Takehiro Imura

Wireless Power TransferUsing Magnetic and Electric Resonance Coupling Techniques

Wireless Power Transfer

Takehiro Imura

Wireless Power TransferUsing Magnetic and Electric ResonanceCoupling Techniques

123

Takehiro ImuraTokyo University of ScienceNoda, Chiba, Japan

ISBN 978-981-15-4579-5 ISBN 978-981-15-4580-1 (eBook)https://doi.org/10.1007/978-981-15-4580-1

[Draft] Original Japanese edition: “Jikai kyoumei niyoru waiyaresu denryoku densou” by TakehiroImura. Copyright © 2017 by Takehiro Imura. Published by Morikita Publishing Co., Ltd. 1-4-11, Fujimi,Chiyoda-ku, Tokyo 102-0071 Japan© Springer Nature Singapore Pte Ltd. 2020This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or partof the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations,recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmissionor information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilarmethodology now known or hereafter developed.The use of general descriptive names, registered names, trademarks, service marks, etc. in thispublication does not imply, even in the absence of a specific statement, that such names are exempt fromthe relevant protective laws and regulations and therefore free for general use.The publisher, the authors and the editors are safe to assume that the advice and information in thisbook are believed to be true and accurate at the date of publication. Neither the publisher nor theauthors or the editors give a warranty, expressed or implied, with respect to the material containedherein or for any errors or omissions that may have been made. The publisher remains neutral with regardto jurisdictional claims in published maps and institutional affiliations.

This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd.The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721,Singapore

Acknowledgements

I am deeply grateful to everyone.Prof. Hori, Prof. Fujimoto, Uchida, Okabe, Ote, Koyanagi, Kato, Moriwaki,

Beh, Parakon, Koh, Paopao, Tsuboka, Tanikawa, Narita, Kimura, G. Yamamoto,Hiramatsu, Pakorn, Hata, Kimura, Gunji, Nagai, M. Sato, Lovison, Kobayashi,Shibata, Furusato, Takeuchi, Nishimura, Cui, Otuka, Yazaki, Suzuki, Takahashi,Hanajiri, Ji, Helanka, Utsu, Katada, Tajima, Nawada, Tokita, Tantan, Chen, Tomii,Y. Ota, Nakajima, Chandrasekaran, Muruga Prashanth, Sasaki, Kuroda, A. Ota,Kaminuma, S. Yamamoto, Akashi, Ikeda, Ichiyanagi, Suita, Taira, Hanawa,A. Sato, Katsunori, Toshiko, Noriko, Makiko, Midori, Karin, Ema, Ruka.

Thank you.

v

Contents

1 Wireless Power Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Types of Wireless Power Transfer . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Detailed Differentiation of Wireless PowerTransfer Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.2 Operating Frequency . . . . . . . . . . . . . . . . . . . . . . . . . 61.2 Outline of Electromagnetic Induction and Magnetic

Resonance Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.2.1 Difference Between Electromagnetic Induction

and Magnetic Resonance Coupling . . . . . . . . . . . . . . . 101.2.2 Types of Circuit Topology . . . . . . . . . . . . . . . . . . . . . 11

1.3 Outline of Electric Field Coupling and Electric ResonanceCoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.4 Radiative-Type Power Transmission . . . . . . . . . . . . . . . . . . . . 131.4.1 Microwave Power Transmission . . . . . . . . . . . . . . . . . 14

1.5 Basic System Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . 16References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2 Basic Knowledge of Electromagnetism and Electric Circuits . . . . . 232.1 Resistance, Coils, and Capacitors . . . . . . . . . . . . . . . . . . . . . . . 23

2.1.1 Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.1.2 Coils Seen from a Circuit Viewpoint . . . . . . . . . . . . . . 242.1.3 Capacitors as Seen from a Circuit Viewpoint . . . . . . . . 27

2.2 Principle of Electromagnetic Induction . . . . . . . . . . . . . . . . . . . 282.2.1 Magnetic Field H, Magnetic Flux Density B,

and Magnetic Flux U . . . . . . . . . . . . . . . . . . . . . . . . . 292.2.2 Ampere’s Law and Biot–Savart Law . . . . . . . . . . . . . . 302.2.3 Faraday’s Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.2.4 Mechanism of Energy Transmission by

Electromagnetic Induction (ElectromagnetismViewpoint) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

vii

2.2.5 Electromagnetic Induction Described from a CircuitViewpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2.2.6 Self-inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422.2.7 Mutual Inductance and Coupling Coefficient . . . . . . . . 442.2.8 Neumann’s Formula (Derivation of Inductance) . . . . . . 46

2.3 High-Frequency Loss (Resistance) . . . . . . . . . . . . . . . . . . . . . . 482.3.1 Copper Loss, Skin Effect, and Proximity Effect . . . . . . 482.3.2 Iron Loss (Hysteresis Loss and Eddy Current Loss) . . . 512.3.3 Radiation Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

2.4 Non-resonant Circuit Transient Phenomena (Pulse) . . . . . . . . . . 562.5 Resonance Circuit and Transient Phenomena . . . . . . . . . . . . . . 58

2.5.1 LCR Series Circuit Transient Phenomena (Pulse) . . . . . 582.5.2 LCR Series Circuit and Q Value . . . . . . . . . . . . . . . . . 592.5.3 Magnetic Field Resonance Transient Phenomena . . . . . 61

2.6 Root Mean Square (RMS) Values, Active Power, ReactivePower, and Instantaneous Power . . . . . . . . . . . . . . . . . . . . . . . 632.6.1 Root Mean Square (RMS) Values . . . . . . . . . . . . . . . . 642.6.2 Instantaneous Power Versus Active Power

and Reactive Power . . . . . . . . . . . . . . . . . . . . . . . . . . 64References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

3 Magnetic Resonance Coupling Phenomenon and BasicCharacteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713.1 Coils and Resonators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

3.1.1 Spiral, Helical, and Solenoid Coils . . . . . . . . . . . . . . . 713.2 Summary of Air Gap and Misalignment Characteristics . . . . . . . 75

3.2.1 Efficiency, Power, and Input Impedancewith Air Gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

3.2.2 Misalignment Characteristics . . . . . . . . . . . . . . . . . . . . 773.3 Near Field of Magnetic Field and Electric Field . . . . . . . . . . . . 803.4 Frequency Determinant (kHz to MHz to GHz) . . . . . . . . . . . . . 85

3.4.1 Resonant Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . 853.4.2 Parameters of Coils for kHz, MHz, and GHz . . . . . . . . 883.4.3 kHz Coil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893.4.4 GHz Coil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

4 Basic Circuit for Magnetic Resonance Coupling (S–S Type) . . . . . . 934.1 Derivation of Equivalent Circuit . . . . . . . . . . . . . . . . . . . . . . . 93

4.1.1 Solution Using Kirchhoff’s Voltage Law . . . . . . . . . . . 944.1.2 Calculation Using the Z-Matrix . . . . . . . . . . . . . . . . . . 97

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4.2 Equivalent Circuit at the Resonant Frequency . . . . . . . . . . . . . . 1014.2.1 Derivation of the Equivalent Circuit at the Resonant

Frequency, and Its Maximum Efficiency . . . . . . . . . . . 1014.2.2 Derivation of Voltage Ratio and Current Ratio . . . . . . . 1034.2.3 Load Characteristics at the Resonant Frequency . . . . . . 1054.2.4 kQ Representation . . . . . . . . . . . . . . . . . . . . . . . . . . . 1074.2.5 Maximum Efficiency Considering Coil Performance . . . 109

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

5 Comparison Between Electromagnetic Induction and MagneticResonance Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135.1 Five Basic Types of Circuits: N–N, N–S, S–N, S–S, S–P . . . . . 1145.2 Non-resonant Circuit (N–N) . . . . . . . . . . . . . . . . . . . . . . . . . . 116

5.2.1 Verification of Equivalent Circuit (N–N) . . . . . . . . . . . 1175.2.2 Efficiency and Power (N–N) . . . . . . . . . . . . . . . . . . . . 121

5.3 Secondary-Side Resonant Circuit (N–S) . . . . . . . . . . . . . . . . . . 1235.3.1 Verification of Equivalent Circuit (N–S) . . . . . . . . . . . 1235.3.2 Efficiency and Power (N–S) . . . . . . . . . . . . . . . . . . . . 126

5.4 Primary-Side Resonant Circuit (S–N) . . . . . . . . . . . . . . . . . . . . 1315.4.1 Verification of Equivalent Circuit (S–N) . . . . . . . . . . . 1315.4.2 Efficiency and Power (S–N) . . . . . . . . . . . . . . . . . . . . 1335.4.3 Design Using Primary-Side Resonance

Conditions (S–N) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1345.4.4 Design with Overall Resonance Condition

(Power Factor = 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . 1355.4.5 Comparison Between Primary-Side and Overall

Resonance Conditions (Power Factor = 1) . . . . . . . . . . 1365.5 Circuit of Magnetic Resonance Coupling (S–S) . . . . . . . . . . . . 139

5.5.1 Verification of Equivalent Circuit (S–S) . . . . . . . . . . . . 1395.5.2 Efficiency and Power (S–S) . . . . . . . . . . . . . . . . . . . . . 1435.5.3 Calculations for Magnetic Resonance Coupling

(S–S Type Circuit) . . . . . . . . . . . . . . . . . . . . . . . . . . . 1445.6 Circuit of Magnetic Resonance Coupling (S–P) . . . . . . . . . . . . 147

5.6.1 Verification of Equivalent Circuit (S–P) . . . . . . . . . . . . 1485.6.2 Efficiency and Power (S–P) . . . . . . . . . . . . . . . . . . . . . 1525.6.3 Calculations for Magnetic Resonance Coupling

(S–P Circuit) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1545.7 Comparison Summary of Five Types of Circuits . . . . . . . . . . . . 1575.8 Evaluation and Transition Across Four Types of Circuits

Along X1 and X2 Axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1585.9 Comparison of Four Types of Circuits During Magnetic

Flux Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1665.10 Role of Primary Magnetic Flux . . . . . . . . . . . . . . . . . . . . . . . . 172References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

Contents ix

6 Feature of P–S, P–P, LCL-LCL, and LCC-LCC . . . . . . . . . . . . . . 1756.1 S–S, S–P, P–S, and P–P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1756.2 LCL and LCC, etc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1806.3 Relay Coil and Gyrator Properties . . . . . . . . . . . . . . . . . . . . . . 1846.4 k = 0 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

7 Open and Short-Circuit-Type Coils . . . . . . . . . . . . . . . . . . . . . . . . 1897.1 Overview of Open- and Short-Circuit Type Coils . . . . . . . . . . . 1897.2 An Intuitive Understanding of the Open-Circuit Type Through

the Dipole Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1927.3 Lumped-Element Model and Distributed Constant Circuit . . . . . 1937.4 Open-Circuit and Short-Circuit Type Coils from the

Perspective of a Distributed Constant Circuit . . . . . . . . . . . . . . 1987.5 The Open-Circuit Type Coil . . . . . . . . . . . . . . . . . . . . . . . . . . 2017.6 The Short-Circuit Type Coil . . . . . . . . . . . . . . . . . . . . . . . . . . 2087.7 Summary of the Open-Circuit Type and Short-Circuit

Type Coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

8 Magnetic Resonance Coupling Systems . . . . . . . . . . . . . . . . . . . . . . 2178.1 Overview of Wireless Power Transfer Systems . . . . . . . . . . . . . 2178.2 Resistive Loads, Constant-Voltage Loads (Secondary

Batteries), and Constant-Power Loads (Motors and ElectricDevices) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2188.2.1 Resistive Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2198.2.2 Constant-Voltage Load (Secondary Battery) . . . . . . . . . 2238.2.3 Constant-Power Load . . . . . . . . . . . . . . . . . . . . . . . . . 224

8.3 High Power Through Frequency Tracking Control . . . . . . . . . . 2268.4 Overview of Achieving Maximum Efficiency Through

Impedance Tracking Control . . . . . . . . . . . . . . . . . . . . . . . . . . 2278.5 Preliminary Knowledge for Discussing Efficiency

Maximization Through Impedance Tracking Control . . . . . . . . . 2318.5.1 AC–DC Conversion Through Rectifiers . . . . . . . . . . . . 2318.5.2 AC/DC Voltage, Current, and Equivalent Load

Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2328.5.3 Concept of Impedance Transformation

by Using a DC/DC Converter . . . . . . . . . . . . . . . . . . . 2488.5.4 Impedance Transform in a Step-Down Chopper,

Step-up Chopper, and Step up/Down ChopperUsing a DC/DC Converter . . . . . . . . . . . . . . . . . . . . . 249

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8.6 Realization of Maximum Efficiency Tracking ControlThrough Impedance Optimization . . . . . . . . . . . . . . . . . . . . . . 2558.6.1 Simple Design for Maximum Efficiency . . . . . . . . . . . 2558.6.2 Strict Design for Maximum Efficiency Control . . . . . . . 258

8.7 Realization of the Maximum Efficiency and the DesiredPower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2728.7.1 Secondary Side Maximum Efficiency and Primary

Side Desired Power . . . . . . . . . . . . . . . . . . . . . . . . . . 2738.7.2 The Primary Side Maximum Efficiency

and the Secondary Side Desired Power . . . . . . . . . . . . 2768.8 Secondary-Side Power on–off Mechanism (Correspondence

to Short Mode and Constant Power Load) . . . . . . . . . . . . . . . . 2788.8.1 Half Active Rectifier (HAR) . . . . . . . . . . . . . . . . . . . . 2818.8.2 Maximum Efficiency Control with HAR . . . . . . . . . . . 283

8.9 Maximum Efficiency and Desired Power Simultaneously bySecondary Side Control Alone . . . . . . . . . . . . . . . . . . . . . . . . . 284

8.10 Estimation of Mutual Inductance . . . . . . . . . . . . . . . . . . . . . . . 288References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290

9 Repeating Coil and Multiple Power Supply (Basic) . . . . . . . . . . . . . 2939.1 Linear Arrangement of Repeating Coil . . . . . . . . . . . . . . . . . . . 294

9.1.1 Linear Arrangements of Three Repeating Coils . . . . . . 2949.1.2 Linear Arrangement of Repeating Coil (N Coils) . . . . . 296

9.2 K-Inverter Theory (Gyrator Theory) . . . . . . . . . . . . . . . . . . . . . 2979.2.1 Calculation Method Using K-Inverter . . . . . . . . . . . . . 2989.2.2 Dead Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

9.3 Calculation Using Z-Matrix Accounting for Cross-Coupling(Three Coils) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

9.4 Positive and Negative Mutual Inductance . . . . . . . . . . . . . . . . . 3099.4.1 Vertical Direction and +Lm . . . . . . . . . . . . . . . . . . . . . 3129.4.2 Horizontal Direction and −Lm . . . . . . . . . . . . . . . . . . . 3159.4.3 Combination of Vertical and Horizontal Directions . . . . 3169.4.4 Multiple Power Supply Equivalent Circuit (Three

Coils) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3179.5 Calculation with Z-Matrix Accounting for Cross-Coupling

(N Coils) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

10 Applications of Multiple Power Supplies . . . . . . . . . . . . . . . . . . . . . 32510.1 Improved Efficiency When Using Multiple Power Supplies . . . . 325

10.1.1 The Model Considered in This Chapter . . . . . . . . . . . . 32610.1.2 Deriving the Formula for Total Efficiency and

Optimal Load Value . . . . . . . . . . . . . . . . . . . . . . . . . . 32810.1.3 When Mutual Inductance Lm Varies . . . . . . . . . . . . . . 332

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10.1.4 When Mutual Inductance Lm Is Constant . . . . . . . . . . . 33310.1.5 Graph of Multiple Power Supply Efficiency

Increase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33510.2 Cross-Coupling Canceling Method (CCC Method) . . . . . . . . . . 342

10.2.1 Formulas for Multiple Loads and Cross-Coupling . . . . 34410.2.2 Verification of the Effects of Cross-Coupling and

Simple Frequency Tracking Method (Method A) . . . . . 34710.2.3 Optimization for Load Resistance Only (Method B)

and Limits Thereof . . . . . . . . . . . . . . . . . . . . . . . . . . . 34910.2.4 Cross-Coupling Canceling Method (Method C) . . . . . . 354

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359

11 Basic Characteristics of Electric Field Resonance . . . . . . . . . . . . . . 36111.1 Capacitors as Electromagnetics and the True Nature of

Displacement Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36111.2 Electric Field Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36411.3 Introduction of Electric Resonance Coupling . . . . . . . . . . . . . . 36611.4 Air Gap Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36811.5 Misalignment Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 36911.6 Near-Field Electric Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37011.7 Comparison of Electric Field Coupling and Electric Resonance

Coupling (N–N, N–S, S–N, S–S, S–P) . . . . . . . . . . . . . . . . . . . 37211.7.1 Electric Field Coupling Type Coupler Structure

and Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37211.7.2 Each Topology of Electric Field Coupling . . . . . . . . . . 37311.7.3 Derivation of Theoretical Formula . . . . . . . . . . . . . . . . 37511.7.4 Validation of Theoretical Formula and Analysis . . . . . . 37911.7.5 Efficiency Comparison . . . . . . . . . . . . . . . . . . . . . . . . 37911.7.6 Conditions for High Efficiency and High-Power

Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384

12 Unified Theory of Magnetic Field Coupling and Electric FieldCoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38512.1 Unified Interpretation of Magnetic Field Coupling and Electric

Field Coupling and Preparation for Comparison . . . . . . . . . . . . 38512.1.1 Magnetic Field Coupling (IPT) . . . . . . . . . . . . . . . . . . 38612.1.2 Electric Field Coupling (CPT) . . . . . . . . . . . . . . . . . . . 38812.1.3 Unified Design Method for Resonance Conditions . . . . 391

12.2 Magnetic Field Coupling, IPT (SS, SP, PS, and PP) . . . . . . . . . 39712.2.1 Derivation of Compensation Capacitor Condition

R1 ¼ R2 ¼ 0ð Þ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398

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12.2.2 Circuit Analysis and CharacterizationR1 6¼ R2 6¼ 0ð Þ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400

12.2.3 Power Transmission Characteristics EvaluationDuring Load Fluctuation . . . . . . . . . . . . . . . . . . . . . . . 408

12.3 Electric Field Coupling, CPT (SS, SP, PS, and PP) . . . . . . . . . 41212.3.1 Derivation of Compensation Inductor Condition

R1 ¼ R2 ¼ 0ð Þ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41312.3.2 Circuit Analysis and Characterization

R1 6¼ R2 6¼ 0ð Þ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41512.3.3 Power Transmission Characteristics Evaluation

During Load Fluctuation . . . . . . . . . . . . . . . . . . . . . . . 42112.4 Comparison and Unified Theory of Magnetic Resonance

Coupling and Electric Resonance Coupling . . . . . . . . . . . . . . . 42612.4.1 Unified Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . 426

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427

Contents xiii

Chapter 1Wireless Power Transfer

Wireless power transfer (WPT) refers to the technologyof transmitting powerwithoutusing any wires, such as electric wires, which are normally used to transmit power.Conventional wireless power transfer has been limited to transmitting power overan air gap (transmission distance) of several centimeters; however, in 2007, for thefirst time, it was proven that highly efficient high-power wireless power transfer isfeasible over a large air gap exceeding 1 m [1]. This technology is referred to asmagnetic resonance coupling.

Before the unveiling of magnetic resonance coupling, it was believed that wirelesspower transfer was feasible only over a distance of 1/10th of a coil diameter; however,after the emergence of magnetic resonance coupling, it was found that power couldactually be transmitted with high efficiency and high power over distances equal to orgreater than a coil diameter. Figure 1.1 illustrates the setup for experiments conductedby the authors. We found that the technology works satisfactorily with a large airgap, even when off-center. This is a major boost for research and development in thefield of wireless power technology.

This technology only recently came to light, making good use of the resonancephenomenon based on coupling through a magnetic field, that is, electromagneticinduction. On the other hand, various other methods of wireless power transfer havebeen studied in addition to magnetic resonance coupling. In this chapter, we describethe basic setup of wireless power transfer.

1.1 Types of Wireless Power Transfer

There are several available methods for wireless power transfer, and one commonfeature that all these methods share is the wireless power transmission using high-frequency alternating current (AC). Broadly speaking, there are two types of wirelesspower transfer: coupling and radiative. The coupling type is further categorized intothe magnetic field and electric field types, whereas the radiative type is categorized

© Springer Nature Singapore Pte Ltd. 2020T. Imura, Wireless Power Transfer,https://doi.org/10.1007/978-981-15-4580-1_1

1

2 1 Wireless Power Transfer

(a) Directly above (b) Off-center (c) To the side

Fig. 1.1 Setup for magnetic resonance coupling light bulb light-up experiment

into microwave (electromagnetic wave) and laser (optical) types. Hence, wirelesspower transfer can be typically classified into four types.1 However, because therehas been relatively little research on lasers, it is often categorized into only threetypes.

1.1.1 Detailed Differentiation of Wireless Power TransferTypes

Figure 1.2 depicts the types of wireless power transfer. First, they are broadly cate-gorized into coupling and radiative types. The radiative type is described in Sect. 1.4,

Fig. 1.2 Types of wireless power transfer

1Depending on the focus of attention, the abovementioned types are sometimes referred to as afour-member group consisting of electromagnetic induction, magnetic resonance coupling, electricfield coupling, and microwaves; however, this is not systematically balanced. Thus, in this book,we have categorized the types as indicated above.

1.1 Types of Wireless Power Transfer 3

Fig. 1.3 Types of couplings

and the coupling type is described in Sect. 1.1. Figure 1.3 depicts the types of cou-plings, and Fig. 1.4 illustrates the corresponding diagrams. First, we refer to thepower-transmitting side as the primary side and the power-receiving side as thesecondary side. Thus, the coupling type can be categorized into ➀ magnetic fieldcoupling (electromagnetic induction) and ➁ electric field coupling (displacementcurrent) depending on whether the coupling is via a magnetic field H or an electricfield E. The coupling type is further categorized into a total of four types based onthe resonance phenomenon.

➀ The magnetic field coupling type generally uses electromagnetic induction.➁ The electric field coupling type uses an electric field instead of a magneticfield. Furthermore, introducing a resonant capacitor during electromagnetic induc-tion will cause the capacitor to resonate with the coil, making the resonant fre-quency on the power-transmitting side the same as that on the power-receivingside. This enables the achievement of high efficiency and high power, as well asa large air gap. In other words, the skillful use of the resonance phenomenon isreferred to as ➂ magnetic resonance coupling. Similarly, while the electric field cou-pling type generally uses electric field coupling, the introduction of a resonant coil,such that it resonates with a capacitor (coupler) and skillfully uses the resonancephenomenon, is referred to as ➃ electric resonance coupling.

Put simply, the resonance phenomenon (electromagnetic induction or electricfield coupling) is frequently used for power factor improvement. However, magneticresonance coupling and electric resonance coupling will not work if the resonantconditions are not right. The specific types are discussed in Chaps. 5 and 11. Powertransmission is possible whenever a power-transmitting side and power-receivingside are coupled with a magnetic field or electric field. The coupling part is generallyreferred to as a resonator, or partly as an antenna, although it is often referred to as acoil when coupled with a magnetic field, and a coupler or plate when coupled withan electric field (see in “[Column] Terms”).

4 1 Wireless Power Transfer

Fig. 1.4 Diagrams ofmagnetic field coupling,magnetic resonance coupling, electric field coupling,and electric resonance coupling

[Column] Terms

Wireless power transfer is a field involving interdisciplinary fusion. As a result of thenear-simultaneous participation of various persons of different backgrounds, variouswords with the same meaning are used for certain terms. Unifying the terminologyin this book would be difficult; however, because it is important to understand theconcept clearly, here, we provide a simple summary of the terms used.

• Wireless power transfer

Wireless power transfer is sometimes referred to as wireless power supply or non-contact power supply. Although these terms have the almost same meanings, some-times it can be wireless but in contact. Thus, to avoid any misunderstanding, theterm “noncontact power supply” is sometimes deliberately used. However, wireless

1.1 Types of Wireless Power Transfer 5

power transfer generally means no contact in most cases, and hence, the three termsare used interchangeably.

• Wireless charging

Wireless charging refers to wireless power transfer involving charging, which is alsoreferred to as noncontact charging.

• Magnetic resonance coupling

Magnetic resonance coupling is a coupling method that uses a magnetic field andresonance. It is also referred to asmagnetic resonance. Here, the word resonancemaybe understood to mean resonating and coupling. More specifically, magnetic reso-nance coupling is a technology that uses the electromagnetic induction phenomenonand accomplishes wireless power transfer with a circuit topology (circuit structure)consisting of resonant circuits on both sides.

• Electromagnetic induction (magnetic coupling)

Electromagnetic induction is a term generally used for coupling methods using amagnetic field. It originally referred to the electromagnetic induction phenomenonitself; however, in wireless power transfer, it is used to indicate power transmission.It is also referred to as magnetic coupling or inductive coupling, as well as inductivepower transfer (IPT).

• Electric resonance coupling (electric resonance)

Electric resonance coupling is a coupling method that uses an electric field andresonance. It is also referred to as electric resonance.

• Electric field coupling (electric coupling)

Electric field coupling is a term generally used for coupling methods that use anelectric field. It is also referred to as electric coupling or capacitive coupling, as wellas capacitive power transfer (CPT).

• Electromagnetic resonance coupling (electromagnetic resonant coupling)

This is a term generally used for both electric resonance coupling and magneticresonance coupling. It is also referred to as electromagnetic resonance.

• Coupling components: coil, plate, resonator, coupler, antenna

The part of wireless power transfer may be referred to as coupler; however, gener-ally, coupling is often achieved through a magnetic field, and hence, it is frequentlyreferred to as a coil. In addition, a resonator refers to the inclusion of both a coil andcapacitor.

The background is described below. Initially, coupling components were alsocalled antennas because power was transmitted wirelessly using resonance [2]. How-ever, because an antenna refers to a component that transmits electromagnetic waves,ifwe focus our attention on the phenomenon of using resonance and coupling to trans-mit power, then the term resonator is more suitable [3, 4]. Hence, the term resonator

6 1 Wireless Power Transfer

has been increasingly used. Resonators have been used in communication, and thus,are more readily recognized for signals than power. However, because weak poweris transmitted during communication, their meaning is unchanged. In addition, it ispossible to couple magnetic fields without resonance, and sometimes only couplingcomponents are indicated. Thus, instead of resonator, the word coil is often used.The word coupler is also used. Hence, for the time being, the word coil is generallyused in reference to magnetic fields. As for electric fields, because of the image of acapacitor electrode plate, the words plate or electrode plate are sometimes used.

1.1.2 Operating Frequency

Figure 1.5 depicts the frequencies and types of wireless power transfer. Accordingto a March 15, 2016 revised ministerial ordinance within Japan, the frequenciesfor electrical vehicles (EV) are in the 85 kHz band and are generally referred toas being in the 79–90 kHz band at 7.7 kW or less.2 Thus, the 6.78 MHz band forconsumer products, such as mobile devices, in the range of 6.765–6.795 MHz at100 W or less,3 generally referred to as the industrial, scientific, and medical (ISM)band, has been added to type designations for the magnetic field coupling type.Similarly, frequencies designated for consumer products, such as mobile devices,

Fig. 1.5 Frequency and types of wireless power transfer

2Details of March 15, 2016, revised the ministerial ordinance written as Ministry of Internal Affairsand Communications No. 15. In ministerial ordinance documents, items for electric vehicles areindicated as “noncontact power devices for electrical vehicles,” and “vehicles fully or partly usingelectricity as a power supply;” thus, hybrid vehicles are also recognized.3ISM is a frequency band made for industry, science, and medicine, and is used relatively freelyworldwide. This includes the 6.78 MHz band. However, it is free of restrictions in all countries. Inthe present revision, the allowable magnetic field strength at a location 10 m away differs between44 dB for 6.765–6.776 MHz and 64 dB for 6.776–6.795 MHz. Here, 1 µA/m is 0 dB. In addition,in ministerial ordinance documents, the expression “6.7 MHz band magnetic field coupling typegeneral noncontact power supply device” is used.

1.1 Types of Wireless Power Transfer 7

in the range of 425–524 kHz at a frequency of 100 W or less,4 referred to as the400 kHz band, have been added as type designations for the electric field couplingtype. Previously, the use of high-frequency wireless devices exceeding 10 kHz at50 W required application on an individual basis according to rules governing high-frequency equipment under the Radio Law. However, if the devices are subject totype designations, it is possible for users to use such devices without following theprocedures for high-frequency equipment. In addition, in general, wireless powertransfer devices may be sold after permission is granted following type-designationprocedures proposed by manufacturers or importers.

From a technical viewpoint, power transmission is possible at a commercial fre-quency of 50/60 Hz; however, in terms of device miniaturization and efficiency, ingeneral, devices are operated at high frequencies. High frequencies also vary, includ-ing the kHz (103) bands, MHz (106) bands, GHz (109) bands, and THz (1012) bands.In general, frequencies up to the MHz bands are of the coupling type, and frequen-cies in the GHz band or higher are of the radiative type, which is also influencedby the wavelength (Table 1.1). The relationship of frequency f and wavelength λ isdescribed by Eq. (1.1), where c is the speed of light.

f = c

λνc = 299,792,458 ≈ 3 × 108 (m/s) (1.1)

Table 1.1 Frequencies andwavelengths

Frequency Wavelength

1 kHz 300 km

10 kHz 30 km

100 kHz 3 km

1 MHz 300 m

10 MHz 30 m

100 MHz 3 m

1 GHz 30 cm

10 GHz 3 cm

20 kHz 15 km

85 kHz 3.5 km

6.78 MHz 44.2 m

13.56 MHz 22.1 m

2.45 GHz 12.2 cm

5.8 GHz 5.2 cm

4The usage frequency is the 400 kHz band, which was established as 425–471, 480–489, 491–494, 506–517, and 519–524 kHz. In addition, in ministerial ordinance documents, the expression“400-kHz-band electric field coupling type general noncontact power supply device” is used.

8 1 Wireless Power Transfer

For example, for the coupling type 13.56 MHz, one wavelength is approximately22 m. However, for the radiative type 2.45 GHz, one wavelength is approximately12 cm.

The coupling type requires the wavelength to be sufficiently larger than the coilsize; thus, at 1 GHz or higher, the coil size is too small for the coupling type. Hence,GHz coil is generally not used except for special applications. For example, whenapproximately a five turns coil generates 13.56MHz, the coil diameter size is approx-imately 30 cm, which is about 1/100 the wavelength of approximately 22 m. That is,the wavelength is sufficiently larger than the coil size at MHz bands or lower; thus,the coupling type is the main wireless power transfer used.

On the other hand, when operating with the radiative type, no electromagneticwaves are emitted at coil sizes that are not close to the wavelength; thus, the wave-length and antenna length are close in size. For example, with a half-wavelengthdipole antenna, which is a typical example of a radiative type, the antenna lengthis approximately 6 cm, which is half the wavelength of approximately 12 cm at2.45 GHz, making it a practical size. Additionally, the half-wavelength of the wave-length at 13.56 MHz is 11 m; therefore, the radiative type is not a practical size forMHz.

[Column] Frequency of wireless power transfer and actual sense of speed

Wireless power transfer occurs at approximately 9 kHz to 13.56 MHz, making eachcycle very short. For example, if the frequency is 100 kHz, then in terms of time,period is 10µs. An example of the difference between the switching speed ofwirelesspower transfer and the speed at which we actually move is a car running at 100 km/htakes 36 ms = 36,000 µs to travel 1 m, and in 10 µs only advances 0.000278 m =0.278 mm.

1.2 Outline of Electromagnetic Induction and MagneticResonance Coupling

As illustrated in Figs. 1.6 and 1.7, the magnetic resonance coupling method narrowsthe conditions found in the electromagnetic induction method [5]. Detailed differ-ences between the two methods are described in Chap. 5, but put simply, both the

Fig. 1.6 Relationship of electromagnetic induction and magnetic resonance coupling

1.2 Outline of Electromagnetic Induction andMagnetic Resonance… 9

(a) Nonresonant electromagnetic induction (b) Magnetic resonance coupling

Fig. 1.7 Schematic diagram of electromagnetic induction and magnetic resonance coupling

magnetic resonance coupling method and electromagnetic induction method involvecoupling through a magnetic field, and the principle of electromagnetic induction isused for coupling components in both the methods. However, in magnetic resonancecoupling, as depicted in Fig. 1.7b, forming a resonant circuit on both the primary sideand secondary side results in a circuit topology (circuit structure) that makes gooduse of magnetic resonance coupling. Thus, highly efficient high power is realizedeven with large air gaps.

The principle of electromagnetic induction is described in Chap. 2. As depicted inFig. 1.8, as a result of the magnetic flux created by current I1 flowing to the primaryside passing through (interlinking) a secondary-side coil loop, energy is transmitted tothe secondary side. Thus, voltage V is induced in a direction countering the magneticflux on the secondary side, and power transmission occurs in the form of the currentI2 flow.When this occurs, energy is propagated not just by themagnetic fieldH alonebut also via changes in the magnetic field dH/dt. In other words, even if immobilemagnets and several magnetic fields H with no fluctuations created by direct currentare strongly present, if there are no changes in the magnetic fields over time or space,then no energy is transmitted.

Fig. 1.8 Power transmission by electromagnetic induction

10 1 Wireless Power Transfer

1.2.1 Difference Between Electromagnetic Inductionand Magnetic Resonance Coupling

The difference between electromagnetic induction and magnetic resonance couplingdepends on whether resonance is being used. Magnetic resonance coupling (S–S)using resonance with series capacitor can achieve high efficiency and high powerover a large air gap, as depicted in Fig. 1.9. On the other hand, with electromagneticinduction (N–N) using no resonance, when an air gap is large, power is hardlytransmitted from the power-transmitting side, and the efficiencyworsens owing to theinability of the power to be received on the receiving side. Thus, power transmissionover a large air gap is not possible. In the example of efficiency η and received powerP2 with air gap g illustrated in Fig. 1.9, this difference is self-evident.

Conventionally, electromagnetic induction predominantly involved resonatingeither the primary side or the secondary side.Moreover, these electromagnetic induc-tion methods were used in very close proximity. When the air gaps are small, effi-ciency is high regardless of the circuit structure, subsequently increasing the power. Insuch cases, when a resonant capacitor is introduced on the primary side, high power isachieved. In addition, the closeness implies that even if there is no resonance, powercan be easily achieved in comparison with large air gaps. If the conditions underwhich minimum power is achievable are satisfied, then it is possible to introduce aresonant capacitor on the secondary side to improve efficiency.

However, in any case, only the power or efficiency is improved, and when theair gap is large, efficiency is poor with primary resonance, and negligible power isreceivedwith secondary-side resonance, implying that realistically speaking, electro-magnetic induction is not useful.5 In contrast, magnetic resonance coupling havingboth primary-side resonance and secondary-side resonance are capable of realizinghigh power and high efficiency over a large air gap.

(a) Efficiency (b) Received power

0

20

40

60

80

100

0 50 100 150 200

η[%

]

g [mm]

S-S N-N

0200400600800

10001200

0 50 100 150 200

P2 [W

]

g [mm]

S-S N-N

Fig. 1.9 Comparison of magnetic resonance coupling (S–S) and non-resonant electromagneticinduction (N–N)

5Strictly speaking, power improves as much as efficiency improves with secondary-side resonance.However, this value is small.

1.2 Outline of Electromagnetic Induction andMagnetic Resonance… 11

1.2.2 Types of Circuit Topology

The mechanism of magnetic resonance coupling achieving high power and highefficiency, as depicted in Fig. 1.9, is described in Chap. 5. Here, we first describe thecircuit topology (circuit structure).

Magnetic resonance coupling is a method of wireless power transfer for cou-pling through a magnetic field by making the primary-side resonance frequency andsecondary-side resonance frequency the same. Figure 1.10 illustrates five typicalcircuits [5]. L1 is the primary-side self-inductance, r1 is the primary-side internalresistance, L2 is the secondary-side self-inductance, r2 is the secondary-side internalresistance, Lm is the mutual inductance, RL is the load resistance, C1 is the primaryresonance capacitor, and C2 is the secondary-side resonance capacitor.

(a) Nonresonance (N-N) (b) Secondary-side resonance (N-S)

(c) Primary-side resonance (S-N) (d) Magnetic resonant coupling (S-S)

(e) Magnetic resonant coupling (S-P)

Fig. 1.10 Five circuit topologies

12 1 Wireless Power Transfer

Figure 1.10a depicts the electromagnetic induction with no resonance, similar toa transformer. However, coupling coefficient k � 1 in a regular transformer; how-ever, for wireless power transfer, the coupling coefficient k becomes smaller than1. Because there is no resonant capacitor, this is a non-resonant circuit (N–N) withno C. Figure 1.10b depicts an electromagnetic induction method in which a reso-nant capacitor C2 is inserted on the secondary side. In other words, it is a C2-onlysecondary-side resonance circuit (N–S: non-resonant–series). Figure 1.10c depictsan electromagnetic induction method in which a resonant capacitor C1 is insertedon the primary side. In other words, it is a C1-only primary-side resonance circuit(S–N: series–non-resonant).

Figure 1.10d depicts a magnetic resonance coupling method in which a reso-nant capacitor C1 is inserted on the primary side, and a resonant capacitor C2 isinserted on the secondary side. This is conditioned such that both the transmittingand receiving resonance frequency are the same. Here, both the transmitting sideresonant capacitor C1 and the power-receiving-side resonant capacitor C2 are con-nected in series, making this an S–S (series–series) magnetic resonance couplingmethod. Figure 1.10e depicts a magnetic resonance coupling method similar to thatin Fig. 1.10d, in which a resonant capacitor C1 is inserted on the primary side anda resonant capacitor C2 is inserted on the secondary side. This is conditioned suchthat both the transmitting and receiving resonance frequencies are the same. How-ever, while the power-transmitting-side resonant capacitor C1 is connected in series,the power-receiving-side resonant capacitor C2 is connected in parallel, making thisan S–P (series–parallel) magnetic resonance coupling method. These methods areexplained in detail in Chap. 5.

1.3 Outline of Electric Field Coupling and ElectricResonance Coupling

Power transmission may be achieved not only by magnetic field coupling but also bycoupling a power-transmitting side and power-receiving side through an electric field.Electric field coupling is caused by time change in an electric field.However, couplingthrough an electric field alone is low in efficiency; thus, electric resonance couplingis used to achieve maximum efficiency and high power. The relationship betweenelectric field coupling and electric resonance coupling is illustrated in Figs. 1.11and 1.12. This relationship is the same as the relationship between electromagneticinduction (magnetic field coupling) and magnetic resonance coupling.

In this book, we describe electric resonance coupling in Chap. 11. As a result ofthe electric field generated on the primary-side coupling the secondary side, energyis transmitted. In addition, the propagation of energy caused not by the electric fieldE itself but by changes in electric field dE/dt is similar to that during magnetic fieldcoupling. In other words, even if several unchanging electric fields E are strongly

1.3 Outline of Electric Field Coupling and Electric Resonance Coupling 13

Fig. 1.11 Relationship between electric field coupling and electric resonance coupling

(a) Nonresonant electric field coupling (b) Electric resonance coupling

Fig. 1.12 Schematic diagram of electric field coupling and electric resonance coupling

present, if there are no changes in the electric fields over time or space, then energyis not transmitted.

1.4 Radiative-Type Power Transmission

Radiative-type power transmission consists of twomethods: microwave power trans-mission and laser power transmission. In this book, only a brief introduction is pro-vided. Figure 1.13 depicts the radiative types, and Fig. 1.14 depicts the microwave-and laser-radiative-type power transmissions, which are the methods that ultimatelybrought wireless power transfer into being. Unlike the coupling type, the radiative-type power transmission truly sends power flying across large distances. In principle,power can reach any distance even with attenuation and scattering. In other words,it is possible to transmit (send) power even if there is no nearby receiving antenna.Thus, the radiative type is the more advantageous type, with the ability to transmitpower over distances exceeding 10 m. Because electromagnetic waves reach intospace, the distances they cover are of a completely different dimension from thoseof the coupling type. Therefore, the radiative type has no comparison with respectto power transmission to flying objects, power transmission from flying objects, and

14 1 Wireless Power Transfer

Fig. 1.13 Radiative types

(a) Microwave method (b) Laser method

Fig. 1.14 Two radiation methods

space-based solar power. Microwave power transmission is in short distance techni-cally possible; thus, the coupling type,which presently is superior in terms of cost andefficiency, may be covered bymicrowave power transmission in the future. Presently,microwave power transmission mainly involves operating within a several GHz bandfrom the viewpoint of beam focusing and demand for device miniaturization. On theother hand, laser power transmission mainly involves operating within a THz band.At present, however, its overall efficiency is just over 50%, and future developmentis expected.

1.4.1 Microwave Power Transmission

Among the radiative types, the electromagnetic wave type generally uses 2.45 GHzor 5.8 GHzmicrowaves and thus, is referred to as microwave power transmission [6].Because it does not negatively affect regular communication, its research and devel-opment has proceeded on the premise of using the ISM bands of 2.45 and 5.8 GHz,which are special frequency bands that can be used relatively freely. Examples of

1.4 Radiative-Type Power Transmission 15

Fig. 1.15 Technology related to microwave power transmission

familiar frequency bands are the 2.45 GHz band used for microwave ovens and the5.8 GHz band used for electronic toll collection system (ETC).

The electromagnetic waves used here as a form of energy are the same as mobilephone electromagnetic waves; however, when they are transmitted as power, thequestion arises as to how much to focus the beams such that the power pinpointsthe receiving side. On the other hand, microwave power transmission includes theenergy harvesting technology that scatters and collects power.

Microwave power transmission operates at high frequencies of 2.45 and 5.8 GHz,transmitting power created by a high-frequency power supply from a transmittingantenna as electromagnetic waves, which are then received by a receiving antennalocated far away. This receiving antenna is referred to as a rectenna (Fig. 1.15a),which consists of an integration of a rectifier and a receiving antenna. Because thefrequencies are high, the wavelength cannot be ignored with regard to the antennaand circuits. Hence, the voltage differs depending upon the circuit position, even onthe same line. Separately constructing the receiving antenna and rectifier and thentrying to combine them is not suitable. Hence, the receiving antenna and rectifiermust be of an integrated type.

Furthermore, it is possible to control the beam direction with a phased-arrayantenna using phase control. This is a form of technology for moving a beam backand forth (Fig. 1.15b). Microwave power transmission consists of a variety of tech-nologies, including one (retrodirective system) that can receive a one-time pilot signalfrom the location to which power is being transmitted, and thereby transmit powerby accurately concentrating the beam in the direction from which the pilot signaloriginates. Microwave power transmission is expected to be used in solar powersatellites (SPS) and space solar power systems.

• Laser power transmission

Laser power transmission is radiated in the form of electromagnetic waves; how-ever, it is in THz and thus, exists as light [7]. A familiar version of this type of trans-mission is a laser pointer. Its frequency is higher than that of microwaves, and poweris transmitted by a laser transmitter and captured by a solar panel. Thus, improvedsolar panel efficiency also leads to improved overall efficiency. In addition, whenwe consider the application of laser power transmission to space-based solar power,although there is attenuation produced by clouds and other factors, owing to its highfrequency and lower beam scattering than that of microwaves, it is being exploredto supply power to a space elevator.

16 1 Wireless Power Transfer

Laser power has a unique characteristic in comparisonwith other forms ofwirelesspower transfer: by using light in the visible range, power transmission locations arevisible. This visibility makes it extremely safe.

[Column] Interdisciplinary study of antennas, resonators, power electronics,physics, power devices, and high voltage

Wireless power transfer technology is a melting pot of various fields, and it issimilar to an ideal specimen of interdisciplinary research and development. Themagnetic resonance coupling proposed by physicists in 2007 was first explainedtheoretically by the couplingmode theory and then described as representingwirelesspower transfer at 10 MHz, three digits higher than before, while the phenomenonitself remained a mystery. One reason why it was a mystery is that open circuitcoil type, which can resonate itself, was used. To elucidate the phenomenon, manyresearchers in antenna engineering and resonators who had studiedGHz bands joinedthe research.

At a glance, electromagnetic induction andmagnetic resonance coupling are com-pletely different and are also high in frequency; thus, it was surprising that researcherswhohadpreviously studied electromagnetic inductionwere so late in joining this fieldof research. Gradually, as studies got closer to the general electromagnetic induc-tion theories, including the equivalent circuit theory, and it was found that operationaround 100 kHz was possible, many experts in power electronics and control theory,and experts researching electromagnetic induction, began to get involved. In addi-tion, at 100 kHz, power devices also requiredmetal-oxide-semiconductor field-effecttransistors (MOSFETs) of silicon carbide (SiC) and not conventional silicon; hence,power device specialists also came aboard.

The present reality of the technology is that there has been a momentary drop infrequency to 85 kHz; however, in the future this may return to high frequencies of6.78 MHz or 13.56 MHz. In addition, it is very likely that gallinium nitride (GaN)and not SiC will be important for power devices. This is because if the problems ofthe power supply and rectifier circuits are solved, then somewhat higher frequencieswill be advantageous in terms of the potential for lightweight coils or larger air gaps.In addition, because there is now a need for high-voltage measures, high-voltage spe-cialists are needed. Thus, there is also a potential growth in the high-frequency powerelectronics field. For example, high-speed control using field-programmable gatearray (FPGA) is also needed. Thus, many fields and industries have come togetherthrough cooperation between researchers and engineers.

1.5 Basic System Configuration

Although wireless power transfer tends to be focused on the coupling components,it is important to have an understanding of the overall system. Therefore, we presentthe basic system configuration for wireless power transfer. Details are providedin Chap. 8. Here, we only introduce the basic concept. Figure 1.16 illustrates theconceptual diagram of the basic configuration of a wireless power transfer system.

1.5 Basic System Configuration 17

Fig. 1.16 Conceptual diagram of basic system configuration

The following is based upon a regular household with a 50/60 Hz, 100 V (AC):AC (50/60Hz)→DC (DCpower supply)→DC/DC converter (voltage variation)

→AC (inverter, high-frequency generation, RF)→ transmit and receive coil→DC(rectification) → DC/DC converter (impedance transformation) → load

Because AC → DC from 50/60 Hz represents the generally established technol-ogy, it is often omitted in explanations 6

(1) DC generation (DC power supply): Creating DC (direct current) from appli-cable 50/60 Hz frequencies with an AC/DC converter; in other words, a DCpower supply. In the case of batteries, they are DC to begin with; hence, thisprocess does not require them. Power-supply voltage adjustment is sometimesaccomplished with a DC/DC converter function included.

(2) DC/AC transformation (inverter, high-frequency power supply): DC/AC trans-formation converts DC performed by a converter or obtained from a battery toAC; in other words, an inverter. At frequencies of MHz or higher, power supplyis often referred to as a high-frequency power supply. The frequencies when theDC/AC transformation occurs are those used in wireless power transfer. Fre-quently used frequencies are 9.9, 20, 85, and 100–200 kHz, and the ISM bandsare 6.78 and 13.56 MHz for the coupling type, and 2.45 and 5.8 GHz for theradiative type.

6DC is direct current, and AC is alternating current. RF (radio frequency) is a high frequency thatis often used at GHz bands or higher. In addition, power-transmitting-side DC/DC converters areoften included in DC power supply functions and are thus omitted at fixed voltages. Thus, we shallsubsequently explain them as part of the DC power supply. Hereinafter, we explain each part indetail.

18 1 Wireless Power Transfer

(a) Square wave (b) Three-level square wave (c) Sine wave

Fig. 1.17 Sine wave and square wave

For the coupling type, although the frequency is high, it is often written as AC rep-resenting regular alternating current, sowhether it is high frequency needs to be deter-mined in context. In addition, in the radiative-type microwave power transmission,this is often written as RF.

When drawing an equivalent circuit, it is often written from here. As depictedin Fig. 1.17a, when operating an inverter, waveform is a square wave. However, asdepicted in Fig. 1.17b, sometimes a three-level square wave is used with a phase shiftto vary the voltage. By creating a 0 V voltage period, it is possible to adjust the timefor which the voltage is applied, and vary the voltage. In other words, because thevoltage can be varied even without a power-transmitting-side converter, a converterdoes not need to be used. In addition, when considering only basic wave componentsor a power supply that creates a sine wave, the power supply is often depicted witha sine wave, as depicted in Fig. 1.17c. In general, square-wave operation is commonwhen using an inverter. A high-frequency power supply for high frequency alsosometimes produces a sine-wave output.

(3) Power transmission coil and receiving coil: Section of space (air gap) in whichwireless power transmission actually occurs. Alternating current is alwaysrequired and generally needs to be high frequency for high efficiency. A resonantcapacitor is connected here.

(4) AC/DC transformation (rectifier): Wireless power transferred AC (RF) is con-verted back toDCwith a rectifier circuit; in otherwords, a rectifier. After passingthrough a rectifier, the voltage becomes positive and the DC is in a rippling state,which needs to be converted to unrippled DC with a smoothing capacitor. Thelarger the capacity of the smoothing capacitor, the lesser the rippling; how-ever, because larger capacitor increases its volume, miniaturization is required.Furthermore, there are also methods using synchronous rectification to changeswitch to active to reduce loss.

(5) DC/DC transformation (DC/DC converter): Has optimal load for maximumefficiency. To reach a voltage and current ratio equal to the optimal load, thatis, impedance adjustment, a DC/DC converter is often used on front side of theload. Otherwise, DC/DC converter is used for adjustments to reach the desiredpower.

(6) Load components (resistance, battery, etc.): For the load, often, the simplestresistance is depicted; however, in actual products, the load components includebatteries, capacitors, and motors. These differ in operation and in the difficultiesthey pose to the overall system. The resistance is the load with the simplest