Analysis and design of an integrated RF energy harvester for ultra low‐power environments

Abstract

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] o C 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 W (−18 dBm). The minimum input power leading to a positive energy balance is −30 dBm with an output voltage of 1.1 V. available in both indoor and outdoor environments, and it does not require movements or frictions (like electro-mechanic and piezoelectric harvesters) that may shorten the device lifetime. On the other hand, the RF energy can be classified as ambient-dependent, noncontrollable, and nonpredictable. 3 Therefore, the prior study of the RF power distribution in the environment is the basis for a fruitful design of an RF harvester circuit. Metrics like Sensitivity (S IN ) of the front-end and Power Conversion Efficiency ( TOT ) are critical and must be optimized to exploit the RF source.It has been shown that in the underground stations of a densely populated and strongly developed metropolis such as London or Boston, the RF field can be a competitive harvesting source in terms of recovered DC power, compared with other scavenging techniques, such as thermal human or vibration. 2,4 However, little information is available on RF energy availability in many other environments. In this paper, we report long-term measurements performed by means of a commercial multi-band dipole array antenna in three different environments (here defined as urban, semi-urban, and rural) of a limited area in Northern Italy. The aim of this measurement campaign is to verify whether the ambient RF power, in these and similar locations, can be an effective source for an RF energy harvester.Many harvesting systems, comprising an antenna, an RF AC-DC converter, a DC-DC converter, and a charge accumulator have already been presented in literature, eg, previous studies 5-7 Usually, a large buffer capacitor (C H ) is placed between the AC-DC and DC-DC converters to gene...

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