Applied Mechanics and Materials Vol.

666 (2014) pp 87-92 Online: 2014-10-20

© (2014) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMM.666.87

Design and Implementation of an Interleaved Boost DC-DC Converter for

PEM Fuel Cells

Sakda Somkun1, Chatchai Sirisamphanwong and Sukruedee Sukchai

School of Renewable Energy Technology, Naresuan University, Phitsanulok, Thailand
1
email: sakdaso@nu.ac.th

Keywords: Fuel cell, hydrogen, power converter, renewable energy.

Abstract. This paper elaborates a design and implementation of a 2-phase interleaved boost DC-DC
converter as the power conditioning circuit for PEM fuel cells. The converter ripple current is
analyzed so as to determine the inductance value, which would draw the fuel cell ripple current less
than 4 % of its nominal value for a longer lifetime and efficient operation. Compact design is
benefited from reduced inductor core volume due to paralleled connection as well as
microprocessor-based control. Practical implementation is also discussed. Selection of simulation
and experimental results is presented to validate the design methodology. The converter has the
maximum output power of 1 kW at the output voltage of 120 V with the efficiency better than 92 %.

Introduction
Hydrogen is expected to be another energy carrier along with electricity [1]. The main distinct feature
of hydrogen is it can be stored for a long period of time without leakage. Hydrogen production from
water electrolysis can be used to stabilize the intermittent nature of renewable energy sources, i.e.
wind, solar energy [2]. So, this is an ultimate goal for carbon-free societies to tackle the global
warming. Using fuel cells is an efficient method to utilize hydrogen energy thanks to their high
efficiency, zero emission, and low audible noise. Polymer electrolyte membrane (PEM) fuel cells are
widely used in transportation, stationary power generation and portable devices due to their simple
construction and low temperature operation, which means they can be started from room temperature
[3].
The output of a PEM fuel cell is unregulated low voltage with the open circuit voltage of
approximately 1 V/cell and decreases with the increasing load current [3]. Thus, there must be a
power conditioning circuit to step-up and regulate the output of the fuel cell stacks to a useable value
before connecting to the load [4], which is typically an AC-DC converter (inverter) in fuel cell electric
vehicles or stationary power supplies. However, the fuel cell current drawn by a power converter
contains a small fluctuation, so called ripple current ( ∆I FC ). It has been reported that the fuel cell
ripple current should be maintained within 4 % of its rated value for a longer lifetime and efficient
operation [5]. The conventional boost DC-DC converter [6] shown in Fig. 1 has been typically
selected as the fuel cell power conditioner because it increases the input voltage and draws a low
ripple input current. The voltage conversion ratio and the input ripple current are given by
VO 1
= , (1)
VFC 1 − D
VO
∆I FC = D(1 − D) (2)
Lf S
, where f S is the switching frequency, and D is the duty ratio of the power switch, S1. However, the
conventional boost converter is suitable for low power applications as the inductor ( L ) and output
capacitor ( CO ) becomes larger with the increasing rated power. Thus, paralleled operation of the

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88 Electronics, Mechatronics and Automation III

boost converters with an interleaved switching technique is an alternative solution so as to raise the
handling power and obtain a compact design [4].
This paper describes a design of a 2-phase interleaved boost DC-DC converter for a 1-kW PEM fuel
cell, model H-1000 from Horizon [7], which has the rated voltage and current of 43.2 and 23.5 A
respectively. The converter has been analyzed and constructed to have the maximum power ( PO ,max )
of 1 kW the output voltage ( VO ) of 120 V and input ripple current less than 0.94 A (4 % of 23.5 A).
Selection of simulation and experimental results are presented to verify the design methodology.
vL(t)
(VFC)
PEM Fuel Cell t
iFC L iO iFC L iO
iFC L iD D1 io iD
+ + vL - + vL - iC + iFC(t) + vL - iC
iC + (VFC-Vo) +
vFC S1 + Ro VO VFC + Ro VO ∆IL +
Co S1=ON CO VFC S1=OFF CO Ro VO
- IFC
- - -
t
TS
ton=DTS toff
Fig. 1 A conventional boost DC-DC converter as a fuel cell power conditioner and its basic operation
in continuous conduction mode.

Analysis of the Interleaved Boost DC-DC Converter

The N-phase interleaved boost converter is shown in Fig. 2(a), where the inductor and switch
networks are connected in parallel with a common output capacitor supplying the load resistor. The
values of the inductors, L1 , L2 , … , LN , are theoretically equal to L . To achieve a low ripple in the fuel
cell current, iFC (t ) , there must be a delay time of TS N in the switching sequence between the
adjacent phases. Figs. 2(b) and 2(c) illustrate the switching patterns and current waveforms of the
2-phase topology in two duty cycle ranges: 0 ≤ D ≤ 0.5 and 0.5 < D ≤ 1 , operating in the continuous
conduction mode (CCM). Each inductor carries a half of fuel cell average current, I FC with the ripple
value defined in Eq. 2. Whenever there is a change in the slopes of the inductor currents, the fuel cell
current will be diversified. Thus, the fluctuating frequency of the fuel cell current is always N times of
f S . The ripple in iFC (t ) can be easily determined from the sum of the inductor current slopes times
the period between the changing slopes, e.g., 0 ≤ t ≤ t1 in Fig. 2(b). Eqs. 3 and 4 summarizes the
ripples in the fuel cell current, ∆I FC and the output voltage, ∆VO . Figs. 3(a) and 3(b) compare ∆I FC
and ∆VO , which show that the maximum ripple components are reduced by N times with respect to
the conventional topology.
 VO
 Lf D (1 − 2 D ), for 0 ≤ D ≤ 0.5
∆I FC = S (3)
V
 O ( D − 0.5)(2 − 2 D), for 0.5 < D ≤ 1
 Lf S
 PO ,max D( D − 0.5)
V C f , for 0 ≤ D ≤ 0.5
D −1
∆VO =  O O S
. (4)
P
 O ,max ( D − 0.5), for 0.5 < D ≤ 1
VO CO f S
The core volume ( VCore ) of the inductors is also smaller, benefited from the decrease in the inductor
current, which can be considered from the energy ( Wm ) stored in each inductor [8] given by
1 ˆ 2 Bˆ 2VCore µ c LIˆL2
Wm = LI L = ⇒ VCore = , (5)
2 2µ c Bˆ 2
 Applied Mechanics and Materials Vol. 666 89

, where B̂ and µ c is the peak flux density and permeability of the core. Thus, the volume of each
inductor is theoretically reduced by the factor of 1 / N 3 compared to the conventional topology. With
N inductors, the total core volume will be 1 / N 2 times smaller.

(a) (b) (c)

Fig. 2 (a) Circuit topology of the N-phase interleaved boost converter, and switching and current
waveforms for (b) 0 ≤ D ≤ 0.5 and (c) 0.5 < D ≤ 1 respectively (N=2).

Fig. 3 (a) Input ripple current of N-phase interleaved boost converter, (b) Output ripple voltage of
N-phase interleaved boost converter.

Design and Construction of an Interleaved Boost DC-DC Converter

In this section, the 2-phase interleaved boost converter is designed for a fuel cell stack from Horizon
model H-1000, of which voltage varies from 43.2-67.8 V and the maximum current of 23.5 A as
shown in Fig. 4 (a). The maximum output power is 1 kW at the output voltage of 120 V. The fuel cell
ripple current must be less than 0.94 A, 4 % of 23.5 A, and the output voltage ripple is 1.2 V. The
converter is assumed to be lossless and operates in the CCM during this stage. The switching
frequency, f S is selected to be 25 kHz. The calculation procedure is described as follows

1) The operating duty cycle, D is calculated using Eq. 1: 43.2 ≤ VFC ≤ 67.8 ⇒ 0.435 ≤ D ≤ 0.64 .

2) According to Fig. 3(a), the maximum value of ∆I FC will be at D = 0.64 . Hence, the minimum
inductance, L can be calculated from this value using Eq. 3, giving L = 0.52 mH.
3) The peak inductor current, IˆL = (I FC + ∆I L ) / 2 is necessary for construction of the inductor.
The inductor ripple current is derived from Eq. 2, resulting in IˆL = (23.5 + 2.12) / 2 = 12.81 A.
90 Electronics, Mechatronics and Automation III

4) From Fig. 3(b), CO is determined using Eq. 4 with D = 0.64 . If ∆Vo = 1.2 V, CO = 39.5 μF.
Simulation of the 2-phase interleaved boost converter with the parameters calculated above was
developed based on the switched-circuit model [9] as follows

di L1
L = v FC − (1− S1 )vo (6)
dt
di
L L 2 = vFC − (1 − S 2 )vo (7)
dt
dv
CO o = (1 − S1 )iL1 + (1 − S 2 )iL 2 − io (8)
dt
iFC = iL1 + iL 2 (10)

, where the possible values of S1 and S2 are 0 and 1, representing the off and on states of the power
switches. The model was implemented in MATLAB/Simulink. Fig. 4(b) displays the simulated
waveforms of iL1 , iL 2 , iFC and vo at the fuel cell rated power. The fluctuating values in the inductor
currents, fuel cell current and voltage are very close to the calculation. This validates the design
methodology.

Fig. 4 (a) Polarization curve of the H-1000 fuel cell [4], (b) Simulation results of L1 i , L2 i , FC i and
Ov of the 2-phase boost converter at the rated fuel cell power.

Fig. 5(a) exhibits the prototype of the 2-phase interleaved boost converter. The inductors were
assembled from EPCOS N-97 ferrite ETD-59 cores, whose base material is MnZn. The winding was
three Litz wires (60/36AWG) twisted together to minimize the skin effect due to the switching
frequency, and wound on the plastic former, 58 turns each. Total air gaps of 3 mm were added to each
inductor to prevent the core from saturation and to adjust the inductance to be approximately 0.7 mH,
causing ∆I FC to be less than 3.5 % of the rated current. A 400-V, 330-μF aluminum electrolytic
capacitor was selected as the output capacitor. The capacitor is a wide temperature class, -25°C to
105°C, which is suitable for a high capacitor ripple current in this application. With this large design
margin, the output voltage ripple can be negligible. MOSFETs model IRFP4232PBF and ultra-fast
recovery diodes model MUR3020WT were used as the power switches.
The prototype converter was controlled using the cascade structure shown in Fig. 5(b).
Proportional-integral (PI) controllers were employed for both the voltage and current control loops,
which were digitally implemented in a TMS320F28035 32-bit microcontroller. The pulse width
modulators (PWM) for generation of switching patterns were also embedded in the microcontroller.
 Applied Mechanics and Materials Vol. 666 91

(a) (b)
Fig. 5 (a) Prototype of the interleaved boost DC-DC converter, (b) Converter control block diagram.

Experimental Results and Discussion

The prototype boost converter was connected to the H-1000 fuel cell. However, the fuel cell
maximum power dropped to 400 W at 40 V due to no operation for long time. Proton conducting
ability of the polymer electrolyte membrane decreases due to dehydration [10]. The stack is now
under a rejuvenation process by water injection into the anode side to hydrate the membrane [7] so
that its rated power could be partly recovered.
Fig. 6(a) verifies the design procedure, which shows that ∆I FC = 0.92 A at D = 0.67 still less than the
target at ∆I FC = 0.94 at D = 0.64 , 4 % of the rated fuel cell current. This guarantees the safety
operation of the fuel cell. The actual inductance calculated backward from ∆I L 2 using (2) was
approximately 0.69 mH which is close to the calculated value. The interaction between the fuel cell
and power converter can be seen in v FC [11]. The AC component of v FC ( v~FC ) is in the opposite
~
phase to iFC due to the voltage drop in the ohmic resistance of the fuel cell stack. The double layer
~
capacitance effect of the stack can be observed in v~FC near the edges of iFC .

v~FC vFC
fS =
~
iFC iFC
D=
vo
vL 2 ∆I FC =

∆I L 2 = io
~
iL 2 200 W 400 W 200 W

(a) (b)
Fig. 6 (a) Steady state waveforms of v L 2 and AC components of v FC , i FC and i L 2 , (b) Dynamic
characteristic of the fuel cell and converter during load changed from 200 W to 400 W to 200 W.

The transient characteristic of the fuel cell and the converter is depicted in Fig. 6(b). The
microcontroller-based control scheme regulated the output voltage at 120 V when the load had even
changed. Slow dynamic in the electrochemical process can be observed in the fuel cell voltage and
current when the load changed from 200 W to 400 W. The output voltage recovered back to 120 V
92 Electronics, Mechatronics and Automation III

within 20 ms to supply the load power of 400 W but v FC and iFC had further changed for about 50 ms
to reach the steady state. The regulation of the output voltage can be improved by hybridization with
ultracapacitors or batteries to compensate the slow dynamic of the fuel cell stack [12].
The converter efficiency was tested with a 600-W 50-V DC power supply as the input voltage, and
found to be in the range of 92.2-94.7 % with the output power of 100-600 W. Higher efficiency can be
achieved with an increasing number of paralleled modules [13], but it may add complexity in the
control system.

Summary
A 2-phase interleaved boost DC-DC converter for fuel cell application was analyzed and
implemented. The converter was designed to step-up and regulate the fuel cell voltage to be 120 V and
also to draw the fuel cell ripple currrent less than 4 % of its rated current with the help of paralleled
connection and interleaved switching technique. Compact construction was also achieved due to
reduced inductor core volume and microcontrller-based control scheme. This converter can be used as
the active front-end circuit for small AC power systems or electric vehicles.

Acknowledgement
This work has been supported by Naresuan University (research grant no. R2557C024).

References
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Electronics, Mechatronics and Automation III
10.4028/www.scientific.net/AMM.666

Design and Implementation of an Interleaved Boost DC-DC Converter for PEM Fuel Cells
10.4028/www.scientific.net/AMM.666.87

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