Integrated power management circuit for piezoelectronic generator in wireless monitoring system of orthopaedic implants

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Abstract
Piezoelectric (PZT) materials are capable of converting the mechanical energy of compression into electrical energy. With the recent advent of extremely low-power electrical devices, PZT generators have become attractive in many kinds of applications, especially for biomedical applications. Piezoelectronic generators are used in a wireless monitoring system of orthopaedic implants. Due to their poor source characteristics, the efficiency of PZT generator is low. A hybrid direct current (DC)–DC, comprising a switched capacitor (SC) DC–DC converter and a low dropout (LDO) linear voltage regulator, is presented to improve conversion efficiency. A bandgap reference (BGR) circuit which works in sub-threshold region is also presented. Because SC DC–DC converter works in the highest voltage region in this system, small power supply current, including supply current through BGR and other auxiliary modules, means low power consumption. BGR's power supply voltage can be varied from 3 to 16 V. Its supply current is only 3.2 µA at 125 °C and its temperature coefficient is 46 ppm. Stacked switches technique is proposed to reduce leakage current in switching process of SC converter. Simulation results show that the efficiency of SC's converter can reach 88%, that of LDO can reach 80% and that of the overall system can reach 66%, including power consumption of all auxiliary components, which is far higher than previous work.

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