Wireless Charging

Wireless Charging consists in transmitting the electrical power without wires. Wireless power transmission is a tehnology that is most used for charging the batteries of different devices such as: phones, tablets and more. Basically there are two ways to wirelessly charge a device: inductive and capacitive.

phone wireless charging pad

Wireless energy or power transmission is useful in cases where power line cables are troublesome, dangerous o imposible to use. The problem with the power transmission is different than that of wireless telecomunication, such as radio where the useful and small power signal must be found in an ocean of electric noise. It must provide high efficiency of energy transmission at the required levels and flexibility of power receiver positioning to the trasmitter.


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Electric power transmission methods

When an electric current passes through a conductor there is an electric field in the dielectric surrounding the conductor.
Energy flow has three main components:

  • Magnetic field, concentric with the conductor.
  • Electric field perpendicular to the surface of the conductor.
  • Power gradient, parallel to the conductor.

Based on the spatial distribution of energy there are four methods of transmitting electrical power to a consumer:

  1. Capacitive coupling. Power transmission is via capacitive coupling of two parallel electrodes located in relatively flat plans.
  2. Electromagnetic induction. Power transmission is via the current induced by a magnetic field between two close coils.
  3. Magnetic resonance. Power transmission through space is done using magnetic resonance phenomenon based on the same principle as electromagnetic induction. Resonance is between two coils when the electromagnetic field around them oscillate at the same frequency. While the two coils are in resonance, power is transferred between them.
  4. Radio waves. Power transmission is via radio waves which are received by an antenna. Using microwaves generated by terrestrial transmitters which will then be taken by a device with a recovery antenna that converts microwaves directly into current.
Comparison of position displacement efficiency in electromagnetic induction and capacitive coupling

Comparison of position displacement efficiency in electromagnetic induction and capacitive coupling (DC input – DC output)

Methods of wireless charging based on components available on the market

Capacitive wireless charging

Capacitive method is an alternative approach to power transmission through magnetic field. Bringing the device into a position forming a capacitor can be used to transfer energy. This concept, which uses a quasi-static electric field to transfer energy through a capacitor formed by electrodes belonging to physically separate devices is adopted by Murata Electronics Europe – and is now widely introduced in new models.

Since there is a wide variety of batteries used in portable devices, battery interface standardization would be a big step towards easily design and will also have to satisfy one requirement, such as faster charging.

Advantages of capacitive wireless charging

One of the main advantages of using electrostatic transfer method to the inductive method is that placing the device to be charged on the load is much less critical.

High efficiency of energy transfer, 80% for any design is maintained due to the large tolerances in the XY positioning design (on the surface), while Z (height) remains a parameter solved by design.

As the main active electrode is simply a thin film of copper, a few microns thick embedded in plastic, it’s much easier to incorporate into the housing consumer device, than a power inductor device.

Since the electric field as an energy carrier in a capacitive coupling configuration has no high current flow, there is no problem of heat removal in the coupling zone.

Inductive wireless charging

Inductive method is already used in a number of consumer devices (such as electric toothbrushes) and is based on Maxwell’s theory: changes in the magnetic field from a coil induce a current in another coupled coil.

The primarily problem is to ensure high efficiency power transmission and coupling coil positioning flexibility without the need for a strict alignment of the two coils (any small difference in the coil alignment can lead to complete loss of capability to transfer power).

The main concern is controlling EMI interference generated by power transmission signal using an inductive field. WiFi, Bluetooth, NFC, cellular systems, FM radio are some devices with wireless connectivity that may suffer interference from these electromagnetic fields.

With higher power wireless chargers, it increases the heat loss level and Li-Ion batteries are extremely sensitive to temperature and unwanted thermal stress might affect electronic components that are extremely compact.

Texas Instruments offers advanced integrated circuits for transmitter and receiver, design tools and resources that help in designing devices using inductive wireless charging.

Note: Wireless Power Consortium sets the international standard for interoperable wireless charging. Interoperable products can be recognized by the logo “Qi”.

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