How is scr turned on

In gate triggering method, a positive voltage applied between the gate and the cathode terminals. We can use three types of gate signals to turn On the SCR. Those are DC signal, AC signal and pulse signal. In this triggering, a sufficient DC voltage is applied between the gate and cathode terminals in such a way that the gate is made positive with respect to the cathode. The gate current drives the SCR into conduction mode.

In this method, a continuous gate signal DC Voltage is applied at the gate and hence it causes internal power dissipation or more power loss.

Another important drawback is there is no isolation between the power and control circuits as they both are DC. With proper isolation between the power and control circuits using transformers , the SCR is triggered by the phase-shift AC voltage derived from the main supply.

The firing angle is controlled by changing the phase angle of the gate signal. However, only one half of the cycle is available for the gate drive to control the firing angle and for the next half of the cycle, a reverse voltage is applied between the gate and cathode. This is one of the limitation of AC triggering and the other is need for separate step down or pulse transformer to supply the voltage to gate drive from the main supply. The most popular method of triggering the SCR is the pulse triggering.

In this method, gate is supplied with single pulse or a train of high frequency pulses. For isolating the gate drive from the main supply, a pulse transformer is used. The static or VI characteristics of the SCR give no indication about the speed at which the SCR has switched into forward conduction mode from forward blocking mode.

Hence, the dynamic characteristics are sometimes more important which gives the switching characteristics of the SCR. The turn ON time of the SCR Ton can be subdivided into three distinct intervals namely delay time t d , rise time t r and spread time t s. The delay time is measured from the instant at which the gate current reaches 90 percent of its final value to the instant at which anode current reaches 10 percent of its final value. It can also be defined as the time taken for the anode voltage to fall from initial anode voltage value V a to 0.

Consider the below figure and observe that, until the time td, the SCR is in forward blocking mode, so the anode current is a small leakage current. Carefully read the name of this method.

What does this mean? This simply means that we will make it forward biased and will increase this bias voltage till SCR gets ON. Therefore, increasing this bias voltage will narrow down the width of the depletion region of junction J2 and at a particular voltage, this depletion region will vanish. At this stage, reversed biased junction J2 is said to have avalanche breakdown and this voltage is called the forward breakover voltage. This is the reason; this critical voltage is called forward breakover voltage.

As soon as avalanche breakdown at junction J2 occurs, current starts flowing from anode to cathode of SCR. The value of this anode current is only limited by the load.

Thus SCR is now in its conduction mode in forward direction i. Normally this method is not used to turn on SCR as it may damage it. Generally the forward breakover voltage is less than reverse breakdown voltage and hence reverse breadwon voltage is considered as final voltage rating while designing SCR. It must also be noted and bear in mind that, once avalanche breakdown take place at junction J2, the blocking capability of J2 is lost.

Therefore if anode voltage is reduced below forward breakover voltage, the SCR will continue to conduct. The SCR can now be turned off by bringing its anode current below a certain value called the holding current.

Gate triggering is the method in which positive gate current is flown in forward biased SCR to make it ON. When the SCR is triggered by applying a gate signal, current flows through the SCR, the secondary of the pulse transformer, and the load. The capacitor is charged to only about 1 V and can be considered a short circuit for the duration for the duration of commutation. 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.

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. The maximum frequency at which this circuit can operate depends on the turnoff time of the SCR.

Search Search. This kind of triggering method is mainly used to increase the voltage among the anode and cathode. So that the width of the depletion layer can be increased and makes to increase the accelerating voltage of minority charge carriers at J2 junction. Further, this can be lead to an avalanche breakdown of J2- junction at a forward break over-voltage.

Throughout the triggering state in the SCR, the range of forwarding voltage drop is 1 to 1. This may be amplified using the load current.

In practical, this method cannot be used as it requires an extremely large anode voltage to the cathode. Once the voltage is high than the break over-voltage, then it offers extremely huge currents. This may cause harm to the thyristor. So, in most of the situations, this kind of SCR triggering method cannot be used.