Bidirectional flyback lithium battery equalizer with diodes clamped in staggered manner

Abstract

The invention discloses a diode staggered clamping bidirectional flyback lithium battery equalizer, and belongs to the technical field of power electronics. The lithium battery equalizer comprises two lithium batteries connected in series, two switch tubes, two diodes and a high-frequency transformer. Balancing of two non-isolated lithium battery monomers is realized by adopting the working principle of the bidirectional flyback converter, and the lithium battery pack has the characteristics of simple structure, convenience in control and low cost. According to the invention, only two diodes are adopted to realize leakage inductance energy recovery, so that the efficiency of the converter is improved, the voltage spike of the switching tube is inhibited, and the voltage stress of the switching tube is reduced. The method is suitable for an occasion where two lithium battery monomers need active equalization, and the equalizer can be expanded to be suitable for active equalization of multiple battery monomers.

Description

Technical Field

[0001] The invention relates to a bidirectional flyback lithium battery equalizer with diode interleaved clamping, belonging to the technical field of battery management. Background Art

[0002] Active lithium battery equalizers not only ensure the safe operation of lithium battery systems, but also maximize the energy utilization of battery systems. The flyback converter has a simple structure and flexible control, so the flyback-based lithium battery equalizer has been widely studied. However, one of the mainstream solutions for flyback balancing is that each battery cell is equipped with a bidirectional flyback converter, one port is connected to the battery cell, and the other port is connected in parallel. The energy transferred by the cell is collected and transferred at the parallel port. Although it is easy to control, it uses many devices, has a long transmission path, and is relatively inefficient. Another mainstream solution is to use the energy transfer method from the cell to the battery pack, using a transformer coil to transfer the transformer magnetization energy to the battery pack. This method simplifies the circuit structure, but the magnetization energy cannot distinguish between strong cells and weak cells in the process of transferring to the battery pack, resulting in low balancing efficiency.

[0003] To improve the balancing speed, the working principle of some balancing topologies integrates the working characteristics of forward and flyback converters, further reducing the complexity of the balancer topology, but the magnetic reset of the transformer still requires additional circuits. To this end, the existing solution is to group the battery cells and use complementary drive signals to control the corresponding diodes respectively. The transformer can undergo magnetization and demagnetization in one switching cycle, ensuring the normal operation of the transformer.

[0004] Due to the leakage inductance of the flyback transformer, there is a voltage spike in the switch tube during the shutdown process. Although active or passive methods can be used to alleviate or eliminate the spike voltage to achieve a switch tube with lower voltage and smaller on-resistance, the number of cells in the lithium battery balancer is large, and the added active or passive voltage clamping elements greatly increase the complexity of the circuit and the complexity of control, resulting in increased cost of the balancer and poor reliability. Summary of the invention

[0005] Purpose of the invention: In view of the problems and shortcomings in the prior art, the present invention provides a bidirectional flyback lithium battery balancer with interleaved diode clamping. Starting from the bidirectional flyback converter, a simple topology of a bidirectional flyback converter that can be used for a lithium battery balancer is established, and by adding a diode in each flyback converter, it is paired with another flyback corresponding battery cell to achieve the clamping of the switch tube voltage spike. The experimental results verify that this topology can achieve efficient balancing and effectively suppress the voltage spike of the switch tube.

[0006] Technical solution: A bidirectional flyback lithium battery equalizer with diode interleaved clamping, including a non-isolated bidirectional flyback converter, which is composed of a first battery cell B1, a second battery cell B2, a first switch tube S1, a second switch tube S2, and a high-frequency transformer T; the high-frequency transformer T includes a first winding W1 and a second winding W2;

[0007] The first switch tube S1 and the second switch tube S2 form a half-bridge structure with the first diode D1 and the second diode D2 respectively; the positive electrode of the first battery cell B1, the cathode of the second diode D2, and the drain of the first switch tube S1 are connected together; the negative electrode of the second battery cell B2, the anode of the first diode D1, and the source of the second switch tube S2 are connected together; the negative electrode of the first battery cell B1, the positive electrode of the second battery cell B2, the opposite end of the first winding W1, and the opposite end of the second winding W2 are connected together; the cathode of the first diode D1, the source of the first switch tube S1 and the same end of the first winding W1 are connected together; the anode of the second diode D2, the drain of the second switch tube S2 and the same end of the second winding W2 are connected together.

[0008] In a non-isolated bidirectional flyback converter, switch tubes S1 and S2 are complementary to control and adjust the duty cycle.

[0009] The present invention changes the traditional isolated bidirectional flyback converter into a non-isolated bidirectional flyback converter and applies it to the lithium battery balancing function, which has the characteristics of high efficiency, simple structure and low cost. The diode interleaved clamping structure can realize the simplest switch tube peak voltage suppression and realize the recovery of transformer leakage inductance energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a circuit diagram of an interleaved clamped flyback equalizer according to an embodiment of the present invention;

[0011] Figure 2 1 are working waveforms and mode diagrams of the interleaved clamped flyback equalizer according to an embodiment of the present invention; (a) working waveform, (b) mode 1, (c) mode 2, (d) mode 3, (e) mode 4;

[0012] Figure 3 is an equivalent circuit diagram of an interleaved clamped flyback equalizer according to an embodiment of the present invention;

[0013] Figure 4 1. The present invention relates to a monolithic balanced current variation curve diagram of an interleaved clamped flyback equalizer according to an embodiment of the present invention; (a) a monolithic current variation curve with line impedance, (b) a monolithic current variation curve with monolithic voltage difference, and (c) a monolithic current variation curve with switching cycle;

[0014] Figure 5It is a topology diagram of a multi-cell interleaved clamped flyback equalizer according to an embodiment of the present invention;

[0015] Figure 6 The waveforms of the equalizer under different conditions are shown in Figure 1. (a) i W1 、i W2 、i m 、i B1 、i B2 Waveform, (b) u1, u2, i in case 1 m 、i B1 、i B2 Waveform, (c) i in case 2 W1 、i W2 、i m 、i B1 、i B2 Waveform, (d) u1, u2, i in case 2 B1 、i B2 Waveform, (e) i in case 3 W1 、i W2 、i m 、i B1 、i B2 Waveform, (f) u1, u2, i in case 3 B1 、i B2 Waveform. DETAILED DESCRIPTION

[0016] The present invention is further explained below in conjunction with specific embodiments. It should be understood that these embodiments 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, various equivalent forms of modifications to the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application.

[0017] like Figure 1As shown, a bidirectional flyback lithium battery balancer with diode interleaved clamping includes a non-isolated bidirectional flyback converter, which is composed of a first battery cell B1, a second battery cell B2, a first switch tube S1, a second switch tube S2, and a high-frequency transformer T; the high-frequency transformer T includes a first winding W1 and a second winding W2; the first switch tube S1 and the second switch tube S2 respectively form a half-bridge structure with the first diode D1 and the second diode D2; the positive electrode of the first battery cell B1, the cathode of the second diode D2, and the first switch tube S1 are connected to the first battery cell B1, and the cathode of the second diode D2 is connected to the first switch tube S2. The drain of S1 is connected together; the negative electrode of the second battery cell B2, the anode of the first diode D1, and the source of the second switch tube S2 are connected together; the negative electrode of the first battery cell B1, the positive electrode of the second battery cell B2, the opposite end of the first winding W1, and the opposite end of the second winding W2 are connected together; the cathode of the first diode D1, the source of the first switch tube S1 and the same end of the first winding W1 are connected together; the anode of the second diode D2, the drain of the second switch tube S2 and the same end of the second winding W2 are connected together.

[0018] In a non-isolated bidirectional flyback converter, switch tubes S1 and S2 are complementary to control and adjust the duty cycle.

[0019] According to the modulation method of complementary conduction of the switch tubes of the bidirectional flyback converter, Figure 1 The working waveform of the interleaved clamped flyback equalizer is shown in Figure 2 As shown in (a), one switching cycle contains four switching modes. Assuming that the energy of battery cell B1 is transferred to cell B2, the corresponding four modes are as follows: Figure 2 As shown in (b)-(e).

[0020] according to Figure 1 The current relationship in the transformer coil, and considering that the number of turns of the transformer coil is equal, the transformer excitation current i m Equal to the center tap current; Figure 1 The leakage inductance of the transformer is taken into account, so the transformer excitation voltage u m is the coil voltage in the figure, then

[0021]

[0022] By utilizing the voltage difference between battery cells and making each switch on for half of the switching cycle, battery cell balancing can be achieved.

[0023] Mode 1 [t0-t1], Figure 2 (b)

[0024] After t0, the switch tube S2 is turned off, S1 is in the on state, the diodes D1 and D2 are cut off, the monomer B1 transmits energy to the transformer, and the transformer excitation current increases.

[0025]

[0026] Where, L m With L r They are the magnetizing inductance and coil leakage inductance of the transformer respectively.

[0027] Mode 2 [t1-t2], Figure 2 (c)

[0028] At t1, S1 is turned off, S2 is turned on, D1 is turned on, and the terminal voltage of S1 is clamped at (U B1 +U B2 ), after time t1,

[0029]

[0030] After time t1, the difference between voltages u1 and u2 is only the conduction voltage drop U of the diode. DF , this voltage acts on the transformer leakage inductance, causing the current i W1 gradually decreases, and i W2 Increase gradually.

[0031] Mode 3 [t2-t3], Figure 2 (d)

[0032] At t2, the current i W1 =0,i W2 =i m After t2, D1 is turned off, and the transformer excitation energy flows to monomer B2 through S2. The excitation current i m Linear decrease.

[0033]

[0034] Mode 4 [t3-t4], Figure 2 (e)

[0035] At t3, the switch tube S2 is turned off and S1 is turned on. r2 The current in the S2 body diode turns on, so

[0036]

[0037] Because u W1 =u W2 , the voltage difference between voltage u1 and voltage u2 acts completely on the two leakage inductances, so i W2 Rapidly drops to zero, i W1 Rapidly increases to the excitation current i m equal, so the period is short.

[0038] according to Figure 2(a) shows the waveform, the monomer B1 outputs energy, B2 absorbs energy; vice versa, and finally the voltage between the monomers is balanced. In addition, due to the use of diode interleaving clamping, the switch tube voltage is clamped at (U B1 +U B2 ).

[0039] For two diodes, reasonable design of the leakage inductance and the parallel capacitance of the switch tube can effectively achieve this.

[0040] Analyzing the energy transfer of the equalizer requires establishing an equivalent model of the converter. Since the voltages u1 and u2 vary in size in stages and their duration is affected by many factors, and the voltage is directly related to the excitation current of the transformer, it is necessary to analyze the relationship between the size and duration of u1 and u2. Figure 3 The equivalent circuit of the interleaved clamped flyback equalizer is given. Considering the high-frequency operation, the leakage inductance is much larger than the line impedance. In the subsequent circuit analysis, the influence of the line impedance on the converter operation is not considered. The expressions of voltage u1 and u2 in one cycle are

[0041]

[0042] In the time period t0-t1, the excitation voltage and excitation current are

[0043]

[0044] In the time period t1-t2, according to the equivalent model, the variables in the converter are obtained as follows:

[0045]

[0046] in

[0047]

[0048] Current i W1 The time required to drop to 0 is

[0049] In the time period t2-t3,

[0050]

[0051] Time period t3-t4,

[0052]

[0053] Current i W2 The time required to drop to 0 is

[0054] Δt 3_4 =t4-t3=-L r im_t3 / u L2 (13) according to Figure 3 The equivalent model shown, the current i W1 The DC component I W1 Determined by the DC component U1 in the u1 voltage, that is,

[0055] I W1 =U1 / R (14)

[0056] Among them, the size of U1 can be obtained according to the instantaneous expression of voltage u1 shown in formula (6), I W1 According to the current i W1 The instantaneous expression of

[0057]

[0058]

[0059] According to equations (6)-(16), we get the equation:

[0060]

[0061] in

[0062] A=2RL r (2L M +L r )

[0063] B=[2RT s (L M +L r )+4L r (2L M +L r )]U DF +4RL r T s U B1 +2RL r T s U B2

[0064]

[0065] According to formula (17), the current i is obtained m_t0 , and according to formula (9) we get i m_t1 , the average charge and discharge currents of the battery cells can be obtained as

[0066]

[0067] According to formula (18), the variation curves of the battery cell output current with line impedance R, different cell voltage differences, and different switching frequencies under different leakage inductance values ​​can be obtained as follows: Figure 4 (a)-(c) are shown. It can be seen that Figure 4 The current curve shown also shows that the staggered clamped flyback equalizer provided by the present invention can effectively achieve balance between monomers. Meanwhile, the smaller the line impedance, the greater the voltage difference between monomers, and the lower the switching frequency, the faster the balance speed.

[0068] The staggered clamped flyback lithium battery equalizer of the present invention can further expand its structure, such as Figure 5 As shown, it can be seen that as the number of battery cells increases, the number of switching tubes and transformer coils only increases in direct proportion, and no additional devices are added for connection.

[0069] In order to verify the performance of the diode interleaved clamped flyback lithium battery equalizer proposed in the present invention, an experimental prototype was established. The main parameters of the prototype are as follows: Transformer self-inductance L m =50μH, line impedance R = 0.1, transformer coil leakage inductance L r =0.2μH, switching frequency f s =50kHz. When the voltage difference between cells is large, the voltage converges faster. When the voltage between cells is small, the voltage converges slower because the balancing current becomes smaller.

[0070] Figure 6 (a)-(f) are the working waveforms of the equalizer under three voltage conditions. The single-cell voltages are: Case 1U B2 =3.950V, U B1 =3.581V; Case 2U B2 =3.915V, U B1 =3.823V; Case 3U B2 =3.885V, U B1 =3.865V.

[0071] Figure 6 (a)-(b) is the transformer coil current i when the single cell voltage is case 1 W1 、i W2 , excitation current i m , the output current i of the monomer after capacitor filtering B1 、i B2 , and the voltage u1, u2 waveforms, it can be seen that i W1 、i W2 Waveform and Figure 2 (a) shows the same waveform, indicating that the equalizer principle analyzed by the present invention is correct. m The DC component is large, such as Figure 6 i in (a) m The DC current is about 0.5A, so the transformer design principle is the same as the flyback transformer. Figure 2 In (a), there is an obvious convex part in the negative half cycle of voltage u1 during the time period t1-t2. Figure 6 (b) can clearly capture this feature. In addition, the equalizer proposed in the present invention only adds a diode to achieve the clamping of the peak voltage and the recovery of the leakage inductance energy in the flyback converter, which can be clearly seen from Figure 6 The voltage u1 and u2 waveforms in (b) show that the proposed equalizer has this feature.

[0072] In case 2, the voltage difference of the battery cells is only 0.092V. The most obvious feature is the current i m The DC component of the transformer decreases, which leads to significant changes in the primary coil current of the transformer. Figure 2 The duration of the time period t1-t2 in (a) is significantly shorter, and the entire equilibrium process is basically similar to Case 1.

[0073] When the cell voltages are approximately equal, in case 3, the current i m The average value is approximately equal to 0. To ensure the normal operation of the equalizer, the current i W1 、i W2 The reactive current component accounts for a large proportion, which leads to low efficiency of the equalizer when the single-cell voltage is similar. This feature is also a feature of all equalizers with natural balancing characteristics.

[0074] The present invention proposes a bidirectional flyback lithium battery equalizer with diode interleaved clamping. The equalizer is derived from a bidirectional flyback converter and realizes the non-isolation characteristics of series-connected monomers. Resonance suppression and energy feedback of leakage inductance energy are achieved by adding diodes, thereby effectively improving the conversion efficiency of the equalizer.

Claims

1. A bidirectional flyback lithium battery equalizer with interleaved diode clamping, characterized in that: A non-isolated bidirectional flyback converter is included; the non-isolated bidirectional flyback converter includes a first battery cell, a second battery cell, a first switch tube, a second switch tube and a high-frequency transformer; the high-frequency transformer T includes a first winding W1 and a second winding W2; The first switch tube and the second switch tube form a half-bridge structure with the first diode and the second diode respectively; the positive electrode of the first battery cell, the cathode of the second diode, and the drain of the first switch tube are connected together; the negative electrode of the second battery cell, the anode of the first diode, and the source of the second switch tube are connected together; the negative electrode of the first battery cell, the positive electrode of the second battery cell, the opposite end of the first winding, and the opposite end of the second winding are connected together; the cathode of the first diode, the source of the first switch tube and the same end of the first winding are connected together; the anode of the second diode, the drain of the second switch tube and the same end of the second winding are connected together.

2. The diode interleaved clamped bidirectional flyback lithium battery equalizer according to claim 1, characterized in that: The non-isolated bidirectional flyback converter controls and adjusts the duty cycle through the complementary control of the first switch tube and the second switch tube.

3. The bidirectional flyback lithium battery equalizer with diode interleaved clamping according to claim 1 or 2, characterized in that: The interleaved clamped flyback lithium battery equalizer can be expanded in structure. As the number of battery cells increases, the number of switch tubes and transformer coils of the equalizer only increases in direct proportion, without adding additional devices for connection.

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