**1. Introduction**

With the spreading of mobile phones, portable and wearable electronic devices and changes in the human lifestyle, the need for WPT technology grows to get rid of the inconvenience due to using power cables. On the other hand, there are some applications where WPT probably the only solution or the most efficient solution for their powering for instance implanted biomedical devices, buried sensors, some sensors found in a severe environment such as very high temperatures, and so forth. One of the first trials for WPT was performed by Nikola Tesla a century ago. He wanted to develop a wireless power distribution system. **Figure 1** illustrates a simplified diagram of a WPT system which simply consists of a transmitter that sends the transmitted power through an RF coil or RF resonator. On the receiver

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 stage for WPT systems.

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

**125**

**Figure 3.**

*WPT using inductive coupling scheme.*

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

**2. Inductive coupling WPT**

closed path with a length of *l* .

varying of the magnetic flux (

cardiac pacemakers and insulin pumps [1].

shows the current and potential applications for WPT systems.

φ

microwaves. Through this chapter, we will cover these technologies with highlights on the recent techniques for improving the power transfer efficiency such as using intermediate resonators, applying metasurface structures, and so on. **Figure 2**

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

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 time-

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

∮*H dl I* . = (1)

) as characterized by Faraday's law as in (2). It is

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

microwaves. Through this chapter, we will cover these technologies with highlights on the recent techniques for improving the power transfer efficiency such as using intermediate resonators, applying metasurface structures, and so on. **Figure 2** shows the current and potential applications for WPT systems.
