chandresh chopper
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05/01/2023 chandresh kumar 1
DC TO DC CONVERTER
Submitted to :Er. Shimi S.L(Assistant Professor) Department of Electrical Engineering , NITTTR Chandigarh
Presentation by:Chandresh kumar (141503)ME-MOD-IC-2014Department of Electrical engineering NITTTR Chandigarh
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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 (as a switch) converter offers greater
efficiency, faster response, lower maintenance, smaller size and smooth control.
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Types of Choppers 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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Principle Of Step-down Chopper
V
i0
V 0
C hopper
R
+
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• A step-down chopper with resistive load. • The thyristor in the circuit acts as a switch.• When thyristor is ON, supply voltage appears
across the load • When thyristor is OFF, the voltage across the
load will be zero.
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V d c
v 0
V
V /R
i0
Id c
t
t
tO N
T
tO F F
verage value of output or load voltage.verage value of output or load current.
Time interval for which SCR conducts.Time interval for which SCR is OFF.
Period of switching
dc
dc
ON
OFF
ON OFF
V AI AttT t t
or chopping period.1 Freq. of chopper switching or chopping freq.fT
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Average Output Voltage
.
duty cycle
ONdc
ON OFF
ONdc
ON
tV Vt t
tV V V dT
tbut dt
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2
0
Average Output Current
RMS value of output voltage
1 ON
dcdc
ONdc
t
O o
VIR
tV VI dR T R
V v dtT
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2
0
2
But during , Therefore RMS output voltage
1
.
.
ON
ON o
t
O
ONO ON
O
t v V
V V dtT
tVV t VT T
V d V
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2
2
Output power
But
Output power
O O O
OO
OO
O
P V IVIR
VPRdVP
R
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Effective input resistance of chopper
The output voltage can be varied by varying the duty cycle.
idc
i
VRIRRd
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Principle Of Step-up Chopper
+
V OV
C hopper
CLOAD
DLI
+
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• The values of L and C are chosen depending upon the requirement of output voltage and current.
• The inductor current ‘I’ rises and the inductor stores energy during the ON time of the chopper, tON.
• Step-up chopper is used to obtain a load voltage higher than the input voltage V.
• When the chopper is ON, the inductor L is connected across the supply.
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• The current tends to decrease resulting in
reversing the polarity of induced EMF in L.
• Therefore voltage across load is given by:-
• When the chopper is off, the inductor current I
is forced to flow through the diode D and load
for a period, tOFF.
. ., O OdIV V L i e V Vdt
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• A large capacitor ‘C’ connected across the
load, will provide a continuous output voltage
• Diode D prevents any current flow from
capacitor to the source.
• Step up choppers are used for regenerative
braking of dc motors
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Expression For Output Voltage
Assume the average inductor current to be during ON and OFF time of Chopper.
Voltage across inductor Therefore energy stored in inductor
= . .Where
When Chopper
period of chopper.
is ON
ON
ON
I
L V
V I tt ON
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(energy is supplied by inductor to load)Voltage across
Energy supplied by inductor
where period of Chopper.Neg
When Chopper
lecting losses, energy stored in inductor
is OFF
O
O OFF
OFF
L V V
L V V It
t OFF
L
= energy supplied by inductor L
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Where T = Chopping period or period
of switching.
ON O OFF
ON OFFO
OFF
OON
VIt V V It
V t tV
t
TV VT t
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1
1
11
Where duty cyle
ON OFF
OON
O
ON
T t t
V VtT
V Vd
td
T
For variation of duty cycle ' ' in the range of 0 1 the output voltage will vary in the range
O
O
dd V
V V
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Control Strategies
• The output dc voltage can be varied by the
following methods.
1. Pulse width modulation control or constant
frequency operation.
2. Variable frequency control
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Pulse Width Modulation
• tON is varied keeping chopping frequency ‘f’ &
chopping period ‘T’ constant.
• Output voltage is varied by varying the ON
time tON
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Pulse Width Modulation
V 0
V
V
V 0
t
ttO N
tO N tO F F
tO F F
T
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Variable Frequency Control
• Chopping frequency ‘f’ is varied keeping either tON or
tOFF constant.
• To obtain full output voltage range, frequency has to
be varied over a wide range.
• This method produces harmonics in the output and for
large tOFF load current may become discontinuous
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Variable Frequency Controlv 0
V
V
v 0
t
t
tO N
tO N
T
T
tO F F
tO F F
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Performance Parameters
• The thyristor requires a certain minimum time to turn ON and turn OFF.
• Duty cycle d can be varied only between a min. & max. value, limiting the min. and max. value of the output voltage.
• Ripple in the load current depends inversely on the chopping frequency, f.
• To reduce the load ripple current, frequency should be as high as possible.
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Classification Of Choppers
• Choppers are classified as –Class A Chopper–Class B Chopper–Class C Chopper–Class D Chopper–Class E Chopper
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Class A Chopper
V
C hopper
FW D
+
v 0
v 0
i0
i0
LOAD
V
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• When chopper is ON, supply voltage V is connected across
the load.
• When chopper is OFF, vO = 0 and the load current
continues to flow in the same direction through the FWD.
• The average values of output voltage and current are
always positive.
• Class A Chopper is a first quadrant chopper
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• Class A Chopper is a step-down chopper in
which power always flows form source to load.
• It is used to control the speed of dc motor.
• The output current equations obtained in step
down chopper with R-L load can be used to
study the performance of Class A Chopper.
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O utput curren t
Thyristo rgate pu lse
O utpu t vo ltage
ig
i0
v 0
t
t
ttO N
T
C H O N
FW D C onduc ts
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Class B Chopper
V
C hopper
+
v 0
v 0
i0
i0
L
E
R
D
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• When chopper is ON, E drives a current through L
and R in a direction opposite to that shown in figure.
• During the ON period of the chopper, the inductance
L stores energy.
• When Chopper is OFF, diode D conducts, and part
of the energy stored in inductor L is returned to the
supply.
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• Average output voltage is positive.
• Average output current is negative.
• Therefore Class B Chopper operates in second quadrant.
• In this chopper, power flows from load to source.
• Class B Chopper is used for regenerative braking of dc
motor.
• Class B Chopper is a step-up chopper.
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O utpu t cu rren t
D con d u cts C h o pp e r
con d u cts
Thyristorgate pu lse
O utpu t vo ltage
ig
i0
v 0
t
t
t
Im in
Im ax
T
tO NtO F F
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Expression for Output Current
min
For the initial condition i.e.,
During the interval diode 'D' conduc
at 0
The solution of the ab
ts voltage equation
ove equation is obtainedalong similar lines as in s
is given by
OO
O
LdiV Ri E
dt
i t I t
tep-down chopper with R-L load
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min
max
max min
During the interval chopper is ON voltage equation is g
1 0
At
1
0
iven by
OFF OFF
R Rt tL L
O OFF
OFF O
R Rt tL L
OO
V Ei t e I e t tR
t t i t I
V EI e I eR
Ldi Ri Edt
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max
max
min
min max
Redefining the time origin, at 0
The solution for the stated initial condition is
1 0
At
1ON ON
O
R Rt tL L
O ON
ON O
R Rt tL L
t i t I
Ei t I e e t tR
t t i t I
EI I e eR
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Class C Chopper
V
C hopper
+
v 0
D 1
D 2C H 2
C H 1
v 0i0
i0
L
E
R
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• Class C Chopper is a combination of Class A and Class B Choppers.
• For first quadrant operation, CH1 is ON or D2 conducts.
• For second quadrant operation, CH2 is ON or D1 conducts.
• When CH1 is ON, the load current is positive.• The output voltage is equal to ‘V’ & the load
receives power from the source. • When CH1 is turned OFF, energy stored in
inductance L forces current to flow through the diode D2 and the output voltage is zero.
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• continues to flow in positive direction.• When CH2 is triggered, the voltage E forces
current t to flow in opposite direction through L and CH2 .
• The output voltage is zero.• On turning OFF CH2 , the energy stored in the
inductance drives current through diode D1 and the supply
• Output voltage is V, the input current becomes negative and power flows from load to source
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• Average output voltage is positive• Average output current can take both positive
and negative values.• Choppers CH1 & CH2 should not be turned
ON simultaneously as it would result in short circuiting the supply.
• Class C Chopper can be used both for dc motor control and regenerative braking of dc motor.
• Class C Chopper can be used as a step-up or step-down chopper.
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G ate pu lseo f C H 2
G ate pu lseof C H 1
O utput curren t
O utpu t vo ltage
ig 1
ig 2
i0
V 0
t
t
t
t
D 1 D 1D 2 D 2C H 1 C H 2 C H 1 C H 2O N O N O N O N
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Class D Chopper
V+ v 0
D 2
D 1 C H 2
C H 1
v 0
i0
L ER i0
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• Class D is a two quadrant chopper.
• When both CH1 and CH2 are triggered simultaneously,
the output voltage vO = V and output current flows
through the load.
• When CH1 and CH2 are turned OFF, the load current
continues to flow in the same direction through load, D1
and D2 , due to the energy stored in the inductor L.
• Output voltage vO = - V.
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• Average load voltage is positive if chopper
ON time is more than the OFF time
• Average output voltage becomes negative if
tON < tOFF .
• Hence the direction of load current is always
positive but load voltage can be positive or
negative.
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G ate pulseof C H 2
G ate pu lseof C H 1
O utpu t curren t
O utpu t voltage
Average v0
ig 1
ig 2
i0
v 0
V
t
t
t
t
C H ,C HO N1 2 D 1,D 2 C onducting
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G ate pulseo f C H 2
G ate pulseo f C H 1
O utpu t curren t
O utput vo ltage
Average v 0
ig 1
ig 2
i0
v 0
V
t
t
t
t
C HC H
1
2
D , D1 2
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Class E Chopper
V
v0
i0L ER
C H 2 C H 4D 2 D 4
D 1 D 3C H 1 C H 3
+
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Four Quadrant Operation
v 0
i0
C H - C H O NC H - D C onduc ts
1 4
4 2
D D2 3 - ConductsC H - D C onduc ts4 2
C H - C H O NC H - D C onducts
3 2
2 4
C H - D C onductsD - D C onducts
2 4
1 4
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• Class E is a four quadrant chopper• When CH1 and CH4 are triggered, output
current iO flows in positive direction through CH1 and CH4, and with output voltage vO = V.
• This gives the first quadrant operation.• When both CH1 and CH4 are OFF, the energy
stored in the inductor L drives iO through D2 and D3 in the same direction, but output voltage vO = -V.
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• Therefore the chopper operates in the fourth
quadrant.
• When CH2 and CH3 are triggered, the load current
iO flows in opposite direction & output voltage vO
= -V.
• Since both iO and vO are negative, the chopper
operates in third quadrant
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• When both CH2 and CH3 are OFF, the load
current iO continues to flow in the same
direction D1 and D4 and the output voltage
vO = V.
• Therefore the chopper operates in second
quadrant as vO is positive but iO is negative.
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धन्यवाद !
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