Wireless charging, also known as wireless power transfer, is the technology that enables a power source to transmit electromagnetic energy to an electrical load across an air gap, without interconnecting cords.


                                       The next question arising in our mind is why are we in need of this tech. Since we are bored with tangling wires all around us and worried about the wear and tear of the wire, we needed a new technology that will get rid of this wires, then came this tech. This technology is attracting a wide range of applications, from low-power toothbrush to high-power electric vehicles because of its convenience and better user experience. Nowadays, wireless charging is rapidly evolving from theories toward standard features on commercial products, especially mobile phones and portable smart devices. Lets discuss in detail in the article below.

                                       The development of wireless charging technologies is advancing toward two major directions, i.e., radiative wireless charging (or radio frequency (RF) based wireless charging) and non-radiative wireless charging (or coupling-based wireless charging). Radiative wireless charging adopts electromagnetic waves, typically RF waves or microwaves, as a medium to deliver energy in a form of radiation. The energy is transferred based on the electric field of an electromagnetic wave, which is radiative. Due to the safety issues raised by RF exposure, radiative wireless charging usually operates in a low power region. For example, omni-directional RF radiation is only suitable for sensor node applications with up to 10mW power consumption. Alternatively, nonradiative wireless charging is based on the coupling of the magnetic-field between two coils within the distance of the coils’ dimension for energy transmission. As the magneticfield of an electromagnetic wave attenuates much faster than the electric field, the power transfer distance is largely limited.

As soon as H. C. Oersted discovered that electric current generates a magnetic field around it the advent of wireless technology evolved. Then Ampere’s Law, Biot-Savart’s Law and Faraday’s Law were derived to model some basic property of magnetic field. They are followed by the Maxwell’s equations introduced in 1864 to characterize how electric and magnetic fields are generated and altered by each other. Later, in 1873, the publication of J. C. Maxwell’s book A Treatise on Electricity and Magnetism unified the study of electricity and magnetism. Since then, electricity and magnetism are known to be regulated by the same force. These historic progress established the modern theoretic foundation of electromagnetism.

                              In 1888, H. R. Herts used oscillator connected with induction coils to transmit electricity over a tiny gap. This first confirmed the existence of electromagnetic radiation experimentally. Nikola Tesla, the founder of alternating current electricity, was the first to conduct experiments of wireless power transfer based on microwave technology. He focused on long-distance wireless power transfer [16] and realized the transfer of microwave signals over a distance about 48 kilometers in 1896.

                                 Another major breakthrough was achieved in 1899 to transmit 108 volts of high-frequency electric power over a distance of 25 miles to light 200 bulbs and run an electric motor [16]. However, the technology that Tesla applied had to be shelved because emitting such high voltages in electric arcs would cause disastrous effect to humans and electrical equipment in the vicinity [17]. Around the same period, 
                                 Tesla also made a great contribution to promote the magnetic-field advance by introducing the famous “Tesla coil”, illustrated in Figure 3a. In 1901, Tesla constructed the Wardenclyffe Tower, shown in Figure 3b to transfer electrical energy without cords through the Ionosphere. However, due to technology limitation (e.g., low system efficiency due to large-scale electric field), the idea has not been widely further developed and commercialized. Later, during 1920s and 1930s, magnetrons were invented to convert electricity into microwaves, which enable wireless power transfer over long distance. However, there was no method to convert microwaves back to electricity. Therefore, the development of wireless charging was abandoned. 

Most used tech are
  • Inductive Coupling
  • Magnetic Resonance Coupling


                                       Inductive coupling is based on magnetic field induction that delivers electrical energy between two coils. Inductive power transfer (IPT) happens when a primary coil of an energy transmitter generates a magnetic field across the secondary coil due to the current flow in the circular primary coil which the secondary coil acts as a energy receiver within the field.  The near-field magnetic power then induces voltage/current across the secondary coil of the energy receiver within the field. This voltage can be used for charging a wireless device or storage system. The operating frequency of inductive coupling is typically in the kilo Hertz range. The secondary coil should be tuned at the operating frequency to enhance charging efficiency.  Due to lack of the compensation of high quality factors, the effective charging distance is generally within 20cm.

                                      The advantages of magnetic inductive coupling include ease of implementation, convenient operation, high efficiency in close distance (typically less than a coil diameter) and ensured safety. Therefore, it is applicable and popular for mobile devices.


                                         Magnetic resonance coupling, as shown above figure, is based on evanescent wave coupling which generates and transfers electrical energy between two resonant coils through varying or oscillating magnetic fields.  High energy transfer efficiency is achieved when both the frequencies of primary and secondary coil must be the same as resonance to occur thereby they are strongly coupled. In practical case it is observed in above condition with the maximum power transfer efficiency of 92.6% over the distance of 0.3cm.   Due to the property of resonance, magnetic resonance coupling this will not affect nearby wireless devices.   Additionally, magnetic resonance coupling can be applied between one transmitting resonator and many receiving resonators. Therefore, it enables concurrent charging of multiple devices. It usually operate in megahertz frequency range hence it can charge a device with longer distances with high efficiency.


                                 The above figure shows the schematic block diagram of a wireless battery charger. On the left, the power transmitter which is connected to the electrical grid, on the right the power receiver which is integrated into the load device is shown. In both the power transmitter and the power receiver, the key element for signal transfer is represented by a resonant tank, comprising both coupled inductors. On the transmitter side, there is the primary coil and on the receiver side, there is the secondary coil. A comparative study of different resonant topologies is proposed in.

                                  As suggested by the above figure, the signal flow does not only consist of the power signal from the power transmitter to the power receiver, but also of communication data streaming in the opposite direction. Since the power transmitter needs to be continuously informed about battery power needs and state of charge, a communication link is required. The communications channel is implemented through an amplitude modulation of the power drawn from the transmitter. In the power transmitter section, an AC-DC stage converts the AC voltage provided by the electrical grid into a DC bus level. A DC-AC converter, supplied by the DC bus level, generates the AC power signal. In the power receiver section, a rectifier converts the AC power signal out of the resonant tank into a DC voltage level, suitable for battery charging. The DC bus rail out of the rectifier has been chosen equal to a 7V value. Information towards the power transmitter is generated through a power modulation of the coupling circuit resonant curve.
                                  The amount of transmitted power is controlled by varying the frequency and duty-cycle of the half-bridge stage. A frequency range of 110k 205kHz and a duty-cycle range of 10% 50% are fixed by the PC standard. The modulation network consists of two parts: one is connected to the AC-side of the rectifier; the other is connected to the DC-side. The load device is modeled through a current generator.


                                      The line voltage is rectified by the full-bridge rectifier, generating a semi-sinusoidal voltage at double the line frequency. The frequency of oscillation then depends mainly upon the size and maximum flux density of the ferrite core used in the feedback transformer, and the storage time of the transistors. When the cycle has started, the current in the feedback transformer increases until the core saturates. At this point the feedback drive of the active transistors is therefore removed, and, once its storage time has passed, it turns off. In this application the oscillation frequency would be  25 to 40 kHz. The dependence upon the storage time is minimized by the RC network at the base of the transistor, which increases the rate of charge extraction from the base at turn-off. The network also serves to decouple the base from the oscillation caused by the base transformer at turn-off, preventing spurious turn-on of the device.

                                         Faraday's law of induction is a basic law of electromagnetism that predicts how a magnetic field will interact with an electric circuit to produce an electromotive force (EMF). It is the fundamental operating principle of transformers, inductors, and many types of electrical motors, generators and solenoids. The production of emfs and currents by the changing magnetic field through a conducting loop is called induction.

The other laws involved are Faraday's laws of Magnetic flux, Mutual and self inductance.


  • It improves user-friendliness as the hassle from connecting cables is removed. Different brands and different models of devices can also use the same charger. 
  •  It renders the design and fabrication of much smaller devices without the attachment of batteries. 
  • It provides better product durability (e.g., waterproof and dustproof) for contact-free devices
  • It enhances flexibility, especially for the devices for which replacing their batteries or connecting cables for charging is costly, hazardous, or infeasible (e.g., bodyimplanted sensors).
  • Wireless charging can provide power requested by charging devices in an on-demand fashion and thus are more flexible and energy-efficient. 

Next Post »