Design of radio frequency energy harvesting system : for use in implantable sensors

Implantable biomedical wireless sensors provide monitoring of vital health signs such as oxygen, temperature and intraocular pressure and may help to analyse and detect diseases in humans and animals. However, one of the design challenges of implantable devices is providing a safe and reliable energy source. Replaceable batteries are one of the most common methods for powering up implantable devices and have been used in e.g.cardiac pacemakers for decades. However, the need for a regular battery replacement may require surgical incisions. Multiple studies have been done on energy harvesting from ambient energy sources to provide the required power for the operation of the implantable sensor and thus reducing the need for battery replacement. In this work, a circuit-level radio frequency (RF) energy harvesting system has been designed and simulated in 65 nm CMOS process technology. The system consists of an AC-DC converter, a DC-DC converter, a Ring oscillator, a Buffer, and a Voltage sensor with comparators, dividers and a reference generator. The rectifier operates at a frequency of 900 MHz and offers a power conversion efficiency (PCE) of 71%. The doubler works at 50 MHz with a voltage conversion efficiency (VCE) of 98%. Additionally, the Voltage sensor monitors the voltage level of the energy-storing unit, that in this project is intended to be an mm-size rechargeable battery. If the voltage level is equal to or higher than a threshold value, Vref, the harvesting system will be in discharging mode. Similarly, if the voltage level is below Vref, then the system will be in charging mode.