Capacitive Coupling Wireless Power Transfer with Quasi-LLC Resonant Converter Using Electric Vehicles’ Windows

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Article Capacitive Coupling Wireless Power Transfer with Quasi-LLC Resonant Converter Using Electric Vehicles’ Windows

KangHyun Yi

School of Electronic and Electric Engineering, Daegu University, Daegudaero 201, Gyeongbuk 712714, Korea; [email protected]; Tel.: +82-53-850-6652

Received: 18 March 2020; Accepted: 18 April 2020; Published: 21 April 2020

Abstract: This paper proposes a new capacitive coupling wireless power transfer method for charging electric vehicles. Capacitive coupling wireless power transfer can replace conventional inductive coupling wireless power transfer because it has negligible eddy-current loss, relatively low cost and weight, and good misalignment performance. However, capacitive coupling wireless power transfer has a limitation in charging electric vehicles due to too small coupling capacitance via air with a very high frequency operation. The new capacitive wireless power transfer uses glass as a dielectric layer in a vehicle. The area and dielectric permittivity of a vehicle’s glass is large; hence, a high capacity coupling capacitor can be obtained. In addition, switching losses of a power conversion circuit are reduced by quasi-LLC resonant operation with two transformers. As a result, the proposed system can transfer large power and has high eﬃciency. A 1.6 kW prototype was designed to verify the operation and features of the proposed system, and it has a high eﬃciency of 96%.

Keywords: capacitive coupling wireless transfer; dielectric layer; quasi-LLC resonant operation; electric vehicles

1. Introduction Power conversion systems are widely researched for transportation electrification and the rise in electric vehicle (EV) development. More than 5.1 million EVs were produced in 2018. In particular, charging technology for batteries is important to expand the EV market, so on-/off-board charging and fast-charging systems have been investigated [1]. Figure1 shows methods of charging an EV.Figure1a,b shows a slow charging method that receives alternative current (AC) input and a quick charging method that receives direct current (DC) input. However, the prior approaches have drawbacks such as galvanic isolation, the size and weight of the charger, and user inconvenience. Wireless power transfer (WPT) charging will be an alternative charging solution to address the drawbacks of wired charging and give the consumers convenience as shown in Figure1c. Furthermore, it has inherent galvanic isolation, low weight, and reduced cost for charging systems in EVs. Generally, the types of the WPT are inductive coupling wireless power transfer (ICWPT) and capacitive coupling wireless power transfer (CCWPT). ICWPT uses the magnetic field between a transmitting coil and receiving coil under the EV as shown in Figure1c. The WPT charging method exchanges information between the charger and the electric vehicle through wireless communication to control charging. While this technology has good efficiency and commercialization techniques, there are disadvantages such as one position power transfer, heat dissipation in the metal barrier, and large coil volume [2]. Furthermore, it fundamentally has power transfer interference by the metal. CCWPT uses the displacement current in two capacitors with four copper plates and a dielectric layer [3–5]. Compared to the magnetic coupling WPT system, the capacitive coupling WPT system

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CCWPT Electronicsuses the2020 displacement, 9, 676 current in two capacitors with four copper plates and a dielectric2 of 12 layer [3–5]. Compared to the magnetic coupling WPT system, the capacitive coupling WPT system does not makedoes significant not make signiﬁcant eddy-current eddy-current losses losses in nearby in nearby metals, metals, and there there is no is concern no concern about the about the temperature rising in metal. In addition, metal plates are used in a capacitive coupling WPT system to temperature rising in metal. In addition, metal plates are used in a capacitive coupling WPT system transfer power, which can reduce the system’s cost and weight. to transfer power, which can reduce the system’s cost and weight.

(a) (b)

(c)

Figure 1.Figure Electric 1. Electricvehicle vehicle charging charging method. method. (a) (Slowa) Slow wired wired charging charging.. ((bb)) Quick Quick wired wired charging. charging. (c) Wireless( cpower) Wireless transfer power transfer(WPT) (WPT)charging. charging. The structure of the capacitive coupler or the characteristics of the dielectric and the compensation method of the power conversion circuit determine the performance of CCWPT. The capacitive coupler Thestructure structure is generally of the a parallel capacitive plate structure, coupler which or can the be characteristics implemented with of rectangular the dielectric or circular and the compensationdiscs [6 –method8]. If the transmissionof the power distance conversion is more circuit than 10 cm,determine the coupling the capacitanceperformance is determined of CCWPT. The capacitiveby coupler the edge estructureffect of air is as generally the dielectric. a parallel This coupling plate capacitance structure, is which relatively can small. be implemented In order to with obtain a larger coupling capacitance, a study was also conducted to form a coupling capacitance using rectangulara vehicle or circular bumper with disc larges [6– relative8]. If the permittivity transmission as a dielectric distance [9]. In is addition, more than a method 10 cm, of using the a coupling capacitanceplurality is determined of plates has also by been the proposed edge effect to improve of air the as coupling the dielectric. of the plate This coupling coupling capacitance capacitance [10]. is relativelyCompensation small. In order circuit to topologiesobtain a larger have been coupling proposed capacitance, to deliver large a study amounts was of also power conducted with very to form a couplingsmall capacitance coupling capacitance using a [vehicle11–18]. A bumper Class-E inverter with large can be relative employed, permittivity and the coupling as a capacitor dielectric is [9]. In addition,used a method as a regular of using resonant a plurality component of [11 plates,12]. However, has also the been disadvantage proposed of Class-Eto improve inverter-based the coupling of topology is its sensitivity to parameter variations such as the coupling capacitance or other reactive the platecomponents, coupling capacitance and it is difficult [10 to]. Compensation increase the system circuit power topologies level. Simple have full-bridge been invertersproposed with to deliver large amountsa series compensated of power with inductor very are small proposed coupling [13–15 ]. capacitance It is very simple [11– but18]. very A Class sensitive-E inverter with the can be employedcoupling, and the capacitance. coupling Anothercapacitor compensation is used as topologya regular is resonant a double-sided componentLCLC, LCL, [11,or12].CLLC However,in the disadvantagewhich of two Class external-E inverter inductors-based and two topology external is capacitors its sensitivity are used to onparameter each side variations of the coupling such as the couplingcapacitor capacitance [16–18 or]. Theseother can reactive deliver component large amountss, ofand power it is but difficult require largeto increase inductors the or system a lot of power passive elements. The disadvantage is that it is difficult to design because many passive elements level. Simpleare used, full and-bridge the operation inverters frequency with a series is very compensated high. In summary, inductor the prior are conventional proposed [13 capacitive–15]. It is very simple butcoupling very sensitive WTP systems with are the sensitive coupling to parameter capacitance. variations Another or have a compensation number of devices topology that use many is a double- sided LCLCpassive, LCL, components. or CLLC Moreover,in which whentwo external air is used inductors as the dielectric, and two a strong external electric capacitors field is formed, are used on each sidewhich of the is a coupling safety problem capacitor [18–20]. [16–18]. These can deliver large amounts of power but require large inductorsIn this or paper,a lot of a new passive CCWPT elements. system is The proposed disadvantage for charging is EVs. that The it is proposed difficul CCWPTt to design uses because copper plates and transparent plates such as indium tin oxide (ITO) between glass dielectric layers of many passivethe EV’s elements windows. are The used proposed, and CCWPT the operation system uses frequency a dielectric layeris very of vehicle’s high. In windows summary, for the the prior conventional capacitive coupling WTP systems are sensitive to parameter variations or have a number of devices that use many passive components. Moreover, when air is used as the dielectric, a strong electric field is formed, which is a safety problem [18–20]. In this paper, a new CCWPT system is proposed for charging EVs. The proposed CCWPT uses copper plates and transparent plates such as indium tin oxide (ITO) between glass dielectric layers of the EV’s windows. The proposed CCWPT system uses a dielectric layer of vehicle’s windows for the coupling capacitor and has the previously studied step-up and step-down transformers to obtain a low quality factor [21]. The glass has large relative permittivity, and the EVs have large areas of glass in front and on the back and sides. This results in large coupling capacitance and output power,

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relatively, and a strong electric field can be trapped in the dielectric, which has the advantage of couplingstability. capacitorSwitching and loss has in thethe previouslypower conversion studied step-upcircuit can and be step-down reduced with transformers the quasi to-LLC obtain resonant a low qualityoperation factor. As [21 a ].result The glass, turn has-on large loss relative of the powerpermittivity, MOSFET and theby EVs zero have voltage large switching areas of glass (ZVS) in front in a andtransmitter on the back and and turn sides.-off loss This of results the rectifier in large by coupling zero current capacitance switching and (ZCS) output in power, rectifier relatively, diodes can and be a strongdecreased. electric The field operation can be and trapped features in the of the dielectric, proposed which system has were the advantage verified with of stability. a 1.6 kWSwitching prototype lossfor charging in the power EVs. conversion circuit can be reduced with the quasi-LLC resonant operation. As a result, turn-on loss of the power MOSFET by zero voltage switching (ZVS) in a transmitter and turn-off loss of2. theThe rectifier Proposed by zeroCCWPT current System switching (ZCS) in rectifier diodes can be decreased. The operation and features of the proposed system were verified with a 1.6 kW prototype for charging EVs. 2.1. Substrates for Dielectric Layers in Vehicle 2. The Proposed CCWPT System Table 1 shows the relative permittivity of material in the vehicle exterior. The bumpers and the 2.1.plastic Substrates exterior for of Dielectric headlights Layers are in made Vehicle of polypropylene or acrylonitrile-butadiene-styrene (ABS) resin, and the windows in the vehicle are glass [22]. As shown in Table 1, the glass has the largest Table1 shows the relative permittivity of material in the vehicle exterior. The bumpers and the relative permittivity and large coupling capacitance that can be obtained with glass because the area plastic exterior of headlights are made of polypropylene or acrylonitrile-butadiene-styrene (ABS) resin, of the front and back windows is large. Therefore, a coupling capacitor can be formed with the and the windows in the vehicle are glass [22]. As shown in Table1, the glass has the largest relative vehicle’s window glass. One electrode outside of the vehicle is copper plate and the other inside the permittivity and large coupling capacitance that can be obtained with glass because the area of the vehicle is a transparent electrode for visibility in not charging the vehicle, as shown in Figure 2. When front and back windows is large. Therefore, a coupling capacitor can be formed with the vehicle’s an electric vehicle is charged, the copper plates mechanically adhere to the front and back window window glass. One electrode outside of the vehicle is copper plate and the other inside the vehicle glass outside of the vehicle with high pressure. Transparent electrodes on the front and back window is a transparent electrode for visibility in not charging the vehicle, as shown in Figure2. When an glass inside the vehicle are formed with physical vapor deposition or chemical vapor deposition. ITO electric vehicle is charged, the copper plates mechanically adhere to the front and back window glass will be a candidate for transparent electrodes. The window glass for the vehicles has a polyvinyl outside of the vehicle with high pressure. Transparent electrodes on the front and back window glass butyral (PVB) film sandwiched between two sheets of glass [23]. If the electrode ITO in the vehicle is inside the vehicle are formed with physical vapor deposition or chemical vapor deposition. ITO will formed after the PVB film, an insulation is possible as shown in Figure 2. ITO’s visible light be a candidate for transparent electrodes. The window glass for the vehicles has a polyvinyl butyral transmittance is more than 80%, so it is possible to ensure visibility even when charging an electric (PVB) film sandwiched between two sheets of glass [23]. If the electrode ITO in the vehicle is formed vehicle with a glass window [24]. Two coupling capacitors can be made with the window glass of the after the PVB film, an insulation is possible as shown in Figure2. ITO’s visible light transmittance is vehicle to transfer power wirelessly as in Figure 3. more than 80%, so it is possible to ensure visibility even when charging an electric vehicle with a glass window [24]. Two coupling capacitors can be made with the window glass of the vehicle to transfer Table 1. Relative permittivity of according to material in vehicle’s exterior. power wirelessly as in Figure3.

Table 1. RelativeMaterial permittivity of according toRelative material inPermittivity vehicle’s exterior.

AirMaterial Relative Permittivity1.0005 GlassAir 1.0005 4–7 PolypropyleneGlass 4–7 2.2–2.4 Polypropylene 2.2–2.4 ABSABS Resin Resin 2.3–2.5 2.3–2.5

Transparent electrode plate Pressure (inside of a vehicle) Copper plate (Outside of a vehicle) Chemical contact Mechanical contact Glass dielectric layer (Window of a vehicle) Inner Glass layer Polyvinyl butyral (PVB) film

Figure 2. Coupling capacitor implementation in a vehicle. Figure 2. Coupling capacitor implementation in a vehicle.

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Figure 3. Proposed capacitive coupling wireless power transfer (CCWPT) charging system using the Figure 3. Proposed capacitive coupling wireless power transfer (CCWPT) charging system using the windows of an electric vehicle. windows of an electric vehicle. 2.2. Quasi-LLC Power Conversion Circuit for Adjusting Impedance of a Coupling Capacitor 2.2. Quasi-LLC Power Conversion Circuit for Adjusting Impedance of a Coupling Capacitor Figure4 shows a proposed CCWPT power conversion circuit for charging EVs. The dielectric layer for theFi couplinggure 4 shows capacitor a proposed in the EVs CCWPT is glass. power A full-bridge conversion inverter circuit makes for charging the AC EVs. voltage, The adielectric step-up transformerlayer for the increases coupling thecapacitor equivalent in the capacitance EVs is glass of. theA full coupling-bridge capacitor,inverter make ands an the output AC voltage, voltage a canstep be-up determined transformer with increases a step-down the equivalent transformer. capacitance Wireless of power the coupling transfers capacitor, by resonance and an between output voltage can be determined with a step-down transformer. Wireless power transfers by resonance a resonant inductor Lr and the equivalent coupling capacitance. Figure5 shows key waveforms of thebetween proposed a resonant CCWPT powerinductor conversion Lr and the circuit. equivalent The step-up coupling transformer capacitance. can make Figure impedance 5 shows of thekey couplingwaveforms capacitance of the proposed small and CCWPT the step-down power transformerconversion circuit. can adjust The the step impedance-up transformer of load resistancecan make andimped qualityance factor. of the The coupling magnetized capacitance inductor small in the and step-down the step transformer-down transformer can make can a zero adjust voltage the switchingimpedance (ZVS) of load of switches resistance in the and full quality bridge factor. inverter. The Moreover, magnetized a zero inductor current in switching the step- (ZCS)down oftransformer rectifier diodes can make in the a zero full voltage bridge rectifierswitching can (ZVS) be achieved of switches by in the the quasi- full bridgeLLC resonant inverter. operation. Moreover, Thea zero operation current isswitching similar to(ZCS) a conventional of rectifier diodesLLC resonant in the full DC bridge/DC converter, rectifier can as shownbe achieved in Figure by the5. Forquasi mode-LLC analysis, resonant the operation. following The assumptions operation are is made. similar to a conventional LLC resonant DC/DC converter, as shown in Figure 5. For mode analysis, the following assumptions are made. (1) All analyses are performed in steady-state operation. (1) All analyses are performed in steady-state operation. (2) The capacitances of Cb and Co are sufficiently large to make their voltages constant. (2) The capacitances of Cb and Co are sufficiently large to make their voltages constant. (3) M1–M4 are ideal except for their internal diodes and output capacitors. (3) M1–M4 are ideal except for their internal diodes and output capacitors. (4) D –D are ideal except for their junction capacitors. (4) D1–1D4 are4 ideal except for their junction capacitors. (5)(5) TheThe inductance inductance of of LmL2m is2 isseveral several times times greater greater than than the the inductance inductance of of LrL. r. (6)(6) ItIt is is assumed assumed that that the the inductance inductance of of LLm1m is1 islarge large enough enough and and is isinfinite. infinite. (7)(7) TheThe turn turn ratio ratio of of the the step step-up-up transformer transformer TT1 1isis n1n. 1. (8)(8) TheThe turn turn ratio ratio of of the the step step-down-down transformer transformer TT2 2isis n2n. 2.

ModeMode 11(t (t00––tt11)):: This mode begins whenwhen MM11 and MM44 are turned ooffff atat tt00.. At thisthis moment,moment, resonantresonant inductorinductorL Lrr currentcurrent isis positive,positive, so so that that it it will will flow flow through through the the output output capacitors capacitors of ofM M2 2and andM M3.3. InIn thisthis mode,mode, the the junction junction capacitors capacitors of of the theD 1Dand1 andD 4Drectifiers4 rectifiers are are charged charged and and the the junction junction capacitors capacitors of the of Dthe2 and D2 andD3 rectifiers D3 rectifiers are discharged.are discharged. ModeMode 22 (t (t1–1–tt22)):: ThisThis modemode beginsbegins whenwhen drain-sourcedrain-source voltagesvoltages ofofM M22 andandM M33 areare zero.zero. AtAt thisthis moment,moment, thethe resonantresonantcurrent current will will flow flow through through the the body body diodes diodes of ofM M2 2and andM M33,, whichwhich createscreates aa ZVSZVS conditioncondition forforM M22 andand MM33.. The gate signals of M2 andand MM33 shouldshould be be applied applied during during this this mode. mode. Mode 2 (t2–t3): Switches M2 and M3 have been turned on in the ZVS condition, and the energy is transferred from the input to the electric vehicle. In this mode, the circuit works like a series resonant converter with resonant inductor Lr and resonant capacitor Cr. This mode ends when Lr current is the

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Mode 2 (t2–t3): Switches M2 and M3 have been turned on in the ZVS condition, and the energy is Electronicstransferred 2020, 9, fromx FOR thePEER input REVIEW to the electric vehicle. In this mode, the circuit works like a series5 resonant of 13 converterElectronics with 2020 resonant, 9, x FOR PEER inductor REVIEWLr and resonant capacitor Cr. This mode ends when Lr current is the5 of 13 samesame as i asLm2 i/nLm1 2current./n1 current. The Theoutput output current current reach reacheses zero. zero. The Theprimary primary inductor inductor current current and and the thevoltage voltage acrossacross thesame themagnetized as magnetized iLm2/n1 current. inductor inductor The can o canutput be expressed be current expressed byreach a by serieses a serieszero. resonance The resonance primary with with aninductor initial an initial valuecurrent value as and follows as follows.the voltage. across the magnetized inductor can be expressed by a series resonance with an initial value as follows.

Rectifier Coss1 C Glasses in EVs oss3 + Rectifier + + D1 + D3 + 푽푪풄ퟏ −Glasses in EVs Coss1 v Coss3 M1 vds1 M3 ds3 + CC1 vd1 v + + 푽 − D1 +d3 D3 + - Lr 푪풄ퟏ - vds3 Step up transformer Step down transformer - - M1 vds1 M3 CC1 vd1 vd3 + L Cj1 Cj3 - - r Step up transformer Step down transformer - - + iLr C C + Cb Cj1 R j3 VO Vin Capacitve + o O + iLr vs Cb Lm1 coupling Lm2 vLm2 C ¯ R VO Vin Capacitve + o O vs - Lm2 v ¯ Lm1 coui pling Lm2 ¯ Lm2 - ¯ + iLm2 + + D2 + D T1, n1=NP1/NS1 T2 ,n2=NP2/NS2 4 v CC2 vd4 M2 ds2 M4 + vds4 vd2 + + D2 + D T1, n1=NP1/N- S1푽 + T2 ,n2=NP2/NS2 - 4 - - 푪풄ퟐ - v M2 vds2 vds4 CC2 C C vd2 d4 Coss2 CoMss44 j2 j4 - - 푽푪풄ퟐ + - - - C C Coss2 Coss4 j2 j4 Figure 4. Proposed quasi-LLC resonant CCWPT power conversion circuit. Figure 4. FigureProposed 4. Proposed quasi-LLC quasi-LLC resonant resonant CCWPT CCWPT power power conversion conversion circuit. circuit.

M1&M4 ON M1&M4 ON Vgs_M1,M4 M1&M4 ON M1&M4 ON Vgs_M1,M4

M2&M3 ON M2&M3 ON Vgs_M2,M3 M2&M3 ON M2&M3 ON Vgs_M2,M3 v v Vin vds1 , vds4 ds2 , ds3 v v Vin vds1 , vds4 ds2 , ds3

n2Vo vLm2 n2Vo vLm2 -n2Vo -n2Vo

iLm2/n1 iLr iLm2/n1 iLr

vcc1, vcc2 vcc1, vcc2

Vo vd1 , vd4 vd2 , vd3 Vo vd1 , vd4 vd2 , vd3

D1&D4 ON D2&D3 ON D1&D4 ON D2&D3 ON iD1,2,3,4 D1&D4 ON D2&D3 ON D1&D4 ON D2&D3 ON iD1,2,3,4

t0 t1 t2 t3 t4 t5 t t t 6 7 0' t0 t1 t2 t3 t4 t5 t t t 6 7 0' Figure 5. Key waveforms of the proposed CCWPT circuit. FigureFigure 5. Key 5. Key waveforms waveforms of the of proposedthe proposed CCWPT CCW circuit.PT circuit. 1 L 1 i   Itcos t t   V  n V t  V t  n n V r sin t t Lrt Lr 2  1 2  in1 Cc 1 2 Cc 2 2 1 2 o  L  1 2  (1) LCrr Cr LCrrr iLrt  I Lrt2cos t t 2   V in  n1 V Cc 1 t 2  V Cc 2 t 2  n 1 n 2 V o  sin  t t 2  (1) LCrr Cr LCrr 1 CC C  CC12 (2) r 2 1 CC n1 CCC 1 C 1 CC12 Cr  2  (2) n1 CCC 1 C 1

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q 1 Lr 1 iLr(t) = ILr(t2) cos (t t2) + ( Vin n1(VCc1(t2) + VCc2(t2)) + n1n2Vo)/ C sin (t t2) (1) √LrCr − − − r √LrCr − ! 1 C C C = C1 C2 (2) r 2 C + C n1 C1 C1

vLm (t) = n Vo (3) 2 − 2 The capacitance of coupling capacitors can be increased by the turn ratio of the T1 transformer. The quality factor of the proposed power CCWPT system can be shown as

q ( ) 2( + ) Lr/ CC1 CC2 / n1 CC1 CC2 Q = × (4) Rac

2 2 8Ro Rac = n n (5) 1 2 π2 where n1 is the turn ratio of the T1 transformer, and n2 is the turn ratio of the T2 transformer. The quality factor can be adjusted by the turn ratios of the two transformers. Mode 4 (t3–t4): At t3, the Lr current and Lm2 current divided by n1 are equal. The output current reaches zero. All of the output rectiﬁer diodes D1 ~D4 are reverse biased. T2 transformer’s secondary voltage is lower than the output voltage. The output is separated from the T2 transformer. During this time, since the output is separated from primary, Lm2 is freed to participate in the resonance. It will form a resonant tank of Lm2 and Lr resonant with Cj1~Cj4.

1 iLr(t) = ILr(t3) cos q (t t3) 2 2 2 4Cj(Lr+n Lm2)/(n n ) − 1 1 2r L +n2L (6) r 1 m2 1 + ( V n (V (t ) + V (t )) )/ sin q (t t ) in 1 Cc1 2 Cc2 2 o C (n2n2) 3 − − 4 j/ 1 2 C (L +n2L ) (n2n2) − 4 j r 1 m2 / 1 2

Because the junction capacitance is very small, the inductor current looks linear. For the next half-cycle from t5 to t0’, the operation is the same as analyzed above. The proposed power conversion circuit has a small coupling capacitance but can be adjusted to have a low quality factor and impedance of the coupling capacitor by the turn ratio of the step-up transformer. By operating the LLC series resonant converters, the ZVS of the primary switch and the ZCS of the rectiﬁer located in the electric vehicle can be obtained even under load ﬂuctuations. Charging control can be done through a wireless communication like the existing ICWPT [25]. Therefore, it can be suitable as a wireless charging power conversion circuit using the glass of an electric vehicle.

3. Design Considerations of the Proposed CCWPT System

3.1. Capacitor Estimation Figure6 shows the simple structure of a capacitor in the transmitter and receiver. The capacitor can be made with electrodes and dielectric layers in the transmitter, receiver, and air. The capacitance can be derived using Gauss’s law, as shown in Figure6. Voltage, V, across the capacitor and capacitance can be expressed in Gauss’s law as follows.

Q/S Q/S V = Erd + E0dair = dair + d (7) ε0 εrε0

Q S S Cc = = εrε0 (8) V d/εrε0 + dair ≈ d Electronics 2020, 9, x FOR PEER REVIEW 6 of 13

vLm22() t n V o (3)

The capacitance of coupling capacitors can be increased by the turn ratio of the T1 transformer. The quality factor of the proposed power CCWPT system can be shown as

L C C n2 C C r C1 C 2  1 C 1 C 2  Q= (4) Rac

8R R =n22 n o (5) ac 1 2 2

where n1 is the turn ratio of the T1 transformer, and n2 is the turn ratio of the T2 transformer. The quality factor can be adjusted by the turn ratios of the two transformers.

Mode 4 (t3–t4): At t3, the Lr current and Lm2 current divided by n1 are equal. The output current reaches zero. All of the output rectifier diodes D1 ~D4 are reverse biased. T2 transformer’s secondary voltage is lower than the output voltage. The output is separated from the T2 transformer. During this time, since the output is separated from primary, Lm2 is freed to participate in the resonance. It will form a resonant tank of Lm2 and Lr resonant with Cj1~Cj4. 1 it  I tcos  t t  Lr Lr 332 2 2 4Cj ( L r n1 L m 2 ) / ( n 1 n 2 )

2 (6) Lrm n12 L 1  Vin  n1 V Cc 1tt22  V Cc 2  sin  t  t 3   o  4C / ( n22 n ) 2 2 2 j 12 4Cj ( L r n1 L m 2 ) / ( n 1 n 2 ) Because the junction capacitance is very small, the inductor current looks linear. For the next half-cycle from t5 to t0′, the operation is the same as analyzed above. The proposed power conversion circuit has a small coupling capacitance but can be adjusted to have a low quality factor and impedance of the coupling capacitor by the turn ratio of the step-up transformer. By operating the LLC series resonant converters, the ZVS of the primary switch and the ZCS of the rectifier located in the electric vehicle can be obtained even under load fluctuations. Charging control can be done Electronicsthrough2020 ,a9 ,wireless 676 communication like the existing ICWPT [25]. Therefore, it can be suitabl7 ofe 12 as a wireless charging power conversion circuit using the glass of an electric vehicle. where S is the area of the electrode, E is the electric field in the glass dielectric layer, E is the electric 3. Design Considerations of the Pr roposed CCWPT System 0 field in the air part, ε0 and εrε0 are the permittivity of the air and glass dielectric substrate, and d and dair 3.1.are theCapacitor widths. Estimation The capacitance is critically determined by the width of the glass dielectric layer. Since the glass dielectric does not induce an edge effect due to fringing fields, Equation (8) is suitable. As can beFigure seen in6 shows Figure 1the, the simple area ofstructure the front of windshield a capacitor andin the rear transmitter windshield and of receiver. the electric The vehicle capacitor is largecan be and made used with as a electrodes dielectric, soand that dielectric a larger layers bonding in the capacity transmitter, can be receiver, obtained and compared air. The to capacitance studies usingcan conventional be derived air. using This Gauss’ is advantageouss law, as shown in terms in of Figure stability 6. Volta sincege, a strongV, across electric the field capacitor can be and limitedcapacitance to a glass can dielectric. be expressed in Gauss’s law as follows.

Area S Copper electrode outside of the EV Q + Air (ε0) dair E0 E Glass dielectric (εr) r V d

-Q - ITO electrode inside the EV Electronics 2020, 9, x FOR PEER REVIEW 7 of 13 FigureFigure 6. Structure 6. Structure of capacitor of capacitor with with glass glass dielectric dielectric layer. layer. QSS 3.2. Output Voltage DC Gain of the ProposedCcr Power  Conversion  Circuit  0 (8) V d /   d d r0 Q/SQ/S air The rectiﬁed output voltage forV charging Er d  E the0 d air battery  varies d air  depending d on the battery capacity(7) where S is the area of the electrode, Er is the electric field00 in the glass r  dielectric layer, E0 is the electric of an electric vehicle. The output voltage DC gain is considered to design parameters of the power field in the air part, ε0 and εrε0 are the permittivity of the air and glass dielectric substrate, and d and conversion circuit in this chapter. Figure7 shows the equivalent circuit for the output voltage gain dair are the widths. The capacitance is critically determined by the width of the glass dielectric layer. T withSince fundamental the glass dielectric harmonic does approximation. not induce an edge Inductance effect due of theto fringing transformer fields, E1 quationmagnetized (8) is inductor suitable. L Asm1 canis very be seen large; in hence,Figure it 1, is the neglected area of the for front the DC windshield voltage gain. and Therear outputwindshield voltage of the DC electric gain is similarvehicle LLC tois thatlarge of and the used conventional as a dielectric,converter. so that a larger The outputbonding voltage capacity gain can can be be obtained expressed compared as follows. to studies using conventional air. This is advantageous in terms of stability since a strong electric field can be 2 2 limited to a glass dielectric. Vo ω n1Lm2Rac M = = (9) Vin ω2 ω2 jω 1 LP + Rac 1 3.2. Output Voltage DC Gain of the Proposed Power− Cωonversiono Circuit− ω p The rectified output voltage for charging the battery1 varies depending on the battery capacity of = = an electric vehicle. The output voltageω DCo gain2π fo is considered to design parameters of the power(10) √LrCr conversion circuit in this chapter. Figure 7 shows the equivalent circuit for the output voltage gain 1 with fundamental harmonic approximation.ωp = 2π fp =Inductanceq of the transformer T1 magnetized inductor(11) 2 m1 + L is very large; hence, it is neglected for the DC voltageLr n 1gain.Lm2 CTher output voltage DC gain is similar to that of the conventional LLC converter. The output voltage gain can be expressed as follows.

2 (CC1+CC2)/(CC1CC2)/n1 Lr +

2 2 2 vin Lm1 n Lm2 1 n1 n2 Rac vR

FigureFigure 7.7. EquivalentEquivalent circuitcircuit forfor thethe proposedproposed CCWPTCCWPT system.system.

Since the wireless charging of the electric vehicle cannot perform accurate feedback control, it is diﬃcult to set the correct operating frequency to obtain the desired output voltage due to load V 22 n L R M o12 m ac 22 (9) Vin  j 11 L  R  P ac  op

1 oo2 f (10) LCrr

1 pp2 f 2 (11) Lr n12 L m C r

Since the wireless charging of the electric vehicle cannot perform accurate feedback control, it is difficult to set the correct operating frequency to obtain the desired output voltage due to load variation or deviation of the coupling capacitance. The proposed power conversion circuit can prevent the sudden change of output voltage due to load fluctuation and coupling capacitance deviation without feedback control using a two transformer turn ratio. As shown in Equation (4), the quality can be adjusted by changing the equivalent resistance of the load and the impedance of the coupling capacitance by the turn ratio of the two transformers. Figure 8 is a graph of the DC voltage gain according to the turn ratio of the transformers. If the operating area is at the same point as the resonant frequency and the switching frequency, the turn ratio of step-up transformer T1 makes the quality factor decrease. When the quality factor is large, the output voltage becomes sensitive to the change in resonant frequency by the coupling capacitance deviation. Even when the resonant

Electronics 2020, 9, 676 8 of 12 variation or deviation of the coupling capacitance. The proposed power conversion circuit can prevent the sudden change of output voltage due to load ﬂuctuation and coupling capacitance deviation without feedback control using a two transformer turn ratio. As shown in Equation (4), the quality can be adjusted by changing the equivalent resistance of the load and the impedance of the coupling capacitance by the turn ratio of the two transformers. Figure8 is a graph of the DC voltage gain according to the turn ratio of the transformers. If the operating area is at the same point as the resonant frequencyElectronics 20 and20, 9, thex FOR switching PEER REVIEW frequency, the turn ratio of step-up transformer T1 makes the quality8 of 13 factor decrease. When the quality factor is large, the output voltage becomes sensitive to the change infrequency resonant is frequency increased byby the10%, coupling the turn capacitance ratio of the step deviation.-up transformer Even when must the be resonant 0.55 or less frequency to obtain is increasedthe desired by output 10%, the voltage turn ratio, as shown of the in step-up Figure transformer 8a. On the other must behand, 0.55 the or lesschange to obtain in output the desiredvoltage outputaccording voltage, to the as change shown in Figureresonance8a. On frequency the other in hand, the operating the change region in output by the voltage turn accordingratio of the to step the - changedown transformer in resonance is frequency small, as inshown the operating in Figure region 8b. It bycan the beturn seen ratio that ofin thethe step-down case of electric transformer vehicle iswireless small, ascharging shown without in Figure precise8b. It canfeedback be seen control, that in the the turn case ratio of electric of the step vehicle-up wirelesstransformer charging can be withoutsolved in precise order feedback to minimize control, the the influence turn ratio on of the the deviation step-up transformer of the resonance can be inductance solved in order and the to minimizecoupling capacitan the inﬂuencece. on the deviation of the resonance inductance and the coupling capacitance.

Lr=63μH,Lr=Lr=16nF, Lm2=1600μH, n2 =2.2, Vo=400V, Ro=100Ω Lr=63μH,Lr=Lr=16nF, Lm2=1600μH, n2 =0.45, Vo=400V, Ro=100Ω 4.5 6

n1 = 1, Q=4.3 n2 = 1, Q=0.08

4 n1 =0.71, Q=1.6 n2 =1.4, Q=0.16 n1= 0.55, Q=0.7 5 n2= 1.8, Q=0.27 3.5 n1=0.45, Q=0.4 n2=2.2, Q=0.4 n1 = 0.38, Q=0.2 n2 = 2.6, Q=0.56

n1 =0.33, Q=0.16 M n2 =3, Q=0.75 M

3 4 n n i i a a 2.5 G G

e 3 e g g 2 a a t t l l o o 1.5 Operation region 2

V Operation region V 1 1 0.5

0 0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 f = f /f s r f = fs/fr (a) (b)

FigureFigure 8. 8. OutputOutput voltagevoltage gain gain according according to to the the transformer transformer turn turn ratios. ratios. ((aa)) Output Output voltage voltage gain gain

accordingaccording to to a a turn turn ratio ratio of of transformer transformer T T1.(1. (b) Output voltage gain according according to to a a turnturn ratio ratio of of transformertransformer TT22..

4.4. ExperimentalExperimental ResultsResults AA prototypeprototype system system was was implemented implemented to to verify verify thethe operation operation and and features features of of the the proposed proposed CCWPTCCWPT system.system. FigureFigure9 9 is is the the experimental experimental set set for for the the prototype prototype system. system. Table Table2 2shows shows parameters parameters ofof thethe prototype.prototype. The measured capacitance of of the the coupling coupling capacitor capacitor w wasas approximately approximately 88 n nFF with with a 2 a240 24000 cm cm2 rectangularrectangular area. area. The The electrode electrodess are are made made of ofcopper copper tape tape.. Even Even if ifthe the electrode electrode is ismade made of ofcopper copper and and transparent transparent electrode electrode ITO, ITO, there there was was no no difference difference in incircuit circuit operation operation and and efficiency efficiency in inprevious previous studies studies [21 [21]. ].The The thickness thickness of of the the glass glass dielectric dielectric layer layer is is about about 2 mm. The area of of thethe couplingcoupling capacitorcapacitor usedused inin thethe experimentexperiment waswas setset consideringconsidering thethe areaarea andand thicknessthickness ofof thethe frontfront andand rearrear windowswindows ofof aa commercialcommercial electricelectric vehicle.vehicle. TheThe transmittingtransmitting circuitcircuit waswas composedcomposed ofof aa full-bridgefull-bridge inverterinverter usingusing fourfour switches,switches, andand thethe receivingreceiving circuitcircuit waswas composedcomposed ofof fourfour diodesdiodes andand aa full-bridgefull-bridge rectifier.rectifier. The The maximum maximum output output power power is 1.6 is kW.1.6 With kW. reference With reference to the design to the considerations design considerations in Section 3 in, theSectionn1 of 3, the the turn n1 of ratio the turn of the ratio step-up of the transformer step-up transformer was set towas 0.45, set andto 0.45 the, andn2 of the the n2 turn of the ratio turn of ratio the step-downof the step- transformerdown transformer was set was to 2.2. set Whento 2.2. theWhen winding the winding ratio of ratio the transformer of the transformer is designed is designed in this way,in this an way, output an voltageoutput voltage of 400 V of or 400 more V or can more be obtainedcan be obtained even if even the load if the changes load chang withes respect with respect to the inputto the voltage input voltage of 400 V. of The 400 resonant V. The inductorresonant was inductor designed was to designed be 63 µH soto thatbe 63 the μH output so that voltage the output could bevoltage 400 V could at a switching be 400 V frequency at a switching of 90 frequency kHz. Figure of 10 90 shows kHz. Figure experimental 10 shows waveforms experimental according waveforms to the loadaccording variation. to the The load experimental variation. The waveform experimental is consistent waveform with is theconsistent theoretical with analysis. the theoretical When analysis. the M1 When the M1 switch is turned on, the output power is transmitted through the resonance of the coupling capacitors and the resonant inductor. Even if the load changes, it can be seen that the switch M1 is turned on after the voltage between the drain source of the switch M1 is completely discharged, and the ZVS operation is performed and the voltage of the magnetizing inductor of the step-down transformer is the output voltage, taking into account the turn ratio as shown in Figure 10. The voltage peak of a coupling capacitor made of the glass depends on the load. This confirms that the proposed CCWPT’s circuit behavior is the same as the quasi-LLC converter operation. Figure 11 shows the efficiency from DC input to DC output with load. It shows high efficiency over 95% at 1.6 kW output with the WT333E power meter. Even if the load is 30% or less, it has an efficiency of 90% or more. The proposed CCWPT system has high efficiency over the entire load. The proposed

Electronics 2020, 9, 676 9 of 12 Electronics 2020, 9, x FOR PEER REVIEW 9 of 13 switch is turned on, the output power is transmitted through the resonance of the coupling capacitors CCWPT systemand the has resonant a low inductor. operating Even if frequency the load changes, because it can betwo seen transformers that the switch M can1 is turned make on the coupling capacitanceafter large the. voltageDue to between the low the operating drain source frequency, of the switch Ma 1typicalis completely silicon discharged,-based MOSFET and the ZVS can be used. The proposedoperation system is performed is capable and o thef delivering voltage of the large magnetizing power inductor using of a the window step-down that transformer is widely is located in the output voltage, taking into account the turn ratio as shown in Figure 10. The voltage peak of a vehicles andcoupling has high capacitor efficiency. made of the glass depends on the load. This confirms that the proposed CCWPT’s circuit behavior is the same as the quasi-LLC converter operation. Figure 11 shows the efficiency from DC input to DC output withTable load. It 2 shows. Specific high Components efficiency over of 95% Prototype at 1.6 kW. output with the WT333E power meter. Even if the load is 30% or less, it has an efficiency of 90% or more. The proposed CCWPT Parameterssystem has high efficiency over the entire load.Symbol The proposed CCWPT system hasValue/Part a low operating frequency because two transformers can make the coupling capacitance large. Due to the low operating Input voltage Vin 400 V frequency, a typical silicon-based MOSFET can be used. The proposed system is capable of delivering Outputlarge power power using a window that is widely locatedPo in vehicles and has high efficiency.1.6 kW Resonant inductor Lr 63 μH Table 2. Specific Components of Prototype. n1 and n2 Np1:Ns1 and Np2:Ns2 1:2.2 and 2.2:1 Parameters Symbol Value/Part Magnetizing inductor of T2 Lm2 1600 μH Input voltage Vin 400 V Coupling capacitor CC1 and CC2 16 nF Output power Po 1.6 kW Transformer coreResonant inductor T1 and T2 Lr 63 µH EI6044 n1 and n2 Np1:Ns1 and Np2:Ns2 1:2.2 and 2.2:1 Primary switches M1,2,3,4 STW13NK100Z Magnetizing inductor of T2 Lm2 1600 µH Diodes Coupling capacitor D1,2,3C,4C 1 and CC2 VS16- nFHFA16PA60C-N3 Transformer core T1 and T2 EI6044 Width of glass Primary switches - M1,2,3,4 STW13NK100Z2 mm Coupling capacitor electrodeDiodes - D1,2,3,4 VS-HFA16PA60C-N3Copper Width of glass - 2 mm Coupling capacitor electrode - Copper

(1) (6) (2) (3) (4)

(1) Transmitter circuit

(5)

(2) Rectifier

(3) Resonant Inductor (4) Transformer T1 (5) Coupling capacitors (6) Transformer T2 Figure 9. Experimental set: transmitter circuit, rectiﬁer, the resonant inductor, two transformers, Figure 9. Experimentalthe coupling capacitors set: transmitter using glass and circuit, copper rectifier, plate. the resonant inductor, two transformers, the coupling capacitors using glass and copper plate.

Electronics 2020, 9, 676 10 of 12 Electronics 2020, 9, x FOR PEER REVIEW 10 of 13

vgs_M1 :10V/div.

vLm2 :500V/div.

vds_M1 :250V/div.

The ZVS of M1 vCc1 :500V/div.

iLr :5A/div. 30% Time base: 2μs/div.

vLm2 :500V/div.

vgs_M1 :10V/div.

vds_M1 :250V/div. The ZVS of M1 vCc1 :500V/div.

iLr :5A/div. 50% Time base: 2μs/div.

vLm2 :500V/div.

vgs_M1 :10V/div.

vds_M1 :250V/div. The ZVS of M1 vCc1 :500V/div.

iLr :5A/div. 80% Time base: 2μs/div.

vLm2 :500V/div.

vgs_M1 :10V/div. vds_M1 :250V/div.

The ZVS of M1 vCc1 :500V/div.

iLr :5A/div. 100% Time base: 2μs/div.

Figure 10. Experimental key waveforms according to load variation: the gate-source voltage of M1, Figure 10. Experimental key waveforms according to load variation: the gate-source voltage of M1, the drain-source voltage of M1, the resonant inductor current, the voltage across the magnetized the drain-source voltage of M1, the resonant inductor current, the voltage across the magnetized inductor of the transformer T2, the voltage of the coupling capacitor, the zero voltage switching (ZVS) inductor of the transformer T2, the voltage of the coupling capacitor, the zero voltage switching (ZVS) operation of M1. operationElectronics of M20201., 9, x FOR PEER REVIEW 11 of 13

96.5

96 ] % [

95.5 y c n e

i 95 c i f f

E 94.5

93.5 400 600 1000 1300 1600 Output power [W]

FigureFigure 11. Power 11. Power conversion conversion efficiencyﬃciency according according to output to output power. power.

5. Conclusions A new CCWPT system is proposed for charging the EVs. The studied CCWPT uses the EVs’ glass dielectric layer for large coupling capacitance, and sufficient output power can be obtained with the two transformers such as the step-up and step-down transformers. Coupling capacitance can be estimated similar to that of a typical flat plate electrode capacitance. The glass has large relative permittivity and the EVs have large areas of glass in the front, back, and on the sides. This results in transferring large output power with a relatively large coupling capacitance, and a strong electric field can be trapped in the dielectric, which has the advantage of stability. The desired output voltage and power can be obtained, and the deviation of the coupling capacitance and the resonance inductance can be compensated by adjusting the turn ratios of the two transformers. The proposed CCWPT system employs quasi-LLC resonant power conversion circuit to reduce the switching loss of power switches. Since a turn-on loss of the power MOSFET by the ZVS in a transmitter and a turn- off loss of the rectifier the ZCS in a receiver can be decreased, high efficiency can be obtained. Because the operating frequency is not high, the proposed CCWPT system can use a low-cost existing silicon- based MOSFET. Therefore, the proposed CCWPT system is suitable for replacing the conventional ICWPT to charge EVs. Funding: This research received no external funding.

Acknowledgments: K. Yi would particularly like to thank Y. You for his help in obtaining experimental results during this work and G. Bang for his help in editing the diagram of the paper. Y. You has worked for HS Haesung Corp. in the Republic of Korea since 2017 and G. Bang has studied for a master’s degree at Daegu Univ. since 2019.

Conflicts of Interest: The author declares no conflict of interest.

References

Electronics 2020, 9, 676 11 of 12

5. Conclusions A new CCWPT system is proposed for charging the EVs. The studied CCWPT uses the EVs’ glass dielectric layer for large coupling capacitance, and sufficient output power can be obtained with the two transformers such as the step-up and step-down transformers. Coupling capacitance can be estimated similar to that of a typical flat plate electrode capacitance. The glass has large relative permittivity and the EVs have large areas of glass in the front, back, and on the sides. This results in transferring large output power with a relatively large coupling capacitance, and a strong electric field can be trapped in the dielectric, which has the advantage of stability. The desired output voltage and power can be obtained, and the deviation of the coupling capacitance and the resonance inductance can be compensated by adjusting the turn ratios of the two transformers. The proposed CCWPT system employs quasi-LLC resonant power conversion circuit to reduce the switching loss of power switches. Since a turn-on loss of the power MOSFET by the ZVS in a transmitter and a turn-off loss of the rectifier the ZCS in a receiver can be decreased, high efficiency can be obtained. Because the operating frequency is not high, the proposed CCWPT system can use a low-cost existing silicon-based MOSFET. Therefore, the proposed CCWPT system is suitable for replacing the conventional ICWPT to charge EVs.

Funding: This research received no external funding. Acknowledgments: K. Yi would particularly like to thank Y. You for his help in obtaining experimental results during this work and G. Bang for his help in editing the diagram of the paper. Y. You has worked for HS Haesung Corp. in the Republic of Korea since 2017 and G. Bang has studied for a master’s degree at Daegu Univ. since 2019. Conﬂicts of Interest: The author declares no conﬂict of interest.

References

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