ISSN 2319-8885
Vol.03,Issue.05,
April & May-2014,
Pages:0745-0753
www.semargroup.org,
www.ijsetr.com
High-Step-Up and High-Efficiency Fuel-Cell Power-Generation System
with ZVS Operation for Grid Connected System
SWETHA ETUKALA1, CH. SHANKAR RAO2, MRS.P.PUSHPA DEEPTHI3
1
Dept of EEE, CMRCET, India, E-mail: swetha.etukala@gmail.com.
2
Dept of EEE, CMRCET, India, E-mail: sankarraoch@gmail.com.
3
Dept of EEE, CMRCET, India, E-mail: Deepthi.ponnuri@gmail.com.
Abstract: A high-efficiency fuel-cell power- generation system with an active-clamp ZVS converter is presented in this paper
to boost a 16-V dc voltage into a 320-V 50-Hz ac voltage. The proposed system includes a high-efficiency high-step-up
interleaved soft-switching flyback–forward converter and a full-bridge inverter. The front-end active-clamp ZVS converter has
the advantages of zero-voltage-switching performance for all the primary switches, reverse-recovery-problem alleviation for the
secondary output diodes, large voltage- conversion ratio, and small input-current ripple. Furthermore, there are two coupled
inductors in the proposed converter. Each coupled inductor can work in the ZVS mode when the corresponding main switch is
in the turn-on state and in the forward mode when it is in the turnoff state, which takes full use of the magnetic core and
improves the power density. In addition, the full-bridge inverter with an LC low-pass filter is adopted to provide low-totalharmonic-distortion ac voltage to the load. Therefore, high-efficiency and high-power density conversion can be achieved in a
wide input-voltage range by employing the proposed system.
Keywords: Active Clamp, ZVS Converter, Fuel-Cell Generation System.
I. INTRODUCTION
Developments and applications of the fuel-cell power
generation system become one of the most effective
solutions to compensate the fossil-fuel energy shortage and
to protect the global because the fuel cell is a clean and
renewable energy source with high efficiency, high
reliability, and easy modularization performance [1], [2].
Commonly, a lot of low-voltage cells are integrated to a
fuel-cell stack to improve the output power level. The
output voltage of the fuel-cell stack is lower than 40 V due
to the cost and reliability issues in the household standalone power generation applications [3],[4]. This means that
a front-end dc/dc converter is necessary to boost the low
voltage of the fuel-cell stack to a standard high bus voltage
before being inverted into a 220-V ac output. The required
dc/dc converter should have the advantages of large voltageconversion ratio, high efficiency, and small input-current
ripple. The widely employed isolated voltage-fed converters
are not the optimal candidates for the high-step-up fuel-cell
generation system because they have a step-down
conversion feature, large input-current ripple, and high
output-diode voltage stress [5]–[8]. The large transformer
turns ratio, an additional LC input filter and the heavy
output-diode losses are the main obstacles for their
efficiency and power density improvement. Compared with
the voltage-fed converters, the isolated current fed
converters have the clear advantages of input-current-ripple
reduction and high-step-up voltage ratio in the low-input
high output-voltage conversion system.
The conventional current-fed push–pull converters are
welcomed in the high-step-up and low-power applications
due to their simple structure and flexible flux balance for the
transformer [9]. Furthermore, some improvements have
been made to realize a soft-switching performance [10].
However, the voltage stress of the primary switches is
relatively high. The current-fed full-bridge converters are
suitable for the large current applications [11], [12].
Unfortunately, the duty cycle of the primary main switches
should be greater than 0.5 because a current path should be
provided for the input inductor in any operation condition.
Therefore, an additional start-up solution should be
designed to reduce the inrush current during the start-up
operation. The dual boost converters can distribute the input
current due to the interleaved operation. In addition, the
active-clamp circuits can be inserted to achieve a zero
voltage-switching (ZVS) soft-switching performance and to
recycle the leakage energy [13]–[15]. Unfortunately, there
are two input inductors and one transformer in the dual
boost converters, which limits the power density
improvements. Some interleaved boost converters with
winding cross-coupled inductors and active-clamp circuits
are proposed for large-current and high-output-voltage
conversion [16], [17].
Copyright @ 2014 SEMAR GROUPS TECHNICAL SOCIETY. All rights reserved.
SWETHA ETUKALA, CH. SHANKAR RAO, MRS.P.PUSHPA DEEPTHI
In this paper, a high-step-up interleaved ZVS converter
with active-clamp circuits is proposed for the household
fuel-cell power-generation system. Boost-type conversion is
realized by employing two coupled inductors to obtain a
large voltage conversion ratio, where each coupled inductor
can work in the fly-back mode when the corresponding
main switch is in the turn-on state and in the forward mode
when it is in the turnoff state. As a result, the magnetic core
is fully utilized and the power density is enhanced.
Furthermore, by introducing the active-clamp scheme, a
ZVS soft-switching operation is carried out for the primary
main and clamp switches. The diode reverse-recovery
problem is alleviated for the secondary rectifier diodes.
Hence, the power-device switching losses are reduced to
improve the circuit efficiency. Moreover, the input current
is distributed and the input-current ripple is minimized by
the interleaved operation and the current-fed configuration.
All the aforementioned distinguished features make the
proposed converter an optimal candidate for the high-stepup, high-efficiency, and high-power-density conversion.
The coupling references are plotted as “∗” and “·.” The
secondary windings of the two coupled inductors are in
series to achieve boost type conversion. LLk is the leakage
inductance summation of the two coupled inductors, which
is reflected to the secondary side. Lm1 and Lm2 are the
magnetizing inductors. N is defined as the turns ratio n2/n1.
The parallel capacitors Cs1 and Cs2 are used to implement
the ZVS soft-switching operation. The voltage-doubler
configuration is employed in the secondary side to achieve
high-step-up conversion, which contains the output diodes
Do1 and Do2 and the output capacitors Co1 and Co2. Each
of the output capacitors sustains half of the output bus
voltage. The single-phase full-bridge inverter is composed
of four MOSFETs S3, S4, S5, and S6 and an LC low-pass
filter.
Fig.1. High-efficiency fuel-cell power-generation system.
Fig. 2. Waveforms of the proposed converter.
II. SYSTEM STRUCTURE AND ANALYSIS
A. Circuit Configuration and Description
The proposed high-efficiency fuel-cell power-generation
system, which is shown in Fig. 1, consists of an interleaved
high-stepup ZVS converter and a single phase full-bridge
inverter. For the inter leaved high-step-up ZVS flyback–
forward converter, the main switches S1 and S2 work in the
interleaved mode to handle the large input current. The
active-clamp circuits are composed of the auxiliary switches
Sc1 and Sc2 and the clamp capacitors Cc1 and Cc2, which
are employed to recycle the leakage energy, suppress the
turnoff voltage spikes on the main switches, and realize
ZVS soft-switching performance for all the primary power
devices. The clamp switches Sc1 and Sc2 are driven
complementarily with the main switches S1 and S2,
respectively. There are two coupled inductors in the
proposed converter, which are named L1 and L2. The
primary inductors L1a and L2a with n1 turns are coupled
with their secondary inductors L1b and L2b with n2 turns.
B. Operational Analysis of Proposed Converter
Two 180◦ out-of-phase gate signals with the same duty
cycle are applied to the main switches S1 and S2. The gate
signals of the clamp switches Sc1 and Sc2 are
complementary with the corresponding main switches S1
and S2. Based on the steady state operation, there are 16
operational stages in switching period. Due to the symmetry
of the circuit, only eight stages are described briefly. The
steady-state wave forms are shown in Fig. 2, and the
corresponding equivalent circuits are shown in Fig. 3.
Stage 1 [t0, t1]: During this stage, the main switches S1 and
S2 are in the turnon state. The clamp switches Sc1 and Sc2
are in the turnoff state, and the output diodes Do1 and Do2
are both reverse-biased. The two coupled inductors operate
in the flyback mode to store the energy. The energy to the
load is provided by the secondary output capacitors Co1 and
Co2.
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.05, April & May-2014, Pages: 0745-0753
High-Step-Up and High-Efficiency Fuel-Cell Power-Generation System with ZVS Operation for Grid Connected System
Stage 2 [t1, t2]: At t1, the turnoff gate signal is applied to
the main switch S2, which makes its drain–source voltage
increase in a nearly linear way due to the parallel capacitor
Cs2. This interval is very short because the primary winding
current is large and the parallel capacitor is small.
Stage 3 [t2, t3]: At t2, the drain–source voltage of S2
increases to make the diode Do1 conduct. During this stage,
the coupled inductor L1 operates in the forward mode and
L2 works in the flyback mode to transfer energy to the load.
Stage 4 [t3, t4]: At t3, the voltage on the parallel capacitor
Cs2 increases to that on the clamp capacitor Cc2. As a
result, the antiparallel diode of the clamp switch Sc2 begins
to conduct.
Stage 5 [t4, t5]: At t4, the turn-on gate driver signal is given
to conduct the clamp switch Sc2 with ZVS turn-on
operation. The current through the anti parallel diode of the
clamp switch Sc2 transfers to Sc2 quickly.
Stage 6 [t5, t6]: At t5, the turnoff gate signal is provided to
the clamp switch Sc2. Due to the parallel capacitor Cs2, the
drain–source voltage of the main switch S2 decreases
linearly and that of the clamp switch Sc2 increases in an
approximately linear way. As a result, Sc2 turns off under
ZVS condition. One part of the leakage energy continues to
be delivered to the load and another part of the leakage
energy is recycled to the input source.
Stage 7 [t6, t7]: At t6, the drain–source voltage of the main
switch S2 decreases to zero. Therefore, its anti parallel
diode starts to conduct. The leakage current falls due to the
voltage on the capacitor Co1.
Stage 8 [t7, t8]: At t7, the main switch S2 turns on with
ZVS soft-switching performance. The secondary diode Do1
still remains in the conduction state. At t8, the leakage
current decreases to zero and the diode Do1 turns off with
zero-current switching operation. The two primary inductors
are charged linearly by the input voltage again.
A similar operation works in the rest stages of a
switching period. The auxiliary switch Sc1 and the clamp
capacitor Cc1 can absorb the turnoff voltage spikes on the
main switch S1 and recycle the leakage energy.
III. STEADY-STATE CIRCUIT PERFORMANCE
ANALYSIS
A. Voltage-Gain Derivation
Under the ideal condition, which means that the leakage
inductance is zero, the power devices are ideal with zero
conduction resistance and conduction voltage drop, the
voltages on the clamp capacitors and output capacitors are
constant, the parallel capacitors are zero, and the voltages on
the main and clamp switches are equal to those on the clamp
capacitors. They are given by
(1)
where D is defined as the duty cycle of the main switches.
Due to the voltage-second balance on the magnetizing
inductor, the output capacitor voltage can be easily obtained
by
(2)
The output voltage is the summation of the voltages on
the two output capacitors. Therefore, the voltage gain of the
proposed converter under the ideal condition is
(3)
Unfortunately, the leakage inductance has some impact
on the voltage gain. Once the leakage inductance is
considered, the voltage gain is given by (4),
(4)
From (4), it can be concluded that the voltage gain of the
proposed converter is determined by the turns ratio of the
coupled inductors, the main switch duty cycle, the leakage
inductance, the switching frequency, and the output load.
The relationship of the voltage gain, the duty cycle, and the
leakage inductance at a certain turns ratio is shown in Fig. 4,
where the turn’s ratio is 6, the output voltage is 380 V, the
output power is 500 W, and the switching frequency is 50
kHz. As the duty cycle increases, the voltage gain extends
greatly. The leakage inductance degrades the voltageconversion ratio a little. The smaller the leakage inductance,
the smaller the voltages gain loss for the proposed fly-back–
forward converter. In addition, the winding resistors of the
coupled inductors, the conduction resistors, and the diode
forward voltage of the power devices have a little impact on
the voltage gain. The leakage inductance is taken as zero to
simplify the voltage gain analysis affected by the parasitic
parameters. The derived voltage gain is given by
(5)
From (5), it can be seen that the voltage gain drops a
little as the winding resistors of the coupled inductors, the
power-device conduction resistors, and the diode forward
voltage increases. Once the circuit components are ideal, (5)
can be simplified into (3). In fact, the parasitic resistors of
the circuit components are rather smaller than the output
resistor.
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.05, April & May-2014, Pages: 0745-0753
SWETHA ETUKALA, CH. SHANKAR RAO, MRS.P.PUSHPA DEEPTHI
B. Current Sharing Performance of Proposed Converter
The secondary windings of the two coupled inductors are
in series to realize the boost-type conversion. The secondary
series configuration is convenient for the primary inputcurrent auto-sharing. From the voltage-gain expression
shown in (4), it can be drawn that the magnetizing inductors
of the coupled inductors are independent of the voltageconversion ratio. As a result, the difference of the
magnetizing inductors is unrelated to the primary current
sharing performance. Although the current ripple on the
magnetizing inductor varies as the magnetizing inductor
changes, the difference of the root mean square (rms)
current on the two magnetizing inductors is small. The
simulated current
Fig. 3. Operational stages of the proposed converter: (a)
Stage 1 [t0−t1]. (b) Stage 2 [t1−t2]. (c) Stage 3 [t2−t3]. (d)
Stage 4 [t3−t4]. (e) Stage 5 [t4−t5]. (f) Stage 6 [t5−t6]. (g)
Stage 7 [t6−t7]. (h) Stage 8 [t7−t8].
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.05, April & May-2014, Pages: 0745-0753
High-Step-Up and High-Efficiency Fuel-Cell Power-Generation System with ZVS Operation for Grid Connected System
LEAKAGE INDUCTANCE (Vin = 12 V, N = 6, D =
0.75,Pout = 500 W, AND fs = 50 kHz)
TABLE III
CURRENT SHARING PERFORMANCE WITH
ASYMMETRICAL DUTY
CYCLE (Vin = 12 V, N = 6, LLk1 = LLk2 = 0.44 μH, Pout
= 500 W, AND fs = 50 kHz)
Fig. 4. Voltage-gain performance of the proposed
converter.
sharing performance induced by the asymmetrical
magnetizing inductor is given in Table I, where Irms1 and
Irms2 represent the rms input currents of each phase. In
spite of large difference on the magnetizing inductors, the
rms primary input current is nearly the same.
TABLE I
CURRENT SHARING PERFORMANCE WITH
ASYMMETRICAL
MAGNETIZING INDUCTOR (Vin = 12 V, N = 6, D =
0.75,P = 500 W, AND fs = 50 kHz)
From the voltage-gain expression shown in (4), it can be
drawn that the magnetizing inductors of the coupled
inductors are independent of the voltage-conversion ratio.
As a result, the difference of the magnetizing inductors is
unrelated to the primary current sharing performance.
Although the current ripple on the magnetizing inductor
varies as the magnetizing inductor changes, the difference of
the root mean square (rms) current on the two magnetizing
inductors is small. The simulated current sharing
performance induced by the asymmetrical magnetizing
inductor is given in Table I, where Irms1 and Irms2
represent the rms input currents of each phase. In spite of
large difference on the magnetizing inductors, the rms
primary input current is nearly the same. The leakage
inductance has a clear variation, owing to the large-scale
industrial manufacture.
TABLE II
CURRENT SHARING PERFORMANCE WITH
ASYMMETRICAL
Thus, the current sharing performance caused by the
asymmetrical leakage inductance should be discussed
carefully to explore the inherent circuit performance. From
the equivalent circuit of the proposed converter shown in
Fig. 1, it can be found that the leakage inductance LLk
represents the reflected leakage inductance summation on
the secondary side. The voltage-gain expression (4) shows
that the leakage inductance has some influence on the
voltage conversion ratio. Fortunately, once the total leakage
inductance is the same, the voltage gain and the current
sharing performance nearly stay the same. The simulation
results of the current sharing performance caused by the
asymmetrical leakage inductance are illustrated in Table II,
where LLk1 and LLk2 are the primary leakage inductances
of the two coupled inductors. It can be found that the
difference on the leakage inductance has a small effect on
the current sharing performance. In practice, the duty cycle
has some variation due to the parasitic parameters of the
power devices and the signal circuits.
The simulation current difference with asymmetrical
duty cycles is introduced in Table III, where D1 and D2 are
the duty cycles of the main switches S1 and S2. As the duty
cycle varies, the energy stored in the magnetizing inductors
changes, which has a little effect on the output voltage gain
and the current sharing performance. When D1 = D2 = 0.75,
the current difference (Irms1 − Irms2)/Irms2 is nearly zero.
When D1 is 0.7 and D2 is 0.8, the current difference is
about 16%. Once D1 is decreased to 0.65 and D2 is
increased to 0.85, the current difference changes to 29%.
Compared with the conventional boost converter [18], the
current sharing performance caused by the asymmetrical
duty cycle is improved due to the cross coupling
configuration of the coupled inductors. Therefore, the
proposed converter has good current sharing performance,
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.05, April & May-2014, Pages: 0745-0753
SWETHA ETUKALA, CH. SHANKAR RAO, MRS.P.PUSHPA DEEPTHI
which can simplify the control circuit design and improve
the system reliability.
C. Performance Comparison
In the classic current-fed full-bridge converters, the duty
cycle of the primary main switches should be greater than
0.5 in any load condition to provide the current path for the
input inductor. As a result, an additional start-up circuit
should be designed to minimize the inrush current during
the start-up operation. Furthermore, the switch conduction
losses in the primary side are large because the primary
current should flow through two primary switches in most
operation stages. In addition, the current stress on the input
inductor is high because it should sustain the whole input
current. Fortunately, a two phase interleaved boost converter
exists in the primary side of the proposed converter, except
that the diodes in the conventional boost converter are
replaced by the active switches. This means that the mainswitch duty cycle of the presented converter can vary from 0
to 1. When the duty cycle is lower than 0.5, the proposed
converter may operate in discontinuous current mode with
light load. As a result, ZVS performance is lost because the
leakage energy is not sufficient to discharge the energy
stored in the parallel capacitor when the corresponding
clamp switch turns off. A wide duty-cycle operation range is
achieved in the proposed converter to remove the additional
start-up circuit compared with the classic current fed fullbridge converters. Moreover, the large input current is
distributed to two interleaved phases, which decreases the
current stress on the magnetizing inductors. Finally, the
primary current flows through only one switch and the
average current stress on the active-clamp switches is
relatively low, which reduces the primary switch conduction
losses. As a result, the proposed converter is more suitable
for high-efficiency, high step-up, and high-power-density
dc/dc conversion compared with the classic current-fed fullbridge converters.
IV. CONTROL STRATEGY
In order to regulate the output voltage of the proposed
interleaved ZVS converter and provide high quality ac
energy to the load, the effective control strategy should be
employed. The control block diagram of the proposed
power-generation system is shown in Fig.5. The
conventional peak-current-mode control scheme cannot be
adopted in the proposed converter, because the coupled
inductors operate in the fly back mode and the forward
mode alternatively, which does not make the peak current of
the main switch occur at its turn-off moment. Fortunately,
the average-current-mode control strategy can be introduced
to obtain a fast dynamic response and an accurate voltage
regulation. Only the total input current, which is the
summation of the current across both the primary coupled
inductors, is required due to the fine current auto sharing
performance. From the analysis in Section III, it can be
concluded that the voltage gain and the small signal model
of the proposed converter are quite similar to those of the
dual boost converters. As a result, the design criterion of the
dual boost converters can be directly employed in this paper
to achieve a good circuit performance [13]–[15]. Generally
speaking, a tradeoff should be made for the consideration of
the current controller between the circuit stability and the
dynamic performance.
Fig.5. Control block diagram of the proposed powergeneration system.
TABLE IV
SYSTEM SPECIFICATIONS
This can be implemented by simulation analysis or Bode
plot derivation [18]. Furthermore, the protect function, such
as the under voltage, the overvoltage, the over current (OC),
and the over temperature, is realized to improve the system
reliability. The gate driver signals are implemented by
SG3525. The output voltage of the inverter Vac and its filter
inductor current Iac are sampled and employed to realize a
high steady and dynamic response sinusoidal voltage. The
published advanced control strategies can be implemented
by the microchip Mega16A to improve the system
performance [13], [19]–[21].
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.05, April & May-2014, Pages: 0745-0753
High-Step-Up and High-Efficiency Fuel-Cell Power-Generation System with ZVS Operation for Grid Connected System
V. SIMULATION RESULTS
The ZVS soft-switching performance of the main and
clamp switches is shown in Fig. 6. ZVS turn-on and turnoff
operations are achieved for both the main and clamp
switches, which reduce the switching losses greatly. The
voltage and current waveforms on the clamp capacitors Cc1
and Cc2 are shown in Fig.6. The voltage ripple on the clamp
capacitors is small, which can suppress the turnoff voltage
spikes on the main switches. The input current iin and the
current through the primary inductors iL1a and iL2a are
shown in Fig. 6. The input current is the current summation
of the two primary inductors. Although the current ripple on
the primary inductors is large, the input-current ripple is
very small due to the current ripple cancelation caused by
the interleaved operation. The voltage stress of the output
diode is equivalent to the converter output voltage. The
detailed turnoff current waveforms of the output diode Do.
The diode reverse-recovery current of the proposed
converter is small because its turnoff current is controlled by
the leakage inductance of the coupled inductors.
Input & Output Voltages:
VI. CONCLUSION
In this paper, an interleaved high-step-up ZVS fly-back–
forward converter has been proposed for the fuel-cell power
generation system. The voltage doubler rectifier structure is
employed to provide a large voltage-conversion ratio and to
remove the output-diode reverse-recovery problem.
Furthermore, ZVS soft-switching operation is realized for
all the primary active switches to minimize the switching
losses. In addition, the input-current ripple is small due to
the interleaved operation and the current-fed-type
configuration. The steady state operation analysis and the
main circuit performance are discussed to explore the
advantages of the proposed converter in a high-efficiency
high-step-up power-generation system. Experimental results
have demonstrated that the proposed system is an excellent
power-converter systemfor fuel-cell applications, featuring
high efficiency, high-step up ratio, and high power density.
Fig.6.High Step-up Fuel Cell Forward.
Fig.7. High Step up Fuel cell Forward.
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.05, April & May-2014, Pages: 0745-0753
SWETHA ETUKALA, CH. SHANKAR RAO, MRS.P.PUSHPA DEEPTHI
Output Voltage & Current:
Fig.8. Measuring Ports.
Pulses
Output:
Fig.9. High Step up Fuel cell Forward R load.
Fig.10. Extension High Step-up Fuel Cell Forward Grid
Connected.
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.05, April & May-2014, Pages: 0745-0753
High-Step-Up and High-Efficiency Fuel-Cell Power-Generation System with ZVS Operation for Grid Connected System
Output:
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International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.05, April & May-2014, Pages: 0745-0753