Wireless power transmission maximum efficiency tracking system and method

Abstract

The invention relates to a wireless power transmission maximum efficiency tracking system and method. The system comprises a DC power supply, a transmitting end Buck-Boost circuit, a full-bridge inverter circuit, an LCC-S compensation topology, a full-bridge rectifier circuit, a receiving end Buck-Boost circuit, a transmitting end controller and a receiving end controller. According to the invention, PI control is adopted to adjust the duty ratio of a Buck-Boost circuit switching tube in a CCM mode, so that stable control of the output voltage of a receiving end is realized; considering that the output voltage of the receiving end can change along with the resistance value when the load resistance value changes, a Buck-Boost circuit in a CCM mode is added to the transmitting end, and the duty ratio of a switching tube is controlled and adjusted through PI, so that the voltage stability of the system under the variable load condition is realized. When the system coupling coefficient and the load resistance change, the duty ratio of the switching tube is dynamically adjusted, and maximum transmission efficiency tracking based on coupling coefficient identification of the wireless charging system under the condition of variable load is realized.

Description

Technical Field

[0001] The present application relates to the field of wireless charging for electric vehicles, and specifically, to a maximum efficiency tracking system and method for wireless power transmission. Background Art

[0002] The application of wireless charging technology for electric vehicles effectively improves the charging convenience and endurance of electric vehicles. However, in the case of variable load and offset, the wireless charging system will generate mutual inductance fluctuations, resulting in unstable transmission efficiency, which seriously affects the efficiency of wireless power transmission for electric vehicles. During the charging process of electric vehicles, the offset of the magnetic coupling structure causes mutual inductance fluctuations, affecting the system transmission efficiency and making it difficult to achieve stable power transmission. The current wireless charging control strategy aims at maximum transmission power and transmission efficiency, and adopts impedance matching or closed-loop control methods for frequency or load tracking. However, it is necessary to detect the coupling coefficient in real time and calculate the load resistance value. Affected by the calculation and measurement accuracy, the response time is relatively long, which affects the wireless transmission efficiency. Summary of the Invention

[0003] In order to overcome at least one deficiency in the prior art, the present application provides a maximum efficiency tracking system and method for wireless power transmission.

[0004] In a first aspect, a maximum efficiency tracking system for wireless power transmission is provided, including: a DC power supply, a transmitter Buck-Boost circuit, a full-bridge inverter circuit, an LCC-S compensation topology, a full-bridge rectifier circuit, a receiver Buck-Boost circuit, a transmitter controller, and a receiver controller;

[0005] The transmitter controller collects the voltage and current of the transmitter coil in the LCC-S compensation topology, and the receiver controller collects the voltage and current of the receiver coil and the load voltage in the LCC-S compensation topology, and transmits the voltage and current of the receiver coil and the load voltage to the transmitter controller through wireless communication;

[0006] The transmitter controller generates a duty cycle for adjusting the switching tube of the transmitter Buck-Boost circuit according to the voltage and current of the transmitter coil, the voltage and current of the receiver coil, and the load voltage, denoted as the first duty cycle, performs pulse modulation on the first duty cycle, and sends the pulse-modulated first duty cycle to the transmitter Buck-Boost circuit. The transmitter Buck-Boost circuit performs PI control and phase-shift control according to the pulse-modulated first duty cycle to make the load voltage reach the set value;

[0007] The transmitter controller calculates the coupling coefficient based on the voltage and current of the transmitter coil and the voltage and current of the receiver coil, calculates the optimal equivalent resistance at the maximum transmission efficiency of the system according to the coupling coefficient, and transmits the optimal equivalent resistance to the receiver controller through wireless communication; the receiver controller generates a duty cycle for adjusting the switching tube of the receiver Buck-Boost circuit according to the optimal equivalent resistance, denoted as the second duty cycle, performs pulse modulation on the second duty cycle, and sends the pulse-modulated second duty cycle to the receiver Buck-Boost circuit. The receiver Buck-Boost circuit adjusts the phase of the switching tube in the receiver Buck-Boost circuit according to the pulse-modulated second duty cycle until the second duty cycle reaches the target value, achieving maximum efficiency tracking.

[0008] In one embodiment, the coupling coefficient is calculated using the following formula:

[0009]

[0010] where k is the coupling coefficient, U1 and I1 are the voltage and current of the transmitter coil respectively, U2 and I2 are the voltage and current of the receiver coil respectively, R1 is the internal resistance of the transmitter coil, R2 is the internal resistance of the receiver coil, L1 is the self-inductance of the transmitter coil, L2 is the self-inductance of the receiver coil, and ω is the angular frequency.

[0011] In one embodiment, the optimal equivalent resistance is calculated using the following formula:

[0012]

[0013] where R eq-ηmax is the optimal equivalent resistance, R1 is the internal resistance of the transmitter coil, R2 is the internal resistance of the receiver coil, ω is the angular frequency, k is the coupling coefficient, L1 is the self-inductance of the transmitter coil, and L2 is the self-inductance of the receiver coil.

[0014] In one embodiment, the second duty cycle is calculated using the following formula:

[0015]

[0016] where d 2-ηmax is the second duty cycle, L b is the self-inductance of the coil in the receiver Buck-Boost circuit, f s2 is the switching frequency of the receiver Buck-Boost circuit, and R eq-ηmax is the optimal equivalent resistance.

[0017] Second, a method for maximum efficiency tracking of wireless power transmission is provided, including:

[0018] Step 1, connect to the DC power supply and perform initialization settings;

[0019] Step 2: The transmitter controller calculates the coupling coefficient based on the voltage and current of the transmitter coil and the voltage and current of the receiver coil.

[0020] Step 3: Calculate the optimal equivalent resistance at the maximum transmission efficiency of the system according to the coupling coefficient.

[0021] Step 4: The transmitter controller generates the duty cycle of the switching tube of the transmitter Buck-Boost circuit according to the voltage and current of the transmitter coil, the voltage and current of the receiver coil, and the load voltage, denoted as the first duty cycle, and performs pulse modulation on the first duty cycle, and sends the pulse-modulated first duty cycle to the transmitter Buck-Boost circuit. The transmitter Buck-Boost circuit performs PI control and phase-shift control according to the pulse-modulated first duty cycle to make the load voltage reach the set value.

[0022] Step 5: Determine whether the coupling coefficient has changed. If so, execute Step 6; if not, execute Step 7.

[0023] Step 6: Determine whether the load voltage has changed. If so, return to Step 4; if not, return to Step 2.

[0024] Step 7: The receiver controller generates the duty cycle of the switching tube of the receiver Buck-Boost circuit according to the optimal equivalent resistance, denoted as the second duty cycle, and performs pulse modulation on the second duty cycle, and sends the pulse-modulated second duty cycle to the receiver Buck-Boost circuit. The receiver Buck-Boost circuit adjusts the phase of the switching tube in the receiver Buck-Boost circuit according to the pulse-modulated second duty cycle until the second duty cycle reaches the target value to achieve maximum efficiency tracking.

[0025] Compared with the prior art, the present application has the following beneficial effects:

[0026] 1. The duty cycle of the switching tube of the Buck-Boost circuit in the CCM mode is adjusted by PI control to achieve stable control of the output voltage at the receiver. Considering that when the load resistance changes, the output voltage at the receiver will change with the resistance. A Buck-Boost circuit in the CCM mode is added at the transmitter, and the duty cycle of the switching tube is adjusted by PI control to achieve voltage stability of the system under variable load conditions.

[0027] 2. A Buck-Boost circuit in the DCM mode is added at the load end. Based on the LCC-S compensation topology circuit, the expression of the optimal impedance is obtained through the transmission efficiency expression, and then the calculation expression of the coupling coefficient is deduced to realize real-time identification of the coupling coefficient.

[0028] 3. When the system coupling coefficient and load resistance change, dynamically adjust the duty cycle of the switching tube to achieve maximum transmission efficiency tracking based on coupling coefficient identification in the wireless charging system under variable load conditions. Brief Description of the Drawings

[0029] This application can be better understood by referring to the description given below in conjunction with the accompanying drawings. The drawings, together with the following detailed description, are included in this specification and form a part of this specification. In the drawings:

[0030] Figure 1 The block diagram of the maximum efficiency tracking system for wireless power transfer is shown;

[0031] Figure 2 The circuit topology of the wireless charging system is shown;

[0032] Figure 3 The LCC-S transmission efficiency characteristic curve is shown;

[0033] Figure 4 The working principle of the buck-boost circuit is shown;

[0034] Figure 5 The CCM inductor current and gate voltage are shown;

[0035] Figure 6 The DCM inductor current and gate voltage are shown;

[0036] Figure 7 The flowchart of the maximum efficiency tracking method for wireless power transfer is shown;

[0037] Figure 8 The voltage and duty cycle change curves when the load changes are shown;

[0038] Figure 9 The real-time identification result of the coupling coefficient is shown;

[0039] Figure 10 The equivalent resistance and coupling coefficient waveforms under variable load conditions are shown;

[0040] Figure 11 The power change curve under variable coupling coefficient is shown;

[0041] Figure 12 The output characteristic curve under variable load is shown. Detailed Description of the Preferred Embodiments

[0042] Exemplary embodiments of the present application will be described below in conjunction with the accompanying drawings. For clarity and conciseness, not all features of the actual embodiments are described in the specification. However, it should be understood that many embodiment-specific decisions may be made during the development of any such actual embodiment to achieve the specific goals of the developer, and these decisions may vary with different embodiments.

[0043] Here, it should also be noted that in order to avoid obscuring the present application with unnecessary details, only the device structures closely related to the solution of the present application are shown in the drawings, while other details less related to the present application are omitted.

[0044] It should be understood that the present application is not limited to the described embodiments due to the following description with reference to the accompanying drawings. In this document, where feasible, embodiments can be combined with each other, features can be replaced or borrowed between different embodiments, and one or more features can be omitted in one embodiment.

[0045] In order to ensure high-efficiency and stable power transmission of wireless charging under constant voltage output, this application analyzes the constant voltage output and maximum efficiency tracking control methods, aims to maintain a constant power output and high-efficiency operation, and reduce the output voltage fluctuation. Considering the system power fluctuation problems under coil offset and variable load conditions, a maximum efficiency tracking system and method for wireless power transmission are designed.

[0046] Figure 1 The structural block diagram of the maximum efficiency tracking system for wireless power transmission is shown. Refer to Figure 1 , the system includes: a DC power supply, a transmitter Buck-Boost circuit, a full-bridge inverter circuit, an LCC-S compensation topology, a full-bridge rectifier circuit, a receiver Buck-Boost circuit, a transmitter controller, and a receiver controller. The specific implementation functions of each module will be introduced in detail below.

[0047] The transmitter controller collects the voltage and current of the transmitter coil in the LCC-S compensation topology, and the receiver controller collects the voltage and current of the receiver coil and the load voltage in the LCC-S compensation topology, and transmits the voltage, current, and load voltage of the receiver coil to the transmitter controller through wireless communication;

[0048] The transmitter controller based on the voltage U1 and current I1 of the transmitter coil, the voltage U2 and current I2 of the receiver coil, and the load voltage U L, generate the duty cycle of the switching tube of the transmitting-end Buck-Boost circuit, denoted as the first duty cycle d1, perform pulse modulation on the first duty cycle d1, and send the pulse-modulated first duty cycle to the transmitting-end Buck-Boost circuit. The transmitting-end Buck-Boost circuit performs PI control and phase-shift control according to the pulse-modulated first duty cycle, so that the load voltage U L reaches the set value; here, the process of generating the first duty cycle d1 is a conventional means in the art, and PI control and phase-shift control include adjusting the phases of the switching tubes in the transmitting-end Buck-Boost circuit, which also belong to the conventional means in the art, such as adjusting the phases of the switching tubes G1, G2, G3, and G4. Figure 2 shows the circuit topology of the wireless charging system, including a DC power supply, a transmitting-end Buck-Boost circuit, a full-bridge inverter circuit, an LCC-S compensation topology, a full-bridge rectifier circuit, and a receiving-end Buck-Boost circuit. Among them, L1 is the self-inductance of the transmitting coil, L2 is the self-inductance of the receiving coil, R1 is the internal resistance of the transmitting coil, R2 is the internal resistance of the receiving coil, U s is the DC power supply voltage, U1 and I1 are the voltage and current of the transmitting coil respectively, U2 and I2 are the voltage and current of the receiving coil respectively, the switching tubes G1, G2, G3, and G4 constitute the full-bridge inverter circuit, the switching tubes D1, D2, D3, and D4 constitute the full-bridge rectifier circuit, U L , I L are the voltage and current on the load resistor R L . Z s is the equivalent output impedance of the receiving-end circuit, R eq is the equivalent input impedance of the rectifier circuit, R in is the equivalent input impedance of the impedance matching network, Z ref is the reflection impedance from the receiving end to the transmitting end, Z T is the equivalent impedance of the transmitting end.

[0049] The transmitting-end controller calculates the coupling coefficient k according to the voltage U1, current I1 of the transmitting coil and the voltage U2, current I2 of the receiving coil, and calculates the optimal equivalent resistance R eq-ηmax at the maximum transmission efficiency of the system according to the coupling coefficient k, and transmits the optimal equivalent resistance R eq-ηmax to the receiving-end controller through wireless communication; the receiving-end controller calculates according to the optimal equivalent resistance R eq-ηmax, generates a duty cycle for adjusting the switch tube of the receiving-end Buck-Boost circuit, recorded as the second duty cycle d2, and pulse modulates the second duty cycle d2, and sends the pulse-modulated second duty cycle to the receiving-end Buck-Boost circuit. The receiving-end Buck-Boost circuit adjusts the switch tube Q in the receiving-end Buck-Boost circuit according to the pulse-modulated second duty cycle. b phase until the second duty cycle reaches the target value to achieve maximum efficiency tracking.

[0050] In this embodiment, the compensation characteristics of four basic resonant compensation networks are analyzed through theoretical models. The LCC-S high-order compensation topology circuit is selected based on the low-order compensation topology. The influencing factors of transmission power and transmission efficiency are analyzed, and LCC-S is selected as the resonant topology structure of the system.

[0051] The four basic compensation networks offer either constant current or constant voltage output characteristics. With the exception of the SS compensation topology, the compensation capacitor in the other three compensation topologies changes with changes in mutual inductance. The SS compensation capacitor value is independent of mutual inductance and load, offering excellent anti-offset performance, and the system output remains unaffected by changes in load and mutual inductance. The SP compensation capacitor value is not only dependent on the resonant frequency but is also affected by coil inductance and mutual inductance. Changes in mutual inductance can cause system frequency offsets, increasing the complexity of closed-loop control. Due to parameter sensitivity, the PP and PS types are typically used in statically coupled systems, and parallel compensation configurations are not suitable for the transmitter.

[0052] To further improve the performance of the wireless charging system, an LCC-S advanced compensation topology circuit is proposed based on the basic compensation topology. Compensation capacitors and inductors are added to meet the system's requirements for constant current and constant voltage charging, thereby improving the system's filtering capability and stability.

[0053] Figure 3 The LCC-S transmission efficiency characteristic curve is shown. The coupling coefficient ranges from 0 to 0.3, and the load resistance ranges from 0 to 100Ω, resulting in a transmission efficiency range of 0 to 1. With the LCC-S compensation topology, transmission efficiency is affected by both the load and the coupling coefficient. A larger coupling coefficient results in higher system transmission efficiency and a smaller effect of load resistance. When the coupling coefficient remains below 0.2 and constant, transmission efficiency decreases as load resistance increases.

[0054] In summary, the LCC-S compensation topology has good circuit characteristics, can achieve constant voltage or constant current output of the wireless charging system. By adding a resonant capacitor to the transmitter circuit, the compensation inductor can be effectively reduced, and the size of the compensation inductor can be reduced. The series topology structure is selected for the receiver, and the circuit structure is simple, which is beneficial to reducing the number of components and lowering the vehicle-mounted energy consumption. When adjusting the compensation capacitor and inductor, the system performance is optimized. Considering the actual application requirements, the LCC-S compensation topology is used as the basis for circuit design.

[0055] In addition, a buck-boost circuit model is established, and the working principle of the circuit is analyzed through the charging process. Figure 4 The working principle of the buck-boost circuit is shown. According to whether the inductor current drops to zero during the freewheeling stage, it is divided into the DCM mode (discontinuous inductor current mode) and the CCM mode (continuous inductor current mode). The DCM mode and CCM mode of the Buck-Boost circuit are theoretically analyzed. Figure 5 The CCM inductor current and gate voltage are shown. Figure 6 The DCM inductor current and gate voltage are shown. Through analysis, it is found that when the Buck-Boost converter operates in the DCM mode, the equivalent impedance is only related to the inductor value, switching period, and duty cycle, and is independent of the input and output voltages and the load. When the load resistance changes, the equivalent impedance of the Buck-Boost circuit will not be affected. Therefore, the Buck-Boost in the DCM mode is more suitable for the impedance matching circuit design under variable loads, and the DCM mode can be selected for the wireless charging impedance matching circuit design.

[0056] Through the analysis of the CCM mode and DCM mode of the Buck-Boost circuit, the voltage output expression in the CCM mode is simpler. The Buck-Boost conversion circuit in the CCM mode is selected for the transmitter of the wireless charging circuit, and PI control is used to control the output voltage. In the constant voltage output control of the wireless charging system, the voltage data at the load end is collected and sent to the controller, compared with the output voltage set value to obtain the error variable, the voltage error variable is normalized, and after being processed by the PI controller, the control quantity is obtained. The control quantity is limited, and compared with the triangular wave to generate PWM pulses, adjusting the duty cycle of the switching tube, and adjusting the voltage to quickly return to the set value, so as to achieve the constant voltage output of the system.

[0057] Therefore, in this embodiment, the transmitter Buck-Boost circuit is a Buck-Boost circuit in the CCM mode, and the receiver Buck-Boost circuit is a Buck-Boost circuit in the DCM mode.

[0058] Specifically, the coupling coefficient is calculated using the following formula:

[0059]

[0060] Among them, k is the coupling coefficient, U1 and I1 are the voltage and current of the transmitting coil respectively, U2 and I2 are the voltage and current of the receiving coil respectively, R1 is the internal resistance of the transmitting coil, R2 is the internal resistance of the receiving coil, L1 is the self-inductance of the transmitting coil, L2 is the self-inductance of the receiving coil, and ω is the angular frequency.

[0061] Specifically, the optimal equivalent resistance is calculated using the following formula:

[0062]

[0063] Among them, R eq-ηmax is the optimal equivalent resistance, R1 is the internal resistance of the transmitting coil, R2 is the internal resistance of the receiving coil, ω is the angular frequency, k is the coupling coefficient, L1 is the self-inductance of the transmitting coil, and L2 is the self-inductance of the receiving coil.

[0064] Specifically, the second duty cycle is calculated using the following formula:

[0065]

[0066] Among them, d 2-ηmax is the second duty cycle, L b is the self-inductance of the coil in the receiving-end Buck-Boost circuit, f s2 is the switching frequency of the receiving-end Buck-Boost circuit, and R eq-ηmax is the optimal equivalent resistance.

[0067] The embodiment of the present application also provides a method for tracking the maximum efficiency of wireless power transmission. Figure 7 The flowchart of the method for tracking the maximum efficiency of wireless power transmission is shown. Refer to Figure 7 , the method includes:

[0068] Step 1: Connect to the DC power supply and perform initialization settings; set the input voltage U s and the load resistance R L , and the system completes wireless power transmission through an inverter circuit, an LCC-S compensation topology, a rectifier circuit, etc.

[0069] Step 2: The transmitting-end controller calculates the coupling coefficient according to the voltage and current of the transmitting coil and the voltage and current of the receiving coil.

[0070] Step 3: Calculate the optimal equivalent resistance at the maximum transmission efficiency of the system according to the coupling coefficient.

[0071] Step 4: The transmitter controller generates the duty cycle of the switching tube of the transmitter Buck-Boost circuit, denoted as the first duty cycle, based on the voltage and current of the transmitter coil, the voltage and current of the receiver coil, and the load voltage. Then, pulse modulation is performed on the first duty cycle, and the pulse-modulated first duty cycle is sent to the transmitter Buck-Boost circuit. The transmitter Buck-Boost circuit performs PI control and phase-shift control according to the pulse-modulated first duty cycle to make the load voltage reach the set value;

[0072] Step 5: Determine whether the coupling coefficient has changed. If so, execute Step 6; if not, execute Step 7;

[0073] Step 6: Determine whether the load voltage has changed. If so, return to Step 4; if not, return to Step 2;

[0074] Step 7: The receiver controller generates the duty cycle of the switching tube of the receiver Buck-Boost circuit, denoted as the second duty cycle, based on the optimal equivalent resistance. Then, pulse modulation is performed on the second duty cycle, and the pulse-modulated second duty cycle is sent to the receiver Buck-Boost circuit. The receiver Buck-Boost circuit adjusts the phase of the switching tube in the receiver Buck-Boost circuit according to the pulse-modulated second duty cycle until the second duty cycle reaches the target value to achieve maximum efficiency tracking.

[0075] In this embodiment, after the initialization settings of the wireless power transfer system circuit are completed, the identification of the coupling coefficient and the calculation of the equivalent optimal impedance are performed. When the system circuit parameters change, that is, in the case of offset, the current coupling coefficient is calculated, the output constant voltage is adjusted, and the equivalent impedance is adjusted to the maximum efficiency tracking point. When the load resistance changes, it is determined whether the load voltage has changed. If the voltage remains unchanged, the current coupling coefficient is calculated, and the equivalent impedance is continuously adjusted to the maximum efficiency tracking point. If the voltage changes, it means that both the output voltage and the transmission efficiency point have changed. The receiver controller calculates the optimal load to adjust the duty cycle of the Buck-Boost switching tube in the CCM mode and the DCM mode to complete the maximum transmission efficiency tracking under constant voltage output.

[0076] For the maximum efficiency tracking method of wireless power transfer in this embodiment, the specific implementation manner can be seen in the embodiment part of the wireless power transfer maximum efficiency tracking system in the previous text, and its technical effects correspond to those of the above method, which will not be elaborated here.

[0077] In summary, the present application has the following technical effects:

[0078] 1. The duty cycle of the switching tube in the Buck - Boost circuit under CCM mode is adjusted by PI control to achieve stable control of the output voltage at the receiving end. Considering that when the load resistance changes, the output voltage at the receiving end will vary with the resistance, a Buck - Boost circuit in CCM mode is added at the transmitting end. By adjusting the duty cycle of the switching tube through PI control, voltage stability of the system under variable load conditions is achieved.

[0079] 2. A Buck - Boost circuit in DCM mode is added at the load end. Based on the LCC - S compensation topology circuit, through the transmission efficiency expression, the expression of the optimal impedance is obtained, and then the calculation expression of the coupling coefficient is derived to achieve real - time identification of the coupling coefficient.

[0080] 3. When the system coupling coefficient and load resistance change, the duty cycle of the switching tube is dynamically adjusted to achieve maximum transmission efficiency tracking based on coupling coefficient identification in the wireless charging system under variable load conditions.

[0081] To further verify the effectiveness of the system and method of this application, a wireless charging maximum transmission efficiency tracking control circuit based on coupling coefficient identification is built, and constant - voltage output tests, coupling coefficient identification tests, and maximum efficiency tracking tests are carried out.

[0082] (1) Build a simulation circuit for maximum transmission efficiency tracking of the wireless charging system

[0083] A wireless charging simulation circuit and a simulation circuit for maximum transmission efficiency tracking of the wireless charging system are built using Simulink, including parts such as PI control, high - frequency inverter circuit, rectifier filter circuit, LCC - S compensation topology, and maximum transmission efficiency tracking control. Based on the simulation circuit of the wireless charging system maximum transmission efficiency tracking simulation circuit, constant - voltage output tests, coupling coefficient identification, and maximum transmission efficiency tracking tests are carried out through Simulink.

[0084] If experimental tests are carried out based on the wireless charging system simulation circuit, the circuit parameters of the wireless charging system are shown in Table 1.

[0085] Table 1 Circuit simulation parameters of the wireless charging system

[0086]

[0087] (2) Constant - voltage output test of the wireless charging system

[0088] During the constant - voltage output test, other circuit parameters are kept unchanged. At this time, the set value of the output voltage at the receiving end is 200V. The initial value of the load resistance is 10Ω, switched to 20Ω at 0.2s, and switched to 30Ω at 0.4s. Figure 8The voltage and duty cycle change curves when the load changes are shown. After 0.5 ms, the voltage and duty cycle values at the load end tend to be stable. The initial load resistance is 10 Ω, which is switched to 20 Ω at 0.2 s and to 30 Ω at 0.4 s. During the switching process of the load resistance from 10 Ω to 20 Ω and then to 30 Ω, the duty cycle value gradually decreases, and the output terminal voltage value stabilizes at the set value of 200 V within a very short time. When the load resistance changes from 10 Ω to 20 Ω at 0.2 s, the output terminal voltage value remains at 200.1 V, and the duty cycle decreases from 0.555 to 0.466; when the load resistance changes from 20 Ω to 30 Ω at 0.4 s, the voltage remains at 200 V, and the duty cycle decreases from 0.466 to 0.415.

[0089] Adopting PI control to adjust the duty cycle of the switching tube in the Buck - Boost circuit under CCM mode can achieve stable output of the receiving - end voltage, maintain the stability of the output terminal voltage under variable load resistance conditions, and the voltage fluctuation is less than 1%. That is, during the change process of the load resistance, the output terminal voltage has good stability, can realize reliable calculation of the optimal load and coupling coefficient, and is applicable to the control strategy design of a wireless charging system with variable load.

[0090] (3) Coupling coefficient identification test of wireless charging system

[0091] When the transmitting and receiving coils are offset, the coupling performance between the coils decreases, and the coupling coefficient changes with the offset distance. For the rounded - rectangle coil, the simulation values and identification values of the coupling coefficient within the offset distance of 0 - 220 mm are obtained through Maxwell and MATLAB, as shown in Table 2.

[0092] Table 2 Coupling coefficient data when the coil is offset

[0093]

[0094] Within the deviation distance of 0 - 220 mm, Figure 9 the real - time identification results of the coupling coefficient are shown. As can be seen from Figure 9 it, during the offset process of the coupling coils, the coupling coefficient gradually decreases with the increase of the offset distance. During the real - time identification process of the coupling coefficient, the coupling coefficient is adjusted through the equivalent impedance, and the theoretical value and the identification value of the coupling coefficient are basically the same, verifying the feasibility of the real - time identification of the coupling coefficient.

[0095] When identifying and testing the coupling coefficient under variable load conditions, that is, when the coupling coefficient is maintained at 0.18, the load resistance is switched from 10 Ω, 20 Ω to 30 Ω, Figure 10 the waveforms of the equivalent resistance and the coupling coefficient under variable load are shown. From Figure 10It can be seen that when the load resistance changes, the coupling coefficient and equivalent resistance value are not affected by the load resistance after adding the Buck-Boost circuit in DCM mode. This control strategy realizes the real-time identification of the coupling coefficient and ensures its accuracy.

[0096] (4) Maximum transmission efficiency tracking test under LCC-S and SS compensation topologies

[0097] Under the conditions of variable load and variable coupling coefficient, the maximum transmission efficiency tracking test of the wireless charging system under LCC-S and SS compensation topologies is compared.

[0098] The load resistor value is fixed at 12Ω, and the transmitting coil is varied within the offset distance of 0 to 200mm. The maximum transmission efficiency tracking test under variable coupling coefficient is performed by changing the offset distance. The changes in the system's equivalent optimal resistance, transmission efficiency, and duty cycle are shown in Table 3.

[0099] Table 3 System simulation data under variable coupling coefficient conditions

[0100]

[0101]

[0102] It can be seen from Table 3 that the coupling coefficient changes during the coil offset process, and the system load equivalent resistance and optimal duty cycle also change. Figure 11 The power variation curve under variable coupling coefficient is shown. Figure 11 It can be seen that during coil deflection, the output power remains at around 3.3kW. The transmission efficiency of the SS compensation topology is around 89%, while the LCC-S compensation topology achieves a maximum transmission efficiency of 92.4%, a 3.4% improvement. During coil deflection, the switching duty cycle is adjusted in real time by identifying the coupling coefficient. The equivalent optimal load is also dynamically adjusted based on the coupling coefficient change, ensuring that the system always operates at maximum transmission efficiency.

[0103] The coupling coefficient was fixed and identified under variable load conditions. When the coupling coefficient was fixed at 0.18 and the load resistance varied from 10 to 60 Ω, the system's equivalent optimal resistance, transmission efficiency, and duty cycle were shown in Table 4.

[0104] Table 4 System simulation data under variable load conditions

[0105]

[0106]

[0107] Keeping the coupling coefficient unchanged, during the change process of the load resistance value among 10Ω, 20Ω and 30Ω, Figure 12 The output characteristic curves under variable load are shown. When the coupling coefficient remains unchanged, the load terminal resistance value is changed at 0.05s and 0.1s. After a short-term fluctuation, the output power and transmission efficiency quickly stabilize. The transmission efficiency remains at 90% throughout the process and can reach up to 92.3% at most, which is about 3% higher than that of the S-S compensation topology circuit. In summary, when the wireless charging system introduces the Buck-Boost impedance matching circuit in the DCM mode under the LCC-S compensation topology structure, keeping the coupling coefficient fixed, the equivalent optimal resistance value is not greatly affected by the load resistance value. As the load resistance value increases, the equivalent optimal duty cycle of the system will change with the resistance value, and the system transmission efficiency is stable at about 90%.

[0108] As described above, it is only various embodiments of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed in the present application can easily think of changes or substitutions, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims described above.

Claims

1. A maximum efficiency tracking system for wireless power transfer, characterized in that, Including: A DC power supply, a transmitter Buck - Boost circuit, a full - bridge inverter circuit, an LCC - S compensation topology, a full - bridge rectifier circuit, a receiver Buck - Boost circuit, a transmitter controller, and a receiver controller; The transmitter controller collects the voltage and current of the transmitter coil in the LCC - S compensation topology, and the receiver controller collects the voltage and current of the receiver coil and the load voltage in the LCC - S compensation topology, and transmits the voltage and current of the receiver coil and the load voltage to the transmitter controller through wireless communication; The transmitter controller generates a duty cycle for adjusting the switching tube of the transmitter Buck - Boost circuit according to the voltage and current of the transmitter coil, the voltage and current of the receiver coil, and the load voltage, denoted as the first duty cycle, performs pulse modulation on the first duty cycle, and sends the pulse - modulated first duty cycle to the transmitter Buck - Boost circuit. The transmitter Buck - Boost circuit performs PI control and phase - shift control according to the pulse - modulated first duty cycle to make the load voltage reach the set value; The transmitter controller calculates the coupling coefficient according to the voltage and current of the transmitter coil and the voltage and current of the receiver coil, calculates the optimal equivalent resistance at the maximum transmission efficiency of the system according to the coupling coefficient, and transmits the optimal equivalent resistance to the receiver controller through wireless communication. The receiver controller generates a duty cycle for adjusting the switching tube of the receiver Buck - Boost circuit according to the optimal equivalent resistance, denoted as the second duty cycle, performs pulse modulation on the second duty cycle, and sends the pulse - modulated second duty cycle to the receiver Buck - Boost circuit. The receiver Buck - Boost circuit adjusts the phase of the switching tube in the receiver Buck - Boost circuit according to the pulse - modulated second duty cycle until the second duty cycle reaches the target value to achieve maximum efficiency tracking.

2. The system according to claim 1, wherein The coupling coefficient is calculated using the following formula: Where k is the coupling coefficient, U1 and I1 are the voltage and current of the transmitter coil respectively, U2 and I2 are the voltage and current of the receiver coil respectively, R1 is the internal resistance of the transmitter coil, R2 is the internal resistance of the receiver coil, L1 is the self - inductance of the transmitter coil, L2 is the self - inductance of the receiver coil, and ω is the angular frequency.

3. The system according to claim 1, characterized in that, The optimal equivalent resistance is calculated using the following formula: Among them, R eq-ηmax is the optimal equivalent resistance, R1 is the internal resistance of the transmitting coil, R2 is the internal resistance of the receiving coil, ω is the angular frequency, k is the coupling coefficient, L1 is the self-inductance of the transmitting coil, and L2 is the self-inductance of the receiving coil.

4. The system according to claim 1, wherein The second duty cycle is calculated using the following formula: where d 2-ηmax is the second duty cycle, L b is the self-inductance of the coil in the receiver Buck-Boost circuit, f s2 is the switching frequency of the receiver Buck-Boost circuit, and R eq-ηmax is the optimal equivalent resistance.

5. A method for tracking the maximum efficiency of wireless power transfer, characterized in that, Including: Step 1: Connect the DC power supply and perform initialization settings; Step 2: The transmitter controller calculates the coupling coefficient according to the voltage and current of the transmitter coil and the voltage and current of the receiver coil; Step 3: Calculate the optimal equivalent resistance at the maximum transmission efficiency of the system according to the coupling coefficient; Step 4: The transmitter controller generates the duty cycle of the switching tube of the transmitter Buck-Boost circuit, denoted as the first duty cycle, based on the voltage and current of the transmitter coil, the voltage and current of the receiver coil, and the load voltage. Then, pulse modulation is performed on the first duty cycle, and the pulse-modulated first duty cycle is sent to the transmitter Buck-Boost circuit. The transmitter Buck-Boost circuit performs PI control and phase-shift control according to the pulse-modulated first duty cycle to make the load voltage reach the set value; Step 5: Determine whether the coupling coefficient has changed. If so, execute Step 6; if not, execute Step 7; Step 6: Determine whether the load voltage has changed. If so, return to Step 4; if not, return to Step 2; Step 7: The receiver controller generates the duty cycle of the switching tube of the receiver Buck-Boost circuit, denoted as the second duty cycle, based on the optimal equivalent resistance. Then, pulse modulation is performed on the second duty cycle, and the pulse-modulated second duty cycle is sent to the receiver Buck-Boost circuit. The receiver Buck-Boost circuit adjusts the phase of the switching tube in the receiver Buck-Boost circuit according to the pulse-modulated second duty cycle until the second duty cycle reaches the target value to achieve maximum efficiency tracking.

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