Implementation method of current continuous soft switching based on switched inductor battery balancer

Abstract

The present invention proposes a method for implementing continuous current soft switching of a switched inductor battery balancer. The method is divided into three cases according to the voltage of the battery cells being balanced, namely, no balancing is required between the battery cells, battery cell B1 transfers energy to battery cell B2, and battery cell B2 transfers energy to battery cell B1. The signal is modulated according to the modulation ratio to obtain a PWM signal for controlling the first switch tube S1 and the second switch tube S2. The method of the present invention can realize soft switching of all the switch tubes in the balancer, reduce the loss of the balancer, and improve the efficiency. In addition, the control of the balancer does not require the participation of the current feedback signal, eliminates the current sensor, saves the cost of the balancer, and thus greatly improves the market competitiveness of the switched inductor lithium-ion battery balancer.

Description

Technical field

[0001] The present invention relates to the technical field of control of power electronic converters, in particular to a method for realizing current continuous soft switching based on a switched inductor battery equalizer. [Background Technology]

[0002] Lithium-ion batteries are widely used in daily production and life, such as in handheld portable power tools, laptop power supplies, smart microgrid energy storage systems, and electric vehicle power batteries. Due to differences in battery characteristics between individual cells, after repeated charge and discharge cycles, lithium-ion battery cells in series can experience inconsistent states of charge (SOC) and voltages. Long-term operation can lead to overcharge and overdischarge, adversely affecting the lifespan, capacity, and safety of the cells. SOC differences are often reflected in differences in cell voltage, making voltage balancing a must-have feature for lithium-ion battery packs used in series.

[0003] At present, lithium-ion battery balancers can be divided into energy-consuming type balancers and non-energy-consuming type balancers. The energy-consuming type balancer uses switch-controlled resistors to connect battery cells with higher SOC to ensure that the voltages of all battery cells tend to be consistent. This method is simple to control, low-cost, and easy to implement, but it has large losses and can easily cause safety hazards such as local overheating of the battery pack.

[0004] Non-energy-consuming balancers are further divided into active and passive balancers. Active balancers primarily use switching transistors to actively balance energy and voltage between battery cells. Common methods include switched capacitors and their derivative topologies, switched inductors and their derivative topologies, bidirectional CUK balancers, and bidirectional flyback balancers. However, these balancers incur significant losses in battery packs with a large number of series cells and high voltages.

[0005] Passive equalizers primarily achieve voltage balancing between battery cells through a rectifier circuit composed of transformer coils and diodes. The principle is that current always flows to the cell with the lowest voltage, so the cell with the lowest voltage always receives the most current distribution, causing its voltage to rapidly approach that of the cell with higher potential. Currently, the most commonly used topologies include a flyback equalizer with a multi-coil transformer, a single-secondary transformer + single-bridge-arm rectifier cascade equalizer, a single-secondary transformer + bridge rectifier cascade equalizer, and a slope voltage doubler rectifier equalizer. The flyback equalizer topology with a multi-coil transformer is relatively simple, but due to the characteristics of the flyback topology, it can only be used for voltage balancing of smaller power battery packs.

[0006] In recent years, switched-inductor battery balancers have evolved into improved cascaded balancing structures. While these cascaded balancers effectively accelerate balancing between battery cells, the design methods, voltage, and current stresses of components such as switches and inductors at different levels are inconsistent, placing pressure on modularity and cost. While traditional switched-inductor battery balancers offer slower balancing speeds, their circuit structure is simple and modularly scalable. Switched-inductor balancers, which transfer energy between adjacent cells, still have significant application value in battery packs with 3-4 cells in series, such as handheld portable power tool batteries and laptop batteries. However, the inability of each switch in the balancer to fully achieve soft switching complicates efficiency improvements and widespread application.

[0007] Therefore, realizing soft switching of all switch tubes in a simple structured, traditional switched inductor-based lithium-ion battery balancer can further expand the application scenarios of traditional switched inductor-based lithium-ion battery balancers. A method for realizing continuous current soft switching of a switched inductor-based battery balancer is now proposed. [Summary of the invention]

[0008] The purpose of the present invention is to solve the problems in the prior art that the equalizer switch tube cannot achieve soft switching, has low efficiency, and has high control implementation cost, and proposes a current continuous soft switching implementation method based on a switched inductor battery equalizer.

[0009] To achieve the above objectives, the present invention proposes a method for realizing continuous current soft switching based on a switching inductor battery balancer. The method is realized based on a switching inductor lithium-ion battery balancer. The switching inductor lithium-ion battery balancer includes two battery cells to be balanced: a first battery cell B1 and a second battery cell B2, two switching tubes: a first switching tube S1 and a second switching tube S2, and an inductor L0; the voltages of the first battery cell B1 and the second battery cell B2 are U B1 、U B2 The current flowing into the first battery cell B1 and the second battery cell B2 are i B1 、i B2 , the inductor current is i L The driving signals of the first switch tube S1 and the second switch tube S2 are complementary, and the conduction time of the first switch tube S1 and the second switch tube S2 are DT s and (1-D)T s , where D is the modulation ratio of the first switch S1, T s is a switching period; the inductance value of the inductor L0 is L; the method comprises the following steps:

[0010] S1. Real-time detection of the switch inductor lithium-ion battery equalizer to obtain the open circuit voltage U of the first battery cell B1 BO1And the open circuit voltage U of the second battery cell B2 BO2 ;

[0011] S2. Determine the open circuit voltage U BO1 、U BO2 with U T The relationship between U T is the start-up voltage of the lithium-ion battery balancer based on the switching inductor, if |U BO1 -U BO2 |≤U T , then balancing is not required, and both the first switch tube S1 and the second switch tube S2 are turned off; otherwise, the process goes to step S3;

[0012] S3. Determine the voltage U again BO1 、U BO2 with U T If U BO1 -U BO2 >U T , then go to step S4; if U BO2 -U BO1 >U T , then proceed to step S5;

[0013] S4. Set the modulation ratio of the first switch S1 to D=D1, and the average value of the inductor current I L >0, in order to achieve soft switching, the minimum inductor current is set to -x, then I L =0.5(Δi L -2x), Δi L is the change in inductor current during a switching cycle; the modulation ratio D1 is obtained using the following formula, and then step S6 is entered.

[0014]

[0015] Where A = R Σ T s (U BO1 +U BO2 ), B=(U BO1 +U BO2 )(2L-R Σ T s ), C=2L(xR Σ -U BO2 ), R ∑ is the equalizer equivalent resistance and Ts is the switching period;

[0016] S5. Set the modulation ratio of the first switch S1 to D=D2, and the average value of the inductor current I L <0, to achieve soft switching, set the maximum value of the inductor current to x, then I L =0.5(2x-ΔiL ), Δi L is the change in inductor current during a switching cycle; the modulation ratio D2 is obtained using the following formula, and then step S6 is entered.

[0017]

[0018] Where a = R Σ T s (U BO1 +U BO2 ), b=-(U BO1 +U BO2 )(2L+R Σ T s ), c=2L(xR Σ +U BO2 ), R ∑ is the equalizer equivalent resistance and Ts is the switching period;

[0019] S6. Modulate the signal according to the modulation ratio to obtain a PWM signal for controlling the first switch tube S1 and the second switch tube S2.

[0020] As a preferred method, the average value of the inductor current I is obtained. L The method is as follows: based on the switching inductor lithium-ion battery balancer, the open circuit voltages of the first battery cell B1 and the second battery cell B2 are detected in real time during operation. BO1 、U BO2 , according to the average value of the inductor voltage in one switching cycle is equal to 0, the average value of the inductor current I is obtained L for

[0021]

[0022] As a preferred method, obtain the equalizer equivalent resistance and R ∑ The method is: detect the internal resistance R of the battery cell B , the on-resistance of the switch tube R S , the equivalent resistance R of the inductor L0 L , find R B 、R S 、R L The sum of the three R ∑ , that is, R Σ =R L +R S +R B .

[0023] As a preference, the change in inductor current during one switching cycle is

[0024]

[0025] As a preferred option, when the average inductor current I L Satisfy I L <0.5Δi L , soft switching is achieved.

[0026] Preferably, each switching cycle includes two switching modes.

[0027] Preferably, in step S4 , the modulation ratio D of the first switch tube S1 is set to D1 , and at this time, the energy of the first battery cell B1 flows to the second battery cell B2 .

[0028] Preferably, in step S5 , the modulation ratio D of the first switch tube S1 is set to D2 , and at this time, the energy of the second battery cell B2 flows to the first battery cell B1 .

[0029] The beneficial effects of the present invention are as follows: the method of the present invention can realize soft switching of all switching tubes in the equalizer, thereby reducing the loss of the equalizer and improving the efficiency; and the control of the equalizer does not require the participation of the current feedback signal in the control, eliminating the current sensor, saving the cost of the equalizer, thereby greatly improving the market competitiveness of the lithium-ion battery equalizer based on the switching inductor.

[0030] The features and advantages of the present invention will be described in detail through embodiments with reference to the accompanying drawings.

Brief Description of the Drawings

[0031] Figure 1 The circuit topology of the lithium-ion battery balancer based on the switched inductor;

[0032] Figure 2 Four possible waveforms of inductor current in the circuit topology of the switched inductor lithium-ion battery balancer;

[0033] Figure 3 is the equivalent circuit of the circuit topology mode 1 of the switched inductor lithium-ion battery balancer;

[0034] Figure 4 is the equivalent circuit of the circuit topology mode 2 of the switched inductor lithium-ion battery balancer;

[0035] Figure 5 is a flow chart of the method of the present invention;

[0036] Figure 6 The theoretical value of duty cycle varies with battery cell voltage;

[0037] Figure 7 The influence curve of duty cycle error on the actual minimum value of inductor current is shown in Fig.

[0038] Symbol names in the figure: B1—first battery cell, B2—second battery cell; U B1—First battery cell voltage, U B2 —Second battery cell voltage; S1—first switch tube, S2—second switch tube; u S1 —The driving signal of the first switch tube, u S2 —Driving signal of the second switch tube; U BO1 —Open circuit voltage of the first battery cell, U BO2 - open circuit voltage of the second battery cell; i B1 —First battery cell current, i B2 —Second battery cell current; L0—inductance; i L —Inductor current; I L —Average value of inductor current; D—Modulation ratio of the first switch tube; T s —Switching cycle; R B —Battery cell internal resistance; R S —Switching tube on-resistance; R L —Inductor equivalent resistance; R ∑ —Equalizer equivalent resistance and. [Specific implementation method]

[0039] Based on the switch inductor lithium ion battery balancer circuit topology Figure 1 As shown in the figure, it is actually a buck-boost converter with bidirectional energy flow. When it is applied to the balancing of battery cells, the two switches are controlled to conduct complementary. According to the size of the voltage difference between the two battery cells and the size of the switch modulation ratio, the operation of the inductor current in the balancer can be divided into four types, as follows: Figure 2 shown. Figure 2 In the figure, the average value of the inductor current corresponding to cases 1 and 2 is greater than zero, at which point the energy in the first battery cell B1 flows to the second battery cell B2; the average value of the inductor current corresponding to cases 3 and 4 is less than zero, at which point the energy in the second battery cell B2 flows to the first battery cell B1. Based on the direction of the inductor current during the switching process of the switch tube, it can be determined that when the inductor current waveform is as shown in cases 1 and 4, the switch tube cannot fully achieve soft switching; while in cases 2 and 3, the switch tube can fully achieve soft switching, so we hope that the inductor current runs at Figure 2 The waveforms in the figure correspond to those in cases 2 and 3. Figure 2 The implementation method of the waveforms shown in Case 2 and Case 3.

[0040] When the inductor current is continuous, the duty cycle of the balancer is strictly constrained by the mathematical relationship between the input and output. However, in practice, due to factors such as the equivalent output impedance of the battery cells, the on-resistance of the switch tube, and the impedance of the inductor line, which are not negligible in balancers with low battery cell counts, the relationship between the input and output voltages and the duty cycle D no longer satisfies the formula for a buck-boost converter with continuous current. To this end, the following derivation process was established.

[0041] Assume that the equivalent impedance of all battery cells is R B , the on-resistance of all electrical switches is R S , the equivalent impedance of the inductor is R L , then R B 、R S 、R L and R ∑ for

[0042] R Σ =R L +R S +R B (1)

[0043] When the first switch tube S1 is turned on and the second switch tube S2 is turned on, the equivalent circuit is as follows: Figure 3 、 Figure 4 As shown in the figure, U BO1 、U BO2 The open circuit voltages of the first battery cell B1 and the second battery cell B2 are respectively written as KVL equations for the two equivalent circuits. The average value is calculated within one switching cycle. Based on the relationship that the average value of the inductor voltage within one cycle is equal to 0, the average value of the inductor current I is obtained. L for

[0044]

[0045] In one switching cycle, the change in inductor current is

[0046]

[0047] To achieve soft switching, the average inductor current I L Must meet

[0048] I L <0.5Δi L (4)

[0049] According to the voltage U of the two battery cells B1 and B2 B1 、U B2 There are three situations:

[0050] I.|U BO1-U BO2 |≤U T

[0051] When|U Bo1 -U Bo2 |<U T When U T The start-up voltage of the lithium-ion battery balancer based on the switching inductor is that the energy between the two battery cells is close and no balancing is required, so the switching tubes S1-S2 are all turned off.

[0052] II.U BO1 -U BO2 >U T

[0053] At this time, the modulation ratio D of the switch tube S1 is set to D1, and the energy of the first battery cell B1 flows to the second battery cell B2. The average value of the inductor current I L >0, in order to achieve soft switching, the minimum inductor current is set to -x, then

[0054] I L =0.5(Δi L -2x) (5)

[0055] Substituting equations (2) and (3) into equation (5), we get

[0056]

[0057] Where A = R Σ T s (U BO1 +U BO2 ), B=(U BO1 +U BO2 )(2L-R Σ T s ), C=2L(xR Σ -U BO2 ).

[0058] III.U BO2 -U BO1 >U T

[0059] At this time, the modulation ratio D of the switch tube S1 is set to D2, and the energy of the second battery cell B2 flows to the first battery cell B1. The average value of the inductor current I L <0, to achieve soft switching, set the maximum value of the inductor current to x, then

[0060] I L =0.5(2x-Δi L ) (7)

[0061] Substituting equations (2) and (3) into equation (7), we get

[0062]

[0063] Where a = R Σ T s (U BO1 +U BO2 ), b=-(U BO1 +U BO2 )(2L+R Σ T s ), c=2L(xR Σ +U BO2 ).

[0064] The signal is modulated according to the modulation ratio obtained in the above-mentioned cases II and III to obtain a PWM signal for controlling the first switch tube S1 and the second switch tube S2.

[0065] Figure 5 The duty cycle prediction control strategy for realizing soft switching does not need to detect the inductor current in the switched inductor balancer, which can further reduce the cost of the balancer.

[0066] To verify Figure 5 The effectiveness of the soft switching strategy of the equalizer shown in the figure requires an analysis of the effect of the actual deviation of the duty cycle on the current (x value) used to set the soft switching. BO1 >U BO2 When the situation, for U BO1 BO2 A similar analysis can be made for the situation when .

[0067] During the actual operation of the balancer, the voltage difference between battery cells generally exceeds a certain value, and the balancer starts balancing. The lithium-ion battery balancer is generally set to 0.05V. Therefore, the following analyzes the situation when the battery cell voltage difference is within this range. Figure 6 Shown is the sum of the resistance R ∑ The curve of duty cycle changing with cell voltage when the resistance is 0.2Ω and the cell voltage difference is 0.01V, 0.03V, and 0.05V respectively. As the cell voltage increases, the duty cycle gradually increases, but the change in the duty cycle is very small; when the cell voltage difference increases, the duty cycle tends to decrease.

[0068] Figure 7 The figure shows that there is a voltage difference between battery cells. Due to the error in the digital chip implementation process, the actual duty cycle D_ real and Figure 6 When there is a difference between the theoretical calculated value shown (D_ real =(1±0.01)D), the absolute value of the minimum value of the actual output inductor current of the balancer is x_ real ​The operating curve shows that when the duty cycle operating error is 1%, the actual minimum inductor current differs from the expected theoretical value by about 20%. Since the minimum inductor current is only responsible for achieving soft switching of the switching device and has no adverse effect on the operation of the equalizer, this error is acceptable.

[0069] Figure 5 The calculation and implementation of the key duty cycle of the control strategy shown are often implemented using digital control chips. Currently, commonly used chips include ARM's STM series microcontrollers and TI's 2000 series DSPs. Taking DSP-F28335 as an example, the maximum main frequency is 150MHz. Taking the switching frequency of 20kHz as an example, the resolution of the duty cycle is 1 / 7500. Taking into account the dead zone and delay, the error of the actual duty cycle should be within 1%, that is, the impact of the actual value error of the duty cycle on the equalizer is acceptable.

[0070] In summary, the current continuous soft switching implementation method based on the switched inductor battery balancer disclosed in the present invention realizes the soft switching of all switching tubes in the balancer, and the method does not use current feedback to participate in the control, which saves the cost of the current sensor, reduces the cost of the balancer, and effectively improves the efficiency of the balancer, increasing the market competitiveness of the balancer.

[0071] The above embodiments are intended to illustrate the present invention, not to limit the present invention. Any solution that is a simple transformation of the present invention falls within the protection scope of the present invention.

Claims

1. A method for implementing continuous current soft switching based on a switched inductor battery balancer. The method is implemented based on a switched inductor lithium-ion battery balancer. The switched inductor lithium-ion battery balancer includes two battery cells to be balanced: a first battery cell B1 and a second battery cell B2, two switching transistors: a first switching transistor S1 and a second switching transistor S2, and an inductor L0. The voltages of the first battery cell B1 and the second battery cell B2 are U B1 、U B2 The current flowing into the first battery cell B1 and the second battery cell B2 are i B1 、i B2 , the inductor current is i L The driving signals of the first switch tube S1 and the second switch tube S2 are complementary, and the conduction time of the first switch tube S1 and the second switch tube S2 are DT s and (1-D)T s , where D is the modulation ratio of the first switch S1, T s is a switching period, and the inductance value of the inductor L0 is L; it is characterized in that, The method comprises the following steps: S1. Real-time detection of the switch inductor lithium-ion battery equalizer to obtain the open circuit voltage U of the first battery cell B1 BO1 And the open circuit voltage U of the second battery cell B2 BO2 ; S2. Determine the open circuit voltage U BO1 、U BO2 with U T The relationship between U T is the start-up voltage of the lithium-ion battery balancer based on the switching inductor, if |U BO1 -U BO2 |≤U T , then balancing is not required, and both the first switch tube S1 and the second switch tube S2 are turned off; otherwise, the process goes to step S3; S3. Determine the voltage U again BO1 、U BO2 with U T If U BO1 -U BO2 >U T , then go to step S4; if U BO2 -U BO1 >U T , then proceed to step S5; S4. Set the modulation ratio of the first switch S1 to D=D1, and the average value of the inductor current I L >0, in order to achieve soft switching, the minimum inductor current is set to -x, then I L =0.5(Δi L -2x), Δi L is the change in inductor current during a switching cycle; the modulation ratio D1 is obtained using the following formula, and then the process goes to step S6. Where A = R Σ T s (U BO1 +U BO2 ), B=(U BO1 +U BO2 )(2L-R Σ T s ), C=2L(xR Σ -U BO2 ), R ∑ is the equalizer equivalent resistance and Ts is the switching period; S5. Set the modulation ratio of the first switch S1 to D=D2, and the average value of the inductor current I L <0, to achieve soft switching, set the maximum value of the inductor current to x, then I L =0.5(2x-Δi L ), Δi L is the change in inductor current during a switching cycle; the modulation ratio D2 is obtained using the following formula, and then step S6 is entered. Where a = R Σ T s (U BO1 +U BO2 ), b=-(U BO1 +U BO2 )(2L+R Σ T s ), c=2L(xR Σ +U BO2 ), R ∑ is the equalizer equivalent resistance and Ts is the switching period; S6. Modulate the signal according to the modulation ratio to obtain a PWM signal for controlling the first switch tube S1 and the second switch tube S2.

2. The method for implementing continuous current soft switching of a switched inductor battery equalizer according to claim 1, wherein: Get the average inductor current I L The method is as follows: based on the switching inductor lithium-ion battery balancer, the open circuit voltages of the first battery cell B1 and the second battery cell B2 are detected in real time during operation. BO1 、U BO2 , according to the average value of the inductor voltage in one switching cycle is equal to 0, the average value of the inductor current I is obtained L for 3. A method for implementing current continuous soft switching based on a switched inductor battery equalizer according to claim 1 or 2, characterized in that: Get the equalizer equivalent resistance and R ∑ The method is: detect the internal resistance R of the battery cell B , the on-resistance of the switch tube R S , the equivalent resistance R of the inductor L0 L , find R B 、R S 、R L The sum of the three R ∑ , that is, R Σ =R L +R S +R B .

4. A method for implementing continuous current soft switching based on a switched inductor battery equalizer according to claim 1 or 2, characterized in that: The change in inductor current during one switching cycle is 5. The method for implementing current continuous soft switching based on a switched inductor battery equalizer according to claim 4, characterized in that: When the average inductor current I L Satisfy I L <0.5Δi L , soft switching is achieved.

6. The method for implementing current continuous soft switching based on a switched inductor battery equalizer according to claim 1, wherein: Each switching cycle contains two switching modes.

7. The method for implementing continuous current soft switching of a switched inductor battery equalizer according to claim 1, wherein: In step S4 , the modulation ratio D of the first switch tube S1 is set to D1 , and the energy of the first battery cell B1 flows to the second battery cell B2 .

8. The method for implementing current continuous soft switching based on a switched inductor battery equalizer according to claim 1, wherein: In step S5 , the modulation ratio D of the first switch tube S1 is set to D2 , and at this time, the energy of the second battery cell B2 flows to the first battery cell B1 .

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