Electrical Engineering (EE) Exam  >  Electrical Engineering (EE) Notes  >  Power Electronics  >  Detailed Notes: DC Choppers

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

Download, print and study this document offline
Please wait while the PDF view is loading
 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
-
-
= + +
? ?
= + - +
? ?
= =
-
= +
? ?
+ +
? ?
? ?
Read More
5 videos|40 docs|66 tests

Up next

FAQs on Detailed Notes: DC Choppers - Power Electronics - Electrical Engineering (EE)

1. What is a DC chopper in electrical engineering?
Ans. A DC chopper, also known as a DC-to-DC converter or a DC-DC converter, is an electronic circuit used to convert a fixed DC voltage into a variable DC voltage. It is commonly used in various applications like motor speed control, power supplies, and battery charging.
2. How does a DC chopper work?
Ans. A DC chopper works by using a switch to control the connection of the input DC voltage to the load. When the switch is closed, the input voltage is directly connected to the load, and when the switch is open, the connection is interrupted. By controlling the timing and duration of the switch operation, the average voltage delivered to the load can be varied, resulting in a variable DC voltage.
3. What are the advantages of using a DC chopper?
Ans. Using a DC chopper offers several advantages, such as: - Voltage control: A DC chopper allows precise control of the output voltage, making it suitable for applications where variable voltage is required, such as motor speed control. - Energy efficiency: By controlling the average voltage delivered to the load, a DC chopper can improve overall energy efficiency. - Compact size: DC choppers are compact in size and lightweight, making them suitable for space-constrained applications. - Bidirectional operation: Some DC choppers can operate in both step-up and step-down modes, allowing bidirectional power flow.
4. Are there any limitations or drawbacks of using a DC chopper?
Ans. Yes, there are certain limitations and drawbacks of using a DC chopper, including: - High switching losses: The switching operation of the chopper introduces losses in the form of heat, which can reduce overall efficiency. - Electromagnetic interference (EMI): The switching action of the chopper can generate high-frequency harmonics that may cause EMI issues if not properly mitigated. - Complexity: The design and implementation of a DC chopper circuit can be complex, requiring careful consideration of factors like switching frequency, voltage ratings, and protection mechanisms. - Cost: Depending on the specific application, the cost of implementing a DC chopper circuit can be relatively high compared to other voltage control methods.
5. What are some common applications of DC choppers?
Ans. DC choppers find applications in various fields, including: - Electric vehicle motor control: DC choppers are used in electric vehicles to control the speed and torque of the motor. - Renewable energy systems: In solar or wind power systems, DC choppers are used to convert the varying DC voltage from the source into a stable and adjustable DC voltage for battery charging or grid integration. - Uninterruptible power supplies (UPS): DC choppers are utilized in UPS systems to convert DC power from the battery into AC power during power outages. - Adjustable speed drives: DC choppers are commonly used in adjustable speed drives to control the speed of industrial motors, providing flexibility and energy efficiency.
5 videos|40 docs|66 tests
Download as PDF

Up next

Explore Courses for Electrical Engineering (EE) exam
Signup for Free!
Signup to see your scores go up within 7 days! Learn & Practice with 1000+ FREE Notes, Videos & Tests.
10M+ students study on EduRev
Related Searches

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

,

Free

,

Viva Questions

,

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

,

Summary

,

Extra Questions

,

Semester Notes

,

past year papers

,

shortcuts and tricks

,

Exam

,

Previous Year Questions with Solutions

,

ppt

,

pdf

,

Sample Paper

,

practice quizzes

,

study material

,

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

,

Important questions

,

mock tests for examination

,

video lectures

,

MCQs

,

Objective type Questions

;