Wide working range multi-objective optimization method and system based on dual active converters

Abstract

The invention relates to the field of dual-active bridge type converters, in particular to a wide-working-range multi-objective optimization method based on a dual-active bridge type converter, and the method comprises the steps: calculating a voltage conversion ratio through an input voltage and an output voltage, and determining an external phase shift angle ratio of a primary side H bridge to a secondary side H bridge of the dual-active bridge type converter according to a relation between the output voltage and a set value. And judging the working mode of the dual-active bridge converter according to the maximum transmission power and the current transmission power. And calculating a current effective value according to the voltage conversion ratio and the external phase shift angle ratio, taking the current effective value as an optimization target, and optimizing the current effective value and a current peak value by using an NSGA-II optimization function in combination with a working mode to obtain an optimal modulation scheme. In addition, based on the optimal modulation scheme, a trigger signal of each switching tube of the dual active bridge converter is generated in the DSP. According to the invention, the soft switching range can be expanded, the switching loss of the switching tube in the dual-active bridge converter is effectively reduced, and the overall efficiency of the converter is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of dual-active bridge converters, and in particular to a wide working range multi-objective optimization method and system based on dual-active converters. Background Art

[0002] In order to achieve the establishment of an environmentally friendly society and the efficient use of sustainable energy, distributed energy storage systems have received widespread attention from many scholars and enterprises. For distributed energy storage systems, the efficient operation of the DC interface circuit will realize the high-efficiency charging and discharging process of batteries or supercapacitor components. Due to the simple circuit structure, high power density, strong bidirectional power transmission capability, strong wide-range voltage regulation capability, and simple control scheme, dual-active bridge DC converters are often used as the interface circuit of energy storage systems. At the same time, due to the presence of high-frequency transformers, current isolation on both sides of the converter can be achieved.

[0003] In recent years, with the rapid development of power electronics technology, isolated DC-DC converters have been widely used in distributed energy storage systems, DC transmission, DC energy conversion, and electric vehicles. Due to its simple modulation structure, strong bidirectional transmission capability, and high power density, the dual-active bridge converter has become one of the most popular topologies in interface circuits. In DC distributed power systems, isolated bidirectional DC-DC converters play an important role in power flow modulation and output voltage maintenance. At the same time, the primary and secondary sides are connected through a high-frequency transformer to improve the power density.

[0004] How to effectively modulate the dual-active bridge DC converter, usually using phase shift modulation to perform power control operations on the dual-active bridge DC converter, including single-phase phase shift modulation, extended phase shift modulation, double phase shift modulation and three-phase phase shift modulation. The optimization objectives usually select peak current, effective current, return power and soft switching range, thereby improving the working efficiency of the dual-active bridge DC converter. However, for traditional phase shift modulation strategies, the optimization process rarely combines a wider soft switching area with lower current stress. Optimizing the current will lead to a narrower soft switching range, and expanding the soft switching range will increase the current stress accordingly. At the same time, the influence of nonlinear changes in junction capacitance loss is ignored, and the analysis of the resonance process and dead time is simple and insufficient. Summary of the invention

[0005] In view of this, the purpose of the present invention is to propose a wide operating range multi-objective optimization method and system based on a dual active converter to solve the problem that the existing optimization method cannot take into account both the soft switching range and the current stress, and the analysis of the resonance process and the dead time is insufficient.

[0006] Based on the above object, the present invention provides a wide operating range multi-objective optimization method based on a dual active bridge converter, comprising the following steps:

[0007] S1, obtaining an input voltage, an output voltage and an output voltage reference value of a dual active bridge converter;

[0008] S2, calculating the voltage matching ratio according to the input voltage, the output voltage and the transformer ratio, and obtaining the external phase shift angle ratio between the primary full bridge and the secondary full bridge of the dual active bridge converter according to the output voltage and the output voltage reference value;

[0009] S3, obtaining the duty cycle of the primary full bridge and the internal phase shift angle ratio of the primary full bridge according to the voltage matching ratio and the external phase shift angle ratio;

[0010] S4, obtaining the maximum transmission power of the dual active bridge converter and the power per unit value currently required to be transmitted, and determining the working mode of the dual active bridge converter according to the power per unit value and the voltage matching ratio requirement;

[0011] S5. According to the working mode of the dual active bridge converter and the basic characteristics of different power switch tubes, the minimum dead zone boundary condition that can realize the soft switching of the power switch tube is analyzed and obtained;

[0012] S6. Calculate the effective current value and the peak-to-peak current value according to the voltage matching ratio and the cycles of different working modes. Take the effective current value and the peak-to-peak current value as the optimization target, and use the NSGA-II optimization function to combine the working mode boundary and the soft switching working boundary to solve and obtain the optimal modulation scheme.

[0013] S7. Based on the obtained optimal modulation scheme, a trigger signal for each switch tube of the dual active bridge converter is generated in the DSP.

[0014] Preferably, in step S2, the formula for calculating the voltage matching ratio is:

[0015]

[0016] Among them, k is the voltage matching ratio; V1 is the input voltage, V2 is the output voltage, and n is the high-frequency transformer ratio.

[0017] Preferably, obtaining the external phase shift angle ratio between the primary full bridge and the secondary full bridge of the dual active bridge converter according to the output voltage and the output voltage reference value includes:

[0018] The output voltage is subtracted from the output voltage reference value, and based on the difference between the output voltage and the output voltage reference value, the external phase shift angle ratio between the primary full-bridge power switch tube drive signal and the secondary full-bridge power switch tube drive signal is obtained through a proportional-integral controller.

[0019] Preferably, obtaining the duty cycle of the primary full bridge and the internal phase shift angle ratio of the primary full bridge includes:

[0020] According to the values ​​of the external phase shift angle ratio and the voltage matching ratio, the duty cycle of the primary full bridge and the internal phase shift angle ratio of the primary full bridge are obtained through an offline lookup table.

[0021] Preferably, in step S4, the working mode of the dual active bridge converter is a working mode under an improved asymmetric modulation strategy, including four types:

[0022] Working mode 1: The switching sequence is D2T S ,D1T S ,(D2+D3)T S ,(1+D2-D3)T S ,(1-D1)T S and T S ;

[0023] Working mode 2: The switching sequence is D2T S ,D1T S ,(D2+D3)T S ,(1-D1)T S ,(1+D2-D3)T S and T S ;

[0024] Working mode three: the switching sequence is D2T S ,(D2+D3)T S ,D1T S ,(1-D1)T S ,(1+D2-D3)T S and T S ;

[0025] Working mode 4: The switching sequence is D2T S ,D1T S ,(1-D1)T S ,(D2+D3)T S ,(1+D2-D3)T S and T S ;

[0026] D1 is the internal phase shift angle ratio between the driving signal of the primary full-bridge switch tube S1 and the driving signal of the switch tube S4; D2 is the external phase shift angle ratio between the primary full-bridge switch tube S1 and the secondary full-bridge switch tube Q1 of the dual active bridge converter; D3 is the duty cycle of the driving signal of the primary full-bridge switch tube S1 in the primary full-bridge;

[0027] S1, S4 and Q1 are the numbers of the primary side full-bridge switch tube and the secondary side full-bridge switch tube respectively.

[0028] Preferably, according to the working mode of the dual active bridge converter and the basic characteristics of different power switch tubes, the minimum dead zone boundary conditions that can realize the soft switching of the power switch tube are analyzed to include:

[0029] According to the working mode division of the dual active bridge converter and the basic characteristics of the power switch tube, the basic conditions for the switch tube to achieve soft switching are analyzed, and the minimum boundary current value that can achieve soft switching of the switch tube is calculated according to the law of conservation of energy. By analyzing the resonance process of the inductor and the junction capacitance on both sides of the power switch tube, the mathematical expression of the resonance time is derived, and the minimum boundary value of the required dead time is obtained.

[0030] Preferably, in step S6, the formula for calculating the effective current value and the peak-to-peak current value is:

[0031]

[0032] Among them, i RMS is the effective value of current, i peak is the peak-to-peak value of the current, k is the voltage matching ratio, D1 is the inner phase shift angle ratio between the driving signal of the primary full-bridge switch tube S1 and the driving signal of the switch tube S4; D2 is the outer phase shift angle ratio between the primary full-bridge switch tube S1 and the secondary full-bridge switch tube Q1 of the dual active bridge converter; D3 is the duty cycle of the driving signal of the primary full-bridge switch tube S1 in the primary full-bridge, S1, S4 and Q1 are the numbers of the primary full-bridge switch tube and the secondary full-bridge switch tube respectively.

[0033] Preferably, the NSGA-II function is as follows:

[0034]

[0035] Among them, f(x) is the optimization target, x is the relevant variable that affects the solution target, Ax≤b is the non-equality constraint of the target to be optimized, A eq x=b eq is the equality constraint of the objective to be optimized.

[0036] Preferably, the NSGA-II function includes inequality constraints and equality constraints;

[0037] The inequality constraints include the minimum current value that can achieve soft turn-on of the switch and the boundary conditions of different working modes:

[0038] The minimum current value that can achieve soft turning on of the switch tube is as follows:

[0039]

[0040] Where V2 is the output voltage value; Q ossis the amount of charge flowing through the two sides of the power switch tube in one switching cycle; k is the voltage matching ratio of the dual active bridge converter; n is the voltage transformer ratio; L is the connection inductance value.

[0041] The equality constraint formula is as follows:

[0042] P=P ref

[0043] Where P is the per-unit value of the transmission power; P ref is the reference value of transmission power.

[0044] The present invention also provides a wide operating range multi-objective optimization system based on a dual active bridge converter, comprising:

[0045] Data sampling module: used to obtain the input voltage, output voltage and output voltage reference value of the dual active bridge converter;

[0046] Data analysis module: used to calculate the voltage matching ratio according to the input voltage, output voltage and transformer ratio, obtain the external phase shift angle ratio between the primary full bridge and the secondary full bridge of the dual active bridge converter according to the output voltage and the output voltage reference value, and obtain the duty cycle of the primary full bridge and the internal phase shift angle ratio of the primary full bridge according to the voltage matching ratio and the external phase shift angle ratio;

[0047] Optimal modulation strategy acquisition module: used to obtain the mathematical expressions of the effective current value and the peak-to-peak current according to the voltage matching ratio and the cycles of different working modes. Taking the effective current value and the peak-to-peak current as the optimization target, the optimal modulation strategy working point is optimized and solved through the NSGA-II optimization function, combined with the working mode boundary conditions and soft switching constraints, to obtain the optimal modulation scheme;

[0048] Driving signal acquisition module: used to obtain the driving signals of all switching tubes of the dual active bridge converter in the DSP according to the optimal modulation scheme.

[0049] Beneficial effects of the present invention: The multi-objective optimization method proposed in the present invention calculates the voltage conversion ratio through the input voltage and the output voltage, and determines the external phase shift angle ratio of the primary H-bridge and the secondary H-bridge of the dual active bridge converter according to the relationship between the output voltage and the set value. The working mode of the dual active bridge converter is judged according to the maximum transmission power and the current transmission power. Then, the effective value of the current is calculated according to the voltage conversion ratio and the external phase shift angle ratio, and the effective value of the current is used as the optimization target. The NSGA-II optimization function is combined with the working mode to optimize the effective value of the current and the current peak value to obtain the optimal modulation scheme. In addition, based on the improved asymmetric duty cycle modulation strategy, the trigger signal of each switch tube of the dual active bridge converter is generated in the DSP. The unilateral asymmetric duty cycle modulation strategy of the present invention can effectively reduce the current stress, expand the working range of the soft switch, and thus significantly reduce the switching loss of the switch tube in the dual active bridge converter, and improve the overall efficiency of the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings in the following description are only for the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0051] Figure 1 A flow chart of a multi-objective optimization method according to an embodiment of the present invention;

[0052] Figure 2 A flowchart of optimization using the NSGA-II function according to an embodiment of the present invention;

[0053] Figure 3 Schematic diagram of the topological structure of a dual active bridge converter according to an embodiment of the present invention;

[0054] Figure 4 Schematic diagram of ZVS analysis at each switching state switching moment under the novel asymmetric modulation of an embodiment of the present invention, wherein: Figure 4 (a) is from time t_1 to time t_1+t_db, Figure 4 (b) is from time t_2 to time t_2+t_db, Figure 4 (c) is from time t_3 to time t_3+t_db, Figure 4 (d) is from time t_4 to time t_4+t_db, Figure 4 (e) is from time t_5 to time t_5+t_db, Figure 4 (f) is from time t_6 to time t_6+t_db;

[0055] Figure 5is a waveform diagram of working mode 1 of an embodiment of the present invention;

[0056] Figure 6 2 is a waveform diagram of working mode 2 of an embodiment of the present invention;

[0057] Figure 7 3 is a waveform diagram of working mode 3 of an embodiment of the present invention;

[0058] Figure 8 is a waveform diagram of working mode 4 of an embodiment of the present invention;

[0059] Fig. 9 A comparison diagram of the effective current values ​​of the present invention and other modulation strategies under different voltage matching ratios of an embodiment of the present invention;

[0060] Fig.10 A comparison diagram of current peak values ​​of the present invention and other modulation strategies under different voltage matching ratios of an embodiment of the present invention;

[0061] Fig.11 This is a comparison diagram of the efficiency of the present invention and other modulation strategies according to an embodiment of the present invention;

[0062] Fig.12 A control execution block diagram of an embodiment of the present invention;

[0063] Fig.13 4 is a system module diagram of an embodiment of the present invention. DETAILED DESCRIPTION

[0064] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments.

[0065] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0066] like Figure 1As shown, the embodiment of this specification provides a wide working range multi-objective optimization method based on a dual active converter, comprising the following steps:

[0067] S1. Determine the input voltage, output voltage and output voltage reference value of the dual active bridge converter;

[0068] S2. Calculate the voltage matching ratio by input voltage, output voltage and transformer ratio;

[0069] The voltage matching ratio formula is as follows:

[0070]

[0071] Among them, k is the voltage matching ratio; V1 is the input voltage, V2 is the output voltage, and n is the high-frequency transformer ratio.

[0072] According to the output voltage and the output voltage reference value, the external phase shift angle ratio between the primary and secondary full bridges of the dual active bridge converter is obtained, which is as follows:

[0073] The output voltage of the secondary full-bridge is collected, and the output voltage is subtracted from the output voltage reference value. Based on this difference, the external phase shift angle ratio between the primary full-bridge power switch tube drive signal and the secondary full-bridge power switch tube drive signal is calculated through a proportional-integral controller.

[0074] S3. According to the voltage matching ratio and the external phase shift angle ratio reference value, the corresponding duty cycle of the primary full bridge and the internal phase shift angle ratio of the primary full bridge are obtained by an offline lookup table, as follows:

[0075] According to the values ​​of the external phase shift angle ratio and the voltage matching ratio, the corresponding duty cycle of the primary full bridge and the internal phase shift angle ratio of the primary full bridge are found in the offline lookup table.

[0076] S4. Calculate the maximum transmission power required by the dual active bridge converter under different working conditions, and determine the working mode of the dual active bridge converter according to the required transmission power and voltage matching ratio as follows:

[0077] Working mode 1: The switching sequence is D2T S ,D1T S ,(D2+D3)T S ,(1+D2-D3)T S ,(1-D1)T S and T S ;

[0078] Working mode 2: The switching sequence is D2T S ,D1T S ,(D2+D3)T S ,(1-D1)T S ,(1+D2-D3)TS and T S ;

[0079] Working mode 3: The switching sequence is D2T S ,(D2+D3)T S ,D1T S ,(1-D1)T S ,(1+D2-D3)T S and T S ;

[0080] Working mode 4: The switching sequence is D2T S ,D1T S ,(1-D1)T S ,(D2+D3)T S ,(1+D2-D3)T S and T S .

[0081] The dual active bridge converter has four working modes under the new asymmetric modulation strategy, and the transmission power of each working mode is shown in the table;

[0082]

[0083] S5. According to the different working modes of the selected dual active bridge converter, in order to achieve soft turn-on of the power switch tube, the non-ideal factors of the incidental junction capacitance are considered, and the charging and discharging process of the switch tube junction capacitance is analyzed in detail to determine the boundary conditions of the minimum dead zone, as follows:

[0084] According to the selected working mode and transmission power of the dual active bridge converter, many non-ideal factors are analyzed. The switching process of each power tube is modeled through the law of conservation of energy and the resonant charging and discharging process of the junction capacitance of each switch tube, so as to obtain the minimum boundary current and circuit resonance time, and determine the minimum boundary value of the dead zone to ensure the realization of soft switching.

[0085] S6. The optimization target is selected as the effective value of the current and the peak-to-peak value of the current. The effective value of the current and the peak-to-peak value of the current are calculated using the voltage matching value and the cycle of different working modes. The optimal modulation strategy is solved by combining the minimum value of the switching current with the NSGA-II optimization function. The formula for the effective value of the current and the peak-to-peak value of the current are as follows, see Figure 2 The figure optimized by using NSGA-II function in the present invention:

[0086]

[0087] Among them, D1 is the inner phase shift angle ratio between the primary full-bridge switch tube S1 drive signal and the switch tube S4 drive signal; D2 is the outer phase shift angle ratio between the primary full-bridge switch tube S1 and the secondary full-bridge switch tube Q1 of the dual active bridge converter; D3 is the duty cycle of the primary full-bridge switch tube S1 drive signal in the primary full bridge.

[0088] The NSGA-II function optimization mathematical model is as follows:

[0089]

[0090] Among them, f(x) is the optimization target, x is the relevant variable that affects the solution target, Ax≤b is the non-equality constraint of the target to be optimized, A eq x=b eq is the equality constraint of the objective to be optimized.

[0091] The NSGA-II function includes inequality constraints and equality constraints;

[0092] The inequality constraints mainly include the minimum current value that can achieve soft turn-on of the switch tube, as shown below:

[0093] Based on the law of conservation of energy, the minimum value of the switching current is as follows:

[0094]

[0095] Where V2 is the output voltage value; Q oss is the amount of charge flowing through the two sides of the power switch tube in one switching cycle; k is the voltage matching ratio of the dual active bridge converter; n is the voltage transformer ratio; L is the connection inductance value.

[0096] The equality constraint formula is as follows:

[0097] P=P ref

[0098] Where P is the per-unit value of the transmission power; P ref is the per-unit reference value of the transmission power;

[0099] S7. According to the novel asymmetric duty cycle modulation strategy, the trigger signals of the various switch tubes of the dual active bridge converter are obtained in the DSP.

[0100] The present invention is based on the NSGA-II function in MATLAB and uses the root mean square current optimization technology to achieve lower peak current and root mean square current under the new asymmetric modulation. The influence of non-ideal factors such as switching transient current value, dead time and nonlinear change of junction capacitance on the soft switching characteristics of the new asymmetric modulation strategy is studied. The proposed optimization strategy can achieve the lowest peak current and effective value current, while widening the soft switching range. Therefore, the present invention can analyze the influence of non-ideal factors on the soft switching characteristics of the switch tube, and effectively optimize the current stress in the dual active bridge converter through the NSGA-II algorithm, thereby improving its working efficiency.

[0101] Embodiment 1:

[0102] Topology Figure 3 As shown, it includes two symmetrical H bridges connected to a high-frequency transformer through an inductor L, where the high-frequency transformer is referred to as HFT. The turns ratio of the transformer is n, the input voltage is denoted as V1, and the output voltage is denoted as V2. In the equivalent circuit model of DAB, the equivalent AC output voltages on the primary and secondary sides are marked as V h1 and V h2 . Ts represents the switching period of the switch tube. Switches S1, S2, S3, and S4 are located on the primary H-bridge, and switches Q1, Q2, Q3, and Q4 are located on the secondary H-bridge. DSi and DQi are the reverse parallel diodes of the primary H-bridge and the secondary H-bridge, respectively, while CSi and CQi are the junction capacitances of the corresponding switches on the primary H-bridge and the secondary H-bridge.

[0103] By controlling the inner phase shift angle D1 between the trigger signal of the primary H-bridge switch tube S1 and the trigger signal of the switch tube S4, and the outer phase shift angle D2 between the primary H-bridge switch tube S1 and the secondary H-bridge switch tube Q1, combined with the duty cycle D3 of the primary H-bridge switch tube S1, the current stress under the same transmission power can be reduced. The new asymmetric modulation strategy helps to expand the zero voltage switching (ZVS) range of the switch tube in the dual active bridge converter, thereby effectively improving the efficiency of the dual active bridge converter and realizing the efficient operation of the dual active bridge controller. See Figure 4 , which shows a schematic diagram of ZVS analysis at each switch state switching moment under unilateral asymmetric modulation of the present invention.

[0104] The output quantity obtained by the present invention through control is the switch trigger signal of the dual active bridge converter, wherein the trigger signals of the two switch tubes on each half bridge are complementary. Therefore, the trigger signals of all switch devices in the dual active bridge converter can be determined by obtaining D1, D2 and D3.

[0105] The dual active bridge converter of the present invention has four working modes, which are distinguished by the sequence of switching moments, specifically including: working mode 1, the switching sequence is D2TS, D1TS, (D2+D3)TS, (1+D2-D3)TS, (1-D1)TS and TS; working mode 2, the switching sequence is D2TS, D1TS, (D2+D3)TS, (1-D1)TS, (1+D2-D3)TS and TS; working mode 3, the switching sequence is D2TS, (D2+D3)TS, D1TS, (1-D1)TS, (1+D2-D3)TS and TS; working mode 4, the switching sequence is D2TS, D1TS, (1-D1)TS, (D2+D3)TS, (1+D2-D3)TS and TS. See Figure 5 This is the waveform of working mode 1; see Figure 6 This is the waveform of working mode 2; see Figure 7 This is the waveform of working mode 3; see Figure 8 This is the waveform of working mode 4.

[0106] According to the working mode of the dual active bridge converter and the basic characteristics of different power switch tubes, in order to achieve the soft turn-on of the power switch tube, various non-ideal factors are analyzed, and the minimum boundary current value at each switching moment is determined, thereby obtaining the minimum dead zone boundary condition that can achieve soft switching. At the same time, the charging and discharging process of the switch tube junction capacitance is analyzed in detail.

[0107] According to the voltage conversion ratio and different modulation degrees of freedom, the effective value and peak-to-peak value of the current are obtained. Taking these two values ​​as optimization targets, combined with the NSGA-II optimization function, the working mode boundary and the soft switching working boundary, the optimal modulation scheme is solved.

[0108] The present invention deeply analyzes the relationship between dead time, junction capacitance loss, current value at the turn-off time, inductance value and voltage gain. On this basis, the resonance process is studied and the range of dead time is estimated. Therefore, by reasonably setting the inductor current and dead time, the charge in the junction capacitance loss can be fully released, thereby achieving ZVS of a certain switch. In addition, the nonlinear change of junction capacitance can also be calculated to obtain the equivalent capacitance at a specific voltage, and the formula is as follows.

[0109]

[0110] Generally speaking, the energy stored in the inductor L should be large enough to charge and discharge the switch junction capacitance. For example, after consuming the energy in the junction capacitance cs1, the drain-source voltage of the switch S1 becomes zero, thereby achieving ZVS of S1. Accordingly, it is necessary to ensure that the minimum iL value when the switch device is turned on or off is I minIn order to provide enough energy for ZVS. At the same time, the dead time should be enough for the junction capacitance to complete charging and discharging.

[0111] To calculate the minimum current I min , the ZVS model at each switching moment is as follows Figure 4 The minimum energy stored in the inductor when the switch is turned off is equal to the energy of the DC power supply V1 and nV2 during the dead time t db Energy consumed internally.

[0112] Depend on Figure 4 From the description, we can see that at time t2, S1 is turned off and S2 is turned on. The direction of the inductor current is positive, and the current flows through the anti-parallel diode Ds3. db During this period, the inductor current iL starts to charge the junction capacitance Cs1 of S1 and discharge the junction capacitance Cs2 of S2. Figure 4 d, in this process, the energy in the junction capacitance Cs2 of S2 is transferred to the junction capacitance Cs1 of S1. The minimum energy stored in L is consumed by the voltage sources V1 and nV2. In addition, at t = t2 and t = t2 + t db When , the voltage in Cs1 and Cs2 is as follows.

[0113]

[0114] Among them, u cs3 The voltage on the junction capacitance Cs3 of S3; u cs4 is the voltage on the junction capacitance Cs4 of S4; u(t) is the junction capacitance voltage of different switch tubes at different times.

[0115] From t2 to t2+t db During this period, the inductor current expression is:

[0116]

[0117] In the formula, C oss is the value of the junction capacitance.

[0118] According to the energy balance principle, from t2 to t2+t db The energy relationship can be expressed as:

[0119]

[0120] in, is the energy stored in the junction capacitance and inductance at time t2 (supplement); E delivered is the energy consumed by the voltage source.

[0121] At t2, for the ZVS model, the total energy including the energy stored in cs2 and inductor L can be expressed as:

[0122]

[0123] in, is the energy stored in cs2; E L (I L ) is the energy stored in the inductor L (supplement).

[0124] At t2, the minimum energy stored in the inductor L can be expressed as:

[0125]

[0126] In the formula, I min To achieve the minimum inductor current value of ZVS.

[0127] At t2+t db When , for the ZVS model, the total energy includes the energy stored in cs1:

[0128]

[0129] in, is the energy stored in the junction capacitance and inductance at time t2 (supplement); is the energy stored in cs1 (replenishment).

[0130] The energy consumed by a voltage source can be expressed as:

[0131]

[0132] The minimum current value to achieve ZVS can be expressed as:

[0133]

[0134] Among them, I min To achieve the minimum current value of ZVS, V2 is the output voltage; k is the voltage conversion ratio; L is the inductance value.

[0135] In order to achieve zero voltage switching of all semiconductors, the current value at each switching moment should meet the limits in Table 1.

[0136] Table 1 ZVS boundaries at each switching moment:

[0137]

[0138] See also Figure 2 , showing the process of using NSGA-II optimization function to optimize the current effective value and current peak in the soft switching boundary establishment and dead zone optimization method of the dual active bridge converter.

[0139] The present invention takes the effective value of current and the peak value of current as the optimization target, and optimizes with the help of NSGA-II optimization function, see Figure 2 Graph optimized using the NSGA-II optimization function in the present invention.

[0140] The optimal modulation scheme is solved by combining the working mode boundary and the soft switching working boundary through the NSGA-II optimization function. The formula of the effective current value and the peak-to-peak current value are as follows. Figure 2 The figure optimized by using NSGA-II function in the present invention:

[0141]

[0142] Among them, k is the voltage matching ratio, D1 is the inner phase shift angle ratio between the primary full-bridge switch tube S1 drive signal and the switch tube S4 drive signal; D2 is the outer phase shift angle ratio between the primary full-bridge switch tube S1 and the secondary full-bridge switch tube Q1 of the dual active bridge converter; D3 is the duty cycle of the primary full-bridge switch tube S1 drive signal in the primary full bridge.

[0143] The NSGA-II function optimization mathematical model is as follows:

[0144]

[0145] Among them, f(x) is the optimization target, x is the relevant variable that affects the solution target, Ax≤b is the non-equality constraint of the target to be optimized, A eq x=b eq is the equality constraint of the objective to be optimized.

[0146] The NSGA-II function includes inequality constraints and equality constraints;

[0147] The inequality constraints mainly include the minimum current value that can achieve soft-turn on of the switch tube and the boundary conditions of different working modes:

[0148] In order to achieve soft turn-on of the dual active bridge converter during operation to reduce switching losses and improve the operating efficiency of the dual active bridge converter, at the switching moment of each switch tube, the current flowing through the switch tube has constraints:

[0149]

[0150] The equality constraint formula is as follows:

[0151] P=P ref

[0152] See also Fig.12 , is a flow chart of the unilateral asymmetric duty cycle modulation method of the present invention, and its specific implementation process includes the following steps:

[0153] Step 1, measuring the input voltage V1 and the output voltage V2 of the dual active bridge converter;

[0154] Step 2: Calculate the voltage conversion ratio k based on the measured input voltage V1 and output voltage V2, and compare the output voltage V2 with the output voltage given value V 2ref , after making a difference, the external phase shift angle ratio D2 of the primary side H-bridge switch tube S1 trigger signal and the secondary side H-bridge switch tube Q1 trigger signal is obtained through the proportional integral controller, and the duty cycle D3 of the primary side full bridge and the internal phase shift angle ratio D1 of the primary side full bridge are obtained through the offline lookup table;

[0155] Step 3: Determine the power that the current converter needs to transmit, and compare it with the maximum transmission power to obtain the transmission power per unit value P ref , based on which the boundary constraints of the current working mode are determined to select the working mode;

[0156] Step 4: Taking the effective current value and the peak current value as the optimization targets, the optimization is solved by the NSGA-II optimization function, where the equality constraint is determined by the per-unit value of the transmission power, and the inequality constraints are the soft switching boundary constraint and the working mode operation boundary constraint, respectively, to obtain the optimal modulation scheme (D1(opt), (D2(opt), (D3(opt));

[0157] Step 5, after obtaining the optimal modulation scheme (D1(opt), (D2(opt), (D3(opt)), according to the improved asymmetric duty cycle modulation strategy, obtain the trigger signals of all switch tubes S1, S2, S3, S4, Q1, Q2, Q3, Q4 of the dual active bridge converter.

[0158] The present invention compares and analyzes the current effective value and efficiency of several traditional symmetrical phase-shift modulation strategies and the unilateral asymmetrical duty cycle modulation strategy proposed by the present invention under the same transmission power conditions. Fig. 9 As shown, under different voltage conversion ratios k, the unilateral asymmetric duty cycle modulation strategy and its optimization method proposed in the present invention exhibit the lowest current effective value. Fig.10 It shows that under different voltage conversion ratios k, the strategy and optimization method of the present invention also have the lowest current peak value. Fig.11 In the efficiency comparison, the unilateral asymmetric duty cycle modulation strategy and its optimization method of the present invention show the highest efficiency, which indirectly verifies that the optimization method can realize the soft switching of the switch tube to the greatest extent. In summary, the unilateral asymmetric duty cycle modulation strategy of the present invention can effectively reduce current stress, expand the working range of soft switching, and thus significantly reduce the switching loss of the switch tube in the dual active bridge converter, and improve the overall efficiency of the converter.

[0159] Based on the above method, the present invention also discloses a multi-objective optimization system based on a dual active bridge converter, such as Fig.13 As shown, it includes a data sampling module, a data analysis module, an optimal modulation strategy acquisition module and a driving signal acquisition module;

[0160] Data sampling module: used to determine the input voltage, output voltage and output voltage reference value of the dual active bridge converter;

[0161] Data analysis module: used to obtain the voltage matching ratio according to the input voltage and the output voltage, obtain the external phase shift angle ratio of the primary full bridge and the secondary full bridge of the dual active bridge converter according to the output voltage and the output voltage given value, and obtain the duty cycle of the primary full bridge and the internal phase shift angle ratio of the primary full bridge through an offline lookup table according to the values ​​of the external phase shift angle ratio and the voltage matching ratio;

[0162] Optimal modulation strategy acquisition module: used to obtain the mathematical expressions of the effective current value and the peak-to-peak current value according to the voltage matching ratio and several different modulation degrees of freedom. Taking the effective current value and the peak-to-peak current value as the optimization target, the optimal modulation strategy working point is optimized and solved through the NSGA-II optimization function, combined with the working mode boundary conditions and soft switching constraints, to obtain the optimal modulation scheme;

[0163] Driving signal acquisition module: used to obtain driving signals of all switch tubes of the dual active bridge converter in DSP according to the improved asymmetric modulation strategy.

[0164] See also Fig.12 In another feasible embodiment of the present invention, the following is adaptively modified according to the situation. It includes a data sampling module, a data analysis module, an optimal modulation strategy acquisition module and a drive signal acquisition module.

[0165] The data acquisition module is used to determine the input voltage, output voltage and output voltage reference value of the dual active bridge converter. The data analysis module calculates the voltage matching ratio according to the input voltage and the output voltage, and determines the external phase shift angle ratio of the primary full bridge and the secondary full bridge of the dual active bridge converter according to the difference between the output voltage and the set reference value. By combining the external phase shift angle ratio and the voltage matching ratio, the duty cycle and the internal phase shift angle ratio of the primary full bridge are obtained by using an offline lookup table. The optimal modulation strategy acquisition module derives the mathematical expressions of the effective current value and the peak-to-peak current value based on the voltage matching ratio and multiple modulation degrees of freedom. Taking these two values ​​as optimization targets, the NSGA-II optimization algorithm is used to optimize the working point of the modulation strategy in combination with the working mode boundary conditions and soft switching constraints to obtain the optimal modulation scheme. The drive signal acquisition module generates the drive signal of each switch tube of the dual active bridge converter in the DSP according to the improved asymmetric modulation strategy. The present invention can analyze the influence of non-ideal factors on the soft switching characteristics of the switch tube, and effectively optimize the current stress in the dual active bridge converter through the NSGA-II algorithm, thereby improving its working efficiency.

[0166] In summary, the method and system of the present invention are intended to optimize switching losses. After comprehensive analysis, a method for accurately establishing the zero voltage switching boundary and optimizing the dead time is proposed. On this basis, the peak current and the root mean square current are optimized using the NSGA-II optimization function, and lower peak current and root mean square current under improved asymmetric modulation are successfully achieved. By comparing with the existing modulation scheme, the proposed optimization method can achieve the lowest peak current, the lowest effective current and the widest ZVS range.

[0167] Those skilled in the art should understand that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of the present invention is limited to these examples; under the concept of the present invention, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other changes in different aspects of the present invention as described above, which are not provided in detail for the sake of simplicity. Any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A multi-objective optimization method for a wide operating range of a dual active bridge converter, characterized in that: The following steps are involved: S1, obtaining an input voltage, an output voltage and an output voltage reference value of a dual active bridge converter; S2, calculating the voltage matching ratio according to the input voltage, the output voltage and the transformer ratio, and obtaining the external phase shift angle ratio between the primary full bridge and the secondary full bridge of the dual active bridge converter according to the output voltage and the output voltage reference value; S3, obtaining the duty cycle of the primary full bridge and the internal phase shift angle ratio of the primary full bridge according to the voltage matching ratio and the external phase shift angle ratio; S4, obtaining the maximum transmission power of the dual active bridge converter and the power per unit value currently required to be transmitted, and determining the working mode of the dual active bridge converter according to the power per unit value and the voltage matching ratio requirement; S5. According to the working mode of the dual active bridge converter and the basic characteristics of different power switch tubes, the minimum dead zone boundary condition that can realize the soft switching of the power switch tube is analyzed and obtained; S6. Calculate the effective current value and the peak-to-peak current value according to the voltage matching ratio and the cycles of different working modes. Take the effective current value and the peak-to-peak current value as the optimization target, and use the NSGA-II optimization function to combine the working mode boundary and the soft switching working boundary to solve and obtain the optimal modulation scheme. S7. Based on the obtained optimal modulation scheme, a trigger signal for each switch tube of the dual active bridge converter is generated in the DSP.

2. The wide operating range multi-objective optimization method based on dual active bridge converter according to claim 1, characterized in that: In step S2, the formula for calculating the voltage matching ratio is: Among them, k is the voltage matching ratio; V1 is the input voltage, V2 is the output voltage, and n is the high-frequency transformer ratio.

3. The wide operating range multi-objective optimization method based on dual active bridge converter according to claim 1, characterized in that: The step of obtaining the external phase shift angle ratio between the primary full bridge and the secondary full bridge of the dual active bridge converter according to the output voltage and the output voltage reference value comprises: The output voltage is subtracted from the output voltage reference value, and based on the difference between the output voltage and the output voltage reference value, the external phase shift angle ratio between the primary full-bridge power switch tube drive signal and the secondary full-bridge power switch tube drive signal is obtained through a proportional-integral controller.

4. The wide operating range multi-objective optimization method based on dual active bridge converter according to claim 1, characterized in that: The step of obtaining the duty cycle of the primary full bridge and the internal phase shift angle ratio of the primary full bridge includes: According to the values ​​of the external phase shift angle ratio and the voltage matching ratio, the duty cycle of the primary full bridge and the internal phase shift angle ratio of the primary full bridge are obtained through an offline lookup table.

5. The wide operating range multi-objective optimization method based on dual active bridge converter according to claim 1, characterized in that: In step S4, the working mode of the dual active bridge converter is a working mode under the improved asymmetric modulation strategy, including four types: Working mode 1: The switching sequence is D2T S ,D1T S ,(D2+D3)T S ,(1+D2-D3)T S ,(1-D1)T S and T S ; Working mode 2: The switching sequence is D2T S ,D1T S ,(D2+D3)T S ,(1-D1)T S ,(1+D2-D3)T S and T S ; Working mode three: the switching sequence is D2T S ,(D2+D3)T S ,D1T S ,(1-D1)T S ,(1+D2-D3)T S and T S ; Working mode 4: The switching sequence is D2T S ,D1T S ,(1-D1)T S ,(D2+D3)T S ,(1+D2-D3)T S and T S ; D1 is the internal phase shift angle ratio between the driving signal of the primary full-bridge switch tube S1 and the driving signal of the switch tube S4; D2 is the external phase shift angle ratio between the primary full-bridge switch tube S1 and the secondary full-bridge switch tube Q1 of the dual active bridge converter; D3 is the duty cycle of the driving signal of the primary full-bridge switch tube S1 in the primary full-bridge; S1, S4 and Q1 are the numbers of the primary side full-bridge switch tube and the secondary side full-bridge switch tube respectively.

6. The wide operating range multi-objective optimization method based on dual active bridge converter according to claim 1, characterized in that: According to the working mode of the dual active bridge converter and the basic characteristics of different power switch tubes, the minimum dead zone boundary conditions that can achieve soft switching of the power switch tube are analyzed and obtained, including: According to the working mode division of the dual active bridge converter and the basic characteristics of the power switch tube, the basic conditions for the switch tube to achieve soft switching are analyzed, and the minimum boundary current value that can achieve soft switching of the switch tube is calculated according to the law of conservation of energy. By analyzing the resonance process of the inductor and the junction capacitance on both sides of the power switch tube, the mathematical expression of the resonance time is derived, and the minimum boundary value of the required dead time is obtained.

7. The wide operating range multi-objective optimization method based on dual active bridge converter according to claim 1, characterized in that: In step S6, the formula for calculating the effective value of the current and the peak-to-peak value of the current is: Among them, i RMS is the effective value of current, i peak is the peak-to-peak value of the current, k is the voltage matching ratio, D1 is the inner phase shift angle ratio between the driving signal of the primary full-bridge switch tube S1 and the driving signal of the switch tube S4; D2 is the outer phase shift angle ratio between the primary full-bridge switch tube S1 and the secondary full-bridge switch tube Q1 of the dual active bridge converter; D3 is the duty cycle of the driving signal of the primary full-bridge switch tube S1 in the primary full-bridge, S1, S4 and Q1 are the numbers of the primary full-bridge switch tube and the secondary full-bridge switch tube respectively.

8. The wide operating range multi-objective optimization method based on dual active bridge converter according to claim 1, characterized in that: The NSGA-II function is as follows: Among them, f(x) is the optimization target, x is the relevant variable that affects the solution target, Ax≤b is the non-equality constraint of the target to be optimized, A eq x=b eq is the equality constraint of the objective to be optimized.

9. The multi-objective optimization method for a wide operating range based on a dual active bridge converter according to claim 8, characterized in that: The NSGA-II function includes inequality constraints and equality constraints; The inequality constraints include the minimum current value that can achieve soft turn-on of the switch and the boundary conditions of different working modes: The minimum current value that can achieve soft turning on of the switch tube is as follows: Wherein, V2 is the output voltage value; Q oss is the amount of charge flowing through the two sides of the power switch tube in one switching cycle; k is the voltage matching ratio of the dual active bridge converter; n is the voltage transformer ratio; L is the connection inductance value. The equality constraint formula is as follows: P=P ref Where P is the per-unit value of the transmission power; P ref is the reference value of transmission power.

10. A wide operating range multi-objective optimization system based on dual active bridge converter, characterized in that: include: Data sampling module: used to obtain the input voltage, output voltage and output voltage reference value of the dual active bridge converter; Data analysis module: used to calculate the voltage matching ratio according to the input voltage, output voltage and transformer ratio, obtain the external phase shift angle ratio between the primary full bridge and the secondary full bridge of the dual active bridge converter according to the output voltage and the output voltage reference value, and obtain the duty cycle of the primary full bridge and the internal phase shift angle ratio of the primary full bridge according to the voltage matching ratio and the external phase shift angle ratio; Optimal modulation strategy acquisition module: used to obtain the mathematical expressions of the effective current value and the peak-to-peak current according to the voltage matching ratio and the cycles of different working modes. Taking the effective current value and the peak-to-peak current as the optimization target, the optimal modulation strategy working point is optimized and solved through the NSGA-II optimization function, combined with the working mode boundary conditions and soft switching constraints, to obtain the optimal modulation scheme; Driving signal acquisition module: used to obtain the driving signals of all switching tubes of the dual active bridge converter in the DSP according to the optimal modulation scheme.

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