Direct-current converter based on asymmetric three-level bridge arm circuit

Abstract

The utility model discloses a direct-current converter based on an asymmetric three-level bridge arm circuit, which comprises a direct-current bus, the asymmetric three-level bridge arm circuit connected with the direct-current bus in parallel, a resonance circuit, an isolation transformer and a rectifying circuit, the asymmetric three-level bridge arm circuit comprises a voltage dividing capacitor, a clamping circuit, a flying capacitor C3 and a half-bridge inverter. According to the direct-current converter, the connection points of the resonant circuits are changed, so that voltage sharing of the voltage-dividing capacitors is realized, the problem of neutral point imbalance caused by characteristic difference of a driving chip and a switching tube in the traditional half-bridge three-level converter is effectively solved, and the direct-current converter is higher in response speed and higher in control precision.

Description

Technical Field

[0001] The utility model relates to the technical field of power electronics, in particular to a DC converter based on an asymmetric three-level bridge arm circuit. Background Technique

[0002] In recent years, half-bridge three-level DC converters have been widely used in fields such as high-voltage DC power transmission and locomotive traction, breaking through the voltage level limitations of existing power devices such as IGBTs, and are one of the hot research topics in the current field of power electronics DC conversion.

[0003] The half-bridge three-level DC converter uses two switching tubes in series to replace one switching tube in the traditional half-bridge converter. The flying capacitor therein plays the role of decoupling the soft-switching processes of the leading arm and the lagging arm and equalizing the voltage of the switching tubes, and the clamping diode not only helps to equalize the voltage of the switching tubes, but also accelerates the charging process of the flying capacitor. The half-bridge three-level DC converter can not only reduce the voltage stress borne by the switching tubes to half of the input voltage, but also achieve ZVS (zero-voltage turn-on) of the main switching tubes and ZCS (zero-current turn-off) of the rectifier diodes within the full voltage range. This can reduce the stress borne by the switching tubes and rectifier diodes, improve the service life, and at the same time can also reduce the switching loss and improve the overall efficiency.

[0004] The conventional three-level symmetric half-bridge LLC topology is as Figure 1 shown. The midpoint of the series-connected first capacitor C1 and second capacitor C2 is defined as point B, and the connection point of the upper half-bridge arm and the lower half-bridge arm is defined as point A. One end of the resonant circuit is connected to the midpoint A of the bridge arm, and the other end is connected to the midpoint B. When the first switching tube Q1 and the second switching tube Q2 of the upper half-bridge arm are turned on, energy is provided by the first capacitor C1, and when the third switching tube Q3 and the fourth switching tube Q4 of the lower half-bridge arm are turned on, energy is provided by the second capacitor C2, as Figure 2 shown. At the same time, the voltages at both ends of the first switching tube Q1 and the second switching tube Q2 of the upper half-bridge arm and the third switching tube Q3 and the fourth switching tube Q4 of the lower half-bridge arm are clamped to half of the input voltage through the first diode D1, the second diode D2 of the clamping circuit and the flying capacitor C3. This topology requires the first capacitor C1 and the second capacitor C2 to have a large capacitance, and the working states of the first capacitor C1 and the second capacitor C2 are inconsistent, which easily leads to the imbalance of the midpoint voltage B. The first capacitor C1, the second capacitor C2, the first switching tube Q1, the second switching tube Q2, the third switching tube Q3 and the fourth switching tube Q4 need to bear a greater voltage stress, affecting the normal operation of the power supply, and there are certain potential safety hazards. Therefore, it is necessary to find a new type of circuit to eliminate the voltage difference between the voltage-dividing capacitors, the first capacitor C1 and the second capacitor C2, so as to achieve the voltage equalization of the voltage-dividing capacitors and ensure the normal operation of the circuit. Content of the Utility Model

[0005] The technical problem to be solved by the present utility model is to provide a DC converter based on an asymmetric three-level bridge arm circuit, so as to solve the problem of midpoint imbalance caused by extracting energy from a large capacitor when the upper half bridge arm or the lower half bridge arm of a three-level symmetric half-bridge LLC circuit is turned on.

[0006] The technical solution of the present utility model for solving the above technical problem is as follows:

[0007] A DC converter based on an asymmetric three-level bridge arm circuit includes a DC bus, an asymmetric three-level bridge arm circuit connected in parallel with the DC bus, a resonant circuit, an isolation transformer, and a rectifier circuit. The asymmetric three-level bridge arm circuit includes a voltage-dividing capacitor, a clamping circuit, a flying capacitor C3, and a half-bridge inverter. The voltage-dividing capacitor and the half-bridge inverter are both connected in parallel between the input ports of the DC bus. The half-bridge inverter includes an upper half bridge arm and a lower half bridge arm connected in series. The connection point of the upper half bridge arm and the lower half bridge arm is A. Each of the upper half bridge arm and the lower half bridge arm includes two switching tubes connected in series. The flying capacitor C3 and the clamping circuit are connected in parallel. The clamping circuit includes a first diode D1 and a second diode D2 connected in series. The anode of the first diode D1 is connected to the midpoint B of the voltage-dividing capacitor, and the cathode is connected to the connection point of the two switching tubes of the upper half bridge arm. The cathode of the second diode D2 is connected to the midpoint B of the voltage-dividing capacitor, and the anode is connected to the connection point of the two switching tubes of the lower half bridge arm. The negative pole C of the DC bus and the connection point A serve as the output end of the asymmetric three-level bridge arm circuit. The two ends of the resonant circuit are connected to the output end of the asymmetric three-level bridge arm circuit. One end of the primary winding of the isolation transformer is connected to the negative pole C of the DC bus, and the other end is connected to one end of the resonant circuit. The secondary winding of the isolation transformer is connected to the input end of the rectifier circuit. The output end of the rectifier circuit serves as the output end of the DC converter.

[0008] Further, the upper half bridge arm includes a first switching tube Q1 and a second switching tube Q2 connected in series. The lower half bridge arm includes a third switching tube Q3 and a fourth switching tube Q4 connected in series. The first switching tube Q1, the second switching tube Q2, the third switching tube Q3, and the fourth switching tube Q4 are sequentially connected in series in the manner of source and drain connection. The drain of the first switching tube Q1 is connected to the positive input of the DC bus. The source of the fourth switching tube Q4 is connected to the connection point of the negative pole C of the DC bus and the power common ground. One end of the flying capacitor C3 is connected between the source of the first switching tube Q1 and the drain of the second switching tube Q2, and the other end is connected between the source of the third switching tube Q3 and the drain of the fourth switching tube Q4. The sources of the second switching tube Q2 and the third switching tube Q3 are connected and then connected to one end of the resonant circuit.

[0009] The beneficial effects of the above further solution are as follows: When each switching tube is turned off, it only needs to bear half of the input voltage, reducing the voltage stress on the switching tube, facilitating the selection of the switching tube. At the same time, the conduction loss of the low-voltage switching tube is also lower, and the overall efficiency of the machine will be improved.

[0010] Further, the voltage dividing capacitor includes two identical first capacitors C1 and second capacitors C2 connected in series.

[0011] The beneficial effects of the above further solution are as follows: Try to ensure equal voltage division.

[0012] Further, the rectifying circuit includes a full-wave rectifying circuit composed of a first rectifying diode D3 and a second rectifying diode D4.

[0013] Further, the first switching tube Q1 and the fourth switching tube Q4 conduct complementarily; the second switching tube Q2 and the third switching tube Q3 conduct complementarily. The control signals of the first switching tube Q1 and the second switching tube Q2 have a phase difference of 180 degrees; the control signals of the third switching tube Q3 and the fourth switching tube Q4 have a phase difference of 180 degrees.

[0014] Further, the resonant circuit includes a series-connected resonant inductor L1, a resonant capacitor C4, and an exciting inductor L2. The two ends of the exciting inductor L2 are connected in parallel with the primary winding of the isolation transformer.

[0015] The beneficial effects of the above further solution are as follows: Adopting the LLC series resonant topology structure can achieve zero-voltage turn-on of the switching tubes Q1, Q2, Q3, Q4 and zero-current turn-off of the rectifying diodes D3, D4. This can reduce the switching stress of the switching tubes and rectifying diodes, reduce the switching loss, improve the overall efficiency of the machine, and at the same time, the electromagnetic compatibility characteristics will be better.

[0016] Further, the first switching tube Q1, the second switching tube Q2, the third switching tube Q3, and the fourth switching tube Q4 have the same structure.

[0017] Further, a filter capacitor C5 is connected in parallel at the output end of the rectifying circuit.

[0018] Further, the isolation transformer adopts a center-tapped transformer. Further, the first switching tube Q1, the second switching tube Q2, the third switching tube Q3, and the fourth switching tube Q4 have the same structure.

[0019] The present application adopts the above technical solutions and has at least the following beneficial effects:

[0020] The utility model adopts an asymmetric three-level bridge arm circuit. By changing the connection points of the resonant circuit, only during the turn-on or turn-off process do the upper and lower bridge arms extract energy from the voltage-dividing capacitors C1 and C2 to clamp the voltage of the bridge arm switching tubes to the voltage at point B. After the upper and lower bridge arms are turned on or off, there is no current in the upper and lower voltage-dividing capacitors C1 and C2, and the current at point B is zero. In this way, energy extraction from the midpoint B is avoided, thus achieving voltage equalization of the voltage-dividing capacitors, effectively solving the midpoint imbalance problem caused by the differences in the characteristics of the driving chips and switching tubes in the traditional half-bridge three-level circuit, and having a faster response speed and higher control accuracy. The utility model is not only applicable to a high-voltage three-level structure for the input voltage, but also has all the advantages of a three-level symmetric half-bridge LLC when the input voltage is a high-voltage two-level. Moreover, compared with the three-level symmetric half-bridge topology, it can save two large input capacitors and solve the midpoint imbalance problem. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 FIG. is a DC converter diagram of the symmetric three-level bridge arm circuit of the present utility model;

[0022] Figure 2 FIG. is a bus capacitor current and driving waveform diagram of the DC converter of the symmetric three-level bridge arm circuit of the present utility model;

[0023] Figure 3 FIG. is a DC converter diagram of the asymmetric three-level bridge arm circuit of the present utility model;

[0024] Figure 4 FIG. is a bus capacitor current and driving waveform diagram of the DC converter of the asymmetric three-level bridge arm circuit of the present utility model. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] Now, exemplary embodiments of the present utility model will be described in detail with reference to the accompanying drawings. It should be understood that the embodiments shown and described in the drawings are merely exemplary, intended to illustrate the principles and spirit of the present utility model, and not to limit the scope of the present utility model.

[0026] Embodiment 1

[0027] As Figure 3As shown in the figure, this embodiment provides a DC converter with an asymmetric three-level bridge arm circuit. The converter includes a DC bus, an asymmetric three-level bridge arm circuit connected in parallel with the DC bus, a resonant circuit, an isolation transformer, and a rectifier circuit. The asymmetric three-level bridge arm circuit includes voltage-dividing capacitors, a clamping circuit, a flying capacitor C3, and a half-bridge inverter. The voltage-dividing capacitors and the half-bridge inverter are both connected in parallel between the DC bus input ports. The half-bridge inverter includes an upper half-bridge arm and a lower half-bridge arm connected in series. The upper half-bridge arm and the lower half-bridge arm each include two switching tubes connected in series. The flying capacitor C3 and the clamping circuit are connected in parallel. The clamping circuit includes a first diode D1 and a second diode D2 connected in series. The anode of the first diode D1 is connected to the midpoint B of the voltage-dividing capacitors, and the cathode is connected to the connection point of the two switching tubes of the upper half-bridge arm. The cathode of the second diode D2 is connected to the midpoint B of the voltage-dividing capacitors, and the anode is connected to the connection point of the two switching tubes of the lower half-bridge arm.

[0028] In the specific circuit, the connection point of the upper half-bridge arm and the lower half-bridge arm is defined as A. The upper half-bridge arm includes a first switching tube Q1 and a second switching tube Q2 connected in series. The lower half-bridge arm includes a third switching tube Q3 and a fourth switching tube Q4 connected in series. The first switching tube Q1, the second switching tube Q2, the third switching tube Q3, and the fourth switching tube Q4 are sequentially connected in series in the way of connecting the source and the drain. The drain of the first switching tube Q1 is connected to the positive pole of the DC bus input, and the source of the fourth switching tube Q4 is connected to the power common ground. The positive pole of the flying capacitor C3 is connected between the source of the first switching tube Q1 and the drain of the second switching tube Q2, and the negative pole of the flying capacitor C3 is connected between the source of the third switching tube Q3 and the drain of the fourth switching tube Q4. The sources of the second switching tube Q2 and the third switching tube Q3 are connected and then connected to one end of the resonant circuit.

[0029] The voltage-dividing capacitors are two capacitors of the same size connected in series, specifically the first capacitor C1 and the second capacitor C2. The positive pole of the first capacitor C1 is connected to the positive pole of the DC bus. The negative pole of the first capacitor C1 is connected to the positive pole of the second capacitor C2, and the connection point is defined as the midpoint B of the converter. The negative pole of the second capacitor C2 is connected to the negative pole of the DC bus.

[0030] The output terminals of the asymmetric three-level bridge arm circuit in the present invention adopt the negative pole of the DC power supply, namely point C and point A defined in the figure. The traditional symmetric three-level bridge arm circuit uses point A and point B as the output terminals. The two input terminals of the resonant circuit are directly connected to the output terminals A and C of the asymmetric three-level bridge arm circuit. In this embodiment, the resonant circuit includes a series-connected resonant inductor L1, a resonant capacitor C4, and an exciting inductor L2. The rectifier circuit includes a full-wave rectifier circuit composed of a first rectifier diode D3 and a second rectifier diode D4. The two ends of the exciting inductor L2 are respectively connected to the two ends of the primary winding P1 of the isolation transformer TX1. The isolation transformer TX1 is a transformer with a center tap. The secondary windings S1 and S2 of the isolation transformer TX1 are respectively connected to the anodes of the first rectifier diode D3 and the second rectifier diode D4. The cathodes of the first rectifier diode D3 and the second rectifier diode D4 are connected together as the positive output of the DC converter. The center tap of the isolation transformer TX1 is used as the negative output of the DC converter.

[0031] In order to make the direct current output by the DC converter more stable, a filter capacitor C5 is connected in parallel at the output terminal of the rectifier circuit. That is, a filter capacitor C5 is connected in parallel between the positive and negative poles of the DC converter, and the positive pole of the filter capacitor C5 is connected to the positive pole of the DC converter.

[0032] The following is a specific analysis of the present invention in conjunction with the attached drawings:

[0033] The first switch tube Q1, the second switch tube Q2, the third switch tube Q3, and the fourth switch tube Q4 on the V1 side of the DC bus power supply work normally, and all four drives adopt a fixed duty cycle of 0.5. The first switch tube Q1 and the fourth switch tube Q4 conduct complementarily, and the second switch tube Q2 and the third switch tube Q3 conduct complementarily. When the DC converter in the present invention adopts duty cycle control, only the phase shift angle between the first switch tube Q1, the second switch tube Q2, the third switch tube Q3, and the fourth switch tube Q4 needs to be changed; when adopting frequency modulation control, only the switching frequency needs to be changed.

[0034] When adopting frequency modulation control, first define the switching frequency as f here s , the first resonant frequency as f r1 , and the second resonant frequency as f r2 , then:

[0035]

[0036] Among them, L1 is the resonant inductor, C4 is the resonant capacitor, and L2 is the exciting inductor. According to the frequency range of the operating frequency of the LLC resonant converter, its operating mode can be divided into three different situations: 1. fr1 < fs < fr2; 2. fs = fr2; 3. fs > fr2.

[0037] When adopting duty cycle control, the switching frequency is constant. Let the switching frequency be equal to the second resonance frequency point, that is, when fs = fr2, the output voltage is adjusted by regulating the duty cycle. At this time, the primary switch tube realizes ZVS turn-on, and the secondary diode realizes ZCS turn-off. The maximum voltage gain is 1, and the voltage gain varies between 0 and 1 by adjusting the duty cycle.

[0038] Figure 4 The main waveforms of the frequency modulation control of this converter in one switching cycle are given. Since the working principles in the positive and negative half-cycles are similar, only the different working states of the circuit in the positive half-cycle are analyzed here, and its working conditions are described as follows:

[0039] State 1 (t0 - t1), the first switch tube Q1 and the second switch tube Q2 are turned on, and the third switch tube Q3 and the fourth switch tube Q4 are turned off. Under the action of the DC bus voltage V1, the resonant inductor L1 and the resonant capacitor C4 are charged, and the current of the excitation inductor L2 increases linearly. At this time, the voltage across the transformer is V1 / 2, and the voltage drop across the third switch tube Q3 and the fourth switch tube Q4 is also V1 / 2.

[0040] State 2 (t1 - t2), the first switch tube Q1 is turned off. The junction capacitance of the first switch tube Q1 receives the charge of the junction capacitance on the fourth switch tube Q4 through the flying capacitor C3. When the voltage across the first switch tube Q1 reaches V1 / 2, the clamping diode D1 conducts, clamping the voltage across the first switch tube Q1 to V1 / 2. The voltage drops across the second switch tube Q2 and the fourth switch tube Q4 are also 0. The voltage across the first switch tube Q1 and the third switch tube Q3 is V1 / 2.

[0041] State 3 (t2 - t3), the second switch tube Q2 is turned off. The junction capacitance of the second switch tube Q2 receives the charge of the junction capacitance on the third switch tube Q3 through the flying capacitor C3. When the voltage across the second switch tube Q2 reaches V1 / 2, the clamping diode D1 conducts, clamping the voltage across the second switch tube Q2 to V1 / 2. The voltage drops across the first switch tube Q1 and the second switch tube Q2 are also V1 / 2. The voltage across the third switch tube Q3 and the fourth switch tube Q4 is 0.

[0042] State 4 (t3 - t4), the third switch tube Q3 and the fourth switch tube Q4 are turned on, and the first switch tube Q1 and the second switch tube Q2 are turned off. Its working mode is similar to that of State 1 and will not be elaborated here.

[0043] It should be noted that: The first switching transistor Q1, the second switching transistor Q2, the third switching transistor Q3, and the fourth switching transistor Q4 are made of metal-oxide-semiconductor transistors (MOSFETs). However, other types of electronically controlled switching devices, such as bipolar junction transistors (BJTs) and insulated-gate bipolar transistors (IGBTs), etc., can also be used as the switching transistors in this embodiment.

[0044] The structures of the above 4 switching transistors are the same; generally, MOS transistors are used.

[0045] The present invention adjusts the duty cycle of the second switching transistor Q2 according to the magnitude relationship between the voltage difference during the charging and discharging processes of the flying capacitor and a preset value, so that the voltage value of the flying capacitor is stabilized at 1 / 2 of the input voltage, thereby reducing the voltage stress that each switching transistor in the DC converter needs to bear, reducing the loss and cost of the circuit, and improving the power supply efficiency.

[0046] In an actual circuit, since the charging and discharging time, that is, the duty cycle, cannot be made exactly equal, it will cause a certain degree of imbalance in the charging and discharging energy, resulting in overcharging or overdischarging of the capacitor. There are two reasons for this consequence: one is that the two clusters of triangular carrier signals used when modulating the control waveform of high-frequency PWM cannot be made exactly equal in amplitude and have a phase difference of 180°; the other is that the switching instantaneous characteristics of the switching transistors cannot be exactly the same, resulting in a delay when the switching transistors turn on and off, and the delay may be different. Therefore, there is a difference in the duty cycle of the switching transistors, and the capacitor charging and discharging are unbalanced. So, it is necessary to track and adjust the capacitor voltage to ensure that the capacitor charging and discharging are balanced. Therefore, the present invention adopts an asymmetric three-level bridge arm circuit. By changing the connection position of the resonant circuit, the symmetric three-level half-bridge LLC circuit is improved to an asymmetric three-level half-bridge LLC circuit. Without the need for a large input capacitor, only about 1uF film capacitors C1 and C2 are connected in series to construct the bus midpoint B, and the voltages at both ends of the first diode D1, the second diode D2, the flying capacitor C3, and the first switching transistor Q1, the second switching transistor Q2, the third switching transistor Q3, and the fourth switching transistor Q4 of the clamping circuit are clamped to the bus midpoint B. Capacitors C1 and C2 only have current flowing through them during the dead time of the switching transistors, as Figure 4 ..., which solves the problem of midpoint imbalance caused by drawing energy from the large capacitor when the upper or lower transistor of the three-level symmetric half-bridge LLC circuit is turned on.

[0047] 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 within the protection scope of the present invention.

Claims

1. A DC converter based on an asymmetric three-level bridge arm circuit, characterized in that, It includes a DC bus, an asymmetric three-level bridge arm circuit connected in parallel with the DC bus, a resonant circuit, an isolation transformer, and a rectifier circuit. The asymmetric three-level bridge arm circuit includes voltage-dividing capacitors, a clamping circuit, a flying capacitor C3, and a half-bridge inverter. The voltage-dividing capacitors and the half-bridge inverter are both connected in parallel between the input ports of the DC bus. The half-bridge inverter includes an upper half-bridge arm and a lower half-bridge arm connected in series. The connection point of the upper half-bridge arm and the lower half-bridge arm is A. Each of the upper half-bridge arm and the lower half-bridge arm includes two switching tubes connected in series. The flying capacitor C3 and the clamping circuit are connected in parallel. The clamping circuit includes a first diode D1 and a second diode D2 connected in series. The anode of the first diode D1 is connected to the midpoint B of the voltage-dividing capacitor, and the cathode is connected to the connection point of the two switching tubes of the upper half-bridge arm. The cathode of the second diode D2 is connected to the midpoint B of the voltage-dividing capacitor, and the anode is connected to the connection point of the two switching tubes of the lower half-bridge arm. The negative electrode C of the DC bus and the connection point A serve as the output terminals of the asymmetric three-level bridge arm circuit. The two ends of the resonant circuit are connected to the output terminals of the asymmetric three-level bridge arm circuit. One end of the primary winding of the isolation transformer is connected to the negative electrode C of the DC bus, and the other end is connected to one end of the resonant circuit. The secondary winding of the isolation transformer is connected to the input terminal of the rectifier circuit. The output terminal of the rectifier circuit serves as the output terminal of the DC converter.

2. The DC converter according to claim 1, wherein The upper half-bridge arm includes a first switching tube Q1 and a second switching tube Q2 connected in series. The lower half-bridge arm includes a third switching tube Q3 and a fourth switching tube Q4 connected in series. The first switching tube Q1, the second switching tube Q2, the third switching tube Q3, and the fourth switching tube Q4 are connected in series in the order of source and drain connection. The drain of the first switching tube Q1 is connected to the positive input of the DC bus. The source of the fourth switching tube Q4 is connected to the connection point of the negative electrode C of the DC bus and the power common ground. One end of the flying capacitor C3 is connected between the source of the first switching tube Q1 and the drain of the second switching tube Q2, and the other end is connected between the source of the third switching tube Q3 and the drain of the fourth switching tube Q4. The sources of the second switching tube Q2 and the third switching tube Q3 are connected and then connected to one end of the resonant circuit.

3. The DC converter according to claim 1 or 2, characterized in that, The voltage-dividing capacitor includes two identical first capacitors C1 and C2 connected in series.

4. The DC converter according to claim 1 or 2, characterized in that, The rectifier circuit includes a full-wave rectifier circuit composed of a first rectifier diode D3 and a second rectifier diode D4.

5. The DC converter according to claim 2, characterized in that, During the power supply process, the first switching tube Q1 and the fourth switching tube Q4 conduct complementary. The second switching tube Q2 and the third switching tube Q3 conduct complementary. The control signals of the first switching tube Q1 and the second switching tube Q2 have a phase difference of 180 degrees. The control signals of the third switching tube Q3 and the fourth switching tube Q4 have a phase difference of 180 degrees.

6. The DC converter according to claim 1 or 2, characterized in that, The resonant circuit includes a resonant inductor L1, a resonant capacitor C4, and an exciting inductor L2 connected in series. The two ends of the exciting inductor L2 are connected in parallel with the primary winding of the isolation transformer.

7. The DC converter according to claim 2, wherein, The first switching tube Q1, the second switching tube Q2, the third switching tube Q3, and the fourth switching tube Q4 have the same structure.

8. The DC converter according to claim 1, characterized in that, A filter capacitor C5 is connected in parallel to the output terminal of the rectifier circuit.

9. The DC converter according to claim 1, wherein The isolation transformer used is a center-tapped transformer.

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