IEEE Microwave Magazine - May 2016 - 38

80

(a)

75

Efficiency

Power Conversion Efficiency (%)

85

70
65
60

CW
1 Ms/s
2 Ms/s
5 Ms/s

55
50
45

0

2

4
6
8
Input Power (dBm)

Three Stages
Two Stages
One Stage

10
(b)

Figure 12. The measured conversion efficiency of QPSKmodulated signals [26].

Vibrational energy is the additional source proposed in [28], while the planar rectenna area is
exploited to host a photovoltaic cell in [29]. In [30], flexible fabrics are shown to be a suitable support for multiple scavenging transducers deploying RF, solar, and
thermal sources. In all these solutions, the cooperative
action of different sources demonstrates an improvement in the total harvester conversion efficiency.
In practical EH applications, the final user of the
power rectified by the rectenna is a device operating discontinuously, e.g., a sensor that needs to be
activated during very short time intervals and for a
few times per day. As a consequence, rectennas must
include an intermediate energy buffer (or PMU) able
Electric Power Extraction from Usual Life Living Environment

EM Wave

Human Body
Temperature

Vibration,
Shaking,
Fluttering

Solar and Light

Grazing

Digital TV

Pressure
Stress

Figure 13. Available environmental sources [27].

38

Pin

Figure 14. (a) A single-stage full-wave rectifier topology
and (b) RF-to-dc efficiency behavior versus input RF
power for different numbers of rectifying stages [33].

to manage the variable workload conditions. Wellestablished solutions consist of dc-to-dc switching
converters capable of dynamically tracking the maximum power point (MPP) condition: a rectified voltage
of about one half the open-circuit one has been shown
to be close to the optimum condition for any frequency
and power level [31], [32].
The proper choice of the rectifier topology can significantly enhance the RF-to-dc conversion efficiency,
as well. For the extremely low RF power budgets
involved, the use of two Schottky diodes arranged in
the single-stage full-wave rectifier (or voltage doubler)
topology shown in Figure 14(a) has been demonstrated
to provide the most convenient choice, because of the
reduced diode losses [33]. An
increase in the number of
stages can be justified only
in different scenarios, such as
in RFID tag applications, that
involve higher power levels,
as shown in Figure 14(b).
A recent study described
in [34] has demonstrated a
significant advantage in using
backward t u n nel diodes
instead of traditional Schottky
ones. The tunnel diodes' operating principle is based on the
quantum mechanical tunnelWi-Fi
ing effect rather than on the
thermionic emission effect of
PC
the Schottky diodes. This has
been proven to be sufficient to
significantly increase the RFto-dc conversion efficiency of

May 2016



Table of Contents for the Digital Edition of IEEE Microwave Magazine - May 2016

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