04-10-2016, 12:37 PM
Realization of Efficient RF Energy Harvesting Circuits Employing Different Matching Technique
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Abstract—Power management and charging of batteries for
wireless sensors become a problem when using them in the field
applications. In this paper, we present RF energy harvesting
circuit with three different approaches: resonator, number of
multiplier stages and low pass filter (LPF). Resonator provide
30 times improvement in amplitude of input (100 mV) AC signal.
In proposed circuit L type network, between input power source
and rectifier, works as resonator as well as matching network at
resonant frequency. It results in maximum efficiency 79% with
50kΩ load at -10 dBm input power. We also present the effect
of multiplier stages on output voltage and RF to DC conversion
efficiency. Optimum efficiency of approximately 80% is achieved
with Dickson topology in input power region 0 to 10dBm for
3
rd
, 5
th and 7
th stages, respectively. Application of LPF is
also introduced with an existing circuit. It provides 140 mV
improvement in output voltage with input power -10dBm. It also
shows that maximum efficiency 75% and 64% is possible with
dielectric constant (r=9) and substrate height (H=0.0004m), for
microstrip line of matching circuit at -10 dBm input power with
10 kΩ load.
INTRODUCTION
Plentiful ambient RF power has recently attracted huge
attention from the industry and research community to power
the nodes of wireless sensor network wirelessly through RF
energy harvesting. This process alleviates the problem of
battery replacement and eliminates the system dependency on
external power supply, which makes the system self powered.
Energy harvesting uses a variety of natural and man-made
sources with a little or no injurious environmental effect[1].
It is a process of extracting ambient energy available in
the environment and converting them into usable form of
energy. It can develop sufficient voltage for driving low power
electronic circuits which requires power in µW or mW.
Energy harvesting from ambient RF signals can be
accomplished by matching and rectifier circuit in which
former is used for matching the antenna impedance with
rectifier impedance for maximum power transfer and later is
used for converting microwave energy into DC. There are
some parameters that have to be considered before designing
an RF energy harvesting circuit. (a) Line of sight between
transmitting and receiving antenna for better reception of
signal (b) Distance between transmitting and receiving
antenna © Types of antenna and its operating frequency
(d) Type of matching network (e) Number of voltage multiplier stages. Performance of RF energy harvesting
circuit can be judged in different approaches, such as DC
output voltage and RF to DC conversion efficiency. These
parameters are highly affected by surrounding environment
and terrain conditions. Signal strength in particular band of
frequency mainly depends on these conditions. Its strength
varies with time, because of blocking or shadowing due to
large obstacles and by reflections also. Another factor that
tremendously influences the received power (Pr) is distance
between transmitter (TX) and receiver (RX). According to
Friis transmission equation [2]:
Pr =
PtGtGrλ
2
(4πR)
2
(1)
where Pr, Pt, Gt, Gr, R are received power, transmitted
power, gain of the transmitted antenna, gain of the receiver
antenna and distance between transmitter and receiver antenna,
respectively. The received signal strength (Pr) reduces
with the square of distance ® between transmitter and
receiver side and operating frequency.
There are two approaches to achieve high conversion
efficiency. First option is to collect the maximum power by
operating in different band simultaneously and deliver it to
the rectifying circuit, second one is to suppress the harmonics
generated by the diode. Considerable amount of research has
been done such as use of multiple antennas in place of single
antenna. In [3], [17], it is shown that by increasing the number
of antenna and multiplier stages, efficiency will increase but
it will also increase the overall circuit size.
There are two working regions for RF energy harvesting,
low power region (-30dBm to 0dBm) and high power region
(0dBm to 20dBm). It is very difficult to harvest high amount
of power with a single and high directive antenna. When
the number of antenna increases to recieve this amount of
power, the size of the circuit increases. Since the RF energy
resources are usually of low power region, so the harvesting
circuit should be designed to provide better performance in
low power region.
Under low input power scenarios, major challenge in the
RF energy harvesting process includes low conversion effi-
ciency and availability of negligible DC voltage. Reduction of
the diode turn on voltage and use of DC to DC booster circuit
at the end of rectifier circuit is also a promising approach
to pull out this problem. In [4], a threshold voltage (Vth)
distributor circuit based rectifier circuit is proposed, in which
MOSFET is used as diode to assign the correct bias voltage.
But additional parasitic capacitances of MOSFET result in
very low efficiency of 1.2% at an input RF power of 1
dBm. The above problem is solved by using CMOS based
full wave rectifier circuit in [5], but additional architecture
(asymmetrical stack architecture) makes its operation much
complicated. Besides these approaches, the threshold voltage
reduction technique uses DC booster in harvesting circuit
and provides better results in terms of enhanced DC voltage
and efficiency [6]-[7]. But requirement of high input power
(63µW to 80µW) to initiate its operation and need of high
impedance of order (≥MΩ), at the time of connection with
harvesting circuit, restrict its operation. When it is connected
as load, harvesting circuit offers very low resistance (≥6Ω)
[8]. An accumulate and use topology, with additional circuit
was proposed to address these existing problems with booster
circuit [9], but it takes more time to store sufficient charge to
turn on the operation of DC booster circuit, that makes this
approach time consuming and less efficient.
Resonator circuit is also a promising approach to strengthen
the weak RF signals. A resonator circuit comprises of discrete
inductor and capacitor. It exhibits resonant behavior for
particular frequencies of interest and the frequency at which
maximum amplification is achieved, called its resonant frequency.
In [10], a separate resonator connected with two stage
voltage multiplier circuit is described, a slight improvement
in the power conversion efficiency (60%) was observed but at
the cost of increased circuit size. In [5] and [11], 0.35µm and
0.25µm CMOS technology based rectifier circuit is proposed
which have isolated matching network, but its size is too
large. In [12] also, a separate resonator coupled with two stage
voltage multiplier circuit is described. Limitation with this
approach is its increased size and low efficiency. To moderate
these circumstances, we summarize our work with different
scheme for RF energy harvesting system:
• We propose a resonator circuit based approach to raise
the amplitude of RF signal as well as efficiency. It works
as matching network and gives resonant behaviour between
RF input power source of internal resistance 50Ω and the
rectifier circuit.
• We demonstrate the effect of number of multiplier stages
on the RF energy harvesting circuit performance, and
• We are introducing LPF in harvesting circuit to reduce the
harmonics level and to increase the output voltage.
This paper is organized as follows. In section 2, we begin with
the design specifications for RF energy harvesting circuit. In
section 3, 4 and 5, harvesting circuit with resonator, number
of multiplier stages, LPF effects are discussed. Finally, this
paper concludes with key findings and some future prospects.
II. DESIGN SPECIFICATIONS FOR RF ENERGY
HARVESTING CIRCUIT
A. Selection of Matching Circuit
One of crucial requirement of the energy harvesting circuit
is to transfer the total received power by antenna to the rectifier circuit. Due to nonlinear behavior of diode, the rectifier
impedance is going to vary with received RF power and its
frequency that affects the circuit performance. In this situation
the circuit performance can be controlled by the introduction
of matching network between rectifier and the antenna. If RF
power source is not matched then some amount of power gets
reflected, this reflected power builds standing waves on the
transmission line between the source and load that results in
reduction in output voltage. Appropriate matching is possible
by selection of proper matching topology and its components
values. A slight change in the matching circuit parameter
alters drastically the frequency range in which the efficiency
of the energy conversion is maximum.
Matching networks can be designed either with lumped
elements (resistor, inductor and capacitor) or distributed elements
(microstrip lines). It can be designed by a single
resistor for impedance matching, but it is not a desirable
solution because most of the power will be lost in the resistor.
Another disadvantage of resistive matching is that, it can
match only the real part of the impedance. In this situation, L
type matching is a promising approach. It comprises of series
capacitor with shunt inductor or series inductor with shunt
capacitor, but the bandwidth obtained by L type network is
not sufficient. It can be increased by adding another section
that forms Π matching network. Introduction of extra element
provides an extra degree of freedom to control the value of
quality factor in addition to perform impedance matching.
Fig.1(a) shows the efficiency of harvesting circuit with L
and Π type matching network. It can be seen that harvesting
circuit with Π type matching circuit attains wider bandwidth
than L type matching network. Fig.1(b) shows the effect
of variable frequency on efficiency for different RF input
power for both type of matching networks. It can be seen
from Fig.1(b) that the rate of change of efficiency for L type
matching network is more in comparison to Π network. Microstrip
line based matching network has emerged as another
viable option for maximum power delivery. The Q factor of
microstrip line based matching network enhances the voltage
level and provides the passive amplification of the input
signal. In this paper, three different matching configurations
(L, Π, microstrip line based topology) are used to design RF
energy harvesting circuits.
B. Selection of Diodes
As the recieved signal strength in GSM band is higher as
compared to other, so the operating frequency of harvesting
circuit is selected in this band. Since the RF energy resources
are usually of low power region so the peak voltage of the
signals in this region is much smaller than rectifier diode turn
on voltage[13]. To fulfill these requirement diode with very
low turn on voltage and high switching speed is required.
Schottky diode offers low forward voltage and high switching
speed, and consider as an ideal component for RF energy
harvesting. In different diode specifications, schottky diode
have metal semiconductor junction in which metal side acts
as the anode and n-type semiconductor acts as the cathode of the diode. Due to low voltage drop, less energy is wasted
as heat, making it most efficient choice for applications
sensitive to efficiency. Besides this, the edges of the schottky
contact are fairly sharp, high electric field gradient occurs
around them which limits the reverse breakdown voltage, low
forward voltage and fast recovery time, that leads to increase
efficiency.
Fig. 1© shows the efficiency comparison of two different
diodes (HSMS-2852, HSMS-2822). It can be seen that the circuit
employing HSMS-2852 diode provides better efficiency
in low power region and with HSMS-2822, it provides better
efficiency in high power region. Since the RF sources are
usually of low power so we focus only in low power region.
In this paper, HSMS-2852 diode from Avago Technologies is
used, it has lowest turn on voltage of 150mV in HSMS-285X
series. It is mounted into a single package from adjacent sites
on the wafer, assuring the highest possible degree of match.
In low input RF power region (-25dBm to 0dBm), it provides
better efficiency in comparisoion to other specification[