Wireless power transfer uses electromagnetic fields to safely transfer power from a transmitting source to a receiving device to charge or recharge a battery. As the name suggests, it does so without needing a physical connection.
Inductive power transfer
All wireless charging is based on the principle of transferring an electrical current between two objects through coils that induce an electromagnetic field. There are two basic types: tightly-coupled and loosely-coupled.
> Read more: An introduction to wireless charging
The process for the type of inductive power transfer known as tightly-coupled, looks like this:
- Mains voltage is converted into high frequency alternating current (AC), which is sent to the transmitter coil where it induces a time varying magnetic field
- AC flowing within the transmitter coil induces a magnetic field which extends to the receiver coil when within a specified distance
- The magnetic field generates current within the receiver coil of the device
- Current flowing within the receiver coil is converted into direct current (DC) by the receiver circuit, which can then be used to charge the battery
The best example of a tightly-coupled inductive transfer is that of an electric toothbrush, which charges only when placed in a specific position. So although the technology could be used to charge smart devices efficiently, the need for a precise alignment of the coils is a usability problem to overcome.
Qi developed by the Wireless Power Consortium is the leading open interface standard for tightly-coupled inductive charging over distances of up to 4cm, although a 2cm distance is more typical.
The basic premise of resonant frequency charging - or loosely coupled - is that the energy tunnels from one coil to the other instead of spreading out from the primary coil in an omnidirectional manner.
The key is finding the frequency at which an object naturally vibrates or rings, such as how a tuning fork can cause another to vibrate when both are tuned to the same pitch. Another good analogy is that of an opera singer who is able to shatter a wine glass. This happens when the singer finds the perfect tone that is the exact resonant frequency of the wine glass diameter. The power transfers, causing the vibration that shatters the wine glass.
This method gives several performance and convenience benefits over tightly-coupled charging, including:
- Greater spatial freedom by not having to precisely align your smartphone or device on top of the transmitter for it to begin charging
- Charging multiple devices simultaneously: perfect for a use-case in a busy café, shopping mall or airport
- Charging speeds: Resonant promises improved efficiency for power transfer between the transmitter and receiver coils
The Airfuel Alliance of which Nordic Semiconductor is part is the leading promoter of resonant charging.
> Read more: How resonant charging can power smart cities
Within the world of resonance charging, two different flavors exist: loosely-coupled and uncoupled.
The loosely-coupled method can transmit power through any non-metallic object that exists between the coils such as wood, plastic or granite. This makes a table or other work surface a good location for a coil, albeit with losses. Uncoupled technology delivers power via radio bands, similar to a Wi-Fi router. Both methods are being developed by Airfuel members.
The standards compared
Tightly-coupled inductive systems have a higher efficiency and therefore use less power and produce less waste heat. This could be an advantage for smart devices such as smartphones where heat needs to be kept low. The disadvantage is the usability factor of having to place the device in a specific position and the limitation of charging just one device at a time.
Loosely-coupled resonant systems allow charging over larger distances and for multiple devices at once, but the charging efficiency is lower. It may be the better choice in applications where inductive charging is impractical or undesirable.
Whilst there are some efficiency gains for pure inductive over resonant, approx. 30%, the ease of use factor almost certainly outweighs this. The power mat or surface supplying the power does of course needs a mains energy supply so power here is not so much of a problem.
Additionally, the power we use to charge our battery-driven mobile devices is a very small fraction of a typical home’s energy usage. To put this in perspective, a typical Android or iOS phone uses approx. 1.5kWh per year, the same amount of energy that a 100W incandescent bulb would use in just 15 hours.
With this in mind, we can live with an efficiency loss. It’s the ease of use and ‘drop-and-charge’ simplicity we crave. This is only truly offered by loosely-coupled inductive resonant based solutions.