20 Dec 2022

firing circuit for Thyristor family. Turn ON Methods.

Thyristor can be turn on and latch into in is of into ON state by applying sufficient galo Current and a forward vig between Anode and cathode

The Method of turning on the tyristor is

called as+199ering.

Ale have to used a geprate triggering circuit to turn on the thyristor.

Different turns on method s

Thermal triggering @ By light Triggering forward vig luggering.

(3) dy/dl Triggering.

Ginte Triggering Gale

Thermal triggering If the temperaturd of the thyristor innenses then

there is increase in no of election hole pairs in PN junction This will Increase the leakage current and hease the value of di and d2 (5-0150

In: dates + (ICO.&co₁)

IP dit d² lends towards unity then thyristor may he turn on this is called as thermal Iuggeving. The breakoves vtg goes on demeases if the lemp is Incentes
In short due to the increase in temp the therma +generated Leckage current increases. this current coill Mulнeved in Internal.

light triggering: (Focussing light) If the light of adicale Frequency and intensity Is aloud to Strike on the thyristor junction then Photons will Strickes and I will increases electron

ehele hole pairs and thyristor may be turn ON due to Increase in Leckage current. and there fore this principal is also used turn on the LASCR

forward vig triggering - As there is increase in forward Vig between Anode and cathada of the trelor leakage curgent of thy- -TISTOT Increases. due to the internal current multiplication this current increases as the forw- -ard utg Reaches to breakoves vtg. (VBO) then the junction 72 colll break and thyristor will be turn on this called as forward Vig triggering even if gale is open

dvldt triggering

Talith Forward vig applied across the SCR the junction J1 33

are fanward biased & Junellon

32 is reverse biased
Space charge exist in the depletion aeg on nearly Junction 12 and hence this Junction J2 behaves like a capacitor. If the Forward vtg is suddenly applied the charging current through the junction

Capacitance fest (ei) this may turn ON the

SCR. The Forward vtg (VA) appred between ancie

and cathode. Now thi Forward Vtg appears

across junction

through the capacitor ic id q


g (ci xv)

Gdva + va da

dt dt As the Junction capacitance is constant the

hence the rate of rise of Forward vig

dua is high and charging current is also high.. dt and this current plays impartant role of gate cument to turn on the thyristor even if gate current is zero.

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