An Optimal Unilateral Asymmetric Three-Degree-of-Freedom Phase-Shift Modulation Method for Dual Active Bridge

Abstract

The present invention discloses an optimal single-sided asymmetric three-degree-of-freedom phase-shift modulation method for a dual-active bridge, which relates to the field of dual-active bridge modulation and includes the following steps: when the current normalized power is greater than 0 and less than the current voltage conversion ratio, it is determined whether the current normalized power is greater than the normalized critical power point. If so, enter the heavy-load mode and obtain the optimal phase-shift solution in the heavy-load mode; otherwise, enter the light-load mode to obtain the optimal phase-shift solution in the light-load mode; drive the switching tubes of the dual-active bridge according to the obtained optimal phase-shift solution to complete the optimal single-sided asymmetric three-degree-of-freedom phase-shift modulation of the dual-active bridge. This method minimizes the peak-to-peak value of the inductor current within the full power range, while increasing the number and range of soft switches. The modulation method of the present invention takes into account the optimization of the switching loss and conduction loss of the dual-active bridge converter, so that the efficiency of the dual-active bridge converter is improved within the full power range.

Description

Technical Field

[0001] The present invention relates to the field of dual-active-bridge modulation, and particularly to an optimal single-sided asymmetric three-degree-of-freedom phase-shift modulation method for a dual-active bridge. Background Art

[0002] The circuit structure of a dual-active-bridge converter (DAB converter) is as Figure 1 shown. The DAB converter consists of a primary full bridge composed of 4 switching tubes (S1 - S4), a secondary full bridge composed of 4 switching tubes (Q1 - Q4), two voltage-stabilizing filter capacitors (C1 and C2), a high-frequency transformer T, and an equivalent inductance L of leakage inductance. The input-side DC voltage source is V1, the output-side DC voltage source is V2, and the transformer turns ratio is n:1.

[0003] The simplest and most traditional control of the DAB converter is single-phase shift control (SPS). SPS control can achieve zero-voltage switching under heavy load and full-range zero-voltage switching when M = nV2 / V1 = 1. However, when M is not equal to 1 and under light load, it will lose the ZVS characteristic, and there are also disadvantages such as large current stress, large reflux power, and low converter efficiency.

[0004] To solve this problem, without changing the topology, H-bridge inner phase shift is introduced on the basis of SPS, and modulation methods such as dual-phase shift (DPS), extended phase shift (EPS), and triple-phase shift (TPS) are proposed to optimize the efficiency of the dual-active-bridge converter. However, the reactive power of EPS and DPS is still very large under light load. At the same time, most of these modulations take the inductor current stress or the effective value of the inductor current as the optimization target, and the existing modulation methods cannot simultaneously reduce the root-mean-square current of the inductor and expand the soft-switching operating range. SPS, EPS, DPS, and TPS modulations all have all the switching tubes conducting with a 50% duty cycle, so they are also called symmetric duty-cycle modulations. The typical waveform diagrams of symmetric duty-cycle modulations are respectively as Figure 2 (a), Figure 2 (b), Figure 2 (c) and Figure 2 (d) shown.

[0005] Since the switches in the dual-active-bridge symmetric duty-cycle modulation method operate at a 50% duty cycle, the operation of the DAB is half-cycle symmetric under steady-state conditions. Therefore, if and only if the switches in the first half of the same cycle meet the soft-switching conditions, the switches in the second half of the cycle can meet the soft-switching conditions. That is, the soft-switching conditions of S1, S4, Q1, and Q4 are equivalent to those of S2, S3, Q2, and Q3. So when one switch cannot achieve soft switching, the other corresponding switch cannot achieve soft switching either. This leads to the existing symmetric duty-cycle modulation method being unable to simultaneously take into account improving the soft-switching operating range to reduce switching losses and limiting the root-mean-square current of the inductor to reduce conduction losses, and thus unable to achieve the optimization of efficiency.

[0006] When the dual-active-bridge converter operates under different working conditions, due to the existence of inductors and capacitors, the operating characteristics of the converter are non-linear. And when there are disturbances in the load and input voltage, the traditional PI control has the phenomena of slow dynamic response speed and large overshoot in the output voltage. Summary of the Invention

[0007] Aiming at the above deficiencies in the prior art, a dual-active-bridge optimal single-sided asymmetric three-degree-of-freedom phase-shift modulation method provided by the present invention solves the problems that the existing dual-active-bridge modulation methods cannot simultaneously take into account improving the soft-switching operating range to reduce switching losses and limiting the root-mean-square current of the inductor to reduce conduction losses, and that the PI control has a slow dynamic response speed and a large overshoot.

[0008] In order to achieve the above invention purpose, the technical solution adopted by the present invention is as follows:

[0009] Provide a dual-active-bridge optimal single-sided asymmetric three-degree-of-freedom phase-shift modulation method. The dual-active bridge includes a primary full bridge and a secondary full bridge. The primary full bridge includes arm L1 and arm L2. Arm L1 includes upper switch S1 and lower switch S2. Arm L2 includes upper switch S3 and lower switch S4. The secondary full bridge includes arm L3 and arm L4. Arm L3 includes upper switch Q1 and lower switch Q2. Arm L4 includes upper switch Q3 and lower switch Q4. The method includes the following steps:

[0010] S1. Obtain the real-time normalized power through a PI controller; obtain the real-time voltage conversion ratio; obtain the normalized critical power point between the light-load mode and the heavy-load mode;

[0011] S2. Judge whether the current normalized power is greater than 0 and less than the current voltage conversion ratio. If so, enter step S3; otherwise, return to step S1;

[0012] S3. Judge whether the current normalized power is greater than the normalized critical power point. If so, enter the heavy-load mode and enter step S4; otherwise, enter the light-load mode and enter step S5;

[0013] S4. Based on the Lagrange multiplier method, taking the peak-to-peak inductor current as the objective function and the transmission power as the equality constraint condition, obtain the optimal phase-shift solution in the heavy-load mode, and enter step S6;

[0014] S5. Based on the Lagrange multiplier method, taking the peak-to-peak inductor current as the objective function and the transmission power as the equality constraint condition, obtain the optimal phase-shift solution in the light-load mode, and enter step S6;

[0015] S6. Drive the switching tubes of the dual-active bridge according to the obtained optimal phase-shift solution to complete the optimal single-sided asymmetric three-degree-of-freedom phase-shift modulation of the dual-active bridge;

[0016] Among them, the specific method for obtaining the real-time benchmark power through the PI controller includes the following sub-steps:

[0017] S1-1. Obtain the difference between the real-time output voltage of the dual-active bridge and the reference voltage to obtain the output voltage deviation, and obtain the change rate of the output voltage deviation;

[0018] S1-2. Take the output voltage deviation and its change rate as the input variables of the fuzzy controller to obtain the correction value and the correction value;

[0019] S1-3. Input the correction value and the correction value into the PI controller and adjust the corresponding parameters of the PI regulator to obtain the PI controller with adjusted parameters;

[0020] S1-4. Take the output voltage deviation as the input of the PI controller with adjusted parameters, and take the output of the PI controller with adjusted parameters as the real-time benchmark power.

[0021] Further, the phase-shift time of switching tube S1 and switching tube S4 is D1T S ; the phase-shift time of switching tube S2 and S3 is (1 - 2D2 - D1)T S ; the four switching tubes of the secondary full-bridge operate with a 50% duty cycle, and the phase-shift time of switching tube Q1 and switching tube Q4 is D1T S ; the phase-shift time of switching tube Q2 and switching tube Q3 is D1T S ; the phase-shift time of switching tube Q1 and switching tube S1 is D3T S .

[0022] Further, the specific method for obtaining the real-time voltage conversion ratio in step S1 is:

[0023] According to the formula:

[0024]

[0025] Obtain the real-time voltage conversion ratio M; where is the real-time output voltage of the dual-active bridge; is the real-time input voltage of the dual-active bridge; n is the turns ratio of the transformer of the dual-active bridge.

[0026] Further, the specific method for obtaining the benchmark critical power point between the light load mode and the heavy load mode in step S1 is:

[0027] According to the formula:

[0028]

[0029] Obtain the benchmark critical power point between the light load mode and the heavy load mode .

[0030] Further, the specific method for step S4 is:

[0031] According to the formula:

[0032]

[0033] Obtain the optimal phase shift solution in the heavy load mode , and ; where is the benchmark transmission power in the heavy load mode.

[0034] Further, the method for obtaining the benchmark transmission power in the heavy load mode is:

[0035] According to the formula:

[0036]

[0037] Obtain the benchmark transmission power in the heavy load mode .

[0038] Further, the specific method for obtaining the optimal phase shift solution in the light load mode in step S5 is:

[0039] According to the formula:

[0040]

[0041] Obtain the optimal phase shift solution in the light load mode , and ; where is the benchmark transmission power in the light load mode.

[0042] Further, the method for obtaining the benchmark transmission power in the light load mode is:

[0043] According to the formula:

[0044]

[0045] Obtain the benchmark transmission power in the light load mode 。

[0046] The beneficial effects of the present invention are as follows:

[0047] 1. This method minimizes the peak-to-peak value of the inductor current within the full power range, and at the same time increases the number and range of soft switches.

[0048] 2. The modulation method of the present invention takes into account the optimization of the switching loss and conduction loss of the dual-active-bridge converter, so that the efficiency of the dual-active-bridge converter is increased within the full power range. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 is the circuit structure diagram of the dual-active bridge (converter);

[0050] Figure 2 is the typical waveform diagram of the symmetric duty cycle modulation method; where Figure 2 (a) is the waveform diagram corresponding to SPS; Figure 2 (b) is the waveform diagram corresponding to EPS; Figure 2 (c) is the waveform diagram corresponding to DPS; Figure 2 (d) is the waveform diagram corresponding to TPS;

[0051] Figure 3 is the flow schematic diagram of this method;

[0052] Figure 4 is the waveform diagram corresponding to the light load mode of this method;

[0053] Figure 5 is the waveform diagram corresponding to the heavy load mode of this method;

[0054] Figure 6 is the control simulation V2 waveform diagram of the traditional PI controller;

[0055] Figure 7 is the control simulation V2 waveform diagram of the PI controller in this method. DETAILED DESCRIPTION OF THE INVENTION

[0056] The following describes the specific embodiments of the present invention to facilitate those skilled in the art to understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions created using the concept of the present invention are within the scope of protection.

[0057] As Figure 3As shown in the figure, the optimal single-sided asymmetric three-degree-of-freedom phase-shift modulation method for the dual-active bridge includes the following steps:

[0058] S1. Obtain the real-time normalized power through a PI controller; obtain the real-time voltage conversion ratio; obtain the normalized critical power point between the light-load mode and the heavy-load mode;

[0059] S2. Determine whether the current normalized power is greater than 0 and less than the current voltage conversion ratio. If so, go to step S3; otherwise, return to step S1;

[0060] S3. Determine whether the current normalized power is greater than the normalized critical power point. If so, enter the heavy-load mode and go to step S4; otherwise, enter the light-load mode and go to step S5;

[0061] S4. Based on the Lagrange multiplier method, with the peak-to-peak inductor current as the objective function and the transmitted power as the equality constraint condition, obtain the optimal phase-shift solution in the heavy-load mode, and enter step S6;

[0062] S5. Based on the Lagrange multiplier method, with the peak-to-peak inductor current as the objective function and the transmitted power as the equality constraint condition, obtain the optimal phase-shift solution in the light-load mode, and enter step S6;

[0063] S6. Drive the switching tubes of the dual-active bridge according to the obtained optimal phase-shift solution to complete the optimal single-sided asymmetric three-degree-of-freedom phase-shift modulation of the dual-active bridge;

[0064] Among them, the dual-active bridge includes a primary full bridge and a secondary full bridge. The primary full bridge includes arm L1 and arm L2. Arm L1 includes upper switching tube S1 and lower switching tube S2. Arm L2 includes upper switching tube S3 and lower switching tube S4; the secondary full bridge includes arm L3 and arm L4. Arm L3 includes upper switching tube Q1 and lower switching tube Q2. Arm L4 includes upper switching tube Q3 and lower switching tube Q4.

[0065] The specific method for obtaining the real-time normalized power through a PI controller includes the following sub-steps:

[0066] S1-1. Obtain the difference between the real-time output voltage and the reference voltage of the dual-active bridge to obtain the output voltage deviation, and obtain the change rate of the output voltage deviation;

[0067] S1-2. Use the output voltage deviation and its change rate as the input variables of the fuzzy controller to obtain the correction value and the correction value;

[0068] S1-3. Input the correction value and the correction value into the PI controller and adjust the corresponding parameters of the PI regulator to obtain the PI controller with adjusted parameters;

[0069] S1-4, using the output voltage deviation as the input of the PI controller after parameter adjustment, and using the output of the PI controller after parameter adjustment as the real-time benchmark power.

[0070] The phase shift time of switch tube S1 and switch tube S4 is D1T S ; The phase shift time of switch tubes S and S3 is (1-2D2-D1)T S The four switches of the secondary full bridge operate at a 50% duty cycle, that is, a symmetrical duty cycle modulation method is adopted. There is a phase shift in the secondary full bridge, and the phase shift in the original secondary full bridge is the same. The phase shift time of switch tube Q1 and switch tube Q4 is D1T S ; The phase shift time between switch tube Q2 and switch tube Q3 is D1T S ; The phase shift time between switch tube Q1 and switch tube S1 is D3T S Where T S Represents a switching cycle.

[0071] The specific method for obtaining the real-time voltage conversion ratio in step S1 is: according to the formula:

[0072]

[0073] Get the real-time voltage conversion ratio M; is the real-time output voltage of the dual active bridge; is the real-time input voltage of the dual active bridge; n is the transformation ratio of the transformer of the dual active bridge.

[0074] The specific method of obtaining the benchmark critical power point between the light load mode and the heavy load mode in step S1 is:

[0075] According to the formula:

[0076]

[0077] Obtain benchmarked critical power points between light-load mode and heavy-load mode .

[0078] The specific method of step S4 is: according to the formula:

[0079]

[0080]

[0081] Obtain the optimal solution for phase shifting in heavy load mode , and ;in is the benchmarked transmission power in heavy load mode.

[0082] The specific method for obtaining the optimal phase shift solution in the light load mode in step S5 is as follows: According to the formula:

[0083]

[0084]

[0085] Obtain the optimal phase shift solution in the light load mode 、 and ; where is the benchmarked transmission power in the light load mode.

[0086] In the specific implementation process, the waveform diagram of this method is as shown in Figure 4 and Figure 5 ; where v p represents the primary side duty cycle alternating current square wave voltage, v s represents the secondary side duty cycle alternating current square wave voltage, i L represents the inductor current. Perform time domain analysis on the light load mode and the heavy load mode to obtain the peak-to-peak value of the inductor current and the expression of the transmission power. The ratio of the actual power to the reference power is the benchmarked power, and the benchmarked power is a value between 0 and 1; where the actual power can be directly measured, and the reference power , is the switching frequency, L is the auxiliary inductor.

[0087] According to Figure 5 obtain the expressions for the benchmarked peak-to-peak value of the inductor current and the benchmarked transmission power in the light load mode:

[0088]

[0089] .

[0090] According to Figure 4 obtain the expressions for the benchmarked peak-to-peak value of the inductor current and the benchmarked transmission power in the heavy load mode:

[0091]

[0092] .

[0093] After obtaining the optimal phase-shift solution in the corresponding mode, it is sent to a pulse-width modulator to generate a corresponding control signal, and the corresponding switching tubes in the dual-active bridge are controlled through a drive circuit, thus realizing the optimal unilateral asymmetric three-degree-of-freedom phase-shift modulation of the dual-active bridge in this method.

[0094] In an embodiment of the present invention, the parameters of the DAB converter simulation platform are shown in Table 1. By giving the same set of normalized power and voltage conversion ratios, the test conditions of this method and the traditional single-phase-shift modulation method are ensured to be the same, as shown in Table 2. The peak-to-peak value and the effective value of the inductor current of the two modulation methods are measured.

[0095] Table 1: Parameters of the DAB converter simulation platform

[0096]

[0097] Table 2: Simulation data of this method and the traditional modulation method

[0098]

[0099] According to the simulation results in Table 2, it can be seen that this method can effectively reduce the peak-to-peak value and the effective value of the inductor current in the full power range compared with the single-phase-shift control, effectively reducing the conduction loss. At the same time, this method realizes the zero-voltage conduction of 5 switches and the zero-current turn-off of 3 switches under light load, while the single-phase-shift modulation loses the soft switching of 4 switches. Therefore, this method effectively reduces the switching loss while reducing the conduction loss.

[0100] As Figure 6 and Figure 7 shown, a load disturbance is added at t = 0.5 s. Compared with the traditional PI control method, the output voltage overshoot of the PI controller (control method) of this method is smaller, and it can stabilize at 90 V more quickly, solving the defect that the traditional PI regulator cannot simultaneously meet the requirements of fast response and reduced overshoot, effectively suppressing the overshoot, reducing the fluctuation of the output voltage, and improving the dynamic response performance, static stability and robustness of the converter.

Claims

1. An optimal single - sided asymmetric three - degree - of - freedom phase - shift modulation method for a dual - active bridge. The dual - active bridge includes a primary full - bridge and a secondary full - bridge. The primary full - bridge includes arm L1 and arm L2. Arm L1 includes upper switch S1 and lower switch S2. Arm L2 includes upper switch S3 and lower switch S4. The secondary full - bridge includes arm L3 and arm L4. Arm L3 includes upper switch Q1 and lower switch Q2. Arm L4 includes upper switch Q3 and lower switch Q4. It is characterized in that, It includes the following steps: S1. Obtain the real-time benchmarked power through a PI controller; obtain the real-time voltage conversion ratio M; obtain the benchmarked critical power point between the light load mode and the heavy load mode; S2. Determine whether the current benchmarked power is greater than 0 and less than the current voltage conversion ratio. If so, proceed to step S3; otherwise, return to step S1; S3. Determine whether the current benchmarked power is greater than the benchmarked critical power point. If so, enter the heavy load mode and proceed to step S4; otherwise, enter the light load mode and proceed to step S5; S4. Based on the Lagrange multiplier method, taking the peak-to-peak inductor current in the heavy load mode as the objective function and the transmission power in the heavy load mode as the equality constraint condition, obtain the optimal phase shift solution in the heavy load mode and proceed to step S6; S5. Based on the Lagrange multiplier method, taking the peak-to-peak inductor current in the light load mode as the objective function and the transmission power in the light load mode as the equality constraint condition, obtain the optimal phase shift solution in the light load mode and proceed to step S6; S6. Drive the switching tubes of the dual-active bridge according to the obtained optimal phase shift solution to complete the optimal single-sided asymmetric three-degree-of-freedom phase shift modulation of the dual-active bridge; Among them, the specific method for obtaining the real-time benchmarked power through a PI controller includes the following sub-steps: S1-1. Obtain the difference between the real-time output voltage and the reference voltage of the dual-active bridge to obtain the output voltage deviation, and obtain the change rate of the output voltage deviation; S1-2. Use the output voltage deviation and its change rate as the input variables of the fuzzy controller to obtain a correction value; S1-3. Input the correction value into the PI controller and adjust the corresponding parameters of the PI regulator to obtain a PI controller with adjusted parameters; S1-4. Use the output voltage deviation as the input of the PI controller with adjusted parameters, and use the output of the PI controller with adjusted parameters as the real-time benchmarked power; The specific method for obtaining the benchmarked critical power point between the light load mode and the heavy load mode in step S1 is: According to the formula: Obtain the benchmark critical power point between the light load mode and the heavy load mode 2. The optimal single - sided asymmetric three - degree - of - freedom phase - shift modulation method for a dual - active bridge according to claim 1, characterized in that, The phase-shift time of switch S1 and switch S4 is D1T S ; The phase-shift time of switch S2 and switch S3 is (1 - 2D2 - D1)T S ; The four switches of the secondary full-bridge operate at a 50% duty cycle, and the phase-shift time of switch Q1 and switch Q4 is D1T S ; The phase-shift time of switch Q2 and switch Q3 is D1T S ; The phase-shift time of switch Q1 and switch S1 is D3T S .

3. The optimal single - sided asymmetric three - degree - of - freedom phase - shift modulation method for a dual - active bridge according to claim 1, characterized in that, The specific method for obtaining the real-time voltage conversion ratio in step S1 is: According to the formula: Obtain the real-time voltage conversion ratio M; where V2 is the real-time output voltage of the dual-active bridge; V1 is the real-time input voltage of the dual-active bridge; and n is the turns ratio of the transformer of the dual-active bridge.

4. The optimal single - sided asymmetric three - degree - of - freedom phase - shift modulation method for a dual - active bridge according to claim 2, characterized in that, The specific method for step S4 is: According to the formula: Obtain the optimal phase shift solutions D1, D2, and D3 in the overload mode; where is the benchmark transmission power in the overload mode.

5. The optimal single - sided asymmetric three - degree - of - freedom phase - shift modulation method for a dual - active bridge according to claim 4, characterized in that, The method for obtaining the benchmarked transmission power in the heavy load mode is: According to the formula: Obtain the benchmark transmission power in the overload mode 6. A method for optimal single - side asymmetric three - degree - of - freedom phase - shift modulation of a dual - active bridge, according to claim 2, characterized in that, The specific method for obtaining the optimal phase shift solution in the light load mode in step S5 is: According to the formula: Obtain the optimal phase-shift solutions D1, D2, and D3 in the light-load mode; where P1 * is the benchmark transmission power in the light-load mode.

7. A method for optimal single - side asymmetric three - degree - of - freedom phase - shift modulation of a dual - active bridge, according to claim 6, characterized in that, The method for obtaining the benchmarked transmission power in the light load mode is: According to the formula: Obtain the benchmark transmission power P1 in the light load mode * .

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