2016 IEEE 8th International Power Electronics and Motion Control Conference (IPEMC-ECCE Asia)
Design and Implementation of High Efficiency Wireless Power Transfer System for On-Board Charger of Electric Vehicle Duc - Hung Tran, Van - Binh Vu, Van – Long Pham, and Woojin Choi Department of Electrical Engineering
Soongsil University, Dongjak-Gu, Soeul, South Korea
[email protected] Abstract— In this paper, a high efficiency Wireless Power Transfer (WPT) system for the on-board charger of Electric Vehicle (EV) is proposed and implemented. The analysis of the proposed system based on the Thevenin equivalent circuit model to come up with a suitable design for the battery charge application is presented. The proposed Wireless Power Transfer method has adopted four-coil system with air core and its superior performance is proved by comparing it to the conventional two-coil system. In addition, since the proposed WPT converter is able to operate at an almost constant frequency regardless of the load, CC/CV charge of the battery can be simply implemented. A 6.6kW prototype is implemented with 200 mm air gap to prove the validity of the proposed method. The prototype was built and tested to verify the performance of constant current and constant voltage (CC/CV) charge. The experimental results show that the maximum efficiency of the proposed system is 97.08 % at 3.7 kW output power in the CV mode charge.
of them is the three-coil system [3]. Nevertheless, the system cannot achieve full soft switching at the turn-on and turn-off of the switches all over the charge process. In this research a novel topology for WPT system using two symmetric intermediate coils is proposed for on-board electric vehicle charger to overcome the drawbacks of the previous system as shown in Fig.1. The mathematical model of the proposed system is presented and analyzed in detail. Necessary conditions to gain zero–phase angle condition in both constant current and constant voltage mode of charge are designed based on the circuit analysis and hence the high efficiency can be achieved. This paper is composed of four sections. In the Section II, a general analysis for series–series compensated four–coil system by Thevenin theorem is presented. The experimental results of the 6.6 kW prototype of WPT system with an air gap of 200 mm are presented in the Section III to validate the proposed idea and the design. Finally, the conclusion is given in the Section IV
Keywords— Wireless Power Transfer (WPT), Electric Vehicles (EVs), Zero - Phase Angle (ZPA), Resonant tank
II. CIRCUIT CONFIGURATION AND OPERATION PRINCIPLE
I. INTRODUCTION Wireless Power Transfer (WPT) topology has got more attention during the last few years since it is able to transfer the electrical energy with no wire connection and also allows recharging their devices more safely and conveniently, thereby eliminating almost issues of the conventional wired counterpart. The well-known approach is the inductive wireless power transfer which employing two–coil resonators for applications in a short distance [1][4]. However, the energy transfer efficiency is declined rapidly if the distance between the transmitter and receiver is increased as a result of weak magnetic coupling. Some studies have proposed to use high permeability material such as ferrite to enhance its magnetic coupling however it contributes to an increase in volume, weight and cost, which makes it unsuitable for electric vehicle applications [2]. Another approaches are to take the advantages of the intermediate coils to improve the coupling factor and one
A. First Harmonic Equivalent Circuit Model of the System Fig. 1 shows the circuit diagram of the proposed symmetric four-coil WPT system. The circuit consists of a full bridge inverter, a series–series compensated loosely coupled transformer with two intermediate resonators and a full bridge diode rectifier. The first harmonic approximated model of the circuit is shown in Fig.2. All coupling factors between resonators are considered in the analysis of the system precisely. To facilitate the analysis, it is assumed that the parameters in the primary side are set similar to those of secondary side L1 = L4; L2 = L3; k12 = k34; and k13 = k24. Resonator coils, L1 and L4 self-resonate at the frequency f1, while L2 and L3 have different self–resonant frequency f2 = n21*f1. Coil resistances are neglected since the voltage drop across the coil inductance is dominant. The simplified two–port network model of the ac circuit is then derived as shown in Fig. 3. By applying the Kirchhoff’s voltage law to the circuit the voltage equations can
978-1-5090-1210-7/16/$31.00 ©2016 IEEE
978-1-5090-1210-7/16/$31.00 ©2016 IEEE
2016 IEEE 8th International Power Electronics and Motion Control Conference (IPEMC-ECCE Asia)
Rectifier
Inverter Resonance Tank
io
S2 C1
Udc
u1 S3
D1
C4
D2 Rb
i4
i1 L1
L4
C2 i2 L2
C3
u4
S4 L3
Cb D3
Battery model
S1
D4
i3
Fig.1.Configuration of the series – series compensated four – coil system
k14
L1
u1
k24 C3
k13 C2
C1
Thevenin impedance ITH
C4
U4
UTH
L4
L3
L2
I4
ZTH
RL,eq
RL,eq Fig. 4. Thevenin equivalent circuit of the proposed system
k12
k34
k23
Fig.2. AC equivalent circuit of the proposed system
I1
Zpri
Zse
Zlkp
Zlks
U1
ITH
I4
Zmag
Fig. 5. Thevenin equivalent circuit of the proposed system if Zpri = 0
U4
RL,eq
Zin
UTH
Fig.3. Two port impedance network of the proposed WPT system
0=
− −
+
(1) ,
Where, Zmag is the magnetizing impedance between transmitter and receiver. Zpri and Zse are impedances of the transmitter and receiver, whose values are expressed as (2). As shown in (2) the impedances Zmag, Zpri and Zse are complex
=
=
1−
−
U4
RL,eq
Fig. 6. Thevenin equivalent circuit of the proposed system when 2 Z 2− =0
be obtained as follows. =
RL,eq
+
functions combined with coupling coefficients, operating frequency ω, frequency ratio n21 and self-inductances L1 and L4. B. Implementation of CC/CV Mode Chagre with the Proposed WPT System To analyze the characteristic of the four–coil resonators after obtaining the first harmonic model of the system, Thevenin theorem is applied for simplified equivalent circuit shown in Fig.4. Thevenin equations for circuit is then derived as
1−
−2
1−
− (2)
=
−
2
1− 1−
−
+ −
2016 IEEE 8th International Power Electronics and Motion Control Conference (IPEMC-ECCE Asia)
−
= =
+
=
+
(3)
,
=
=
,
−
−
(4)
+
(5)
,
=
−
+
50
0
-50
-100 40
, ,
Zpri = Zmag Ro = 17 Ω Ro = 20 Ω Ro = 27 Ω Ro = 40 Ω
100
Phase of Zin in four coil system(ᵒ)
=
fCC 45
50
fCV2 55
Frequency (kHz)
(6)
60
65
70
a)
Where ZTH and UTH represent the Thevenin impedance and Thevenin voltage of the proposed circuit, respectively. It can be easily deduced from the (3) and (4) that it is possible to achieve the CC/CV mode charge by the suitable design of the compensation network. From Fig. 4 the charge current in the CC mode and the charge voltage in the CV mode can be expressed as (5) and (6), respectively. It can be noticed from (7) that the charge current I4 would be almost constant regardless of the load if the Thevenin impedance ZTH is designed to have a quite larger value than that of , . It can be achieved by making Zpri in (2) as 0 at the resonant frequency. Since the Thevenin impedance ZTH and Thevenin voltage UTH both become almost infinity, the CC mode charge can be implemented (Fig. 5). In additioin, if the input impedance Zin of the system is a real number, Zero Phase Angle condition can be obtained as shown in (8). =
=
= =
−
−
=−
+
,
−
+
+ +
, ,
(7) (8)
− ,
From the equation (6), the charge voltage U4 is almost constant regardless of the load if the Thevenin impedance ZTH is equal to zero. It could be satisfied if Z2pri – Z2mag = 0 at the resonant frequency. Then the charge voltage U4 is equal to the reflected output voltage U1*Zmag/Zpri, which makes it possible to implement the CV mode charge as shown in Fig. 6. The input impedance of the proposed system in this case is calculated as (9). =
, ,
(9)
Since the input impedance Zin is a function of load RL,eq as shown in (8), the phase of Zin, which is determined by the arg[Im{Zin}/Re{Zin}] = arg[-R/Zmag], would be almost zero if the value of Zmag is sufficiently large. Therefore, the most important aspect of the design is to maximize the magnetizing inductance Zmag..
b) Fig. 7. Phase of input impedance Zin and magnetizing impedance Zmag .
C. ZPA Condition at CV Mode Charge and the Efficiency Optimization of the Proposed WPT System. The WPT system consists of three major components, a full bridge inverter, a compensation network, and a diode rectifier. The total efficiency is primarily dependent on the switching loss on MOSFETs of the full bridge inverter and the efficiency of the resonant tank. In order to obtain higher efficiency, the proposed system needs to be designed to achieve ZPA condition for the soft switching. Since it can be achieved naturally at CC mode charge in the proposed system, it is required to derive the condition to achieve it at the CV mode charge. It is possible to achieve ZPA condition at the constant voltage mode charge if the Equ. (10) is satisfied |
|=|
| (10)
→∞
By using (9), the relationship between the coupling factors and the frequency ratio n21 can be derived as in (10). 1+
= 1−
+
− 2
(11)
If the system is designed to satisfy Equ. (11), the ZPA condition can be achieved regardless of the load at the frequency fCV2 due to the large value of Zmag as shown in Fig. 7(a). In order to achieve the high efficiency of the system, an equation has been derived as shown in (12). Here the resistances of the transmitter and receiver coils are included and the resistances of the intermediate coils are neglected since they are
2016 IEEE 8th International Power Electronics and Motion Control Conference (IPEMC-ECCE Asia)
(a) Fig. 9. CC/ CV profile and its equivalent impedance
(b) Fig. 8. Input waveforms in both CC and CV charging mode a) 6.6 kW in CC mode b) 3.3 kW in CV mode. Fig. 10. Efficiency of the proposed four coil system
relatively small. = =
| | | | Re
,
+
,
|
,
IV. CONCLUSION
Re
1
, ,
=|
+r
1+
r r + Im
,
(12)
It is clear from the Eqn. (12) that the high efficiency can be achieved by maximizing Zmag. In Fig. 7 (b), the magnetizing impedance Zmag of the proposed four–coil system is improved in comparison to the traditional two–coil system. As a result, very high efficiency can be achieved with the proposed fourcoil system. III. EXPERIMENTAL RESULTS In order to prove the validity of the proposed WPT system a 6.6 kW prototype was implemented. The distance between the primary coil and the secondary coil is 200 mm. Fig.8 (a) and (b) shows the voltage and current waveforms of the MOSFETs in both CC and CV charge mode, respectively. As shown in Fig. 9 the ZVS turn-on and ZCS turn-off of the switch are achieved in the entire range of charge process. Fig.9 shows the CC/CV charge profile of the battery module for the EV application and its equivalent impedance. Fig. 10 shows the overall efficiency plots of the dc-dc converter with proposed four-coil WPT system and the peak efficiency of 97.09 % was obtained at 3.7 kW in the CV charge mode.
This paper proposes the new topology and a detailed design procedure of a high-efficiency 6.6-kW WPT system for the on– board charger of EV applications. The necessary conditions for the frequency ratio between transmitter and intermediate coils to achieve the ZPA condition in both CC and CV mode for a loosely coupled wireless system are given. By achieving ZVS and ZCS for all of the primary switches, the switching loss can be eliminated over the entire load range, thereby achieving the high efficiency. A peak efficiency of 97.08 % at 3.7 kW with 200 mm air gap has been achieved. References [1]
[2]
[3]
[4]
C. Zheng, J.-S. Lai, R. Chen, W. E. Faraci, Z. U. Zahid, B. Gu, L. Zhang, G. Lisi, and D. Anderson, “High Efficiency Contactless Power Transfer System for Electric Vehicle Battery Charging Application,” IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 6777, no. c, pp. 1–1, 2014. J. Deng and S. Member, “Magnetic Integration of LCC Compensated Resonant Converter for Inductive Power Transfer Applications,” pp. 660– 667, 2014. S. Moon, B.C. Kim, S.Y. Cho, C.H. Ahn, and G.W. Moon, “Analysis and design of a wireless power transfer system with an intermediate coil for high efficiency,” Industrial Electronics, IEEE Transactions on, vol. 61, no. 11, pp. 5861–5870, Nov 2014. R. Bosshard, S. S. Member, J. W. Kolar, J. M.uhlethaler, S. S. Member, and B. Wunsch, “Power Transfer Coils for Electric Vehicles,” vol. 3, no. 1, pp. 50–64, 2015
