3. Introduction
•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.
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4. • 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.
Basic DC Chopper
Choppers
Fixed DC Supply Variable DC Output
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5. • Instantaneous voltage across load is either zero (S off) or
Vi (S on).
• Average (DC) output voltage over a cycle is:
V0= TON Vi V0=d Vi
TON+TOFF
V0=TON Vi
T
Basic DC Chopper
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6. 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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7. Methods Of Control
•The output dc voltage can be varied by
the following methods.
•Pulse width modulation control or
constant frequency operation.
•Variable frequency control.
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8. Methods for varying Average
Output Voltage
Pulse-Width Modulation
• Pulse width TON is varied
while overall switching
period is kept constant.
Pulse-Frequency Modulation
• Pulse width TON is kept
constant while the period
(frequency) is varied.
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9. Choppers are of Two Types
Step-down choppers.
Step-up choppers.
In step down chopper output voltage is less than
input voltage.
In step up chopper output voltage is more than
input voltage.
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11. Operating Principle
The key principle that drives the boost converter is the
tendency of an inductor to resist changes in current.
Mode 1 (On State):
When the switch is closed, current flows through the inductor in
clockwise direction and the inductor stores the energy. Polarity
of the left side of the inductor is positive.
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12. Mode 2 (Off State)
When the switch is opened, current will be reduced as the
impedance is higher. Therefore, change or reduction in current
will be opposed by the inductor. Thus the polarity will be
reversed (means left side of inductor will be negative now). As
a result two sources will be in series causing a higher voltage
to charge the capacitor through the diode D.
Here the switch is open and the only path offered to inductor
current is through the flyback diode D, the capacitor C and the
load R.
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13. Continuous Current Mode
As elements are ideal,
DC power drawn from
source must equal the
DC power absorbed by
load.
P0 = Pi
V0 I0 = Vi Ii
I0 =Vi * Ii
V0
=Vi * Ii
Vi d
I0 = Ii_
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16. On-State
• When S is on (D is off),
capacitor energy supplies the
load voltage.
Vo=Vc (if capacitor is
charged)
• 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.
• For very first time, when S is
closed Vo=0, as capacitor is
not charged.
Off-State
• 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
• During off state of S, capacitor
charges and voltage at it gradually
build up to Vi+VL
(This capacitor voltages serves as
load voltage when next time S in on)
• If S is off forever, inductor acts as
short circuit. It does not develop any
voltage and
Vo= Vi
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17. Voltage and current waveforms
for a 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 dissipated during off-
state.
If duty cycle increases above 0.5,
inductor will not dissipate 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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18. Application
Chopper are used for DC motor control( battery - supplied
vehicles),
Solar and Wind energy conservation
It’s also used in electric cars.
Airplane and spaceships ,where onboard-regulated DC
power supplies are required.
Chopper circuits are used as power supplies in computers,
commercial electronics, electronics instruments.
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