Wireless power pad with local power activation for portable devices

Wireless power transfer by magnetic induction offers a simple to use way to recharge mobile devices like e.g. mobile phone, music players or medical sensors. As shown by a previous report and an existing Power Pad demonstrator, wireless inductive power transfer is possible with a good power efficiency and low magnetic radiation only on a surface and with local activation underneath the mobile device. However, compared to the existing Power Pad, an improved detection method for the local activation is needed. This report investigates the use of RF-ID tags for the position detection. It addresses the problem to multiplex the RF-ID signal to a number of neighboured power cells to locate the device and the simultaneous transfer of power and information. To investigate the solution, a demonstrator is designed and built. This first circuit can activate 4 neighboured and overlapping cells. The circuit consists of a NFC/RF-ID PN511controller by NXP, an analogue multiplexer to direct the RF-ID signal to the cells and FETs as switches to switch the power signal to each cell. The system operates at 500 kHz for the power transmission and at 13.56 MHz for the RF ID data transmission. The circuit is controlled by an external computer using a dedicated software developed with LabView. Conclusions: The results show that RF/ID is suitable for the position detection and local activation of an inductive power pad. The designed circuit can detect the position of a mobile device and switch on the power of the related cell. The operation with simultaneous power and data transmission needs a dedicated filtering effort. Less filtering is necessary, if data and power transmission are done sequentially. Based on the results of this work, an extended version to support up to 52 cells for a complete pad is in preparation.