**2. Inductive coupling WPT**

*Wireless Power Transfer – Recent Development, Applications and New Perspectives*

side, there is a receiving resonator which can be an antenna or coil to receive the incoming wave from the transmitter. Afterward, an impedance matching circuit is inserted to ensure maximum power transfer between the receiving resonator and the rectifying circuit. Then, the rectifying stage is connected. Many combinations could be used for the rectification purpose such as half-wave, full-wave, or any series/parallel diodes combinations. All these rectification circuits are used for converting RF power into DC power. In order to achieve smoothing DC output voltage as well as blocking the higher-order modes, the rectifying circuit is followed by a DC pass filter. The final stage is the device (load) that needs to be charged wirelessly. In this chapter, we will focus on the coupled resonators which is the first

Wireless power transfer technologies can be divided into different categories such as inductive coupling, resonant inductive coupling, capacitive coupling,

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**Figure 2.** *WPT applications.*

stage for WPT systems.

**Figure 1.** *WPT system.*

Conventional coils of wire are the simplest way to transmit a wireless power between transmitter and receiver. In this case, the system can be represented as a transformer where a transmitting coil is analogous to the primary coil, while the received coil is equivalent to the secondary coil as revealed in **Figure 3**. An inductive power transfers between the two coils in a form of a magnetic field. The intensity of the magnetic field follows Ampere's law as in (1), where *H* is the magnetic field intensity that is generated when an electric current, I, passes through an electric closed path with a length of *l* .

$$
\Phi \,\overline{H}.dl = I \tag{1}
$$

When the Transmitter has a time-varying current and mounted at an appropriate position from the receiver. Receiver's coil cuts the magnetic field lines, and an induced electromotive force (emf) is generated between the terminals of the receiver's coil as shown in **Figure 3**. The value of the emf depends on the timevarying of the magnetic flux (φ ) as characterized by Faraday's law as in (2). It is clear that this WPT technology is valid only for short-range applications for example wireless charging pads to recharge cellphones and handheld wireless devices such as laptops and tablets, electric toothbrush, shaver's battery charging, induction stovetops and industrial heaters, charging implanted prosthetic devices such as cardiac pacemakers and insulin pumps [1].

**Figure 3.** *WPT using inductive coupling scheme.*

*Wireless Power Transfer – Recent Development, Applications and New Perspectives*

$$emf = -\frac{d\phi}{dt}\tag{2}$$

WPT system performance can be estimated by the power transfer efficiency (PTE) which depends on the KQ product. K is the coupling coeffect between transmitter and receiver, it is a ratio and varies from 0 to 1 as a maximum value at totally power coupling. Q is the unloaded quality factor of the transmitter's or receiver's coil; Q can be calculated from the coil inductance as in (3), where ω is the angular frequency, L is the coil inductance, and R the loss resistance of the coil. While PTE is calculated from (4) [2]. It is clear that increasing the transfer efficiency needs a high value of the KQ product.

$$Q = \frac{\alpha L}{R} \tag{3}$$

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**Figure 5.**

**Figure 6.**

*Design of a pair of printed spiral coils [8].*

*WPT, Recent Techniques for Improving System Efficiency DOI: http://dx.doi.org/10.5772/intechopen.96003*

**3. Resonant inductive coupling WPT**

*Alternative-winding coils geometry and its current distribution [6].*

Resonant inductive coupling or magnetic resonance coupling is another form of the WPT technologies in which power is transferred between two tuned resonant circuits, one in the transmitter and the other tuned circuit in the receiver as depicted in **Figure 7**. Each resonant circuit comprises an inductor connected to a capacitor to resonate and couple the transmitted power at their resonance frequency. This resonance is responsible for emphasizing the quality factor (Q-factor) for the resonant circuit. Therefore, the coupling and the power transfer efficiency between the transmitter and receiver increase due to the directly proportional relationship between them. Magnetic resonance coupling scheme is applied in midrange applications such as charging electric vehicles, charging portable devices,

Several studies have invested the resonant inductive coupling technique for enhancement the power transfer efficiency of WPT systems [11–13]. In [14], we proposed dual open-loop spiral resonators (OLSRs) to improve the magnetic field for WPT system. OLSRs are fed through Metal–Insulator–Metal (MIM) capacitive coupling using a 50 Ohm microstrip transmission line as shown in **Figure 8**. A series resonance model is used to achieve resonant inductive as illustrated in the equivalent circuit model in **Figure 9**. The open-loop spiral resonator (OLSR) includes the series combination between the MIM capacitor and the spiral-loop inductor. Dual OLSRs are used instead of a single OLSR to strengthen the surface current on the spiral resonators. Therefore, it helps to intensify the electromagnetic

biomedical implants, powering busses, trains, RFID, smartcards.

$$PTE = \frac{k^2 Q\_s Q\_r}{\left[1 + \sqrt{1 + k^2 Q\_s Q\_r}\right]^2} \times 100\,\% \tag{4}$$

Numerous studies were introduced in the inductive coupling approach [3–9]. In [10], a multi-layer spiral inductor is proposed for biomedical applications at a frequency of 13.56 MHz which is the license-free industrial, scientific, and medical (ISM) band. It uses a stacked structure to achieve a compact WPT, where the stacked inductors occupying an area of 10 mm × 10 mm with 1 cm separation between transmitter and receiver. The inductance is further increased by stacking the printed spiral inductors on top of each other in such a way that the flow of the current always takes the same direction as shown in **Figure 4**. In [8], a pair of printed spiral coils, as illustrated in **Figure 5**, used in biomedical implanted microelectronic devices to maximize the inductive power transmission efficiency. Zixuan et al. [6] introduced an analysis of alternative-winding coils for getting high-efficiency inductive power for mid-range WPT. Alternativewinding coils structure is demonstrated in **Figure 6**.

**Figure 4.** *Multi-layer stacked inductor; (a) top view (b) 3D geometry [10].*

*WPT, Recent Techniques for Improving System Efficiency DOI: http://dx.doi.org/10.5772/intechopen.96003*

**Figure 5.** *Design of a pair of printed spiral coils [8].*

*Wireless Power Transfer – Recent Development, Applications and New Perspectives*

*<sup>d</sup> emf dt*

WPT system performance can be estimated by the power transfer efficiency (PTE) which depends on the KQ product. K is the coupling coeffect between transmitter and receiver, it is a ratio and varies from 0 to 1 as a maximum value at totally power coupling. Q is the unloaded quality factor of the transmitter's or receiver's coil; Q can be calculated from the coil inductance as in (3), where

angular frequency, L is the coil inductance, and R the loss resistance of the coil. While PTE is calculated from (4) [2]. It is clear that increasing the transfer effi-

> *<sup>L</sup> <sup>Q</sup> <sup>R</sup>* ω

> > <sup>2</sup> <sup>2</sup>

*t r*

*t r*

Numerous studies were introduced in the inductive coupling approach [3–9]. In [10], a multi-layer spiral inductor is proposed for biomedical applications at a frequency of 13.56 MHz which is the license-free industrial, scientific, and medical (ISM) band. It uses a stacked structure to achieve a compact WPT, where the stacked inductors occupying an area of 10 mm × 10 mm with 1 cm separation between transmitter and receiver. The inductance is further increased by stacking the printed spiral inductors on top of each other in such a way that the flow of the current always takes the same direction as shown in **Figure 4**. In [8], a pair of printed spiral coils, as illustrated in **Figure 5**, used in biomedical implanted microelectronic devices to maximize the inductive power transmission efficiency. Zixuan et al. [6] introduced an analysis of alternative-winding coils for getting high-efficiency inductive power for mid-range WPT. Alternative-

*kQQ* = × + +

2

1 1

*kQQ PTE*

winding coils structure is demonstrated in **Figure 6**.

*Multi-layer stacked inductor; (a) top view (b) 3D geometry [10].*

ciency needs a high value of the KQ product.

φ

= − (2)

= (3)

100%

ωis the

(4)

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**Figure 4.**

**Figure 6.** *Alternative-winding coils geometry and its current distribution [6].*
