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Wireless charging of medical electronics

Posted: 19 Mar 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Wireless charging? electric vehicles? electro dynamic induction?

Wireless charging may soon take over plugs and wires, just as Wi-Fi and Bluetooth have modernized personal communication. Wireless charging with inductive coupling uses an electromagnetic field that transfers energy from the transmitter to the receiver, and this technology is a suitable method to charge medical devices.

Wireless transfer of power is not new. In 1831, Michael Faraday discovered induction and stated that electromagnetic forces can travel through space. In the late 1800s and early 1900s, Nicola Tesla began demonstrating wireless broadcasting and power transmission. Early experiments in Colorado Springs in 1899 lead to the Wardenclyffe Tower in New YorkTesla was adamant to prove that electrical power could be transmitted without wires, but lack of funding halted the project.

It was not until the 1920s that public broadcasting began, and Europe built massive AM transmitters with signal strengths to penetrate many countries. The transmitter at Beromnster in Switzerland could have transmitted at 600 kW, but legislation on electro-smog and protests from the local population limited the power to 180 kW. Smaller FM stations have since replaced these large national transmitters.

How does wireless charging relate to radio transmission? Both are similar in that they transmit power by electro-magnetic waves. Wireless charging operates in a near field condition in which the primary coil produces a magnetic field that's picked up by the secondary coil in close proximity. The radio transmitter works on the far-field principle by sending waves that travel through space. While the receiving coil of the wireless charger captures most of the energy generated, the receiving antenna of the radio needs only a few microvolts to raise the signal above the noise level and receive clear intelligence when amplified.

Types of wireless charging
Wireless charging is classified into three categories: radio charging, inductive charging and resonance charging. Radio charging will serve low-power devices operating within a 10-m radius from the transmitter to charge batteries in medical implants, hearing aids, watches, and entertainment devices. Radio charging can also activate advanced RFID (radio-frequency identification) chips through resonantly enhanced induction. The transmitter sends a low-power radio wave at a frequency of 915MHz (frequency for microwave ovens) and the receiver converts the signal to energy. The radio charging method is closest to a regular radio transmitter; it offers high flexibility but has low power capture and exposes people to electro-smog.

Most of today's wireless chargers use inductive charging, featuring a transmit and receive coil in close proximity. Electric toothbrushes were one of the first devices to use this charging method, and mobile phones are the largest growing sector to charge without wires. To retrofit an existing mobile phone for mobile charging, simply attach a "skin" that contains the receiver and provides interconnection to the charger socket. Many new devices will have this feature built in.

For larger batteries, such as those in electric vehicles (EVs), resonance charging, or electro dynamic induction, is being developed. Resonance charging works by making a coil ring. The oscillating magnetic field works within a 1-m radius; the distance between the transmit and receive coils must be well within the 1/4 wavelength (915MHz has a wavelength of 0.328 m). Currently, resonance charging in trials can deliver about 3000 W at a transfer efficiency of 80% to 90%.

The success of wireless charging was subject to adapting a global standard, and the WPC (Wireless Power Consortium) accomplished this in 2008. With the "Qi" norm, device manufacturers can now build charger platforms to serve a broad range of Qi-compatible devices. The first release limits the power to 5 W and works as follows.

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