Dc dc converter for ultracapacitor boosted electric vehicle

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN IN –
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH 0976
ENGINEERING AND TECHNOLOGY (IJARET)
6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME

ISSN 0976 - 6480 (Print) IJARET
ISSN 0976 - 6499 (Online)
Volume 3, Issue 2, July-December (2012), pp. 71-81
© IAEME: www.iaeme.com/ijaret.html
©IAEME
Journal Impact Factor (2012): 2.7078 (Calculated by GISI)
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DC-DC CONVERTER FOR ULTRACAPACITOR BOOSTED
ELECTRIC VEHICLE

Mohamed HediChabchoub and HafedhTrabelsi
Computer Embedded System (CES),SfaxIngineering School, University of Sfax, Sfax,
Tunisia
chabchoub_medhedi@yahoo.frandhafedh.trabelsi@yahoo.fr

ABSTRACT

For traction system, when there is a transient high power requirement, such as an acceleration
or cold start, the battery can be boosted with UltraCapacitor UC. The UC show an extremely
high power density. So the UC is capable of supplying pulse energy within a short time.
While the battery can provide high power, just not as high as the UC with the same size.
Given The UC and the batteryon board are rechargeable, the kinetic energy won’t be wasted
but can be transferred into electric energy and recaptured by the UC or battery. That will save
the energy and improve the system’s efficiency by using the phase shift modulation angle of
Dual Active Bridge Isolated Bidirectional Converter.
Keywords: UltraCapacitor, Dual Active Bridge Isolated Bidirectional Converter, phase shift
modulation angle, EV.

I- INTRODUCTION

Research on transport systems is constantly developing; and it aims at solving problems of air
pollution, the replacement of fossil energy resources and improving the overall performance
for conventional vehicles which does not exceed 30% . However, much energy is lost during
braking. Electric Cars and EVs and Hybrids Electric Cars VEHs give adequate solutions to
the problems of energy loss and pollution, particularly in urban areas. The use of UCs in
these vehicles reinforces the main source of energy providing power peaks of short duration
during starting and acceleration.The UCs are characterized by a high power density and low
Equivalent Series Resistance ESR, and they have several advantages: the ability reaches
thousands of Farads per cell, they loaded and unloaded quickly with currents of several
hundred amperes. At room temperature, the UCs may reach one million cycles of loading and
unloading. Themost remarkable disadvantage of UCs is the low cell voltage (2.5 to 2.7V) [1]
[2]. It is therefore necessary to connect in series, a large number of UCs cells so that the

71

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
voltages at its terminals reach the main source of energy. This connection requires the
addition of a circuit balancing the voltages at the terminals of the cells [3] [4].
II- DESIGN OF ELECTRIC TRACTION SYSTEMS
The electric car is an ecological growing concern for the industrialists in order to adequately
meet the needs of road users. The electric traction system may include apart from the battery,
the main energy source, a bidirectional DC-DC converter, an UC, an inverter and an electric
traction machine Fig.1.

Fig.1 System ofelectric traction

In the last decade, research on high performance of bidirectional DC-DC converters has
gained momentum. This type of converter plays a key role to establish a conversion and an
efficient supply of energy in such systems [2] [5]. It plays the role of the interface circuit,
connecting the latter to SCs, to achieve a two-way exchangeof power flow.To control the
flow of energy, while regulating the DC bus voltage and load or unload the UCs, several
topologies of DC-DC converters of the type insulated or not insulated have been
implemented [2] [5] [6] [7].The not isolated topology has the advantage of being simple and
lightweight. But since the voltage of a cell UC is relatively low, the series connection of cells
causes an increase in the ESR and a decrease in equivalent capacity. Bidirectional topology
of multiple inputs is used to connect multiple energy sources of different voltages. This
topology has been used for small electric cars [2]. To overcome these drawbacks, the isolated
topologies can transfer more energy at reduced voltage [6] [7]. A high frequency transformer
(THF), is incorporated in the converter. Thus, loading and unloading of the UC can be done
at low voltage with high current. Side of the battery voltage is higher with a lower
current.The bidirectional converter and isolated DC-DC may be, half-bridge, has two bridges
[6], or a push-pull circuit [5]. For the management of high power, double-bridge
configuration is most favorable.

III- ARCHITECTURE OF DC-DC CONVERTER

In Fig. 2, battery, main source of energy, provided in the normal state, the power is in charge
(Load). The SC, temporary stored of energy recovered during braking supports peak powers
required by the load (traction motor) during start-up and acceleration.In the mode of loading
of the UC, the bridge, the DC bus side, operates as an inverter, operates in the secondary
bridge rectifier and provides power to the UC. In the discharge mode, the secondary bridge
operates as an inverter, while the primarybridge operates as a rectifier.

72


Fig.2 isolated bidirectional DC-DCdouble-bridge converter for the management of the UC
power
In the configuration considered:
• Because the operating frequency is high, the size of the THF binding is very small the
load isa synchronous machine, combined with athree-phase inverter.
• L0 Includes the leakage inductance of THF reduced the secondary side and any other
series inductance.
We denote by:
• vpAnd vs respectively the primary and secondary voltages of THF.
• m=Ns/Np=Vs/Vp The transformation ratio of THF,
• v2 The supply voltage of the second bridge.
• vL0= vs – v2 the voltage across the inductor L0.
• Tc The half-switching period, the frequency being is fc=1/2Tc
• Φ The phase shift between vs=mvp voltages and v2 (this is the phase shift between the
commands of the two bridges).
Viewedinhandlingrelatively low voltagesearch of the twobridgesis based onMOSFETsQ1,
Q2, Q3 and Q4 inthe primary side and Q5, Q6,Q7 and Q8 in the secondary side[8].
Withaconstantswitching frequencyfcandduty cyclesD1=D2=0.5, the primary
voltagevp(t)is+Vbor-Vbandthe secondary voltagev2(t)(feeding the second bridge) is+Vscor-
Vsc. Sincevs(t+Tc)=-vs(t), the secondary currentis(t)in the inductorL0rebuilt duringeach half
cyclewithoppositesigns. The modeof operation is basedon the shapeof this current.
Thecontrol signalsof the two bridgesare generated by aDSP, a
microprocessororaspecialized chiplikeUC3875generatesaPWM control.
1- UCcharge mode.

Analytically and with electronic switches ideal, gaits, current is(t), voltage vL0(t) and
control signals in this mode are given in Fig.3.
This mode of operation covers six segments. Down the same Fig.3, gives a summary table,
over a period, the status of each electronic switch.
• Between t0 and t2 the current is(t) increases linearly with the following expression:
1
i s (t ) = i s (t 0 ) + (mVb + Vsc )t (1)
L0
• At t=t1the current passes throughzero.
• Fromt2 to t3thecurrentis(t) increased linearly witha smaller slope,

73


Fig.3: Waveformidealcharging modeUC, Fig.4:WaveformidealdischargemodeUC,
control signals, vs(t) andv2(t). vL0(t) and control signals, vs(t)and v2(t).vL0(t) and
is(t): is(t):
a)mvb>Vsc, b) mvb=Vsc, a )mvb>Vsc, b) mvb=Vsc,
c) mvb<Vsc. c) mvb<Vsc.

74


Its expression is:
1
is (t ) = is (t2 ) + (mVb − Vsc )(t − t2 ) (2)
L0
• Att3=Tcthe current is (t3 ) = −is (t0 ) = I s max

• Fromt3 to t5thecurrentis(t) decreases linearlyaccording tothefollowing expression:
1
is (t ) = I s max + (−mVb − Vsc )(t − Tc ) (3)
L0
• At t=t4the current passes throughzero.
• The last segment fromt5 tot6thecurrent has continued to decreaselinearlyin the words:
1
is (t ) = is (t5 ) + (−mVb + Vsc )(t − t5 ) (4)
L0
• Att6,end of theperiod: i s (t 6 ) = i s (t 0 ) = − I s max
During loadingof the UC, the waveformof the current is(t) changesevery moment.The
voltage VSC increases, it goestoavaluelower than mVbFig.3.a, to a greater value Fig.3.c.
Attwo voltagesequal, obtainthe shape of theFig.3.b.

2- UC discharge mode

It is worth noting that the same reasoning of analytical development of current expression
applies to discharge modeof theUC. However we must to distinguish thedifferences
between charge and discharge modes (seeFig.4).
3- Thepower transferred
The average powertransfercan becalculated on thehalfperiodby:
Tc
P = T1c ∫
0
vs (t ) × is(t )dt (5)
we obtain:
mVbVscφ (π − φ )
P= (6)
2π 2 f c L0
Depending on thephase angle Φ, the power transferredas shown inFig.5has twoextrema at
±π/2.It is easy tocontrol the transfer ofpowerby modulating thephase angle Φ.
For 0<Φ<π the poweris positive, itis transferred to theSC for theload.
For -π<Φ<0 the poweris negative, it is transferred tothe DC busto restore the
energyrecovered.
Thepower transferredis highestfor Φ=±π/2 and it is equal to:
mV V
Pmax = ± b sc (7)
8 f c L0
Depending on the system,to managethe amount of energyto be transferred,we have
thepossibility of choosing, thetransformation ratio of theTHF, thebattery voltage, the
number of cells of UCs and the inductance L0.

75


2000 L0=2e-6H L0=1e-6H L0=0.5e-6H

1500

1000

500

P (W )
0

-500

-1000

-1500

-2000
-pi -5pi/6 -4pi/6 -2pi/6 -3pi/6 -pi/6 0 pi/6 2pi/6 3pi/6 4pi/6 5pi/6 pi
φ

Fig.5: Power transferredas a function ofphase shift Φ
Fordifferentinductances L0
Furthermore we havethe opportunity to actonthe switching frequencyfcandphase angle Φ.
It is necessary to note that the action of the duty ratios D1 and D2 modifies the waveform
of the inductor current,which ensures a zero current switching and to make them soft. Thus
the efficiency of the converter for applications with low power will be significantly
improved [9] [10].

According toexpression(5), the phasemodulationhasthe advantage of beingsimple.
Butitworks bestforhigh power.

Wenote thatthedualbridgeconverterisusedfor systemsofgreat powers, but it is not suitable
for low power.

IV- SIMULATION RESULTS

The model parametersused(Fig.2) are:
• Forthemainvoltagesource(battery) Vb=50V
• For UC, the"BCAP1500" has a capacityper cellCsc=1500F rated voltage Vsc=2.7V,
withinternal resistance ESR=0.47m . Since weneed to reacha charging voltageof the
order of 11V, we used fourunitsin series.The total capacity is then 375F
withanESR=0.18m andofa nominal voltage of 10.8V.
• Forthe THFferrite magnetic circuit; theN27for example, thetransformation ratiois
m=Ns/N2=0.2: theseleakageinductances: primary L1=1.2µH and secondary L2=0.2µH,
itsmagnetizing inductanceisLm=1.8mH: [17].
• Forthe series inductance L0=2µH.
• The switching frequencyis fc=20 KHz: with a duty cycleof 50%. [4].

76


15
10
Vs
Vsc<mVb Vs V2
8
V2 10
Vsc>mVb
6

4 Φ>0 Φ<0
5

2
t
0 0
t
-2

-5
-4

-6
-10

-8

-10 -15

VL VL Vsc>mVb
Is
Vsc<mVb Is

t
t

VL
Vsc=mVb Is VL Vsc=mVb
Is

t
t

Vsc>mVb
VL VL Vsc<mVb
Is Is

t
t

Fig.6: AllurevS,v2,vL0and isduring Fig.7: AllurevS,v2,vL0and isduring
chargingtheUCfor:a)mvb>Vsc, dischargetheUCfor:a)mvb<Vsc,
b) mvb=Vsc,c) mvb<Vsc b) mvb=Vsc, c) mvb>Vsc

77


To reducethe computation time, the margin of voltage change across the UCV scislimited
to a range starting from6 to10V. Fig.6 shows the phase angleφbetween these
condaryvoltagevs=±mVb,and one thatfeeds the secondbridgev2=±Vsc.
Fig.6givestheappearance of avoltagevL0(t)andthecurrent flowing through it(secondary
current)is(t).We can remark that the simulation results are similar to the analytical results
(see Figs. 3, 4, 6 and 7). Normally,we should notoverload thedifferentUCcells. Avoid
operation corresponding to the Fig.6.c.T hat is tosay,Vscmust always be≤mVb,otherwisea
severe degradation ofdevice characteristics U Csappears with the riskof destroying them.
In the case considered, we mustlimit theloadingof allunitsto 11Vandprovidestrict oversight
of the individual state of charge of
different units[3] [4].
Analyticallyand at constant switching frequency, the expression(12)shows, in accordance
with theFig.8that the increaseof the maximum powertransferred(with π±/2)can be achieved
bydecreasingthe value ofseries inductanceL0. InFig.9, we note that the
increaseinthispoweris accompaniedbya decrease in thecorrespondingphase shiftΦ.
200
L0=1e-6H 200
L=2e-6 f=10KHz
180 L0=2e-6H
L=2e-6 f=50KHz
180

160
160

140
140

120
120
P (W)

100
P(W)

100

80
80

60
60

40
40

20 1.413rd 1.965rd
1.256rd 2.9rd 20
1.256rd 1.413rd
0
2.82rd 0
1.9rd
0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3
φ φ

Fig.8: power transferredto theSCas a Fig.9: power transferredto theSCas a
function ofphase shift function ofphase shift
fordifferentinductance forvariousdifferentswitching frequencies.

Similarly, the decrease of the switching frequency, you can increase thepowerbutit is
always withacorrespondingdecrease in thephase shiftΦFig.9.

In practice, thephase shift Φgivesa maximumtransferofpoweralwaysinferiortoπ±/2 based
onthetransfer direction. The choiceof this angledepends on the
loadingorunloadingneedsslow or fast.

Modeofloading, the phase shiftwhich allows the fastestloadingin the interval
[70° 90°], otherwise the voltage gainand the energy storedinthis modewill be reduced.
Indischarge mode,the phase shiftsΦ<60°, cause thefastestunloadingUCFig.11.

78


8 11
φ=0.4∗π φ=0.2∗π φ=0.45∗π φ=0.5∗π
0.2π 0.4π 0.45π 0.5π 0.95π 10
7
9
6
8

5
7

V s c (V )
V s c (V )

4 6

5
3
4
2
3

1
2

0 1
2 3 4 5 6 7 8 9 10 0.5 1 1.5 2 2.5 3
t 5 t x 10
6
x 10

Fig.10: voltage, fordifferentphase Fig.11: voltage, fordifferentphase
shiftsduring loadingof theUC. shiftsduring the unloadingof theUC.

(c)

(a) Φ=0.2π VL
Is
VL
Φ=0.3π Vsc=7V
Is

Vsc=4V
t

t

Fig.12: Alluresof vL0and isduring
loadingtothe UCfor different values
(b) ofphase shiftΦto keepa triangular
VL shapeallowingcurrentsoft switching
Φ=0.25π Is

Vsc=6V

t

Fig.13: Improvement ofefficiency of the
converterby modulating thephase shiftΦ

79


With the modulation phase angle Φcan make in soft switching Fig.12 (zero current), well
the switching losses are reduced and the efficiencyincreases Fig.13

V- CONCLUSION

With the aim to design an electric ecological vehicle, a study of a traction system involving
UCs is conducted. This paper carries out the simulation to verify the normal operation of
the converters and the effectiveness of control method thanks to phase shift angle.
Simulation results confirm those calculated analytically. The consolidation of the battery
by the UC by providing peak power is even more interesting by acting on the phase shift
between Φ orders of the two bridges sometimes associated with an action on the cyclical
order of the two bridges.
Indeed the appropriate choice of the phase angle Φ minimizes switching losses and hence
improved performance of exchange between the battery and the SC is provided.

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Dc dc converter for ultracapacitor boosted electric vehicle

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