What are Wireless Charging Coils?
Wireless charging coils are the coils used to wirelessly transmit and receiver power. These coils are made of copper. There are two coils required by a wireless charging system - a transmitter coil in the charging pad and a receiver coil in the device that needs to be charged. Coils which can perform both transmission and reception are also available.
An alternating current passed through the transmitter coil creates an oscillating magnetic field which fluctuates in strength as the electric current’s amplitude fluctuates. When a receiver coil is placed near the transmitter coil, the changing magnetic field induces an alternating electric current in it. This induced current is passed through a rectifier to convert it to direct current that can charge the device’s battery. Capacitors are added to both coils so that they resonate at the same frequency i.e. the frequency of the alternating current. Doing this increases the amount of induced current in the receiver coil thus improves the efficiency of power transmission.
These coils are generally used for low power applications usually from 5 to 15 W for consumer electronics such as smartphones, handheld devices. For higher power applications like electric vehicle charging etc. which requires power over 1 kilowatt, bigger and more coils are used. Coil size also affects the distance of power transfer. The bigger the coil, or the more coils there are, the greater the distance a charge can travel. In the case of small handheld devices like smartphones, smart home devices, wireless charging pads etc. the copper coils are only a few inches in diameter, severely limiting the distance over which power can travel efficiently. As higher power applications use bigger and more coils, power can be transmitted at distances of up to a couple of meters.
Qi is the wireless charging standard maintained by the Wireless Power Consortium (WPC) for powering devices in the range of 5-65 W. It states specifications like power regulation with a digital control loop and coil dimensions of 5mm to 40 mm. Qi specification is the most popular standard in the world, with more than 200 million devices supporting this interface. For higher power applications Magne charge, SAE J2954, AirFuel Alliance etc. are some examples of standards currently being used.
