Power Electronics
By: Dr. Shahram Javadi
Department of Electrical Engineering
Sh.javadi@iauctb.ac.ir
1
By: Dr. Shahram Javadi
Syllabus Outline
1.
2.
3.
4.
5.
6.
Introduction To Power Electronic
Power Diodes
Diode Rectifiers
Thyristors
Control Rectifiers
AC Voltage Controllers
7. Thyristor Commutation Techniques
8. Power Transistors
9. DC to DC Choppers
10. DC to AC Inverters
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By: Dr. Shahram Javadi
Introduction
 Commutation
 Process of turning off a conducting thyristor.
 Current Commutation
 Voltage Commutation
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By: Dr. Shahram Javadi
Methods of Commutation
 Natural Commutation
 Forced Commutation
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By: Dr. Shahram Javadi
Natural Commutation
 Occurs in AC circuits
T
+
vs
~

R
 vo

5
By: Dr. Shahram Javadi
Sinusoidal
Supply voltage vs

0
3
2
t
Gate Pulse

t

Load voltage vo
Turn off
occurs here

0


3
2
Voltage across SCR
By: Dr. Shahram Javadi
t
tc
t
6
Natural Commutation
 Natural Commutation of Thyristors
takes place in
 AC voltage controllers.
 Phase controlled rectifiers.
 Cyclo converters.
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By: Dr. Shahram Javadi
Forced Commutation
 Applied to dc circuits
 Commutation achieved by reverse
biasing the SCR or by reducing the SCR
current below holding current value.
 Commutating elements such as
inductance and capacitance are used
for commutation purpose.
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By: Dr. Shahram Javadi
Methods of Forced Commutation
1) Self commutation.
2) Impulse commutation.
3) Resonant pulse commutation.
4) Complementary commutation.
5) External pulse commutation.
6) Load Side Commutation.
7) Line Side Commutation.
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By: Dr. Shahram Javadi
Forced Commutation
Forced Commutation is applied to:
 Choppers.
 Inverters.
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By: Dr. Shahram Javadi
1) Self Commutation
Self Commutation
Or
Load Commutation
Or
Class A Commutation
(Commutation By Resonating The Load)
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By: Dr. Shahram Javadi
1) Self Commutation
 Circuit is under damped by including suitable
values of L & C in series with load.
 Oscillating current flows.
 SCR is turned off when current is zero.
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By: Dr. Shahram Javadi
1) Self Commutation
T
i
R
Load
L
Vc(0)
+ C
V
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By: Dr. Shahram Javadi
1) Self Commutation
Expression for Current
T
R I(S)
SL
1 VC(0)
CS
S
+ - +
C
V
S
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By: Dr. Shahram Javadi
1) Self Commutation
V  VC 0 
S
I S  
1
R  SL 
CS
CS V  VC 0 
I S  
S

2
RCS  S LC  1
C V  VC 0 
 2
R 1 
LC  S  S 

L LC 
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By: Dr. Shahram Javadi
1) Self Commutation
V  VC  0 
L
I S  
R 1
2
S S 
L LC
V  VC  0  
L
I S  
2
2
R 1  R   R 
2
S S 
   
L LC  2 L   2 L 
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By: Dr. Shahram Javadi
1) Self Commutation
V  V  0  
C
I S  
L

R 
1  R 

 
S 
 
2 L   LC  2 L 


2
2



2

A
S   
2
Where,
A
V  VC  0  
L
R
, 
,
2L

1  R 
 
LC  2 L 
2
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By: Dr. Shahram Javadi
2
1) Self Commutation
 is called the natural frequency
A

I S  
2
 S   2
Taking inverse Laplace transforms
A  t
i  t   e sin  t

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By: Dr. Shahram Javadi
1) Self Commutation
 Expression for current
V  VC  0  2 RL t
i t  
e sin  t
L
Peak value of current
V  V  0


C
L
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By: Dr. Shahram Javadi
1) Self Commutation
Expression For Voltage Across Capacitor At The Time Of
Turn Off
T
i
R
Load
L
Vc(0)
+ C
V
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By: Dr. Shahram Javadi
1) Self Commutation
Applying KVL to figure
vc t   V  vR  VL
di
vc t   V  iR  L
dt
Substituting for i,
vc t   V  R
A

e
 t
d  A  t

sin t  L  e sin t 

dt 
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By: Dr. Shahram Javadi
1) Self Commutation
vc t   V  R
vc t   V 
vc t   V 
vc t   V 
A

A

A

A

e
 t
sin t  L
A

 t
 t
e

cos

t


e
sin t 

e t  R sin t  L cos t  L sin t 
e
e
 t
 t

R

 R sin t  L cos t  L 2 L sin t 
R

 2 sin t  L cos t 
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By: Dr. Shahram Javadi
1) Self Commutation
Substituting for A,
vc  t 
V  V  0 

V 
e
C
 t
L
V  V  0 

V 
e
R

 2 sin  t   L cos  t 
R

vc  t 
sin

t


cos

t
 2 L


SCR turns off when current goes to zero.
C
 t
i.e., at  t  
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By: Dr. Shahram Javadi
1) Self Commutation
Therefore at turn off
vc t   V 
V  VC 0

 
e
vc t   V  V  VC 0 e

vc t   V  V  VC 0 e

0  cos 
 

 R
2 L
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By: Dr. Shahram Javadi
1) Self Commutation
Note: For effective commutation
the circuit should be under damped.
2
1
 R 
That is   
LC
 2L 
With R = 0, and the capacitor initially uncharged
that is VC  0   0
V
t
i t  
sin
L
LC
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By: Dr. Shahram Javadi
1) Self Commutation
But  
1
LC
V
t
C
t
i t  
LC sin
V
sin
L
L
LC
LC
and capacitor voltage at turn off is equal to 2V
Fig. shows the waveforms for the above conditions.
Once the SCR turns off voltage across it is
negative voltage.
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By: Dr. Shahram Javadi
1) Self Commutation
Current i
0
/2
t

2V
Capacitor voltage
V

Conduction time of SCR 

t
Gate pulse
t
t
V
Voltage across SCR
By: Dr. Shahram Javadi
27
2) Impulse Commutation
IL
T1
T3
V
VC(O)
L

+
C
T2
FWD
L
O
A
D
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By: Dr. Shahram Javadi
2) Impulse Commutation
 ‘C’ charged to a voltage VC(O) with polarity
as shown.
 T1 is conducting and carries load current IL.
 To turn off T1, T2 is fired.
 Capacitor voltage reverse biases T1 and
turns it off.
 ‘C’ Charges through load.
 T2 self commutates.
 To reverse capacitor voltage T3 is turned ON.
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By: Dr. Shahram Javadi
2) Impulse Commutation
Gate pulse
of T3
Gate pulse
of T2
VS
Gate pulse
of T1
t
Capacitor
voltage
t
VC
tC
Voltage across T1
t
-VC
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By: Dr. Shahram Javadi
2) Impulse Commutation
Expression For Circuit Turn Off Time tC
tc depends on I L & is given by the expression
tc
1
VC   I L dt
C0
(assuming the load current to be constant)
I L tc
VC 
C
VC C
tc 
seconds
IL
For proper commutation tc  tq , turn off time of T1
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By: Dr. Shahram Javadi
2) Impulse Commutation
 T1 is turned off by applying a negative
voltage across its terminals. Hence this
is voltage commutation.
 tC depends on load current. For higher
load currents tC is small. This is a
disadvantage of this circuit.
 When T2 is fired, voltage across the load
is V+VC; hence the current through load
shoots up and then decays as the
capacitor starts charging.
55
By: Dr. Shahram Javadi
2) Impulse Commutation
An Alternative Circuit For Impulse Commutation
i
+
T1
VC(O)
IT 1
T2
V
_
C
D
L
IL
RL
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By: Dr. Shahram Javadi
2) Impulse Commutation
 Initially ‘C’ is charged to VC with top
plate positive.
 T1 is fired, load current IL flows.
 ‘C’ discharges at the same time &
reverses its polarity.
 ‘D’ ensures bottom plate remains
positive.
 To turn off T1, T2 is fired.
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By: Dr. Shahram Javadi
2) Impulse Commutation
Gate pulse
of T2
Gate pulse
of T1
t
VC
Capacitor
voltage
t
-V
tC
This is due to i
IT 1
IL
Current through SCR
V
RL
t
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By: Dr. Shahram Javadi
2) Impulse Commutation
2V
RL
IL
Load current
t
V
Voltage across T1
t
tC
59
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
L
T
i
a
b
C
IL
V
Load
FWD
74
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
 Series LC circuit connected across
thyristor ‘T’.
 Initially ‘C’ is charged to ‘V’ volts with
plate ‘a’ as positive.
 Current in LC oscillates when SCR is
ON.
 ‘T’ turns off when capacitor
discharges through thyristor in a
direction opposite to IL
75
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
Gate pulse
of SCR
t1
V
t

Capacitor voltage
vab
t
tC
Ip
i
IL


t
t
76
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
ISCR
Thyristor Current
t
Voltage across
SCR
t
77
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
Expression For tC , the Circuit Turn Off Time
Assume that at the time of turn off of the SCR
the capacitor voltage vab  V and a constant
load current I L flows.
tc is the time taken for the capacitor
voltage to reach 0 volts from  V volts and
it is derived as follows
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By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
tc
I L tc
1
V   I L dt 
C0
C
VC
tc 
seconds
IL
For proper commutation tc should be greater than tq .
Magnitude of I p the peak value of i should be
greater than the load current I L .
Expression for i is derived as follows
79
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
The LC Circuit During The Commutation Period
I(S)
SL
L
T
i
C
LC Circuit
+
VC(0)
 =V
T
1
CS
+
-
V
S
Transformed Circuit
80
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
V 
  CS
s
I S  
 2
1
S
LC

1
SL 
Cs
V
S
VC
V
1
I S  
 
1
 2 1  L
2
S 
LC  S 

LC

LC 
81
By: Dr. Shahram Javadi
3) Resonant Pulse
 1 


V  LC 
I S   

L S2  1 

LC 
Commutation
1
1 

LC 
 1 


C  LC 
I S   V

L S2  1
LC
Taking inverse LT
C
i t   V
sin t
L
82
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
1
Where  
LC
V
or
i t  
sin  t  I p sin  t
L

C
Ip V
amps
L
83
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
Expression For Conduction Time Of SCR
Conduction time of SCR

  t

 IL 
sin  


I

p 

 
1


84
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
Alternate Circuit For Resonant Pulse Commutation
C
ab
T1
iC(t)
L
iC(t)
IL
T2
 +
VC(0)
V
T3
FWD
L
O
A
D
85
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
 Initially C charged with polarity as
shown in figure.
 T1 is conducting & IL is constant.
 To turn off T1, T2 is fired.
 iC(t) flows opposite to IL & T1 turns off at
iC(t) = IL
86
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
ic  t   I p sin  t
Where I p  VC  0 
C
& and the capacitor voltage
L
1
vc  t    iC  t .dt
C
1
C
vc  t    VC  0 
sin  t.dt
C
L
vc  t   VC  0  cos  t
87
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
Instant at
which the thyristor turns off is at t1
&
t1
I L  I p sin
LC
I p  VC  0 
C
L
88
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
 IL
L
or
t1  LC sin 
 VC  0  C
& the corresponding capacitor voltage is
1



vc  t1   V1  VC  0  cos  t1
89
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
Expression For tC
Assuming a constant load
current I L which charges the capacitor
CV1
tc 
seconds
IL
Normally
V1  VC  0 
For reliable commutation tc  tq
tC depends on I L & becomes smaller for
higher values of I L
90
By: Dr. Shahram Javadi
Current iC(t)
t
V
Capacitor
voltage vab
t
t1
V1
tC
VC(0)
By: Dr. Shahram Javadi
91
3) Resonant Pulse Commutation
Resonant Pulse Commutation
With Accelerating Diode
92
By: Dr. Shahram Javadi
D2
C
T1
L
iC(t)
IL
iC(t)
T2
+
VC(0)
V
T3
FWD
L
O
A
D
93
By: Dr. Shahram Javadi
3) Resonant Pulse Commutation
 Diode D2 connected as shown to
accelerate discharge.
 T2 turned on to turn off T1.
 Once T1 is off at t1. iC(t) flows through
D2 until current reduces to IL at time t2.
 From t = t2 , ‘C’ charges through load,
T2 self commutates.
 But thyristor recovery process low
hence longer reverse bias time.
94
By: Dr. Shahram Javadi
3) Resonant
Pulse Commutation
iC
IL
0
t
VC
0
V1
VC(O)
t1
t2
t
tC
95
By: Dr. Shahram Javadi
4) Complementary Commutation
IL
R1
R2
ab
iC
V
C
T1
T2
118
By: Dr. Shahram Javadi
4) Complementary Commutation
 Two SCRs are used, turning ON one SCR
turns off the other.
 T1 is fired, IL flows through R1.
 At same time ‘C’ charges towards ‘V’
through R2 with plate ‘b’ positive.
 To turn off T1, T2 is fired resulting in
capacitor voltage reverse biasing T1 and
turning it off.
 When T2 is fired current through load
shoots up as voltage across load is V+VC
119
By: Dr. Shahram Javadi
4) Complementary Commutation
Gate pulse
of T1
IL
Gate pulse
of T2

t
2V
R1
V
Current through R 1
V
R1
t
2V
R2
Current through T 1
V
R1
t
Current through T2
2V
R1
V
R2
t
120
By: Dr. Shahram Javadi
4) Complementary Commutation
Current
through R2
t
V
Voltage across
capacitor vab
t
-V
tC
tC
Voltage across T1
t
tC
121
By: Dr. Shahram Javadi
4) Complementary Commutation
Expression For Circuit Turn Off Time tC
vc  t   V f  Vi  V f  e t 
Where V f is the final voltage, Vi is the initial
voltage and  is the time constant.
At t  tc , vc  t   0, V f  V ,   R1C, & Vi  V

0  V   V  V  e
 tc
R1C
 V  2Ve
 tc
R1C
122
By: Dr. Shahram Javadi
4) Complementary Commutation

V  2Ve
 tc
R1C
; 0.5  e
 tc
R1C
Taking natural logarithms on both sides
tc
ln 0.5 
R1C
tc  0.693R1C
This time should be greater than the turn off time tq of T1
Similarly when T2 is commutated
tc  0.693R2C
And this time should be greater than tq of T2
Usually R1  R2  R
123
By: Dr. Shahram Javadi
5)External Pulse Commutation
T1
VS
T2
RL
L
2VAUX
+

C
T3
VAUX
131
By: Dr. Shahram Javadi
5)External Pulse Commutation
 T1 is conducting & RL is connected
across supply.
 T3 is fired & ‘C’ is charged to 2VAUX with
upper plate positive.
 T3 is self commutated.
 To turn off T1, T2 is fired.
 T2 ON results in a reverse voltage VS –
2VAUX appearing across T1.
132
By: Dr. Shahram Javadi
6) Load Side Commutation
 In load side commutation the
discharging and recharging of capacitor
takes place through the load. Hence to
test the commutation circuit the load
has to be connected. Examples of load
side commutation are Resonant Pulse
Commutation and Impulse
Commutation.
133
By: Dr. Shahram Javadi
7) Line Side Commutation
 In this type of commutation the
discharging and recharging of capacitor
takes place through the supply.
134
By: Dr. Shahram Javadi
7) Line Side Commutation
L
T1
+
IL
+
T3
VS
_C
FWD
Lr
L
O
A
D
T2
_
135
By: Dr. Shahram Javadi
7) Line Side Commutation
 Thyristor T2 is fired to charge the
capacitor ‘C’. When ‘C’ charges to a
voltage of 2V, T2 is self commutated.
To reverse the voltage of capacitor to
-2V, thyristor T3 is fired and T3
commutates by itself. Assuming that
T1 is conducting and carries a load
current IL thyristor T2 is fired to turn
off T1.
136
By: Dr. Shahram Javadi
7) Line Side Commutation
 The turning ON of T2 will result in
forward biasing the diode (FWD) and
applying a reverse voltage of 2V
acrossT1. This turns off T1, thus the
discharging and recharging of
capacitor is done through the supply
and the commutation circuit can be
tested without load.
137
By: Dr. Shahram Javadi