Achim Hilgers, Dries van Wageningen and Eberhard Waffenschmidt, Philips Research
The considerations in Maximum power transfer into space - limited by ICNIRP public exposure are based upon the reference values of the magnetic field. However, they relate to the case that a complete body is exposed. In the case of a proximity power, which is intended by the wireless power consortium, only parts of the body, especially extremities are exposed. Therefore, the reference level gives only a rough indication and the compliance to the limits have to be checked against the basic restrictions.
Contrary to inductive power transmission into a space, at wireless proximity power transmission the magnetic field is concentrated in the small volume between transmitter and receiver, which is not accessible by the user. The user will be exposed to leakage fields only. Furthermore, shielding on top of the receiver and on bottom of the transmitter can be applied, which further reduces magnetic field emissions. Figure 11 gives an impression: it shows a mobile device with receiver coil and ferrite shielding on top, which is put on a transmitter with transmitter coil (not visible) with a ferrite layer underneath.
Because of these reasons, the usable magnetic field strength between transmitter and receiver can be orders of magnitude higher than in the case of inductive power into space. A power transmission of 5 W to mobile device is shown to be possible without violating ICNIRP basic restrictions (reference to related FEM-text).