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  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    1

    Chapter 3

    DC to DC CONVERTER(CHOPPER)

    General Buck converter Boost converter Buck-Boost converter Switched-mode power supply Bridge converter Notes on electromagnetic compatibility

    (EMC) and solutions.

  • DC-DC Converter (Chopper)

    DEFINITION: Converting the unregulated DC input to a controlled DC output with a desired voltage level.

    General block diagram:

    LOAD

    Vcontrol(derived from

    feedback circuit)

    DC supply(from rectifier-filter, battery,fuel cell etc.)

    DC output

    APPLICATIONS: Switched-mode power supply (SMPS), DC

    motor control, battery chargers

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    3

    Linear regulator Transistor is

    operated in linear (active) mode.

    Output voltage

    The transistor can be conveniently modelled by an equivalent variable resistor, as shown.

    Power loss is high at high current due to:

    TLo RIP2=

    TLo RIV =

    +

    VoRL

    + VCE IL

    MODEL OF LINEARREGULATOR

    RT

    EQUIVALENTCIRCUIT

    Vs

    RL

    + VCE IL

    VsVo

    +

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    4

    Switching Regulator Power loss is zero

    (for ideal switch): when switch is

    open, no current flow in it,

    when switch is closed no voltage drop across it.

    Since power is a product of voltage and current, no losses occurs in the switch.

    Power is 100% transferred from source to load.

    Switching regulator is the basis of all DC-DC converters

    +

    Vo

    RL

    + VCE IL

    MODEL OF LINEARREGULATOR

    EQUIVALENT CIRCUIT

    Vs

    RL

    IL

    VsVo+

    (ON)closed

    (OFF)open

    (ON)closed

    DT T

    OUTPUT VOLTAGE

    Vo

    SWITCH

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    5

    Buck (step-down) converter

    Vd

    L

    D C RL

    S+

    Vo

    Vo

    +

    CIRCUIT OF BUCK CONVERTER

    CIRCUIT WHEN SWITCH IS CLOSED

    CIRCUIT WHEN SWITCH IS OPENED

    Vo

    +

    iL

    Vd D RL

    S

    Vd D RL

    S

    + vL

    + vL

    iL

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    6

    Circuit operation when switch is turned on (closed)

    Diode is reversed biased. Switch conducts inductor current

    This results in positive inductor voltage, i.e:

    It causes linear increase in the inductor current

    odL VVv =

    ==

    dtvL

    i

    dtdiLv

    LL

    LL

    1

    Vd VD

    + vL -

    C RL

    +

    Vo

    VdVo

    Vo

    closedopened

    closedopened

    t

    DT Tt

    iLmin

    iLmaxIL

    vL

    iL

    iL

    +

    S

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    7

    Operation when switch turned off (opened)

    Because of inductive energy storage, iL continues to flow.

    Diode is forward biased

    Current now flows through the diode and

    oL Vv =

    Vd

    + vL -

    C RL

    +

    Vo

    VdVo

    Vo

    closedopened

    closedopened

    t

    DT Tt

    iLmin

    iLmaxIL

    vL

    iL

    iL

    S

    D

    (1-D)T

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    8

    Analysis for switch closed

    ( ) DTL

    VVi

    LVV

    DTi

    ti

    dtdi

    i

    i

    LVV

    dtdi

    dtdiL

    VVv

    odclosedL

    odLLL

    L

    L

    odL

    LodL

    =

    ===

    =

    ==

    Figure From

    linearly. increase must Therefore

    constant. tive-posi a is of vative

    -deri thesince :Note

    oltage,inductor v The

    IL

    iL max

    DT T

    iL

    Vd Vo

    vL

    t

    t

    iL min

    closed

    iL

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    9

    Analysis for switch opened

    ( ) TDLVi

    LV

    TDi

    ti

    dtdi

    ii

    LV

    dtdi

    dtdiLVv

    oopenedL

    oLLL

    LL

    oL

    LoL

    )1(

    )1(

    Figure From

    linearly. decreasemust constant, tive

    -nega a is of vative-deri thesince :Note

    opened,switch For

    =

    ==

    =

    ===

    IL

    iL max

    DT T

    iL

    Vd Vo

    vL

    t

    t

    iL min

    opened

    iL

    (1 D)T

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    10

    Steady-state operation

    ( ) ( )

    do

    so

    sod

    openedLclosedL

    L

    L

    DVV

    TDLVDT

    LVV

    ii

    i

    i

    =

    =

    =+

    0)1(

    0

    :i.e zero, is period oneover of change theisThat cycle.next theof begining

    at the same theis cycle switching of endat the that requiresoperation state-Steady

    iL Unstable current

    Decaying current

    Steady-state current

    t

    t

    t

    iL

    iL

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    11

    ==

    +=

    +=+=

    ===

    LfD

    RViII

    LfD

    RV

    TDL

    VR

    ViII

    RVII

    oL

    L

    o

    ooLL

    oRL

    2)1(1

    2

    :current Minimum

    2)1(1

    )1(21

    2

    :current Maximum

    Rin current Average currentinductor Average

    min

    max

    L

    Average, Maximum and Minimum inductor current

    IL

    Imax

    Imin

    iL

    iL

    t

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    12

    Continuous current operationiL

    Imax

    Imin t0

    min

    min

    min

    min

    be bechosen is Normally operation. of mode continous ensure

    current toinductor minimum theis This2

    )1(

    02

    )1(1

    ,0 operation, continuousFor

    2)1(1

    2

    analysis, previous From

    LL

    RfDLL

    LfD

    RV

    I

    LfD

    RViII

    o

    oL

    L

    >>

    =

    ==

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    13

    Output voltage ripple

    2

    2

    8)1(

    factor, ripple theSo,8

    )1(8

    82221

    :formula area triangleuse figure, From

    LCfD

    VVr

    LCfD

    CiTV

    iTiTQ

    CQVVCQCVQ

    iii

    o

    o

    Lo

    LL

    ooo

    RLc

    ==

    ==

    =

    =

    ===+=

    iRiLL

    iC

    iLiL=IR

    imax

    imin

    0

    0Vo

    Vo/R+

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    14

    Design procedures for Buck

    Vd(inputspec.)

    SWITCH

    f = ?D = ?TYPE ?

    D

    L

    Lmin= ?L = 10Lmin

    Cripple ?

    RLPo = ?Io = ?

    Calculate D to obtain required output voltage.

    Select a particular switching frequency: preferably >20KHz for negligible acoustic

    noise higher fs results in smaller L, but higher device

    losses. Thus lowering efficiency and larger heat sink. Also C is reduced.

    Possible devices: MOSFET, IGBT and BJT. Low power MOSFET can reach MHz range.

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    15

    Design procedures for Buck

    Determine Lmin. Increase Lmin by about 10 times to ensure full continuos mode.

    Calculate C for ripple factor requirement.

    Capacitor ratings: must withstand peak output voltage must carry required RMS current. Note RMS

    current for triangular w/f is Ip/3, where Ip is the peak capacitor current given by iL/2

    Wire size consideration: Normally rated in RMS. But iL is known as

    peak. RMS value for iL is given as:

    22

    , 32

    += LLRMSL iII

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    16

    Examples of Buck converter

    A buck converter is supplied from a 50V battery source. Given L=400uH, C=100uF, R=20 Ohm, f=20KHz and D=0.4. Calculate: (a) output voltage (b) maximum and minimum inductor current, (c) output voltage ripple.

    A buck converter has an input voltage of 50V and output of 25V. The switching frequency is 10KHz. The power output is 125W. (a) Determine the duty cycle, (b) value of L to limit the peak inductor current to 6.25A, (c) value of capacitance to limit the output voltage ripple factor to 0.5%.

    Design a buck converter such that the output voltage is 28V when the input is 48V. The load is 8Ohm. Design the converter such that it will be in continuous current mode. The output voltage ripple must not be more than 0.5%. Specify the frequency and the values of each component. Suggest the power switch also.

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    17

    Boost (step-up) converter

    Vd

    L D

    C

    RL

    S

    Vd

    L D

    CRLS

    Vd

    LD

    C RLS

    + vL

    +

    Vo

    + vL -

    Vo

    +

    CIRCUIT OF BOOST CONVERTER

    CIRCUIT WHEN SWITCH IS CLOSED

    CIRCUIT WHEN SWITCH IS OPENED

    Vo

    +

    iL

  • Power Electronics and Drives (Version 2): Dr.

    Zainal Salam

    18

    Boost analysis:switch closed

    Vd

    L D

    CS

    + vL

    iL

    +vo

    ( )LDTVi

    LV

    dtdi

    DTi

    ti

    dtdi

    LV

    dtdi

    dtdiLVv

    dclosedL

    dL

    LLL

    dL

    LdL

    =

    =

    ==

    ==

    =

    DT T

    iL

    vL

    CLOSED