Turn-ON methods of a Thyristor / SCR

The thyristor or a silicon controlled converter (SCR) is the most widely used power semiconductor device in power electronics. It can be turned ON by the following methods with maintaining positive voltage across the thyristor from
anode to cathode.

1. Gate triggering

2. Forward voltage triggering

3. Thermal triggering

4. dv/dt triggering

5. light triggering

1. Gate Triggering:

It is the most usual method of firing the forward biased thyristor due to its simplicity and efficient way of triggering. A positive voltage across the gate and cathode is applied to turn ON the SCR. The gate current injects the charges in to the inner P layer and reduces the forward breakover voltage. Thus the thyristor is turned on before the occurrence of the actual forward breakdown voltage. With increase in the gate current the forward breakover voltage decreases and SCR comes into conduction.

Forward voltage triggering:

In this method of triggering the thyristor a positive voltage called forward voltage is applied across the SCR from anode to cathode keeping the gate circuit open. As the voltage gradually increased up to the forward breakover voltage the device latches into conduction. As the forward voltage is applied the junctions J­1 and J3 are forward biased and the inner junction J2 is reverse biased. The junction J2 forms the depletion region. By increasing the forward voltage, the depletion layer at junction J2 is completely removed and avalanche breakdown occurs. Thus the SCR comes into conduction. The voltage at which the SCR comes into the conduction mode is called forward breakover voltage.

Thermal or Temperature Triggering:

In thermal triggering during forward blocking mode of a silicon controlled rectifier the reverse biased junction J2 form the depletion region. The junction allows only small leakage current. As the temperature of the junction is increased the width of depletion region decreases and more leakage current flow through the junction. The leakage current in turn increases the junction temperature further and thus at a point of temperature the reverse biased junction vanishes completely and the device turns ON allowing the full current.

dv/dt triggering:

The reverse biased inner junction J2 during the forward operation a thyristor has the characteristics that of a capacitor due to the junction capacitance. The junction capacitance is due the presence of space charges across the junction. For sudden changes in voltage across a capacitor it behaves as short circuit and current flows. Similarly if a forward voltage across the anode and cathode of an SCR is applied suddenly it may turn ON the SCR.

Light Triggering:

Triggering through application of light energy is possible through a type of SCRs called light-activated SCR or shortly LASCR. Light source is applied to the LASCR instead of a gate signal to turn ON. The light pulse of required wavelength is passed through optical fibres. The technique is used in HVDC conversion systems.