IEEE Circuits and Systems Magazine - Q3 2018 - 29

II. Piezoelectric Conversion
The goal of electronic energy harvesting interfaces is to
efficiently extract energy from electromechanical harvesting structures. To maximize the energy extraction,
the power management unit itself should not dissipate
much of the input source power available. Therefore, it
must be designed and optimized considering stringent
low-power requirements. To provide useful power levels
to the load, usually higher than those available directly
from the rectification stage, piezoelectric energy harvesting interfaces should be able to continuously store
the available energy and, when required, release the energy accumulated to the load.
The main criteria adopted to evaluate the performance of an energy harvesting system were described
by Tabesh Frechette [41] as following:
1) Efficiency: The power loss of the harvesting rectifiers should be small compared to the input power
available. Efficiency in piezoelectric energy harvesting systems is further discussed in Section II-B.
2) Standalone operation: This criterion describes
how independent the harvesting interface is from
the other components or target application. This
concept is very important in energy harvesting interfaces since the load conditions, in most of the
applications, may vary over time, cohering, for instance, in a variation on the state of the connected
sensor from sleep to active mode.
3) Circuit complexity: This criterion is tightly connected to efficiency and is particularly relevant for
passive piezoelectric harvesting techniques without an auxiliary power supply. Since the available

Piezoelectric mechanical structures, exploiting ambient vibrational energy, generate an AC voltage that cannot
be used directly to power WSNs. Therefore, energy management circuits are required to rectify, store and regulate the power produced by the piezoelectric transducer.
Regarding energy-management circuits, custom Integrated Circuit (IC) solutions obtain a better performance
than implementations with off-the-shelf discrete components, for two main reasons: a) due to the unavoidable
trade-off between complexity of the control scheme (and
thus efficiency) and power consumption, discrete component solutions tend to favor a simplified control scheme to
reduce the power loss [36]: and b) the designer has more
freedom on the sizing, positioning and routing choices for
the components adopted in the design.
Emerging implementations of Piezoelectric Energy
Harvesting (PEH) technology can be found in different
application fields, such as wireless industrial monitoring [37], [38], health monitoring [39], and automotive
technology [40].
This article is organized as follows: Section II introduces the principles of piezoelectric energy harvesting,
along with an introduction of the main techniques adopted to rectify the input AC voltage of the piezoelectric
transducer. In Section III, a topological analysis of and
discussion on resonant energy harvesting rectifiers is
presented, with comments on the results obtained. In
Section IV, PEH circuit implementations using Maximum
Power Point Tracking (MPPT) algorithms are analyzed
and discussed. In Section V, the performance achieved
by the selected PEH implementations are discussed and
compared. Section VI concludes the paper.

32 kHz
Oscillator

Electronic
Watch

Pacemaker

Hearing
Aid

100 nW

1 µW

10 µW

100 µW

Neural Activity
Monitoring
Bluetooth
1 mW

10 mW

Zigbee

Phone
Charger

100 mW

Thermal
Thermoelectric
Pyroelectric
RF
Antennas
Kinetic
Piezoelectric
Electrostatic
Electromagnetic
Solar
Photovoltaic

Figure 2. energy harvesting approaches and possible applications [6].

THIRD quaRTeR 2018

Ieee cIRcuITs anD sysTems magazIne

Table of Contents for the Digital Edition of IEEE Circuits and Systems Magazine - Q3 2018