Transformer core anti-saturation algorithm based on phase-shift + direct current control equalizer

Abstract

In the active half-bridge equalizer based on phase-shifted + direct current flow control, the output voltage of the half-bridge converter contains a direct current component, which causes the transformer core magnetization curve to operate asymmetrically, and in a severe case, the transformer core magnetic saturation will cause the equalizer to not operate normally. Therefore, the application discloses a transformer core anti-saturation algorithm based on a phase-shifted + direct current flow control equalizer, estimates the maximum magnetic induction intensity of the transformer core according to circuit operation parameters, determines whether to change the duty cycle of each half-bridge converter according to whether the maximum magnetic induction intensity is within a reasonable range, and re-estimates the maximum magnetic induction intensity until a reasonable range is reached. The application solves the adverse conditions caused by the transformer core saturation operation in the active half-bridge equalizer based on phase-shifted + direct current flow control, and ensures the safe operation of the active half-bridge equalizer based on phase-shifted + direct current flow control.

Description

Technical Field

[0001] This invention relates to a transformer core anti-saturation algorithm based on a phase-shifting + DC flow control equalizer, belonging to the field of power electronic converter control. Background Technology

[0002] In recent years, lithium batteries have been widely used in aviation, aerospace, smart microgrid energy storage systems, and electric vehicle power batteries. To obtain sufficiently high voltage and capacity, lithium battery cells need to be connected in series to form a lithium battery pack. Due to differences in characteristics between individual battery cells, after multiple charge and discharge cycles, the state of charge (SOC) and voltage of the series-connected lithium battery cells become inconsistent. Long-term operation can lead to overcharging and over-discharging, which will adversely affect the lifespan, capacity, and safety of the individual battery cells. SOC differences are usually reflected in differences in individual cell voltages; therefore, voltage balancing is an essential function of series-connected lithium battery packs.

[0003] In recent years, researchers have studied dual active half-bridge equalizers, which couple four cells on both sides of the primary and secondary sides through a high-frequency transformer. This scheme has a simple circuit structure and the lowest cost among all current solutions. However, it uses open-loop control with a fixed duty cycle, resulting in minimal energy transfer between the primary and secondary sides of the transformer, leading to extremely slow equalization speed. Furthermore, the operating frequency is only acceptable at very low frequencies, resulting in a large transformer size and weight. To overcome this problem, researchers proposed a phase-shifting control strategy between the half-bridges. This strategy can ensure rapid equalization by increasing the switching frequency, but it only achieves rapid energy transfer between battery cells within the half-bridges; the cells within the half-bridges remain under open-loop control, failing to guarantee rapid equalization of all battery cells. Building on phase-shifting control, researchers further developed a control strategy with adjustable DC output voltage for the half-bridges, achieving rapid equalization of all lithium battery cells in the equalizer. However, the DC voltage output by the half-bridge converter, acting on the transformer windings, can easily cause transformer core saturation, leading to the equalizer's malfunction.

[0004] Therefore, based on the use of an easy-to-implement and low-cost active half-bridge lithium battery equalizer, after achieving rapid equalization of all lithium battery cells by using phase-shift control and DC flow regulation, it is necessary to ensure that the transformer core in the equalizer is not saturated. This solution is thus derived. Summary of the Invention

[0005] Purpose of the invention: To address the issue that transformer core saturation is prone to occur after implementing phase-shift control and DC flow control strategies in active half-bridge lithium battery equalizers, this invention proposes an anti-saturation algorithm for transformer cores based on a phase-shift + DC flow control equalizer.

[0006] Technical Solution: A transformer core anti-saturation algorithm based on a phase-shifting + DC flow control equalizer. This method is implemented using an active half-bridge lithium battery equalizer, which includes n active half-bridge converters and a transformer with n windings. The i-th active half-bridge converter contains two equalized lithium battery cells B. i1 With B i2 2 switching transistors S i1 With S i2 The lithium battery unit B i1 With B i2 The voltage is U Bi1 with U Bi2 In the above parameters, i is any value from 1 to n; in the i-th active half-bridge converter, battery cell B i1 The negative terminal and the switching transistor S i1 The source terminal is connected and serves as the negative terminal of the i-th active half-bridge converter; the switching transistor S i1 The drain and the switching transistor S i2 The source terminal is connected and serves as one of the AC output terminals of the i-th active half-bridge converter; battery unit B i1 The positive electrode and battery cell B i2 The negative terminal 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 terminal and the switching transistor S i2 The drain of the i-th active half-bridge converter is connected and serves as the positive terminal 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 terminal 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 terminal of the i-th transformer winding; the positive terminal of the i-th active half-bridge converter is connected to the negative terminal of the (i+1)-th active half-bridge converter; the method is characterized by comprising the following steps:

[0007] S1. Real-time detection of the voltage U of each lithium battery cell. Bi1 with U Bi2 ;

[0008] S2. Based on the phase-shift + DC control strategy, two control quantities are obtained for each active half-bridge converter, namely the phase shift ratio Φ. i With duty cycle D i ;

[0009] S3. According to equation (a), the maximum magnetic flux density B of the transformer core is obtained. m ;

[0010] In equation (a), the number of turns in each of the n windings of the transformer is equal to N; L m T is the magnetizing inductance value of the transformer; sR is the switching cycle of the equalizer; R is the equivalent impedance of the transformer coil; A e A is the cross-sectional area of ​​the transformer core. i =0.5(U Bi1 +U Bi2 );

[0011] S4. Discrimination B m With 0.5B s The relationship between B s The saturation magnetic induction intensity of the transformer core; such as B. m ≥0.5B s If so, proceed to step S5; such as B m <0.5B s Then proceed to step S6;

[0012] S5. Optimize the duty cycle of the half-bridge converter to kD. i The newly obtained duty cycle D i Replace the original duty cycle value;

[0013] S6. Based on the duty cycle D i Compared to Φ i Modulation signal of active half-bridge converter.

[0014] A transformer core anti-saturation algorithm based on a phase-shift + DC flow control equalizer is proposed to determine the maximum magnetic induction intensity B of the transformer core. m Whether the limit is exceeded, by optimizing the duty cycle, the transformer core is kept within the linear range, avoiding saturation caused by the DC current in the output voltage of the active half-bridge, and improving the performance of the active half-bridge equalizer based on phase shift + DC current control.

[0015] Beneficial effects: The transformer core anti-saturation algorithm proposed in this invention, based on a phase-shifting + DC flow control equalizer, determines the maximum magnetic induction intensity B of the transformer core. m Whether the limits are exceeded ensures that the transformer core operates within the linear range, avoiding saturation caused by the DC current in the active half-bridge output voltage, thus improving the performance of the active half-bridge equalizer based on phase shifting and DC current control. These two beneficial effects can significantly enhance the market competitiveness of the active half-bridge equalizer. Attached Figure Description

[0016] Figure 1 This is the main circuit topology based on an active half-bridge lithium battery equalizer.

[0017] Figure 2 The main circuit topology is based on an active half-bridge 4-lithium battery cell equalizer.

[0018] Figure 3The control strategy is based on phase-shifting + DC regulation of an active half-bridge lithium battery cell equalizer.

[0019] Figure 4 The output voltage waveform of the half-bridge based on the phase-shift control strategy of the equalizer of the four lithium battery cells in the active half-bridge and the DC flow regulation strategy.

[0020] Figure 5 This is a flowchart of the transformer core anti-saturation algorithm based on a phase-shifting + DC flow control equalizer disclosed in this invention;

[0021] Symbol name in the figure: S i1 -S i2 (i = 1, 2, ..., n) — the first and second switches in the i-th active half-bridge converter; B i1 -B i2 (i = 1, 2, ..., n) — The first and second lithium battery cells 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; U Bi1 -U Bi2 (i = 1, 2, ..., n) — the cell voltage of the lithium battery unit in the i-th active half-bridge converter; D i (i = 1, 2, ..., n) — Duty cycle of the i-th active half-bridge converter; Φ i (i = 1, 2, ..., n) — the shift ratio of the i-th active half-bridge converter; B m —Maximum magnetic flux density of the transformer core; B s —Saturation magnetic induction intensity of transformer core. Detailed Implementation

[0022] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art will fall within the scope defined by the appended claims.

[0023] The main circuit topology of the active half-bridge lithium battery equalizer studied in this invention is as follows: Figure 1 As shown, it includes n active half-bridge converters and a transformer with n windings; the i-th active half-bridge converter contains two balanced lithium battery cells B. i1 With B i2 2 switching transistors S i1 With S i2The lithium battery unit B i1 With B i2 The voltage is U Bi1 with U Bi2 In the above parameters, i is any value from 1 to n; in the i-th active half-bridge converter, battery cell B i1 The negative terminal and the switching transistor S i1 The source terminal is connected and serves as the negative terminal of the i-th active half-bridge converter; the switching transistor S i1 The drain and the switching transistor S i2 The source terminal is connected and serves as one of the AC output terminals of the i-th active half-bridge converter; battery unit B i1 The positive electrode and battery cell B i2 The negative terminal 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 terminal and the switching transistor S i2 The drain of the i-th active half-bridge converter is connected and serves as the positive terminal 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 terminal 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 terminal of the i-th transformer winding; the positive terminal of the i-th active half-bridge converter is connected to the negative terminal of the (i+1)-th active half-bridge converter. Energy transfer between lithium-ion battery cells includes energy transfer within the half-bridge and energy transfer between half-bridge cells.

[0024] For ease of explanation, the following text uses an equalizer with four battery cells and two active half-bridge converters as an example to illustrate its working principle. The corresponding equalizer topology is as follows: Figure 2 As shown.

[0025] In an active half-bridge lithium battery equalizer, based on the phase-shift control strategy, the half-bridge output voltage u is further adjusted. i DC flow U i By adjusting the size, the balance between the units within the half-bridge and the units between the half-bridges can be achieved simultaneously. Therefore, related research has proposed a corresponding control strategy block diagram, as shown below. Figure 3 As shown in the figure. Only the control block diagram of the i-th equalizer is given, with the average of the sum of the n active half-bridge voltages as the reference. The shift ratio of each half-bridge drive signal is Φ. i The DC output is adjusted solely based on the voltage difference between the two units within the half-bridge, achieving rapid balancing of the units within the half-bridge. The control variable is the duty cycle D. i The above two control quantities control the balancing between half-bridge units and the balancing within half-bridge units, respectively. The controllers used are both proportional regulators, because the voltage change of the battery unit is very slow, and a proportional regulator + limiter can fully cope with the slow-changing unit voltage.

[0026] The operating waveform of the active half-bridge 4-lithium battery cell equalizer using phase shifting and DC flow regulation is as follows: Figure 4 As shown, the DC component in the output voltage of each half-bridge varies with the duty cycle D. i The voltage changes with the transformer coil, and when applied to the transformer coil, it causes the transformer core to become biased in magnetism. In severe cases, this can lead to the loss of transformer function. Therefore, it is essential to ensure that the transformer core operates in the linear region.

[0027] Under the phase-shift + DC current regulation control strategy, the half-bridge output voltage and current values ​​are respectively

[0028] u i =U i +u aci (i=1…n) (1)

[0029] in

[0030] U i =D i U Bi1 -(1-D i )U Bi2 (i=1…n) (2)

[0031]

[0032] Where A i =U Bi1 +U Bi2 ,

[0033] i i =I i +i aci (i=1…n) (4)

[0034]

[0035] AC component i of the balancing current aci It is quite complex and is related to many factors such as phase shift angle, resistance, leakage inductance, and unit voltage. It is not related to the analysis of DC bias magnetism of transformer.

[0036] Because DC flow regulation is introduced in the half-bridge converter control, the saturation condition of the transformer core must be considered. The saturation condition of the transformer core is mainly determined by its maximum magnetic flux density B. m According to the magnetization curve relationship, B m With the maximum value of magnetizing current i Lm Directly related, according to Figure 1 The direction of the current in the transformer is equal to the number of turns in each winding, thus...

[0037]

[0038] The excitation current consists of two parts: a DC component i Lmd and communication components i Lma The corresponding AC component is caused by the AC voltage component acting on the magnetizing inductor. This occurs when the equivalent resistance and leakage inductance of each branch are equal, i.e., R1 = ... = R n =R, L1=…=L n =L, excitation voltage u m for

[0039]

[0040] Generally, the half-bridge offset is set to be within a relatively small range, u m The amplitude is a square wave with the average value of the battery cell voltage; therefore, according to the relationship between inductor voltage and current, we get...

[0041]

[0042] The DC component of the transformer's excitation current is caused by the balancing current of the units within the half-bridge, i.e.

[0043]

[0044] Because the battery cell voltages are random, as the number of equalizer battery cells increases, the effects of the equalization current of each half-bridge cell on the excitation current can cancel each other out, making i Lmd The value is not too large.

[0045] Based on basic magnetic circuit knowledge, the maximum magnetic induction intensity B of the iron core can be obtained. m for

[0046]

[0047] In the formula, N is the number of turns in a single winding of the transformer, and A e Let i be the cross-sectional area of ​​the transformer core. Lm_max The instantaneous maximum value of the excitation current is, and its magnitude is

[0048]

[0049] In the formula, considering different combinations of current directions, an absolute value sign is required. According to equations (10) and (11), the maximum magnetic induction intensity B of the iron core is obtained. m for

[0050]

[0051] If the maximum magnetic flux density B obtained from equation (12) m Approaching saturation magnetic induction intensity B s If so, the duty cycle needs to be readjusted.

[0052] Based on the above analysis, the transformer core anti-saturation control algorithm based on a phase-shifting + DC flow control active half-bridge equalizer disclosed in this invention patent is obtained, as follows: Figure 5 As shown. The method includes the following steps:

[0053] S1. Real-time detection of the voltage U of each lithium battery cell. Bi1 with U Bi2 (i = 1, 2, ..., n);

[0054] S2. Based on the phase-shift + DC control strategy, two control quantities are obtained for each active half-bridge converter, namely the phase shift ratio Φ. i With duty cycle D i ;

[0055] S3. According to equation (12), the maximum magnetic induction intensity B of the transformer core is obtained. m ;

[0056] S4. Discrimination B m With 0.5B s The relationship between B s The saturation magnetic induction intensity of the transformer core; such as B. m ≥0.5B s If so, proceed to step S5; such as B m <0.5B s Then proceed to step S6;

[0057] S5. Optimize the duty cycle of the half-bridge converter to kD. i The newly obtained duty cycle D i Replace the original duty cycle value;

[0058] S6. Based on the duty cycle D i Compared to Φ i Modulation signal of active half-bridge converter.

[0059] After adopting the above control algorithm, if B is determined... m ≥0.5B s By continuously reducing the duty cycle value, i.e., the magnitude of the DC current in the half-bridge output voltage, the operating voltage (B) of the transformer core is adjusted. m Value, until B is satisfied. m <0.5B s until.

[0060] In summary, the transformer core anti-saturation algorithm based on phase-shift + DC flow control equalizer disclosed in this invention optimizes the duty cycle to ensure that the transformer core operates within the linear range, thus avoiding saturation caused by the DC flow in the output voltage of the active half-bridge and improving the performance of the active half-bridge equalizer based on phase-shift + DC flow control.

Claims

1. A transformer core anti-saturation algorithm based on a phase-shifting + DC-current control equalizer, the method being implemented based on an active half-bridge lithium battery equalizer, wherein the active half-bridge lithium battery equalizer comprises n active half-bridge converters and a transformer with n windings; the i-th active half-bridge converter contains two equalized lithium battery cells B. i1 With B i2 2 switching transistors S i1 With S i2 The lithium battery unit B i1 With B i2 The voltage is U Bi1 with U Bi2 In the above parameters, i is any value from 1 to n; in the i-th active half-bridge converter, battery cell B i1 The negative terminal and the switching transistor S i1 The source terminal is connected and serves as the negative terminal of the i-th active half-bridge converter; the switching transistor S i1 The drain and the switching transistor S i2 The source terminal is connected and serves as one of the AC output terminals of the i-th active half-bridge converter; battery unit B i1 The positive electrode and battery cell B i2 The negative terminal 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 terminal and the switching transistor S i2 The drain of the i-th active half-bridge converter is connected and serves as the positive terminal 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 terminal 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 terminal of the i-th transformer winding; the positive terminal of the i-th active half-bridge converter is connected to the negative terminal of the (i+1)-th active half-bridge converter; characterized in that... The method includes the following steps: S1. Real-time detection of the voltage U of each lithium battery cell. Bi1 with U Bi2 ; S2. Based on the phase-shift + DC control strategy, two control quantities are obtained for each active half-bridge converter, namely the phase shift ratio Φ. i With duty cycle D i ; S3. According to equation (a), the maximum magnetic flux density B of the transformer core is obtained. m ; In equation (a), the number of turns in each of the n windings of the transformer is equal to N; L m T is the magnetizing inductance value of the transformer; s R is the switching cycle of the equalizer; R is the equivalent impedance of the transformer coil; A e A is the cross-sectional area of ​​the transformer core. i =0.5(U Bi1 +U Bi2 ); S4. Discrimination B m With 0.5B s The relationship between B s The saturation magnetic induction intensity of the transformer core; such as B. m ≥0.5B s If so, proceed to step S5; such as B m <0.5B s Then proceed to step S6; S5. Optimize the duty cycle of the half-bridge converter to kD. i The newly obtained duty cycle D i Replace the original duty cycle value; S6. Based on the duty cycle D i Compared to Φ i Modulation signal of active half-bridge converter.

2. A transformer core anti-saturation algorithm based on a phase-shifting + DC flow control equalizer, characterized in that, Determine the maximum magnetic flux density B of the transformer core given by formula (a) in claim 1. m Whether the limit is exceeded, by optimizing the duty cycle, the transformer core is kept within the linear range, avoiding saturation caused by the DC current in the output voltage of the active half-bridge, and improving the performance of the active half-bridge equalizer based on phase shift + DC current control.

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