For light triggered scrs a slot niche is made in the

•
For light-triggered SCRs, a slot (niche) is made in the
inner p-layer.
•
When it is irradiated, free charge carriers are
generated just like when gate signal is applied b/w
gate and cathode.
•
Pulse light of appropriate wavelength is guided by
optical fibers for irradiation.
•
If the intensity of this light thrown on the recess
exceeds a certain value, forward-biased SCR is turned
on. Such a thyristor is known as light-activated SCR
(LASCR).
•
Light-triggered thyristors is mostly used in high-
voltage direct current (HVDC) transmission systems.
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5. dv/dt triggering
•
With forward voltage across anode & cathode of a thyristor, two
outer junctions (A & C) are forward biased but the inner junction
(J2) is reverse biased.
•
The reversed biased junction J2 behaves like a capacitor because of
the space-charge present there.
•
As p-n junction has capacitance, so larger the junction area the
larger the capacitance.
•
If a voltage ramp is applied across the anode-to-cathode, a current
will flow in the device to charge the device capacitance according to
the relation:
•
If the charging current becomes large enough, density of moving
current carriers in the device induces switch-on.
•
This method of triggering is not desirable because high charging
current (Ic) may damage the thyristor.
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