Introduction to Thyristors and Triacs
Brief History on Thyristors and Triacs:
- Although the large semiconductor diode was a predecessor to thyristors, the modern power electronics area truly began with advent of thyristors.
- One of the first developments was the publication of the P-N-P-N transistor switch concept in 1956 by J.L. Moll and others at Bell Laboratories, probably for use in Bell’s Signal application.
- However, engineers at General Electric quickly recognized its significance to power conversion and control and within nine months announced the first commercial Silicon Controlled Rectifier in 1957. This had a continuous current carrying capacity of 25A and a blocking voltage of 300V.
- Thyristors (also known as the Silicon Controlled Rectifiers or SCRs) have come a long way from this modest beginning and now high power light triggered thyristors with blocking voltage in excess of 6kv and continuous current rating in excess of 4kA are available.
- They have reigned supreme for two entire decades in the history of power electronics. Along the way a large number of other devices with broad similarity with the basic thyristor (invented originally as a phase control type device) have been developed.
- They include, inverter grade fast thyristor, Silicon Controlled Switch (SCS), light activated SCR (LASCR), Asymmetrical Thyristor (ASCR) Reverse Conducting Thyristor (RCT), Diac, Triac and the Gate turn off thyristor (GTO).
- From the construction and operational point of view a thyristor is a four layer, three terminals, minority carrier semi-controlled device. It can be turned on by a current signal but cannot be turned off without interrupting the main current.
- It can block voltage in both directions but can conduct current only in one direction. During conduction it offers very low forward voltage drop due to an internal latch-up mechanism.
- Thyristors have longer switching times (measured in tens of μs) compared to a BJT.
- This, coupled with the fact that a thyristor cannot be turned off using a control input, have all but eliminated thyristors in high frequency switching applications involving a DC input (i.e, choppers, inverters).
- However in power frequency ac applications where the current naturally goes through zero, thyristor remain popular due to its low conduction loss its reverse voltage blocking capability and very low control power requirement.
- In fact, in very high power (in excess of 50 MW) AC – DC (phase controlled converters) or AC – AC (cyclo-converters) converters, thyristors still remain the device of choice.