If an SCR is forward-biased and a gate signal is applied, the device turns on. However, once the anode current IA is above the holding current, the gate loses control. The only way to turn the SCR off is to reduce the anode current below the holding current value or to make the anode negative with respect to the cathode. The process of turnoff is known as commutation. in AC applications, the required condition for turnoff is achieved when the source reverses during the negative half-cycle. This method is called natural or line commutation. For DC applications. additional circuitry must be used to turn the SCR off. These circuits first force a reverse current through the SCR for a short period to reduce. The anode current to zero. They then maintain the reverse bias for the necessary time. to complete the turnoff. This process is called forced commutation. It should be noted that if a forward voltage is applied instantly after the anode current is decreased to zero. the SCR will not block the forward voltage and will start conducting again. even though it is not triggered by a gate pulse. It is therefore. important to keep the device reverse-biased for a finite time, called the tum of time (tOFF), before a forward anode voltage can be applied. The turnoff time of an SCR is specified as the minimum period between the instant the anode current becomes zero and the instant the device is able to block the forward voltage. SCR turn off can be accomplished in the following ways:
- diverting the anode current to an alternate path
- Shorting the SCR from anode to cathode
- applying a reverse voltage by Making the cathode to the anode) across the SCR positive with respect
- forcing the anode current to zero for a brief period
- Opening the external path from its anode supply voltage
- Momentarily reducing the supply voltage to zero
The most common methods of achieving commutation are discussed briefly in the following subsections.
In DC circuits. the SCR can be turned off by switching the anode current to an alternate path for sufficient time to allow the SCR to recover its blocking ability. A simple circuit using a transistor switch Q is shown in Figure 1. When the SCR is on, the transistor is in the off state. To turn the SCR off, a positive pulse is applied to the base of Q, turning it on. The anode current is diverted to the transistor. When the anode current falls below the holding current, the SCR turns off. The transistor is held on just long enough to turn off the SCR. This is not a useful method for repetitive on-off operations since the SCR is not actually reverse biased and turn off is therefore slow.
In another method, the SCR can be turned off by applying a reverse bias for enough time to allow the SCR to recover its forward blocking ability. A typical commutation circuit includes a commutation capacitor C and an auxiliary SCR2, as shown in Figure 2. when the main SCR1 is conducting, capacitor C at this instant, SCR2 is off. To turn SCR1 off, SCR2 is triggered. When SCR2 turns on, the capacitor is switched across SCR1, applying a reverse voltage across it. If SCR1 is reverse biased long enough, it will turn off.
To ensure successful commutation, the value of the capacitance C can be determined by:
C ≥ tOFF / 0.693 RL
C = Commutation capacitor in µF
RL = Load resistance in Ω
tOFF = turnoff time in µs
In the circuit in Figure 4.28, the source voltage is 220V and the load resistance is 10 Ω. If the turnoff time for the SCR is 10 µs, find the minimum value of capacitance that will ensure commutation.
The minimum value of C is:
C = tOFF / /0.693 RL = 10 (10-6)/ 0.693 x 10 = 1.44 µF
A suitable value of C would be 1.5 µF.
Commutation by External Source
In this type of commutation circuit, the commutation energy is obtained from an external source in the form of a pulse. A simple circuit is shown in Figure 3. The pulse generator reverse biases the SCR and thus turns it off. The pulse width must be such that the SCR is reverse biased for a period greater than the turnoff time of the SCR.
Figure 3: Commutation by external source (a) Circuit (b) waveforms
When the SCR is triggered by applying a gate signal, current flows through the SCR, the secondary of the pulse transformer, and the load. To turn the SCR off, a positive pulse transformer is applied to the cathode of the SCR. The capacitor is charged to only about 1 V and can be considered a short circuit for the duration for the duration of commutation.
Commutation by Resonance
The natural resonance set up in an LC circuit can be used directly to turn off an SCR, eliminating the need for an external source. Figure 4 shows a simple series resonant turnoff circuit. The underdamped LC resonating circuit in series with the load applies a reverse voltage to the SCR to turn it off.
The parallel resonance commutation circuit shown in Figure 5 can also be used to turn an SCR off. The capacitor C is initially charged during the SCR off period to the source voltage with the polarity indicated. When the SCR is turned on, the capacitor discharges through the LC resonant circuit and applies a reverse voltage across the SCR to turn it off. Once the SCR is turned on, it conducts till the capacitor charges again to VS and starts discharging through the SCR and L. then it automatically turns off.
AC Line Commutation
This commutation method is used in circuits with an AC source. Figure 6 shows a typical line commutated circuit and its associated waveforms. Load current flows in the circuit during the positive half-cycle. The SCR is reverse biased during the negative half-cycle of the input voltage. With zero gate signal, the SCR will turn off if the turnoff the SCR is less than the duration of the half-cycle, that is, for a period T/2. The maximum frequency at which this circuit can operate depends on the turnoff time of the SCR.