IEEE Circuits and Systems Magazine - Q3 2018 - 41

The advantage of charge pumps with respect to conventional
switching converters is that magnetic passive
components are not needed.

Pmax, VD = C p (VOC - Vd) 2 ~
Pmax, FB = C p (VOC - 2Vd) 2 ~

when Vrect = VOC - Vd
when Vrect = VOC /2 - Vd (15)

MPPT algorithms are used to keep VRect at its optimal
value, usually driving a DC-DC converter used as a lossless resistor. In the literature, both VD and FB rectifiers
have been employed to implement MPPT PEH systems.
VD rectifier implementations can be found in [72], [88],
and [89], while implementations of FB rectifiers can be
found in [44], [90], and [91]. One technique, that can
be used to reduce the diode voltage drop is to employ
active diodes in the design; three implementation examples using this approach can be found in [92], [93],
and [47].
MPPT algorithms have been also applied to custom
rectification schemes; two examples can be found in
[47], and [48]. They have also been used to bias-flip circuits that are directly connected to the piezoelectric
transducer, as in [94] and, [86].
DC-DC converters are typically used to implement variable resistors adjusting the duty cycle and the switching
frequency. Ottman et al. [67], [68] adopted a buck converter in Discontinuous Current Mode (DCM) as matching
impedance. Using a buck converter, the rectified voltage
must be lower than the peak voltage of the AC piezoelectric energy source. A buck-boost converter in DCM can
be used to overcome this limitation, as proposed in [57],
and [95]. DCM is usually the preferred switching-mode
operation when implementing piezoelectric harvesting
systems, because it helps to limit dynamic power losses
compared to the Continuous Current Mode (CCM) [96].
Furthermore, CCM requires a higher capacitor voltage
compared to DCM, particularly in systems in which there
is a large difference in the input and output power levels
[97], [98]. Buck-boost converters operated in DCM are a
popular choice for piezoelectric energy harvesting for
two main reasons: 1) flexibility, because they can handle
different input voltage levels, above and below the target
rectification voltage, and 2) because they behave as lossless resistors [99].
THIRD quaRTeR 2018

Other standard DC-DC converters have been employed to implement energy harvesting systems. Rao
and Arnold [100] employed a boost converter to rectify
the piezoelectric voltage, while Kong et al. [46] proposed a
flyback converter implementation. The flyback converter
is a buck-boost converter that uses a transformer as a
storage magnetic component. The use of the transformer,
as in the previously discussed cases of hybrid SSHI (Section III-D) and of MR-SSHI (Section III-C), provides electrical isolation from the piezoelectric transducer and, by
varying the turns ratio, the output voltage can be adjusted according to the application requirements. The main
sources of power dissipation using a flyback converter,
when the converter runs at steady state and the resistive
matching impedance is achieved, were analyzed in [46].
According to the conducted analytical derivations, one of
the main sources of power dissipation is the transformer
itself, due to its intrinsic parasitic resistance. Another
technique, employed to step-up the AC piezoelectric voltage, is the use of charge pumps. The advantage of charge
pumps with respect to conventional switching converters
is that magnetic passive components are not needed. In
many implementations of switching converters, off-chip
bulky inductors and transformers are needed to rectify
the voltage. Kawai et al. [89] implemented a fully integrated 35 µm Complementary Metal-Oxide-Semiconductor
(CMOS) power management unit, employing a MPPT algorithm to drive a charge pump circuit.
PEH interfaces with MPPT usually require two DCDC converters: one is adopted to achieve the matching
load seen by the piezoelectric element, and the other to

4
3.5
Power (µW)

The maximum power that can be extracted using a
VD is higher than the maximum power that can be extracted using a FB rectifier, as shown in Fig. 22. The maximum power in the case of the FB rectifier occurs for a
lower value of the rectified voltage.

VD
FB

3
2.5
2
1.5
1
0.5
0

0.5

1

1.5

2
Vrect

2.5

3

3.5

4

Figure 22. Theoretical maximum output power estimation
considering a voltage doubler and a full bridge rectifier.

Ieee cIRcuITs anD sysTems magazIne

41



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