IEEE Solid-States Circuits Magazine - Fall 2021 - 107

converter is adjusted to set VRECT
value, achieving MPPT for
to
this V /2OC
the FBR. Lu et al. [40] reported a power-harvesting
efficiency of more than
95%, and Shim et al. [41] reported a fast
tracking time of 9.09 ms/V with more
than 97% power-harvesting efficiency.
Peng et al. [42] proposed a " senseand-set "
approach that oversamples IS
and, assuming RS
calculates the instant VMPP
is known a priori,
with a mixedsignal
circuit. In this proposed method,
IS
is measured by charging CP
for a
short period of time. The very small
voltage is then amplified by transferring
the charge to a small capacitor, indicating
I
.S
in a regulated manner,
the proposed method maintains
VMPP
Combined with a high-frequency
open-circuit interface operation that extracts
energy on CP
across CP dynamically. The proposed
method can be applied to both
periodic and nonperiodic cases and has
shown a 5.41- and 5.56-times power extraction
improvement compared to an
FBR. Peng et al. [42] exemplified how,
with advancements in IC technology,
complicated control schemes can be realized
to boost the extracted power for
vibrational energy harvesting.
Perturb and Observe
Closed-loop MPPT methods, like P&O,
maximize the output power by meaIS
VCP
PET
D1
D2
CBUF
RLOAD
CP
D3
D4
(a)
IS
IS
VCP
VCP
PET
CP
Vdc
CBUF RLOAD
C CT
′P
VMPP
Time
Time
VRECT
Vdc
suring a physical quantity that indicates
the extracted power directly.
With an FBR followed by a flyback
converter that operates with constant
ON time modulation, [43] measured
the extracted power by measuring the
peak inductor current of the flyback
converter through a small sensing resistor.
An analog-to-digital converter
digitizes the sensing voltage, and the
P&O algorithm is implemented with
digital circuits in a low-power microcontroller
unit (MCU). The reported
MPPT efficiency stays above 94%.
Stanzione et al. [50] proposed a
fully mixed-signal implementation,
where the extracted power is evaluated
by multiplying the converter's
input voltage and current. In this
design, the multiplication function is
performed by biasing the MOSFET in
the subthreshold region to take advantage
of the logarithmic current-
voltage characteristics. As a result,
an output indicating the power level
can be easily obtained by current
summation. Measurement results
have shown a peak end-to-end efficiency
of 88%, with 99.8% MPPT peak
efficiency. While the harvester in [50]
is designed for electrostatic energy
harvesting, the MPPT solution is
general and can be used for piezoelectric
energy harvesting.
Nonresonant Operation
All of the interface circuits discussed so
far achieve resistive loading, as the specific
interface voltage and current are in
phase [31], [43]. It is assumed that the
impedance of a piezoelectric cantilever
vibrating around the resonant frequency
is mostly resistive, and a resistive
matching works well in extracting
power. However, in applications such
as infrastructure or factory monitoring,
the operating frequency is determined
by the structure or machine vibrations
and may not match the tiny piezo structure's
resonant frequency.
Considering the more realistic circuit
model shown in Figure 1(b), due
to the high-quality nature of the mechanical
structure, a slight frequency
offset can result in a very reactive ,ZS
and a pure resistive loading can degrade
the extracted power significantly.
From the MPT theorem, while PAVL
from the transducer does not change
with the operating frequency, it can
only be extracted with complex-conjugate
impedance matching.
Hsieh et al. [31] proposed achieving
complex impedance matching by
introducing a delay into the conventional
switching methods. This causes
a phase offset between the interface
voltage and current, and the interface
circuit effectively presents a complex
(b)
(c)
FIGURE 6: The schematics and waveforms of (a) a VOC sensing schematic [41], (b) a sense-and-set schematic [42], and (c) sense-and-set waveforms.
IEEE SOLID-STATE CIRCUITS MAGAZINE
FALL 2021
107

IEEE Solid-States Circuits Magazine - Fall 2021

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