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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 74-88 © IAEME
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SOLAR POWERED SYNCHRONOUS BUCK CONVERTER FOR LOW
VOLTAGE APPLICATIONS
Lopamudra Mitra
Asst. Professor, Dept. of Electrical & Electronics Engineering,
Silicon Institute of Technology, Bhubaneswar, India.
ABSTRACT
The current resurgence of interest in the use of renewable energy is driven by the need to
reduce the high environmental impact of fossil based energy systems. Future sustainability depends
on use of different renewable energy sources. A photovoltaic generation system is becoming one of
the important renewable energy due to absence of fuel cost, low maintenance and environment
friendliness. This paper presents synchronous buck converter based PV energy system for portable
applications; especially low power device applications such as charging mobile phone batteries are
considered. Here, the converter topology used uses soft switching technique to reduce the switching
losses which is found prominently in the conventional buck converter, thus efficiency of the system
is improved and the heating of MOSFETs due to switching losses reduce and the MOSFETs have a
longer life. The DC power extracted from the PV array is directly fed to the synchronous buck
converter to suit the load requirements. The whole system is simulated using MATLAB-Simulink
environment.
Keywords: MOSFET, ZVS, ZCS, Synchronous Buck Converter, PV Module.
I. INTRODUCTION
India imports more than 80% of its oil; hence it has a huge dependency on external sources
for development. With depleting fossil reserves worldwide, there has been a threat to India’s future
energy security. Hence, the government of India is investing huge capital on development of
alternative sources of energy such as solar, small hydroelectric, biogas and wind energy systems
apart from the conventional nuclear and large hydroelectric systems.
For environmental concern and increase of peak power demand PV solar cells has become an
alternative energy source for green and clean power generation. Solar cells are steadily gaining
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acceptance in our society. These are usually adapted for either grid connected or standalone
applications. It is becoming a boon for the rural community for whom electricity had become only an
imaginary thing. Due to a sudden up rise of mobile usage, and it’s cheaper availability, it has become
an affordable thing to have. But its recharging is cause of concern for the rural counterparts for
whom electricity is not so abundant. These lesser electrical demand can be met with these PV solar
cells.
But these PV cells are not so popular due to their high initial cost. But due to stiff
competition among the manufacturers these cost are also scaling down. After building such an
expensive renewable energy system, the user naturally wants to operate the PV array at its highest
energy conversion output by continuously utilizing the solar power developed by it at different time.
For low voltage applications such as mobile charging and laptop power supply etc, the output of the
PV array should be regulated in order to match the dynamic energy requirement of the load [3]. In
addition, the modulation process should be very efficient so that the system losses can be decreased
considerably. For this efficient regulation of DC voltage, synchronous buck converter is proposed in
the paper.
1.1 PV ENERGY SYSTEMS FOR PORTABLE APPLICATIONS
This energy generation system consists mostly of capacities below 100W. They have a huge
range of applications ranging from powering calculators, educational toys, solar lamps, traffic
signals, mobile chargers, etc. They are usually made up of poly crystalline material of solar cells due
to their higher energy density over a small area and fits in the portable applications. However, this
system is not highly commercialised due to battery technology required to store the power generated
and high cost of poly crystalline silicon solar cells. They generally use lithium ion batteries [4] to
store energy due to its high energy capacity and light in weight. These systems come handy when
power is required on move and has a potential to revolutionise the current era of electronics with free
power on move. The simple mobile charger based on PV energy system consists of a small solar
module generally made of poly crystalline silicon, connected to the electrical load through a
buck/boost converter for regulation of voltage at the load end [5]. This regulation is usually done
using a feedback loop that senses the output voltage and tries to keep it at the desired output voltage
required.[21].
However, higher input voltages and lower output voltages have brought about very low duty
cycles, increasing switching losses and decreasing conversion efficiency. So in this paper, the
efficiency of the synchronous buck converter is optimised by eliminating switching losses using soft
switching technique. The voltage-mode soft-switching method that has attracted most interest in
recent years is the zero voltage transition [1],[2], [4]-[8], [10], [11], [13]-[20], [22]-[24], [26]-[27],
[29].This is because of its low additional conduction losses and because its operation is closest to the
PWM converters. The auxiliary circuit of the ZVT converters is activated just before the main switch
is turned on and ceases after it is accomplished. The auxiliary circuit components in this circuit have
lower ratings than those in the main power circuit because the auxiliary circuit is active for only a
fraction of the switching cycle; this allows a device that can turn on with fewer switching losses than
the main switch to be used as the auxiliary switch.
Various converter topologies have been proposed in the literature [4]-[6]. In the conventional
buck converter usually switching losses are higher due to high switching frequency of
operation of MOSFET and losses in the freewheeling diode is more due to larger forward voltage
drop (0.4V). Consequently, it reduces the overall efficiency of the converter systems (typically less
than 90%). The possible solutions are to increase the efficiency of the converter system is described
as follows. First solution is to replace the freewheeling diode by MOSFET switch. Here MOSFET
acts as a rectifier. So forward voltage drop in the switch can be reduced. Second solution is to
incorporate the auxiliary MOSFET across the main MOSFET along with resonant circuits (Lr& Cr)
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[7].This combinations constitute a soft switching technique, so that the switching loss can be reduced
in the main switch. The resultant dc-dc converter topology is said to be synchronous buck converter.
Here main MOSFET “s” is switched on and off synchronously with the operation of the MOSFET
switch ‘s2.[24-25]
In this paper an attempt has been taken to analyze such converter for PV energy system based
low power applications especially to charge the batteries used in mobile phones. This converter
topology enables to provide simple and cost effective solution in the charging circuit. This converter
using soft switching technique for low power application [20-25 ] is found in literatures but in this
paper this converter is directly connected to the modelled PV module for low voltage application not
known to be present in literature is presented.
1.2 OPERATION OF A SYNCHRONOUS BUCK CONVERTER
Fig 1: Synchronous Buck Converter.
The operation of synchronous buck converter with ZVS and ZCS technique for reducing the
switching loss of main switch is described as follows [9]
Mode 1: Before starting of this mode diode of S2 was conducting and at time t1, mosfet S1 is
turned on through ZCT which is caused by the current passing through Lr. In this mode Lr and Cr are
resonance with each other and it ends when diode of S2 stops conducting and when current through Lr
reaches I0.
Fig 2: Mode I
Mode2: Lr and Cr continue to resonate. At t1 the synchronous switch S2 is turned on under ZVS.
This mode ends when S2 is switched of and iLr reaches its maximum value.
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Fig 3: Mode 2
Mode 3: At the starting of this mode, iLr reaches its peak value iLrmax. Since iLr is more than load
current I0, the capacitor Cs will be charged and discharge through body diode of main switch S,
which leads to conduction of body diode. This mode ends when resonant current iLr falls to load current
I0. So current through body diode of main switch S becomes zero which results turned off of body
diode. At the same time the main switch S is turned on under ZVS. The voltage and current
expressions for this mode are:
ILr = I0; VCr = VCr1; VCr is some voltage which can found basing on other modes.
Fig 4: Mode3
Mode 4: In this mode, the main switch is turned on under ZVS. During this mode growth
rate of iS is determined by the resonance between Lr and Cr. The resonance process continues
and iLr starts to decrease. This mode ends when iLr falls to zero and S1 is turned off through
ZCS.
Fig 5: Mode4
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Mode 5: In the previous mode, S1 is turned off. The body diode of S1 begins to conduct because of
discharging of Cr. The resonant current iLr starts increasing in reverse direction and finally becomes
zero. The mode ends when body diode of S1 is turned off.
Fig 6: Mode 5
Mode 6: Since in the previous mode, body diode of S1 is turned off, the MOSFET S alone carries the
current now. There is no resonance in this mode and circuit operation is same as conventional PWM
buck converter.
Fig 7: Mode 6
Mode 7: At starting of this mode, the main switch S is turned off with ZVS. The schotkey diode D
starts conducting. The resonant energy stored in the capacitor Cr starts discharging to the load
through the high frequency schottky diode DS for a very short period of time, hence body – diode
conduction losses and drop in output voltage is too low. This mode finishes when Cr is fully
discharged.[26]
Fig 8: Mode 7
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Mode 8: Before starting of this mode, the body diode of switch S2 is conducting. But as soon as
resonant capacitor Cr is fully discharged, the schottky diode is turned off under ZVS.
During this mode, the converter operates like a conventional PWM buck converter until the
switch S1 is turned on in the next switching cycle.
Fig 9: Mode 8
II. MODELLING OF PV ARRAY
The use of equivalent electric circuits makes it possible to model characteristics of a PV cell.
The method used here is implemented in MATLAB Simulink for simulations. The same modeling
technique is also applicable for modeling a PV module.
The simplest model of a PV cell is shown as an equivalent circuit below that consists of an
ideal current source in parallel with an ideal diode. The current source represents the current
generated by photons (often denoted as Iph or IL), and its output is constant under constant
temperature and constant incident radiation of light.
Fig 10: Equivalent Circuit of ideal cell with load
There are two key parameters frequently used to characterize a PV cell. Shorting together the
terminals of the cell, as shown in Figure 5, the photon generated current will follow out of the cell as
a short-circuit current (Isc). Thus, Iph = Isc. As shown in Figure, when there is no connection to the
PV cell (open-circuit), the photon generated current is shunted internally by the intrinsic p-n junction
diode. This gives the open circuit voltage (Voc). The PV module or cell manufacturers usually
provide the values of these parameters in their datasheets.
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The output current (I) from the PV cell is found by applying the Kirchoff’s current law
(KCL) on the equivalent circuit shown in Figure.10.
I = Isc − Id (1.1)
where: Isc is the short-circuit current that is equal to the photon generated current, and
Id is the current shunted through the intrinsic diode.
The diode current Id is given by the Shockley’s diode equation:
Id = Io exp( qVd /kT −1) (1.2)
where: Io is the reverse saturation current of diode (A),
q is the electron charge (1.602×10-19 C),
Vd is the voltage across the diode (V),
k is the Boltzmann’s constant (1.381×10-23 J/K),
T is the junction temperature in Kelvin (K)
Replacing Id of the equation (1.1) by the equation (1.2) gives the current-voltage relationship of the
PV cell.
Ι = Isc − Io (exp(qV/kT) −1) (1.3)
where: V is the voltage across the PV cell, and I is the output current from the cell.
The reverse saturation current of diode (Io) is constant under the constant temperature and found by
setting the open-circuit condition as shown in Figure 6. Using the equation (1.3),
let I = 0 (no output current) and solve for Io.
0 = I sc − I o(exp (qVsc / kT )−1) (1.4)
I sc= I o(exp (qVsc / kT )−1) (1.5)
I o = I sc/(exp (qVsc / kT )−1) (1.6)
To a very good approximation, the photon generated current, which is equal to Isc, is directly
proportional to the irradiance, the intensity of illumination, to PV cell . Thus, if the value, Isc, is
known from the datasheet, under the standard test condition, Go=1000W/m2 at the air mass (AM) =
1.5, then the photon generated current at any other irradiance, G (W/m2), is given by:
I scIG = (G/Go) I scIGo (1.7)
Figure shows that current and voltage relationship (often called as an I-V curve) of an ideal
PV cell simulated by MATLAB using the simplest equivalent circuit model. The PV cell output is
both limited by the cell current and the cell voltage, and it can only produce a power with any
combinations of current and voltage on the I-V curve. It also shows that the cell current is
proportional to the irradiance.
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Fig 11: I-V Characteristics of a PV Cell
A single PV cell produces an output voltage less than 1V, about 0.6V for crystalline silicon
(Si) cells, thus a number of PV cells are connected in series to archive a desired output voltage.
When series-connected cells are placed in a frame, it is called as a module. Most of commercially
available PV modules with crystalline-Si cells have either 36 or 72 series-connected cells. A 36-cell
module provides a voltage suitable for charging a 12V battery, and similarly a 72-cell module is
appropriate for a 24V battery. This is because most of PV systems used to have backup batteries,
however today many PV systems do not use batteries; for example, grid-tied systems. Furthermore,
the advent of high efficiency DC-DC converters has alleviated the need for modules with specific
voltages. When the PV cells are wired together in series, the current output is the same as the single
cell, but the voltage output is the sum of each cell voltage, as shown in Figure 12.
Fig 12: PV Cells connected in series to make up a PV Module
Also, multiple modules can be wired together in series or parallel to deliver the voltage and
current level needed. The group of modules is called an array.
III. SIMULATION RESULTS AND DISCUSSIONS
The simple diode equivalent model is take into considered and PV module is modelled and
various effects of temperature and irradiance are shown below.
0 5 10 15 20 25 30 35 40 45 500
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
module voltage
module
current
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Fig 13: I-V Characteristics with varying Irradiance for G =400 W/m2, 600W/m2 and 1000 W/m2
Fig 14: I-V Characteristics with varying Temperature for T=25ºC, 35ºC and 50 ºC
The following parameters are considered for design:
Vin = 30V ,Vout = 12volts, Iload = 1 amps Fsw = 200 kHz Duty ratio (D) = Vin / Vout = 0.43.
Assume Iripple = 0.3*Iload (typically 30%). The switching frequency is selected at 200 kHz. The
current ripple will be limited to 30% of maximum load.
Fig 15: Simulink model of synchronous buck converter without PV
0 5 10 15 20 25 30 35 40 45 500
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
module voltage
module current
0 5 10 15 20 25 30 35 40 45 500
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
module voltage
module current
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Fig 16: Output Voltage of synchronous buck converter not connected to PV
Fig 17: Output current
Fig 18
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age of synchronous buck converter not connected to PV
current of synchronous buck converter not connected to PV
18: Voltage across main switch S
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age of synchronous buck converter not connected to PV
of synchronous buck converter not connected to PV
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Fig
Fig
Fig
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Fig 19: Current across switch S
Fig 20: Voltage across switch S1
Fig 21: Current across switch S1
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Fig
The voltage waveform of MOSFET ‘S’in fig.
which means the MOSFET is switched on when the voltage
zero power loss across MOSFET ‘S’
resonant inductor (Lr) is used as an auxiliary circuit for causing ZVS for MOSFET ‘S’. The
waveforms shown in fig.18 and fig.
indicates the zero current turn off of MOSFET ‘S1’ (ZCT). It is
resonant inductor.
Fig 23: Synchronous buck conv
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Fig 22: Current across switch S2
of MOSFET ‘S’in fig. reveals the zero voltage switching (ZVS),
the MOSFET is switched on when the voltage across MOSFET is zero, thereby causing
zero power loss across MOSFET ‘S’. The MOSFET ‘S1’along with resonant capacitor (Cr) and
resonant inductor (Lr) is used as an auxiliary circuit for causing ZVS for MOSFET ‘S’. The
nd fig.19&20 describe the current and voltage across MOSFET ‘S1’
indicates the zero current turn off of MOSFET ‘S1’ (ZCT). It is turned off by ZCT because of
Synchronous buck converter connected with PV.
ional Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
reveals the zero voltage switching (ZVS),
is zero, thereby causing
The MOSFET ‘S1’along with resonant capacitor (Cr) and
resonant inductor (Lr) is used as an auxiliary circuit for causing ZVS for MOSFET ‘S’. The
describe the current and voltage across MOSFET ‘S1’
turned off by ZCT because of
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976
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Fig 24: Output Current of synchronous buck converter connected with PV
Fig 25: Output voltage of synchronous buck converter connected with PV
IV. CONCLUSION
The waveforms depict the soft switching phenomena. This converter is used as a DC
converter between PV array and load. Since the switching and conduction losses are reduced, the
system can be used as a high efficient portable device. Besides the main switch ZVS turned
turned-off, the auxiliary switch ZCS turned
and turned-off under ZVS. Hence switching losses ar
current stresses on the main devices do not take place, and the
allowable voltage and current values.
connecting the PV module and with connection of the PV module in MATLAB Simulink
environment and for input voltage of 30V the output voltage of 12V is obtained which can be used
for any low power application fed from PV module and in most cases the of PV is around 15V to
40V depending on temperature and irradiance, hence this converter connected with PV can be used
for portable applications.
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Output Current of synchronous buck converter connected with PV
Output voltage of synchronous buck converter connected with PV
The waveforms depict the soft switching phenomena. This converter is used as a DC
between PV array and load. Since the switching and conduction losses are reduced, the
system can be used as a high efficient portable device. Besides the main switch ZVS turned
off, the auxiliary switch ZCS turned-on and turned-off, the synchronous switch also turned
off under ZVS. Hence switching losses are reduced and the additional voltage and
stresses on the main devices do not take place, and the auxiliary devices are subjected to
es. In this paper the simulation is done for two cases i.e without
connecting the PV module and with connection of the PV module in MATLAB Simulink
environment and for input voltage of 30V the output voltage of 12V is obtained which can be used
power application fed from PV module and in most cases the of PV is around 15V to
40V depending on temperature and irradiance, hence this converter connected with PV can be used
ional Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
Output Current of synchronous buck converter connected with PV
Output voltage of synchronous buck converter connected with PV
The waveforms depict the soft switching phenomena. This converter is used as a DC-DC
between PV array and load. Since the switching and conduction losses are reduced, the
system can be used as a high efficient portable device. Besides the main switch ZVS turned-on and
ronous switch also turned-on
additional voltage and
auxiliary devices are subjected to
In this paper the simulation is done for two cases i.e without
connecting the PV module and with connection of the PV module in MATLAB Simulink
environment and for input voltage of 30V the output voltage of 12V is obtained which can be used
power application fed from PV module and in most cases the of PV is around 15V to
40V depending on temperature and irradiance, hence this converter connected with PV can be used
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87
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[28] Ramjee Prasad Gupta and Dr. Upendra Prasad, “Design of a Pwm Based Buck Boost Dc/Dc
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[29] Arun Kumar Pandey, Prof. S. K. Misra snd Sumit Kumar Misra, “Transient Response
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ISSN Online: 0976-6553.