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    Assignment on

    Buck converter

    Submitted to

    Dr. RAJNEESH KUMAR

    By

    Lokesh 2008A3PS074P

    Ekansh Nayal 2008A3PS143P

    Savil Gupta 2008A3PS199P

    Ishan Mishra 2008A3PS111P

    Aman Sehgal 2008A8PS252P

    In partial fulfillment of course EEEC461

    POWER ELECTRONICS

    April 25, 2011

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    Abstract-A buck converter is simply is

    a particular type of power converter with

    an average output DC voltage smallerthan the input DC voltage. This type of

    circuit is used to step-down a source

    voltage to a higher, regulated voltage,

    allowing one power supply to provide

    different driving voltages. Its main

    application is in regulated dc power

    supplies and dc motor speed control. The

    purpose of this document is for the reader

    to become familiar with the function and

    implementation of a buck converter. A

    basic design will be discussed along with

    waveforms of voltages and current

    generated during its operation.

    Keywords Buck Converter, Switched-Mode dc-dcconversion, Pulse Width Modulation, RippleVoltage, Snubbers.

    I. INTRODUCTIONA buck converter is part of a subset of DC-

    DC converters called switch-mode

    converters. The circuits belonging to this

    class, including boost, flyback, buck-

    boost, and push-pull converters are very

    similar. They generally perform the

    conversion by applying a DC voltage

    across an inductor or transformer for aperiod of time (usually in the 100 kHz to 5

    MHz range) which causes current to flowthrough it and store energy magnetically,

    then switching this voltage off and causing

    the stored energy to be transferred to the

    voltage output in a controlled manner.

    The basic buck converter circuit consists

    of only a switch (typically a transistor), a

    diode, an inductor, and a capacitor.Conceptually, the basic circuit constitutes

    a step-down converter for a purely

    resistive load. The specific connections areshown in Figure 1.

    Fig 1: Step-down dc-dc Converter

    II. DISCUSSIONBy varying the duty ratio (= ton/Ts) of the

    switch, Vo can be controlled. Another

    important observation is that the average

    output voltage Vo varies linearly with the

    control voltage as in the case of linear

    amplifiers. In actual applications, the

    foregoing circuit has two drawbacks:-

    1. In practice the load is inductive, evenwith resistive load there is always

    some stray inductance.

    2. Output voltage fluctuates between 0and Vd which is eliminated by

    using a Low Pass Filter consisting

    of inductor and capacitor.

    Diode solves the problem of stored inductive

    energy. The filter capacitor at the output is

    assumed to be very large since constantinstantaneous voltage is required at the output.

    Fig 2: Switched Mode dc-dc Conversion

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    Figure 3 shows the waveforms for the

    continuous-conduction mode of operation

    where the inductor current flows continuously.

    When the switch is on for a time ton, the

    switch conducts the inductor current and

    diode becomes reverse-biased. This results

    in a positive voltage VL=Vd-Vo across

    inductor. This voltage causes a linear

    increase in inductor current. When the

    switch is turned off, because of the

    inductive energy storage, IL continues to

    flow.

    Fig 3: Circuit states assuming ILflowing

    continuously (a) Switch On (b) Switch Off

    Neglecting the power losses associated with all

    the circuit elements, the input power Pd equals

    the output power Po. Therefore

    Vd/Vo=1/D proving that in continuous

    conduction mode, the step-down

    converter is equivalent to a dc

    transformer where the turns ratio can be

    controlled electronically in a range of 0-1

    by controlling the duty ratio of the switch.

    Fig 4: Current at the boundary of

    continuous-discontinuous conduction (a)

    current waveform (b) ILB versus D keeping

    Vd constant.

    Being at the boundary of continuous-

    discontinuous mode, by definition the

    inductor current IL goes to zero at the end

    of the off period. At this boundary, the

    average inductor current, where the

    subscript B refers to the boundary is

    ILB=DTs (Vd-Vo)/2L. Therefore, during an

    operating condition (with a given set of Ts,

    Vd, Vo, L and D), if the average output

    current becomes less than the ILB value,

    the IL will become discontinuous as shown

    in figure 5.

    Fig 5: Discontinuous condition in step-

    down converter.

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    III. CALCULATIONSThe different values used are

    Input current = output current =1.3 A

    Output Voltage = 10.28A

    D = 0.42

    Input Voltage = 0.34V and f= 11.2 kHz

    1. Inductor Voltage

    . of the switch

    3. Drain current

    4. Output Voltage Ripple

    5. Upper graph , lower graph

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    6. PWM output

    IV. CONCLUSIONBuck converters are used to lower the dc

    voltage to user-demanded required voltages.

    The parasitic elements in a step-up converter

    are due to the losses associated with the

    same inductor, the capacitor, the switch and

    the diode. Unlike the ideal characteristic, in

    practice, Vo/Vd declines as the duty ratio

    approaches unity. Without the use of

    snubbers, the power dissipated during turn on

    and turn off can be quite high and can lead to

    damage of devices. With the use of snubbers,

    the current or voltage waveforms are pulled

    for a longer time hence the overlap is reduced

    and hence lesser heat is dissipated. The

    increase in efficiency isnt significant but it

    makes our circuits safer to operate.

    REFERENCES

    1. Ned Mohan, Tore M. Undeland, William p.

    Robbins Power Electronics converters,applications and design, Third edition.

    2. Analog Electronics, L.K. Maheshwari and

    MMS Anand.

    3. Power Electronics Lab Manual, BITS

    Pilani.