Antenna and rectifier designs for miniaturized radio frequency energy scavenging systems
Abstract
With ample radio transmitters scattered throughout urban landscape, RF
energy scavenging emerges as a promising approach to extract energy from
propagating radio waves in the ambient environment to continuously charge low
power electronics. With the ability of generating power from RF energy, the need for
batteries could be eliminated. The effective distance of a RF energy scavenging
system is highly dependent on its conversion efficiency. This results in significant
limitations on the mobility and space requirement of conventional RF energy
scavenging systems as they operate only in presence of physically large antennas and
conversion circuits to achieve acceptable efficiency. This thesis presents a number of
novel design strategies in the antenna and rectifier designs for miniaturized RF energy
scavenging system.
In the first stage, different energy scavenging systems including solar energy
scavenging system, thermoelectric energy scavenging system, wind energy
scavenging system, kinetic energy scavenging system, radio frequency energy
scavenging system and hybrid energy scavenging system are investigated with
regard to their principle and performance. Compared with the other systems, RF
energy scavenging system has its advantages on system size and power density with
relatively stable energy source. For a typical RF energy scavenging system, antenna
and rectifier (AC-DC convertor) are the two essential components to extract RF
energy and convert to usable electricity.
As the antenna occupies most of the area in the RF energy scavenging system,
reduction in antenna size is necessary in order to design a miniaturized system.
Several antennas with different characteristics are proposed in the second stage.
Firstly, ultra-wideband microstrip antennas printed on a thin substrate with a
thickness of 0.2 mm are designed for both half-wave and full-wave wideband RF
energy scavenging. Ambient RF power is distributed over a wide range of frequency
bands. A wideband RF energy scavenging system can extract power from different
frequencies to maximize the input power, hence, generating sufficient output power
for charging devices. Wideband operation with 4 GHz bandwidth is obtained by the
proposed microstrip antenna. Secondly, multi-band planar inverted-F antennas with
low profile are proposed for frequency bands of GSM 900, DCS 1800 and Wi-Fi 2.4
GHz, which are the three most promising frequency bands for RF energy scavenging.
Compared with previous designs, the triple band antenna has smaller dimensions
with higher antenna gain. Thirdly, a novel miniature inverted-F antenna without
empty space covering Wi-Fi 2.4 GHz frequency band is presented dedicated for
indoor RF energy scavenging. The antenna has dimensions of only 10 × 5 × 3.5
mm3 with appreciable efficiency across the operating frequency range.
In the final stage, a passive CMOS charge pump rectifier in 0.35 μm CMOS
technology is proposed for AC to DC conversion. Bootstrapping capacitors are
employed to reduce the effective threshold voltage drop of the selected MOS
transistors. Transistor sizes are optimized to be 200/0.5 μm. The proposed rectifier
achieves improvements in both power conversion efficiency and voltage conversion
efficiency compared with conventional designs.
The design strategies proposed in this thesis contribute towards the realization of
miniaturized RF energy scavenging systems.