Auxiliary power module with lithium battery equalization function and control method thereof

Abstract

The invention discloses an auxiliary power module (APM) with a lithium battery equalization function and a control method thereof, and belongs to the field of DC-DC converters, the APM comprises a high-voltage side, n half-full bridge type converters, a filter capacitor C and a low-voltage side, and the high-voltage side comprises 2n lithium battery units; and the low-voltage side comprises a 12V low-voltage storage battery BLV. Aiming at the problem of high cost of the existing active equalizer, the active equalization function of the lithium battery is integrated in the APM, and the control method comprises the following steps of: balancing adjacent battery units on a high-voltage side and simultaneously solving the problem of magnetic saturation of a transformer by using an asymmetric duty ratio modulation strategy; two-way flow of energy between the high-voltage side and the low-voltage side is achieved through a phase shift control strategy, four different operation modes are achieved through the control method, and the market competitiveness of the product is effectively improved.

Description

Technical Field

[0001] The invention relates to an auxiliary power module with a lithium battery balancing function and a control method thereof, belonging to the technical field of power electronic converters and control thereof. Background Art

[0002] Since the 21st century, the global environment has been polluted while traditional fossil energy has been increasingly depleted. The energy transition issue has become an important challenge facing global social and economic development. Therefore, the global new energy industry has been developed in large quantities, especially the electric vehicle industry. New energy vehicles usually use lithium batteries as power batteries. In order to meet their voltage levels, multiple battery cells are usually connected in series and parallel. Due to subtle differences in manufacturing processes and materials, after multiple charges and discharges, the state of charge (SOC) and voltage of each battery cell are usually inconsistent, which greatly affects the battery life and safety of use. Therefore, studying a lithium battery equalizer with fast balancing speed, high modularity and low cost has positive and substantial significance for solving the above problems.

[0003] At present, lithium battery balancers can be divided into energy-consuming balance technology and non-energy-consuming balance technology. Energy-consuming balance technology is also called passive balance. It generally converts excess electric energy into heat through energy-consuming resistors for consumption. Its advantages are simple circuit structure and control method. However, its excessive heat generation leads to a high working environment temperature of the battery pack, affecting its normal operation and even causing explosion and other dangers. Non-energy-consuming balancers are also called active balancers. They complete energy transfer between batteries through energy storage elements. They can basically meet the requirements of high balance efficiency, fast balance speed and large balance current, but their cost is often high. Therefore, active balancers are almost not implemented in practical applications. Summary of the invention

[0004] Purpose of the invention: In view of the problem that the existing active equalizers are high in cost and difficult to use in practice, the present invention proposes an auxiliary power module composed of a modularized half-full-bridge converter, integrating the lithium battery balancing function into the auxiliary power module, reducing the cost of active balancing and reducing the volume of the auxiliary power module; a single half-full-bridge converter solves the problem of transformer magnetic saturation through an asymmetric modulation strategy to achieve balancing of adjacent battery cells, and realizes the bidirectional flow of energy between the half-bridge side and the full-bridge side through a phase shift control method, thereby ensuring the function of the auxiliary power module.

[0005] Technical solution: An auxiliary power module with lithium battery balancing function includes a high-voltage side, n half-full-bridge converters, a filter capacitor C and a low-voltage side.

[0006] The high voltage side includes 2n lithium battery cells (B 11 , B 12 , B21 , B 22 ……B n1 , B n2 ) are connected in series with the positive and negative electrodes connected;

[0007] The low voltage side includes a 12V low voltage battery B LV ;

[0008] The semi-full-bridge converter includes: a half-bridge side, a high-frequency link, and a full-bridge side.

[0009] The half-bridge side includes a first bridge circuit; the high-frequency link includes an auxiliary inductor L i And high frequency transformer T i ; The full bridge side includes a second bridge path and a third bridge path;

[0010] The two ends of the first bridge circuit are respectively connected to the high-voltage side battery unit B i1 (i=1,2……n) and the positive electrode of the high voltage side battery unit B i2 The midpoint a of the first bridge circuit is connected to the auxiliary inductor L i connected at one end.

[0011] The auxiliary inductor L i The other end of the transformer T i The same-name ends of the first winding are connected; the transformer T i The opposite end of the first winding is connected to the midpoint b of the two battery cells connected in series;

[0012] The midpoint c of the bridge arm of the second bridge circuit is connected to the transformer T i The same-name end of the second winding is connected; the transformer T i The opposite end of the second winding is connected to the midpoint d of the bridge arm of the third bridge circuit; the output end of the third bridge circuit is connected to the filter capacitor C; the low-voltage battery B LV In parallel with the filter capacitor C.

[0013] The first bridge circuit includes a first switch tube Q i1 And the second switch tube Q i2 The first switch tube Q i1 The source of the second switch tube Q i2 The drain of the first bridge circuit is connected to form a first bridge circuit, and the midpoint a of the bridge arm of the first bridge circuit is connected to the auxiliary inductor L i After connecting with the transformer T i The same-name end of the first winding is connected; the battery cell B i1 The positive electrode of the first switch tube Q i1 Drain connection, battery cell B i2 The negative electrode of the second switch tube Qi2 Source connection.

[0014] The second bridge circuit includes a third switch tube Q i3 And the fourth switch tube Q i4 The third bridge circuit includes a fifth switch tube Q i5 And the sixth switch tube Q i6 The third switch tube Q i3 The source of the fourth switch tube Q i4 The drain of the second bridge circuit is connected to form a second bridge circuit, and the midpoint c of the bridge arm of the second bridge circuit is connected to the transformer T i The same-name end of the second winding is connected; the fifth switch tube Q i5 The source of the sixth switch tube Q i6 The drain of the third bridge circuit is connected to form a third bridge circuit; the midpoint d of the bridge arm of the third bridge circuit is connected to the transformer T i The opposite end of the second winding is connected; the fifth switch tube Q i5 is connected to one end of the filter capacitor C, and the sixth switch tube Q i6 Connected to the other end of the filter capacitor C; the low-voltage battery B LV Connected in parallel with the filter capacitor C.

[0015] The control method applied to the semi-full bridge converter includes an asymmetric modulation strategy and a phase shift control strategy;

[0016] The asymmetric modulation strategy includes the following contents:

[0017] Since the voltage of the battery cell changes slowly in a short time, the real-time DC regulation duty cycle θ can be introduced according to equation (4). At this time, the first switch tube Q of the first bridge circuit is i1 The duty cycle of the first bridge is 50% + θ, and the second switch tube Q i2 The duty cycle is 50% + θ.

[0018] Through the asymmetric modulation strategy, the inductor current i i The DC component I i Keep it within a reasonable range; thereby achieving DC regulation control and avoiding transformer magnetic saturation; the DC component I in the inductor current i When the value exceeds a certain value, the transformer will be magnetically saturated. Assuming that I i The maximum value of I i-max =4A, therefore, the DC component I is converted to i Keeping it within 4A can avoid the occurrence of transformer magnetic saturation.

[0019] The phase shift control strategy includes the following contents:

[0020] The switch tube Q i3 , switch tube Q i4 , switch tube Q i5 And the switch tube Q i6 The duty cycle of the driving signal is 0.5, and the switch tube Q i3 And the switch tube Q i6 The switch tube Q i4 And the switch tube Q i5 At the same time, the switch tube Q i3 , switch tube Q i6 And the switch tube Q i1 Between and switch tube Q i4 , switch tube Q i5 And the switch tube Q i2 There is a phase shift d between them. When the phase shift d>0, the high-voltage side battery unit provides energy for the low-voltage side 12V battery. When the phase shift d<0, the energy flow direction is opposite.

[0021] Beneficial effect: The electric vehicle auxiliary power module composed of modularized semi-full-bridge converters adopted by the present invention reduces the cost of active equalizers in actual use and reduces the size of APM. The n semi-full-bridge converters in APM can operate independently. After adopting the above-mentioned asymmetric modulation strategy, each converter can avoid transformer magnetic saturation caused by DC bias, thereby achieving balance between adjacent battery cells; bidirectional energy flow between the high-voltage side and the low-voltage side is achieved through the phase shift control method, and four operating modes are achieved according to the actual different energy flow modes. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The main circuit of the semi-full bridge converter in the embodiment of the present invention;

[0023] Figure 2 Schematic diagram of main waveforms of a semi-full-bridge converter in a phase-shifting state according to an embodiment of the present invention;

[0024] Figure 3 1 is a working principle diagram of mode 1 of a semi-full-bridge converter in phase-shift control according to an embodiment of the present invention;

[0025] Figure 4 Schematic diagram of the working principle of mode 2 of the semi-full bridge converter in phase shift control according to an embodiment of the present invention;

[0026] Figure 5 Schematic diagram of the working principle of mode 3 of the semi-full bridge converter in phase shift control according to an embodiment of the present invention;

[0027] Figure 6 Schematic diagram of the working principle of mode 4 of the semi-full bridge converter in phase shift control according to an embodiment of the present invention;

[0028] Figure 7 is an equivalent circuit model of the half-bridge side of the semi-full-bridge converter under the action of a DC signal in an embodiment of the present invention;

[0029] Figure 8 is an equivalent circuit model of the filter circuit of the embodiment of the present invention;

[0030] Fig. 9 A block diagram of a semi-full bridge converter control system in an embodiment of the present invention;

[0031] Fig.10 is the inductor current i of the half-full bridge converter in the embodiment of the present invention i Simulation waveform diagram in Matlab / Simulink under traditional symmetrical duty cycle control;

[0032] Fig.11 is the inductor current i of the half-full bridge converter in the embodiment of the present invention i Simulation waveform diagram in Matlab / Simulink under asymmetric modulation strategy;

[0033] Fig.12 It is a schematic diagram of a specific application of an embodiment of the present invention in a simplified electric vehicle system. DETAILED DESCRIPTION

[0034] 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.

[0035] The main circuit topology of the semi-full bridge converter used in the present invention is as follows: Figure 1 As shown. Including the half-bridge side, auxiliary inductor L i 、Transformer T i and full bridge side.

[0036] The half-bridge side includes a first bridge circuit; the input end of the first bridge circuit is connected to two battery cells connected in series, and the output end a of the first bridge circuit is connected to the auxiliary inductor L i One end is connected;

[0037] The high frequency link includes an auxiliary inductor L i With transformer T i , auxiliary inductor L i The other end of the transformer T i The same-name ends of the first winding are connected, and the transformer T i The opposite end of the first winding is connected to the midpoint b of the two battery cells connected in series;

[0038] The full bridge side includes the second bridge circuit and the third bridge circuit. The midpoint of the second bridge circuit is connected to the transformer T i The same-name end of the second winding is connected to the transformer T i The opposite-name end of the second winding is connected to the midpoint d of the third bridge circuit, and the output end of the third bridge circuit is connected to the low-voltage battery.

[0039] The first bridge circuit includes a first switch tube Q i1 And the second switch tube Q i2 , the first switch tube Q i1 The source of the second switch tube Q i2 The drain of the first bridge circuit is connected to form a first bridge circuit, and the midpoint a of the bridge arm of the first bridge circuit is connected to the auxiliary inductor L i After connecting with transformer T i The same-name end of the first winding is connected to the battery cell B i1 The positive electrode of the first switch tube Q i1 Drain connection, battery cell B i2 The negative electrode of the second switch tube Q i2 The source connection of

[0040] The full-bridge switch network includes a third switch tube Q i3 , the fourth switch tube Q i4 、The fifth switch tube Q i5 And the sixth switch tube Q i6 ; The third switch tube Q i3 The source and the fourth switch tube Q i4 The drain of the second bridge circuit is connected to form a second bridge circuit, and the midpoint c of the bridge arm of the second bridge circuit is connected to the transformer T i The same-name end of the second winding is connected; the fifth switch tube Q i5 The source and the sixth switch tube Q i6 The drain of the third bridge is connected to form a third bridge circuit, and the midpoint d of the third bridge circuit is connected to the transformer T i The opposite end of the second winding is connected, and the fifth switch tube Q i5 Connected to the positive electrode of the low-voltage battery, the sixth switch tube Q i6 Connect to the negative terminal of the low-voltage battery.

[0041] Figure 1 The waveform diagram of the auxiliary power circuit converter with balancing function disclosed in the present invention under the full bridge phase shift control strategy is shown in FIG. i1 And the third switch tube Q of the second bridge i3 The phase shift control strategy is adopted to realize the bidirectional flow of power. Taking the phase shift ratio d>0 as an example, the corresponding switch tube Q i1 -Q i6 The driving signal waveform is as follows Figure 2As shown in the figure, through the phase shift control strategy, the average value of the current flowing into the low-voltage battery is not 0 in one switching cycle, thereby realizing the energy flow from the high-voltage side to the low-voltage side. When the phase shift ratio d<0, the energy flow direction is opposite to the above situation. If no energy flow is required, the phase shift ratio d=0 is selected. Figure 1 Taking the driving signals of the switches in as an example, in one cycle, the converter operation can be divided into four modes, corresponding to Figures 3 to 6 .

[0042] Mode 1-Mode 4 correspond to Figure 3-6 , where mode 1 and mode 2 are phase-shift operations, and the auxiliary inductor current i i The slopes at each stage can be expressed as:

[0043]

[0044] Among them, U x1 For transformer T i1 Primary voltage, L is the auxiliary inductance L i Inductance value, ideally, the transformer primary voltage and auxiliary inductance value in each half-bridge converter are equal, so the auxiliary inductor current i i In each stage it can be expressed as:

[0045]

[0046] From formula (2), we can know that the auxiliary inductor current i i The slopes in each stage are determined by the battery cell voltage U Bi1 , U Bi2 and transformer T i The voltage of the first winding determines the transformer T i The voltage of the first winding can be assumed to be constant. Therefore, when the two battery cells are unbalanced, the auxiliary inductor current i i The net change rate in one cycle is not 0, and the auxiliary inductor current i i Continuously shifting to the high voltage battery cell side, causing the transformer T i Produce phenomena such as magnetic saturation.

[0047] To solve the above problems, the half-bridge switching network adopts an asymmetric modulation strategy, and the half-bridge output voltage u i and the auxiliary inductor current i i Can be expressed as the superposition of their respective DC components and AC components

[0048]

[0049] Among them U i ,I i 、u i-ac 、ii-ac are the DC component and AC component of the half-bridge output voltage and auxiliary inductor current respectively. Under the asymmetric modulation strategy, I i It can be expressed as

[0050]

[0051] In which, it is assumed that the value of the equivalent resistance in each loop is equal to R. Since the battery voltage changes slowly in a short time, it can be seen from formula (4) that the DC component in the half-bridge output current can be controlled by the DC adjustment duty cycle θ to control each stage I i The size of the transformer can be avoided to avoid the problem of transformer magnetic saturation caused by DC bias.

[0052] Since the transformer T i It has an isolation effect. There is no DC flow between the half-bridge side and the full-bridge side. Therefore, the equivalent circuit model of the half-bridge side under DC signal is as follows: Figure 7 As shown, from Kirchhoff's current law, we know

[0053] I i +I Bi2 =I Bi1 (5)

[0054] Among them I i ,I Bi1 ,I Bi2 The energy flow direction and speed between two battery cells can be determined by I i The size of the control, when I i >0, in one cycle, battery B i1 Relative to B i2 More energy is needed, in I i >I Bi1 , that is I Bi2 <0, battery cell B i2 Through the above analysis, control I i The size of can not only avoid transformer magnetic saturation but also achieve balance between battery cells.

[0055] Under the asymmetric modulation strategy and phase shift modulation strategy, the output power of the converter can be expressed as

[0056]

[0057] in

[0058]

[0059] In order to provide a relatively stable current to the load, a filter capacitor C is connected in parallel on the low-voltage battery side, such as Figure 8As shown, ideally, the output capacitor C will absorb i o The AC component in .

[0060] By combining the asymmetric modulation strategy with the phase-shift modulation strategy, the converter can achieve four operating modes. The specific conditions of each DC component in different modes are shown in Table 1.

[0061] Table 1 Specific conditions of the four equilibrium modes

[0062] <![CDATA[I i 、d]]> Charge Discharge <![CDATA[I i >I Bi1 ,d=0]]> <![CDATA[B i2 ]]> <![CDATA[B i1 ]]> <![CDATA[I Bi1 >I i >0,d>0]]> <![CDATA[B LV ]]> <![CDATA[B i1 、B i2 ]]> <![CDATA[I i >0,d>0]]> <![CDATA[B LV ]]> <![CDATA[B i1 、B i2 ]]> <![CDATA[I i >I Bi1 >0,d<0]]> <![CDATA[B i2 ]]> <![CDATA[B i1 、B LV ]]>

[0063] From formula (4), we can know that the DC component I in the half-bridge output current i is i Mainly affected by the voltage of 2 battery cells U Bi1 and U Bi2 And the influence of DC regulation duty cycle θ, because the lithium battery voltage changes slowly, so, Fig. 9 The figure shows a control system structure schematic diagram of an auxiliary power circuit with lithium battery balancing function.

[0064] The controller includes a half-bridge side control module and a full-bridge side control module, and the two control modules are operated simultaneously;

[0065] The specific execution steps of the half-bridge side control module are:

[0066] Step S11: The detected voltages of the two battery cells U Bi and U Bi2 The given value and the comparison value are compared by a comparator respectively;

[0067] Step S12: the comparator output value is passed through the DC regulator to obtain the half-bridge side DC regulation duty cycle θ;

[0068] Step S13: the DC regulation duty cycle θ is pulse-width modulated after passing through a limiter to generate a corresponding pulse-width modulation signal;

[0069] Step S14: The driving module drives the first switch tube Q on the half-bridge side according to the modulation signal generated by the modulation module. i1 And the second switch tube Q i2 To exercise control;

[0070] The specific execution steps of the full-bridge side control module are:

[0071] Step S21: The detected output voltage U o and the output current i o Perform multiplication to obtain its active power P o , and with a given value P o-ref Comparison is performed by a comparator;

[0072] Step S22: the comparator output value is passed through the output power regulator to obtain a shift phase d;

[0073] Step S23: the phase shift ratio d is subjected to phase shift modulation by a limiter to generate a corresponding phase shift modulation signal;

[0074] Step S24: The driving module controls the full-bridge side switch tube Q according to the modulation signal generated by the modulation module. i3～ Q i6 Take control.

[0075] Build a simulation model in Matlab / Simulink, such as Fig.10 and Fig.11 As shown in the figure, the output current i is respectively ab From the simulation waveform, it can be concluded that asymmetric modulation can effectively prevent current deviation, thereby avoiding magnetic saturation of the transformer.

[0076] like Fig.12 As shown, a simplified electric vehicle system architecture is shown. After the main circuit topology of an auxiliary power circuit with a lithium battery balancing function disclosed in the present invention is modularized, the high-voltage battery is connected to the traction system, the charging system, the low-voltage battery and its load through an isolated step-down DC-DC converter. The lithium battery balancing module is integrated in the APM, which can not only reduce the cost of active balancing, but also replace the traditional auxiliary power converter that is large in size and expensive due to its high step-down ratio and rated voltage.

[0077] In summary, the main circuit topology of an auxiliary power circuit with a lithium battery balancing function disclosed in the present invention not only combines lithium battery balancing with APM, while reducing the production costs of both, but also solves the transformer magnetic saturation problem caused by DC bias through joint control of an asymmetric modulation strategy and a phase-shift modulation strategy, and different operating modes can be selected according to the usage scenario, thereby ensuring the practicability of the disclosed converter.

Claims

1. An auxiliary power module with lithium battery balancing function, characterized in that: It includes a high voltage side, n half-full bridge converters, a filter capacitor C and a low voltage side.

2. The high voltage side includes 2n lithium battery cells (B 11 , B 12 , B 21 , B 22 ……B n1 , B n2 ) are connected in series with the positive and negative electrodes connected; The low voltage side includes a 12V low voltage battery B LV ; The semi-full bridge converter comprises: The half-bridge side, the high-frequency link, and the full-bridge side.

3. The auxiliary power module with lithium battery balancing function according to claim 1, characterized in that: The half-bridge side includes a first bridge circuit; the high-frequency link includes an auxiliary inductor L i And high frequency transformer T i ; The full bridge side includes a second bridge path and a third bridge path; The two ends of the first bridge circuit are respectively connected to the high-voltage side battery unit B i1 The positive electrode and the high voltage side battery cell B i2 The negative electrode of the first bridge circuit is connected, i=1,2...n; the midpoint a of the first bridge circuit is connected to the auxiliary inductor L i One end of is connected; The auxiliary inductor L i The other end of the transformer T i The same-name ends of the first winding are connected; the transformer T i The opposite end of the first winding is connected to the midpoint b of the two battery cells connected in series; The midpoint c of the bridge arm of the second bridge circuit is connected to the transformer T i The same-name end of the second winding is connected; the transformer T i The opposite end of the second winding is connected to the midpoint d of the bridge arm of the third bridge circuit; the output end of the third bridge circuit is connected to the filter capacitor C; the low-voltage battery B LV In parallel with the filter capacitor C.

4. The auxiliary power module with lithium battery balancing function according to claim 1, characterized in that: The first bridge circuit includes a first switch tube Q i1 And the second switch tube Q i2 The first switch tube Q i1 The source of the second switch tube Q i2 The drain of the first bridge circuit is connected to form a first bridge circuit, and the midpoint a of the bridge arm of the first bridge circuit is connected to the auxiliary inductor L i After connecting with the transformer T i The same-name end of the first winding is connected; the battery cell B i1 The positive electrode of the first switch tube Q i1 Drain connection, battery cell B i2 The negative electrode of the second switch tube Q i2 Source connection.

5. The auxiliary power module with lithium battery balancing function according to claim 1, characterized in that: The second bridge circuit includes a third switch tube Q i3 And the fourth switch tube Q i4 The third bridge circuit includes a fifth switch tube Q i5 And the sixth switch tube Q i6 The third switch tube Q i3 The source of the fourth switch tube Q i4 The drain of the second bridge circuit is connected to form a second bridge circuit, and the midpoint c of the bridge arm of the second bridge circuit is connected to the transformer T i The same-name end of the second winding is connected; the fifth switch tube Q i5 The source of the sixth switch tube Q i6 The drain of the third bridge circuit is connected to form a third bridge circuit; the midpoint d of the bridge arm of the third bridge circuit is connected to the transformer T i The opposite end of the second winding is connected; the fifth switch tube Q i5 is connected to one end of the filter capacitor C, and the sixth switch tube Q i6 Connected to the other end of the filter capacitor C; the low-voltage battery B LV Connected in parallel with the filter capacitor C.

6. A control method applied to a semi-full bridge converter as claimed in any one of claims 1 to 4, characterized in that: Including asymmetric modulation strategy and phase shift control strategy; The asymmetric modulation strategy includes the following contents: Introducing real-time DC regulation duty cycle θ, the first switch tube Q of the first bridge i1 The duty cycle of the first bridge is 50%-θ, and the second switch tube Q i2 The duty cycle is 50%+θ; The asymmetric duty cycle control strategy is used to make the inductor current i i The DC component I i Keep within the maximum threshold; thus achieving DC regulation control; The phase shift control strategy includes the following contents: The switch tube Q i3 , switch tube Q i4 , switch tube Q i5 And the switch tube Q i6 The duty cycle of the driving signal is 0.5, and the switch tube Q i3 And the switch tube Q i6 The switch tube Q i4 And the switch tube Q i5 At the same time, the switch tube Q i3 , switch tube Q i6 And the switch tube Q i1 Between and switch tube Q i4 , switch tube Q i5 And the switch tube Q i2 There is a phase difference between the switch tube Q i3 , switch tube Q i6 And the switch tube Q i1 Between and switch tube Q i4 , switch tube Q i5 And the switch tube Q i2 The shift ratio between them is d. When the shift ratio d>0, the high-voltage side battery unit provides energy for the low-voltage side 12V battery. When the shift ratio d<0, the energy flow direction is opposite.

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