Knowledge

Wireless charger working principle



With the release of the iPhone 8, the discussion of wireless charging technology is also more and more rampant. We generally think that electric and magnetic are related, so most of the questions about the principle of wireless charging are based on the electromagnetic induction phenomenon or derived from it.


At present, there are mainly four ways to achieve wireless power transmission:

Magnetic induction

To the primary coil of a certain frequency of alternating current, the electromagnetic induction in the secondary coil to produce a certain amount of current, thereby transferring energy from the transmitter to the receiver. This principle is similar to the common transformer principle in power system. When the primary side (primary coil) of the transformer is connected with alternating current, the secondary side (secondary coil) will induce electromotive force due to the electromagnetic induction principle. If the secondary circuit is connected, that is, Induction current can occur, so that you can achieve the power from the transmitter coil to the receiver coil wireless transmission. At present, the way of transferring electric energy by using this method has been widely applied in the market of low-power and short-distance wireless charging, such as small portable electronic devices such as electric toothbrushes, mobile phones and cameras, which are generally charged by a charging base. The power transmitting coil is installed in the charging base, and the receiving coil is installed in the electronic device.


Resonance

This wireless power transmission and wireless communication principle is similar to the transmitter resonance circuit of the full open-loop electromagnetic wave filled the entire space, the receiver circuit resonant in the specific frequency, in order to achieve energy transfer. This mode of transmission in the receiver output power is relatively small, you can get a higher transmission efficiency. However, the existence of electromagnetic radiation, transmission power, the greater the distance, the lower the efficiency, the more serious the radiation.


Magnetic coupling resonance

This approach can be seen as a reinforced version of the resonant, it needs to launch and receive two resonant system, respectively, made of induction coil. By adjusting the transmitting frequency, the transmitting end vibrates at a certain frequency, which produces not ordinary electromagnetic waves diffused everywhere, but a non-radiative magnetic field, that is, the electric energy is converted into a magnetic field to form an energy channel between the two coils . The natural frequency of the receiving end is the same as the transmitting end frequency, so that resonance occurs. With each resonance, there is more voltage in the receiver's sensor. After several resonances, the sensor surface will gather enough energy that the receiving end receives energy in this non-radiative magnetic field, thus completing the conversion from magnetic energy to electrical energy and realizing the wireless transmission of electric energy. One of MIT's labs demonstrated an experiment in 2008 that lit a 60-watt light bulb at two meters and called the technology Witricity, and the project is still in the lab. Of course, this non-radiative electromagnetic field is limited and not suitable for long distances, requiring the transmitter and receiver to be within 8 times the radius of the induction coil.


Radio waves

We all know that electromagnetic waves can be used to transmit information, and in theory, energy can be transmitted as long as the frequency is high enough. This wireless charging method may be the closest to what people think of wireless charging. An electromagnetic wave generator is placed at the power source, which transmits the energy to the receiving antenna through the transmitting antenna and then converts the electromagnetic wave signal back into electrical energy for use by the device. This power transmission can also have obvious weaknesses, such as electromagnetic interference by large, low transmission efficiency, radiation on the human body.