Comparison of various wireless charging principles

The research on wireless charging technology originated in the 1830s when Michael Faraday discovered the phenomenon of electromagnetic induction, that is, the change of magnetic flux generates induced electromotive force, thereby generating current in the wire. However, the earliest idea of ​​wireless power transmission was proposed by Nikola Tesla in the 1890s, and he was called the father of wireless power transmission by some people.

Technical principle

In terms of specific technical principles and solutions, there are currently four basic wireless charging technologies: electromagnetic induction, magnetic resonance, radio wave, and electric field coupling. These technologies are respectively suitable for short-range, medium-range, and long-range power transmission.

Various wireless charging methods have their own characteristics, and the specific comparison is shown in Table 1.

Table 1 Comparison of various wireless charging principles

The most mature and common type is electromagnetic induction. Its fundamental principle is to use the principle of electromagnetic induction, similar to a transformer, with a coil at the transmitting end and a coil at the receiving end. The primary coil is connected to an alternating current of a certain frequency, and a certain current is generated in the secondary coil due to electromagnetic induction, thereby transferring energy from the transmitting end to the receiving end, as shown in Figure 1. Powermat, the initiator of the PWC Alliance, has launched a WiCC charging card using electromagnetic induction. It is about the same size as an SD card, with components such as coils and electrodes embedded inside, and can be used by inserting it next to the battery of an existing smartphone.

The magnetic resonance type is also called the near-field resonance type. It consists of an energy transmitting device and an energy receiving device. When the two devices are adjusted to the same frequency, or resonate at a specific frequency, they can exchange energy with each other. The principle is the same as the resonance principle of sound. Coils with the same vibration frequency arranged in a magnetic field can supply power from one to another, as shown in Figure 2. The technical difficulties are miniaturization and high efficiency, and it is considered to be the most promising way to be widely used in wireless charging of electric vehicles in the future.

Radio wave type, the basic principle is similar to the crystal radio used in the early days, mainly composed of microwave transmitting device and microwave receiving device. A typical example is the microwave power transmission system of William C. Brown in the 1960s, as shown in Figure 3. The entire transmission system consists of three parts: microwave source, transmitting antenna, and receiving antenna; there is a magnetron in the microwave source, which can control the source to output a certain power in the 2.45 GHz frequency band; the transmitting antenna is an antenna array with 64 slots, and the receiving antenna has a collection and conversion efficiency of 25%. The mobile wireless charging system of Ryukoku University in Japan also transmits power through microwaves with a frequency of 2.45GHz, lighting up the lights of a model police car in motion.

The electric field coupling type uses the induced electric field generated by two sets of asymmetric dipoles coupled in the vertical direction to transmit electric energy. Its basic principle is to transfer electric energy from the transmitting end to the receiving end through the electric field. This method is mainly used by Murata Manufacturing and has the characteristics of strong resistance to horizontal misalignment.