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1Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
DC Choppers
2Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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.
3Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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.
4Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Principle Of Step-down Chopper
V
i0
V 0
C hopper
R
+
5Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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.
6Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
V d c
v 0
V
V /R
i0
Id c
t
t
tO N
T
tO F F
7Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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
8Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Average Output Voltage
.
duty cycle
ONdc
ON OFF
ONdc
ON
tV Vt t
tV V V dT
tbut dt
9Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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
11Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
2
2
Output power
But
Output power
O O O
OO
OO
O
P V IV
IR
VPRdVP
R
12Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Effective input resistance of chopper
The output voltage can be varied by varying the duty cycle.
idc
i
VRIRRd
13Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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.
14Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Pulse Width Modulation
• tON is varied keeping chopping frequency ‘f’ & chopping period ‘T’ constant.
• Output voltage is varied by varying the ON time tON
15Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
V 0
V
V
V 0
t
ttO N
tO N tO F F
tO F F
T
16Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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
17Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
v 0
V
V
v 0
t
t
tO N
tO N
T
T
tO F F
tO F F
18Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Step-down ChopperWith R-L Load
V
i0
V 0
C hopper
R
LFW D
E
+
19Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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’.
20Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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.
21Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
O utputvo ltage
O utputcurrent
v 0
V
i0
Im a x
Im in
t
t
tO N
T
tO F F
C ontinuouscurrent
O utputcurrent
t
D iscon tinuouscurrent
i0
22Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Expressions For Load Current
iO For Continuous Current Operation When
Chopper Is ON (0 t tON)
23Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
V
i0
V 0
R
L
E
+
-
24Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
min
min
Taking Laplace Transform
. 0
At 0, initial current 0
OO
O O O
O
O
diV i R L Edt
V ERI S L S I S iS S
t i I
IV EI S RR SLS SLL
25Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
min
Taking Inverse Laplace Transform
1
This expression is valid for 0 ,i.e., during the period chopper is ON.At the instant the chopper is turned off, load c
R Rt tL L
O
ON
V Ei t e I eR
t t
maxurrent is O ONi t I
26Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
When Chopper is OFFi0
R
L
E
27Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
max
When Chopper is OFF 0
0
Talking Laplace transform
0 0
Redefining time origin we have at 0,
initial current 0
OFF
OO
O O O
O
t t
diRi L Edt
ERI S L SI S iS
t
i I
28Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
max
max
Taking Inverse Laplace Transform
1
O
R Rt tL L
O
I EI S R RS LS SL L
Ei t I e eR
29Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
min
The expression is valid for 0 ,i.e., during the period chopper is OFF
At the instant the chopper is turned ON or at the end of the off period, the load current is
OFF
O OFF
t t
i t I
30Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
min
max
max
max min
min
From equation
1
At ,
To Find &
1
R Rt tL L
O
ON O
dRT dRTL L
V Ei t e I eR
t t dT i t I
V EI e I e
I I
R
31Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
max
min
From equation
1
At ,
1
R Rt tL L
O
OFF ON O
OFF
Ei t I e eR
t t T t i t I
t t d T
32Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
1 1
min max
min
max min
max
1
Substituting for in equation
1
we get,
1
1
d RT d RTL L
dRT dRTL L
dRTL
RTL
EI I e eR
I
V EI e I eR
V e EIR R
e
33Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
max
1 1
min max
min
max min
Substituting for in equation
1
we get,
1
1
is known as the steady state ripple.
d RT d RTL L
dRTL
RTL
I
EI I e eR
V e EIR R
e
I I
34Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
max min
max min
Therefore peak-to-peak ripple current
Average output voltage.
Average output current
2
dc
dc approx
I I I
V d V
I II
35Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
min max
min
max minmin
Assuming load current varies linearlyfrom to instantaneous load current is given by
. 0O ON
O
I I
I ti I for t t dT
dTI Ii I t
dT
36Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
20
0
2
max minmin
0
2min max min2 2max min
min0
RMS value of load current
1
1
21
dT
O RMS
dT
O RMS
dT
O RMS
I i dtdT
I I tI I dt
dT dT
I I I tI II I t dtdT dT dT
37Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
12 2
max min2min min max min
20
0
2
max minmin
0
RMS value of output current
3
RMS chopper current
1
1
O RMS
dT
CH
dT
CH
I II I I I I
I i dtT
I II I t dt
T dT
38Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
12 2
max min2min min max min3
Effective input resistance is
CH
CH O RMS
iS
I II d I I I I
I d I
VRI
39Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Where Average source currentS
S dc
idc
I
I dI
VRdI
40Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Principle Of Step-up Chopper
+
V OV
C hopper
CLOAD
DLI
+
41Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Step-up chopper is used to obtain a load voltage higher than the input voltage V.
• The values of L and C are chosen depending upon the requirement of output voltage and current.
• When the chopper is ON, the inductor L is connected across the supply.
• The inductor current ‘I’ rises and the inductor stores energy during the ON time of the chopper, tON.
42Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• When the chopper is off, the inductor current I is forced to flow through the diode D and load for a period, tOFF.
• The current tends to decrease resulting in reversing the polarity of induced EMF in L.
• Therefore voltage across load is given by
. ., O OdIV V L i e V Vdt
43Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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.
44Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Expression For Output VoltageAssume 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
45Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
(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
46Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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
47Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
1
1
11
Where duty cyle
ON OFF
OON
O
ON
T t t
V V tT
V Vd
tdT
48Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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
49Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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.
50Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Classification Of Choppers
• Choppers are classified as – Class A Chopper– Class B Chopper– Class C Chopper– Class D Chopper– Class E Chopper
51Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class A Chopper
V
C hopper
FW D
+
v 0
v 0
i0
i0
LOAD
V
52Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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 .
53Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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.
54Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
O utput curren t
Thyristo rgate pu lse
O utpu t vo ltage
ig
i0
v0
t
t
ttO N
T
C H O N
FW D C onduc ts
55Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class B Chopper
V
C hopper
+
v 0
v 0
i0
i0
L
E
R
D
56Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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.
57Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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.
58Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
O utpu t curren t
D con d u cts C h o pp e r
con d u cts
Thyristo rgate pu lse
O utpu t vo ltage
ig
i0
v 0
t
t
t
Im in
Im ax
T
tO NtO F F
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Expression for Output Current
60Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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 Edt
i t I t
tep-down chopper with R-L load
61Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
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
63Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class C Chopper
V
C hopper
+
v0
D 1
D 2C H 2
C H 1
v 0i0
i0
L
E
R
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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.
65Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Current continues to flow in positive direction.• When CH2 is triggered, the voltage E forces
current 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.
66Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
G ate pu lseof 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
68Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class D Chopper
V+ v 0
D 2
D 1 C H 2
C H 1
v 0
i0
L ER i0
69Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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 .
70Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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.
71Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
G ate pu lseof C H 2
G ate pu lseof 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 H ,C HO N1 2 D 1,D 2 Conducting
72Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
G ate pu lseo f C H 2
G ate pulseof C H 1
O utpu t curren t
O utpu t vo ltage
Average v0
ig 1
ig 2
i0
v 0
V
t
t
t
t
C HC H
1
2
D , D1 2
73Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class E Chopper
V
v 0
i0L ER
C H 2 C H 4D 2 D 4
D 1 D 3C H 1 C H 3
+
74Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Four Quadrant Operationv 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 onduc ts
3 2
2 4
C H - D C onduc tsD - D C onducts
2 4
1 4
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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.
76Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• 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.
78Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Effect Of Source &Load Inductance
• The source inductance should be as small as possible to limit the transient voltage.
• Also source inductance may cause commutation problem for the chopper.
• Usually an input filter is used to overcome the problem of source inductance.
79Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• The load ripple current is inversely proportional to load inductance and chopping frequency.
• Peak load current depends on load inductance. • To limit the load ripple current, a smoothing
inductor is connected in series with the load.
80Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
V
i0
v 0
C hopper
FW D
+
LOAD
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Average output voltage,
Average output current,
The thyristor is commutated at the instant
output current at the instant of commutation is
since V is the output v
ONdc
dcdc
ON
tV V dVT
V dVIR R
t tVR
oltage at that instant.
Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
20
0
Free wheeling diode (FWD) will never conduct in a resistive load.
Average & RMS free wheeling diode currents are zero.
1
But during
ONt
O RMS
O ON
V v dtT
v V t
83Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
2
0
2
1
Where duty cycle,
ONt
O RMS
ONO RMS
O RMS
ON
V V dtT
tV VT
V dV
tdT
84Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
RMS value of thyristor current = RMS value of load current
Average value of thyristor current= Average value of load current
O RMSV
RdVR
dVR