An introduction to wireless charging

The automotive sector could also use this technology for in-car charging of personal devices, or on a larger scale for charging electric cars. Because no connectors are required, it’s also well suited to hermetically sealed products such as portable medical devices. In fact, it’s been used since the early 1990s in electric toothbrushes, but moving forward may be used in futuristic applications such as implantable devices.

So, how does it work? Let’s consider a smartphone lying on a charging pad or base station. Both the base station and the smartphone contain coils. Putting an AC current through the primary coil (in the base station) causes an alternating magnetic field (flux), which in turn induces a current in the secondary coil (in the smartphone). This induced current is effectively energy being transferred via a magnetic field between the two coils. The output from the secondary coil can be rectified and used to charge the device’s battery, and the smartphone communicates separately with the base station to control charging levels and stop charging when the battery is full.

A block diagram for a typical wireless charging system

The efficiency of charging is dependent on the quality of the coils, as well as how much of the magnetic flux from the primary coil reaches the secondary coil.

The quality of coils is usually expressed as a figure of merit called Q. Q is proportional to operating frequency and inductance, and inversely proportional to the coil’s equivalent series resistance. A high quality coil therefore has high inductance and low equivalent series resistance.

Wireless charging coils from Abracon's AWCCA family. This series of coils offers high quality Litz windings, low DC-resistance and high Q at competitive cost.

How much of the magnetic flux from the primary coil reaches the secondary coil depends on how far apart and well aligned they are, and on their relative diameters. The coupling factor (theoretically between 0 and 1, in practice between 0.3 and 0.6) is a measure of how much of the magnetic flux reaches the secondary coil; a high coupling factor means the coils are ‘tightly coupled’ and the transfer is efficient, while a low coupling factor means they are ‘loosely coupled’ which is less efficient. In practice, some charging base stations have alignment methods such as auto-alignment with a permanent magnet or a simple marking on the surface of the charging pad to ensure good alignment, while some modify the charging currents to compensate for poor alignment.

TDK's Tx and Rx modules include an auto-alignment system using a permanent magnet. TDK also offers shieldig materials, Qi Tx and Rx coils, and Rx plus NFC combo coils

Currently there two different industry bodies promoting wireless charging in different forms.

The Alliance for Wireless Power (A4WP) and the Power Matters Alliance (PMA) joined forces in early 2014. They will both use the A4WP’s Rezence standard going forward. Rezence is based on resonant inductive wireless charging, in which both the primary and secondary coils are at resonance. The coils have a low coupling factor, but the distance between coils can be much greater than for non-resonant setups, suiting certain application types.

The Wireless Power Consortium’s Qi standard uses tight coupling between the coils. Counter-intuitively, the best results with tight coupling are achieved by operating the transmitting coil at a slightly different frequency to the receiving coil (two tightly coupled coils cannot both be in resonance at the same time). The consortium argues that this setup offers the highest amount of power at the best efficiency.

When choosing a wireless charging standard for your application, you’ll want to consider which one fits your application best by looking at the trade-off between efficiency, EMI, horizontal and vertical freedom of alignment, and whether multiple devices will be charged simultaneously from one base station.

When choosing coil hardware, you need ones that are suitable for whichever charging standard you’ve chosen. Q-factor is the key figure of merit, with a high Q value being most desirable for high efficiency. Note also that some are designed for transmitting and receiving (Tx and Rx), while some are specific to Tx or Rx, and of course, they come in different sizes. Some companies also offer coil modules with a permanent magnet at the centre of the Tx coil and a corresponding magnetic material at the centre of the Rx coil, for automatic placement systems.

As an example, Vishay’s Qi-compliant IWTX-4646BE-50 Tx coil has a Q of 185 (pictured). When combined with the company’s IWAS-4832FF-50 Rx coil with Q of 30, at 2.7mm spacing with 19V input voltage, efficiency is greater than 70%.

Vishay's Qi transmitter coil (part number IWTX-4646BE-50)

Avnet Abacus offers wireless charging coils from Abracon, TDK and Vishay as well as a wireless charging module available from Murata.

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