Download - DC-Choppers
Power Electronics by Prof. M. Madhusudhan Rao 11
DC Choppers
Power Electronics by Prof. M. Madhusudhan Rao 22
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.
Power Electronics by Prof. M. Madhusudhan Rao 33
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.
Power Electronics by Prof. M. Madhusudhan Rao 44
Principle Of Step-down Chopper
V
i0
V 0
C hopper
R
+
Power Electronics by Prof. M. Madhusudhan Rao 55
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 66
V d c
v 0
V
V /R
i0
Id c
t
t
tO N
T
tO F F
Power Electronics by Prof. M. Madhusudhan Rao 77
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
Power Electronics by Prof. M. Madhusudhan Rao 88
Average Output Voltage
.
duty cycle
ONdc
ON OFF
ONdc
ON
tV V
t t
tV V V d
T
tbut d
t
Power Electronics by Prof. M. Madhusudhan Rao 99
2
0
Average Output Current
RMS value of output voltage
1 ON
dcdc
ONdc
t
O o
VI
RtV V
I dR T R
V v dtT
Power Electronics by Prof. M. Madhusudhan Rao 1010
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 V
T T
V d V
Power Electronics by Prof. M. Madhusudhan Rao 1111
2
2
Output power
But
Output power
O O O
OO
OO
O
P V I
VI
R
VP
R
dVP
R
Power Electronics by Prof. M. Madhusudhan Rao 1212
Effective input resistance of chopper
The output voltage can be varied by
varying the duty cycle.
idc
i
VR
I
RR
d
Power Electronics by Prof. M. Madhusudhan Rao 1313
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.
Power Electronics by Prof. M. Madhusudhan Rao 1414
Pulse Width Modulation
• tON is varied keeping chopping frequency ‘f’ & chopping period ‘T’ constant.
• Output voltage is varied by varying the ON time tON
Power Electronics by Prof. M. Madhusudhan Rao 1515
V 0
V
V
V 0
t
ttO N
tO N tO F F
tO F F
T
Power Electronics by Prof. M. Madhusudhan Rao 1616
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
Power Electronics by Prof. M. Madhusudhan Rao 1717
v 0
V
V
v 0
t
t
tO N
tO N
T
T
tO F F
tO F F
Power Electronics by Prof. M. Madhusudhan Rao 1818
Step-down ChopperWith R-L Load
V
i0
V 0
C hopper
R
LFW D
E
+
Power Electronics by Prof. M. Madhusudhan Rao 1919
• 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’.
Power Electronics by Prof. M. Madhusudhan Rao 2020
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 2121
O utpu tvo ltage
O utpu tcurrent
v 0
V
i0
Im a x
Im in
t
t
tO N
T
tO F F
C ontinuouscurrent
O utpu tcurrent
t
D iscon tinuouscurrent
i0
Power Electronics by Prof. M. Madhusudhan Rao 2222
Expressions For Load Current
iO For Continuous Current Operation When
Chopper Is ON (0 t tON)
Power Electronics by Prof. M. Madhusudhan Rao 2323
V
i0
V 0
R
L
E
+
-
Power Electronics by Prof. M. Madhusudhan Rao 2424
min
min
Taking Laplace Transform
. 0
At 0, initial current 0
OO
O O O
O
O
diV i R L E
dt
V ERI S L S I S i
S S
t i I
IV EI S
RR SLS SLL
Power Electronics by Prof. M. Madhusudhan Rao 2525
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 t
L LO
ON
V Ei t e I e
R
t t
maxurrent is O ONi t I
Power Electronics by Prof. M. Madhusudhan Rao 2626
When Chopper is OFF
i0
R
L
E
Power Electronics by Prof. M. Madhusudhan Rao 2727
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 E
dt
ERI S L SI S i
St
i I
Power Electronics by Prof. M. Madhusudhan Rao 2828
max
max
Taking Inverse Laplace Transform
1
O
R Rt t
L LO
I EI S
R RS LS SL L
Ei t I e e
R
Power Electronics by Prof. M. Madhusudhan Rao 2929
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
Power Electronics by Prof. M. Madhusudhan Rao 3030
min
max
max
max min
min
From equation
1
At ,
To Find &
1
R Rt t
L LO
ON O
dRT dRT
L L
V Ei t e I e
R
t t dT i t I
V EI e I e
I I
R
Power Electronics by Prof. M. Madhusudhan Rao 3131
max
min
From equation
1
At ,
1
R Rt t
L LO
OFF ON O
OFF
Ei t I e e
R
t t T t i t I
t t d T
Power Electronics by Prof. M. Madhusudhan Rao 3232
1 1
min max
min
max min
max
1
Substituting for in equation
1
we get,
1
1
d RT d RT
L L
dRT dRT
L L
dRT
L
RT
L
EI I e e
R
I
V EI e I e
R
V e EI
R Re
Power Electronics by Prof. M. Madhusudhan Rao 3333
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 RT
L L
dRT
L
RT
L
I
EI I e e
R
V e EI
R Re
I I
Power Electronics by Prof. M. Madhusudhan Rao 3434
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
Power Electronics by Prof. M. Madhusudhan Rao 3535
min max
min
max minmin
Assuming load current varies linearly
from to instantaneous
load current is given by
. 0O ON
O
I I
I ti I for t t dT
dTI I
i I tdT
Power Electronics by Prof. M. Madhusudhan Rao 3636
20
0
2
max minmin
0
2min max min2 2max min
min
0
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 dt
dT dT dT
Power Electronics by Prof. M. Madhusudhan Rao 3737
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
Power Electronics by Prof. M. Madhusudhan Rao 3838
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
VR
I
Power Electronics by Prof. M. Madhusudhan Rao 3939
Where
Average source currentS
S dc
idc
I
I dI
VR
dI
Power Electronics by Prof. M. Madhusudhan Rao 4040
Principle Of Step-up Chopper
+
V OV
C hopper
CLOAD
DLI
+
Power Electronics by Prof. M. Madhusudhan Rao 4141
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 4242
• 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 O
dIV V L i e V V
dt
Power Electronics by Prof. M. Madhusudhan Rao 4343
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 4444
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 t
t ON
Power Electronics by Prof. M. Madhusudhan Rao 4545
(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
Power Electronics by Prof. M. Madhusudhan Rao 4646
Where
T = Chopping period or period
of switching.
ON O OFF
ON OFFO
OFF
OON
VIt V V It
V t tV
t
TV V
T t
Power Electronics by Prof. M. Madhusudhan Rao 4747
1
1
1
1
Where duty cyle
ON OFF
OON
O
ON
T t t
V Vt
T
V Vd
td
T
Power Electronics by Prof. M. Madhusudhan Rao 4848
For variation of duty cycle ' ' in the
range of 0 1 the output voltage
will vary in the range O
O
d
d V
V V
Power Electronics by Prof. M. Madhusudhan Rao 4949
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.
Power Electronics by Prof. M. Madhusudhan Rao 5050
Problem
• A Chopper circuit is operating on TRC at a frequency of 2 kHz on a 460 V supply. If the load voltage is 350 volts, calculate the conduction period of the thyristor in each cycle.
Power Electronics by Prof. M. Madhusudhan Rao 5151
3
460 V, = 350 V, f = 2 kHz
1Chopping period
10.5 sec
2 10
Output voltage
dc
ONdc
V V
Tf
T m
tV V
T
Power Electronics by Prof. M. Madhusudhan Rao 5252
3
Conduction period of thyristor
0.5 10 350
4600.38 msec
dcON
ON
ON
T Vt
V
t
t
Power Electronics by Prof. M. Madhusudhan Rao 5353
Problem• Input to the step up chopper is 200 V. The
output required is 600 V. If the conducting time of thyristor is 200 sec. Compute
– Chopping frequency,
– If the pulse width is halved for constant frequency of operation, find the new output voltage.
Power Electronics by Prof. M. Madhusudhan Rao 5454
6
200 , 200 , 600
600 200200 10
Solving for
300
ON dc
dcON
V V t s V V
TV V
T t
T
T
T
T s
Power Electronics by Prof. M. Madhusudhan Rao 5555
6
6
Chopping frequency
1
13.33
300 10Pulse width is halved
200 10100
2ON
fT
f KHz
t s
Power Electronics by Prof. M. Madhusudhan Rao 5656
6
6
Frequency is constant
3.33
1300
Output voltage =
300 10200 300 Volts
300 100 10
ON
f KHz
T sf
TV
T t
Power Electronics by Prof. M. Madhusudhan Rao 5757
Problem
• A dc chopper has a resistive load of 20 and input voltage VS = 220V. When chopper is ON, its voltage drop is 1.5 volts and chopping frequency is 10 kHz. If the duty cycle is 80%, determine the average output voltage and the chopper on time.
Power Electronics by Prof. M. Madhusudhan Rao 5858
220 , 20 , 10
0.80
= Voltage drop across chopper = 1.5 volts
Average output voltage
0.80 220 1.5 174.8 Volts
S
ON
ch
ONdc S ch
dc
V V R f kHz
td
TV
tV V V
T
V
Power Electronics by Prof. M. Madhusudhan Rao 5959
33
3
3
Chopper ON time,
1Chopping period,
10.1 10 secs 100 μsecs
10 10Chopper ON time,
0.80 0.1 10
0.08 10 80 μsecs
ON
ON
ON
ON
t dT
Tf
T
t dT
t
t
Power Electronics by Prof. M. Madhusudhan Rao 6060
Problem
• In a dc chopper, the average load current is 30 Amps, chopping frequency is 250 Hz, supply voltage is 110 volts. Calculate the ON and OFF periods of the chopper if the load resistance is 2 ohms.
Power Electronics by Prof. M. Madhusudhan Rao 6161
3
30 , 250 , 110 , 2
1 1Chopping period, 4 10 4 msecs
250
&
30 20.545
110
dc
dcdc dc
dc
dc
I Amps f Hz V V R
Tf
VI V dV
RdV
IR
I Rd
V
Power Electronics by Prof. M. Madhusudhan Rao 6262
3
3 3
3
Chopper ON period,
0.545 4 10 2.18 msecs
Chopper OFF period,
4 10 2.18 10
1.82 10 1.82 msec
ON
OFF ON
OFF
OFF
t dT
t T t
t
t
Power Electronics by Prof. M. Madhusudhan Rao 6363
• A dc chopper in figure has a resistive load of R = 10 and input voltage of V = 200 V. When chopper is ON, its voltage drop is 2 V and the chopping frequency is 1 kHz. If the duty cycle is 60%, determine
– Average output voltage
– RMS value of output voltage
– Effective input resistance of chopper
– Chopper efficiency.
Power Electronics by Prof. M. Madhusudhan Rao 6464
V
i0
C hopper
+
R v 0
200 , 10 , 2
0.60, 1 .chV V R Chopper voltage drop V V
d f kHz
Power Electronics by Prof. M. Madhusudhan Rao 6565
Average output voltage
0.60 200 2 118.8 Volts
RMS value of output voltage
0.6 200 2 153.37 Volts
dc ch
dc
O ch
O
V d V V
V
V d V V
V
Power Electronics by Prof. M. Madhusudhan Rao 6666
220
0 0
Effective input resistance of chopper is
118.811.88 Amps
10200
16.8311.88
Output power is
1 1
iS dc
dcdc
iS dc
dT dTch
O
V VR
I I
VI
RV V
RI I
V VvP dt dt
T R T R
Power Electronics by Prof. M. Madhusudhan Rao 6767
2
2
0
0
0.6 200 22352.24 watts
10Input power,
1
1
chO
O
dT
i O
dTch
O
d V VP
R
P
P Vi dtT
V V VP dt
T R
Power Electronics by Prof. M. Madhusudhan Rao 6868
0.6 200 200 22376 watts
10Chopper efficiency,
100
2352.24100 99%
2376
chO
O
O
i
dV V VP
R
P
P
P
Power Electronics by Prof. M. Madhusudhan Rao 6969
Problem• A chopper is supplying an inductive load with a
free-wheeling diode. The load inductance is 5 H and resistance is 10.. The input voltage to the chopper is 200 volts and the chopper is operating at a frequency of 1000 Hz. If the ON/OFF time ratio is 2:3. Calculate – Maximum and minimum values of load current
in one cycle of chopper operation.– Average load current
Power Electronics by Prof. M. Madhusudhan Rao 7070
5 , 10 , 1000 ,
200 , : 2 : 3
Chopping period,
1 11 msecs
1000
2
3
2
3
ON OFF
ON
OFF
ON OFF
L H R f Hz
V V t t
Tf
t
t
t t
Power Electronics by Prof. M. Madhusudhan Rao 7171
3
2
35
33
53
1 10 0.6 msec5
ON OFF
OFF OFF
OFF
OFF
T t t
T t t
T t
t T
T
Power Electronics by Prof. M. Madhusudhan Rao 7272
3
3
3
max
1 0.6 10 0.4 msec
Duty cycle,
0.4 100.4
1 10Maximum value of load current is given by
1
1
ON OFF
ON
ON
dRT
L
RT
L
t T t
t
td
T
V e EI
R Re
Power Electronics by Prof. M. Madhusudhan Rao 7373
3
3
max
0.4 10 1 10
5
max 10 1 10
5
Since there is no voltage source in
the load circuit, E = 0
1
1
200 1
101
dRT
L
RT
L
V eI
Re
eI
e
Power Electronics by Prof. M. Madhusudhan Rao 7474
3
3
0.8 10
max 2 10
max
min
120
1
8.0047A
Minimum value of load current with E = 0
is given by
1
1
dRT
L
RT
L
eI
e
I
V eI
Re
Power Electronics by Prof. M. Madhusudhan Rao 7575
3
3
0.4 10 1 10
5
min 10 1 10
5
max min
200 17.995 A
101
Average load current
28.0047 7.995
8 A2
dc
dc
eI
e
I II
I
Power Electronics by Prof. M. Madhusudhan Rao 7676
Problem• A chopper feeding on RL load is shown in
figure, with V = 200 V, R = 5, L = 5 mH, f = 1 kHz, d = 0.5 and E = 0 V. Calculate – Maximum and minimum values of load
current.
– Average value of load current.
– RMS load current.
– Effective input resistance as seen by source.
– RMS chopper current.
Power Electronics by Prof. M. Madhusudhan Rao 7777
33
V = 200 V, R = 5 , L = 5 mH,
f = 1kHz, d = 0.5, E = 0
Chopping period is
1 11 10 secs
1 10T
f
i0
v 0
C hopper
R
LFW D
E
+
Power Electronics by Prof. M. Madhusudhan Rao 7878
3
3
3
3
max
0.5 5 1 10
5 10
max 5 1 10
5 10
0.5
max 1
Maximum value of load current is given by
1
1
200 10
51
140 24.9 A
1
dRT
L
RT
L
V e EI
R Re
eI
e
eI
e
Power Electronics by Prof. M. Madhusudhan Rao 7979
3
3
3
3
min
0.5 5 1 10
5 10
min 5 1 10
5 10
0.5
min 1
Minimum value of load current is given by
1
1
200 10
51
140 15.1 A
1
dRT
L
RT
L
V e EI
R Re
eI
e
eI
e
Power Electronics by Prof. M. Madhusudhan Rao 8080
1 2
12 2
max min2min min max min
Average value of load current is
2for linear variation of currents
24.9 15.120 A
2RMS load current is given by
3
dc
dc
O RMS
I II
I
I II I I I I
Power Electronics by Prof. M. Madhusudhan Rao 8181
12 2
2
1
2
24.9 15.115.1 15.1 24.9 15.1
3
96.04228.01 147.98 20.2 A
3
RMS chopper current is given by
0.5 20.2 14.28 A
O RMS
O RMS
ch O RMS
I
I
I d I
Power Electronics by Prof. M. Madhusudhan Rao 8282
Effective input resistance is
= Average source current
0.5 20 10 A
Therefore effective input resistance is
20020
10
iS
S
S dc
S
iS
VR
I
I
I dI
I
VR
I
Power Electronics by Prof. M. Madhusudhan Rao 8383
Classification Of Choppers
• Choppers are classified as – Class A Chopper– Class B Chopper– Class C Chopper– Class D Chopper– Class E Chopper
Power Electronics by Prof. M. Madhusudhan Rao 8484
Class A Chopper
V
C hopper
FW D
+
v 0
v 0
i0
i0
LOAD
V
Power Electronics by Prof. M. Madhusudhan Rao 8585Prof. 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 .
Power Electronics by Prof. M. Madhusudhan Rao 8686
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 8787
O utpu t cu rren 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
Power Electronics by Prof. M. Madhusudhan Rao 8888
Class B Chopper
V
C hopper
+
v 0
v 0
i0
i0
L
E
R
D
Power Electronics by Prof. M. Madhusudhan Rao 8989
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 9090
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 9191
O utpu t cu rren 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
Power Electronics by Prof. M. Madhusudhan Rao 9292
Expression for Output Current
Power Electronics by Prof. M. Madhusudhan Rao 9393
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 obtained
along 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
Power Electronics by Prof. M. Madhusudhan Rao 9494
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 t
L LO OFF
OFF O
R Rt t
L L
OO
V Ei t e I e t t
R
t t i t I
V EI e I e
R
LdiRi E
dt
Power Electronics by Prof. M. Madhusudhan Rao 9595
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 t
L LO ON
ON O
R Rt t
L L
t i t I
Ei t I e e t t
R
t t i t I
EI I e e
R
Power Electronics by Prof. M. Madhusudhan Rao 9696
Class C Chopper
V
C hopper
+
v 0
D 1
D 2C H 2
C H 1
v 0i0
i0
L
E
R
Power Electronics by Prof. M. Madhusudhan Rao 9797
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 9898
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 9999
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 100100
G ate pu lseof C H 2
G ate pu lseof C H 1
O utput cu rren 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 2
O N O N O N O N
Power Electronics by Prof. M. Madhusudhan Rao 101101
Class D Chopper
V+ v 0
D 2
D 1 C H 2
C H 1
v 0
i0
L ER i0
Power Electronics by Prof. M. Madhusudhan Rao 102102
• 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 .
Power Electronics by Prof. M. Madhusudhan Rao 103103
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 104104
G ate pu lseof C H 2
G ate pu lseof C H 1
O utpu t curren t
O utpu t 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
Power Electronics by Prof. M. Madhusudhan Rao 105105
G ate pu lseof C H 2
G ate pu lseof C H 1
O utpu t curren t
O utpu t 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
Power Electronics by Prof. M. Madhusudhan Rao 106106
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
+
Power Electronics by Prof. M. Madhusudhan Rao 107107
Four Quadrant Operationv 0
i0
C H - C H O NC H - D C onduc ts
1 4
4 2
D D2 3 - C onductsC 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
Power Electronics by Prof. M. Madhusudhan Rao 108108
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 109109
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 110110
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 111111
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.
Power Electronics by Prof. M. Madhusudhan Rao 112112
• 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.
Power Electronics by Prof. M. Madhusudhan Rao 113113
Problem• For the first quadrant chopper shown in figure,
express the following variables as functions of V, R and duty cycle ‘d’ in case load is resistive.– Average output voltage and current– Output current at the instant of commutation– Average and RMS free wheeling diode current.– RMS value of output voltage– RMS and average thyristor currents.
Power Electronics by Prof. M. Madhusudhan Rao 114114
V
i0
v 0
C hopper
FW D
+
LOAD
Power Electronics by Prof. M. Madhusudhan Rao 115115
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 dV
T
V dVI
R Rt t
V
R
oltage at that instant.
Power Electronics by Prof. M. Madhusudhan Rao 116116
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
Power Electronics by Prof. M. Madhusudhan Rao 117117
2
0
2
1
Where duty cycle,
ONt
O RMS
ONO RMS
O RMS
ON
V V dtT
tV V
T
V dV
td
T
Power Electronics by Prof. M. Madhusudhan Rao 118118
RMS value of thyristor current
= RMS value of load current
Average value of thyristor current
= Average value of load current
O RMSV
R
dV
R
dV
R
Power Electronics by Prof. M. Madhusudhan Rao 119119
Impulse Commutated Chopper
• Impulse commutated choppers are widely used in high power circuits where load fluctuation is not large.
• This chopper is also known as
– Parallel capacitor turn-off chopper
– Voltage commutated chopper
– Classical chopper.
Power Electronics by Prof. M. Madhusudhan Rao 120120
LOAD
L
C
IL
L S
V S
+
_
+
_
T 2
T 1
D 1
a
biC
iT 1
v O
+
_
FW D
Power Electronics by Prof. M. Madhusudhan Rao 121121
• To start the circuit, capacitor ‘C’ is initially charged with polarity (with plate ‘a’ positive) by triggering the thyristor T2.
• Capacitor ‘C’ gets charged through VS, C, T2 and load.
• As the charging current decays to zero thyristor T2 will be turned-off.
• With capacitor charged with plate ‘a’ positive the circuit is ready for operation.
• Assume that the load current remains constant during the commutation process.
Power Electronics by Prof. M. Madhusudhan Rao 122122
• For convenience the chopper operation is divided into five modes.– Mode-1
– Mode-2
– Mode-3
– Mode-4
– Mode-5
Power Electronics by Prof. M. Madhusudhan Rao 123123
Mode-1 Operation
LOADL
C
IL
L S
V S
+
_
+
_
T 1
D 1
V C iC
Power Electronics by Prof. M. Madhusudhan Rao 124124
• Thyristor T1 is fired at t = 0.
• The supply voltage comes across the load.
• Load current IL flows through T1 and load.
• At the same time capacitor discharges through T1, D1, L1, & ‘C’ and the capacitor reverses its voltage.
• This reverse voltage on capacitor is held constant by diode D1.
Power Electronics by Prof. M. Madhusudhan Rao 125125
Capacitor Discharge Current
sin
1Where
& Capacitor Voltage
cos
C
C
Ci t V t
L
LC
V t V t
Power Electronics by Prof. M. Madhusudhan Rao 126126
Mode-2 Operation
LOAD
C
L S
V S+
_+
_
T 2
V C
IL
IL
Power Electronics by Prof. M. Madhusudhan Rao 127127
• Thyristor T2 is now fired to commutate thyristor T1.
• When T2 is ON capacitor voltage reverse biases T1 and turns if off.
• The capacitor discharges through the load from –V to 0.
• Discharge time is known as circuit turn-off time.
Power Electronics by Prof. M. Madhusudhan Rao 128128
C
Circuit turn-off time is given by
Where is load current.
t depends on load current, it must be designed
for the worst case condition which occur at the
maximum value of load current and mini
CC
L
L
V Ct
I
I
mum
value of capacitor voltage.
Power Electronics by Prof. M. Madhusudhan Rao 129129
• Capacitor recharges back to the supply voltage (with plate ‘a’ positive).
• This time is called the recharging time and is given by
• The total time required for the capacitor to discharge and recharge is called the commutation time and it is given by
Sd
L
V Ct
I
r C dt t t
Power Electronics by Prof. M. Madhusudhan Rao 130130
• At the end of Mode-2 capacitor has recharged to VS and the free wheeling diode starts conducting.
Power Electronics by Prof. M. Madhusudhan Rao 131131
Mode-3 Operation
LOAD
C
L S
V S
+_
+
_
T 2V S
FW D
IL
IL
Power Electronics by Prof. M. Madhusudhan Rao 132132
• FWD starts conducting and the load current decays.
• The energy stored in source inductance LS is transferred to capacitor.
• Hence capacitor charges to a voltage higher than supply voltage, T2 naturally turns off.
Power Electronics by Prof. M. Madhusudhan Rao 133133
The instantaneous capacitor voltage is
sin
Where
1
SC S L S
S
S
LV t V I t
C
L C
Power Electronics by Prof. M. Madhusudhan Rao 134134
Mode-4 Operation
LOAD
C
L S
V S
+_
+
_
D 1
LFW D
IL
V C
Power Electronics by Prof. M. Madhusudhan Rao 135135
• Capacitor has been overcharged i.e. its voltage is above supply voltage.
• Capacitor starts discharging in reverse direction.
• Hence capacitor current becomes negative. • The capacitor discharges through LS, VS, FWD,
D1 and L. • When this current reduces to zero D1 will stop
conducting and the capacitor voltage will be same as the supply voltage
Power Electronics by Prof. M. Madhusudhan Rao 136136
Mode-5 Operation
• Both thyristors are off and the load current flows through the FWD.
• This mode will end once thyristor T1 is fired.
LOAD
IL
FW D
Power Electronics by Prof. M. Madhusudhan Rao 137137
C a p a c i to r C u rr e n tI L
t
t
I p C u r r e n t t h r o u g h T 1
ic
0I p
iT1
0
I L
Power Electronics by Prof. M. Madhusudhan Rao 138138
t
t
t
Vo l ta g e a c r o s s T 1
O u tp u t Vo l t a g e
C a p a c i to r Vo l t a g e
tctd
v T1
V c
0v o
V s c+ V
V s
v c
V c
- V c
Power Electronics by Prof. M. Madhusudhan Rao 139139
Disadvantages• A starting circuit is required and the starting circuit
should be such that it triggers thyristor T2 first.
• Load voltage jumps to almost twice the supply voltage when the commutation is initiated.
• The discharging and charging time of commutation capacitor are dependent on the load current and this limits high frequency operation, especially at low load current.
Power Electronics by Prof. M. Madhusudhan Rao 140140
• Chopper cannot be tested without connecting load.
• Thyristor T1 has to carry load current as well as resonant current resulting in increasing its peak current rating.