Disclosure of Invention
The invention relates to a bidirectional three-level H-bridge non-isolated DC-DC converter, wherein the converter topology mainly comprises a power supply, a voltage division capacitor, a diode clamp type three-level bridge arm, an energy storage inductor and a filter capacitor. The voltage of the power supply is equally divided by the voltage dividing capacitor to form a half power supply voltage potential, and a basic condition is provided for reducing the voltage borne by the switching tube. The two bridge arms, the two groups of energy storage inductors and the filter capacitor form an H-bridge main power loop. The H bridge can be regarded as formed by reversely connecting two sets of BUCK-BOOST circuits in series in the working process, a left three-level bridge arm, a left filter capacitor and an energy storage inductor form a set to form a half-bridge circuit H1, and the right side of the H bridge is provided with another set of half-bridge circuit H2.
A three-level bridge arm uses four switching tubes, the four switching tubes are connected in series end to end, and two sides of each switching tube are connected with a diode in an anti-parallel mode. And the other diode is connected in series in the forward direction, the cathode of the diode is connected between the two upper switching tubes, the anode of the diode is connected between the two lower switching tubes, and the connection position of the diode is connected to the middle point Z of the two voltage division capacitors. The connecting point of the middle two switching tubes is connected to an energy storage inductor, the other side of the energy storage inductor is connected with the positive electrode of a filter capacitor, and the negative electrode of the filter capacitor is connected to the negative electrode of a power supply.
When the switch works, two of the switch tubes are conducted at the same time each time, and the other two non-conducted switch tubes bear half of the power supply voltage. Compared with a two-level structure, the voltage borne by the switching tube during turn-off is reduced by half, so that the withstand voltage value of the switching tube is also reduced by half during device type selection. The method comprises the steps that when two upper switching tubes of a three-level bridge arm are conducted simultaneously, a P state is defined, the middle two switching tubes are conducted simultaneously, an O state is defined, and the lower two switching tubes are conducted simultaneously, an N state is defined. The switching tube operates mainly in the P and N states and the two states operate complementarily. The O state is a transition state in the process of switching the two switch states with each other. By calculating the action time of the P or N state and carrying out triangular wave carrier modulation, the corresponding trigger pulse of each switching tube can be obtained. When the circuit works in the BUCK mode, the current of the voltage division capacitor flows out from the middle point when in the O state, and when the circuit works in the BOOST mode, the current flows into the voltage division capacitor when in the O state. The current flowing in and out is difficult to reach the natural balance, so that the voltage of the lower voltage dividing capacitor is difficult to maintain at half of the power supply voltage. The O-state action time of the H1 and H2 bridge arms is adjusted according to the deviation amount of the voltages of the two voltage-dividing capacitors, the dynamic balance of the current flowing out of the midpoint and the current flowing into the midpoint can be controlled, and the midpoint voltage balance is realized.
The innovation of the application mainly comprises the following points:
1. the output voltage and the current can be switched between positive and negative polarities smoothly.
2. The output voltage and current can be changed in a wide range and with high precision.
3. The voltage balance of the midpoint potential of the voltage-sharing capacitor can be realized.
Detailed Description
Hereinafter, specific embodiments of the present application will be described with reference to the accompanying drawings.
As shown in fig. 1, two voltage dividing capacitors C1 and C2 are connected in series end to end and connected in parallel between the positive and negative poles of the power supply DC, so as to divide the voltage of the power supply. The H bridge is composed of two three-level half-bridge circuits H1 and H2, and H1 and H2 are connected between the positive electrode and the negative electrode of the power supply in parallel.
In the half-bridge circuit H1, four switching tubes T1-T4 are connected in series end to end, and a diode D1-D4 is connected in parallel to each tube in reverse, which is called diode clamp type three-level topology. In addition, two diodes D9 and D10 are connected in series in the forward direction, the cathode is connected between the T1 and T2 switch tubes, the anode is connected between the T3 and T4 switch tubes, and the junction of D9 and D10 is connected to the midpoint Z of two voltage-dividing capacitors C1 and C2. Then the energy storage inductor L1 is connected to the connecting point of the T2 and the T3 switching tube, the other side of the L1 is connected with the anode of the filter capacitor C3, and the cathode of the C3 is connected to the cathode of the power supply DC.
The half-bridge circuit H2 is built exactly in the same way as the half-bridge circuit H1. The half-bridge circuit H2 has four switching tubes T5-T8 connected in series end to end, and a diode D5-D8 connected in parallel at two sides of each tube in reverse, which is called diode clamp type three-level topology. In addition, two diodes D11 and D12 are connected in series in the forward direction, the cathode is connected between the T5 and T6 switch tubes, the anode is connected between the T7 and T8 switch tubes, and the junction of D11 and D12 is connected to the midpoint Z of two voltage-dividing capacitors C1 and C2. Then the energy storage inductor L2 is connected to the connecting point of the T6 and the T7 switching tube, the other side of the L2 is connected with the anode of the filter capacitor C4, and the cathode of the C4 is connected to the cathode of the power supply DC.
Thus, the basic topology of the three-level H-bridge is well established. The control principle of the topology is explained below by taking the application of the H-bridge to charging and discharging the battery as an example.
The three-level H bridge is controlled, and charging and discharging of a battery and balance control of midpoint voltage of a capacitor are mainly achieved according to a preset value. For convenience of analysis and simplification of the circuit, the H1 and H2 circuits can be equivalent to two direct current sources, the cathodes of the two direct current sources are connected together, and a battery is connected between the anodes. Ideally, both dc source output voltages can range from zero to the bus voltage. Therefore, through the matching of the left and right circuits, the output of any voltage value between zero volt and the bus voltage can be realized, and only the kirchhoff voltage law is satisfied.
The positive electrode of the battery is connected to the half-bridge circuit H1, the negative electrode of the battery is connected to the half-bridge circuit H2, namely, the positive electrode and the negative electrode of the battery are respectively connected to the connection parts of the two energy storage inductors and the filter capacitor, and H1 and H2 are matched with each other to charge and discharge the battery. The battery is connected between the two groups of circuits, when the positive pole and the negative pole of the battery are respectively connected with H1 and H2: if the battery is charged, H1 works in BUCK mode to obtain energy from the power supply and flow to the battery, H2 works in BOOST mode to reversely transmit the current flowing out of the battery to the power supply, so that the current flows out of the positive electrode of the battery into the negative electrode of the battery; when the battery is discharged, H1 operates in BOOST mode to return the energy discharged from the battery to the power supply, and H2 operates in BUCK mode to supply the current from the power supply to the battery so that the current flows from the negative electrode to the positive electrode of the battery. When the positive and negative electrodes of the battery are reversely connected, the operation modes of H1 and H2 are opposite to the above operation modes.
The half-bridge circuits H1 and H2 respectively use four switching tubes, when in work, two switching tubes are conducted at the same time, and the other two non-conducting switching tubes bear half of the power supply voltage. Compared with a two-level structure, the voltage borne by the switching tube during turn-off is reduced by half, so that the withstand voltage value of the switching tube is also reduced by half during device type selection. The upper two switching tubes T1-T2 or T5-T6 of the three-level bridge arm are defined as P state when being simultaneously conducted, the middle two switching tubes T2-T3 or T6-T7 are simultaneously conducted and defined as O state, and the lower two switching tubes T3-T4 or T7-T8 are simultaneously conducted and defined as N state. The switching tube mainly works in the P state and the N state, and the two states work in a complementary mode. The O state is a transition state in the process of switching the two switch states with each other. By calculating the action time of the P or N state and carrying out triangular wave carrier modulation, the corresponding trigger pulse of each switching tube can be obtained.
When the circuit works in the BUCK mode, the current of the voltage division capacitor flows out from the middle point when in the O state, and when the circuit works in the BOOST mode, the current flows into the voltage division capacitor when in the O state. The current flowing in and out is difficult to reach the natural balance, so that the voltage of the lower voltage dividing capacitor is difficult to maintain at half of the power supply voltage.
The O-state action time of the H1 and H2 bridge arms is adjusted according to the deviation amount of the voltages of the two voltage-dividing capacitors, the dynamic balance of the current flowing out of the midpoint and the current flowing into the midpoint can be controlled, and the midpoint voltage balance is realized.
In order to simplify the control steps, a fixed duty ratio is given to a switching tube of H2, on the premise of not carrying out midpoint control, the action time of P, O, N state of the bridge arm in H2 accounts for 0.2, 0.1 and 0.7 of the switching period respectively, the action time of O state of the bridge arm in H1 accounts for 0.1 of the switching period, and the action time of P and N states needs to be regulated in a closed loop mode so as to charge and discharge the battery according to a preset voltage or current value.
In order to realize the balance control of the midpoint voltage, the action time of O states of H1 and H2 needs to be adjusted, and the action time range of the O states is set to be 0.05-0.15 times of the switching period. When the battery is charged, if the lower capacitor voltage is less than one half of the power supply voltage, the O state action time delta T of H2 needs to be increased, and the O state action time delta T of H1 needs to be reduced; if the lower side capacitor voltage is greater than one-half of the power supply voltage, the O state action time delta T of H2 needs to be reduced, the O state action time delta T of H1 needs to be increased, and when the capacitor voltage deviation value is smaller than the allowable deviation value, the O state action time on the two sides is kept unchanged. The changes of the O state action time of H1 and H2 during the discharging of the battery are opposite, and in the O state action time change process, the P state action time and the N state action time are respectively changed by delta T/2. The midpoint voltage adjusting process is shown in fig. 2: the P, O, N state action time of H1 and H2 is controlled respectively according to the voltage magnitude relation of the voltage dividing capacitors C1 and C2. According to the principle, the voltage initial values of the voltage-dividing capacitors are respectively 200V and 800V to be simulated, the voltages of the two voltage-dividing capacitors reach an equilibrium state by adjusting the action time of O states on two sides, the effectiveness of the control method is explained, and the voltage change waveforms of the voltage-dividing capacitors are shown in FIG. 3.