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					          INTERNATIONAL JOURNAL OF ADVANCED RESEARCH 0976
  International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN IN –
                     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

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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
           6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME
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.




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
        6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME




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,




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
        6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME




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.




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
        6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


    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.




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
        6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


                                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].




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
        6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


                                                      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




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
        6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


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.




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
        6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


             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




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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
        6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME




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.


REFERENCES

1.   Z. Pfof, P. Vaculík, 2010, "Ultracapacitors Utilization for Automotive Applications»
     ActaPolytechnica Vol.50 No.1
2.    L. Zhihao, O. Omer, K. Alireza, 2009 "Design and Control of a Multiple Input
     DC/DC Converter for Battery/Ultracapacitor Based Electric Vehicle Power System"
     IEEE, Power Electronics Applied Power Electronics Conference and
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3.   A. Vivek Ravi, V. John2010, "Ultracapacitor Based Ride Through System for Control
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4.    V.V. Haerri, D. Martinovic 2007 "Supercapacitor Module SAM for Hybrid Busses:
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6.   B. Hua, C.M. Chunting,G.Sonya 2008, "The Short-Time-Scale Transient Processes in
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9.   Y. Wang, S. W. H. de Haan, and J. A. Ferreira 2009 "Optimal Operating Ranges Of
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International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
        6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME


    International Power Electronics and Motion Control Conference (IPEMC '09), China,
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