RBSK/EEE/PE
8/21/2015
SNS COLLEGE OF ENGINEERING
SNS KALVI NAGAR, SATHY MAIN ROAD,
COIMBATORE – 641 107
DEPARTMENT OF ELECTRICAL AND ELECTRONICS
ENGINEERING
THYRISTOR
EE 6503
POWER ELECTRONICS
Mr.R.B.SELVAKUMAR
CHOPPERS
ASSISTANT PROFESSOR
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
SNS COLLEGE OF ENGINEERING, COIMBATORE
rbsk.snsce@gmail.com
Mobile : 9942483127
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THYRISTOR CHOPPER CIRCUITS
THYRISTOR CHOPPER CIRCUITS
• SCR IS USED AS SWITCH FOR HIGH
• SCR IS USED AS SWITCH FOR HIGH
POWER LEVEL
POWER LEVEL
• SEPARATE COMMUTATION CIRCUIT IS
• SEPARATE COMMUTATION CIRCUIT IS
ESSENTIAL
ESSENTIAL
• COMMUTATION – PROCESS OF TURNING-
• COMMUTATION ?
OFF, A CONDUCTING THYRISTOR
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THYRISTOR CHOPPER CIRCUITS
TURNING-OFF A THYRISTOR
• SCR IS USED AS SWITCH FOR HIGH
• REDUCING IA < IH
POWER LEVEL
• APPLYING REVERSE VOLTAGE ACROSS IT
• SEPARATE COMMUTATION CIRCUIT IS
• SO, IT REGAIN ITS FORWARD BLOKING
ESSENTIAL
CAPABILITY
• COMMUTATION – PROCESS OF TURNINGOFF, A CONDUCTING THYRISTOR
• HOW TO TURN-OFF A THYRISTOR?
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CLASSIFICATION OF COMMUTATION
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1. FORCED COMMUTATION
• External Elements L & C are used
1. FORCED COMMUTATION
• Achieved in two ways
a) Voltage Commutation
a) Voltage Commutation
b) Current Commutation
- Application of pulse of large reverse voltage
2. LOAD COMMUTATION
- Applied by previously charged capacitor
- Reduces IA to zero
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1. FORCED COMMUTATION
2. LOAD COMMUTATION
b) Current Commutation
- Application of pulse of current > load current IL
in reverse direction
- When pulse current = IL ; IA reduces to zero
- Diode is connected in antiparallel with main
SCR
- Voltage drop across diode reverse biases SCR
- Commutation time is greater than in Voltage
commutation
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THYRISTOR CHOPPER CIRCUITS
• Achieved by either
a) The IL = 0 due to the nature of load circuit
parameters
b) IL is transferred to another device from
conducting SCR
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VOLTAGE COMMUTATED CHOPPER
• Used in high power circuits where load
• VOLTAGE COMMUTATED CHOPPER
fluctuation is not large
• CURRENT COMMUTATED CHOPPER
• Other names
• LOAD COMMUTATED CHOPPER
Parallel-capacitor turn-off chopper
Impulse-commutated chopper
Classical chopper
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TYPE –A CHOPPER
TYPE –A CHOPPER
MAIN POWER SWITCH
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TYPE –A CHOPPER
TYPE –A CHOPPER
CAPACITOR
AUXILIARY THYRISTOR
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TYPE –A CHOPPER
TYPE –A CHOPPER
DIODE
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INDUCTOR
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TYPE –A CHOPPER
TYPE –A CHOPPER
RLE LOAD
FREE WHEELING DIODE
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WORKING
HOW TO CHARGE C?
• Working of this chopper starts only if the C
charged with marked polarities
WAY - 1
• Close Switch S
• How to achieve this?
• So C charged to VS through
VS ,C,S & RC
CHARGING RESISTOR
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HOW TO CHARGE C?
WAY - 2
• Auxiliary Thyristor TA is
Now Chopper is ready for
operation with marked
polarities as positive
triggered
• C gets charged through
VS ,C,TA & load
• Charging current through C
decays & as it reaches zero,
vc = VS ; TA turned-off
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MODE - I
ASSUMPTIONS
T1 Triggered at t = 0
Two currents
it1= ic + io
Commutation Current
• Load Current is Constant
• Thyristor and Diodes are Ideal
Load Current
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MODE - II
MODE -I
• ic changes sinusoidally
• vc changes cosinusoidally
• vTA changes cosinusoidally
Main SCR T1 only
conducting
• At t = t1
•
ic = 0
•
it1= io
•
vc = -VS
•
vTA = VS
•
vo = VS
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MODE - III
To turn off T1, TA is
Triggered at t = t2
MODE -II
For t1 < t < t2
ic = 0
it1= io
vc = -VS
vTA = VS
vo = VS
id = 0
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Commutation Current
Load Current
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vc = -VS
appears across T1
Reverse Biases it
it1= 0
vo = VS + vc = 2 VS
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MODE - IV
MODE – III
At t = t3
ic = 0
iTA= 0
vc = vT1 = VS
vo = 0
For t2 < t < t3
C discharges through load
vc & VT1 Changes linearly
from –VS to zero at t2 + tc
vo starts falling towards
zero
ic = -io
iTA = io
TA turned-off naturally
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MODE – IV
DISADVANTAGES
For t3 < t < T
• A starting circuit is required
FD forward biased
Load current free wheels
ic = 0
iT1 = 0
ifd = io
iTA = 0
vT1 = VS
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• At the instant of commutation vo = 2 VS , so
FD is subjected to twice the supply voltage
• It can’t work at no load
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CURRENT COMMUTATED CHOPPER
ASSUMPTIONS
TYPE –A CHOPPER
• Load Current is Constant
• Thyristor and Diodes are Ideal
• RC is so large, treated as open circuit
during the commutation interval
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FOR 0 < t < t1
WORKING
• Working of this chopper starts only if the C
charged with marked polarities
• vo = VS
• The energy for current
commutation comes from capacitor
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• T1 fired at t = 0
• io = IO
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MODE - I
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MODE -I
To turn off T1,
TA Triggered at t = t1
• T1 remains uneffected
• Ic & vc varies
sinusoidally
ic = (Vs / woL) Sin wo t
• vo = VS
• io = IO
OSCILATTORY
CURRENT
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MODE - II
MODE - II
For t2 < t < t3
TA turned-off at t2
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EXCESS
CURRENT
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TA turned-off at t2
Oscillatory current
flow through T1
At t3, iT1 = 0 ; T1
turned-off
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MODE - III
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MODE -III
Ic > Io
D1 Conducts
Ic is at its peak value
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MODE - IV
MODE - V
Ic = Io , D1 off
FD forward biased
Load current free
wheels
During the time
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LOAD COMMUTATED CHOPPER
MERITS
TYPE –A CHOPPER
• Commutation is reliable
• Capacitor is always charged with correct
polarity
• TA is naturally commutated
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THYRISTORS – T1 to T4
CAPACITOR
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FREE WHEELING DIODE
• If T1,T2 – Main Thyristors, then T3,T4 & C
forms commutating elements
• If T3,T4 – Main Thyristors , then T1,T2 & C
forms commutating elements
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ASSUMPTIONS
WORKING
• Working of this chopper starts only if the C
charged with marked polarities
• Load Current is Constant
• Thyristor and Diodes are Ideal
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MODE - I
vc charges from VS to -VS
At t = 0
T1, T2 Triggered
T3,T4 Reverse biased
vT3 = vT4 = - VS
vo = VS + Vc
= 2 VS
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Load voltage falls to zero.
At t = t1
T3,T4 Forward biased
vT3 = vT4 = VS
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MODE - II
During (t2-t1)
vc = -VS
vo = 0
ic = 0
ifd = io
iT1 = iT2 = 0
vT3 = vT4 = VS
vT1 = vT2 = -ΔVS
At t = t1
C slightly
overcharged by ΔVS
FD Forward biased
Load current Freewheels
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MODE - III
During (t3-t2)
vc = VS
ic = -io
iT3 = iT4 = io
At t = t3
vT1 = vT2 = VS
T1,T2 Forward biased
At t = t2
T3, T4 Triggered
T1,T2 Reverse biased &
turned-off
vo = VS + Vc
= 2 VS
Capacitor charges to VS
vT1 = vT2 = -VS
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MERITS
After t3
FD Forward biased
• Capable of commutating any amount of load
Load current Freewheels
upto t4
current
• No commutating inductor is required
At t = t4
T1,T2 Triggered
Mode I repeats
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• As it can work in high frequencies (KHz),
Filtering requirements are minimal
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DEMERITS
• Peak load voltage is twice the supply voltage
• For high power applications – higher switching losses
at high operating frequencies – efficiency may low
• FD is subjected to twice the supply voltage
• Capacitor current which is alternating to be sensed to
turn on one pair of SCRs when the other pair is
commutated
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