1. Gandhinagar Institute Guided By: Prof. Purva Mistry
of Technology
Power Electronics( )
Topic:
“Thyristors”
Guided By: Prof. Purva Mistry
Branch : Electrical Engineering
Div: B
Batch : B1
Presented By:
Aditya Mehra( )
Rijay Doshi( )
Jay Kared ( )

2. PRESENTATION ON : I-V characteristics
Turn on methods, Switching characteristics
Two transistor model

3. Silicon Controlled Rectifier
A Silicon Controlled Rectifier (or thyristor) is a four layer solid state device that controls current flow
The name “silicon controlled rectifier” is a trade name for the type of thyristor commercialized at General Electric in 1957
The name thyristor has been derived from words “thyratron tube ” and ‘transistor’ because it has characteristics which is combination of both.
It is unidirectional
It has a rating of 1200V / 1500A with switching frequencies ranging from 1KHz to 20KHz.

4. Construction of different semiconductor devices(for extra information)

5. Thyristor Volt-Ampere Characteristic

6. Modes of operation of SCR
Reverse blocking mode
Forward blocking mode
Forward conduction mode
Reverse blocking mode:
When the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place.

7. Now if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor starts conducting.
If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of VG, the thyristor can be switched into the on state suddenly.

8. Forward Blocking mode: When anode is given positive voltage and cathode is given negative voltage, scr is forward biased , junctions J1 and J2 are forward biased but J2 is reverse biased.
Forward conduction mode: When scr is in forward conduction mode and the anode voltage is increased then reverse voltage breakdown for junction J2 takes place , unlike diode the breakdown occurs in forward region that is reason for calling it forward break over voltage . It can also be achieved by applying gate pulse in forward conduction mode.

9. THYRISTOR TURN-ON METHODS
Forward voltage triggering : The SCR can be turned on by increasing forward biasing voltage until forward break over point but this method is not employed because it can damage SCR.
Gate triggering:
Simple , reliable and efficent.
When gate pulse is applied , electrons are injected into p layer of SCR and magnitude of break over voltage is reduced.

10. Effects of Increasing Gate Current

11. Since n layer is heavily doped e’s go from n to p layer
Since n layer is heavily doped e’s go from n to p layer. This causes reduction in depletion layer of Junction J2 and forward break over voltage is reduced.
Higher gate pulse , lower is break over voltage.
dv/dt triggering:
In forward conduction mode junction J2 is reverse biased due to depletion layer.
There are space charges around depletion layer, so J2 behaves like capacitance.

12. Junction capacitance is constant, Therefore,
Ic = current due to capacitance charging
Va = Voltage due to capacitance
Junction capacitance is constant,
Therefore,
The charging current of capacitance plays role of gate when rate of change of voltage is high and SCR is triggered , even when actual gate current is zero.

13. Thermal triggering: In forward blocking state reverse leakage current is present , the magnitude of reverse leakage current increases with increase in temperature. This reverse leakage current can trigger conduction in SCR
Light triggering : LASCR(Light Activated SCR)
In this type of SCR , the triggering is done by light instead of gate pulse. Light plays role of gate current.

14. Two-Transistor Model

15. For a PnP transistor, Ic1 = α1 Ia + IcBO1
When transistor is in off state,
Ic = α Ie + IcBO
For a PnP transistor,
Ic1 = α1 Ia + IcBO1
For a PnP transistor, Ic2 = α Ik + IcBO2
Ia = Ic1+ Ic2
Ia= α2 Ik + IcBO2+ α1 Ia + IcBO1
Ia= α2 (Ia+Ig) + IcBO2+ α1 Ia + IcBO1

17. Switching Characteristics of Thyristor
Switching Characteristics during turn-on:
Total turn on time=
Delay time td +Rise time tr + Spread time tp
Delay time : The time measured from instant at which gate current reaches 0.9Ig to the instant at which anode current reaches 0.1 Ia.
Rise time : The time taken by anode current to tise from 0.1 Ia to 0.9Ia.

18. Spread time : The time taken by anode current to rise from 0.9Ia to Ia.
During turn on time SCR should be considered
Charge controlled device.
Greater the magnitude of gate pulse lesser, time it takes to inject charge.
The maximum power loss takes place during on time.
The turn on time ranges from 1to 4 microseconds.
Switching characteristics during turn-off:
The process of SCR from conduction state to forward blocking state is called commutation process or turn-off process.

19. Turn off time = Reverse recovery time + Gate recovery time.
Reverse recovery time: Time taken by SCR to remove holes from p layer and electrons from n layer.
Gate recovery time :The electrons and holes of depletion layer are trapped around junction J2.The only way to terminate them is by recombination of holes and electrons. The time taken by recombination of holes and electrons is called Gate recovery time.
The SCR is truly off only when depletion layer has been eliminated.

21. References: POWER ELECTRONICS by P.S Bhimbra.
POWER ELECTRONICS by Dr.K.B Khanchandani.

23. Thank You…