Experimental Validation of High-V oltage-Ratio LowInput- Current-Ripple DC-DC Converters for Fuel Cell Applications

Experimental Validation of High-Voltage-Ratio Low
Input-Current-Ripple DC-DC Converters for Fuel Cell
Applications
R. Bekhouchel, F. Khoucha1,2, M.E.H. Benbouzid2, A. Kheloui1
JEcole Militaire Polytechnique, UER ELT, Algiers, Algeria
2University o.fBrest, FRE CNRS 37441RDL, Brest, France
jkhoucha04�yahoojr
Abstract-This work describes the practical implementation of
a Floating-Interleaving Boost Converter (FIBC) for fuel cell
applications. The paper aims to validate the concept of digitally
controlled from four-phases-FIBC for fuel cell applications. FIBC
exhibits interesting performance in terms of magnetics, input, and
output current ripple, part count and distributed power losses. A
potential field of application is indeed medium and higher power fuel
cell front-end converters, where minimizing input current ripple is
significant but also redundancy and reliability are crucial. Actually,
this approach covers all these aspects since provide module and
device redundancy with real-time and flexible digital control
reconfiguration. Relevant aspects related to design; modeling,
simulation and experimental verification of lOOW, Arduino
controlled, 4-phases-FIBC are treated in this paper.
Keywords-floating-interleaving boost converter (FIBC); fuel cell;
Arduino
I. INTRODUCTION
A fuel cell (FC) may be one of the promising solutions to
decrease carbon dioxide emissions under the assumption that the
hydrogen can be produced from renewable-energy sources such as
photovoltaic and wind energy [1].
In automotive applications, proton exchange membrane FCs
appear to be the most suitable, because their working conditions at
low temperature allow the system to start up faster than those
technologies using high-temperature FCs; moreover, the solid
state of their electrolyte (no leakage and low corrosion) and their
high power density make them fit for transport applications [2].
A single cell produces a voltage of approximately IV;
therefore, several cells must be stacked to achieve high voltage
output. FC stacking reduces its reliability and lifetime as a chain
of series-connected cells is as strong as the weakest cell. Due to
reliability and lifetime reasons, practical FC stack output voltage
is reduced to approximately 100 V. On the other hand, the vehicle
powertrain dc bus has a high voltage of a few hundred volts.
Therefore, a dc-dc converter is required to interface the FC stack
with the powertrain dc-bus voltage and to achieve good power
management of the input power source [3], [4]. The FC dc-dc
converter is also required for voltage conditioning as the FC output
voltage strongly varies with variable load.
There is a rising demand for non-isolated, high gain converters
in Photovoltaic (PV) systems, FC systems and battery powered
systems such as electric vehicles where a low voltage source must
be convened to high voltages. In some applications, the
conventional boost converter proves to be an easy option due to its
simple structure and its continuous input current. However it is not
possible to achieve a high gain with this structure. The
performance of a boost converter starts to deteriorate as duty cycle
is increased to obtain high gain. Therefore, converters providing
978-1-4673-9768-1/16/$31.00 ©2016 IEEE
high gain while operating at moderate duty cycles arc necessary
[5].
The most important requirements expected from dc-dc
converter for FC applications are high voltage ratio and low
current ripple [6]. The lower the current ripple, the longer the FC
lifetime [7], [8]. However, in an FC electric vehicle (FCEV), the
high voltage ratio and low current ripple, which are associated with
volume, weight, reliability, and efficiency constraints, are very
important requirements [9].
In this paper, a dc-dc converter topology defined as the
floating-interleaving boost converter (FIBC) will be presented.
The operational modes of the circuit, analysis and design of the
converter, simulation results and test results of the prototype
converter are presented in the subsequent sections.
To conclude this section, the paper is structured as follows.
Introduction is given in Section: Introduction, the 4-phase FIBC
converter is described in Section: Topology Description, the 4-
phase FIBC converter is studied and analyzed in Section: Steady
State Analysis Design, the 4-phase FIBC converter modeling and
control loop design are detailed in Section: 4-Phases-FIBC:
Modeling And Control Loop Design, the simulation results of 4-
phase FIBC are given in Section: Simulation Results, the
experimental results are given in Section: Experimental
Validation, and conclusions summarized in Section: Conclusion.
II. TOPOLOGY DESCRIPTION
In high-power FCEV applications, the major drawbacks of
using conventional boost converters are the difficulty in the design
of magnetic components and high input current ripple, which may
lead to reduce the FC stack lifetime. Reducing the rating current
and voltage applied to passive and power electronic components
(keeping the same system rated power) is the proposed solution.
This makes the magnetic component construction easier, giving
further flexibility for the selection of power electronic components
used in the converters. Fig. I shows the topology of 4-phases
FIBC.
These topologies have a floating output and interleaving input,
which permits reduction in not only current stress but also voltage
stress, unlike conventional interleaved topologies. The benefits of
the N-phase FIBC are the following:

Fig.1. 4-phases-FIBC
1. Increasing the overall converter efﬁciency;
2. Increasing the input and output ripple frequency without
increasing the switching frequency;
3. Decreasing the input ripple current;
4. Enhancing the system reliability by paralleling phases and
not by paralleling multiple devices;
5. Decreasing current and voltage ratings of power electronic
devices;
6. Reducing the size and weight of the passive components.
III. STEADY STATE ANALYSIS DESIGN
Table I shows that the current and voltage ratings of the power
electronic devices of FIBC are smaller than their of the basic boost
converters.
TABLEAU IIII:
CURRENT AND VOLTAGE RATINGS OF POWER ELECTRONIC DEVICES
Voltage ratings Current ratings
4-phase
FIBC 1 2 1
The voltages of the two condensers are given by the following
relation:
2
1
The Voltage Gain of the presented converter is expressed as
follows:
1
1
2
On the other hand, the basic boost converter Voltage Gain is
given by
1
1
3
Fig. 2 shows the plot of Voltage Gain various Duty Cycle for
FIBC and basic boost converter.
Fig.2 Voltage Gain Plot Various Duty Cycle for FIBC and basic boost
converter.
For the input current ripple the mathematical expressions are
derived under six assumptions.
1. The resistances of inductor and capacitor are negligible.
2. Stray inductor and capacitor are negligible.
3. Switches are ideal.
4. Passive components are identical.
5. Switches in parallel operate (360/N)° out of phase.
6. The converters operate in continuous conduction mode.
The ratio of the input current ripple to the inductor current
ripple is given by
4
For basic boost converter
5
The input current slope of the N-phase FIBC is expressed as
follows:
6
The generalized expression of the ratio of the input current
ripple to the inductor current ripple of the N-phase FIBC as a
function of duty cycle MN(D) is
1
1
7
Where n is the interval between two duty cycles values,
resulting in zero current ripple.
The ratio of the input current ripple to the inductor current
ripple of a four-phase FIBC as a function of duty cycle M4(D) is:
0.3 0.4 0.5 0.6 0.7 0.8 0.9
0
1
2
3
4
5
6
7
8
9
10
Duty Cycle(D)
VoltageGain(G)
basic boost
topology of FIBC

1 4
1
0
1
4
4 1 2 4
4 1

1
4
1
2
4 2 3 4
4 1

1
2
3
4
4 3

3
4
1
8
The variation of the ratio of input current ripple to inductor
current ripple as a function of duty cycle is shown in Fig. 3.
Fig.3 Ratio between the input current ripple and the inductor current ripple
versus duty cycle
On the one hand, by studying Fig. 3, it can be observed that
input current ripple cancelation occurs at speciﬁc duty cycles,
which are multiple duties of 1/N, such as 0.25, 0.5, and 0.75 in a
4-phase FIBC. On the other hand, it is clear that the input current
ripple is always less than the inductor current ripple.
IV. 4-PHASES-FIBC: MODELING AND CONTROL LOOP DESIGN
The equations relating to the converter are given for the two
sequences of operation represented Fig 4. In order to simplify the
analysis, we take into account only resistance parasitizes r of
inductor.
(a) 0 < t < DT (b) DT < t < T
Fig.4. Equivalent electric diagram of topology 4-phase FIBC
For the sequence of operation (0<t<DT), the equations are
given below:

9
For the sequence of operation (DT<t<T), the equations are
given below:

10
The averaged-circuit model is:
1
1
1
1
1 1
1 1
11
By admitting the assumption that the duty cycles (D1, D2, D3
and D4) are identical, the equations of this converter with a
resistive load R are given below.
1
2 1
1

12
From the equation (1) we find:
2 1 2 1
4 1
2

13
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Duty Cycles(D)
RatioM(D)

The averaged small signal model is:
2
̃
1 2 ̃ 1
4 1 ̃
2

14
In permanent mode:
1
1 1
1
2 1

15
In variable mode with the first order:
̃
1 2 ̃ 1
4 1 ̃ 4
2

16
By applying the transform of Laplace, we obtain:
1 1 2 ̃
2 1
1 1
2
4 1 ̃
2
1

17
The different transfer functions of this converter are given
below
̃ 1 1 3
2 4 1 4
̃
1 4 1 4
1 1 3

18
The diagram of control integrating the two control loops in loop
closed for a phase is shown
Fig.5. Diagram of control integrating the two control loops
The parameters of two correctors PI (C1 and C2) were
determined by the Sisotool®
available in the Matlab/Simulink
software.
V. SIMULATION RESULTS
The design specifications are input voltage Vfc=20V, output
voltage Vdc=40V, output Power P=100W and switching frequency
f=10 kHz.
By using our preceding results we can determine the inductor
value of 4-phase FIBC.
. . .
4 1 1 2
2 1
19
The critical value of capacitance is obtained by:
Fig.6. Photograph of the emulator.

2
20
The validation of the performances of control is given through
the following results of simulation:
Fig. 7 illustrates the behavior of controlled system with an
output voltage reference Vdc=40V (which represents the DC bus
voltage) and successive load step changes, the resistance can
change between 16Ω and 32Ω yielding variation of 50% of the
power of the DC bus. As it can be seen, despite the load resistor
uncertainty, the controller behavior is satisfactory.
The curve in blue shows a tight voltage regulation under step
load changes.
The curve in red shows the change of operation point of the fuel
cell voltage, showing its high dependence on the current.
The curve in green shows the load current.
Fig. 8 shows the FC current, and inductor currents of the 4-
phase FIBC. One can observe that the FC ripple current is 1/4 the
individual inductor ripple currents. So, the FC ripple current of the
4-phase FIBC converter is nearly zero. It means that the FC mean
current is close to the FC RMS current. In addition, it can be seen
the FC ripple frequency is 4 times the switching frequency of 10-
kHz.
Fig. 7. Controller behavior in response to a step reference and changes in the
load resistance.
Fig.8. Waveform of inductor currents and input current.
0 1 2 3 4 5
0
5
10
15
20
25
30
35
40
45
50
Temps(s)
Tension(V)
2.48 2.49 2.5 2.51 2.52 2.53 2.54
0
5
10
15
20
25
30
35
40
45
50
Temps(s)
Tension(V)
Vdc
Vfc
Ich
0 0.5 1 1.5 2
0
1
2
3
4
5
6
7
Temps(s)
Courant(A)
0.9999 1 1.0001 1.0002
0
1
2
3
4
5
6
7
Temps(s)
Courant(A)
Ifc
IL1
IL2
IL3
IL4

VI. EXPERIMENTAL VALIDATION
A 100W, 20V/40V prototype was constructed to validate the
operation of the 4-phase FIBC converter. An Arduino card was
suitably programmed to generate the gate pulse at a frequency of
10 kHz.
Because of the non-availability of a fuel cell on the level of the
Laboratory, we realized an emulator of its characteristic.
The emulator is a buck converter controlled in voltage and
which reproduces the same characteristic voltage-current of the
FC. The output current of this buck is measured to obtain a
reference voltage using a model of the FC is developed and stored
previously in the memory of an Arduino card DUE. The regulation
of this voltage gives the control signal of the buck.
Figure 8: Photograph of the emulator.
To evaluate the performances of the control, an experimental
set up of an emulator of PEMFC associated to a 4-phase FIBC
converter was applied. The experimental tests have been carried
out by connecting the DC bus to an adjustable resistor. The
Arduino DUE card is used to create and ensured a shift of the
PWM signals generated. Fig. 9 shows an example of four shifted
control signals of a quarter of period for 4-phase-FIBC
Fig. 10 shows The DC bus voltage regulation under load
changes.
The curve in green shows the load current [1A/Div].
Fig.9. Four shifted control signals of a quarter of period.
Fig. 10. The DC bus voltage regulation under load changes.
Fig. 11. Inductor currents and PEMFC emulator current.
On the basis of the figure Fig.10, it can be observed that the
controller of the converter offers good performances in terms of
stability and precision. Concerning the response time, it can be
observed that the DC bus voltage follows its reference perfectly
Fig. 11 shows the experimental results of the PEMFC emulator
current and the inductor currents waveforms [1A/Div].
During the permanent mode represented by Fig.11, we can
observe on the one hand that the currents of phase are perfectly
shifted as wished by the objectives of the control and on the other
hand, it can be observed that the ripple of the current of the FC is
very weak, which confirms the advantage of this topology.
The experimental results are in agreement with those obtained
with theoretically simulation.
VII. CONCLUSION
In this paper the four-phase FIBC has been designed,
simulated and experimentally verified. This is an interesting
alternative dc-dc structure to the IBC basic converter because it’s
provides significant part count reduction, size and eventually
converter cost without penalizing performance.
Converter design guidelines have been provided and
implementation of 100 W 4-phase FIBC has also been performed,
including the power converter and the Arduino card based control
system. Experimental validation has also been reported showing
the most interesting converter features, gate driving scheme, main
inductor and input currents, dynamic response and efficiency
measurements.
Finally, because of its interesting features, such as low input
current ripple, high current capabilities, modularity, power losses
distribution and high efficiency, this converter could adapt
attractively for medium and high power fuel cell dc-dc converters.

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Experimental Validation of High-V oltage-Ratio LowInput- Current-Ripple DC-DC Converters for Fuel Cell Applications

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