4. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Thyristor Turn-off Methods
Thyristor can be turned-off by the following ways:
Natural Commutation
Load Commutation
Forced Commutation
Gate Turn-off
Other commutation methods:
Opening the line switch
Increasing the load impedance
Shunting the load current in parallel to
thyristor that is short circuiting the device
The turn-off process of SCR is known as
commutation
5. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Line or Phase Commutation
This is also known as natural commutation or source
commutation or class F commutation.
In a.c circuit due to natural cycling of supply voltage
the current necessarily falls to zero at some point in the
cycle.
From that instant up to some time the forward voltage
on thyristor will be negative.
During this period, if the gate current is zero and the
positive supply voltage is not re-applied, until the turn
off time is elapsed, the thyristor will turn off.
Line commutated rectifiers and inverters use natural
commutation for turning off the tyristor. Power Electronics :
Devices and Circuits by:V. Jagannathan
6. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Load Commutation:
This type of commutation depends on the nature of load
and can be applied if the load is such that the current
through it falls to zero at some time after the thyristor is
turned on.
This type of commutation is applied to some d.c circuits
where as in a.c circuits’ line commutation is preferred.
When thyristor is turned on, because of
oscillatory action, the thyristor will turn
off.
7. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Forced Commutation
It is not always convenient or economical to bring load
commutation of a typical RL load by connecting a series
capacitor large enough to carry the load current.
Under such circumstances forced commutation is
employed capacitors are mainly used in forced
commutation circuits.
If the power supply is d.c. (there is no natural current
zero for thyristor), or if the conducting semiconductor is
to be turned off at the non-zero part of an a.c. cycle, then it
must be commutated by forcing the current through the
device to zero, and this is called forced commutation.
Choppers and inverters
Forced commutation methods are classified as A B C D
and E.
8. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
When switch Sw, is closed the load current, flowing
via the thyristor, is diverted through the bypass
circuit, which also applies a reverse voltage across
the device, turning it off.
Often this bypass system consists of a capacitor,
which has been charged during a previous cycle to
the polarity shown.
9. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
(i) The time for which the thyristor is reverse biased must
exceed its turn-off time.
(ii) The rate at which the forward voltage is re-applied
across the device must be less than its dv/dt rating.
(iii)It is probable that the bypass circuit will need to be
reset again, so as to be able to apply the required
reverse voltage across the thyristor, if it is refired and
needs to be turned off.
For successful commutation several conditions must be
satisfied:
11. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Gate Turn-Off Thyristors (GTOs)
GTOs can be turned off
by applying a negative
gate current.
iA
vAK
reverse
breakdown
voltage
reverse
blocking
iA
vAK
Real Ideal
reverse
blocking
ON
forward
breakdown
voltage
OFF ON if positive
gate voltage applied OFF
forward
blocking
ON-state
Circuit Symbol:
ON OFF if negative
gate voltage applied
OFF-state
-
Vak
+ ia
G
K
A
15. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
TURN-OFF PROCESS OF THYRISTOR
When the thyristor has been triggered the gate loses
control.
A thyristor can be brought to non-conducting state if its
anode current is reduced below a value called
“Holding Current, IH” for certain time known as
“Turn off time”.
During this turn off process the internal recombination
and carrier sweep out remove the stored charges.
Both the transistors, of two transistor model, go from
saturation region to active region and the regenerative
connection of transistors causes turn off.
16. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Turn off time (tq)
Turn off time is time, which must elapse after forward
current through the thyristor has ceased, before
forward voltage of specified rate may be re-applied
without turn-on.
This is because even when the anode current falls to
zero, the current carriers are present at junction J2,
which will not block forward voltage unless sufficient
time is allowed to let them to recombine naturally.
Typical value of turn off time is between 3-100 μs.
18. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Classification commutation methods
The six distinct classes by which the SCR can be
turned off are:
Class A Commutation by a load resonance.
Class B Self commutated by an L-C circuit
Class C Charge capacitor switched by another load
carrying SCR
Class D Charge capacitor switched by an auxiliary
SCR
Class E An external pulse source for commutation
Class F AC line commutation
19. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Class A Commutation (Commutation by a
load resonance)
When SCR is triggered by applying gate pulse the
capacitor charges upto a voltage higher than V
(voltage boosting),
The current ISCR falls to zero and SCR is reverse
biased & therefore the SCR is turned off.
The overall circuit in this case
should be under damped so that
so that natural zero is obtained.
The capacitor supplies the
commutation energy.
20. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Class B or Self Commutation
(Self commutated by an L-C circuit)
LC circuit is across the SCR & not in series with the
load.
Initially the capacitor get charged to battery voltage V
with upper plate being positive.
In this method, the SCR is
turned off by the action of
resonating but the
commutation component LC
do not carry the load current.
Therefore L and C don’t
form resonant circuit with
the load.
21. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Class B or Self Commutation
(Self commutated by an L-C circuit)
Once the SCR is turned on, it conducts for a definite period,
and then it automatically turns off.
Due to resonance tendency, the capacitor again tends to
discharge & makes SCR reverse biased & turns it off.
When SCR is triggered, the
capacitor discharge through
SCR & L & capacitor is
charged in reverse direction.
25. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Class E Commutation
An external pulse source for commutation
This class of commutation circuit uses an external pulse
source to reverse bias the current carrying SCR and thereby
turns it off.
The width of the pulse has to be chosen such that the SCR
is reverse biased for a period greater than the turn-off time
of SCR.
34. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
The process of commutation of a thyristor involves :
The reduction of current through the conducting
thyristor to zero
the current may become zero either due to the load
current itself falling to zero
or by being transferred to an alternative path
consisting of another thyristor, a diode, a
capacitor, or a capacitor and inductor combination
Subjecting the thyristor to a reverse bias long enough
for it to regain forward voltage blocking capability.
35. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Class A Commutation (Commutation by a
load resonance)
The overall circuit in this case should be under damped so
that so that natural zero is obtained.
The capacitor supplies the commutation energy.
When SCR is triggered by applying gate pulse the capacitor
charges upto a voltage higher than V (voltage boosting),
The current ISCR falls to zero and SCR is reverse biased &
therefore the SCR is turned off.
The resonating circuit (L and C
component) in series with the load ,
SCR and power supply applies a
reverse voltage to SCR to turn it off.
The load may be in series or parallel
with L and C.
36. M
U
E
T
JAMSHORO
Department
of
Electrical
Engineering
Class C Commutation
Charge capacitor switched by another load
carrying SCR
Thyristor Th1 is fired it carries
load current iL as well as
capacitor charging current.
Plate y of capacitor becomes
almost to ground potential. If
energy stored in capacitor is
great enough so that when TH2
is fired, capacitor C can reverse
bias TH1 for longer period than
its turn off time the thyristor
TH1 turns off. This system is
repetitive. Switching one
thyristor on will switch the
other thyristor off.
