Multilevel conversion circuit with flying capacitor and its pre-charge method

Abstract

The application provides a multi-level conversion circuit with flying capacitor and a pre-charging method thereof. The multi-level conversion circuit comprises: a first bridge arm comprising a plurality of series-connected switching devices; a second bridge arm comprising a plurality of series-connected switching devices and a flying capacitor group; a midpoint of the two bridge arms is connected to a power supply and an inductor to form a series branch; a DC bus capacitor, to which the two bridge arms are connected in parallel; a first voltage clamping module connected between a first end of the flying capacitor group and a first end of the DC bus capacitor; and a second voltage clamping module connected between a second end of the flying capacitor group and a second end of the DC bus capacitor. The application only needs to add two voltage clamping modules to meet the application requirements of the multi-level conversion circuit, pre-charges the flying capacitor before the normal operation of the multi-level conversion circuit, prevents the overvoltage damage of the switching devices on both sides of the flying capacitor, and ensures the normal operation of the multi-level conversion circuit.

Description

Technical Field

[0001] This invention relates to the field of power electronics, and in particular to a multilevel conversion circuit with a flying capacitor and its pre-charging method. Background Technology

[0002] For the power factor correction (PFC) circuit, the startup sequence is as follows: the power supply is turned on, the DC bus capacitor is charged through the switching device of the PFC circuit, and when the voltage of the DC bus capacitor reaches a certain preset value, the auxiliary power supply connected in parallel with the DC bus capacitor starts to work and supplies power to the controller. After that, the PFC circuit starts to work normally.

[0003] When a PFC circuit employs a multilevel converter circuit with a flying capacitor, a switching device with a low withstand voltage is typically chosen as the main power device. During the power-on phase of the multilevel converter circuit and its charging of the DC bus capacitor, the circuit formed by the flying capacitor, its external switching device, and the DC bus capacitor experiences a voltage drop. Since the flying capacitor's voltage is zero, the DC bus capacitor's voltage is applied to the external switching device, posing a risk of damage. Furthermore, after the multilevel converter circuit begins normal operation, the controller activates the switching devices to charge the flying capacitor. Because the initial voltage of the flying capacitor is zero, when the controller turns on one of the external switching devices, the DC bus capacitor's voltage is entirely applied to the other external switching device, potentially damaging that device. Therefore, when using a flying capacitor multilevel converter circuit, the switching devices outside the flying capacitor must be protected before the controller operates, and the flying capacitor must be pre-charged before the multilevel converter circuit with the flying capacitor operates normally to prevent overvoltage damage to the switching devices outside the flying capacitor and ensure the normal operation of the multilevel converter circuit with the flying capacitor. Summary of the Invention

[0004] The purpose of this invention is to provide a multilevel conversion circuit with a flying capacitor and its pre-charging method, which can solve one or more defects of the prior art.

[0005] To achieve the above objectives, according to an embodiment of the present invention, a multilevel converter circuit with a flying capacitor is provided, comprising: a first bridge arm including a plurality of switching devices connected in series; a second bridge arm including a plurality of switching devices and a flying capacitor bank connected in series, wherein the midpoint of the second bridge arm and the midpoint of the first bridge arm are connected to a power supply and an inductor to form a series branch; a DC bus capacitor, wherein both the first bridge arm and the second bridge arm are connected in parallel to the DC bus capacitor; a first voltage clamping module connected between a first terminal of the flying capacitor bank and a first terminal of the DC bus capacitor; and a second voltage clamping module connected between a second terminal of the flying capacitor bank and a second terminal of the DC bus capacitor.

[0006] In one embodiment of the present invention, the multilevel conversion circuit with a flying capacitor further includes an auxiliary power supply and a controller; the auxiliary power supply is connected to the DC bus capacitor and is used to power the controller; the controller is coupled to the auxiliary power supply and multiple switching devices of the second bridge arm.

[0007] In one embodiment of the present invention, after the multilevel conversion circuit is powered on, the power supply charges the DC bus capacitor through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm.

[0008] In one embodiment of the present invention, the multilevel converter circuit with flying capacitor further includes a third bridge arm, which includes a plurality of switching devices connected in series, and the midpoint of the third bridge arm is connected between the power supply and the inductor.

[0009] In one embodiment of the present invention, after the multilevel conversion circuit is powered on, the power supply charges the DC bus capacitor through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; and / or the power supply charges the DC bus capacitor through the corresponding switching devices of the third bridge arm and the corresponding switching devices of the first bridge arm.

[0010] In one embodiment of the present invention, when the multilevel conversion circuit is powered on, and the voltage between the midpoint of the first bridge arm and the midpoint of the second bridge arm is greater than the clamping voltage of the first voltage clamping module or the second voltage clamping module, the power supply charges the flying capacitor bank through the corresponding switching device of the second bridge arm, the corresponding switching device of the first bridge arm, and the first voltage clamping module or the second voltage clamping module.

[0011] In one embodiment of the present invention, when the voltage of the DC bus capacitor reaches the preset value of the start-up voltage, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset value of the working voltage, the controller operates and controls the corresponding switching device of the second bridge arm to be turned on, and the power supply charges the flying capacitor bank through the corresponding switching device of the first bridge arm and the corresponding switching device of the second bridge arm.

[0012] In one embodiment of the present invention, when the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching device of the second bridge arm to conduct, and the power supply charges the flying capacitor bank through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; or the power supply charges the flying capacitor bank through the corresponding switching devices of the first bridge arm, the corresponding switching devices of the second bridge arm and the corresponding switching devices of the third bridge arm.

[0013] In one embodiment of the present invention, the multilevel conversion circuit with flying capacitor further includes a current limiting circuit connected in series on the series branch.

[0014] In one embodiment of the present invention, the multilevel conversion circuit with a flying capacitor further includes a current limiting circuit connected in series with the DC bus capacitor.

[0015] In one embodiment of the present invention, the current limiting circuit includes a current limiting resistor and a switch, wherein the current limiting resistor and the switch are connected in parallel.

[0016] In one embodiment of the present invention, the multilevel conversion circuit is a 3-level conversion circuit; the second bridge arm includes a first switching device, a second switching device, a third switching device, and a fourth switching device connected in series; the flying capacitor bank includes a flying capacitor, which is connected between the connection point of the first and second switching devices and the connection point of the third and fourth switching devices; the first voltage clamping module includes a first voltage clamping unit; the first voltage clamping unit is connected between the first terminal of the flying capacitor and the first terminal of the DC bus capacitor; the second voltage clamping module includes a second voltage clamping unit; the second voltage clamping unit is connected between the second terminal of the flying capacitor and the second terminal of the DC bus capacitor.

[0017] In one embodiment of the present invention, after the multilevel conversion circuit is powered on, when the voltage between the midpoint of the second bridge arm and the midpoint of the first bridge arm is greater than the clamping voltage of the second voltage clamping unit, the power supply charges the flying capacitor through the corresponding switching device of the second bridge arm, the second voltage clamping unit, and the corresponding switching device of the first bridge arm; or when the voltage between the midpoint of the first bridge arm and the midpoint of the second bridge arm is greater than the clamping voltage of the first voltage clamping unit, the power supply charges the flying capacitor through the corresponding switching device of the first bridge arm, the first voltage clamping unit, and the corresponding switching device of the second bridge arm.

[0018] In one embodiment of the present invention, when the voltage of the DC bus capacitor reaches the preset operating voltage value, the voltage of the flying capacitor is the maximum value among the difference between the preset operating voltage value and the clamping voltage of the first voltage clamping unit, the difference between the preset operating voltage value and the clamping voltage of the second voltage clamping unit, and 0.

[0019] In one embodiment of the present invention, when the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching devices of the second bridge arm to conduct, and the power supply charges the flying capacitor through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; or the power supply charges the flying capacitor through the corresponding switching devices of the first bridge arm, the corresponding switching devices of the second bridge arm and the corresponding switching devices of the third bridge arm. When the voltage of the flying capacitor reaches half of the preset operating voltage value, the controller controls all the switching devices of the second bridge arm to turn off, and the flying capacitor completes charging.

[0020] In one embodiment of the present invention, when the voltage of the DC bus capacitor is lower than the voltage of the flying capacitor, the flying capacitor discharges through the first voltage clamping unit and the second voltage clamping unit, and / or the first switching device and the fourth switching device discharge.

[0021] In one embodiment of the present invention, the first voltage clamping module includes at least one first voltage clamping unit and at least one first impedance circuit; the first impedance circuit and the corresponding first voltage clamping unit are connected in series; the second voltage clamping module includes at least one second voltage clamping unit and at least one second impedance circuit; the second impedance circuit and the corresponding second voltage clamping unit are connected in series.

[0022] In one embodiment of the present invention, the first voltage clamping module includes at least one first voltage clamping unit and at least one first diode, wherein the first diode and the corresponding first voltage clamping unit are connected in series, and the forward conduction direction of the first diode is opposite to the forward conduction direction of the first voltage clamping unit; the second voltage clamping module includes at least one second voltage clamping unit and at least one second diode, wherein the second diode and the corresponding second voltage clamping unit are connected in series, wherein the forward conduction direction of the second diode is opposite to the forward conduction direction of the second voltage clamping unit.

[0023] In one embodiment of the present invention, the multilevel conversion circuit is an n-level conversion circuit; the second bridge arm includes (2n-2) switching devices connected in series; the flying capacitor bank includes (n-2) flying capacitors; the i-th flying capacitor is connected across the i-th switching device and the (2n-1-i)-th switching device of the second bridge arm, where i is 1, 2, 3, 4 … The first voltage clamping module includes (n-2) first voltage clamping units; the first first voltage clamping unit is connected between the first terminal of the first flying capacitor and the first terminal of the DC bus capacitor, and the j-th first voltage clamping unit is connected between the first terminal of the j-th flying capacitor and the first terminal of the (j-1)-th flying capacitor; the second voltage clamping module includes (n-2) second voltage clamping units; the first second voltage clamping unit is connected between the second terminal of the first flying capacitor and the second terminal of the DC bus capacitor, and the j-th second voltage clamping unit is connected between the second terminal of the j-th flying capacitor and the second terminal of the (j-1)-th flying capacitor, where j is 2, 3, 4, …, n-2.

[0024] In one embodiment of the present invention, the first voltage clamping circuit further includes (n-2) first impedance circuits; the second voltage clamping circuit further includes (n-2) second impedance circuits; each first impedance circuit is connected in series with a corresponding first voltage clamping unit; each second impedance circuit is connected in series with a corresponding second voltage clamping unit.

[0025] In one embodiment of the present invention, the first voltage clamping circuit further includes (n-2) first diodes; the second voltage clamping circuit further includes (n-2) second diodes; each first diode is connected in series with a corresponding first voltage clamping unit; and each second diode is connected in series with a corresponding second voltage clamping unit.

[0026] In one embodiment of the present invention, when the multilevel conversion circuit is powered on, and the voltage between the midpoint of the first bridge arm and the midpoint of the second bridge arm is greater than the sum of the clamping voltages of the first to the i-th first voltage clamping units or the sum of the clamping voltages of the first to the i-th second voltage clamping units, the power supply charges the i-th flying capacitor through the corresponding switching device of the second bridge arm, the first to the i-th first voltage clamping unit or the first to the i-th second voltage clamping unit, and the corresponding switching device of the first bridge arm.

[0027] In one embodiment of the present invention, when the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching device of the second bridge arm to conduct, and the power supply charges (n-2) flying capacitors through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; or the power supply charges (n-2) flying capacitors through the corresponding switching devices of the first bridge arm, the corresponding switching devices of the second bridge arm and the corresponding switching devices of the third bridge arm.

[0028] In one embodiment of the present invention, when the voltage of the i-th flying capacitor reaches (n-1-i) / (n-1) times the preset value of the working voltage, the controller controls the corresponding switching device of the second bridge arm to turn off. After (n-2) flying capacitors have completed charging, the controller controls all the switching devices of the second bridge arm to turn off.

[0029] In one embodiment of the present invention, when the voltage of the i-th flying capacitor is lower than the voltage of the (i+1)-th flying capacitor, the (i+1)-th flying capacitor discharges through the (i+1)-th first voltage clamping unit and the (i+1)-th second voltage clamping unit, and / or the (i+1)-th switching device and the (2n-2-i)-th switching device; or, when the voltage of the DC bus capacitor is lower than the voltage of the first flying capacitor, the first flying capacitor discharges through the first first voltage clamping unit and the first second voltage clamping unit, and / or the first switching device and the (2n-2)-th switching device.

[0030] In one embodiment of the present invention, the multilevel conversion circuit is an n-level conversion circuit; the second bridge arm includes (2n-2) switching devices connected in series; the flying capacitor bank includes (n-2) flying capacitors; the i-th flying capacitor is connected across the i-th switching device and the (2n-1-i)-th switching device of the second bridge arm, where i is 1, 2, 3, ..., n-2, and n is a natural number greater than 3; the first voltage clamping module includes (n-2) first voltage clamping units; each first voltage clamping unit is connected between the first terminal of the corresponding flying capacitor and the first terminal of the DC bus capacitor; the second voltage clamping module includes (n-2) second voltage clamping units; each second voltage clamping unit is connected between the second terminal of the corresponding flying capacitor and the second terminal of the DC bus capacitor.

[0031] In one embodiment of the present invention, the multilevel conversion circuit is an n-level conversion circuit; the second bridge arm includes (2n-2) switching devices connected in series; the flying capacitor bank includes (n-2) flying capacitors; the i-th flying capacitor is connected across the i-th switching device and the (2n-1-i)-th switching device of the second bridge arm, where i is 1, 2, 3, ..., n-2, and n is a natural number greater than 3; the first voltage clamping module includes at least one first voltage clamping unit; at least one first voltage clamping unit is connected between the first terminal of the corresponding flying capacitor and the first terminal of the DC bus capacitor; the second voltage clamping module includes at least one second voltage clamping unit; at least one second voltage clamping unit is connected between the second terminal of the corresponding flying capacitor and the second terminal of the DC bus capacitor.

[0032] To achieve the above objectives, the present invention also provides a pre-charging method for a multilevel converter circuit with a flying capacitor. The multilevel converter circuit includes a first bridge arm, a second bridge arm, a DC bus capacitor, a first voltage clamping module, and a second voltage clamping module. The first bridge arm includes multiple switching devices connected in series. The second bridge arm includes multiple switching devices and a flying capacitor bank connected in series. The midpoint of the second bridge arm and the midpoint of the first bridge arm are connected to a power supply and an inductor to form a series branch. Both the first and second bridge arms are connected in parallel to the DC bus capacitor. The first voltage clamping module is connected between the first terminal of the flying capacitor bank and the first terminal of the DC bus capacitor. The second voltage clamping module is connected between the second terminal of the flying capacitor bank and the second terminal of the DC bus capacitor. The pre-charging method includes: after the multilevel converter circuit is powered on, the power supply charges the DC bus capacitor through the corresponding switching devices of the first and second bridge arms.

[0033] In another embodiment of the invention, the multilevel conversion circuit further includes a third bridge arm; the third bridge arm includes a plurality of switching devices connected in series; the midpoint of the third bridge arm connects the power supply and the inductor. When the multilevel conversion circuit is powered on, the power supply charges the DC bus capacitor through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; and / or the power supply charges the DC bus capacitor through the corresponding switching devices of the third bridge arm and the corresponding switching devices of the first bridge arm.

[0034] In another embodiment of the present invention, when the multilevel conversion circuit is powered on, and the voltage between the midpoint of the first bridge arm and the midpoint of the second bridge arm is greater than the clamping voltage of the first voltage clamping module or the second voltage clamping module, the power supply charges the flying capacitor bank through the corresponding switching device of the second bridge arm, the first voltage clamping module or the second voltage clamping module, and the corresponding switching device of the first bridge arm.

[0035] In another embodiment of the present invention, the multilevel conversion circuit further includes an auxiliary power supply and a controller; the auxiliary power supply is connected to the DC bus capacitor and is used to power the controller; the controller is coupled to the auxiliary power supply and multiple switching devices of the second bridge arm. When the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching devices of the second bridge arm to conduct, and the power supply charges the flying capacitor bank through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm.

[0036] In another embodiment of the present invention, the multilevel conversion circuit further includes an auxiliary power supply and a controller; the auxiliary power supply is connected to the DC bus capacitor and is used to power the controller; the controller is coupled to the auxiliary power supply and multiple switching devices of the second bridge arm. When the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching devices of the second bridge arm to conduct, and the power supply charges the flying capacitor bank through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; or the power supply charges the flying capacitor bank through the corresponding switching devices of the first bridge arm, the corresponding switching devices of the second bridge arm, and the corresponding switching devices of the third bridge arm.

[0037] The present invention can meet the application requirements of multilevel conversion circuits with flying capacitors by adding only two voltage clamping units, such as Zener diodes.

[0038] In addition, after the multi-level conversion circuit is powered on, when the DC bus capacitor is charged by the switching devices of the bridge arm, the switching devices outside the flying capacitor can be clamped and protected by a voltage clamping module (e.g., a Zener diode). After the auxiliary power supply is started and supplies power to the controller, the controller can pre-charge the flying capacitor by controlling the operation of the corresponding switching devices in the bridge arm. At this time, the switching devices outside the flying capacitor can still be protected by the voltage clamping module to prevent them from being damaged.

[0039] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0040] The above and other features and advantages of the present invention will become more apparent from a detailed description of exemplary embodiments thereof with reference to the accompanying drawings.

[0041] Figure 1 The topology of a 3-level conversion circuit with a flying capacitor is shown in the first preferred embodiment of the present invention.

[0042] Figure 2A and Figure 2BThey are respectively in Figure 1 In the illustrated embodiment, after the multi-level conversion circuit is powered on, the positive and negative half-cycle DC bus capacitor charging circuits formed (as shown by the dashed lines in the figure) and before the controller starts working, when the voltage V ab A schematic diagram of the positive and negative half-cycle flying capacitor charging circuit formed when the voltage is higher than the clamping voltage of the Zener diode (as shown by the double horizontal lines in the figure);

[0043] Figure 3A and Figure 3B They are respectively in Figure 1 In the embodiment shown, after the controller starts working, when the controller controls the switching device to form a charging circuit for charging, the charging circuit formed by the positive and negative half-cycles is shown as a schematic diagram (as shown by the dotted line in the figure).

[0044] Figure 4 The topology of a 4-level conversion circuit with a flying capacitor is shown in the second preferred embodiment of the present invention.

[0045] Figure 5A and Figure 5B They are respectively in Figure 4 In the illustrated embodiment, before the controller starts operating, when the voltage V ab A schematic diagram of the positive and negative half-cycle flying capacitor charging circuit formed when the voltage is higher than the clamping voltage of the Zener diode D1 or D2 (as shown by the double horizontal lines in the figure).

[0046] Figure 6A and Figure 6B They are respectively in Figure 4 In the embodiment shown, when the voltage V ab A schematic diagram of the charging circuit formed by the positive and negative half-cycles when the voltage is greater than the sum of the clamping voltages of Zener diodes D1 and D3 or D2 and D4 (as shown by the double horizontal lines in the figure).

[0047] Figure 7A and Figure 7B They are respectively in Figure 4 In the embodiment shown, after the controller starts working, when the controller controls the switching device to form a charging circuit for charging, the charging circuit formed by the positive and negative half-cycles is shown as a schematic diagram (as shown by the dotted line in the figure).

[0048] Figure 8 for Figure 4 The illustrated embodiment is a variation;

[0049] Figure 9 The topology of the n-level converter circuit with a flying capacitor is shown in the third preferred embodiment of the present invention.

[0050] Figure 10A for Figure 9The embodiment shown is a schematic diagram of the charging circuit of each flying capacitor during the positive half-cycle (as shown by the dashed line in the figure).

[0051] Figure 10B for Figure 9 The embodiment shown is a schematic diagram of the charging circuit of each flying capacitor during the negative half-cycle (as shown by the dashed line in the figure).

[0052] Figure 11 for Figure 9 The illustrated embodiment is a variation;

[0053] Figure 12 This is a topology of a 3-level conversion circuit with a flying capacitor, according to another preferred embodiment of the present invention;

[0054] Figure 13 This is the topology of a 3-level conversion circuit with a flying capacitor in another preferred embodiment of the present invention;

[0055] Figure 14 This is the topology of a 3-level conversion circuit with a flying capacitor in another preferred embodiment of the present invention;

[0056] Figure 15 This is a schematic flowchart of the pre-charging method for a multilevel conversion circuit with a flying capacitor according to the present invention. Detailed Implementation

[0057] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, they are provided so that the invention will be thorough and complete, and the concept of the exemplary embodiments will be fully conveyed to those skilled in the art. The same reference numerals in the drawings denote the same or similar structures, and therefore their detailed description will be omitted.

[0058] In describing the elements / components / etc. described and / or illustrated herein, the terms “a,” “an,” “the,” “the,” and “at least one” are used to indicate the presence of one or more elements / components / etc. The terms “comprising,” “including,” and “having” are used to indicate an open-ended inclusion and to mean that additional elements / components / etc. may exist in addition to those listed. Relative terms, such as “upper” or “lower,” may be used in the embodiments to describe the relative relationship of one component of the icon to another component. It is understood that if the device of the icon is flipped so that it is upside down, the component described as being on the “upper” side will become the component on the “lower” side. Furthermore, the terms “first,” “second,” etc., in the claims are used only as illustrative marks and are not intended to limit the number of objects to which they apply.

[0059] like Figure 1As shown, it illustrates a preferred circuit topology of a multilevel converter circuit 100 with a flying capacitor according to the present invention. Figure 2A and 2B As shown, they are respectively shown in Figure 1 In the illustrated embodiment, after the multi-level conversion circuit is powered on, the positive and negative half-cycle DC bus capacitor charging circuits formed (as shown by the dashed lines in the figure) and before the controller starts working, when the voltage V ab A schematic diagram of the positive and negative half-cycle flying capacitor charging circuit formed when the voltage is higher than the clamping voltage of the Zener diode (as shown by the double horizontal lines in the figure). Figure 3A and Figure 3B They are respectively in Figure 1 In the illustrated embodiment, after the controller starts working, when the controller controls the switching devices to form a charging circuit for charging, a schematic diagram of the positive and negative half-cycle charging circuits is shown (as shown by the dotted lines in the figure). It should be noted that in... Figure 1 The illustrated embodiment uses a 3-level conversion circuit as an example. This 3-level conversion circuit can be a power factor correction (PFC) circuit, but the invention is not limited thereto. In this invention, the multi-level conversion circuit 100 may include a first bridge arm 10, a second bridge arm 20, a DC bus capacitor 30, a first voltage clamping module 40, and a second voltage clamping module 50. The DC bus capacitor 30 may include a single capacitor C. p Alternatively, multiple capacitors may be used, depending on the specific requirements. The first bridge arm 10 may include multiple switching devices 11 connected in series, wherein the switching device 11 may include a controllable switch, a diode, or a controllable switch and a diode connected in antiparallel to the controllable switch. The second bridge arm 20 may include multiple switching devices 21 connected in series and a flying capacitor bank 22. In this embodiment, the switching device 21 may include a controllable switch and a diode connected in antiparallel to the controllable switch, wherein the controllable switch and the diode are two separate components, or the controllable switch and the diode are integrated components, or the diode is the body diode of the controllable switch. The midpoint a of the second bridge arm 20 and the midpoint b of the first bridge arm 10 are connected to the power supply V. ac The inductor L forms a series branch 102. Both the first bridge arm 10 and the second bridge arm 20 are connected in parallel to the DC bus capacitor C. p The first voltage clamping module 40 is connected to the first terminal of the flying capacitor bank 22 and the DC bus capacitor C. p Between the first terminals. The second voltage clamping module 50 is connected to the second terminal of the flying capacitor bank 22 and the DC bus capacitor C. p Between the second and third ends.

[0060] exist Figure 1In the illustrated embodiment, the first bridge arm 10 includes a series-connected switching device S a and switching device S b Among them, the switching device S a and S b It can be a slow switch. The second bridge arm 20 includes a first switching device S4, a second switching device S2, a third switching device S1, and a fourth switching device S3 connected in series, wherein the switching devices S1-S4 can be fast switches, such as Si, GaN, or SiC MOSFETs. The flying capacitor bank 22 includes a flying capacitor C. fly1 The first voltage clamping module 40 includes a first voltage clamping unit 41, such as a Zener diode D1, connected to the flying capacitor C. fly1 The first terminal is connected to the DC bus capacitor C p Between the first terminals. The second voltage clamping module 50 includes a second voltage clamping unit 51, such as a Zener diode D2, connected to the flying capacitor C. fly1 The second terminal is connected to the DC bus capacitor C p Between the second terminals. In some embodiments, the first voltage clamping unit 41 is used to clamp the voltage across the first switching device S4 to prevent the first switching device S4 from being damaged by overvoltage; and the second voltage clamping unit 51 is used to clamp the voltage across the fourth switching device S3 to prevent the fourth switching device S3 from being damaged by overvoltage.

[0061] like Figure 1 As shown, the multilevel conversion circuit 100 may further include an auxiliary power supply 60 and a controller (not shown). The auxiliary power supply 60 may be connected to the DC bus capacitor C. p The controller is used to power the controller. The controller can be coupled to the auxiliary power supply 60 and multiple switching devices 21 of the second bridge arm 20. Specifically, the controller is coupled to switching devices S1-S4 of the second bridge arm 20 and is used to control the switching devices S1-S4 to be turned on or off. When the switching device 11 includes a controllable switch, the controller can also be used to turn multiple switching devices 11 on or off. Specifically, the controller can also control the switching devices S1-S4 of the first bridge arm 10. a and S b To turn on or off.

[0062] In some embodiments, the controller is used to control the corresponding controllable switches on the first and second bridge arms to turn on or off. In some embodiments, before the controller is started, power is supplied to the flying capacitor and the DC bus capacitor via corresponding diodes on the first and second bridge arms.

[0063] like Figure 1 , 2AAs shown in 2B, after the multi-level conversion circuit 100 is powered on, the power supply V ac The DC bus capacitor C can be supplied through the corresponding switching device 11 of the first bridge arm 10 and the corresponding switching device 21 of the second bridge arm 20. p Charging. Specifically, after the multi-level conversion circuit 100 is powered on and the power supply V... ac During the positive half-cycle, the power supply V ac Switching devices S2 and S4, as well as switching device S b DC bus capacitor C p Charging; after the multi-level conversion circuit 100 is powered on and the power supply V ac During the negative half-cycle, the power supply V ac Switching devices S1 and S3, as well as switching device S a DC bus capacitor C p Charge.

[0064] like Figure 1 , 2A As shown in Figure 2B, the multilevel conversion circuit 100 may further include a third bridge arm 80. The third bridge arm 80 may include a plurality of switching devices 81 connected in series, for example in… Figure 1 The diode D shown in the embodiment is s1 and D s2 The midpoint of the third bridge arm 80 is connected to the power supply V. ac Between the inductor L and the power supply V. Thus, when the multi-level conversion circuit 100 is powered on, the power supply V... ac The DC bus capacitor C can be supplied through the corresponding switching device 11 of the first bridge arm 10 and the corresponding switching device 21 of the second bridge arm 20. p Charging; and / or, the power source V ac The DC bus capacitor C can also be supplied with power through the corresponding switching device 81 of the third bridge arm 80 and the corresponding switching device 11 of the first bridge arm 10. p Charging. In some embodiments, after the multilevel conversion circuit 100 is powered on and the power supply V... ac During the positive half-cycle, the power supply V ac Inductor L, switching devices S2 and S4, switching device S b and the DC bus capacitor C p This constitutes a charging circuit, namely the power supply V. ac It can be achieved through inductor L, switching devices S2 and S4, and switching device S b The DC bus capacitor C p Charging; and / or, the power source V ac Diode D s1Switching device S b and the DC bus capacitor C p It can also form a charging circuit, that is, the power supply V ac Through diode D s1 and switching device S b The DC bus capacitor C p Charging. After the multi-level conversion circuit 100 is powered on and the power supply V... ac During the negative half-cycle, the power supply V ac Inductor L, switching devices S1 and S3, switching device S a and the DC bus capacitor C p This constitutes a charging circuit, namely the power supply V. ac It can be achieved through inductor L, switching devices S1 and S3, and switching device S a The DC bus capacitor C p Charging; and / or, the power source V ac Diode D s2 Switching device S a and the DC bus capacitor C p It can also form a charging circuit, namely the power supply V ac It can also be achieved through diode D s2 and switching device S a The DC bus capacitor C p Charging. It is understood that in this embodiment, after the multi-level conversion circuit 100 is powered on, the selection of the charging circuit is related to the voltage drop within the charging circuit, for example, diode D. s1 and D s2 The voltage drop is 2V, while the voltage drop of the diode in the switching device is 0.7V. Therefore, when the multilevel conversion circuit 100 is powered on, it will select the charging circuit composed of the corresponding switching device of the first bridge arm 10 and the corresponding switching device of the second bridge arm 20. Then, when the switching device of the second bridge arm 20 is GaN, since the voltage drop of the GaN equivalent body diode is related to the magnitude of the current flowing through it, when the multilevel conversion circuit 100 is powered on, it will select the charging circuit composed of the corresponding switching device of the first bridge arm 10 and the corresponding switching device of the third bridge arm 80.

[0065] like Figure 1 , 2A As shown in 2B, when the multi-level conversion circuit 100 is powered on, and the voltage between the midpoint b of the first bridge arm 10 and the midpoint a of the second bridge arm 20 is greater than the clamping voltage of the first voltage clamping module 40 or the second voltage clamping module 50, the power supply V acThe voltage can be supplied to the flying capacitor bank 22 (e.g., flying capacitor C) via the corresponding switching device 21 of the second bridge arm 20, the first voltage clamping module 40 or the second voltage clamping module 50, and the corresponding switching device 11 of the first bridge arm 10. fly1 Charging. In this embodiment, after the multi-level conversion circuit 100 is powered on and the power supply V... ac During the positive half-cycle, if the voltage between the midpoint a of the second bridge arm 20 and the midpoint b of the first bridge arm 10 is greater than the clamping voltage of the second voltage clamping module 50, such as the clamping voltage of the Zener diode D2, the power supply V ac Inductor L, Switching device S2, Flying capacitor C fly1 Zener diode D2 and switching device S b This forms a charging circuit, i.e., the power supply V ac Through inductor L, switching device S2, Zener diode D2, and switching device S b Give the flying capacitor C fly1 Charging. In this embodiment, after the multi-level conversion circuit 100 is powered on and the power supply V... ac During the negative half-cycle, if the voltage between the midpoint b of the first bridge arm 10 and the midpoint a of the second bridge arm 20 is greater than the clamping voltage of the first voltage clamping module 40, such as the clamping voltage of the Zener diode D1, the power supply V ac Inductor L, switching device S1, flying capacitor C fly1 Zener diode D1 and switching device S a This forms a charging circuit, i.e., the power supply V ac Through inductor L, switching device S1, Zener diode D1, and switching device S a Give the flying capacitor C fly1 Charge.

[0066] like Figure 1 , 3A And as shown in 3B, when the DC bus capacitor C p When the auxiliary power supply 60 is charged to a voltage that reaches the preset startup voltage value, it starts to supply power to the controller. When the DC bus capacitor C... p As charging continues until its voltage reaches the preset operating voltage value, the controller starts working and controls the corresponding switching device 21 of the second bridge arm 20 to turn on, and the power supply V... ac The flying capacitor bank 22 (e.g., flying capacitor C) can be supplied with power through the corresponding switching device 11 of the first bridge arm 10 and the corresponding switching device 21 of the second bridge arm 20. fly1 Charging. In this embodiment, when the power supply V... acDuring the positive half-cycle, the controller controls switch S3 to conduct, and controls switch S2 and / or S... b The power supply V is not conducting. ac Inductor L, Switching device S2, Flying capacitor C fly1 Switching device S3 and switching device S b This constitutes a charging circuit, namely the power supply V. ac Through inductor L, switching device S2, switching device S3 and switching device S b Give the flying capacitor C fly1 Charging. In other embodiments, when the power source V... ac During the positive half-cycle, the controller controls switching devices S3, S2, and S... b All are on. In this embodiment, when the power supply V... ac During the negative half-cycle, the controller controls switching device S4 to conduct, and controls switching devices S1 and / or S... a The power supply V is not conducting. ac Inductor L, Switching device S4, Flying capacitor C fly1 Switching device S1 and switching device S a This constitutes a charging circuit, namely the power supply V. ac Through inductor L, switching device S4, switching device S1 and switching device S a Give the flying capacitor C fly1 Charging. In other embodiments, when the power source V... ac During the negative half-cycle, the controller controls the switching devices S4, S1, and S... a All are conducting.

[0067] In some embodiments, the power supply V ac The flying capacitor bank 22 (e.g., flying capacitor C) can be supplied with power through the corresponding switching device 11 of the first bridge arm 10, the corresponding switching device 21 of the second bridge arm 20, and the corresponding switching device 81 of the third bridge arm 80. fly1 Charging. When the power source V ac During the positive half-cycle, the controller controls the switching devices S4 and S3 to conduct, and the power supply V... ac Switching device D s1 Switching device S4, flying capacitor C fly1 Switching device S3 and switching device S b This constitutes a charging circuit, namely the power supply V. ac Through switching device D s1 Switching device S4, switching device S3 and switching device S b Give the flying capacitor C fly1 Charging. When the power source V acDuring the negative half-cycle, the controller controls switching devices S4 and S3 to conduct, and the power supply V... ac Switching device D s2 Switching device S4, flying capacitor C fly1 Switching device S3 and switching device S a This constitutes a charging circuit, namely the power supply V. ac Through switching device D s2 Switching device S4, switching device S3 and switching device S a Give the flying capacitor C fly1 Charge.

[0068] In some embodiments, the preset startup voltage value may differ from the preset operating voltage value; for example, the preset startup voltage value may be less than the preset operating voltage value. In this case, the auxiliary power supply 60 is started, and the controller can respond, but the controller does not operate immediately.

[0069] like Figure 1 As shown, the multilevel conversion circuit 100 may further include a current limiting circuit 71, connected in series with the series branch 102. The current limiting circuit 71 may, for example, include a current limiting resistor R. s With switch RL, the current-limiting resistor R s It is connected in parallel with the switch RL.

[0070] like Figure 12 As shown, this illustrates another preferred circuit topology of the multilevel converter circuit with a flying capacitor according to the present invention. The multilevel converter circuit 100 may further include components connected to the DC bus capacitor C. p A current-limiting circuit 72 connected in series. The current-limiting circuit 72 may include, for example, a current-limiting resistor R. s1 With switch RL1, the current-limiting resistor R s1 It is connected in parallel with the switch RL1.

[0071] like Figure 1 and 12 As shown, at the beginning of the multi-level conversion circuit, the DC bus capacitor C is supplied with... p Before charging, switches RL and RL1 are in the off state; and after the flying capacitor 22 of the multi-level conversion circuit has finished charging, the controller controls switches RL and RL1 to turn on.

[0072] like Figure 1 , 2AAs shown in Figure 2B, when the multi-level conversion circuit 100 is powered on, and when the voltage between the midpoint b of the first bridge arm 10 and the midpoint a of the second bridge arm 20 is greater than the clamping voltage of the first voltage clamping unit 41 (e.g., Zener diode D1) or the second voltage clamping unit 51 (e.g., Zener diode D2), the power supply V... ac The flying capacitor C can be supplied with voltage through the corresponding switching device 21 of the second bridge arm 20, the first voltage clamping unit 41 or the second voltage clamping unit 51, and the corresponding switching device 11 of the first bridge arm 10. fly1 Charge.

[0073] When the DC bus capacitor C p When the voltage reaches the preset operating voltage value, the flying capacitor C fly1 The voltage is the maximum value among the following: the difference between the preset operating voltage and the clamping voltage of the first voltage clamping unit 41 (e.g., Zener diode D1), the difference between the preset operating voltage and the clamping voltage of the second voltage clamping unit 51 (e.g., Zener diode D2), and 0.

[0074] When the DC bus capacitor C p When the voltage reaches the preset starting voltage value, the auxiliary power supply 60 starts to supply power to the controller; when the DC bus capacitor C... p When the voltage reaches the preset operating voltage value, the controller operates and controls the corresponding switching device 21 of the second bridge arm 20 to turn on, and the power supply V... ac The flying capacitor C can be supplied with power through the corresponding switching device 11 of the first bridge arm 10 and the corresponding switching device 21 of the second bridge arm 20. fly1 Charging. Or, the power source V ac The flying capacitor C can be supplied with power through the corresponding switching device 11 of the first bridge arm 10, the corresponding switching device 21 of the second bridge arm 20, and the corresponding switching device 81 of the third bridge arm 80. fly1 Charging, when the flying capacitor C fly1 When the voltage reaches half of the preset operating voltage value, the controller controls all the switching devices 21 of the second bridge arm 20 to turn off, and the flying capacitor C fly1 Charging complete.

[0075] When the DC bus capacitor C p The voltage is lower than that of the flying capacitor C fly1 When the voltage is [value], the flying capacitor C fly1 The voltage can be discharged through the first voltage clamping unit 41 (e.g., Zener diode D1) and the second voltage clamping unit 51 (e.g., Zener diode D2), and / or, the flying capacitor Cfly1 Discharge can be achieved through the first switching device S4 and the fourth switching device S3.

[0076] The following will combine Figure 1 The three-level conversion circuit and its pre-charging method are shown in the figure. The principle of the present invention is described in detail and explained.

[0077] like Figure 1 , 2A As shown in Figure 2B, a 3-level converter circuit with a flying capacitor is illustrated. After the converter circuit is powered on, the DC bus capacitor C... p Power supply V ac Switching devices (e.g., diode D) via the third bridge arm 80 s1 and D s2 The corresponding switching device 11 on the first bridge arm 10 and the corresponding switching device 12 on the second bridge arm 20 are charged to establish a DC bus voltage, and the auxiliary power supply 60 is powered by this DC bus voltage.

[0078] Before the controller starts working, if the input voltage V ab A clamping voltage higher than that of the voltage clamping unit (e.g., Zener diodes D1 and D2) constitutes... Figure 2A , 2B The double horizontal lines in the middle indicate the positive and negative half-cycle flying capacitor charging circuit. When the power supply V... ac During the positive half-cycle, the power supply V ac Through the current-limiting resistor R s Inductor L, switching device S2, switching device S3 and switching device S b For flying capacitor C fly1 Charging begins; when the power supply V... ac During the negative half-cycle, the power supply V ac Through the current-limiting resistor R s Inductor L, Switching device S a Switching devices S4 and S1 are paired with the flying capacitor C. fly1 Charging begins. When the power supply V... ac During the positive and negative half-cycles, the flying capacitor C fly1 voltage V Cfly1 As shown in equations (1) and (2):

[0079] Positive half-cycle: V Cfly1 =V ab -V D2 (V ab >V D2 (1)

[0080] V Cfly1 =0(V ab <=V D2 )

[0081] Negative half-cycle: V Cfly1 =V ab -V D1 (V ab >V D1 (2)

[0082] V Cfly1 =0(V ab <=V D1 )

[0083] Where V Cfly1 For the flying capacitor C fly1 Voltage; V ab V is the voltage between the midpoint a of the second bridge arm 20 and the midpoint b of the first bridge arm 10. D2 V is the clamping voltage of the second voltage clamping module 51; D1 The clamping voltage of the first voltage clamping unit 41 is .

[0084] In this process, the DC bus capacitor C p voltage V Cp The voltage will stabilize at a preset operating voltage value, for example, the voltage V between the midpoint a of the second bridge arm 20 and the midpoint b of the first bridge arm 10. ab peak V ab_peak And the flying capacitor C fly1 Voltage V on Cfly1 The voltage will be stabilized at the difference between the preset operating voltage and the clamping voltage of the second voltage clamping unit 51 (V). ab_peak -V D2 The difference between the preset operating voltage and the clamping voltage of the first voltage clamping unit 41 (V) ab_peak -V D1 The maximum value in ) and 0V.

[0085] With the DC bus capacitor C p voltage V Cp Once stable, the controller starts and begins operation, controlling the corresponding switching devices to form a charging circuit. The charging circuit for both positive and negative half-cycles is as follows: Figure 3A , 3B As shown by the dashed line. Typically, the clamping voltage V of a voltage clamping unit (such as Zener diodes D1 and D2) is... D It should be slightly less than the maximum voltage stress V that the selected switching device in the second bridge arm 20 can withstand. BRDSS Therefore, after the controller starts, it controls the flying capacitor C. fly1 When the outer switching device S4 is turned on, it will not cause overvoltage damage to the switching device S3, nor will it control the flying capacitor C. fly1 When the outer switching device S3 is turned on, it will not cause overvoltage damage to the switching device S4.

[0086] like Figure 3A As shown, at power supply V ac During the positive half-cycle, the switching device S b Together with S2 and S3, they form a flying capacitor C. fly1 The charging circuit, in which the controller controls the switching device S3 to conduct, and the power supply V ac After passing through the current-limiting resistor R s Inductor L, Switching device S2, Switching device S3, Switching device S b For the flying capacitor C fly1 Charging begins. At this time, V... S4 +V Cfly1 =V Cp , where V Cp V is the voltage across the DC bus capacitor. S4 This is the voltage across the switching device S4. If there were no Zener diode D1, then V... S4 =V Cp It is highly likely that the voltage will exceed the withstand voltage of switching device S4, causing damage to switching device S4. Connecting a Zener diode D1 in parallel across switching device S4 can prevent it from breaking down. When the voltage V between the midpoint b of the first bridge arm and the midpoint a of the second bridge arm... ab The voltages V of the Zener diodes D1 and D2 are respectively greater than the clamping voltages V of the Zener diodes. D1 and V D2 At that time, the flying capacitor C fly1 voltage V Cfly1 For (V) ab - V D1 ) and (V ab - V D2 The larger of the two. Through the flying capacitor C fly1 The Zener diode D1 can prevent the switching device S4 from being damaged.

[0087] like Figure 3B As shown, at power supply V ac During the negative half-cycle, the switching device S a Together with S1 and S4, they form a flying capacitor C. fly1 The charging circuit, in which the controller controls the switching device S4 to conduct, and the power supply V ac After the switching device S a Switching device S4, switching device S1, inductor L, current-limiting resistor R s For the flying capacitor C fly1 Charging. V S3 +V Cfly1 =V Cp , where V S3 This is the voltage across the switching device S3. If there were no Zener diode D2, then V...S3 =V Cp It is highly likely that the voltage will exceed the withstand voltage of switching device S3, causing damage to switching device S3. Connecting a Zener diode D2 in parallel across switching device S3 can prevent it from breaking down. When the voltage V between the midpoint b of the first bridge arm and the midpoint a of the second bridge arm... ab The voltages V of the Zener diodes D1 and D2 are respectively greater than the clamping voltages V of the Zener diodes. D1 and V D2 At that time, the flying capacitor C fly1 voltage V Cfly1 For (V) ab - V D1 ) and (V ab - V D2 The larger of the two. Through the flying capacitor C fly1 The Zener diode D2 can prevent the switching device S3 from being broken down.

[0088] In this embodiment, when the flying capacitor C fly1 voltage V Cfly1 Reaching DC bus capacitance C p voltage V Cp At / 2, all switching devices are turned off, and the flying capacitor C fly1 Pre-charging is complete.

[0089] In this embodiment, when the multilevel conversion circuit is powered down, due to the forward voltage drop of the diode, the DC bus capacitor C... p voltage V Cp It will drop until it falls below the flying capacitor C. fly1 voltage V Cfly1 At this time, the flying capacitor C fly1 By connecting in series with the flying capacitor C fly1 The voltage clamping units on both sides are discharging.

[0090] In other embodiments, the DC bus capacitor C p voltage V Cp It will drop until it falls below the flying capacitor C. fly1 voltage V Cfly1 At this time, the flying capacitor C fly1 Discharge is achieved through switching devices S4 and S3.

[0091] In other embodiments, the DC bus capacitor C p voltage V Cp It will drop until it falls below the flying capacitor C. fly1 voltage V Cfly1 At this time, the flying capacitor C fly1 By connecting in series with the flying capacitor C fly1The voltage clamping modules on both sides, as well as switching devices S4 and S3, are discharged.

[0092] like Figure 13 and 14 As shown, Figure 13 This is a topology of a 3-level conversion circuit with a flying capacitor, according to another preferred embodiment of the present invention. Figure 14 This is a topology of a 3-level converter circuit with a flying capacitor, according to another preferred embodiment of the present invention. Figure 13 The topology of the 3-level converter circuit with flying capacitor shown is similar to that of... Figure 1 The topology of the 3-level converter circuit with flying capacitor shown differs from that of the first voltage clamping module 40, which includes a first voltage clamping unit 41 and a first impedance circuit 42; the second voltage clamping module 50 includes a second voltage clamping unit 51 and a second impedance circuit 52. The first voltage clamping unit 41 can be a Zener diode D1, and the second voltage clamping unit 51 can be a Zener diode D2. The first impedance circuit 42 can be, but is not limited to, a resistor, and the second impedance circuit 52 can be, but is not limited to, a resistor. The first voltage clamping unit 41 and the first impedance circuit 42 are connected in series; the second voltage clamping unit 51 and the second impedance circuit 52 are also connected in series. When the switching device of the second bridge arm 20 is a fast-switching device, such as GaN, its reverse conduction voltage drop is much higher than that of a silicon transistor. Therefore, after adding the first voltage clamping unit 41 and the second voltage clamping unit 51, the first voltage clamping unit 41 and the second voltage clamping unit 51 will conduct during the freewheeling phase, and their reverse recovery current will severely affect the normal operation of the multi-level converter circuit. By adding a first impedance circuit 42 and a second impedance circuit 52 to the first voltage clamping module 40 and the second voltage clamping module 50 respectively, the reverse recovery current of the first voltage clamping unit 41 and the second voltage clamping unit 51 is limited to prevent it from affecting the normal operation of the multi-level change circuit.

[0093] like Figure 14 As shown, the topology of a 3-level PFC circuit with a flying capacitor is similar to that of... Figure 1The topology of the 3-level PFC circuit with flying capacitor shown differs from that of the first voltage clamping module 40, which includes a first voltage clamping unit 41 and a first diode 43; and the second voltage clamping module 50, which includes a second voltage clamping unit 51 and a second diode 53. The first voltage clamping unit 41 can be a Zener diode D1, and the second voltage clamping unit 51 can be a Zener diode D2. The forward conduction direction of the first diode 43 is opposite to that of the first voltage clamping unit 41; and the forward conduction direction of the second diode 53 is opposite to that of the second voltage clamping unit 51. In this embodiment, the forward conduction direction of the first diode 43 is opposite to that of the Zener diode D1, and the forward conduction direction of the second diode 53 is opposite to that of the Zener diode D2, ensuring that Zener diodes D1 and D2 are not turned on except during clamping operations, thus preventing interference with the normal operation of the multi-level conversion circuit.

[0094] In some embodiments, the multilevel conversion circuit 100 can also be an n-level conversion circuit, where n is a natural number greater than 3, i.e., it can be, for example, a 4-level conversion circuit (such as...). Figure 4 , Figure 8 (as shown) or more level conversion circuits (such as) Figure 9 , Figure 11 (As shown).

[0095] With Figure 1 Unlike the 3-level conversion circuit shown, the second bridge arm 20 of the n-level conversion circuit of this invention may include (2n-2) series-connected switching devices 21, and the flying capacitor bank 22 may include (n-2) flying capacitors. The i-th flying capacitor is connected across the i-th switching device and the (2n-1-i)-th switching device in the second bridge arm 20, where i is 1, 2, 3, 4…, n-2, and n is a natural number greater than 3. Furthermore, the first voltage clamping module 40 may include (n-2) first voltage clamping units 41; the first first voltage clamping unit 41 is connected to the first terminal of the first flying capacitor and the DC bus capacitor C. p Between the first terminals, the j-th first voltage clamping unit 41 is connected between the first terminal of the j-th flying capacitor and the first terminal of the (j-1)-th flying capacitor. The second voltage clamping module 50 may include (n-2) second voltage clamping units 51; the first second voltage clamping unit 51 is connected between the second terminal of the first flying capacitor and the DC bus capacitor C. p Between the second terminals, the j-th second voltage clamping unit 51 is connected between the second terminal of the j-th flying capacitor and the second terminal of the (j-1)-th flying capacitor, where j is 2, 3, 4 … , n-2.

[0096] by Figure 4Taking the 4-level PFC circuit 100-1 shown as an example, the second bridge arm 20 may include six switching devices connected in series, such as the first switching device S6, the second switching device S4, the third switching device S2, the fourth switching device S1, the fifth switching device S3, and the sixth switching device S5. The flying capacitor bank 22 may include two flying capacitors, such as flying capacitor C. fly1 and C fly2 Among them, the first flying capacitor C fly1 The second flying capacitor C is connected across the first switching device S6 and the sixth switching device S5. fly2 It is connected across the second switching device S4 and the fifth switching device S3. The first voltage clamping module 40 may include two first voltage clamping units 41, such as Zener diodes D1 and D3. Zener diode D1 is connected to the first flying capacitor C. fly1 The first terminal and DC bus capacitor C p Between the first terminals, the Zener diode D3 is connected to the second flying capacitor C. fly2 The first terminal and the first flying capacitor C fly1 Between the first terminals. The second voltage clamping module 50 may include two second voltage clamping units 51, such as Zener diodes D2 and D4. Zener diode D2 is connected to the first flying capacitor C. fly1 The second terminal and DC bus capacitor C p Between the second terminals, the Zener diode D4 is connected to the second flying capacitor C. fly2 The second terminal and the first flying capacitor C fly1 Between the second and third ends.

[0097] Specifically, when the n-level conversion circuit is powered on, and when the voltage between the midpoint b of the first bridge arm 10 and the midpoint a of the second bridge arm 20 is greater than the sum of the clamping voltages of the first to the i-th first voltage clamping units 41 or the sum of the clamping voltages of the first to the i-th second voltage clamping units 51, the power supply V... ac The voltage can be applied to the i-th flying capacitor C via the corresponding switching device 21 of the second bridge arm 20, the first to i-th first voltage clamping units 41 or the first to i-th second voltage clamping units 51, and the corresponding switching device 11 of the first bridge arm 10. flyi Charge.

[0098] In some embodiments, when the DC bus capacitor C p When the voltage reaches the preset starting voltage value, the auxiliary power supply 60 starts; when the DC bus capacitor C pWhen the voltage reaches the preset operating voltage value, the controller operates and controls the corresponding switching device 21 of the second bridge arm 20 to turn on, and the power supply V... ac The (n-2) flying capacitors can be charged via the corresponding switching device 11 of the first bridge arm 10 and the corresponding switching device 21 of the second bridge arm 20. In other embodiments, the power supply V ac (n-2) flying capacitors can be charged through the corresponding switching device 11 of the first bridge arm 10, the corresponding switching device 21 of the second bridge arm 20, and the corresponding switching device 81 of the third bridge arm 80.

[0099] When the voltage of the i-th flying capacitor reaches (n-1-i) / (n-1) times the preset value of the operating voltage, the controller controls the corresponding switching device 21 of the second bridge arm 20 to turn off. After (n-2) flying capacitors have finished charging, the controller controls all the switching devices 21 of the second bridge arm 20 to turn off.

[0100] When the voltage of the i-th flying capacitor is lower than the voltage of the (i+1)-th flying capacitor, the (i+1)-th flying capacitor discharges through the (i+1)-th first voltage clamping unit 41 and the (i+1)-th second voltage clamping unit 51, and / or the (i+1)-th switching device 21 and the (2n-2-i)-th switching device 21 discharge. In some embodiments, when the DC bus capacitor C p When the voltage is lower than the voltage of the first flying capacitor, the first flying capacitor discharges through the first first voltage clamping unit 41 and the first second voltage clamping unit 51, and / or the first switching device 21 and the (2n-2)th switching device 21 discharge.

[0101] by Figure 4 Taking the 4-level conversion circuit 100-1 shown as an example, when the first flying capacitor C fly1 The voltage is lower than that of the second flying capacitor C. fly2 When the voltage is [value], the second flying capacitor C fly2 It can be discharged through the Zener diodes D3 and D4, and / or through the switching devices S4 and S3. When the DC bus capacitance C... p The voltage is lower than that of the first flying capacitor C. fly1 When the voltage is [value], the first flying capacitor C fly1 It can be discharged through the Zener diodes D1 and D2, and / or through the switching devices S6 and S5.

[0102] The following will combine Figures 4-7BThe 4-level conversion circuit and its pre-charging method shown in the figure further describe and explain the principle of the present invention.

[0103] like Figure 4 , 5A As shown in Figure 5B, a 4-level conversion circuit 100-1 with a flying capacitor according to the present invention is illustrated. In some embodiments, after the 4-level conversion circuit is powered on and the power supply V... ac During the positive half-cycle, the DC bus capacitance C p Power supply V ac Through switching devices S2, S4, S6 and switching device S b Charging; and / or DC bus capacitor C p Power supply V ac Through switching device D s1 and switching device S b Charging. In some embodiments, after the 4-level conversion circuit is powered on and the power supply V... ac During the negative half-cycle, the DC bus capacitance C p Power supply V ac Through switching devices S1, S3, S5 and switching device S a Charging; and / or DC bus capacitor C p Power supply V ac Through switching device D s2 and switching device S a Charging. When the DC bus capacitor C p When charged to a certain voltage value, a DC bus voltage is established to supply power to the auxiliary power supply 60.

[0104] Before the controller starts working, if the input voltage V ab If the clamping voltage is higher than that of Zener diodes D1 or D2, then it constitutes... Figure 5A , 5B The double horizontal lines in the middle indicate the positive and negative half-cycle flying capacitor charging circuit. When the power supply V... ac During the positive half-cycle, the power supply V ac It can be limited by the current-limiting resistor R s Inductor L, switching devices S2 and S4, Zener diode D2, and switching device S b For flying capacitor C fly1 Charging begins. When the power supply V... ac During the negative half-cycle, the power supply V ac It can be limited by the current-limiting resistor R s Inductor L, switching devices S1 and S3, Zener diode D1, and switching device S a For flying capacitor C fly1 Charging is in progress. Flying capacitor C fly1 voltage V Cfly1As shown in equations (3) and (4):

[0105] Positive half-cycle: V Cfly1 =V ab -V D2 (V ab >V D2 (3)

[0106] V Cfly1 =0(V ab <=V D2 )

[0107] Negative half-cycle: V Cfly1 =V ab -V D1 (V ab >V D1 (4)

[0108] V Cfly1 =0(V ab <=V D1 )

[0109] Where V Cfly1 For the flying capacitor C fly1 Voltage; V ab V is the voltage between the midpoint a of the second bridge arm 20 and the midpoint b of the first bridge arm 10. D2 V is the clamping voltage of the Zener diode D2; D1 This is the clamping voltage of the Zener diode D1.

[0110] If the voltage V ab When the voltage exceeds the sum of the clamping voltages of Zener diodes D1 and D3, or the sum of the clamping voltages of D2 and D4, then it constitutes... Figure 6A , 6B The double horizontal lines in the middle indicate the positive and negative half-cycle flying capacitor charging circuit.

[0111] When the power supply V ac During the positive half-cycle, the power supply V ac It can be limited by the current-limiting resistor R s Inductor L, switching device S2, Zener diodes D4 and D2, and switching device S b For flying capacitor C fly2 Charging begins. When the power supply V... ac During the negative half-cycle, the power supply V ac It can be limited by the current-limiting resistor R s Inductor L, switching device S1, Zener diodes D1 and D3, and switching device S a For flying capacitor C fly2 Charging is in progress. Flying capacitor C fly2 voltage V Cfly2 As shown in equations (5) and (6):

[0112] Positive half-cycle: V Cfly2 =V ab -V D2 -V D4 (V ab >V D2 +V D4 (5)

[0113] V Cfly2 =0 (V ab <=V D2 +V D4 )

[0114] Negative half-cycle: V Cfly2 =V ab -V D1 -V D3 (V ab >V D1 +V D3 (6)

[0115] V Cfly2 =0 (V ab <=V D1 +V D3 )

[0116] Where V Cfly2 For the flying capacitor C fly2 Voltage; V ab V is the voltage between the midpoint a of the second bridge arm 20 and the midpoint b of the first bridge arm 10. D2 V is the clamping voltage of the Zener diode D2; D1 V is the clamping voltage of the Zener diode D1; D3 V is the clamping voltage of the Zener diode D3; D4 This is the clamping voltage of the Zener diode D4.

[0117] In this process, the DC bus capacitor C p voltage V Cp It will stabilize at the preset operating voltage value V. ab_peak And the flying capacitor C fly1 Voltage V on Cfly1 It will stabilize at V ab_peak -V D2 V ab_peak -V D1 The maximum value at 0V, while the flying capacitor C fly2 Voltage V on Cfly2 It will stabilize at V ab_peak -V D2 -V D4 V ab_peak -V D1-V D3 The maximum value in 0V.

[0118] As the voltage of the DC bus capacitor reaches the preset starting voltage, the auxiliary power supply 60 starts; when the voltage of the DC bus capacitor stabilizes at the preset operating voltage, the controller starts working and controls the corresponding switching devices to form a charging circuit for the flying capacitor C. fly1 and C fly2 During charging, the charging circuit of positive and negative half cycles is as follows: Figure 7A , 7B As shown by the dashed line.

[0119] like Figure 7A As shown, at power supply V ac During the positive half-cycle, the switching device S b Together with S2, switching devices S3 and S5, they form a flying capacitor C. fly2 The charging circuit, power supply V ac After passing through the current-limiting resistor R s Inductor L, Switching device S2, Switching device S3, Switching device S5, Switching device S b For the flying capacitor C fly2 Charging begins. At this time, V... S4 +V Cfly2 =V Cfly1 , where V S4 This is the voltage across the switching device S4. If there were no Zener diode D3, then V... S4 =V Cfly1 It is highly likely that the voltage will exceed the withstand voltage of switching device S4, causing damage to switching device S4. Zener diode D3 can prevent switching device S4 from breaking down. When the voltage V between the midpoint b of the first bridge arm and the midpoint a of the second bridge arm... ab The clamping voltages V of Zener diodes D2 and D4 are respectively greater than those of Zener diode D4. D2 and V D4 The sum of these voltages, plus the clamping voltage V of Zener diodes D1 and D3. D1 and V D3 When the sum is equal to the capacitance C, the flying capacitor C fly2 The voltage on is (V) ab -V D2 -V D4 ) and (V ab -V D1 -V D3 The larger of the two is passed through the flying capacitor C. fly2 The Zener diode D3 can prevent the switching device S4 from being damaged.

[0120] Switching device S b S2, S4, and switching device S5 constitute the flying capacitor C. fly1The charging circuit, power supply V ac After passing through the current-limiting resistor R s Inductor L, switching device S2, switching device S4, switching device S5, and switching device S b For the flying capacitor C fly1 Charging begins. At this time, V... S6 +V Cfly1 =V Cp , where V S6 V is the voltage across the switching device S6. Cp DC bus capacitor C p The voltage. If there is no Zener diode D1, then V S6 =V Cp It is highly likely that the voltage will exceed the withstand voltage of switching device S6, causing damage to switching device S6. Zener diode D1 can prevent switching device S6 from breaking down. When the voltage V between the midpoint b of the first bridge arm and the midpoint a of the second bridge arm... ab The clamping voltage V greater than that of Zener diodes D1 and D2 D1 and V D2 At that time, the flying capacitor C fly1 The voltage on is (V) ab -V D2 ) and (V ab -V D1 The larger of the two is passed through the flying capacitor C. fly1 The Zener diode D1 can prevent the switching device S6 from being broken down.

[0121] In other embodiments, diode D s1 Switching devices S6 and S5, flying capacitor C fly1 Switching device S b This forms a charging circuit, with power supply V ac Through resistor R s Diode D s1 Switching devices S6 and S5, and switching device S b Give the flying capacitor C fly1 Charging. Power supply V ac Through resistor R s Diode D s1 Switching devices S6, S4, S3 and S5, and switching device S b Give the flying capacitor C fly2 Charge.

[0122] like Figure 7B As shown, at power supply V ac During the negative half-cycle, the switching device S a Together with S1, switching devices S4 and S6, they form a flying capacitor C. fly2 The charging circuit, power supply Vac After the switching device S a Switching device S6, switching device S4, switching device S1, inductor L, current-limiting resistor R s For the flying capacitor C fly2 Charging begins. At this time, V... S3 +V Cfly2 =V Cfly1 , where V S3 This is the voltage across switching device S3. If there is no Zener diode D4, then V... S3 =V Cfly1 It is highly likely that the voltage will exceed the withstand voltage of switching device S3, causing damage to switching device S3. The Zener diode D4 can prevent switching device S3 from breaking down. When the voltage V between the midpoint b of the first bridge arm and the midpoint a of the second bridge arm... ab The clamping voltages V of Zener diodes D2 and D4 are respectively greater than those of Zener diode D4. D2 and V D4 The sum of these voltages, plus the clamping voltage V of Zener diodes D1 and D3. D1 and V D3 When the sum is equal to the capacitance C, the flying capacitor C fly2 The voltage on is (V) ab -V D2 -V D4 ) and (V ab -V D1 -V D3 The larger of the two is passed through the flying capacitor C. fly2 The Zener diode D4 can prevent the switching device S3 from being broken down.

[0123] Switching device S a Together with S1, S3, and switching device S6, they form a flying capacitor C. fly1 The charging circuit, power supply V ac After the switching device S a Switching device S6, switching device S3, switching device S1, inductor L, current-limiting resistor R s Give the flying capacitor C fly1 Charging begins. At this time, V... S5 +V Cfly1 =V Cp , where V S5 V is the voltage across the switching device S5. Cp DC bus capacitor C p The voltage. If there is no Zener diode D2, then V S5 =V Cp It is highly likely that the voltage will exceed the withstand voltage of switching device S5, causing damage to switching device S5. Zener diode D2 can prevent switching device S5 from breaking down. When the voltage V between the midpoint b of the first bridge arm and the midpoint a of the second bridge arm...ab The clamping voltage V greater than that of Zener diodes D1 and D2 D1 and V D2 At that time, the flying capacitor C fly1 The voltage on is (V) ab -V D2 ) and (V ab -V D1 The larger of the two is passed through the flying capacitor C. fly1 The Zener diode D2 can prevent the switching device S5 from being damaged.

[0124] In other embodiments, diode D s2 Switching devices S6 and S5, flying capacitor C fly1 Switching device S a This forms a charging circuit, with power supply V ac Through resistor R s Diode D s2 Switching devices S6 and S5, and switching device S a Give the flying capacitor C fly1 Charging. Power supply V ac Through resistor R s Diode D s2 Switching devices S6, S4, S3 and S5, and switching device S a Give the flying capacitor C fly2 Charge.

[0125] When the capacitor C is flying across fly2 voltage V Cfly2 Reaching DC bus capacitance C p voltage V Cp At / 3, switching devices S3 and S4 are disconnected, and the flying capacitor C fly1 Pre-charge complete; when the flying capacitor C fly1 voltage V Cfly1 Reaching 2V Cp At / 3, switching devices S5 and S6 are disconnected, and the flying capacitor C fly1 Pre-charging is complete. At this point, the flying capacitor C of the 4-level PFC circuit... fly1 and C fly2 Once pre-charging is complete, the controller is used to disconnect all switching devices in the second bridge arm.

[0126] It should be noted that during the operation of the 4-level conversion circuit, when V Cfly1 It is always less than the clamping voltage V of the Zener diode D3. D3 and the clamping voltage V of Zener diode D4 D4In this case, the Zener diodes D3 and D4 can be omitted to protect the switching devices S3 and S4. Only two Zener diodes are needed: D1 and D2. In this embodiment, the switching devices of the second bridge arm can be low-voltage switching devices, for example, those with a maximum withstand voltage V. BRDSS =250V, the voltage clamping module can select a Zener diode with a clamping voltage of 200V, at power supply V ac When the voltage is 264V, through Figure 5A and 5B The DC bus charging circuit shown by the dashed line supplies power to the DC bus capacitor C. p During charging, the DC bus capacitor C p The peak voltage is 264 × sqrt(2), at which point the flying capacitor C fly1 When charged, its voltage C fly1 The voltage is approximately 173.4V, which is significantly lower than the maximum withstand voltage of the switching device. BRDSS Therefore, the Zener diodes D3 and D4 can be omitted.

[0127] When the 4-level converter circuit is powered off, due to the forward voltage drop of the diode, when the DC bus capacitor C... p voltage V Cp Reduced to below the flying capacitance C fly1 voltage V Cfly1 At that time, the series capacitor C is connected in series. fly1 Zener diodes D1 and D2 on both sides are forward-biased, and the flying capacitor C... fly1 Discharge begins through Zener diodes D1 and D2. In some embodiments, when the DC bus capacitance C... p voltage V Cp Reduced to below the flying capacitance C fly1 voltage V Cfly2 At that time, the flying capacitor C fly1 Discharge begins through switching devices S5 and S6.

[0128] When the capacitor C is flying across fly1 voltage V Cfly1 Reduced to below the flying capacitance C fly2 voltage V Cfly2 At that time, the series capacitor C is connected in series. fly2 The Zener diodes D3 and D4 on both sides are forward-biased, and the flying capacitor C... fly2 Discharge begins through Zener diodes D3 and D4. In some embodiments, discharge occurs when the flying capacitor C... fly1 voltage V Cfly1 Reduced to below the flying capacitance C fly2 voltage V Cfly2 At that time, the flying capacitor C fly2 Discharge begins through switching devices S4 and S3.

[0129] In other embodiments, Figure 4 The position of the Zener diode in the 4-level converter circuit 100-1 shown can be changed to Figure 8 The structure shown in the 4-level converter circuit 100-2 is related to the flying capacitor C. fly2 The series-connected Zener diodes D3 and D4 are respectively connected to the DC bus capacitor C. p The two ends are connected, that is, the Zener diode D3 is connected to the flying capacitor C. fly2 and DC bus capacitor C p The first terminal is between the two terminals, while the Zener diode D4 is connected across the flying capacitor C. fly2 and DC bus capacitor C p Between the second terminals; at this time, Zener diodes D1, D2 and D3, D4 can be of different specifications.

[0130] Before the 4-level conversion circuit is powered on and the controller starts working, if the voltage V between the midpoint b of the first bridge arm 10 and the midpoint a of the second bridge arm 20 is... ab The voltage is higher than the clamping voltage of Zener diodes D3 or D4, at which point the power supply V... ac During the positive half-cycle, the power supply V ac Through the current-limiting resistor R s Inductor L, switching device S2, Zener diode D4, and switching device S b For flying capacitor C fly2 Charging; and when the power supply V ac During the negative half-cycle, the power supply V ac Through the current-limiting resistor R s Switching device S1, Zener diode D3, and switching device S a For flying capacitor C fly2 Charging is in progress. Flying capacitor C fly2 voltage V Cfly2 As shown in equations (7) and (8):

[0131] Positive half-cycle: V Cfly2 =V ab -V D4 (V ab >V D4 (7)

[0132] V Cfly2 =0(V ab <=V D4 )

[0133] Negative half-cycle: V Cfly2 =V ab -V D3 (V ab >V D3(8)

[0134] V Cfly2 =0(V ab <=V D3 )

[0135] When the 4-level converter circuit is powered off, due to the forward voltage drop of the diodes, when the voltage V of the DC bus capacitor... Cp Drop until below the flying capacitor C fly1 voltage V Cfly1 At that time, the series capacitor C is connected in series. fly1 The Zener diodes D1 and D2 on both sides are forward-biased, and the flying capacitor C... fly1 The discharge begins through Zener diodes D1 and D2; or through the flying capacitor C. fly1 Discharge begins through switching devices S6 and S5. When the flying capacitor C... fly1 voltage V Cfly1 Drop until below the flying capacitor C fly2 voltage V Cfly2 At that time, the series capacitor C is connected in series. fly2 The Zener diodes D3 and D4 on both sides are forward-biased, and the flying capacitor C... fly2 The discharge begins through Zener diodes D3 and D4; or through the flying capacitor C. fly2 Discharge begins through switching devices S6, S4, S3, and S5.

[0136] like Figure 9 As shown, an n-level converter circuit 100-3 with a flying capacitor is illustrated. After the n-level converter circuit is powered on, the DC bus capacitor C... p Power supply V ac Through diode D s1 Or D s2 The corresponding switching device 11 of the first bridge arm 10 is charged; or it is charged by the power supply V. ac The DC bus voltage is established by charging the corresponding switching devices 11 of the first bridge arm 10 and the corresponding switching devices 21 of the second bridge arm 20, and the auxiliary power supply 60 is powered by this voltage.

[0137] Clearly, for the n-level converter circuit 100-3 with a flying capacitor, a total of 2×(n-2) Zener diodes are needed to protect the corresponding switching devices, and the flying capacitor C... flym voltage V Cflym As shown in equations (9) and (10):

[0138] Positive half-cycle: V Cflym =V Cfly(m-1) -V D(2m) (V Cfly(m-1) >V D(2m)(m=2,3….n-2) (9)

[0139] Negative half-cycle: V Cflym =V Cfly(m-1) -V D(2m-1) (V Cfly(m-1) >V D(2m-1) (m=2,3….n-2) (10)

[0140] In some embodiments, if the voltage stress of the switching device of the second bridge arm 20 is not expected to meet the above formulas (9) and (10) based on the actual working state, it can be adjusted only when the flying capacitor C is in operation. fly1 Both sides and DC bus capacitor C p The two ends of the capacitor C are connected to Zener diodes D1 and D2 respectively. fly1 External switching device S (2n-2) and S (2n-3) To provide protection, the flying capacitor C fly1 voltage V Cfly1 As shown in equations (11) and (12):

[0141] During the positive half-cycle:

[0142] V Cfly1 =V Cp -V D2 (V ab >V D2 (11)

[0143] V Cfly1 =0 (V ab <=V D2 )

[0144] In the negative half-cycle:

[0145] V Cfly1 =V Cp -V D1 (V ab >V D1 (12)

[0146] V Cfly1 =0 (V ab <=V D1 )

[0147] In some embodiments, the number of Zener diodes can be 2, and at most 2×(n-2).

[0148] In some embodiments, such as Figure 9As shown in the embodiment, the first voltage clamping module 40 includes n first voltage clamping units 41 and n first impedance circuits; the first impedance circuits and the corresponding first voltage clamping units 41 are connected in series; the second voltage clamping module 50 includes n second voltage clamping units 51 and n second impedance circuits; the second impedance circuits and the corresponding second voltage clamping units 51 are connected in series. By adding the first impedance circuits and the second impedance circuits, the reverse recovery current of the first voltage clamping units 41 and the second voltage clamping units 51 is limited to prevent interference with the normal operation of the multi-level switching circuit.

[0149] In some embodiments, such as Figure 9 As shown in the embodiment, the first voltage clamping module 40 includes n first voltage clamping units 41 and n first diodes; the first diodes and their corresponding first voltage clamping units 41 are connected in series; the second voltage clamping module 50 includes n second voltage clamping units 51 and n second diodes; the second diodes and their corresponding second voltage clamping units 51 are connected in series. The forward conduction direction of the first diodes is opposite to the forward conduction direction of the corresponding first voltage clamping unit 41; and the forward conduction direction of the second diodes is opposite to the forward conduction direction of the corresponding second voltage clamping unit 51. By adding first and second diodes, the first and second voltage clamping units are not turned on except during clamping operations, preventing interference with the normal operation of the multi-level conversion circuit.

[0150] like Figure 10A As shown by the dashed line, when the controller starts working and the power supply V... ac During the positive half-cycle, the controller controls the switching devices S3~S 2n-3 On, power supply V ac It can be achieved through switching devices S2~S 2n-4 Switching devices S3~S 2n-3 and switching device S b Give the flying capacitor C fly1 ~C fly(n-2) Charging. In other embodiments, the controller can control the switching devices S2~S 2n-4 S3~S 2n-3 and S b On, power supply V ac It can be achieved through switching devices S2~S 2n-4 Switching devices S3~S 2n-3 and switching device S b Give the flying capacitor C fly1 ~C fly(n-2) Charging. In other embodiments, the controller can control the switching devices S2~S 2n-4and S3~S 2n-3 On, power supply V ac Through D s1 Switching devices S2~S 2n-4 Switching devices S3~S 2n-3 and switching device S b Give the flying capacitor C fly1 ~C fly(n-2) Charge.

[0151] like Figure 10B As shown by the dashed line, when the controller starts working and the power supply V... ac During the negative half-cycle, the controller controls the switching devices S2~S 2n-4 On, power supply V ac It can be achieved through switching devices S2~S 2n-4 Switching devices S3~S 2n-3 and switching device S a Give the flying capacitor C fly1 ~C fly(n-2) Charging. In other embodiments, the controller can control the switching devices S2~S 2n-4 S3~S 2n-3 and S a On, power supply V ac It can be achieved through switching devices S2~S 2n-4 Switching devices S3~S 2n-3 and switching device S a Give the flying capacitor C fly1 ~C fly(n-2) Charging. In other embodiments, the controller can control the switching devices S2~S 2n-4 and S3~S 2n-3 On, power supply V ac Through D s2 Switching devices S2~S 2n-4 Switching devices S3~S 2n-3 and switching device S a Give the flying capacitor C fly1 ~C fly(n-2) Charge.

[0152] When the capacitor C is flying across flyi The voltage reaches (n-1-i) / (n-1) times V. Cp When (i=1,2,3,4,5,…, n-2), the corresponding switching device of the second bridge arm 20 is turned off; when all flying capacitors C fly1 ~ C fly(n-2) The voltage values ​​all reach (n-1-i) / (n-1) times V. CpAll switching devices in the second bridge arm 20 are disconnected. Simultaneously, the charging circuit of the flying capacitor is unaffected by the presence or absence of a Zener diode.

[0153] When the n-level converter circuit is powered off, due to the forward voltage drop of the diode, when the DC bus voltage V... Cp Drop until below the flying capacitor C fly1 voltage V Cfly1 At that time, the series capacitor C is connected in series. fly1 The Zener diodes D1 and D2 on both sides are forward-biased, and the flying capacitor C... fly1 The discharge begins through Zener diodes D1 and D2; or through the flying capacitor C. fly1 via the flying capacitor C fly1 The two switching devices on the outer side begin to discharge. When the flying capacitor C... flyi voltage V Cflyi Drop until below the flying capacitor C fly(i+1) voltage V Cfly(i+1) At that time, the series capacitor C is connected in series. fly(i+1) The Zener diodes on both sides are forward-biased, and the flying capacitor C... fly(i+1) Start discharging (i=1,2,3…n-2); or fly-through capacitor C fly(i+1) via the flying capacitor C fly(i+1) The external switching device begins to discharge.

[0154] In other embodiments, Figure 9 The Zener diode connection method described above can be changed to Figure 11 The connection method shown in the n-level conversion circuit 100-4 is illustrated. (And...) Figure 9 The difference is, Figure 9 The Zener diode is connected across the switching device that needs protection, while Figure 11 Then connect the Zener diode to the corresponding flying capacitor and the DC bus capacitor C respectively. p Between the two ends, the Zener diode has more clamping voltage levels and more flexible configuration.

[0155] for Figure 11 For the n-level converter circuit 100-4 with flying capacitor shown, a total of 2×(n-2) Zener diodes are needed to protect the switching devices of the second bridge arm 20. The flying capacitor C... flyi voltage V Cflyi As shown in equations (11) and (12):

[0156] Positive half-cycle: V Cflyi =V Cp -V D(2i) (V Cp >V D(2i) (i=1,2,3….n-2) (11)

[0157] Negative half-cycle: V Cflyi =V Cp -V D(2i-1) (V Cp >V D(2i-1) (i=1,2,3….n-2) (12)

[0158] In some embodiments, if the voltage stress of the switching device of the second bridge arm 20 is not expected to satisfy the above formulas (11) and (12) based on the actual working state, it can be adjusted only when the flying capacitor and the DC bus capacitor C satisfy the relationship. p A Zener diode is connected between the two ends of the capacitor to protect a group of switching devices outside the flying capacitor.

[0159] In some embodiments, the number of Zener diodes can be at least 2 and at most 2×(n-2).

[0160] When the n-level converter circuit is powered off, due to the forward voltage drop of the diode, when the DC bus voltage V... Cp Drop until below the flying capacitor C flyi voltage V Cflyi At that time, the series capacitor C is connected in series. flyi The Zener diodes on both sides are forward-biased, and the flying capacitor C... flyi via the flying capacitor C flyi The Zener diodes on both sides begin to discharge; or the flying capacitor C... flyi via the flying capacitor C flyi The external switching device begins to discharge.

[0161] This invention also provides a pre-charging method for a multilevel converter circuit with a flying capacitor. The multilevel converter circuit may include a first bridge arm, a second bridge arm, a DC bus capacitor, a first voltage clamping module, and a second voltage clamping module. The first bridge arm may include multiple switching devices connected in series. The second bridge arm may include multiple switching devices and a flying capacitor bank connected in series; the midpoint of the second bridge arm and the midpoint of the first bridge arm may be connected to a power supply and an inductor to form a series branch. Both the first and second bridge arms are connected in parallel to the DC bus capacitor. The first voltage clamping module may be connected between the first terminal of the flying capacitor bank and the first terminal of the DC bus capacitor. The second voltage clamping module may be connected between the second terminal of the flying capacitor bank and the second terminal of the DC bus capacitor.

[0162] like Figure 15 As shown, the pre-charging method 200 of the present invention includes:

[0163] Step S201: After the multilevel conversion circuit is powered on, the power supply charges the DC bus capacitor through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm.

[0164] In some embodiments of the present invention, the multilevel conversion circuit may further include a third bridge arm. The third bridge arm may include a plurality of switching devices connected in series; the midpoint of the third bridge arm may connect the power supply and the inductor. When the multilevel conversion circuit is powered on, the power supply may charge the DC bus capacitor through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; and / or the power supply may charge the DC bus capacitor through the corresponding switching devices of the third bridge arm and the corresponding switching devices of the first bridge arm.

[0165] In some embodiments of the present invention, when the multilevel conversion circuit is powered on, and when the voltage between the midpoint of the first bridge arm and the midpoint of the second bridge arm is greater than the clamping voltage of the first voltage clamping unit or the second voltage clamping unit, the power supply can charge the flying capacitor bank through the corresponding switching device of the second bridge arm, the first voltage clamping module or the second voltage clamping module, and the corresponding switching device of the first bridge arm.

[0166] In some embodiments of the present invention, the multilevel conversion circuit may further include an auxiliary power supply and a controller. The auxiliary power supply may be connected to the DC bus capacitor to supply power to the controller. The controller may be coupled to the auxiliary power supply and multiple switching devices of the second bridge arm. When the voltage of the DC bus capacitor reaches a preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches a preset operating voltage value, the controller operates and controls the corresponding switching devices of the second bridge arm to conduct, and the power supply can charge the flying capacitor bank through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm.

[0167] In some embodiments of the present invention, the multilevel conversion circuit may further include an auxiliary power supply and a controller. The auxiliary power supply may be connected to the DC bus capacitor and used to power the controller. The controller may be coupled to the auxiliary power supply and multiple switching devices of the second bridge arm. When the voltage of the DC bus capacitor reaches a preset starting voltage value, the auxiliary power supply starts. When the voltage of the DC bus capacitor reaches a preset operating voltage value, the controller operates and controls the corresponding switching devices of the second bridge arm to conduct. The power supply can charge the flying capacitor bank through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; or, the power supply can charge the flying capacitor bank through the corresponding switching devices of the first bridge arm, the corresponding switching devices of the second bridge arm, and the corresponding switching devices of the third bridge arm.

[0168] In summary, this invention proposes a pre-charging technique using a multilevel converter circuit with a flying capacitor. This requires only a minimum of two additional voltage clamping units to meet the application requirements of 3~n level multilevel converter circuits. The above embodiments are for illustrative purposes only. The structure, configuration, and corresponding control methods of each circuit can vary. Any improvements or equivalent transformations made to individual circuits based on the principles of this invention should not be excluded from the scope of protection of this invention.

[0169] Exemplary embodiments of the present invention have been specifically illustrated and described above. It should be understood that the present invention is not limited to the disclosed embodiments; rather, the present invention is intended to cover various modifications and equivalent arrangements contained within the spirit and scope of the appended claims.

Claims

1. A multilevel converter circuit with a flying capacitor, characterized in that, include: The first bridge arm includes multiple switching devices connected in series; The second bridge arm includes multiple switching devices and a flying capacitor bank connected in series. The midpoint of the second bridge arm and the midpoint of the first bridge arm are connected to a power supply and an inductor to form a series branch. DC bus capacitor, with the first bridge arm and the second bridge arm both connected in parallel to the DC bus capacitor; A first voltage clamping module is connected between the first terminal of the flying capacitor bank and the first terminal of the DC bus capacitor; and The second voltage clamping module is connected between the second terminal of the flying capacitor bank and the second terminal of the DC bus capacitor. The multilevel converter circuit with flying capacitor further includes an auxiliary power supply and a controller; the auxiliary power supply is connected to the DC bus capacitor and is used to power the controller; the controller is coupled to the auxiliary power supply and the plurality of switching devices of the second bridge arm.

2. The multilevel conversion circuit with a flying capacitor according to claim 1, characterized in that, After the multilevel conversion circuit is powered on, the power supply charges the DC bus capacitor through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm.

3. The multilevel conversion circuit with a flying capacitor according to claim 1, characterized in that, It also includes a third bridge arm, which comprises a plurality of switching devices connected in series, and the midpoint of the third bridge arm connects the power supply and the inductor.

4. The multilevel conversion circuit with a flying capacitor according to claim 3, characterized in that, When the multilevel conversion circuit is powered on, the power supply charges the DC bus capacitor through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; and / or the power supply charges the DC bus capacitor through the corresponding switching devices of the third bridge arm and the corresponding switching devices of the first bridge arm.

5. The multilevel converter circuit with a flying capacitor according to claim 2 or 4, characterized in that, When the multilevel conversion circuit is powered on, and the voltage between the midpoint of the first bridge arm and the midpoint of the second bridge arm is greater than the clamping voltage of the first voltage clamping module or the second voltage clamping module, the power supply charges the flying capacitor bank through the corresponding switching device of the second bridge arm, the corresponding switching device of the first bridge arm, and the first voltage clamping module or the second voltage clamping module.

6. The multilevel conversion circuit with a flying capacitor according to claim 2, characterized in that, When the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching device of the second bridge arm to conduct, and the power supply charges the flying capacitor bank through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm.

7. The multilevel conversion circuit with a flying capacitor according to claim 4, characterized in that, When the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching device of the second bridge arm to conduct, and the power supply charges the flying capacitor bank through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; or the power supply charges the flying capacitor bank through the corresponding switching devices of the first bridge arm, the corresponding switching devices of the second bridge arm and the corresponding switching devices of the third bridge arm.

8. The multilevel conversion circuit with a flying capacitor according to claim 1, characterized in that, It also includes a current limiting circuit, which is connected in series to the series branch.

9. The multilevel conversion circuit with a flying capacitor according to claim 1, characterized in that, It also includes a current limiting circuit, which is connected in series with the DC bus capacitor.

10. The multilevel conversion circuit with a flying capacitor according to claim 8 or 9, characterized in that, The current limiting circuit includes a current limiting resistor and a switch, wherein the current limiting resistor and the switch are connected in parallel.

11. The multilevel conversion circuit with a flying capacitor according to claim 2 or 4, characterized in that, The multilevel conversion circuit is a 3-level conversion circuit; the second bridge arm includes a first switching device, a second switching device, a third switching device, and a fourth switching device connected in series; the flying capacitor bank includes a flying capacitor, which is connected between the connection point of the first switching device and the second switching device and the connection point of the third switching device and the fourth switching device; The first voltage clamping module includes a first voltage clamping unit; the first voltage clamping unit is connected between the first terminal of the flying capacitor and the first terminal of the DC bus capacitor; The second voltage clamping module includes a second voltage clamping unit; the second voltage clamping unit is connected between the second terminal of the flying capacitor and the second terminal of the DC bus capacitor.

12. The multilevel conversion circuit with a flying capacitor according to claim 11, characterized in that, When the multilevel conversion circuit is powered on, if the voltage between the midpoint of the second bridge arm and the midpoint of the first bridge arm is greater than the clamping voltage of the second voltage clamping unit, the power supply charges the flying capacitor through the corresponding switching devices of the second bridge arm, the second voltage clamping unit, and the corresponding switching devices of the first bridge arm; or if the voltage between the midpoint of the first bridge arm and the midpoint of the second bridge arm is greater than the clamping voltage of the first voltage clamping unit, the power supply charges the flying capacitor through the corresponding switching devices of the first bridge arm, the first voltage clamping unit, and the corresponding switching devices of the second bridge arm.

13. The multilevel conversion circuit with a flying capacitor according to claim 12, characterized in that, When the voltage of the DC bus capacitor reaches the preset operating voltage value, the voltage of the flying capacitor is the maximum value among the difference between the preset operating voltage value and the clamping voltage of the first voltage clamping unit, the difference between the preset operating voltage value and the clamping voltage of the second voltage clamping unit, and 0.

14. The multilevel conversion circuit with a flying capacitor according to claim 2, characterized in that, When the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching devices of the second bridge arm to turn on, and the power supply charges the flying capacitor through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; when the voltage of the flying capacitor reaches half of the preset operating voltage value, the controller controls all the switching devices of the second bridge arm to turn off, and the flying capacitor completes charging.

15. The multilevel conversion circuit with a flying capacitor according to claim 4, characterized in that, When the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching devices of the second bridge arm to turn on, and the power supply charges the flying capacitor through the corresponding switching devices of the first bridge arm, the corresponding switching devices of the second bridge arm, and the corresponding switching devices of the third bridge arm; when the voltage of the flying capacitor reaches half of the preset operating voltage value, the controller controls all the switching devices of the second bridge arm to turn off, and the flying capacitor completes charging.

16. The multilevel converter circuit with a flying capacitor according to claim 11, characterized in that, When the voltage of the DC bus capacitor is lower than the voltage of the flying capacitor, the flying capacitor discharges through the first voltage clamping unit and the second voltage clamping unit, and / or the first switching device and the fourth switching device discharge.

17. The multilevel conversion circuit with a flying capacitor according to claim 1, characterized in that, The first voltage clamping module includes at least one first voltage clamping unit and at least one first impedance circuit; the first impedance circuit and the corresponding first voltage clamping unit are connected in series. The second voltage clamping module includes at least one second voltage clamping unit and at least one second impedance circuit; the second impedance circuit and the corresponding second voltage clamping unit are connected in series.

18. The multilevel conversion circuit with a flying capacitor according to claim 1, characterized in that, The first voltage clamping module includes at least one first voltage clamping unit and at least one first diode, wherein the first diode and the corresponding first voltage clamping unit are connected in series, and the forward conduction direction of the first diode is opposite to the forward conduction direction of the corresponding first voltage clamping unit; the second voltage clamping module includes at least one second voltage clamping unit and at least one second diode, wherein the second diode and the corresponding second voltage clamping unit are connected in series, and the forward conduction direction of the second diode is opposite to the forward conduction direction of the corresponding second voltage clamping unit.

19. The multilevel converter circuit with a flying capacitor according to claim 2 or 4, characterized in that, The multilevel conversion circuit is an n-level conversion circuit; the second bridge arm includes (2n-2) switching devices connected in series; the flying capacitor bank includes (n-2) flying capacitors; the i-th flying capacitor is connected across the i-th switching device and the (2n-1-i)-th switching device in the second bridge arm, where i is 1, 2, 3, 4 … , n-2, and n is a natural number greater than 3; The first voltage clamping module includes (n-2) first voltage clamping units; the first first voltage clamping unit is connected between the first terminal of the first flying capacitor and the first terminal of the DC bus capacitor, and the j-th first voltage clamping unit is connected between the first terminal of the j-th flying capacitor and the first terminal of the (j-1)-th flying capacitor; The second voltage clamping module includes (n-2) second voltage clamping units; the first second voltage clamping unit is connected between the second terminal of the first flying capacitor and the second terminal of the DC bus capacitor, and the j-th second voltage clamping unit is connected between the second terminal of the j-th flying capacitor and the second terminal of the (j-1)-th flying capacitor, where j is 2, 3, 4, …, n-2.

20. The multilevel conversion circuit with a flying capacitor according to claim 19, characterized in that, The first voltage clamping circuit further includes (n-2) first impedance circuits; the second voltage clamping circuit further includes (n-2) second impedance circuits; each first impedance circuit is connected in series with the corresponding first voltage clamping unit; each second impedance circuit is connected in series with the corresponding second voltage clamping unit.

21. The multilevel conversion circuit with a flying capacitor according to claim 19, characterized in that, The first voltage clamping circuit further includes (n-2) first diodes; the second voltage clamping circuit further includes (n-2) second diodes; each first diode is connected in series with the corresponding first voltage clamping unit; each second diode is connected in series with the corresponding second voltage clamping unit.

22. The multilevel conversion circuit with a flying capacitor according to claim 19, characterized in that, When the multilevel conversion circuit is powered on, and the voltage between the midpoint of the first bridge arm and the midpoint of the second bridge arm is greater than the sum of the clamping voltages of the first to the i-th first voltage clamping units or the sum of the clamping voltages of the first to the i-th second voltage clamping units, the power supply charges the i-th flying capacitor through the corresponding switching device of the second bridge arm, the first to the i-th first voltage clamping unit or the first to the i-th second voltage clamping unit, and the corresponding switching device of the first bridge arm.

23. The multilevel conversion circuit with a flying capacitor according to claim 2, characterized in that, The multilevel conversion circuit is an n-level conversion circuit; the second bridge arm includes (2n-2) switching devices connected in series; the flying capacitor bank includes (n-2) flying capacitors; the i-th flying capacitor is connected across the i-th switching device and the (2n-1-i)-th switching device in the second bridge arm, where i is 1, 2, 3, 4 … , n-2, and n is a natural number greater than 3; When the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching device of the second bridge arm to conduct, and the power supply charges the (n-2) flying capacitors through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm.

24. The multilevel converter circuit with a flying capacitor according to claim 4, characterized in that, The multilevel conversion circuit is an n-level conversion circuit; the second bridge arm includes (2n-2) switching devices connected in series; the flying capacitor bank includes (n-2) flying capacitors; the i-th flying capacitor is connected across the i-th switching device and the (2n-1-i)-th switching device in the second bridge arm, where i is 1, 2, 3, 4 … , n-2, and n is a natural number greater than 3; When the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching device of the second bridge arm to conduct, and the power supply charges the (n-2) flying capacitors through the corresponding switching devices of the first bridge arm, the corresponding switching devices of the second bridge arm, and the corresponding switching devices of the third bridge arm.

25. The multilevel converter circuit with a flying capacitor according to claim 23 or 24, characterized in that, When the voltage of the i-th flying capacitor reaches (n-1-i) / (n-1) times the preset value of the operating voltage, the controller controls the corresponding switching device of the second bridge arm to turn off. After the (n-2) flying capacitors have finished charging, the controller controls all the switching devices of the second bridge arm to turn off.

26. The multilevel converter circuit with a flying capacitor according to claim 19, characterized in that, When the voltage of the i-th flying capacitor is lower than the voltage of the (i+1)-th flying capacitor, the (i+1)-th flying capacitor discharges through the (i+1)-th first voltage clamping unit and the (i+1)-th second voltage clamping unit, and / or the (i+1)-th switching device and the (2n-2-i)-th switching device discharge; or When the voltage of the DC bus capacitor is lower than the voltage of the first flying capacitor, the first flying capacitor discharges through the first first voltage clamping unit and the first second voltage clamping unit, and / or the first switching device and the (2n-2)th switching device discharge.

27. The multilevel converter circuit with a flying capacitor according to claim 2 or 4, characterized in that, The multilevel conversion circuit is an n-level conversion circuit; the second bridge arm includes (2n-2) switching devices connected in series; the flying capacitor bank includes (n-2) flying capacitors; the i-th flying capacitor is connected across the i-th switching device and the (2n-1-i)-th switching device in the second bridge arm, where i is 1, 2, 3, …, n-2, and n is a natural number greater than 3; The first voltage clamping module includes (n-2) first voltage clamping units; each first voltage clamping unit is connected between the first terminal of the corresponding flying capacitor and the first terminal of the DC bus capacitor; The second voltage clamping module includes (n-2) second voltage clamping units; each second voltage clamping unit is connected between the second terminal of the corresponding flying capacitor and the second terminal of the DC bus capacitor.

28. The multilevel converter circuit with a flying capacitor according to claim 2 or 4, characterized in that, The multi-level conversion circuit is an n-level conversion circuit; the second bridge arm includes (2n-2) switching devices connected in series; the flying capacitor bank includes (n-2) flying capacitors; the i-th flying capacitor is connected across the i-th switching device and the (2n-1-i)-th switching device in the second bridge arm, where i is 1, 2, 3, …, n-2, and n is a natural number greater than 3; The first voltage clamping module includes at least one first voltage clamping unit; the at least one first voltage clamping unit is connected between the first terminal of the corresponding flying capacitor and the first terminal of the DC bus capacitor; The second voltage clamping module includes at least one second voltage clamping unit; the at least one second voltage clamping unit is connected between the second terminal of the corresponding flying capacitor and the second terminal of the DC bus capacitor.

29. A pre-charging method for a multilevel converter circuit with a flying capacitor, characterized in that, The multilevel conversion circuit includes a first bridge arm, a second bridge arm, a DC bus capacitor, a first voltage clamping module, and a second voltage clamping module. The first bridge arm includes multiple switching devices connected in series. The second bridge arm includes multiple switching devices connected in series and a flying capacitor bank. The midpoint of the second bridge arm and the midpoint of the first bridge arm are connected to a power supply and an inductor to form a series branch. Both the first and second bridge arms are connected in parallel to the DC bus capacitor. The first voltage clamping module is connected between the first terminal of the flying capacitor bank and the first terminal of the DC bus capacitor. The second voltage clamping module is connected between the second terminal of the flying capacitor bank and the second terminal of the DC bus capacitor. The multilevel conversion circuit also includes an auxiliary power supply and a controller. The auxiliary power supply is connected to the DC bus capacitor and is used to power the controller. The controller is coupled to the auxiliary power supply and multiple switching devices of the second bridge arm, and the pre-charging method includes: After the multilevel conversion circuit is powered on, the power supply charges the DC bus capacitor through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm.

30. The pre-charging method according to claim 29, characterized in that, The multilevel conversion circuit further includes a third bridge arm; the third bridge arm includes multiple switching devices connected in series; the midpoint of the third bridge arm connects the power supply and the inductor; When the multilevel conversion circuit is powered on, the power supply charges the DC bus capacitor through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; and / or the power supply charges the DC bus capacitor through the corresponding switching devices of the third bridge arm and the corresponding switching devices of the first bridge arm.

31. The pre-charging method according to claim 29 or 30, characterized in that, When the multilevel conversion circuit is powered on, and the voltage between the midpoint of the first bridge arm and the midpoint of the second bridge arm is greater than the clamping voltage of the first voltage clamping module or the second voltage clamping module, the power supply charges the flying capacitor bank through the corresponding switching device of the second bridge arm, the first voltage clamping module or the second voltage clamping module, and the corresponding switching device of the first bridge arm.

32. The pre-charging method according to claim 29, characterized in that, When the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching device of the second bridge arm to conduct, and the power supply charges the flying capacitor bank through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm.

33. The pre-charging method according to claim 30, characterized in that, When the voltage of the DC bus capacitor reaches the preset starting voltage value, the auxiliary power supply starts; when the voltage of the DC bus capacitor reaches the preset operating voltage value, the controller operates and controls the corresponding switching device of the second bridge arm to conduct, and the power supply charges the flying capacitor bank through the corresponding switching devices of the first bridge arm and the corresponding switching devices of the second bridge arm; or the power supply charges the flying capacitor bank through the corresponding switching devices of the first bridge arm, the corresponding switching devices of the second bridge arm and the corresponding switching devices of the third bridge arm.

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