Flying capacitor three-level Buck / Boost converter and pre-charge method

Abstract

This application discloses a flying capacitor three-level Buck / Boost converter and a pre-charging method, relating to the field of DC-DC conversion. The converter includes a first power supply, a second power supply, a first capacitor connected in parallel across the two ends of the first power supply, and a second capacitor connected in parallel across the two ends of the second power supply; a first switch, a second switch, a third switch, and a fourth switch connected in series and then in parallel with the first power supply; one end of the second capacitor is electrically connected to the series terminals of the second and third switches through an inductor, and the other end of the second capacitor is electrically connected to the other end of the fourth switch; a pre-charging circuit is connected in parallel with the first power supply, the pre-charging circuit including a first switch, a first resistor, a flying capacitor, a second resistor, and a second switch connected in series; the first switch and the second switch are used to control the first power supply or the second power supply to pre-charge the flying capacitor. This application can avoid the problem of easy damage to power devices during normal startup of the converter.

Description

Technical Field

[0001] This application relates to the field of DC-DC conversion, and more specifically, to a flying capacitor three-level Buck / Boost converter and a pre-charging method. Background Technology

[0002] Non-isolated DC-DC converters are suitable for photovoltaic DC energy storage systems with a wide voltage range due to their fewer components, lower cost, and higher voltage gain. Among them, the flying capacitor three-level Buck / Boost converter has the advantages of low switching voltage stress and small output filter inductor size, making it widely used in high-power energy storage systems.

[0003] When analyzing and studying three-level Buck / Boost converters with flying capacitors, most literature assumes that the flying capacitor is already at a suitable voltage before the circuit starts operating. In reality, the initial voltage of the flying capacitor is zero before the circuit starts operating. If the converter is started at this time, the power devices will bear the entire bus voltage, which can easily lead to damage. Summary of the Invention

[0004] This application aims to at least address the problem in existing technologies where the switching transistors in a three-level Buck / Boost converter with a flying capacitor are easily damaged during charging. To this end, this application provides an example of a three-level Buck / Boost converter with a flying capacitor and a charging method, which solves the problem of excessive voltage stress on the power devices during normal startup of the three-level Buck / Boost converter with a flying capacitor by using two switching switches.

[0005] The solution presented in this application is implemented through the following steps.

[0006] In a first aspect, this application provides a flying capacitor three-level Buck / Boost converter, comprising:

[0007] A first power source, a second power source, a first capacitor connected in parallel across the two ends of the first power source, and a second capacitor connected in parallel across the two ends of the second power source.

[0008] The first switch transistor, the second switch transistor, the third switch transistor, and the fourth switch transistor are connected in series and then in parallel with the first power supply.

[0009] One end of the second capacitor is electrically connected to the series terminal of the second and third switching transistors via an inductor, and the other end of the second capacitor is electrically connected to the other end of the fourth switching transistor.

[0010] A pre-charging circuit connected in parallel with the first power supply, the pre-charging circuit comprising a first switch, a first resistor, a flying capacitor, a second resistor, and a second switch connected in series.

[0011] Wherein, the series terminal of the flying capacitor and the first resistor is electrically connected to the series terminal of the first switch and the second switch, and the series terminal of the flying capacitor and the second resistor is electrically connected to the series terminal of the third switch and the fourth switch; the first switch and the second switch are used to control the first power supply or the second power supply to precharge the flying capacitor.

[0012] In the above-described flying capacitor three-level Buck / Boost converter, optionally, the voltage of the first power supply is greater than the voltage of the second power supply, and the first power supply and the second power supply precharge the flying capacitor respectively, so that the converter operates in Buck mode and Boost mode.

[0013] In the above-described flying capacitor three-level Buck / Boost converter, optionally, the first switch is off and the second switch is on, and the first power supply precharges the flying capacitor to enable the converter to operate in Buck mode.

[0014] In the above-described flying capacitor three-level Buck / Boost converter, optionally, the first switch is open and the second switch is closed, and the second power supply precharges the flying capacitor to enable the converter to operate in Boost mode.

[0015] In the above-described flying capacitor three-level Buck / Boost converter, optionally, the preset charging voltage of the flying capacitor is half the voltage of the first power supply.

[0016] In the above-described flying capacitor three-level Buck / Boost converter, optionally, the conduction direction of the first switch, the second switch, the third switch, and the fourth switch is from the negative terminal to the positive terminal of the first power supply.

[0017] In the flying capacitor three-level Buck / Boost converter described above, optionally, the first switch and the second switch include one or more of relays, semiconductor switches, and power devices.

[0018] In the flying capacitor three-level Buck / Boost converter described above, optionally, the first, second, third, and fourth switching transistors include one or more of gallium nitride field-effect transistors, metal-oxide-semiconductor transistors, and bipolar junction transistors.

[0019] A second aspect of this application provides a charging method for a flying capacitor three-level Buck / Boost converter, used to charge the flying capacitor using the flying capacitor three-level Buck / Boost converter described in any of the first aspects above, the method comprising:

[0020] When the first switch is closed and the second switch is opened, the positive terminal of the first power supply outputs the first current;

[0021] The first current sequentially passes through the first switch, the first resistor, the flying capacitor, the third switch, the inductor, the second capacitor, and the negative terminal of the first power supply to form a first circuit, pre-charging the flying capacitor;

[0022] When the voltage of the flying capacitor reaches half of the voltage of the first power supply, the first switch is turned on to complete the pre-charging.

[0023] A third aspect of this application provides a charging method for a flying capacitor three-level Buck / Boost converter, used to charge the flying capacitor using the flying capacitor three-level Buck / Boost converter described in any one of the first aspects above, the method comprising:

[0024] When the second switch is closed and the first switch is opened, the positive terminal of the second power supply outputs a second current.

[0025] The second current sequentially passes through the inductor, the second switch, the flying capacitor, the second resistor, the second switch, and the negative terminal of the second power supply to form a second circuit, pre-charging the flying capacitor;

[0026] When the voltage of the flying capacitor reaches half of the voltage of the first power supply, the second switch is turned on to complete the pre-charging.

[0027] This application has the following beneficial effects:

[0028] In this embodiment, the pre-charge circuit can quickly pre-charge the flying capacitor by controlling the first and second switches before the converter operates normally, charging the voltage of the flying capacitor to a preset value. Then, the first and second switches are turned off to perform a soft start on the converter, avoiding the problem of excessive voltage stress on the power devices and damage to the power devices during normal startup of the Buck / Boost converter with a three-level flying capacitor. In addition, the first and second resistors on the pre-charge circuit can limit the charging current output by the first and second power supplies, preventing overcurrent damage to the power devices. Attached Figure Description

[0029] The above and other objects, features, and advantages of exemplary embodiments of this application will become readily understood by reading the following detailed description with reference to the accompanying drawings. In the drawings, several embodiments of this application are illustrated by way of example and not limitation, and the same or corresponding reference numerals denote the same or corresponding parts, wherein:

[0030] Figure 1 This is a circuit diagram of a flying capacitor three-level Buck / Boost converter according to an embodiment of this application;

[0031] Figure 2 This is a flowchart illustrating a pre-charging method for a flying capacitor three-level Buck / Boost converter according to an embodiment of this application.

[0032] Figure 3 This is a circuit diagram of the current loop of a pre-charging method according to an embodiment of this application;

[0033] Figure 4 This is a flowchart illustrating another pre-charging method for a flying capacitor three-level Buck / Boost converter according to an embodiment of this application.

[0034] Figure 5 This is a circuit diagram of the current loop for another pre-charging method according to an embodiment of this application.

[0035] Explanation of reference numerals in the attached figures:

[0036] First Power Supply - U H Second power supply -V bus First capacitor -C H Second capacitor -C bus First switching transistor - Q1; Second switching transistor - Q2; Third switching transistor - Q3; Fourth switching transistor - Q4; Flying capacitor - C fly First switch - S1; Second switch - S2; First resistor - R1; Second resistor - R2; Inductor - L. Detailed Implementation

[0037] The technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this disclosure. All other embodiments obtained by those skilled in the art based on the embodiments of this disclosure without creative effort are within the scope of protection of this disclosure.

[0038] For three-level converters with cross-capacitors, to avoid overvoltage issues in power devices during charging of the flying capacitor, a soft-start method is typically used to pre-charge the flying capacitor. The converter is then started only after the flying capacitor has been pre-charged to a set voltage value. This embodiment provides a method that uses a combination of opening and closing dual switches to pre-charge the flying capacitor, enabling the converter to operate in different modes.

[0039] like Figure 1The circuit diagram shown is of a flying capacitor three-level Buck / Boost converter. The converter includes a main circuit and a pre-charge circuit. Specifically, the main circuit includes a first power supply U. H Second power supply V bus First power supply U H The first capacitor C connected in parallel across its two ends H Second power supply V bus The second capacitor C is connected in parallel across its two ends. bus And connected in series with the first power supply U H The first switch Q1, the second switch Q2, the third switch Q3, and the fourth switch Q4 are connected in parallel, along with the inductor L; wherein, the second capacitor C bus One end of the second capacitor Q2 is electrically connected to the series connection of the second switch Q2 and the third switch Q3 via an inductor L, and the other end of the second capacitor Q2 is electrically connected to the other end of the fourth switch Q4. This converter can achieve both Buck and Boost operating modes through two power supplies.

[0040] In specific connection, the other end of the first switching transistor Q1 is connected in parallel to the first power supply U. H On the positive electrode, the other end of the fourth switch Q4 is connected in parallel to the first power supply U. H The negative electrode and the second power supply V bus On the negative electrode. When the transformer is working, the first switch Q1 is the first active switch, the second switch Q2 is the second active switch, the third switch Q3 is the first synchronous rectifier, and the fourth switch Q4 is the second synchronous rectifier.

[0041] In addition, the pre-charge circuit and the first power supply U H The parallel pre-charging circuit includes a first switch S1, a first resistor R1, and a flying capacitor C connected in series. fly The second resistor R2 and the second switch S2; wherein, the flying capacitor C fly The series terminal of the first resistor R1 is electrically connected to the series terminal of the first switch Q1 and the second switch Q2, and the flying capacitor C is connected to the series terminal of the second switch Q2. fly The series terminal of the second resistor R2 is electrically connected to the series terminal of the third switch Q3 and the fourth switch Q4; the first switch S1 and the second switch S2 are used to control the first power supply U. H Or a second power supply V bus For flying capacitor C fly Perform pre-charging.

[0042] The first resistor R1 is connected in series with the first switch S1 and the flying capacitor C. fly Between the second resistor R2 and the flying capacitor C, the second resistor R2 is connected in series with the second switch S2. fly Between, it can be used for the first power supply U H Output current and second power supply Vbus The output current is limited to prevent excessive current from damaging the power devices. The values ​​of the first resistor R1 and the second resistor R2 are selected according to the specific operating scenario, and will not be elaborated further in this embodiment.

[0043] In this embodiment, the pre-charging circuit can quickly pre-charge the flying capacitor before the converter operates normally by controlling the first and second switches, charging the voltage of the flying capacitor to a preset value. Then, the first and second switches are turned off to perform a soft start on the converter, avoiding the problem of excessive voltage stress on the power devices during normal startup of the three-level converter with flying capacitor, which could damage the power devices. In addition, the first and second resistors on the pre-charging circuit can limit the charging current output by the first and second power supplies, preventing overcurrent damage to the power devices.

[0044] In this embodiment, the first power supply U H The voltage is greater than that of the second power supply V. bus The voltage of the first power supply U H As the high-voltage side power supply, the first capacitor C H High-voltage bus capacitors are used, and the second power supply V bus As a low-voltage side power supply, the second capacitor C bus Low-voltage bus capacitors are used. First power supply U H和 Second power supply V bus The flying capacitor C fly Pre-charging is performed to enable the converter to operate in Buck and Boost modes. The first power supply U... H For flying capacitor C fly After pre-charging, the converter undergoes a soft start, entering Buck mode; the second power supply V... bus For flying capacitor C fly After pre-charging, the converter is soft-started and enters Boost mode.

[0045] Specifically, the first switch S1 is closed and the second switch S2 is opened, at which point the first power supply U... H For the flying capacitor C fly Pre-charge, and in the flying capacitor C fly After pre-charging, the first switch S1 is also turned on to put the converter into Buck mode; the first switch S1 is turned on and the second switch S2 is closed, at which point the second power supply V... bus For the flying capacitor C fly Pre-charge, and in the flying capacitor C fly After pre-charging, the second switch S2 is also turned on to enable the converter to operate in Boost mode.

[0046] Through the coordinated control of the first switch S1 and the second switch S2, the first power supply U is supplied.H Or a second power source V bus For flying capacitor C fly During pre-charging, the flying capacitor C fly The preset charging voltage is set to the first power supply U. H Half of the voltage.

[0047] Furthermore, the conduction direction of the first switch Q1, the second switch Q2, the third switch Q3, and the fourth switch Q4 connected in series is always the same as that of the first power supply U. H The negative terminal to the first power supply U H The positive direction.

[0048] In this embodiment, the first switch S1 and the second switch S2 include one or more of a relay, a semiconductor switch, and a power device; the first switch Q1, the second switch Q2, the third switch Q3, and the fourth switch Q4 include one or more of a gallium nitride field-effect transistor, a metal-oxide-semiconductor transistor, and a bipolar junction transistor, or any other suitable type of transistor or combination of transistors.

[0049] like Figure 2 This embodiment also provides a charging method for a flying capacitor three-level Buck / Boost converter, used to charge the flying capacitor using any of the above-mentioned flying capacitor three-level Buck / Boost converters. The method includes the following steps, in which the current flows as follows: Figure 3 As indicated by the dashed line and the arrow.

[0050] Pre-charging begins; the first switch S1 is closed and the second switch S2 is opened; the first power supply U... H The positive terminal outputs the first current; the first current passes sequentially through the first switch S1, the first resistor R1, and the flying capacitor C. fly Third switch Q31, inductor L, second capacitor C bus First power supply U H The negative terminal forms the first circuit, which is connected to the flying capacitor C. fly Pre-charging is performed, and current is limited by the first resistor R1 connected in series; in the flying capacitor C fly The voltage reaches the first power supply U H When the voltage is half, the first switch S1 is turned on to complete the pre-charging. The first power supply U... H The output current can be applied to the flying capacitor C. fly Precharge until the first power supply U is reached. H Half of the voltage. This method is used for the flying capacitor C. fly After pre-charging, the converter is started and enters Buck operating mode.

[0051] like Figure 4 This embodiment also provides a charging method for a flying capacitor three-level Buck / Boost converter, used to charge the flying capacitor using any of the above-mentioned flying capacitor three-level Buck / Boost converters. The method includes the following steps, in which the current flows as follows: Figure 5 As indicated by the dashed line and the arrow.

[0052] Pre-charging begins; the second switch S2 is closed and the first switch S1 is opened; the second power supply V... bus The positive terminal outputs a second current; the second current flows sequentially through inductor L, the second switch Q2, and flying capacitor C. fly Second resistor R2, second switch S2, second power supply V bus The negative terminal forms a second circuit, which is used for the flying capacitor V. bus Pre-charging is performed, and current is limited by the second resistor R2 connected in series; in the flying capacitor C fly The voltage reaches the first power supply U H When the voltage reaches half of the rated voltage, the second switch S2 is opened to complete the pre-charging. The second power supply V... bus The output current can be applied to the flying capacitor C. fly Precharge until the first power supply U is reached. H Half of the voltage. This method is used for the flying capacitor C. fly After pre-charging, the converter is started and enters Boost mode.

[0053] In the foregoing description of this application, unless otherwise expressly specified and limited, the terms "fixed," "installed," "connected," or "linked" should be interpreted broadly. For example, the term "linked" can refer to a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; or it can refer to the internal communication of two components or the interaction between two components. Therefore, unless otherwise expressly limited in this application, those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0054] Based on the above description of this application, those skilled in the art will also understand that terms used, such as "upper," "lower," "length," "width," "top," "bottom," "inner," "outer," "axial," "longitudinal," "transverse," "clockwise," or "counterclockwise," indicating orientation or positional relationships, are based on the orientation or positional relationships shown in the accompanying drawings of this application. These terms are used only for the purpose of facilitating the explanation of the application and simplifying the description, and are not intended to explicitly or implicitly suggest that the device or element involved must have the stated specific orientation, or be constructed and operated in a specific orientation. Therefore, the aforementioned orientation or positional relationship terms should not be understood or interpreted as limitations on the application.

[0055] Furthermore, the terms "first" or "second," etc., used in this application to refer to numbers or ordinal numbers are for convenience of description only and should not be construed as explicitly or implicitly indicating relative importance or specifying the number of indicated technical features. Also, a feature specified as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, or more, unless otherwise explicitly specified.

[0056] While numerous embodiments of this application have been shown and described herein, it will be appreciated by those skilled in the art that such embodiments are provided by way of example only. Many modifications, alterations, and alternatives will arise in the mind and spirit of this application without departing from its intent. It should be understood that various alternatives to the embodiments of this application described herein may be employed in the practice of this application. The appended claims are intended to define the scope of protection of this application and therefore cover equivalents or alternatives within the scope of these claims.

Claims

1. A flying capacitor three-level Buck / Boost converter, characterized in that, include: A first power source, a second power source, a first capacitor connected in parallel across the two ends of the first power source, and a second capacitor connected in parallel across the two ends of the second power source. The first switch transistor, the second switch transistor, the third switch transistor, and the fourth switch transistor are connected in series and then in parallel with the first power supply. One end of the second capacitor is electrically connected to the series terminal of the second and third switching transistors via an inductor, and the other end of the second capacitor is electrically connected to the other end of the fourth switching transistor. A pre-charging circuit connected in parallel with the first power supply, the pre-charging circuit comprising a first switch, a first resistor, a flying capacitor, a second resistor, and a second switch connected in series. Wherein, the series terminal of the flying capacitor and the first resistor is electrically connected to the series terminal of the first switch and the second switch, and the series terminal of the flying capacitor and the second resistor is electrically connected to the series terminal of the third switch and the fourth switch; the first switch and the second switch are used to control the first power supply or the second power supply to precharge the flying capacitor. The voltage of the first power supply is greater than the voltage of the second power supply. The first switch is closed and the second switch is open. The first power supply precharges the flying capacitor and, after the flying capacitor is precharged, also opens the first switch to make the converter work in Buck mode. The first switch is open and the second switch is closed. The second power supply precharges the flying capacitor and, after the flying capacitor is precharged, also opens the second switch to make the converter work in Boost mode.

2. The flying capacitor three-level Buck / Boost converter according to claim 1, characterized in that, The preset charging voltage of the flying capacitor is half the voltage of the first power supply.

3. The flying capacitor three-level Buck / Boost converter according to claim 1, characterized in that, The conduction direction of the first switch, the second switch, the third switch, and the fourth switch is from the negative terminal of the first power supply to the positive terminal of the first power supply.

4. The flying capacitor three-level Buck / Boost converter according to claim 1, characterized in that, The first switch and the second switch include one or more of relays, semiconductor switches, and power devices.

5. The flying capacitor three-level Buck / Boost converter according to claim 1, characterized in that, The first switch, the second switch, the third switch, and the fourth switch include one or more of gallium nitride field-effect transistors, metal oxide semiconductor transistors, and bipolar junction transistors.

6. A pre-charging method for a flying capacitor three-level Buck / Boost converter, characterized in that, The method for pre-charging a flying capacitor using a three-level Buck / Boost converter as described in any one of claims 1-5 includes: When the first switch is closed and the second switch is opened, the positive terminal of the first power supply outputs the first current; The first current sequentially passes through the first switch, the first resistor, the flying capacitor, the third switch, the inductor, the second capacitor, and the negative terminal of the first power supply to form a first circuit, pre-charging the flying capacitor; When the voltage of the flying capacitor reaches half of the voltage of the first power supply, the first switch is turned on to complete the pre-charging.

7. A pre-charging method for a flying capacitor three-level Buck / Boost converter, characterized in that, The method for pre-charging a flying capacitor using a three-level Buck / Boost converter as described in any one of claims 1-5 includes: When the second switch is closed and the first switch is opened, the positive terminal of the second power supply outputs a second current. The second current sequentially passes through the inductor, the second switch, the flying capacitor, the second resistor, the second switch, and the negative terminal of the second power supply to form a second circuit, pre-charging the flying capacitor; When the voltage of the flying capacitor reaches half of the voltage of the first power supply, the second switch is turned on to complete the pre-charging.

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