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Realisation of stable output for contact-less energy transfer system with multiple loads

A contact-less energy transfer system (CETS) delivers electric energy to loads without direct physical connection, showing many advantages over conventional method, while its output, either voltage or current, varies simultaneously with the load. In this paper, a novel method is proposed to realise stable output voltage for systems with multiple receiving coils. As the detachable transformer has weak magnetic coupling, resonant compensation is adopted to improve the power factor and reduce the voltage and current stresses on devices. Mutual inductance model is introduced and later simplified by proposing some key parameters, that is, coupling coefficient k, self-inductance ratio between primary and secondary coil n, secondary circuit’s quality factor Qs. Primary-parallel-secondary-series compensated CETS is suitable for multi-load application as it shows stable output voltage characteristics under certain conditions, which are studied. A prototype of contact-less cell-phone charging platform with up to four loads is implemented to verify the design theory. Experimental results indicate that the charging voltage can keep stable when the load number varies, and the transfer efficiency of the separable transformer is high even though the coupling coefficient is fairly low.