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; [email protected]
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
Proud Sponsors
Skywell
Hardworking Students!
Q&A
Thanks!