Generalized maximum efficiency theory on multi-stage inductive coupling

Inductive coupled coils have been implemented to transmit power wirelessly for many applications such as micro-systems, RFID, and many biomedical implants. All these appli- cations benefit from the distinct advantage to power these devices without any physical connec- tions comparing to conventional energy transmission method using wires and connectors. Despite their different modes, geometries, coupling distances and power consumption requirements, they share the same rules which govern the design of the coils in order to achieve better efficiency of power transfer. The efficiency is even more critical in biomedical implants since any power loss due to the inefficient power link will finally be transformed into heat accumulated in the body which leads to safety problem. A novel feature on maximizing the efficiency is derived in this paper and it is further generalized to multi-stages inductive links which may be of special interest to be used in a high-density 1024-electrode epi-retinal prosthesis. Although multi-stage coupling include more coils with power loss internally, it is still possible to attain higher overall efficiency by improving each coupling stage individually whenever wireless connection is required. This is especially in the case of weakly-coupled coils where the air gap between transmitter and receiver is too large. Such multi-stages coupling configuration is also suitable for surgery in epi-retinal prosthesis.