Power converter and photovoltaic system

Abstract

The application relates to the new energy technology field, and provides a power converter and a photovoltaic system. The FC-TL conversion circuit provided with a flying capacitor is arranged in the power converter. The original switch tube in the FC-TL conversion circuit is multiplexed and matched with a simple current limiting circuit, and the pre-charging of the flying capacitor is completed before the FC-TL conversion circuit performs power conversion. Therefore, when the conversion circuit performs the same power conversion, the voltage borne by the switched-off switch tube does not exceed the rated voltage.

Description

Technical Field

[0001] This application relates to the field of new energy technology, and in particular to power converters and photovoltaic systems. Background Technology

[0002] With the development of new energy sources, higher demands are being placed on the performance of energy systems (such as photovoltaic systems). A key component of an energy system is the power converter. A power converter includes at least one power conversion circuit. Switches in the power conversion circuit are alternately turned on to achieve power conversion. Common power conversion circuits include, but are not limited to: direct current to direct current (DC / DC) conversion circuits, direct current to alternating current (DC / AC) conversion circuits, etc.

[0003] To improve the performance of power converters and enable energy systems to handle high-voltage, high-power applications, power converters are constantly evolving. One such evolution is the flying capacitor three-level (FC-TL) DC / DC converter circuit (hereinafter referred to as the FC-TL converter circuit), which includes multiple switching transistors and a flying capacitor. The two ends of the flying capacitor are connected between specific switching transistors. Because of the introduction of the flying capacitor, the FC-TL converter circuit supports the generation of three voltage levels (or three levels, three voltages), thus enabling the power converter containing the FC-TL converter circuit to achieve higher quality power conversion.

[0004] However, while the introduction of flying capacitors brings significant advantages, it also presents unique technical challenges. During power conversion in an FC-TL converter circuit, if the voltage across the flying capacitor is too low (e.g., close to 0V) and fails to reach the safe voltage, the flying capacitor will exhibit a low-resistance state, equivalent to a short circuit, triggering a huge inrush current. This inrush current can not only burn out the flying capacitor itself but also cause some switching transistors to withstand the full input voltage and break down, seriously threatening the safety of the devices. Summary of the Invention This application provides a power converter and a photovoltaic system. The power converter includes an FC-TL conversion circuit with a flying capacitor. By reusing the original switching transistors in the FC-TL conversion circuit and using a simple current-limiting circuit, the flying capacitor is pre-charged before the FC-TL conversion circuit performs power conversion. This ensures that the voltage across the switched transistors does not exceed their rated voltage during power conversion.

[0005] In a first aspect, embodiments of this application provide a power converter, including a DC / DC conversion circuit, a DC bus, and a controller. The DC / DC conversion circuit includes a current limiting circuit, a flying capacitor, a first switch, a second switch, a third switch, and a fourth switch. The first switch, the second switch, the third switch, and the fourth switch are connected in series between the positive and negative terminals of the DC bus. The first and second terminals of the flying capacitor are respectively connected to the connection points of the first and second switches and the connection points of the third and fourth switches. The first terminal of the flying capacitor is also connected to the positive terminal of the DC bus through the current limiting circuit, or the second terminal of the flying capacitor is also connected to the negative terminal of the DC bus through the current limiting circuit. The controller is used to control the DC / DC conversion circuit to perform power conversion when the voltage of the flying capacitor is greater than or equal to a voltage threshold. The voltage threshold is equal to the voltage of the DC bus minus a first rated voltage. The first rated voltage is the minimum rated voltage between the rated voltage of the first switch and the rated voltage of the fourth switch.

[0006] The DC-DC converter circuit involved in the above embodiments can be the FC-TL converter circuit mentioned in the specification. By reusing the original switching transistors in the FC-TL converter circuit and adding a current limiting circuit, the flying capacitor is pre-charged before the FC-TL converter circuit performs power conversion. This prevents the switching transistors in the FC-TL converter circuit from experiencing overvoltage when turned off due to excessively low voltage across the flying capacitor. Compared to an FC-TL converter circuit without pre-charging, an FC-TL converter circuit with pre-charging ensures that the voltage across the turned-off switching transistor does not exceed its rated voltage when performing the same power conversion. This allows the use of switching transistors with lower rated voltages, thereby reducing the cost of the switching transistors.

[0007] In conjunction with the first aspect, in some embodiments, the controller is specifically configured to control the DC / DC converter circuit to perform power conversion after the flying capacitor is charged to a voltage greater than or equal to a voltage threshold and less than or equal to a second rated voltage; the second rated voltage is the minimum rated voltage among the rated voltage of the second switch and the rated voltage of the third switch.

[0008] In the above embodiment, because switching transistors Q2 and Q3 need to withstand the voltage of the flying capacitor when turned off, charging of the flying capacitor needs to be stopped when it reaches the second rated voltage. This is to prevent overcharging of the flying capacitor, which could cause overvoltage in the second switching transistor (Q2) and the third switching transistor (Q3) when turned off.

[0009] In conjunction with the first aspect, in some embodiments, the current limiting circuit includes a resistor.

[0010] In the above embodiments, the resistor's resistance to current is used to provide a simple, reliable, and low-cost current-limiting charging circuit for the flying capacitor, effectively suppressing the surge current during pre-charging.

[0011] In conjunction with the first aspect, in some embodiments, the current limiting circuit further includes a first switch; the first switch is connected to a resistor.

[0012] In the above embodiments, the controller can control the first switch to turn on when the flying capacitor in the FC-TL conversion circuit is in the pre-charging stage, so as to utilize the resistor for current limiting.

[0013] In conjunction with the first aspect, in some embodiments, the current limiting circuit includes a second switch, the resistance of which, when turned on, is greater than or equal to a resistance threshold.

[0014] In the above embodiments, the controller can control the second switch to turn on when the flying capacitor in the FC-TL conversion circuit is in the pre-charging stage, so as to utilize the resistance when it is turned on to limit the current.

[0015] In conjunction with the first aspect, in some embodiments, the controller is also configured to control the first switch or the second switch to open when the DC / DC conversion circuit is performing power conversion.

[0016] In embodiments employing a second switch, the controller can open the second switch during power conversion in the FC-TL converter circuit to prevent continuous power loss during power conversion. In embodiments employing a first switch in series with a resistor, the controller can open the first switch during power conversion in the FC-TL converter circuit, thereby removing the current-limiting resistor from the power path and preventing continuous power loss during power conversion.

[0017] In conjunction with the first aspect, in some embodiments, the current limiting circuit includes a diode, a resistor, and a capacitor; the diode and resistor are connected in series, and the capacitor is connected in parallel across the diode and resistor connected in series.

[0018] In the above embodiments, a unidirectional pre-charge circuit is formed by the diodes in the current limiting circuit, which guides the current to charge the flying capacitor.

[0019] In conjunction with the first aspect, in some embodiments, the first terminal of the flying capacitor is connected to the positive terminal of the DC bus via a current-limiting circuit, which is connected between the first terminal of the flying capacitor and the positive terminal of the DC bus; the anode of the diode is connected to the positive terminal of the DC bus, and the cathode of the diode is connected to the first terminal of the flying capacitor.

[0020] In the above embodiments, the diodes in the current-limiting circuit form a unidirectional pre-charging circuit, guiding the current to charge the flying capacitor. Furthermore, in this embodiment, the capacitor in the current-limiting circuit can be connected in parallel across the first switching transistor to absorb voltage spikes generated by the parasitic inductance of the circuit during the first switching transistor's turn-off process. This controls the voltage across the first switching transistor during turn-off within a safe range, preventing damage due to overvoltage.

[0021] In conjunction with the first aspect, in some embodiments, the second terminal of the flying capacitor is connected to the negative terminal of the DC bus via a current-limiting circuit, which is connected between the second terminal of the flying capacitor and the negative terminal of the DC bus; the anode of the diode is connected to the second terminal of the flying capacitor, and the cathode of the diode is connected to the negative terminal of the DC bus.

[0022] In the above embodiments, the diodes in the current-limiting circuit form a unidirectional pre-charging circuit, guiding the current to charge the flying capacitor. Furthermore, in this embodiment, the capacitor in the current-limiting circuit can be connected in parallel across the fourth switching transistor. This capacitor can absorb voltage spikes generated by the parasitic inductance of the circuit during the turn-off process of the fourth switching transistor, thereby controlling the voltage across the fourth switching transistor during turn-off within a safe range and preventing damage due to overvoltage.

[0023] In conjunction with the first aspect, in some embodiments, the first, second, third, and fourth switching transistors are all connected in parallel with diodes. The anodes of the parallel diodes are connected to the negative terminal of the DC bus, and the cathodes of the parallel diodes are connected to the positive terminal of the DC bus. When the voltage of the flying capacitor is less than the voltage threshold, the first, second, third, and fourth switching transistors are all turned off. The diodes connected in parallel with some of the first, second, third, and fourth switching transistors are used to charge the flying capacitor. Some of the switching transistors include the second or third switching transistor.

[0024] In the above embodiments, when the switching transistor is naturally turned off, it can be naturally turned on by the parallel diodes, without the need for controller control, thus simplifying the control logic.

[0025] In conjunction with the first aspect, in some embodiments, the power converter further includes a DC / AC conversion circuit and a pre-stage DC / DC conversion circuit for the DC / DC conversion circuit; the pre-stage DC / DC conversion circuit is connected to the first and fourth switching transistors in the DC / DC conversion circuit via a DC bus; the DC / AC conversion circuit is connected to the DC bus and the connection point between the second and third switching transistors in the DC / DC conversion circuit.

[0026] In the above embodiments, the power converter has a three-stage system architecture. By sequentially connecting the front-end DC / DC converter circuit, the DC / DC converter circuit (i.e., the FC-TL circuit), and the DC / AC converter circuit, each stage of the circuit can work collaboratively, including: the front-end DC / DC converter circuit can achieve initial voltage matching and electrical isolation; the intermediate FC-TL circuit utilizes a pre-charge mechanism and its multi-level topology to efficiently and with low stress complete the DC-DC conversion of the main power and provide a high-quality DC bus voltage for the subsequent stages; and the final-stage DC / AC converter, based on this optimization, efficiently generates high-quality sinusoidal AC power. This architecture fully leverages the advantages of the FC-TL circuit, achieving efficient, reliable, and high-performance energy conversion from a DC source to the AC grid at the system level. At this point, the electrical energy for charging the flying capacitor can come from the electrical energy on the DC bus.

[0027] In conjunction with the first aspect, in some embodiments, the power converter includes a pre-stage DC / DC converter circuit of the DC / DC converter circuit; the pre-stage DC / DC converter circuit is connected to the first and fourth switching transistors in the DC / DC converter circuit via a DC bus; the third and fourth switching transistors in the DC / DC converter circuit are connected in series between the positive and negative terminals of the energy storage device.

[0028] In the above embodiments, by directly connecting the energy storage device between the third and fourth switches of the FC-TL circuit and connecting it to the front-end DC / DC converter circuit via the DC bus, this architecture constructs a highly efficient and compact bidirectional energy channel. The FC-TL circuit here directly serves as a high-performance interface between the energy storage device and the system DC bus, and its pre-charging mechanism ensures the safety of the switches during system startup. Combined with the precise control and isolation functions of the front-end DC / DC converter circuit, the entire system achieves safe, efficient, and intelligent bidirectional energy management of the energy storage device. At this time, the electrical energy for charging the flying capacitor can come from the energy storage device or from the electrical energy on the DC bus.

[0029] Secondly, embodiments of this application provide a photovoltaic system, which includes a power converter and an energy storage device as described in the first aspect, wherein the energy storage device is used to provide direct current to the power converter. Attached Figure Description

[0030] Figure 1 An exemplary photovoltaic system and power converter are shown; Figure 2 A schematic diagram of the application of the FC-TL converter circuit in a power converter is shown; Figure 3 This diagram illustrates an application of an FC-TL converter circuit in an energy storage system. Figure 4This diagram shows the flying capacitor in the FC-TL converter circuit when it is equivalent to a short circuit. Figure 5 A schematic diagram of an FC-TL converter circuit with a current limiting circuit is shown. Figure 6 The turn-on timing diagram of the FC-TL converter circuit is shown; Figure 7 This diagram shows a partial charging circuit with the current limiting circuit positioned between the flying capacitor and the positive terminal of the DC bus. Figure 8 This diagram shows a partial charging circuit with the current limiting circuit positioned between the flying capacitor and the negative terminal of the DC bus. Figure 9 This diagram shows a current limiting circuit with the current limiting circuit positioned between the flying capacitor and the positive terminal of the DC bus. Figure 10 A schematic diagram of a current limiting circuit is shown when the current limiting circuit is set between the flying capacitor and the negative terminal of the DC bus. Detailed Implementation

[0031] To facilitate understanding of the power converter and photovoltaic system provided in the embodiments of this application, firstly based on... Figure 1 and Figure 2 This example illustrates the application of power converters in photovoltaic systems and the FC-TL conversion circuit within the power converter.

[0032] refer to Figure 1 The power converter in a photovoltaic system can include a photovoltaic inverter. In addition, a photovoltaic system can also include photovoltaic modules connected to the photovoltaic inverter and a prefabricated substation.

[0033] Optionally, in addition to photovoltaic modules, the primary energy source in a photovoltaic system may also include energy storage devices. For photovoltaic systems with energy storage batteries (or photovoltaic-storage systems), their power converters may include, in addition to photovoltaic inverters, power conversion circuits (such as DC / DC conversion circuits) connected to the energy storage devices.

[0034] The process of a photovoltaic (PV) system supplying power to external devices (such as the power grid) involves: Direct current (DC) from the PV modules is transmitted to a DC / DC converter circuit connected to the PV modules for voltage conversion. The converted DC is then further transmitted to a DC / AC converter circuit. This DC / AC converter converts the DC into alternating current (AC) and transmits it to a prefabricated substation. The prefabricated substation is used to convert the received low-voltage AC into medium-voltage AC, which is then further transmitted to a step-up substation. This step-up substation then boosts the AC voltage again to high-voltage AC before supplying it to the power grid. The DC power provided by the PV modules is generated by the PV modules converting solar energy through the photovoltaic effect.

[0035] Energy storage devices in photovoltaic (PV) systems can address the issue of unstable power supply from PV modules. When the PV modules provide excess DC power, the DC / DC converter circuit connected to the energy storage device converts the excess DC power into voltage and then transmits it to the energy storage device for storage. When the PV modules generate insufficient DC power, the DC / DC converter circuit connected to the energy storage device converts the DC power provided by the energy storage device into voltage and then provides it to the DC / AC converter circuit. In this case, the DC power transmitted to the DC / AC converter circuit includes not only the DC power converted by the DC / DC converter circuit connected to the PV modules but also the DC power converted by the DC / DC converter circuit connected to the energy storage device.

[0036] It should be noted that the photovoltaic system mentioned above is merely an example and should not be construed as limiting the embodiments of this application. In practice, a photovoltaic system may include more or fewer components. For example, in addition to the DC / DC conversion circuit and the DC / AC conversion circuit, the photovoltaic inverter of a photovoltaic system may also include some or all of the components such as the input electromagnetic interference (EMI) filter circuit, the DC trip switch, the AC filter circuit, and the output EMI filter circuit.

[0037] The aforementioned DC / DC converter circuit and DC / AC converter circuit are connected to the controller. The controller is used to control the on and off of the switching transistors in the DC / DC converter circuit and DC / AC converter circuit, thereby enabling the DC / DC converter circuit to perform DC voltage conversion and the DC / AC converter circuit to perform DC to AC conversion.

[0038] In some cases, photovoltaic systems incorporate FC-TL conversion circuits in their power converters to improve converter performance. For example... Figure 2 As shown, the aforementioned FC-TL converter circuit (flying capacitor three-level DC / DC converter circuit) can be specifically set in... Figure 1 The DC / DC conversion circuit of the photovoltaic inverter shown is an example. A DC / DC conversion circuit with an FC-TL converter can be called a two-stage DC / DC conversion circuit. The two-stage DC / AC conversion circuit, through the FC-TL converter, allows the connected DC / AC conversion circuit to better utilize the DC power from the DC bus, resulting in higher waveform quality of the AC power output from the DC / AC conversion circuit.

[0039] refer to Figure 2 The FC-TL converter circuit includes four switching transistors (Q1-Q4) and a flying capacitor (C). flyThe two ends of the flying capacitor are connected to the connection points between switching transistors Q1 and Q2, and between switching transistors Q3 and Q4, respectively. The switching transistors Q1-Q4 of the FC-TL converter circuit are connected in series on the DC bus. Figure 2 The positive and negative terminals of the lines marked BUS+ and BUS- are connected.

[0040] Continue to refer to Figure 2 , mentioned above Figure 1 The DC / DC conversion circuit involved in the photovoltaic inverter may include a front-end DC / DC conversion circuit and an FC-TL conversion circuit (also referred to here as the rear-end DC / DC conversion circuit). The FC-TL conversion circuit serves as the conversion circuit between the front-end DC / DC conversion circuit and the DC / AC conversion circuit. The front-end DC / DC conversion circuit is connected to the switching transistors Q1 and Q4 in the FC-TL conversion circuit via the DC bus, and the DC / AC conversion circuit is connected to the connection point between the DC bus and the switching transistors Q2 and Q3.

[0041] When a photovoltaic system supplies power to external devices (such as the power grid), the front-end DC / DC converter circuit is used to convert the DC power supplied by the photovoltaic modules into voltage and transmit it to the DC bus. Figure 2 On lines marked BUS+ and BUS-, the voltage on the DC bus is denoted as V. DC The DC bus is also connected to a DC / AC converter circuit, providing DC power (voltage V) to the DC / AC converter circuit. DC Optionally, the energy storage device can also obtain DC power from or supply DC power to the DC bus through its connected DC / DC converter circuit to stabilize the voltage of the DC bus.

[0042] The FC-TL converter circuit is used to obtain DC power (voltage V) from the DC bus. DC ), and convert it to "high-level voltage (V) DC ), intermediate level voltage (e.g., V) DC / 2), a DC current with alternating low-level voltage (0) and high-level voltage (0). The three levels of DC current alternate and act on the DC / AC converter circuit. Specifically, they act on some of the switching transistors in the DC / AC converter circuit, reducing the voltage that these transistors withstand when they are turned off. This significantly reduces the losses of the switching transistors and reduces electromagnetic interference, thereby improving the performance of the DC / AC converter circuit so that it can provide higher quality AC power to external loads such as the power grid.

[0043] Optionally, capacitors (e.g., capacitor C1) can be connected between the connection points of switching transistors Q2 and Q3 in the FC-TL converter circuit and between the positive terminal of the DC bus, and capacitors (e.g., capacitor C2) can also be connected between the connection points of switching transistors Q2 and Q3 in the FC-TL converter circuit and between the negative terminal of the DC bus. Capacitors C1 and C2 can be used by the FC-TL converter circuit to provide a more stable intermediate level voltage (e.g., V) to the DC / AC converter circuit. DC / 2).

[0044] The three voltage levels applied to the DC / AC converter circuit reduce the voltage across the switching transistors in the DC / AC converter circuit, including: Since the DC / AC converter circuit draws DC current from the DC bus at a voltage of V... DC (High-level voltage) If no FC-TL converter circuit is set up, the voltage that some switches in the DC / AC converter circuit need to withstand when turned off is V. DC However, the DC / AC converter circuit with an FC-TL converter circuit still draws DC voltage from the DC bus at a voltage of V. DC However, due to the existence of the three voltage levels mentioned above, the voltage that this part of the switching transistor withstands when turned off can be adjusted to be less than V. DC For example, V DC / 2.

[0045] It should be noted here that... Figure 2 This explanation uses the application of an FC-TL converter circuit in a photovoltaic system as an example. In this case, the DC bus provides DC power to the FC-TL converter circuit, and the FC-TL converter circuit outputs DC power after power conversion. In practical applications, the FC-TL converter circuit can also obtain DC power from energy storage devices, convert it, and then output the converted DC power back to the DC bus. For details, please refer to the following... Figure 3 The description.

[0046] like Figure 3 An example of an FC-TL converter circuit applied in an energy storage system is shown. The energy storage system includes an energy storage device and a power converter. The power converter of the energy storage system includes an FC-TL converter circuit and a connected pre-stage DC / DC converter circuit. The pre-stage DC / DC converter circuit is connected to switches Q1 and Q4 in the FC-TL converter circuit via a DC bus. Switches Q3 and Q4 in the FC-TL converter circuit are connected between the positive and negative terminals of the energy storage device.

[0047] When the energy storage system supplies power to the grid, the energy storage device provides DC power to the FC-TL converter circuit. The FC-TL converter circuit converts the DC power from the energy storage device into AC power through the charging and discharging of the flying capacitor, and then outputs the converted DC power to the upstream DC / DC converter circuit via the DC bus. The upstream DC / DC converter circuit converts the DC power again and then transmits it to the energy storage converter to be converted into AC power, which is then transmitted to the grid. At this time, the voltage of the energy storage device is V. BAT By controlling the conduction sequence of switches Q1-Q4, combined with the flying capacitor (C) in the FC-TL converter circuit... fly The FC-TL converter circuit supports alternating generation of DC voltages at various levels (e.g., V) based on the charging and discharging characteristics of the FC-TL converter. BAT 2 V BAT It also provides a pre-stage DC / DC converter circuit connected to the FC-TL converter circuit to provide a more stable DC power supply to the pre-stage DC / DC converter circuit.

[0048] It should be noted here that the aforementioned Figure 2 and Figure 3 The example shown is merely a schematic application of the FC-TL converter circuit. Other applications of the FC-TL converter circuit exist, which will not be elaborated upon here. The three voltage levels mentioned above (e.g., V...) DC V DC / 2、0) are just examples. In reality, there are other combinations of input and output of the FC-TL converter circuit. The specific combination depends on the charging and discharging state of the flying capacitor in the FC-TL converter circuit and the conduction sequence of the four switching transistors (switching transistors Q1-Q4). This application will not elaborate on this.

[0049] It should be noted that all switches in the FC-TL converter circuit are connected to a controller. The controller is used to control the alternating conduction of the switches in the FC-TL converter circuit, enabling the FC-TL converter to perform power conversion. The voltage supplied by the FC-TL converter circuit during power conversion is controlled by the conduction sequence of the switches in the FC-TL converter circuit.

[0050] When an FC-TL converter performs power conversion through partially conducting switching transistors, if the voltage across the flying capacitor is too low (e.g., close to 0V), it will not reach the safe voltage. This will cause the flying capacitor to exhibit a low-resistance state, equivalent to a short circuit. If the input voltage or input current to the FC-TL converter is too high, it will cause the off-state switching transistor to withstand the full input voltage and break down, or cause the switching transistor and / or the flying capacitor to be damaged due to overcurrent.

[0051] like Figure 4As shown, switches Q1 and Q3 are turned on, while switches Q2 and Q4 are turned off. The input current (which can be DC from the DC bus) in the FC-TL converter circuit can flow through switches Q1 and the flying capacitor (C). fly The input current flows through the circuit containing Q1, Q3, and Q4, as well as the circuit containing Q3. If the input current is too large at this time and the flying capacitor is equivalent to a short circuit, it will cause Q1, Q3, or the flying capacitor to be damaged due to overcurrent. Alternatively, if the input current continues to flow through the circuit containing Q1, the flying capacitor, and Q3, the turn-off voltage V of Q4 will be increased. Q4 =V DC -V Cfly Among them, V DC V is the voltage of the DC bus connected to the FC-TL converter circuit. Cfly This is the voltage across the flying capacitor. Because V Cfly The voltage is too low (e.g., close to 0V), causing V Q4 equals V DC In V DC Exceeding the rated voltage of switch Q4 will directly cause it to break down due to excessive voltage. Generally speaking, the higher the rated voltage of a switch, the higher its cost. In practice, when FC-TL converter circuits are used in power converters, to save on switch costs, the rated voltage of the switches in the FC-TL converter circuit is usually slightly higher than Vin / 2. This allows the turn-off voltage of the switch to be lower than its rated voltage when the voltage across the flying capacitor is within a safe range. Here, Vin is the maximum voltage input to the FC-TL converter circuit (a value determined based on the application scenario, for example, it could be V0). DC However, when the flying capacitor is equivalent to a short circuit and V DC When the voltage exceeds Vin / 2, the switching transistor Q4 will be damaged due to excessively high voltage during turn-off, seriously threatening the safety of the device.

[0052] To address the safety issues arising from the flying capacitor's equivalent short circuit during power conversion in FC-TL converter circuits, such as... Figures 2-4 As shown in the figure, this application provides a power converter in which a current-limiting circuit is incorporated into the FC-TL conversion circuit. By reusing the original switching transistors in the FC-TL conversion circuit and combining them with the current-limiting circuit, the flying capacitor is pre-charged before the FC-TL conversion circuit performs power conversion. This effectively overcomes the safety problem caused by the flying capacitor being equivalent to a short circuit during power conversion in the FC-TL conversion circuit. The flying capacitor is pre-charged to a voltage greater than or equal to a voltage threshold (denoted as Vth) before power conversion can proceed through the FC-TL conversion circuit. Where Vth = V DC -Vr1. V DCVr1 (or the first rated voltage) is the voltage of the DC bus, and Vr1 is the minimum rated voltage between the rated voltages of switching transistors Q1 and Q4 in the FC-TL converter circuit. It should be noted that the rated voltages of the four switching transistors in the FC-TL converter circuit can be the same or different.

[0053] In this way, when the FC-TL converter performs power conversion, the switching transistor in the FC-TL converter will not experience excessively high voltage when turned off due to the low voltage across the flying capacitor.

[0054] Meanwhile, since switching transistors Q2 and Q3 are connected across the flying capacitor, the voltage across them when they are turned off is equal to the voltage across the flying capacitor (V). Cfly To prevent overcharging of the flying capacitor from causing overvoltage in switching transistors Q2 and Q3 during turn-off, the conditions for the flying capacitor to complete pre-charging, in addition to the aforementioned pre-charging voltage being greater than or equal to the voltage threshold, may also include a voltage less than or equal to Vr2 (or the second rated voltage). Vr2 is the lower of the rated voltages of switching transistors Q2 and Q3.

[0055] Because the voltage (denoted as Vb) between the connection point of switching transistors Q2 and Q3 and the negative terminal of the DC bus needs to be greater than or equal to the voltage of the flying capacitor in order to drive current to flow through the flying capacitor and charge it, Vb must also be greater than or equal to the voltage threshold (Vth) in order for the flying capacitor to be pre-charged. The relevant details regarding the conditions for ending the pre-charging of the flying capacitor can be found in Table 1 below, and will not be elaborated here. It should be noted that when performing power conversion in an FC-TL converter circuit, it is not necessary to limit Vb to be greater than or equal to the voltage threshold (Vth).

[0056] Based on the foregoing, this application provides a power converter. The power converter includes an FC-TL conversion circuit, a DC bus, and a controller. For example... Figure 5 As shown, the FC-TL converter circuit includes a current limiting circuit and a flying capacitor (C). fly The switching transistors are Q1 (or the first switching transistor), Q2 (or the second switching transistor), Q3 (or the third switching transistor), and Q4 (or the fourth switching transistor).

[0057] Switches Q1, Q2, Q3, and Q4 are connected in series between the positive and negative terminals of the DC bus.

[0058] One end (terminal A1, or the first end) and the other end (terminal A2, or the second end) of the flying capacitor are connected to the connection points of switching transistors Q1 and Q2, and the connection points of switching transistors Q3 and Q4, respectively.

[0059] like Figure 5 As shown in (1), one end of the flying capacitor (terminal A1) is also connected to the positive terminal of the DC bus (the terminal marked BUS+) through a current limiting circuit, or, as shown in Figure 5 As shown in (2), the other end of the flying capacitor (terminal A2) is also connected to the negative terminal of the DC bus (the terminal marked BUS﹣) through a current limiting circuit.

[0060] The controller is used to control the FC-TL converter circuit to perform power conversion after the flying capacitor is pre-charged to a voltage greater than or equal to a voltage threshold (Vth). The voltage threshold is equal to the DC bus voltage minus a first rated voltage, which is the minimum rated voltage between the rated voltages of switching transistors Q1 and Q4.

[0061] The controller is also used to prevent the FC-TL converter circuit from performing power conversion when the voltage across the flying capacitor is less than a voltage threshold.

[0062] Specifically, the controller is used to control the switching transistors in the FC-TL converter circuit to alternately turn on, so that the FC-TL converter circuit can perform power conversion. Conversely, it controls all the switching transistors in the FC-TL converter circuit to turn off, so that the FC-TL converter circuit does not perform power conversion.

[0063] For exemplary applications of FC-TL converter circuits in power converters, please refer to the aforementioned section. Figure 2 and Figure 3 The description will not be repeated here.

[0064] The aforementioned conditions for ending pre-charge (including pre-charging the flying capacitor to a voltage greater than or equal to the voltage threshold) are to ensure that, during power conversion, the voltage across the switching transistors in the FC-TL converter circuit does not exceed their rated voltage when turned off, and to prevent overcurrent flowing through the flying capacitor and the conducting switching transistors. For details on this part, please refer to Table 1 and... Figure 6 The description.

[0065] Table 1 illustrates the on-time and flying capacitor (C) of the FC-TL converter circuit during power conversion. fly The charging and discharging states of the DC bus and the voltage of the DC bus (denoted as V). DC The relative magnitudes of Vb (the voltage between the connection point of switching transistors Q2 and Q3 and the negative terminal of the DC bus) and the voltage that the switched transistors withstand when turned off.

[0066] Table 1

[0067] Based on the information in the last column of Table 1, we can conclude that under various turn-on sequences, the voltage across the turn-off switch must be less than or equal to the switch's rated voltage (Vr). Therefore, the following conditions must be met: V Cfly ≤Vr(Q3) and V Cfly ≤Vr(Q2), V DC V Cfly ≤Vr(Q1) ​​and V DC -V Cfly ≤Vr(Q4). Where Vr(Qi) is the rated voltage of the switching transistor Qi, and i is equal to 1, 2, 3, or 4.

[0068] By V DC -V Cfly ≤Vr(Q1) ​​and V DC -V Cfly From ≤Vr(Q4), we can conclude that in order to simultaneously protect both switching transistors Q1 and Q4 from overvoltage due to the flying capacitor effectively acting as a short circuit, V needs to be such that... DC -V Cfly ≤min(Vr(Q1), Vr(Q4)). That is, V Cfly ≥V DC - min(Vr(Q1), Vr(Q4)). Where V DC - min(Vr(Q1), Vr(Q4)) is the aforementioned voltage threshold (Vth).

[0069] To protect switching transistors Q2 and Q3 from overvoltage due to overcharging of the flying capacitor, V needs to be made... Cfly ≤min(Vr(Q2), Vr(Q3)). min(Vr(Q2), Vr(Q3)) is the aforementioned Vr2 (second voltage threshold).

[0070] Therefore, to ensure that the voltage across the switching transistors in the FC-TL converter circuit does not exceed their rated voltage when turned off during power conversion, the controller needs to precharge the flying capacitor until the voltage is greater than or equal to the voltage threshold (Vth) and less than or equal to the aforementioned Vr2.

[0071] This means that once the flying capacitor is pre-charged to a voltage greater than or equal to the voltage threshold (Vth), the controller can control the flying capacitor to perform power conversion. This is the "safety baseline" to ensure that the switching transistors do not experience overvoltage due to turn-off during power conversion. If further charging of the flying capacitor is required, the voltage should be stopped when the flying capacitor voltage reaches Vr2. Stopping the charging at Vr2 effectively establishes a "voltage reserve" for the flying capacitor. When the FC-TL converter circuit performs power conversion and requires a large current, the flying capacitor can buffer voltage drops, thereby improving the stability of the FC-TL converter circuit and preventing current distortion caused by voltage drops.

[0072] The following is combined with Figure 6 The conduction sequences 1 to 4 in Table 1 are explained.

[0073] On-time sequence 1, reference Figure 6 In step (1), switches Q1 and Q2 are turned on, while switches Q3 and Q4 are turned off. Point A and point B are connected via switches Q1 and Q2, therefore the voltage at point B (Vb) is equal to the voltage at point A (V). DC The same applies. Switches Q1 and Q2 do not need to withstand voltage because they are conducting. The voltage across switch Q3 is determined by the flying capacitor (C). fly The voltage across the switch Q3 is determined by the voltage across the switch, therefore the voltage across Q3 when it is turned off is V. Cfly The voltage across switch Q4 is the voltage between points C and D, with point D connected to ground and its voltage at point D being 0. At this time, the voltage at point C is the same as the voltage at point B (Vb = V). DC Subtract the voltage of switching transistor Q3 (V) Cfly Therefore, the voltage at point C is Vb-V. Cfly Equivalent to V DC -V Cfly Therefore, the voltage across switch Q4 when it is turned off is V. DC -V Cfly .

[0074] On-time sequence 2, reference Figure 6 In step (2), switching transistors Q1 and Q3 are turned on, while switching transistors Q2 and Q4 are turned off. Points A and B are connected by switching transistor Q1 and the flying capacitor (C). fly The circuit is open, therefore the voltage at point B (Vb) is equal to V. DC - V Cfly Switches Q1 and Q3 are conducting and do not require voltage. When switch Q2 is turned off, it bears the voltage between points E and B. At this time, the voltage at point E is V. DC The voltage at point B is Vb, therefore the voltage across switch Q2 when it is turned off is V. DC-Vb=V Cfly When switch Q4 is turned off, the voltage it withstands is the voltage between points C and D. Point D is connected to ground, so the voltage at point D is 0. The voltage at point C is equal to the voltage at point B (Vb = V). DC -V Cfly Therefore, at this time, the voltage across the switch Q4 when it is turned off is V. DC -V Cfly .

[0075] On-time sequence 3, reference Figure 6 In step (3), switching transistors Q2 and Q4 are turned on, while switching transistors Q1 and Q3 are turned off. Flying capacitor (C) fly Discharge, the voltage at point B is V Cfly (C) fly The voltage at point A). Switches Q2 and Q4 are conducting and do not need to withstand voltage. When switch Q1 is turned off, the voltage it withstands is the voltage between points A and E. The voltage at point A is V. DC At this moment, the voltage at point E is Vb, therefore the voltage across switch Q1 when it is turned off is V. DC -Vb= V DC -V Cfly When switch Q3 is turned off, the voltage across it is the voltage between points B and C. At this time, the voltage at point C is 0 because switch Q4 is conducting and connected to the reference ground, and the voltage at point B is Vb. Therefore, the voltage across switch Q3 when it is turned off is Vb = V Cfly .

[0076] On-time sequence 4, reference Figure 6 In step (4), switches Q3 and Q4 are turned on, while switches Q1 and Q2 are turned off. Switches Q3 and Q4 do not need to withstand voltage because they are turned on. The voltage across switch Q2 is supplied by the flying capacitor (C). fly The voltage across the switch Q3 is determined by the voltage across the switch, therefore the voltage across Q3 when it is turned off is V. Cfly When switch Q1 is turned off, the voltage it withstands is the voltage between points A and E. The voltage at point A is V. DC At this moment, since the voltage (Vb) at point B is 0, the voltage at point E is V. Cfly The voltage V that the switch Q1 withstands when it is turned off. DC - V Cfly .

[0077] It should be noted here that in Table 1, V DC An example where Vb is the input and Vb is the output can be found in the aforementioned section. Figure 2 The description. At this time, V DC Equal to the aforementioned V DC In Table 1, Vb is the input terminal, V... DC An example of the output end can be found in the aforementioned pair. Figure 3 The description. At this point, Vb equals the V mentioned above. BAT In addition, the FC-TL converter circuit can be applied to other scenarios, which will not be elaborated further in the embodiments of this application.

[0078] It should also be noted that the turn-on sequences shown in Table 1 represent a portion of the turn-on timing involved in the power conversion of the FC-TL converter circuit. If the voltage across the turned-off switch is equal to the turn-off voltage shown in Table 1 under turn-on timings not listed in Table 1, it should also be included within the scope of protection of this application.

[0079] It should also be noted that although the FC-TL converter circuit can operate under various conduction sequences to achieve different power conversions, in practical applications, only some conduction sequences can be selected for power conversion, depending on the design. This application does not limit this. Furthermore, when switching transistors Q3 and Q4 are connected in series between the positive and negative terminals of power supply devices such as capacitors and energy storage devices, an inductor can be connected in series at the connection point between switching transistors Q2 and Q3 to prevent short circuits in the FC-TL converter circuit during power conversion. For example, in conduction sequences 1 and 4 shown in Table 1 above, without an inductor, a short circuit would occur through the circuit containing the conducting switching transistors Q3 and Q4 when the power supply device discharges. Therefore, if it is necessary for the FC-TL converter circuit to operate under conduction sequences 1 and 4 during power conversion, an inductor is connected in series at the connection point between switching transistors Q2 and Q3.

[0080] The foregoing content refers to the pre-charging of the flying capacitor, which includes: supplying electrical energy from the DC bus and / or capacitors, energy storage devices, etc., to the flying capacitor for charging through a current-limiting circuit. Any circuit formed by the flying capacitor and the current-limiting circuit can serve as a charging circuit for the flying capacitor. Some charging circuits (charging circuits) can be referenced below. Figure 7 and Figure 8 The description is as follows. Other charging circuits will not be described in detail in the embodiments of this application.

[0081] like Figure 2 , Figure 3 and Figure 5 As shown, the current limiting circuit can be placed between the flying capacitor and the positive terminal of the DC bus. Alternatively, the current limiting circuit can also be placed between the flying capacitor and the negative terminal of the DC bus. This can also be understood as the flying capacitor being connected in parallel across either switching transistor Q1 or Q4.

[0082] It should also be noted that, Figure 7 and Figure 8 Therefore, the FC-TL converter circuit is used in Figure 2The photovoltaic inverter shown is used as an example for illustration. Optionally, the capacitor (e.g., capacitor C1) connected between the connection point of the switching transistors Q2 and Q3 in the FC-TL converter circuit and the positive terminal of the DC bus, and the capacitor (e.g., capacitor C2) connected between the connection point of the switching transistors Q2 and Q3 in the FC-TL converter circuit and the negative terminal of the DC bus, can also be used to provide power to charge the flying capacitor.

[0083] Figure 7 The diagram shows a circuit used to charge the flying capacitor when the current limiting circuit is positioned between the flying capacitor and the positive terminal of the DC bus.

[0084] like Figure 7 Neutral (1) and Figure 7 As shown in (2), the flying capacitor can be pre-charged using a current-limiting circuit and switch Q2. Electrical energy passes sequentially through the connection point of switches Q2 and Q3, switch Q2, and the flying capacitor (C). fly The current limiting circuit then returns to the negative terminal of the DC bus.

[0085] In some possible cases, such as Figure 7 As shown in (1), the electrical energy flowing through the connection point of switching transistors Q2 and Q3 can be provided by the power supply device connected to the connection point of switching transistors Q2 and Q3 (e.g., Figure 7 (e.g., capacitor C2, or energy storage battery).

[0086] In other possible cases, such as Figure 7 As shown in (2), the electrical energy flowing through the connection point of switching transistors Q2 and Q3 can be provided by the DC bus connected to the DC bus. The electrical energy on the DC bus can be transferred to the connection point of switching transistors Q2 and Q3 through capacitor C1.

[0087] Figure 8 The diagram shows a circuit used to charge the flying capacitor when the current limiting circuit is positioned between the flying capacitor and the negative terminal of the DC bus.

[0088] like Figure 8 Neutral (1) and Figure 8 As shown in (2), the flying capacitor can be pre-charged using electrical energy from the DC bus. The electrical energy passes sequentially through the positive terminal of the DC bus, the current-limiting circuit, and the flying capacitor (C). fly The connection point of the switching transistors Q2 and Q3 returns to the negative terminal of the DC bus.

[0089] In some possible cases, the electrical energy from the DC bus can be provided by a DC bus connected to the DC bus. Specifically, the electrical energy on the DC bus can be provided by the aforementioned photovoltaic modules.

[0090] In other possible cases, such as Figure 7 As shown in (2), the electrical energy from the DC bus can be provided by the DC bus connected to the DC bus. Specifically, the electrical energy on the DC bus can be provided by capacitor C1.

[0091] Optionally, the switching transistor in the FC-TL converter circuit can be a unidirectional switching transistor or a bidirectional switching transistor.

[0092] like Figure 7 or Figure 8 As shown, the switching transistors in the FC-TL converter circuit are bidirectional switching transistors, including: each of the switching transistors (Q1-Q4) in the FC-TL converter circuit has a diode connected in parallel. The anode of the parallel diode is connected to the negative terminal of the DC bus, and the cathode of the parallel diode is connected to the positive terminal of the DC bus.

[0093] When the voltage across the flying capacitor is less than the voltage threshold, all the switches in the FC-TL converter circuit are turned off, and the diodes connected in parallel with some of the switches in the FC-TL converter circuit are used to charge the flying capacitor.

[0094] In this context, the "all switches in the FC-TL converter circuit are off" means that the controller does not need to generate control signals to turn on the switches in the FC-TL converter circuit. The switches in the FC-TL converter circuit are naturally turned off, and the charging circuit for the flying capacitor pre-charging is automatically formed through parallel diodes (without controller control). This simplifies the controller's control logic.

[0095] It should be noted that all the switching transistors (Q1-Q4) in the FC-TL converter circuit have diodes connected in parallel, including either diodes connected in parallel inside the switching transistors or diodes connected in parallel outside the switching transistors.

[0096] Whether the parallel diode is located inside or outside the switching transistor depends on the type of switching transistor. For example, for a MOSFET (a type of switching transistor), the parallel diode is integrated inside the MOSFET, specifically connected in parallel to the source and drain of the MOSFET. Specifically, the cathode of the parallel diode is connected to the drain of the MOSFET, and the anode of the parallel diode is connected to the source of the MOSFET. As another example, for an IGBT (another type of switching transistor), the parallel diode is located outside the IGBT. Specifically, the cathode of the parallel diode is connected to the collector of the IGBT, and the anode of the parallel diode is connected to the emitter of the IGBT.

[0097] For switching transistors with diodes integrated in parallel, switch-off means that the main channel of the switching transistor (e.g., the channel of a MOSFET) is in a high-impedance state, preventing current from flowing. Current is allowed to flow only when the main channel of the switching transistor is in a low-impedance state.

[0098] It should be noted that the current-limiting circuit in the FC-TL converter circuit contains a resistor. This resistor limits the current flowing into the flying capacitor, preventing damage to the flying capacitor and / or the switching transistor in the charging circuit due to overcurrent during pre-charging. The structure of the current-limiting circuit can be referenced below. Figure 9 and Figure 10 The description.

[0099] Figure 9 This diagram illustrates an exemplary structure of a current-limiting circuit when it is positioned between the flying capacitor and the positive terminal of the DC bus. This includes, but is not limited to, the following four structures.

[0100] Current limiting circuit structure 1, reference Figure 9 As shown in Figure (1), the current limiting circuit includes a resistor R. By utilizing the resistance to current, a simple, reliable, and low-cost current-limiting charging circuit is provided for the flying capacitor, effectively suppressing the surge current during pre-charging.

[0101] Current limiting circuit structure 2, reference Figure 9 As shown in Figure (2), the current limiting circuit includes a switch T1 (or first switch) in addition to the resistor. Switch T1 is connected to the resistor. The controller can control switch T1 to turn on when the flying capacitor in the FC-TL converter circuit is in the pre-charging stage, so as to use the resistor to limit the current. When the FC-TL converter circuit is performing power conversion, the controller can control switch T1 to turn off, thereby removing the current limiting resistor from the power path and avoiding the continuous loss of the resistor during power conversion.

[0102] Current limiting circuit structure 3, see reference Figure 9 As shown in (3), the current limiting circuit includes switch T2 (or the second switch). The resistance of switch T2 after being turned on is greater than or equal to the resistance threshold to achieve current limiting. Optionally, the resistance threshold is set by combining the rated current of the DC bus and the safe operating area (SOA) of the switch in the FC-TL converter circuit. The setting principle is to ensure that during pre-charging, the current and voltage in the pre-charging circuit will not cause damage to the flying capacitor and switch in the FC-TL converter circuit.

[0103] The controller can turn on switch T2 when the flying capacitor in the FC-TL converter circuit is in the pre-charging phase, thereby utilizing its resistance to limit current during conduction. During power conversion in the FC-TL converter circuit, the controller can turn off switch T2 to prevent continuous power loss during power conversion.

[0104] Current limiting circuit structure 4, see reference Figure 9As shown in Figure (4), the current limiting circuit includes a diode D, a resistor R, and a capacitor Cs (or buffer capacitor). The diode and the resistor are connected in series, and the buffer capacitor is connected in parallel across the diode and the resistor connected in series.

[0105] It should be noted that the switching types of the switches (switch T1 or switch T2) in the current limiting circuit include, but are not limited to, semiconductor switching transistors, relays, etc.

[0106] It should be noted that when the current-limiting circuit is placed between the flying capacitor and the positive terminal of the DC bus, the anode of the diode in the current-limiting circuit is connected to the positive terminal of the DC bus, and the cathode of the diode is connected to the flying capacitor. In this way, a unidirectional pre-charging circuit can be formed through the diode in the current-limiting circuit to guide the current to charge the flying capacitor.

[0107] It should also be noted that the buffer capacitor in the current limiting circuit is connected in parallel across the switching transistor Q1. It can be used to absorb the voltage spikes generated by the parasitic inductance of the line during the turn-off process of the switching transistor Q1, thereby controlling the voltage that the switching transistor Q1 bears when it is turned off within a safe range and preventing it from being damaged due to overvoltage.

[0108] Figure 10 An exemplary structure of the current limiting circuit is shown when it is positioned between the flying capacitor and the negative terminal of the DC bus. Figure 10 Middle (1) - Figure 10 The current limiting circuit and its related descriptions involved in (3) can be found in the aforementioned sections. Figure 9 Middle (1) - Figure 9 The description in (3) will not be repeated here.

[0109] refer to Figure 10 As shown in Figure (4), the current limiting circuit includes a diode, a resistor, and a buffer capacitor. The diode and the resistor are connected in series, and the buffer capacitor is connected in parallel across the diode and the resistor connected in series.

[0110] It should be noted that when the current-limiting circuit is placed between the flying capacitor and the negative terminal of the DC bus, the anode of the diode in the current-limiting circuit is connected to the flying capacitor, and the cathode of the diode is connected to the cathode of the DC bus. In this way, a unidirectional pre-charging circuit can be formed through the diode in the current-limiting circuit to guide the current to charge the flying capacitor.

[0111] Optionally, once the voltage across the flying capacitor is charged to a level greater than or equal to the voltage threshold, the controller can control the FC-TL converter circuit to perform power conversion.

[0112] It should also be noted that the buffer capacitor in the current limiting circuit is connected in parallel across the switching transistor Q4. It can be used to absorb the voltage spikes generated by the parasitic inductance of the line during the turn-off process of the switching transistor Q4, thereby controlling the voltage that the switching transistor Q4 bears when it is turned off within a safe range and preventing it from being damaged due to overvoltage.

[0113] It should also be noted that the DC bus involved in the embodiments of this application can also be referred to as a DC line, which is used to transmit DC power.

[0114] The various embodiments of this application can be combined arbitrarily to achieve different technical effects.

[0115] In the description of the embodiments of this application, unless otherwise stated, " / " means "or". For example, A / B can mean A or B. The "and / or" in the text is merely a description of the relationship between related objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist simultaneously, and B exists alone. In addition, in the description of the embodiments of this application, "multiple" means two or more.

[0116] In summary, the above description is merely an embodiment of the technical solution of this application and is not intended to limit the scope of protection of this application. Any modifications, equivalent substitutions, improvements, etc., made based on the disclosure of this application should be included within the scope of protection of this application.

Claims

1. A power converter, characterized in that, It includes a DC / DC converter circuit, a DC bus, and a controller. The DC / DC converter circuit includes a current limiting circuit, a flying capacitor, a first switch, a second switch, a third switch, and a fourth switch. The first switch, the second switch, the third switch, and the fourth switch are connected in series between the positive and negative terminals of the DC bus. The first and second ends of the flying capacitor are respectively connected to the connection points of the first and second switching transistors and the connection points of the third and fourth switching transistors. The first terminal of the flying capacitor is also connected to the positive terminal of the DC bus through the current limiting circuit, or the second terminal of the flying capacitor is connected to the negative terminal of the DC bus through the current limiting circuit. The controller is configured to control the DC / DC converter circuit to perform power conversion after the voltage of the flying capacitor is greater than or equal to a voltage threshold; the voltage threshold is equal to the voltage of the DC bus minus a first rated voltage, the first rated voltage being the minimum rated voltage between the rated voltage of the first switch and the rated voltage of the fourth switch.

2. The power converter according to claim 1, characterized in that, The controller is specifically used to control the DC / DC conversion circuit to perform power conversion after the flying capacitor is charged to a voltage greater than or equal to the voltage threshold and less than or equal to the second rated voltage. The second rated voltage is the minimum rated voltage between the rated voltage of the second switch and the rated voltage of the third switch.

3. The power converter according to claim 1 or 2, characterized in that, The current limiting circuit includes a resistor.

4. The power converter according to claim 2, characterized in that, The current limiting circuit also includes a first switch, which is connected to the resistor.

5. The power converter according to claim 1 or 2, characterized in that, The current limiting circuit includes a second switch, the resistance of which is greater than or equal to a resistance threshold after being turned on.

6. The power converter according to claim 4 or 5, characterized in that, The controller is also configured to, when the DC / DC conversion circuit is performing power conversion, control the first switch or the second switch to open.

7. The power converter according to claim 1 or 2, characterized in that, The current limiting circuit includes diodes, resistors, and capacitors; The diode and the resistor are connected in series, and the capacitor is connected in parallel across the diode and the resistor connected in series.

8. The power converter according to claim 7, characterized in that, The first terminal of the flying capacitor is connected to the positive terminal of the DC bus through the current limiting circuit, and the current limiting circuit is connected between the first terminal of the flying capacitor and the positive terminal of the DC bus. The anode of the diode is connected to the positive terminal of the DC bus, and the cathode of the diode is connected to the first terminal of the flying capacitor.

9. The power converter according to claim 7, characterized in that, The second terminal of the flying capacitor is connected to the negative terminal of the DC bus through the current limiting circuit, and the current limiting circuit is connected between the second terminal of the flying capacitor and the negative terminal of the DC bus. The anode of the diode is connected to the second terminal of the flying capacitor, and the cathode of the diode is connected to the negative terminal of the DC bus.

10. The power converter according to any one of claims 1-9, characterized in that, The first switch, the second switch, the third switch, and the fourth switch are all connected in parallel with diodes. The anode of the parallel diodes is connected to the negative terminal of the DC bus, and the cathode of the parallel diodes is connected to the positive terminal of the DC bus. When the voltage of the flying capacitor is less than the voltage threshold, the first switch, the second switch, the third switch, and the fourth switch are all turned off. A diode connected in parallel among some of the first switch, the second switch, the third switch, and the fourth switch is used to charge the flying capacitor. The partial switch includes either the second switch or the third switch.

11. The power converter according to any one of claims 1-10, characterized in that, The power converter also includes a DC / AC conversion circuit and a pre-stage DC / DC conversion circuit for the DC / DC conversion circuit. The front-end DC / DC converter circuit is connected to the first switch and the fourth switch in the DC / DC converter circuit through the DC bus; The DC / AC conversion circuit is connected to the DC bus and the connection point between the second and third switching transistors in the DC / DC conversion circuit.

12. The power converter according to any one of claims 1-10, characterized in that, The power converter includes a pre-stage DC / DC converter circuit of the DC / DC converter circuit; The front-end DC / DC converter circuit is connected to the first switch and the fourth switch in the DC / DC converter circuit through the DC bus; The third and fourth switching transistors in the DC / DC converter circuit are connected in series between the positive and negative terminals of the energy storage device.

13. A photovoltaic system, characterized in that, The photovoltaic system includes a power converter and an energy storage device as described in any one of claims 1-12, wherein the energy storage device is used to provide direct current to the power converter.

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