Inductive Power Transmission Shielding

- shielding necessary to prevent inductive power transmission and wireless battery charging from causing interference to other radio users.

Using wireless battery charging and power transfer techniques, significant amounts of power are transferred across what is effectively an open interface. Power could be radiated and cause interference. It could also cause interference to the equipments being used.

To prevent interference inductive power transmission shielding or wireless power transmission shielding measures must be incorporated into the system.

Inductive power transmission shielding will ensure that the power is transferred where it is required, without causing undue interference to any item of electronic equipment in the vicinity which is a prime requirement.

Inductive power transmission shielding basics

Inductive power transmission shielding can take a variety of forms. Typically it is located around the transmitter or primary coil to prevent power be transmitted in a region where it is not required. It is also located above the receiver or secondary, i.e. the side facing away from the transmitter coil to prevent any of the power entering the item having power transferred to it. This will prevent interference that could affect its operation.

Stray magnetic flux for wireless power transmission may have a number of adverse effects:

  • The primary reason is that the magnetic field used for wireless power transmission and battery charging may interfere with the device being powered or charged, or other devices.
  • The magnetic field may cause battery heating. This can reduce battery life as many types of batteries, especially types like nickel metal hydride and Lithium ion batteries do not like operating at very high temperatures
  • The stray magnetic field may cause eddy currents in metallic parts on the devices or elsewhere. This may give rise to local heating or other unwanted effects.

Shielding methods

There are two main ways in which shielding can be provided for wireless power transmission systems.

  • Magnetic flux diversion:   In most cases the external medium will be free space that will have its permeability of µ equally to µ0. If the shield is constructed using material with a permeability much greater than µ0, then it will concentrate the magnetic flux in the low reluctance path. This can then be used to contain the magnetic flux in the areas where it is required.

    To achieve its aim, the high permeability material must be thick enough otherwise the reluctance of the path may be high and not work. It must also be positioned correctly to "capture" the magnetic flux and divert it out of the way.

    To provide the magnetic shielding or screening either ferrite with its very high level of permeability of a mu-metal may be used.

    A mu-metal is a nickel-iron alloy that also contains small percentages of copper, chromium, and molybdenum, and it provides a very high level of permeability - typically of the order of 80 000 to 100 000. This makes it ideal for concentrating the magnetic flux and allowing it to take a low reluctance path away from areas where it could otherwise give rise to various forms of interference.
  • Generation of an opposing flux:   Using faraday's law it is possible to generate and equal and opposite flux. This will remove the effects of the magnetic flux where this can be achieved, and thereby provide inductive power transmission screening.

The use of inductive power transmission screening is needed to ensure that the power transmission does not affect the performance of any item of electronic equipment, either that associated with power charging, or elsewhere.

By Ian Poole

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