Thyristors

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What is Thyristor?

The term Thyristor came from its gas tube equivalent, thyratron. Its family name includes,
  • Silicon Controlled Rectifiers(SCR)
  • Bidirectional Triode Thyristors(TRIAC)
  • Gate Turn-off Thyristors(GTO)
  • MOS Controlled Thyristors(MCT)
  • Integrated Gate-Commutated Thyristors(IGCT)
Thyristors are operated as bi-stable switches, operating from non conducting state to conducting state. Thyristor can recover from its conducting state to non conducting state only when the current is brought to zero. It has four or more layers and three or more junctions. This device blocks voltage in both forward and reverse direction. The SCR is almost referred to as thyristor. 




 Thyristor(SCR) Characteristics

SCR is a four-layer semiconductor device of PNPN structure with three p-n junctions. It has three terminals namely, anode, cathode and gate. The cross section of thyristor can be split into sections of NPN and PNP transistors. 
When the anode voltage is made positive with respect to cathode, the junction J1 and J3 are forward biased and J2 is reverse biased. Only a small leakage current flows from anode to cathode. Then the thyristor is said to be in forward blocking mode or off-state and the leakage current is said to be off-state current. If anode-to-cathode voltage is increased to a large value, the junction J2 breaks. This is known as avalanche breakdown. And the corresponding voltage is called forward breakdown voltage. As J1 and J3 are already forward biased, there is free movement of carrier across the three junctions, resulting in a large forward anode current. The device is then in a conducting state or on-state. 
Latching current is the minimum anode current required to maintain the thyristor in the on-state after the thyristor has been turned on and gate signal has been removed. The anode current must be more than latching current to maintain the required amount of carrier flow across the junction. 
Holding current is the minimum anode current to maintain the thyristor in on-state. Once a thyristor conducts, it behaves like a conducting diode and there is no control over the device. If the forward anode current is reduced below holding current, a depletion region develops around junction J2 and the thyristor goes to blocking state. The holding current is less than latching current. 



A thyristor can be turned ON by increasing the forward voltage beyond breakdown voltage. But, such a turn ON could be destructive. The forward voltage is maintained below breakdown voltage and the thyristor is turned on by applying positive gate pulse. Once the thyristor is turned on, it continues to conduct even after the gating signal is removed. It is a latching device(once triggered means device will be in ON condition).

Bidirectional Triode Thyristors(TRIAC)

A TRIAC is a bi-directional device as it can conduct in both directions and is normally used in ac-phase control. It is equivalent to two SCRs connected in anti-parallel with a common gate connection. The terminals of TRIAC cannot be designated as anode and cathode as it is a bidirectional device.
It can be turned ON with either positive or negative gate signal. If terminal MT2 is positive with respect to MT1, the TRIAC can be turned on by applying a positive gate signal between gate G and terminal MT1. If terminal MT2 is negative with respect to MT1, the TRIAC can be turned ON by applying a negative gate signal between gate G and terminal MT1. TRIACs are normally operated in Ist quadrant(positive gate voltage and gate current) or IIIrd quadrant(negative gate voltage and current). The terminal MT1 is taken as the point for measuring the voltage and current at the gate and MT2 terminal.
   

Applications
  • AC voltage stabilizers
  • Switch
  • Choppers
  • Inverters
  • DC circuit breaker                                                                                                                                                                                                 Download PDF
References

1] M.D. Singh and K.B. Khanchandani, "Power Electronics", Mc Graw Hill India, 2013.
2] Rashid M.H., "Power Electronics Circuits, devices and Applications", Prentice Hall, India, Third            Edition, New Delhi, 2004.
3] P.S.Bimbra, "Power Electronics", Khanna Publishers, Eleventh Edition, 2003.

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