Turn off and Turn on Characteristics of Thyristor

The characteristics are as follows:

Turn ON Switching Characteristics of Thyristor

A forward biased thyristor is turned ON by applying a positive gate voltage between the gate and the cathode, as shown in figure(1).

Figure(2) , shows the waveforms of the gate current(IG), anode current(IA) and anode to cathode voltage(VAK). The total switching period being much smaller compared to the cycle time, IA and VAK before and after switching will appear flat.

As shown in figure , there is a transition time “Toff” from forward OFF state to forward ON state. This transition time is called the thyristor turn ON time and can be divided into three separate intervals namely, They are

• Delay time (Td)
• Rise time (Tr)
• Spread time ( Tp)

Delay Time (Td)

It is the time between the instant at which the gate current reaches 90% of its final value and the instant at which the anode current reaches 10% of its final value. It is the time taken by the anode voltage to fall from VAK to 0.9 VAK

Rise Time (Tr)

For a resistive load, “rise time” is the time taken by the anode current to rise from 10% of its final value to 90% of its final value. At the same time, the voltage VAK falls from 90% of its initial value to 10% of its initial value. However, current rise and voltage fall characteristics are strongly influenced by the type of the load. For inductive load the voltage falls faster than the current. While, for a capacitive load, current rises rapidly.

Spread Time ( Tp)

It is the time taken by the anode current to rise from 90% of its final value to 100%. During this time conduction spreads over the entire cross-section of the cathode of the thyristor. The spreading interval depends on the area of the cathode and on the gate structure of the thyristor.

Turn OFF Switching Characteristics of Thyristor

• Once the thyristor is ON and its anode current is above the latching current level, the gate losses control. It can be turned OFF only by reducing the anode current below the holding current. The OFF time tq of a thyristor is defined as the time between the instant anode current becomes zero and the instant the thyristor regains forward blocking capability. If forward voltage is applied across the device, during this period the thyristor turns ON again.
• During turn OFF time, excess minority carriers from all the four layers of the thyristor must be removed. Accordingly, tq is divided into two intervals, the reverse recovery time(tRR) and the gate recovery time(tGR), figure shows the variation of the anode current and the anode to cathode voltage with time during turn OFF operation for an applied sinusoidal voltage(VI).

The anode current becomes zero at time t1 and starts growing in the negative direction with the same DiA / Dt till time t2. This negative current removes excess carriers from the junctions J1 and J3. At time t2 excess carriers densities at these junctions are not sufficient to maintain the reverse current. The value of the anode current at time t2 is called as the reverse recovery current(IRR). The reverse anode current reduces to the level of reverse saturation current. The total charge removed from the junctions between t1 and t3 is called the reverse recovery charge(QRR). Fast decaying reverse current during the interval t2 – t3 coupled with the di / dt limiting inductor may cause a large reverse voltage spike to appear across the device. This voltage must be limited below the VRRM rating of the device. Up to time t2 the voltage across the device(VAK) does not change substantially from its state value. However, after the reverse recovery time, the thyristor regions the reverse blocking capacity and VAK starts following the supply voltage. At the end of the reverse recovery period(trr) trapped charges still exist at the junction J2 which prevents the device from blocking forward voltage just after trr. These trapped charges are removed only by the process of recombination. The time taken for this recombination process to complete between t3 and t4 is called the gate recovery time(tgr). The time interval tq = trr + tgr is called “device turn OFF time” of the thyristor.

In electronic components, Power Semiconductor Devices are electronic devices that need an external power source to operate actively. Materials used to make semiconductor devices are neither excellent insulators nor conductors. They mainly manipulate, amplify, switch, or control the flow of electric current or voltage in a circuit. Power Semiconductor Devices, like diodes, transistors, thyristors, and sensors require power to do their job. A circuit is an interconnection of components. These components are capable of performing active functions like amplification, rectification, and switching they are called Power Semiconductor Devices.

In this article, we will be going through Semiconductor Devices Which are mainly divided into Diodes, Thyristors and Transistors. Semiconductor Devices are Classified into Two-Terminals, Three-Terminals and Four-Terminals Devices, We will go in-depth about these Devices. We will go through working of diodes, Thyristors, Transistors and Look at Its vi characteristics, At last we will conclude our Article with Advantages, Disadvantages, Applications, and Some FAQs.