*3.2. Dynamic Shielding Scheme*

As the application of EVs gradually spreads, different EVs have been presented with different structures and the distance between the vehicle chassis and the ground varies. This results in different strengths of EMF leakage when charging different EVs. If each EV needs to be designed with one active shielding mechanism, it would occupy much of the ground charging area.

Therefore, a double-coil dynamic shielding scheme based on active shielding technology is proposed in this paper. According to the power transmission distance of different EVs, the power supply of active shielding coils installed on the ground is adjusted, so that the EMF leakage of different EVs can be dynamically shielded. Regardless of the variation in the transmission distance of the WPT system, the EMF leakage level is always guaranteed to be within the safety range specified by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) standards and guidelines.

With a constant charging power supply of the WPT system, the magnetic flux density varies with the transmission distance. A magnetic flux density detection module is used to detect the magnetic flux density at different transmission distances. This involves controlling the power supply *V*<sup>A</sup> of the active shielding coil, so high *SE* of the shielding system is maintained while adapting to changes in the transmission distance, resulting in dynamic and good shielding effectiveness. varies with the transmission distance. A magnetic flux density detection module is used to detect the magnetic flux density at different transmission distances. This involves controlling the power supply *V*<sup>A</sup> of the active shielding coil, so high *SE* of the shielding system is maintained while adapting to changes in the transmission distance, resulting in dynamic and good shielding effectiveness. trolling the power supply *V*<sup>A</sup> of the active shielding coil, so high *SE* of the shielding system is maintained while adapting to changes in the transmission distance, resulting in dynamic and good shielding effectiveness. A DC-AC inverter circuit with an adjustable duty cycle is added to the control loop so as to control the power supply of the active shielding coil. By adjusting *VA*, high *SE* for

Non-Ionizing Radiation Protection (ICNIRP) standards and guidelines.

Non-Ionizing Radiation Protection (ICNIRP) standards and guidelines.

needs to be designed with one active shielding mechanism, it would occupy much of the

needs to be designed with one active shielding mechanism, it would occupy much of the

Therefore, a double-coil dynamic shielding scheme based on active shielding technology is proposed in this paper. According to the power transmission distance of different EVs, the power supply of active shielding coils installed on the ground is adjusted, so that the EMF leakage of different EVs can be dynamically shielded. Regardless of the variation in the transmission distance of the WPT system, the EMF leakage level is always guaranteed to be within the safety range specified by the International Commission on

Therefore, a double-coil dynamic shielding scheme based on active shielding technology is proposed in this paper. According to the power transmission distance of different EVs, the power supply of active shielding coils installed on the ground is adjusted, so that the EMF leakage of different EVs can be dynamically shielded. Regardless of the variation in the transmission distance of the WPT system, the EMF leakage level is always guaranteed to be within the safety range specified by the International Commission on

With a constant charging power supply of the WPT system, the magnetic flux density

With a constant charging power supply of the WPT system, the magnetic flux density varies with the transmission distance. A magnetic flux density detection module is used to detect the magnetic flux density at different transmission distances. This involves con-

*Energies* **2021**, *14*, x FOR PEER REVIEW 8 of 20

*Energies* **2021**, *14*, x FOR PEER REVIEW 8 of 20

ground charging area.

ground charging area.

A DC-AC inverter circuit with an adjustable duty cycle is added to the control loop so as to control the power supply of the active shielding coil. By adjusting *VA*, high *SE* for EMF leakage at varying distances is achieved in the active shielding system. The circuit design is shown in Figure 5. A DC-AC inverter circuit with an adjustable duty cycle is added to the control loop so as to control the power supply of the active shielding coil. By adjusting *VA*, high *SE* for EMF leakage at varying distances is achieved in the active shielding system. The circuit design is shown in Figure 5. EMF leakage at varying distances is achieved in the active shielding system. The circuit design is shown in Figure 5.

**Figure 5.** Circuit design of active shielding system. **Figure 5.** Circuit design of active shielding system. A DC-AC inverter circuit is a conversion device that transforms the DC input voltage

A DC-AC inverter circuit is a conversion device that transforms the DC input voltage and then outputs the AC voltage. It realizes the orderly closure of switching elements by controlling the driving voltage for on/off. Thus, the high DC voltage is converted to AC output voltage according to different circulation paths. It is controlled by pulse width modulation (PWM), which further changes the magnitude of the output voltage by shift-A DC-AC inverter circuit is a conversion device that transforms the DC input voltage and then outputs the AC voltage. It realizes the orderly closure of switching elements by controlling the driving voltage for on/off. Thus, the high DC voltage is converted to AC output voltage according to different circulation paths. It is controlled by pulse width modulation (PWM), which further changes the magnitude of the output voltage by shifting the trigger signal of the power electronic switch in the circuit. and then outputs the AC voltage. It realizes the orderly closure of switching elements by controlling the driving voltage for on/off. Thus, the high DC voltage is converted to AC output voltage according to different circulation paths. It is controlled by pulse width modulation (PWM), which further changes the magnitude of the output voltage by shifting the trigger signal of the power electronic switch in the circuit. When the power transmission distance changes, the EMF leakage also changes ac-

ing the trigger signal of the power electronic switch in the circuit. When the power transmission distance changes, the EMF leakage also changes accordingly. As a result, the current flowing through the coil alters as well. In the DC-AC inverter circuit, the magnitude of output voltage *V*<sup>A</sup> is controlled by regulating the duty When the power transmission distance changes, the EMF leakage also changes accordingly. As a result, the current flowing through the coil alters as well. In the DC-AC inverter circuit, the magnitude of output voltage *V*<sup>A</sup> is controlled by regulating the duty cycle of the PWM signal, so that the EMF leakage level is limited within ICNIRP. cordingly. As a result, the current flowing through the coil alters as well. In the DC-AC inverter circuit, the magnitude of output voltage *V*<sup>A</sup> is controlled by regulating the duty cycle of the PWM signal, so that the EMF leakage level is limited within ICNIRP. The topologies often used in high-frequency inverter circuits are class E [34], double

cycle of the PWM signal, so that the EMF leakage level is limited within ICNIRP. The topologies often used in high-frequency inverter circuits are class E [34], double The topologies often used in high-frequency inverter circuits are class E [34], double E [35], half-bridge, and full-bridge [36], and their circuit structures are shown in Figure 6. E [35], half-bridge, and full-bridge [36], and their circuit structures are shown in Figure 6.

(c) Half-Bridge Inverter (d) Full-Bridge Inverter **Figure 6. Figure 6.**  Circuit structures of high-frequency inverters. Circuit structures of high-frequency inverters.

**Figure 6.** Circuit structures of high-frequency inverters. A class E inverter is a simple driving circuit with only one switching device, and it has the advantages of low switching losses and high conversion efficiency. However, the circuit is unlikely to provide high output power when the duty cycle is changed.

A double E inverter consists of two switching devices, each with half the input voltage of a class E inverter. In this case, it lowers the requirement of the DC power supply and switching devices and provides an improvement in power. However, it has a large current ripple in the input inductance and a high loss in the paralleled inductor, which reduces the efficiency of the inverter system.

A half-bridge inverter works through controlling two switching devices to alternate their conduction. It features a simple structure and requires fewer switching devices. However, the maximum AC output voltage is only half of the DC output, and the lower DC voltage utilization reduces the efficiency of the inverter system.

A full-bridge inverter consists of four bridge arms, which can be seen as a combination of two half-bridge inverters. At the same DC voltage and load, the output of a full-bridge inverter is two times that of a half-bridge inverter. Moreover, it has only one capacitor on the DC side, so there is no problem of voltage balance. It can be applied in a wider range and is more flexible to control.

In summary, a full-bridge inverter is more applicable to the WPT system in this work because of its simple structure, high voltage utilization, wide power range, flexible control, and no special requirements for the transmission distance. Therefore, the full-bridge inverter is selected in this work.

With the addition of a full-bridge inverter, it is possible to adjust *V*<sup>A</sup> by changing the duty cycle *α* of the pulse signal. In this way, the EMF leakage can be kept at a stable value while the transmission distance changes, which achieves the dynamic shielding effectiveness of the WPT system.

To further implement the dynamic shielding scheme, an MCU is added to the WPT system to guide the adjustment of *α* according to the detected magnetic flux density, and finally to enable the dynamic adjustment of *VA*. The algorithm flowchart of the proposed dynamic shielding scheme is presented in Figure 7. *Energies* **2021**, *14*, x FOR PEER REVIEW 10 of 20

**Figure 7.** Flowchart of proposed dynamic shielding scheme. **Figure 7.** Flowchart of proposed dynamic shielding scheme.

The dynamic shielding scheme allows the power supply of the active shielding coils to be adjusted so that EMF leakage is restricted to the safe level of ICNIRP. The ICNIRP The dynamic shielding scheme allows the power supply of the active shielding coils to be adjusted so that EMF leakage is restricted to the safe level of ICNIRP. The ICNIRP

reference levels for magnetic field exposure for WPT systems in different operating bands (1 Hz–100 kHz) are listed in Table 1. This work refers to the ICNIRP 2010 version [37].

> *f B***(T)** 1 Hz–8 Hz 0.04/*f*

When the operating frequency is fixed at a certain frequency band, a standard value of the referenced magnetic flux density is fixed. However, the transmission distance may not remain the same when charging different EVs. Therefore, the magnetic flux density will increase or decrease accordingly with the distance. As the transmission distance is shortened, the EMF leakage of the WPT system will increase accordingly. To ensure that the EMF leakage is within the safe range of ICNIRP, the adoption of the proposed dynamic shielding scheme can adjust *V*<sup>A</sup> accordingly. This causes the adjusted EMF leakage to drop below the reference level of ICNIRP 2010. While the transmission distance increases, the

8 Hz–25 Hz 0.005/*f* 25 Hz–50 Hz 0.0002 50 Hz–300 Hz 0.0002 300 Hz–400 Hz 0.0002 400 Hz–3 kHz 0.08/*f* 3 kHz–100 kHz 0.000027

2

kHz in ICNIRP-2010.

reference levels for magnetic field exposure for WPT systems in different operating bands (1 Hz–100 kHz) are listed in Table 1. This work refers to the ICNIRP 2010 version [37]. *Energies* **2021**, *14*, x FOR PEER REVIEW 11 of 20

**Table 1.** Reference level of magnetic field exposure in the operating frequency range of 1 Hz–100 kHz in ICNIRP-2010. EMF leakage will become lower accordingly. From the perspective of energy saving and cost saving, *V*<sup>A</sup> should be reduced accordingly, which can help to ensure that the EMF


For the verification of the proposed dynamic shielding scheme based on double-coil

When the operating frequency is fixed at a certain frequency band, a standard value of the referenced magnetic flux density is fixed. However, the transmission distance may not remain the same when charging different EVs. Therefore, the magnetic flux density will increase or decrease accordingly with the distance. As the transmission distance is shortened, the EMF leakage of the WPT system will increase accordingly. To ensure that the EMF leakage is within the safe range of ICNIRP, the adoption of the proposed dynamic shielding scheme can adjust *V*<sup>A</sup> accordingly. This causes the adjusted EMF leakage to drop below the reference level of ICNIRP 2010. While the transmission distance increases, the EMF leakage will become lower accordingly. From the perspective of energy saving and cost saving, *V*<sup>A</sup> should be reduced accordingly, which can help to ensure that the EMF leakage is within the safety limit of the ICNIRP 2010 standard and prevent excessive waste of resources. active shielding, a WPT system is built using ANSYS Maxwell simulation software. The WPT system operates at 85 kHz, and the safety limit of ICNIRP at this operating frequency is 27 μT. The simulated structure of the WPT coils is depicted in Figure 8. The transmitting and receiving coils have the same structure, both of which have an external radius of 100 mm; the number of turns *N* is 10, and the transmission distance *Z*<sup>2</sup> is initialized to 100 mm. The double-coil active shielding structure consists of two half-loops with outer radius *r*out = 150 cm and inner radius *r*in = 130 cm. The circuit for the combined simulation by 3D Maxwell and Simplorer is illustrated in Figure 9. The transmitting power supply *V*<sup>S</sup> of the WPT system is set to 30 V. The internal resistance of power supply *R*<sup>S</sup> is 8 mΩ, and the resistance of the load resistor *R*<sup>L</sup> = 10 Ω. The other simulation parameters of the circuit are listed in Table 2. In addition, the variation of the power supply of the active shielding coil *V*<sup>A</sup> with the variation of the dis-

The proposed dynamic shielding scheme is applicable to cases in which the transmission distance changes. It has a certain directive significance for future research on shielding technology for subsequent WPT systems. tance between the transmitting coil and receiving coil is given in the subsequent discussion. In addition, the changes in the power supply of active shielding coil *V*<sup>A</sup> with transmission distance are given in the subsequent discussion.

In this paper, a dynamic shielding scheme is used to dynamically adjust the power

#### **4. Simulation and Experiments** supply of active shielding coils *V*<sup>A</sup> at transmission distances of 50 mm, 100 mm, and 150

#### *4.1. Simulation Verification* mm. The EMF leakage of the WPT system is measured to ensure that its value is lower

For the verification of the proposed dynamic shielding scheme based on double-coil active shielding, a WPT system is built using ANSYS Maxwell simulation software. The WPT system operates at 85 kHz, and the safety limit of ICNIRP at this operating frequency is 27 µT. The simulated structure of the WPT coils is depicted in Figure 8. The transmitting and receiving coils have the same structure, both of which have an external radius of 100 mm; the number of turns *N* is 10, and the transmission distance *Z*<sup>2</sup> is initialized to 100 mm. The double-coil active shielding structure consists of two half-loops with outer radius *r*out = 150 cm and inner radius *r*in = 130 cm. than the ICNIRP standard. The corresponding *V*<sup>A</sup> at different transmission distances is shown in Table 3. When the transmission distance is shortened, the magnetic flux density increases accordingly. Therefore, to meet the shielding requirements, the proposed dynamic shielding scheme is used to increase the *V*<sup>A</sup> to 41.61 V and to shield from excessive EMF leakage. When the distance increases, the magnetic flux density becomes smaller correspondingly. At this point, based on the consideration of saving resources and economic costs, *V*<sup>A</sup> is reduced to 20.06 V consequently, so that the EMF leakage is exactly within the safety limit of the ICNIRP standard, avoiding the unnecessary waste of resources.

**Figure 8.** Simulated structure of the WPT coils. **Figure 8.** Simulated structure of the WPT coils.

The circuit for the combined simulation by 3D Maxwell and Simplorer is illustrated in Figure 9. The transmitting power supply *V*<sup>S</sup> of the WPT system is set to 30 V. The internal resistance of power supply *R*<sup>S</sup> is 8 mΩ, and the resistance of the load resistor *R*<sup>L</sup> = 10 Ω. The other simulation parameters of the circuit are listed in Table 2. In addition, the variation of the power supply of the active shielding coil *V*<sup>A</sup> with the variation of the distance between the transmitting coil and receiving coil is given in the subsequent discussion. In addition, the changes in the power supply of active shielding coil *V*<sup>A</sup> with transmission distance are given in the subsequent discussion. *Energies* **2021**, *14*, x FOR PEER REVIEW 12 of 20

**Figure 9.** Circuit for the combined simulation by 3D Maxwell and Simplorer. **Figure 9.** Circuit for the combined simulation by 3D Maxwell and Simplorer.

**Table 2.** Simulation parameters of WPT circuit.


*L*<sup>3</sup> 0.90 μH *C*<sup>1</sup> 4.89 μF *C*<sup>2</sup> 4.89 μF *K*<sup>12</sup> 0.422 In this paper, a dynamic shielding scheme is used to dynamically adjust the power supply of active shielding coils *V*<sup>A</sup> at transmission distances of 50 mm, 100 mm, and 150 mm. The EMF leakage of the WPT system is measured to ensure that its value is lower than the ICNIRP standard. The corresponding *V*<sup>A</sup> at different transmission distances is shown in Table 3.

*K*<sup>13</sup> 0.041

*K*<sup>23</sup> 0.041

Transmission distance *V*<sup>A</sup>

50 mm 41.61 V

100 mm 24.32 V

150 mm 20.06 V

The magnetic flux density distribution of the WPT system at different distances is

displayed in in Figures 10 and 11, demonstrating the EMF leakage with no shielding and double-coil active shielding. The comparison indicates that the magnetic flux density increases with the shortening of the transmission distance and decreases with the increase in transmission distance, when no shielding is available. Moreover, the EMF leakage outside the system is higher. However, with the addition of double-coil active shielding, the EMF leakage outside the WPT system is significantly reduced by adjusting *V*A, as illus-

trated in Figure 11.

100 mm; (**c**) 150 mm.


**Table 3.** *V*<sup>A</sup> at different transmission distances.

When the transmission distance is shortened, the magnetic flux density increases accordingly. Therefore, to meet the shielding requirements, the proposed dynamic shielding scheme is used to increase the *V*<sup>A</sup> to 41.61 V and to shield from excessive EMF leakage. When the distance increases, the magnetic flux density becomes smaller correspondingly. At this point, based on the consideration of saving resources and economic costs, *V*<sup>A</sup> is reduced to 20.06 V consequently, so that the EMF leakage is exactly within the safety limit of the ICNIRP standard, avoiding the unnecessary waste of resources.

The magnetic flux density distribution of the WPT system at different distances is displayed in in Figures 10 and 11, demonstrating the EMF leakage with no shielding and double-coil active shielding. The comparison indicates that the magnetic flux density increases with the shortening of the transmission distance and decreases with the increase in transmission distance, when no shielding is available. Moreover, the EMF leakage outside the system is higher. However, with the addition of double-coil active shielding, the EMF leakage outside the WPT system is significantly reduced by adjusting *V*A, as illustrated in Figure 11. *Energies* **2021**, *14*, x FOR PEER REVIEW 13 of 20 *Energies* **2021**, *14*, x FOR PEER REVIEW 13 of 20

**Figure 10.** The magnetic flux density distribution of WPT system with no shielding: (**a**) 50 mm; (**b**) **Figure 10.** The magnetic flux density distribution of WPT system with no shielding: (**a**) 50 mm; (**b**) 100 mm; (**c**) 150 mm. **Figure 10.** The magnetic flux density distribution of WPT system with no shielding: (**a**) 50 mm; (**b**) 100 mm; (**c**) 150 mm.

**Figure 11.** The magnetic flux density distribution of WPT system with double-coil active shielding: (**a**) 50 mm; (**b**) 100 mm; (**c**) 150 mm. **Figure 11.** The magnetic flux density distribution of WPT system with double-coil active shielding: (**a**) 50 mm; (**b**) 100 mm; (**c**) 150 mm. **Figure 11.** The magnetic flux density distribution of WPT system with double-coil active shielding: (**a**) 50 mm; (**b**) 100 mm; (**c**) 150 mm.

distances after adjusting *V*<sup>A</sup> are presented in Table 4.

**Table 4.** *SE* and *η* at different transmission distances.

**Table 4.** *SE* and *η* at different transmission distances.

shielding scheme is successfully verified.

shielding scheme is successfully verified.

*4.2. Experimental Verification*

*4.2. Experimental Verification*

To further prove the shielding effectiveness of the dynamic shielding scheme and the

To further prove the shielding effectiveness of the dynamic shielding scheme and the impact of double-coil active shielding on the power transfer efficiency of the WPT system,

Transmission distance 50 mm 100 mm 150 mm

Transmission distance 50 mm 100 mm 150 mm

*η* (no shielding) 96.5% 93.8% 92.7%

*η* (no shielding) 96.5% 93.8% 92.7%

*η* (double-coil active shielding) 92.4% 91.9% 90.7%

*η* (double-coil active shielding) 92.4% 91.9% 90.7%

It can be seen that the *SE* of this dynamic shielding scheme reaches more than 69.4%. Moreover, with the increase in distance, the shielding effectiveness becomes more prominent. At 150 mm, it achieves 77.4% shielding effectiveness for EMF leakage in the WPT system. Furthermore, by comparing the *η* of the WPT system with double-coil active shielding and no shielding, it can be found that the addition of double-coil active shielding employed in the paper allows the power transfer efficiency to be essentially above 90%. Therefore, the involvement of this double-coil active shielding structure has not caused a significant impact on the transmission efficiency, and the degradation of the performance of the WPT system is mitigated. In summary, the effectiveness of the proposed dynamic

It can be seen that the *SE* of this dynamic shielding scheme reaches more than 69.4%. Moreover, with the increase in distance, the shielding effectiveness becomes more prominent. At 150 mm, it achieves 77.4% shielding effectiveness for EMF leakage in the WPT system. Furthermore, by comparing the *η* of the WPT system with double-coil active shielding and no shielding, it can be found that the addition of double-coil active shielding employed in the paper allows the power transfer efficiency to be essentially above 90%. Therefore, the involvement of this double-coil active shielding structure has not caused a significant impact on the transmission efficiency, and the degradation of the performance of the WPT system is mitigated. In summary, the effectiveness of the proposed dynamic

In order to validate the correctness of the simulation results and the effectiveness and feasibility of the proposed dynamic shielding scheme, the circuit of the WPT system with

In order to validate the correctness of the simulation results and the effectiveness and feasibility of the proposed dynamic shielding scheme, the circuit of the WPT system with

*SE* 69.4% 76.2% 77.4%

*SE* 69.4% 76.2% 77.4%

To further prove the shielding effectiveness of the dynamic shielding scheme and the impact of double-coil active shielding on the power transfer efficiency of the WPT system, the shielding effectiveness *SE* and power transfer efficiency *η* at different transmission distances after adjusting *V*<sup>A</sup> are presented in Table 4.



It can be seen that the *SE* of this dynamic shielding scheme reaches more than 69.4%. Moreover, with the increase in distance, the shielding effectiveness becomes more prominent. At 150 mm, it achieves 77.4% shielding effectiveness for EMF leakage in the WPT system. Furthermore, by comparing the *η* of the WPT system with double-coil active shielding and no shielding, it can be found that the addition of double-coil active shielding employed in the paper allows the power transfer efficiency to be essentially above 90%. Therefore, the involvement of this double-coil active shielding structure has not caused a significant impact on the transmission efficiency, and the degradation of the performance of the WPT system is mitigated. In summary, the effectiveness of the proposed dynamic shielding scheme is successfully verified. *Energies* **2021**, *14*, x FOR PEER REVIEW 14 of 20
