Wireless power transfer has a broad range of applications ranging from mobile phone chargers to biomedical implants. For cochlear implants [1] and retinal prostheses [2], having a miniaturized form factor and being battery-less are highly desirable. Such devices require real-time power transfer in the range of 10 to 100mW [3], and as human tissue specific absorption rate (SAR) increases with frequency, inductively-coupled power links that operate at 13.56MHz or lower in ISM bands are commonly used, as shown in Fig. 4.2.1. However, lower transmission frequency means larger matching and filtering capacitors that are bulky. In addition, the received AC input amplitude VAC,Peak would fluctuate due to changes in distance and orientation between the coupling coils. Hence, comparator- controlled power switches (active diodes) are used to replace diodes so that the rectifier could work at a lower VAC,Peak and still achieve a high voltage conversion ratio (VCR) and power conversion efficiency (PCE) [4]. In this research, we present the first fully integrated 1X/2X active rectifier in the 30mW range with all capacitors fabricated on-chip, also shown in Fig. 4.2.1. This is made possible by a switching arrangement that avoids connecting the output capacitors in series in the 2X mode. Reverse current is reduced for VAC,Peak that ranges from 1.25 to 4V by a bias current that is quasi-inversely proportional to the output DC voltage, as explained later; and efficiency is carefully measured by the insertion of a sensing resistor plus an additional capacitor to reduce distortion.
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