Many state-of-the-art battery-powered technological devices provide for the possibility of recharging without the physical connection to a power supply, but by simple support to a special base station, which moreover does not necessarily have to be provided by the same manufacturer of the device. If appliances from different manufacturers are able to interoperate with each other, it is due to the existence of open standards, very often developed by a consortium of various brands interested in the development of that given technology.

The concept of wireless energy transfer has been known for some time, over 100 years to be precise, and dates back to the invention of the Tesla coil. A key factor in wireless energy transmission is efficiency: to be able to effectively define the system, a large portion of the energy transmitted by the generator must reach the receiving device. The two types of inductive coupling processes that can be used for near-field wireless transfer are the standard inductive coupling and the resonant inductive coupling.

Generally, the standard inductive coupling is practicable at a relatively short communication distance, since most of the magnetic flux is not connected between the coils and the magnetic fields decay quickly. The inductive resonant coupling offers greater efficiency (up to 95%) and works even at relatively long distances (several meters), given that the resonant coil significantly reduces energy losses allowing the transfer of energy from one coil to another.

Applications Wireless power transfer (WPT) can be used either to directly power the device such as LED lights or a TV and to recharge a battery such as a mobile phone by simply placing it on aboard. Communication between medical devices implanted in the human body and external equipment has long been known. An example is given by the diagnostics parameters transmitted by a peacemaker toward the outside. In this application, an inductive coupling between a small turn placed in the device case and a larger one positioned on the patient's chest allows communication. However, implanted medical devices need to be properly powered and, although the use of lithium-ion batteries allows them to operate autonomously, their replacement requires invasive operations with relative risks to the patient's health. WPT technology can remedy this problem through wireless charging systems. In recent years, the application of WPT technology to the sustainable e-mobility field has had a growing interest in research institutions, especially in Asia. Today, electric vehicles need to be linked, through a connector, to an electrical socket for recharging the batteries. Wireless power transfer allows the elimination of such connectors and enables automatic recharging (figure 1).

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Figure 1: Wireless charging for Automotive

Technology The electromagnetic field that radiates from an antenna takes on characteristics that depend on the distance from the radiating element. In particular, we can distinguish two areas: near field area and far-field area.

An example we all know is the transformer, which transfers energy from a primary coil to a secondary one without direct electrical connection, but using the magnetic inductive coupling. Transformers are made with ferrite cores and require a precise alignment between the primary and secondary side to achieve a strong coupling. Figure 2 shows the block diagram of a typical circuit implementing an inductive magnetic coupling.

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Figure 2: schematics of an inductive magnetic coupling circuit

The first stage is represented by an inverter, which converts direct current (DC) into alternating current (AC) at the appropriate frequency (typically in the range between hundreds of kilohertz and several megahertz). After that, an impedance matching network adjusts the impedance seen by the transmitting coil according to the load, so that an efficiency of about 90% can be achieved. The next stage is composed of the transmitting and receiving coils, respectively, used to generate the magnetic field and to intercept it. A second impedance matching network ensures that the load sees the appropriate impedance and, finally, a rectifier converts the alternating current to a stable DC current thanks to a voltage regulator.

The use of this technology in portable electronic devices is conditioned by the limited freedom of movement due to the need for high efficiency and by the weight of bulk magnetic materials. For the coupling to be efficient, the primary and secondary sides must be well-aligned, and also the distance between them shall not exceed lengths of the order of tens of centimeters. For these reasons, inductive coupling is often used for powering electric vehicles.

Starting from the basic principles of the inductive coupling, it is possible to increase the transmission distances through the technique of resonant magnetic coupling. The concept behind the resonant magnetic coupling is the following: a large inductive spiral excited by a radiofrequency source can exploit its resonance to induce a resonant mode in another similar structure, placed at a certain distance. This allows obtaining a transfer of power without using a radiative field, on a distance that can even be four times the size of the spiral (figure 3).

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Figure 3: a wireless power transfer system based on the resonant magnetic coupling. The system consists of 4 power stages, namely the power factor correction (PFC) converter, the RF amplifier, the coils or resonators, and the onboard rectifier.

The 50-60 Hz alternating current is rectified and converted into direct current by the rectifier block. The continuous signal then supplies the RF block, an amplifier that converts the DC voltage into radio frequency voltage used to drive the loop into the transmission. On the receiving side, the incoming resonance loop transmits the RF signal to the rectifier, which supplies the load with a suitably regulated direct current. Although not shown in the figure, these systems often include impedance matching networks for achieving an acceptable transmissive efficiency between source and load.

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Figure 4: RLC circuit for WPT technology based on resonant magnetic coupling

The systems can be represented as an RLC circuit (figure 4) in which, at the resonance frequency, the energy oscillates between the inductor L where it is stored in the magnetic field and the capacitor C where it is accumulated in the electric field. The quality with which the resonator accumulates energy is defined by the quality factor Q, which is a function of the resonance frequency w0 and of the loss factor Γ: Picture1

When two similar resonators are placed close to each other at the resonance frequency, a coupling occurs between them, enabling a transfer of energy. The following formula gives the optimal efficiency with which the power transfer takes place:

Picture2

As can be seen, it depends solely on the merit factor U which indicates the goodness of the coupling.

Compared to the magnetic inductive coupling, the resonant magnetic coupling has considerable advantages:

  • the absence of ferrite cores makes them lighter and therefore more integrable;
  • the distances between transmitter and receiver can reach up to 4 meters without the highly limiting constraint of a perfect alignment between the two loops;

The alignment of the receiving and transmission coils in the flow field and the distance between the coils determine the efficiency with which the energy is transmitted. The resonance frequency, the ratio between the dimensions of the transmission coils, and those of the receiving coils, the coupling factor, the winding impedance, and the parasitic currents of the coil are other factors that have a great impact on the transmission efficiency energy.

Qi Protocol

The Qi system is a standard for wireless power transfer. It consists of two basic modules, namely the base station and the mobile device. Its architecture of the highest level is represented in Figure 5.

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Figure 5: Qi architecture

The base station includes one or more power transmitters: each of them can provide wireless power transfer functionality to a single mobile device at a time and consists in principle of a power conversion unit and a control unit and communication. The Qi standard is already present on the consumer market, aboard a wide range of mobile devices. But even the developed world can benefit from this technology thanks to projects like the recent TIDA-00881, a Texas Instruments board designed to add to other TI low-power boards (including those of the Launchpad series) the power supply functionality wireless Qi-compliant.

Infineon offers power MOSFETs for many wireless charging standards and is an active member of the Wireless Power Consortium (WPC) and AirFuel Alliance, the two leading corporate consortiums for wireless charging technology. The AirFuel Alliance defined a standard for resonant WPT, which operates at a frequency of 6.78 MHz and allows charging of multiple devices simultaneously. In particular, BSZ0909ND is suitable for wireless charging architectures or piloting components (for example, in drones or multi-engines) where designers need to simplify the layout and significantly save space, without compromising efficiency.

Conclusion Wireless power transfer technology has been developed to support the future of electronic devices and more. From consumer electronics to automotive, the range of opportunities seems to be truly endless.

Different mobile solutions, such as mobile phones, tablets, media players, and mobile TVs, require different adapters with different interface connectors. The user must carry several connectors and adapters even if the purpose is only one: to recharge a mobile device. A universal wireless adapter, equipped with powerful support infrastructure and related ecosystem, could solve this problem.