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Charging without Wires

Published:6/3/2011 9:59:32 AM    View:2672
Detailed information:

Charging without Wires
Cellular phones, cameras, MP3 players and headsets can be charged without wire by adding a transmitting and receiving coil to transfer magnetic power from the base to the receiver. The Wireless Power Consortium (WPC) calls the transmitter the Base Station and the receiver on the mobile device the Power Receiver. The base station contains one or more transmitter coils to provide inductive coupling to the receiver(s). This is similar to a transformer with a primary and secondary coil. Smaller base stations can deliver up to 5 watts; larger systems for appliances can go up to 300 watts. We focus on the smaller station for mobile devices.

The base station is mostly in form of a charging mat. The power receiver consists of a sleeve that fits snug over the housing of a mobile device with connection to the auxiliary port for charging. Besides converting the magnetic power to electricity, the power receiver also provides communication to the base station. This is in form of modulating the load with an 8-bit data string to control power requirements.

The transmitter first sends a control signal to sense the presence of an object on the charging mat. The receiver reacts and requests power, to which the transmitter complies. With no object on the mat, or when the battery is fully charged, the transmitter is in standby mode. Detection occurs by noticing a change in capacitance or resonance when placing an object on the mat.

The transmitter and receiver must be tuned to achieve the most efficient transfer of power. Resonant circuits are not new and already Nikola Tesla used this technology in his experiments with inductive power more than hundred years ago. Capacitors assist in tuning the coils.

The mat achieves good coupling by matching the coils, adding shielding on the opposite side of the coil, keeping the distances small and aligning them with the receiver. Guiding the receiver into position accomplishes this, and the electric toothbrush in the cradle is a typical example. Another system uses a free moving transmit coil that mates with the receiver. A third design uses an array transmit coils. The transmitter on this method only activates the coils that are in close proximity with the receiver coil.

Inductive charging offers the ultimate in convenience. With the 2010 Qi-agreement by WPC on a global standard, many manufacturers of portable devices will include the power receiver with the devices. This frees up the auxiliary jack for other uses, an advantage that is not possible when the retrofit sleeve occupies it. Offering the receiver as part of the finished product also reduces price. The after-market sleeve is expensive and makes the mobile devices bulky.

Wireless charging is also feasible for larger appliances and the electric car. Charging on an EV occurs by driving on top of the inductive coils embedded on the floor. The power transfer is limited to 3,000 to 4,000 watts.

Charging by inductive coupling is not without drawbacks. With an efficiency factor of 70 to 80 percent, some of the lost energy turns to heat. Higher frequencies and tuned circuits promise to improve the transfer efficiency, and even allow distance. Concerns with electromagnetic field (EMF) and increased power consumption on a cell phone are negligible. The power is low and intelligent management only requests current when needed, and in the right amount.

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