Chris Mi, Ph.D, Fellow IEEEProfessor and Chair, Dept. Electrical and Computer Engineering

Director, DOE GATE Center for Electric Drive TransportationSan Diego State University, (619)594-3741; mi@ieee.org

High Power Capacitive Power Transfer for Electric Vehicle

Charging Applications

1

Contents

1.Motivation

2.Double-sided LCLC Compensation Topology

3.Capacitive Coupler Structure

4.Prototypes and Experimental Results

5.System Improvements

6.Conclusion and Future Work2

San Diego State University (SDSU)

• Located in San Diego, California• Ranked the best city in the world in terms of

climate• Full time equivalent students ~30,000, head

count over 50,000• Joint Ph.D program with UC San Diego• Ranked 149 in the US News Report for all US

Universities• Some programs ranked top 20 in the US• ECE department has 4 IEEE Fellows

3

GATE Center for Electric Drive Transportation

• US Department funded Graduate Automotive Technology Education Center of Excellence

• There are total 7 centers in the US• Funds of DOE is supplemented by industrial

support• Produce the best automotive graduates by

offering fellowship and research opportunities

Sustainable Power and Energy Research Center (SPARC)

4

1. Motivation of High Power Capacitive Power

Transfer for Electric Vehicles

5

Capacitive vs. Inductive Power Transfer CPT System Structure

IPT System Structure

Electric field is not sensitive to metal material nearby Electric field does not generate eddy-current loss in the metal Capacitive coupler uses metal plates, instead of Litz-wire, to reduce system c6

Limitations of Recent CPT Systems Low Power Capacity (Desk Lamp[1] and Soccer Robot[2])

Short Distance (Vehicle Charging[3] and Motor Excitation[4])

[1] C. Liu, A.P. Hu, N.C. Nair, “Coupling Study of a Rotary Capacitive Power Transfer System,” 2009 IEEE ICIT, 1-6.[2] A.P. Hu, C. Liu, H. Li, “A Novel Contactless Battery Charging System for Soccer Playing Robot,” 2008 IEEE M2VIP, 646-650.[3] J. Dai, D.C. Ludios, “Wireless Electric Vehicle Charging via Capacitive Power Transfer through a Conformal Bumper,” 2015 IEEEAPEC, 3307-3313.[4] D.C. Ludois, M.J. Erickson, J.K. Reed, “Aerodynamic Fluid Bearing for Translational and Rotating Capacitors in NoncontactCapacitive Power Transfer Systems,” IEEE Transactions on Industrial Electronics, Vol 50, 2014, 1025-1033.

7

Reason of Limitations Series Compensation[5]

Class-E Compensation[6]

PWM Converter[7]

Pros: simplicity Cons: capacitance is limited by fsw

Pros: frequency can be 10’s MHz Cons: power level is limited

Pros: power can be kW level Cons: capacitance is limited by fsw

[5] C. Liu, A.P. Hu, G.A. Covic, N.C. Nair, “ComparativeStudy of CCPT Systems with Two Different Inductor TuningPositions,” IEEE Transactions on Power Electronics, Vol 27,2012, 294-306.

[6] H. Liang, A.P. Hu, A. Swain, X. Dai, “Comparison of TwoHigh Frequency Converters for Capacitive Power Transfer,”2014 IEEE ECCE, 5437-5443.

[7] J. Dai, D.C. Ludios, “Single Active Switch PowerElectronics for KiloWatt Scale Capacitive Power Transfer,”IEEE Journal of Emerging and Selected Topics in PowerElectronics, Vol 3, 2015, 315-323. 8

Challenges of CPT for EV Charging Small Coupling Capacitance

20.891 1

1[1 2.343 ( / ) ] 36.7pFsl

C d ld

An Example: Plates Size l1=610mm (24in) Distance d=150mm Coupling capacitance of parallel plates is[8]:

[8] H. Nishiyama, M. Nakamura, “Form and Capacitance of Parallel Plate Capacitor,” IEEE Transactions on Components, Packing, andManufacturing Tech-Part A, Vol 17, 1994, 477-484.

The former compensation topologies are not suitable to transfer HIGH power with so SMALL coupling capacitance Series topology: requires too large inductance or too high switching frequency Class-E topology: cannot provide enough output power PWM topology: cannot provide high enough switching frequency

NEW Compensation Topology is Required! 9

2. Double-sided LCLC Compensation Circuit

Topology

10

Double-sided LCLC Circuit Topology

Full-bridge inverter provides square-wave excitation

Full-bridge rectifier feeds dc current to battery load

Two inductors are two capacitors are used at each side

P1 and P2 are at the primary side, P3 and P4 are at the secondary side

P1 and P3 form a coupling capacitor, P2 and P4 form the other capacitor11

Fundamental Harmonics Approximation

Square-wave input and output contains fundamental and high-order

harmonics

Lf1-Cf1, Lf2-Cf2 work as low pass filters

Input and output are represented by sinusoidal waveforms

12

Superposition Theorem Excited Only by Primary (Contains Two Parallel Resonances)

1 2 1 2

1 1 2 2

1 11 2

1 1 0

2 2 20

0

/ ( )/ ( )

1

1

2

s s s s s

p s s

f p

f p

f f

sw

C C C C CC C C C C C

C CL

C C

L C

f

L1, Cf1, Cp1 form one parallel resonance

Lf2, Cf2 form the other parallel resonance

13

Superposition Theorem Excited Only by Primary (Contains Two Parallel Resonances)

Output Current I2 only depends on Input Voltage V1

12121

11

1

1

2

21

2

22

11

11

1

11

11

11

11

/1/1/1

/1/1/1

)/(1)/(1

)/(1)/(1

VCCCCCC

CCV

CC

CCCV

CCCV

VCC

VCj

CjV

CjLjCj

V

VV

ss

fs

p

f

sC

sC

p

f

f

p

p

pC

Cf

Output Current 121212

1

2

22 )(

VCCCCCCLj

CCLj

VI

ssf

fs

f

Cf

14

Superposition Theorem Excited Only by Secondary (Contains Two Parallel Resonances)

1 2 1 2

2 2 1 1

2 22 2

2 2 0

1 1 20

0

/ ( )/ ( )

1

1

2

s s s s s

p s s

f p

f p

f f

sw

C C C C CC C C C C C

C CL

C C

L C

f

L2, Cf2, Cp2 form one parallel resonance

Lf1, Cf1 form the other parallel resonance

15

Superposition Theorem Excited Only by Secondary (Contains Two Parallel Resonances)

Input Current I1 only depends on Output Voltage V2

22121

22

2

2

1

12

1

11

22

21

2

22

22

22

22

/1/1/1

/1/1/1

)/(1)/(1

)/(1)/(1

VCCCCCC

CCV

CC

CCCV

CCCV

VCC

VCj

CjV

CjLjCj

V

VV

ss

fs

p

f

sC

sC

p

f

f

p

p

pC

Cf

Input Current 221211

2

1

12 )(

VCCCCCCLj

CCLj

VI

ssf

fs

f

Cf

16

System Power of FHA Analysis

At input inverter side, V1 and I1 are in phase At output rectifier side, V2 and (–I2) are in phase Neglect passive components losses, the system power is expressed as:

0 1 2 0 1 21 2

1 2 1 2 1 2 1 2

2 2 2 2s f f s f fin out in out

s s s s

C C C C C CP P V V V V

C C C C C C C C C C C C

If there exists C1,2>>Cs1 2

01 2

2 2 2 2f fin out s in out

C CP P C V V

C C

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3. Plates Structure Design

18

3D Dimensions of the Plates

An Example: Square plates are used and the size l1=610mm The plates distance d=150mm The thickness of the plate is 2mm The separation between two pairs dc=500mm to reduce the cross-coupling19

X-Y Direction Misalignment Each pair of plates are modeled by a

coupling capacitor

The cross-couplings of P1-P4, P2-P3

are small and neglected

The variation of Cs1,2 with the X

direction misalignment:

Coupling capacitor is not sensitive to misalignment

0 50 100 150 200 250 30030

32

34

36

38

Misalignment(mm)

Capa

cita

nce(

pF)

20

Distance Variation at Z Direction All the cross-couplings are neglected

The distance d increase to 300mm,

double the original distance

Coupling capacitance decrease to 67% of the original value,

when distance is doubled

150 200 250 30020

25

30

35

40

Distance(mm)

Capa

cita

nce(

pF)

21

4. Prototype and Experimental Results

22

0 0.5 1 1.5 2-300-150

0150300

V 1(V)

Time(s)

0 0.5 1 1.5 2-300-150

0150300

V 2(V)

Time(s)

0 0.5 1 1.5 2

-10

0

10

I 1(A)

V1

I1

0 0.5 1 1.5 2

-10

0

10

I 2(A)

V2

I2

Parameter Design and Simulation A 2.4kW CPT system is designed with parameters in the following table.

Vin Vout fsw Lf1 (Lf2) Cf1 (Cf2) C1 (C2) Cs1(Cs2) L1 L2

265V 280V 1MHz 11.6μH 2.18nF 100pF 36.7pF 231μH 242μH

Circuit simulation is shown below

23

Voltage and Current Stress Voltage are current stress are calculated by the previous FHA analysis

Components Lf1 (Lf2) Cf1 (Cf2) C1 (C2) L1(L2) Plates

Voltage 1.0 kV 1.0 kV 7.2 kV 7.2 kV 3.2 kV

Current 15.5 A 15.0 A 4.8 A 5.2 A 0.7 A

Inductors are wound with multiple turns to reduce voltage stress

between turns

Multiple capacitors are connected in series to form C1 and C2

Multiple capacitors are connected in parallel to form Cf1 and Cf2

The electric field in this system is 3.2kV/150mm, and the breakdown

field of air is about 3.0kV/mm. Therefore, there is no concern of arcing24

Leakage Field Strength

At 1MHz, the human exposure to electric field should be lower than

614V/m for safety concern[9]

The safe area for this system is about 0.6m away from the plates

Future study will reduce the radiation of electric field to the environment[9] IEEE Stand for safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3kHz to 300GHz, C95.1, 2005.

25

Prototype Design

Plates are made by aluminum sheets

Inductors are wound by AWG46 Litz-wire without magnetic core

High-power-frequency thin film capacitors resonate with the inductors

Silicon Carbide (SiC) MOSFETs C2M0025120D are used in the inverter

SiC diodes IDW30G65C5 are used in the rectifier 26

Experimental Results Pout=2.4kW at designed input/output

The experimental waveform is the

same with the simulations

Soft-switching is achieved

There is high frequency noise on the

driver signal

Most of the power losses distribute on

the capacitors and plates

If the inductors are wound on magnetic

core, the system efficiency will drop

1%-3%.

27

Tolerance to Misalignment and Distance Output Power maintains 2.1 kW at 300 mm X axis misalignment

0 0.5 1 1.5 2 2.586

87

88

89

90

91

Pout (kW)

Effic

ienc

y (%

)

No Mis100mm200mm300mm

Output Power maintains 1.7 kW at 300 mm Z axis distance

0 0.5 1 1.5 2 2.586

87

88

89

90

91

Pout (W)

Effic

ienc

y (%

)

150mm200mm250mm300mm

28

5. System Improvements

29

Vertical Plate Structure

A compact plate structure

includes four plates that are

vertically arranged

This structure is robust to the

rotation misalignment

The distance d=150mm

The plate distance dc is much

smaller than d

The cross-couplings between

the plates should be considered

30

LCL Compensation Topology

Compensation topology is simplified

The LCL compensation circuit is used to resonate with the

plates, instead of the LCLC topology31

IPT+CPT Combined System

The two inductors L1 are L2 are inductively coupled

The system utilized both electric and magnetic fields to transfer power

32

Experiments of IPT+CPT Combined System

Coil size: 320×320mm

Plate size: 610×610mm

Only LC compensation

network is required at each

side

At nominal input and

output condition,

PIPT=2000W, and

PCPT=800W

The system also has good

misalignment ability 0 500 1000 1500 2000 2500 3000

90

92

94

Output Power (W)

Effic

ienc

y (%

)

No Mis10cm Mis15cm Mis20cm Mis

33

Conclusion and Future Work

1. Capacitive power transfer has become practical for

electric vehicle charging application

2. The double-sided LCLC compensation circuit topology

is suitable to increase the power level of CPT system

3. The plate structure can be improve to enhance the

misalignment ability of CPT system

4. Future work will study the radiation of the CPT system

and come up with effective shielding method,

integrated IPT+CPT, etc.34

Thank you!

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