Comparison of DC-DC Converter Interfaces for Fuel

Cells in Electric Vehicle Applications

A. Lachichi and N. Schofield
School of Electrical and Electronic Engineering
The university of Manchester, M60 1QD, UK
Tel: +44 (0) 161 306 4793, Fax: +44 (0)161 306 4774
E-mail: Amel.Lachichi manchesterXac.uk
Abstract-- Due to low stack voltages and voltage regulation, fuel cell life, as discussed in [6-8], although the effects of
converters must, in general, be used to interface fuel cell to the such phenomena is still not completely understood, and is an
power-train of electric vehicles. This paper considers three area of active research. It is, however clear that a main design
converter topologies based, principally, on interleaving objective is to minimise such ripple and harmonic content.
techniques. Analysis of steady state operation is used to
compare the topologies such that design choices may be assessed
prior to the prototyping. 1.00 Activation polarisation 0.70
0.90 < (reaction rate loss) Peak power 0.63
Index Terms-- Coupled inductors, DC-DC converters, > -.0.80 0.56 0
interleaving techniques, fuel cells. 25 0.70- 0.49
0.60 Ohmic polarisation 0.42 :
:) (resistance loss),
I. INTRODUCTION U 0.50 - i l 0.35

espite efforts to series a number of basic cells, fuel cell X 040.0.28 u

stacks typically deliver current at low voltage which is, a

0.30 Concentransport loss)n 021
generally, not easy to exploit on electric vehicle power- ¢ 020 ( t r
trains. In fact, if the technological limits to-date are 0.10 - Voltage 0.07
considered, the number of cells which one can put in series is 0.00 0.00
of the order of a hundred resulting in an upper DC voltage of 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
IOOV [1]. Additionally, fuel cells also exhibits a non-linear Current density / Acm -2
performance characteristic that can significantly influence
vehicle drive system operation and component optimisation Fig. 1. PEMFC voltage and power density curves.
if not considered at the system design stage. For example,
the three main fuel cell loss mechanisms, activation, This paper presents results of a simulation study
concentration and ohmic, all influence fuel cell performance investigating various candidate DC-DC converter structures
in terms of voltage drop or load regulation [2,3], as for fuel cell interface applications. The study was undertaken
illustrated in Figure 1, showing measured voltage and power to assess design options prior to the prototyping of a
density curves with load current density variation for a converter system for an electric vehicle application
proton exchange membrane fuel cell (PEMFC) [4]. A wide variety of DC-DC converter topologies, including
For vehicle power-train applications above 10's kW, this basic structures with direct energy conversion, structures
low voltage becomes impractical. Indeed, voltage levels with intermediate storage components (with or without
considered for urban electric vehicles at, and in excess of, 1.0 transformer coupling), have previously been published [9-12]
tonne curb mass are around 700V [5]. Hence, the use of a and are among the possible DC-DC converter solutions
power converter is required to interface the fuel cell system investigated. However, some design considerations are
to the vehicle power-train, the primary function of which is essential for the application considered:
to boost the voltage delivered by the fuel cell. Further, the
conversion stage is essential because of the variation of the * the step-up function of the converter, which is
power-train DC link voltage which can vary by at least 20 % particularly high at 8 to 10:1;
during acceleration and regenerative braking as a * control of the DC-DC converter power flow subject to
consequence of other energy sources connected in parallel the wide voltage variation on the converter input due to
with the fuel cell system [5]. the fuel cell regulation (-35% of open circuit) and output
Therefore, the control of fuel cell power necessitates voltage variation due to the battery regulation (+1-35%
power electronic conditioning via systems that consist of nominal);
predominantly of DC-DC conversion, the design of which is * low current ripple drawn from the fuel cell;
a crucial to the vehicle since the DC-DC converter
performance directly influences the fuel cell stack. Indeed, DC-DC converter structures that facilitate control of the
the ripple and harmonic content of the current drawn from output current delivered by the fuel cells are interesting for
the fuel cell stack is one of various phenomena influencing this type of application. Indeed, in this case, it is possible to

1-4244-01 59-3/06/$20.00 ©2006 IEEE

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improve the total efficiency of the whole system by the direct basic step-up converter level and has a ripple content of
control of the quantity of hydrogen fuel consumed via the period T/N.
control of the fuel cell output current [13]. The simplest
converter structure with this functionality is the basic, single
level, step-up converter. However, this topology suffers D2
from excessive component stress because of the high current N
on the fuel cell side. Interleaving the basic, single level, step-
up converter makes it possible to improve the quality of the D
fuel cell output current in terms of current ripple magnitude N
n
and harmonics. ,JW

This paper presents the primary concepts of basic, single KN

level, and interleaved step-up converters to highlight the !LN
input current ripple reduction, firstly, when inductances are I+LI
L
independent of each other and secondly when inductances 1K( T V2
are combined in one magnetic core. These structures, l
however, cannot work efficiently when a high voltage step- _
up ratio is required since the duty cycle, a, is limited by VI
circuit impedance leading to a maximum step-up ratio of
approximately 4 as illustrated in Figure 2. Hence, two series l
connected step-up converters would be required to achieve
the specific voltage gain of the application specification. Fig. 3. Interleaved N identical step-up converters.
The principle of interleaved step-up converters is then
extended to structures that use a step-up transformer. The The input current iL has a peak-to-peak ripple expressed by:
main feature of the transformer is not one of galvanic
isolation (which is not necessary in this type of application), V
but to add a higher step-up ratio than could be achieved by Al/= 1 a T XN (1)
the basic interleaved structures. L1 .N
V2 V2 where:
actual

4 2 theollvatl J I XN = )
( VI
(k-N.u).(N.a+l-k) (2)
theorical XN Nc I-a

3 k,/1, Nltakes into account the number of devices

conducting at the same time in each interval and a is the duty
cycle.
2 . Hence, the converter silicon VA rating, which will be
used to compare candidate structures, is expressed by:
1~~~~~~~
1
VA= Vl iL (3)

0 0.2 0.4 0.6 0.8 1 The advantage of the interleaving technique may be
Duty cycle, -
demonstrated by normalising the fuel cell output current
Fig. 2. Theoretical and actual output/input ratio. ripple to that due to a basic, single level step-up converter:
Al
=100 XN % (4)
II. BASIC INTERLEAVED STEP-UP CONVERTERS A ind N
In this section we consider N interleaved basic, step-up
converters for two cases dependent on the way the N where Al .d is the peak-to-peak current ripple of the simple
inductances are manufactured. level step-up converter.
A. Individual inductances Figure 4 depicts the current ripple reduction ratio for
Figure 3 depicts a basic interleaved step-up converter of N values of N from 2 to 4. It can be observed that the peak
identical levels where the inductances L1 to LN are realised current ripple is reduced by a factor of 1/N for N interleaved
by a separate magnetic core. The gate signals to the power converters and that the minimum current ripple occurs at
switching devices K1 to KN are successively phase shifted by discrete duty cycles given by ac = a/N; where a = 1 to N-i,
T/N where T is the switching period. Thus, the current giving the theoretical case for complete current ripple
delivered by the fuel cell is shared equally between each cancellation.

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 50

30 \
. <, \ 1 ,> , ~ ~ ~ ~ ~ ~ ~ ~j,.j where

windings.
vj =Lj. dtddt'L

L1 and M1
di Lj N
diLji,
dt

are the self and mutual inductances of the

winding respectively, these being considered identical for all
,To obtain the expression of L1 and M1, consideration is
(6)

0~~~~~~~~~~ given to the simple magnetic equivalent circuit depicted in

r 20 Figure 6, where the winding orientations are such they create
positive MMF. RL and RC are the reluctances of the external
A
4 . columns and the central column respectively. If the resultant
z 10 magnetic circuit is perfectly symmetric, the external columns
and the lateral parts will have the same length, lo, as that of
0 '. ,. i; 8 the central column. The external column has a surface area
0 0.5 1 equal to so and an equivalent permeability that accounts for
Duty cycle distributed air gaps, whereas the central column has a surface
equal to N so.
Fig. 4. Reduction ratio of the current ripple.

The energy stored in the DC-DC converter inductance RL RL RL RL Rc

determines the volume of the magnetic core. In the general
case of N interleaved converters, the energy stored in one + + + +
inductance can be expressed as [14]: - fl. - fl.1 - .1u
nJiL n.iL2 n.iL3 n. iLN cl

WN =k.
1 -SO (5) LI L2 L3 LN

Fig. 6. Equivalent magnetic circuit.

where lo, so are the effective length and surface area
respectively of one magnetic core; k is a factor that is To highlight the benefit of the combined core
dependent on core material [14]. It can be seen that the arrangement, the normalised ratio between the current ripple
inductor volume is reduced by a factor of N, hence there is for independent (ind) and combined (com) inductances is
no inductance volumetric penalty in moving to an N-level considered [14] viz:
interleaved topology.
B. Inductances on a combined core A\iL(com) - Lind - (RL(com) + N* RC ) (7)
Theoretically, the N inductances can be combined on a AiL(ind) L1 RL(ind)
single magnetic core, consisting of N external columns, on to
which each inductor is wound and a single central column, as Compared to the independent inductance case, the value
illustrated in Figure 5 [15]. of the current ripple will depend strongly on the reluctance of
the central column Rc as summarised in Table I for three
Winding and
current direction possible configurations and where and FL are the jtc
permeability of the central and the external columns
respectively.
If we use same material for the central and the external
columns, the inductance LI has approximately the same value
as the independent inductances Lind. The value of the fuel cell
input current ripple is thus unchanged. If we want to decrease
External RC without changing the dimensions of this column, it is
Flux ~~~~~~~~colunm necessary to increase its permeability. If this permeability is
direction larger than the permeability ut of the external columns, and
if the N.R_ term is negligible compared to RL, the inductance
! o !Z !| ~~~~L1 will tend towards a value that is twice Lifld. Hence, the
current ripple delivered by the fuel cell decreases by a half.
Fig. 5. Converter inductances realised on one combined magnetic core. However, if we decrease the permeability Rc of the central
column, which means, in the worst case, moving out the
The voltage across the winding of the jt column is expressed central column (air), the reluctance RC is higher than RL,
by: reducing the inductance L1. This leading to an increase of the
current ripple drawn by the fuel cell.

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 Note that whilst the active volume of the magnetic circuit arrangement the external cores will saturate unless their
remains the same, the N inductances realised on a combined dimensions are increased. This arrangement is not of interest.
magnetic core will occupy lower space than the N-individual Another arrangement of basic interleaved, single step-up
inductances. converters can be considered as discussed in [16]. Here, the
authors have considered two interleaved structures where the
TABLE I inductors are replaced by an interphase transformer (IPT).
IMPACT OF CENTRAL COLUNM ON INPUT CURRENT RIPPLE However, this IPT performs the interleaving function but
1c << 11L It = 11L c >>» L does not contribute to an improved step-up voltage ratio
(No central (Same (Different which is limited, as in the basic and multiple level
column) material) material) interleaved structures, to approximately 4.
R L(ind) 2 lo 2 lo 2 lo Thus, when a high step-up voltage is required, the
FL -S sL SO IL SO previous structures show a limitation as depicted in Figure 2.
RL(com) lo 1 1 One solution is to cascade the converters to reach the desired
ItL -So 'SOItL 0tL'SO step-up ratio of 8 to 10:1. Alternatively, full-bridge
topologies can be used.
Rc 0lo lo lo
| *c N SO |Lc N So Ic N s0 II. INTERLEAVED FULL-BRIDGE DC-DC CONVERTER
(R L(com) + N Rc lo 2.1oo
In this part, we consider interleaving structures that
*c SO FL SO OL
contain a transformer to achieve the desired voltage step-up
So
AiL(com) increased 1 greater than 4. The principle of operation can be analysed
1
AhL(independant) under steady state conditions and shown to be similar to the
2
previous circuit topologies without transformer, hence we
Figure 7 illustrates simulation results for the case of three will not present the case of N interleaved structures. Also,
interleaved converters having independent inductances (a) note here that, only voltage-fed full-bridges converters are
combined inductances with [t = It L (b), and combined considered. In fact, current-fed, full-bridge converters need
ItLc the 7highlextra circuitry to make them work properly, otherwise, they
c

inductances with p »> L (C). Figure 7 highlights benefit can be damaged by overvoltages appearing at switching
indctncewtht
Figure

of combining inductances in the same magnetic core. Indeed, instants due to the transformer leakage inductance.
with the same magnetic core between the central and external Additionally, structures with voltage clamping snubbers
columns, we do not change the current ripple seen by the fuel seem to be unsuitable for fuel cell applications because
cell. However, the ripple in each converter level is reduced. power sometimes needs to be reinjected to the supply, which
In the case where different material is used between the is unacceptable for fuel cell systems.
central and external columns, i.e. when the condition of The considered structure is depicted in Figure 8 and
»c >>L is satisfied, the current ripple seen by both the fuel consists of three H-bridge converters, interleaved on the
cell and each converter level is reduced. primary side of each transformer. The transformer secondary
windings are star connected and supply a three phase diode-
'LI 'L2 'L3 bridge rectifier.
will notgetconsiderinsight
To first of the behaviour of the
the leakage inductance of eachstructure,
we
1 V,\ / a T IL *Vl a-T transformer.
L\\ \ \'.,\'\. \ '\\, ind Lind (a) In fact, with the parasitic capacitance of each semi-
conductor, they can avoid apparition of high di/dt. Besides,
they can allow commutation at zero voltage.

v K
~~~~~~~~V
-2xT- 1+
~~~~~~~~_
LX' /\\/'\,//'\ I'''X3
LIRL
(b)K
1 a24
L

K"
FiL.7.2 Individual converter cub2ents (left) and fuel cell cur2en (right).

Fog. 7.tIndiacovertera clurents()ndfuell cowellvcuen(ight) Fig.8.Exampleoirc

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A. Principle of operation III. THREE-PHASE PWM INVERTER-RECTIFIER
Control of the three individual full-bridge converters is STRUCTURE
delayed by 1/3 of the switching period in order to obtain The three-phase PWM inverter-rectifier is depicted in fig.
three-phase balanced (fundamental) voltages. For each 10. In this case a star-star three-phase transformer is used.
converter, phase shift control is adopted. It consists of Two types of control signals can be applied to the power
holding the gate signal of the top switches of one leg on semi-conductors; 1800 conduction (a) or 1200 conduction (b).
during half of the period and delaying the top switches of the The secondary phase voltage obtained for each control
other leg by an angle a, this considered to be the duty cycle schematic is shown in Fig. 11.
of the structure. A complementary control is adopted for
switches situated on the bottom of the structure. Three cases A Inverter switch ratings
arise, depending on the duty cycle. In fact, the phase voltage The maximum current rating of each semi-conductor is
in the secondary side of the transformer has a particular equal to lfc for 1200 conduction control and lfc /2 for 1800
shape depending on the phase shift and hence the DC output conduction. The peak voltage Vp across each semi-conductor
voltage of the rectifier [17], as illustrated in Figure 9, is equal to Vfc in both cases. Hence, the total converter
showing the case for 0< a <.2/3(a), n/3 < a < 2 2r/3 (b), switch rating is given by:
2 .2/3 < a < fC (c). 6 I
The main feature of this configuration is that the voltage VA = fc,k V = 3- Ifc Vfc 180°conduction (10)
can be doubled depending on the duty cycle and hence allow k=l 2 p,k
the use of a transformer with a lower turn ratio. Besides, by 6
choosing appropriate control, the DC inductor L is subject to VA = E I V = 6 -fc* Vfc 120conduction (11)
no current ripple (Fig. 9-c), allowing a reduced size. k=l
B. Inverter switch ratings
For the three cases, the maximum current flowing through L
the semi-conductor is giving by:

Imax 2c
S=2 (8)a v2
lf being the nominal current delivered by the fuel cells. Co
The peak voltage VP across the semi-conductor is equal to H H H
the fuel cell output voltage Vfc. Thus, the total converter
switch rating is then given by:
Fig. 10. Three-phase PWM inverter-rectifier structure.

VA =1A2
, 1 fc,kV
2 Vpk
k12 .

k=1 (9)
= 6Ifc Vp Vs,i
2.n.VfJ3
vn.Vfc/3
Vs3-1V'iVQ ~~~~~~~~VDC
nl.Vrc flK .Vrc
o
/2 '17l^

_ ,_
0
.T/I
l|I

_(01a)_s_7, T
(a)
X /3 X ~~~~~~/6 1 13 '172

2.n.V-c
nl.V~
, vs 1
v

O (b.ratT i I T ) 1 1 f lVf

-n.Vfc X I1I

ll1 13 | | |2 n.Vf, 176 1 /3 '12 VDC T12l || (b)

lla6 v 1 j l1
T
0 >I (c)
-n.Vrc
V Fig. 11. Secondary phase voltages. (a) 1800 conduction. (b) 1200 conduction.

Fig. 9. Simulated voltage waveforms.

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 IV. COMPARISON OF THE STRUCTURES converters allows for boosting of converter the output
The main feature of interleaving basic step-up converters voltage when three individuals transformers are used. It is an
is that it reduces the fuel cell input current ripple according to attractive structure but, for vehicle prototype design has a
the number of basic converters being interleaved. trade-off in terms of core volume. The three-phase PWM
Besides, the volume of each inductance is reduced by the inverter-rectifier improves on transformer volume. Moreover,
number of the interleaved converter compared to one basic with proper control, it allows for lower total installed silicon.
converter. Moreover, when inductances are combined and
with appropriate material, we can further reduce the current
ripple, and gain space. However, the voltage step-up is VI. ACKNOWLEDGEMENTS
constrained to approximately 4. For a high voltage bus one The authors acknowledge the support of the UK
can use cascaded converters. Engineering and Physical Science Research Council
Another solution is to use structures with a transformer. (EPSRC), via Grant No. GR/S81971/01.
An interleaved, three individual, full-bridge converter has the
advantage of boosting the output voltage by a factor of two
without increasing the transformer turns ratio and with a VII. REFERENCES
proper control, the output current can exhibit low current [1] Rodatz P., Buchi F., Onder C., Guzzella L. "Operational aspects of a
ripple. However, this topology has the disadvantage of large PEFC stack under practical conditions," Journal of power
requiring higher total switch utilisation compared to the sources
PWM three-phase, inverter-rectifier. Besides, in the PWM [2] A. J. Appleby, and F. R. Foulkes. "Fuel Cell Handbook," Book, Van
three-phase, inverter-rectifier structure, we can reduce the Nostrand Reinhold, 1989.R. S.
number of switches, whic allw[3] EG&G Technical Services Inc. "Fuel Cell Handbook
(Sixth Edition),"
number of switches, which allows a reduction of their Book, U.S. Department of Energy, 2002.
associated gate components. Moreover, the use of a three- [4] MES-DEA, Fuel cell technical datasheet, http://wwwcebide.
phase transformer in the PWM three-phase inverter-rectifier [5] N. Schofield, H.T. Yap and C.M. Bingham. "Hybrid energy sources
structure is the best choice since it requires a smaller core. for electric and fuel cell vehicle propulsion," IEEE VPP Conf., (2005).
more adequate since we [6] Gemmen
The 1800 conduction control is also moreadequate "Analysis for the Effect of Inverter Ripple Current on Fuel
obTain the lowerchotal switchntroliS
ation
obtain the lower total switch utilisation.
Cell Operating Condition," ASME Inter. Mech. Eng. Congress and
Exposition, pp. 279-289, Nov. 2001.
Normalised to Vfc Ifc and considering 1:10 step [7] s. K. Mazumder, K. Acharya, C. L. Haynes, R. Jr. Williams, M. R.
von Spakovsky, D. J. Nelson, D. F. Rancruel, J. Hartvigsen, R. S.
requirement, Table II summarises the silicon requirement for
presented in the paper.
Gemmen "Solid-oxide-fuel-cell performance and durability: resolution
each structure presented in the paper.
each structure
of the effects of power-conditioning systems and application loads,"
IEEE trans. On Power Electronics, pp. 1263 -1278, Vol. 19, Issue
5, Sept. 2004.
TABLE II [8] K. Acharya, S. K. Mazumder, R. K. Burra, "Impact of power-
AUDIT OF CONVERTER SILICION REQUIREMENTS electronics systems on the performance and durability of tubular solid-
oxide fuel cell," Proc. of IEEE Applied Power Electronics Conference
Structure No. of No. of Maximum silicon VA and Exposition, Vol. 3, pp. 1515 -1520, 2004.
switches diodes rating [9] F. Profumo, A. Tenconi, M. Cerchio, R. Bojoi, G. Gianolio "Fuel Cells
for Electric Power Generation: Peculiarities and Dedicated Solutions
2x Basic 2 2 8 for Power Electronic Conditioning Systems," EPE 2004.
converter [10] Iftikhar A. Khan, "DC-To-DC Converters for Electric and Hybrid
Interleaved N 2N 2N 8 Vehicules".
basic converters [11] Liang Yan, Brad Lehman "An Integrated Magnetic Isolated Two-
Inductor Boost Converter: Analysis, Design and Experimentation,"
Interleaved Full- 12 18 9 IEEE Trans. On Power Electronics, Vol. 20, No. 2, pp. 332-42, March
Bridge converter (3 diodes, 6 switches) 2005.
[12] Rik W. A. A. De Doncker, D. M. Divan, M. H. Kheraluwala "A Three-
Three-phase 6 12 6 1200 (diodes/switches) Phase Soft-Switched High-Power-Density dc/dc Converter for High-
PWM inverter- Power Applications," IEEE Trans. On Industry Applications, Vol. 27,
rectifier converter 5 1800 (2 diodes, 3 No. 1, pp. 63-73, January/February 1991.
switches) [13] K. Rajashekara "Propulsion System Strategies for Fuel Cell Vehicles,"
SAE Technical paper series.
[14] A. Lachichi "Mod6lisation et stabilit6 d'un r6gulateur hybride de
courant/Application aux convertisseurs pour piles a combustible," Phd
thesis, Universit6 of Franche-Comt6, 2005, http 5//te1ccsd.cnrsfr/tel-
V. CONCLUSIONS 00083112/en/
Because of their intrinsic characteristics, fuel cells cannot [15] S. Chandrasekaran, L. U. Gokdere "Integrated Magnetics for
be used without the use of static inverters to feed an electric Interleaved DC-DC Boost Converter," IEEE PESC'04.
[16] G. Calderon-Lopez, A. J. Forsyth, D. R. Nuttall "Design and
load. The paper has presented three examples of DC-DC PerformanceEvalationofa O-kWInterleavedBoost Converterfora
converter topologies based on interleaving techniques. Fuwel Cell Electric Vehicle," IPEMC 2006.
The first structure considers an interleaved, basic, single [17] c. Liu, A. Johnson, J-S. Lai "Modeling and Control of a Novel Six-Leg
ste-upconertr.This
step-upconverter. ~
structure, even simple, improves the
~ '
Thlree-phlase High-Power Converter for Low Voltage Fuel Cell
Applications," IEEE Power Electronics Specialists Conference, pp.
quality of the current drawn by the fuel cell, especially when 4715-21, 2004.
all inductances are realised in the same magnetic core with
appropriate material. However, it presents limits when a high
voltage step-up is required. Structures with transformers have
then been investigated. The use of three, full-bridge DC-DC

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