A dynamic wireless charging resonance coil design method for electric vehicles

Abstract

The present invention discloses a method for designing a dynamic wireless charging resonant coil for electric vehicles in the field of wireless charging, comprising the following steps: Step 1, setting target parameters: setting the front-stage input voltage U of the wireless charging system according to the front-stage and rear-stage circuits and power level requirements of the wireless charging system; in And output voltage U out , system output power P out , the vertical distance between the primary and secondary coils; Step 2, select the resonant compensation network and resonant frequency of the dynamic wireless charging system; Step 3, determine the resonant inductance value in the resonant compensation network; Step 4, determine the mutual inductance coefficient of the resonant coil; Step 5, select the Litz wire; Step 6, determine the constraints; Step 7, determine the minimum size of the coil and the maximum length of the winding; Step 8, wind the resonant coil; Step 9, optimize the core structure; Step 10, determine the placement spacing of the primary coil; Under the premise of meeting the given requirements, optimize the electrical and physical properties of the resonant coil.

Description

TECHNICAL FIELD

[0001] The present application relates to a resonant coil design method in the field of wireless charging. BACKGROUND

[0002] With the development of the times, energy shortage and environmental pollution problems are increasingly prominent, and the large use of fossil fuels has exacerbated this phenomenon. Traditional diesel vehicles are one of the main ways to use fossil fuels. In order to solve the above problems, it is urgent to improve the energy structure.

[0003] Electric vehicles are considered by many researchers as an effective alternative to traditional diesel vehicles. The related field has also been extensively studied. At present, one of the reasons why electric vehicles are difficult to popularize is that the power battery cannot support long-time driving, and there is a lack of reliable and fast charging solutions. Dynamic wireless charging (Dynamic wireless Power Transmission, DWPT) technology is to lay charging equipment on the ground and charge the vehicle during driving. This technology can effectively improve the endurance mileage of electric vehicles.

[0004] Due to the immaturity of the technology, the performance of the dynamic wireless charging technology of the electric vehicle cannot meet the use requirements, and this technology has not been widely used. The resonant coil is one of the main reasons that limits the performance of wireless charging. As the core component of DWPT, the design of the resonant coil plays a decisive role in the transmission efficiency, transmission power and offset tolerance of the system. The current design scheme of the resonant coil mostly focuses on its static performance, and ignores the influence of the coil parameters on the resonant compensation network. Therefore, an optimized scheme of the resonant coil with high power, high efficiency, good dynamic performance and adaptability to the resonant network is urgently needed. SUMMARY

[0005] The purpose of the present application is to provide a dynamic wireless charging resonant coil design method for electric vehicles, which optimizes the electrical and physical performance of the resonant coil under the premise of meeting the given requirements.

[0006] In order to achieve the above purpose, the present application also provides a dynamic wireless charging resonant coil design method for electric vehicles, which comprises the following steps:

[0007] Step 1, given the target parameters:

[0008] According to the front and rear circuits of the wireless charging system and the power level requirements, the input voltage U in and the output voltage U out of the wireless charging system, the system output power P out , and the vertical distance of the primary and secondary coils are set.

[0009] Step 2, select the dynamic wireless charging system resonance compensation network and resonance frequency;

[0010] Step 3, determine the resonance inductance value in the resonance compensation network;

[0011] Step 4, determine the mutual inductance coefficient of the resonance coil;

[0012] Step 5, Litz wire selection;

[0013] Step 6, determine the constraint condition;

[0014] Step 7, determine the minimum size of the coil and the maximum length of the winding;

[0015] Step 8, winding of the resonance coil;

[0016] Step 9, core structure optimization;

[0017] Step 10, determine the primary coil placement spacing.

[0018] Compared with the prior art, the beneficial effects of the present application are that, from the given design target and constraint condition, the parameters of the resonance coil are designed, so that the electrical performance and physical performance of the resonance coil are optimized under the premise of meeting the given requirements.

[0019] As a further improvement of the present application, the specific content of step 2 is as follows,

[0020] The resonance frequency of the resonance compensation network is set to 85KHz, according to the characteristic that the relative position between the resonance coils is not constant, the relative position is always changing frequently and quickly, and the position change direction is mainly in the moving direction of the automobile, the LCC-LCC topology is selected as the resonance compensation network of the coil, and the DD type coil is selected as the resonance coil structure.

[0021] As a further improvement of the present application, the specific content of step 3 is as follows,

[0022] The voltage type inverter is adopted, and the harmonic third harmonic component in the circuit accounts for the fundamental content, which can be calculated by the following formula (1),

[0023]

[0024] Here, the value of the resonance inductance needs to meet

[0025] I 1st is the fundamental wave of the inverter output current, I 3st is the third harmonic of the inverter output current, L f is the resonance inductance in the resonance compensation network, M is the mutual inductance of the resonance coil, U out is the output voltage;

[0026] The primary and secondary resonant coils and the resonant compensation network are symmetrical, that is, the resonant inductor L f =L1=L4, resonant coil self-inductance L=L2=L3, resonant capacitor C f =C1=C4, compensation capacitor C P =C2=C3.

[0027] As a further improvement of the present invention, the specific contents of step 4 are as follows:

[0028] According to the power formula (2) of the wireless charging system, the mutual inductance required to meet the power level can be obtained by combining the given input voltage, output voltage and the determined inductance coefficient; where formula (2) is as follows:

[0029]

[0030] M is the mutual inductance of the resonant coil, ω0 is the resonant angular frequency, U in is the input voltage, L f is the resonant inductance value, and Uout is the output voltage.

[0031] As a further improvement of the present invention, the specific contents of step 5 are as follows:

[0032] Based on the resonant inductance parameters and input voltage calculated above, the current in the resonant coil is calculated by formula (3). Based on the system operating frequency of 85KHz, copper is selected as the conductor material, and its skin depth is calculated. Based on the skin depth, a single-strand Litz wire with a wire diameter of 0.1mm is selected. Based on the current in the resonant coil, the number of Litz wire strands is selected according to a current margin of 1.5 to 2 times. Formula (3) is as follows:

[0033]

[0034] I L is the resonant coil current.

[0035] As a further improvement of the present invention, the specific content of step 6 is as follows:

[0036] Determine the size requirements of the resonant coil based on the actual installation conditions. The size requirements include the length, width, and height of the resonant coil. The length and width mainly limit the plane size of the resonant coil, while the height limits the thickness of the resonant coil's magnetic core and the thickness of the shielding plate.

[0037] The primary and secondary side coils and the resonance compensation network adopt a symmetrical structure, and it is considered that the related parameters are completely consistent. According to the formula (4) and the formula (5), in combination with the input and output power and the resonance inductance value, the coupling coefficient and the self-induction parameter that meet the voltage stress in the resonance compensation network can be calculated respectively; according to the formula (6), the maximum alternating current resistance of the resonance coil that meets the demand of transmission efficiency can be obtained; wherein the formula (4), the formula (5) and the formula (6) are as follows:

[0038]

[0039]

[0040]

[0041] In the formula (6):

[0042]

[0043]

[0044] Z1=jωL1;

[0045]

[0046]

[0047] In the formula (4) and the formula (5), U c1 and U c2 are the resonance capacitor voltage of the transmitting end and the compensation capacitor voltage of the transmitting end respectively, and k is the coupling coefficient of the resonance coil.

[0048] In the formula (6), R3 and R4 are the internal resistances of the resonance coils L3 and L4 respectively.

[0049] Up to now, all the electrical parameters of the resonance coil and the constraint conditions thereof have been determined.

[0050] As a further improvement of the application, the specific content of the step 7 is as follows,

[0051] According to the characteristics of dynamic wireless charging, a DD type coil with a length-width ratio of 1.5 is selected, which is considered to be able to balance the dynamic performance and static performance under this ratio. The magnetic core structure of the coil adopts a flat plate type, which is covered on the top of the DD type coil, and the size is slightly larger than the window size of the DD type coil, but smaller than the external size of the coil; at this time, the setting condition of the minimum size of the coil is that when winding a coil from inside to outside under the transmission distance and the size of the resonance coil, the coupling coefficient of the resonance coil just meets the above constraint condition;

[0052] Based on the selected Litz wire type, calculate its AC resistance per unit length. Then calculate the maximum winding length of the resonant coil. This length is defined as: when the resonant coil is wound to this length, the AC resistance of the resonant coil is equal to the maximum AC resistance of the resonant coil that meets the transmission efficiency.

[0053] At this point, all the dimensional parameter constraints of the resonant coil are determined.

[0054] As a further improvement of the present invention, the specific contents of step 8 are as follows:

[0055] Take the Litz wire with the maximum length of the winding of the above-mentioned resonant coil, constrain the maximum size of the coil to the outer size of the DD-type coil, and wind it from the outside to the inside in a planar spiral dense winding manner. After winding is completed, its mutual inductance value is measured; if the mutual inductance is less than the required value, reduce the size of the outer ring of the resonant coil and wind it again until the mutual inductance requirement value is met; if the coil windings are all within the minimum coil minimum size at this time, it is necessary to change the constraint conditions to design a resonant coil that meets the requirements.

[0056] As a further improvement of the present invention, the specific contents of step 9 are as follows:

[0057] The finite element simulation software Maxwell is used to simulate the coil structure obtained above. The core is thickened at the position where the magnetic flux density is greater than 0.3T, thereby achieving the purpose of reducing loss and preventing core saturation.

[0058] As a further improvement of the present invention, the specific contents of step 10 are as follows:

[0059] According to the circulation constraint between the primary coils and formula (2), the minimum mutual inductance coefficient between the primary coils can be obtained. When the mutual inductance coefficient between the primary coils is equal to this value, the primary coil spacing is defined as the minimum primary coil spacing. According to the resonant coil obtained above, the relationship curve between the mutual inductance coefficient and the offset of the resonant coil in the direction of vehicle travel at a given transmission distance is measured, with the offset as the X-axis and the mutual inductance coefficient as the Y-axis. The curve is moved in the positive direction of the X-axis by a, 2a, 3a, ..., na (the a value is greater than the minimum primary coil spacing), and then these curves are summed with the original curve to obtain a new curve as the relationship between the equivalent mutual inductance coefficient and the position. The a value that minimizes the fluctuation rate of the equivalent mutual inductance curve and the position relationship curve is taken as the spacing of the primary coils. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 Schematic diagram of the resonant compensation circuit of the present invention. DETAILED DESCRIPTION

[0061] The present invention will be further described below in conjunction with the accompanying drawings:

[0062] As Figure 1 shown in one kind of long class parts end automatic forging device, the specific steps are as follows,

[0063] Step 1, given the target parameters:

[0064] According to the front and rear circuit of wireless charging system and power level demand, set the input voltage U in of the front stage of wireless charging system (namely the voltage of external charging port) and the output voltage U out (namely the battery charging voltage), the system output power P out , the vertical distance of the primary and secondary coils.

[0065] Step 2, select dynamic wireless charging system resonance compensation network and resonance frequency;

[0066] According to the wireless charging related standard SAE J2954 released by SAE (Society of Automotive Engineers), set the resonance frequency of the resonance compensation network to 85KHz. All the following parameters are designed based on this frequency. When selecting the resonance topology compensation network and resonance coil structure of the system, the characteristics of the dynamic wireless charging of electric vehicles are considered. The main feature of dynamic wireless charging system is that the relative position between the resonance coils is not constant, and the relative position is always changing frequently and quickly, and the position change direction is mainly in the moving direction of the car. According to the above characteristics, LCC-LCC topology is selected as the resonance compensation network of the coil. Select DD type coil as the resonance coil structure.

[0067] Step 3, determine the resonance inductance value in the resonance compensation network;

[0068] Adopt voltage type inverter, the harmonic three harmonic components in the circuit account for the fundamental content can be calculated by the following formula (1),

[0069]

[0070] Here the value of the resonance inductance needs to meet

[0071] I 1st is the fundamental of the inverter output current, I 3st is the third harmonic of the inverter output current, L f is the resonance inductance in the resonance compensation network, M is the mutual inductance of the resonance coil, U out is the output voltage;

[0072] It should be noted that the primary and secondary resonance coils and the resonance compensation network of the system are symmetrical. Therefore, the electrical parameters of the primary and secondary sides are completely consistent and are not distinguished. That is, the resonance inductance L f= L1 = L4, resonance coil self-inductance L = L2 = L3, resonance capacitor C f = C1 = C4, compensation capacitor C P = C2 = C3. Where the parameter definition reference attached Figure 1 .

[0073] Step 4, determine the resonance coil mutual inductance coefficient;

[0074] According to the power formula (2) of the wireless charging system, combined with the given input voltage, output voltage and the determined inductance coefficient, the mutual inductance coefficient required to meet the power level can be obtained; wherein formula (2) is as follows:

[0075]

[0076] M is the resonance coil mutual inductance, ω0 is the resonance angular frequency, U in is the input voltage, L f is the resonance inductance value, Uout is the output voltage.

[0077] Step 5, Litz wire selection;

[0078] According to the resonance inductance parameters and input voltage calculated above, the current in the resonance coil is calculated by formula (3), according to the system working frequency 85KHz, copper is selected as the wire material, the skin depth is calculated, according to the skin depth, Litz wire with single wire diameter of 0.1mm is selected; according to the current in the resonance coil, the number of Litz wire strands is selected according to 1.5-2 times current margin; wherein formula (3) is as follows:

[0079]

[0080] I L is the resonance coil current.

[0081] Step 6, determine the constraint condition;

[0082] According to the actual installation conditions, the size requirements of the resonance coil are determined, including the length, width and height of the resonance coil; the length and width mainly limit the plane size of the resonance coil, and the height mainly limits the thickness of the magnetic core and the thickness of the shielding plate;

[0083] In the wireless charging system, the primary and secondary coils and the resonant compensation network adopt a symmetrical structure, and it is considered that the related parameters are completely consistent, so the capacitor voltage in the resonant compensation network of the receiving end and the formula of the transmitting end are similar. According to formula (4) and formula (5), combined with the input and output power and the resonant inductance value, the coupling coefficient and the self-induction parameter that meet the voltage stress in the resonant compensation network can be calculated respectively. According to formula (6), the maximum alternating current resistance of the resonant coil that meets the transmission efficiency requirement can be obtained; wherein formula (4), formula (5) and formula (6) are as follows:

[0084]

[0085]

[0086]

[0087] In formula (6):

[0088]

[0089]

[0090] Z1=jωL1;

[0091]

[0092]

[0093] In formula (4) and formula (5), U c1 and U c2 are the resonant capacitor voltage of the transmitting end and the compensation capacitor voltage of the transmitting end respectively, k is the coupling coefficient of the resonant coil, and other parameters are defined with reference to Figure 1 ;

[0094] In formula (6), R3 and R4 are the internal resistance of the resonant coils L3 and L4 respectively, and other parameters are defined with reference to Figure 1 ;

[0095] Up to now, all the electrical parameters of the resonant coil and the constraint conditions thereof have been determined.

[0096] Step 7, determination of the minimum size of the coil and the maximum length of the winding;

[0097] According to the characteristics of dynamic wireless charging, the length-width ratio of the DD type coil is selected as 1.5, which is considered to balance the dynamic performance and static performance. The magnetic core structure of the coil is a flat plate, which is covered on the DD type coil, and its size is slightly larger than the window size of the DD type coil, but smaller than the external size of the coil. At this time, the minimum size of the coil is set as follows: when the coil is wound one turn from inside to outside under the transmission distance and the size of the resonant coil, the coupling coefficient of the resonant coil just meets the above constraint condition;

[0098] According to the selected Litz wire type, the alternating current resistance per unit length is calculated. Then the maximum winding length of the resonant coil is calculated, which is defined as: when the resonant coil winding is this length, the alternating current resistance of the resonant coil is equal to the maximum alternating current resistance of the resonant coil that meets the transmission efficiency;

[0099] At this time, all the size parameter constraint conditions of the resonant coil are determined.

[0100] Step 8, winding of the resonant coil;

[0101] Take the Litz wire with the maximum winding length of the resonant coil, and wind it from outside to inside in a planar spiral dense winding manner with the maximum size constraint of the coil as the external size of the DD type coil. After winding, the mutual inductance value is measured. If the mutual inductance is less than the required value, the outer size of the resonant coil is reduced and wound again until the mutual inductance requirement is met. If the coil winding is within the minimum size of the coil at this time, the constraint condition needs to be changed, so as to design a resonant coil that meets the requirements.

[0102] Step 9, optimization of the magnetic core structure;

[0103] The finite element simulation software Maxwell is used to simulate the above obtained coil structure. The position where the magnetic flux density in the magnetic core is greater than 0.3T is thickened, so as to reduce the loss and prevent the saturation of the magnetic core.

[0104] Step 10, determine the placement distance of the primary coil;

[0105] According to the inter-loop constraint between the primary side coils and formula (2), the minimum mutual inductance coefficient between the primary side coils can be obtained, and when the mutual inductance coefficient between the primary side coils is equal to the value, the distance between the primary side coils is defined as the minimum primary side coil distance; according to the obtained resonant coil, the relationship curve between the mutual inductance coefficient and the offset amount of the resonant coil at a given transmission distance in the driving direction of the automobile is measured, taking the offset amount as the X-axis and the mutual inductance coefficient as the Y-axis; the curve is moved in the positive direction of the X-axis by a, 2a, 3a, …, na (a value is greater than the minimum primary side coil distance), and then the sum of these curves and the original curve is obtained, and the new curve is the equivalent mutual inductance coefficient and the position relationship, and the a value of the equivalent mutual inductance curve and the position relationship curve is taken, that is, the minimum fluctuation rate, that is, the distance between the primary side coils.

[0106] In the present application, 1) a target parameter is given: according to the application requirement of the wireless charging system, the design target of the wireless charging system is given, and the target includes input voltage, output voltage, output power, transmission efficiency and transmission distance.

[0107] 2) Select the dynamic wireless charging system resonant compensation network and resonant frequency: according to the characteristics of dynamic wireless charging, the resonant compensation network and the resonant coil structure of the system are selected. LCC-LCC topology is selected as the resonant compensation network of the coil, which has the characteristics of resonant capacitor and coil coupling state independent of load size, output constant current characteristic, which is suitable for the characteristics of fast and frequent change of coupling state in dynamic wireless charging, and constant current characteristic is suitable for electric vehicle battery charging. Select DD type coil as the resonant coil structure, which has good offset tolerance and is suitable for dynamic wireless charging. According to the relevant standards of electric vehicles, the resonant frequency of the resonant compensation network is set to 85KHz.

[0108] 3) Set the constraint condition: according to the use scene of the wireless charging system, set the constraint condition of the wireless charging system, including:

[0109] Resonant coil size: the resonant coil size constraint includes the constraint of the length, width and height of the resonant coil, which depends on the installation condition of the resonant coil.

[0110] Voltage stress of capacitor in resonant compensation network: the capacitor voltage stress includes the voltage stress on the resonant capacitor in LCC-LCC resonant compensation network and the voltage stress on the compensation capacitor.

[0111] Inter-loop between primary side coils: for dynamic wireless charging, too close primary side coil distance will lead to too large mutual inductance coefficient between primary side coils, which will cause induced current between primary side coils. The loop current is defined as: for a primary side coil, the current flowing through the resonant inductor generated by the adjacent coil.

[0112] 4) Determine the resonant inductance value in the resonant compensation network: for voltage source inverter, the resonant inductance value determines the harmonic content in the compensation network. Therefore, this value needs to ensure that the harmonic content is less than 10%.

[0113] 5) Calculate the resonant coil mutual inductance value: according to the system transmission power, the resonant network parameter calculates the resonant coil mutual inductance value.

[0114] 6) Calculate the primary and secondary coil current: according to the system transmission power, combined with the input and output voltage and the resonant compensation network parameter, the primary and secondary coil current is calculated.

[0115] 7) Determine the resonant coil litz wire selection: according to the system working frequency, the litz wire material, the skin depth is calculated, and the single wire diameter is selected. According to the primary and secondary coil current, the litz wire number is selected.

[0116] 8) Calculate the resonant coil mutual inductance value and coupling coefficient: according to the system transmission power, the resonant compensation network parameter, the resonant coil mutual inductance value is calculated. According to the resonant compensation network capacitor voltage stress constraint, the coupling coefficient constraint is obtained.

[0117] 9) Determine the DD coil aspect ratio: considering the static and dynamic performance of the resonant coil, the coil aspect ratio is determined to be 1.5:1.

[0118] 10) Determine the minimum size of the coil: according to the coupling coefficient constraint, the minimum size constraint of the resonant coil is obtained.

[0119] 11) Determine the maximum length of winding: according to the efficiency requirement, the maximum ac resistance allowed by the resonant coil is obtained, and the maximum length of winding is determined.

[0120] 12) Determine the coil core structure: according to the window size of the DD type coil, the core size is set.

[0121] 13) Optimize the core loss: according to the magnetic flux density distribution of the above core structure obtained by simulation software, for the manganese zinc ferrite core commonly used in electric vehicles, the core is thickened or laminated when the magnetic flux density is greater than 0.3 tesla.

[0122] 14) Set the primary coil spacing: according to the constraint of circulating current value, the minimum spacing of the primary coil can be obtained. According to the mutual inductance curve of the above coil when the vehicle moves in the direction, define the equivalent mutual inductance coefficient of the secondary coil to all primary coils, find out the primary coil spacing that makes the equivalent mutual inductance coefficient of the secondary coil fluctuate the least when the secondary coil position deviates, and define this point as the optimal primary coil spacing.

[0123] The application is based on the basic actual use demand, and in the premise of meeting the basic power transmission and input / output level, the wire gauge of the litz wire, the resonance compensation network topology of the wireless charging system and the voltage stress of the compensation elements are covered, the length-width ratio of the coil and the magnetic core structure are optimized, the placement distance of the primary coil is optimized, and the coil of the dynamic wireless charging system is more suitable for optimization. The resonance compensation network and the design method are more suitable for dynamic wireless charging, and the dynamic performance of the wireless charging system can be optimized.

[0124] The application is not limited to the above-mentioned embodiments, and based on the technical solutions of the present disclosure, those skilled in the art can make some substitutions and modifications to some technical features according to the disclosed technical content without creative labor, and these substitutions and modifications are all within the protection scope of the application.

Claims

1. A design method for a dynamic wireless charging resonant coil for an electric vehicle, characterized by: The following steps are included: Step 1: Given the target parameters: According to the front and rear circuits and power level requirements of the wireless charging system, set the front input voltage U of the wireless charging system. in And output voltage U out , system output power P out , the vertical distance between the primary and secondary coils; Step 2: Select the resonant compensation network and resonant frequency of the dynamic wireless charging system; Step 3, determining the resonant inductance value in the resonant compensation network; Step 4, determining the mutual inductance coefficient of the resonant coil; Step 5, Litz wire selection; Step 6: Determine the constraints; Determine the size requirements of the resonant coil based on the actual installation conditions. The size requirements include the length, width, and height of the resonant coil. The length and width mainly limit the plane size of the resonant coil, while the height mainly limits the thickness of the resonant coil's magnetic core and the thickness of the shielding plate. The primary and secondary coils and the resonant compensation network adopt a symmetrical structure, and their related parameters are assumed to be completely consistent. Therefore, the capacitor voltage formula in the receiving end resonant compensation network is similar to that of the transmitting end. Based on the formula, combined with the input and output power and the resonant inductance value, the coupling coefficient and self-inductance parameters that meet the voltage stress in the resonant compensation network are calculated respectively. Finally, the AC resistance of the maximum resonant coil that meets the transmission efficiency requirements is obtained. Step 7: Determine the minimum size of the coil and the maximum length of the winding; Step 8, winding the resonant coil; Step 9, core structure optimization; Step 10: Determine the spacing of the primary coils.

2. The method for designing a dynamic wireless charging resonant coil for an electric vehicle according to claim 1, characterized in that: The specific contents of step 2 are as follows: The resonant frequency of the resonant compensation network is set to 85KHz. Based on the characteristics that the relative positions between the resonant coils are not constant, their relative positions are always changing frequently and rapidly, and the direction of their position changes is mainly in the direction of vehicle movement, the LCC-LCC topology is selected as the resonant compensation network of the coil, and the DD type coil is selected as the resonant coil structure.

3. The method for designing a dynamic wireless charging resonant coil for an electric vehicle according to claim 2, characterized in that: The specific contents of step 3 are as follows: Using a voltage-type inverter, the third harmonic component of the harmonic in the circuit is calculated as the fundamental wave content by the following formula (1): Here the value of the resonant inductor must satisfy I 1st is the fundamental wave of the inverter output current, I 3st is the third harmonic of the inverter output current, L f is the resonant inductance in the resonant compensation network, M is the mutual inductance of the resonant coil, U out is the output voltage; The primary and secondary resonant coils and the resonant compensation network are symmetrical, that is, the resonant inductor L f =L1=L4, resonant coil self-inductance L=L2=L3, resonant capacitor C f =C1=C4, compensation capacitor C P =C2=C3.

4. The method for designing a dynamic wireless charging resonant coil for an electric vehicle according to claim 3, wherein: The specific contents of step 4 are as follows: According to the power formula (2) of the wireless charging system, the mutual inductance required to meet the power level is obtained by combining the given input voltage, output voltage and the determined inductance. Formula (2) is as follows: M is the mutual inductance of the resonant coil, ω0 is the resonant angular frequency, U in is the input voltage, L f is the resonant inductance value, and Uout is the output voltage.

5. The method for designing a dynamic wireless charging resonant coil for an electric vehicle according to claim 4, characterized in that: The specific contents of step 5 are as follows: Based on the resonant inductance parameters and input voltage calculated above, the current in the resonant coil is calculated by formula (3). Based on the system operating frequency of 85KHz, copper is selected as the conductor material, and its skin depth is calculated. Based on the skin depth, a single-strand Litz wire with a wire diameter of 0.1mm is selected. Based on the current in the resonant coil, the number of Litz wire strands is selected according to a current margin of 1.5 to 2 times. Formula (3) is as follows: I L is the resonant coil current.

6. The method for designing a dynamic wireless charging resonant coil for an electric vehicle according to claim 5, characterized in that: The specific contents of step 6 are as follows: Determine the size requirements of the resonant coil based on the actual installation conditions. The size requirements include the length, width, and height of the resonant coil. The length and width mainly limit the plane size of the resonant coil, while the height limits the thickness of the resonant coil's magnetic core and the thickness of the shielding plate. The primary and secondary coils and the resonant compensation network adopt a symmetrical structure, and their related parameters are considered to be completely consistent. According to formula (4) and formula (5), combined with the input and output power and the resonant inductance value, the coupling coefficient and self-inductance parameters that meet the voltage stress in the resonant compensation network are calculated respectively; According to formula (6), the AC resistance of the maximum resonant coil that meets the transmission efficiency requirement is obtained; formula (4), formula (5) and formula (6) are as follows: In formula (6): Z1=jωL1; In formula (4) and formula (5), U c1 and U c2 are the voltage of the transmitting end resonant capacitor and the voltage of the transmitting end compensation capacitor respectively, and k is the coupling coefficient of the resonant coil; In formula (6), R3 and R4 are the internal resistances of the resonant coils L3 and L4 respectively; At this point, all electrical parameters of the resonant coil and their constraints have been determined.

7. The method for designing a dynamic wireless charging resonant coil for an electric vehicle according to claim 6, characterized in that: The specific contents of step 7 are as follows: Based on the characteristics of dynamic wireless charging, a DD coil with an aspect ratio of 1.5 was selected, as this ratio is believed to provide a good balance between dynamic and static performance. The coil's magnetic core structure is a flat plate, covering the top of the DD coil, making its size slightly larger than the DD coil's window size but smaller than the coil's external dimensions. The minimum coil size is set such that, at this transmission distance and resonant coil size, the coupling coefficient of the resonant coil just meets the aforementioned constraints when the coil is wound one turn from the inside out. Based on the selected Litz wire type, calculate its AC resistance per unit length. Then, calculate the maximum winding length of the resonant coil. This length is defined as the length at which the AC resistance of the resonant coil is equal to the maximum AC resistance of the resonant coil that meets transmission efficiency. At this point, all the dimensional parameter constraints of the resonant coil are determined.

8. The method for designing a dynamic wireless charging resonant coil for an electric vehicle according to claim 7, characterized in that: The specific contents of step 8 are as follows: Take the Litz wire with the maximum length of the winding of the above-mentioned resonant coil, constrain the maximum size of the coil to the outer size of the DD-type coil, and wind it from the outside to the inside in a planar spiral dense winding manner. After winding is completed, its mutual inductance value is measured; if the mutual inductance is less than the required value, reduce the size of the outer ring of the resonant coil and wind it again until the mutual inductance requirement value is met; if the coil windings are all within the minimum coil minimum size at this time, it is necessary to change the constraint conditions to design a resonant coil that meets the requirements.

9. The method for designing a dynamic wireless charging resonant coil for an electric vehicle according to claim 8, characterized in that: The specific contents of step 9 are as follows: The finite element simulation software Maxwell is used to simulate the coil structure obtained above. The core is thickened at the position where the magnetic flux density is greater than 0.3T, thereby achieving the purpose of reducing loss and preventing core saturation.

10. A method for designing a dynamic wireless charging resonant coil for an electric vehicle according to claim 9, characterized in that: The specific contents of step 10 are as follows: According to the circulation constraint between the primary coils and formula (2), the minimum mutual inductance coefficient between the primary coils is obtained. When the mutual inductance coefficient between the primary coils is equal to this value, the primary coil spacing is defined as the minimum primary coil spacing. According to the resonant coil obtained above, the relationship curve between the mutual inductance coefficient and the offset of the resonant coil in the direction of vehicle travel at a given transmission distance is measured, with the offset as the X-axis and the mutual inductance coefficient as the Y-axis. The curve is moved in the positive direction of the X-axis by a, 2a, 3a, ..., na, where the a value is greater than the minimum primary coil spacing. Then, these curves are summed with the original curve to obtain a new curve as the relationship between the equivalent mutual inductance coefficient and the position. The a value that minimizes the fluctuation rate of the equivalent mutual inductance curve and the position relationship curve is taken as the spacing of the primary coils.

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