High Efficiency Wireless Power Transfer for EV Charging and Other Applications

First Prepared on Jan 1, 2007. Last Revised on April 8, 2018

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

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

Contents • Introduction of Wireless Power Transfer

• Safety of WPT Systems

• Case Study of a Wireless Bus System

• Double-sided LCC Compensation

• Capacitive Power Transfer

• Other Developments

• Summary of Recent Achievements in WTP

2

San Diego State University

Introduction to Wireless Power Transfer

4

Possible Solution:

Wireless Charging

Issues of Conductive Charging and Battery Swapping

Electric safety is of concern: electric shock due to rain, etc.

Charge station, plug and cable can be easily damaged, stolen

Charge/swap station takes a lot of space and affect the views

Definition of Wireless Power• The essential principles of WPT are

– given a distances over which the power is transferred through air or other non-conductive medium

– The coupling is almost always less than a quarter wavelength, so the fundamental operation of all of these systems can be described by simple coupled models

• Wireless power transfer (WPT)• Contactless power system (CPS)• Inductive power transfer (IPT)• Capacitive wireless power transfer• Strongly coupled magnetic resonance• Wireless energy transfer

Ref: Grant Covic and John Boys, “Modern Trends in Inductive Power Transfer for Transportation Applications,” IEEE journal of emerging and selected topics in power electronics, vol. 1, no. 1, march 2013

History of Wireless Power Transfer• 1830’s: Faraday's law of induction• 1890’s: Tesla had a dream to send energy

wirelessly• 1990’s: GM EV1 used an Inductive charger • 2007: MIT demonstrated a system that can

transfer 60W of power over 2 m distance at very low efficiency

• 2010: Wireless/inductive chargers are available: electronics, factories, medical

• 2012: Qualcomm, Delphi (Witricity), Plugless Power, KAIST, etc. have developed EV wireless charger prototypes

• 2014: in-motion charging demonstration: Daejoeng, Vienna, London

The Predicted Wireless Charging Market: $17 Billion by 2019, including applications in consumer electronics, home appliance, industrial robots, and EV charging

"Tesla Broadcast Tower 1904" by Unattributed(Life time: Unattributed) - Original publication: UnknownImmediate source:

http://www.sftesla.org/images/Tesla_Broadcast_Tower.JPG. Licensed under Public domain via Wikimedia Commons -

http://commons.wikimedia.org/wiki/File:Tesla_Broadcast_Tower_1904.jpeg#mediaviewer/File:Tesla_Broadcast_Tower_1904.jpeg

Application of WPT in EV Charging

MIT 2007

Tokyo 2009

Intel 2008

Korea KAIST

Leaf 2012

Witricity/Delphi

Japan

conductix-wampfler

Other Application of WPT• Integrated wireless power for portable equipment

- Phones- Laptops- Hand tools

• Specialty products, body implants- Pacemaker- Neurostimulator- Cochlea Hearing Implants- Conference Tables

700,000 heart pacemakers are implanted globally each year, each last 5 to 10 years.

Methods of Wireless Power Transfer

• Most WPT is to effectively transfer heat• Microwave has been used in our homes/offices• Induction heating is popular in industrial applications

Wireless Power Transfer

电能的无线传输

Wireless Power Transfer

电能的无线传输

Electromagnetic Induction

电磁感应式

Electromagnetic Induction

电磁感应式

Electromagnetic Resonance

电磁谐振式

Electromagnetic Resonance

电磁谐振式

Radiation

辐射式

Radiation

辐射式

Microwave

微波

Microwave

微波

Laser

激光

Laser

激光

Ultrasound

超声波

Ultrasound

超声波

Radio wave

无线电波

Radio wave

无线电波

High cost

Limitations of Current WPT

Large size

Need of:

• Novel designs

• Novel topologies

• Novel methods

• New materials

• New control methods

Low efficiency

Limited distance

Sensitive to vehicle alignment

Transformer Theory• Inductive WPT systems works like a transformer• But loosely coupled between the primary and

secondary• Result: mutual coupling coefficient is only 10~20%

• Conventional Transformer• Leakage is ~2%• Operate at 50/60Hz

• Wireless less power• Leakage is >80%• Operate at KHz ~ MHz

Capacitor Compensation - Resonance

1 111 1

22 2

2

1( )

0 1( )

m

m L

R j L j LCV I

Ij L R R j LC

Series-Series Resonance Structure

L1, L2 – Self inductance

Lm – Mutual inductance

L1= L1σ +Lm; L2= L2σ +Lm

Efficiency

• Input Power

• Output Power

• Efficiency2

22

1 2 1 2

( )| [ ( ) ] | cos

m L

m

P L RP Z Z Z L

2 22 1

2 2 2 21 2

( )| || [ ( ) ] |

m LL

m

V L RP I RZ Z L

21 2

1 1 1 21 2

| || | | | cos cos| ( ) |m

V ZP V IZ Z L

0 1 2 3 4 5 6 7 8 9 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 15 30 45 60 75 90 105 120 135 kHz

Effic

ienc

y

Safety Issues of WPT Systems

Safety Issues• 6.6 kW charging

power• Coil is 500×500 mm• Worst case is

human lay down next to the car and facing the car

• The worst radiation is well below the ICNIRP regulation

Vehicle model

concrete

Human model at the worst radiation position

Max. B field = 1.36 μT

Max. E field = 2.83 Vm-1

ICNIRP Guideline:B field of human tissue < 27μT

B field of pacemaker < 6.25μT

E field < 83 Vm-1

@85kHz

EM Field in Humans

Max. B field = 0.89 μT Max. E field = 3.14 Vm-1

EM Field Inside Car

ICNIRP Guideline:B field of human tissue < 27μT

B field of pacemaker < 6.25μT

E field < 83 Vm-1

@85kHz

Maximum B field = 1156 μT

Animal Model

• Cat under the car

• 1.156 mT of magnetic field is observed which is not longer safe

Simulation tool: Maxwell+HFSS

Measuring Equipment: Narda EHP – 200A

EM Field Measurement

Results of Foreign Object Test #1

Experiment Result: the gum wrapper was burned and there left an imprint, which means the temperature is high.

Case Study #1

Electric Bus

23

Electric Bus Project• Charge points are located in the bus stop area

Bus drives in 10 s; Bus stop 20s; bus drive out 10s

Total charging time: 30s

Power from Grid: Rectifier+PFC+DC/DC+Inverter

Battery Rectifier

Total energy delivered:

30s/3600s * 120kW

= 999Wh 1.6km

Initial Savings – cover initial investment

Annual Savings - $ 250k

Economics/Benefits of a Bus Project• Saving on board battery

- Savings of investment of battery: $100k/bus- Savings of weight >1 T/bus = 200Wh/mile/bus

• Savings of operating cost- Two operators/station is no longer needed: $200k/year

• No need of new land for charge station installations• Increase battery life due to narrow SOC band is used

- Top off every time at bus stops, no full discharge of the battery• More reliable; does not have to deal with hundred of

amperes of currents, eliminate spark, eliminate electric shock

• Less maintenance: no tear and wear of cable, plug,

Fleet Savings

• Fleet- 10 buses; 30 miles round trip; 10 trips/day-bus- Total 300 miles per bus-day

• Total battery savings: $1 MM• Total energy savings: 220 MWh/year-fleet• Total saving of labor cost: $200k/year• Using high efficiency charger; 10% more

efficient, then savings of 250MWh/year-fleet• 20 years maintenance-free; further savings

Case Study #2

Light Rail

27

Light Rail Project• Charge points are located in the Train stops

area

Light Rail Coil Segmentation

Double‐Sided LCC Topology for Inductive WPT

30

Double-Sided LCC Topology• Key inventions:

- Optimized multi-coil design for maximum coupling, with bipolar architecture- LCC topology for soft switching to further increase efficiency and frequency- Distributed circuit parameters to minimize the capacitor size and voltage

rating- Foreign object detection and electromagnetic field emissions for human and

animal safety for the developed system.

=

DC/DC DC/AC

Multi-phase Interleaved PFCRectifier

DC Link

High Frequency Inverter

BatteryAC 1Φ

Integrated LCC Compensated Coil Struture

=

Rectifier & FilterMulti-phase Interleaved Buck

DC/DC

Open CircuitProtection

Main Controller

Position Detection

Wireless Comms.

Position Detection

Secondary Controller

Wireless Comms.

Vo, Io Monitoring

PFC-Buck Maximum Efficiency: 97% DC to DC Maximum Efficiency: 95.3%

AC to DC Maximum Total Efficiency: 92% (6kW)

Double-sided LCC Compensated Wireless Power Transfer

Topology

• Important Characteristic:

• The output current at resonant frequency:

• The output power can be expressed as:

2 2 _1 120 0

mLf Lf

f f

U LI I k UL L

2 2 _1 1 220

Lff

LP U I k U UL

Experiment Results: DC-DC Efficiency

Xmis=0mm, Gap =200mm Xmis=300mm, Gap =200mm Xmis=125mm, Gap =400mm

System Efficiency

=

DC/DC DC/AC

Multi-phase Interleaved PFCRectifier

DC Link

High Frequency Inverter

BatteryAC 1Φ

Integrated LCC Compensated Coil Struture

=

Rectifier & FilterMulti-phase Interleaved Buck

DC/DC

Open CircuitProtection

Main Controller

Position Detection

Wireless Comms.

Position Detection

Secondary Controller

Wireless Comms.

Vo, Io Monitoring

PFC-Buck Maximum Efficiency: 97% DC to DC Maximum Efficiency: 95.3%

AC to DC Maximum Total Efficiency: 92% (6kW)

Capacitive Wireless Power Transfer

38

Analogy of CPT and IPT

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 CPT coupler uses metal plates, instead of Litz-wire, reduce system cost

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 or parallel topology: requires too large inductance or too high switching

frequency

NEW Compensation Topology is Required! 40

Double-sided LCLC Circuit Topology

Two inductors and 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 capacitor

41

F. Lu, H. Zhang, H. Hofmann and C. Mi, "A Double-Sided LCLC-Compensated Capacitive Power Transfer System for Electric Vehicle Charging," in IEEE Transactions on Power Electronics, vol. 30, no. 11, pp. 6011-6014, Nov. 2015. doi: 10.1109/TPEL.2015.2446891

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

42

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-coupling43

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 46

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%.

47

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

48

Misalignment Performance x Direction Misalignment

Air-gap Variation

CM remains 84.2% of the well-aligned value, LM remains 30% of the well-aligned value

CM remains 66.5% of the well-aligned value, LM remains 41% of the well-aligned value

Advantage: Better Misalignment Performance

Other Developments

50

Dynamic Capacitive Power Transfer [11]

[11] F. Lu, H. Zhang, H. Hofmann, Y. Mei, C. Mi, “A Dynamic Capacitive Power Transfer System with Reduced Power Pulsation,” Proc. IEEE Workshop Emerg. Tech. Wireless Power Trans. (WoW), pp. 60-64, 2016.

Reduce system cost using metal plates as capacitive couplers

Reduce stand-by power loss because of small circulating current in the coupler

Experimental Prototype

Transmitter size: 1200mm×300mm

Receiver size: 300mm×300mm

Airgap distance: 50mm

Dynamic Experiment Power pulsation

When y=[0mm, 900mm], the power pulsation is within ±4.0%

Misalignment

Output power drops to 72.3 W at 200 mm misalignment (46.7% of well-aligned power)

Single Ended CPT System

Two plates only

Chassis and the earth

are the third and forth

plates

Safety of CPT Systems

58

Safety Issue and Impact of Foreign Object High voltages on plates

M

MC C

PV

1

An example: PM=3.0kW, fsw=1MHz

Voltages are in kV level

Solution: reliable insulation is required on plate surface

2CMM VCP

Electric Field Emissions |VC1|=|VC2|=5.2 kV, |V14|=|V23|=4.2 kV, and |V13|=|V24|=3.1 kV

Safe range is 700mm from the edge of plates

Solution: Six-Plate Coupler[12]

P5 is grounded, and P6 is equivalently grounded

Safe range is 120mm from the edge

Further research shows safe range is 400 mm from the edge with 300mm misalignment

[12] H. Zhang, F. Lu, H. Hofmann, C. Mi, “A Six-Plate Capacitive Coupler to Reduce Electric Field Emission in Large Air-gap Capacitive Power Transfer,” IEEE Trans. Power Electron., 2017, doi: 10.1109/ TPEL.2017.2662583.

Metallic Foreign Object Influence

Positions A5, A8, A9, A10, and A11 are sensitive positions

The influence becomes significant with increasing metal size

Dielectric Foreign Object Influence

Dielectric foreign object can increase the same-side capacitance

Capacitance variation can change resonances and affect power transfer process

CPT Influence to Metallic Foreign Object

Metallic foreign object has almost same potential as the nearby plate

Human touch can induce current flowing through the body

Circuit Model of Human Touch

At high frequency, human model is approximated to be 500Ω resistor (IEC)

High-frequency (above 100 kHz) current has no neurological and cardiac problem (IEEE 95.1)

Circuit simulation shows Ib=2.2 A, which may cause heating problem

IEEE C95.1 requires everage energy density lower than 144J/kg in 6 minutes

Safe with 100 ms protection mechanism

kgJkgJm

tRID bb 14403.4

601.05002.2 22

Final Comparison of CPT and IPT

66

IPT CPTSwitching frequency 85kHz 1MHz

Coupling field Magnetic Electric

Foreign objects (metal) Will generate heat  Will not generate heat

Material Litz wires, ferrites Copper/Aluminum plates

Cost High Low

Safety  Good ExcellentSize Small  Large

Misalignment Poor Good

Efficiency Excellent Excellent

Voltage stress Medium High 

Power level High  Medium 

Stationary or dynamic  Better for stationary Both

Wireless Charging of AGVs

67

AGV Charging System Structure

68

Properties of an AGV system Low chassis height: around 10’s of mm Low battery voltage due to safety reason: around 10’s of volts

Motivations of wireless charging Increase effective working time Reduce the size of the onboard battery

Solution II: LCC-LCC Compensation

69

Resonant relationship

22

20

211

20

1

220

21

20

1

1,1

1,1

ff

ff

ff

LC

LLC

L

CL

CL

System output power21

210

VVLL

LPff

Mout

Operate in constant current mode, and less sensitive to airgap variation

Benefit

Solution III: Series-Series Compensation

70

Self-inductances are compensated, resulting in a current source

220

21

20

1

11L

CL

C

， Resonant relationship

Output power

210

1 VVL

PM

out Benefit

Vehicle-side space and weight is reduced

Measured Output Power and Efficiency Property

71

Relatively high efficiency range is achieved

As long as Pout reaches 400W, the efficiency is higher than 89%

We are committed to conduct research to improve performance, efficiency and safety

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