SILICON CONTROLLED RECTIFIER (SCR)

INTRODUCTION:-

A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a four-layer solid state

device that controls current.

The name "silicon controlled rectifier" or SCR is Generel electronics's trade name for a type of thyrister.

The SCR was developed by a team of power engineers led by Gordon Hall and commercialised by

Frank W. "Bill" Gutzwiller in 1957.

SCR...

SYMBOLS:-

OPERATION:-

An SCR is a type of rectifier, controlled by a logic gate signal. It is a four-layer, three-terminal device.

A p-type layer acts as an anode and an

n-type layer as a cathode; the p-type layer closer to the n-type (cathode) acts as a gate. It is

unidirectional in nature.

CONSTRUCTION OF SCR:-

It consists of a four layers pellet of P and N type semiconductor materials. Silicon is used as the

intrinsic semiconductor to which the proper

impurities are added. The junctions are either diffused or alloyed. The Planar construction is used

for low power SCR's, here all the junctions

are diffused.

The Mesa type construction is used for high power SCR's. In this case junction J2 is obtained by

diffusion method and then the outer two

layers

are alloyed to it because the PNPN pellet is required to handle large currents. It is properly braced

with tungsten or molybdenum plates to

provide

greater mechanical strength. One of these plates is hard soldered to a copper stud, which is

threaded for attachment of heat sink.

The doping

of PNPN will depend on the application of SCR.

MODES OF OPERATION:-

In the normal "off" state, the device restricts current to the leakage current. When the

gate-to-cathode voltage exceeds a certain threshold,

the device turns "on" and conducts current. The device will remain in the "on" state even after gate

current is removed so long as current

through the device remains above the holding coupling. Once current falls below the holding

current for an appropriate period of time,

the device will switch "off". If the gate is pulsed and the current through the device is below the

holding current, the device will remain in the

"off" state.

If the applied voltage increases rapidly enough, capacitive coupling may induce enough charge

into the gate to trigger the device into the "on"

state; this is referred to as "dv/dt triggering." This is usually prevented by limiting the rate of voltage

rise across the device, perhaps by using

asnubber. "dv/dt triggering" may not switch the SCR into full conduction rapidly and the

partially-triggered SCR may dissipate more power

than is usual, possibly harming the device.

SCRs can also be triggered by increasing the forward voltage beyond their rated break down

voltage (also called as break ver voltage),

but again, this does not rapidly switch the entire device into conduction and so may be harmful

so this mode of operation is also usually

avoided. Also, the actual breakdown voltage may be substantially higher than the rated breakdown

voltage, so the exact trigger point will

vary from device to device.

SCRs are made with voltage ratings of up to 7,500 V, and with current ratings up to 3,000 RMS

amperes per device. Some of the larger ones

can take over 50 kA in single-pulse operation. SCRs are used in power switching, phase control,

chopper, battery charger, and inverter

circuits. Industrially they are applied to produce variable DC voltages for moters (from a few to

several thousand HP) from AC line voltage.

They control the bulk of the dimmers used in stage lighted, and can also be used in some electric

vehicles to modulate the working

voltage in a jacabson circuit. Another common application is phase control circuits used with

inductive loads. SCRs can also be found in

welding power suplies where they are used to maintain a constant output current or voltage.

Large silicon-controlled rectifier assemblies

with many individual devices connected in series are used in high voltage DC converter stations.

Two SCRs in "inverse parallel" are often used in place of a TRIAC for switching inductive loads

on AC circuits.

Because each SCR only conducts for half of the power cycle and is reverse-biased for the

other half-cycle, turn-off of the SCRs is assured.

By comparison, the TRIAC is capable of conducting current in both directions and assuring that

it switches "off" during the brief

zero-crossing of current can be difficult.

Typical electrostatic discharge (ESD) protection structures in integrated circuits produce a

parasitic SCR. This SCR is undesired;

if it is triggered by accident, the IC can go into latch up and potentially be destroyed.

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