Lumped Impedance Transformers for Compact and Robust Coupled Magnetic Resonance Systems

An innovative coupled magnetic resonance system (CMRS), introducing two lumped impedance transformers, is proposed. There are three major magnetic couplings between coils in CMRS: source-transmitter (Tx), Tx-receiver (Rx), and Rx-load couplings. Except for Tx-Rx coupling, other couplings do not directly contribute to wireless power transfer. Hence, in this paper, this miscellaneous coupling is replaced with a lumped transformer with ferrite core. Because there is only a Tx-Rx coupling, the CMRS becomes compact in size and robust to ambient changes. Moreover, the design of CMRS is drastically simplified without complicated multiresonance tunings due to little magnetic flux linkage from the source coil or load coil. Coreless coils are used for Tx and Rx coils to examine the characteristics of CMRS with lumped transformers. A detailed static analysis on the explicit circuit model of the proposed CMRS and design procedures are fully established. Experiments for 1- and 10-W prototype CMRSs with a class-E inverter at the switching frequency of 500 kHz, where the quality factors are less than 100, verified the usefulness of the proposed model, achieving 80% of the maximum Tx coil-to-load efficiency. It is concluded in this paper that the conventional CMRS, in general, is just a special form of an inductive power transfer system where the quality factor is extremely high.