Basics, Charectarastics and Triggering types

1. THYRISTORS
SHANKAR
Radel Electronics Pvt ltd

2. Introduction
Semiconductor devices
 Diode
 Transistor
 Thyristors

3. Introduction
 Thyristor is the most important type of power
semiconductor devices.
 A Thyristor is a four layer, semiconductor of p-n-p-
n structure with three p-n junctions. It has three
terminals, the anode, cathode and the gate.
 They are operated as bi-stable switches from non-
conducting to conducting state.
 This also called uni-lateral switch
 The word Thyristor is coined from Thyratron and
transistor. It was invented in the year 1957 at Bell
Labs.

4. Thyristors family
 SCR
 TRIAC
 DIAC
 GTO
 SUS
 SBS

5. Silicon Controlled Rectifier
(SCR)

6. Constructional Features

7. V-I Characteristics of SCR

8. LATCHING CURRENT (IL)
The minimum value of anode current
required to turn ON a Thyristors from it’s OFF
state and is usually greater than holding
current.
HOLDING CURRENT (IH)
The minimum value of anode current required
for a Thyristors to maintain itself in conduction
state

9. Forward Blocking Mode
 With anode to cathode voltage being small, only
leakage current flows through the device. The SCR is
then said to be in the forward blocking state.

10. Forward Conduction Mode
Thyristor is brought from forward blocking mode
to forward conduction mode by turning it on by
exceeding the forward break over voltage or by
applying a gate pulse between gate and
cathode

11. Reverse Blocking Mode
 When cathode is made positive with respect to
anode with Gate is open, Thyristors is reverse
biased as shown in below figure. Junctions J1 J3
are seen to be reverse biased whereas junction
J2 is forward biased. The device behaves as if
two diodes are connected in series with reverse
voltage applied across them.

12. Effects on gate current on forward
blocking voltage

13. THYRISTOR Triggering methods
 Thermal Turn on or High Temperature
Triggering
 Light Triggering
 High Voltage Triggering
 dv/dt Triggering
 Gate Triggering

14. Thermal Turn on or High
Temperature
 The width of depletion layer of SCR decreases
with increase in junction temperature.
 By increasing the junction temperature the
reverse biased junction collapses thus the device
starts to conduct.
 This type of turn on many cause thermal run
away and is usually avoided.

15. Light Triggering
 For light triggered SCRs a special terminal is made
inside the inner P layer instead of gate terminal.
 When light is allowed to strike this terminal, free charge
carriers are generated.
 When intensity of light becomes more than a normal
value, the Thyristor starts conducting.
 This type of SCRs are called as LASCR

16. High Voltage Triggering
 In this mode, an additional forward voltage is applied
between anode and cathode.
 When the anode terminal is positive with respect to
cathode(VAK) , Junction J1 and J3 is forward biased
and junction J2 is reverse biased.
 No current flows due to depletion region in J2 is
reverse biased (except leakage current).
 As VAK is further increased, at a voltage VBO (Forward
Break Over Voltage) the junction J2 undergoes
avalanche breakdown and so a current flows and the
device tends to turn ON(even when gate is open)
 This type of turn on is destructive and should be
avoided.

17. dv/dt Triggering
 When the device is forward biased, J1 and J3 are
forward biased, J2 is reverse biased.
 Junction J2 behaves as a capacitor, due to the
charges existing across the junction.

18. Gate Triggering
 This is most widely used SCR triggering method.
 Applying a positive voltage between gate and
cathode can Turn ON a forward biased Thyristor.
 When a positive voltage is applied at the gate
terminal, charge carriers are injected in the inner
P-layer, thereby reducing the depletion layer
thickness.
 As the applied voltage increases, the carrier
injection increases, therefore the voltage at which
forward break-over occurs decreases

19. Selection of SCR
 A-K voltage
 A-K current
 Gate Current
 Ton time
 Toff time
 Package
 Holding current
 Operational Temparature

20. Applications
 Used in AC-DC convertors
 Used in Invertors.
 High power switches.

21. TRIAC
 TRIAC from triode for alternating current
 It is Bi-directional device

22. Characteristics

23. DIAC
 DIAC diode for alternating current.
 Bi-directional device

24. V-I characteristics

25. END