Continuous and discontinuous mode of conduction of buck converter

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2. CCoonntteennttss
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3. IInnttrroodduuccttiioonn
Chopper 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.
3

4. TTyyppeess ooff cchhooppppeerrss
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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5. BBUUCCKK CCOONNVVEERRTTEERR
Two modes of operation :
Continuous conduction mode(CCM)
Discontinuous conduction mode(DCM)
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Fig 1.Buck Converter

6. II.. CCCCMM
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7. 7 Fig 2. Equivalent Circuit Diagrams

8. WWaavveeffoorrmmss ffoorr CCCCMM ::
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Fig 3. Continuous conduction mode waveforms

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11. Calculation ooff ccaappaacciittoorr ccuurrrreenntt ::
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Fig 4.

12. IIII..DDiissccoonnttiinnuuoouuss mmooddee ooff ccoonndduuccttiioonn
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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13. 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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t DT
on
=
t =
D '
T
I V V
D = - ´
L
t
I Vo
D = ´
l off
L
t
t T DT D T
idle
on
s o
l
off
'
( )
= - -

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i Vs DT VsDD '
T l 2
L
D = – Vo ´ =
2L

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Fig 5 Discontinuous conduction mode waveforms

18. DD TVs
< '
L
DVo
R
2
18
(I)
L <
2 D'
RT
(II)
crit (III) K < K
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.

19. 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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R R
>
< For CCM
crit
R R
crit
For DCM
R =2
L crit '
D T

20. The complete buck converter characteristics,
including both continuous and discontinuous
conduction modes, are therefore :
M(Conversion ratio)
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= For K>Kcrit
K
2
2
1 1 4
D
D
+ +
=
For K<Kcrit

21. EEffffiicciieennccyy ffaaccttoorrss
• 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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23. Power electronics by P. S. Bhimbhra
Modern power electronics by Ned
Mohan
Buck Converter,Wikipedia
DC DC Buck converter
applications,International rectifier.
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