A Multi-Output Capacitive Charger for Electric Vehicles

A Multi-Output Capacitive Charger for Electric Vehicles Van-Binh Vu*, Luqman Bin Mohamad Kamal, Jasper Tay, Volker Pickert, Mohamed Dahidah, Thillainathan Logenthiran, Van-Tung Phan*, Senior Member, IEEE School of Electrical and Electronic Engineering, Newcastle University, UK *Email: [email protected], [email protected] advantages to the users. Firstly, the cost is reduced because only an inverter is required for several outputs. Secondly, MOCs is very appropriate to applications, where the space for installation is limited. The other important reason is that the charging time for several batteries is shortened. However, due to the complexity to regulate the outputs accurately and independently, MOCs are rarely used for EV application, where high power is required. In order to simplify the control and optimize the system efficiency, the load-independent characteristic is utilized in several previous researches [6], [7].

Abstract— Multi-output converters are widely used in various electronic devices and Electric Vehicles (EVs) due to their benefit in terms of cost, efficiency and space for installation. On the other hand, Wireless Power Transfer (WPT) methods including Inductive Power Transfer (IPT) and Capacitive Power Transfer (CPT) have been applied increasingly in EVs since they are more convenient and safe as compared to conventional conductive chargers. Due to replacing copper wires and permeable materials of inductive coupling method with cheap metal plates, implementing a CPT system is more cost effective and structurally simple system as compared to IPT. However, researches on multi-output chargers relating to CPT are rarely presented. This paper proposes a new concept for a multi-output charger system by adopting CPT for EVs application. The proposed system can charge independently several batteries of EVs at one time by adopting only one full bridge inverter at the primary side. The mathematical analysis supported by simulation results are presented.

This paper proposes a new approach to implement a multioutput charger system adopting capacitive power transfer for EV battery charging application. It is contained in four sections. At first, the concept of MOCs adopting CPT is proposed and presented in Section II. Next, double-sided LCL resonant topology is selected to implement MOCs system with the analysis of load-independent current characteristic in Section III. The design procedure and selections of components are mentioned in Section IV. Finally, simulation results are provided in Section V to verify the validity of the proposal method.

Keywords—Capacitive power transfer; multi-output charger; electric vehicles.

I. INTRODUCTION Inductive power transfer has been widely used in battery charger for various electronic devices and electric vehicles [1]. It adopts magnetic fields to transfer power with the air-gap from several centimeters to several meters. It is more beneficial to users in terms of convenience and safety as compared to the wired charging method. However, IPT technology has several drawbacks in its sensitivity to metal objects, high cost and complex structures [2]. In order to overcome the above disadvantages of IPT, Capacitive Power Transfer is an alternative solution. Differentiating from IPT, CPT utilizes electric fields to transfer power. The electric fields do not generate any losses in the metal objects nearby CPT system. Moreover, a CPT system usually embraces several cheap metal plates to transfer power, instead of expensive Litz-wire and ferrites in the case of IPT. As a result, the cost of a CPT system is normally lower than IPT. Due to these above advantages, the CPT becomes a host research trend recently [2], [3].

Fig. 1. Conceptual multi-output charger with capacitive power transfer (CPT) schematic

On the other hand, multi-output chargers (MOCs) method is broadly applied in various applications such as household equipment, portable electronics devices, LED drivers and telecommunication due to its benefits in terms of cost and space for installation [4], [5]. From a DC input voltage source, a MOCs system can provide several outputs. Each battery of output device is connected to the output port of MOCs system and is charged independently at one time. MOCs bring several

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II. THE PROPOSED CHARGING METHOD USING CPT A. Concept of MOCs with CPT The concept of the proposed multi-output charger is shown in Fig.1. N coupling plates are used to transfer energy from the primary source to secondary load. In order to transfer high power and high efficiency for EVs, resonant tanks are essential



Fig. 2.

The proposed multi-output CPT charger with double-sided LCL resonant circuit

hundreds of watt, LCL topology is proposed in [9], where the voltage for the coupling capacitor is boosted for higher power. In these topologies, the series inductance is very large due to a small value of capacitance. One of the solutions is to increase the switching frequency to MHz range. Unfortunately, the switching devices are limited by output power rating when operating in MHz range. Therefore, series and LCL topologies are not satisfactory for high power and large air-gap distance applications. Recently, double-sided LCLC is proposed to transfer the output power of 2.4 kW with an efficiency of 91 % at 150 mm [10]. Two capacitors are connected in parallel with coupling plate, which helps to reduce the series inductance value and switching frequency to 1 MHz. To reduce the number of resonant components and save space for installation, four-plate structure and associated double-sided LCL resonant topology is presented in [11]. In this work, this topology is selected to implement in each channel of two channels CPT system as shown in Fig. 2.

in both sides. The principle here is similar to an IPT system when the coupling capacitor is very small at large air-gap. The proposed system can theoretically charge different ݊ output batteries at the same time. In the proposed system, ݊ different output circuits share an inverter, which converts the DC input voltage into an AC voltage. Because the capacitances of plates are in a range of picofarad, the switching frequency of the inverter is boosted to operate in MHz range to limit the value of resonant inductors. Next, AC power from inverter is supplied to primary resonant tank, which is directly connected to the primary plates. The electric field is created by primary plates while the power from this electric fields is received by secondary plates. In secondary side of each channel, AC power from receiver plates is passed through the secondary resonant tank. This will be converted into DC power by a rectifier and adjusted by a regulator to charge the output battery. Depending on the required charging battery’s voltage and current, the regulator can be assigned as buck or boost converter. The selection of inverter and resonant tank strongly depends on the power level. The relays ܵ௔ଵǡ ǥ ܵ௔௜ǡ ǥ ܵ௔௡ are added at primary side of each channel to enable the output of each channel to charge the battery. Whenever there is an EV parking at output number ݅ to charge its battery, the relay ܵ௔௜ is in ON state, otherwise, it is in OFF state.

III. ANALYSIS OF THE PROPOSED CPT SYSTEM The simplified ࡭࡯ equivalent circuit of one channel in Fig. 2 is shown in Fig. 3. The concise output characteristics of doubled-side LCL resonant tank is analyzed by adopting the fundamental harmonics method. In most of battery charging systems, the constant current (CC) mode is prerequisite [12]. Therefore, conditions to achieve load-independent characteristic is utilized in this section. To simplify, the components excited by the input and output voltage is separately analyzed, as shown in Fig. 4(a) and Fig. 4(b), respectively.

B. Selection of CPT resonant topologies In this section, different CPT resonant topologies are compared to select the suitable one for high power rating and large air-gap applications, which are normally needed by Electric Vehicle Battery Charge. To compensate high leakage capacitance in a CPT system, resonant inductors are usually inserted in primary or secondary side. Depending on the position of these inductors, several topologies are reported [8][9]. The simplest method is added an inductor in series with coupling capacitors in primary side [8]. However, this method is only suitable to low power applications, where the capacitance can achieve high value. In order to transfer tens or

Fig. 3.



Equivalent circuit model of one channel multi-output CPT system.

From the AC equivalent circuit of one channel as shown in Fig.3, ‫ܥ‬ெ is defined as the mutual capacitance between the primary and secondary, while ‫ܥ‬ଵ and ‫ܥ‬ଶ are the selfcapacitance of primary and secondary, respectively. Similar to that of the inductive coils, the coupling capacitive coefficient of plates can be defined as follows: ݇ൌ

‫ܥ‬ெ ඥ‫ܥ‬ଵ ‫ܥ‬ଶ

to the analysis when applyingܸ௜௡̴஺஼ . The following equations can be achieved by using the condition in Eqns. (4) and (6): ‫ۓ‬ ۖ ௢ ௢ ‫ܸ۔‬஼௘௤ଶ ൌ ܸ஼ଵ௦ ۖ ‫ە‬

ሺͳሻ

In the primary side, ‫ܮ‬ଵ௣ is resonated with ‫ܥ‬ଵ௣ according to Eq. (6). It results in no current through ‫ܮ‬ଵ and the input current, which is contributed byܸ௢̴஺஼ , can be written as:

In order to determine the resonant condition, two equivalent input capacitance are defined as Eq. (2)  ‫ ܥۓ‬ൌ ሺ‫ ܥ‬െ ‫ ܥ‬ሻ ൅ ‫ܥ‬ெ ሺ‫ܥ‬ଶ െ ‫ܥ‬ெ ሻ ଵ ெ ۖ ௘௤ଵ ‫ܥ‬ଶ ሺʹሻ ሺ‫ܥ‬ ‫ܥ‬ ெ ଶ െ ‫ܥ‬ெ ሻ ‫۔‬ ሻ ሺ‫ܥ‬ ൅ ൌ െ ‫ܥ‬  ‫ܥ‬ ۖ ௘௤ଶ ଶ ெ ‫ܥ‬ଵ ‫ە‬

௢ ‫ܫ‬௜௡̴஺஼ ൌ

ቐ

ͳ ͳ ൅ ൌ ͲሺͶሻ ݆߱௢ ‫ܮ‬ଶ ൅ ݆߱௢ ‫ܥ‬ଵ௦ ݆߱௢ ‫ܥ‬௘௤ଶ ͳ ൌ Ͳሺͷሻ ݆߱௢ ‫ܥ‬ଵ௦

݆߱௢ ‫ܮ‬ଵ௣ ൅

ͳ ൌ Ͳሺ͸ሻ ݆߱௢ ‫ܥ‬ଵ௣

ቐ

௜௡ ௜௡ ൌ ܸ஼ଵ௣ ‫ܸ۔‬஼௘௤ଵ ۖ ‫ە‬

 (11)

మ௅ ஼ ൯ ௝ఠ೚ ௅భೞ ൫ଵିఠ೚ భ ೐೜

ܲ௜௡ ൌ ܸ௜௡̴஺஼ ‫ܫ כ‬௜௡̴஺஼ ൌ ܸ௜௡̴஺஼ ‫ܸ כ‬௢̴஺஼ ܲ௢ ൌ ܸ௢̴஺஼ ‫ܫ כ‬௢̴஺஼ ൌ ܸ௢̴஺஼ ‫ܸ כ‬௜௡̴஺஼

ଵ మ௅ ஼ ൯ ௝ఠ೚ ௅భ೛ ൫ଵିఠ೚ మ ೐೜ ଵ

 (12)

మ௅ ஼ ൯ ௝ఠ೚ ௅భೞ ൫ଵିఠ೚ భ ೐೜

1.

The multi-output charger is designed to always operate at constant resonant frequency߱௢ and provides an approximately load-independent current to the battery of each channel, it does not depend on the battery’s state.

2.

A regulator is inserted at secondary side of each channel as the charging controller, it regulates the output current when there is a variation of coupling capacitance or input voltage from PFC stage.

௜௡ ܸ௜௡̴஺஼ ൌ ܸ஼ଵ௣ ͳ ݆߱௢ ‫ܥ‬௘௤ ͳ ሺ͹ሻ ൌ ܸ௜௡̴஺஼ ଶ ͳ ͳ െ ߱௢ ‫ܮ‬ଵ ‫ܥ‬௘௤ ݆߱௢ ‫ܮ‬ଵ ൅ ݆߱௢ ‫ܥ‬௘௤

Moreover, there is also no current through‫ܮ‬ଶ , because ‫ܮ‬ଵ௦ and ‫ܥ‬ଵ௦ are formed another resonance according to Eq. (5). Then, the load current can be illustrated as: ௜௡ ‫ܫ‬௢̴஺஼ ൌ

௜௡ ‫ܫ‬௢̴஺஼ ൌ ‫ܫ‬௢̴஺஼ ൌ ܸ௜௡̴஺஼

ଵ

మ௅ ஼ ൯ ௝ఠ೚ ௅భ೛ ൫ଵିఠ೚ మ ೐೜ ଵ

Based on above analysis and assumptions, a new charging method is proposed to charge several EVs at the one time in this paper

The circuit characteristics will be analyzed at the resonant frequency ߱௢ under the conditions from Eqns. (3)-(6). The effect of ܸ௜௡̴஺஼ and ܸ௢̴஺஼ can be analyzed separately according to superposition theorem. When ܸ௜௡̴஺஼ is applied, the current through ‫ܮ‬ଵ௣ is equal 0 due to a parallel resonance between‫ܮ‬ଵ , ‫ܥ‬ଵ௣ and ‫ܥ‬௘௤ଵ according Eq. (3). The voltage on ‫ܥ‬ଵ௣ is equal to ܸ௜௡̴஺஼ and the voltage on‫ܥ‬௘௤ଵ can be obtained as follows: ‫ۓ‬ ۖ

௢ ‫ܫ‬௜௡̴஺஼ ൌ ‫ܫ‬௜௡̴஺஼ ൌ ܸ௢̴஺஼

The output current Io_AC becomes independent to battery’s charging state, it results in simplifying the control. The input and output power can be expressed as (12):

ͳ ͳ ൅ ൌ Ͳሺ͵ሻ ݆߱௢ ‫ܥ‬ଵ௣ ݆߱௢ ‫ܥ‬௘௤ଵ

݆߱௢ ‫ܮ‬ଵ௦ ൅

௢ ܸ஼ଵ௣ ͳ ൌ ܸ௢̴஺஼ ሺͳͲሻ ݆߱௢ ‫ܮ‬ଵ௣ ݆߱௢ ‫ܮ‬ଵ௣ ൫ͳ െ ߱௢ଶ ‫ܮ‬ଶ ‫ܥ‬௘௤ ൯

Hence, the input and output current can be expressed as follow based on the superposition theorem:

To simplify the analysis and design of the CPT system, it is assumed that the self-capacitance of primary is equal to that of the secondary side. As a result, ‫ܥ‬௘௤ଵ is equal to‫ܥ‬௘௤ଶ . The circuit parameters are designed by the following equations to achieve a constant output current and constant resonant frequency for the topology: ݆߱௢ ‫ܮ‬ଵ ൅

௢ ܸ௢̴஺஼ ൌ ܸ஼ଵ௦ ͳ ݆߱௢ ‫ܥ‬௘௤ ͳ ሺͻሻ ൌ ܸ௢̴஺஼ ଶ‫ܥ ܮ‬ ͳ ͳ െ ߱ ௢ ଶ ௘௤ ݆߱௢ ‫ܮ‬ଶ ൅ ݆߱௢ ‫ܥ‬௘௤

(a)

ͳ ܸ஼ଵ௦̴஺஻ ൌ ܸ௜௡̴஺஼ ሺͺሻ ݆߱௢ ‫ܮ‬ଵ௦ ݆߱௢ ‫ܮ‬ଵ௦ ൫ͳ െ ߱௢ଶ ‫ܮ‬ଵ ‫ܥ‬௘௤ ൯

When ܸ௢̴஺஼ is applied, the simplified circuit is illustrated in Fig. 4(b). ‫ܮ‬ଶ , ‫ܥ‬ଵ௦ and ‫ܥ‬௘௤ଵ forms a parallel resonant according to Eq. (4). There is no current passing inductor ‫ܮ‬ଵ௦ . It is similar

(b) Fig. 4. The AC equivalent circuit of LCL-LCL tank (a) When Vin_AC applies (b) When Vo_AC applies.



At one time, the different load conditions of each battery are carried out, which can be summarized in Table II with several cases. It is required that proposed multi-output CPT charger needs to supply an accurate current for each battery.

IV. DESIGN PROCEDURE The parameters of resonant tank are designed based on Eqns. (3)-(6), (11)-(12) and detailed reported in Table I together with the multi-output capacitive charger specification. The charging system consisting 2 channels is designed with power rating of 1.8 kW for each channel and shares the same input voltage of 250 V. The switching frequency is selected as 1 MHz in this paper. TABLE I.

TABLE II.

SPECIFICATION AND PARAMETERS FOR ONE CHANNEL OF CPT SYSTEM Parameter Input Voltage Output Voltage Output Current Self-capacitance Mutual capacitance Series inductors Additional inductors Parallel capacitors Switching frequency Distance

Symbol Vin Vo Io C 1, C 2 CM L1 ,L2 L1p ,L1s C1p, C1s fo D

Value 250 V 120~270 V 6.8 A 400 pF 20.5 pF 66.5 μH 3 μH 8.42 nF 1 MHz 10 cm

SEVERAL CASES OF DIFFERENT LOAD CONDITIONS OF TWO BATTERIES Battery I

Battery II

(1)

100% load

50% load

(2)

50% load

100% load

(3)

50% load

50% load

(4)

100% load

100% load

The metal plates for multi-output CPT system are set up as Fig. 5.a. Each channel is composed of 4 plates, 2 for primary side underground and the others for secondary sides in vehicles. The plates of each channel are symmetrical in their structure. The distance DC between two primary plates P11 and P21 should be design to eliminate the effect of crosscoupling between two channels. In this work, DC is selected as 150 cm. As a result, there is almost no effect between two channels according to the electric fields distribution shown in Fig 5.b.

(a)

(b) Fig. 5. (a) Three dimensional view of metal plates coupling capacitors. (b) Electric fields distribution around plates.

Fig. 6. Simulation waveform when the condition load of 50% for channel 1 and 100% for channel 2.

V. SIMULATION RESULTS

The main operation waveforms of the proposed charger are provided in Fig. 6 and Fig. 7 under different load condition of each channel. It can be clearly seen that there is almost no

In order to validate the performance of proposed method, the system of two output batteries is simulated in this Section.



reactive power in the primary side because the input voltage Vpri is almost in phase with the input current Ipri. When battery I requires 50% load and battery II requires 100% load as shown in Fig. 6, the voltage’s diode of battery I (Vsec1) is half as battery II (Vsec2) because the impedance of battery II is twice as battery I. The output currents Io1 and Io2 almost keep the same.

VI. CONCLUSION This paper suggests a new method to implement a multioutput CPT charger for EV’s application. A conceptual multioutput charger with capacitive power transfer is proposed. The condition to achieve load-independent current is provided by analyzing the frequency characteristic of double-sided LCL resonant circuit. With the proposal charging method, a stable charge current of IPT system can be guaranteed at resonant condition. The distance between metal plates is also mentioned to cancel the effect of cross-coupling between two channels. Simulation results under different load condition verify the validity of the proposal charging method. For future works, the design of regulator in secondary side is conducted in order to charge the output batteries in both Constant Current (CC) and Constant Voltage (CV) modes. Experimental results are also updated to verify the validation of proposed method.

When two batteries have same load conditions, then the voltage’s diode have the same value and secondary side’s waveform is similar between two channels, which is depicted in Fig. 7. The proposed charger provides the exact same current for each channel.

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Fig. 7. Simulation waveform when the condition load of 100% for channel 1 and 100% for channel 2.