HIGH EFFICIENT BRIDGELESS BOOST RECTIFIER FOR LOW VOLTAGE ENERGY HARVESTING APPLICATIONS

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International Journal of Electrical Engineering & Technology (IJEET)
Volume 7, Issue 3, May–June, 2016, pp.93–105, Article ID: IJEET_07_03_008
Available online at
http://www.iaeme.com/ijeet/issues.asp?JType=IJEET&VType=7&IType=3
ISSN Print: 0976-6545 and ISSN Online: 0976-6553
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© IAEME Publication
HIGH EFFICIENT BRIDGELESS BOOST
RECTIFIER FOR LOW VOLTAGE ENERGY
HARVESTING APPLICATIONS
K. Ajith
Assistant Professor,
Department of Electrical and Electronics Engineering,
Kakatiya Institute of Technology & Science,
Warangal, Andhra Pradesh, India
T. Praveen Kumar
Assistant Professor
V. Ramaiah
Professor
Sai Teja
PG Scholar
ABSTRACT
A single phase ac-dc bridgeless boost rectifier for low voltage energy
harvesting applications is proposed in this paper. The conventional bridge
type boost converters for low voltage energy harvesting requires more
components hence they suffer from high power loss and require more number
of energy storage components like inductors and capacitors. Conventional
converters can be modeled for boost operation or buck-boost operation alone.
The proposed converter overcomes the above mentioned draw backs of
conventional converter. Detailed analysis of proposed convertor is also
presented under boost, buck-boost mode operations. The proposed converter

K. Ajith, T. Praveen Kumar, V. Ramaiah and Sai Teja
operation is analyzed using MATLAB/SIMULINK environment both open loop
and closed loop conditions.
Keywords: Bridge Less Boost Rectifier, Buck-Boost Converter, Electro-
Magnetic Generator, Energy Harvesters
Cite this Article: K. Ajith, T. Praveen Kumar, V. Ramaiah and Sai Teja, High
Efficient Bridgeless Boost Rectifier For Low Voltage Energy Harvesting
Applications. International Journal of Electrical Engineering & Technology,
7(3), 2016, pp. 93–105.
http://www.iaeme.com/ijeet/issues.asp?JType=IJEET&VType=7&IType=3
1. INTRODUCTION
Wireless sensor systems are receiving increasing interest since they offer flexibility,
ease of implementation and the ability to retrofit systems without the cost and
inconvenience of cabling. Furthermore, by removing wires there is the potential for
embedding sensors in previously inaccessible locations. At present, the majority of
wireless sensor nodes are simply battery-powered. Despite measures such as low
power techniques for communicating (e.g. IEEE 802.15.4 and Zigbee protocols) and
the intelligent management of the sensor node’s power consumption, batteries will
still require periodical replacement. Replacing batteries is not compatible with
embedded applications nor it is feasible for networks with large numbers of nodes.
The advances made in low power wireless systems present an opportunity for
alternative types of power source. Solutions such as micro fuel cells and micro turbine
generators are capable of high levels of energy and power density. However, they
involve the use of chemical energy and require refueling. Energy harvesting
approaches that transform light, heat and kinetic energy available in the sensor’s
environment into electrical energy offer the potential of renewable power sources
which can be used to directly replace or augment the battery. Such renewable sources
could increase the lifetime and capability of the network and mitigate the
environmental impact caused by the disposal of batteries [1-2].
Kinetic energy harvesters convert mechanical energy present in the environment
into electrical energy. The past decade has seen an increasing focus in the research
community on kinetic energy harvesting devices. Typically, Kinetic energy is
converted into mechanical energy using electromagnetic, piezoelectric, or electrostatic
transduction mechanisms. In comparison to electrostatic and piezoelectric transducers,
electromagnetic transducers outperform in terms of efficiency and power density. The
limitation of the energy harvesters is low voltage output. In order to overcome this,
we employ bridge boost rectifier to step up the voltage. However, the bridge rectifier
would incur considerable voltage drop, making the low voltage rectification
infeasible. Though we employ CMOS diodes with low-voltage drops, they require
additional bias networks or external comparators. Thus, both complexity and power
loss of the circuitry would increase. Another approach to maximize the conversion
efficiency in low-voltage rectification is to use bridgeless direct ac-dc converters,
which employ split capacitors, bidirectional switches or two parallel dc-dc converters
to condition positive and negative input voltages separately. The above topologies
employ large capacitors to suppress ripple voltage, which makes these topologies
impractical due to limited size of energy harvesters. Also, the additional switches
would incur extra switching losses and driving circuit dissipations. To overcome
above drawbacks, a new bridgeless boost rectifier is considered with unique
integration of boost and buck-boost converters. The circuit employs only two power

High Efficient Bridgeless Boost Rectifier For Low Voltage Energy Harvesting Applications
electronic switches, simplifying the circuit and thereby reducing cost and size of the
rectifier. In addition, power losses are also minimized leading to high efficiency [1-5].
ELECTRO-MAGNETIC GENERATOR
A general diagram of an electromagnetic generator is demonstrated in Fig. 1, where k
is spring stiffness constant; m is the proof-mass; DE and DP represent electrical and
parasitic dampers, respectively. Essentially, the energy harvesting system consists of a
spring, a proof mass, and an electrical damper. The extrinsic vibrations excite the
internal oscillation between the proof mass (magnet) and electrical damper (coils) [2].
Figure 1 General diagram of electromagnetic micro-generator
The internal oscillation produces a periodically variable magnetic flux in the coil,
which induces a corresponding alternating output voltage. The output voltage level of
the micro scale and meso scale energy harvesting devices is usually in the order of a
few hundred millivolts depending on the topology of device. The output ac voltage
should be rectified, boosted, and regulated by power converters to fulfill the voltage
requirement of the loads. Nonetheless, miniature energy harvesting systems have rigid
requirement on the size and weight of power electronic interfaces. In energy
harvesting systems, power electronic circuit forms the key interface between
transducer and electronic load, which might include a battery. The micro generator
output has to be processed by a suitable power converter before providing to the
electronic loads. The electrical and physical characteristics of the power conditioning
interfaces determine the functionality, efficiency, and the size of the integrated
systems. The power electronic circuits are employed to
1. Regulate the power delivered to the load, and
2. Actively manage the electrical damping of the transducers so that maximum power
could be transferred to the load.
2. MATHEMATICAL MODELING
I. Large Signal Modeling
In order to develop a large signal model and analyze the steady-state operation of the
considered bridgeless converter circuit, several assumptions are made during one
switching cycle [3-6].
1. The output filter capacitor C is large enough to keep the output voltage Vo constant.
2. The input is a sinusoidal voltage source. The switching frequency is much higher than
the input voltage frequency. During each switching cycle, the input voltage could be
treated as a constant voltage source and be expressed as (1), where Ti is the period of
input voltage.
(t) = Vmsin (2πt/Ti) (1)
Internal series resistances of passive components are not taken into account for
convenience in calculations.

According to Fig. 2, during the boost operation cycle [t0,t3), the average transistor
voltage , can be expressed as
= (1-d1-d2) (t) + d2 (t) (2)
The average switch current is2a (t) is derived by integrating the is2 (t) waveform as
depicted in Fig. 6(a) during one switching period
is2a(t)= (3)
The average diode voltage (t), and the average diode current id2a (t) can be
expressed, respectively, as
(t) = d1 (t) + (1-d1-d2) ( (t) - (t)) (4)
Figure 2 Boost Operation Waveform
id2a (t) = (5)
According to the inductor volt-second balance, d2(t) can be related to d1(t) as
d2 = d1 (6)
From (2) - (6), by manipulation we obtain the following,
= (7)
From (7), we can derive that the average switch voltage is proportional to
the averageswitch current is2a (t).In other words the switch behaves like an equivalent
resistor with resistance Re (d1), where
Re (d1) = (8)
Also we obtain,
(t) (t) = (9)
Substituting (8) into (9), the equivalent power out of the diode can be presented as

p (t) = (t) (t) = (10)
According to (10), the diode behaves like a controlled power source, the power of
which is equals to the power consumption of the equivalent resistor Re (d1).Similar
derivations can also be applied to the buck-boost operation cycle [ ), and the same
expressions can be derived. So, The switch behaves like an equivalent resistor Re (d1)
and the diode behaves like a controlled power source, the power of which is equals to
the power consumption of the equivalent resistor Re (d1).Thus, the switch network
could be modeled by a loss-free resistor and a dependent power source, as illustrated
in Fig. 3.
Figure 3 Averaged switch model of general two-switch network in Discontinuous
Conduction Mode (DCM)
The equivalent circuit of the topology in the steady state can be modeled by
replacing the switch network with its averaged model. In the steady-state condition, in
which the inductor and capacitor can be, respectively, replaced with short circuit and
open circuit, the circuit can be simplified to the circuits of Fig. 4, where d1(t) = D.
Figure 4 DCM dc equivalent circuits representing
(a) Boost operation and (b) Buck-boost operation
II. Converter Analysis
In boost operation mode [0, Ti /2), the instantaneous power delivered to the equivalent
resistor in dc equivalent circuit model can be expressed as
p(t) = = (11)
The instantaneous current flowing through the load resistor R can be expressed as
i(t) = (12)
Hence, the instantaneous power delivered to the load is expressed as
pL(t) = Vo i(t) = (13)

For each boost cycle [0, Ti /2), the total energy delivered to the load is obtained as
Ein = (14)
Now, the energy consumed by the load is determined by
E0= (15)
According to the principle of conservation of energy, the energy delivered to the load
is equal to the energy provided by the power supply.
Eo= Ein
= (16)
The integral part of (16) is a function of boost ratio (Vm/Vo), which would be
infinitely close to Ti/2, if the ratio is much larger than 1. For low-voltage energy
harvesting, the input voltage is usually smaller than 0.5 V. However, most of the
electronic loads should be driven by a 3.3 V voltage. With the boost ratio larger than
6.6, the integral would become equal to Ti/2. Thus, (16) becomes
(17)
Similar equations can also be driven for the buck-boost operation (Ti/2 to Ti)
according to the model provided in Fig. 4(b).Different from the boost operation, the
power consumed by the loss-free resistor is all delivered to the load. The derivations
of the buck boost dc equivalent model lead to
(18)
From equations (16) and (18), the required relations of the converter can be
established. If the gain boost ratio is much higher than unity; equation (17) can be
preferred.
III. Design Procedure
If the voltage gain is much larger than unity, according to (17) and (18), the boost and
buck-boost operations share the same duty cycle D, which is determined by the boost
ratio (Vo/Vm), the inductance of the inductor L, and the switching frequency fs.
Equation (19) defines the commutation relationship of the duty cycle of the proposed
boost/buck-boost converter [13].
D = d1 = = (19)
The boost ratio is defined according to the specific application, while the load
resistance R is dependent on the output power level. With the specified power and
voltage demands, the inductance is designed according to the desired range of duty
cycle and switching frequency. The larger the switching frequency is, the smaller the
inductance would be. In order to design a smaller inductor with purpose of obtaining
smaller size and weight, a higher switching frequency is preferred. However, the
higher the switching frequency is, the higher the switching loss would be. A tradeoff
between the size of inductor and the switching loss should be taken into account in the
design process. Both the boost and buck-boost operations of the converter provide the
same inductor current ripple, which can be expressed as

(20)
The maximum current ripple corresponds to the peak input voltage. According to
previous analyses, the inductor, diodes, and MOSFETs share the same value of
current ripple, which is defined in the following equation given by
(21)
From (21), the current ratings of all those components could be found. Thus the
value of inductor can be designed as follows: Let us assume maximum current flows
through inductor given by ipk.. Therefore the peak or maximum value of inductor
current is given by
D2 TS = ipk (22)
We have, Condition for having discontinuous conduction is
d2< 1- d1max (23)
Let us assume d1max=0.8 and then d2<0.2 for designing for discontinuous
conduction. From (22) we have
d2 = < 0.2 (24)
L < (25)
The voltage ratings of the MOSFETs and diodes are normally chosen higher than
V0 with an appropriate margin for safe operation. The turn-on resistances of
MOSFETs and the forward voltage drop of diodes are the major components, which
impact the efficiency. The output capacitor C is assumed to be very high so that it
filters out the voltage ripple and also maintains the output voltage V0 constant. The
charging and discharging currents of the capacitor decides the voltage ripple. The
value of bus capacitance can be calculated from the relation given below
C = (26)
Where, = Peak inductor current
dV = Ripple voltage (0-10% of output voltage)
The validity of the assumptions can be checked by using the two given equalities
mentioned
Ts<< RC and
3. CONTROL STRATEGY
For a dynamic Electromagnetic energy harvester system, if the external excitation
frequency is different from the intrinsic resonance frequency, the Piezo Electric
Instrument (PEI) should be able to match its input impedance with the internal
impedance of the harvester so that maximum power point (MPP) could be tracked.
This paper proposed a new topology, which has the maximum power point tracking
(MPPT) capability. However, the main objective of this paper is to introduce the
circuit topology, which is capable of satisfying the voltage requirement (3.3 V) of an
electronic load. Thus, a voltage feedback control loop is utilized to regulate the load

voltage. The simplified scheme of the controller and power stage is illustrated in Fig.
5. The converter is designed to operate in DCM [13].
Figure 5 Control Circuit for the Boost Converter
The output voltage is filtered by a passive low-pass filter and then fed to the
analog-to-digital converter (ADC) of the controller. The difference between the ADC
output and the desired voltage is calculated and compensated through the PI algorithm
to generate an adjustable duty cycle signal. The switching signals of S1 and S2 are
dependent on the polarity of the input voltage. A sign detector is used to determine the
input voltage polarity. The sign detector is composed of a voltage reference, an op-
amp, and the on-chip analog comparator. The op-amp operates as an analog adder,
where a dc bias (voltage reference) is added to the input voltage. The signal
summation is compared with the voltage reference to detect the polarity [13].
4. SIMULATION & RESULTS
The proposed direct AC to DC converter circuit with boost and buck-boost converter
is simulated using MATLAB. The converter is simulated for both open loop and
closed loop operation. A closed loop operation of the circuit is modeled and
simulated. Here, the actual voltage is compared with the reference voltage (3.3V) and
the error signal is given to a PI controller. Then, a hysteresis control is given by using
a saturation block which limits the PI controller output (control voltage) to remain
within certain limits. The control voltage is again compared with a saw tooth
waveform from the signal generator so that gate pulses are being given to the boost
and buck-boost converter switches according to the detection of positive and negative
peaks, respectively. The simulation is done for obtaining a dc output voltage, VO
=3.3V from the electromagnetic micro generator which is modeled as a sinusoidal
voltage source with the specification, Vin = 400mV and frequency 100Hz. For
converter operation the SIMULINK model of boost and buck-boost converters
created, circuit parameters used for the simulation are given in the following Table 2.
Table 2 Components and Parameters in the Simulink
PARAMETER VALUE
VIN 0.4 V ,100 HZ
VO 3.3 V
Power Inductor 4.7 µH
Switching Frequency 50 kHz
Filter Capacitor 100 µF
Load 200 ohm

Theoretical Calculations: The corresponding duty ratio and theoretical values of the
converter for R= 200Ω and R = 300Ω are calculated using mathematical modeling
and designing procedure analyzed in previous chapters and are as follows in the Table
3 given below.
Table 3 Theoretical Calculations for Different Loads
Vin(V) V
O
(V) F
S
(kHZ) L(µH) R
L
(Ω ) D iL,max(A)
0.4 3.3 50 4.7 200 .5656 0.9532
0.4 3.3 50 4.7 300 .4618 0.786
The simulation has been performed for both conventional bridge converter and
considered bridgeless boost converter. The results of the conventional and new
converter considered for boosting can be compared. The results show that the gain of
bridgeless converter is much more than that of the conventional bridge converter.
I. Conventional Bridge Boost Converter
The boost converter Simulink model and simulation results are shown in Fig 6.1 and
Fig 6.2 .The operation is performed for 4 V ac, 100 Hz and output settles at 5 V dc.
The gate signals to the transistor are given using the 555 timer at 50 KHz and 60%
duty ratio. The boost converter employs a diode bridge rectifier, which converts input
ac to dc and then presents the boost circuit which undergoes a second stage i.e.
boosting the rectified dc to the level of load requirement.
II. Open Loop Operation
The Simulink model and simulation results of open loop operation of the bridgeless
boost converter employed are shown in Fig. 8. The output is settled at 3 Volts. But the
requirement of the load is 3.3 V, so irrespective of any variations in the input the
output voltage must be constant to feed the load.
Fig. 6 Simulation Model of Conventional Bridge
Converter
Fig.7 Simulation Results of Conventional Converter

Fig. 8 Simulation model of open loop operation
of bridgeless converter
Fig. 9 Open loop simulation results
We can observe from above, open loop simulation does not yield the expected
output as shown in the results in Fig. 9. Also, as input to the vibrational source vary
depending on the environment and surroundings, the output voltage will also vary
accordingly. In order to maintain constant voltage irrespective of any variations in the
input and other circuit parameters, a controlled operation is essential. Therefore,
closed loop operation is preferred which controls the output as per the requirement of
the load.
III. Closed Loop Operation
The Simulink model of closed loop operation of the bridgeless boost converter
employed is shown in Fig. 10 and simulation results are also presented. We can now
clearly observe that after reaching steady state the output voltage is constant and as
per requirement of the load. The pulses in the simulation circuit are generated by
comparing carrier wave with a reference wave, according to the required frequency
and duty ratio. The duty ratio automatically settles to the required in order to maintain
voltage constant.
 The actual voltage is compared with the reference voltage (3.3V) and the error signal
is given to a PI controller
 Then, a hysteresis control is given by using a saturation block which limits the PI
controller output (control voltage) to remain within certain limits.
 The control voltage is again compared with a saw tooth waveform from the signal
generator so that gate pulses generated are being given to the boost and buck-boost
converter switches according to the detection of positive and negative peaks,
respectively.
With 200 Ω resistive load the converter is capable of tightly regulating output
voltage and delivering 52 mW to the load. Fig. 11 shows the waveforms of input
voltage, gate signals of both switches, as well as the input current. During the positive
input cycle, S1 is turned ON, while S2 is driven by the boost control scheme. When
the circuit operates in the negative input cycle, S2 is turned ON, while S1 is controlled
under the buck-boost conditioning strategy. The waveforms of inductor current are
depicted in Fig. 12. We can observe that from Fig. 6.8 the conduction of diodes alter
every half cycle according to the boost and buck boost operation.

Fig. 14 and Fig.15 demonstrate the boost and buck-boost operations
correspondingly. The gate pulses of the switches can be observed in accordance with
the input current. Closed-loop voltage control successfully stabilizes the duty cycle at
0.72 at the steady state. As seen from Fig. 16, the output voltage is regulated at 3.3-V
dc with approximately 0.2 V (i.e., 12%) voltage ripple. Closed-loop voltage control
successfully stabilizes the duty cycle at 0.78 at the steady state. The output voltage for
R =300 Ω is demonstrated in Fig. 17.
Fig. 10 Closed loop Simulink model of the
converter
Fig. 11Closed Loop Simulation Results (R=200
Ω)
Fig. 12Inductor Current of Closed Loop Operation Fig. 13. Diode Voltages and Currents

Fig. 14 Input Current and Buck Boost Gate Pulse
Vg1
Fig. 15 Input Current and Boost Gate Pulse Vg2
Fig. 16 Input and Output Voltages(R=200Ω) Fig. 17 Input and Output Voltages(R=300Ω)
5. CONCLUSION
A single stage ac–dc topology for low-voltage low-power energy harvesting
applications is proposed in this project. The topology uniquely combines a boost
converter and a buck-boost converter to condition the positive input cycles and
negative input cycles, respectively. Only one inductor and one filter capacitor are
required in this topology. The closed loop simulation has been carried out in power
electronic simulation software MATLAB. Output voltage is tightly regulated at 3.3 V
with 12% ripple through closed-loop voltage control. Using a low pass filter at the
output the waveform can be smoothened by reducing the ripple content. The
measured conversion efficiency is 66% at 52.5mW. In comparison to state-of-the-art
low-voltage bridgeless rectifiers; this study employs the minimum number of passive
energy storage components, and achieves the maximum conversion efficiency. The
future research will be focused on investigating and designing integrated three-phase
power electronic interfaces for electromagnetic energy harvesting.

REFERENCES
[1] Beeby, S. P., M. J. Tudor, et al. (2006). Energy harvesting vibration sources for
microsystems applications. Measurement Science and Technology 17(12): R175-
R195.
[2] S. P. Beeby, R. N. Torah, M. J. Tudor, P. Glynne-Jones, T. O’Donnell, C.R.
Saha, and S. Roy, A micro electromagnetic generator for vibration energy
harvesting,” J. Micromech. Microeng., 17(7), pp. 1257–1265, Jul. 2007.
[3] P. D. Mitcheson, T. C. Green, and E. M. Yeatman, Power processing circuits for
electromagnetic, electrostatic and piezoelectric inertial energy scavengers,
Microsyst. Technol., 13(11–12), pp. 1629–1635, Jan.2007.
[4] Anoop D Nath, K Radhakrishnan and Eldhose K A, Low-Voltage Direct AC-DC
Boost Converter for Microgenerator Based Energy Harvesting, International
Journal of Advanced Research in Electrical, Electronics and Instrumentation
Engineering 2(3), March 2013.
[5] G. D. Szarka, B. H. Stark, and S. G. Burrow, Review of power conditioning for
kinetic energy harvesting systems, IEEE Trans. Power Electron, 27(2), pp. 803–
815, Feb. 2012.
[6] E. Lefeuvre, D. Audigier, C. Richard, and D. Guyomar, Buck-boost converter for
sensorless power optimization of piezoelectric energy harvester, IEEE Trans.
Power Electron., 22(5), pp. 2018–2025, Sep.2007.
[7] Y.-H. Lam, W.-H. Ki, and C.-Y. Tsui, Integrated low-loss CMOS active rectifier
for wirelessly powered devices, IEEE Trans. Circuits Syst. II: Express Briefs,
53(12), pp. 1378–1382, Dec. 2006.
[8] C. Peters, J. Handwerker, D. Maurath, and Y. Manoli, A sub-500 mV highly
efficient active rectifier for energy harvesting applications, IEEE Trans. Circuits
Syst. I: Regular Papers, 58(7), pp. 1542–1550, Jul.2011.
[9] T. T. Le, J. Han, A. V. Jouanne, K. Mayaram, and T. S. Fiez, Piezoelectric micro-
power generation interface circuits, IEEE J. Solid-State Circuits, 41(6), pp. 1411–
1420, Jun. 2006.
[10] S. Cheng, R. Sathe, R. D. Natarajan, and D. P. Arnold, A voltage multiplying
self-powered AC/DC converter with 0.35-V minimum input voltage for energy
harvesting applications, IEEE Trans. Power Electron, 26(9), pp. 2542–2549, Sep.
2011.
[11] P. D. Mitcheson, T. C. Green, and E. M. Yeatman, Power processing circuits for
MEMS inertial energy scavengers, in Proc. Des., Test, Integr. Packag. MEMS/
MOEMS, Stresa, Italy, Apr. 2006, pp. 1629–1635.
[12] D. Kwon and G. A. Rincon-Mora, A rectifier-free piezoelectric energy harvester
circuit, in Proc. IEEE Int. Symp. Circuits Syst., Taipei, Taiwan, Jun. 2009, pp.
1085–1088.
[13] M.Gopinath, Hardware Implementation of Bridgeless PFC Boost Converter Fed
Dc Drive. International Journal of Electrical Engineering & Technology, 3(1),
2012, pp. 131–137.
[14] L.Raguraman and P.Sabarish, Integrated Bridgeless PWM Based Power
Converters. International Journal of Advanced Research in Engineering and
Technology, 4(5), 2013, pp. 17–23.
[15] Alireza Khaligh, Haoyu Wang, Yichao Tang, A Bridgeless boost rectifier for
low voltage energy harvesting applications, IEEE Transactions on power
electronics 28(11), pp. 5206–5214.

HIGH EFFICIENT BRIDGELESS BOOST RECTIFIER FOR LOW VOLTAGE ENERGY HARVESTING APPLICATIONS

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HIGH EFFICIENT BRIDGELESS BOOST RECTIFIER FOR LOW VOLTAGE ENERGY HARVESTING APPLICATIONS