Thyristor Commutation Techniques in Power Electronics

Most of the converter equipment and switch-mode power supplies use power electronics components like thyristors, MOSFET and other power semiconductor devices for high frequency switching operations at high-power ratings. Consider the thyristors that we use very frequently as bistable switches in several applications. These thyristors use switches needed to be switched on and off. For switching on the thyristors, there are some thyristor turn on methods called as thyristor triggering methods. Similarly, for switching off thyristors, there are methods called as thyristors commutation techniques. Before discussing about thyristor commutation techniques, we must know something about the thyristors basics such as thyristor, thyristor operation, different types of thyristors and thyristor turn on methods.


Two to four lead semiconductor devices consisting of four layers of alternating N and P-type materials are called as thyristors. These are generally used as bi-stable switches which will conduct only when the gate terminal of thyristor is triggered. Thryistor is also called as silicon controlled rectifier or SCR.


Thyristor Commutation Techniques

As we have studied above, a thyristor can be turned on by triggering gate terminal with low voltage short duration pulse. But after turning on, it will conduct continuous until the thyristor is reverse biased or the load current falls to zero. This continuous conduction of thyristors causes problems in some applications. The process used for turning off a thyristor is called as commutation. By the commutation process, the thyristor operating mode is changed from forward conducting mode to forward blocking mode. So, the thyristor commutation methods or thyristor commutation techniques are used to turn off.

The commutation techniques of thyristors are classified into two types:

  • Natural Commutation
  • Forced Commutation

Natural Commutation

Generally, if we consider AC supply, the current will flow through the zero crossing line while going from positive peak to negative peak. Thus, a reverse voltage will appear across the device simultaneously, which will turn off the thyristor immediately. This process is called as natural commutation as thyristor is turned off naturally without using any external components or circuit or supply for commutation purpose.

Natural Commutation
Natural Commutation

Natural commutation can be observed in AC voltage controllers, phase controlled rectifiers and cycloconverters.


Forced Commutation

The thyristor can be turned off by reverse biasing the SCR or by using active or passive components. Thyristor current can be reduced to a value below the value of holding current. Since, the thyristor is turned off forcibly it is termed as a forced commutation process. The basic electronics and electrical components such as inductance and capacitance are used as commutating elements for commutation purpose.

Forced commutation can be observed while using DC supply; hence it is also called as DC commutation. The external circuit used for forced commutation process is called as commutation circuit and the elements used in this circuit are called as commutating elements.

Classification of Forced Commutation Methods

The forced commutation can be classified into different methods as follows:

  • Class A: Self commutated by a resonating load
  • Class B: Self commutated by an LC circuit
  • Class C: Cor L-C switched by another load carrying SCR
  • Class D: C or L-C switched by an auxiliary SCR
  • Class E: An external pulse source for commutation
  • Class F: AC line commutation

Class A: Self Commutated by a Resonating Load

Class A is one of frequently used thyristor commutation techniques. If  thyristor is triggered or turned on, then anode current will flow by charging capacitor C with dot as positive. The second order under-damped circuit is formed by the inductor or AC resistor, capacitor and resistor. If the current builds up through SCR and completes the half cycle, then the inductor current will flow through the SCR in the reverse direction which will turn off  thyristor.

Class A-Commutation
Class A-Commutation

After the thyristor commutation or turning off the thyristor, the capacitor will start discharging from its peak value through the resistor is an exponential manner. The thyristor will be in reverse bias condition until the capacitor voltage returns to the supply voltage level.

Class B: Self Commutated by an L-C Circuit

The major difference between the class A and class B thyristor commutation techniques is that the LC is connected in series with thyristor in class A, whereas in parallel with thyristor in class B. Before triggering on the SCR, the capacitor is charged up (dot indicates positive). If the SCR is triggered or given triggering pulse, then the resulting current has two components. The constant load current flowing through the R-L load is ensured by the large reactance connected in series with the load which is clamped with freewheeling diode. If sinusoidal current flows through the resonant L-C circuit, then the capacitor C is charged up with dot as negative at the end of the half cycle.

Class B-Commutation
Class B-Commutation

The total current flowing through the SCR becomes zero with the reverse current flowing through the SCR opposing the load current for a small a small fraction of the negative swing. If the resonant circuit current or reverse current becomes just greater than the load current, then the SCR will be turned OFF.

Class C: C or L-C Switched by another Load Carrying SCR

In the above thyristor commutation techniques we observed only one SCR but in these class C commutation techniques of thyristor there will be two SCRs. One SCR is considered as main thyristor and the other as auxiliary thyristor. In this classification both may act as main SCRs carrying load current and they can be designed with four SCRs with load across the capacitor by using a current source for supplying an integral converter.

Class C-Commutation
Class C-Commutation

If the thyristor T2 is triggered, then the capacitor will be charged up. If the thyristor T1 is triggered, then the capacitor will discharge and this discharge current of C will oppose the flow of load current in T2 as the capacitor is switched across T2 via T1.

Class D: L-C or C Switched by an Auxiliary SCR

The class C and class D thyristor commutation techniques can be differentiated with the load current in class D: only one of the SCR’s will carry the load current while the other acts as an auxiliary thyristor whereas in class C both SCRs will carry load current. The auxiliary thyristor consists of resistor in its anode which is having resistance of approximately ten times the load resistance.

Class D-Commutation
Class D-Commutation

By triggering the Ta (auxiliary thyristor) the capacitor is charged up to supply voltage and then the Ta will turn OFF. The extra voltage if any, due to substantial inductance in the input lines will be discharged through the diode-inductor-load circuit.

If the Tm (main thyristor) is triggered, then the current will flow in two paths: commutating current will flow through the C-Tm-L-D path and load current will flow through the load. If the charge on the capacitor is reversed and held at that level using the diode and if Ta is re-triggered, then the voltage across the capacitor will appear across the Tm via Ta. Thus, the main thyristor Tm will be turned off.

Class E: External Pulse Source for Commutation

For the class E thyristor commutation techniques, a transformer which can not saturate (as it is having a sufficient iron and air gap) and capable to carry the load current with small voltage drop compared with the supply voltage. If the thyristor T is triggered, then the current will flow through the load and pulse transformer.

Class E-Commutation
Class E-Commutation

An external pulse generator is used to generate a positive pulse which is supplied to the cathode of the thyristor through pulse transformer. The capacitor C is charged to around 1v and it is considered to have zero impedance for the turn off pulse duration. The voltage across the thyristor is reversed by the pulse from the electrical transformer which supplies the reverse recovery current, and for the required turn off time it holds the negative voltage.

Class F: AC Line Commutated

In class F thyristor commutation techniques, an alternating voltage is used for supply and, during the positive half cycle of this supply, load current will flow. If the load is highly inductive, then the current will remain until the energy stored in the inductive load is dissipated. During the negative half cycle as the load current becomes zero, then thyristor will turn off. If voltage exists for a period of rated turn off time of the device, then the negative polarity of the voltage across the outgoing thyristor will turn it off.

Class F-Commutation
Class F-Commutation

Here, the duration of the half cycle must be greater than the turn off time of thyristor. This commutation process is similar to the concept of three phase converter. Let us consider, primarily T1 and T11 are conducting with the triggering angle of the converter, which is equal to 60 degrees, and is operating in continuous conduction mode with highly inductive load.

If the thyristors T2 and T22 are triggered, then instantaneously the current through the incoming devices will not rise to the load current level. If the current through the incoming thyristors reaches the load current level, then the commutation process of outgoing thyristors will be initiated. This reverse biasing voltage of thyristor should be continued until the forward blocking state is reached.

Thyristor can be simply called as a controlled rectifier. There are different types of thyristors, which are used for designing power electronics based innovative electrical projects. The process of turning on thyristor by providing triggering pulses to gate terminal is called as triggering. Similarly, the process of turning off thyristor is called as commutation. Hope this article give brief information about different commutation techniques of the thyristor. Further technical assistance will be provided based on your comments and queries in the comments section below.


  1. Narsing Das says:

    I have to prepare lab manual on experiment topic
    1. Output characteristics of Synchro Transmitter.
    2. To use Synchro transmitter pair as remote control device.
    Can you provide me all the necessary data for lab manual
    please sir/mam


    Thanks a lot for this. Really helped…:)

  3. Nirdosh Yadav says:

    Thanks sir for this ………..
    This is really useful….

    1. Tarun Agarwal says:

      Hi Nirdosh Yadav
      Most well come
      For any assistance or for customization of projects please email us on

  4. Which longer is the fastest technique to turn off the SCR

    1. Tarun Agarwal says:

      Hi Mahii,
      Thanks for your Appreciation.

  5. Good stuff…Happy with this explination

    1. Tarun Agarwal says:

      Hi Ganesh, Thank you for your appreciation.
      For customization of projects or for any assistance please email us on

  6. nic site…digrams are very useful

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