Download Academic paper: A High Voltage Gain DC

Global Science and Technology Journal
Vol. 3. No. 1. March 2015 Issue. Pp.64-76
A High Voltage Gain DC-DC Boost Converter for PV Cells
Md. Al Muzahid*, Md. Fahmi Reza Ansari**, K. M. A. Salam*** and
Hasan U. Zaman****
Regular small scale photovoltaic (PV) cells do not provide enough high voltage. As a
result, a high voltage gain converter is essential. By using traditional boost converters,
we cannot achieve the required high voltage gain, even with an extreme duty cycle.
Therefore, a DC-DC boost converter is proposed in this paper for achieving high voltage
gain by coupling inductors and the voltage lift technique. It is designed especially for
high voltage conversion ratio applications such as providing power to an electric motor.
By combining coupled inductors and the voltage lift technique, the energy stored in
leakage inductor is recycled. As a result, a reduction of the switch turn-off voltage and
implementation of soft switching turn-on operation are accomplished. In this paper, the
operating principle and steady-state analysis of the continuous-conduction mode are
discussed in detail. The clock frequency of 50 KHz and 24V DC from the photovoltaic
cells can be converted to a 240V DC voltage output, which can be implemented using
this converter. The simulation work was done using PSIM software.
Keywords: DC/DC converter, Coupling Inductor, Continuous conduction mode.
Field of Research: Electrical and Electronic Engineering.
1. Introduction:
In recent years, renewable energy has become increasingly important in the power
distribution system. It opens up to the options to electric consumers whether they want
to receive power from the main electricity source or renewable sources which may
have enough capacity not only to fulfill their own demand but also to provide electric
power to other or alternatively supply to a microgrid.
A microgrid includes various microsources and loads. Microsources are comprised of
various renewable energy applications, such as solar cell modules, wind energy and
fuel cell stacks. Figure 1, shows the schematic diagram of a regular microgrid system
supplied by various microsources. The boost converter is used to increase the output
voltage of the microsources such as PV cells to 220-240V for the dc interface to the
dc load or through the dc-ac inverter (Kwasinski, 2005; Fleming, 2007; Carrasco,
2006). The solar cell module, wind energy and the fuel cell stacks are low-voltage
sources, thus a high step-up voltage gain dc-dc converter is required to regulate the
voltage of the dc-dc interface. In many applications, high-efficiency, high gain step-up
dc-dc converters are required as an interface between the available low voltage
sources and the output loads, which are operated at much higher voltages. (Wai, 2007;
Dwari, 2011; Wu, 2008; Hsieh, 2011)
_____________________________
*, **Dept.
of ECE, North South University, Basundhara, Dhaka, Bangladesh.
E-mail: [email protected]*, [email protected] **
***Associate Professor, Dept. of ECE, North South University, Basundhara, Dhaka, Bangladesh.
Email: [email protected]
****Associate Professor, Dept. of ECE, North South University, Basundhara, Dhaka, Bangladesh
Email: [email protected]
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Figure 1: General Power Conversion System with a High Voltage Gain Boost
Converter
The traditional boost converters cannot provide such a high dc voltage gain, even for
an extreme duty cycle. It also may cause reverse-recovery problem and increase the
rating of all devices, which may eventually cause the conversion efficiency to decrease
and the electromagnetic interference (EMI) can affect the whole system. (Mohan 1995)
The coupled-inductor boost converter can be a substitute for the traditional boost
converters to solve this issue. Because the turn’s ratio of the primary inductor (L1) to
the secondary inductor (L2) of the coupled inductor can be efficient to reduce the duty
ratio and the voltage stress of the switch. (Do, 2011; Tseng, 2013; Lee, 2013; Li, 2012;
Chen, 2013; Zhao, 2013)
To reduce the voltage stress of the primary-side active switches and secondary-side
rectifier diodes, a new voltage-multiplier circuit has been integrated. Based on the
capacitor-divider concept, voltage-multiplier circuit is used to store energy in the output
clamping capacitors and to share voltage stresses of both the active switches and
rectifier diodes, which improves the conversion efficiency and reduces the reverserecovery problem of the rectifier diodes. (Kwon, 2009)
In the proposed converter, the turn’s ratio of the coupled inductors can be designed to
extend the voltage gain. A voltage-lift capacitor offers an extra voltage conversion
ratio. The advantages of the proposed converter are as follows:
1. The converter is characterized by a low input current ripple and low conduction
loss, making it suitable for high power applications.
2. The converter achieves the high step-up voltage gain that renewable energy
systems require.
3. Leakage energy is recycled and sent to the output terminal, which improves
large voltage spikes on the main switch.
4. The voltage stress in the main switch of the converter is substantially lower than
the output voltage.
5. Low cost and high efficiency are achieved by the low rds. (on) and low voltage
rating of the power switching device. Primary windings of the coupled inductors
with Np turns are employed to decrease input current ripple, and secondary
windings of the coupled inductors with Ns turns are connected in series to
extend voltage gain. (Tseng, 2013; Wai, 2005)
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2. Projected Boost Converter and Operating Principle:
Figure 2 shows the schematic diagram of the high voltage gain DC-DC boost converter
for PV cell applications. The equivalent circuit of the coupled inductor includes the
magnetizing inductor Lm, leakage inductors Lk1 and Lk2 and an ideal transformer. Other
components of this converter are a dc input voltage Vin, one power switch, one coupled
inductor, five diodes and three capacitors. The boost converter is designed to generate
a stable voltage VC1 and to supply the energy for the load. Additionally, the diode D4
is turned on when switch S is in turned off period, the voltage across switch S is
clamped at a low voltage level and the energy stored in the leakage inductance is
recycled into C1. Since switch S has a low voltage rating and low conduction
resistance rds (on), the proposed converter has high efficiency. Capacitors VC01 and
VC02 provide the stable energy to output to ensure proper implementation of the
voltage lifting technique. In addition, the turn’s ratio of the coupled inductor is adjusted
to achieve a high step-up voltage gain. In this proposed converter, the duty ratio is
designed to be 0.55 by adjusting the turn’s ratio of the coupled inductor. Thus the
converter can be operated under Continuous Conduction Mode (CCM) and use lowrating switches and diodes to minimize the cost.
Figure 2: Circuit Diagram of the Proposed Converter
To simplify the circuit analysis, the following conditions are assumed:
1. Capacitors C1, C01 and C02 are large enough that V C1, VC01, and VC02 are
constant during one switching period.
2. All semiconductor components are ideal.
3. Most of the energy is stored in the magnetizing inductance L m, which is larger
than the leakage inductances Lk1 and Lk2.
4. Turns ratio of the coupled inductor is n=Ns/Np.
2.1 Operation of the proposed converter:
The proposed converter operating in continuous conduction mode has been analyzed
using the PSIM software. Figure 3, illustrates some typical waveforms under CCM
operation in one switching period. The operating principle of CCM is divided into five
modes during each switching period. The operating modes are described below:
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Figure 3: Some Typical Waveforms under CCM Operation for Boost Converter
Mode 1: (t0-t1)
In this mode, the switch S is turned on. Diodes D1 and D3 are conducting but diodes
D2, D4 and D5 are turned off. The path of the current flow is shown in Figure 4. The
energy of the dc source Vin is transferred to the input inductor L1, the voltage across
the input inductor L1 is Vin and the input current iin is equal to iD1 and is increased. The
capacitor C1 delivers its energy to the magnetizing inductor Lm and the primary
leakage inductor Lk1 and it induces voltage VC1 across them.
Figure 4: The Mode 1 Circuit Diagram
But the magnetizing inductor Lm keeps on transferring its energy through the
secondary leakage inductor Lk2 to inductor L2 and charge capacitor C02. As a result
both currents iLk2 and iLm decrease, until the increasing iLk1 becomes equal to the
decreasing iLm. The energies stored in capacitors C01 and C02 are constantly
discharged to the load R. When the current iLk2 is down to zero at t =t1, the mode 1 is
ended.
Mode 2: (t1-t2)
In this mode, the switch S remains on and only the diode D1 is conducting while rest
of other diodes D2, D3, D4 and D5 are turned off. The path of the current flow is shown
in Figure 5. The energy of the dc source Vin is still stored into the input inductor L1.
The capacitor C1 delivers the energy to the magnetizing inductor Lm and primary
leakage inductor Lk1. The voltage across magnetizing inductor Lm and primary leakage
inductor Lk1 is VC1.Thus, currents iin, iD1, iLm and iLk1 are increased. Both capacitors C01
and C02 are discharged to the load R. When switch S is turned off at t = t2, the mode
2 is ended.
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Figure 5: The Mode 2 Circuit Diagram
Mode 3: (t2-t3)
In this mode, switch S and diode D1 are turned off and the rest of diodes D2, D3, D4
and D5 are conducting. The path of the current flow is shown in Figure 6. The capacitor
C1 is charged by the energy supplied from the dc source Vin and the input inductor L1
connected in series. On the other hand, the primary leakage inductor Lk1 is in series
with capacitor C1 as a voltage source (VC1) to the magnetizing inductor Lm which
delivers its energy to the charge capacitor C01.
The magnetizing inductor Lm also transfers the magnetizing energy through the
coupled inductor T1 to the secondary leakage inductor Lk2 and to the charge capacitor
C02. As a result, currents iin, iD2, iD4, iLm and iLk1 are decreased. But currents iC1, iLk2,
iD3 and iD5 are increased. The energy stored in capacitors C01 and C02 are discharged
to the load R. When the current iC01 is dropped to zero at t = t3, the mode 3 is ended.
Figure 6: The Mode 3 Circuit Diagram
Mode 4: (t3-t4)
In this mode, switch S and diode D1 remain off and diodes D2, D3, D4 and D5 continue
conducting. The path of the current flow is shown in Figure 7. Almost all conditions
remain as Mode 3, except primary leakage inductor Lk1 is in series with capacitor C1
as a voltage source VC1 through the magnetizing inductor Lm, then it is discharged to
the load. Thus, currents iin, iD2, iD4, iLm and iLk1 are gradually decreased, but currents
iC02, iLk2, iD3 and iD5 are still increased. The energy stored in capacitors C01 and C02
is discharged to the load R. When current iLk1 is decreased to zero at t = t4, the mode
4 is ended.
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Figure 7: The Mode 4 Circuit Diagram
Mode 5: (t4-t5)
In this mode, switch S and diode D1 remain off, while diode D4 is turned off and diodes
D2, D3 and D5 are conducting. The path of the current flow is shown in Figure 8. The
dc source Vin and input inductor L1 are connected serially and continues to charge the
capacitor C1.
Figure 8: The Mode 5 Circuit Diagram
The magnetizing inductor Lm continuously transfers magnetizing energy through the
coupled inductor T1 and diode D3 to the secondary leakage inductor Lk2 to inductor L2
and charged capacitor C02. As a result, currents iin, iD2, iD3, iD5, iLk2 and im are
decreased. The energies stored in capacitors C01 and C02 are discharged to the load.
When switch S is turned on at the beginning of the next switching period, the mode 5
is ended.
3. Steady-State Analysis of the Proposed Converter:
The proposed converter is operating in continuous conduction mode (CCM). The
formula derivation, design specification and parameters of the proposed converter are
described below.
3.1 Formula Derivation:
When the switch S is conducting, the voltage across inductor L1 and L2, magnetizing
inductor Lm and Leakage inductors Lk1 and Lk2 are written as,
VL1 = Vin --------------------------------------------------------------------------- (1)
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VL2 = nVin ------------------------------------------------------------------------- (2)
Where, n is the turn’s ratio,
Ns
n=
Np
VLm = L
Lm
m + Lk1
VC1 = kVC1 --------------------------------------------------- (3)
Where, k is the coupling coefficient,
Lm
k=
Lm + Lk1
Applying KVL,
VLk1 = VC1 – VLm = VC1 – kVC1 = VC1 (1-k) -------------------------------- (4)
VLk2 = nVLm ----------------------------------------------------------------------- (5)
When the switch S is turned off, the voltage across inductor L1, magnetizing inductor
Lm and Leakage inductor Lk2 are written as,
Applying KVL,
VL1 = Vin – VC1 ------------------------------------------------------------------ (6)
VLm = VC1 - VC01 – VLk1 -------------------------------------------------------- (7)
VLk2 = nVLm + VL2 – VC02 ------------------------------------------------------ (8)
By using Voltage-Second Balance Principle on Np and Ns of the coupled inductor, the
following equations can be written,
The voltage across the inductor L1 is,
DT
T
∫0 Vin dt + ∫DT(Vin − VC1 )dt = 0 ------------------------------------------ (9)
The voltage across the magnetizing inductor Lm is,
DT
T
∫0 kVC1 dt + ∫DT(VC1 − VC01 − VLk1 )dt = 0 ---------------------------- (10)
From equation (9), VC1 is derived as,
Vin
VC1 = 1−D
----------------------------------------------------------------------- (11)
From equations (4), (10) and (11), VC01 is derived as,
kVin
VC01 = (1−D)
2 ----------------------------------------------------------------- (12)
From equations (2), (5) and (8), VC02 is derived as,
VC02 = nVin --------------------------------------------------------------------- (13)
The output voltage VR express as,
VR = VC01 + VC02 -------------------------------------------------------------- (14)
From equations (12), (13) and (14), VR is derived as,
VR = {
n(1−D)2 +k
}Vin
(1−D)2
--------------------------------------------------------- (15)
As a result, from equation (15), the voltage gain of the proposed boost converter can
be represented as,
V
Vgain = V R =
in
n(1−D)2 +k
(1−D)2
------------------------------------------------------ (16)
So, equation (16) indicates that, the proposed converter achieves a high voltage gain,
if the value of duty cycle is increased or the turn ratio of the coupled inductor is
increased.
From equation (15), the output current iR is derived as,
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iR =
VR
=
R
VC01 + VC02
R
={
n(1−D)2 +k
R(1−D)2
}Vin ------------------------------------- (17)
3.2 Design Specification and Parameters of the Proposed Converter:
Based on the above circuit specification, the circuit design parameters can be
considered as follows.
3.2.1 Duty cycle:
Maximum duty cycle is given by,
V ∗η
D = 1 − in
--------------------------------------------------------------------- (18)
V
o
Where,
Vin = Input voltage
Vo = Output voltage
η = Efficiency of the converter.
From the above equation it can be observed that the output voltage is always greater
than the input voltage and with the increase in the value of duty cycle, the output
voltage is also increased. Therefore, the maximum operating duty cycle is selected
nearly 0.55.
3.2.2 Design of Coupled Inductor and Other Inductors:
Coupled inductor includes the magnetizing inductor Lm, leakage inductors Lk1 and Lk2
and an ideal transformer. Therefore the turn’s ratio n and coupling coefficient k of the
ideal transformer are defined as,
N
n = N s ----------------------------------------------------------------------------- (19)
p
k=L
Lm
m +Lk
------------------------------------------------------------------------ (20)
The boost converter is operated in the continuous current mode i.e. the inductor
current never falls to zero. So the inductor value is given by,
L1 =
Vin (Vo −Vin )
ΔiL1 ∗f∗Vo
nDkVin
L2 = Δi
= Δi
L2 ∗f∗(2D−1)
DkVin
L1 ∗f∗(2D−1)
---------------------------------------------- (21)
--------------------------------------------------------------- (22)
ΔiL1 = Ripple current of the inductor L1.
ΔiL2 = Ripple current of the inductor which should be maintained between 20% to 40%
of the output current and is given by,
(.02 to .04)∗iR ∗Vo
ΔiL2 =
---------------------------------------------------------- (23)
V
in
iR = maximum output current.
From the equations (19), (20), (21), (22) and (23) we calculate the values of L1 = 25µH
and L2 = 100µH. here Lm and Lk values are chosen in such a way that the value of k
is almost equal to 1.
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3.2.3 Design of the Capacitors:
The minimum value of the capacitance is maintained in order to keep the output
voltage when the load value exceeds. The capacitance value is given by the Filter
Capacitor selection,
iR ∗D
C01 or C02 = f∗ΔV
---------------------------------------------------------------- (24)
o
To find the value of capacitor C1 we use the following equation,
Δi ∗(1−D)
C1 = L1f∗V
------------------------------------------------------------------- (25)
in
From the equations (24) and (25) we calculate the values of C01 and C02 = 220µF and
C1 = 1000µF.
3.2.4 Selection of the Switching Device:
Power MOSFET is chosen as switch because of its cost effectiveness, high speed,
low on state resistance, improved gating and high speed power switching. Square
pulse waves operating at a frequency of 50 KHz drive the Power MOSFET.
4. Simulation Results:
The proposed boost converter was simulated using the PSIM Simulator with the
following specifications:
Input DC voltage = 24V
Output DC voltage = 240V
Switching Frequency = 50 KHz
Duty Cycle = 0.55
Inductors L1 and L2 = 25µH and 100µH
Capacitors C1, C01 and C02 = 100µF, 220µF and 220µF
Resistance R = 500Ω
The photovoltaic cell supplies 24V DC to the circuit after being stabilized by a voltage
stabilizer. Power MOSFET operating at 50 KHz frequency is used as the switching
device. A resistive load is used for ideal operation of the circuit.
The input voltage vs. time waveform is illustrated in figure 9 which is supplied by the
solar cell.
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Figure 9: Input Voltage vs. Time Waveform for Proposed Converter
The output voltage of the high voltage gain DC/DC converter vs. time waveform is
demonstrated in figure 10.
Figure 10: Output Voltage vs. Time Waveform for Proposed Converter
The output current vs. time waveform is observed in figure 11. After initial starting
period, the current became almost constant.
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Figure 11: Output Current vs. Time Waveform for Proposed Converter
The voltage across Inductor L1 vs. time waveform is shown in figure 12. Changing
between energized and drained states, the voltage stays positive.
Figure 12: Voltage across Inductor L1 vs. Time Wave form for Proposed
Converter
The voltage across Inductor L2 vs. time waveform is illustrated in figure 13. Changing
of the states are more rapid compared to Inductor L1.
Figure 13: Voltage across Inductor L2 vs. Time Waveform for Proposed
Converter
Based on the above analysis and output waveform, the proposed converter is capable
of obtaining high voltage gain which is required for the voltage rating of the photovoltaic
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Muzahid, Ansari, Salam & Zaman
(PV) cell power applications. An output voltage of 220 to 240 V is achieved with an
input voltage of 24 V at a duty ratio of 0.55. This result translates into a voltage boost
ratio of approximately 10 times, thereby confirming the high voltage gain boost
capability without an extreme duty ratio.
5. Conclusion:
Renewable solar energy sources are mostly used for small industrial and large
household loads, as photovoltaic cells are not suitable as larger power sources. In
such a system, as an alternative to stepping up the ac voltage, which requires
transformers and therefore introduces core and copper losses, raising the level of dc
voltage is a good alternative which avoids complicacy in this type of systems. It also
enables low scale solar power systems to supply to large loads that require stable
power supply. Achieving high voltage gain requires adjusting the turn’s ratio of the
coupled inductor and varying the capacitor with proper rating. By recycling the energy
stored in the leakage inductor of the new coupled inductor, high efficiency and gain
have been obtained. It can be used in medium power range industrial dc machineries
and to feed into high voltage dc power supply system. The high voltage gain DC-DC
boost converter is successfully simulated and the result shows the expected
functionality. As the result is achieved by simulation, the practical application of this
model may show some percentage of deviation from the ideal characteristics. If
properly implemented, this converter may be used to get high voltage gain for any
range of low input voltages which may be an important innovation for future power
transmission systems. Future work about this converter will involve high power output
from low voltage input units and adding an inverter to supply power to ac load
applications.
6. Acknowledgment:
The authors would like to express their heartfelt gratitude to their parents and their
family for invaluable help and support all over this work.
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