To turn-off or commutate a thyristor
To turn-off a thyristor, anode current must be reduced below holding current and a reverse bias must be applied across thyristor for a finite period of time. If all these two conditions are not met simultaneously then forced commutation can be used to turn-off the thyristor.
Most SCRs can be turned-off by voltage reversal during negative half cycle of the ac supply for
Turn-off time of a thyristor
Turn-on time (ton) of an SCR is related to its turn-off time (toff) in which of the following way?
The turn-on time of an SCR depends on time constant (L/R) of the load.
Assertion (A) : It is necessary to keep the thyristor reverse biased for a finite period before a forward anode voltage can be reapplied.
Reason (R) : Once the SCR starts conducting an appreciable forward current, the gate has no control on it and the device can be brought back to the blocking state only by reducing the forward current level below that of the holding current.
It is necessary to keep the thyristor reverse biased for a finite period before a forward anode voltage can be reapplied because if a forward voltage is applied immediately after reducing the anode current to zero, it will not block the forward voltage and will start conducting again, although it is not triggered by a gate pulse. Reason is also a true statement but not the correct explanation of assertion.
Match List-I (Commutation techniques) with List-ll (Alternative name) and select the correct answer using the codes given below the lists:
C. Class-B, C and D
1. Forced commutation
2. Load commutation
3. External-pulse commutation
4. Line commutation
Consider the following statements associated with various commutation techniques of a thyristor:
1. In forced commutation, the commutating components L and C carry load current continuously.
2. Forced commutation is usually employed in dc choppers.
3. In load commutation, L and Care connected in parallel with the load or C in series with the load.
4. Load commutation is employed in series inverters.
5. In class-C commutation, the main thyristor that is to be commutated is connected in parallel with the load.
Which of the statements given above are correct?
Thus, statements 2 and 4 are only correct.
For the class-B commutation circuit shown below, the initial voltage across capacitor is 230 volt. For a constant load current of 300 A, the conduction time for the auxiliary thyristor is approximately equal to
Given, L = 5μH, C = 20μF
∴ Conduction time for auxiliary thyristor
For the circuit shown below, the peak value of the current through thyristors T1 and T2 are
Given, Vs = 200 V, R1 = 10 Ω, and R2 = 100 Ω
Given circuit represents a class-C commutation circuit.
The peak value of current through thyristor T1 is
The peak value of current through thyristor T2 is
The SCR in the circuit shown below is forced commutated by a circuitry not shown in the figure. The SCR has minimum charging current of 5 mA to turn it on and its junction capacitance is 25 pF.
Under steady-state condition, voltage across capacitor is Vs = 200 V and SCR conducts having a current through it equal to
When the SCR is forced commutated, the capacitor voltage at time t is
Let Cj be the junction capacitance of SCR. Now, when SCR get turned-on, the
= 0.02 μF
Thus, the minimum value of C so that SCR does not get turn-on is 0.02 μF.
If C > 0.02 μF, SCR will be turn-on.