THYRISTORS AND IC FABRICATION

SILICON CONTROLLED RECTIFIERS (SCR)

A silicon controlled rectifier is a semiconductor device that acts as a true electronic switch. it can change alternating current and at the same time can control the amount of power fed to the load. SCR combines the features of a rectifier and a transistor.

CONSTRUCTION
When a pn junction is added to a junction transistor the resulting three pn junction device is called a SCR. ordinary rectifier (pn)  and a junction transistor (npn) combined in one unit to form pnpn device. three terminals are taken : one from the outer p- type material called anode a second from the outer n- type material called cathode K and the third from the base of transistor called Gate. GSCR is a solid state equivalent of thyratron. the gate anode and cathode of SCR correspond to the grid plate and cathode of thyratron SCR is called thyristor

WORKING
Load is connected in series with anode the anode is always kept at positive potential w.r.t cathode.

WHEN GATE IS OPEN

No voltage applied to the gate, j2 is reverse biased while j1 and j3 are FB . J1 and J3 is just in npn transistor with base open .no current flows through the load RL and SCR is cut off. if the applied voltage is gradually increased a stage is reached when RB junction J2 breakdown .the SCR now  conducts heavily and  is said to be ON state. the applied voltage at which SCR conducts heavily without gate voltage is called Break over Voltage.

WHEN GATE IS POSITIVE W.R.T CATHODE.

The SCR can be made to conduct heavily at smaller applied voltage by applying small positive potential to the gate.J3 is FB and J2 is RB the electron from n type material start moving across J3 towards left holes from p type toward right. electrons from j3 are attracted across junction J2 and gate current starts flowing. as soon as gate current flows anode current increases. the increased anode current in turn makes more electrons available at J2  breakdown and SCR starts conducting heavily. the gate looses all control if the gate voltage is removed anode current does not decrease at all. The only way to stop conduction is to reduce the applied voltage to zero.

BREAKOVER VOLTAGE
I t is the minimum forward voltage gate being open at which SCR starts conducting heavily i.e turned on

PEAK REVERSE VOLTAGE( PRV)
It is the maximum reverse voltage applied to an SCR without conducting in the reverse direction.

HOLDING CURRENT

It is the maximum anode current gate being open at which SCR is turned off from on conditions.

FORWARD CURRENT RATING
It is the maximum anode current that an SCR is capable of passing without destruction

CIRCUIT FUSING RATING
It is the product of of square of forward surge current and the time of duration of the surge

 VI CHARACTERISTICS OF SCR

FORWARD CHARCTERISTICS

When anode is +ve w.r.t cathode the curve between V & I is called Forward characteristics. OABC is the forward characteristics of the SCR at Ig =0. if the supplied voltage is increased from zero point A is reached .SCR starts conducting voltage across SCR suddenly drops (dotted curve AB) most of supply voltage appears across RL

REVERSE CHARCTERISTICS

When anode is –ve w.r.t.cathode the curve b/w V&I is known as reverse characteristics reverse voltage come across SCR when it is operated with ac supply reverse voltage is increased anode current remains small avalanche breakdown occurs and SCR starts conducting heavily is known as reverse breakdown voltage

SCR as a switch
SCR Half and Full wave rectifier

Application

SCR as a static contactor
SCR for power control
SCR for speed control of d.c.shunt motor
Over light detector