IEEE Circuits and Systems Magazine - Q3 2018 - 44

There is a research trend in the design of harvesting
interfaces that is focused mainly on the adaptivity
and on standalone operation criteria.

The above-mentioned works evaluate the efficiency
in different ways. The conversion efficiency is the most
common metric for evaluating the performance of piezoelectric harvesting systems. In [75], [105], the achieved
output power is compared to the maximum output
power achievable using the DET technique, while in [47],
[111], [113] the power extraction efficiency metric was
adopted to measure the overall efficiency of the harvesting interface.
The conversion efficiency is also used to measure
the efficiency of the rectification stage, as in [95], [100],
[113]. In [48] the MPPT efficiency was evaluated considering the ratio of the optimum operating voltage V MPP
and half of the open circuit voltage, 2V MPP /VOC, since the
ideal operating voltage V MPP in the implementation described in [48] is equal to VOC /2.
A very positive result in terms of active chip area and
overall packaging integration was achieved by Aktakka
et al. [111] where the active area of the chip is 0.3 mm 2
and the packaged volume occupies only 0.25 cm 3. Darmayuda et al. [95] also achieved good results in terms
of integration by implementing a harvesting interface
with an active area of 0.05 mm 2. Rao and Arnold [100]
implemented a harvesting interface with an overall chip
active area of 0.143 mm 2. However, the harvesting interface has been realized using two chips in separate dies
and external components, a Schottky diode, an inductor
and two resistors, are employed in the design.
Environmental conditions and the fabrication process
have a great impact on the efficiency of the proposed
rectifiers. Two harvesting interfaces purposely designed
to harvest energy from low input voltages ^10.35 V h can
be found in [49], [93]. The efficiency achieved in these
implementations is limited; namely 24% peak efficiency
in [49] and an average efficiency of 41% in [93]. Furthermore, in [49] a 1 MX load is required to achieve the performances presented in the table. On the other hand,
Chao et al. achieved the highest power extraction efficiency (93-96%) with a relatively high minimum start-up
voltage, Vin 2 1.8 V. Elliot and Mitcheson [112] achieved
a conversion efficiency of 86% using discrete components, but the harvesting system relies on an external
power supply.
Some of the aforementioned implementations have
been tested considering a wide range of vibrations. Specifically, two remarkable achievements can be found
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Ieee cIRcuITs anD sysTems magazIne

in [43], and [100]. Moreover, Xu et al. [43] tested the
harvesting system considering a 10 Hz to 1 kHz input
frequency range, achieving an average conversion efficiency of 60% using a 8 mm 3 off-the-shelf inductor. The
efficiency analysis performed by Rao and Arnold [100]
was not conducted considering the different vibration
frequencies (from 1 Hz to 100 kHz) but instead considering different input voltage and output power levels,
achieving a peak conversion efficiency of 60%.
VI. Conclusions
In this paper, we have presented a review of state-ofthe-art power management circuits for PEH, addressing
the principles of the piezoelectric conversion and the
main criteria that can be used to evaluate the performance of a harvesting interface. The rectification techniques employed in PEH systems were discussed and
compared emphasizing the advantages and disadvantages of each approach.
The rectifiers recently proposed in the literature
have mainly been implemented by employing MPPT algorithms to achieve a high PCE. Nevertheless, resonant
rectification techniques have also been proposed to
harvest efficiently energy from wide-band excitations.
Based on the seven criteria used to evaluate the performance of a harvesting interface that were discussed
in Section II, there is a research trend in the design of
harvesting interfaces that is focused mainly on the adaptivity and on standalone operation criteria, since the efficiency of most of the proposed harvesting systems is
usually evaluated considering different load conditions
and excitation frequencies. Recent designs optimized to
achieve a high peak efficiency performance considering
a constant output load and a single excitation frequency
are less common.
When comparing self-powered harvesting interfaces
considering both the adaptivity and the standalone operation criteria, a high conversion efficiency (>80%) can
be achieved by employing MPPT algorithms with custom rectification solutions.
To ameliorate the performance of self-powered harvesting systems, significant research should be conducted focusing on the minimum operating voltage criterion, since the efficiency achieved in implementations
focusing on this aspect is still limited (<50%). The input
power and voltage levels generated by the piezoelectric
THIRD quaRTeR 2018

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