Stability improvement method for cascaded system of dual active bridge converter based on fixed-ratio impedance remodeling

Abstract

The application provides a kind of based on the stability improvement method of series system of dual active bridge converter with constant ratio impedance remodeling, for the low-frequency oscillation problem caused by the interaction of dual active bridge converter as constant power load and input LC filter, the method establishes the accurate small signal model of converter containing full control dynamics by using the control mode of constant ratio impedance remodeling, deduces the closed-loop input admittance expression, and identifies the potential unstable frequency band in combination with the output impedance Bode diagram of the front-stage Buck circuit;Further design filter, remodel the input impedance of converter in key frequency band to meet the cascade stability criterion, while avoiding interference to voltage loop high-frequency dynamics.The application only needs to increase feedback path and low-pass filter on the basis of existing voltage loop, without additional sensors, simple to realize and strong universality.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and more specifically, to a method for improving the stability of a cascaded system containing dual active bridge converters based on constant ratio impedance reshaping. Background Technology

[0002] With the rapid development of DC microgrids, new energy vehicles, and distributed energy systems, dual active bridge (DAB) converters are widely used in DC voltage conversion and power interface applications due to their advantages such as electrical isolation, bidirectional power transmission, high efficiency, and modularity. In practical applications, to suppress the interference of switching frequency subharmonics on the front-end power supply, an LC filter is usually connected in series at the input of the DAB converter. However, when the DAB converter operates as a constant power load (CPL), its input impedance exhibits negative resistance characteristics. This resistance interacts with the output impedance of the LC filter, easily causing low-frequency oscillations or even instability in the system, seriously affecting power quality and equipment safety.

[0003] In existing technologies, methods for addressing the stability issues of such cascaded systems are mainly divided into two categories: passive damping and active control. Passive damping suppresses resonance by adding resistive elements to the LC filter, but it introduces significant additional losses and reduces system efficiency. Active control methods typically shape the output impedance of the source converter or inject control signals that enhance system stability into the load converter control loop using complex algorithms. However, these methods are difficult to implement when the source converter is an uncontrollable LC filter or a simple power supply; moreover, existing load impedance shaping methods often ignore the dynamic characteristics of the controller, simplifying the load input impedance to an ideal negative resistance, resulting in insufficient design accuracy and failure to fully utilize the control effect. Therefore, there is an urgent need for a method that can fully consider the dynamics of the controller, requires no modification to the source end, and achieves a simple stability improvement for cascaded systems.

[0004] Reference 1: Y. Guan, Y. Xie, Y. Wang, Y. Liang and X. Wang, "An Active DampingStrategy for Input Impedance of Bidirectional Dual Active Bridge DC–DCConverter: Modeling, Shaping, Design, and Experiment," in IEEE Transactionson Industrial Electronics, vol. 68, no. 2, pp. 1263-1274, Feb. 2021. This paper addresses the oscillation and instability issues caused by negative impedance characteristics in a cascaded dual active bridge converter (DBC) and input LC filter system, proposing an active damping control strategy based on input impedance reshaping. This strategy employs a three-loop control structure, adding an input voltage loop to the traditional output voltage and current loops, using the input voltage as the control target. By establishing a complete small-signal model of the DAB converter encompassing all closed-loop dynamics, the precise expression for its closed-loop input admittance is derived. However, this scheme, by adding an independent input voltage loop to generate parallel virtual admittance and using the input voltage as a direct control target, requires additional sensors and a complex controller design.

[0005] Reference 2: X. Zhang, X. Ruan and Q. -C. Zhong, "Improving the Stability ofCascaded DC / DC Converter Systems via Shaping the Input Impedance of the LoadConverter With a Parallel or Series Virtual Impedance," in IEEE Transactionson Industrial Electronics, vol. 62, no. 12, pp. 7499-7512, Dec. 2015. This paper addresses the stability issues arising from the interaction between the source converter and the load converter in a cascaded DC / DC converter system. It proposes a control method to reshape the input impedance of the load converter by connecting a virtual impedance in parallel or series at the load converter's input port. The core idea is to modify the magnitude or phase of the load input impedance by introducing a parallel virtual impedance (PVI) or a series virtual impedance (SVI) within a narrow frequency range where the source output impedance intersects the load input impedance. This satisfies the Middlebrook stability criterion while maximizing the dynamic performance of the load converter. The paper derives the conditions that the PVI and SVI must satisfy and proposes a corresponding control implementation strategy, namely, applying control within a specific frequency band using an input impedance regulator (IIR) and a bandpass filter. However, this scheme requires the additional introduction of input voltage or current sensors to construct the virtual impedance in the control loop using the IIR and bandpass filter, and it simplifies the load input impedance to an ideal negative resistance.

[0006] Patent application CN111817570A discloses a wide-range control method for DAB converters based on multi-mode model prediction, relating to the fields of new energy and DC microgrid conversion technology. Employing multi-mode control enables DAB converters to operate over a wider voltage range. Optimization of different control objectives through pattern recognition is more suitable for complex operating conditions, maintaining the normal operation of the DAB converter. Using modes A, B, and C for wide-range control of the DAB converter improves its transmission efficiency and dynamic response, reducing computational load under steady-state operation without the influence of Kalman filters. This effectively improves the dynamic performance, steady-state transmission efficiency, and robustness of the DAB converter over a wide range. However, this patent cannot completely solve the existing technical problems, nor can it meet the needs of this invention. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for improving the stability of cascaded systems containing dual active bridge converters based on constant ratio impedance reshaping.

[0008] According to the present invention, a method for improving the stability of a cascaded system containing a dual active bridge converter based on constant ratio impedance reshaping is provided. The cascaded system includes a Buck circuit in the front stage and a dual active bridge converter as the load in the rear stage. The method includes: Step 1: Constructing a closed-loop small-signal model of the dual active bridge converter and obtaining the closed-loop input admittance expression of the dual active bridge converter; Step 2: Constructing a closed-loop small-signal model of the Buck circuit and obtaining the closed-loop output impedance expression of the Buck circuit; Step 3: Based on the closed-loop input admittance of the dual active bridge converter and the closed-loop output impedance of the Buck circuit... Impedance, plot Bode plot, identify phase difference within the frequency range where amplitude curves intersect, and determine the potential unstable frequency band of the cascaded system; Step 4: Design the transfer function of the filter based on the potential unstable frequency band; Step 5: Sample the input and output voltage signals of the dual active bridge converter in real time, and generate the output voltage reference value after processing by a constant ratio path and filter; Step 6: Input the difference between the output voltage reference value and the actual sampled output voltage signal to the output voltage loop controller, and the output voltage loop controller generates a phase shift signal for controlling the power switching transistors of the dual active bridge converter.

[0009] Preferably, step 1 includes: Based on the single-phase-shift modulation principle, the steady-state power transfer characteristic of the dual active bridge converter is determined as follows:

[0010] in, , These are the input and output voltages, respectively. For transformer turns ratio, For switching frequency, It is a resonant inductor. For comparison; By applying a small-signal perturbation near the steady-state operating point and linearizing it, we obtain the open-loop small-signal model:

[0011] In the formula, This refers to the small-signal disturbance in the input current of the dual active bridge converter. This refers to the small-signal disturbance in the output current of the dual active bridge converter. This represents the small-signal disturbance in the phase shift of the dual active bridge converter. , These are the small-signal disturbances in the input and output voltages of the dual active bridge converter, respectively. To move relative to the disturbance To input current disturbance The open-loop transfer function, To reduce output voltage disturbances To input current disturbance The open-loop transfer function, To move relative to the disturbance To output current disturbance The open-loop transfer function, To reduce input voltage disturbances To output current disturbance The open-loop transfer function is expressed as follows:

[0012] Introducing an output voltage loop controller With a constant ratio path, the filter transfer function is: The closed-loop control equation is determined as follows:

[0013] Where n is the ratio, which is determined by the output-side relationship. ,in For output impedance, For load resistance, The output capacitor is s, and s is the complex frequency variable in the Laplace transform; Solving the above equations simultaneously, we obtain the closed-loop input admittance of the dual active bridge converter:

[0014] in, For the filter transfer function, This is the transfer function for the output voltage loop controller; The input impedance of the dual active bridge converter is: .

[0015] Preferably, step 2 includes: Under voltage-mode control, the open-loop output impedance expression of the Buck circuit is:

[0016] in, For filtering inductors, For filtering capacitors, For load resistance; The open-loop gain of the voltage loop in the Buck circuit is:

[0017] in, For the voltage controller, transfer function For PWM modulator gain, For voltage sampling coefficients, The control-output transfer function of the Buck circuit is expressed as follows:

[0018] In the formula, This is the input voltage for the Buck circuit; The closed-loop output impedance expression of the Buck circuit is: .

[0019] Preferably, step 3 includes: Within the selected frequency range, calculate the input impedance of the dual active bridge converter under impedance reshaping control. and the closed-loop output impedance of the Buck circuit The amplitude and phase; Determined to meet frequency Let the frequencies corresponding to all intersection points be denoted as . And determine the continuous interval in which it is located. ; Calculate the phase difference at each intersection point ; in, Indicates the source-side equivalent output impedance at frequency The phase angle below, This indicates the equivalent input impedance on the load side at frequency [frequency missing]. The phase angle below; The frequency bands corresponding to intersections with a phase difference exceeding 150° are marked as potentially unstable frequency bands.

[0020] Preferably, step 4 includes: Choose a first-order low-pass filter, whose transfer function is:

[0021] in, The cutoff frequency, ; The cutoff frequency The frequency should be selected based on the following criteria: greater than or equal to the upper limit of the potentially unstable frequency band. And less than the voltage loop bandwidth of the dual active bridge converter. 0.3 to 0.5 times; The transfer function of the designed filter Substituting the closed-loop input admittance expression of the dual active bridge converter, calculate the input impedance after impedance reshaping. And verify its relationship with the closed-loop output impedance of the Buck circuit. If the phase difference at the amplitude intersection point is less than 180°, the system is considered stable.

[0022] Preferably, the turns ratio n in the constant turns ratio path is determined by the steady-state output voltage of the dual active bridge converter. With steady-state input voltage The ratio is determined. .

[0023] Preferably, the input voltage signal of the dual active bridge converter undergoes constant ratio path processing, which means multiplying the input voltage signal by the transformation ratio n; the filter processing means passing the multiplied signal through a filter with a transfer function of n. The filter is used for filtering.

[0024] Preferably, the output voltage reference value of the dual active bridge converter With actual output voltage The error is sent to the voltage controller. The process of generating the shift signal d satisfies the following relationship: .

[0025] Preferably, the shift ratio signal d is used to generate a corresponding PWM drive signal, which is used to control the on and off of the power switching transistors in the dual active bridge converter.

[0026] The cascaded system containing dual active bridge converters provided by the present invention includes: A Buck circuit, the output of which is used to provide bus voltage; A dual active bridge converter, wherein the input terminal of the dual active bridge converter is connected to the output terminal of the Buck circuit, and the output terminal of the dual active bridge converter is used to connect the load; An input voltage sampling module is connected to the input terminal of the dual active bridge converter and is used to acquire the input voltage signal of the dual active bridge converter. An output voltage sampling module is connected to the output terminal of the dual active bridge converter and is used to acquire the output voltage signal of the dual active bridge converter. A constant ratio module, the input terminal of which is connected to the output terminal of the input voltage sampling module, is used to multiply the input voltage signal by the transformation ratio n according to the ratio of the steady-state output voltage to the steady-state input voltage of the dual active bridge converter; A filter module, the input of which is connected to the output of the constant ratio module, has the following transfer function: The transfer function Based on the design of the potentially unstable frequency band of the cascaded system, the filter module is used to filter the signal processed by the constant ratio module and output a voltage reference value signal. An error generation module is provided, wherein the first input terminal of the error generation module is connected to the output terminal of the filter module, and the second input terminal of the error generation module is connected to the output terminal of the output voltage sampling module. The error generation module is used to obtain the difference between the output voltage reference value signal and the actual sampled output voltage signal. An output voltage loop controller is provided, the input of which is connected to the output of the error generation module. The transfer function of the output voltage loop controller is: The output voltage loop controller is used to generate a shift ratio signal based on the difference; A PWM drive signal generation module is provided, the input of which is connected to the output of the output voltage loop controller. The PWM drive signal generation module is used to generate a PWM drive signal based on the shift ratio signal. The PWM drive signal is used to control the on and off of the power switching transistors in the dual active bridge converter.

[0027] Compared with the prior art, the present invention has the following beneficial effects: (1) By accurately modeling and designing filters, the input impedance of the DAB converter can be effectively reshaped in potentially unstable frequency bands, suppressing low-frequency oscillations caused by negative resistance characteristics and ensuring stable system operation; (2) The filter limits the control action to the critical frequency band, avoiding interference with the high-frequency dynamics of the voltage loop. The system can still maintain good load transient response and input voltage disturbance suppression capability. (3) Only a feedback path and a low-pass filter need to be added to the existing voltage loop, without increasing hardware costs, and it is easy to implement in a digital controller; (4) This method does not depend on the characteristics of the source converter and is applicable to various cascaded systems with Buck circuits or other DC / DC converters at the front end. At the same time, the established modeling and design process can be extended to other types of load converters. Attached Figure Description

[0028] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a cascaded Buck circuit with a DAB topology. Figure 2 The simulation results are for constant voltage control under light load. Figure 3 The simulation results are for selective frequency impedance reshaping control under light load. Figure 4 Impedance Bode plot under light load; Figure 5 The simulation results are for constant voltage control under heavy load. Figure 6 The simulation results are for selective frequency impedance reshaping control under heavy load. Figure 7 This is the impedance Bode plot under heavy load. Detailed Implementation

[0029] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0030] Example This invention provides a method for improving the stability of a cascaded system with dual active bridge converters based on constant ratio impedance reshaping. This method establishes an accurate small-signal model of the DAB converter that includes full control dynamics, quantifies and analyzes the impact of control on the input impedance, and then designs filters based on the system's potential unstable frequency bands to reshape the DAB input impedance in the critical frequency bands, thereby satisfying the cascade stability criterion and avoiding negative impacts on high-frequency dynamics.

[0031] This invention addresses the technical solution of DC microgrid systems, providing a method for improving the stability of cascaded systems based on constant transformer ratio impedance reshaping. The core control law proposed in this invention is: based on traditional constant voltage control, the steady-state output voltage reference value of the DAB is... Revised to: .in, This is the instantaneous sampled value of the DAB input voltage; It is the ratio of steady-state output voltage to steady-state input voltage (constant transformer ratio). This is a first-order low-pass filter. The corrected reference value is related to the actual output voltage. After the difference is calculated, the data is fed into the existing voltage loop PI controller. Generate a shift ratio signal .

[0032] by Figure 1 Taking the DC microgrid composed of a Buck circuit cascaded with a DAB as an example, the specific technical implementation scheme is explained.

[0033] Step 1: Derivation of DAB closed-loop input admittance.

[0034] Based on the single-phase-shift modulation principle, the steady-state power transfer characteristics of the DAB converter are as follows:

[0035] in, , These are the input and output voltages, respectively. For transformer turns ratio, For switching frequency, It is a resonant inductor. To compare the shifts, a small-signal perturbation is applied near the steady-state operating point and linearized to obtain the open-loop small-signal model:

[0036] In the formula, This refers to the small-signal disturbance in the input current of the dual active bridge converter. This refers to the small-signal disturbance in the output current of the dual active bridge converter. This represents the small-signal disturbance in the phase shift of the dual active bridge converter. , These are the small-signal disturbances in the input and output voltages of the dual active bridge converter, respectively. To move relative to the disturbance To input current disturbance The open-loop transfer function, To reduce output voltage disturbances To input current disturbance The open-loop transfer function, To move relative to the disturbance To output current disturbance The open-loop transfer function, To reduce input voltage disturbances To output current disturbance The open-loop transfer function, , , , This is a constant determined by the operating point, and its specific expression is:

[0037] Consider the output voltage loop controller And a path with a constant ratio, wherein the ratio The filter transfer function is Then the closed-loop control equation is:

[0038] Utilizing output-side relationships ,in For output impedance, For load resistance, Let be the output capacitor, and s be the complex frequency variable in the Laplace transform. Solving the above equations simultaneously, we obtain the closed-loop input admittance of the DAB converter:

[0039] in, For the filter transfer function, This is the transfer function for the output voltage loop controller; The input impedance of the DAB converter is:

[0040] Step 2: Derivation of Buck's output impedance.

[0041] A. Working Principle Analysis In the cascaded system addressed in this invention, the preceding stage of the DAB converter is a Buck circuit. The output impedance of the Buck circuit depends on its topology and control method. Under voltage-mode control, the output impedance of the Buck circuit can be expressed as: First, establish the small-signal model of the Buck circuit. The Buck circuit topology consists of a switching transistor, a diode, and a filter inductor. and filter capacitor Composition, load resistance is Using the state-space averaging method, the open-loop control-output transfer function and open-loop output impedance can be derived in continuous conduction mode. The open-loop output impedance is defined as the ratio of output voltage disturbance to output current disturbance; its expression is as follows:

[0042] This expression reflects the inherent characteristics of the Buck circuit output filter.

[0043] In practical applications, the Buck circuit regulates the output voltage through a voltage closed-loop control. Let the voltage controller transfer function be... A PI controller is typically used. The PWM modulator gain is It is usually taken as the reciprocal of the sawtooth wave amplitude. Typically, it is 1. Therefore, the open-loop gain of the voltage loop is:

[0044] in The control-output transfer function of the Buck circuit is expressed as follows:

[0045] In the formula This is the input voltage for the Buck circuit.

[0046] According to the principles of automatic control, the closed-loop output impedance can be expressed as:

[0047] This equation shows that closed-loop control can effectively reduce the output impedance in the low-frequency range, but may cause peaks near the crossover frequency.

[0048] To identify potentially unstable frequency bands, it is necessary to adjust the input impedance of the DAB converter under impedance reshaping control. With Buck circuit closed-loop output impedance Compare them. Draw the Bode plots for both, the specific steps are as follows: (a) Calculate within the selected frequency range and The amplitude and phase of the frequency range are typically chosen from 10 Hz to half the switching frequency.

[0049] (b) Find the frequency points where the amplitude curves intersect, i.e., satisfy the condition. frequency Since there may be more than one intersection point, let the frequencies corresponding to all intersection points be denoted as . And determine the continuous interval in which it is located. .

[0050] (c) Calculate the phase difference at each intersection point According to the extended form of the Middlebrook stability criterion, when the phase difference is close to ±180° near the amplitude intersection frequency, the system will exhibit unstable oscillations. It is generally considered that if... If the phase difference exceeds 150°, the system faces a serious stability risk. Therefore, the frequency bands corresponding to intersections with phase differences exceeding 150° are marked as potentially unstable frequency bands.

[0051] (d) If multiple intersection points exist, the frequency band closest to a 180° phase difference should be selected as the primary design target. Furthermore, gain margin should also be considered: when Greater than When the ratio is greater than 0 dB, the phase condition is more stringent.

[0052] The above analysis clarifies the frequency range in which the system is most prone to oscillation, providing a basis for subsequent filter design.

[0053] Step 3: Design the filter .

[0054] Based on the potentially unstable frequency bands identified in step 2, select the filter type. For most applications, a first-order low-pass filter is recommended.

[0055] Cutoff frequency Determine the cutoff frequency. The selection criteria are: It should be greater than or equal to the upper limit frequency of the potentially unstable frequency band. To ensure that impedance reshaping is effective in the critical frequency band; It should be less than the voltage loop bandwidth of the DAB converter. 0.3 to 0.5 times that of the voltage loop to avoid interference with high-frequency dynamics; If the potential unstable frequency band is wide or there are multiple frequency bands, a high-order filter or a multi-band combined filter can be used.

[0056] Design Substitute the input admittance expression from step 1 to calculate the DAB input impedance after impedance reshaping. Verify its relationship with Does the stability criterion hold? If not, adjust. Alternatively, adjust the filter order until the requirements are met.

[0057] Real-time sampling of the input voltage of the DAB converter and output voltage , input voltage Through the transformation ratio and filter After processing, it is used as a reference value for the output voltage:

[0058] Output voltage reference value With actual output voltage The error is sent to the voltage controller. , generating a shifted signal :

[0059] according to The corresponding PWM drive signal is generated to control the on and off of the power switching transistors of the DAB converter, thereby controlling the output voltage.

[0060] For simplicity, the proposed technical solution is illustrated using a DC microgrid system composed of Buck circuits cascaded with dual active bridge converters as an example. The detailed parameters of the DC microgrid system are shown in Table 1.

[0061] Application scenario: Busbar voltage stabilization.

[0062] Based on the stability improvement method of cascaded systems with dual active bridge converters based on constant ratio impedance reshaping proposed in this invention, and in accordance with actual engineering requirements, the simulation circuit parameters are set as shown in Table 1.

[0063] Table 1 Simulation Circuit Parameters

[0064] A. Simulation verification under light load conditions With the Buck output voltage controlled at 100V and the DAB output voltage at 50V, and the load resistance changed to 5Ω, under these conditions, the DAB output voltage is controlled sequentially using a constant voltage method and a selective frequency impedance reshaping method. The waveforms of the bus voltage and output voltage and current obtained under the constant voltage method are as follows: Figure 2 As shown. The waveforms of the bus voltage and output voltage and current obtained under selective frequency impedance reshaping mode are as follows. Figure 3 As shown, the voltage and current waveforms are stable under light load conditions.

[0065] The Bird diagram under this operating condition is as follows Figure 4 As shown in the figure, it can be seen that the equivalent output impedance of the Buck circuit and the equivalent input impedance of the DAB circuit under constant voltage control have no intersection point, so the system is stable. Although the equivalent output impedance of the Buck circuit and the equivalent input impedance of the DAB circuit under selective frequency impedance reshaping control have an intersection point, selective frequency impedance reshaping control compensates for the phase. Therefore, within the intersection point range, the phase difference between the equivalent output impedance of the Buck circuit and the equivalent input impedance of the DAB under selective impedance reshaping control is less than 180° within the intersection point range, so the system is stable.

[0066] B. Simulation verification under heavy load conditions With the Buck output voltage controlled at 100V and the DAB output voltage at 50V, and the load resistance changed to 3Ω, under these conditions, the DAB output voltage is controlled sequentially using a constant voltage method and a selective frequency impedance reshaping method. The waveforms of the bus voltage and output voltage and current obtained under the constant voltage method are as follows: Figure 5 As shown. The waveforms of the bus voltage and output voltage and current obtained under selective frequency impedance reshaping mode are as follows. Figure 6 As shown, under heavy load conditions, the voltage and current waveforms using constant current voltage control are unstable, while the voltage and current waveforms using selective frequency impedance reshaping control are stable.

[0067] The Bird diagram under this operating condition is as follows Figure 7 As shown in the figure, the equivalent output impedance of the Buck circuit and the equivalent input impedance of the DAB circuit under constant voltage control intersect at a point. Within this intersection range, the phase difference between the input impedance of the DAB circuit under constant voltage control and the equivalent output impedance of the Buck circuit exceeds 180°, indicating that the system is unstable. Within the intersection range of the equivalent output impedance of the Buck circuit and the equivalent input impedance of the DAB circuit under selective frequency impedance reshaping control, the selective frequency impedance reshaping control compensates for the phase difference. Therefore, within this intersection range, the phase difference between the equivalent output impedance of the Buck circuit and the equivalent input impedance of the DAB circuit under selective impedance reshaping control is less than 180°, indicating that the system is stable.

[0068] As can be seen from the two examples above, selective frequency impedance reshaping control can compensate for the phase, thereby enhancing the stability of the system.

[0069] Those skilled in the art will understand that, in addition to implementing the system, apparatus, and their modules provided by this invention in purely computer-readable program code, the same program can be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system, apparatus, and their modules provided by this invention can be considered a hardware component, and the modules included therein for implementing various programs can also be considered structures within the hardware component; alternatively, modules for implementing various functions can be considered both software programs implementing the method and structures within the hardware component.

[0070] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A method for improving the stability of a cascaded system containing dual active bridge converters based on constant ratio impedance reshaping, characterized in that, The cascaded system includes a Buck circuit in the front stage and a dual active bridge converter as the load in the rear stage. The method includes: Step 1: Constructing a closed-loop small-signal model of the dual active bridge converter and obtaining the closed-loop input admittance expression of the dual active bridge converter; Step 2: Constructing a closed-loop small-signal model of the Buck circuit and obtaining the closed-loop output impedance expression of the Buck circuit; Step 3: Based on the closed-loop input admittance of the dual active bridge converter and the closed-loop output impedance of the Buck circuit, plotting a Bode plot, identifying the phase difference within the frequency range where the amplitude curves intersect, and determining the potential unstable frequency band of the cascaded system; Step 4: Designing the transfer function of the filter based on the potential unstable frequency band; Step 5: Real-time sampling of the input and output voltage signals of the dual active bridge converter, and generating an output voltage reference value after processing by a constant ratio path and filter; Step 6: Inputting the difference between the output voltage reference value and the actual sampled output voltage signal to the output voltage loop controller, which generates a phase shift signal for controlling the power switching transistors of the dual active bridge converter.

2. The method for improving the stability of a cascaded system with dual active bridge converters based on constant ratio impedance reshaping according to claim 1, characterized in that, Step 1 includes: The closed-loop input admittance expression includes the transfer function of the output voltage loop controller and the transfer function of the constant ratio path; Based on the single-phase-shift modulation principle, the steady-state power transfer characteristic of the dual active bridge converter is determined as follows: in, , These are the input and output voltages, respectively. For transformer turns ratio, For switching frequency, It is a resonant inductor. For comparison; By applying a small-signal perturbation near the steady-state operating point and linearizing it, we obtain the open-loop small-signal model: In the formula, This refers to the small-signal disturbance in the input current of the dual active bridge converter. This refers to the small-signal disturbance in the output current of the dual active bridge converter. This represents the small-signal disturbance in the phase shift of the dual active bridge converter. , These are the small-signal disturbances in the input and output voltages of the dual active bridge converter, respectively. To move relative to the disturbance To input current disturbance The open-loop transfer function, To reduce output voltage disturbances To input current disturbance The open-loop transfer function, To move relative to the disturbance To output current disturbance The open-loop transfer function, To reduce input voltage disturbances To output current disturbance The open-loop transfer function is expressed as follows: Introducing an output voltage loop controller With a constant ratio path, the filter transfer function is: The closed-loop control equation is determined as follows: Where n is the ratio, which is determined by the output-side relationship. ,in For output impedance, For load resistance, The output capacitor is s, and s is the complex frequency variable in the Laplace transform; Solving the above equations simultaneously, we obtain the closed-loop input admittance of the dual active bridge converter: in, For the filter transfer function, This is the transfer function for the output voltage loop controller; The input impedance of the dual active bridge converter is: 。 3. The method for improving the stability of a cascaded system with dual active bridge converters based on constant ratio impedance reshaping according to claim 2, characterized in that, Step 2 includes: Under voltage-mode control, the open-loop output impedance expression of the Buck circuit is: in, For filtering inductors, For filtering capacitors, For load resistance; The open-loop gain of the voltage loop in the Buck circuit is: in, For the voltage controller, transfer function For PWM modulator gain, For voltage sampling coefficients, The control-output transfer function of the Buck circuit is expressed as follows: In the formula, This is the input voltage for the Buck circuit; The closed-loop output impedance expression of the Buck circuit is: 。 4. The method for improving the stability of a cascaded system with dual active bridge converters based on constant ratio impedance reshaping according to claim 3, characterized in that, Step 3 includes: Within the selected frequency range, calculate the input impedance of the dual active bridge converter under impedance reshaping control. and the closed-loop output impedance of the Buck circuit The amplitude and phase; Determined to meet frequency Let the frequencies corresponding to all intersection points be denoted as . And determine the continuous interval in which it is located. ; Calculate the phase difference at each intersection point ; in, Indicates the source-side equivalent output impedance at frequency The phase angle below, This indicates the equivalent input impedance on the load side at frequency [frequency missing]. The phase angle below; The frequency bands corresponding to intersections with a phase difference exceeding 150° are marked as potentially unstable frequency bands.

5. The method for improving the stability of a cascaded system with dual active bridge converters based on constant ratio impedance reshaping according to claim 4, characterized in that, Step 4 includes: Substitute the transfer function of the filter into the closed-loop input admittance expression of the dual active bridge converter to reshape the input impedance of the dual active bridge converter in the potentially unstable frequency band. Choose a first-order low-pass filter, whose transfer function is: in, The cutoff frequency, ; The cutoff frequency The frequency should be selected based on the following criteria: greater than or equal to the upper limit of the potentially unstable frequency band. And less than the voltage loop bandwidth of the dual active bridge converter. 0.3 to 0.5 times; The transfer function of the designed filter Substituting the closed-loop input admittance expression of the dual active bridge converter, calculate the input impedance after impedance reshaping. And verify its relationship with the closed-loop output impedance of the Buck circuit. If the phase difference at the amplitude intersection point is less than 180°, the system is considered stable.

6. The method for improving the stability of a cascaded system with dual active bridge converters based on constant ratio impedance reshaping according to claim 1, characterized in that, The turns ratio n in the constant turns ratio path is determined by the steady-state output voltage of the dual active bridge converter. With steady-state input voltage The ratio is determined. .

7. The method for improving the stability of a cascaded system with dual active bridge converters based on constant ratio impedance reshaping according to claim 1, characterized in that, The input voltage signal of the dual active bridge converter is processed via a fixed transformation ratio path, which means multiplying the input voltage signal by the transformation ratio n. The filter processing refers to passing the multiplied signal through a filter with a transfer function of... The filter is used for filtering.

8. The method for improving the stability of a cascaded system with dual active bridge converters based on constant ratio impedance reshaping according to claim 1, characterized in that, The output voltage reference value of the dual active bridge converter With actual output voltage The error is sent to the voltage controller. The process of generating the shift signal d satisfies the following relationship: 。 9. The method for improving the stability of a cascaded system containing dual active bridge converters based on constant ratio impedance reshaping according to claim 1, characterized in that, The shift ratio signal d is used to generate a corresponding PWM drive signal, which is used to control the on and off of the power switching transistors in the dual active bridge converter.

10. A cascaded system containing dual active bridge converters, characterized in that, The stability improvement method for a cascaded system with dual active bridge converters based on constant ratio impedance reshaping, as described in any one of claims 1 to 9, includes: A Buck circuit, the output of which is used to provide bus voltage; A dual active bridge converter, wherein the input terminal of the dual active bridge converter is connected to the output terminal of the Buck circuit, and the output terminal of the dual active bridge converter is used to connect the load; An input voltage sampling module is connected to the input terminal of the dual active bridge converter and is used to acquire the input voltage signal of the dual active bridge converter. An output voltage sampling module is connected to the output terminal of the dual active bridge converter and is used to acquire the output voltage signal of the dual active bridge converter. A constant ratio module, the input terminal of which is connected to the output terminal of the input voltage sampling module, is used to multiply the input voltage signal by the transformation ratio n according to the ratio of the steady-state output voltage to the steady-state input voltage of the dual active bridge converter; A filter module, the input of which is connected to the output of the constant ratio module, has the following transfer function: The transfer function Based on the design of the potentially unstable frequency band of the cascaded system, the filter module is used to filter the signal processed by the constant ratio module and output a voltage reference value signal. An error generation module is provided, wherein the first input terminal of the error generation module is connected to the output terminal of the filter module, and the second input terminal of the error generation module is connected to the output terminal of the output voltage sampling module. The error generation module is used to obtain the difference between the output voltage reference value signal and the actual sampled output voltage signal. An output voltage loop controller is provided, the input of which is connected to the output of the error generation module. The transfer function of the output voltage loop controller is: The output voltage loop controller is used to generate a shift ratio signal based on the difference; A PWM drive signal generation module is provided, the input of which is connected to the output of the output voltage loop controller. The PWM drive signal generation module is used to generate a PWM drive signal based on the shift ratio signal. The PWM drive signal is used to control the on and off of the power switching transistors in the dual active bridge converter.

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