Switching Characteristics of Thyristor: The thyristor is subjected to various voltages or currents during its turn-on and turn-off processes. Due to the applied voltage and current variation, we obtain the switching or dynamic or ON off characteristics of the thyristor. For a reliable and economic design of converter circuits incorporating thyristor, the study of switching characteristics of the thyristor is of vital importance.
Here, we will discuss the switching or ON-OFF characteristics of thyristor under two sections. The first discussion would be on the thyristor switching characteristics during turn on and then during turn off.
Switching Characteristics of Thyristor During Turn On
For triggering a thyristor or to make thyristor operate in the conduction mode, there are various methods. One of the methods of thyristor triggering is gate triggering where a forward-biased thyristor is made to work in forward-biased thyristor is made to work in forwarding conduction mode by applying a positive gate voltage across the gate terminal and cathode.
While triggering a thyristor, there is a certain amount of time delay between the transition from forward blocking mode to forward conduction mode. The time delay encountered is termed thyristor turn-on time.
For discussing the switching characteristics of thyristor during turn-on, the turn-on time is further divided into delay time(td), rise time(tr), and spread time(tp).
1. Delay Time (td)
Time is taken by gate current to reach 0.9 times Ig from the instant at which the gate current is applied. The ultimate value of gate current is Ig.
In other words, it is the time taken by the anode current to reach 0.1 times of Ia. Here, Ia is the final value of the anode current.
Also, delay time(td) is the time it takes for the anode voltage to decline from Vo to 0.9 times Va, where Va is the anode voltage’s starting value.
This can also be defined in terms of forwarding leakage current to rise from forwarding leakage current to 0.1 times of Ia.
From figure (a) we can observe that the thyristor is initially in forwarding blocking mode where the anode voltage is 0 A and the anode current is a forward leakage current that is very small.
The starting of the turn-on process is specified by the rise of anode current and fall of anode voltage. The delay time(td) can be decreased either by the application of a high gate current and the more forward voltage across the anode and cathode. This time delay is in order of few microseconds(µs).
2. Rise Time (tr)
The rise time is the duration during which the anode current rises from 0.1 Ia to 0.9 Ia. Ia is the final value of the anode current.
Alternatively, it is the time duration during the forward blocking voltage or anode voltage that falls from 0.9 Va to 0.1 Va and Va is the initial value (OA in figure a) of anode voltage.
Rise time can be reduced by the application of the high amount of gate current as it is inversely proportional to the magnitude and build-up rate of gate current.
Moreover applying a steep gate current pulse to the gate terminal of SCR reduces the rise time.
The nature of the anode circuit mainly determines the rise time.
On the condition of a series RL circuit, the inductive effect causes the rate of rising current (di/dt) to be slow, and delay time(tr) is more for such a circuit. However, for the RC series circuit, the rate of rising of current is high and hence tr is less.
The power is high during this period which can be observed in figure a. This is due to the higher value of both Va and Ia during this period.
3. Spread Time (tp)
The time duration during which anode current rises from 0.9Ia to Ia or the anode voltage falls from 0.1 Va to the on-state voltage drop. The on-state voltage drop is about 1 to 5V. Spread time depends on the structure of the gate and the area of the cathode.
Switching Characteristics of Thyristor During Turn-OFF
Turning off of a thyristor indicates the change in the operating state of thyristor from forwarding conductor mode to forward blocking mode or in simple words from on state to the off state. When the thyristor is brought back to the off state it must be capable of blocking the forward voltage.
The turn-off process is also termed as commutation process and it is the process of bringing the thyristor from forwarding conduction mode to forwarding blocking mode.
Turning off the thyristor is only possible if the anode current is reduced below the holding current.
At the instant when the anode current is brought to zero and if there exists forward voltage across SCR, then the SCR will go into the conduction mode although the gate current is not supplied. As during such instant, the SCR will not be able to block the forward voltage as the four layers of SCR are still in favorable condition for the conduction of current.
In order to turn off the thyristor and eliminate such a situation, the thyristor must be subjected to a reverse-biased voltage for a certain duration after the anode current has reached zero.
Turn-Off Time (tq)
The time duration from the instant when the anode current falls to zero and the instant when the thyristor regains its forward blocking capability is known as turn-off time(tq).
The turn-off time can be further studied under two classifications: reverse recovery time(trr) and gate recovery time(tgr).
During the turn-off period, the excess carriers from the four layers of the thyristors are removed such that it regains its forward blocking capability.
1. Reverse Recovery Time (trr)
From the figure, we observe that the recovery time trr is divided into instants t1, t2, and t3.
At the instant t1, the anode current is zero and starts to build in the negative direction with a slope same to that during the commutation process. The reverse recovery current flowing after t1 is due to the sweeping out of the carriers from the top p-layer and bottom n-layer of SCR. This reverse recovery current is responsible for the removal of the charge carriers from the junctions J1 and J3.
At the instant of t2, about 60% of carriers are swept from the J1 and J3 and carrier density starts decreasing thereby the current starts to decay.
The decay of the reverse recovery current causes a voltage spike across the SCR which may damage it. Hence, for protection, a snubber circuit is used.
During the time interval t1 to t3 i.e the reverse recovery time(trr), the excess carriers of the junctions J1 and J3 are swept. However, charge carriers are still trapped around junction J2 which are removed during the interval of gate recovery time(tgr).
2. Gate Recovery Time (tgr)
Around the junction(J2), there are charge carriers trapped. This junction is in the inner layer of the thyristor and the carriers cannot outflow to the external circuit and these extra charge carriers must be removed by the process of recombination.
The time span between t3 and t4 during which extra charge carriers around the J2 recombines is known as gate recovery time(tgr).
The total turn-off time of the thyristor is in order of 3 to 100µs. Thyristors are classified based on their turn-off time into inverter grade and converter grade SCRs.
Inverter grade SCR has turn-off time(tq) in the order of 3 to 50µs and converter grade SCR has turn-off time(tq) of order 50 to 100µs.
Actually, SCRs are part of the power electronics circuit or power circuit. The turn-off time(tq) is only applicable to a single SCR but in a true power circuit, the turn-off time(tq) is provided by the circuit turn-off time(tc).
The circuit turn-off time(tc) is greater than the turn-off time(tq) for proper commutation otherwise we may encounter failure commutation.
i.e tc > tq.