary switch tube Q5 and the control signal waveform of the third secondary switch tube Q7 are both at a low level during the secondary dead time corresponding to the zero crossing point, that is, there is no overlap (see Figure 4 ).

[0057] When the real-time AC voltage Vac is negative, during the secondary dead time, the controller 22 controls the second secondary switch tube Q6 and the fourth secondary switch tube Q8 to be turned off, and the first secondary switch tube Q5 and the third secondary switch tube Q7 are always on. Overall, the control signal waveform of the second secondary switch tube Q6 and the control signal waveform of the fourth secondary switch tube Q8 are both at a low level during the secondary dead time corresponding to the zero crossing point, that is, there is no overlap (see Figure 4 ).

[0058] However, due to the error in voltage sampling, the control signal switching of the power conversion circuit may be inaccurate, or the voltage polarity switching delay may be caused by the existence of the filter circuit or the filter, so that the secondary side switch tubes Q5~Q8 are not accurately switched at the voltage zero crossing point, and then, Figure 5 As shown in the dotted ellipse box, the AC current will oscillate near the voltage zero point (Vac=0). Current oscillation will increase the current overstress risk of the device and increase the power loss of the device, and will cause current waveform distortion, affecting the power quality, which is not conducive to device safety and normal operation of the circuit.

[0059] Therefore, the controller 22 of the embodiment of the present application can be used to execute the control method of the power conversion circuit provided in the embodiment of the present application to solve the problem of current oscillation at the voltage zero point caused by the secondary side switch tube not switching at the voltage zero point.

[0060] Next, based on Figure 2 The structural topology of the power conversion circuit 21 shown in FIG. Figure 6 and Fig.10 The control method of the power conversion circuit in the embodiment of the present application is described in detail. Figure 6 A flow chart showing a control method of a power conversion circuit according to an embodiment of the present application is shown. Fig.10 The working waveform diagram corresponding to the power conversion circuit 21 in the embodiment of the present application within the preset transition region of the voltage zero crossing point is shown.

[0061] See also Figure 6 The control method of the embodiment of the present application includes the following steps: Step S10: obtaining a real-time AC voltage on the second end of the secondary bridge circuit.

[0062] Specifically, the controller 22 may obtain the real-time AC voltage Vac by sampling through a voltage sampling circuit.

[0063] Step S20: determine whether the real-time AC voltage is in a preset transition region of the voltage zero crossing point.

[0064] The preset transition region of the voltage zero crossing point refers to the region near the voltage zero crossing point of Vac, that is, the region where Vac is close to 0. For example, Figure 5 The area within the elliptical dotted frame shown can be used as a preset transition area.

[0065] The size of the preset transition region can be set accordingly according to actual conditions and is not specifically limited here. For example, (-5V, +5V) can be used as the preset transition region of the voltage zero crossing point of Vac. When the real-time AC voltage Vac is at (-5V, +5V), it means that Vac is in the preset transition region and Vac is about to cross the zero point to switch the voltage polarity.

[0066] Step S30A: when the real-time AC voltage is in the preset transition region of the voltage zero crossing point, each switch tube in the secondary bridge circuit is controlled to perform on-off action within the secondary switching time, so that the secondary bridge circuit performs power conversion.

[0067] Specifically, in Figure 2 In the circuit shown, the process of step S30A may be: If the real-time AC voltage Vac is in the preset transition region and is positive, then during the secondary switching time, the first secondary switch tube Q5 and the third secondary switch tube Q7 are controlled to be turned on alternately, and the first secondary switch tube Q5 and the second secondary switch tube Q6 are controlled to be turned on synchronously, and the third secondary switch tube Q7 and the fourth secondary switch tube Q8 are controlled to be turned on synchronously.

[0068] If the real-time AC voltage Vac is in the preset transition region and is negative, then during the secondary switching time, the second secondary switch tube Q6 and the fourth secondary switch tube Q8 are controlled to be turned on alternately, and the first secondary switch tube Q5 and the second secondary switch tube Q6 are controlled to be turned on synchronously, and the third secondary switch tube Q7 and the fourth secondary switch tube Q8 are controlled to be turned on synchronously.

[0069] Correspondingly, such as Fig.10 As shown, within the secondary side switching time corresponding to the preset transition area, the control signal S5 of the first secondary side switch tube Q5 and the control signal S7 of the third secondary side switch tube Q7 are complementary, the control signal S6 of the second secondary side switch tube Q6 and the control signal S8 of the fourth secondary side switch tube Q8 are complementary, the control signal S5 of the first secondary side switch tube Q5 and the control signal S6 of the second secondary side switch tube Q6 are the same, and the control signal S7 of the third secondary side switch tube Q7 and the control signal S8 of the fourth secondary side switch tube Q8 are the same.

[0070] It should be understood that when the voltage sampling error causes inaccurate switching of the control signal of the power conversion circuit 21, or the voltage polarity switching is delayed, the four secondary side switches under the conventional driving scheme will deviate from the voltage zero point and switch states instantaneously at a certain moment near the voltage zero point, causing the power polarity near the voltage zero point to jump instantaneously, thereby causing current oscillations near the voltage zero point, causing overstress damage to the secondary side switch tube and current waveform distortion.

[0071] Compared with the conventional driving scheme, the embodiment of the present application adds a preset transition region and proposes a control logic executed in the preset transition region. Once the real-time AC voltage Vac enters the preset transition region, no matter whether the real-time AC voltage Vac is positive or negative, the controller 22 controls the first secondary switch tube Q5 and the third secondary switch tube Q7 to be turned on alternately, and the second secondary switch tube Q6 and the fourth secondary switch tube Q8 to be turned on alternately within the secondary switch time according to step S30A, wherein the first secondary switch tube Q5 and the second secondary switch tube Q6 are turned on synchronously, and the third secondary switch tube Q7 and the fourth secondary switch tube Q8 are turned on synchronously.

[0072] In this way, when Vac enters the preset transition area of ​​the voltage zero crossing point, there is always a half bridge arm connected in the first secondary bridge arm 2141 for power transmission, thereby reducing the power variation amplitude near the voltage zero crossing point, allowing the power to smoothly transition between different polarities, thereby avoiding current oscillations during the secondary switching time near the voltage zero crossing point, thereby protecting the secondary switch tube and optimizing the current waveform.

[0073] Furthermore, if Figure 7 As shown, it is assumed that the controller 22 controls the first secondary switch tube Q5 to the fourth secondary switch tube Q8 to be turned off within the secondary dead time corresponding to the preset transition region, that is, Figure 7 The control signals S5~S8 in the preset transition region are all at low level during the secondary side dead time corresponding to the preset transition region. Figure 8 As shown, if the secondary current is flows into the first end of the secondary bridge circuit at this time, the secondary current is cannot flow through the channel and body diode of the second secondary switch tube Q6, nor can it flow through the channel and body diode of the third secondary switch tube Q7, and can only flow through the parasitic capacitance of the second secondary switch tube Q6 and the parasitic capacitance of the third secondary switch tube Q7. Similarly, if Fig. 9 As shown, if the secondary current is flows into the second end of the secondary bridge circuit at this time, the secondary current is cannot flow through the channel and body diode of the first secondary switch tube Q5, nor can it flow through the channel and body diode of the fourth secondary switch tube Q8, and can only flow through the parasitic capacitance of the second secondary switch tube Q6 and the parasitic capacitance of the third secondary switch tube Q7.

[0074] Since the parasitic capacitor has a small capacity and limited freewheeling capability, Figure 8 and Fig. 9 In this scenario, the power conversion circuit 21 is prone to current oscillation during the secondary side dead time near the voltage zero crossing point. When the secondary side dead time is relatively long, it is likely to cause overstress damage to the secondary side switch tube.

[0075] Therefore, to avoid overstress damage to the secondary switch tube during the secondary dead time, please continue to refer to Figure 6 and Fig.10 , the control method of the embodiment of the present application also includes: Step S30B, when the real-time AC voltage is in the preset transition region of the voltage zero crossing point, some switches in the secondary bridge circuit are controlled to be turned on within the secondary dead time, so that the secondary bridge circuit and the secondary side of the transformer form a freewheeling loop.

[0076] Specifically, in Figure 2 In the circuit shown, the process of step S30B may be: When the real-time AC voltage is in a preset transition region of the voltage zero crossing point, within the secondary side dead time, the first secondary side switch tube and the third secondary side switch tube are controlled to be turned on, or the second secondary side switch tube and the fourth secondary side switch tube are controlled to be turned on.

[0077] If the real-time AC voltage is in the preset transition region and is positive, then during the secondary dead time, the second secondary switch tube and the fourth secondary switch tube are controlled to be turned on, and the first secondary switch tube and the third secondary switch tube are controlled to be turned off. Fig.10 As shown, within the secondary side dead time corresponding to the preset transition region, the control signals S6 and S8 are both high level, and the control signals S5 and S7 are both low level.

[0078] At this time, if Fig.11 As shown, the channel of the second secondary switch tube Q6, the body diode of the first secondary switch tube Q5, the first capacitor Cr1 and the secondary side of the transformer 212 are connected to form a freewheeling loop, that is, the upper half bridge arm of the first secondary bridge arm 2141 and the first capacitor Cr1 and the secondary side of the transformer 212 form a freewheeling loop. The channel of the third secondary switch tube Q7, the body diode of the fourth secondary switch tube Q8, the second capacitor Cr2 and the secondary side of the transformer 212 are connected to form another freewheeling loop, that is, the lower half bridge arm of the first secondary bridge arm 2141 and the second capacitor Cr2 and the secondary side of the transformer 212 form another freewheeling loop.

[0079] Therefore, if the secondary current is flows into the first end of the secondary bridge circuit at this time, the secondary current is can flow to the second end of the secondary bridge circuit through the channel of the second secondary switch tube Q6 and the body diode of the first secondary switch tube Q5, or flow to the second end of the secondary bridge circuit through the channel of the third secondary switch tube Q7 and the body diode of the fourth secondary switch tube Q8. Since the current carrying capacity of the channel of the secondary switch tube is stronger than the parasitic capacitance, Fig.11 and Figure 8 For comparison, see Fig.13 , which can effectively reduce the current oscillation of the power conversion circuit 21 during the secondary side dead time near the voltage zero crossing point, thereby avoiding overstress damage to the secondary side switch tube and improving the current sinusoidality.

[0080] Similarly, if the real-time AC voltage is in the preset transition region and is negative, then during the secondary dead time, the first secondary switch tube and the third secondary switch tube are both turned on, and the second secondary switch tube and the fourth secondary switch tube are both turned off. Accordingly, during the secondary dead time corresponding to the preset transition region, the control signals S5 and S7 are both high level, and the control signals S6 and S8 are both low level.

[0081] At this time, if Fig.12 As shown, the channel of the first secondary switch tube Q5, the body diode of the second secondary switch tube Q6, the first capacitor Cr1 and the secondary side of the transformer 212 are connected to form a freewheeling loop, that is, the upper half of the first secondary bridge arm, the first capacitor Cr1 and the secondary side of the transformer 212 form a freewheeling loop. The channel of the third secondary switch tube Q7, the body diode of the fourth secondary switch tube Q8, the second capacitor Cr2 and the secondary side of the transformer 212 are connected to form another freewheeling loop, that is, the lower half of the first secondary bridge arm, the second capacitor Cr2 and the secondary side of the transformer 212 form another freewheeling loop.

[0082] Therefore, if the secondary current is flows into the second end of the secondary bridge circuit at this time, the secondary current is can flow to the first end of the secondary bridge circuit through the channel of the first secondary switch tube Q5 and the body diode of the second secondary switch tube Q6, or flow to the second end of the secondary bridge circuit through the channel of the fourth secondary switch tube Q8 and the body diode of the third secondary switch tube Q7. Since the current carrying capacity of the channel of the secondary switch tube is stronger than the parasitic capacitance, Fig.12 and Fig. 9 For comparison, see Fig.13 The power conversion circuit 21 is not prone to current oscillation during the secondary side dead time near the voltage zero crossing point, thereby avoiding overstress damage to the secondary side switch tube and improving the current sinusoidality.

[0083] In addition, if Fig.10 As shown, overall, the control signal waveform of the second secondary switch tube Q6 and the control signal waveform of the fourth secondary switch tube Q8 overlap within the secondary dead time corresponding to the preset transition area (that is, both are high level). Alternatively, the control signal waveform of the first secondary switch tube Q5 and the control signal waveform of the third secondary switch tube Q7 overlap within the secondary dead time corresponding to the preset transition area (that is, both are high level). Moreover, as Fig.13 As shown, the waveform of the real-time AC current outputted from the second end of the secondary bridge circuit can be presented as a basically smooth sine wave, and the distortion amplitude is significantly smaller than Figure 5 The waveform shown.

[0084] In addition, in case that the real-time AC voltage Vac is not in the preset transition region of the voltage zero crossing point, the method of the embodiment of the present application further includes the following steps: Step S40A: when the real-time AC voltage is not in the preset transition region of the voltage zero crossing point, each switch tube in the secondary bridge circuit is controlled to perform on-off action within the secondary switching time, so that the secondary bridge circuit performs power conversion.

[0085] Specifically, in Figure 2 In the circuit shown, you can refer to Figure 4 , the process of step S40A may be: If the real-time AC voltage Vac is not in the preset transition region and is positive, then during the secondary switching time, the first secondary switch tube Q5 and the third secondary switch tube Q7 are controlled to be alternately turned on, and the second secondary switch tube Q6 and the fourth secondary switch tube Q8 are both kept turned on.

[0086] Furthermore, during the secondary side dead time, the first secondary side switch tube Q5 and the third secondary side switch tube Q7 are controlled to be turned off, and the second secondary side switch tube Q6 and the fourth secondary side switch tube Q8 are kept turned on.

[0087] If the real-time AC voltage Vac is not in the preset transition region and is negative, then during the secondary switching time, the first secondary switch tube Q5 and the third secondary switch tube Q7 are controlled to remain turned on, and the second secondary switch tube Q6 and the fourth secondary switch tube Q8 are alternately turned on.

[0088] Furthermore, during the secondary side dead time, the second secondary side switch tube Q6 and the fourth secondary side switch tube Q8 are controlled to be turned off, and the first secondary side switch tube Q5 and the third secondary side switch tube Q7 are kept turned on.

[0089] That is, when the real-time AC voltage Vac is not in the preset transition region (that is, the real-time AC voltage Vac is outside the preset transition region at the voltage zero crossing point), the controller 22 is Figure 4 The conventional driving scheme shown is used to control the states of the secondary side switches Q5 to Q8.

[0090] It is understandable that the controller 22 of the embodiment of the present application controls the operation of the secondary bridge circuit 214 while also controlling the operation of the primary bridge circuit 211. Therefore, the control method of the embodiment of the present application further includes: During the primary switching time, each switch tube in the primary bridge circuit is controlled to perform on-off actions so that the primary bridge circuit can perform power conversion.

[0091] Specifically, in Figure 2 In the circuit shown, the controller 22 can control the first primary switch tube Q1 and the second primary switch tube Q2 to be alternately turned on during the primary switching time, and control the third primary switch tube Q3 and the fourth primary switch tube Q4 to be alternately turned on during the primary switching time.

[0092] Furthermore, when there is a primary dead time between two adjacent primary switching times, the controller 22 can control all the switch tubes Q1 to Q4 in the primary bridge circuit 211 to be turned off within the primary dead time.

[0093] It should be understood that in other embodiments, if the primary bridge circuit 211 of the power conversion circuit 21 is a primary half-bridge circuit, that is, Figure 2 When some of the primary switching tubes in the primary full-bridge circuit are replaced with capacitors, the primary switching tubes in the primary half-bridge circuit are switched on and off according to the on-off logic of the same primary switching tubes in the primary full-bridge circuit, so they will not be repeated here.

[0094] To sum up, the control method of the power conversion circuit in the embodiment of the present application does not require the addition of additional hardware circuit structure. Through the on-off logic control of the switch tube, soft switching of the switch tube can be achieved near the voltage zero point, effectively reducing the current oscillation near the voltage zero point, thereby avoiding overstress damage to the secondary side switch tube, and optimizing the current sinusoidality, thereby improving the quality of power output.

[0095] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application and are not intended to limit it. Although the present application has been described in detail with reference to the preferred embodiments, a person of ordinary skill in the art should understand that the technical solution of the present application may be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present application.

Claims

1. A control method for a power conversion circuit, characterized in that: The power conversion circuit includes a primary bridge circuit, a transformer and a secondary bridge circuit, wherein a first end of the primary bridge circuit is connected to a primary side of the transformer, a second end of the primary bridge circuit is used to connect a DC power supply or a DC load, a first end of the secondary bridge circuit is connected to a secondary side of the transformer, and a second end of the secondary bridge circuit is used to connect an AC load or an AC power supply; the control method includes: Acquiring a real-time AC voltage on the second end of the secondary bridge circuit; When the real-time AC voltage is in a preset transition region of the voltage zero crossing point, each switch tube in the secondary bridge circuit is controlled to perform on-off action within the secondary switching time, so that the secondary bridge circuit performs power conversion; wherein there is a secondary dead time between two adjacent secondary switching times; When the real-time AC voltage is in a preset transition region of the voltage zero crossing point, part of the switch tubes in the secondary bridge circuit are controlled to be turned on within the secondary dead time, so that the secondary bridge circuit and the secondary side of the transformer form a freewheeling loop.

2. The control method according to claim 1, characterized in that: The secondary bridge circuit comprises a first secondary bridge arm and a second secondary bridge arm connected in series, the first secondary bridge arm comprises an upper half bridge arm and a lower half bridge arm connected in series, the upper half bridge arm comprises a first secondary switch tube and a second secondary switch tube connected in reverse series, the lower half bridge arm comprises a third secondary switch tube and a fourth secondary switch tube connected in reverse series, and the second secondary bridge arm comprises a first capacitor and a second capacitor connected in series; When the real-time AC voltage is in a preset transition region of a voltage zero crossing point, controlling some of the switch tubes in the secondary side bridge circuit to be turned on within the secondary side dead time includes: When the real-time AC voltage is in a preset transition region of the voltage zero crossing point, within the secondary side dead time, the first secondary side switch tube and the third secondary side switch tube are controlled to be turned on, or the second secondary side switch tube and the fourth secondary side switch tube are controlled to be turned on.

3. The control method according to claim 2, characterized in that: When the real-time AC voltage is in a preset transition region of a voltage zero crossing point, within the secondary side dead time, controlling the first secondary side switch tube and the third secondary side switch tube to be turned on, or controlling the second secondary side switch tube and the fourth secondary side switch tube to be turned on, comprises: If the real-time AC voltage is within the preset transition region and is positive, then during the secondary dead time, the second secondary switch tube and the fourth secondary switch tube are controlled to be turned on, and the first secondary switch tube and the third secondary switch tube are controlled to be turned off; If the real-time AC voltage is within the preset transition region and is negative, then within the secondary dead time, the first secondary switch tube and the third secondary switch tube are controlled to be turned on, and the second secondary switch tube and the fourth secondary switch tube are controlled to be turned off.

4. The control method according to claim 1, characterized in that: The secondary bridge circuit comprises a first secondary bridge arm and a second secondary bridge arm connected in series, the first secondary bridge arm comprises an upper half bridge arm and a lower half bridge arm connected in series, the upper half bridge arm comprises a first secondary switch tube and a second secondary switch tube connected in reverse series, the lower half bridge arm comprises a third secondary switch tube and a fourth secondary switch tube connected in reverse series, and the second secondary bridge arm comprises a first capacitor and a second capacitor connected in series; When the real-time AC voltage is in a preset transition region of a voltage zero crossing point, each switch tube in the secondary bridge circuit is controlled to perform on-off action within the secondary switching time so that the secondary bridge circuit performs power conversion, including: When the real-time AC voltage is in a preset transition region of the voltage zero crossing point, during the secondary switching time, the first secondary switch tube and the third secondary switch tube are controlled to be alternately turned on, and the second secondary switch tube and the fourth secondary switch tube are controlled to be alternately turned on, wherein the first secondary switch tube is turned on synchronously with the second secondary switch tube, and the third secondary switch tube is turned on synchronously with the fourth secondary switch tube.

5. The control method according to claim 1, characterized in that: The control method further comprises: When the real-time AC voltage is not in a preset transition region of the voltage zero crossing point, each switch tube in the secondary bridge circuit is controlled to perform on-off action within the secondary switching time, so that the secondary bridge circuit performs power conversion.

6. The control method according to claim 5, characterized in that: The secondary bridge circuit comprises a first secondary bridge arm and a second secondary bridge arm connected in series, the first secondary bridge arm comprises an upper half bridge arm and a lower half bridge arm connected in series, the upper half bridge arm comprises a first secondary switch tube and a second secondary switch tube connected in reverse series, the lower half bridge arm comprises a third secondary switch tube and a fourth secondary switch tube connected in reverse series, and the second secondary bridge arm comprises a first capacitor and a second capacitor connected in series; When the real-time AC voltage is not in the preset transition region of the voltage zero crossing point, controlling each switch tube in the secondary bridge circuit to perform on-off action within the secondary switching time so that the secondary bridge circuit performs power conversion, includes: If the real-time AC voltage is not in the preset transition region and is positive, then during the secondary switching time, the first secondary switch tube and the third secondary switch tube are controlled to be alternately turned on, and the second secondary switch tube and the fourth secondary switch tube are both kept turned on; If the real-time AC voltage is not in the preset transition region and is negative, then during the secondary switching time, the first secondary switch tube and the third secondary switch tube are controlled to remain turned on, and the second secondary switch tube and the fourth secondary switch tube are controlled to be turned on alternately.

7. The control method according to claim 1, characterized in that: The control method further comprises: During the primary switching time, each switch tube in the primary bridge circuit is controlled to perform on-off actions, so that the primary bridge circuit performs power conversion.

8. The control method according to claim 7, characterized in that: The primary bridge circuit comprises a first primary bridge arm and a second primary bridge arm connected in series, the first primary bridge arm comprises a first primary switch tube and a second primary switch tube connected in series, and the second primary bridge arm comprises a third primary switch tube and a fourth primary switch tube connected in series; The method of controlling each switch tube in the primary bridge circuit to perform on-off action during the primary switching time so that the primary bridge circuit performs power conversion includes: During the primary switching time, the first primary switching tube and the second primary switching tube are controlled to be alternately turned on, and the third primary switching tube and the fourth primary switching tube are controlled to be alternately turned on.

9. A power conversion device, characterized in that: The power conversion device comprises a power conversion circuit and a controller, wherein the power conversion circuit comprises a primary bridge circuit, a transformer and a secondary bridge circuit, wherein a first end of the primary bridge circuit is connected to a primary side of the transformer, a second end of the primary bridge circuit is used to connect a DC power supply or a DC load, a first end of the secondary bridge circuit is connected to a secondary side of the transformer, and a second end of the secondary bridge circuit is used to connect an AC load or an AC power supply; The controller is connected to the power conversion circuit, and the controller is used for: When the real-time AC voltage on the second end of the secondary bridge circuit is in a preset transition region of the voltage zero crossing point, each switch tube in the secondary bridge circuit is controlled to perform on-off action within the secondary switching time, so that the secondary bridge circuit performs power conversion; wherein there is a secondary dead time between two adjacent secondary switching times; When the real-time AC voltage is in a preset transition region of the voltage zero crossing point, part of the switch tubes in the secondary bridge circuit are controlled to be turned on within the secondary dead time, so that the secondary bridge circuit and the secondary side of the transformer form a freewheeling loop.

10. The power conversion device according to claim 9, characterized in that The secondary bridge circuit comprises a first secondary bridge arm and a second secondary bridge arm connected in series, the first secondary bridge arm comprises an upper half bridge arm and a lower half bridge arm connected in series, the upper half bridge arm comprises a first secondary switch tube and a second secondary switch tube connected in reverse series, the lower half bridge arm comprises a third secondary switch tube and a fourth secondary switch tube connected in reverse series, and the second secondary bridge arm comprises a first capacitor and a second capacitor connected in series; The controller is used to: when the real-time AC voltage is in a preset transition area of ​​the voltage zero point, within the secondary side dead time, control the first secondary side switch tube and the third secondary side switch tube to be turned on, or control the second secondary side switch tube and the fourth secondary side switch tube to be turned on, so that the upper half bridge arm, the first capacitor and the secondary side of the transformer form a freewheeling loop, and the lower half bridge arm, the second capacitor and the secondary side of the transformer form another freewheeling loop.

11. The power conversion device according to claim 10, characterized in that The controller is used to generate a control signal, and the control signal is used to control the on-off state of each switch tube in the primary bridge circuit and the secondary bridge circuit; Among them, when the real-time AC voltage is in a preset transition area of ​​the voltage zero crossing point, the control signal waveform of the second secondary side switch tube and the control signal waveform of the fourth secondary side switch tube overlap within the secondary side dead time, or the control signal waveform of the first secondary side switch tube and the control signal waveform of the third secondary side switch tube overlap within the secondary side dead time.

12. The power conversion device according to claim 9, characterized in that The waveform of the real-time alternating current outputted from the second end of the secondary bridge circuit is a substantially smooth sine wave.

13. A power supply system, characterized in that: The power supply system comprises: a DC power supply and a power conversion device as described in any one of claims 9 to 12, wherein the DC power supply is connected to the second end of the primary bridge circuit, the second end of the secondary bridge circuit is connected to a common coupling point, the DC power supply comprises at least one of an energy storage device and a photovoltaic power generation device, and the common coupling point is used to connect at least one of a power grid and an AC load.

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