Bidirectional dc-dc converter, lithium battery formation system and control method

Abstract

The application belongs to the technical field of direct current converters, and particularly relates to a bidirectional direct current converter, a lithium battery formation system and a control method. The bidirectional direct current converter comprises two switching tubes, two inductors and two capacitors. The bidirectional direct current converter can realize bidirectional step-up and step-down conversion, allows wide-range variation of the battery voltage, has few power devices and low cost, has the same polarity and common ground at the input and output, has small electromagnetic interference, has a simple sampling circuit structure, can realize zero-voltage turn-on of the two switching tubes in the whole working range, has small switching loss and high conversion efficiency, can realize efficient recovery and reuse of lithium battery energy storage, and reduces energy loss and production cost in battery formation.

Description

Technical Field

[0001] This invention belongs to the field of DC-DC converter technology, specifically relating to a bidirectional DC-DC converter, a lithium battery formation system, and a control method. Background Technology

[0002] Lithium-ion batteries, due to their small size, large capacity, high operating voltage, long cycle life, and lack of memory effect, have become the most promising high-efficiency rechargeable batteries and the fastest-growing chemical energy storage devices. Based on the chemical characteristics of lithium-ion batteries, they require multiple constant-voltage, constant-current charge-discharge cycles before leaving the factory to enhance their energy storage capacity; this process is called formation. Currently, most formation equipment lacks a reliable and efficient energy recovery system. During lithium-ion battery discharge, energy is directly dissipated through power resistors, resulting in high heat generation and low energy utilization. Furthermore, to lower the temperature in the formation workshop, high-power air conditioning systems must be installed and operated, further increasing energy consumption. Therefore, traditional battery formation systems significantly increase battery production costs and result in serious energy waste. Using supercapacitors to recover the energy from lithium-ion battery discharge and then recharge it can improve energy utilization, avoid energy waste, and reduce battery production costs. The bidirectional DC-DC converter is a key technological component in supercapacitor-based lithium-ion battery formation systems, responsible for voltage matching between the lithium-ion battery pack and the supercapacitor, and for controlling the bidirectional flow of energy. Because lithium batteries have a wide output voltage range and their lifespan is closely related to their current ripple, bidirectional DC-DC converters in supercapacitor-based lithium battery formation systems need a wide voltage gain range and low current ripple. Currently, bidirectional DC-DC converters are mainly divided into two categories: isolated and non-isolated. Compared to the former, non-isolated bidirectional DC-DC converters have many advantages such as smaller size and weight, simpler structure, lower cost, and higher efficiency. Traditional Buck / Boost bidirectional DC-DC converters are widely used due to their advantages such as continuous low-voltage current, common ground for high and low voltage sides, and fewer components. However, this converter cannot achieve bidirectional buck-boost. Four-switch buck-boost converters can achieve bidirectional boost and buck, and have advantages such as low voltage stress on power devices, fewer passive components, and input and output with the same polarity and common ground. However, their input and output currents are discontinuous, they have a large number of power devices, and their cost is higher. Summary of the Invention

[0003] In view of this, the present invention provides a bidirectional DC-DC converter, a lithium battery formation system and a control method. The bidirectional DC-DC converter contains only two switching transistors. When applied to a lithium battery formation system, it can realize bidirectional boost / buck conversion and zero voltage switching (ZVS) of all switching transistors. It can operate efficiently across the entire operating range and has the characteristics of continuous lithium battery-side current, fewer components, small size, low cost, and allowing for a wide range of lithium battery voltage variations.

[0004] To achieve the above objectives, the following solution is proposed:

[0005] A bidirectional DC-DC converter includes: a lithium battery-side capacitor C b First capacitor C1, first inductor L1, second inductor L2, first switch S1, second switch S2;

[0006] The lithium battery side capacitor C b The positive terminal is connected to one end of the first inductor L1;

[0007] The other end of the first inductor L1 is connected to the positive terminal of the first capacitor C1 and the drain of the first switching transistor S1;

[0008] One end of the second inductor L2 is connected to the negative terminal of the first capacitor C1 and the source terminal of the second switch S2;

[0009] The lithium battery side capacitor C b The negative terminal is connected to the source of the first switching transistor S1 and the other end of the second inductor L2;

[0010] The lithium battery side capacitor C b The positive terminal is the positive electrode on the lithium battery side;

[0011] The drain of the second switch S2 serves as the positive terminal on the supercapacitor side;

[0012] The lithium battery side capacitor C b The negative electrode serves as the negative terminal for both the lithium battery side and the supercapacitor side.

[0013] Furthermore, the first inductor L1 must satisfy:

[0014]

[0015] In the formula, U b U dc These are the terminal voltages of the lithium battery and the supercapacitor, respectively; f s,min Minimum switching frequency; I L1 δ% represents the average current of the first inductor L1; δ% represents the ripple rate of the first inductor current, which is typically taken as 20%-30%.

[0016] The second inductor L2 needs to satisfy:

[0017]

[0018] Where ΔI is the current margin, i.e., the peak value of the second inductor current i in the lithium battery discharge mode. L2,peak With the first inductor current valley value i L1,val The difference is usually taken as 2A-4A, Po,max This represents the maximum output power.

[0019] The present invention also provides a control method for the bidirectional DC-DC converter used in the above-mentioned lithium battery formation system, specifically including the following steps:

[0020] S1. Sample the first inductor current value i L1,f Compared with the benchmark value i L1,fef By comparison, the error signal i is obtained. L1,e ;

[0021] S2. The error signal i L1,e The signal is sent to the lithium battery current controller, where it passes through a unidirectional limiting circuit to obtain the adjustment signal u. r ;

[0022] S3. Sample the lithium battery side voltage value U b The first inductor current sampling value i L1,f The average value I L1 and the supercapacitor side voltage sampling value U dc The signal is fed into the switching frequency calculation stage to obtain the switching frequency f. s This results in a frequency of f. s Unipolar triangular carrier u c The calculation formula for the switching frequency calculation step is as follows: ;

[0023] S4. The adjustment signal u r With unipolar triangular carrier u c Intersection generates a PWM signal u PWM ;

[0024] S5. Sample the lithium battery side voltage value U b The upper threshold U of the lithium battery side voltage b,upp The signal is fed into voltage comparator 1 to obtain the lithium battery side overvoltage protection signal U. pro,up ;

[0025] S6. Sample the lithium battery side voltage value U b The lower threshold U of the lithium battery side voltage b,low The signal is fed into voltage comparator 2 to obtain the lithium battery side undervoltage protection signal U. pro,low ;

[0026] S7. The lithium battery side overvoltage protection signal U pro,up and lithium battery side voltage undervoltage protection signal U pro,low PWM signal u PWM ANDing yields the drive signal u for the first switch S1. gs,S1 , will u gs,S1The inverted signal is used as the drive signal u for the second switch S2. gs,S2 ;

[0027] The present invention also provides a lithium battery formation system, comprising the above-mentioned bidirectional DC-DC converter for lithium battery formation system, a lithium battery and a supercapacitor, wherein the lithium battery is connected to the lithium battery side of the bidirectional DC-DC converter for lithium battery formation system, and the supercapacitor is connected to the supercapacitor side of the bidirectional DC-DC converter for lithium battery formation system.

[0028] The present invention also provides a control method for the above-mentioned lithium battery formation system, specifically:

[0029] By changing the reference value of the first inductor current i L1,fef The polarity is used to set the operating mode of the lithium battery. When i L1,fef When i > 0, the formation system operates in lithium battery discharge mode, and the energy of the lithium battery is transferred to the supercapacitor; when i L1,fef When <0, the formation system operates in lithium battery charging mode, and the energy stored in the supercapacitor is transferred to the lithium battery;

[0030] By changing the reference value of the first inductor current i L1,fef The size of the current is used to set the constant current charging and discharging current value of the lithium battery;

[0031] By changing the upper threshold value U of the lithium battery side voltage b,upp To set the charging cutoff voltage;

[0032] By changing the lower threshold value U of the lithium battery side voltage b,low To set the discharge cutoff voltage.

[0033] Compared with the prior art, the present invention has the following technical effects:

[0034] 1) This converter can realize bidirectional buck-boost conversion, allowing for a wide range of lithium battery voltage variations. It has advantages such as fewer power devices, low cost, and input and output polarity and common ground.

[0035] 2) Zero-voltage switching (ZVS) of the first switch S1 and the second switch S2 can be achieved throughout the entire operating range, resulting in lower switching losses and higher conversion efficiency.

[0036] 3) It can realize the efficient recovery and reuse of lithium battery energy storage, reducing energy loss and production costs during battery formation. Attached Figure Description

[0037] Figure 1 A schematic diagram of a circuit structure for a lithium battery formation system based on a supercapacitor, provided for an embodiment of the present invention;

[0038] Figure 2 for Figure 1 The diagram shows the system control strategy block diagram of the bidirectional DC-DC converter in the lithium battery formation system.

[0039] Figure 3 The diagram shows the modal analysis of the bidirectional DC-DC converter in lithium battery discharge mode.

[0040] Figure 4 The main waveforms of the bidirectional DC-DC converter in lithium battery discharge mode are shown below.

[0041] Figure 5 Modal analysis diagram of the bidirectional DC-DC converter in lithium battery charging mode;

[0042] Figure 6 The main waveforms of the bidirectional DC-DC converter in lithium battery charging mode are shown below.

[0043] Figure 7 The steady-state simulation waveform of the bidirectional DC-DC converter in lithium battery discharge mode is shown.

[0044] Figure 8 The steady-state simulation waveform of the bidirectional DC-DC converter in lithium battery charging mode is shown.

[0045] Figure 9 Transient simulation waveforms of the bidirectional DC-DC converter in lithium battery discharge mode;

[0046] Figure 10 The reference value of the first inductor current is determined by i L1,ref Transient simulation waveform of the bidirectional DC-DC converter when the current is changed from 2.5A to -2.5A. Detailed Implementation

[0047] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0048] Figure 1 This is a schematic diagram of the circuit structure of a lithium battery formation system provided in an embodiment of the present invention. Figure 1 The dashed box in the middle shows a bidirectional DC-DC converter, including: a lithium battery-side capacitor C. b The components include: a first capacitor C1, a first inductor L1, a second inductor L2, a first switching transistor S1, and a second switching transistor S2; the lithium battery-side capacitor C... bThe positive terminal of the first inductor L1 is connected to one end of the first inductor L1; the other end of the first inductor L1 is connected to the positive terminal of the first capacitor C1 and the drain of the first switching transistor S1; one end of the second inductor L2 is connected to the negative terminal of the first capacitor C1 and the source of the second switching transistor S2; the lithium battery side capacitor C b The negative terminal is connected to the source of the first switching transistor S1 and the other end of the second inductor L2; the lithium battery side capacitor C b The positive terminal of the first switch is the positive terminal on the lithium battery side; the drain of the second switch S2 serves as the positive terminal on the supercapacitor side; the lithium battery side capacitor C b The negative terminal serves as the negative terminal for both the lithium battery side and the supercapacitor side. The lithium battery side of the bidirectional DC-DC converter is connected to the lithium battery, and the supercapacitor side is connected to the supercapacitor.

[0049] like Figure 2 As shown, the control method for the bidirectional DC-DC converter in this lithium battery formation system specifically includes the following steps:

[0050] S1. Sample the first inductor current value i L1,f Compared with the benchmark value i L1,fef By comparison, the error signal i is obtained. L1,e ;

[0051] S2. The error signal i L1,e The signal is sent to the lithium battery current controller, where it passes through a unidirectional limiting circuit to obtain the adjustment signal u. r ;

[0052] S3. Sample the lithium battery side voltage value U b The first inductor current sampling value i L1,f The average value I L1 and the supercapacitor side voltage sampling value U C The signal is fed into the switching frequency calculation stage to obtain the switching frequency f. s This results in a frequency of f. s Unipolar triangular carrier u c ;

[0053] S4. The adjustment signal u r With unipolar triangular carrier u c Intersection generates a PWM signal u PWM ;

[0054] S5. Sample the lithium battery side voltage value U b The upper threshold U of the lithium battery side voltage b,upp The signal is fed into voltage comparator 1 to obtain the lithium battery side overvoltage protection signal U. pro,up ;

[0055] S6. Sample the lithium battery side voltage value U b The lower threshold U of the lithium battery side voltage b,low The signal is fed into voltage comparator 2 to obtain the lithium battery side undervoltage protection signal U. pro,low ;

[0056] S7. The lithium battery side overvoltage protection signal U pro,up and lithium battery side voltage undervoltage protection signal U pro,low PWM signal u PWM ANDing yields the drive signal u for the first switch S1. gs,S1 , will u gs,S1 The inverted signal is used as the drive signal u for the second switch S2. gs,S2 .

[0057] The control method for the above-mentioned lithium battery formation system is as follows:

[0058] By changing the reference value of the first inductor current i L1,fef The polarity is used to set the operating mode of the lithium battery. When i L1,fef When i > 0, the formation system operates in lithium battery discharge mode, and the energy of the lithium battery is transferred to the supercapacitor; when i L1,fef When <0, the formation system operates in lithium battery charging mode, and the energy stored in the supercapacitor is transferred to the lithium battery;

[0059] By changing the reference value of the first inductor current i L1,fef The size of the value determines the constant current value for charging and discharging the lithium battery.

[0060] By changing the upper threshold value U of the lithium battery side voltage b,upp To set the charging cutoff voltage;

[0061] By changing the lower threshold value U of the lithium battery side voltage b,low To set the discharge cutoff voltage.

[0062] The following is about Figure 1 The working principle of the bidirectional DC-DC converter in the lithium battery formation system shown is explained.

[0063] To simplify the analysis, the following assumptions are made: First switch S1, second switch S2, lithium battery side capacitor C... b The first capacitor C1, the first inductor L1, and the second inductor L2 are all ideal components; the lithium battery side capacitor C b The first capacitor C1 is large enough that voltage ripple can be ignored; the lithium battery side capacitor C... b The negative terminal is the zero potential reference point; the body diodes of the first switch S1 and the second switch S2 are D and D, respectively. S1 D S2Supercapacitor C dc The capacity is very large, therefore C dc It can be regarded as a constant pressure source U dc .

[0064] Based on the above assumptions, the steady-state operation of the bidirectional DC-DC converter for the lithium battery formation system based on supercapacitors can be divided into four modes in both the lithium battery discharge and charging modes, as described below.

[0065] (1) Lithium battery discharge mode

[0066] When the first inductor current reference value i L1,fef When the value is greater than 0, the formation system operates in lithium battery discharge mode, and the energy of the lithium battery is transferred to the supercapacitor.

[0067] In this mode, the equivalent circuits for each mode are as follows: Figure 3 As shown in (a)-(d), the main waveforms within one switching cycle are as follows: Figure 4 As shown.

[0068] Before time t0, the body diode D of the first switching transistor S1 S1 The continuous current has been activated.

[0069] Mode 1, t0~t1 stage: (equivalent circuit as shown) Figure 3 (a) shown)

[0070] At time t0, the first switch S1 is turned on by ZVS, and its body diode D... S1 The circuit is automatically switched off, and mode 1 begins. First, the second inductor L2 charges the first capacitor C1 through the first switch S1, while the lithium battery and the second inductor L2 simultaneously charge the first inductor L1. Then, the lithium battery charges the first inductor L1 through the first switch S1. The first capacitor C1 first absorbs energy from the lithium battery and the second inductor L2, and then charges the second inductor L2 through the first switch S1; the first inductor current i... L1 The second inductor current i increases linearly in the positive direction. L2 First, it decreases linearly in the positive direction to zero, then it increases linearly in the negative direction. At this point, we have:

[0071] (1)

[0072] In the formula, L1 is the inductance of the first inductor, L2 is the inductance of the second inductor, and U C1 U is the terminal voltage of the first capacitor C1. b This refers to the terminal voltage of the lithium battery.

[0073] Mode 2, t1~t2 stage: (equivalent circuit as follows) Figure 3 (b)

[0074] At time t1, the first switch S1 is turned off, and the body diode D of the second switch S2... S2 Conduction begins, mode 2 starts. Through body diode D... S2 The lithium battery and the first inductor L1 simultaneously supply power to the first capacitor C1 and the supercapacitor C. dc Charging, the second inductor L2 is connected to the supercapacitor C dc Release energy; first inductor current i L1 The positive linear decreases, and the second inductor current i L2 If the linear decrease is reversed, then:

[0075] (2)

[0076] Mode 3, t2~t3 stage: (equivalent circuit as follows) Figure 3 (c) shown)

[0077] At time t2, the second switch S2 is turned on by ZVS, and its body diode D... S2 The circuit is automatically shut off, and mode 3 begins. First, through the second switch S2, the lithium battery and the first inductor L1 simultaneously supply power to the first capacitor C1 and the supercapacitor C. dc Charging, the second inductor L2 is connected to the supercapacitor C dc Energy is released; then, the lithium battery and the first inductor L1 simultaneously charge the first capacitor C1 and the second inductor L2, and the supercapacitor C... dc First, energy is released from the lithium battery and the first inductor L1, and then energy is released to the second inductor L2; the current i of the first inductor... L1 The positive linear decreases, and the second inductor current i L2 First, the current decreases linearly in the reverse direction to zero, and then increases linearly in the forward direction. Its current expression is the same as that in equation (2).

[0078] Mode 4, t3~t4 stage: (equivalent circuit as follows) Figure 3 (d)

[0079] At time t3, the second switch S2 is turned off, and the body diode D of the first switch S1 is switched off. S1 When the circuit is turned on, mode 4 begins. Through the first switch S1, the second inductor L2 charges the first capacitor C1, and simultaneously, the lithium battery and the second inductor L2 charge the first inductor L1. The first inductor current i L1 The second inductor current i increases linearly in the positive direction. L2 The current decreases linearly in the positive direction, and its expression is the same as that in equation (1).

[0080] Ignoring the dead time, based on the volt-second balance of the first inductor L1 and the second inductor L2, we can obtain from equations (1) and (2):

[0081] (3)

[0082] In the formula, T s D is the switching cycle, and D is the duty cycle of the drive signal for the first switching transistor S1.

[0083] In addition, by Figure 3 (a) We can obtain:

[0084] (4)

[0085] According to equations (3) and (4), the voltage gain of the bidirectional DC-DC converter proposed in this invention in lithium battery discharge mode is:

[0086] (5)

[0087] (2) Lithium battery charging mode

[0088] When the first inductor current reference value i L1,fef When <0, the formation system operates in lithium battery charging mode, and the energy stored in the supercapacitor is transferred to the lithium battery.

[0089] In this mode, the equivalent circuits for each mode are as follows: Figure 5 As shown in (a)-(d), the main waveforms within one switching cycle are as follows: Figure 6 As shown.

[0090] Mode 1, t0~t1 stage: (equivalent circuit as shown) Figure 5 (a) shown)

[0091] Before time t0, the body diode D of the second switch S2 S2 The continuous current has been activated.

[0092] At time t0, the second switch S2 is turned on by ZVS, and its body diode D... S2 The circuit is automatically shut off, and mode 1 begins. First, the second inductor L2 and the first capacitor C1 together charge the first inductor L1 and the lithium battery. The supercapacitor C... dc After the second switch S2 absorbs energy from the second inductor L2, it releases energy to the first inductor L1 and the lithium battery; then, the first capacitor C1 and the supercapacitor C dc Together, they release energy from the first inductor L1 and the lithium battery, while the second inductor L2 draws energy from the supercapacitor C. dc Energy absorption; first inductor current i L1 The reverse linear increase, the second inductor current i L2 First, the current decreases linearly to zero in the reverse direction, and then increases linearly in the forward direction. At this time, the current expressions of inductors L1 and L2 are as shown in equation (2).

[0093] Mode 2, t1~t2 stage: (equivalent circuit as follows) Figure 5(b)

[0094] At time t1, the second switch S2 is turned off, and the body diode D of the first switch S1... S1 Conduction begins, mode 2 starts. Through body diode D... S1 The first inductor L1 charges the lithium battery, and the second inductor L2 releases energy to the first capacitor C1; the first inductor current i L1 The reverse linear decreases, and the second inductor current i L2 The current decreases linearly in the positive direction. At this time, the current expressions of inductors L1 and L2 are as shown in equation (1).

[0095] Mode 3, t2~t3 stage: (equivalent circuit as follows) Figure 5 (c) shown)

[0096] At time t2, the first switch S1 is turned on by ZVS, and its body diode D... S1 The circuit is naturally shut off, and mode 3 begins. First, through the first switch S1, the first inductor L1 continues to release energy to the lithium battery. The second inductor L2 first releases energy to the first capacitor C1, and then absorbs energy from both the first inductor L1 and the first capacitor C1. Then, the first inductor L1 and the first capacitor C1 charge the lithium battery, while the second inductor L2 absorbs energy from the first capacitor C1. The first inductor current i... L1 The reverse linear decreases, and the second inductor current i L2 First, the current decreases linearly to zero in the forward direction, and then increases linearly in the reverse direction. Its current expression is the same as that of equation (1).

[0097] Mode 4, t3~t4 stage: (equivalent circuit as follows) Figure 5 (d)

[0098] At time t3, the first switch S1 is turned off, and the body diode D of the second switch S2... S2 Conduction begins, mode 4 starts. Through body diode D... S2 The second inductor L2 and the first capacitor C1 work together to charge the first inductor L1 and the lithium battery; at the same time, the second inductor L2 also charges the supercapacitor C. dc Charging; First inductor current i L1 The reverse linear increase, the second inductor current i L2 The current decreases linearly in the reverse direction, and its expression is the same as that in equation (2).

[0099] Ignoring dead time, based on the volt-second balance of the first inductor L1 and the second inductor L2, the voltage gain of the bidirectional DC-DC converter proposed in this invention in lithium battery charging mode can be obtained as follows:

[0100] (6)

[0101] Furthermore, modal analysis shows that:

[0102] (7)

[0103] In the formula, U S1 U S2 These represent the voltage stresses borne by the first switch S1 and the second switch S2, respectively.

[0104] Lithium battery side capacitor C b The voltage stress on the first capacitor C1 is:

[0105] (8)

[0106] The average current stress of each inductor is:

[0107] (9)

[0108] In the formula, I L1 I L2 These are the average currents of the first inductor L1 and the second inductor L2, respectively; I b I dc Lithium batteries and supercapacitors, respectively. dc The average current.

[0109] According to equations (1), (2), and (5), we can obtain:

[0110] (9)

[0111] First inductor current i L1 The peak value is:

[0112] (10)

[0113] Second inductor current i L2 The peak value is:

[0114] (11)

[0115] To achieve ZVS turn-on of the first switch S1 and the second switch S2 in lithium battery discharge mode, the peak value I of the second inductor current in this mode needs to be increased. L2,peak The value I is greater than the valley value of the first inductor current. L1,val At this point, we have:

[0116] (12)

[0117] To achieve ZVS turn-on of the first switch S1 and the second switch S2 in lithium battery charging mode, the absolute value of the valley of the second inductor current in this mode needs to be |I L2,val|Greater than the absolute value of the peak value of the first inductor current|I L1,peak At this time, we have:

[0118] (13)

[0119] In the formula, ΔI is called the current margin, and its value ranges from 2A to 4A.

[0120] It can be seen that equations (13) and (14) are the same, which indicates that the soft switching conditions are the same in the lithium battery charging mode and the discharging mode.

[0121] Substituting equations (11) and (12) into equation (13), we get:

[0122] (14)

[0123] To ensure ZVS turn-on of all switches across the entire operating range, it is necessary to achieve the minimum lithium battery voltage U b,min and maximum load P o,max Under the condition that equation (14) is satisfied, we have:

[0124] (15)

[0125] In the formula, f s,min Minimum switching frequency;

[0126] The first inductance L1 is designed to have a current ripple rate not exceeding δ%. In this case, we have:

[0127] (16)

[0128] Furthermore, if the switching frequency of the converter remains constant, the trough value I of the first inductor current will increase as the lithium battery voltage rises or the load decreases. L1,val The peak value of the second inductor current, I, will decrease. L2,peak The change is small, resulting in an excessively large current margin ΔI. A large current margin helps achieve ZVS turn-on of the switch, but it also brings large conduction losses, causing the conversion efficiency to decrease. Therefore, in order to ensure that the switch reliably achieves ZVS turn-on while avoiding excessive conduction losses, it is necessary to calculate and change the switching frequency in real time according to the operating conditions of the converter, so that the current margin ΔI remains approximately constant. According to equation (18), the formula for calculating the switching frequency can be obtained:

[0129] (17)

[0130] After adopting the variable switching frequency control method, the switching frequency of the proposed bidirectional converter is at the lowest lithium battery voltage U. b,low and maximum load P o,maxIt reaches its lowest level under certain conditions.

[0131] Specific embodiments of the present invention are given below. Their design specifications are shown in Table 1.

[0132] Table 1 Design Indicators

[0133] parameter Parameter value <![CDATA[Cut-off discharge voltage U of lithium battery b,low > 80V <![CDATA[Cut-off voltage U of lithium battery charging b,upp > 120V <![CDATA[Supercapacitor side voltage U dc > 100V <![CDATA[Maximum output power P o,max > 300W <![CDATA[Minimum switching frequency f s > 100kHz First inductor current ripple rate δ% 30% <![CDATA[Dead time T d > 100ns

[0134] The present invention designs the relevant inductance based on the design parameters shown in Table 1.

[0135] Substituting the parameters shown in Table 1 into equation (16), we get:

[0136] (19)

[0137] The actual value of the first inductance is: L1 = 750μH.

[0138] Substituting the parameters shown in Table 1 and L1=750μH into equation (15), we get:

[0139] (20)

[0140] The actual value of the second inductance is: L2 = 22μH.

[0141] The feasibility of the proposed bidirectional converter was verified using Saber simulation. Specific technical specifications are shown in Table 1, and the circuit parameters are as follows: first capacitor C1 = 10μF, lithium battery side capacitor C... b =47μF; First inductor L1=750μH, second inductor L2=22μH. In the simulation, the reference current value i of the first inductor is changed. L1,fef The polarity is used to set the operating mode of the lithium battery. If i L1,ref =2.5A indicates that the formation system is set to lithium battery constant current discharge mode, with a constant current value of 2.5A. If i L1,ref = -2.5A indicates that the formation system is set to lithium battery constant current charging mode, with a constant current value of 2.5A. Furthermore, the discharge cutoff voltage is set to U. b,low =80V, charging cut-off voltage set to U b,up =120V. When the voltage of the lithium battery exceeds 120V, charging stops; when the voltage of the lithium battery falls below 80V, discharging stops immediately. Therefore, the operating voltage range of the lithium battery during the formation process is 80V-120V.

[0142] Figure 7 For lithium battery discharge mode (first inductor current reference value set to i) L1,ref Steady-state simulation waveform of the proposed bidirectional converter at 2.5A.

[0143] Figure 7(a) gives the first inductor current i in this mode. L1 Second inductor current i L2 Lithium battery side voltage U b and supercapacitor side voltage U dc The simulated waveform shows that the lithium battery voltage U... b =80V, supercapacitor voltage U dc =100V, actual voltage gain is U dc / U b =100 / 80=1.25, the measured duty cycle is approximately 0.534, which is basically consistent with the theoretical duty cycle D=0.56. First inductor current i L1 The average value I L1 Approximately 2.5A, second inductor current i L2 The average value I L2 =-1.93A. Peak value I of the second inductor current. L2,peak =5.31A, which is greater than the valley value I of the first inductor current. L1,val =2.22A, ΔI≈3A, which satisfies the zero-voltage turn-on condition for the first switch S1 and the second switch S2.

[0144] Figure 7 (b) The drive signal u of the first switch S1 is given. gs,S1 Terminal voltage u S1 and current i S1 The simulated waveform. Figure 7 (c) The drive signal u of the second switch S2 is given. gs,S2 Terminal voltage u S2 and current i S2 The simulated waveform. (From...) Figure 7 (b) and Figure 7 (c) It can be seen that the switching frequency f s ≈114kHz; the voltage stress of both the first switch S1 and the second switch S2 is approximately 180V, consistent with the theoretical value; the drive signal u of the first switch... gs,S1 Second switch drive signal u gs,S2 Before it becomes high, its terminal voltage u S1 and u S2 Both have dropped to zero, indicating that both the first switch S1 and the second switch S2 have achieved ZVS turn-on.

[0145] Figure 8 Lithium battery charging mode (first inductor current reference value set to i) L1,ref Steady-state simulation waveform of the proposed bidirectional converter at -2.5A.

[0146] Figure 8 (a) gives the first inductor current i in this mode.L1 Second inductor current i L2 Lithium battery side voltage U b and supercapacitor side voltage U dc The simulated waveform shows that the lithium battery voltage U... b =80V, supercapacitor voltage U dc =100V, actual voltage gain is U b / U dc =80 / 100≈0.4, the measured duty cycle is approximately 0.531, which is basically consistent with the theoretical duty cycle D=0.56. First inductor current i L1 The average value I L1 Approximately -2.5A, second inductor current i L2 The average value I L2 =2.00A. Absolute value of the valley current of the second inductor |I L2,val |=5.14A, greater than the peak absolute value of the first inductor current|I L1,peak |=2.17A, ΔI≈3A, which satisfies the zero-voltage turn-on condition for the first switch S1 and the second switch S2.

[0147] Figure 8 (b) The drive signal u of the first switch S1 is given. gs,S1 Terminal voltage u S1 and current i S1 The simulated waveform. Figure 8 (c) The drive signal u of the second switch S2 is given. gs,S2 Terminal voltage u S2 and current i S2 The simulated waveform. (From...) Figure 8 (b) and Figure 8 (c) It can be seen that the switching frequency f s ≈115 kHz; the voltage stress of both the first switch S1 and the second switch S2 is approximately 180V, consistent with the theoretical value; the drive signal u of the first switch... gs,S1 Second switch drive signal u gs,S2 Before it becomes high, its terminal voltage u S1 and u S2 Both have dropped to zero, indicating that both the first switch S1 and the second switch S2 have achieved ZVS turn-on.

[0148] Figure 9 In lithium battery discharge mode, the lithium battery voltage U b Transient simulation waveform of the converter during the change. It can be seen that 200ms ago, the lithium battery voltage U... b =120V, first inductor current i L1 The average value I L1The voltage is approximately 2.5A, indicating that the converter is in constant current discharge mode for the lithium battery. At 200ms, the lithium battery voltage U... b The voltage suddenly drops from 120V to 80V. It can be seen that after a 10ms adjustment process, the first inductor current i... L1 The average value I L1 ≈ 2.5A, switching frequency f s The change from 124.3kHz to 115.4kHz indicates that the converter can still achieve constant current discharge of the lithium battery when the lithium battery voltage changes, and can automatically adjust the switching frequency.

[0149] Figure 10 The figure shows the reference value of the first inductor current, i. L1,ref The transient simulation waveform of the proposed bidirectional converter when the current changes from 2.5A to -2.5A is shown. It can be seen that before 200ms, the first inductor current i... L1 The average value I L1 ≈2.5A indicates that the converter is in the lithium battery constant current discharge mode; at 200ms, the current reference value i L1,ref The current was switched from 2.5A to -2.5A. It can be seen that after a 5ms adjustment process, the first inductor current i... L1 The average value I L1 = -2.5A indicates that the converter has entered the lithium battery constant current charging mode, thus verifying the feasibility of the present invention.

[0150] The simulation results above demonstrate that the bidirectional DC-DC converter, lithium battery formation system, and control method proposed in this invention have the following technical advantages: 1) The converter can achieve bidirectional buck-boost conversion, allowing for a wide range of lithium battery voltage variations, and has advantages such as fewer power devices, low cost, and input / output polarity and common ground; 2) It can achieve zero-voltage switching (ZVS) of the first switch S1 and the second switch S2 throughout the entire operating range, exhibiting lower switching losses and higher conversion efficiency; 3) It enables efficient recovery and reuse of lithium battery energy storage, reducing energy loss and production costs during battery formation.

[0151] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes the element.

[0152] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention, and is not intended to limit it. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the present invention.

Claims

1. A bidirectional DC-DC converter for a lithium battery formation system based on a supercapacitor, characterized in that, Including battery-side capacitor C b First capacitor C1, first inductor L1, second inductor L2, first switch S1, second switch S2; The battery-side capacitor C b The positive terminal is connected to one end of the first inductor L1; The other end of the first inductor L1 is connected to the positive terminal of the first capacitor C1 and the drain of the first switching transistor S1; One end of the second inductor L2 is connected to the negative terminal of the first capacitor C1 and the source terminal of the second switch S2; The battery-side capacitor C b The negative terminal is connected to the source of the first switching transistor S1 and the other end of the second inductor L2; The battery-side capacitor C b The positive terminal is the positive terminal on the battery side; The drain of the second switch S2 serves as the positive terminal on the supercapacitor side; The battery-side capacitor C b The negative terminal serves as the negative terminal for both the battery side and the supercapacitor side. The first inductor L1 satisfies: ， In the formula, U b U is the voltage at the battery side. dc f is the voltage across the supercapacitor. s,min For the lowest switching frequency, I L1 δ% represents the average current of the first inductor L1, and δ% represents the current ripple rate of the first inductor. The second inductor L2 satisfies: ， In the formula, ΔI is the current margin, i.e., the peak value of the second inductor current i in the battery discharge mode. L2,peak With the first inductor current valley value i L1,val The difference, take 2A-4A, P o,max For maximum output power, U b,min This is the lowest lithium battery voltage.

2. A control method for a bidirectional DC-DC converter used in a lithium battery formation system as described in claim 1, characterized in that, The control method includes: S1. Sample the first inductor current value i L1,f Compared with the benchmark value i L1,fef By comparison, the error signal i is obtained. L1,e ; S2. The error signal i L1,e The signal is sent to the battery current controller, where it passes through a unidirectional limiting circuit to obtain the adjustment signal u. r ; S3. Sample the battery side voltage value U b The first inductor current sampling value i L1,f The average value I L1 and the sampled value of the supercapacitor side voltage U dc The signal is fed into the switching frequency calculation stage to obtain the switching frequency f. s This results in a frequency of f. s Unipolar triangular carrier u c The calculation formula for the switching frequency calculation step is as follows: ; S4. The adjustment signal u r With unipolar triangular carrier u c Intersection generates a PWM signal u PWM ; S5. Sample the battery side voltage value U b The upper threshold U of the battery side voltage b,upp The signal is fed into voltage comparator 1 to obtain the battery-side overvoltage protection signal U. pro,up ; S6. Sample the battery side voltage value U b The lower threshold value U of the battery side voltage b,low The signal is fed into voltage comparator 2 to obtain the undervoltage protection signal U on the battery side. pro,low ; S7. The battery side voltage overvoltage protection signal U pro,up and the battery side voltage undervoltage protection signal U pro,low PWM signal u PWM ANDing yields the drive signal u for the first switch S1. gs,S1 The driving signal u gs,S1 The inverted signal is used as the drive signal u for the second switch S2. gs,S2 .

3. A lithium battery formation system, characterized in that, The system includes a bidirectional DC-DC converter for a lithium battery formation system as described in claim 1, a lithium battery, and a supercapacitor, wherein the lithium battery is connected to the battery side of the bidirectional DC-DC converter for the lithium battery formation system, and the supercapacitor is connected to the supercapacitor side of the bidirectional DC-DC converter for the lithium battery formation system.

4. A control method for the lithium battery formation system according to claim 3, characterized in that, The control method includes: By changing the first inductor current reference value i L1,fef The polarity is used to set the working mode of the lithium battery, which includes a discharge mode and a charging mode. By changing the first inductor current reference value i L1,fef The size of the lithium battery is used to set the charging rate and discharging rate. By changing the upper threshold value U of the battery side voltage b,upp To set the charging cutoff voltage; By changing the lower limit threshold U of the battery side voltage b,low To set the discharge cutoff voltage.

5. The control method according to claim 4, characterized in that, The method involves changing the first inductor current reference value i L1,fef The polarity is used to set the operating mode of the lithium battery, specifically: when i L1,fef When the value is greater than 0, the lithium battery formation system operates in lithium battery discharge mode, and the energy of the lithium battery is transferred to the supercapacitor. when i L1,fef When the value is less than 0, the lithium battery formation system operates in lithium battery charging mode, and the energy stored in the supercapacitor is transferred to the lithium battery.

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