A Minimum Current Stress Optimization Method for Dual Active Bridge DC-DC Converters

Abstract

The present invention provides a method for optimizing the minimum current stress of a dual active bridge DC-DC converter. In a high-power application scenario, in order to enable a DAB converter to obtain the minimum current stress within the full power range, the dynamic relationship between the change of inductance and the change of current stress is considered, and an appropriate inductance is selected accordingly. In addition, by analyzing a triple phase-shift control strategy and establishing a corresponding optimization model, the optimal phase-shift ratio combination is sought with the goal of global minimization of current stress to achieve current stress optimization, ensure that the current stress of the DAB converter is always kept within the theoretical expected range during the entire operation process, and ensure the reliability of the switch tube when the converter is in stable operation.

Description

Technical Field

[0001] The present invention relates to the field of converters, and more specifically, to a method for optimizing minimum current stress of a dual active bridge DC-DC converter. Background Art

[0002] In recent years, with the gradual escalation of the global energy crisis and environmental problems, the energy structure is gradually shifting from traditional fossil fuels to renewable energy such as photovoltaics and wind power. This trend has given rise to an urgent need for efficient power conversion technology. Dual active bridge DC-DC converters (DAB) are widely used in high-power applications such as DC microgrids, electric vehicles, and energy storage systems due to their advantages in energy recovery, battery charging, discharge management, and power quality improvement. At the same time, dual active bridge DC-DC converters have shown their potential in the field of efficient power conversion due to their electrical isolation, high power density, bidirectional energy flow, bidirectional symmetry of circuit structure, and easy soft switching. However, in actual high-power application scenarios, it is a challenge to design and control high-power DAB converters under the condition of device current limitation to reduce the current stress of switching devices. Problems such as current stress, switching loss, electromagnetic interference, and system stability need to be solved through careful design and perfect control strategies.

[0003] In order to reduce the current stress of DAB converter, increase the service life of switching devices, reduce converter loss and improve efficiency. DAB converter mainly changes the phase shift ratio between the primary bridge and the secondary bridge and the phase shift ratio inside the full bridge through the phase shift control strategy to optimize multiple objectives, such as current stress. Among them, the most commonly used phase shift strategy is single phase shift (SPS) control, which is usually simple to control and has a fast dynamic response time. SPS has only one control degree of freedom. When the input and output voltages do not match, it is easy to generate large current stress under light load conditions, and it is impossible to ensure that the soft switching characteristics are fully realized in the full power range. Therefore, considering the limitations of single phase shift itself, many researchers have proposed extended phase shift (EPS) control, dual phase shift (DPS) control, and triple phase shift (TPS) control. DPS control and TPS control aim to reduce current stress by increasing the number of phase shift control degrees of freedom, optimizing the phase shift ratio and finding the optimal combination. Among them, extended phase shift and dual phase shift control have only two phase shift control degrees of freedom. While improving efficiency, the operation of the two phase shift strategies is not too complicated, but it has less freedom than triple phase shift control. Triple phase shifting has three degrees of freedom, and the implementation of the phase shifting strategy is relatively complex, but the optimization effect on current stress is very obvious. The current TPS control proposed phase shifting control strategy only considers reducing current stress by adjusting the phase shift ratio and ignores the influence of inductor inductance change on current stress, making it difficult to obtain the minimum current stress. Some TPS control also proposed theoretical analysis of the influence of triple phase shifting inductor magnetic parameter design method on current stress, without considering the influence of inductor inductance change in high power converter. Summary of the invention

[0004] The present invention aims at the fact that traditional optimization control strategies often only focus on the phase shift optimization under low power conditions and ignore the influence of the change of inductance on the current stress. It provides a minimum current stress optimization method for a dual active bridge DC-DC converter, which solves the shortcomings of current stress optimization of high-power DAB converters in practical applications.

[0005] The present invention provides a method for optimizing the minimum current stress of a dual active bridge DC-DC converter, comprising: Obtain three effective working modes of the DAB converter in each transmission direction under the triple phase-shift control strategy, as well as the phase-shift ratio constraints in each effective working mode; For each transmission direction, based on the voltage transmission ratio K, the inductance value and the three shift ratios, the current stress and the transmission power of the DAB converter are calculated, and the current stress and the transmission power are normalized to obtain the current stress normalized value and the transmission power normalized value; Obtain the soft switching constraints in each valid working mode; Based on the transmission power expression of the single phase-shift control strategy and the transmission power expression of the triple phase-shift control strategy, the inductance value of the DAB converter is determined; Constructing a current stress optimization equation for a DAB converter, wherein the current stress optimization equation takes minimization of current stress as an objective function, takes transmission power as an equality constraint, and takes a shift ratio constraint and a soft switching constraint as inequality constraints; Determining a target operating mode of the DAB converter according to the transmission power per unit value and the voltage transmission ratio K, wherein the target operating mode is one of three effective operating modes; The current stress optimization equation of the target working mode is solved to obtain the optimal combination of the shift ratio of the target working mode and calculate the minimum current stress.

[0006] The present invention provides a dual active bridge DC-DC converter minimum current stress optimization method, which takes into account the dynamic relationship between the change of inductance and current stress, and selects the appropriate inductance accordingly. In addition, by analyzing the triple phase shift control strategy and establishing a corresponding optimization model, the optimal phase shift ratio combination is sought with the global minimization of current stress as the goal, so as to achieve current stress optimization, ensure that the current stress of the DAB converter is always kept within the theoretical expected range during the entire operation process, and ensure the reliability of the switch tube when the converter is in stable operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 This is the topological structure diagram of the DAB converter circuit; Figure 2 A method flow chart of a dual active bridge DC-DC converter minimum current stress optimization method provided by the present invention; Figure 3 It is the primary side equivalent circuit diagram of DAB converter; Figure 4 It is a schematic diagram showing the correlation between the change of equivalent inductance and inductor current; Figure 5-1 It is the waveform diagram of single phase shift control strategy; Figure 5-2 It is the waveform diagram of triple phase shift control strategy; Figure 6-1 The waveform diagram of triple phase shift control strategy of mode 1; Figure 6-2 The waveform diagram of the triple phase-shift control strategy of mode 4; Figure 6-3 The waveform diagram of triple phase shift control strategy of mode 5; Figure 7 This is the steady-state working waveform of TPS phase-shift control in mode 1; Figure 8 This is the block diagram of the TPS phase-shift control strategy; Fig. 9 This is the simulation waveform of the traditional single phase shift (SPS) control under the first working condition; Fig.10 This is a schematic diagram of the optimized three-phase shift (TPS) control simulation waveform under the first working condition; Fig.11 This is the simulation waveform of the traditional single phase shift (SPS) control under the second working condition; Fig.12 This is a schematic diagram of the simulation waveform of optimized three-phase shift (TPS) control under the second working condition; Fig.13 This is the simulation waveform of the traditional single phase shift (SPS) control under the third working condition; Fig.14 This is a schematic diagram of the simulation waveform of optimized three-phase shift (TPS) control under the third operating condition. DETAILED DESCRIPTION

[0008] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. In addition, the technical features in the various embodiments or single embodiments provided by the present invention can be arbitrarily combined with each other to form a feasible technical solution. This combination is not subject to the constraints of the sequence of steps and / or the structural composition mode, but must be based on the ability of ordinary technicians in this field to achieve. When the combination of technical solutions is contradictory or cannot be achieved, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0009] In order to solve the shortcomings of current stress optimization in practical applications of high-power DAB converters, the present invention proposes to consider the dynamic relationship between the change of inductance and current stress, select appropriate inductance, ensure that the current stress is always kept within the theoretical expected range, ensure the reliability of the switch tube under the stable operation conditions of the converter, and improve the service life of the switch tube. Using a triple phase-shift control strategy, an optimization model of DAB converter transmission power and current stress is constructed. With the goal of global minimization of current stress, the current stress is optimized by seeking the optimal phase-shift ratio combination in combination with the KKT condition, and compared with the current stress under the traditional single phase-shift control method. Finally, the feasibility of the proposed method is experimentally verified by building an experimental engineering prototype of a DAB converter with a rated power of 100kW.

[0010] See also Figure 1, which is the basic topology of the DAB converter. The DAB converter is mainly composed of a bilateral symmetrical H-bridge connected through a high-frequency transformer. Among them, the DC side input and output voltages are and , the supporting capacitors on both sides are , , It is a full bridge Output AC voltage, It is a full bridge Output AC voltage, is the equivalent inductance, the inductance is the sum of the transformer leakage inductance and the inductance, and the high-frequency transformer ratio is , is the current flowing through the inductor, defining the voltage transfer ratio , the operating frequency of the switch tube is , the switch tube working cycle is .

[0011] By controlling the magnitude and direction of the phase shift between the four bridge arms in the bilateral H-bridge, the phase shift ratio between the two AC square wave voltages is changed, so that energy can flow freely between the two DC power supplies, and all switch tubes operate at a 50% duty cycle. There are three independent phase shift control variables in the phase shift control. is the half cycle duration, represents the internal shift ratio of the primary side, The external phase shift ratio is the ratio between the primary side and the secondary side. Compared with the internal shift phase on the secondary side, the output voltages on both sides of the bilateral H-bridge transformer are three-level waveforms.

[0012] Figure 2 A dual active bridge DC-DC converter minimum current stress optimization method provided by the present invention comprises the following steps: Step 1: Obtain three effective working modes of the DAB converter in each transmission direction under the triple phase-shift control strategy, and the phase-shift ratio constraints in each effective working mode.

[0013] It can be understood that the high-frequency AC square wave voltage output at both ends of the bilateral H-bridge makes it equivalent to two AC sources, acting together at both ends of the equivalent inductor. Figure 3 The primary equivalent circuit of the transformer is given.

[0014] Primary voltage Equivalent to , secondary voltage Converted to the primary side voltage is , the sum of the two voltages is applied across the equivalent inductor, so the current slope during this interval can be approximated by the formula (1).

[0015] (1) According to the inductance tester under the conditions of voltage 100V and current 600A, Figure 4 It can be seen that the equivalent inductance is closely related to the change of the inductor current. The reduction of the inductance will lead to greater current stress. The greater the current stress, the magnetic saturation of the inductor magnetic element will occur, further leading to a decrease in the inductance. Therefore, it is necessary to stabilize the inductance by reducing the current stress.

[0016] in, Figure 5-1 This is the waveform diagram of the single phase-shift control strategy. Figure 5-2 The triple phase shift control strategy waveform diagram is shown in Figure 2. Figure 5-1 and Figure 5-2 Analysis shows that under different phase-shift control strategies, by adjusting the size of the phase shift ratio, the duration of the inductor current in a specific stage can be changed to control the size of the current stress, so that the inductor inductance is maintained in a stable stage and the stable operation of the converter is ensured.

[0017] Among the phase-shift control strategies for high-power dual active bridge converters, the triple phase-shift control strategy has attracted widespread attention due to its three independent internal and external phase-shift angles. , The sequence of the rising and falling edges of the TPS phase shift control is divided into 12 unique operating modes. These operating modes are symmetrical in the forward and reverse power transmission processes, and each transmission direction has 6 different operating modes. This article takes forward power transmission as an example for analysis. , , The relationship between can divide the six working modes of forward power transmission into as shown in Table 1.

[0018] Table 1: Phase shift constraints in six TPS forward transmission modes

[0019] Figure 6-1 , Figure 6-2 and Figure 6-3 Three effective working modes under TPS control during forward transmission are shown, namely mode 1, mode 4 and mode 5.

[0020] Step 2: For each transmission direction, based on the voltage transfer ratio K, the inductance value and the three shift ratios, calculate the current stress and the transmission power of the DAB converter, and normalize the current stress and the transmission power to obtain the current stress per unit value and the transmission power per unit value.

[0021] It is understandable that during the forward power transmission process, since there is a non-overlapping area between the primary and secondary voltages, that is, At this time, energy transfer cannot be directly from the input side to the output side, but must be achieved through inductive energy storage, which will slowly increase the current on the equivalent inductor. Under the same power transmission conditions, the stress of the inductor current is greater than that of other modes. In order to improve the energy transmission efficiency of high-power DAB converters, reduce current stress, and ensure stable operation within the full power range, the analysis focuses on the voltage overlap area between the primary and secondary sides of the transformer. There are three working modes, namely mode 1, mode 4 and mode 5. Therefore, the non-overlapping parts of the primary and secondary voltages of the transformer are not considered, and only these three working modes are focused on. Mode 1 of the DAB converter under the TPS phase-shift control strategy during forward power transmission is selected to deeply analyze its current stress and transmission power characteristics. Figure 7 The steady-state operating waveform of TPS phase-shift control in mode 1 is shown.

[0022] According to the working state of the converter in mode 1, the inductor current waveform of the high-power DAB converter is symmetrical in one cycle, so only the inductor current at four time nodes in the first half cycle of the DAB converter in steady-state operation is analyzed. The four time nodes can be represented by shift phase and half switching cycle: , , , , where the half-switching duty cycle of the DAB converter is .

[0023] Therefore, the inductor current value at the node can be calculated based on the half-cycle time node and the working state of mode 1. At the same time, according to the symmetry of the converter inductor current in one cycle, it can be known that in the half-cycle According to the converter working state in mode 1, the inductor current expression is derived as follows: (2) Furthermore, the inductor current expression at four time nodes is derived as follows: (3) The maximum value of the inductor current in one cycle is defined as the current stress of the DAB converter, as shown in equation (4).

[0024] (4) According to the above expression relationship, the inductor current stress expression in mode 1 (0<K<1) is: (5)

[0025] Similarly, the inductor current stress expression in mode 1 (K>1) is: (6) in is the current stress reference value, as shown in formula (7):

[0026] (7) The maximum transmission power of single phase shift control is taken as the transmission power reference value, as shown in formula (8): (8) The transmission power of the DAB converter depends on the input port voltage and the inductor current Calculation shows that: (9) According to equations (5) and (6), the current stress under different voltage transfer ratios is calculated respectively. The transmission power of the DAB converter is calculated by equation (9). The calculated current stress and transmission power of the DAB converter are normalized based on the current stress reference value and the transmission power reference value respectively to obtain the current stress per unit value and the transmission power per unit value.

[0027] Among them, the per unit value expression of the current stress of the DAB converter in mode 1 is: ; The per-unit transmission power expression of the DAB converter in mode 1 is: .

[0028] The same analysis method as mode 1 is used to analyze other working modes. During the forward energy transmission process, the per-unit values ​​of current stress in the three modes under the TPS control strategy are listed in Table 2.

[0029] Table 2: Normalized current stress of TPS in forward transmission

[0030] At the same time, the per-unit transmission power expression of the DAB converter in mode 1 is: (10)

[0031] Similarly, the normalized transmission power of Mode 4 and Mode 5 can be expressed as: (11)

[0032] Step 3: Obtain the soft switching constraint conditions in each valid working mode.

[0033] Since the switching devices of high-power DAB converters often operate under high-frequency conditions. Therefore, achieving zero-voltage turn-on of the switch tube can significantly reduce the conduction loss, thereby improving the energy transmission efficiency of the DAB converter. The soft switching characteristics can be mainly described as: a reverse-parallel diode is connected at both ends of the switch tube. When the switch device is turned on, the voltage at both ends is clamped to 0, and then the current is commutated from the reverse-parallel diode to the switch tube, which allows the switch tube to be turned on under zero voltage conditions. At this time, the switch tube is in a ZVS state. Therefore, at the moment the switch tube is turned on, , , , The rising edge requires current ≤0; for , , , The rising edge requires current ≥0. Therefore, according to the analysis of different working states under mode 1, the soft switching constraint conditions under mode 1 are as shown in the formula: (12)

[0034] The soft switching constraint range of the converter in mode 4 and mode 5 can be obtained. The soft switching constraint range in the three modes is shown in Table 3.

[0035] Table 3: Soft switching constraints

[0036] Step 4: Determine the inductance value of the DAB converter based on the transmission power expression of the single phase-shift control strategy and the transmission power expression of the triple phase-shift control strategy.

[0037] It is understandable that in a DAB converter, the equivalent inductance is a key component of converter energy transfer, which directly affects the magnitude of the current stress during steady-state operation of the converter. In addition, in the phase shift control strategy, the equivalent inductance also determines the phase shift time, which in turn affects the zero voltage turn-on characteristics of the converter. Therefore, in order to ensure that the high-power DAB converter can operate safely and reliably, it is necessary to reasonably design the equivalent inductance and select the appropriate inductance.

[0038] When selecting the equivalent inductance of a high-power DAB converter, the size of the converter's transmission power and the zero voltage turn-on condition must be comprehensively considered. This ensures that while meeting the requirements of efficient energy conversion, the reliability of the switch tube is improved, thereby improving the overall performance and reliability of the converter. According to formula (9), the expression of the transmission power under single phase shift control is as follows: (13) when When it is equal to 0.5, the transmission power of the DAB converter can reach the maximum at this moment. The maximum inductance at the maximum transmission power is The expression is as follows: (14) (15) Similarly, consider the minimum value of the inductance of the magnetic component when zero voltage is turned on under three-phase shift control The expression is as follows: (16) According to the main circuit parameter requirements of the high-power converter, the appropriate inductance is selected. When the input voltage is 950V, the output voltage is 800V, the maximum transmission power is 167kW, and the voltage matching ratio is is 0.72, the converter operating frequency for Under this condition, the voltage mismatch is large and the working condition is relatively bad. According to equations (15) and (16), the range of equivalent inductance is shown in equation (17): (17) (18) Therefore, the minimum value of the equivalent inductance under this harsh working condition is for , the maximum value for Considering the transmission efficiency and power density of high-power DAB converters, Figure 4 From the relationship curve between inductance and current stress, it can be seen that if the inductance is too small, the current stress will be too large and the equivalent inductance will be magnetically saturated. Select and leave a certain margin, the inductance can be selected .

[0039] Step 5, constructing a current stress optimization equation for the DAB converter, wherein the current stress optimization equation takes current stress minimization as the objective function, transmission power as the equality constraint, and shift ratio constraint and soft switching constraint as the inequality constraint.

[0040] It is understandable that in TPS phase-shift control, the inductor current stress, transmission power and soft switching conditions are all directly affected by the phase shift ratio. Since TPS phase-shift control has three independent phase-shift control variables, it is possible to achieve flexible control.

[0041] In order to minimize the inductor current stress, the inductor current stress should be minimized as the optimization target under the premise of ensuring that the soft switching conditions are met. To this end, an optimization model is established, and the current stress minimization is set as the objective function, and the transmission power is used as the equality constraint. At the same time, the shift phase ratio range and soft switching conditions are used as inequality constraints. This optimization problem uses the KKT (Karush-kuhn-tucker) function as the optimization algorithm for obtaining the minimum current stress, aiming to find the shift phase ratio combination when the current stress is minimum. Therefore, the basic form of the solution equation can be expressed as: (19) in, represents the Lagrange polynomial function; It represents the three shift ratios , and The optimal combination of is the per-unit value of transmission power; are the equality constraint and inequality constraint multipliers respectively, is the soft switching constraint of mode i, for The multiplier of is the equality constraint.

[0042] Step 6: Determine the target operating mode of the DAB converter according to the transmission power per unit value and the voltage transmission ratio K, wherein the target operating mode is one of the three effective operating modes.

[0043] It is understandable that, firstly, it is necessary to determine the current working mode of the DAB converter according to the transmission power per unit value and the voltage transmission ratio K.

[0044] Specifically, if the voltage transfer ratio K ≥ 1, when the transmission power per unit value satisfy When the transmission power per unit value is satisfy , the target working mode is mode 5.

[0045] Similarly, if the voltage transfer ratio is 0<K<1, when the transmission power per unit value satisfy , the target working mode is mode 1; when the transmission power per unit value satisfy , the target working mode is mode 5.

[0046] Step 7, solving the current stress optimization equation of the target working mode, obtaining the optimal combination of the shift ratio of the target working mode, and calculating the minimum current stress.

[0047] It is understandable that after the target operating mode of the DAB converter is determined, the equality constraints and inequality constraints under the target operating mode of the current voltage transfer ratio K are obtained. The equality constraints and inequality constraints under the target operating mode are substituted into the current stress optimization equation to obtain the current stress optimization equation under the target operating mode, and the current stress optimization equation under the target operating mode is solved to obtain the optimal combination of the shift ratio, and the minimum current stress of the DAB converter under the optimal combination of the shift ratio is calculated.

[0048] Among them, when the high-power DAB converter works in mode 1, 4, and 5, since the transmission power range is the same under mode 4 and mode 5, only mode 5 is considered. According to the above equation, the optimal combination of different voltage matching ratios under mode 1 and mode 5 can be solved.

[0049] Under the condition of mode 1, K≥1 is the optimal combination of time shift comparison: (20) At this time, the minimum current stress is related to the voltage transfer ratio K and the expected power value Relationship: (twenty one) Mode 5 is the optimal combination of time-shift comparison under the condition of K≥1: (twenty two) In this mode, the minimum current stress is related to the voltage transfer ratio K and the expected power value when K ≥ 1. Relationship: (twenty three)

[0050] Similarly, the optimal combination of the shift ratios of Mode 1 and Mode 5 when 0<K<1 as well as the minimum current stress and transmission power range can be calculated.

[0051] Mode 1: When 0<K<1: (twenty four) Mode 5: When 0<K<1: (25) According to the above minimum current stress optimization scheme, the TPS phase shift control closed loop block diagram is as follows: Figure 8 As shown. According to the input voltage , output voltage , output current These three quantities are then sampled through the output voltage With reference voltage For comparison, the PI controller output is shifted outward compared to Then the transmission power per unit value is calculated by the voltage matching ratio K between the input and output voltages , the optimal combination of displacement is calculated according to the minimum current stress optimization strategy , , achieving minimum current stress optimization and improving the overall efficiency of high-power DAB converters.

[0052] The simulation waveforms of traditional single-phase shift (SPS) control and optimized three-phase shift (TPS) control under the conditions of input voltage 950V, output voltage 800V and power 100kW are as follows: Fig. 9 and Fig.10 shown.

[0053] The simulation waveforms of traditional single-phase shift (SPS) control and optimized three-phase shift (TPS) control under the conditions of input voltage 950V, output voltage 900V and power 100kW are as follows: Fig.11 and Fig.12 shown.

[0054] The simulation waveforms of traditional single-phase shift (SPS) control and optimized three-phase shift (TPS) control under the conditions of input voltage 800V, output voltage 1035V and power 167kW are as follows: Fig.13 and 14 shown.

[0055] from Figures 9 to 14 It can be seen that under the same conditions, by comparing the optimized three-phase shift (TPS) control with the traditional single-phase shift (SPS) control, the optimized three-phase shift (TPS) has smaller current stress than the traditional phase shift control. Therefore, the optimized three-phase shift (TPS) can stabilize the current stress within a reliable range and improve the transmission efficiency of the converter.

[0056] The present invention provides a method for optimizing the minimum current stress of a dual active bridge DC-DC converter. In a high-power application scenario, in order to enable a DAB converter to obtain the minimum current stress within a full power range, the dynamic relationship between the change of inductance and the change of current stress is considered, and an appropriate inductance is selected accordingly. In addition, by analyzing a triple phase shift control strategy and establishing a corresponding optimization model, the optimal phase shift ratio combination is sought with the global minimization of current stress as the goal, so as to achieve current stress optimization, ensure that the current stress of the DAB converter is always kept within the theoretical expected range during the entire operation process, and ensure the reliability of the switch tube when the converter is in stable operation.

[0057] It should be noted that in the above embodiments, the description of each embodiment has its own emphasis, and for parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0058] Although the preferred embodiments of the present invention have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0059] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for optimizing the minimum current stress of a dual active bridge DC-DC converter, characterized in that: include: Obtain three effective working modes of the DAB converter in the forward and reverse transmission directions under the triple phase-shift control strategy, as well as the phase-shift ratio constraints in each effective working mode; For each transmission direction, based on the voltage transmission ratio K, the inductance value and the three shift ratios, the current stress and the transmission power of the DAB converter are calculated, and the current stress and the transmission power are normalized to obtain the current stress normalized value and the transmission power normalized value; Obtain the soft switching constraints in each valid working mode; Based on the transmission power expression of the single phase-shift control strategy and the transmission power expression of the triple phase-shift control strategy, the inductance value of the DAB converter is determined; Constructing a current stress optimization equation for a DAB converter, wherein the current stress optimization equation takes minimization of current stress as an objective function, takes transmission power as an equality constraint, and takes a shift ratio constraint and a soft switching constraint as inequality constraints; Determining a target operating mode of the DAB converter according to the transmission power per unit value and the voltage transmission ratio K, wherein the target operating mode is one of three effective operating modes; The current stress optimization equation of the target working mode is solved to obtain the optimal combination of the shift ratio of the target working mode and calculate the minimum current stress.

2. The dual active bridge DC-DC converter minimum current stress optimization method according to claim 1, characterized in that: The method of obtaining three effective working modes of the DAB converter in each transmission direction under the triple phase shift control strategy and the phase shift constraint conditions in each effective working mode includes: According to the voltage across the equivalent inductor in the primary equivalent circuit of the DAB converter and the primary and secondary voltages of the DAB transformer , The triple phase shift control is divided into 12 working modes according to the rising and falling edge sequence. The 12 working modes are symmetrical in the process of forward and reverse power transmission, and each transmission direction has 6 different working modes. Each transmission direction has three effective working modes, which are recorded as mode 1, mode 4 and mode 5. Among them, the phase shift ratio constraint of mode 1 is: , the shift constraint of mode 4 is: , the shift constraint of mode 5 is: .

3. The method for optimizing the minimum current stress of a dual active bridge DC-DC converter according to claim 1, characterized in that: The method of calculating the current stress and transmission power of the DAB converter based on the voltage transfer ratio K, the inductance value and the three shift ratios includes: The inductor current at four time nodes in the first half cycle of the steady-state operation of the DAB converter in each effective working mode is calculated, where the four time nodes are represented by shift phases and half switching cycles: , , , , where the half-switching duty cycle of the DAB converter is ; According to the DAB converter inductor current has symmetry in one cycle, in half cycle , according to the working state of the DAB converter in mode 1, the inductor current expression is derived as follows: ； The inductor current expression at four time nodes is further derived as follows: ； The maximum value of the inductor current in one cycle is defined as the current stress of the DAB converter, which is expressed as follows: ； Then, when the voltage transfer ratio is 0<K<1, the expression of the inductor current stress in mode 1 is: ； When the voltage transfer ratio K>1, the expression of the inductor current stress in mode 1 is: ； According to the input port voltage and the inductor current Calculate the transmission power of the DAB converter: ； The calculation methods of the current stress and the transmission power in other modes are the same as those in mode 1.

4. The method for optimizing the minimum current stress of a dual active bridge DC-DC converter according to claim 3, characterized in that: The current stress and the transmission power are normalized to obtain a per-unit value of the current stress and a per-unit value of the transmission power, including: Calculate the current stress baseline value , as follows: ； The maximum transmission power of single phase shift control is taken as the transmission power reference value, as shown in the following formula: ； The per-unit value expression of the current stress of the DAB converter in mode 1 is: ； The per-unit transmission power expression of the DAB converter in mode 1 is: 。 5. The dual active bridge DC-DC converter minimum current stress optimization method according to claim 1, characterized in that: The soft switching constraint conditions in each effective working mode are constructed, including: The soft switching constraints of mode 1 are: ； The soft switching constraints of mode 4 are: ； The soft switching constraints of mode 5 are: 。 6. The dual active bridge DC-DC converter minimum current stress optimization method according to claim 1, characterized in that: The transmission power expression based on the single phase shift control strategy and the transmission power expression based on the triple phase shift control strategy, determining the inductance value of the DAB converter, includes: The expression of transmission power under single phase shift control is as follows: ； when When it is equal to 0.5, the transmission power of the DAB converter can reach the maximum, and the maximum inductance at the maximum transmission power is The expression is as follows: ； ； in, and are the DC input voltage and output voltage of the DAB converter respectively, and the high-frequency transformer ratio is , L is the inductance value, represents the internal shift ratio of the primary side of the DAB converter, It indicates the displacement ratio between the primary side and the secondary side, also called external displacement ratio. is the internal transfer ratio of the secondary side of the DAB converter, K is the voltage transfer ratio, is the operating frequency of the switch tube, is the maximum transmission power; The minimum inductance of the magnetic component is achieved when zero voltage is turned on under the triple phase shift control strategy The expression is as follows: ； According to the maximum inductance Expression and minimum value of inductance The range of equivalent inductance obtained by the expression is as follows: ； ； in, is the proportionality coefficient, 0.5≤ <1.

7. The method for optimizing the minimum current stress of a dual active bridge DC-DC converter according to claim 1, characterized in that: The constructing of the current stress optimization equation under the target working mode includes: ； in, represents the Lagrange polynomial function; It represents the three shift ratios , and The optimal combination of is the per-unit value of transmission power; are the equality constraint and inequality constraint multipliers respectively, is the soft switching constraint of mode i, for The multiplier of is the equality constraint.

8. The dual active bridge DC-DC converter minimum current stress optimization method according to claim 1, characterized in that: Determining the target operating mode of the DAB converter according to the transmission power per unit value and the voltage transmission ratio K includes: If the voltage transfer ratio K ≥ 1, when the transmission power per unit value satisfy When the transmission power per unit value is satisfy , the target working mode is mode 5; If the voltage transfer ratio is 0<K<1, when the transmission power per unit value satisfy , the target working mode is mode 1; when the transmission power per unit value satisfy , the target working mode is mode 5.

9. The dual active bridge DC-DC converter minimum current stress optimization method according to claim 8, characterized in that: The step of solving the current stress optimization equation of the target working mode, obtaining the optimal combination of the shift ratio of the target working mode, and calculating the minimum current stress includes: Under mode 1, when the voltage transfer ratio K≥1, the optimal combination of the shift ratio is: The minimum current stress is: ； Under mode 5, when the voltage transfer ratio K≥1, the optimal combination of the shift ratio is: The minimum current stress is: ； Under mode 1, when the voltage transfer ratio is 0<K<1, the optimal combination of the shift ratio is: ； The minimum current stress is: ； Under mode 5, when the voltage transfer ratio is 0<K<1, the optimal combination of the shift ratio is: ； The minimum current stress is: 。

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