This document discusses various methods for turning off SCRs (silicon controlled rectifiers), which are semiconductor devices that can only be switched from conduction to non-conduction by reducing the current below a threshold. It describes natural commutation, which occurs when the AC supply reaches zero crossings, and forced commutation methods used in DC circuits, including class A-E commutation that use external circuit components like inductors and capacitors to force the current to zero. The key forced commutation methods are classified based on the circuit arrangements and how zero current is achieved to turn off the SCR.

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THYRISTOR
Turn off/Commutation
methods
Dr. A. Ravi
Professor/EEE
FRANCIS XAVIER ENGINEERING COLLEGE
TIRUNELVELI
Tamil Nadu - India

2. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN OFF METHODS
What is Commutation
Process of Turn OFF
the SCR
The SCR can be brought back to the forward
blocking state from the forward conduction state
by reducing the anode or forward current below
the holding current level.

3. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN OFF METHODS
The term commutation means the transfer of
currents from one path to another.
conditions
The anode or
forward current of
SCR must be
reduced to zero or
below the level of
holding current
A sufficient
reverse voltage
must be applied
across the SCR to
regain its forward
blocking state.

4. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN OFF METHODS
The reverse voltage which causes to commutate the SCR.
Commutation voltage.
The commutation methods are classified into two major types.
 Forced Commutation
 Natural commutation

5. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN OFF METHODS
Natural commutation.
The source of commutation voltage is the supply source itself.
If the SCR is connected to an AC supply, at every
end of the positive half cycle the anode current
goes through the natural current zero.
and also immediately a reverse voltage is applied
across the SCR. These are the conditions to turn
OFF the SCR.

6. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN OFF METHODS
Natural commutation.

7. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN OFF METHODS
Forced commutation.
In DC Circuits, there is no natural current
zero to turn OFF the SCR.
There is external circuit called commutation
circuit which will make the forward current
forced to zero to turn OFF the SCR.
Commutation circuit inductors and capacitors are
available called Commutating components.

8. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN OFF METHODS
Forced commutation.
Forced commutation is classified into different types Based
on the arrangement of the commutating components and the
way by which the zero current achieved
This commutation is mainly used in chopper and
inverter circuits.
Class A Commutation
Class B Commutation
Class C Commutation
Class D Commutation
Class E Commutation

9. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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SCR TURN OFF METHODS
Class A Commutation
self commutation, or resonant commutation, or load commutation.
The source of commutation voltage is in the load.
This load must be an under damped R-L-C supplied with a DC supply so
that natural zero is obtained.
Load is Parallel with capacitor
Load is Series with capacitor

10. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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Class B Commutation self commutation
SCR is triggered, the current
flows in two directions, one is
through Vdc+-SCR-R- Vdc-
and another one is the
commutating current through L
and C components.
Once the SCR is turned ON, the capacitor is starts
discharging through C+ -L-SCR-C-. When the capacitor is
fully discharged, it starts charging with a reverse polarity.
Hence a reverse voltage applied across the SCR hence SCR
is OFF.

11. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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Class B Commutation self commutation
• Once the SCR is turned ON, the capacitor is
starts discharging through C+ – L – SCR – C-.
• When the capacitor is fully discharged, it starts
charging with a reverse polarity.
• Hence a reverse voltage applied across the SCR
which causes the commutating current IC to
oppose load current IL.
•When the commutating current Ic is higher than
the load current, the SCR will automatically turn
OFF and the capacitor charges with original
polarity.

12. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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Class C Commutation
The main SCR is to be commutated is connected in series
with the load and an additional or complementary SCR is
connected in parallel with main SCR.
Complimentary commutation
SCR TURN OFF METHODS

13. Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli
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Class C Commutation
Complimentary commutation
MODE 1
When the SCR1 or main SCR is
triggered, current starts flowing in two
directions, one path is Vdc+-R1-SCR1-
Vdc and another path is the charging
current Vdc+ -R- C+ -C- SCR1-Vdc- .
Therefore, the capacitor starts charging
up to the value of Vdc.
When the SCR2 is triggered, SCR2 is turned ON and voltage in the
capacitor a negative polarity is applied across the SCR1.
So this reverse voltage across the SCR1 immediately causes to turn
OFF the SCR1
MODE 2

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SCR TURN OFF METHODS
Class D Commutation Auxiliary commutation
When the supply voltage Vdc is
applied, both SCRs are in OFF
state and hence the capacitor
voltage is zero. In order to
charge the capacitor, SCR2
must be triggered first. So the
capacitor charges through the
path Vdc+ -C+ -C- – SCR2- R-
Vdc-.
When the capacitor is fully charged the SCR2 becomes turned OFF
because no current flow through the SCR2 when capacitor is
charged fully. If the SCR1 is triggered, the current flows in two
directions; one is the load current path Vdc+ – SCR1- R- Vdc- and
another one is commutation current path C+ – SCR1- L- D- C.

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SCR TURN OFF METHODS
Class D Commutation Auxiliary commutation
As soon as the capacitor completely discharges, its polarities
will be reversed but due to the presence of diode the reverse
discharge is not possible. When the SCR2 is triggered
capacitor starts discharging through C+ – SCR2- SCR1- C-.
When this discharging current is more than the load current
the SCR1 becomes turned OFF.
Again, the capacitor starts charging through the SCR2 to a
supply voltage E and then the SCR2 is turned OFF.
Therefore, both SCRs are turned OFF
Used in Inverter and Chopper
Circuits

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SCR TURN OFF METHODS
Class E Commutation External pulse commutation
External pulse source is used to produce the
reverse voltage across the SCR
Dr.A.Ravi, Francis Xavier Engineering College,Tirunelveli

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TURN-OFF-METHODS/
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Reference

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