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Power Electronics Chopper (dc – dc converter)

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  • Chopper (DC DC Converter)




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  • Contents:


    Elementary Principles


    Principle of Chopper Action

    Buck Chopper

    Boost Chopper

    Buck Boost Chopper

    Different Quadrant Operations of Chopper


  • Features of Choppers

    Chopper is a static device that converts fixed DC input voltage to a variable DC output

    voltage directly.

    DC equivalent of an AC transformer as it behaves in an identical manner.

    More efficient than AC transformers as they involve in one stage conversion.

    They offer smooth control, high efficiency, fast response and regeneration.

    A chopper can act as a step up or step down DC device.

    Chopper can be operated in either a continuous or continuous current conduction mode.

    Choppers can be built with and without electrical isolation.

    They find application in trolley cars, marine hoists, forklift trucks and mine haulers.


  • Elementary Principle

    High speed on/off semiconductor switch.

    It connects source to load and disconnects the load from source at high speed.

    Continuous triggering ON and triggering OFF at rapid switching speed.

    By varying ON and OFF time of the SCR, the average voltage across the load can be



  • Classification of Choppers

    Depending upon the direction of the output current and voltage, choppers are classified

    into five classes viz.

    Class A (One Quadrant Operation)

    Class B (One Quadrant Operation)

    Class C (Two Quadrant Operation)

    Class D (Two Quadrant Operation)

    Class E (Four Quadrant Operation)

    Based on the output voltage, choppers are classified into

    Step Up (Boost) Chopper

    Step Down (Buck) Chopper

    Buck Boost Chopper


  • Further Classification of Choppers

    Depending upon the power loss occurred during turn ON/OFF of the switching device,

    the choppers are classified into two categories viz.

    1. Hard switched Converter: Here the power loss is high during the switching (ON to OFF

    and OFF to ON) as a result of the non zero voltage and current on the power switches.

    2. Soft switched or resonant Converters: In this type of choppers, the power loss is low at

    the time of switching as a result of zero voltage and/or zero current on the switches.


  • DC/DC Converter Technology

    Linear Regulators

    Switching Regulators

    Charge Pumps


    DC/DC Converter Technology Comparison

    Parameter Linear regulatorSwitching regulator

    Charge pump

    Efficiency Low High Medium

    EMI Noise Low High Medium

    Output current Low to medium Low to High Low

    Boost (step-up) No Yes Yes

    Buck (step-down) Yes Yes Yes

    Solution size Small Large Medium

  • Chopper: Methods of Control

    The output dc voltage can be verified by the following methods.

    1. Constant Frequency Control or Pulse Width Modulation (PWM) Control

    2. Variable Frequency Control


    In PWM, the pulse width ton of the output waveform is varied keeping chopping frequency

    f and hence chopping period T constant. Therefore output voltage is varied by varying the

    ON time, ton . Figure shows the output voltage waveform for different ON times.


    In this method of control, chopping frequency f is varied keeping either ton or t off constant.

    This method is also known as frequency modulation.

    In frequency modulation to obtain full output voltage, range of frequency has to be varied

    over a wide range. This method produces harmonics in the output and for large t off load

    current may be discontinuous.


  • Chopper: Methods of Control 9







    tON tOFF















    PWM Control Variable Frequency Control

  • Principle of Chopper Operation:

    A switch is connected in series with a DC voltage source and load.

    The switch can be a power transistor, an SCR or a GTO.

    Ideal switch will offer zero resistance (ON), infinite resistance (OFF) and instantaneous

    switching (no lag).

    The switch can be turned ON and turned OFF with the help of triggering circuit and

    commutating circuit respectively.

    It can be turned on or turned off as desired at very high frequency.

    During the TON period, chopper is in ON condition and load voltage is equal to source

    voltage Vs.

    During the TOFF period, chopper is OFF condition, load current flows through the

    freewheeling diode FD.


  • Principle of Chopper Operation:

    As a result, load terminals are short circuited by freewheeling diode FD.

    Thus the load voltage is zero during TOFF period.

    In this manner, a chopped DC voltage is produced across the load terminals.

    From the T=TON +TOFF equation, it is clear that the load voltage depends on two factors.

    a) The supply voltage.

    b) The duty cycle of the chopper

    Since the supply voltage is constant, load voltage is governed by the duty-cycle of the


    In other words, the load voltage is dependent on two factors TON and TOFF.


  • Principle of Chopper Operation:

    Hence it is concluded that the average load voltage can be controlled by varying the value

    of TON and/of TOFF in the following two ways.

    a) Varying TON and keeping the periodic time T constant. This is called constant frequency


    b) Variable frequency system. i.e. keeping either TON constant and varying TOFF or keeping

    TOFF constant and varying TON.

    VO = (TON/[TON+TOFF] )VS = (TON/T )VS = . VS

    Here = Duty cycle = (TON/T); where T = TON +TOFF ;Chopper frequency f = 1/T

    () =2




  • Principle of Chopper Operation:

    The representations of the chopper circuit during different phases are shown below.


  • Step Down (Buck) Chopper

    Output Voltage is less than input voltage.

    The thyristor in the circuit acts as a switch.

    When thyristor is ON, supply voltage appears across the load

    When thyristor is OFF, the voltage across the load will be zero.

    Practical arrangement includes an inductor (L) and a diode which are used to eliminate

    current pulsations providing a smooth DC current.

    With S closed, D is Off and it remains Off as long as S in On.

    The i/p current builds up exponentially and flows through L and load.

    V O equals V I.

    With S OFF or open, the current through L decays to zero.

    This causes an inductive voltage with opposite polarity across L.


  • Step Down (Buck) Chopper

    V L forward biases diode D.

    Current flows through L, Load and D.

    This arrangement permits the use of simple filter inductance L to provide a satisfactorily

    smooth DC load current.

    With higher switching frequency, smaller inductance is sufficient to get desired O/P.


  • Step Down (Buck) Chopper

    The o/p voltage is equal to the i/p voltage when the switch is ON and D is reverse biased.

    Diode current is same as the load current during T OFF.

    During T ON, I O is same as I I.


  • Step Down (Buck) Chopper Continuous Current

    The average value of inductor current is:

    = +

    2= =


    = =

    = =

    With Switch Open,

    = = =



    ; =2

    = .

    ; =

    ; =


  • Step Down (Buck) Chopper Continuous Current

    As elements are ideal, DC power drawn from source must equal the DC power absorbed

    by load.

    = =


  • Step Down (Buck) Chopper Discontinuous Current

    For low value of d with low L, I L decreases and may fall to zero during T OFF.

    It again builds up with T ON, and hence it is called discontinuous current.


  • Step Down (Buck) Chopper Discontinuous Current

    This mode is undesirable and is avoided by proper selection of chopping frequency and L.

    The minimum value of L for continuous current mode is ensured by setting I min = 0.

    = 0 =

    = .2

    2 =



  • Types of Choppers:

    Type A Chopper or FirstQuadrant Chopper

    Type B Chopper or Second-Quadrant Chopper

    Type-C chopper or Two-quadrant type-A Chopper

    Type-D Chopper or Two-Quadrant TypeB Chopper

    Type-E chopper or the Fourth-Quadrant Chopper


  • Step Up (Boost) Chopper:

    The output voltage is more than the input voltage by several times.

    L is used to provide a smooth i/p current.

    The SCR (S) acts as the switch which works in the PWM mode.

    With S On, the L is connected to the supply.

    Load voltage V L jumps instantaneously to V I, but current through L increases linearly &

    stores energy.

    When S is Open, the current collapses and energy stored in L is transferred to C through


    The induced voltage across the inductor reverses and adds to the source voltage increasing

    the O/P voltage.

    The current that was flowing through S now flows through L, D and C to the load.

    Energy stored in the inductor is released to the load.


  • Step Up (Boost) Chopper:

    With S

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