Detailed Notes: DC Choppers | Power Electronics - Electrical Engineering (EE) PDF Download

 Page 1


 
 
 
 
DC Choppers 
 
 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.  
 
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. 
 
 Principle of Step-down Chopper 
 
V
i
0
V
0
Chopper
R
+
- 
 
 
• 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.  
Page 2


 
 
 
 
DC Choppers 
 
 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.  
 
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. 
 
 Principle of Step-down Chopper 
 
V
i
0
V
0
Chopper
R
+
- 
 
 
• 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.  
 
 
V
dc
v
0
V
V/R
i
0
I
dc
t
t
t
ON
T
t
OFF
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 A
I A
t
t
T t t
=
=
=
=
= + = or chopping period.
1
 Freq. of chopper switching or chopping freq. f
T
= =
Average Output Voltage
.
 duty cycle
ON
dc
ON OFF
ON
dc
ON
t
V V
t t
t
V V V d
T
t
but d
t
? ?
=
? ?
+
? ?
? ?
= =
? ?
? ?
? ?
= =
? ?
? ?
2
0
Average Output Current 
RMS value of output voltage
1
ON
dc
dc
ON
dc
t
O o
V
I
R
t V V
I d
R T R
V v dt
T
=
? ?
= =
? ?
? ?
=
?
2
0
2
But during   ,  
Therefore RMS output voltage
1
.
.
ON
ON o
t
O
ON
O ON
O
t v V
V V dt
T
t V
V t V
T T
V d V
=
=
= =
=
?
Page 3


 
 
 
 
DC Choppers 
 
 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.  
 
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. 
 
 Principle of Step-down Chopper 
 
V
i
0
V
0
Chopper
R
+
- 
 
 
• 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.  
 
 
V
dc
v
0
V
V/R
i
0
I
dc
t
t
t
ON
T
t
OFF
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 A
I A
t
t
T t t
=
=
=
=
= + = or chopping period.
1
 Freq. of chopper switching or chopping freq. f
T
= =
Average Output Voltage
.
 duty cycle
ON
dc
ON OFF
ON
dc
ON
t
V V
t t
t
V V V d
T
t
but d
t
? ?
=
? ?
+
? ?
? ?
= =
? ?
? ?
? ?
= =
? ?
? ?
2
0
Average Output Current 
RMS value of output voltage
1
ON
dc
dc
ON
dc
t
O o
V
I
R
t V V
I d
R T R
V v dt
T
=
? ?
= =
? ?
? ?
=
?
2
0
2
But during   ,  
Therefore RMS output voltage
1
.
.
ON
ON o
t
O
ON
O ON
O
t v V
V V dt
T
t V
V t V
T T
V d V
=
=
= =
=
?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
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. 
–  
Pulse Width Modulation 
• tON is varied keeping chopping frequency ‘f’ & chopping period ‘T’ constant. 
• Output voltage is varied by varying the ON time t
ON
 
V
0
V
V
V
0
t
t
t
ON
t
ON
t
OFF
t
OFF
T
 
 
Variable Frequency Control 
• Chopping frequency ‘f’ is varied keeping either t
ON
  or  t
OFF
  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 
2
2
Output power
But
 Output power
O O O
O
O
O
O
O
P V I
V
I
R
V
P
R
dV
P
R
=
=
?
=
=
Effective input resistance of chopper
The output voltage can be varied by 
varying the duty cycle.
i
dc
i
V
R
I
R
R
d
=
=
Page 4


 
 
 
 
DC Choppers 
 
 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.  
 
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. 
 
 Principle of Step-down Chopper 
 
V
i
0
V
0
Chopper
R
+
- 
 
 
• 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.  
 
 
V
dc
v
0
V
V/R
i
0
I
dc
t
t
t
ON
T
t
OFF
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 A
I A
t
t
T t t
=
=
=
=
= + = or chopping period.
1
 Freq. of chopper switching or chopping freq. f
T
= =
Average Output Voltage
.
 duty cycle
ON
dc
ON OFF
ON
dc
ON
t
V V
t t
t
V V V d
T
t
but d
t
? ?
=
? ?
+
? ?
? ?
= =
? ?
? ?
? ?
= =
? ?
? ?
2
0
Average Output Current 
RMS value of output voltage
1
ON
dc
dc
ON
dc
t
O o
V
I
R
t V V
I d
R T R
V v dt
T
=
? ?
= =
? ?
? ?
=
?
2
0
2
But during   ,  
Therefore RMS output voltage
1
.
.
ON
ON o
t
O
ON
O ON
O
t v V
V V dt
T
t V
V t V
T T
V d V
=
=
= =
=
?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
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. 
–  
Pulse Width Modulation 
• tON is varied keeping chopping frequency ‘f’ & chopping period ‘T’ constant. 
• Output voltage is varied by varying the ON time t
ON
 
V
0
V
V
V
0
t
t
t
ON
t
ON
t
OFF
t
OFF
T
 
 
Variable Frequency Control 
• Chopping frequency ‘f’ is varied keeping either t
ON
  or  t
OFF
  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 
2
2
Output power
But
 Output power
O O O
O
O
O
O
O
P V I
V
I
R
V
P
R
dV
P
R
=
=
?
=
=
Effective input resistance of chopper
The output voltage can be varied by 
varying the duty cycle.
i
dc
i
V
R
I
R
R
d
=
=
 
 
v
0
V
V
v
0
t
t
t
ON
t
ON
T
T
t
OFF
t
OFF
 
 
 Step-down Chopper with R-L Load 
 
V
i
0
V
0
Chopper
R
L
FWD
E
+
-
 
 
• When chopper is ON, supply is connected across load. 
• Current flows from supply to load. 
• When chopper is OFF, load current continues to flow in the same direction 
through FWD due to energy stored in inductor ‘L’. 
• Load current can be continuous or discontinuous depending on the values of ‘L’ 
and duty cycle ‘d’ 
• For a continuous current operation, load current varies between two limits Imax 
and Imin 
• When current becomes equal to Imax the chopper is turned-off and it is turned-on 
when current reduces to Imin. 
Page 5


 
 
 
 
DC Choppers 
 
 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.  
 
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. 
 
 Principle of Step-down Chopper 
 
V
i
0
V
0
Chopper
R
+
- 
 
 
• 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.  
 
 
V
dc
v
0
V
V/R
i
0
I
dc
t
t
t
ON
T
t
OFF
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 A
I A
t
t
T t t
=
=
=
=
= + = or chopping period.
1
 Freq. of chopper switching or chopping freq. f
T
= =
Average Output Voltage
.
 duty cycle
ON
dc
ON OFF
ON
dc
ON
t
V V
t t
t
V V V d
T
t
but d
t
? ?
=
? ?
+
? ?
? ?
= =
? ?
? ?
? ?
= =
? ?
? ?
2
0
Average Output Current 
RMS value of output voltage
1
ON
dc
dc
ON
dc
t
O o
V
I
R
t V V
I d
R T R
V v dt
T
=
? ?
= =
? ?
? ?
=
?
2
0
2
But during   ,  
Therefore RMS output voltage
1
.
.
ON
ON o
t
O
ON
O ON
O
t v V
V V dt
T
t V
V t V
T T
V d V
=
=
= =
=
?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
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. 
–  
Pulse Width Modulation 
• tON is varied keeping chopping frequency ‘f’ & chopping period ‘T’ constant. 
• Output voltage is varied by varying the ON time t
ON
 
V
0
V
V
V
0
t
t
t
ON
t
ON
t
OFF
t
OFF
T
 
 
Variable Frequency Control 
• Chopping frequency ‘f’ is varied keeping either t
ON
  or  t
OFF
  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 
2
2
Output power
But
 Output power
O O O
O
O
O
O
O
P V I
V
I
R
V
P
R
dV
P
R
=
=
?
=
=
Effective input resistance of chopper
The output voltage can be varied by 
varying the duty cycle.
i
dc
i
V
R
I
R
R
d
=
=
 
 
v
0
V
V
v
0
t
t
t
ON
t
ON
T
T
t
OFF
t
OFF
 
 
 Step-down Chopper with R-L Load 
 
V
i
0
V
0
Chopper
R
L
FWD
E
+
-
 
 
• When chopper is ON, supply is connected across load. 
• Current flows from supply to load. 
• When chopper is OFF, load current continues to flow in the same direction 
through FWD due to energy stored in inductor ‘L’. 
• Load current can be continuous or discontinuous depending on the values of ‘L’ 
and duty cycle ‘d’ 
• For a continuous current operation, load current varies between two limits Imax 
and Imin 
• When current becomes equal to Imax the chopper is turned-off and it is turned-on 
when current reduces to Imin. 
 
 
Output
voltage
Output
current
v
0
V
i
0
I
max
I
min
t
t
t
ON
T
t
OFF
Continuous
current
Output
current
t
Discontinuous
current
i
0
 
 
 
Expressions for Load Current Io for Continuous Current Operation When Chopper 
is ON (0 = = = = T = = = =  Ton) 
 
V
i
0
V
0
R
L
E
+
- 
 
 
( ) ( ) ( )
( )
( )
min
min
Taking Laplace Transform
. 0
At 0, initial current 0
O
O
O O O
O
O
di
V i R L E
dt
V E
RI S L S I S i
S S
t i I
I V E
I S
R R
S LS S
L L
-
-
= + +
? ?
= + - +
? ?
= =
-
= +
? ?
+ +
? ?
? ?