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Piezoelectric energy harvester impedance matching using a piezoelectric transformer
Accepted Manuscript Title: Piezoelectric Energy Harvester Impedance Matching Using a Piezoelectric Transformer Authors: Hamid Jabbar, Hyun Jun Jung, Nan Chen, Dae Heung Cho, Tae Hyun Sung PII: DOI: Reference:
S0924-4247(16)30488-5 http://dx.doi.org/doi:10.1016/j.sna.2017.07.036 SNA 10231
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
15-9-2016 17-7-2017 18-7-2017
Please cite this article as: Hamid Jabbar, Hyun Jun Jung, Nan Chen, Dae Heung Cho, Tae Hyun Sung, Piezoelectric Energy Harvester Impedance Matching Using a Piezoelectric Transformer, Sensors and Actuators: A Physicalhttp://dx.doi.org/10.1016/j.sna.2017.07.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Piezoelectric Energy Harvester Impedance Matching Using a Piezoelectric Transformer Hamid Jabbar a, Hyun Jun Jung b, Nan Chen c, Dae Heung Cho d and Tae Hyun Sung e,* a,b,d,e c
Electrical Engineering Department, Hanyang University, Seoul, Korea
School of Computer Science and Technology, Northwestern Polytechnical University, Xi`an, China.
a
Hamid Jabbar: (e-mail: [email protected]). Hyun Jun Jung: (e-mail: [email protected]). c Nan Chan: (e-mail: [email protected]). d Dae Heung Cho: (e-mail: [email protected]). b
e,
* Tae Hyun Sung (corresponding author) (e-mail: [email protected] , Phone: +82-2-2220-2317) Address: 408, HIT, 222 Wangsimni-ro, Seongdong-Gu, Seoul, 133-791, Korea
Graphical abstract
Highlights
Piezoelectric Energy Harvester impedance matching is performed. Piezoelectric transformer also lowers high piezoelectric energy harvester voltage. Circuit generates no electromagnetic interference. The impedance matching control is simple. Piezoelectric transformer and harvester can be made on same substrate.
Abstract—To harvest maximum power from a piezoelectric energy harvester requires conjugate impedance matching, consisting of both resistive and inductive load. In practical circuits, dc-dc converters working in discontinuous conduction mode are used for emulated resistive impedance matching. These converters contain a large and expensive electromagnetic component to reduce the wire conduction losses. A new approach toward piezoelectric energy harvester resistive impedance matching is presented by using a step-down piezoelectric
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transformer with a bi-directional inductor-less half-bridge circuit working under zero voltage switching conditions. The large and expensive magnetic transformer and inductor in previous works are replaced with a ceramic component with no electromagnetic interference. The presented technique can reduce the circuit size and the ceramic transformer can be manufactured on the harvester cantilever.
Keywords— Piezoelectric, Energy harvesting, piezoelectric transformer, impedance matching, harvesting circuit.
1.
INTRODUCTION
Piezoelectric energy harvesting technology is derived from structural damping where piezoelectric material is used to suppress the mechanical vibrations in the structure by dissipating the energy in the electrical load circuit [1]–[2]. In a piezoelectric energy harvesting system, instead of dissipating the energy in the load circuit, it is utilized for useful work in different electrical and electronics applications. Piezoelectric energy harvesters in the form of cantilevers beams are used for generating energy from the low frequency vibrating structures. At low vibration frequencies, the reactance part of the loosely coupled piezoelectric energy harvester (PEH) impedance is capacitive because its impedance phase is well below zero degrees. To harvest the maximum energy from the piezoelectric material, conjugate impedance matching comprising of both resistive and inductive load components is required where the purpose of the inductor is to match the piezoelectric material inherent capacitance [3]. To estimate the energy available from the PEH, researchers utilize the passive electrical components such as resistor and inductor to find the relationship between the vibration frequencies, power generated and optimum load. The high value of the inductor is required to match the inherent piezoelectric capacitance; therefore, usually, the PEH is tested with the resistive load only. Also, as the real part of the PEH impedance increases between the fundamental resonance and anti-resonance frequencies of the harvesting system, the resistive impedance matching technique becomes feasible [4]. Using the resistive impedance matching allows for maximum power transfer when the resistive load impedance matches the modulus of the PEH equivalent output impedance [4].
In practical circuits, to transfer the optimum power from the piezoelectric energy harvester to the application load requires voltage rectification, resistive impedance matching, and voltage level converter circuits as shown in Fig. 1. For resistive impedance matching converter, the discontinuous conduction mode (DCM) dc-dc converters whose input emulates a resistor (Re), such as a buck, boost, flyback, or buck-boost, can be used [4, 5]. In the DCM dc-dc converters, considering the ideal components, neglecting the small ripple, the relationship between the emulated resistance, inductance, switching frequency, and duty-cycle can be easily obtained [6]. As the Vrect is the rectified dc input voltage of the DCM dc-dc converter in Fig. 1, the power is drawn by the dcdc converter from the source is obtained by multiplying Vrect with the average value of the pulsating dc-link current irect. Therefore, a dc-dc converter that can be controlled to change the dc-link voltage and current will be seen as effective load resistance to the piezoelectric energy harvester. In real world applications, the vibration amplitude and frequency can change requiring an adaptive control circuit to operate the impedance matching converter at the optimum point with varying input conditions [5, 7, 8]. These control system needs to have simple control technique with low power consumption. In [5], the DCM flyback converter and in [4], the DCM buck-boost converter is used for resistive impedance matching. The switching frequency in the DCM dc-dc converter is kept low to reduce the switching losses. To reduce the wire resistance losses in the magnetic components of these converters, thicker wires are used which results in large size inductor and transformer. When designing the harvesting circuit, a trade-off is reached between the switching frequency, switching losses and the magnetic component size [4, 5, 9, 10]. The alternate of a large inductive component in DCM dc-dc converters can be a piezoelectric material working in its resonance frequency range. In this range, the piezoelectric material exhibits inductive behavior. This principle is utilized in the piezoelectric transformer, whose input has a complex inductive impedance which allows power to be transferred to an isolated application load [11]. Also, the piezoelectric transformer can be manufactured on the same substrate as the piezoelectric energy harvester, thus reducing the size of the electrical circuit. In [12], the PT was used in a power converter for driving piezoelectric actuators. In [13] the PT output was used for impedance matching with a piezoelectric transducer. Usually, the piezoelectric transformer output impedance matching with its application-specific load is discussed. But in this paper, we try to match the output of piezoelectric energy harvester output impedance with the input impedance of the piezoelectric transformer and its driving circuit. In this paper, a piezoelectric transformer (PT) based switching converter is used for performing the impedance matching and to step-down the voltage of piezoelectric energy harvester, simultaneously transferring the harvested power to the isolated load without any electromagnetic interference (EMI). To operate the PT at
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its resonance frequency, an inductor-less half bridge driving circuit operating at zero voltage switching (ZVS) condition is used without any inductive component. The switching frequency and duty-cyle of the converter can be controlled so that the dc-link voltage and current can be varied and the converter can act as a varying emulated resistive impedance load to the piezoelectric energy harvester. In the presented system, during one switching cycle, the current flow is bi-directional, it flows from the dc-link to the PT based half bridge converter circuit and part of it is also returned back to the dc-link. The average current flow is always from the dc-link to the impedance matching converter. The presented circuit is similar to the bi-directional converter for piezoelectric energy harvesting presented in [14] and [15], so this technique can be used to replace that converter. In section 2 and 3, the piezoelectric energy harvester and the piezoelectric transformer used in this work are discussed. In Section 4 the new impedance matching converter is presented. Results are given in Section 5 and Section 6 concludes this paper. 2.
PIEZOELECTRIC ENERGY HARVESTER
The widely used cantilever type piezoelectric energy harvester (PEH) made of soft lead zirconate titanate (PZT) piezoceramic material was selected for this research work, and its dimensions are given in Table 1. The piezoelectric patch was mounted at the center of the cantilever beam. To enhance the displacement and harvesting energy, a tip mass of 3 g was used.
The PEH was mounted on the shaker to measure its output electrical parameters. The PEH impedance magnitude and its phase angle response were measured at different frequencies using the impedance analyzer (model E4990A) as shown in Fig. 2 along with its Van Dyke equivalent circuit. Using the scheme shown in Fig. 3, the shaker vibration frequency was varied and the PEH open-circuit voltage variation was measured, as shown in the same figure. Under open circuit conditions, the sinusoidal peak to peak voltage reaches the maximum value of about 74 Volts at 37.8 Hz. The PEH output voltage was then connected with the rectification and smoothing circuit also known as the standard circuit as shown in Fig. 4 [16]. Resistive impedance matching was performed at a vibration frequency of 37.8 Hz to find the relationship between the matching load, rectified dc voltage (Vrect), and harvested power (Prect) under different load conditions. The rectified voltage and the power were measured as shown in Fig. 4 using an oscilloscope (model DSO-X 4034A).
The maximum power of 5.64 mW was harvested at 30 kΩ of resistive load and 13.01 volts of dc voltage. These voltages and power values were used to select the electrical components for the piezoelectric transformer driving circuits in section-5. Also, the power harvested by the proposed technique will be compared with this standard circuit result in section-5.
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PIEZOELECTRIC TRANSFORMER
A step-down PT can be selected from three widely studied designs: thickness mode, radial mode, and contour-extensional mode [17, 18]. The PT operates in its resonance frequency range for optimum power conversion and for the thickness-mode PT, it is in the megahertz range, while for other two types, it is in the kilohertz range. The radial-mode PT is used in this work as it offers high efficiency and its multi-layer manufacturing techniques can be relatively easily achieved [11, 18]. A step-down piezoelectric transformer design as presented in [19] and made of hard PZT piezoceramic material was used in this work. The PT is of a square shape, with the internal circular ring as input. The PT input and output were isolated by a dielectric gap. The piezoelectric transformer (PT) physical properties, shape and its well accepted equivalent circuit along with its parameters values are shown in Fig. 5. The resistance RS represents the mechanical vibration losses and series LS and CS represent the resonance circuit. The input to output transformation ratio is represented as n. The Input and output capacitances, Cd1 and Cd2 respectively, are measured at 1 kHz [20].
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The PT input impedance magnitude and its phase were measured using the impedance analyzer as shown in Fig. 6. It can be seen that between the resonance (fr) and anti-resonance (fa) frequency, the impedance phase inverts showing the inductive behavior. The impedance magnitude is low at fr but increases rapidly and reaches a maximum at the fa. Between fr and fa both real (R) and imaginary part of impedance (X) show a non-linear increasing behavior. In manufactured PT, some small spurious modes were also observed near the resonance frequency [21]. The optimum matched load of PT is given by ( RL = 1/ωCd2) where it has maximum efficiency. For this PT, the optimum resistive matching load is 1 kΩ.
4.
PEH IMPEDANCE MATCHING WITH PT
Piezoelectric material has an inherent capacitance. The harvesters based on this material are tuned to work at low vibration frequencies as most real-world application frequencies are near or less than 100 Hz range. To harvest optimum power, usually, the PEH output voltage is first rectified. Then, the power converter circuits perform two tasks: 1) impedance matching, and 2) step-down the PEH output voltage to the level required by the application load circuit. To achieve the above two tasks together, a step-down piezoelectric transformer is utilized in this research paper. Fig. 5 and 6 show the PT’s complex, inductive and varying input impedance. While working in its resonance frequency range, PT will work as an inductor and efficiently transfer the power from its input to the output. Using the step-down PT will result in the lower output voltage as compared to its input. Thus step-down PT can be used to accomplish the task of impedance matching and simultaneously step down the input voltage.
To efficiently transfer the input power to the output, PT is operated at its resonance frequency range. A switching circuit is required to convert the PEH rectified dc voltage to drive the PT at kilohertz frequency. Different techniques can be used to drive PT in resonance such as push-pull, class-E, and half-bridge [22, 23]. The half-bridge driving circuit technique is the simplest and also generates the highest conversion efficiency [22]. The proposed scheme for emulated resistive impedance matching is presented in Fig. 7 where the rectified PEH voltage (Vrect) is used to drive the PT in its resonance frequency using a half-bridge circuit. The output of PT has to be rectified again for usage in electrical or electronics load applications. By controlling the frequency, duty-cycle and dead-time between the switches of the half-bridge converter, the PT input, and output power can be controlled. As the PEH can generate a finite amount of power, this will affect the current (irect) drawn from the dc-link and change in dc-link voltage (Vrect) level. The piezoelectric energy harvester will see the change in dc-link voltage and current as an emulated resistance. While working, some part of the current will flow back to the dc-link from the PT based half-bridge converter, but the average flow of current is always in to the presented PT based impedance matching converter. The PT is operated at first vibration mode and its driving frequency is between the resonance and antiresonance frequency in this vibration mode. Optimum performance from PT is obtained when it is driven with sinusoidal input. This can be achieved with the half-bridge circuit using the inductor before the PT input. The inductor acts as a filter to generate a sine voltage signal for the PT input [24]. This technique will make PT working in Zero Voltage Switching (ZVS) condition to reduce the circuit losses, but it requires an inductor and again using an inductor to lower the wire resistance will result in a relatively large size inductor. An inductorless half-bridge driving circuit can be used where PT is driven with trapezoidal wave (soft-square voltage) in ZVS condition [25]. However, if ZVS technique is not implemented, power dissipation will occur in MOSFET switches due to hard-switching. Removing inductor and driving PT with soft square voltage will result in some loss of PT efficiency [22]. The analytical model of an inductor-less ZVS piezoelectric transformer driven by half-bridge converter circuit is given in [26]. The half-bridge (HB) driving circuit uses MOSFET switches, whose internal capacitance adds up with the PT input capacitance, which is determined by the thickness and number of the PT input layers. The MOSFET switches have parasitic capacitances, among them the drain to source parasitic capacitance. COSS, as shown in Fig. 7, plays a major role in half-bridge circuits switching energy loss [27]. In the inductor-less half-bridge driving circuit operating in ZVS condition, to discharge the parasitic and PT input capacitances; a higher dead-time between the switches is required [28-30]. When the sufficient dead-time is achieved, the current spikes due to hard-switching that result in switching losses are removed. The switches are turned off and on at PT resonance frequency by the driving circuit, which also controls the dead-time between the switches. The driving circuit controller has to adjust frequency and dead-time simultaneously to achieve the ZVS condition [31].
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The ZVS will occur when the voltage across the PT input capacitance is the same as Vrect when high-side MOSFET (S1) is turned ON, and it is zero volts when low-side MOSFET (S2) is turned ON. The PT input capacitance design plays an important role in achieving ZVS condition as explained in [31]. The experimental steady-state current and voltage waveforms of the proposed converter in ZVS with PT operating at its fundamental resonance frequency are shown in Fig. 8. The dead-time between the high-side (S1) and low-side (S2) switches is almost comparable with the switches on time and the significant current changes can be observed in this time period. The half-bridge converter operation is divided into different time steps in Fig. 8, and the behavior of the proposed impedance matching converter during each time step is shown in Fig. 9 [26, 28, 29, 32]. The direction of the current arrows (irect and iin-PT) shows the direction of the oscilloscope current probe used for measuring the waveforms in Fig. 8.
Step-1 [t0 - t1]: To achieve ZVS, the voltage across low-side switch (Vin,PT) was zero. At time t0 the Lo-side MOSFET switch is turned ON at zero voltage condition as Cd1 is fully discharged. At t0 the forward current flows into the PT, then it changes the direction. The current freewheels through the low-side switch. No current flows into the HB circuit from the PEH and resonance is achieved in this period. Step-2 [t1 - t2]: When the dead-time starts, both switches are OFF. During the dead-time, the parasitic capacitor COSS-Hi and COSS-Lo and Cd1 are charged by the reverse resonant current. Therefore, the voltage across the lowside MOSFET increases until it is slightly higher than the Vrect voltage level, till t2 when high-side switch body diode starts to be conducted. Step-3 [t2 – t3]: Both switches are OFF. The Voltage across the high-side switch is zero, so its body diode is forward biased. Current flows from the PT to the Crect capacitor. In this case, the current flows-back to the dclink side and the converter is simply represented by its equivalent circuit in Fig. 9(e) where the value of Cx and Rx are presented in [26, 28]. Step-4 [t3 – t4]: The Hi-side switch turns ON. The voltage across PT is the same as the Vrect voltage. The current flows from Crect to PT. This current reaches the peak when the switch is turned OFF. It is the only time duration when the current is supplied by the dc-link directly to the PT. The supplied current (irect) is the same as the current through the PT inductor (iLm). The slope of the current also shows an inductive component behavior. Step-5 [t4 – t5]: Dead-time starts. Both switches are off. The current flow from Crect gradually reduces to zero at the end of this cycle. The current from the capacitance COSS-Hi and COSS-Lo flows to the PT, due to which the Vin,PT reduces to zero volts. The dead time has to be large so that Vin,PT can go to zero by discharging COSS-Lo to achieve ZVS. Step-6 [t5 – t6]: The dead time continues. As the COSS-Lo is discharged to 0 V, its body diode is now forwardbiased. The output of PT under ZVS condition will be sinusoidal in-phase current and voltage, which needs to be rectified and smooth again for the mostly dc voltage operated load applications. The PT offers high efficiency with optimum matched load, but higher resistive load values can also be used with reduced efficiency [31]. If inductor to be used with a half-bridge circuit for the PT discussed in section-3 and 4, a value of 1.5 mH is required to achieve ZVS. 5.
EXPERIMENTAL RESULTS
To evaluate the proposed system, the circuit presented in Fig. 7 was constructed on PCB. For AC signal rectification BAS3007A diode bridge rectifiers were used at PEH and PT output. For the S1 and S2 switches, 2N7002 MOSFET was utilized. The IR21091S half-bridge driver IC was used with 10 V external power supply and TTL level square wave signal. The 10 Vpeak-peak signals were generated by the half-bridge driver IC for S1 and S2 switches gate to source voltage (VGS). The Dead-time of up to 5 µs was controlled manually using a 200 kΩ variable resistor with the driver IC. The externally powered half-bridge driver chip current consumption was 700 µA at 10 V in a steady-state condition. A. Impedance matching converter efficiency To find the performance of the proposed impedance matching converter presented in Fig. 7, PEH was implemented by its equivalent circuited presented in Fig. 2 and driven by the AC current source (iS). The output of the PT was directly connected with the resistive load (RL,ac). The impedance matching converter (inductor-
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less half-bridge piezoelectric transformer based resonant converter) efficiency at different input power levels is shown in Table 2. The impedance matching converter efficiency is highly dependent on the PT load resistance (RL,ac) and the input power. For each input condition, the minimum load resistance (RL,ac) was found at which ZVS can be achieved. When RL,ac is near the PT matching load value of 1 kΩ, the efficiency increases. For the given input conditions, the ZVS was not achieved at RL below 2 kΩ. At iS of 1 mA and RL,ac of 3 kΩ the efficiency of 82% was achieved and maximum efficiency of 86% was achieved for input power greater than 10 mW and RL,ac of 2 kΩ. The efficiency graph for different RL,ac under iS of 1 mApeak is shown in Fig. 10. The efficiency of the converter falls as the frequency is increased. The power consumption of the externally powered driving chip is not considered in the efficiency calculations.
B. Testing with Single Cantilever PEH The piezoelectric harvester discussed in section-2 was mounted on a shaker with a base acceleration of 6.80 m/s2 at 37.8 Hz vibration frequency. The input vibration conditions are similar as mentioned in section 2. The output of the PEH was the rectifier, which was then connected with inductor-less half-bridge PT driving circuit as shown in Fig. 7. The output of the PT was rectified, smoothed, and connected with a resistance decade box. The proposed system was experimentally tested under different load conditions, and the optimum resistive load (RL) range where ZVS can be achieved was found. Under each resistive load condition, the driving circuit switching frequency was varied in the resonance frequency range. For each frequency, the dead-time resistor was manually set to achieve the ZVS condition.
The PEH rectified voltage (Vrect) under different load conditions and varying driving frequency is shown in Fig. 11(a). The voltage rises linearly to a peak value, after which the ZVS condition cannot be maintained and it falls gradually. The output voltage (Vout) and power harvested (Pout) at different load conditions are shown in Fig. 11 (b) and (c). The ZVS condition was achieved under shown RL load, if the load resistance is kept lower, the frequency bandwidth for ZVS reduces. The ZVS can be achieved at lower RL value but at a cost of reduced bandwidth. The minimum load value PT was able to drive in ZVS condition was 3 kΩ and 1.97 mW of power (Pout) was extracted, which is 35 % efficiency as compared to the standard interface circuit maximum generated power shown in Fig. 4. When the ZVS condition is achieved, the PT output voltage and current are in-phase and voltage is stepped-down. The driving chip frequency, switches on time and dead-time between them, are shown in Fig. 11 (d) under a fixed load condition. In the experimentally tested ZVS range, the single PEH impedance was not optimally matched. In the single PEH standard circuit testing case, the maximum power of 5.64 mW was harvested at 30 kΩ of resistive load and 13.01 volts dc voltage. The minimum load value (optimum PT matched load is 1 kΩ) PT was able to drive in ZVS condition was 3 kΩ and 1.97 mW of power (Pout) and at this condition, the Vrect was about 4 V. This result was obtained at single PT driving frequency point of 76.4 kHz, a low ZVS band-width was obtained near the optimum PT matching load (RL). This Vrect value was not at an optimum position, much lower as achieved in standard circuit case in Fig. 4. It was observed, that to achieve higher overall efficiency, Vrect in proposed impedance matching converter should be same as in the standard circuit (resistive impedance matching) testing case. Also, as the single PEH was not able to generate the enough power, the optimum load could be connected. C. Testing with four parallel connected PEHs To analyze the presented system performance with multiple harvesters, four PEHs, the same as presented in section 2 were mounted on a shaker, and each harvester output was first rectified and then connected in parallel. For four PEHs connected in parallel, the standard circuit open circuit rectified voltage was 11.976 V. To find the optimum power from the four PEHs in parallel, standard circuit testing resulted in 2.31 mW of optimum power at Vrect of 5.04 Volts at 37.8 Hz of vibration frequency and a matching load resistance of 9 kΩ. The vibration amplitude was set to lower the optimum Vrect voltage in four parallel-connected PEHs so that it can be achieved by the PT based Half-bridge converter in the ZVS condition. The four parallel PEHs were used to generate enough power so that the optimum resistive load (RL) can be attached to the PT output.
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The experimental results of proposed impedance matching technique with four parallel connected PEH are presented in Fig. 12. As Vrect increases with a frequency under ZVS condition to attain the value in case of optimum resistive impedance matching, the efficiency of the proposed impedance matching decreases. However, the efficiency of the proposed system increases as compared to the resistive impedance matching with the standard interface circuit. The half-bridge PT based impedance matching converter input (dc-link) to the PT output load (RL) maximum efficiency remained between 70 to 75 % under different RL conditions. The proposed system achieved near 63% efficiency at 4 kΩ of PT output resistive load as compared to the standard circuit.
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CONCLUSIONS
A new technique is presented in which the output impedance of the piezoelectric energy harvester is matched with a piezoelectric transformer based inductor-less half-bridge converter circuit working in ZVS condition. The presented system matches the piezoelectric energy harvester impedance, and the power harvested is transmitted to the isolated load without any magnetic component. Experimental results show that the presented inductor-less half-bridge driving circuit operating the piezoelectric transformer in ZVS condition has a maximum efficiency of 80%. About 60% efficiency is achieved when the output of presented technique is compared to the optimum resistive load condition of the standard interface circuit. The power harvested is low as the piezoelectric transformer output load and optimum size is not achieved as required for high-efficiency power conversion. Also, the presence of small spurious modes in the vicinity of PT resonance frequency affects its efficiency. By using the linear Vrect trend in proposed impedance matching technique, the adaptive controller can also be implemented easily to control the half-bridge switches dead-time to harvest the maximum power. The converter presented in this paper can also replace the bi-directional converter presented in [14, 15]. Future work involves an improved piezoelectric transformer design, with optimum dimensions, matched output load and without any spurious modes. The analytical and simulation techniques will be used to study the effects of the piezoelectric energy harvester parameters on the presented impedance matching converter design. The implementation of the circuit at integrated circuit level will also be explored for overall increased circuit efficiency and reduce losses. Among the goal of the further study will be to manufacture the piezoelectric transformer alongside the piezoelectric energy harvester cantilever beam. ACKNOWLEDGEMENT This work was supported by the *Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20142020103970)
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Note, A More Realistic Characterization of Power MOSFET Output Capacitance Coss, International Rectifier. [28] E. Horsley, N. Nguyen-Quang, M. Foster, D. Stone, Achieving ZVS in inductor-less half-bridge piezoelectric transformer based resonant converters, Power Electronics and Drive Systems, 2009 PEDS 2009 International Conference on, IEEE2009, pp. 446-51. [29] M.P. Foster, J.N. Davidson, E.L. Horsley, D.A. Stone, Critical Design Criterion for Achieving Zero Voltage Switching in Inductorless Half-Bridge-Driven Piezoelectric-Transformer-Based Power Supplies, IEEE Trans Power Electron, 31(2016) 5057-66. [30] S. Bronstein, S. Ben-Yaakov, Design considerations for achieving ZVS in a half bridge inverter that drives a piezoelectric transformer with no series inductor, Power Electronics Specialists Conference, 2002 pesc 02 2002 IEEE 33rd Annual, IEEE2002, pp. 585-90. [31] M. Foster, E. Horsley, D. Stone, Predicting the zero-voltage switching profiles of half-bridge driven inductor-less piezoelectric transformer-based inverters, IET Power Electronics, 5(2012) 1068-73. [32] M. Ekhtiari, T. Andersen, M.A. Andersen, Z. Zhang, Dynamic optimum dead time in piezoelectric transformer-based switch-mode power supplies, IEEE Trans Power Electron, 32(2017) 783-93.
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Authors Biography Hamid Jabbar received his Bachelor of Mechatronics Engineering degree from National University of Sciences and Technology, Pakistan in 2002 and MS in Electronics and Communications Engineering from Myongji University Korea in 2008. He completed his PhD in electrical engineering from Hanyang University, Seoul in Feb. 2017. He is Research Assistant Professor in department of electrical engineering, Hanyang University, Seoul, Korea and his research area includes the application of piezoelectric materials for energy harvesting and power conversion. He has over 10 years of experience is design and development of mechatronics systems. His research interests include circuit designing and mechatronics systems. Hyun Jun Jung received the Bachelor of engineering (electrical) degree from Seoul National University of Science and Technology, Seoul, South Korea in 2012. He received his Master of electrical engineering degree from Hanyang University, Seoul, South Korea in 2014 and Ph.D. degree from Hanyang University in the field of energy harvesting system in Feb. 2017. His research interest includes mechanical and electrical modeling of energy harvesting system and wireless sensor network.
Nan Chen was born in Xi`an, P. R. China, in 1988. She received her B.S. degree in Electronic Information Engineering from Xinyang Normal University, Xinyang, P. R. China, in 2010; and her M.S. degree in Software Engineering from Northwestern Polytechnical University, Xi’an, P. R. China, in 2013; where she has been working towards her Ph.D. degree since 2013. Her current research interests include analog and mixed-signal integrated circuit designs, and power electronics such as digitally controlled DC-DC switching converters and energy harvesting circuits. Dae Heung Cho was born in Seoul, Republic of Korea in 1988. He received the B.Sc. degree from Hanyang University, Seoul, Republic of Korea. He undergoes M.Sc in same university now. His major is electronic, electrical and communication engineering.
Tae Hyun Sung received a B.A. (1982), an M.S. (1987) in inorganic material engineering from Hanyang University and a Ph.D. (1991) in Material Science and Engineering from Tokyo Institute of Technology. He worked at International Superconductivity Technology Center (ISTEC) as a researcher (1992). He was in Massachusetts Institute of Technology (MIT) as a Post doc. (1995). He worked at the Korea Electric Power Research Institute (KEPRI) as a group leader of superconductivity group. Also, he was selected as a “Top 100 Engineers” by IBC (International Biographic Centre), is the member of the National Academy of Engineering of Korea (NAEK). Since 2009, he has been professor of Department of Electrical Engineering, Hanyang University. His research interests include superconductivity, electrical material, energy storage device, and piezoelectric energy harvesting technology.
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Fig. 1: A typical system for impedance matching and power conversion from a piezoelectric energy harvester.
Fig. 2: Piezoelectric energy harvesting impedance magnitude and phase along with its equivalent circuit.
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Fig. 3: Piezoelectric energy harvesting open circuit voltage with the experimental setup.
Fig. 4: Piezoelectric energy harvester rectified voltage and power harvested under different resistive loads.
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Fig. 5: Piezoelectric transformer along with its equivalent circuit and parameters when the output is open-circuited.
Fig. 6: a) Impedance magnitude and phase, b) real (R) and imaginary (X) part of the PT impedance at first resonance frequency mode range measured with output terminals open.
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Fig. 7: Proposed scheme to connect the piezoelectric energy harvester with step-down piezoelectric transformer
Fig. 8: Experimental steady-state waveforms at the ZVS conditions.
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Fig. 9: Half-bridge PT based converter circuit operation in ZVS conditions at different time steps.
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Fig. 10: Impedance matching converter efficiency at iS = 1 mApeak
Fig. 11: Experimental result with single PEH under varying PT resonance frequency and PT output resistive load; a) rectified PEH output voltage (Vrect), b) PT rectified output voltage, c) PT output power (Pout) response, d) switches on-time and deadtime between them.
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Fig. 12: Experimental result with four parallel connected PEH under varying PT resonance frequency and PT output resistive load; a) rectified PEH output voltage (Vrect), b) PT rectified output voltage, c) PT output power (Pout) response, d) switches on-time and dead-time between them.
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Table 1: Specifications of the piezoelectric energy harvester
Material Piezoelectric Cantilever substrate
Dimensions length x width x thickness (mm) 38×38×0.18 60×40×0.18
Soft-PZT steel
Table 2: Impedance matching converter efficiency iS
Vrect
mApeak 1 2 3 4
V 3.11 4.52 6.96 9.79
Pin = Vrect x irect mW 1.315 4.68 10.57 20.06
Vout,ac
RL,ac
Pout,ac=
V 1.81 3.08 4.31 4.18
kΩ 3 2.5 2 2
mW 1.09 3.79 9.29 17.35
𝑽𝒐𝒖𝒕,𝒂𝒄 𝑹𝑳,𝒂𝒄
Converter Efficiency = Pout,ac/Pin % 82.82 80.98 86.73 86.46
S0924-4247(16)30488-5 http://dx.doi.org/doi:10.1016/j.sna.2017.07.036 SNA 10231
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
15-9-2016 17-7-2017 18-7-2017
Please cite this article as: Hamid Jabbar, Hyun Jun Jung, Nan Chen, Dae Heung Cho, Tae Hyun Sung, Piezoelectric Energy Harvester Impedance Matching Using a Piezoelectric Transformer, Sensors and Actuators: A Physicalhttp://dx.doi.org/10.1016/j.sna.2017.07.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Piezoelectric Energy Harvester Impedance Matching Using a Piezoelectric Transformer Hamid Jabbar a, Hyun Jun Jung b, Nan Chen c, Dae Heung Cho d and Tae Hyun Sung e,* a,b,d,e c
Electrical Engineering Department, Hanyang University, Seoul, Korea
School of Computer Science and Technology, Northwestern Polytechnical University, Xi`an, China.
a
Hamid Jabbar: (e-mail: [email protected]). Hyun Jun Jung: (e-mail: [email protected]). c Nan Chan: (e-mail: [email protected]). d Dae Heung Cho: (e-mail: [email protected]). b
e,
* Tae Hyun Sung (corresponding author) (e-mail: [email protected] , Phone: +82-2-2220-2317) Address: 408, HIT, 222 Wangsimni-ro, Seongdong-Gu, Seoul, 133-791, Korea
Graphical abstract
Highlights
Piezoelectric Energy Harvester impedance matching is performed. Piezoelectric transformer also lowers high piezoelectric energy harvester voltage. Circuit generates no electromagnetic interference. The impedance matching control is simple. Piezoelectric transformer and harvester can be made on same substrate.
Abstract—To harvest maximum power from a piezoelectric energy harvester requires conjugate impedance matching, consisting of both resistive and inductive load. In practical circuits, dc-dc converters working in discontinuous conduction mode are used for emulated resistive impedance matching. These converters contain a large and expensive electromagnetic component to reduce the wire conduction losses. A new approach toward piezoelectric energy harvester resistive impedance matching is presented by using a step-down piezoelectric
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transformer with a bi-directional inductor-less half-bridge circuit working under zero voltage switching conditions. The large and expensive magnetic transformer and inductor in previous works are replaced with a ceramic component with no electromagnetic interference. The presented technique can reduce the circuit size and the ceramic transformer can be manufactured on the harvester cantilever.
Keywords— Piezoelectric, Energy harvesting, piezoelectric transformer, impedance matching, harvesting circuit.
1.
INTRODUCTION
Piezoelectric energy harvesting technology is derived from structural damping where piezoelectric material is used to suppress the mechanical vibrations in the structure by dissipating the energy in the electrical load circuit [1]–[2]. In a piezoelectric energy harvesting system, instead of dissipating the energy in the load circuit, it is utilized for useful work in different electrical and electronics applications. Piezoelectric energy harvesters in the form of cantilevers beams are used for generating energy from the low frequency vibrating structures. At low vibration frequencies, the reactance part of the loosely coupled piezoelectric energy harvester (PEH) impedance is capacitive because its impedance phase is well below zero degrees. To harvest the maximum energy from the piezoelectric material, conjugate impedance matching comprising of both resistive and inductive load components is required where the purpose of the inductor is to match the piezoelectric material inherent capacitance [3]. To estimate the energy available from the PEH, researchers utilize the passive electrical components such as resistor and inductor to find the relationship between the vibration frequencies, power generated and optimum load. The high value of the inductor is required to match the inherent piezoelectric capacitance; therefore, usually, the PEH is tested with the resistive load only. Also, as the real part of the PEH impedance increases between the fundamental resonance and anti-resonance frequencies of the harvesting system, the resistive impedance matching technique becomes feasible [4]. Using the resistive impedance matching allows for maximum power transfer when the resistive load impedance matches the modulus of the PEH equivalent output impedance [4].
In practical circuits, to transfer the optimum power from the piezoelectric energy harvester to the application load requires voltage rectification, resistive impedance matching, and voltage level converter circuits as shown in Fig. 1. For resistive impedance matching converter, the discontinuous conduction mode (DCM) dc-dc converters whose input emulates a resistor (Re), such as a buck, boost, flyback, or buck-boost, can be used [4, 5]. In the DCM dc-dc converters, considering the ideal components, neglecting the small ripple, the relationship between the emulated resistance, inductance, switching frequency, and duty-cycle can be easily obtained [6]. As the Vrect is the rectified dc input voltage of the DCM dc-dc converter in Fig. 1, the power is drawn by the dcdc converter from the source is obtained by multiplying Vrect with the average value of the pulsating dc-link current irect. Therefore, a dc-dc converter that can be controlled to change the dc-link voltage and current will be seen as effective load resistance to the piezoelectric energy harvester. In real world applications, the vibration amplitude and frequency can change requiring an adaptive control circuit to operate the impedance matching converter at the optimum point with varying input conditions [5, 7, 8]. These control system needs to have simple control technique with low power consumption. In [5], the DCM flyback converter and in [4], the DCM buck-boost converter is used for resistive impedance matching. The switching frequency in the DCM dc-dc converter is kept low to reduce the switching losses. To reduce the wire resistance losses in the magnetic components of these converters, thicker wires are used which results in large size inductor and transformer. When designing the harvesting circuit, a trade-off is reached between the switching frequency, switching losses and the magnetic component size [4, 5, 9, 10]. The alternate of a large inductive component in DCM dc-dc converters can be a piezoelectric material working in its resonance frequency range. In this range, the piezoelectric material exhibits inductive behavior. This principle is utilized in the piezoelectric transformer, whose input has a complex inductive impedance which allows power to be transferred to an isolated application load [11]. Also, the piezoelectric transformer can be manufactured on the same substrate as the piezoelectric energy harvester, thus reducing the size of the electrical circuit. In [12], the PT was used in a power converter for driving piezoelectric actuators. In [13] the PT output was used for impedance matching with a piezoelectric transducer. Usually, the piezoelectric transformer output impedance matching with its application-specific load is discussed. But in this paper, we try to match the output of piezoelectric energy harvester output impedance with the input impedance of the piezoelectric transformer and its driving circuit. In this paper, a piezoelectric transformer (PT) based switching converter is used for performing the impedance matching and to step-down the voltage of piezoelectric energy harvester, simultaneously transferring the harvested power to the isolated load without any electromagnetic interference (EMI). To operate the PT at
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its resonance frequency, an inductor-less half bridge driving circuit operating at zero voltage switching (ZVS) condition is used without any inductive component. The switching frequency and duty-cyle of the converter can be controlled so that the dc-link voltage and current can be varied and the converter can act as a varying emulated resistive impedance load to the piezoelectric energy harvester. In the presented system, during one switching cycle, the current flow is bi-directional, it flows from the dc-link to the PT based half bridge converter circuit and part of it is also returned back to the dc-link. The average current flow is always from the dc-link to the impedance matching converter. The presented circuit is similar to the bi-directional converter for piezoelectric energy harvesting presented in [14] and [15], so this technique can be used to replace that converter. In section 2 and 3, the piezoelectric energy harvester and the piezoelectric transformer used in this work are discussed. In Section 4 the new impedance matching converter is presented. Results are given in Section 5 and Section 6 concludes this paper. 2.
PIEZOELECTRIC ENERGY HARVESTER
The widely used cantilever type piezoelectric energy harvester (PEH) made of soft lead zirconate titanate (PZT) piezoceramic material was selected for this research work, and its dimensions are given in Table 1. The piezoelectric patch was mounted at the center of the cantilever beam. To enhance the displacement and harvesting energy, a tip mass of 3 g was used.
The PEH was mounted on the shaker to measure its output electrical parameters. The PEH impedance magnitude and its phase angle response were measured at different frequencies using the impedance analyzer (model E4990A) as shown in Fig. 2 along with its Van Dyke equivalent circuit. Using the scheme shown in Fig. 3, the shaker vibration frequency was varied and the PEH open-circuit voltage variation was measured, as shown in the same figure. Under open circuit conditions, the sinusoidal peak to peak voltage reaches the maximum value of about 74 Volts at 37.8 Hz. The PEH output voltage was then connected with the rectification and smoothing circuit also known as the standard circuit as shown in Fig. 4 [16]. Resistive impedance matching was performed at a vibration frequency of 37.8 Hz to find the relationship between the matching load, rectified dc voltage (Vrect), and harvested power (Prect) under different load conditions. The rectified voltage and the power were measured as shown in Fig. 4 using an oscilloscope (model DSO-X 4034A).
The maximum power of 5.64 mW was harvested at 30 kΩ of resistive load and 13.01 volts of dc voltage. These voltages and power values were used to select the electrical components for the piezoelectric transformer driving circuits in section-5. Also, the power harvested by the proposed technique will be compared with this standard circuit result in section-5.
3.
PIEZOELECTRIC TRANSFORMER
A step-down PT can be selected from three widely studied designs: thickness mode, radial mode, and contour-extensional mode [17, 18]. The PT operates in its resonance frequency range for optimum power conversion and for the thickness-mode PT, it is in the megahertz range, while for other two types, it is in the kilohertz range. The radial-mode PT is used in this work as it offers high efficiency and its multi-layer manufacturing techniques can be relatively easily achieved [11, 18]. A step-down piezoelectric transformer design as presented in [19] and made of hard PZT piezoceramic material was used in this work. The PT is of a square shape, with the internal circular ring as input. The PT input and output were isolated by a dielectric gap. The piezoelectric transformer (PT) physical properties, shape and its well accepted equivalent circuit along with its parameters values are shown in Fig. 5. The resistance RS represents the mechanical vibration losses and series LS and CS represent the resonance circuit. The input to output transformation ratio is represented as n. The Input and output capacitances, Cd1 and Cd2 respectively, are measured at 1 kHz [20].
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The PT input impedance magnitude and its phase were measured using the impedance analyzer as shown in Fig. 6. It can be seen that between the resonance (fr) and anti-resonance (fa) frequency, the impedance phase inverts showing the inductive behavior. The impedance magnitude is low at fr but increases rapidly and reaches a maximum at the fa. Between fr and fa both real (R) and imaginary part of impedance (X) show a non-linear increasing behavior. In manufactured PT, some small spurious modes were also observed near the resonance frequency [21]. The optimum matched load of PT is given by ( RL = 1/ωCd2) where it has maximum efficiency. For this PT, the optimum resistive matching load is 1 kΩ.
4.
PEH IMPEDANCE MATCHING WITH PT
Piezoelectric material has an inherent capacitance. The harvesters based on this material are tuned to work at low vibration frequencies as most real-world application frequencies are near or less than 100 Hz range. To harvest optimum power, usually, the PEH output voltage is first rectified. Then, the power converter circuits perform two tasks: 1) impedance matching, and 2) step-down the PEH output voltage to the level required by the application load circuit. To achieve the above two tasks together, a step-down piezoelectric transformer is utilized in this research paper. Fig. 5 and 6 show the PT’s complex, inductive and varying input impedance. While working in its resonance frequency range, PT will work as an inductor and efficiently transfer the power from its input to the output. Using the step-down PT will result in the lower output voltage as compared to its input. Thus step-down PT can be used to accomplish the task of impedance matching and simultaneously step down the input voltage.
To efficiently transfer the input power to the output, PT is operated at its resonance frequency range. A switching circuit is required to convert the PEH rectified dc voltage to drive the PT at kilohertz frequency. Different techniques can be used to drive PT in resonance such as push-pull, class-E, and half-bridge [22, 23]. The half-bridge driving circuit technique is the simplest and also generates the highest conversion efficiency [22]. The proposed scheme for emulated resistive impedance matching is presented in Fig. 7 where the rectified PEH voltage (Vrect) is used to drive the PT in its resonance frequency using a half-bridge circuit. The output of PT has to be rectified again for usage in electrical or electronics load applications. By controlling the frequency, duty-cycle and dead-time between the switches of the half-bridge converter, the PT input, and output power can be controlled. As the PEH can generate a finite amount of power, this will affect the current (irect) drawn from the dc-link and change in dc-link voltage (Vrect) level. The piezoelectric energy harvester will see the change in dc-link voltage and current as an emulated resistance. While working, some part of the current will flow back to the dc-link from the PT based half-bridge converter, but the average flow of current is always in to the presented PT based impedance matching converter. The PT is operated at first vibration mode and its driving frequency is between the resonance and antiresonance frequency in this vibration mode. Optimum performance from PT is obtained when it is driven with sinusoidal input. This can be achieved with the half-bridge circuit using the inductor before the PT input. The inductor acts as a filter to generate a sine voltage signal for the PT input [24]. This technique will make PT working in Zero Voltage Switching (ZVS) condition to reduce the circuit losses, but it requires an inductor and again using an inductor to lower the wire resistance will result in a relatively large size inductor. An inductorless half-bridge driving circuit can be used where PT is driven with trapezoidal wave (soft-square voltage) in ZVS condition [25]. However, if ZVS technique is not implemented, power dissipation will occur in MOSFET switches due to hard-switching. Removing inductor and driving PT with soft square voltage will result in some loss of PT efficiency [22]. The analytical model of an inductor-less ZVS piezoelectric transformer driven by half-bridge converter circuit is given in [26]. The half-bridge (HB) driving circuit uses MOSFET switches, whose internal capacitance adds up with the PT input capacitance, which is determined by the thickness and number of the PT input layers. The MOSFET switches have parasitic capacitances, among them the drain to source parasitic capacitance. COSS, as shown in Fig. 7, plays a major role in half-bridge circuits switching energy loss [27]. In the inductor-less half-bridge driving circuit operating in ZVS condition, to discharge the parasitic and PT input capacitances; a higher dead-time between the switches is required [28-30]. When the sufficient dead-time is achieved, the current spikes due to hard-switching that result in switching losses are removed. The switches are turned off and on at PT resonance frequency by the driving circuit, which also controls the dead-time between the switches. The driving circuit controller has to adjust frequency and dead-time simultaneously to achieve the ZVS condition [31].
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The ZVS will occur when the voltage across the PT input capacitance is the same as Vrect when high-side MOSFET (S1) is turned ON, and it is zero volts when low-side MOSFET (S2) is turned ON. The PT input capacitance design plays an important role in achieving ZVS condition as explained in [31]. The experimental steady-state current and voltage waveforms of the proposed converter in ZVS with PT operating at its fundamental resonance frequency are shown in Fig. 8. The dead-time between the high-side (S1) and low-side (S2) switches is almost comparable with the switches on time and the significant current changes can be observed in this time period. The half-bridge converter operation is divided into different time steps in Fig. 8, and the behavior of the proposed impedance matching converter during each time step is shown in Fig. 9 [26, 28, 29, 32]. The direction of the current arrows (irect and iin-PT) shows the direction of the oscilloscope current probe used for measuring the waveforms in Fig. 8.
Step-1 [t0 - t1]: To achieve ZVS, the voltage across low-side switch (Vin,PT) was zero. At time t0 the Lo-side MOSFET switch is turned ON at zero voltage condition as Cd1 is fully discharged. At t0 the forward current flows into the PT, then it changes the direction. The current freewheels through the low-side switch. No current flows into the HB circuit from the PEH and resonance is achieved in this period. Step-2 [t1 - t2]: When the dead-time starts, both switches are OFF. During the dead-time, the parasitic capacitor COSS-Hi and COSS-Lo and Cd1 are charged by the reverse resonant current. Therefore, the voltage across the lowside MOSFET increases until it is slightly higher than the Vrect voltage level, till t2 when high-side switch body diode starts to be conducted. Step-3 [t2 – t3]: Both switches are OFF. The Voltage across the high-side switch is zero, so its body diode is forward biased. Current flows from the PT to the Crect capacitor. In this case, the current flows-back to the dclink side and the converter is simply represented by its equivalent circuit in Fig. 9(e) where the value of Cx and Rx are presented in [26, 28]. Step-4 [t3 – t4]: The Hi-side switch turns ON. The voltage across PT is the same as the Vrect voltage. The current flows from Crect to PT. This current reaches the peak when the switch is turned OFF. It is the only time duration when the current is supplied by the dc-link directly to the PT. The supplied current (irect) is the same as the current through the PT inductor (iLm). The slope of the current also shows an inductive component behavior. Step-5 [t4 – t5]: Dead-time starts. Both switches are off. The current flow from Crect gradually reduces to zero at the end of this cycle. The current from the capacitance COSS-Hi and COSS-Lo flows to the PT, due to which the Vin,PT reduces to zero volts. The dead time has to be large so that Vin,PT can go to zero by discharging COSS-Lo to achieve ZVS. Step-6 [t5 – t6]: The dead time continues. As the COSS-Lo is discharged to 0 V, its body diode is now forwardbiased. The output of PT under ZVS condition will be sinusoidal in-phase current and voltage, which needs to be rectified and smooth again for the mostly dc voltage operated load applications. The PT offers high efficiency with optimum matched load, but higher resistive load values can also be used with reduced efficiency [31]. If inductor to be used with a half-bridge circuit for the PT discussed in section-3 and 4, a value of 1.5 mH is required to achieve ZVS. 5.
EXPERIMENTAL RESULTS
To evaluate the proposed system, the circuit presented in Fig. 7 was constructed on PCB. For AC signal rectification BAS3007A diode bridge rectifiers were used at PEH and PT output. For the S1 and S2 switches, 2N7002 MOSFET was utilized. The IR21091S half-bridge driver IC was used with 10 V external power supply and TTL level square wave signal. The 10 Vpeak-peak signals were generated by the half-bridge driver IC for S1 and S2 switches gate to source voltage (VGS). The Dead-time of up to 5 µs was controlled manually using a 200 kΩ variable resistor with the driver IC. The externally powered half-bridge driver chip current consumption was 700 µA at 10 V in a steady-state condition. A. Impedance matching converter efficiency To find the performance of the proposed impedance matching converter presented in Fig. 7, PEH was implemented by its equivalent circuited presented in Fig. 2 and driven by the AC current source (iS). The output of the PT was directly connected with the resistive load (RL,ac). The impedance matching converter (inductor-
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less half-bridge piezoelectric transformer based resonant converter) efficiency at different input power levels is shown in Table 2. The impedance matching converter efficiency is highly dependent on the PT load resistance (RL,ac) and the input power. For each input condition, the minimum load resistance (RL,ac) was found at which ZVS can be achieved. When RL,ac is near the PT matching load value of 1 kΩ, the efficiency increases. For the given input conditions, the ZVS was not achieved at RL below 2 kΩ. At iS of 1 mA and RL,ac of 3 kΩ the efficiency of 82% was achieved and maximum efficiency of 86% was achieved for input power greater than 10 mW and RL,ac of 2 kΩ. The efficiency graph for different RL,ac under iS of 1 mApeak is shown in Fig. 10. The efficiency of the converter falls as the frequency is increased. The power consumption of the externally powered driving chip is not considered in the efficiency calculations.
B. Testing with Single Cantilever PEH The piezoelectric harvester discussed in section-2 was mounted on a shaker with a base acceleration of 6.80 m/s2 at 37.8 Hz vibration frequency. The input vibration conditions are similar as mentioned in section 2. The output of the PEH was the rectifier, which was then connected with inductor-less half-bridge PT driving circuit as shown in Fig. 7. The output of the PT was rectified, smoothed, and connected with a resistance decade box. The proposed system was experimentally tested under different load conditions, and the optimum resistive load (RL) range where ZVS can be achieved was found. Under each resistive load condition, the driving circuit switching frequency was varied in the resonance frequency range. For each frequency, the dead-time resistor was manually set to achieve the ZVS condition.
The PEH rectified voltage (Vrect) under different load conditions and varying driving frequency is shown in Fig. 11(a). The voltage rises linearly to a peak value, after which the ZVS condition cannot be maintained and it falls gradually. The output voltage (Vout) and power harvested (Pout) at different load conditions are shown in Fig. 11 (b) and (c). The ZVS condition was achieved under shown RL load, if the load resistance is kept lower, the frequency bandwidth for ZVS reduces. The ZVS can be achieved at lower RL value but at a cost of reduced bandwidth. The minimum load value PT was able to drive in ZVS condition was 3 kΩ and 1.97 mW of power (Pout) was extracted, which is 35 % efficiency as compared to the standard interface circuit maximum generated power shown in Fig. 4. When the ZVS condition is achieved, the PT output voltage and current are in-phase and voltage is stepped-down. The driving chip frequency, switches on time and dead-time between them, are shown in Fig. 11 (d) under a fixed load condition. In the experimentally tested ZVS range, the single PEH impedance was not optimally matched. In the single PEH standard circuit testing case, the maximum power of 5.64 mW was harvested at 30 kΩ of resistive load and 13.01 volts dc voltage. The minimum load value (optimum PT matched load is 1 kΩ) PT was able to drive in ZVS condition was 3 kΩ and 1.97 mW of power (Pout) and at this condition, the Vrect was about 4 V. This result was obtained at single PT driving frequency point of 76.4 kHz, a low ZVS band-width was obtained near the optimum PT matching load (RL). This Vrect value was not at an optimum position, much lower as achieved in standard circuit case in Fig. 4. It was observed, that to achieve higher overall efficiency, Vrect in proposed impedance matching converter should be same as in the standard circuit (resistive impedance matching) testing case. Also, as the single PEH was not able to generate the enough power, the optimum load could be connected. C. Testing with four parallel connected PEHs To analyze the presented system performance with multiple harvesters, four PEHs, the same as presented in section 2 were mounted on a shaker, and each harvester output was first rectified and then connected in parallel. For four PEHs connected in parallel, the standard circuit open circuit rectified voltage was 11.976 V. To find the optimum power from the four PEHs in parallel, standard circuit testing resulted in 2.31 mW of optimum power at Vrect of 5.04 Volts at 37.8 Hz of vibration frequency and a matching load resistance of 9 kΩ. The vibration amplitude was set to lower the optimum Vrect voltage in four parallel-connected PEHs so that it can be achieved by the PT based Half-bridge converter in the ZVS condition. The four parallel PEHs were used to generate enough power so that the optimum resistive load (RL) can be attached to the PT output.
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The experimental results of proposed impedance matching technique with four parallel connected PEH are presented in Fig. 12. As Vrect increases with a frequency under ZVS condition to attain the value in case of optimum resistive impedance matching, the efficiency of the proposed impedance matching decreases. However, the efficiency of the proposed system increases as compared to the resistive impedance matching with the standard interface circuit. The half-bridge PT based impedance matching converter input (dc-link) to the PT output load (RL) maximum efficiency remained between 70 to 75 % under different RL conditions. The proposed system achieved near 63% efficiency at 4 kΩ of PT output resistive load as compared to the standard circuit.
6.
CONCLUSIONS
A new technique is presented in which the output impedance of the piezoelectric energy harvester is matched with a piezoelectric transformer based inductor-less half-bridge converter circuit working in ZVS condition. The presented system matches the piezoelectric energy harvester impedance, and the power harvested is transmitted to the isolated load without any magnetic component. Experimental results show that the presented inductor-less half-bridge driving circuit operating the piezoelectric transformer in ZVS condition has a maximum efficiency of 80%. About 60% efficiency is achieved when the output of presented technique is compared to the optimum resistive load condition of the standard interface circuit. The power harvested is low as the piezoelectric transformer output load and optimum size is not achieved as required for high-efficiency power conversion. Also, the presence of small spurious modes in the vicinity of PT resonance frequency affects its efficiency. By using the linear Vrect trend in proposed impedance matching technique, the adaptive controller can also be implemented easily to control the half-bridge switches dead-time to harvest the maximum power. The converter presented in this paper can also replace the bi-directional converter presented in [14, 15]. Future work involves an improved piezoelectric transformer design, with optimum dimensions, matched output load and without any spurious modes. The analytical and simulation techniques will be used to study the effects of the piezoelectric energy harvester parameters on the presented impedance matching converter design. The implementation of the circuit at integrated circuit level will also be explored for overall increased circuit efficiency and reduce losses. Among the goal of the further study will be to manufacture the piezoelectric transformer alongside the piezoelectric energy harvester cantilever beam. ACKNOWLEDGEMENT This work was supported by the *Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20142020103970)
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Authors Biography Hamid Jabbar received his Bachelor of Mechatronics Engineering degree from National University of Sciences and Technology, Pakistan in 2002 and MS in Electronics and Communications Engineering from Myongji University Korea in 2008. He completed his PhD in electrical engineering from Hanyang University, Seoul in Feb. 2017. He is Research Assistant Professor in department of electrical engineering, Hanyang University, Seoul, Korea and his research area includes the application of piezoelectric materials for energy harvesting and power conversion. He has over 10 years of experience is design and development of mechatronics systems. His research interests include circuit designing and mechatronics systems. Hyun Jun Jung received the Bachelor of engineering (electrical) degree from Seoul National University of Science and Technology, Seoul, South Korea in 2012. He received his Master of electrical engineering degree from Hanyang University, Seoul, South Korea in 2014 and Ph.D. degree from Hanyang University in the field of energy harvesting system in Feb. 2017. His research interest includes mechanical and electrical modeling of energy harvesting system and wireless sensor network.
Nan Chen was born in Xi`an, P. R. China, in 1988. She received her B.S. degree in Electronic Information Engineering from Xinyang Normal University, Xinyang, P. R. China, in 2010; and her M.S. degree in Software Engineering from Northwestern Polytechnical University, Xi’an, P. R. China, in 2013; where she has been working towards her Ph.D. degree since 2013. Her current research interests include analog and mixed-signal integrated circuit designs, and power electronics such as digitally controlled DC-DC switching converters and energy harvesting circuits. Dae Heung Cho was born in Seoul, Republic of Korea in 1988. He received the B.Sc. degree from Hanyang University, Seoul, Republic of Korea. He undergoes M.Sc in same university now. His major is electronic, electrical and communication engineering.
Tae Hyun Sung received a B.A. (1982), an M.S. (1987) in inorganic material engineering from Hanyang University and a Ph.D. (1991) in Material Science and Engineering from Tokyo Institute of Technology. He worked at International Superconductivity Technology Center (ISTEC) as a researcher (1992). He was in Massachusetts Institute of Technology (MIT) as a Post doc. (1995). He worked at the Korea Electric Power Research Institute (KEPRI) as a group leader of superconductivity group. Also, he was selected as a “Top 100 Engineers” by IBC (International Biographic Centre), is the member of the National Academy of Engineering of Korea (NAEK). Since 2009, he has been professor of Department of Electrical Engineering, Hanyang University. His research interests include superconductivity, electrical material, energy storage device, and piezoelectric energy harvesting technology.
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Fig. 1: A typical system for impedance matching and power conversion from a piezoelectric energy harvester.
Fig. 2: Piezoelectric energy harvesting impedance magnitude and phase along with its equivalent circuit.
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Fig. 3: Piezoelectric energy harvesting open circuit voltage with the experimental setup.
Fig. 4: Piezoelectric energy harvester rectified voltage and power harvested under different resistive loads.
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Fig. 5: Piezoelectric transformer along with its equivalent circuit and parameters when the output is open-circuited.
Fig. 6: a) Impedance magnitude and phase, b) real (R) and imaginary (X) part of the PT impedance at first resonance frequency mode range measured with output terminals open.
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Fig. 7: Proposed scheme to connect the piezoelectric energy harvester with step-down piezoelectric transformer
Fig. 8: Experimental steady-state waveforms at the ZVS conditions.
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Fig. 9: Half-bridge PT based converter circuit operation in ZVS conditions at different time steps.
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Fig. 10: Impedance matching converter efficiency at iS = 1 mApeak
Fig. 11: Experimental result with single PEH under varying PT resonance frequency and PT output resistive load; a) rectified PEH output voltage (Vrect), b) PT rectified output voltage, c) PT output power (Pout) response, d) switches on-time and deadtime between them.
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Fig. 12: Experimental result with four parallel connected PEH under varying PT resonance frequency and PT output resistive load; a) rectified PEH output voltage (Vrect), b) PT rectified output voltage, c) PT output power (Pout) response, d) switches on-time and dead-time between them.
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Table 1: Specifications of the piezoelectric energy harvester
Material Piezoelectric Cantilever substrate
Dimensions length x width x thickness (mm) 38×38×0.18 60×40×0.18
Soft-PZT steel
Table 2: Impedance matching converter efficiency iS
Vrect
mApeak 1 2 3 4
V 3.11 4.52 6.96 9.79
Pin = Vrect x irect mW 1.315 4.68 10.57 20.06
Vout,ac
RL,ac
Pout,ac=
V 1.81 3.08 4.31 4.18
kΩ 3 2.5 2 2
mW 1.09 3.79 9.29 17.35
𝑽𝒐𝒖𝒕,𝒂𝒄 𝑹𝑳,𝒂𝒄
Converter Efficiency = Pout,ac/Pin % 82.82 80.98 86.73 86.46