Voltage boost and voltage drop control device and method for bidirectional three-level DC-DC converter

Abstract

The present invention relates to a boost and buck control device and method for a bidirectional three-level DC-DC converter, wherein the boost control device comprises a bus voltage outer loop, an inductor current inner loop and a first signal output terminal connected in sequence, and a flying capacitor voltage loop and a second signal output terminal connected in sequence, wherein the inductor current inner loop outputs a first duty cycle signal; the flying capacitor voltage loop outputs a second duty cycle signal; the first signal output terminal comprises a subtractor and a PWM unit connected in sequence, and the second signal output terminal comprises an adder and a PWM unit connected in sequence, wherein the subtractor and the adder are respectively connected to the first duty cycle signal and the second duty cycle signal, and the PWM unit is used to output two control signals of opposite phases. Compared with the prior art, the present invention utilizes a hybrid control method of variable duty cycle and variable phase, which can stably maintain the balance of the flying capacitor, and has the advantages of being stable and reliable, low cost, and improving the running mileage of electric vehicles.

Description

Technical Field

[0001] The present invention relates to the field of bidirectional three-level DC-DC converter control, and in particular to a voltage step-up and voltage step-down control device and method for a bidirectional three-level DC-DC converter. Background Art

[0002] The lithium battery in the car is boosted by DC-DC to provide the motor controller with a voltage higher than the lithium battery to improve the efficiency of the motor. When the motor of the electric car is in power generation mode, the electricity generated by the motor can be stepped down by bidirectional DC-DC to charge the lithium battery, thereby improving the battery life. At the same time, the number of lithium batteries connected in series to obtain high voltage can be reduced, which can reduce the cost of lithium batteries. Compared with the two-level DC-DC, the use of three-level bidirectional DC-DC can reduce the IGBT switch stress, reduce the inductance and volume of the energy storage inductor, and increase the power density of the DC-DC. Since the three-level DC-DC introduces a flying capacitor, the control algorithm needs to maintain the balance of the flying capacitor voltage. Only when the flying capacitor voltage is maintained at half of the high-voltage side voltage can the entire system remain stable. If the flying capacitor voltage is unbalanced, it may cause damage to the IGBT.

[0003] The existing method of controlling the voltage balance of the flying capacitor takes BOOST (boost mode) as an example to maintain the voltage balance of the flying capacitor by controlling the on and off time of Q1 and Q2. The core is that if the flying capacitor voltage is lower than the target value, the on time ΔD1 of Q1 is increased, that is, the charging time of the flying capacitor is increased, and the on time ΔD2 of Q2 is reduced, that is, the discharge time of the flying capacitor is reduced. Since the voltage of the flying capacitor and the output voltage are coupled, that is, changing the voltage of the flying capacitor may cause a change in the output voltage, the existing method decouples the flying capacitor voltage and the output voltage through ΔD1=ΔD2. The existing method is feasible when the load variation range is not large, but the existing method for DC-DC applied to the front stage of the motor cannot maintain the voltage balance of the flying capacitor within the full power range. And due to the existence of the flying capacitor, the flying capacitor needs to be pre-charged when the DC-DC is started, and the existing pre-charging cost through hardware is relatively high. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a boost and buck control device and method for a bidirectional three-level DC-DC converter that can more effectively maintain the voltage balance of the flying capacitor.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A boost control device for a bidirectional three-level DC-DC converter, the bidirectional three-level DC-DC converter comprising a lithium battery, an inductor, four IGBT switch tubes, a flying capacitor and a bus capacitor, the boost control device comprising a bus voltage outer loop, an inductor current inner loop and a first signal output end connected in sequence, and a flying capacitor voltage loop and a second signal output end connected in sequence,

[0007] The bus voltage outer loop is respectively connected to the standard bus voltage value and the actual bus voltage value through a subtractor;

[0008] The inductor current inner loop is connected to the output of the bus voltage outer loop and the current value of the inductor through a subtractor, and outputs a first duty cycle signal;

[0009] The flying capacitor voltage loop is connected to half of the actual bus voltage value and the voltage of the flying capacitor through a subtractor, and outputs a second duty cycle signal;

[0010] The first signal output end includes a subtractor and a PWM unit connected in sequence, the positive input end of the subtractor is connected to the first duty cycle signal, and the negative input end is connected to the second duty cycle signal; the second signal output end includes an adder and a PWM unit connected in sequence, the adder is respectively connected to the first duty cycle signal and the second duty cycle signal, and the PWM unit is used to output two inverted control signals.

[0011] Furthermore, a duty cycle limiter is provided in the connection line between the subtractor and the PWM unit, and a duty cycle limiter is provided in the connection line between the adder and the PWM unit.

[0012] Furthermore, the bus voltage outer loop includes a subtractor, a voltage proportional integral regulator and a reference value limiter connected in sequence, the positive input terminal of the subtractor is connected to the standard bus voltage value, and the negative input terminal is connected to the actual bus voltage value;

[0013] The inductor current inner loop comprises a subtractor, a current proportional integral regulator and a duty cycle limiter which are connected in sequence, wherein the positive input terminal of the subtractor is connected to the output of the bus voltage outer loop, and the negative input terminal is connected to the current value of the inductor;

[0014] The flying capacitor voltage loop includes a multiplier, a subtractor and a flying proportional integral regulator connected in sequence, the multiplier is also connected to the actual bus voltage value, the positive input end of the subtractor is connected to the multiplier, and the negative input end is connected to the flying capacitor voltage value, and the value of the multiplier is 0.5.

[0015] The present invention also provides a step-down control device for a bidirectional three-level DC-DC converter, wherein the bidirectional three-level DC-DC converter comprises a lithium battery, an inductor, four IGBT switch tubes, a flying capacitor and a bus capacitor, and the step-up control device comprises a lithium battery voltage outer loop, an inductor current inner loop and a third signal output end connected in sequence, and a flying capacitor voltage loop and a fourth signal output end connected in sequence.

[0016] The lithium battery voltage outer ring is respectively connected to the standard lithium battery voltage value and the actual lithium battery voltage value through a subtractor;

[0017] The inductor current inner loop is connected to the output of the lithium battery voltage outer loop and the current value of the inductor through a subtractor, and outputs a third duty cycle signal;

[0018] The flying capacitor voltage loop is connected to half of the actual bus voltage value and the voltage of the flying capacitor through a subtractor, and outputs a fourth duty cycle signal;

[0019] The third signal output end includes an adder and a PWM unit connected in sequence, and the adder is respectively connected to the third duty cycle signal and the fourth duty cycle signal; the fourth signal output end includes a subtractor and a PWM unit connected in sequence, the positive input end of the subtractor is connected to the third duty cycle signal, and the negative input end is connected to the fourth duty cycle signal, and the PWM unit is used to output two inverted control signals.

[0020] Furthermore, a duty cycle limiter is provided in the connection line between the adder and the PWM unit, and a duty cycle limiter is provided in the connection line between the subtractor and the PWM unit.

[0021] Furthermore, the lithium battery voltage outer loop includes a subtractor, a voltage proportional integral regulator and a reference value limiter connected in sequence, the positive input terminal of the subtractor is connected to the standard lithium battery voltage value, and the negative input terminal is connected to the actual lithium battery voltage value;

[0022] The inductor current inner loop includes a subtractor, a current proportional integral regulator and a duty cycle limiter connected in sequence, the positive input end of the subtractor is connected to the output of the lithium battery voltage outer loop, and the negative input end is connected to the current value of the inductor;

[0023] The flying capacitor voltage loop includes a multiplier, a subtractor and a flying proportional integral regulator connected in sequence, the multiplier is also connected to the actual bus voltage value, the positive input end of the subtractor is connected to the multiplier, and the negative input end is connected to the flying capacitor voltage value, and the value of the multiplier is 0.5.

[0024] The present invention also provides a control method for a bidirectional three-level DC-DC converter, wherein the bidirectional three-level DC-DC converter includes a lithium battery, an inductor, four IGBT switch tubes, a lithium battery capacitor, a flying capacitor and a bus capacitor, and the method includes the following steps:

[0025] Capacitor pre-charging step: pre-charge the lithium battery capacitor, flying capacitor and bus capacitor;

[0026] Boost mode control step: when the bidirectional three-level DC-DC converter is in boost mode, it is controlled by the boost control device of the bidirectional three-level DC-DC converter as described above, and the bus voltage is monitored in real time. If the bus voltage is greater than a preset high voltage threshold, it is switched to buck mode;

[0027] Buck mode control step: when the bidirectional three-level DC-DC converter is in the buck mode, it is controlled by the buck control device of the bidirectional three-level DC-DC converter as described above, and the bus voltage is monitored in real time. If the bus voltage is greater than a preset low voltage threshold, it is switched to the boost mode.

[0028] Furthermore, the four IGBT switch tubes are a first switch tube, a second switch tube, a third switch tube and a fourth switch tube connected in sequence, the first switch tube is connected to the lithium battery, a second relay switch is connected in the connection line between the lithium battery and the inductor, and the second relay switch is connected in parallel with the first relay switch and the resistor connected in series in sequence,

[0029] In the capacitor pre-charging step, the flying capacitor is pre-charged, comprising the following steps:

[0030] S201: Initializing the system of the bidirectional three-level DC-DC converter;

[0031] S202: Turn on the first switch tube and turn off the second switch tube;

[0032] S203: Close the first relay switch;

[0033] S204: monitor and determine in real time whether the flying capacitor voltage is not less than half of the lithium battery voltage. If so, execute step S205;

[0034] S205: Turn off the first switch tube and the second switch tube;

[0035] S206: Real-time monitoring and determination of whether the actual bus voltage is equal to the lithium battery voltage. If so, the pre-charging of the flying capacitor is completed.

[0036] Furthermore, the high voltage threshold is 700V, and the low voltage threshold is 270V.

[0037] Furthermore, the method further comprises a fault control step: diagnosing in real time whether a fault occurs, and if a fault occurs, stopping the current mode of the bidirectional three-level DC-DC converter and entering a fault mode.

[0038] Compared with the prior art, the present invention has the following advantages:

[0039] (1) A hybrid control method of variable duty ratio and variable phase is used in a boost control device and a buck control device of a bidirectional three-level DC-DC converter to maintain flying capacitor balance, so that the bidirectional three-level DC-DC converter has a wide range of applications and can maintain output voltage stability and flying capacitor voltage balance when the load changes over a wide range;

[0040] (2) The flying capacitor precharging method utilizes the existing hardware circuit and implements flying capacitor precharging through software control, without the need for an additional precharging circuit, thus saving costs;

[0041] (3) The present invention proposes a bidirectional three-level DC-DC converter control method for mode management, which smoothly realizes the switching between BOOST (boost mode) and BUCK (buck mode), thereby not only being able to drive the motor to run, but also being able to recover energy, thereby increasing the mileage of the electric vehicle. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 A schematic diagram of the topological structure of a bidirectional three-level DC-DC converter used in the present invention;

[0043] Figure 2 This is a flying capacitor pre-charging flow chart of the present invention;

[0044] Figure 3 This is a schematic diagram of a flying capacitor pre-charging circuit of the present invention;

[0045] Figure 4 It is a structural schematic diagram of the boost control device of the present invention;

[0046] Figure 5 It is a structural schematic diagram of the voltage reduction control device of the present invention;

[0047] Figure 6 Schematic diagram of mode management of a bidirectional three-level DC-DC converter control method of the present invention;

[0048] In the figure, 1. bus voltage outer loop, 2. inductor current inner loop, 3. first signal output terminal, 4. flying capacitor voltage loop, 5. second signal output terminal, 6. lithium battery voltage outer loop, 7. third signal output terminal, 8. fourth signal output terminal, 9. subtractor, 10. adder, 11. voltage proportional integral regulator, 12. current proportional integral regulator, 13. flying proportional integral regulator, 14. reference value limiter, 15. duty cycle limiter, 16. multiplier, 17. PWM unit. DETAILED DESCRIPTION

[0049] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0050] Example 1

[0051] The present embodiment provides a boost control device for a bidirectional three-level DC-DC converter. The bidirectional three-level DC-DC converter includes a lithium battery, an inductor, four IGBT switch tubes, a flying capacitor and a bus capacitor. The boost control device includes a bus voltage outer loop 1, an inductor current inner loop 2 and a first signal output terminal 3 connected in sequence, and a flying capacitor voltage loop 4 and a second signal output terminal 5 connected in sequence.

[0052] Each component is described in detail below.

[0053] 1. Bidirectional three-level DC-DC converter

[0054] like Figure 1 As shown, the bidirectional three-level DC-DC converter includes a lithium battery, an inductor, four IGBT switch tubes, a flying capacitor and a bus capacitor. The four IGBT switch tubes are a first switch tube, a second switch tube, a third switch tube and a fourth switch tube connected in sequence. The first switch tube is connected to the lithium battery. A second relay switch is connected in the connection line between the lithium battery and the inductor. The second relay switch is connected in parallel with the first relay switch and a resistor connected in series in sequence.

[0055] 2. Boost control device

[0056] 2.1. Bus voltage outer loop 1

[0057] like Figure 4 As shown, the bus voltage outer loop 1 is connected to the standard bus voltage value and the actual bus voltage value through a subtractor 9;

[0058] Specifically, the bus voltage outer loop 1 includes a subtractor 9, a voltage proportional integral regulator 11 and a reference value limiter 14 connected in sequence, the positive input terminal of the subtractor 9 is connected to the standard bus voltage value, and the negative input terminal is connected to the actual bus voltage value.

[0059] Figure 4 In FIG. 1 , V_PI is a voltage proportional integral regulator 11 of a bus voltage outer loop 1 , and Iref_Limit is a reference value limiter 14 of an output current loop of the voltage proportional integral regulator 11 .

[0060] 2.2 Inductor Current Inner Loop 2

[0061] The inductor current inner loop 2 is connected to the output of the bus voltage outer loop 1 and the current value of the inductor through a subtractor 9, and outputs a first duty cycle signal;

[0062] Specifically, the inductor current inner loop 2 includes a subtractor 9, a current proportional integral regulator 12 and a duty cycle limiter 15 connected in sequence, the positive input terminal of the subtractor 9 is connected to the output of the bus voltage outer loop 1, and the negative input terminal is connected to the current value of the inductor;

[0063] Figure 4 In the figure, I_PI is the current proportional integral regulator 12, and duty_Limit is the duty cycle limiter 15.

[0064] 2.3. First signal output terminal 3 and second signal output terminal 5

[0065] The first signal output terminal 3 includes a subtractor 9 and a PWM unit 17 connected in sequence, the positive input terminal of the subtractor 9 is connected to the first duty cycle signal, and the negative input terminal is connected to the second duty cycle signal; the second signal output terminal 5 includes an adder 10 and a PWM unit 17 connected in sequence, the adder 10 is respectively connected to the first duty cycle signal and the second duty cycle signal, and the PWM unit 17 is used to output two inverted control signals.

[0066] A duty cycle limiter 15 is also provided in the connection line between the subtractor 9 and the PWM unit 17 , and a duty cycle limiter 15 is also provided in the connection line between the adder 10 and the PWM unit 17 .

[0067] 2.4 Flying capacitor voltage loop 4

[0068] The flying capacitor voltage loop 4 is connected to half of the actual bus voltage value and the flying capacitor voltage through a subtractor 9, and outputs a second duty cycle signal;

[0069] Specifically, the flying capacitor voltage loop 4 includes a multiplier 16, a subtractor 9 and a flying proportional integral regulator 13 connected in sequence, the multiplier 16 is also connected to the actual bus voltage value, the positive input end of the subtractor 9 is connected to the multiplier 16, and the negative input end is connected to the flying capacitor voltage value, and the value of the multiplier 16 is 0.5.

[0070] The flying capacitor voltage loop 4 can also be connected to Uf_Limit after the flying proportional integral regulator 13 is connected to the line. It is a flying capacitor voltage limiter, which is not added in this embodiment.

[0071] 2.5 Working Principle

[0072] The duty cycle and phase difference of the four IGBT (IGBT switch tube) drive signals are calculated by adjusting the bus voltage outer loop 1, the flying capacitor voltage loop 4, and the inductor current inner loop 2 to achieve the purpose of boosting the lithium battery to power the motor driver. Figure 4 As shown. The bus voltage outer loop 1 can keep the output voltage stable, reduce the fluctuation of the actual bus voltage value, and provide a stable voltage to the motor controller. The inductor current inner loop 2 can improve the dynamic performance of the system. At the same time, it can play a role in current limiting to prevent large currents from damaging components. The flying capacitor voltage loop 4 stabilizes the flying capacitor voltage value at half of the actual bus voltage value. This embodiment maintains the flying capacitor voltage balance through mixed control of variable duty cycle and variable phase, and the specific steps are as follows. D is the duty cycle calculated by the bus voltage outer loop 1 and the inductor current inner loop 2. ΔD is the output of the flying capacitor voltage loop 4, D2=D-ΔD, and D2 is the duty cycle of Q2. A pair of complementary drive signals of Q2 and Q3 are obtained through the PWM unit 17. D1=D+ΔD, D1 is the duty cycle of Q1, and the phase difference between Q1 and Q2 signals is 180°-ΔD°. Drive signals Q1 and Q4 are obtained through the PWM unit 17, and the drive signals of Q1 and Q4 are complementary. Through this hybrid control of variable duty cycle and variable phase, the voltage stability of the flying capacitor can be well maintained, and the flying capacitor voltage will not be unbalanced due to sudden changes in load.

[0073] like Figure 5 As shown, this embodiment also provides a step-down control device for a bidirectional three-level DC-DC converter, the bidirectional three-level DC-DC converter includes a lithium battery, an inductor, four IGBT switch tubes, a flying capacitor and a bus capacitor, and the boost control device includes a lithium battery voltage outer loop 6, an inductor current inner loop 2 and a third signal output terminal 7 connected in sequence, and a flying capacitor voltage loop 4 and a fourth signal output terminal 8 connected in sequence;

[0074] The following is a detailed description of each component.

[0075] 3.1. Lithium battery voltage outer ring 6

[0076] The lithium battery voltage outer ring 6 is connected to the standard lithium battery voltage value and the actual lithium battery voltage value through a subtractor 9;

[0077] The lithium battery voltage outer loop 6 includes a subtractor 9, a voltage proportional integral regulator 11 and a reference value limiter 14 which are connected in sequence, the positive input terminal of the subtractor 9 is connected to the standard lithium battery voltage value, and the negative input terminal is connected to the actual lithium battery voltage value;

[0078] 3.2 Inductor Current Inner Loop 2

[0079] The inductor current inner loop 2 is connected to the output of the lithium battery voltage outer loop 6 and the current value of the inductor through a subtractor 9, and outputs a third duty cycle signal;

[0080] The inductor current inner loop 2 includes a subtractor 9, a current proportional integral regulator 12 and a duty cycle limiter 15 which are connected in sequence, the positive input end of the subtractor 9 is connected to the output of the lithium battery voltage outer loop 6, and the negative input end is connected to the current value of the inductor;

[0081] 3.3. Third signal output terminal 7 and fourth signal output terminal 8

[0082] The third signal output terminal 7 includes an adder 10 and a PWM unit 17 connected in sequence, and the adder 10 is respectively connected to the third duty cycle signal and the fourth duty cycle signal; the fourth signal output terminal 8 includes a subtractor 9 and a PWM unit 17 connected in sequence, the positive input terminal of the subtractor 9 is connected to the third duty cycle signal, and the negative input terminal is connected to the fourth duty cycle signal, and the PWM unit 17 is used to output two inverted control signals.

[0083] A duty cycle limiter 15 is also provided in the connection line between the adder 10 and the PWM unit 17 , and a duty cycle limiter 15 is also provided in the connection line between the subtractor 9 and the PWM unit 17 .

[0084] 3.4 Flying capacitor voltage loop 4

[0085] The flying capacitor voltage loop 4 is connected to half of the actual bus voltage value and the flying capacitor voltage through a subtractor 9, and outputs a fourth duty cycle signal;

[0086] The flying capacitor voltage loop 4 includes a multiplier 16, a subtractor 9 and a flying proportional integral regulator 13 connected in sequence. The multiplier 16 is also connected to the actual bus voltage value. The positive input end of the subtractor 9 is connected to the multiplier 16, and the negative input end is connected to the flying capacitor voltage value. The value of the multiplier 16 is 0.5.

[0087] 3.5 Working Principle

[0088] These three loops realize energy recovery through DC / DC when the motor generates electricity. Figure 4As shown. The lithium battery voltage outer loop can keep the electromagnetic charging voltage stable, and can charge the lithium battery in constant voltage mode. The inductor current inner loop 2 can improve the dynamic performance of the system, and at the same time can charge the lithium battery in constant current mode to avoid high current charging and damage to the battery. The flying capacitor voltage loop 4 stabilizes the flying capacitor voltage at half of the bus voltage. As in the boost mode, the flying capacitor voltage balance is maintained through a mixed control of variable duty cycle and variable phase, but the specific duty cycle calculation is different from the boost mode. In the buck mode, D2=D+ΔD, D1=D-ΔD, and the phase difference between Q1 and Q2 is 180°+ΔD°.

[0089] like Figure 6 As shown, this embodiment also provides a control method for a bidirectional three-level DC-DC converter, comprising the following steps:

[0090] Capacitor pre-charging step: pre-charge the lithium battery capacitor, flying capacitor and bus capacitor;

[0091] The pre-charging process requires the microcontroller to collect the battery voltage Uin, flying capacitor voltage Uf, bus voltage Uo, and inductor current IL. When the capacitor is charged, it is equivalent to a short circuit, and a pre-charging resistor is required to limit the current. There are three capacitors Cin, Co, and Cf in the circuit that need to be pre-charged. When the relay K1 is closed, the current passes through the pre-charging resistor R to pre-charge Cin and Co.

[0092] The pre-charging of the flying capacitor Cf requires turning on Q1 and turning off Q2 after the DC / DC controller is powered on and initialized, and then waiting for K1 to close. When K1 is closed, the current passes through the pre-charging resistor R, the energy storage inductor L, and the diode in parallel with Q3. The flying capacitor Cf and IGBT Q1 circuit pre-charges Cf. The pre-charging circuit of the flying capacitor Cf is as follows: Figure 3 The pre-charge flow chart of the flying capacitor Cf is shown in Figure 2 shown.

[0093] like Figure 2 Specifically, the flying capacitor is pre-charged, including the following steps:

[0094] S201: Initializing the system of the bidirectional three-level DC-DC converter;

[0095] S202: Turn on the first switch tube Q1 and turn off the second switch tube Q2;

[0096] S203: Close the first relay switch K1;

[0097] S204: monitor in real time and determine whether the flying capacitor voltage Uf is not less than half of the lithium battery voltage. If the condition is not met, continue waiting. If the condition is met, execute step S205;

[0098] S205: Turn off the first switch tube Q1 and the second switch tube Q2;

[0099] S206: Real-time monitoring and judging whether the actual bus voltage Uo is equal to the lithium battery voltage Uin, if the condition is not met, continue to wait, if it is met, the pre-charging of the flying capacitor is completed. After the pre-charging is completed, wait for the system command to enter the boost mode or the open-loop mode.

[0100] Boost mode control step: when the bidirectional three-level DC-DC converter is in boost mode, it is controlled by the boost control device of the bidirectional three-level DC-DC converter as described above, and the bus voltage is monitored in real time. If the bus voltage is greater than a preset high voltage threshold, it is switched to buck mode;

[0101] Buck mode control step: when the bidirectional three-level DC-DC converter is in the buck mode, it is controlled by the buck control device of the bidirectional three-level DC-DC converter as described above, and the bus voltage is monitored in real time. If the bus voltage is greater than a preset low voltage threshold, it is switched to the boost mode; in this embodiment, the high voltage threshold is 700V and the low voltage threshold is 270V.

[0102] Fault control steps: diagnose in real time whether a fault occurs. If a fault such as overvoltage, overcurrent, or overtemperature occurs, stop the current mode of the bidirectional three-level DC-DC converter and enter the fault mode.

[0103] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A boost control device for a bidirectional three-level DC-DC converter, the bidirectional three-level DC-DC converter comprising a lithium battery, an inductor, four IGBT switch tubes, a flying capacitor and a bus capacitor, characterized in that: The boost control device comprises a bus voltage outer loop (1), an inductor current inner loop (2) and a first signal output terminal (3) connected in sequence, and a flying capacitor voltage loop (4) and a second signal output terminal (5) connected in sequence. The bus voltage outer loop (1) is respectively connected to the standard bus voltage value and the actual bus voltage value via a subtractor (9); The inductor current inner loop (2) is connected to the output of the bus voltage outer loop (1) and the current value of the inductor through a subtractor (9), and outputs a first duty cycle signal; The flying capacitor voltage loop (4) is connected to half of the actual bus voltage value and the voltage of the flying capacitor through a subtractor (9), and outputs a second duty cycle signal; The first signal output end (3) comprises a subtractor (9) and a PWM unit (17) connected in sequence, the positive input end of the subtractor (9) of the first signal output end (3) being connected to the first duty cycle signal, and the negative input end being connected to the second duty cycle signal; the second signal output end (5) comprises an adder (10) and a PWM unit (17) connected in sequence, the adder (10) being connected to the first duty cycle signal and the second duty cycle signal respectively, and the PWM unit (17) of the second signal output end (5) being used to output two control signals of opposite phases; A duty cycle limiter (15) is also provided in the connection line between the subtractor (9) and the PWM unit (17), and a duty cycle limiter (15) is also provided in the connection line between the adder (10) and the PWM unit (17); The bus voltage outer loop (1) comprises a subtractor (9), a voltage proportional integral regulator (11) and a reference value limiter (14) which are connected in sequence, wherein the positive input end of the subtractor (9) is connected to the standard bus voltage value, and the negative input end is connected to the actual bus voltage value; The inductor current inner loop (2) comprises a subtractor (9), a current proportional integral regulator (12) and a duty cycle limiter (15) connected in sequence, the positive input end of the subtractor (9) being connected to the output of the bus voltage outer loop (1), and the negative input end being connected to the current value of the inductor; The flying capacitor voltage loop (4) comprises a multiplier (16), a subtractor (9) and a flying proportional integral regulator (13) which are connected in sequence, the multiplier (16) is also connected to an actual bus voltage value, the positive input end of the subtractor (9) is connected to the multiplier (16), and the negative input end is connected to the flying capacitor voltage value, and the value of the multiplier (16) is 0.

5.

2. A step-down control device for a bidirectional three-level DC-DC converter, wherein the bidirectional three-level DC-DC converter comprises a lithium battery, an inductor, four IGBT switch tubes, a flying capacitor and a bus capacitor, characterized in that: The voltage reduction control device comprises a lithium battery voltage outer loop (6), an inductor current inner loop (2) and a third signal output terminal (7) connected in sequence, and a flying capacitor voltage loop (4) and a fourth signal output terminal (8) connected in sequence, The lithium battery voltage outer ring (6) is respectively connected to a standard lithium battery voltage value and an actual lithium battery voltage value via a subtractor (9); The inductor current inner loop (2) is connected to the output of the lithium battery voltage outer loop (6) and the current value of the inductor through a subtractor (9), and outputs a third duty cycle signal; The flying capacitor voltage loop (4) is connected to half of the actual bus voltage value and the voltage of the flying capacitor through a subtractor (9), and outputs a fourth duty cycle signal; The third signal output terminal (7) comprises an adder (10) and a PWM unit (17) connected in sequence, the adder (10) being connected to the third duty cycle signal and the fourth duty cycle signal respectively; the fourth signal output terminal (8) comprises a subtractor (9) and a PWM unit (17) being connected in sequence, the positive input terminal of the subtractor (9) of the fourth signal output terminal (8) being connected to the third duty cycle signal, and the negative input terminal being connected to the fourth duty cycle signal, the PWM unit (17) of the fourth signal output terminal (8) being used to output two control signals of opposite phases; A duty cycle limiter (15) is also provided in the connection line between the adder (10) and the PWM unit (17), and a duty cycle limiter (15) is also provided in the connection line between the subtractor (9) and the PWM unit (17); The lithium battery voltage outer loop (6) comprises a subtractor (9), a voltage proportional integral regulator (11) and a reference value limiter (14) which are connected in sequence, wherein the positive input end of the subtractor (9) is connected to a standard lithium battery voltage value, and the negative input end is connected to an actual lithium battery voltage value; The inductor current inner loop (2) comprises a subtractor (9), a current proportional integral regulator (12) and a duty cycle limiter (15) which are connected in sequence, the positive input end of the subtractor (9) being connected to the output of the lithium battery voltage outer loop (6), and the negative input end being connected to the current value of the inductor; The flying capacitor voltage loop (4) comprises a multiplier (16), a subtractor (9) and a flying proportional integral regulator (13) which are connected in sequence, the multiplier (16) is also connected to an actual bus voltage value, the positive input end of the subtractor (9) is connected to the multiplier (16), and the negative input end is connected to the flying capacitor voltage value, and the value of the multiplier (16) is 0.

5.

3. A control method for a bidirectional three-level DC-DC converter, wherein the bidirectional three-level DC-DC converter comprises a lithium battery, an inductor, four IGBT switch tubes, a lithium battery capacitor, a flying capacitor and a bus capacitor, characterized in that: The method comprises the following steps: Capacitor pre-charging steps: pre-charge lithium battery capacitors, flying capacitors and bus capacitors; Boost mode control step: when the bidirectional three-level DC-DC converter is in boost mode, it is controlled by the boost control device of a bidirectional three-level DC-DC converter as claimed in claim 1, and the bus voltage is monitored in real time. If the bus voltage is greater than a preset high voltage threshold, it is switched to buck mode; Buck mode control step: when the bidirectional three-level DC-DC converter is in the buck mode, the bidirectional three-level DC-DC converter is controlled by the buck control device of the bidirectional three-level DC-DC converter as claimed in claim 2, and the bus voltage is monitored in real time. If the bus voltage is greater than a preset low voltage threshold, it is switched to the boost mode.

4. The method according to claim 3, characterized in that The four IGBT switch tubes are a first switch tube, a second switch tube, a third switch tube and a fourth switch tube connected in sequence, the first switch tube is connected to the lithium battery, a second relay switch is connected in the connection line between the lithium battery and the inductor, and the second relay switch is connected in parallel with the first relay switch and the resistor connected in series in sequence, In the capacitor pre-charging step, the flying capacitor is pre-charged, including the following steps: S201: Initializing the system of the bidirectional three-level DC-DC converter; S202: Turn on the first switch tube and turn off the second switch tube; S203: Close the first relay switch; S204: monitor and determine in real time whether the flying capacitor voltage is not less than half of the lithium battery voltage. If so, execute step S205; S205: Turn off the first switch tube and the second switch tube; S206: Real-time monitoring and determination of whether the actual bus voltage is equal to the lithium battery voltage. If so, the pre-charging of the flying capacitor is completed.

5. The method according to claim 3, characterized in that: The high voltage threshold is 700V, and the low voltage threshold is 270V.

6. The method according to claim 3, characterized in that The method further comprises a fault control step: diagnosing in real time whether a fault occurs, and if a fault occurs, stopping the current mode of the bidirectional three-level DC-DC converter and entering a fault mode.

Images