A Flipping Active-Diode Rectifier for PiezoelectricVibration Energy-Harvesting
1
Wan-Ling Wu, 1 Ching-Yuan Yang and 2 Dung-An Wang
1
2
Department of Electrical Engineering, National Chung Hsing University
Graduate Institute of Precision Engineering, National Chung Hsing University
145 Xingda Rd., South Dist., Taichung City 402, Taiwan (R.O.C.)
Abstract—In the paper, we discuss piezoelectric-vibration energyharvesting flipping active-diode rectifiers, including the full bridge
rectifier (FBR), the active diode rectifier, the switch only rectifier
(SOR), and the flipping-capacitor rectifier (FCR). The energy
harvesting circuits were implemented in 0.35-μm CMOS process.
Compared to the transferred-power ability of the conventional
FBR, the simulation improved factors of an active diode rectifier,
SOR and FCR can reach up to 2x, 3.2x and 4.6x at an excitation
frequency of 100 Hz, respectively.
Keywords: Piezoeletrics; energy harvesting circuit; full bridge
rectifier; switch-only rectifier (SOR); flipping-capacitor rectifier
(FCR)
I.
INTRODUCTION
In recent years, energy harvesting has become an important
technique for environmental awareness. Instead of other energy
harvesting components, piezoelectric energy harvesters (PEHs)
are common used owing to its high power density [1], high
scalability, high output voltage generation, and compatibility
with integrated circuit (IC) technologies [2]. Since the
piezoelectric transducer (PT) cannot be used to power load
electronics directly, an interface circuit is able to rectify the
output power from PT and provide a stable supply. The
equivalent model of a PEH can be represented as a dependent
current source Ip and a parallel parasitic capacitor Cp. Generally,
conventional FBR circuits can convert PEH’s ac current into a
dc output voltage [3]. However, the voltage inversion affect the
energy extraction efficiency because of the charging loss of the
PEH parasitic capacitor. When a current flows into a capacitor,
it may cause a phase difference between voltage and current.
The virtual power is the main reason of the power loss.
Therefore, some kinds of energy-harvesting circuits are
proposed to improve the loss on FBR. An active diode rectifier
is mainly used to reduce the reverse current consumption [4]. In
addition, some kinds of energy-harvesting circuits are proposed
to improve the charging loss caused by the capacitor. The SOR
improves this defect by briefly shorting the PEH when the Ip
value cross over zero [5]. As a result, the loss of Cp is cut in
half, effectively enhance the extracted power. To extract more
energy from the PEH, the FCR is proposed [6, 7]. The FCR
circuit uses capacitors to store the energy from the current
source and give back to Cp on time. The process can flip the
voltage and make the loss less.
This paper discusses the design and analysis of the above
circuits, also organizes these four approaches and compares
each output power and voltage .
II.
CIRCUIT DESCRIPTION
A. Full Bridge Rectifiers
Full bridge rectifiers (FBRs) are widely used due to their
simplicity and stability. Fig. 1 shows the schematic of
traditional FBR, which can be sub-divided into positive half
cycle and negative half cycle of input current Ip. During positive
half cycle, the current source Ip charges the capacitor Cp . When
the voltage of Cp higher than the Vth of the power MOS in the
rectifier, the current flows into output capacitor CL from node
Va. During the negative half cycle, the current source Ip charges
the Cp in the opposite direction. When the voltage of Cp is
flipped higher than the Vth of the power MOS, the current flows
into output capacitor from node Vb. Their power efficiencies are
relatively low as they set high-voltage thresholds for the input
voltage to overcome prior to any energy extraction. However,
the charge loss of Cp is the main cause which limits the energy
extraction. The black part of the current waveform in Fig. 1 is
the loss during rectification [3].
Vrect
IP
MP2
MP1
Qloss
Va
RP
IP
Vb
CP
CL
RL
Vab
Vrect
-Vrect
MN2
MN1
Fig. 1.Schematic of the full bridge rectifier.
B. Active diode Rectifier
If the output voltage of the FBR is greater than the signal
source, it will cause the output energy to flow backward, the
circuit may cause additional reverse current consumption,
resulting in a reduction of conversion efficiency. In order to
avoid this situation, reference [4] proposed an active diode to
prevent the energy backflow, the architecture is called an active
diode rectifier, as shown in Fig. 2.
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When the current IP is positive, Va is greater than Vb, the
output of comparator CMP1 is able to turn ON the MN1. The
current flows through MN1 and MP1 to charge the output
capacitor. On the contrary, if the current IP is negative, the
comparator CMP2 turns the MN2 ON, the current changes to flow
through MN2, MP2 and the output capacitor.
The comparator and NMOS are act as active diode, NMOS
will quickly turned OFF if the current flows back. When IP starts
to turn into negative, Vb rises to a positive value, the output of
comparator 1 turns OFF the MN1, and vice versa. Therefore, the
current cannot flows back into the input source.
Vrect
MP2
multiple switched capacitors instead of external high-Q
inductor, thus significantly reduces the required circuit area and
is suitable for deep-tissue implant applications. Fig. 4 shows the
basic form of FCR circuit that uses only one capacitor C1 and
five switches. The FCR operation can be sub-divided into three
steps. In the beginning, the voltage of Cp maintains Vrect at the
end of the positive cycle. First, ph1p signal is generated before
Ip current cross over zero, and makes C1 get a part of charge
from Cp. Second, the ph00 clears the charge in Cp just at the time
when Ip current is zero. Finally, Cp is charged back from C1 in
opposite direction. If the capacitance value Cp is same as C1,
after calculating several times of flipping, the flipping voltage
Vr in steady state can be calculated as
MP1
RP
Vb
IP
+ ⋯+
+
=∑
(1)
and
Va
CP
CMP1
| |=
CL
lim | | =
RL
CMP2
As a result, this method makes the loss decrease because of the
partially voltage flipped.
MN2
MN1
(2)
⟶
IP
Qloss
Fig. 2.Schematic of the active diode rectifier
C. Switch Only Rectifier[5]
In order to solve the power loss caused by the phase shift
between the voltage Vab and the current Ip, the simplest way is to
add a switch across the input. In Fig. 3, a switch parallel with
PEH makes Va and Vb short at the time when Ip changes
direction. The capacitor Cp discharges immediately to ground
when the switch is ON. Once Cp has been discharged, the switch
turned OFF. Therefore, there is no need to waste the power on
pulling the voltage down to zero and the power that we saved
can be used to charge the Vab from 0 to a higher ±Vrect during
each half cycle. The voltage and the current waveforms of the
switch only rectifier is shown in Fig. 3. It can be seen that the
power loss during rectification is getting smaller than active
diode rectifier.
IP
Qloss
IP
Vrect
Va
PEH
AC-DC
rectifier
CP
Vab
CL RL
Va
PEH
Vrect
ph 1p
IP
ph 1n
AC-DC
rectifier
ph00
CP
ph 1n
C1
Vab
Vrect
CL RL
Vr
-Vrect
ph 1p
Vb
ph1p
ph00
ph1n
Fig. 4. Flipping-capacitor rectifier.
We can increase the numbers of flipping capacitors in order
to get higher flipping voltage Vr. Fig. 5 shows the voltage flip
efficiency with different numbers of flipping capacitors. The
transient response with one to four sets of flipping capacitors is
shown in Fig. 6. As can been seen, increasing the numbers of
flipping capacitors can achieve better efficiency but at the cost
of circuit. Considering about the chip area and the flipping
voltage efficiency, we finally adopted FCR with 4C in the
following discussion.
Vrect
-Vrect
Vb
VSW
Fig. 3. Switch-only rectifier
D. Flipping-Capacitor Rectifier[6] [7]
An alternative approach is proposed to synchronously flip
the voltage across the piezoelectric transducer using one or
Fig. 5. Simulation results of voltage flip efficiency with MATLAB.
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III.
MEASUREMENT RESULTS
The energy harvesting circuits were designed with 0.35-μm
CMOS foundry process. Fig. 9 shows the die photograph of the
test chip and the total chip area is about 1.66 mm2. We used
function generator generating two input voltage sine wave
signals, the peak to peak voltage is about 12V at the frequency
of 500Hz.The two coupling capacitors is used to block DC
components. The capacitor Cp is used to play as the parallel
parasitic capacitor of piezoelectric energy harvesters.
Fig. 6. Simulation results of voltage flip efficiency with HSPICE.
Fig. 7 shows the simulation results of conventional FBR, the
active diode rectifier, SOR, and FCR with 4C for comparison.
The piezoelectric element is modeled by a current source about
30 A and a parallel parasitic capacitor about 12 nF with the
input frequency of 100 Hz, a load capacitance of 1 F and a
load resistance of 5 MΩ, respectively. The simulated output
power between difference rectifiers is shown in Fig.8. Table I
shows the different output power comparing with a
conventional FBR, the active diode rectifier reaches up to 2x,
the SOR reaches up to 3.2x and the FCR reaches up to 4.6x.
30uA
IP
Fig. 9. Micrograph of the test chip.
-30uA
3.29v
FBR Va
Active
diode
0.8v
Va
Vb
Vb
FCR
Va
3.2v
3.75v
5.05v
Vab
sw_only
Vb
Fig. 7. Simulation results of the energy harvester circuits.
PH00
Fig. 10. Measured transient waveforms of active diode rectifier.
Fig. 8.Simulated power at the output of rectifiers.
TABLE Ⅰ PERFORMANCE COMPARISON BETWEEN
DIFFERENCE RECTIFIERS
Rectifiers
Full bridge
Active
Switch only
Flipping capacitors
Max. output power
9.41μW
18.4μW
30.5μW
43.5μW
Normalize
1
2
3.2
4.6
Fig 10 shows the measured output voltage of an active diode
rectifier, the output voltage is about 1.8V. Fig 11 shows the
measured output voltage of switch only rectifier, the output
voltage is about 2V.As the result shows the ph00 signal clears the
charge in Cp and pulls the voltage down to zero. Fig 12 shows
the measured output voltage of flipping-capacitor rectifier, the
output voltage is about 2.8V. The zoom-in voltage flipping
instants for the input voltage flipped from negative to positive is
shown in Fig. 13 with the switch signal ph00. It can be seen that
the power transit into switching capacitor and empty the
remaining charge while the ph00 signal pulse turns high. After
the signal turns low, Cp is charged back to an opposite
polarization during the following pulses.
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Va
TABLE II PERFORMANCE COMPARISON WITH STATE-OF-THEART INTERFACE CIRCUITS
Vb
This work
JSCC2014
[8]
JSSC2010
[7]
TPEL[9]
0.35 m
0.35μm
0.35μm
0.32μm
Extraction
Technique
SSHC
Energy
Pile-Up
SSHI
SECE
Frequency
100 Hz
100 Hz
225 Hz
60 Hz
Inductor
No
10mH
47μH
560μH
PIC/PFBR
4.6
4.2
2.8
3
Technology
Vab
(CMOS)
PH00
Fig. 11. Measured transient waveforms of switch only rectifier.
Va
Vb
IV.
CONCLUSION
This paper presented a fully integrated flipping-capacitor
rectifier for piezoelectric energy harvesting without requiring
large external inductor. The improvement of extracted output
voltage between active diode rectifier, switch only rectifier and
flipping-capacitor rectifier reveals the proposed flippingcapacitor rectifier as a promising solution for piezoelectric
energy harvesting applications, especially for deep-tissue
implant implementations.
Vab
PH00
V.
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Fig. 12. Measured transient waveforms of FCR.
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Va
Vb
[4]
Vab
[5]
[6]
[7]
PH00
Fig. 13. Measured transient waveforms during flipping.
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Table II shows the performance comparisons among state-ofthe-art interface circuits for piezoelectric energy harvesters and
the proposed circuit.
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