Pre-charging Circuit Based on Negative-Pole Pre-charging and Flying Capacitor Three-Level Converter

Abstract

The present invention provides a pre-charge circuit based on negative electrode pre-charging, including a charging module, which includes a first clamping diode, a second clamping diode, a first switch, a soft start unit and a second switch. The positive electrode of the first clamping diode is used to be connected between a first capacitor and a second capacitor, the negative electrode of the first clamping diode is used to be connected to the positive electrode of a flying capacitor, the negative electrode of the second clamping diode is connected to the positive electrode of the first clamping diode, the positive electrode of the second clamping diode is used to be connected to the negative electrode of the flying capacitor, one end of the first switch is used to be connected to the negative electrode of the flying capacitor, the other end of the first switch is connected to the negative bus, the second switch is connected in series on both sides of the positive bus, the soft start unit is connected in parallel with the second switch, and after the soft start unit is closed, it is used to charge the buses on both sides. After the first switch and the second switch are closed, they are used to charge the flying capacitor. After the present invention charges the buses on both sides through the soft start unit, the first switch and the second switch are closed to charge the flying capacitor.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and particularly relates to a pre-charging circuit based on negative pre-charging and a flying capacitor three-level converter. Background Art

[0002] Flying capacitor three-level converters are commonly used in energy storage systems to achieve bidirectional conversion of energy between energy storage devices and DC power grids. Generally, before the system of the converter operates, it is necessary to pre-charge the flying capacitors in the converter to half of the input voltage value to complete soft start operation, and to avoid overvoltage damage to the power semiconductor devices in the circuit topology when they are turned on instantaneously.

[0003] In related technologies, the commonly used soft start methods include two categories. The first is to add an external charging power supply to charge the flying capacitors separately using the external power supply. When the voltage value of the flying capacitors reaches half of the input side voltage, stop charging the flying capacitors and disconnect the added external charging circuit from the main circuit of the converter to complete the soft start of the converter system. The second is to directly use the energy of the power grid or energy storage device to pre-charge the flying capacitors, which requires improving the circuit topology and adding a soft start switch. Usually, a switch or power semiconductor device needs to be added in the connection line between the flying capacitors and the main circuit. The first method is simple to implement but requires adding an additional charging circuit, resulting in the complication of the design and structure of the converter system. The switches or power semiconductor devices added in the second method become part of the circuit topology after the soft start is completed. Therefore, there are requirements for the power level of the added switches or power semiconductor devices, which will affect the service life of the converter system in the long run. Summary of the Invention

[0004] The present invention provides a pre-charging circuit based on negative pre-charging, which can pre-charge all flying capacitors simultaneously without changing the structure of the converter itself, and the pre-charging circuit has a simple structure.

[0005] The present invention is implemented as follows. A pre-charging circuit based on negative pre-charging includes:

[0006] Charging module, where the charging module includes two groups. Each group of the charging module includes a first clamping diode, a second clamping diode, a first switch, a soft start unit, and a second switch. The positive electrodes of the two first clamping diodes are respectively used to connect between a first capacitor and a second capacitor on both sides of the flying capacitor three-level converter. Among them, the first capacitor and the second capacitor on the same side of the flying capacitor three-level converter are connected in series and then connected to the positive bus and the negative bus. The negative electrodes of the two first clamping diodes are respectively used to connect to the positive electrodes of the flying capacitors on both sides of the flying capacitor three-level converter. The negative electrodes of the two second clamping diodes are respectively and correspondingly connected to the positive electrodes of the two first clamping diodes. The positive electrodes of the second clamping diodes are respectively used to connect to the negative electrodes of the flying capacitors. One ends of the two first switches are respectively used to connect to the negative electrodes of the flying capacitors. The other ends of the first switches are connected to the negative bus. The two second switches are respectively connected in series on both sides of the positive bus. The soft start unit is respectively connected in parallel with the two second switches. After the soft start unit is closed, it is used to charge the buses on both sides. After the first switch and the second switch are closed, they are used to charge the flying capacitors.

[0007] Furthermore, the charging module further includes a third switch. One end of the third switch is used to connect between the first capacitor and the second capacitor, and the other end of the third switch is connected between the first clamping diode and the second clamping diode of the same group of the charging module.

[0008] Furthermore, the soft start unit includes a fourth switch and a first resistor, and the fourth switch and the first resistor are connected in series.

[0009] Furthermore, the first switch is a first switching tube. The charging module further includes a second resistor. The emitter of the first switching tube is connected to the negative bus. The collector of the first switching tube is connected to one end of the second resistor, and the other end of the second resistor is used to connect to the negative electrode of the flying capacitor.

[0010] The present invention also provides a flying capacitor three-level converter, including:

[0011] Converter module, where the converter module includes two groups. The converter module includes a switching tube unit, a flying capacitor, a first capacitor, and a second capacitor. The flying capacitors are arranged in one-to-one correspondence with the switching tube units. The switching tube units are respectively connected to the positive bus and the negative bus. The two groups of switching tube units are connected by an inductor. The flying capacitors are connected in parallel with the corresponding switching tube units. The negative electrode of the first capacitor is connected to the positive electrode of the second capacitor. The positive electrode of the first capacitor is connected to the positive bus. The negative electrode of the second capacitor is connected to the negative bus. The converter module is symmetrically arranged on both sides of the positive bus and the negative bus;

[0012] Charging module, the charging module includes two groups, each group of the charging module includes a first clamping diode, a second clamping diode, a first switch, a soft start unit and a second switch. The anodes of the two first clamping diodes are respectively connected between the first capacitor and the second capacitor of the two groups of converter modules. The cathodes of the two first clamping diodes are respectively connected to the anodes of the flying capacitors of the two groups of converter modules. The cathodes of the two second clamping diodes are respectively and correspondingly connected to the anodes of the two first clamping diodes. The anodes of the second clamping diodes are respectively connected to the cathodes of the flying capacitors. One ends of the two first switches are respectively connected to the cathodes of the flying capacitors. The other ends of the first switches are connected to the negative bus. The two second switches are respectively connected in series on both sides of the positive bus. The soft start unit is respectively connected in parallel with the two second switches. After the soft start unit is closed, it is used to charge the buses on both sides. After the first switch and the second switch are closed, it is used to charge the flying capacitor.

[0013] Furthermore, the flying capacitor three-level converter further includes a plurality of third resistors, and the third resistors are respectively connected in parallel with the first capacitor, the second capacitor and the flying capacitor.

[0014] Furthermore, the converter module further includes a discharge module, and the discharge module is used to release the electric energy stored in the first capacitor and the second capacitor. Both ends of the discharge module are respectively connected to the positive bus and the negative bus.

[0015] Furthermore, the discharge module includes a fifth switch and a fourth resistor, and the fifth switch is connected in series with the fourth resistor.

[0016] Furthermore, the switch tube unit includes four second switch tubes, and each of the second switch tubes is connected in series. One end of the first second switch tube is connected to the positive bus, and one end of the fourth second switch tube is connected to the negative bus. The anode of the flying capacitor is connected between the first second switch tube and the second second switch tube. The cathode of the flying capacitor is connected between the third second switch tube and the fourth second switch tube.

[0017] Furthermore, one end of the inductor is connected to the second second switch tube and the third second switch tube of one group of the switch tube units, and the other end of the inductor is connected to the second second switch tube and the third second switch tube of the other group of the switch tube units.

[0018] The pre-charge circuit based on negative pre-charge provided by the present invention charges the busbars on both sides by closing the soft-start unit. When the voltages of the busbars on both sides reach the predetermined values, the soft-start unit is disconnected, the soft-start switch is disconnected, the second switch is closed, and the first switch is closed to form a charging loop, and the flying capacitor starts to be charged. By using the voltage difference between the busbars on the left and right sides, different charging paths are formed to pre-charge the flying capacitors on both sides respectively, and the soft start is completed. There is no need to add switches or power semiconductor devices in the connection line between the flying capacitor and the main circuit. Compared with the traditional method of pre-charging the positive electrode of the flying capacitor (i.e., adding a switching device on the positive electrode), the pre-charge circuit of the present invention is based on negative pre-charge, without adding a switching device on the positive electrode, which can effectively reduce the number of devices in the pre-charge circuit, simplify the structure of the pre-charge circuit, and the pre-charge circuit is provided with a first clamping diode and a second clamping diode, which can make the pre-charge process safer. Moreover, the pre-charge circuit of the present invention does not require adding switches or power semiconductor devices in the connection line between the flying capacitor and the switch tube unit, and will not become a part of the circuit topology after the soft start is completed, and will not affect the service life of the converter system. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 FIG. is a schematic circuit diagram of a flying-capacitor three-level converter according to an embodiment of the present invention.

[0020] Figure 2 FIG. is a schematic diagram of the current path from the first moment to the second moment during the pre-charge process of a flying-capacitor three-level converter according to an embodiment of the present invention.

[0021] Figure 3 FIG. is a schematic diagram of the current path from the fifth moment to the sixth moment during the pre-charge process of a flying-capacitor three-level converter according to an embodiment of the present invention.

[0022] Figure 4 FIG. is a schematic circuit diagram of a flying-capacitor three-level converter according to another embodiment of the present invention.

[0023] Figure 5 FIG. is a schematic circuit diagram of a flying-capacitor three-level converter according to another embodiment of the present invention.

[0024] REFERENCE SIGNS:

[0025] Converter module 100, switch tube unit 110, second switch tube Sa1, flying capacitor C1; first capacitor C2, second capacitor C3, first clamping diode D1, second clamping diode D2, second switch K1, first switch K2, soft-start unit 200, fourth switch K3, first resistor R1, second resistor R2, third switch K4; inductor L1; third resistor R3; discharge module 300, fifth switch K5, fourth resistor R4. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0027] The present invention provides a pre-charging circuit based on negative pre-charging and a flying capacitor three-level converter. The pre-charging circuit based on negative pre-charging can be applied to the flying capacitor three-level converter to pre-charge capacitors such as flying capacitors, so that the voltage value of the flying capacitor is half of the input voltage value, and it can effectively avoid overvoltage damage of the power semiconductor devices in the circuit topology of the two-phase interleaved converter at the moment of conduction.

[0028] Embodiment 1

[0029] Please refer to Figure 1 , the present invention provides a pre-charging circuit based on negative pre-charging, including a charging module. The charging module includes two groups. Each group of the charging module includes a first clamping diode D1, a second clamping diode D2, a first switch K2, a soft-start unit 200 and a second switch K1. The anodes of the two first clamping diodes D1 are respectively used to be connected between a first capacitor C2 and a second capacitor C3 on both sides of the flying capacitor three-level converter. Among them, the first capacitor C2 and the second capacitor C3 on the same side of the flying capacitor three-level converter are connected in series and then connected to the positive bus and the negative bus. The cathodes of the two first clamping diodes D1 are respectively used to be connected to the anodes of the flying capacitors C1 on both sides of the flying capacitor three-level converter. The cathodes of the two second clamping diodes D2 are respectively correspondingly connected to the anodes of the two first clamping diodes D1. The anodes of the second clamping diodes D2 are respectively used to be connected to the cathodes of the flying capacitors C1. One ends of the two first switches K2 are respectively used to be connected to the cathodes of the flying capacitors C1. The other ends of the first switches K2 are connected to the negative bus. The two second switches K1 are respectively connected in series on both sides of the positive bus. The soft-start unit 200 is respectively connected in parallel with the two second switches K1. After the soft-start unit 200 is closed, it is used to charge the buses on both sides. After the first switch K2 and the second switch K1 are closed, they are used to charge the flying capacitor C1.

[0030] An embodiment of the present invention provides a pre - charge circuit based on negative - terminal pre - charge. After connecting to a battery or the power grid, the soft - start unit 200 is closed to charge the busbars on both sides respectively. When the voltages of the busbars on both sides reach a predetermined value, the soft - start unit 200 is disconnected, the second switch K1 is closed, and the first switch K2 is closed to form a charging circuit. At the same time, using the voltage difference between the busbars on the left and right sides, the flying - capacitor C1 starts to be charged, forming different charging paths to pre - charge the flying - capacitors C1 on both sides respectively, and completing the soft - start. Compared with the traditional positive - terminal pre - charge of the flying - capacitor C1 (i.e., adding a switching device on the positive terminal), the pre - charge circuit of the present invention is based on negative - terminal pre - charge, without adding a switching device on the positive terminal, which can effectively reduce the number of devices in the pre - charge circuit, simplify the structure of the pre - charge circuit, and the pre - charge circuit is provided with a first clamping diode D1 and a second clamping diode D2, which can make the pre - charge process safer. Moreover, the pre - charge circuit of the present invention does not need to add a switch or a power semiconductor device in the connection line between the flying - capacitor C1 and the switching - tube unit 110, and will not become a part of the circuit topology after the soft - start is completed, and will not affect the service life of the converter system.

[0031] Among them, in this embodiment, the soft - start unit 200 includes a fourth switch K3 and a first resistor R1, and the fourth switch K3 and the first resistor R1 are connected in series.

[0032] In this embodiment, the first switch K2 is a first switching tube, and the charging module further includes a second resistor R2. The emitter of the first switching tube is connected to the negative busbar, the collector of the first switching tube is connected to one end of the second resistor R2, and the other end of the second resistor R2 is connected to the negative terminal of the flying - capacitor C1. By setting the second resistor R2, the first switch K2 can be protected.

[0033] Embodiment 2

[0034] Please refer to Figure 5 , the pre - charge circuit based on negative - terminal pre - charge further includes a third switch K4. One end of the third switch K4 is used to connect between the first capacitor C2 and the second capacitor C3, and the other end of the third switch K4 is connected between the first clamping diode D1 and the second clamping diode D2. In this embodiment, setting the third switch K4 between the first clamping diode D1, the second clamping diode D2, and the first capacitor C2 and the second capacitor C3 can eliminate the current spikes generated by the first clamping diode D1 and the second clamping diode D2.

[0035] Embodiment 3

[0036] The present invention also provides a flying - capacitor three - level converter, including: a converter module and a charging module.

[0037] The converter module includes two groups. The converter module includes a switching tube unit 110, a flying capacitor C1, a first capacitor C2, and a second capacitor C3. The flying capacitor C1 is arranged in one-to-one correspondence with the switching tube unit 110. The switching tube unit 110 is respectively connected to the positive bus and the negative bus. The two groups of switching tube units 110 are connected by an inductor L1. The flying capacitor C1 is connected in parallel with the corresponding switching tube unit 110. The negative electrode of the first capacitor C2 is connected to the positive electrode of the second capacitor C3. The positive electrode of the first capacitor C2 is connected to the positive bus. The negative electrode of the second capacitor C3 is connected to the negative bus. The converter module is symmetrically arranged on both sides of the positive bus and the negative bus;

[0038] The charging module includes two groups. Each group of charging modules includes a first clamping diode D1, a second clamping diode D2, a first switch K2, a soft start unit 200, and a second switch K1. The positive electrodes of the two first clamping diodes D1 are respectively connected between the first capacitor C2 and the second capacitor C3 of the two groups of converter modules. The negative electrodes of the two first clamping diodes D1 are respectively connected to the positive electrodes of the flying capacitors C1 of the two groups of converter modules. The negative electrodes of the two second clamping diodes D2 are respectively and correspondingly connected to the positive electrodes of the two first clamping diodes D1. The positive electrodes of the second clamping diodes D2 are respectively used to be connected to the negative electrodes of the flying capacitors C1. One ends of the two first switches K2 are respectively used to be connected to the negative electrodes of the flying capacitors C1. The other ends of the first switches K2 are connected to the negative bus. The two second switches K1 are respectively connected in series on both sides of the positive bus. The soft start unit 200 is respectively connected in parallel with the two second switches K1. After the soft start unit 200 is closed, it is used to charge the buses on both sides. After the first switch K2 and the second switch K1 are closed, it is used to charge the flying capacitor C1.

[0039] Among them, a flying capacitor three-level converter according to an embodiment of the present invention is a bidirectional converter. When the converter is in use, one side of the positive bus and one side of the negative bus can be used as the input end or the output end, and the other sides of the positive bus and the negative bus are correspondingly the output end or the input end. The input end and the output end are not limited to a certain side, but can be adjusted according to the usage situation.

[0040] In this embodiment, the first capacitor C2 and the second capacitor C3 are connected in series for voltage division. By setting the first clamping diode D1 and the second clamping diode D2, double clamping of the flying capacitor C1 can be achieved, making the pre-charging process safer.

[0041] It can be understood that, in this embodiment, the flying capacitor three-level converter includes a left bus and a right bus. One group of converter modules is arranged on the left bus and is respectively connected to the positive bus and the negative bus on the left side. The other group of converter modules is arranged on the right bus and is respectively connected to the positive bus and the negative bus on the right side. The two groups of converter modules have the same composition and the same structure.

[0042] Embodiment 4

[0043] Please refer to Figure 1 , the switching transistor unit 110 includes four second switching transistors Sa1, each of the second switching transistors Sa1 is connected in series, one end of a second switching transistor Sa1 at both ends is connected to the positive bus, and one end of another second switching transistor Sa1 at both ends is connected to the negative bus.

[0044] Specifically, the second switching transistors Sa1 of a group of switching transistor units 110 include four, and in the direction from the positive bus to the negative bus are the first second switching transistor Sa1, the second second switching transistor Sa1, the third second switching transistor Sa1, and the fourth second switching transistor Sa1 in sequence. The collector of the first second switching transistor Sa1 is connected to the positive bus, the emitter of the first second switching transistor Sa1 is connected to the collector of the second second switching transistor Sa1, the emitter of the second second switching transistor Sa1 is connected to the collector of the third second switching transistor Sa1, the emitter of the third second switching transistor Sa1 is connected to the collector of the fourth second switching transistor Sa1, and the emitter of the fourth second switching transistor Sa1 is connected to the negative bus. The two groups of switching transistor units 110 are both connected in the above form and are respectively arranged on the positive bus and the negative bus on both sides. A diode is connected to the collector and emitter of each second switching transistor Sa1, and this diode can prevent device damage caused by overvoltage due to the absence of a path in the second switching transistor Sa1 when the current reverses.

[0045] Among them, in this embodiment, the second switching transistor Sa1 is an IGBT (Insulated Gate Bipolar Transistor), that is, an insulated gate bipolar transistor. It can be understood that the second switching transistor Sa1 can also adopt other semiconductor power devices, and the T4 power supply is used as the driving circuit power supply for the semiconductor power device. The semiconductor power device has a low operation failure rate and a fast switching speed. When charging the flying capacitor C1, the PWM working mode is adopted to accurately control the voltage of the flying capacitor C1.

[0046] More specifically, the two groups of switching transistor units 110 are connected by an inductor L1. One end of the inductor L1 is connected between the second second switching transistor Sa1 and the third second switching transistor Sa1 of one group of switching transistor units 110 (that is, between the emitter of the second second switching transistor Sa1 and the collector of the third second switching transistor Sa1), and the other end of the inductor L1 is connected between the second second switching transistor Sa1 and the third second switching transistor Sa1 of the other group of switching transistor units 110 (that is, between the emitter of the second second switching transistor Sa1 and the collector of the third second switching transistor Sa1). The inductor L1 plays a role in filtering and current limiting. It can be understood that when the number of switching transistor units 110 corresponding to the buses on both sides is greater than one group respectively, the corresponding inductor L1 is set.

[0047] Please refer to Figure 1 , the positive electrode of the flying capacitor C1 is connected between the first second switching tube Sa1 and the second second switching tube Sa1, and the negative electrode of the flying capacitor C1 is connected between the third second switching tube Sa1 and the fourth second switching tube Sa1. The connection methods of the flying capacitors C1 of the two converter modules 100 and the switching tube unit 110 are the same, and will not be elaborated here.

[0048] Among them, the switching tube unit 110 includes two groups. Correspondingly, the number of flying capacitors C1 is also two. In this embodiment, the number of flying capacitors C1 is adjusted according to the number of the switching tube units 110. The pre-charging circuit based on negative electrode pre-charging for charging the flying capacitor C1 is not limited to the two groups of switching tube units 110.

[0049] Embodiment 5

[0050] Please refer to Figure 4 , the flying capacitor three-level converter further includes a plurality of third resistors R3, and the third resistors R3 are respectively connected in parallel with the first capacitor C2, the second capacitor C3 and the flying capacitor C1. In this embodiment, each of the first capacitor C2, the second capacitor C3 and the flying capacitor C1 is respectively connected in parallel with a third resistor R3. Through the third resistor R3, after the flying capacitor three-level converter is used up, the first capacitor C2, the second capacitor C3 and the flying capacitor C1 can be discharged through the third resistor R3.

[0051] Embodiment 6

[0052] Please refer to Figure 1 , the converter module further includes a discharge module 300, and the discharge module 300 is used to release the electric energy stored in the first capacitor C2 and the second capacitor C3. Both ends of the discharge module 300 are respectively connected to the positive bus and the negative bus. After the flying capacitor three-level converter is used up, it is necessary to discharge the capacitors such as the first capacitor C2 and the second capacitor C3. By setting the discharge module 300, the capacitors such as the first capacitor C2 and the second capacitor C3 can be discharged, avoiding damage to the flying capacitor three-level converter.

[0053] In this embodiment, the discharge module 300 includes a fifth switch K5 and a fourth resistor R4, and the fifth switch K5 is connected in series with the fourth resistor R4. Specifically, one end of the fifth switch K5 is connected to the positive bus, the other end of the fifth switch K5 is connected to one end of the fourth resistor R4, and the other end of the fourth resistor R4 is connected to the negative bus. More specifically, a discharge module 300 is provided on each of the two sides of the bus. One end of the fifth switch K5 of one set of discharge modules 300 is connected to the positive bus on the left side, the other end of the fifth switch K5 is connected to one end of the fourth resistor R4, and the other end of the fourth resistor R4 is connected to the negative bus on the left side. The discharge module 300 is located between the soft start unit 200 on the left side and the first capacitor C2 and the second capacitor C3. One end of the fifth switch K5 of the other set of discharge modules 300 is connected to the positive bus on the right side, the other end of the fifth switch K5 is connected to one end of the fourth resistor R4, and the other end of the fourth resistor R4 is connected to the negative bus on the right side. The discharge module 300 is located between the soft start unit 200 on the right side and the first capacitor C2 and the second capacitor C3.

[0054] After using the flying capacitor three-level converter, after closing the second switch K1 and the fifth switch K5, all the capacitors such as the first capacitor C2 and the second capacitor C3 in the flying capacitor three-level converter can be discharged through the fourth resistor R4, and the first clamping diode D1 can provide a discharge loop for the pre-charging of the flying capacitor C1. By setting the third resistor R3, the capacitors connected in parallel thereto can be discharged. It can be ensured that after using the flying capacitor three-level converter, the capacitors can be discharged to avoid damage to the flying capacitor three-level converter.

[0055] It can be understood that Figure 4 The third resistor R3 in the embodiment can be selectively set or not set as needed. If the third resistor R3 is set, the discharge module 300 discharges all the capacitors such as the first capacitor C2 and the second capacitor C3, while the third resistor R3 discharges the capacitors connected in parallel thereto. If the third resistor R3 is not set, the discharge module is used to discharge all the capacitors such as the first capacitor C2 and the second capacitor C3.

[0056] It can be understood that the second switch K1 to the fifth switch K5 can adopt low-power switches, which can effectively reduce costs, and the second switch K1 and the fifth switch K5 can also adopt common switch devices such as relays, contactors, circuit breakers or power semiconductor devices. The first resistor R1 and the fourth resistor R4 can adopt power resistors or other common resistors, and are selected according to requirements such as design cost or power.

[0057] Embodiment 6

[0058] Please refer to Figure 1, an embodiment of the present invention provides a pre-charge circuit based on negative terminal pre-charging and a flying capacitor three-level converter. Each second switch tube Sa1 in the switch tube unit 110 is connected in series in sequence. One end of the first second switch tube Sa1 is connected to the positive bus, and one end of the fourth second switch tube Sa1 is connected to the negative bus. The positive electrode of the flying capacitor C1 is connected between the first second switch tube Sa1 and the second second switch tube Sa1, and the negative electrode of the flying capacitor C1 is connected between the third second switch tube Sa1 and the fourth second switch tube Sa1. The connection modes of the second switch tubes Sa1 and the flying capacitor C1 in the two groups of switch tube units 110 are the same. One group of switch tube units 110 is located on the left side of the positive and negative buses, and the other group of switch tube units 110 is located on the right side of the positive and negative buses. One end of the inductor L1 is connected between the second second switch tube Sa1 and the third second switch tube Sa1 of the same group of switch tube units 110, and the other end of the inductor L1 is connected between the second second switch tube Sa1 and the third second switch tube Sa1 of the other group of switch tube units 110. When using the pre-charge circuit based on negative terminal pre-charging, the left side of the positive and negative buses can be used as the input terminal or the output terminal. Correspondingly, the right side of the positive and negative buses can be used as the output terminal or the input terminal. The pre-charge circuit based on negative terminal pre-charging has the ability of bidirectional energy flow, and the positions of the input terminal and the output terminal can be swapped. When the left side of the positive and negative buses is used as the input terminal and the right side of the positive and negative buses is used as the output terminal, the flying capacitor C1 on the left side is the flying capacitor C1 of the input terminal, and the flying capacitor C1 on the right side is the flying capacitor C1 of the output terminal. The pre-charge circuit based on negative terminal pre-charging of the present invention can charge the flying capacitor C1 of the input terminal and the flying capacitor C1 of the output terminal simultaneously, without separately designing pre-charge circuits for the flying capacitor C1 of the input terminal and the flying capacitor C1 of the output terminal, which can simplify the structure of the entire pre-charge circuit.

[0059] A first capacitor C2 and a second capacitor C3 are provided on the left side of the positive and negative busbars. The first capacitor C2 and the second capacitor C3 are connected in series. A discharge module 300 is provided on the left side of the positive and negative busbars. The fifth switch K5 and the fourth resistor R4 in the discharge module 300 are connected in series. The first capacitor C2 and the second capacitor C3 are located between the discharge module 300 and the switch unit 110. Similarly, a first capacitor C2, a second capacitor C3 and a discharge module 300 are provided on the right side of the positive and negative busbars, and the connection method and the setting position are the same as those on the left side of the positive and negative busbars. The two ends of one second switch K1 are connected to the left side of the positive busbar, and the two ends of the other second switch K1 are connected to the right side of the positive busbar. Correspondingly, a group of soft start units 200 are connected in parallel with the second switch K1 on the left side, and another group of soft start units 200 are connected in parallel with the second switch K1 on the right side. More specifically, the fourth switch K3 in the soft start unit 200 is connected in series with the first resistor R1 and then connected in parallel with the second switch K1. The positive electrode of the first clamping diode D1 is connected between the first capacitor C2 and the second capacitor C3. The negative electrode of the first clamping diode D1 is connected to the positive electrode of the flying capacitor C1. The negative electrode of the second clamping diode D2 is connected to the positive electrode of the first clamping diode D1. The positive electrode of the second clamping diode D2 is connected to the negative electrode of the flying capacitor C1. One end of the first switch K2 is connected to the negative electrode of the flying capacitor C1, and the other end of the first switch K2 is connected to the negative busbar. Among them, the connection methods of the first clamping diode D1, the second clamping diode D2 and the first switch K2 on both sides of the busbars are the same, and will not be elaborated here.

[0060] Please refer to Figure 2 and Figure 3 , when performing pre-charging, it is divided into the following 8 moments:

[0061] At the first moment, the fourth switch K3 of the soft start unit 200 on the left side is closed to charge the left busbar. At this time, the voltages of the flying capacitor C1 on the right side and the first capacitor C2 and the second capacitor C3 on the right side increase;

[0062] At the second moment, when the voltage of the left busbar is charged to a predetermined value, the second switch K1 is closed and the fourth switch K3 is opened. Among them, the predetermined value is set manually and can be preset according to the parameters of the pre-charging process.

[0063] Among them, during the process from the first moment to the second moment, when the left busbar is charged, the voltages of the flying capacitors C1 on both sides will also be slightly charged. The charging current path at this time is as Figure 2As shown. During the first moment to the second moment, the charging path is as follows: from the first capacitor C2 on the left to the second capacitor C3. Part of the current flows from the first capacitor C2 on the left to the first clamping diode D1 on the left, the flying capacitor C1 on the left, the diode anti-parallel to the third second switch Sa1 of the switching tube unit 110 on the left, the inductor L1, the diode anti-parallel to the second second switch Sa1 of the switching tube unit 110 on the right. Part of the current flows to the diode anti-parallel to the first second switch Sa1, and part of the current flows to the flying capacitor C1 on the right. The current flowing through the diode anti-parallel to the first second switch Sa1 flows to the first capacitor C2 and the second capacitor C3 on the right. The current flowing through the flying capacitor C1 on the right flows through the second clamping diode D2 on the right and then through the second capacitor C3.

[0064] At the third moment, close the fourth switch K3 of the soft start unit 200 on the right to charge the right bus.

[0065] At the fourth moment, when the voltage of the right bus is charged to a predetermined value, close the second switch K1 and open the fourth switch K3. Here, the predetermined value is set manually and can be preset according to the parameters of the pre-charging process.

[0066] At the fifth moment, close the first switch K2 on the left to charge the flying capacitor C1 on the left until the flying capacitor C1 on the left is charged to a predetermined value (i.e., the voltage of the flying capacitor C1 is equal to half of the input voltage). At the same time, after charging the flying capacitor C1 on the left, the voltage of the right bus will be higher than that on the left, and the flying capacitor C1 on the right will also be charged to a certain extent.

[0067] At the sixth moment, close the first switch K2 on the right. The flying capacitor C1 on the left is pre-charged to a predetermined value. At the same time, since the voltage of the right bus is higher than that of the left bus, a charging path is formed for the flying capacitor C1 on the right to charge the flying capacitor C1 on the right.

[0068] Among them, from the fifth moment to the sixth moment, when the flying capacitor C1 on the left is pre-charged, since the voltage of the right bus is higher than that of the left bus, the midpoint potential of the first clamping diode D1 and the second clamping diode D2 on the right is higher than the midpoint potential of the first clamping diode D1 and the second clamping diode D2 on the left, and the voltage of the flying capacitor C1 on the right also rises. A charging path is formed for the flying capacitor C1 on the right, and the charging path is as Figure 3As shown in the figure, the charging path on the right side is as follows: from the first clamping diode D1 on the right side, the flying capacitor C1 on the right side, the diode anti-parallel to the third second switching tube Sa1 on the right side, the inductor L1, the diode anti-parallel to the second second switching tube Sa1 on the left side, the flying capacitor C1 on the left side to the first switch K2 on the left side, forming a charging loop. When the bus voltage on the right side is lower than the bus voltage on the left side, there is no charging path for the flying capacitor C1 on the right side. The charging path on the left side is from the first capacitor C2, the first clamping diode D1 on the left side, the flying capacitor C1 on the left side to the first switch K2 on the left side, forming a charging loop.

[0069] At the seventh moment, the circuit of the converter module (i.e., the main power circuit) starts to work, adjusting the flying capacitor C1 on the right side to a predetermined value, that is, the voltage of the flying capacitor C1 on the right side is equal to half of the input voltage.

[0070] At the eighth moment, the current of the inductor L1 reaches the rated value, and the flying capacitor three-level converter enters a steady state.

[0071] After the pre-charging is completed, by closing the second switch K1, the flying capacitor three-level converter can operate normally. After use, closing the fifth switch K5 discharges the capacitors such as the first capacitor C2 and the second capacitor C3.

[0072] The above pre-charging circuit based on negative pole pre-charging charges the busbars on both sides by closing the soft-start unit 200 respectively. When the busbar voltages on both sides reach the predetermined values, the soft-start unit 200 is disconnected, the second switch K1 is closed, and the first switch K2 is closed to form a charging loop. At the same time, using the voltage difference between the busbars on the left and right sides, the flying capacitor C1 starts to be charged, forming different charging paths to pre-charge the flying capacitors C1 on both sides respectively and complete the soft start. Compared with the traditional positive pole pre-charging of the flying capacitor C1 (i.e., adding switching devices on the positive pole), the pre-charging circuit of the present invention is based on negative pole pre-charging, without adding switching devices on the positive pole, which can effectively reduce the number of devices in the pre-charging circuit, simplify the structure of the pre-charging circuit, and the pre-charging circuit is provided with the first clamping diode D1 and the second clamping diode D2, which can make the pre-charging process safer. And the pre-charging circuit of the present invention does not need to add switches or power semiconductor devices in the connection line between the flying capacitor C1 and the switching tube unit 110, and will not become a part of the circuit topology after the soft start is completed, and will not affect the service life of the converter system.

[0073] Please refer to Figure 4 This invention provides a pre-charging circuit based on negative pole pre-charging and a flying capacitor three-level converter in another embodiment. Compared with Figure 1Differently, a third switch K4 is added. One end of the third switch K4 is connected between the first capacitor C2 and the second capacitor C3, and the other end of the third switch K4 is connected between the first clamping diode D1 and the second clamping diode D2. In this embodiment, by arranging the third switch K4 between the first clamping diode D1, the second clamping diode D2, the first capacitor C2 and the second capacitor C3, the current spikes generated by the first clamping diode D1 and the second clamping diode D2 can be eliminated.

[0074] The operation mode of the pre-charging circuit based on negative pole pre-charging in this embodiment is similar to Figure 1 the operation mode in the embodiment, and will not be elaborated here.

[0075] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A pre - charging circuit based on negative - pole pre - charge, characterized in that, it includes: A charging module, the charging module includes two groups. Each group of the charging module includes a first clamping diode, a second clamping diode, a first switch, a soft - start unit, and a second switch. The anodes of the two first clamping diodes are respectively used to be connected between a first capacitor and a second capacitor on both sides of a flying - capacitor three - level converter. Wherein, the first capacitor and the second capacitor on the same side of the flying - capacitor three - level converter are connected in series and then connected to the positive bus and the negative bus. The cathodes of the two first clamping diodes are respectively used to be connected to the anodes of the flying capacitors on both sides of the flying - capacitor three - level converter. The cathodes of the two second clamping diodes are respectively and correspondingly connected to the anodes of the two first clamping diodes. The anodes of the second clamping diodes are respectively used to be connected to the cathodes of the flying capacitors. One ends of the two first switches are respectively used to be connected to the cathodes of the flying capacitors. The other ends of the first switches are connected to the negative bus. The two second switches are respectively connected in series on both sides of the positive bus. The soft - start unit is respectively connected in parallel with the two second switches. After the soft - start unit is closed, it is used to charge the buses on both sides. After the first switch and the second switch are closed, they are used to charge the flying capacitors. The first switch and the second switch are IGBTs.

2. The pre - charging circuit based on negative - pole pre - charge according to claim 1, characterized in that, the charging module further includes a third switch. One end of the third switch is used to be connected between the first capacitor and the second capacitor, and the other end of the third switch is connected between the first clamping diode and the second clamping diode of the same group of the charging module.

3. The pre - charging circuit based on negative - pole pre - charge according to claim 1, characterized in that, the soft - start unit includes a fourth switch and a first resistor, and the fourth switch and the first resistor are connected in series.

4. The pre - charging circuit based on negative - pole pre - charge according to claim 1, characterized in that, the first switch is a first switch tube. The charging module further includes a second resistor. The emitter of the first switch tube is connected to the negative bus. The collector of the first switch tube is connected to one end of the second resistor, and the other end of the second resistor is used to be connected to the cathode of the flying capacitor.

5. A flying - capacitor three - level converter, characterized in that, it includes: A converter module, the converter module includes two groups. The converter module includes a switch - tube unit, a flying capacitor, a first capacitor, and a second capacitor. The flying capacitor is arranged in one - to - one correspondence with the switch - tube unit. The switch - tube unit is respectively connected to the positive bus and the negative bus. The two groups of switch - tube units are connected by an inductor. The flying capacitor is connected to the corresponding switch - tube unit. The cathode of the first capacitor is connected to the anode of the second capacitor. The anode of the first capacitor is connected to the positive bus. The cathode of the second capacitor is connected to the negative bus. The converter module is symmetrically arranged on both sides of the positive bus and the negative bus; Charging module, the charging module includes two groups, each group of the charging module includes a first clamping diode, a second clamping diode, a first switch, a soft start unit and a second switch. The anodes of the two first clamping diodes are respectively connected between the first capacitor and the second capacitor of the two groups of the converter modules. The cathodes of the two first clamping diodes are respectively connected to the anodes of the flying capacitors of the two groups of the converter modules. The cathodes of the two second clamping diodes are respectively and correspondingly connected to the anodes of the two first clamping diodes. The anodes of the second clamping diodes are respectively connected to the cathodes of the flying capacitors. One ends of the two first switches are respectively connected to the cathodes of the flying capacitors. The other ends of the first switches are connected to the negative bus. The two second switches are respectively connected in series on both sides of the positive bus. The soft start unit is respectively connected in parallel with the two second switches. After the soft start unit is closed, it is used to charge the buses on both sides. After the first switch and the second switch are closed, it is used to charge the flying capacitor. The first switch and the second switch are IGBTs.

6. The flying capacitor three-level converter according to claim 5, characterized in that, it further includes a plurality of third resistors, and the third resistors are respectively connected in parallel with the first capacitor, the second capacitor and the flying capacitor.

7. The flying capacitor three-level converter according to claim 5, characterized in that, the converter module further includes a discharging module, and the discharging module is used to release the electric energy stored in the first capacitor and the second capacitor. Both ends of the discharging module are respectively connected to the positive bus and the negative bus.

8. The flying capacitor three-level converter according to claim 7, characterized in that, the discharging module includes a fifth switch and a fourth resistor, and the fifth switch is connected in series with the fourth resistor.

9. The flying capacitor three-level converter according to claim 5, characterized in that, the switching tube unit includes four second switching tubes, and each of the second switching tubes is connected in series. One end of the first second switching tube is connected to the positive bus, and one end of the fourth second switching tube is connected to the negative bus. The anode of the flying capacitor is connected between the first second switching tube and the second second switching tube, and the cathode of the flying capacitor is connected between the third second switching tube and the fourth second switching tube.

10. The flying capacitor three-level converter according to claim 9, characterized in that, one end of the inductor is connected to the second second switching tube and the third second switching tube of one group of the switching tube units, and the other end of the inductor is connected to the second second switching tube and the third second switch of the other group of the switching tube units.

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