Digital implementation method of phase shift control based on active half-bridge lithium battery equalizer

Abstract

The present invention discloses a digital implementation method for phase shift control based on an active half-bridge lithium battery equalizer, including a lithium battery cell voltage detection and calculation function, a voltage discrimination and phase shift ratio calculation function, and a drive signal determination function. Existing active half-bridge lithium battery equalizers adopt an open-loop control strategy, which has a low switching frequency, slow balancing speed, and large volume and weight. To overcome these shortcomings, a phase shift control strategy for the output voltage of each active half-bridge is proposed based on the voltage conditions of the lithium battery cells in the active half-bridge. The phase shift ratio of the active half-bridge output voltage is determined based on the difference between the voltage of the active half-bridge lithium battery cell and the average voltage of all lithium battery cells. The active half-bridge lithium battery equalizer that introduces phase shift control can operate at a higher switching frequency, reducing the volume and weight of the equalizer and accelerating the balancing speed.

Description

Technical Field

[0001] The invention relates to a digital implementation method of phase shift control based on an active half-bridge lithium battery equalizer, and belongs to the field of power electronic converter control. Background Art

[0002] In recent years, lithium batteries have been widely used in aviation, aerospace, smart microgrid energy storage systems, and electric vehicle power lithium batteries. To achieve sufficiently high voltage and capacity, lithium battery cells need to be connected in series to form a lithium battery pack before use. Due to differences in the characteristics of lithium battery cells, after multiple charge and discharge cycles, the series-connected lithium battery cells experience inconsistent states of charge (SOC) and voltages. Long-term operation can lead to overcharge and over-discharge, which can adversely affect the lifespan, capacity, and safety of the lithium battery cells. SOC differences are often reflected in differences in cell voltage, making voltage balancing a must-have feature in series-connected lithium battery packs.

[0003] Active half-bridge lithium-ion battery balancers have the simplest topology of all current balancers. Each lithium-ion battery cell corresponds to only one active switch, and the entire balancer requires only one multi-winding transformer to achieve direct energy transfer channels for all lithium-ion battery cells, making the balancer relatively efficient and low-cost. However, these balancers currently use open-loop control with a fixed duty cycle. The energy transfer speed between the transformer windings is slow, resulting in extremely slow balancing speeds. Furthermore, the operating frequency is acceptable only at very low levels, resulting in a relatively large transformer size and weight, which directly leads to the low market competitiveness of these balancers.

[0004] Therefore, based on the active half-bridge lithium battery balancer, an appropriate control strategy is adopted to reduce the weight and volume of the balancer at a higher operating frequency, and a faster balancing speed between lithium batteries can also be achieved. This is how this solution was developed. Summary of the Invention

[0005] Purpose of the invention: In view of the phenomenon that the active half-bridge lithium battery balancer can only achieve a faster balancing speed at a lower switching frequency during open-loop control, which in turn causes the disadvantages of large size and weight of the balancer, the present invention proposes a digital implementation method of phase shift control based on the active half-bridge lithium battery balancer, which not only improves the operating frequency of the balancer, reduces the size and weight of the balancer, but also ensures the balancing speed of the balancer.

[0006] Technical solution: A digital implementation method for phase shift control based on an active half-bridge lithium battery equalizer. The implementation of this method is based on an active half-bridge lithium battery equalizer. The active half-bridge lithium battery equalizer includes n active half-bridge converters and a transformer with n windings. The i-th active half-bridge converter contains two lithium battery cells B to be balanced.i1 With B i2 , 2 switch tubes S i1 With S i2 ; The lithium battery unit B i1 With B i2 The voltage is U Bi1 with U Bi2 ; i in the above parameters is any value of 1, 2, ... n; in the i-th active half-bridge converter, the battery unit B i1 The negative electrode and the switch tube S i1 The source of the switch S is connected and serves as the negative electrode of the i-th active half-bridge converter; i1 The drain and switch tube S i2 The source of the battery cell B is connected and serves as the AC output terminal of the i-th active half-bridge converter; i1 The positive electrode of battery cell B i2 The negative electrode is connected and serves as the other end of the AC output of the i-th active half-bridge converter; battery unit B i2 The positive electrode and the switch tube S i2 The drain of the i-th active half-bridge converter is connected and serves as the positive electrode of the i-th active half-bridge converter; one end of the AC output of the i-th active half-bridge converter is connected to the same-name end of the i-th transformer winding; the other end of the AC output of the i-th active half-bridge converter is connected to the opposite-name end of the i-th transformer winding; the positive electrode of the i-th active half-bridge converter is connected to the negative electrode of the (i+1)-th active half-bridge converter; the method comprises the following steps:

[0007] S1. Let i = 1;

[0008] S2. Detection U Bi1 with U Bi2 ; Find the voltage and U of the lithium battery cell containing two equalizers in the i-th active half-bridge converter Bi =U Bi1 +U Bi2 ; Cumulatively calculate the voltage and U of the lithium battery cells in all active half-bridge converters Bs =U Bs +U Bi , the newly obtained U Bs Will overwrite the previous U Bs value; i = i + 1, the newly obtained i will overwrite the previous i value; determine whether i is equal to n + 1, if not, re-execute step S2, otherwise go to step S3;

[0009] S3. Calculate the average value U of the voltage sum of all lithium battery cells in the active half-bridge converter Ba =U Bs / n; execute i=1; execute step S4;

[0010] S4. Judgment | U Bi -UBa |With U Bth The relationship, where U Bth is the threshold voltage for whether the active half-bridge converter works. If |U Bi -U Ba |>U Bth , then execute step S5, otherwise execute step S6;

[0011] S5. Calculate the phase shift of the output voltage of the i-th active half-bridge converter Φ i =k Φ (U Bi -U Ba ), where k Φ is the shift ratio coefficient; jump to step S7;

[0012] S6. Directly set the output voltage shift of the i-th active half-bridge converter to Φ i =0; jump to step S7;

[0013] S7. Execute i=i+1, the newly obtained i will overwrite the previous i value; determine whether i is equal to n+1. If not, re-execute step S4, otherwise proceed to step S8;

[0014] S8. Determine the drive signal of the corresponding switch tube according to the shift ratio of all the active half-bridge converters obtained; i =0, the corresponding active half-bridge converter switch is turned off.

[0015] Based on the digital implementation method of phase shift control of the active half-bridge lithium battery equalizer, when the lithium battery cell voltage in the i-th active half-bridge converter corresponding to the phase shift obtained in step S5 has deviated from the average value by more than the specified threshold voltage, the lithium battery cell needs to be balanced; in step S6, when the lithium battery cell voltage deviates from the average value by less than the specified threshold voltage, the lithium battery cell does not need to be balanced, and the switch tube in the corresponding i-th active half-bridge converter is turned off.

[0016] Beneficial effects: After the implementation of the digital implementation method of phase shift control based on the active half-bridge lithium battery equalizer disclosed in the present invention, the energy transmission speed between the lithium battery cells contained in each active half-bridge becomes controllable, and a faster balancing speed can still be achieved at a higher switching frequency, which not only realizes the miniaturization and lightweight of the lithium battery equalizer, but also ensures the balancing speed under any lithium battery conditions; the above beneficial effects can greatly enhance the market competitiveness of the lithium battery equalizer proposed by the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is the main circuit topology based on active half-bridge lithium battery balancer;

[0018] Figure 2It is the main circuit topology based on active half-bridge 4-lithium battery cell equalizer;

[0019] Figure 3 This is the main working waveform of the active half-bridge 4-lithium battery cell equalizer in open-loop control;

[0020] Figure 4 The active half-bridge 4-lithium battery cell balancer operates in mode 1 under open-loop control;

[0021] Figure 5 The active half-bridge 4-lithium battery cell balancer operates in mode 2 under open-loop control;

[0022] Figure 6 The equivalent circuit of the active half-bridge 4-lithium battery cell balancer is shown in Figure 2.

[0023] Figure 7 This is a working waveform diagram of the active half-bridge 4-lithium battery cell equalizer disclosed in the present invention when using a phase-shift control strategy;

[0024] Figure 8 This is a control block diagram of the active half-bridge lithium battery cell equalizer disclosed in the present invention when using a phase-shift control strategy;

[0025] Figure 9 This is a flow chart of the digital implementation method for phase shift control based on an active half-bridge lithium battery equalizer disclosed in the present invention;

[0026] Figure 10 The waveforms of u1, u2 and i1 when the active half-bridge lithium battery equalizer phase shift control digital implementation method disclosed in the present invention is controlled when Φ=0;

[0027] Figure 11 The waveforms of u1, u2 and i1 when the active half-bridge lithium battery equalizer phase shift control digital implementation method disclosed in the present invention is controlled when Φ=0.05;

[0028] Figure 12 The waveforms of u1, u2 and i1 when the digital implementation method for phase shift control of an active half-bridge lithium battery equalizer disclosed in the present invention is controlled when Φ=0.1;

[0029] Figure 13 The waveforms of u1, u2 and i1 when the active half-bridge lithium battery equalizer phase shift control digital implementation method disclosed in the present invention is controlled when Φ=0.15;

[0030] Figure 14 The waveforms of u1, u2 and i1 when the active half-bridge lithium battery equalizer phase shift control digital implementation method disclosed in the present invention is controlled when Φ=0.2;

[0031] Symbol name in the figure: Si1 -S i2 (i=1,2,…n)——the first and second switching tubes in the i-th active half-bridge converter; B i1 -B i2 (i=1, 2, ... n) - the first lithium battery cell and the second lithium battery cell in the i-th active half-bridge converter; T - high-frequency transformer; W i (i=1,2,…n)——the i-th winding in the high-frequency transformer T; u i (i=1,2,…n)——output voltage of the i-th active half-bridge converter; i i (i=1,2,…n)——output current of the i-th active half-bridge converter; i m ——High-frequency transformer excitation current; L1, L2——Transformer primary and secondary coil leakage inductance; R1, R2——Transformer primary and secondary coil equivalent resistance; L m —Transformer excitation inductance; u m —Transformer excitation inductance voltage; u ac1 、u ac2 ——AC component of the output voltage of half-bridge 1 and half-bridge 2; i ac1 ——AC component of transformer winding 1 output current. DETAILED DESCRIPTION

[0032] The present invention is further illustrated below with reference to specific examples. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, modifications of various equivalent forms of the present invention made by those skilled in the art all fall within the scope defined by the claims attached to this application.

[0033] The main circuit topology of the active half-bridge lithium battery equalizer disclosed in the present invention is as follows: Figure 1 As shown in the figure, it contains n active half-bridge converters, 2n lithium battery cells that need to be balanced, and a transformer with n windings. The energy transfer between lithium battery cells includes the energy transfer between lithium battery cells within the half-bridge and the energy transfer between lithium battery cells between half-bridges. For the sake of convenience, the following uses the equalizer with 4 battery cells and 2 active half-bridge converters as an example to illustrate its working principle. The corresponding equalizer topology is as follows: Figure 2 shown.

[0034] Figure 2 The 4-battery cell balancer shown in the figure is in open-loop control, that is, the two switches in each half-bridge converter are operated complementary with a duty cycle of 0.5, and the odd-numbered switches (S 11 / S 21 ), even-numbered switching tubes (S 12 / S 22) are switched synchronously. Since the voltage of the battery cell is directly related to the current waveform, only the battery cell voltage U is given here. B11 >U B12 >U B21 >U B22 The corresponding waveform and modal diagram are as follows: Figure 3-Figure 5 shown.

[0035] The modal diagram shows the transformer primary and secondary coil leakage inductance L1, L2 and equivalent resistance R1, R2. Generally, L1 = L2 = L, R1 = R2 = R. A switching cycle is divided into two working modes, namely mode 1 (corresponding to time t0-t3, the modal diagram is shown as follows Figure 4 As shown), mode 2 (corresponding to time t3-t6, the modal diagram is as shown Figure 5 (as shown). Within the same mode (half a switching cycle), the polarity of the half-bridge converter's output currents i1 and i2 changes, effectively charging and discharging the battery cells. The voltage differences between the battery cells create a DC component in the half-bridge output voltage, which in turn creates a DC component in the transformer's primary and secondary currents i1 and i2. This, in turn, causes a DC component in the transformer's magnetizing current. Therefore, transformer design must consider the impact of bias magnetization on core saturation.

[0036] according to Figure 3-Figure 5 Analysis of the waveforms and modal diagrams of the equalizer during open-loop control shows that the energy transfer speed of the lithium battery cells within the half-bridge is affected by the equivalent resistance and the cell voltage difference; while the energy transfer of the lithium battery cells between half-bridges is affected by the equivalent resistance, the leakage inductance of the transformer winding, and the voltage difference between the cells. When the switching frequency is very high, the inductive reactance becomes larger, resulting in a very slow balancing speed. The voltage difference between the lithium battery cells is originally very small, so a very low switching frequency is required to achieve faster energy transfer of the lithium battery cells between the half-bridges, resulting in a large size and weight of the equalizer.

[0037] Figure 6 The equivalent circuit of the equalizer, L m is the transformer excitation inductance, u m is the excitation inductor voltage. Since different coils of the transformer are connected to battery cells with nearly equal voltages. In order to make the half-bridge output voltage u i There is a certain voltage difference between the two, which can be i A phase difference is set between them, and the size of the phase difference is determined according to the voltage difference. Figure 7 The main waveforms of the half-bridge 4-lithium battery cell balancer under phase shift control are given. ac1 、u ac2 、i ac1They are the AC components corresponding to u1, u2, and i1 respectively. The DC components in u1 and u2 only affect the energy transfer of the lithium battery unit within the half-bridge, and the AC components in u1 and u2 only affect the energy transfer of the lithium battery unit between the half-bridges. Therefore Figure 7 The AC component analyzed in this paper means analyzing the energy transfer capability of lithium battery cells between the half-bridges of the balancer.

[0038] In phase-shift control, the switches in the two half-bridge converters still operate complementary with a duty cycle of 0.5, and the two half-bridge output voltages u ac1 、u ac2 The phase shift is defined as Ф

[0039]

[0040] According to the response of the first-order circuit and t a0 , t a2 The current at the moment i ac1 The characteristics of opposite polarity and equal magnitude give formula (16)

[0041]

[0042] Where, τ=(L / R), 0.5(U B11 +U B12 )=A1,0.5(U B21 +U B22 )=A2,

[0043]

[0044] i ac1 (t a2 )=-i ac1 (t a0 ), i ac1 (t a3 )=-i ac1 (t a1 ).

[0045] According to the above analysis, the current and power between the half bridges are greatly increased after phase shift control, overcoming the disadvantages of open-loop control. Therefore, the switching frequency of the equalizer can be increased and the volume and weight of the transformer can be reduced. The corresponding control block diagram is as follows: Figure 8 The half-bridge converter obtains the phase shift angle Φ through the proportional regulator based on the voltage sum of the two battery cells in the half-bridge and the average value of the voltage sum of all half-bridge battery cells. i ,

[0046] Φ i =k Φ (U Bi -U Bs ) (3)

[0047] Among them, U Bi =U Bi1 +U Bi2 , Since the battery cell voltage changes very slowly, the use of a P regulator will not affect the performance of the balancer. Figure 8 The equalizer control block diagram shown in the figure is used to obtain the phase shift digital control method based on the half-bridge lithium battery equalizer disclosed in the patent of this invention, such as Figure 9 shown.

[0048] A digital implementation method for phase shift control based on an active half-bridge lithium battery equalizer. The implementation of the method is based on an active half-bridge lithium battery equalizer. The active half-bridge lithium battery equalizer includes n active half-bridge converters and a transformer with n windings. The i-th (i=1, 2, ... n) active half-bridge converter contains two lithium battery cells B to be balanced. i1 With B i2 , 2 switch tubes S i1 With S i2 ; The lithium battery unit B i1 With B i2 The voltage is U Bi1 with U Bi2 ; The method comprises the following steps:

[0049] S1. Let i = 1;

[0050] S2. Detection U Bi1 with U Bi2 ; Find the voltage and U of the lithium battery cell containing two equalizers in the i-th active half-bridge converter Bi =U Bi1 +U Bi2 ; Cumulatively calculate the voltage and U of the lithium battery cells in all active half-bridge converters Bs =U Bs +U Bi , the newly obtained U Bs Will overwrite the previous U Bs value; i = i + 1, the newly obtained i will overwrite the previous i value; determine whether i is equal to n + 1, if not, re-execute step S2, otherwise go to step S3;

[0051] S3. Calculate the average value U of the voltage sum of all lithium battery cells in the active half-bridge converter Ba =U Bs / n; execute i=1; execute step S4;

[0052] S4. Judgment | U Bi -U Ba |With U Bth The relationship, where U Bthis the threshold voltage for whether the active half-bridge converter works. If |U Bi -U Ba |>U Bth , then execute step S5, otherwise execute step S6;

[0053] S5. Calculate the phase shift of the output voltage of the i-th active half-bridge converter Φ i =k Φ (U Bi -U Ba ), where k Φ is the shift ratio coefficient; jump to step S7;

[0054] S6. Directly set the output voltage shift of the i-th active half-bridge converter to Φ i =0; jump to step S7;

[0055] S7. Execute i=i+1, the newly obtained i will overwrite the previous i value; determine whether i is equal to n+1. If not, re-execute step S4, otherwise proceed to step S8;

[0056] S8. Determine the drive signal of the corresponding switch tube according to the shift ratio of all the active half-bridge converters obtained; i =0, the corresponding active half-bridge converter switch is turned off.

[0057] Based on the digital implementation method of phase shift control of the active half-bridge lithium battery equalizer, when the lithium battery cell voltage in the i-th active half-bridge converter corresponding to the phase shift obtained in step S5 has deviated from the average value by more than the specified threshold voltage, the lithium battery cell needs to be balanced; in step S6, when the lithium battery cell voltage deviates from the average value by less than the specified threshold voltage, the lithium battery cell does not need to be balanced, and the switch tube in the corresponding i-th active half-bridge converter is turned off.

[0058] Figure 10-14 Given U B11 =U B12 =3.85V, U B21 =U B22 When =3.78V, the waveforms of the primary and secondary voltages u1, u2 and the primary and secondary currents i1 of the transformer under different shift ratios Ф. Figure 10 、 Figure 11 、 Figure 12 、 Figure 13 、 Figure 14 The waveforms of u1, u2 and i1 are respectively when Ф=0, Ф=0.05, Ф=0.1, Ф=0.15 and Ф=0.2. Figure 10It can be seen that when Φ = 0, the inter-group voltage difference is small and the balancer switching frequency is high, the inter-group balancing current is extremely small, and within a switching cycle, the energy absorbed and released by the battery cells between the groups is almost equal, which means that energy is not transferred between the groups, resulting in energy imbalance between the battery cells between the groups. However, by simply increasing the phase shift angle between u1 and u2, even when the inter-group voltage difference is small and the balancer switching frequency is high, there will be a significant difference in the energy absorbed and released by the battery cells between the groups within a switching cycle. This difference increases with the increase in the phase shift angle. The larger the difference, the greater the energy transferred between the battery cells between the groups, and the faster the balancing speed of the battery cells between the groups.

[0059] In summary, the digital implementation method of phase shift control based on the active half-bridge lithium battery equalizer disclosed in the present invention greatly expands the energy transfer capability of the battery cells between the half-bridges in the active half-bridge lithium battery equalizer, thereby greatly improving the switching frequency of the switch tube in the equalizer, reducing the volume and weight of the equalizer, ensuring that the balancing speed between the battery cells is adjustable, and increasing the market competitiveness of the active half-bridge lithium battery equalizer.

Claims

1. A digital implementation method for phase shift control based on an active half-bridge lithium battery equalizer. The implementation of the method is based on an active half-bridge lithium battery equalizer, wherein the active half-bridge lithium battery equalizer includes n active half-bridge converters and a transformer with n windings; the i-th active half-bridge converter contains two lithium battery cells B to be balanced. i1 With B i2 , 2 switch tubes S i1 With S i2 ; The lithium battery unit B i1 With B i2 The voltage is U Bi1 with U Bi2 ; i in the above parameters is any value of 1, 2, ... n; in the i-th active half-bridge converter, the battery unit B i1 The negative electrode and the switch tube S i1 The source of the switch S is connected and serves as the negative electrode of the i-th active half-bridge converter; i1 The drain and switch tube S i2 The source of the battery cell B is connected and serves as the AC output terminal of the i-th active half-bridge converter; i1 The positive electrode of battery cell B i2 The negative electrode is connected and serves as the other end of the AC output of the i-th active half-bridge converter; battery unit B i2 The positive electrode and the switch tube S i2 The drain of the i-th active half-bridge converter is connected and serves as the positive electrode of the i-th active half-bridge converter; one end of the AC output of the i-th active half-bridge converter is connected to the same-name end of the i-th transformer winding; the other end of the AC output of the i-th active half-bridge converter is connected to the opposite-name end of the i-th transformer winding; the positive electrode of the i-th active half-bridge converter is connected to the negative electrode of the i+1-th active half-bridge converter; characterized in that, The method comprises the following steps: S1. Let i = 1; S2. Detection U Bi1 with U Bi2 ; Calculate the sum of the voltages U of the two balanced lithium battery cells in the i-th active half-bridge converter Bi =U Bi1 +U Bi2 ;Grand total Calculate the sum of the voltages of all lithium battery cells in the active half-bridge converter, U Bs =U Bs +U Bi , the newly obtained U Bs Will overwrite the previous U Bs value; i = i + 1, the newly obtained i will overwrite the previous i value; determine whether i is equal to n + 1, if not, re-execute step S2, otherwise go to step S3; S3. Calculate the average value U of the sum of the voltages of the lithium battery cells in all active half-bridge converters Ba =U Bs / n; execute i=1; execute step S4; S4. Judgment | U Bi -U Ba |With U Bth The relationship, where U Bth is the threshold voltage for whether the active half-bridge converter works. If |U Bi -U Ba |>U Bth , then execute step S5, otherwise execute step S6; S5. Calculate the phase shift of the output voltage of the i-th active half-bridge converter Φ i =k Φ (U Bi -U Ba ), where k Φ It is the shift ratio coefficient, which is set to a reasonable constant in actual control; Jump to step S7; S6. Directly set the output voltage shift of the i-th active half-bridge converter to Φ i =0; Jump to step S7; S7. Execute i=i+1, the newly obtained i will overwrite the previous i value; determine whether i is equal to n+1. If not, re-execute step S4, otherwise proceed to step S8; S8. Determine the drive signal of the corresponding switch tube according to the shift ratio of all active half-bridge converters obtained; i =0, the switch tube in the corresponding active half-bridge converter is turned off.

2. The digital implementation method for phase shift control based on an active half-bridge lithium battery equalizer according to claim 1, characterized in that: In the i-th active half-bridge converter corresponding to the shift ratio obtained in step S5, when the lithium battery cell voltage has deviated from the average value by more than the specified threshold voltage, the lithium battery cell needs to be balanced; in step S6, when the lithium battery cell voltage deviates from the average value by less than the specified threshold voltage, the lithium battery cell does not need to be balanced, and the switch tube in the corresponding i-th active half-bridge converter is turned off.

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