What is Thyristor? – Thyristor Family and its Applications

Thyristor is the general name given to a family of semiconductor devices having four layers with a control mechanism, although this term is most commonly applied to the SCR (silicon control­led rectifier). This term is derived from thyratron and transistor because the device combines the rectification action of thyratron and control action of transistor. Thyristor stands for THYRatron transISTOR.

The device was first developed at the Bell Laboratories in the USA and was commercially introduced by the General Electrical Company in the USA in 1957. The device appeared in the market under different names such as SCR, thyristor, thyrode etc. Since its inception the thyristor has come to stay as a basic building block in many industrial and power system applications. Its ability to be controlled, compactness, fast response, high reliability, better effi­ciency, large power handling capacity, high voltage and current ratings, good trigger sensitivity, static operation, large power gain, sturdy construction, long life, very little maintenance and low cost of fabrication due to advancement in the field of fabrication—have given the thyristor a colorful reception in every field.

Today thyristors are finding applications in the control of dc/ac motors; for the improvement of power factor; and as switching devices. They have become an integral part of HVDC transmission. They are so advantageous that our major steel plants had decided to mod­ernise their plants by replacing MG sets and mercury-arc rectifiers by thyristor converters. Thyristors have helped in further cost re­duction and in the development of drive system by changing the emphasis from dc motors to ac motors. With cycle converters and inverters, the speed of an ac motor can also be controlled with ease and reliability. Apart from these main applications it finds use as a switching device, particularly in the improvement of power factor of transmission lines and mains. Thyristor can be used as a power switching device with a power handling capacity ranging from a few watts to as high as 4 MW (2,500 A at 1,600 V). Some thyristors have a rating as high as 400 A, 10,000 V for use in hv dc transmis­sion lines.

Thyristors, with its large number of advantages and tremen­dous control capabilities, have numerous applications and have completely replaced the electromagnetic control systems. Thyristor basically serves two functions viz. electronic switching and electronic control.

Some of the applications of thyristors are listed below:

1. Speed control of dc and ac motors.
2. As rectifier for conversion of ac into dc.
3. As inverter for conversion of dc into ac.
4. As dc chopper or dc to dc converter for converting dc at one level to dc at another level.
5. As cycloconverter for converting ac of one frequency into ac of another frequency.
6. Control of temperature, level, position and illumination.
7. Power switches (dc and ac circuit breakers).
8. HVDC transmission lines.
9. Improvement of power factor in transmission lines.
10. As static switches.
11. Control of induction heating.
12. Relay control.
13. Phase control.
14. As special power supplies for aircraft and computers etc.

SCRs and triacs having high voltage and current ratings are widely employed for power control applications whereas other members of thyristor family are employed for small power applications and for switching in control and digital circuits.

Thyristor Family:

The P-N-P-N devices with zero, one or two gates constitute the basic thyristor. But today the thyristor family, includes other similar multilayer devices also. The complete list of thyristor family members include diac (bidirectional diode thyristor), triac (bidirectional triode thyristor), SCR (silicon controlled rectifier), Shockley diode, SCS (silicon controlled switch), SBS (silicon bilateral switch), SUS (silicon unilateral switch) also known as complementary SCR or CSCR, LASCR (light activated SCR), LAS (light activated switch) and LASCS (light activated SCS).

The most important member of the thyristor family is the silicon controlled rectifier (SCR). SCR is a four layer (P-N-P­-N), three junction semiconductor device with three terminals, namely, the anode, the cathode and the gate. It is a unilateral device and conduction takes place from anode to cathode under proper bias conditions (forward bias).

Diacs and triacs are bidirectional devices. The diac is a two terminal, three layer device and is commonly used for triggering triacs. The triac is a 3 terminal semiconductor device and may be considered equivalent to two SCRs connected in antiparallel. Shockley diode is a two terminal reverse blocking diode thyristor having no gate. SCS (silicon controlled switch) is similar to SCR except that it has two gates and can be turned on or off by either gate. SUS (silicon unilateral switch) has gate on the anode side and can be employed as a programmable unijunction transistor (PUJT). SBS (silicon bilateral switch) is a device consisting of two identical SUS structures arranged in antiparallel but having only one gate, that is used only for external synchronization or for proper biasing.

LASCR is the light activated SCR which is turned on by photon bombardment.

UJT (unijunction transistor), unlike a bipolar transistor has only one junction, and like other conventional transistors, it processes the transistor action and operates like a switch. The characteristics of UJT are similar to those of a SUS. Its construction is, however, different and it does not belong to thyristor family.