Silicon Controlled Rectifier (SCR)

SILICON CONTROLLED RECTIFIER (SCR)

Introduction

The SCR stand for Silicon Control Rectifier, it is used in industries because it can handle high values of current and voltage.

Three terminals

·        Anode - P-layer

·        Cathode - N-layer (opposite end)

·        Gate - P-layer near the cathode

Three junctions - four layers

Connect power such that the anode is positive with respect to the cathode - no current will flow

A silicon controlled rectifier i s 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.

Figure 5.7 Basic Structure, equivalent transistor model and sy mbol of SCR

Construction

When a pn junction i s added to a junction transistor the resulting three pn junction device is called a SCR. ordin ary 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 o f transistor called Gate. GSCR is a solid state e quivalent of thyratron. The gate anode and cathode of SCR correspond to the grid plate and cathode of thyratron SCR is called thyristor.

Working Principle

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

1 SCR Operation / Working

The Silicon Control Rectifier SCR start conduction when it is forwa rd biased. For this purpose the cathode is kept at negative and anode at positive. When positivee clock pulse is applied at the gate the SCR turns ON.

When forward bias volt age is applied to the Silicon Control Rectifier SCR, the junction J1 and J3 become forward bias w hile the junction J2 become reverse bias.

When we apply a clock pulse at the gate terminal, the junction J2 become forward bias and the Silicon Control Rectifier SCR start conduction.The Silicon Control Rectifier SCR turn ON and OFF very quickly, At the OFF state the Silicon Control Rectifier SCR provide infinity resistance and in ON state, it offers very low resistance, which is in the range of 0.01O to 1O.

2 SCR Firing & Triggering

The Silicon Control Rectifier SCR is normally operated below the forward break over voltage (VBO). To turn ON the Silicon Control Rectifier SCR we apply clock pulse at the gate terminal which called triggering of Silicon Control Rectifier, but when the Silicon Control Rectifier SCR turned ON, now if we remove the triggering voltage, the Silicon Control Rectifier SCR will remain in ON state. This voltage is called Firing voltage.

3 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.

4 When Gate is Positive w.r.to 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 loses 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.

Break over Voltage

It 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.

Turning OFF methods of Silicon Control Rectifier - SCR

There are two methods through which Silicon Control Rectifier SCR can be turned OFF,

1 Anode current interruption method

Figure 5.8 SCR turn-off by anode Current

In this method a paralle l or a series switch is used to turn OFF the S ilicon Control Rectifier (SCR electronics) by turning O FF the switch.

2 Forced Commutation method

Figure 5.8 SCR turn-off by forced communicatio n

In this method a revers ed polarity battery is connected, so the cur rent through the Silicon Control Rectifier SCR is reduced and it turn OFF.

V-I Characteristics of SCR

Figure 5.9 V-I Characteristics of SCR

1 Forward Characteristics

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 zer o point A is reached .SCR starts conducti ng voltage across SCR suddenly drops (dotted curve AB) most of supply voltage appears across RL.

2 Reverse Characteristi cs

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

3 Application

·        SCR as a switch

·        SCR Half and Full wave rectifier

·        SCR as a static contactor

·        SCR for power control

·        SCR for speed control of d.c.shunt motor

·        Over light detector