A Piezoelectric Energy Harvester with High Efficiency and...

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.15, NO.3, JUNE, 2015 ISSN(Print) 1598-1657 http://dx.doi.org/10.5573/JSTS.2015.15.3.319 ISSN(Online) 2233-4866

Manuscript received Sep. 23, 2014; accepted Dec. 25, 2014 1 KAIST, Daejeon 305-732, South Korea 2 Department of Electrical and Computer Engineering, Virginia Tech. 3 KAIST, Daejeon, Korea, and is now with the SamSung Co., Ltd, Korea E-mail : [email protected]

A Piezoelectric Energy Harvester with High Efficiency and Low Circuit Complexity

Xuan-Dien Do1, Huy-Hieu Nguyen3, Seok-Kyun Han1, Dong Sam Ha2, and Sang-Gug Lee1

Abstract—This paper presents an efficient vibration energy harvester with a piezoelectric (PE) cantilever. The proposed PE energy harvester increases the efficiency through minimization of hardware complexity and hence reduction of power dissipation of the circuit. Two key features of the proposed energy harvester are (i) incorporation synchronized switches with a simple control circuit, and (ii) a feed-forward buck converter with a simple control circuit. The chip was fabricated in 0.18 µm CMOS processing technology, and the measured results indicate that the proposed rectifier achieves the efficiency of 77%. The core area of the chip is 0.2 mm2. Index Terms—Active rectifier, feed-forward buck converter, high efficiency rectifier, piezoelectric energy harvester, synchronized switch rectifier.

I. INTRODUCTION

Elimination of batteries is highly useful for some applications, where replacement of a battery is costly or impractical. Piezoelectric (PE) energy harvesting offers high energy density ranging from 10 uW to 100 uW/cm3 and is a viable solution for those applications [1]. Low power dissipation of the circuit as well as maximum extraction of the power is important for any small scale energy harvesters, which leads to high efficiency. A PE

transducer exhibits an AC voltage or current source with a source capacitance. Two major circuit bocks of a PE energy harvester are a rectifier and a voltage regulator. Conventional full bridge rectifiers with passive diodes suffer from low efficiency due to (i) large power dissipation across the passive diodes and (ii) inability to extract large power from the capacitive PE source. Various configurations of active diodes were investigated to reduce power dissipation of passive diodes, and a rectifier based on cross-coupled PMOS transistor and comparators in [5] achieves high efficiency. To extract maximum power from the capacitive source of a PE transducer, two nonlinear approaches were investigated [1-5]. The first approach is to discharge the capacitor with a switch when the PE transducer current changes its direction from positive to negative or vice versa [1, 2, 4, 10]. The approach called synchronized switch (SS) shortens the time period to charge the capacitor, but fails to harvest the energy stored in the capacitor prior to the zero-crossing point of the current. The SS rectifiers reported in [1, 4] short the two terminals of the transducer directly with a switch(es). The method requires complex circuitry to detect the zero-crossing point and two external supply voltages. The second approach called synchronized switch harvesting on inductor (SSHI) relies on an LC resonator to flip the polarity of the capacitor voltage at each current zero-crossing point [1, 8]. An SSHI rectifier can extract more power than an SS rectifier, as it scavenges the energy stored in the capacitor prior to the zero-crossing point. An SSHI rectifier requires typically a large off-chip inductor and a complex the control circuit to fine tune the timing of the resonator [1].

Since the voltage generated by a PE transducer is

320 XUAN-DIEN DO et al : A PIEZOELECTRIC ENERGY HARVESTER WITH HIGH EFFICIENCY AND LOW CIRCUIT COMPLEXITY

typically higher than that for rechargeable batteries, a buck or buck-boost converter is usually used to regulate the output voltage. The power dissipation of a converter as well as its efficiency is critical to the efficiency of small scale PE energy harvesters, whose power ranges typically 10µW to 100µW. Hence, a DC-DC converter with a sophisticated control scheme does not necessary lead to higher efficiency due to larger power dissipation of the control circuitry itself.

This paper presents an efficient PE energy harvester, which adopts an SS rectifier and a buck converter. The control circuitry for the SS rectifier and the buck converter is much simpler compared with other existing PE energy harvesters, which results in high efficiency. The proposed SS rectifier adds two switches to the rectifier reported in [5] for synchronized switching, but unlike existing SS rectifiers it does not require external supply voltages.

II. PROPOSED ENERGY HARVESTING CIRCUIT

1. Proposed Rectifier Lam et. al proposed a rectifier which implements

diodes with two cross-coupled PMOS transistors and comparator-based NMOS transistors [5], and the rectifier called Lam rectifier in this paper is highly efficient. The proposed rectifier incorporates synchronized switching to Lam rectifier to realize an SS rectifier, which will further improve the efficiency. Fig. 1(a) shows the proposed rectifier, in which two switches, MP3 and MP4, are added to the two cross-coupled pMOS diodes in parallel. The PE transducer in the figure is modeled as a current source iP(t) = IPsin(2πfPt), in which IP = 125 uA, in parallel with a capacitor CP = 125 µF, and resistor RP = 1 MΩ [1]. Frequency fP = 200 Hz is the excitation frequency. When current iP is negative, capacitor CP is charged to -(Vrect+VD), where Vrect is the rectifier output voltage and VD is the total voltage drop across the diodes [1]. In Lam rectifier, when the current crosses zero and becomes positive, the negative charge of CP is discharged first before it starts to be charged positively. The PE current starts to flow to the load only when the capacitor voltage reaches at (Vrect+VD). In other words, the generated PE energy is not delivered to the load during the period for the capacitor voltage rising from -(Vrect+VD) to (Vrect+VD) .

The proposed rectifier senses the zero-crossing point of PE current iP and turns on MP3 and MP4 momentarily to discharge the capacitor (i.e., synchronized switching operation), so that the capacitor voltage rises from -(Vrect+VD) to zero instantly. Consequently, it shortens the time for the capacitor voltage to reach (Vrect+VD). The control circuit to sense zero- crossing point and to turn on the two transistors is shown in Fig. 1(b). The exclusive-OR operation of the control voltage G1 or G2 and its delayed one generates a short negative pulse, which turns on the two transistors MP3 and MP4 momentarily.

Fig. 2 shows relevant waveforms of the proposed rectifier. The capacitor voltage VBA becomes 0 from -(Vrect+VD) instantly due to the synchronized switching operation. When VBA reaches +(Vrect+VD), transistor MP1 is turned on, and control voltage VG2 transitions from low to high to turn on MN2. The harvested current, irect flows into the load while VBA being held at constant. The bottom shaded waveform indicates the periods when the energy is transferred to the load. When the transducer current crosses zero to be negative, MP1 is turned off, and control voltage VG2 drops from high to low to turn

iP

CP

MP1MP2

MN2 MN1

A B

Vrect

G2 G1

MP4 MP3?S

RP

Transducer

irect

?S

COM1COM2

(a)

OR INV INV INVOR

G1

G2

φS

ti

(b)

Fig. 1. (a) Proposed active cross-coupled rectifier with SS, (b) The circuit of synchronized switches control.

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.15, NO.3, JUNE, 2015 321

off MN2. The change of VG2 leads to the gate voltage φS to be zero momentarily to conduct MP3 and MP4.

For the source current, the maximum extracted power of the proposed SS rectifier with synchronized switching and that of the one without a synchronized switching scheme (i.e., Lam Rectifier) are obtained as follows for ideal components [1, 3]:

2

max, rectifier_ SS 24P

P P

IPC fp

= (1)

2

max, rectifier_without_ SS 24P

P P

IPC fp

= (2)

The SS rectifier doubles the extracted power compared

with Lam Rectifier. In contrast, the maximum extracted power of a conventional full bridge rectifier with passive diodes is given as [1, 3].

2

max, conventional 2

4(1 )

4P P DropP

PP P

C f VIPIC f

pp

= - (3)

where VDrop is the diode voltage drop of passive diodes. Considering IP = 125 u, CP = 25 nF, fP = 200 Hz, and VDrop = 0.4 V, the extracted power of the proposed SS rectifier is about 3.5 times than that for a conventional full-bridge rectifier with passive diodes.

2. Proposed DC-DC Buck Converter

A buck converter is used for the proposed PE energy

harvester to regulate the output voltage. Fig. 3(a) shows the proposed buck converter and the associated controller, which adopt a feed-forward control to reduce the power consumption. Fig. 3(b) shows the timing diagram of the controller. Input voltage Vrect (equally the output voltage

VBA

ip

t

t

t

VG2

t

tVG1

Sj

Extrated Power

t

Vrect

High

Low

Low

High

Fig. 2. Waveforms of the proposed rectifier.

(a)

(b)

Fig. 3. (a) Feed-forward buck converter, (b) timing diagram.


of the rectifier) is applied to the sawtooth signal generator, which generates a sawtooth waveform with the peak value of:

2

1 2

*peak rectRV V

R R=

+ (4)

Two power switches PSW and NSW are controlled by

a PWM signal with duty cycle D. The duty cycle D for PCLK is:

1

2

1 * ref

rect

VRDR V

æ ö= +ç ÷è ø

(5)

with the switching frequency

1

2

11

*

RR

fT R C

æ ö+ç ÷

è ø= = (6)

III. SIMULATION AND MEASUREMENT

RESULTS

Fig. 4 shows current source, iP, synchronized switches control signal, φS, transducer capacitor voltage VBA, and harvested current irect, with variation of frequency from 100 Hz to 400 Hz with a 100 Hz step. The synchronized switches control signal φS, is low for a short period (15 µs) at transducer current zero crossing point, thereby transducer capacitor voltage VBA to zero instantaneously.

The proposed PE energy harvester is fabricated in 0.18 µm CMOS processing technology, and a die photo is shown in Fig. 5. The core area of the chip is 0.2 mm2. The test set up to simulate and measure the performance of the chip is as follows: PE transducer parameters: IP = 125 uA; fP = 200 Hz; CP = 25 nF; RP = 1 MΩ, and load resistor: adjusted from 20 kΩ to 80 kΩ.

Fig. 6 shows the power efficiency of system as a

5V

5V

125uA iP

ϕS

VBA

0

0

0125uA irect

5V

5V

125uA iP

ϕS

VBA

0

0

0125uA irect

00

(a) (b)

5V

5V

125uA iP

ϕS

VBA

0

0

0125uA irect

5V

5V

125uA iP

ϕS

VBA

0

0

0125uA

irect

00

(c) (d)

Fig. 4. Piezoelectric current source, iP, switches control signal, φS, transducer capacitor voltage VBA, and harvested current irect (a) 100 Hz, (b) 200 Hz, (c) 300 Hz, (d) 400 Hz.


function of load current. With the above parameters, the maximum extracted load current is 60 uA at 1.8 V output. In Fig. 6, the peak power efficiency of the system is 77 % at the load current of about 52 uA.

Fig. 7 shows waveforms of capacitor voltage VAB and

rectifier output voltage Vrect of the proposed rectifier with synchronized switching and the one without synchronized switching (i.e. Lam rectifier) under the same load resistance. The waveform of the proposed rectifier indicates that VAB is reset at the zero-crossing point and charged rapidly. It is interesting to note that Vrect of the proposed SS rectifier is higher (3.6 V) than Lam rectifier (2.3 V) due to less wasted power. Fig. 8 indicates that the output voltage VO is regulated to 1.8 V with the duty cycle of about 0.5. It was observed that the converter operates in continuous conduction mode (CCM).

The maximum extracted power of the proposed harvester under the optimal resistor load of 100 KΩ is 125 µW, and the output power of the rectifier is 143 µW. The proposed harvester achieves 77% efficiency when compared against the theoretical maximum extracted power of 158 µW [1]. The efficiency of the buck converter is 87%, which is quite high due to the simple control circuitry. Lam rectifier extracts only 72 µW, which demonstrates the effectiveness of the synchronized switching. The proposed DC-DC converter consumes quiescent current of only 350 nA.

Table 1 compares the performance of the proposed PE harvester with recently reported PE energy harvesting systems. [7] occupies small area chip by using simple

Fig. 5. Die photo of the proposed system.

Unreachable

Fig. 6. Efficiency of system as functions of output current.

Vol

tage

(V)

-4

-2

0

2

4

Vrect with synchronized switches

Vrect without synchronized switches

VAB with synchronized switches

VAB without synchronized switches

5ms

Fig. 7. VAB and Vrect of a pMOS cross-coupled rectifier with and without synchronized switches.

10us

Vo

Vrect

PWM

Fig. 8. Waveforms of Vo and the converter control signal. Table 1. Compare with Previous Works

Publication [7] [1] [9] This work Technology (µm) 0.18 0.35 0.18 0.18

fC (Hz) 95 225 419 200 Area (mm2) 0.05 4.25 0.25 0.2

Rectifier Efficiency (%) 92 NA NA 90.5

DC-DC Converter Buck- Boost Buck Buck Buck

Overall Efficiency (%) 54 62.7 50-60 77


control, however, the output voltage of system is high ripple. Therefore, a LDO is added to regulator output voltage. [1] achieves high overall efficiency by using bias-flip rectifier but chip area is big. [9] achieves small area chip by using simple control, however, this circuit has uses two off-chip Schottky diodes for rectifier. This work proposes system that includes fully integrated high efficiency synchronous switches rectifier and low circuit complexity DC-DC buck converter. Although it is difficult to compare the performance fairly due to different processing technologies and features, the proposed PE harvester achieves the highest efficiency among existing PE energy harvesting systems.

V. CONCLUSIONS

This paper presents an efficient PE energy harvester, which minimizes the hardware complexity to reduce the power dissipation and hence increase the efficiency. Two key features of the proposed energy harvester are (i) incorporation synchronized switches with a simple control circuit, and (i) a feed-forward buck converter with a simple control circuit. The chip was fabricated in 0.18 µm CMOS processing technology, and the measured results indicate that the proposed rectifier achieves the efficiency of 77%.

ACKNOWLEDGMENT

This work was supported by the Ministry of Education, Science Technology (MEST) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation. Fabrication was supported by IDEC, KAIST.

REFERENCES

[1] Y. Ramadass, and A. Chandrakasan “An efficient piezoelectric energy harvesting interface circuit using a bias-flip rectifier and share inductor,” in IEEE J. Solid-State Circuit, vol. 45, no. 1, pp. 189-204, Jan. 2010.

[2] S. Guo, and H. Lee “An efficiecncy-enhanced CMOS rectifier with unbalanced-biased comparators for transcutanceous–powered high-current implants,” in IEEE J. Solid-State Circuit, vol. 44, no. 6, Jun. 2009.

[3] X-D Do, Y-H Ko, H-H Nguyen, H-B Le, and S-G Lee “An Efficient Parallel SSHI Rectifer for Piezoelectric Energy Scavenging Systems,” in IEEE Int. Conf. on Advanced Com. Tech., pp.1394,1397, 13-16 Feb. 2011

[4] E. Dallage, D.Miaton, G.Venchi, G.Frattini, and G.Ricotti “Self-supplied integrable high- efficiency AC-DC converter for piezoelectric Energy scavenging systems,” in IEEE Int. Symposium on Circuit and System Conf., 2007, pp.1633-1636.

[5] Y. Sun, Nguyen, H.-H, C.J. Jeong, and S.G. Lee, "A Integrated High Performance Active Rectifier for Piezoelectric Vibration Energy Harvesting Systems," in IEEE Trans. on Power Elec., 2012

[6] Y. H. Lam, W. H. Ki, C. Y. Tsui " Integrated Low-Loss CMOS ctive Rectifier for Wirelessly Powered Devices", in IEEE Trans. on Cir. and Sys. II: Vol. 53, NO.12, December 2006

[7] I. M. Darmayuda, Y. Gao; M.Tong Tan; S.-J. Cheng; Y. Zheng; M. Je; C.-H. Heng. "A self-powered power conditioning IC for piezoelectric energy harvesting from short-duration vibrations" IEEE Transactions on Circuits and Systems II: Express Briefs, Vol59, No.9, Sept.2012.

[8] X.- D. Do, H.- H. Nguyen, S.- K. Han, & S.-G. Lee,” A rectifier for piezoelectric energy harvesting system with series Synchronized Switch Harvesting Inductor.” In Proc. IEEE Asian In Solid-State Circuits Conference (A-SSCC), 2013, pp. 269-272.

[9] EE. Aktakka, R.L Peterson, K. Najafi, "A self-supplied inertial piezoelectric energy harvester with power-management IC," Solid-State Circuits Conference Digest of Technical Papers (ISSCC), 2011 IEEE International , pp.120,121, Feb. 2011

[10] X.-D. Do, H.-H. Nguyen, S.-K. Han, D.S. Ha, S.-G. Lee, "A Self-Powered High-Efficiency Rectifier With Automatic Resetting of Transducer Capacitance in Piezoelectric Energy Harvesting Systems," Very Large Scale Integration (VLSI) Systems, IEEE Transactions on , to be published.

Xuan-Dien Do was born in Vietnam, in 1985. He received the B.S degree from Hanoi University of Technology, VietNam in 2008. He is now working toward the M.S and PhD degree at Nice Laboratory, Korea Advanced


Institute of Science and Technology, Daejeon, Korea. His current research interests are PMIC, DC-DC converter, and analog circuit for energy harvesting and mobile phone.

Huy-Hieu Nguyen was born in Vietnam, in 1980. He received the B.S. degree from Hanoi University of Technology, Hanoi, Vietnam in 2003 and the M.S. and Ph.D. degrees in electric engineering from Korea Advanced Institute of Science and

Technology, South Korea in 2010. He is now with Samsung Electronics where he is engaged in CMOS analog and power management integrated circuit design.

Seok-Kyun Han was born in Cheonnam, Korea in 1964. He received the B.S. degree in electronic engineering from the Kwangju University, Korea, in 1995, and the M.S. and Ph.D. degrees in electrical engineering Mokpo University and

Mokpo Maritime University, Korea, in 1998 and 2004, respectively. From 1995 to 1998, he was with Information and Communications University, Daejeon, Korea, as a research assistant professor. Since 2010, he is with Korea Advanced Institute of Science and Technology, Daejeon, Korea, where he is now a research associate professor. His research interests the silicon technology-based RFIC, Microwave circuits designs.

Dong Sam Ha (M’86–SM’97–F’08) received a B.S. degree in Electrical Engineering from Seoul National University, Seoul, Korea, in 1974, and M.S. and Ph.D. degrees in Electrical and Computer Engineering from the University of Iowa, Iowa

City, IA, USA, in 1984 and 1986, respectively. Since Fall 1986, he has been a faculty member with the Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA, where he is a professor and Director of the Multifunctional Integrated Circuits and Systems (MICS) Group composed of four faculty

members. Along with his students, he has developed four computer-aided design tools for digital circuit testing and CMOS standard cell libraries. The source code for the four tools and the cell libraries have been distributed to about 370 universities and research institutions worldwide each. His research interests include power conditioning circuits for energy harvesting, power management aCs for smart cameras, transceivers for high-speed fiber optical communications, and high-temperature circuits and systems for down hole communications.

Sang-Gug Lee was born in Gyungnam, Korea in 1958. He received the B.S. degree in electronic engineering from Gyungbook National University, Daegue, Korea, in 1981, and the M.S. and Ph.D. degrees in electrical engineering from University

of Florida, Gainesville, Florida, in 1989 and 1992, respectively. In 1992, he joined Harris Semiconductor, Melbourne, Florida, USA, where he was engaged in silicon-based RF IC designs. From 1995 to 1998, he was with Handong Global University, Pohang, Korea, as an assistant professor in the school of computer and electronic engineering. From 1998 to 2009, he was with Information and Communications University, Daejeon, Korea as an assistant, associate, and professor in the school of engineering. Since March 2009, he is with KAIST, Daejeon, Korea, as a professor in the department of electrical engineering. His research interest is the silicon technology-based (especially in CMOS) radio transceiver designs, lately focusing on ultra-low power and extreme high frequencies (that is, Tera Hertz) implementations. In the past few years, his research interests also include other analog integrated circuit designs such as display semiconductors, power management ICs, and automotive ICs. From 2005 to 2009, he served as a technical committee member of IEEE ISSCC in the wireless communication technology committee. From 2005 to 2010, he served as a research director of Auto-ID Lab Korea. In 2007, his laboratory was selected as a National Research Laboratory. Since 2012, he is serving as a director of Future Promising Fusion Technology Pioneer Center, leading a research group in the area of silicon technology based THz IC design.

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