Wireless Power Transfer for Electric Vehicles Charging Based on Hybrid Topology Switching With a Single Inverter

Abstract

In wireless power transfer (WPT) system, the conventional compensation topologies only can provide a constant current (CC) or constant voltage (CV) output under their resonant conditions. It is difficult to meet the CC and CV hybrid charging requirements without any other schemes. In this study, a switching hybrid topology (SHT) is proposed for CC and CV electric vehicle (EV) battery charging. By utilizing an additional capacitor and two AC switches (ACSs), a double-side LCC (DS-LCC) and an inductor and double capacitors-series (LCC-S) topologies are combined. According to the specified CC and CV charging profile, the CC and CV charging modes can be flexibly converted by the two additional ACSs. In addition, zero phase angle (ZPA) also can be achieved in both charging modes. In this method, because the operating frequency is fixed, without using PWM control, and only a small number of devices are added, it has the benefits of low-cost, easy-controllability and high efficiency. A 3.3-kW experimental prototype is configured to verify the proposed switching hybrid charger. The maximum DC efficiencies (at 3.3-kW) of the proposed SHT is 92.58%.

1. Introduce

Electric vehicle (EV) battery charging methods include contact charging and wireless charging [1], [2]. Compared with contact charging, wireless charging has the advantages of no contact loss, no mechanical wear, safety, and reliability [3]-[4]. Thus, wireless charging has received increasing attention.

EV battery charging has several charging strategies, e.g., CC charging, CV charging and CC/CV hybrid charging [5]. According to the characteristics of battery, and combining the advantages of CC and CV charging. Generally, CC/CV hybrid charging is widely utilized in EV industrial [6]. The typical profile of CC/CV hybrid charging is shown in Fig. 1. The entire charging process is divided into two stages, which are CC mode and CV mode. In CC mode, the charging current is constant and charging voltage gradually increases. The CV mode begins when the charging voltage increases to specified voltage, the charging voltage remains unchanged and the charging current slowly decreases until zero. The equivalent resistance continues to rise throughout the entire charging process.

Fig. 1 Typical profile of EV battery charging

In wireless power transfer (WPT) system, normally, two conventional methods are often used to realize CC and CV hybrid charging. First, by adding an additional DC-DC converter behind the secondary rectifier and using PI control method to regulate the outputs directly [7], however, this method increases the cost and volume of the whole WPT system, and lower the total systeme efficiency. Another method is using phaseshift (PS) and frequency modulation (FM) hybrid control to adjust the effective input voltage [8]. Even though the outputs can be indirectly regulated, the operating frequency should be continuously changed to satisfy the zero voltage switching (ZVS) condition, thus, the control process is very complex, and the DC-link voltage utilization is low.

In order to achieve the CC and CV outputs for hybrid battery charging and ZPA for high efficiency, simultaneously. In WPT system, the numerous compensation topologies which have a CC/CV output can be reconfigured and combined by using ACSs. In this method, because the operating frequency is fixed, without using PWM control and only a small number of devices are required, it attracts increasing attentions of researchers. In [9], a composite topology, which combines a series-series (S-S) and a series-parallel (SP) topology, was proposed. CC and CV output can be realized by switching; however, because a center-tapped transformer and three ACSs are required, the topology sturcture is complex. In [10], a switching hybrid LCC-S compensation topology was proposed. Based on a conventional LCC-S topology, two additional capacitors were added to the topology for the implementation of CC and ZPA, and CC and CV charging modes can be converted by using two ACSs, however, the two ACSs were placed in both primary and secondary sides, therefore, wireless communication is required for synchronous control. In this study, an SHT is proposed for CC and CV hybrid charging, the combination of a DS-LCC and a LCC-S topologies can be implemented only through an additional capacitor and two ACSs. In addition, all the additional resonant devices are plaeced in the secondary side, thus, the battery information can be directly obtained by the secondary controller, and wireless communication is not needed. A 3-kW experimental prototype is configured, and comparative experiments are conducted to verify the proposed SHT.

2. Theoretical Analysis

2.1 Fundamental Analysis of the WPT system

Fig. 2 shows the structure of the proposed WPT system based on SHT. U_DC is the DC-link voltage. A full-bridge inverter (FBI) is applied to convert the DC components into AC components. U_AB and I_IN are the output voltage and current of the FBI, respectively. If U_AB is expanded by the Fourier series, it can be expressed as 

#(1)

where n is the number of harmonics and φ is the phase angle. S₁ and S₂ are the corresponding ACSs to switch the CC and CV charging modes. The bridge rectifier (BR) is applied to transform the AC output voltage Uab and current Iab to battery charging voltage U_Bat and current I_Bat, respectively. While the fundamental harmonic analysis (FHA) method is utilized to analyze, and the output low-pass filter (LPF) consists only of the capacitance CO, the following equations can be derived according to [11]:

#(2)

where U_ab and Ia_b are the RMS values of U_ab and I_ab. The output AC and DC equivalent resistance are defined as R_Ac = U_ab / I_S, and R_Bat = U_Bat / I_Bat, respectively.

In WPT system, the loosely coupled transformer (LCT) is a key component, Generally, there are two analytical models utilized to analyze LCT, which are named M model and T model, as shown in Fig. 3. L_p and L_s are the self-inductances of primary coil and secondary coil, respectively. M is the mutual inductance of LCT. On the premise that the M model and the T model are equivalent, and the following equations can be derived

#(3)

where L_T and L_R are the leakage inductance of the primary coil and secondary coil when the turn ratio is same.

Fig. 2 WPT system based on proposed switching hybrid compensation topology

Fig. 3 Equivalent models of loosely coupled transformer. (a) M model. (b) T model.

2.2 CV Mode Analysis of the Proposed Switching Hybrid Compensation Topology

In Figure 2, if the ACS_S S₁ and S₂ both off, the equivalent T model for CC mode can be shown in Fig. 4(a). If the series-connections of L_R and L₂ are considered as a whole inductor L’, while the series-connections of C₁ and C_f2 are considered as a whole capacitor C’, there are

#(4)

Fig. 4 Equivalent T models of the proposed switching hybrid compensation topology

(a) for CV mode. (b) for CC mode

when C’ is large enough, the seriesconnection of L’ and C’ can be regarded as an equivalent capacitance C₀, i.e.,

#(5)

The angular frequency of U_AB is defined as ω₀ and is referred to as the operation angular frequency of the resonant network hereafter. In the V-C resonant network, when L₁ resonate with C_p1, i.e., ω₀ ² = 1 / (L₁C_p1), the primary coil current Ip can be obtained as

#(6)

The RMS value of Ip is a constant and is irrelevant to the other parameters of the topology. In the C-V resonant network, when M resonate with C₀, i.e., ω₀² = 1 / (MC₀), and by combining (2) the battery charging voltage U_Bat can be derived as

#(7)

From (7), it can be seen that the battery charging voltage U_Bat is constant and independent of load in CV mode. In Fig. 3(a), the total input impedance Z_in can be given as

#(8)

Under the condition of ω₀² = 1 / (L₁C_p1) = 1 / (MC₀), and substituting (3) into (8), the condition to implement ZPA can be derived as

#(9)

Under the condition as given in (9), and combining (2), the input impedance of ZPA in CV mode can be given as

#(10)

2.3 CC Mode Analysis of the Proposed Switching Hybrid Compensation Topology

In Figure 2, if the ACSs S₁ and S₂ both on, the equivalent T model for CC mode can be shown in Fig. 4(b). If L_T and L_R are much larger than C_f1 and C_f2, their seriesconnections can be considered as two equivalent inductors L_T’ and L_R’, i.e.,

#(11)

When L₁ resonate with C_p1, the primary coil current I_p is same with that in CV mode. If the C-C resonant network operates under its resonant condition, i.e.,

#(12)

Thus, the AC output current I_ab can be derived as

#(13)

by substituting (2) to (13), the battery charging current I_Bat can be derived as

#(14)

From (14), it can be seen that the battery charging voltage I_Bat is constant and independent of load in CC mode. From Fig. 3(b) and according to the previous analysis, the total input impedance Z_in in CC mode can be expressed as (8). To realize ZPA in CC mode, the following equations should be satisfied

ω₀²(LP-L1)Cf1 = ω₀²(L_s-L₂)C_f2 = 1       (15)

Under the condition as given in (15), and combining (2), the input impedance of ZPA in CC mode can be given as

#(16)

2.4 Parameter Modification-based CC and CV Mode Implementation for WPT system

For the proposed switching hybrid compensation topology, the parameter design method for CC/CV output implementation is produced in detail below. Under the condition that LCT parameters have been given, the parameters of the proposed switching hybrid compensation topology are designed according to the specified CC and CV values. Because the primary side compensation devices are shared in both CC and CV charging modes, and the circuit in CV mode is more simple than that in CC mode, the primary side devices should be determined according to Fig. 4(a). Under the resonant condition and according to (7), primary compensation inductance L₁ can be designed as

#(17)

After the determination of L₁, the primary compensation capacitances C_p1 and C_f1 can be determined as

#(18)

In addition, according to the specified CC value and (14), the secondary compensation inducatance L₂ can be derived as

#(19)​​​​​​​

In Figure 4(b), similarly, the secondary compensation capacitances C_p2 and C_f2 can be calculated according to the secondary resonant conditions

#(20)​​​​​​​

In CV mode, according to the CV output condition, i.e., ω₀² = 1 / (MC₀), and by combining (4) and (5), the additional compensation capacitance C₁ can be derived as

#(21)​​​​​​​

After the determination of LCT and the specification of CC and CV values, all the compensation devices of the proposed SHT can be determined according to (17)–(21). In this research, the charging current and voltage are designed to 20 A and 165 V, respectively. The designed system parameters are shown in Table 1. The input phase angle of the compensation topology is defined as

#(22)​​​​​​​

where the operator “Im” represents the imaginary part, and “Re” represents the real part of the input impedance. The voltage gain G_V and current gain G_I of the proposed SHT are defined as follows:

#(23)​​​​​​​

#(24)​​​​​​​

#(25)​​​​​​​

By using the designed parameters listed in Table 1, the voltage/current gain and phase angle of the porposed SHT are caluculated, as shown in Fig.5, the ZPA can be achieved at designed resonant frequency (85 kHz) both in CC mode and CV mode. In addition, the output voltage in CV mode and output current of CC mode both are constants at 85 kHz.

Fig. 5 Input phase angle and voltage/current gain of the proposed SHT

(a) for CV mode. (b) for CC mode.​​​​​​​

Table 1. Specifications and parameters of the WPT system​​​​​​​

Fig. 6 Charging characteristics of the proposed hybrid charger.

(a) Measured charging profile; (b) Measured DC efficiency and charging power.

3. Experimental Verification

In order to verify the reasonability and feasibility of the proposed SHT-based WPT system, a 3.3-kW experimental WPT prototype is configured according to the parameters listed in Table 1. The deviation between the designed parameters and practical parameters is found to be less than 1%.

In this study, the specified values of I_Bat and U_Bat are designed to 20 A and 165 V, respectively. The charging characteristics of the proposed hybrid charger are shown in Fig. 6. In Fig.6 (a), it can be observed that as R_Bat increases, the charge current in CC mode and charge voltage in CV mode are almost constants. Measured DC efficiency and charge power are shown in Fig. 4(b); as the R_Bat increases, both DC efficiency andcharge power increase in CC mode and decrease in CV mode. The maximum DC efficiency (92.35%) and charge power (3.3 kW) are observed

The experimental waveforms are shown in Fig. 7. With the fluctuations of equivalent DC output resistance, charge current I_Bat in CC mode as well as the charge voltage U_Bat in CV mode are nearly constants. The plausibility of the proposed SHT is verified via the above experimental results.

Fig. 7 Experimental waveforms of CC and CV modes with load fluctuations.

(a) In CC mode (b) In CV mode​​​​​​​

4. Conclusion

In this study, an SHT for EV wireless charging is proposed. The CC/CV charging and ZPA character can be implemented just via the switching between two ACSs. Due to the two ACSs are placed in primary side, the system can be easily controlled via only one primary controller and without synchronous wireless communication between transmitter and receiver. Thus, the cost and control complexity of the whole WPT system are significantly reduced. A 3.3-kW experimental prototype is configured to verify the charging performances of the proposed hybrid charger. The maximum DC efficiency (92.28%) and charge power (3.3 kW) are achieved at the critical charge point. The charging characteristics also remained stable during load fluctuation.