On this tutorial, we are going to study SCR Flip OFF Strategies. There are a number of methods to correctly implement the SCR Flip OFF strategies like Pure, Pressured. In Pressured Commutation, there are once more a number of sub-categories like Class A, B, C, D, E. We are going to study all these completely different strategies to correctly flip OFF an SCR, completely different courses of Commutation and likewise the Dynamic Flip OFF Traits of SCR.


Within the earlier article, we now have seen completely different SCR Turn ON Methods, the place we discovered the right way to flip ON an SCR by making use of applicable constructive gate voltage between the gate and cathode terminals.

An essential level to notice right here is that not like a transistor, the place the collector to empty present flows so long as there’s a small base present flowing (gate voltage in case of a MOSFET), an SCR doesn’t want any gate voltage to remain turned ON.

Even when the gate voltage is eliminated (after the SCR turns ON), the SCR doesn’t cease conducting. So, the gate has no management movement of present from anode to cathode and the SCR can’t be turned OFF by the gate terminal. The SCR might be introduced again to the ahead blocking state from the ahead conduction state solely by lowering the anode or ahead present beneath the holding present degree.

The flip OFF strategy of an SCR known as Commutation. Commutation means switch of present from one circuit to a different. The place have you ever heard of the time period commutation and commutators earlier than? In brushed DC Motors. Commutators are a part of the comb meeting and are answerable for reversing the course of present within the winding.

So, the commutation circuit for an SCR does s comparable job by lowering the ahead present to zero with a purpose to flip OFF the SCR or Thyristor. Even after lowering the anode present to zero, the SCR will as soon as once more conduct if there’s a direct ahead voltage. So, to show OFF a conducting SCR correctly, the next situations have to be happy:

  • The anode or ahead present of SCR have to be decreased to zero or beneath the extent of holding present after which,
  • A ample reverse voltage have to be utilized throughout the SCR to regain its ahead blocking state.

When the SCR is turned OFF by lowering ahead present to zero, extra cost carriers exists in numerous layers. To regain the ahead blocking state of an SCR, these extra carriers have to be recombined. Due to this fact, with a purpose to speed up this recombination course of, a reverse voltage is utilized throughout the SCR.

We all know that an SCR, primarily, has solely two states: ON State (or Conduction State) and OFF State (Blocking State). Additionally, the SCR doesn’t have any management on the load voltage and present (as it’s only a swap). So, the one solution to management the extent of voltage and present is thru the method of Flip ON and Flip OFF of the SCR.

Commutation is a crucial course of used within the flip OFF of SCR.

SCR Flip OFF Strategies

An SCR is alleged to be ‘turned OFF’ if there is no such thing as a movement of ahead present and even when the SCR is as soon as once more ahead biased (constructive voltage at anode), the SCR won’t conduct with none Gate Sign (utilizing one of many SCR Flip ON Strategies).

The reverse voltage, which causes to commutate the SCR, known as the Commutation Voltage. Relying on the kind of switching of SCR (Cyclic or Sequential), the commutation strategies are categorised into two main varieties. They’re:

  • Pure Commutation
  • Pressured Commutation

Earlier than discussing about several types of ACR Flip OFF Strategies, there is a vital amount referred to as the Flip OFF Time of SCR which we now have to know.

Flip OFF Time tOFF of an SCR is the time between the second anode present turns into zero and the second SCR begins to dam the ahead voltage.

Pure Commutation

In pure commutation, the supply of commutation voltage is the availability supply itself. If the SCR is related to an AC provide, at each finish of the constructive half cycle, the anode present naturally turns into zero (because of the alternating nature of the AC Provide). As the present within the circuit goes by the pure zero, a reverse voltage is utilized instantly throughout the SCR (because of the destructive half cycle). These situations flip OFF the SCR.

This technique of commutation can be referred to as as Supply Commutation or AC Line Commutation or Class F Commutation. This commutation is feasible with line commutated inverters, managed rectifiers, cyclo converters and AC voltage regulators as a result of the availability is the AC supply in all these converters.


Throughout the constructive half cycle of the AC Provide, the load present flows usually. However, throughout the destructive cycle, the SCR will flip OFF (because of momentary zero present and instant destructive polarity). For profitable pure commutation, the flip OFF time tOFF have to be lower than the period of half cycle of the availability.

Pressured Commutation

In case of DC circuits, there is no such thing as a pure present zero to show OFF the SCR. In such circuits, ahead present have to be pressured to zero with an exterior circuit (referred to as Commutating Circuit) to commutate the SCR. Therefore the identify, Pressured Commutation.

This commutating circuit encompass parts like inductors and capacitors and they’re referred to as Commutating Elements. These commutating parts trigger to use a reverse voltage throughout the SCR that instantly convey the present within the SCR to zero.

Relying on the method for attaining zero present within the SCR and the association of the commutating parts, Pressured Commutation is classed into differing kinds. They’re:

  • Class A – Self Commutation by Resonating the Load
  • Class B – Self Commutation by Resonating the Load
  • Class C – Complementary Commutation
  • Class D – Auxiliary Commutation
  • Class E – Pulse Commutation

similar to class A, B, C, D, and E. This commutation is principally utilized in chopper and inverter circuits.

Class A Commutation

That is also referred to as Self Commutation by Resonating the Load or just the Resonant Commutation. On this commutation, the supply of commutation voltage is within the load. The commutating parts are L and C and the Capacitor might be related both in parallel or in collection with the load resistance RL as proven beneath.

There are additionally waveforms of SCR present, voltage and capacitor voltage.


The worth of load resistance and the commutating parts are chosen in such a approach that they kind an under-damped RLC resonant circuit. When the circuit is utilized with a DC Supply, the ahead currents begins flowing by the SCR and through this era, the capacitor is charged as much as the worth of Vdc. The present within the circuit might be both of the 2 waveforms proven beneath, relying on how the load is related to the capacitor (parallel or collection).


When conducting, the present within the SCR is the charging present of the capacitor. From the waveforms, it’s clear that the present turns into zero on the level ‘Okay’. At this level, the SCR turns OFF.

The resonant frequency of the circuit, which relies on the Commutation Elements L and C and likewise on the load resistance, determines the time for switching OFF the SCR.

Class A Commutation technique is easy and dependable and is normally utilized in excessive frequency operations i.e., frequencies within the vary of 1000 Hz and above because of the excessive values of L and C parts (as they carry the complete load present). Such a commutation is usually utilized in Collection Inverters.

Class B Commutation

That is additionally a self commutation circuit by which commutation of SCR is achieved by a resonating LC Circuit. However the primary distinction between Class A and Class B Commutation is that the LC resonant circuit is related throughout the SCR however not in collection with the load as in case of Class A Commutation. In consequence, the commutating circuit and the L and C parts in it doesn’t carry the load present.

The next picture exhibits the Commutating Circuit and likewise the waveforms related to Class B Commutation.


When a DC provide is utilized to the circuit, the capacitor costs as much as Vdc, with an higher plate constructive and decrease plate destructive. When the SCR is triggered, the present flows in two instructions: one is thru Vdc+ – SCR – R – Vdc– and the one other one is the commutating present (IC) by L and C parts.

When the SCR is turned ON, the capacitor begins discharging within the path C+ – L – SCR – C. When the capacitor is totally discharged, it begins charging with a reverse polarity. On account of the reverse voltage, a commutating present IC, will movement in the other way of the load present IL.

When the commutating present IC turns into greater than the load present, the SCR will routinely flip OFF and the capacitor costs with its authentic polarity (by Inductor and Load).

From the above clarification, we are able to perceive that the SCR is turned ON for a while after which routinely turned OFF for a while. This can be a repeatedly repeating course of. The frequency of ON/OFF state relies on the values of L and C within the commutating circuit. Such a commutation is usually utilized in chopper circuits.

Class C Commutation

On this commutation technique, the primary SCR (which is to be commutated) is related in collection with the load and a further or complementary SCR is related in parallel with the primary SCR. Therefore, this technique can be referred to as as Complementary Commutation.

On this, SCR turns OFF with a reverse voltage of a charged capacitor. The determine beneath exhibits the complementary commutation with applicable waveforms.


Initially, each SCRs are in OFF state so the capacitor voltage can be zero. When the SCR1 or major SCR is triggered (by making use of a pulse to its gate), present begins flowing in two paths: one is the load present IL by Vdc+ – RL – SCR1 – Vdc– and the opposite is the charging present of the capacitor IC by Vdc+ – R1 – C+ – C – SCR1 – Vdc–. Due to this fact, the capacitor begins charging as much as the worth of Vdc, with the polarity as proven within the above picture.

When the SCR2 is triggered (by making use of a pulse to its gate), it’s turned ON. In consequence, the destructive polarity of the Capacitor is utilized throughout the anode of SCR1 and the constructive polarity is utilized to cathode of SCR1.

This may trigger a reverse bias throughout the primary SCR (SCR1) and therefore, it turns OFF. Now, the capacitor costs by the load and the trail is Vdc+ – RL – C+ – C – SCR2 – Vdc–. The polarity of the capacitor is now reversed.

If the SCR 1 is triggered as soon as once more, the discharging present of the capacitor turns OFF the SCR2 and the method repeats.

This commutation is principally utilized in single section inverters with a middle tapped transformers. The McMurray Bedford inverter is the most effective instance of this commutation circuit. This can be a very dependable technique of commutation and it’s also helpful even at frequencies beneath 1000 Hz.

Class D Commutation

That is additionally referred to as as auxiliary commutation as a result of it makes use of an auxiliary SCR to modify the charged capacitor. On this, the primary SCR is commutated by the auxiliary SCR. The principle SCR with load resistance types the facility circuit whereas the diode D, inductor L and SCR2 types the commutation circuit.


When the availability voltage Vdc is utilized, each SCRs are in OFF state and therefore the capacitor voltage is zero. With a purpose to cost the capacitor, SCR2 have to be triggered first and the capacitor costs by the trail Vdc+ – C+ – C – SCR2 – RL – Vdc–.

When the capacitor is totally charged, the present movement decreases and for the reason that SCR2 is in collection with the capacitor, it turns OFF. If the SCR1 is triggered, the present flows in two instructions: one is the load present IL by Vdc+ – SCR1 – RL – Vdc– and one other one is commutation present (because of capacitor discharge) by C+ – SCR1 – L – D – C.

As quickly because the capacitor discharges utterly, its polarities might be reversed however the presence of diode prevents the reverse discharge. When the SCR2 is triggered, capacitor begins discharging by C+ – SCR2 – SCR1 – C. When this discharging present is greater than the load present, the SCR1 will get turned OFF.

Once more, the capacitor begins charging by the SCR2 to a provide voltage Vdc after which the SCR2 is turned OFF (after capacitor is totally charged). Each the SCRs are actually turned OFF and the above course of is repeated.

This commutation technique is principally utilized in inverters and likewise used within the Jone’s Chopper Circuit.

Class E Commutation

Such a Commutation is also referred to as Exterior Pulse Commutation. On this, an exterior pulse supply is used to supply the reverse voltage throughout the SCR. The circuit beneath exhibits the Class E commutation circuit, which makes use of a pulse transformer to supply the commutating pulse. The transformer is designed with tight coupling between the first and secondary and likewise there’s a small air hole within the transformer in order that it doesn’t get saturated when the heartbeat is utilized.

If the SCR must be commutated, pulse is utilized whose period is the same as or better than the flip OFF time of the SCR.

If the SCR is triggered, load present flows by the heartbeat transformer. If a pulse (of potential VP) is utilized to the first of the heartbeat transformer, a voltage is induced within the secondary of the heartbeat transformer.

This induced voltage is utilized throughout the SCR with a reverse polarity (–VP) and therefore the SCR is turned OFF. The capacitor presents a really low or zero impedance to the excessive frequency pulse.


Dynamic Flip OFF Switching Traits

The transition of an SCR from ahead conduction state to ahead blocking state known as as flip OFF or commutation of SCR. As we all know that when the SCR begins conducting, the gate has no management over it to convey again to ahead blocking or OFF state.

To show OFF the SCR, the present have to be decreased to a degree beneath the holding present of SCR. We now have mentioned varied strategies above to show OFF the SCR by which SCR flip OFF is achieved by lowering the ahead present to zero. But when we apply the ahead voltage instantly after the present zero of SCR, it begins conducting once more even with out gate triggering.

That is because of the presence of cost carriers within the 4 layers. Due to this fact, it’s essential to use the reverse voltage, over a finite time throughout the SCR to take away the cost carriers.

Therefore, the flip OFF time is outlined because the time between the moment the anode present turns into zero and the moment at which the SCR retains the ahead blocking functionality. The surplus cost carriers from the 4 layers have to be eliminated to convey again the SCR to ahead conduction mode.

This course of takes place in two levels. In a primary stage, the surplus carriers from outer layers are eliminated and in second stage extra carriers within the interior two layers are to be recombined. Therefore, the whole flip OFF time tOFF is split into two intervals: reverse restoration time trr and gate restoration time tgr.

 tOFF = trr + tgr 

The determine beneath exhibits the switching traits of SCR throughout flip ON and OFF. The time t1 to t3 known as as reverse restoration time. On the occasion t1, the anode present is zero and builds up within the reverse course, which known as as reverse restoration present. This present removes the surplus cost carriers from outer layers throughout the time t1 to t3.

Dynamic Turn OFF Switching Characteristics

At occasion t3, junctions J1 and J3 are in a position to block the reverse voltage however, the SCR just isn’t but in a position to block the ahead voltage because of the presence of extra cost carriers in junction J2. These carriers might be disappeared solely by the best way of recombination and this may very well be achieved by sustaining a reverse voltage throughout the SCR.

Therefore , throughout the time t3 to t4, the recombination of costs takes place and on the instantaneous t4, junction J2 utterly recovers. This time known as gate restoration time tgr.

  • From the determine, the flip OFF time is the time interval between the t4 and t1. Usually, this time varies from 10 to 100 microseconds. This flip OFF time tOFF is relevant to the person SCR.
  • The time required by the commutation circuit to use the reverse voltage to commutate the SCR known as the circuit commutation time (tC). For a security margin or dependable commutation, this tC have to be better than the tOFF in any other case commutation failure happens.
  • The SCRs, which have sluggish flip OFF time, normally between 50 to 100 microseconds, are referred to as as converter grade SCRs. These are utilized in section managed rectifiers, cyclo converters, AC voltage regulators, and so on.
  • The SCRs, which have quick flip OFF time, normally between 3 to 50 microseconds, are inverter grade SCRs. These are costlier in comparison with converter grade and are utilized in choppers, power commutated converters and inverters.


An entire tutorial on SCR Flip OFF Strategies. You discovered the necessity for turning OFF SCR, other ways to show OFF an SCR, commutation and several types of commutation.