Modeling and circuit design for intra-body power transfer

Abstract

The human eye contains rich information such as glucose, intraocular pressure, proteins, and antibodies, and this information can be used to diagnose various diseases. Therefore, smart lens, which is a contact lens integrated with a sensor and a sensor interface integrated circuit (IC), is emerging as a promising minimally invasive diagnostic platform. In this paper, a new wireless power transfer method for the smart lens, intra-body power transfer (IBPT) is proposed. The system using IBPT can be implemented in a smaller form factor than the system based on conventional power transfer methods, thereby making it more comfortable to be worn by the patients, and offers the advantage of high degree of freedom in designing the power transmitter. The feasibility of IBPT is verified by simulation and experiment, and new circuit structures which are well suited for the IBPT system are proposed.

An equivalent circuit model of the IBPT system is proposed and verified by some experiment results. Through the system modeling, the optimum power transmission frequency is determined as 1 MHz. At 1 MHz, the AC-DC converter of the power receiver can operate efficiently because it has a large voltage gain over a wide range of the load resistance.

A new AC-DC converter structure for the IBPT system is proposed, where a voltage doubler using threshold-voltage-compensated MOS diodes and a series-parallel charge pump are combined. Since the capacitance between the tissue and the electrode can be used as an input coupling capacitor of the voltage doubler circuit, the use of a large off-chip capacitor can be avoided. In addition, a dynamic threshold-voltage-compensated MOS diode is proposed to be used in the voltage doubler. Compared to the conventional static threshold-voltage-compensated MOS diode, the reverse leakage current is reduced significantly, and the efficiency of the AC-DC converter is therefore increased. Since the size of the dynamic threshold-voltage-compensated MOS diode is smaller than that of the static threshold-voltage-compensated MOS diode, the reduced silicon area can be used to increase the flying capacitance. As a result, the efficiency of the proposed AC-DC converter can be further improved. The proposed AC-DC converter achieves the peak power conversion efficiency of 69.3% and the peak voltage conversion ratio of 3.5 using only one on-chip flying capacitor.