A three-phase LLC converter resonant cavity current sharing control method

Abstract

The application provides a three-phase LLC converter resonant cavity current sharing control method in the technical field of three-phase LLC converters, and comprises the following steps: step S1, based on a preset sampling frequency, input currents of three phases of a three-phase LLC converter are collected respectively, and discrete Fourier transforms are performed on the input currents of each phase to extract real parts and imaginary parts of switching frequency components; step S2, based on the real parts and imaginary parts, an unbalance degree of the input currents is calculated; step S3, based on a preset balance degree threshold, the unbalance degree is checked, and based on a checking result, phase shift adjustment is performed on the input currents by a gradient descent method to perform current sharing control on the input currents. The application has the advantages that the efficiency and reliability of three-phase LLC converter resonant cavity current sharing are greatly improved without adding additional sensors, and the system robustness and power density are improved.

Description

Technical Field

[0001] This invention relates to the field of three-phase LLC converter technology, and in particular to a method for current sharing control of the resonant cavity of a three-phase LLC converter. Background Technology

[0002] The three-phase LLC converter is a common high-frequency isolated DC-DC converter topology, in which both the primary and secondary sides are star-connected. In this structure, the three resonant cavities on the primary side are connected in a star configuration via resonant capacitors, with the star point connected to the negative terminal of the input voltage; the star connection on the secondary side is in a floating state. If the parameters of the three resonant cavities are inconsistent—especially if the resonant inductor deviates due to manufacturing processes, component tolerances, etc. (the resonant inductor is usually formed by connecting an external inductor and the leakage inductance of the transformer in series, and its parameters are prone to fluctuation)—it will lead to uneven current distribution in each resonant cavity, resulting in uneven heat distribution, and in severe cases, may cause converter failure.

[0003] To address this problem, traditional methods typically involve arranging current sensors (such as current transformers (CTs) or Hall effect sensors (HALLs) and corresponding detection circuits in three resonant cavities to monitor the current value in real time. Once a significant current deviation is detected, current sharing is achieved by adjusting the phase of the control signals in the three bridge arms. However, this traditional method has the following drawbacks: 1. Additional sensors and detection circuits are required, which occupy space on the main power board and reduce the system power density; 2. The detection circuit is located in the high-voltage main power circuit, which increases the number of components and leads to a decrease in system reliability; 3. The requirement to meet the high and low voltage isolation spacing increases the complexity of PCB layout and routing; 4. Increased hardware costs and system debugging time.

[0004] Figure 2 This paper presents a typical three-phase LLC converter topology with synchronous rectification. The primary-side resonant cavity is connected in a star configuration to the negative input terminal N1 via a resonant capacitor, while the secondary side is connected in a star-floating configuration with N2 as the reference point. Each resonant cavity consists of a resonant inductor Lr, a transformer Tr, and a resonant capacitor Cr. The secondary side comprises a three-phase half-bridge synchronous rectification circuit consisting of S1–S6. This three-phase LLC converter is equipped with input voltage Vin, input current Iin, output voltage, and output current detection functions, and achieves voltage regulation and power output through frequency conversion control. Ideally, the parameters of the three resonant cavities are symmetrical, and the current should be consistent. However, in practice, due to component parameter deviations, especially the dispersion of the resonant inductor, current imbalance often occurs, leading to uneven thermal stress in the resonant cavity and half-bridge SiC-MOSFETs, and even overheating damage.

[0005] Therefore, how to provide a current sharing control method for the resonant cavity of a three-phase LLC converter, so as to improve the efficiency and reliability of current sharing in the resonant cavity of the three-phase LLC converter without adding additional sensors, thereby improving the system robustness and power density, has become an urgent technical problem to be solved. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a current sharing control method for the resonant cavity of a three-phase LLC converter, which improves the efficiency and reliability of current sharing in the resonant cavity of the three-phase LLC converter without adding additional sensors, thereby improving the system robustness and power density.

[0007] This invention is implemented as follows: a method for current sharing control of the resonant cavity of a three-phase LLC converter, comprising the following steps: Step S1: Based on the preset sampling frequency, the input current of the three phases of the three-phase LLC converter is collected respectively, and the discrete Fourier transform is performed on the input current of each phase to extract the real part and imaginary part of the switching frequency component. Step S2: Calculate the imbalance of the input current based on the real and imaginary parts of each part; Step S3: Verify the imbalance based on a preset balance threshold. Based on the verification result, adjust the phase shift of the input current using the gradient descent method to control the current sharing of the input current.

[0008] Furthermore, in step S1, the sampling frequency is an integer multiple of the switching frequency; the switching frequency is the operating frequency of the switching devices in the three-phase LLC converter.

[0009] Furthermore, in step S1, the discrete Fourier transform employs synchronous sampling technology and locks the switching frequency through a phase-locked loop.

[0010] Furthermore, in step S2, the formula for calculating the degree of imbalance is: ; in, Indicates the degree of imbalance; Indicates the real part; Indicates the imaginary part.

[0011] Furthermore, step S3 specifically includes: The imbalance is verified based on a preset balance threshold. When the imbalance is greater than the balance threshold, a current imbalance verification result is generated; when the imbalance is less than or equal to the balance threshold, a current balance verification result is generated. When the verification result indicates current balance, monitoring continues; when the verification result indicates current imbalance, the input current is phase-shifted using the gradient descent method to achieve current sharing control.

[0012] Furthermore, in step S3, the phase shift adjustment amount of the gradient descent method is proportional to the degree of imbalance, and the phase shift adjustment direction is determined based on gradient information.

[0013] Furthermore, the gradient information is obtained through the perturbation observation method, specifically: a perturbation is applied to the phase shift based on a preset perturbation amount, and the change in the cost function is measured to obtain the gradient information.

[0014] Furthermore, in step S3, during the phase shift adjustment process, one phase is used as a reference to perform phase shift operations on the other two phases.

[0015] Furthermore, step S3 also includes: The three-phase resonant cavity current is reconstructed using a Luneburger observer, and the imbalance coefficient is calculated based on the three-phase resonant cavity current. The phase shift adjustment is then guided based on the imbalance coefficient.

[0016] Furthermore, in step S3, the phase shift adjustment employs adaptive integral control to generate the required phase difference.

[0017] The advantages of this invention are: First, a Discrete Fourier Transform is performed on the three-phase input current to accurately extract the switching frequency components and calculate the current imbalance. Then, the phase shift is adaptively adjusted using the gradient descent method, dynamically adjusting the phases of the other two phases based on one phase. This effectively suppresses current imbalance caused by component parameter deviations without adding any additional sensors. This eliminates the reliability risks and space occupation caused by additional hardware, and significantly enhances the system's robustness against parameter fluctuations and power density through closed-loop adaptive control, while shortening the commissioning time. In other words, without adding additional sensors, the efficiency and reliability of current sharing in the resonant cavity of the three-phase LLC converter are greatly improved, thereby improving the system's robustness and power density. Attached Figure Description

[0018] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0019] Figure 1 This is a flowchart of a current sharing control method for the resonant cavity of a three-phase LLC converter according to the present invention.

[0020] Figure 2 This is a topology diagram of a three-phase LLC converter paired with synchronous rectification. Detailed Implementation

[0021] The technical solution in this application embodiment has the following general idea: perform discrete Fourier transform on the three-phase input current, accurately extract the switching frequency component and calculate the current imbalance, and then adaptively adjust the phase shift through the gradient descent method. In this way, without adding any additional sensors, the current imbalance caused by component parameter deviation can be effectively suppressed. This not only eliminates the reliability risks and space occupation caused by additional hardware, but also significantly enhances the robustness against parameter fluctuations and power density through closed-loop adaptive control, and shortens the debugging time.

[0022] Please refer to Figures 1 to 2 As shown, a preferred embodiment of the current sharing control method for the resonant cavity of a three-phase LLC converter according to the present invention includes the following steps: Step S1: Based on the preset sampling frequency, the input current of the three phases of the three-phase LLC converter is collected respectively, and the discrete Fourier transform is performed on the input current of each phase to extract the real part and imaginary part of the switching frequency component (fundamental component). The extraction process of the fundamental component is as follows: Let the sampling sequence (input current) be: i in [n], n=0,1,...,N−1, corresponding to time t n =n˙Ts, where Ts is the sampling period. The Discrete Fourier Transform (DFT) coefficients of the fundamental component (switching frequency fsw) are: ; but: ; ; Define the cost function: ; J is minimized by adjusting phase shifts Δϕb and Δϕc (with phase A as the reference, and phases B and C adjusted separately). Δϕb and Δϕc are calculated using the gradient descent method. ; ; in, The learning rate is represented by n; the sample number is represented by N; the total number of samples is represented by e; the natural constant is represented by j; the imaginary unit is represented by Re(·); the operator for taking the real part of a complex number is represented by Im(·); This represents the phase shift adjustment amount of the second phase in the k-th iteration; This represents the phase shift adjustment amount for the third phase in the k-th iteration; This indicates the phase shift adjustment amount for the second phase; This indicates the phase shift adjustment amount for the third phase; The cost function J is expressed with respect to The partial derivatives; The cost function J is expressed with respect to... The partial derivatives; The gradient is calculated using the perturbation and observation (PO) method: At the current phase shift, the cost function J is measured; a small perturbation δϕ is applied to Δϕb, and Jb is measured again, then: ; ; The disturbance observation method requires multiple adjustments, which may affect system stability. Therefore, the disturbance amplitude should be small and the adjustment speed should be slow. In practice, one phase can be adjusted in two cycles, alternating between the two phases.

[0023] Step S2: Calculate the imbalance of the input current based on the real and imaginary parts of each part; Step S3: Verify the imbalance based on a preset balance threshold. Based on the verification result, adjust the phase shift of the input current using the gradient descent method to control the current sharing of the input current.

[0024] This invention only requires the detection of the input current, eliminating the need for traditional three-phase resonant cavity current detection. Based on a state-space model, a Luneburger observer is designed and discretized to reconstruct the amplitude of the three-phase resonant cavity current. Through an adaptive integral phase shift compensation algorithm, inductive current sharing of the three-phase resonant cavity current is achieved, maintaining the thermal stability of the power supply system and improving the system efficiency.

[0025] In step S1, the sampling frequency is an integer multiple of the switching frequency (e.g., 10 times); the switching frequency is the operating frequency of the switching devices in the three-phase LLC converter.

[0026] In step S1, the discrete Fourier transform employs synchronous sampling technology and locks the switching frequency through a phase-locked loop.

[0027] In step S2, the formula for calculating the degree of imbalance is: ; in, Indicates the degree of imbalance; Indicates the real part; Indicates the imaginary part.

[0028] When a three-phase LLC converter is operating in steady state, the current in each resonant cavity can be approximated as a sine wave (fundamental wave approximation method). Let the currents in the three-phase resonant cavities be: ; ; ; ; in, as well as Both represent the instantaneous current of the three-phase current; as well as Both represent the amplitude of the three-phase current; as well as Both represent the phase angles of the three-phase currents; Indicates angular frequency; Indicates the switching frequency; Input current The sum of the three-phase currents: ; In an ideal situation, when the three phases are balanced (i.e., Ia=Ib=Ic=I, and ϕa=0, ϕb=−120º, ϕc=120º), the DC component of the input current is provided by the rectifier section, while the AC ripple components will theoretically cancel each other out when the three phases are balanced. However, in reality, due to the switching action, the input current will have ripple. When the three-phase current is unbalanced, the AC component of the input current will contain harmonics of a specific frequency.

[0029] Let the fundamental component (switching frequency component) of the three-phase current be: ; Since the drive signal of a three-phase LLC converter has a fixed phase difference (120 degrees), the phase difference of the three-phase current is also 120 degrees, but the amplitudes may differ. When the circuit operates near the resonant frequency, the resonant cavity exhibits resistive behavior, and the current and voltage are in phase. At this time: ; ; ; but:

[0030] make: ; ; Imbalance: ; When the three-phase currents are balanced, Ia=Ib=Ic=I, then: A=0, B=0, Δ=0; the magnitude of Δ can reflect the degree of imbalance, that is, based on the degree of imbalance Δ, the current imbalance of the three-phase resonant cavity can be judged.

[0031] Step S3 specifically involves: The imbalance is verified based on a preset balance threshold. When the imbalance is greater than the balance threshold, a current imbalance verification result is generated; when the imbalance is less than or equal to the balance threshold, a current balance verification result is generated. When the verification result indicates current balance, monitoring continues; when the verification result indicates current imbalance, the input current is phase-shifted using the gradient descent method to achieve current sharing control.

[0032] In step S3, the phase shift adjustment amount of the gradient descent method is proportional to the degree of imbalance, and the phase shift adjustment direction is determined based on gradient information.

[0033] The gradient information is obtained through the perturbation observation method, specifically: a perturbation is applied to the phase shift based on a preset perturbation amount, and the change in the cost function is measured to obtain the gradient information.

[0034] In step S3, during the phase shift adjustment process, one phase is used as a reference to perform phase shift operations on the other two phases.

[0035] Step S3 further includes: The three-phase resonant cavity current is reconstructed using a Luneburger observer, and the imbalance coefficient is calculated based on the three-phase resonant cavity current. The phase shift adjustment is then guided based on the imbalance coefficient.

[0036] That is, the three-phase resonant cavity current is reconstructed based on the state-space model built using the Luneburg observer. The state vector of the state-space model is defined as X, and the system state equation is: ; ; ; in, as well as Both represent system matrices; Both represent the resonant inductor current; Both represent the resonant capacitor voltage; Indicates system input; Indicates the input voltage; According to modern control theory, using the Luneburg observer, its expression is: ; ; in, This represents the gain matrix of the Romberg observer; This represents the estimated value of the state vector X; express The estimated value; Discretizing the above continuous system yields the discretized state equations: ; ; ; in, The sampling period is ; Represents the state transition matrix of a discrete-time system; Represents the input matrix of the discrete-time system; represents the matrix exponent; k represents the discrete-time index; Based on the reconstructed three-phase resonant cavity current, the unbalance coefficient is defined. : ; The phase difference that needs to be adjusted is generated through adaptive phase shift control, also known as adaptive integral control.

[0037] if rho > 0.1 K_i = 0.5; else K_i = 0.1; % Integral control: delta_phi = zeros(1,3); for k=1:3 error = I_avg - I_pk(k); integral_gain(k)=integral_gain(k)+K_i*error*Ts; delta_phi(k)=integral_gain(k); end In step S3, the phase shift adjustment employs adaptive integral control to generate the required phase difference.

[0038] The core idea of ​​this invention is to oversample the input current, perform a digital Fourier transform on the sampled sequence using synchronous sampling technology, extract the real and imaginary parts of the fundamental component (switching frequency component), calculate the imbalance degree to determine the degree of imbalance in the three-phase current, and adjust the phase shift of two phases using the gradient descent method of the cost function to ultimately achieve three-phase current balance. This algorithm is simple and easy to implement, but it cannot identify which phase is unbalanced. Furthermore, based on state-space reconstruction theory, an innovative three-phase resonant cavity current reconstruction algorithm is proposed, which can obtain the amplitude of the three-phase resonant cavity current and quickly achieve three-phase current balance by adjusting the phase shift, resulting in higher system efficiency.

[0039] In summary, the advantages of this invention are as follows: First, a Discrete Fourier Transform is performed on the three-phase input current to accurately extract the switching frequency components and calculate the current imbalance. Then, the phase shift is adaptively adjusted using the gradient descent method, dynamically adjusting the phases of the other two phases based on one phase. This effectively suppresses current imbalance caused by component parameter deviations without adding any additional sensors. This eliminates the reliability risks and space occupation caused by additional hardware, and significantly enhances the system's robustness against parameter fluctuations and power density through closed-loop adaptive control, while shortening the commissioning time. In other words, without adding additional sensors, the efficiency and reliability of current sharing in the resonant cavity of the three-phase LLC converter are greatly improved, thereby improving the system's robustness and power density.

[0040] While specific embodiments of the present invention have been described above, those skilled in the art should understand that the specific embodiments described are merely illustrative and not intended to limit the scope of the present invention. Equivalent modifications and variations made by those skilled in the art in accordance with the spirit of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for current sharing control of the resonant cavity in a three-phase LLC converter, characterized in that: Includes the following steps: Step S1: Based on the preset sampling frequency, the input current of the three phases of the three-phase LLC converter is collected respectively, and the discrete Fourier transform is performed on the input current of each phase to extract the real part and imaginary part of the switching frequency component. Step S2: Calculate the imbalance of the input current based on the real and imaginary parts of each part; Step S3: Verify the imbalance based on a preset balance threshold. Based on the verification result, adjust the phase shift of the input current using the gradient descent method to control the current sharing of the input current.

2. The current sharing control method for the resonant cavity of a three-phase LLC converter as described in claim 1, characterized in that: In step S1, the sampling frequency is an integer multiple of the switching frequency; the switching frequency is the operating frequency of the switching devices in the three-phase LLC converter.

3. The current sharing control method for the resonant cavity of a three-phase LLC converter as described in claim 1, characterized in that: In step S1, the discrete Fourier transform employs synchronous sampling technology and locks the switching frequency through a phase-locked loop.

4. The current sharing control method for the resonant cavity of a three-phase LLC converter as described in claim 1, characterized in that: In step S2, the formula for calculating the degree of imbalance is: ； in, Indicates the degree of imbalance; Indicates the real part; Indicates the imaginary part.

5. The current sharing control method for the resonant cavity of a three-phase LLC converter as described in claim 1, characterized in that: Step S3 specifically involves: The imbalance is verified based on a preset balance threshold. When the imbalance is greater than the balance threshold, a current imbalance verification result is generated; when the imbalance is less than or equal to the balance threshold, a current balance verification result is generated. When the verification result indicates current balance, monitoring continues; when the verification result indicates current imbalance, the input current is phase-shifted using the gradient descent method to achieve current sharing control.

6. The current sharing control method for the resonant cavity of a three-phase LLC converter as described in claim 1, characterized in that: In step S3, the phase shift adjustment amount of the gradient descent method is proportional to the degree of imbalance, and the phase shift adjustment direction is determined based on gradient information.

7. The current sharing control method for the resonant cavity of a three-phase LLC converter as described in claim 6, characterized in that: The gradient information is obtained through the perturbation observation method, specifically: a perturbation is applied to the phase shift based on a preset perturbation amount, and the change in the cost function is measured to obtain the gradient information.

8. The current sharing control method for the resonant cavity of a three-phase LLC converter as described in claim 1, characterized in that: In step S3, during the phase shift adjustment process, one phase is used as a reference to perform phase shift operations on the other two phases.

9. The current sharing control method for the resonant cavity of a three-phase LLC converter as described in claim 1, characterized in that: Step S3 further includes: The three-phase resonant cavity current is reconstructed using a Luneburger observer, and the imbalance coefficient is calculated based on the three-phase resonant cavity current. The phase shift adjustment is then guided based on the imbalance coefficient.

10. The current sharing control method for the resonant cavity of a three-phase LLC converter as described in claim 1, characterized in that: In step S3, the phase shift adjustment employs adaptive integral control to generate the required phase difference.

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