Dries van Wageningen and Eberhard Waffenschmidt, Philips Research
The power that can be taken from a homogeneous magnetic field B is dependent on the induced voltage Uind in the used receiver coil. Considering it as a loop, for sinusoidal signal shape it results as:
Uind = 2πf.B.A
where f = frequency and A = loop area.
With the same flux density, a higher power can be transferred at higher frequencies. This means that the product of maximum magnetic flux density times the frequency is relevant for the power transmission.
An application like a wireless power space requires a magnetic field at any arbitrary position in that space. To achieve a functionality like W-LAN for power, the whole working space of a user has to be covered by the magnetic field. The whole body of the user is exposed to the magnetic field, without any time restriction. Therefore, the strict ICNIRP guidelines for public exposure have to be applied without any relaxes.
Taking the reference values of these guidelines into account, the maximum power transfer based on the given magnetic field can be calculated. Using resonance in the receiver has been taken into account. The result is shown in Figure 10. For the technically interesting frequency range between 100 kHz and 10 MHz the maximum power is independent of the operating frequency. For frequencies lower than 150 kHz, the maximum power reduces with reducing frequency. For frequencies above 10 MHz it should be possible to transmit more power, however, the assumptions to calculate the maximum power transfer may be no longer valid.
The figure shows that for a receiver size which would fit a mobile device (loop diameter 0.04 m to 0.1 m) the power transmission must stay below 2 mW to 30 mW in order not to harm any person. This is about two orders of magnitude too low for general power applications.