Power Electronics

4 Year Electrical
th

Engineering (Power &

Control)
Lecture No. 4
 Thyristor
 Thyristors are usually three-terminal devices
with four layers of alternating p- and n-type
material in their main power handling
section.
 The three terminals are:
 Anode
 Cathode
 Gate
 A Anode
A Anode

p
J1 Anode Junction
G n
p J2 Block Junction
G Gate J3 Gate Junction
Gate n
K Cathode
K Cathode
 iT

Forward volt-drop
(conducting

Forward
Latching Gate break over
current triggered voltage
Reverse
breakdown Holding IL
voltage current IH
VAK
VBO
Forward
leakage
Reverse
current
leakage
current

Thyristor v-i characteristic

 Thyristor Commutation
Techniques
 A thyristor is normally switched on by
applying a pulse of gate signal.
 Once the thyristor is turned on and the output
requirements are satisfied, it is usually
necessary to turn it off.
 Commutation is the process of turning off a
thyristor, ant it normally causes transfer of
current flow to other parts of circuit.
 A commutation circuit normally uses
additional components to accomplish the
turn-off.
 With the development of thyristors, many
commutation circuits have been developed
and the objective of all the circuits is to
reduce the turn-off process of the
thyristors.
 There are many techniques to commutate a
thyristor. However, thses can be broadly
classified into two types:
Natural commutation
Forced commutation
 Natural Commutation
 If the source voltage is AC, the thyristor
current goes through a natural zero, and a
reverse voltage appears across the thyristor.
 The device is then automatically turned of due
to the natural behavior of the source voltage.
 This is known as natural commutation or line
commutation.
 This type of commutation is applied in ac
voltage controller, phase-controlled rectifiers
and cycloconverters.
 T1 io
+ +

Vs=Vmsinwt R Vo

- -

Circuit diagram and waveform of natural

commutation technique
 100
0
Vs

-100
0 1 2 3 4 5 6
wt

100
Load voltage
50
vo &io

Load current
0
0 1 2 3 4 5 6
wt
 Forced commutation
 In some thyristor circuits, the input voltage is
dc and the forward current of the thyristor is
forced to zero by an additional circuitry called
commutation circuit to turn off the thyristor.

 This technique is called forced commutation

and normally applied in dc-dc converter and
dc-ac converter.
 The forced commutation of a thyristor can
be achieved by seven ways and can be
classified as:
Self commutation
Impulse commutation
Resonant pulse commutation
Complementary commutation
External pulse commutation
Load-side commutation
Line commutation
 This classification of forced commutations
is based on the arrangement of the
commutation circuit components and the
manner in which the current of a thyristor
is forced to be zero.

The commutation circuit normally consists

of a capacitor, an inductor and one or more
thyristors and diode.
 Self commutation
150

i 100

+ +

Current i (A)
50

T1
L VL 0
0 1 2 3
Time (s)
4 5
-5
x 10

Vs - 500
+ 400

C VC 300

200

- Capacitor voltage vc (V)

- 100

0
0 1 2 3 4 5
Time (s) -5
x 10
 When thyristor T1 is switched on, the capacitor
charging current i is given by
di 1
Vs  L  idt  vc t 0
dt C
The initial conditions i(t=0)=0 and vc(t=0)=0, the above
equation can be solved for the capacitor current, i, as:
C .
i t  Vs
.

sin t  I p sin t
L
The voltage, vc, across the capacitor can be derived
as:
t
1
vc t   idt Vs 1  cos t 
C 0
 200

100

Current (A)
T1 0
L 0 1 2 3 4 5 6
i wt
+ 200
-C
Vo VC
+ - 0
Voltage (V)

-200
0 1 2 3 4 5 6
wt
Figure above shows a typical circuit where the
capacitor has an initial voltage of –Vo. When the
thyristor T1 is fired, the current that will flow through
the circuit is given by.
di 1
L  idt  vc t 0  0
dt C
With initial voltage vc(t=0)=-Vo and i(t=0)=0, then the
above equation the capacitor current as
C
i t  Vo sin t  I p sin t
L
t
1
And the capacitor voltage as vc t   idt  V o cos t
C 0
 Impulse Commutation
T1
Im
+
+
-
Vo C
Vc L
T3 + O
Vs Dm
- A
L D
T2
-
 discharging time will depend on the load
current and assuming a constant load
current of Im, toff is given by

t off
1 I mtoff CVo
Vo 
C I
0
m dt 
C
, therefore, toff 
Im
 Resonant pulse
commutation
T1
Im
+
+ Vc - T2 L
C L O
Dm
A
Vs - Vo + i(t) D

T3

-
 When commutation thyristor T2 is fired, a
resonant circuit is formed by L, C, T1 and T2.
The resonant current can be derived as
C
it  Vo sin t I p sin t
L
And the capacitor voltage is
t
1
vc t   idt  Vo cos t
C0
Due to the resonant current, the forward
current of thyristor T1 is reduced to zero at
t=t1, when the resonant current equal the
load current Im. The time t1 must satisfy the
condition i(t=t1)=Im and is found as
 Im L 
t1  LC sin 1
 
V C 
 o 
The corresponding value of the capacitor
voltage is

vc t t1   Vo cos t1