Analysis and Design of an Integrated RF Energy Harvester for Ultra Low-Power Environments

Radio-frequency (RF) energy harvesting must cope with the limited availability and high variability of the energy source. In this paper, the available RF power in three typical environments (urban, semi-urban, and rural) is investigated. Measurements show that in the surveyed urban and semi-urban environments, an average input power above −22 and −29 dBm, respectively, is available in the [700, 1,000] MHz band. A mathematical model of the interface between the RF rectifier and the DC-DC converter is provided. The analysis demonstrates that the energy can be efficiently transferred to the external accumulator coupling the rectifier with a strobed, input control DC-DC converter. Based on the measurements and the analysis, an RF harvester architecture has been designed in 65 nm Complementary Metal-Oxide Semiconductor (CMOS) technology to operate over the [−40, 85]oC temperature and the [1.1, 2.5] V battery voltage ranges. The input control strategy adopted for the converter allows the adaptation of the harvester to the available RF power and enables a real maximum power point tracking (MPPT). Post-layout simulation of the harvester, recharging a large capacitor, precharged at 2 V, at 950 MHz of input frequency returned a 33.4% peak efficiency with an input power of 15 μW (−18 dBm). The minimum input power leading to a positive energy balance is −30 dBm with an output voltage of 1.1 V.