**3. Resonant inductive coupling WPT**

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

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

**Figure 7.** *Resonant inductive coupling WPT structure.*

field in order to get a high transmission distance or higher power transfer efficiency. **Figure 10** displays a comparison between the power transfer efficiency for using a single and double OLSR. The results show the improvement in PTE in double OLSR. The OLSRs WPT system operates at 438.5 MHz with a measured PTE of 70.8% at a transmission distance of 31 mm and a design area of 576 mm2 . While PTE for a single OLSR is 56% at 487 MHz at the same transmission distance.

A printed spiral coil with a planar interdigital capacitor is proposed in [15] as shown in **Figure 11**. It studies the misalignment issues between transmitter and

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**Figure 11.**

**Figure 9.**

**Figure 10.**

*Equivalent circuit model for OLSR [14].*

*PTE versus frequency of a single and double OLSR [14].*

*Geometry of a printed spiral coil with planar interdigital capacitor [15].*

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

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

**Figure 9.**

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

field in order to get a high transmission distance or higher power transfer efficiency. **Figure 10** displays a comparison between the power transfer efficiency for using a single and double OLSR. The results show the improvement in PTE in double OLSR. The OLSRs WPT system operates at 438.5 MHz with a measured PTE of 70.8% at

A printed spiral coil with a planar interdigital capacitor is proposed in [15] as shown in **Figure 11**. It studies the misalignment issues between transmitter and

. While PTE for a

a transmission distance of 31 mm and a design area of 576 mm2

single OLSR is 56% at 487 MHz at the same transmission distance.

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**Figure 8.**

*OLSR WPT geometry [14].*

**Figure 7.**

*Resonant inductive coupling WPT structure.*

*Equivalent circuit model for OLSR [14].*

**Figure 10.** *PTE versus frequency of a single and double OLSR [14].*

**Figure 11.** *Geometry of a printed spiral coil with planar interdigital capacitor [15].*
