Turn-off Methods (Commutation) of SCR

Turn-off Methods (Commutation) of SCR

The Silicon Controlled Rectifier (SCR) is a unidirectional three-terminal device. When gate pulses are given to the SCR, it starts conducting. There are several methods to turn on (trigger) SCR. Once the SCR starts to conduct, it will not turn off even if we remove the gate pulses. So, there are various methods to turn-off SCR.

Once SCR comes in forward conducting mode, turn off methods are used to bring again in forward blocking mode. These methods are known as the commutation method of SCR.

The SCR can be turned off by reducing the anode current (forward current) below the holding current. The commutation means to transfer the path of current.

Hence, the commutation circuits are used to reduce the forward current to zero to turn-off SCR. There are two methods to turning off the SCR;

  • Natural Commutation
  • Forced Commutation

When the anode current is reduced to zero, it consists of excess charge carriers in different layers. These excess carriers must be recombined to regain the forward blocking mode. This recombination process accelerated by applying a reverse voltage across the SCR.

Natural Commutation

This method is also known as source commutation or line commutation. Because, in this method, the source of commutation voltage is a supply source itself.

If it is connected to an AC supply, the anode current goes zero naturally at every end of the positive half cycle. Now immediately reverse the voltage across the SCR and turn-off SCR naturally.

This type of commutation is possible in the application where the source of supply is AC source like controlled rectifier, cyclo-converter, and AC voltage regulators.

The below figure shows the circuit diagram and waveform of natural commutation.

Natural Commutation
Natural Commutation

Forced Commutation

The current cannot go naturally zero in the application of DC supply. Hence, in the case of DC, the anode current is forced to zero. The external circuit used to force current is known as a forced commutation circuit.

This commutating circuit consists of inductors and capacitors. The components used in the commutation circuit is known as commutating components. The commutating circuit is used to apply a reverse voltage across SCR the immediately reduce current to zero.

This type of commutation circuit is used in the application where the device is dealing with DC supply like chopper and inverter.

The commutation circuit is classified into different types;

  • Class-A Commutation (Self or Load Commutation)
  • Class-B Commutation (Resonant-Pulse Commutation)
  • Class-C Commutation (Complementary Commutation)
  • Class-D Commutation (Impulse Commutation)
  • Class-E Commutation (External Pulse Commutation)

Class-A Commutation (Self or Load Commutation)

In this type of commutation, the inductor, capacitor, and resistor are used to make a commutation circuit. This circuit is a second-order underdamped circuit. The circuit diagram of the class-A commutation circuit is as shown in the below figure.

Class-A Commutation
Class-A Commutation

When the SCR is connected in forward, it won’t start conducting unless the gate current is not given. The value of inductor, capacitor, and load resistance is selected in such a way that the circuit behaves as an underdamped circuit.

Once it starts conducting, the current will flow through the inductor. And this current charge the capacitor.

When the capacitor is fully charged to peak value or equal to an input voltage, the inductor polarity gets reversed and SCR becomes in reverse bias. And capacitor discharges through the load resistance.

This method is reliable and simple. The high values of L and C are preferred for high-frequency operation above 1000 Hz.

Class-B Commutation (Resonant-Pulse Commutation)

In this type of commutation, the LC resonant circuit is connected in antiparallel. This is a very similar circuit compared to the class-A commutation where the LC circuit is connected in series.

The LC resonant circuit is not directly connected with a load. It is connected with the SCR. Hence, the full load current will not pass through the SCR. The circuit diagram of Resonant-Pulse Commutation is as shown in the below figure.

Class-B Commutation
Class-B Commutation

When SCR is turned on, the current will divide into two parts. In one part, a current will flow through the SCR, and in the second part, the current will flow through the LC circuit.

When the input voltage is applied to the circuit, the capacitor starts charging. The SCR remains in reverse bias as the gate pulses are not given. Once the SCR starts conducting, the current will divide into two parts.

Now, the constant current will flow through the load resistance. And sinusoidal current will flow through the LC circuit and the capacitor charge with reverse polarity. Hence, a reverse voltage appears across the thyristor, which opposes the anode current. And when the anode current is less than the holding current the SCR will turn off.

In this process, the SCR will turn on for some time and automatically turn off after some time. The frequency of the ON/OFF process depends on the value of L and C. This is a continuous process and mostly used in the chopper.

Class-C Commutation (Complementary Commutation)

In this type of commutation circuit, two SCR is used. One SCR is the main SCR that is connected in series with the load. Second SCR is known as complementary or auxiliary SCR. Hence, this commutation method is also known as complementary commutation. The circuit diagram of this commutation circuit is as shown in the below figure.

Class-C Commutation
Class-C Commutation

At starting, both SCR are in non-conducting mode. When the gate pulse is given to SCR1, it starts conducting. In this condition, the current will flow in two directions. One is through E – RL – T1 – E and second is through E – R2 – C – T1 – E. Hence, the capacitor starts charging and polarity of a capacitor is as shown in the figure.

Now, the gate pulses are given to the SCR2. It turns on and negative polarity current flow through the SCR1 and it turned off.

After that, the capacitor starts charging with opposite polarity and current flow through E – R2 – C – T2 – E. And SCR2 starts conducting.

Now if again, the SCR1 triggers, the capacitor discharge through the SCR2. And SCR2 turned off. This is a very reliable method and can work on a higher frequency of up to 1000hz.

Class-D Commutation (Impulse Commutation)

Similar to class-c commutation, in this type of commutation also, two Thyristors T1 and T2 are used. One is the main SCR and the second is auxiliary SCR. The circuit diagram of impulse commutation is as shown in the below figure.

Class-D Commutation
Class-D Commutation

As shown in the above figure, the SCR1 or main SCR and Load resistance make power circuit and Diode, inductor, and SCR2 make an auxiliary circuit. At starting, both SCR are in off position. Hence, the voltage across the capacitor is zero. First SCR2 will trigger. And the capacitor will charge through the path E – C – T2 – RL – E.

Once the capacitor is charged, no current flow through the SCR2 and it turn off. If the gate pulse is given to the SCR1, the current will flow in direction of E – SCR1 – RL – E.

During this period, the capacitor will discharge through C – SCR1 – L – D – C. When the capacitor is discharged, it starts charging in the reverse direction but due to the presence of diode this will not happen.

Now, the SCR2 is triggered and starts discharging through C – SCR2 – SCR1 – C. When discharging current is more than the load current the SCR1 will turn off.

Class-E Commutation (External Pulse Commutation)

In this type of commutation, the external pulse source is used to turn-off SCR. Hence, this method is also known as external pulse commutation. The circuit diagram of external pulse commutation is as shown in the below figure.

Class-E Commutation
Class-E Commutation

As shown in the above figure, the circuit consists of a pulse transformer. In this method, the pulse transformer is used to turn-off SCR and a capacitor is used for dv/dt protection.

When the thyristor triggers the load current will flow through the pulse transformer and load resistance. The load current is passed through the primary winding of the pulse transformer and the emf induced in the secondary winding.

This induced emf is given to the thyristor that reverses the polarity and anode current start reducing. When the anode current is less than the holding current, turn-off SCR.

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