Erez Falkenstein

Low-Power Far-Field Wireless Powering for Wireless Sensors

This paper discusses far-field wireless powering for low-power wireless sensors, with applications to sensing in environments where it is difficult or impossible to change batteries and where the exact position of the sensors might not be known. With expected radio-frequency (RF) power densities in the 20-200- μW/cm2 range, and desired small sensor overall size, low-power nondirective wireless powering is appropriate for sensors that transmit data at low duty cycles. The sensor platform is powered through an antenna which receives incident electromagnetic waves in the gigahertz frequency range, couples the energy to a rectifier circuit which charges a storage device (e.g., thin-film battery) through an efficient power management circuit, and the entire platform, including sensors and a low-power wireless transmitter, and is controlled through a low-power microcontroller. For low incident power density levels, codesign of the RF powering and the power management circuits is required for optimal performance. Results for hybrid and monolithic implementations of the power management circuitry are presented with integrated antenna rectifiers operating in the 1.96-GHz cellular and in 2.4-GHz industrial-scientific-medical (ISM) bands.

Low-Power Wireless Power Delivery

This paper addresses design and implementation of integrated rectifier-antennas (rectennas) for wireless powering at low incident power densities, from 25 to 200 μW/cm2. Source-pull nonlinear measurement of the rectifying devices is compared to harmonic-balance simulations. Optimal diode RF and dc impedances for most efficient rectification, as a function of input power, are obtained. This allows optimized antenna design, which can eliminate or simplify matching networks and improve overall efficiency. As an example of the design methodology, Schottky diodes were characterized at 1.96 GHz and an antenna is matched to the optimal complex impedance for the most efficient rectifier. For incident power density range of interest, the optimal impedance is 137 + j149 Ω, with an RF to dc conversion efficiency of the rectifying circuit alone of 63% and total rectenna efficiency of 54%.

High-Efficiency Harmonically Terminated Diode and Transistor Rectifiers

This paper presents a theoretical analysis of harmonically terminated high-efficiency power rectifiers and experimental validation on a class-C single Schottky-diode rectifier and a class- F-1 GaN transistor rectifier. The theory is based on a Fourier analysis of current and voltage waveforms, which arise across the rectifying element when different harmonic terminations are presented at its terminals. An analogy to harmonically terminated power amplifier (PA) theory is discussed. From the analysis, one can obtain an optimal value for the dc load given the RF circuit design. An upper limit on rectifier efficiency is derived for each case as a function of the device on-resistance. Measured results from fundamental frequency source-pull measurement of a Schottky diode rectifier with short-circuit terminations at the second and third harmonics are presented. A maximal device rectification efficiency of 72.8% at 2.45 GHz matches the theoretical prediction. A 2.14-GHz GaN HEMT rectifier is designed based on a class-F-1 PA. The gate of the transistor is terminated in an optimal impedance for self-synchronous rectification. Measurements of conversion efficiency and output dc voltage for varying gate RF impedance, dc load, and gate bias are shown with varying input RF power at the drain. The rectifier demonstrates an efficiency of 85% for a 10-W input RF power at the transistor drain with a dc voltage of 30 V across a 98-Ω resistor.

Scalable RF Energy Harvesting

This paper discusses harvesting of low-power density incident plane waves for electronic devices in environments where it is difficult or impossible to change batteries and where the exact locations of the energy sources are not known. As the incident power densities vary over time and space, distributed arrays of antennas with optimized power-management circuits are introduced to increase harvested power and efficiency. Scaling in array size, power, dc load, frequency, and gain is discussed through three example arrays: a dual industrial-scientific-medical band Yagi-Uda array with a low-power startup circuit; a narrowband 1.96-GHz dual-polarized patch rectenna array with a reconfigurable dc output network designed for harvesting base-station power; and a broadband dual-polarized 2-18-GHz array with multi-tone performance. The efficiency of rectification and power management is investigated for incident power densities in the 1-100-μW/cm2 range.

Custom IC for Ultralow Power RF Energy Scavenging

This letter presents a custom IC that provides an efficient interface between an ultralow power RF rectifying antenna (rectenna) power source and a microbattery for maximum power scavenging. The energy scavenger IC operates a boost converter in pulsed fixed-frequency discontinuous conduction mode to present a positive resistance to the rectenna. It uses current-starved circuitry, a nonoverlapping gate drive, and a subthreshold current source to achieve a nominal supply current in the 200-nA range for V DD = 2.5 V. Experimental results are given with the IC scavenging energy from a 1.93-GHz patch rectenna to a battery with voltages ranging from 2.5 to 4.15 V. Overall conversion efficiency including all control losses is demonstrated at over 35% at an input power of just 1.5 μW and at over 70% at input power levels over 30 μW. The IC is fabricated in a 5-V, 0.35-μm CMOS process. Although the IC was designed for RF energy scavenging, the low-power boost converter can be applied to other power sources such as wind, vibration, and temperature.

Custom IC for Ultra-low Power RF Energy Harvesting

This paper presents a custom IC that provides an efficient interface between an ultra-low power RF rectifying antenna (rectenna) source and a microbattery, with the goal of maximum power harvesting. The energy harvester IC uses current-starved circuitry, a non-overlapping gate-drive, and a sub-threshold current source to achieve a nominal supply current in the 200 nA range for VDD = 2.5 V. Experimental results are given with the IC harvesting energy from a 1.93 GHz patch rectenna to a battery with voltages ranging from 2.5 V to 4.15 V. Overall conversion efficiency including all control losses is demonstrated at over 35 % at an input power of just 1.5 ¿W and at over 70 % at input power levels over 30 ¿W. The IC is fabricated in a 5 V, 0.35 ¿m CMOS process. Although the IC was designed for RF energy harvesting, the low power boost converter can be applied to other power sources such as wind, vibrations, and temperature.

High-efficiency harmonically-terminated rectifier for wireless powering applications

In wireless powering, the rectifier efficiency has a large effect on overall system efficiency. This paper presents an approach to high-efficiency microwave rectifier design based on reduced conduction angle power amplifier theory. The analysis for an ideal rectifying device is derived to predict efficiency dependence on optimal dc load. A class-C 2.45 GHz Schottky-diode rectifier with short-circuit 2nd and 3rd harmonic terminations is designed using source-pull measurements, and demonstrates a maximum RF-DC conversion efficiency of 72.8% when matched to 50Ω. The approach is applied to integration of a rectifier with a dual-polarization patch antenna in a non 50Ω environment and free-space measurements demonstrate a lower bound on efficiency of 56% at 150 µW\cm2 power density which includes matching circuit and mismatch losses.

Far-Field RF-Powered Variable Duty Cycle Wireless Sensor Platform

This brief discusses a low-power wireless sensor based on commercial components for sensing and data transmission. The sensor is wirelessly powered from the far field through an integrated single or dual-polarization antenna, rectifier, and power management module. Since the unit is intended for mobile use, the variable available power is monitored, and the duty cycle for wireless data transmission adaptively adjusted through the use of a low-power microcontroller and a custom power management circuit. In sleep mode, the circuit consumes 1 μA at 2.5 V.

Low-power density wireless powering for battery-less sensors

This paper presents an overview of far-field wireless powering for low-power wireless sensors. With expected RF power densities in the 20 to 200μW/cm2 range, and desired small sensor overall size, low-power non-directive wireless powering is appropriate for sensors that transmit data at low duty cycles. The sensor platform is powered through an antenna which receives incident electromagnetic waves in the GHz frequency range, couples the energy to a rectifier circuit which charges a storage device (e.g. thin-film battery) through an efficient power management circuit, and the entire platform, is controlled through a low-power micro-controller. For low incident power density levels, co-design of the RF powering and the power management circuits is required for optimal performance. Results are presented with integrated antenna-rectifiers operating in the 2-GHz cellular band with over 50% efficiency for an incident power density of 75μW/cm2.

Bow-tie rectenna arrays

This paper presents experimental results on several self-complementary bow-tie antenna arrays populated with rectifier diodes. Results with two different diodes are compared, and the number of diodes, antenna polarization, DC load, DC connection (parallel/series) and incident power density are varied. The transmitter is in the far field and the incident power density ranges from 10 to 125μW/cm2 at 2.3GHz. The main conclusion of this work is that it is possible to transmit useful power wirelessly, using low and safe power density levels (tens of μW/cm2), and receive it with a scalable device.