Kf2417451751

S.Sreevidya, A. Hemashekar / International Journal of Engineering Research and Applications
(IJERA) ISSN: 2248-9622 www.ijera.com
Vol. 2, Issue4, July-August 2012, pp.1745-1751
Novel Soft-Switching Techniques Using A Full Bridge Topology In
Fuel Cell Power Conversion
*S.Sreevidya, **A. Hemasekhar,
*PG student, SVPCET,PUTTUR,
**M.tech.,(ph.D) Associate professor, HOD,
SVPCET,PUTTUR,

Abstract
This paper presents a set of novel soft- Several approaches to realize dc–dc isolated power
switching techniques to increase the power conversion for FC power sources have been proposed
conversion efficiency in fuel cell (FC) systems based on full bridge, push–pull, and current-fed
using a full-bridge topology. For this purpose, a topologies. Some of the key contributions in the area
special right-aligned modulation sequence is include the study outlined in the following. A FC
developed to minimize conduction losses while power converter based on a controlled voltage
maintaining soft-switching characteristics in the doubler was introduced, which uses phase-shift
MOSFETs. Traditional auxiliary elements in the modulation to control the power flow through the
primary, such as series inductors that are transformer leakage inductance [3]. This interesting
impractical for realizing due to the extreme input topology proved to be less efficient than other
current, are avoided and reflected to the output of traditional topologies [4], but presents the advantage
the rectifier to minimize circulating current and of low component count. A FC inverter based on a
generate soft transitions in the output diodes. As a traditional push–pull dc–dc converter was presented
result, the proposed combined techniques featuring low cost, low component count, and DSP
successfully reduce conduction losses, minimize control [5]. For example, maintaining ZVS (full-
reverse-recovery losses in the output rectifiers, bridge) is difficult due to the poor voltage regulation
minimize transformer ringing, and ensure low of the FC and the wide range of loading conditions,
stress in all the switches. The high efficiency is which creates excessive conduction losses due to
maintained in the entire range of loading circulating current in the primary. The push pull
conditions (0%–100%) while taking into topology reduces transformer utilization ,
consideration remarkable challenges associated compromises magnetizing balance as the power
with FC power conversion: high input current, rating increases as well as limiting the possibilities
low voltage and poor regulation, and wide range for soft-switching operation.
of loading conditions. A detailed analysis of the Current-fed-based topologies need bulky
techniques for efficiency gains are presented and a input inductors (high current), present oscillations
phase-shift zero-voltage switching topology is produced by the interaction between parasitics
employed as a reference topology to highlight the (leakage inductance, intra winding capacitance, and
mechanisms for performance enhancement and the input inductor), and could present excessive
the advantages in the use of the special degrading high-frequency ripple current in the output
modulation. capacitors due to the absence of filter inductor. While
the trend for high-input-voltage converters (e.g.,
I. I NTRODUCTION connected to the line) has been to minimize switching
FUEL Cells (FC) are power sources that losses and deal with relatively small line regulation,
convert electrochemical energy into electrical energy FC power conversion presents the opposite scenario
with high efficiency, low emissions, and quiet with low input voltage, poor regulation, and very
operation. A basic proton exchange membrane high input current. Unlike applications with high
(PEM) single-cell arrangement is capable of input voltage, achieving ZVS with low voltage does
producing unregulated voltage below1Vand consists not lead to substantial efficiency gains, given the
of two electrodes (anode and cathode) linked by small energy stored in the MOSFETs output
electrolyte [1]. The output current capability of a capacitance (Coss). The power dissipated in a
single cell depends on the electrode effective area, MOSFET due to the output capacitance during turn
and several single cells are connected in series to on is a function of the square of the FC voltage vfc2 .
form a FC stack. Due to the mechanical challenges Since FC are low-voltage high-current power
associated with stacking several single cells, FC are sources, the relative importance of switching losses
typically low-voltage high current power sources and can be outweighed by conduction losses in the
can continuously run while reactant is fed into the MOSFETs that are a function of ifc2 . By taking into
system [2]. consideration the aforementioned technical
challenges, it becomes critical to address the

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following relevant points in FC power conversion: 1) successful design of power conditioning stages. Both
need for reduction in conductions losses, and thus, PEMFC and direct methanol FC (DMFC) belong to
unnecessary circulating current in the primary; 2) this category. The factors that mainly contribute to
impracticality of realizing high-current inductors the output voltage behavior in a DMFC are fuel
(costly and bulky) and using series capacitors in the (methanol concentration), fuel flow rate (supplied to
primary (reduced lifetime due to ripple current); 3) the anode), air/oxygen flow rate (supplied to the
minimize substantial reverse-recovery losses in the cathode), and operating temperature [1]. As well, the
output rectifiers (due to the high output voltage); 4) output current is a significant factor that affects the
minimize the associated transformer oscillations output voltage and, hence, its output power. To better
(ringing); and 5) ensure that the high efficiency, by illustrate this behavior, a set of experiments were
combining points 1) to 5), is maintained under the FC performed with a Nafion115 membrane (Aldrich) in a
wide input voltage range and 0%–100% loading 5.3-cm2 active area cell fed with 1 mol aqueous
conditions. This paper addresses the challenges 1) to MeOH and oxygen at variable flow rates.Fig. 2
5) by proposing a set of soft-switching techniques in shows a family of polarization curves (voltage versus
a full-bridge forward topology. For this purpose, a current) under different operating conditions. The
special modulation sequence is developed to figure shows how the output voltage and power
minimize conduction losses while maintaining soft- availability of the FC are modified by the operating
switching characteristics in the MOSFETs and soft conditions (e.g., operating temperature, oxygen flow
transitions in the output rectifiers. Auxiliary elements rate, and output current using a fixed methanol
in the primary, such as series inductors and capacitors concentration). It is interesting to note how the output
that are impractical to realize due the extreme input voltage of this DMFC is greatly affected by its
current are avoided by reflecting them to the operating temperature and output current (fuel and
secondary of the circuit to minimize circulating oxygen flow rates are close to optimal in this case).
current and generate soft transitions in the switches. This results in a significant change of the available
These variations are conceptually depicted in Fig. 1 output power, the area under the polarization curve.
indicating three major modifications suited for FC Therefore, in order to obtain a desired output power,
power conversion. The proposed combined it is first necessary to modify the operating conditions
techniques have the ability to maintain high to increase the area under the polarization curve (for
efficiency in the entire operating range of the FC example, by increasing the operating temperature). It
(wide input voltage) and under any loading condition. should be pointed out that the transition from a given
Detailed analysis of the techniques for efficiency polarization curve to another through variation in
gains is presented and a phase-shift ZVS topology is operating conditions is very slow.
employed as a reference topology to highlight the
mechanisms for performance enhancement and the
advantages in the use of the special modulation.

Fig. 2. Steady-state characteristic of a 5.3-cm2
polymer-electrolyte single FC (DMFC). Polarization
curves as a function
(mL/min).
Fig. 1. Conceptual schematic and gate waveforms
illustrating Lzvs inductor reflection to the output of The main reasons for this behavior are the
the rectifier_1, right-aligned gate signals for the high heat capacity of the cell, and the slow mass
upper switches _2, and +50% duty cycle in the lower transport processes in the flow fields and electrodes.
switches _3. However, a fast dynamic response exists when the
output current changes for fixed operating condition.
As a result of this example, the poor voltage
II. FC VOLTAGE REGULATION
This section briefly revisits the regulation regulation, high current, and low-voltage
characteristic of a polymer-electrolyte FC under characteristics are highlighted. The same principle
different operating conditions, providing the basis for follows for larger electrode areas required to produce

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high currents, and a number of singles cells in series For example, the IRFB4110 has 3.7 mΩ at
to conform a FC stack (i.e.,20 V/60 A for a 25◦C and 6 mΩ at 100◦C (typical), resulting in 35-W
commercial Ballard Nexa PEMFC). conduction losses under 75-A rms at 100◦C. When
the switching losses are
III. RIGHT-ALIGNED MODULATION analyzed, the same power device experiences less
AND PRIMARY INDUCTOR than 6.5 W during the turn- ON transition due to its
ELIMINATION IN THE FULL-BRIDGE output capacitance Coss when switching at 40 kHz
TOPOLOGY with vfc = 22 V as given in the following:
This section presents in a sequential and
conceptual manner the steps taken to fulfill the
requirements toward increasing the efficiency of the
full-bridge forward converter in FC power Therefore, it can be inferred that in this
conversion. A description of the power-loss particular low-voltage high-current application, the
mechanisms in the input stage is first presented, efficiency gain resulting from reducing circulating
followed by the analysis of the output rectifier. Each current in four switches outweighs those of switching
design goal is addressed by the combined effects of losses, especially under heavy loading conditions.
the proposed soft-switching techniques. When the lower switches are considered, the scenario
is even more favorable, as M2 and M4 not only
A. Full-Bridge Input Stage benefit from lower conduction losses, but also
The conduction losses in the MOSFETs due operate in ZVS due to the modification_3 in the
to circulating current [design goal (a)] and the high- modulation (+50% duty cycle). In addition, the
current bulky inductor in the primary are eliminated reduction in the conduction interval also helps to
[goal (b)] by removing the traditional Lzvs inductor reduce copper losses in the transformer windings and
in the primary and by forcing a right-aligned favors the use of planar magnetics with their inherent
sequence of pulses in the upper switches as illustrated low leakage inductance to increase power transfer.
in Fig. 1 (modifications _1 and _2). In order to
illustrate the gains of the two changes with a practical B. Output Rectifier Stage
example, Fig. 3 presents the conduction losses of a The output rectifiers contribute to power
commercial MOSFET (IRFB4110) with low RdsON losses due to conduction and reverse recovery. Since
as a function of duty cycle for the voltage the output voltage of the power converter is high (i.e.,
polarization curve of a commercial hydrogen FC 220 V to supply a single-phase inverter), the
(Ballard Nexa 1.2 kW). conduction current is typically a few amperes per
It can be seen in Fig. 3 that the total kilowatt of output power (i.e., 4.54 A), making the
conduction losses under phase-shift ZVS (+ curve reverse-recovery losses the dominant factor. Reverse-
that includes circulating current) are considerably recovery charge is a function of the forward
higher than losses only associated with power conduction current (IF ) and the rate of change of
transferred to the secondary current (di/dt), as well as operating temperature of the
device.The reverse-recovery losses can be estimated
by using the recovery charge, switching frequency
(Fsw ), and reverse applied voltage (VR), including
the peak ringing value as follows

Fig. 3. Conduction losses of a commercial MOSFET
(IRFB4110) due to circulating current as a function
of duty cycle for the voltage polarization curve of a
commercial FC vfc .
Fig. 4. Reverse-recovery losses: conceptual
The losses have been calculated using the relationship between rate of change of current di/dt,
rms value of the current through switchM1 and the initial forward current IF and reverse-recovery charge
MOSFET ON-resistance RdsON , which is a function Qrr for IF 3 > IF 2 > IF 1 .
of the device temperature As a simple review of this combined effects,
Fig. 4 shows a conceptual relationship among di/dt,
the IF , and Qrr in which the initial forward current is
given by IF 3 > IF 2 > IF 1. As indicated in (3) the

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reverse-recovery losses can be reduced by means of producing a Lzvs like effect. Once the primary
controlling di/dt [design goal (c)] and by reducing the current reaches the current level of the filter inductor
reverse peak voltage VR produced by transformer reflected to the primary iL interval T1 ends.
oscillations[design goal (d)]. For this purpose, the
Lzvs inductor is reflected to the secondary and placed
at the output of each upper rectifier D5 and D7
(modification _3 ). This technique limits the di/dtin
the upper rectifiers, eliminates reverse recovery in the
lower diodes D6 and D8 , and reduces significantly
the transformer oscillations by preventing a zero-
voltage state at the secondary. As will be seen, the
technique avoids simultaneous conduction of D5 , D6
, D7 , and D8 , thus reducing undesirable ringing that
occurs when the primary current matches the inductor
output current, which results in a severe voltage step
in the secondary that creates ringing, and therefore,
electromagnetic interference (EMI). In the following
section, the operation of the full-bridge forward Fig. 5. Switching sequence for MOSFETsM1 ,M2
converter and the effect of the proposed ,M3 , andM4 (transitions intervals have been
modifications for efficiency improvements are exaggerated for clarity).
presented in detail over the various switching
intervals. Interval T2 : MOSFETs M1 and M4 are in
the ON-state and their current continue to ramp up
IV. OPERATION INTERVALS AND LOSS- with slope voltage, auxiliary inductor, and output
REDUCTION EFFECTS filter inductor reflected to the primary. Here, V`o,
The combination of the proposed L`a, and L` denote, respectively, the output voltage,
techniques, Lzvs inductor reflection to the output of auxiliary inductor, and output filter inductor reflected
the rectifier (1), right-aligned gate signals for the to the primary. Interval T3 : Begins when gate
upper switches (2), and +50% duty cycle in the lower signal G1 drops causing M1 to turn off and D2 to
switches (3) are investigated in detail in this section. start conduction. This interval is a short
Fig. 5 shows the switching sequence for MOSFETs blanking/dead time betweenM1 andM2 , and finishes
M1 , M2 , M3 , andM4 along with the main when the gate signal G2 rises. The behavior ofM2
waveforms for the techniques under study. Transition and D2 is the same as M4 and D4 in the next
intervals have been exaggerated for clarity. The switching cycle. Interval T4 : M2 turns on with
structures acquired by the power converter during the ZVS, given that D2 was forward-biased during T3 .
switching intervals are depicted in Fig. 6. The This interval is also brief and extending G4 slightly
switching sequence results in 12 different intervals beyond 50% is to ensure ZVS on both lower
T1–T12, which are used to explain the behavior of switches. Interval T5 : The energy stored in the
the converter. In order to examine various efficiency- leakage inductance Llk is returned to the dc-bus
gain mechanisms, a detailed analysis of the current capacitors through D3 body diode. Since the
and voltage waveform is presented for the upper and traditional Lzvs has been reflected to the secondary,
lower MOSFETs, followed by the upper and lower the inductance in the primary Llk can be dramatically
output rectifiers. minimized by using a planar transformer. The
transformer primary current ip is, therefore, reset to
A .Detailed Analysis of the MOSFETs Operation zero during this short interval.
The waveforms for MOSFETs M1 and M4 Interval T6 : The circulating current in the primary is
and their respective body diodes D1 and D4 are eliminated, translating into tangible efficiency gains
shown in Fig. 7 during a full-cycle period, including given the high input current that is characteristic in
the gate signals G1 and G4, drain to- source voltages FC power conversion. The remaining intervals T7–
vM1 and vM4 , currents for the MOSFETs n-channel T12 repeats the same behavior for M2 and M3 and
iM1 and iM4 , and the body diodesiD1andiD4. their corresponding body diodes D2 and D3 .
Interval T1 : The right-aligned modulation, which
ensures no circulating current in the primary, starts B. Output Rectifier Operation
with interval T1. The upperMOSFETM1 turns on In order to complete the analysis of the
with zero-current switching (ZCS), and the current waveforms and efficiency gains, the output rectifier
path is through M4 that is already in the ON state. An should be investigated. The current and voltage
interesting effect in the primary current rate of waveforms for D7 (upper) and D8 (lower) diodes are
change di/dt can be identified during the T1 interval, presented in Fig. 8, where both conduction losses and
which is inherently limited by the action of the reverse-recovery instants can be identified
inductors La and Llk reflected to the primary,

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Fig. 6. Power-converter structures for intervals T1 –T12 .
.
Interval T1 : The current in diode D7 1) The auxiliary inductors La and Lb shape the
initiates the recovery process and di/dt is limited by current waveforms of D5 and D7 during reverse
Lb , like in phase-shift ZVS. However, due to the recovery. Therefore, the inductor values can be
interleaving action of La and Lb , only three diodes selected to achieve a desired Qrr in the upper diodes
are forward-biased (rather than four), preventing a and, hence, control the total reverse recovery power
zero-state in the transformer secondary. Recall that losses.
in traditional full-bridge converters, all four diodes 2) DiodesD6 andD8 experience negligible reverse-
are in conduction in the absence of a pulse in the recovery losses, unlike the phase-shift ZVS topology,
primary. This modification will lead to reduced which is explained by near-zero forward current
ringing during T2 . when the reduced reverse voltage is applied.
Interval T2 : The interval T2 begins once D5 3) The presence of La and Lb reduce oscillations and
reaches the current level of the filter inductor. Diode the peak reverse voltage applied to D6 and D8 that
D7 recovers with a soft transitions due to the results from transformer ringing. Transformer
moderate di/dt and also experiences a reduced oscillation results in undesirable effect, such as high
blocking voltage in the presence of low transformer maximum reverse voltage rating for the diodes, EMI,
ringing, both helping to reduce reverse-recovery over voltage between windings, and power losses in
losses. This can be explained due to the fact that the auxiliary snubber circuits. The concept of avoiding a
zero voltage condition in the secondary of the zero-voltage condition on the transformer secondary
transformer is eliminated with the interleaving action is addressed by preventing simultaneous conduction
of La and Lb , thus preventing an abrupt voltage step of D5 , D6 , D7 , and D8 . As a result, the turn-ON
in the transformer secondary that excites the pulse is partially reflected to the secondary of the
parasitics that cause self-resonance (leakage transformer as if the converter were operating in
inductance, intra-/inter windings). While traditional discontinuous conduction mode. Hence, the
full-bridge ZVS requires a snubber to limit the oscillations are reduced under any loading condition.
ringing, the proposed technique eliminates the These combined improvements increase the
snubber while reducing the overall reverse-recovery efficiency of the rectifier stage in addition to the
losses in the upper diodes. efficiency gains of the MOSFET. The behavior of the
Interval T3: defines the conduction interval of D7. converter highlights the advantages of the proposed
Intervals T4–T5 : Initiates the reverse recovery in D8 techniques in full-bridge topology for FC power
. conversion
Interval T6 : The recovery of D8 does not .
experience reverse voltage due to the interleaving C. Frequency Response and Dynamic Behavior
effect of La and Lb Therefore, the transition has The frequency response of the control-to-output
negligible losses. This effect leads to substantial characteristic of the full-bridge topology, which is a
efficiency gains in the lower diodes D6 and D8 . For buck-derived topology, is dominated by the transfer
the proposed soft-switching techniques reveal the function of the output filter (L and C). When the
following improvements. converter is operated in phase-shift ZVS, a series
inductance is required to limit the current rate of

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change in the primary to generate soft transitions in VI. Simulation Results
the switches. This limitation, reduces the effective In order to improve the efficiency in full
duty cycle reflected to the secondary, therefore, bridge fuel cell power conversion by using soft
affecting the control-to-output characteristic. In switching techniques, the system has been simulated.
closed-loop operation using traditional compensation The converter was built using the parameters and
(small signal), the artificial dumping does not have parts in Table I, and a phase-shift ZVS topology was
any noticeable effect in phase and gain margins. A employed as a reference topology. For the ZVS
similar behavior is experienced when the proposed operation, the inductor Lzvt was included and La and
techniques are employed using traditional Lb were removed.
compensators, therefore, showing a dynamic On the other hand, the proposed
response similar to that of a phase-shift ZVS. modifications were tested using La and Lb and
In this study, in order to facilitate the removing Lzvt . The practical implementation of the
efficiency evaluation process, multiple measurements converter follows current trends by using DSP
were performed with a closed loop controller (small- control and modulation , in particular to simplify the
signal) in steady-state operation. The controller was task of generating the right-aligned modulation. As
realized with an inner current loop (inductor current) well, as part of the requirements to realize the
and an outer voltage loop. Both control loops and the proposed techniques, the drivers of the upper
modified modulation were implemented digitally MOSFETs was arranged to produce actual pulse
using TMS320F2808 fixed-point DSP. Validation of width modulation, rather than the fixed 50% duty
the waveforms and comparative efficiency cycle employed in the phase-shift ZVS counterpart.
measurements are presented in the following section. Operation of proposed circuit is described in
V Conventional Circuit figures as follows. In this case input voltage is
The conventional circuit for efficiency gains 24volts. Fig 8(a) shows the input voltage of proposed
in full-bridge fuel cell power conversion is shown in circuit and fig 8(b) shows the switching pulse and
figure7 below. A fixed DC is converted into AC by drain-source voltage. Pulse width is made equal to
inverter. It is boosted by using a high frequency soft switching time to ensure zero voltage switching.
transformer then this boosted ac voltage is passed Fig.9(a) shows the output voltage and Fig.9(b) shows
through a rectifier to convert it as DC voltage. the output current. The dc motor output voltage
MOSFETS will have high switching speed and initially increases linearly and becomes constant.
frequency.N-Channel MOSFETs will have 3xtimes The Dc output voltage and output current will have
high switching speed compared to than that of P- less ripples. Fig.10(a) shows the dc motor speed in
channel. Faster switching speeds can be obtained rpm and Fig.10(b) shows the dc motor torque in N-m.
with well-designed gate driver circuits. TO charge The dc motor speed increases linearly then becomes
the gate capacitance at turn ON large current (1-2A) constant and also torque decreases slightly and
is required also switching times will be small. becomes constant.

(a)
Fig 7. Circuit Diagram for conventional circuit

Due to gate leakage current, Nano-amps are
needed to maintain the gate voltage once the device is
ON. A negative voltage is often applied at turn OFF
to discharge the gate for speedy switch OFF.The
switching pulses given to M3 and M4 are similar to
that of M1&M2. The dc output voltage and current
will have ripples because of the convertor circuit had
used a half wave rectifier. In order to reduce the
ripples will go for a proposed circuit with full wave
rectifier.
(b)

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Fig8. (a) Input voltage (b) Switching pulse and impractical high-current inductors in the primary; 3)
Drain-Source voltage (Vds) reduction of reverse-recovery losses in the output
rectifiers; 4) minimization of transformer oscillations
(ringing); and 5) improved efficiency under the FC
wide input voltage range and 0%–100% loading
conditions. As anticipated by the analysis, by taking
advantage of the opportunities for performance
enhancements, considerable efficiency gains were
observed in the entire range of operation of the
system while maintaining the simplicity and
ruggedness of the full-bridge topology.
(a) REFERENCES
[1] M. Nymand and M. A. E. Andersen, “High-
efficiency isolated boost DC-DC converter
for high-power low-voltage fuel-cell
applications,” IEEE Trans. Ind. Electron.,
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[2] M. Ordonez, P. Pickup, J. E. Quaicoe, and
M. T. Iqbal, “Electrical dynamic response of
a direct methanol fuel cell,” IEEE Power
(b) Electron. Soc. Newslett., vol. 19, no. 1, pp.
Fig9. (a) Output voltage (b) Output current 10–15, Jan. 2007.
[3] J. Wang, F. Z. Peng, J. Anderson, A. Joseph,
and R. Buffenbarger, “Low cost fuel cell
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generation,” IEEE Trans. Power Electron.,
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[4] J. Wang, M. Reinhard, F. Z. Peng, and Z.
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(a) [5] R. Gopinath, S. Kim, J. Hahn, P. N. Enjeti,
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[6] L. Palma [19] J.-M. Kwon and B.-H. Kwon,
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the full-bridge topology were investigated in this [7] D. G. Holmes, P. Atmur, C. C. Beckett, M.
study including the reflection of the traditional Lzvs P. Bull,W. Y. Kong,W. J. Luo, D. K. C. Ng,
inductor to the output of the rectifier, right-aligned N. Sachchithananthan, P. W. Su, D. P.
modulation for the upper switches, and +50% duty Ware, and P. Wrzos, “An innovative,
cycle in the lower switches. The study presented in efficient current-fed push-pull grid
this paper identified the main power-loss mechanism connectable inverter for distributed
in full-bridge FC power conversion in the presence of generation systems,” in Proc. IEEE Power
the poor voltage regulation of polymer-electrolyte FC Electron. Spec. Conf., 2006, pp. 1504–1510.
and the wide range of loading conditions.
The combined techniques successfully
addressed the most important power loss effects and
issues in FC power conversion: 1) reduction in
unnecessary high circulating currents in the primary,
and thus, conduction losses; 2) elimination of

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