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ContentsContents

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IntroductionIntroductionChopper is a static device.A variable dc voltage is obtained from a

constant dc voltage source.Also known as dc-to-dc converter.Widely used for motor control.Also used in regenerative braking. Thyristor converter offers greater

efficiency, faster response, lower maintenance, smaller size and smooth control.

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Types of choppers Types of choppers

Buck(Step-down) choppers:• In step down chopper output voltage is

less than input voltage.Boost(Step-up) choppers:• In step up chopper output voltage is

more than input voltage.Buck-Boost chopper

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BUCK CONVERTERBUCK CONVERTERTwo modes of operation :Continuous conduction mode(CCM)Discontinuous conduction mode(DCM)

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Fig 1.Buck Converter

I. CCMI. CCM

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7Fig 2. Equivalent Circuit Diagrams

Waveforms for CCM :Waveforms for CCM :

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Fig 3. Continuous conduction mode waveforms

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Calculation of capacitor current :Calculation of capacitor current :

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Fig 4.

II.Discontinuous mode of conductionII.Discontinuous mode of conduction

The discontinuous conduction mode arises when the switching ripple in an inductor current or capacitor voltage is large enough to cause the polarity of the applied switch current or voltage to reverse, such that the current- or voltage-unidirectional assumptions made in realizing the switch with semiconductor devices are violated. The DCM is commonly observed in dc-dc converters and rectifiers, and can also sometimes occur in inverters or in other converters containing two-quadrant switches..

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The discontinuous conduction mode typically occurs with large inductor current ripple in a converter operating at light load and containing current-unidirectional switches. Some converters are purposely designed to operate in DCM for all loads.

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TDDTTt

tL

VoI

tL

VVI

TDt

DTt

idle

offl

onos

l

off

on

'

)(

'

−−=

×=∆

×−=∆

==

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L

TVsDDDT

Vsil 2

'

2L

Vo– =×=∆

17Fig 5 Discontinuous conduction mode waveforms

L

TVsDD

R

DVo

2

'<

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(I)

'2

DRT

L <(II)

critKK <(III)

The dimensionless parameter K is a measure of the tendency of a converter to operate in the discontinuous conduction mode.

Large values of K lead to continuous mode operation, while small values lead to the discontinuous mode for some values of duty cycle. The critical value of K at the boundary between modes, Kcrit(D), is a function of duty cycle, and is equal to D’ for the buck converter.

It is natural to express the mode boundary in terms of the load resistance R, rather than the dimensionless parameter K.

(iv)

Where,

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crit

crit

RR

RR

>< For CCM

For DCM

TD

LRcrit '

2=

The complete buck converter characteristics, including both continuous and discontinuous conduction modes, are therefore :

M(Conversion ratio)

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2

411

2

DK

D

++=

= For K>Kcrit

For K<Kcrit

Efficiency factorsEfficiency factors

• Resistance when the transistor or MOSFET switch is conducting.

• Diode forward voltage drop (usually 0.7 V or 0.4 V for schottky diode)

• Inductor winding resistance• Capacitor equivalent series resistance

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Power electronics by P. S. BhimbhraModern power electronics by Ned

MohanBuck Converter,WikipediaDC DC Buck converter

applications,International rectifier.

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