Wireless Power Transfer: Using Magnetic and Electric Resonance Coupling Techniques

By Takehiro Imura

This book describes systematically wireless power transfer technology using magnetic resonant coupling and electric resonant coupling and presents the latest theoretical and phenomenological approaches to its practical implementation, operation and its applications. It also discusses the difference between electromagnetic induction and magnetic resonant coupling, the characteristics of various types of resonant circuit topologies and the unique features of magnetic resonant coupling methods. Designed to be self-contained, this richly illustrated book is a valuable resource for a broad readership, from researchers to engineers and anyone interested in cutting-edge technologies in wireless power transfer.

• Language: English
• Publisher: Springer
• Release date: Jun 16, 2020
• ISBN: 9789811545801

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© Springer Nature Singapore Pte Ltd. 2020

T. ImuraWireless Power Transferhttps://doi.org/10.1007/978-981-15-4580-1_1

1. Wireless Power Transfer

Takehiro Imura¹  

(1)

Tokyo University of Science, Noda, Chiba, Japan

Takehiro Imura

Email: [e-mail]

Wireless power transfer (WPT) refers to the technology of transmitting power without using any wires, such as electric wires, which are normally used to transmit power. Conventional wireless power transfer has been limited to transmitting power over an air gap (transmission distance) of several centimeters; however, in 2007, for the first time, it was proven that highly efficient high-power wireless power transfer is feasible over a large air gap exceeding 1 m [1]. This technology is referred to as magnetic resonance coupling.

Before the unveiling of magnetic resonance coupling, it was believed that wireless power 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 could actually be transmitted with high efficiency and high power over distances equal to or greater than a coil diameter. Figure 1.1 illustrates the setup for experiments conducted by the authors. We found that the technology works satisfactorily with a large air gap, even when off-center. This is a major boost for research and development in the field of wireless power technology.

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Fig. 1.1

Setup for magnetic resonance coupling light bulb light-up experiment

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

1.1 Types of Wireless Power Transfer

There are several available methods for wireless power transfer, and one common feature that all these methods share is the wireless power transmission using high-frequency alternating current (AC). Broadly speaking, there are two types of wireless power transfer: coupling and radiative. The coupling type is further categorized into the magnetic field and electric field types, whereas the radiative type is categorized into microwave (electromagnetic wave) and laser (optical) types. Hence, wireless power transfer can be typically classified into four types.¹ However, because there has been relatively little research on lasers, it is often categorized into only three types.

1.1.1 Detailed Differentiation of Wireless Power Transfer Types

Figure 1.2 depicts the types of wireless power transfer. First, they are broadly categorized into coupling and radiative types. The radiative type is described in Sect. 1.4, and the coupling type is described in Sect. 1.1. Figure 1.3 depicts the types of couplings, and Fig. 1.4 illustrates the corresponding diagrams. First, we refer to the power-transmitting side as the primary side and the power-receiving side as the secondary side. Thus, the coupling type can be categorized into ① magnetic field coupling (electromagnetic induction) and ② electric field coupling (displacement current) depending on whether the coupling is via a magnetic field H or an electric field E. The coupling type is further categorized into a total of four types based on the resonance phenomenon.

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Fig. 1.2

Types of wireless power transfer

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Fig. 1.3

Types of couplings

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Fig. 1.4

Diagrams of magnetic field coupling, magnetic resonance coupling, electric field coupling, and electric resonance coupling

① The magnetic field coupling type generally uses electromagnetic induction. ② The electric field coupling type uses an electric field instead of a magnetic field. Furthermore, introducing a resonant capacitor during electromagnetic induction will cause the capacitor to resonate with the coil, making the resonant frequency on the power-transmitting side the same as that on the power-receiving side. This enables the achievement of high efficiency and high power, as well as a large air gap. In other words, the skillful use of the resonance phenomenon is referred to as ③ magnetic resonance coupling. Similarly, while the electric field coupling type generally uses electric field coupling, the introduction of a resonant coil, such that it resonates with a capacitor (coupler) and skillfully uses the resonance phenomenon, is referred to as ④ electric resonance coupling.

Put simply, the resonance phenomenon (electromagnetic induction or electric field coupling) is frequently used for power factor improvement. However, magnetic resonance coupling and electric resonance coupling will not work if the resonant conditions are not right. The specific types are discussed in Chaps. 5 and 11. Power transmission is possible whenever a power-transmitting side and power-receiving side are coupled with a magnetic field or electric field. The coupling part is generally referred to as a resonator, or partly as an antenna, although it is often referred to as a coil when coupled with a magnetic field, and a coupler or plate when coupled with an electric field (see in [Column] Terms).

[Column] Terms

Wireless power transfer is a field involving interdisciplinary fusion. As a result of the near-simultaneous participation of various persons of different backgrounds, various words with the same meaning are used for certain terms. Unifying the terminology in this book would be difficult; however, because it is important to understand the concept 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 noncontact power supply. Although these terms have the almost same meanings, sometimes it can be wireless but in contact. Thus, to avoid any misunderstanding, the term noncontact power supply is sometimes deliberately used. However, wireless power transfer generally means no contact in most cases, and hence, the three terms are used interchangeably.

Wireless charging

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

Magnetic resonance coupling

Magnetic resonance coupling is a coupling method that uses a magnetic field and resonance. It is also referred to as magnetic resonance. Here, the word resonance may be understood to mean resonating and coupling. More specifically, magnetic resonance coupling is a technology that uses the electromagnetic induction phenomenon and 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 a magnetic field. It originally referred to the electromagnetic induction phenomenon itself; 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 inductive power transfer (IPT).

Electric resonance coupling (electric resonance)

Electric resonance coupling is a coupling method that uses an electric field and resonance. 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 an electric field. It is also referred to as electric coupling or capacitive coupling, as well as capacitive power transfer (CPT).

Electromagnetic resonance coupling (electromagnetic resonant coupling)

This is a term generally used for both electric resonance coupling and magnetic resonance 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, generally, coupling is often achieved through a magnetic field, and hence, it is frequently referred to as a coil. In addition, a resonator refers to the inclusion of both a coil and capacitor.

The background is described below. Initially, coupling components were also called antennas because power was transmitted wirelessly using resonance [2]. However, because an antenna refers to a component that transmits electromagnetic waves, if we focus our attention on the phenomenon of using resonance and coupling to transmit power, then the term resonator is more suitable [3, 4]. Hence, the term resonator has been increasingly used. Resonators have been used in communication, and thus, are more readily recognized for signals than power. However, because weak power is transmitted during communication, their meaning is unchanged. In addition, it is possible to couple magnetic fields without resonance, and sometimes only coupling components 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 generally used in reference to magnetic fields. As for electric fields, because of the image of a capacitor 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. According to a March 15, 2016 revised ministerial ordinance within Japan, the frequencies for electrical vehicles (EV) are in the 85 kHz band and are generally referred to as being in the 79–90 kHz band at 7.7 kW or less.² Thus, the 6.78 MHz band for consumer products, such as mobile devices, in the range of 6.765–6.795 MHz at 100 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, in the range of 425–524 kHz at a frequency of 100 W or less,⁴ referred to as the 400 kHz band, have been added as type designations for the electric field coupling type. Previously, the use of high-frequency wireless devices exceeding 10 kHz at 50 W required application on an individual basis according to rules governing high-frequency equipment under the Radio Law. However, if the devices are subject to type designations, it is possible for users to use such devices without following the procedures for high-frequency equipment. In addition, in general, wireless power transfer devices may be sold after permission is granted following type-designation procedures proposed by manufacturers or importers.

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Fig. 1.5

Frequency and types of wireless power transfer

From a technical viewpoint, power transmission is possible at a commercial frequency of 50/60 Hz; however, in terms of device miniaturization and efficiency, in general, devices are operated at high frequencies. High frequencies also vary, including 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 frequencies in the GHz band or higher are of the radiative type, which is also influenced by the wavelength (Table 1.1). The relationship of frequency f and wavelength λ is described by Eq. (1.1), where c is the speed of light.

Table 1.1

Frequencies and wavelengths

$$ f = \frac{c}{\lambda }\quad \nu c = 299{,}792{,}458 \approx 3 \times 10^{8} \,({\text{m/s}}) $$

(1.1)

For example, for the coupling type 13.56 MHz, one wavelength is approximately 22 m. However, for the radiative type 2.45 GHz, one wavelength is approximately 12 cm.

The coupling type requires the wavelength to be sufficiently larger than the coil size; 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, when approximately a five turns coil generates 13.56 MHz, the coil diameter size is approximately 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 electromagnetic waves are emitted at coil sizes that are not close to the wavelength; thus, the wavelength and antenna length are close in size. For example, with a half-wavelength dipole antenna, which is a typical example of a radiative type, the antenna length is approximately 6 cm, which is half the wavelength of approximately 12 cm at 2.45 GHz, making it a practical size. Additionally, the half-wavelength of the wavelength at 13.56 MHz is 11 m; therefore, the radiative type is not a practical size for MHz.

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

Wireless power transfer occurs at approximately 9 kHz to 13.56 MHz, making each cycle very short. For example, if the frequency is 100 kHz, then in terms of time, period is 10 μs. An example of the difference between the switching speed of wireless power transfer and the speed at which we actually move is a car running at 100 km/h takes 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 Magnetic Resonance Coupling

As illustrated in Figs. 1.6 and 1.7, the magnetic resonance coupling method narrows the conditions found in the electromagnetic induction method [5]. Detailed differences between the two methods are described in Chap. 5, but put simply, both the magnetic resonance coupling method and electromagnetic induction method involve coupling through a magnetic field, and the principle of electromagnetic induction is used for coupling components in both the methods. However, in magnetic resonance coupling, as depicted in Fig. 1.7b, forming a resonant circuit on both the primary side and secondary side results in a circuit topology (circuit structure) that makes good use of magnetic resonance coupling. Thus, highly efficient high power is realized even with large air gaps.

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Fig. 1.6

Relationship of electromagnetic induction and magnetic resonance coupling

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Fig. 1.7

Schematic diagram of electromagnetic induction and magnetic resonance coupling

The principle of electromagnetic induction is described in Chap. 2. As depicted in Fig. 1.8, as a result of the magnetic flux created by current I1 flowing to the primary side passing through (interlinking) a secondary-side coil loop, energy is transmitted to the secondary side. Thus, voltage V is induced in a direction countering the magnetic flux on the secondary side, and power transmission occurs in the form of the current I2 flow. When this occurs, energy is propagated not just by the magnetic field H alone but also via changes in the magnetic field dH/dt. In other words, even if immobile magnets and several magnetic fields H with no fluctuations created by direct current are strongly present, if there are no changes in the magnetic fields over time or space, then no energy is transmitted.

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Fig. 1.8

Power transmission by electromagnetic induction

1.2.1 Difference Between Electromagnetic Induction and Magnetic Resonance Coupling

The difference between electromagnetic induction and magnetic resonance coupling depends on whether resonance is being used. Magnetic resonance coupling (S–S) using resonance with series capacitor can achieve high efficiency and high power over a large air gap, as depicted in Fig. 1.9. On the other hand, with electromagnetic induction (N–N) using no resonance, when an air gap is large, power is hardly transmitted from the power-transmitting side, and the efficiency worsens owing to the inability of the power to be received on the receiving side. Thus, power transmission over a large air gap is not possible. In the example of efficiency η and received power P2 with air gap g illustrated in Fig. 1.9, this difference is self-evident.

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Fig. 1.9

Comparison of magnetic resonance coupling (S–S) and non-resonant electromagnetic induction (N–N)

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

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

1.2.2 Types of Circuit Topology

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

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

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Fig. 1.10

Five circuit topologies

Figure 1.10a depicts the electromagnetic induction with no resonance, similar to a transformer. However, coupling coefficient k ≒ 1 in a regular transformer; however, for wireless power transfer, the coupling coefficient k becomes smaller than 1. Because there is no resonant capacitor, this is a non-resonant circuit (N–N) with no C. Figure 1.10b depicts an electromagnetic induction method in which a resonant capacitor C2 is inserted on the secondary side. In other words, it is a C2-only secondary-side resonance circuit (N–S: non-resonant–series). Figure 1.10c depicts an electromagnetic induction method in which a resonant capacitor C1 is inserted on 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 resonant capacitor C1 is inserted on the primary side, and a resonant capacitor C2 is inserted on the secondary side. This is conditioned such that both the transmitting and receiving resonance frequency are the same. Here, both the transmitting side resonant capacitor C1 and the power-receiving-side resonant capacitor C2 are connected in series, making this an S–S (series–series) magnetic resonance coupling method. Figure 1.10e depicts a magnetic resonance coupling method similar to that in Fig. 1.10d, in which a resonant capacitor C1 is inserted on the primary side and a resonant capacitor C2 is inserted on the secondary side. This is conditioned such that both the transmitting and receiving resonance frequencies are the same. However, while the power-transmitting-side resonant capacitor C1 is connected in series, the power-receiving-side resonant capacitor C2 is connected in parallel, making this an S–P (series–parallel) magnetic resonance coupling method. These methods are explained in detail in Chap. 5.

1.3 Outline of Electric Field Coupling and Electric Resonance Coupling

Power transmission may be achieved not only by magnetic field coupling but also by coupling 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, coupling through an electric field alone is low in efficiency; thus, electric resonance coupling is used to achieve maximum efficiency and high power. The relationship between electric field coupling and electric resonance coupling is illustrated in Figs. 1.11 and 1.12. This relationship is the same as the relationship between electromagnetic induction (magnetic field coupling) and magnetic resonance coupling.

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Fig. 1.11

Relationship between electric field coupling and electric resonance coupling

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Fig. 1.12

Schematic diagram of electric field coupling and electric resonance coupling

In this book, we describe electric resonance coupling in Chap. 11. As a result of the electric field generated on the primary-side coupling the secondary side, energy is transmitted. In addition, the propagation of energy caused not by the electric field E itself but by changes in electric field dE/dt is similar to that during magnetic field coupling. In other words, even if several unchanging electric fields E are strongly present, if there are no changes in the electric fields over time or space, then energy is not transmitted.

1.4 Radiative-Type Power Transmission

Radiative-type power transmission consists of two methods: microwave power transmission and laser power transmission. In this book, only a brief introduction is provided. 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 ultimately brought 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 transmit power over distances exceeding 10 m. Because electromagnetic waves reach into space, the distances they cover are of a completely different dimension from those of the coupling type. Therefore, the radiative type has no comparison with respect to power transmission to flying objects, power transmission from flying objects, and space-based solar power. Microwave power transmission is in short distance technically possible; thus, the coupling type, which presently is superior in terms of cost and efficiency, may be covered by microwave power transmission in the future. Presently, microwave power transmission mainly involves operating within a several GHz band from the viewpoint of beam focusing and demand for device miniaturization. On the other hand, laser power transmission mainly involves operating within a THz band. At present, however, its overall efficiency is just over 50%, and future development is expected.

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Fig. 1.13

Radiative types

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Fig. 1.14

Two radiation methods

1.4.1 Microwave Power Transmission

Among the radiative types, the electromagnetic wave type generally uses 2.45 GHz or 5.8 GHz microwaves and thus, is referred to as microwave power transmission [6]. Because it does not negatively affect regular communication, its research and development 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 familiar frequency bands are the 2.45 GHz band used for microwave ovens and the 5.8 GHz band used for electronic toll collection system (ETC).

The electromagnetic waves used here as a form of energy are the same as mobile phone electromagnetic waves; however, when they are transmitted as power, the question arises as to how much to focus the beams such that the power pinpoints the receiving side. On the other hand, microwave power transmission includes the energy 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 transmitting antenna as electromagnetic waves, which are then received by a receiving antenna located 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 the frequencies are high, the wavelength cannot be ignored with regard to the antenna and circuits. Hence, the voltage differs depending upon the circuit position, even on the same line. Separately constructing the receiving antenna and rectifier and then trying to combine them is not suitable. Hence, the receiving antenna and rectifier must be of an integrated type.

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Fig. 1.15

Technology related to microwave power transmission

Furthermore, it is possible to control the beam direction with a phased-array antenna using phase control. This is a form of technology for moving a beam back and forth (Fig. 1.15b). Microwave power transmission consists of a variety of technologies, including one (retrodirective system) that can receive a one-time pilot signal from the location to which power is being transmitted, and thereby transmit power by accurately concentrating the beam in the direction from which the pilot signal originates. Microwave power transmission is expected to be used in solar power satellites (SPS) and space solar power systems.

Laser power transmission

Laser power transmission is radiated in the form of electromagnetic waves; however, it is in THz and thus, exists as light [7]. A familiar version of this type of transmission is a laser pointer. Its frequency is higher than that of microwaves, and power is transmitted by a laser transmitter and captured by a solar panel. Thus, improved solar panel efficiency also leads to improved overall efficiency. In addition, when we 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 high frequency and lower beam scattering than that of microwaves, it is being explored to supply power to a space elevator.

Laser power has a unique characteristic in comparison with other forms of wireless power transfer: by using light in the visible range, power transmission locations are visible. 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 is similar to an ideal specimen of interdisciplinary research and development. The magnetic resonance coupling proposed by physicists in 2007 was first explained theoretically by the coupling mode theory and then described as representing wireless power transfer at 10 MHz, three digits higher than before, while the phenomenon itself remained a mystery. One reason why it was a mystery is that open circuit coil type, which can resonate itself, was used. To elucidate the phenomenon, many researchers in antenna engineering and resonators who had studied GHz bands joined the research.

At a glance, electromagnetic induction and magnetic resonance coupling are completely different and are also high in frequency; thus, it was surprising that researchers who had previously studied electromagnetic induction were so late in joining this field of research. Gradually, as studies got closer to the general electromagnetic induction theories, including the equivalent circuit theory, and it was found that operation around 100 kHz was possible, many experts in power electronics and control theory, and experts researching electromagnetic induction, began to get involved. In addition, at 100 kHz, power devices also required metal-oxide-semiconductor field-effect transistors (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 in frequency to 85 kHz; however, in the future this may return to high frequencies of 6.78 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 of the power supply and rectifier circuits are solved, then somewhat higher frequencies will 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 specialists are needed. Thus, there is also a potential growth in the high-frequency power electronics field. For example, high-speed control using field-programmable gate array (FPGA) is also needed. Thus, many fields and industries have come together through 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 present the basic system configuration for wireless power transfer. Details are provided in Chap. 8. Here, we only introduce the basic concept. Figure 1.16 illustrates the conceptual diagram of the basic configuration of a wireless power transfer system.

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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 (50/60 Hz) → DC (DC power 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 technology, it is often omitted in explanations ⁶

(1)

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

(2)

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

For the coupling type, although the frequency is high, it is often written as AC representing regular alternating current, so whether it is high frequency needs to be determined 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 depicted in Fig. 1.17a, when operating an inverter, waveform is a square wave. However, as depicted in Fig. 1.17b, sometimes a three-level square wave is used with a phase shift to vary the voltage. By creating a 0 V voltage period, it is possible to adjust the time for which the voltage is applied, and vary the voltage. In other words, because the voltage can be varied even without a power-transmitting-side converter, a converter does not need to be used. In addition, when considering only basic wave components or a power supply that creates a sine wave, the power supply is often depicted with a sine wave, as depicted in Fig. 1.17c. In general, square-wave operation is common when using an inverter. A high-frequency power supply for high frequency also sometimes produces a sine-wave output.

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Fig. 1.17

Sine wave and square wave

(3)

Power transmission coil and receiving coil: Section of space (air gap) in which wireless power transmission actually occurs. Alternating current is always required and generally needs to be high frequency for high efficiency. A resonant capacitor is connected here.

(4)

AC/DC transformation (rectifier): Wireless power transferred AC (RF) is converted back to DC with a rectifier circuit; in other words, a rectifier. After passing through 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. The larger the capacity of the smoothing capacitor, the lesser the rippling; however, because larger capacitor increases its volume, miniaturization is required. Furthermore, there are also methods using synchronous rectification to change switch to active to reduce loss.

(5)

DC/DC transformation (DC/DC converter): Has optimal load for maximum efficiency. To reach a voltage and current ratio equal to the optimal load, that is, impedance adjustment, a DC/DC converter is often used on front side of the load. Otherwise, DC/DC converter is used for adjustments to reach the desired power.

(6)

Load components (resistance, battery, etc.): For the load, often, the simplest resistance is depicted; however, in actual products, the load components include batteries, capacitors, and motors. These differ in operation and in the difficulties they pose to the overall system. The resistance is the load with the simplest operation. The batteries are treated as rated voltages. Capacitors raise the voltage as energy is stored. As for motors, power requirements fluctuate in real time according to operation; hence, their rated power load is momentary and requires accurate transmission of the necessary power, making it the most problematic load.

Dividing a system into the six transformation components above is useful system. The simplest division involves three steps: combine (1) and (2) into the power-transmitting-side power-supply components, and divide the rest into the transmit and receive coil components in (3), and the load components from rectification to impedance adjustment in (4)–(6), that is, the three parts of the power-transmitting side, coil components, and power-receiving side.

Efficiency can be categorized according to usage as intercoil efficiency (AC–AC efficiency), DC–DC efficiency, and AC–DC efficiency. Considering the system efficiency and overall efficiency originally from 50/60 Hz, the AC–DC (50/60 Hz load) efficiency is

AC (50/60 Hz) → DC (DC power supply) → DC/DC transformation (DC/DC converter) → AC (high-frequency power supply) → AC (power transmission coil) → AC (receiving coil) → DC (rectification) → DC/DC transformation (DC/DC converter) → DC (load).

However, it is also possible to classify DC–DC (DC power supply-load) into system efficiency and overall efficiency.

Control signals are transmitted through wireless communication. Considering power and signals as separate systems is referred to as out-of-band (out-band), and considering them as the same system is referred to as in-band. In-band is not realistic because the signals are treated the same as with wireless equipment and likely require license acquisition. In general, control signals are generally transmitted wirelessly out-of-band in a separate system.⁷ In Fig. 1.16, there is a deliberate direct change in the DC power-supply voltage on the power-transmitting side, and control signals are entered into a DC/DC converter; however, if a three-level waveform is used instead of a simple square wave, direct inverter control is performed so that the voltage can be changed. In addition, although there is deliberate impedance transformation on the power-receiving side and DC/DC converter control, if the rectifier components are an active circuit, then it is possible to control the rectifier components.

[Column] Reflection and efficiency

Although not addressed in this book, a phenomenon referred to as reflection occurs at high frequency. A problem begins to occur in which a distribution constant circuit region appears in which the wavelength cannot be ignored. This problem is defined by whether this reflection results in a loss. This phenomenon is not discussed because it exceeds the scope of the book; however, the equations and obtained efficiency characteristics are as follows: In cases in this book in which reflection does not represent loss, the equation for efficiency η is defined by Eq. (1.2) using the S parameter. S21 represents penetration, and S11 represents reflection. η21 is the square of penetration, and η11 is the square of reflection. P1 is the transmitted power, and P2 is the received power. On the other hand, if efficiency is strictly defined as even reflection