Dual receiver series topology for bipolar segmented DWPT system

With global energy exhaustion and environmental pollution becoming increasingly prominent, EVs have developed rapidly with the advantages of being clean, energy-saving, and pollution-free.^1–3 The traditional wired charging mode of EVs has many potential problems, such as high loss of charging line, mechanical wear, contact with sparks, etc. Wireless charging technology for EVs can effectively solve these problems.^4–6 Stationary Wireless Power Transfer (SWPT) and Dynamic Wireless Power Transfer (DWPT) are the main wireless charging modes. For SWPT, fixed charging position and poor anti-offset are the main factors against the popularity of EVs.^7–10 As a result, DWPT technology for EVs came into being and became the research focus of many scholars and institutions, which could provide power for EVs when they are moving. Meanwhile, it can reduce the battery capacity requirement or even no batteries with a long mileage.^11–14 

In the EVs DWPT system, the transmitting coil buried under the road generates a high-frequency alternating magnetic field, and then the induced electromotive force could be obtained in the receiving coil located in EVs.^15–17 The existing literature summarized the development of wireless power supply technology.^18,19 Existing literature also studied the magnetic coupling coefficient of EV wireless charging systems, which provided a theoretical basis for the design and optimization of the magnetic coupling coils.^20,21 Due to the range and battery capacity issues of electric vehicles, there are many existing studies on dynamic wireless charging. However, the existing dynamic wireless charging modes also have some issues. The long-track DWPT system was originally proposed by Kim et al.²² Due to having only one long guide rail as the transmitter, the coupling of long-track is relatively stable. However, most of the magnetic field of the long rail coil cannot be received by the receiver, resulting in severe magnetic leakage. Moreover, the long-track coil has high internal resistance, which causes severe losses.^23–25 For the traditional unipolar segmented DWPT system, if the segmented transmitting coils are turned on one by one, the magnetic field generated by adjacent coils will cancel out, and the mutual inductance will drop dramatically when the EV reaches the position near the adjacent transmitting coils, which would affect the transfer power and the transmission efficiency.^26–28 The Electric field Coupled-Wireless Power Transfer (EC-WPT) can avoid the problem of field cancellation between adjacent emitters to some extent. However, adjacent plates will introduce two new problems: (1) If adjacent plates are too close, the two transmitting plates will couple and affect system resonance. (2) If adjacent plates are placed too far apart, the resulting power drop will be significant.^29,30 For existing segmented DWPT designs, Song et al. designed a multi-primary winding parallel topology for DWPT but did not analyze the receiving characteristics of the corresponding receiving coils.³¹ Some studies proposed a multi-phase wireless power supply system, which can effectively turn on the appropriate transmitting coil to supply power to the load.^32–34 Still, its design was so cumbersome as to make the operation complicated.

To address the above-mentioned problem, this paper proposes a novel dual receiver topology for the segmented DWPT system. The topology designed can solve the power loss and fluctuation caused by magnetic field cancellation. In addition, the main contributions of the paper are as follows:

1. This paper uses finite element simulation to analyze the magnetic field cancellation problem of adjacent coils in traditional segmented DWPT. Based on this, a design method of changing the direction of magnetic field and current is proposed.

2. A dual receiver series topology (DRST) is designed to work in three working modes and six working states. The receivers can receive the optimal power in three stages by this topology. In addition, this paper explains the current flow direction under various working conditions.

3. The experiments are carried out, and the results show that the proposed DRST is effective to improve the average output power from 2.007 to 5.657 W and significantly reduce the power fluctuation compared to the single receiver.

The rest of the paper is organized as follows: Sec. II analyzes the magnetic field characteristics at the near of coils under unipolar and bipolar power supply modes and gives the mutual inductance equivalent circuit diagram of the double receiving-coil. Section III describes the working principle of the proposed DRST and elaborates on its three working modes and six working states. Finally, the experimental results verify the theoretical analysis and show that the proposed system topology is effective and reliable in Sec. IV.

The typical segmented DWPT system for EVs is shown in Fig. 1, which mainly consists of two parts, called transmitter and receiver devices, respectively. The transmitter device comprises multiple-segmented guide rail coils and converts electric energy into magnetic field energy. The receiver device includes a receiving coil, rectifier unit, and battery pack, which could pick up the magnetic field energy and supply for EVs. The transmitting coils are orderly arranged under the road. When EVs pass the two adjacent transmitting coils, the relative position of the receiving and transmitting coils will go through three stages, as shown in Fig. 2.

In the traditional segmented DWPT system with unipolar power supply mode, the currents of the adjacent sides of two transmitting coils are opposite. Adjacent wires will generate opposite magnetic fields, which will weaken the magnetic field, as shown in Fig. 3. Bipolar power supply mode is an effective measure to reduce the magnetic field mutual cancellation by tuning the currents of the adjacent coils with a 180° phase difference. The currents and the magnetic field are shown in Fig. 4.

In Fig. 4(b), the magnetic field near two adjacent sides has been significantly enhanced. However, the current phase difference between the two transmitting coils is also 180°. Furthermore, to improve the performance of the segmented DWPT system, a dual receiver on the same plane is adopted to pick up the power, and its equivalent circuit is shown in Fig. 5.

In Fig. 5, the SS resonant compensation topology transfers power. u_in stands for the high-frequency power source. For the convenience of expression, L_p and L_s represent the two coils with the inductances of L_p and L_s, and the subscripts “P” and “S” stand for the transmitting and receiving coils, respectively. C_p1 and C_p2, R_p1, and R_p2, i_p1, and i_p2 stand for the inductances, capacitances, resistances, and currents of two adjacent transmitting coil circuits, while C_s1 and C_s2, R_s1, and R_s2, i_s1, and i_s2 represent the corresponding parameters of two receiving coil circuits. M_i (i = 1,2,3,4) stands for the mutual inductance of L_p and L_s, as in Fig. 5.

To solve the problem of the 180° phase difference between the current of the two receiving coils mentioned in Sec. II, a DRST for the bipolar segmented DWPT system is proposed, and its circuit topology is shown in Fig. 6.

In Fig. 6, S_i (i = 1,2,3,4,5,6) and D_i (i = 1,2,3,4) represent fully controlled components and diodes, respectively. When the S₅ and S₆ are turned on simultaneously, the currents in the adjacent sides of two transmitting coils are in-phase, and the enhanced magnetic field can be achieved, as shown in Fig. 4(b).

According to the three stages mentioned in Fig. 2, when EV is driving from transmitting coil 1 to coil 2, the waveforms of main parameters and the status of S₁–S₆ are shown in Fig. 7. In addition, the DWPT system has three working modes, including six working states, as shown in Fig. 8, where the red dotted line with the arrow indicates the actual flow direction of the current.

Mode 1: t₀ ≤ t $<t1$⁠. In the first stage, the receiving coils L_S1 and L_S2 are above the transmitting coil 1. S₁, S₄, and S₅ are turned on, while S₂, S₃, and S₆ are turned off. u_in supplies power to L_P1, and the working states during the positive and negative half cycle of u_in are shown in Figs. 8(a) and 8(b), respectively.

Mode 2: t₁ ≤ t $<t2$⁠. In the second stage, L_S1 is above the transmitting coil 1 and L_S2 is above the transmitting coil 2. S₂, S₃, S₅, and S₆ are turned on, while S₁ and S₄ are turned off. u_in supplies power to L_P1 and L_P2, and the working states during the positive and negative half cycles of u_in are shown in Figs. 9(a) and 9(b), respectively.

Mode 3: t₂ ≤ t $<t3$⁠. In the third stage, L_S1 and L_S2 are above the transmitting coil 2. S₁, S₄, and S₆ are turned on, while S₂, S₃, and S₅ are turned off. u_in supplies power to L_P2, and the working states during the positive and negative half cycle of u_in are shown in Figs. 10(a) and 10(b), respectively.

According to the above-mentioned analysis, the status of S₁–S₆ in the different stages can be seen in Table I. Where 1 and 0 stand for on and off of the switch tubes, respectively.

Figure 11 shows the experimental device for the DWPT system, and the critical system parameters are given in Table II. In Fig. 11, the power amplifier AG1019 is used to provide high-frequency power. As the control unit, DSP is adopted to generate PWM wave to drive S₁–S₆. Figure 12 shows the single receiver and dual receiver used in the experiment. Figure 13 shows the specific dimensions of the coupling mechanism.

For the convenience of description, a point P (0, 2, 8) on the receiving coil is taken as the reference point, as shown in Fig. 13. So that the x-coordinate component of P (Px) can be used to represent the real-time position of EV. For the double-receiving-coil segmented DWPT system, when 0 ≤ Px $<$ 24 and 34 ≤ Px, the two receiving coils are powered by one transmitting coil, corresponding to Stage 1 and Stage 3 in Fig. 2. When 24 ≤ Px $<$ 34, EV is powered by two transmitting coils, corresponding to Stage 2 Fig. 2. For different power supply modes, the output voltage of the system varies greatly only in stage 2. Figures 14 and 15 show the output voltage waveform when Px = 24 and Px = 29 for unipolar and bipolar power supply modes.

Furthermore, the comparison experiment on receiving power between the single-receiving-coil system and the double-receiving-coil system is carried out. In addition, to simplify the description, the double-receiving-coil system and the single-receiving-coil system are labeled as DCS and SCS. In the experiment, the output power is recorded when Px moves from 24 to 35 cm in steps of 1 cm, as shown in Fig. 13.

In Fig. 16, it is obvious that the output power of DCS is higher than that of SCS, and the maximum output power of 5.73 W at Px = 29 is 5.43 W higher than that of SCS. When 24 < Px < 34, the average output power of DCS is about 5.657 W, and that of CSC is 2.007 W. Meanwhile, the power fluctuation of DCS with the maximum of 6.31% is far less than that of CSC with the maximum of 85.05%. The experimental results fully prove that the proposed DRST is effective to improve the output power and reduce the power fluctuation when EVs passing the adjacent two transmitting coils.

This paper mainly investigates the optimal design of a segmented DWPT system for EVs. In the traditional unipolar power supply mode, the DWPT system has a large fluctuation of receiving power when EVs move over two adjacent transmitting coils. Consequently, this paper selects the bipolar power supply mode achieved by two fully controlled components (S₅ and S₆). By turning the current with the same phase in the two adjacent lines of two transmitting coils, the magnetic field at the junction of two adjacent coils is obviously enhanced, and it is conducive to improve the stability of transmission power. Furthermore, a DRST is proposed to pick up the power. By controlling S₁, S₂, S₃, and S₄, three working modes with six working states could be achieved to obtain a small fluctuation of receiving power. The experimental results show that DRST is effective to output the power with a higher average value and a minor fluctuation.

This work was supported by the Postgraduate Research & Practice Innovation Program of Jiangsu Province under Grant No. KYCX23−3091.

The authors have no conflicts to disclose.

Yaqiang Zou: Conceptualization (lead); Supervision (lead); Validation (lead). Tian Xu: Formal analysis (lead); Investigation (lead); Methodology (lead); Software (lead); Validation (equal); Writing – original draft (lead); Writing – review & editing (lead). Zhong Li: Funding acquisition (equal); Methodology (equal); Validation (equal); Visualization (equal). Hao Qiang: Conceptualization (equal); Data curation (lead); Project administration (lead); Resources (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.