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The Islamia University of Bahawalpur
University College of Engineering & Technology
EEN-324
Power Electronics
DC Choppers
Chp#9
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Engr. Affifa Adeeb
INTRODUCTION
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DC-DC converter (chopper) is used to convert constant DC
voltage into variable DC voltage.
In DC-DC conversion circuits, thyristors are used as
switching elements. Here thyristors must be turned off using
forced commutation as they lack facility of natural
commutation that is available in AC circuits.
Buck chopper produces output that is less than or equal to
input voltage.
Boost chopper provides an output voltage that is greater than
or equal to input voltage.
Typical application of DC choppers is DC motor speed
control.
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PRINCIPLES OF BASIC DC CHOPPERS
Basic DC Chopper
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Switch is turned on and off periodically. In this way constant
voltage can be connected to and disconnected from the load.
By a periodic application of constant voltage at a particular
frequency across the load, variable voltage can be achieved
by controlling the on period of the switch.
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Instantaneous voltage across load is either zero (S off) or
Vi (S on).
Average (DC) output voltage over a cycle is:
V0=
TON
Vi
TON+TOFF
V0=TON Vi
T
V0=d Vi
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Output voltage as function of duty cycle
Output voltage varies linearly with duty cycle.
It is possible to control output voltage from zero to Vi
as duty cycle varies from zero to 1.
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METHODS FOR VARING AVERAGE OUTPUT
VOLTAGE
Pulse-Width
Modulation
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Pulse width TON is
varied while overall
switching period is kept
constant.
Pulse-Frequency
Modulation
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Pulse width TON is kept
constant while the
period (frequency) is
varied.
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STEP-DOWN (BUCK)
CHOPPER
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Basic Step-Down Chopper circuit
Equivalent circuit for on state
Equivalent Circuit for off state
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Continous Current Mode
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As elements are ideal,
DC power drawn from
source must equal the
DC power absorbed by
load.
P0 = Pi
V0 I 0 = Vi I i
I0 =Vi * Ii
V0
=Vi * Ii
Vi d
I0 = Ii_
d
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Discontinous Current Mode
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STEP-UP (BOOST) CHOPPER
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Basic Step-Up Chopper Circuit
On State
Off state
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On-State
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When S is on (D is off),
capacitor energy supplies the
load voltage.
Off-State
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Vo=Vc (if capacitor is
charged)
When S is off (D is on),
inductor voltage reverses its
polarity and adds in input
voltage to provide output
voltage which is equal to:
V0=Vi+VL
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During on-state of switch S,
voltage across inductor
instantly becomes equal to
input supply voltage. Current
through it increases gradually
and stores energy in its
magnetic field.
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(This capacitor voltages
serves as load voltage when
next time S in on)
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For very first time, when S is
closed Vo=0, as capacitor is
not charged.
During off state of S,
capacitor charges and voltage
at it gradually build up to
Vi+VL
If S is off forever, inductor
acts as short circuit. It does
not develop any voltage and
Vo= Vi
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Voltage and current
waveforms for duty cycle
50%
d= 0.5 means Switch is on and
off for equal time intervals.
Energy that inductor develops
during on-state is completely
dessipated during off-state.
If duty cycle increases above
0.5, inductor will not
dessipate its energy
completely in off-states. The
remaining inductor voltage
(due to left-over energy) adds
up next time when switch is
off and more increased
voltage appears at output.
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If duty cycle increases above 0.5, inductor will not dessipate its
energy completely in off-states. The remaining inductor voltage (due
to left-over energy) adds up next time when switch is off and more
increased voltage appears at output.
Neglecting losses, energy transferred by inductance during TOFF
must equal the energy gained by it during period TON
Final expression for output load voltage is:
Vo=Vi [1/(1-d)]
If switch is open (d=0), output voltage is equal to input
voltage. As d increases, output voltage becomes larger than
input voltage.
So output voltage is always higher than input voltage if switch
is operated at an appropriately high frequency.
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BUCK-BOOST CHOPPER
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On State
Off State
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