15 |
P a g e
(b) Thermal (or) Temperature Triggering:-
The width of depletion layer of SCR decreases with increase in junction temperature.
Therefore in SCR when V
AR
is very near its breakdown voltage, the device is triggered by increasing
the junction temperature.
By increasing the junction temperature the reverse biased junction collapses thus the device starts to
conduct.
(c) Radiation Triggering (or) Light Triggering:-
For light triggered SCRs a special terminal niche 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
(d) 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.
If
voltage across the device is V, the charge
by Q and capacitance by C then,
i
c
=dQ/dt
Q=CV
i
c
=d(CV)/dt
=CdV/dt+VdC/dt
as dC/dt = 0
i
c
= CdV/dt
Therefore when the rate of change of voltage across the device becomes large, the device may turn
ON, even if the voltage across the device is small.
(e) 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.
