CMOS Active Full Wave Rectifier for Wirelessly Powered Biomedical Implants

Biomedical prosthetic devices, which are wearable and implantable, draw power from an energy harvester followed by a rectifier circuit. The AC power is transferred wirelessly by means of an inductive coupling link to the rectifier circuit. The power transfer efficiency of such a system is very low due to skin resistance and the movements of the primary coil placed on the skin. The analog front end circuits of biomedical implants, however, require power in the mW range, and this DC power must be delivered by the rectifier circuit. Therefore, rectifiers with high power conversion efficiency are in great demand in wirelessly powered devices such as cochlear implants and brain-machine interface systems for recording and stimulation. Full wave rectifiers using diodes suffer from poor efficiency and crossover delays due to the forward voltage drop of the diodes. An active CMOS full wave rectifier addresses these issues even though the transferred power in the secondary coil is low. A CMOS active full-wave rectifier with offset-controlled high speed comparators with an operating frequency of 13.56 MHz and 300 pF capacitive filter in standard TSMC 0.18 μm CMOS technology is presented here. Two comparators, controlled NFETs and a pair of cross-coupled PFETs replace the four diodes traditionally used in the diode rectifier. By using active elements, the diode’s overdrive voltage is removed, allowing much higher voltage and power conversion ratios. The inherent switching delay of the diodes is reduced using comparators and switched-offset biasing which do not allow the reverse current to flow. The bias voltage for comparators is generated using a peaking current source, which is inversely proportional to the DC output voltage.