SCR Triggering (Turn-on) Methods

SCR Triggering (Turn-on) Methods

When SCR anode terminal of SCR is connected with a positive terminal of SCR and cathode terminal of SCR is connected with a negative terminal of SCR. In this type of connection, the anode is positive with respect to the cathode. Hence, the SCR is connected as forward bias. As we have seen in the working of SCR, it has three operating regions;

  • Forward Blocking Mode
  • Forward Conducting Mode
  • Reverse Blocking Mode

Generally, SCR is connected in forward bias. But in forward bias also, it has two modes of regions. Hence, the methods of switching of SCR from forward blocking mode to forward conducting mode is known as Triggering of SCR or Turning ON the SCR.

The triggering of SCR depends on the temperature, supply voltage, and gate current. There are several methods to turn-on SCR. These methods are listed below.

  • Forward Voltage Triggering
  • Temperature Triggering
  • dv/dt Triggering
  • Light Triggering
  • Gate Triggering

Forward Voltage Triggering

When the SCR is connected in forward bias and supplied at a rated voltage between anode and cathode, but the gate current is not given.

In this condition, the SCR remains in forward blocking mode and current cannot flow. Because junction J2 is in reverse bias and supply voltage is not enough to cross the depletion region.

But if we gradually increase the supply voltage and it increases than the breakover voltage, avalanche breakdown occurs in junction J2. And large current starts flowing through the SCR.

In this condition, due to high current and high voltage, large voltage drop (power loss) occurs in the SCR. And this method is not useful because it draws high current which can damage the SCR. Therefore, this method of triggering the SCR is not useful in most of the cases.

V-I Characteristic of SCR
V-I Characteristic of SCR

Temperature Triggering

This type of triggering is also known as thermal triggering. The reverse leakage current can be increased by increasing the temperature of junction J2.

This causes to decrease in the depletion width and at a certain temperature, the leakage starts conducting the SCR.

This type of triggering of SCR uses in special circumstances. And it is also considering while designing the thyristor.

The device uses the SCR is also capable to sustain for temperature. Practically, this method is not used because it causes thermal runaways and it will damage the SCR.

dv/dt Triggering

When SCR is connected in forward bias, the junction J1 and J3 are in forward bias and junction J2 is in reverse bias.

In this condition, it behaves as a capacitor where two plates J1 and J3 and dielectric is J2. The charging current of the capacitor as below equation.

    \[ I_C = C \frac{dv}{dt} \]

Where C is capacitance and dv/dt is the rate of change of applied voltage.

From the above equation, if we increase the rate of change of voltage (dv/dt), the charging current also increases. And SCR starts conducting.

This is not suitable for the practical triggering of SCR. Because a high rate of change of voltage causes high spikes in voltage waveform. These spikes are high enough to damage the SCR.

Light Triggering

In this method, the SCR is triggered by the Light. Hence, this method is also known as radiation triggering.

When light strikes on the junction J2, it creates more electrons-holes pair at the junction J2. Which causes an additional charge carrier at a junction that is used to generate current. And SCR starts conducting.

This method is used in the power system for triggering SCR in converts for the HVDC transmission line. This type of SCR is specially designed and it is known as Light Activated SCR (LASCR).

Gate Triggering SCR

This is the most used method for triggering the SCR. Because this method is more efficient to turn-on SCR.

In this method, the SCR is connected in forward bias and Gate current is supplied at junction J2. For this purpose, a positive gate voltage is applied between the gate and the cathode terminal of SCR.

The gate current causes to increase the reverse leakage current. And it causes the breakdown of junction J2 even if the applied voltage is less than the breakover voltage.

If we give more gate current, the SCR can conduct at low voltage also. The value of the gate current depends on the size of SCR. The value of gate current is in the range of few milliamperes.

There are three types of gate signals used to trigger the SCR.

  • DC Gate Triggering
  • AC Gate Triggering
  • Pulse Triggering

DC Gate Triggering

A proper value of DC source is connected between the gate and cathode terminal. The gate terminal is positive with respect to the cathode terminal.

When supplied DC voltage is sufficient to produce gate current, it causes the breakdown of junction J2 and SCR stats conducting.

In this method, the DC source is used to generate a gate current. DC supply is a continuous source of supply and is connected continuously with the device. Hence, it causes more power loss during conduction.

AC Gate Triggering

In this method, the AC source is used to generate gate pulses. It requires a very low voltage level. Hence, it is necessary to use the step-down transformer to reduce the voltage level.

There are two methods to trigger the SCR with the AC Gate supply. In one method, only a resistor and diode are used and in the second method, two diodes, one resistor, and one capacitor are required. The first method is known as R Triggering and the second method is known as RC triggering. Hence, there are two methods of triggering SCR with AC source and that is;

  • R Triggering
  • RC Triggering

R Triggering

The connection diagram for R triggering is as shown in the below figure.

R Triggering of SCR
R Triggering of SCR

The variable resistor is used to control the value of the gate current. By changing the value of the resistor, we can generate a sufficient amount of gate current which is used to trigger the SCR.

The diode D is connected in series with a variable resistor. This diode is known as a blocking diode. And it is used to prevent the current in the negative half cycle.

This gate circuit is a purely resistive circuit. Hence, the gate current is in phase with the applied voltage. By this method, we can get a maximum 90-degree firing angle.

RC Triggering

The connection diagram for RC triggering is as shown in the below figure.

RC Triggering of SCR
RC Triggering of SCR

In this method, the capacitor is charged up to the peak value of the supplied voltage through variable resistor R, in a positive half cycle.

The variable resistor controls the amount of gate current. When a sufficient amount of gate current is generated, the SCR starts conducting. In this method, we can achieve a firing angle of more than 90-degree.

In the negative half cycle, the capacitor is discharged through the diode D2. The diode D1 is used to prevent the reverse breakdown of gate cathode junction in a negative half cycle.

Pulse Triggering

In this method, the gate current is generated by a single pulse or a sequence of high-frequency pulses.

This method is the most popular method for triggering the SCR because it does not require continuous pules. Hence, the loss occurred in the gate circuit is very less. The pulse transformer is used to make the isolation between the main source and gate circuit.

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