International Journal on Advanced Electrical and Computer Engineering (IJAECE)
______________________________________________________________________________________________
A Modified Sepic Converter With High Static Gain For Renewable
Applications
1
M.Muthukumaran, 2S.Sankarkumar, 3M.Sureshkumar, 4M.Murugapandi
1,2,3,4
Department of Electrical and Electronics Engineering, Sree Sowdambika College of Engineering, Aruppukottai.
1
Email: Gyanmuthu89@gmail.com, 2smsankar.kumar@gmail.com, 3sureshgym1@gmail.com
Abstract – A high step-up DC-DC converter based on the
modified SEPIC converter is presented in this paper. The
proposed topology presents low switch voltage and high
efficiency for low input voltage and high output voltage
applications. Two alternatives with and without magnetic
coupling are analyzed. The magnetic coupling allows to
increase the static gain with a reduced switch voltage. The
theoretical analysis and experimental results are presented.
Two experimental prototypes were developed with an
input voltage equal to 12 V and an output power equal to
100 W. The efficiency obtained with the prototype without
magnetic coupling was equal to 91.2% with an output
voltage equal to 120 V and nominal output power.
Efficiency equal to 95% was obtained with the prototype
with magnetic coupling operating with an output voltage
equal to 240 V and nominal output power.
Keywords – DC-DC converter, high static gain and
renewable applications.
I. INTRODUCTION
High static gain DC-DC converters are nowadays an
important research focus due to the crescent demand of
this technology for some applications as renewable
energy sources, fuel-cells, embedded systems, portable
electronic equipments, uninterrupted power supply,
battery powered systems and others applications
supplied by low DC voltage energy sources [1]-[5].
High step -up ratio is necessary when some loads
operate with and DC or AC peak voltage higher than ten
times the input source voltage.
When a high step-up ratio is necessary, the usual
solution is the use of isolated DC-DC converters.
However, the isolated solution presents some problems
as the efficiency reduction due to the power transformer
losses and intrinsic parameters as leakage inductance.
The power transformer also presents an important
contribution in the converter volume. The power
converters used with renewable energy sources must
present a very high efficiency due to the high cost of the
energy source, as photovoltaic module or fuel-cells.
Also in embedded systems and portable equipments the
converter power density is an important design
parameter. Therefore, the solutions that allow the
elimination of the power transformer can improve the
efficiency and power density of the power conversion
system. However, the classical non-isolated DC-DC
converters present a limited step-up gain (q=Vo/V i). The
boost converter is the classical non isolated step-up DCDC converter and normally can operate with and
adequate static and dynamic performance with an output
voltage around five times the input voltage with a dutycycle equal to D=0.8. A static gain around to q=5 is a
limited value for the applications considered in this
work and this range is considered as a standard staticgain in this paper. A converter operating with static gain
equal or higher q=10 is considered a high-static gain
solution and an operation with static gain higher than
q=20 is considered a very high static gain solution in
this paper.
Many techniques were developed in order to increase
the static gain of the non-isolated structure for the
implementation of high efficiency and high power
density solutions. A review of the main techniques
proposed is presented in [1]. The main characteristics
desired in the applications considered are a static gain
equal or higher than ten times, low switch voltage, low
input current ripple, reduced weight and volume and
high efficiency. New alternatives for high and very high
step-up ratio applications are proposed and are based on
a new DC-DC non-isolated topology called Modified
SEPIC converter that presents a static gain close to the
double of the classical boost converter. The proposed
converter allows the inclusion of an auxiliary winding
for the implementation of the magnetic coupling
technique.
II. PROPOSED CONVERTERS
A. Power Circuit without Magnetic Coupling
The power circuit of the classical SEPIC converter is
presented in Fig.1. The step-up and step-down static
gain of the SEPIC converter is an interesting operation
characteristic for a wide input voltage range application.
However the switch voltage is equal the sum of the input
and output voltage. The modification of the SEPIC
converter is accomplished with the inclusion of the
diode DM and the capacitor CM, as presented in Fig.2.
Many operational characteristics of the classical SEPIC
converter are changed with the proposed modification.
Fig. 1. Classical SEPIC converter.
_______________________________________________________________________________________________
ISSN(Online): 2349-9338, ISSN(Print): 2349-932X Volume -2, Issue -1, 2015
16
International Journal on Advanced Electrical and Computer Engineering (IJAECE)
______________________________________________________________________________________________
Fig. 4. Second operation stage.
Fig. 2. Modified SEPIC converter without magnetic
coupling.
The capacitor CM is charged with the output voltage of
the classical boost converter. Therefore, the voltage
applied to the L2 inductor during the conduction of the
power switch (S) is higher than in the classical SEPIC,
increasing the static gain. The polarity of the C S
capacitor voltage is inverted in the proposed converter
and the expressions of the capacitors voltages and others
operation characteristics are presented in the theoretical
analysis.
The continuous conduction mode operation of the
modified SEPIC converter presents two operation
stages. All capacitors are considered as a voltage source
for the theoretical analysis.
1) First Stage ([t0, t1] Fig. 3) - At the instant t0, switch S
is turned-off and the energy stored in the input inductor
L1 is transferred to the output through the C S capacitor
and output diode Do and also is transferred to the CM
capacitor through the diode DM. Therefore, the switch
voltage is equal to the CM capacitor voltage. The energy
stored in the inductor L2 is transferred to the output
through the diode Do.
2) Second Stage ([t1, t2] Fig. 4) - At the instant t1, switch
S is turned-on and the diodes DM and Do are blocked and
the inductors L1 and L2 store energy. The input voltage
is applied to the input inductor L1 and the voltage VCSVCM is applied to the inductor L2. The VCM voltage is
higher than the VCS voltage.
The main theoretical waveforms operating with hardswitching commutation are presented in Fig. 5.
The voltage in all diodes and in the power switch is
equal to the CM capacitor voltage. The output voltage is
equal to the sum of the CS and CM capacitors voltage.
The average L1 inductor current is equal to the input
current and the average L2 inductor current is equal to
the output current.
Fig. 5. Main theoretical waveforms.
The static gain of the proposed converter can be
obtained considering that the average inductor voltage is
zero at the steady-state and is presented in (1). The static
gain of the proposed converter is higher than the
obtained with the classical boost.
(1)
The CM capacitor voltage is calculated by (2) that is the
same output voltage of the classical Boost converter.
The switch voltage is equal to the VCM voltage.
Therefore the switch voltage will be lower than the
converter output voltage.
(2)
The voltage across the CS capacitor is calculated by (3).
Fig. 3. First operation stage.
(3)
_______________________________________________________________________________________________
ISSN(Online): 2349-9338, ISSN(Print): 2349-932X Volume -2, Issue -1, 2015
17
International Journal on Advanced Electrical and Computer Engineering (IJAECE)
______________________________________________________________________________________________
The static gain of the classical SEPIC, Boost and
modified SEPIC converters are presented in Fig. 6. As
can be observed in this figure, with a duty-cycle equal to
D=0.818, a static gain equal to 10 is obtained and the
switch voltage is equal to 5.5 times the input voltage.
Therefore, the switch voltage is close to half of the
output voltage.
Fig. 7. Modified SEPIC converter with magnetic
coupling.
Fig. 6. Converters static gain.
B. Power Circuit with Magnetic Coupling
The modified SEPIC converter without magnetic
coupling can operate with the double of the static gain of
the classical boost converter for a high duty-cycle.
However, a very high static gain is necessary in some
applications. A practical limitation for the modified
SEPIC converter in order to maintain the converter
performance is a duty-cycle close to D=0.85, resulting in
a static gain equal to q=12.3. A simple solution to
increase the static gain without increases the duty-cycle
and the switch voltage is to include a secondary winding
in the L2 inductor. The L2 inductor operation is similar
to a buck-boost inductor and a secondary winding can
increases the output voltage by the transformer turns
ratio (n). Figure 7 shows this circuit alternative.
However, this converter structure presents the problem
of overvoltage at the output diode Do due to the
existence of the transformer L2 leakage inductance. The
energy stored in the leakage inductance due to the
reverse recovery current of the output diode results in
voltage ring and high reverse voltage at the diode Do.
This overvoltage is not easily controlled with classical
snubbers or dissipative clamping. A simple solution for
this problem is the inclusion of a voltage multiplier at
the secondary side as presented in Fig. 8. This voltage
multiplier increases the converter static gain, the voltage
across the output diode is reduced to a value lower than
the output voltage and the energy stored in the leakage
inductance is transferred to the output. Therefore the
secondary voltage multiplier composed by the diode
DM2 and capacitor CS2 is also a non-dissipative clamping
circuit for the output diode. The circuit presented in Fig.
8 is the power circuit studied in this paper.
Fig. 8. Proposed converter - Modified SEPIC converter
with magnetic coupling and output diode voltage
clamping.
The continuous conduction mode operation of the
modified SEPIC converter with magnetic coupling and
output diode clamping presents five operation stages.
All capacitors are considered as a voltage source for the
theoretical analysis.
1) First Stage ([t0, t1] Fig. 9) – The power switch S is
conducting and the input inductor L1 stores energy. The
capacitor CS2 is charged by the secondary winding L2S
and diode DM2. The leakage inductance limits the
current and the energy transference occurs in a resonant
way. The output diode is blocked and the maximum
diode voltage is equal to (Vo-VCM). At the instant t1 the
energy transference to the capacitor CS2 is finished and
the diode DM2 is blocked.
2)
Second Stage ([t1, t2] Fig. 10) – From the instant t1
when the diode DM2 is blocked to the instant t2 when the
power switch is turned off, the inductors L1 and L2 store
energy and the inductor currents increase linearly.
3)
Third Stage ([t2, t3] Fig. 11) - At the instant t2 the
power switch S is turned off. The energy stored in the L1
inductor is transferred to the CM capacitor. Also there is
the energy transference to the output thought the
capacitors CS1, CS2 inductor L2 and output diode Do.
4)
Fourth Stage ([t3, t4] Fig. 12) - At the instant t3, the
energy transference to the capacitor CM is finished and
the diode DM1 is blocked. The energy transference to the
output is maintained until the instant t4, when the power
switch is turned on.
Fig. 9. First operation stage.
_______________________________________________________________________________________________
ISSN(Online): 2349-9338, ISSN(Print): 2349-932X Volume -2, Issue -1, 2015
18
International Journal on Advanced Electrical and Computer Engineering (IJAECE)
______________________________________________________________________________________________
The static gain variation as a function of the duty-cycle
is presented in Fig.15.
Fig. 10. Second operation stage.
The static gain can be increased by the transformer turns
ratio (n ) without to increase the switch voltage. The
switch voltage is calculated by (2) and is the lowest
curve presented in Fig. 15.
Fig. 11. Third operation stage.
Fig. 12. Fourth operation stage.
Fig. 13. Fifth operation stage.
5) Fifth Stage ([t4, t5] Fig. 13) – When the power switch
is turned on at the instant t4, the current at the output
diode Do decreases linearly and the di/dt is limited by
the transformer leakage inductance, reducing the diode
reverse recovery current problems. When the output
diode is blocked, the converter returns to the first
operation stage.
Fig. 14. Main theoretical waveforms of the modified
SEPIC converter with magnetic coupling and voltage
multiplier at the secondary side.
The main theoretical waveforms of the modified SEPIC
converter with magnetic coupling and with the voltage
multiplier at the secondary side are presented in Fig. 14.
The switch voltage and the voltage across all diodes are
lower than the output voltage. The power switch turn on
occurs with almost zero current reducing significantly
the switching loss. The current variation ratio (di/dt)
presented by all diodes is limited due to the presence of
the transformer leakage inductance, reducing the
negative effects of the diode reverse recovery current.
The static gain of the modified SEPIC converter with
magnetic coupling and voltage multiplier is equal to:
Fig. 15. Static gain variation as a function of the dutycycle and transformer turns ratio.
Where the transformer turns ratio (n) is calculated by:
III. EXPERIMENTAL RESULT
The experimental validation is obtained with the
_______________________________________________________________________________________________
ISSN(Online): 2349-9338, ISSN(Print): 2349-932X Volume -2, Issue -1, 2015
19
International Journal on Advanced Electrical and Computer Engineering (IJAECE)
______________________________________________________________________________________________
implementation of two prototypes. The circuits
presented in Fig. 2 and in Fig. 8 were implemented
following the parameters presented in Table I. The
components used in the prototypes are shown in Table
II.
TABLE I
Prototypes Parameters
Parameter
Input Voltage (Vi)
Output voltage (Vo)
Output Power (Po)
Switching Frequency (f)
Duty-cycle (D)
Switch Voltage (Vs)
Static Gain (q)
Modified
SEPIC
without
magnetic
coupling
12 V
120 V
100 W
30 kHz
0.82
67 V
10
Modified
SEPIC
with magnetic
coupling
12 V
240 V
100 W
30 kHz
0.82
67 V
20
Figure 19 shows the input current, the switch voltage
and current and the output voltage of the modified
SEPIC converter with magnetic coupling, presented in
Fig. 8.
The output voltage is equal to 240 V and the switch
voltage is close to 70 V. The experimental results are
similar to the theoretical waveforms presented in Fig.
14.
The switch commutation is presented in Fig. 20. The
turn on commutation loss is reduced due to the presence
of the transformer leakage inductance.
TABLE II
Prototypes Components
Modified
SEPICModified
SEPIC
Component without
with
magnetic coupling
magnetic coupling
S
IRFP90N200
IRFP90N200
DM
MUR860
DM1
MUR860
DM2
MUR860
Do
MUR860
MUR860
CM
3.3 µF/250 V
3.3 µF/250 V
CS
3.3 µF/250 V
CS1
3.3 µF/250 V
CS2
3.3 µF/250 V
L1
EE42/15
EE42/15
L2
EE30/14
EE30/14
Co
100 µ/400 V
100 µ/400 V
Fig.16. Input current (CH4), output voltage (CH3) and
switch current (CH2) and voltage (CH1) of the Modified
SEPIC converter without magnetic coupling (5 A/div,
40 V/div,10 µs/div).
The experimental results obtained with the modified
SEPIC converter without magnetic coupling are
presented from Fig. 16 to Fig. 18. The results obtained
with the proposed converter with magnetic coupling and
voltage multiplier are presented from Fig. 19 to Fig. 21.
Figure 16 shows the input current, the switch voltage
and current and the output voltage. The output voltage is
equal to 120 V and the switch voltage is close to 70 V.
There is a peak switch voltage at the switch turn off due
to a jump connection necessary to measure the switch
current waveform. Eliminating the current probe
connection at the power switch terminals, the switch
voltage is clamped to the VCM capacitor voltage close to
70 V.
Fig. 17. Switch current (CH2) and voltage (CH1) of the
Modified SEPIC converter without magnetic coupling (5
A/div, 40 V/div, 1 µs/div).
The switch commutation is presented in Fig. 17. The
layout problem commented above and the reverse
recovery current of the output diode can increase the
commutation losses in the hard switching operation. A
non dissipative snubber can be included to reduce the
commutation loss but is not included in this paper. The
output diode reverse recovery current is presented in
Fig. 18.
_______________________________________________________________________________________________
ISSN(Online): 2349-9338, ISSN(Print): 2349-932X Volume -2, Issue -1, 2015
20
International Journal on Advanced Electrical and Computer Engineering (IJAECE)
______________________________________________________________________________________________
The efficiency curves of the proposed converters are
presented in Fig. 21. The modified SEPIC converter
without magnetic coupling (black line) presents
efficiency equal to 91.2% at the nominal power and the
modified SEPIC converter with magnetic coupling and
voltage multiplier (blue line) presents efficiency equal to
95% at the nominal output power. The maximum
efficiency is equal to 96.5% and occurs at 70% of the
nominal output power.
Fig.18. Reverse recovery current of the output diode (2
A/div, 100 ns/div).
The presence of the transformer leakage inductance
reduces the commutation losses of the magnetic
coupling converter increasing the efficiency. The
efficiency of the proposed converter without magnetic
coupling can be improved including a non dissipative
snubber, but 91.2% is a good efficiency value for the
hard switching operation and high static gain converter.
Fig. 21. Efficiency curve of the proposed converters as a
function of the output power.
Fig.19. Input current (CH2), output voltage (CH3),
switch current (CH4) and switch voltage (CH1) of the
Modified SEPIC converter with magnetic coupling and
voltage multiplier (5 A/div, 100 V/div,10 µs/div).
Fig. 20. Switch current (CH4) and switch voltage (CH1)
of the Modified SEPIC converter with magnetic
coupling and voltage multiplier (2.5 A/div, 20 V/div, 5
µs/div).
The negative effects of the reverse recovery current of
the output diode are minimized. The peak switch voltage
presented in Fig. 20 can be reduced eliminating the
current probe connection used for the switch current
waveform acquisition.
IV. CONCLUSION
Two new topologies of non isolated high static gain
converters are presented in this paper. The first topology
without magnetic coupling can operate with a static gain
higher 10 with a reduced switch voltage. The structure
with magnetic coupling can operate with static gain
higher 20 maintaining low the switch voltage. The
efficiency of proposed converter without magnetic
coupling is equal to 91.2% operating with input voltage
equal to 12 V, output voltage equal 120 V and output
power equal 100 W.
The efficiency of proposed converter with magnetic
coupling is equal to 95% operating with input voltage
equal to 12 V, output voltage equal 240 V and output
power equal 100 W. The commutation losses of the
proposed converter with magnetic coupling are reduced
due to the presence of the transformer leakage
inductance and the secondary voltage multiplier that
operates as a non dissipative clamping circuit to the
output diode voltage.
REFERENCES
[1]
C W. Li, X. He, “Review of Non-Isolated High
Step-Up DC/DC Converters in Photovoltaic
Grid-Connected
Applications”,
IEEE
Transactions on Industrial Electronics, vol. 58,
no. 4, April 2011.
_______________________________________________________________________________________________
ISSN(Online): 2349-9338, ISSN(Print): 2349-932X Volume -2, Issue -1, 2015
21
International Journal on Advanced Electrical and Computer Engineering (IJAECE)
______________________________________________________________________________________________
[2]
Q. Zhao and F. C. Lee, “High-efficiency, high
step-up DC–DC converters,” IEEE Transaction
on Power Electronics, vol. 18, no. 1, pp. 65–73,
Jan.2003.
[3]
R.-J. Wai and R.-Y. Duan, “High step-up
converter
with
coupled-inductor,”
IEEE
Transaction on Power Electronics, vol. 20, no. 5,
pp. 1025–1035, Sep. 2005.
[4]
G. Henn, R. Silva, P. Praça, L. Barreto D.
Oliveira, “Interleaved Boost Converter with High
Voltage Gain”, IEEE Transaction on Power
Electronics, vol. 25, no. 11, pp. 2753–2761, Nov.
2010.
[5]
R. J. Wai, R. Y. Duan, “High-efficiency Power
Conversion for Low Power Fuel Cell Generation
System”, IEEE Transactions on Power
Electronics, vol. 20, no.4, pp. 847-856, Jul 2005.

_______________________________________________________________________________________________
ISSN(Online): 2349-9338, ISSN(Print): 2349-932X Volume -2, Issue -1, 2015
22