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Limitation of inductive power transfer for consumer applications

Inductive power transmission is proposed more and more also for consumer applications. In this work, limitations with respect to efficiency of the whole magnetic system are investigated. The power efficiency of a given structure is dependent on resonant matching and on the load impedance. First, the matching conditions for optimal power efficiency are derived. Then the achievable efficiency for inductive transmission structures with varying distance and size ratios are investigated. Recent publications on inductive power transmission are evaluated and discussed based on these results. As a conclusion, inductive power transmission in a larger space (e.g. a whole room) is very inefficient. On the other hand, inductive power transmission at a surface can be efficient as conventional power supplies. Based on this insight, an inductive power transmission pad has been designed and built, with the purpose to charge mobile devices like mobile phones. It can charge an arbitrary number of devices and allows free positioning of the devices on the pad. It consists of an array of planar transmitter coils and has a size of 20 cm x 26 cm. It can detect the position of a receiver and activates only the coils underneath a receiver.