ISSN: 2319-5967
ISO 9001:2008 Certified
International Journal of Engineering Science and Innovative Technology (IJESIT)
Volume 3, Issue 3, May 2014
Design and Implementation of a Single Stage
Ac-Dc Energy Harvesting Converter for Micro
generator
L.Bala kumar, Pheba Cherian, S.Joyal Isac
Abstract— This paper presents a single stage ac-dc energy harvesting converter topology for efficient and optimum
energy harvesting from low-voltage microgenerator. The proposed converter utilizes the bidirectional current conduction
capability of MOSFETs in order to avoid the use of a first –end of the bridge rectifier. It is working in discontinuous
conduction mode and offers a resistive load to the microgenerator. Analysis and modeling of the converter explained in
detail is presented. For low power applications, the power consumption of gate drive and control circuits should be
minimal. In this paper, they are specifically designed to consume very low power. A well suitable startup circuit and
auxiliary dc supply circuit is proposed for the implementation of the converter. A low voltage microgenerator is used to
verify the performance and operation of the converter and the gate drive circuits and finally simulated by PSIM software
package.
Index Terms— Energy harvesting, ac-dc conversion, boost converter, low power, low voltage, P SIM
I. INTRODUCTION
The development of energy-efficient semiconductor devices has reduced the power requirements of electronic
circuits. This has led to the development of wireless electronic devices like sensor nodes, medical implants, etc.,
which require only a few milliwatts for their operation. They can be powered by harvesting ambient energy from
the environment in the form of light, vibration, heat, etc. [1]–[8]. Vibration, being a ubiquitous source of low
power, has been a research focus for many years [1]–[10]. Electromagnetic microgenerators are particularly
popular due to high energy density and are hence considered for this work.
Fig 1. Resonance-based inertial electromagnetic microgenerator
Such micro generators are typically spring–mass systems, in which mechanical energy is converted to electrical
energy by electromagnetic damping (Fig. 1). The output of an inertial microgenerator is typically around a few
hundred millivolts of ac. The conventional power converters [Fig. 2(a)] reported for vibration energy harvesting
mostly consists of a front-end diode bridge rectifier followed by a standard buck or boost converter. This
arrangement of two-stage power conversion has several disadvantages for the electromagnetic microgenerator.1)
Diode voltages in a bridge rectifier are difficult to overcome for low input voltage; 2) input current is much higher
than output current, leading to more losses in diodes and 3) a rectifier offers a nonlinear load, which makes the
converter unsuitable for energy harvesting.
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Fig.2. Block diagram: (a) Two-stage power conversion
(b) Reported dual-polarity boost converter
A dual-polarity boost converter [Fig. 2(b)] topology for such ac–dc conversion was recently reported in above.
This converter uses two inductors, and the output dc bus is split into two series-connected capacitors. In [9], the
authors presented a direct ac–dc converter which utilizes the bidirectional current-conduction capability of
MOSFETs for direct ac–dc conversion. The converter utilizes only one inductor and charges the output capacitor
paper presented only open-loop simulation and experimental results using an ideal sinusoidal voltage source. The
reported converter also utilized continuously without any line voltage sensing. The paper presented only open-loop
simulation and experimental results using an ideal sinusoidal voltage source. The reported converter also utilized
two n-channel MOSFETs to form a bidirectional switch. However, floating gate drivers and indirect feedback
control were needed for closed-loop operation. For a low-power application, it is difficult to realize such drivers. In
this paper, the proposed split-capacitor topology utilizes an n- and p-MOSFET pair to form the bidirectional switch
which does not need floating gate drivers. This paper also includes the detailed analysis and modeling for the
converter.
The rest of this paper is organized as follows. Section II presents a brief review of the converter. In Section III, the
analysis and modeling of the converter is described. Section IV presents the design of the control and driver
circuits. A prototype of the converter is developed to verify its operation and performance with the designed
controller. A suitable startup circuit along with a dual dc supply for stand-alone operation of the energy-harvesting
system is also presented. The simulation and experimental results are presented in Section V. Finally this paper is
concluded.
II. REVIEW OF THE CONVERTER
The converter topology utilizes a bidirectional switch for its operation. However, a single semiconductor device
capable of both bidirectional conduction and blocking capability does not exist. A MOSFET channel is typically
capable of conduction in both directions when it is sufficiently turned ―ON.‖ However, due to the presence of the
inherent body diode, it cannot block current in the reverse direction. A bidirectional switch in the present case is
realized by connecting the drain of an n-MOSFET to the source of a p-MOSFET so that their body diodes block the
current in the opposite direction. The MOSFETs are turned on and off at the same instants and thus can conduct and
block currents in both directions. This bidirectional switch is referred as S1 throughout this paper. It should also be
noted that the converter is operated in discontinuous conduction mode (DCM). This operation has many
advantages: 1) A constant duty cycle extracts constant power from the source, enabling a simple control; 2) A
converter operating with a constant duty cycle has only fundamental and switching harmonic frequency
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International Journal of Engineering Science and Innovative Technology (IJESIT)
Volume 3, Issue 3, May 2014
components and thus offers a resistive load to the microgenerator; and 3) DCM operation reduces switching losses
which are significant in such low-power applications
The circuit diagram for the split-capacitor topology is shown in Fig. 3. A single inductor L is used for the boost
operation in both half cycles. The converter utilizes three capacitors to boost the low ac voltage. Split capacitors C2
and C3 are charged in alternate half cycles as explained next.
Positive half cycle: The inductor current increases linearly from zero when switch S1 turns ON. When S1 is turned
OFF, the body diodes block the circulating current. Diode D1 is forward biased, and the current flows into
capacitor C2 to complete the charging process.
Negative half cycle: In the negative half cycle, the current rises in the opposite direction when S1 is turned ON.
However, this time, when S1 is turned OFF, diode D1 remain OFF and diode D2 is forward biased. The inductor
energy is transferred to capacitor C3.
Fig. 3. Proposed direct ac–dc converter: Split-capacitor topology.
The three capacitors share energy through charge recycling. It should be noted that even though voltages across
capacitors C2 and C3 show large variations, the duty cycle can be effectively controlled to maintain a steady
voltage across output capacitor C1.
III. CONVERTER ANALYSIS
A. Input Side Analysis
A bidirectional switch in the input side is used to build the inductor current. The input current waveform of the
converter can be considered as shown in Fig. 4(a). Consider any kth switching cycle of the boost converter as
shown in Fig. 4(b), where Ts is the time period of the switching cycle, D is the duty cycle of the converter, dfTs is
the boost-inductor current fall time (or the output diode conduction time), Vi is the voltage of the microgenerator
with amplitude Vp, and V0 is the converter output voltage. The converter switching frequency is much higher than
the generator output ac voltage frequency. Therefore, Ts is much smaller than the time period of the input ac cycle
(Ti). In the analysis presented henceforth, circuit parasitics are ignored, and it is assumed that output voltage
remains constant over a switching cycle. The peak value of the inductor current (ipk) for a general switching cycle
can be obtained as in
IPk = m1 · DTs = VikDTs/L (1)
The inductor current fall time can be found as
DfTs = iPk/m2 = iPk/ (Vo − Vik). (2)
During this switching cycle, the energy (Ek) transferred from the input to the output can be obtained as
Ek = Vik · iPk · Ts (D + df)/2 …. (3)
Where Vik = Vp sin (2π · k · Ts/Ti). Defining N = Ti/Ts, the average input power Pi of the converter can be
obtained in
Pi= (1/Ti)·_Nk=1Ek =(1/Ti)·Nk=1 Vik · iPk · T(D +df)/2….(4)
Therefore, the average input power Pi can be derived as
Pi = V 2pD2Ts β
4L1
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2π
β = ∫ (1/π) ∫sin2 θ · [1 − (Vp/V0) · sin θ]-1dt …..(5)
0
Fig.4. Converter input current: (a) Input current over one cycle of microgenerator output voltage. (b) Input current
over one switching cycle
Fig. 5. DCM switch model
Fig.6. (a) Port 1: Voltage V1 and current I1 (b) Port 2: Voltage V2 and current I2
B. Output Side Analysis
A switch-diode low-frequency model can be formed by averaging over the switching frequency, as shown in Fig. 5.
This allows us to study the input-current- and output-voltage-related characteristics.
The port currents I1 (t) and I2 (t) can be averaged over the switching cycle according to the DCM operation. Their
waveforms are shown in Fig. 6. The three time periods DTs, dfTs, and dsTs correspond to the three states of the
converter in each switching cycle. During period DTs, switch S1 is ON. The period dfTs corresponds to the time
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when S1 is turned off but the inductor current is still nonzero, and dsTs is the discontinuous state of the converter.
The waveforms for the port 1 voltage and current are shown in Fig. 6(a). Their averaged values can be derived as
V1avg (t) =df [Vo (t) − Vi (t)] + (1 − D)Vi(t)
I1avg (t) =0.5ipeak (D + df) Ts.
. . (6)
Ts
The current I2 (t) corresponds to the diode current. The voltages V2 (t) and I2 (t) are similarly averaged as
V2avg (t) = Vo (t)
I2avg (t) =0.5ipeakdfTs
Ts
. . (7)
Fig.7. Converter equivalent circuit
Fig. 8.Proposed energy-harvesting system using PSIM
Fig. 9.Bidirectional Switch S1.
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IV. CONVERTER IMPLEMENTATION
In low-power applications, the implementation of auxiliary circuits (gate driver and control circuits) is very
important. They should be chosen such that they consume very low power and are able to drive the circuit in steady
state. In this section, a gate driver circuit and an auxiliary dc supply with startup circuit are presented. A control
strategy with its analog implementation is also described.
Fig.10. Proposed gain values for P MOSFET & N MOSFET
A. Feedback and Control Circuit
A simple PI controller is implemented to regulate the output voltage. As can be seen in Fig. 11, the negative rail of
the output voltage is not the same as the ground of the control circuit which is the common node of the split
capacitors. Therefore, a specific feedback circuit for the converter has to be designed. For the split-capacitor
topology, capacitors C2 and C3 are chosen to have equal value. The voltages across these two capacitors are shown
in Fig. 11. It can be found from previous analysis that the average voltage across any of these split capacitors is half
of the output voltage. Therefore, this voltage can be used as feedback to the controller
B. Gate Driver Circuit
The bidirectional switch is realized using an n-MOSFET (Mn) and a p-MOSFET (Mp) connected in series (Fig.
13).The source of the n-MOSFET is connected to the ground. The source of the p-MOSFET is connected to the
drain of the n-MOSFET. Such an arrangement gives switch S1 bidirectional current capability and ability to block
the reverse conduction through body diodes. The schematic of the gate driver circuits is shown in Fig. 11 along
with the overall converter system. In this converter, the MOSFETs are driven with respect to the common node of
the split capacitors, i.e., C2 and C3. Comparator CP1 is used to drive the n-MOSFET. Since the source of the
n-MOSFET Mn is connected to the ground, it can be driven with a conventional low-side driver. Comparator CP2
is used to drive the p-MOSFET Mp using a negative gate pulse. It should be noted that the MOSFET Mp is driven
with respect to ground instead of its source Sp. However, since the voltage drop across MOSFET Mn is very small
during conduction, the gate drive voltage can turn on the MOSFET Mp properly. The inputs to the comparator CP2
are connected in such a way that the output is negative when the value of the duty cycle is higher than the sawtooth
waveform. Therefore, both the MOSFETs are turned on at the same time.
C. Startup Circuit
It can be observed that the controller and driver circuits require a dual dc supply for their operation. The low ac
input voltage cannot be used to start the converter. A suitable circuit, as shown in Fig. 11, is used as startup circuit.
The voltage nodes V+ A and V− A denote the positive and negative dc voltages which power the controller and gate
driver in the converter system. Batteries E1 and E2 provide the startup power to charge capacitors Ca and Cb
through diodes Dc and Dd. With the controller and driver circuits operating, capacitors Ca and Cb start getting
charged by the microgenerator through diodes Da and Db. This boost mechanism is similar to the charging of
capacitors C2 and C3 in the split-capacitor topology. Furthermore, these capacitors are designed to maintain steady
dc voltage while powering the auxiliary circuits. In steady state, the voltages across capacitors Ca and Cb can be
approximately related as V + A = V0 and V −A = −V0. The nominal value of battery voltages are chosen to be less
than steady-state voltages of capacitors Ca and Cb. Therefore, when the converter is operating, capacitors Ca and
Cb are charged by the energy harvesting converter. Under this condition, diodes Dc and Dd become reverse biased
and batteries E1 and E2 are cutoff from the circuit.
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V. RESULTS
The prototype for the converter was developed to verify its operation. The values of the key components of the
converter are presented in Table I.
Table 1.Converter Circuit Components
PARAMETER
VALUE
PARASITIC
Switching frequency
Output voltage
Capacitor C1
Capacitor (C2 &C3)
Inductor (L)
N & P MOSFET
Diode(D1,D2,Da,Db,Dc,Dd)
10000Hz
3.3V
22µF
4.7µF
10 µH
20V,6A
23V,1A
Resr=33mΩ
Resr=5mΩ
Resr=80mΩ
Rdson(N&P MOSFET)=30mΩ40mΩ
-
The auxiliary circuits were developed using micro power components. The gate driver circuit is realized using a
low voltage comparator TLV2760 from Texas Instruments which has a nominal current requirement of 20 μA per
channel.
A resonance-based linear microgenerator capable of producing 400-mV ac at 108 Hz is used as input
to the converter. An electro dynamic shaker is used to produce vibrations for the electromagnetic microgenerator.
The converter is operated in DCM to reduce switching losses.
Fig.11. PWM waveforms for MOSFETs Mn and Mp (1 V/div)
A resonance-based linear micro generator capable of producing 400-mV ac at 108 Hz is used as input to the
converter. An electro dynamic shaker is used to produce vibrations for the electromagnetic microgenerator. The
converter is operated in DCM to reduce switching losses.
Fig.12. Input waveform from microgenerator in PSIM
The gate pulses for MOSFETs Mn and Mp generated by comparators Cp1 and Cp2 are shown in Fig. 13. It can be
seen that the gate voltage of the n-MOSFET (Vgn) is positive when the p-MOSFET gate voltage (Vgp) in negative.
Therefore, both the MOSFETs are turned ON at the same instant.
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Fig.13. Gate pulse waveform in Bi directional switch in PSIM
Fig.14.Required output constant voltage in PSIM
ACKNOWLEDGEMENT
I would like to say thanks to beloved God, my parents, my guide Ms. V.Bharanigha and Mr. M.Ram Kumar.
VI. CONCLUSION
This paper has presented a split-capacitor-based ac–dc boost converter for low-power low-voltage energy
harvesting. A bidirectional switch, based on series-connected n- and p MOSFETs, has been proposed in this paper.
The converter utilizes this bidirectional switch to boost the low ac microgenerator voltage to a steady dc voltage in
both the input half cycles. The modeling and analysis for the converter has been presented. The auxiliary circuits in
the energy-harvesting converter—the gate driver circuits and the control circuit—have been designed for
low-power operation. A suitable startup circuit, an auxiliary dc supply, and a feedback circuit are proposed for the
implementation of the converter. Experimental results for a low-voltage microgenerator have been presented to
verify the operation of the converter and the proposed auxiliary circuits. The designed auxiliary circuits draw
minimal power and are able to operate the converter at a high efficiency.
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AUTHOR BIOGRAPHY
L.Bala Kumar received the B.E.degree in Electrical and Electronics Engineering from the Sasurie College of
Engineering, Tirupur, Tamil Nadu, India, in 2009 and completed M.Tech degree in Power Electronics and Drives in BS
Abdur Rahman University, Chennai, India and currently working for the post of Assistant Professor at Electrical and
Electronics Engineering department in Saveetha Engineering College, Chennai, Tamil Nadu, Chennai
Pheba Cherian received the B.Tech.degree in Electrical and Electronics Engineering from the Amal Jyothi College of
Engineering, Kottayam, Kerala, India, in 2010 and completed M.Tech degree in Electronics and Control Engineering in
SRM University, Chennai, India and currently working for the post of Assistant Professor at Electrical and Electronics
Engineering Department in Saveetha Engineering College, Chennai, Tamil Nadu, Chennai
S,Joyal Isac received the B.E.degree in Electrical and Electronics Engineering from the Sree Sowdambika Engineering
College, Arupukottai, Tamil Nadu, India, in 2009 and completed M.E degree in Power Systems Engineering,
Velammal Engineering College, Chennai, India and currently working for the post of Assistant Professor at Electrical
and Electronics Engineering Department in Saveetha Engineering College, Chennai, Tamil Nadu, Chennai.
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