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An SCR consists of four layers of alternating P and N type semiconductormaterials. Silicon is used as the

intrinsic semiconductor, to which the properdopants are added. The junctions are either diffused or

alloyed. The planar construction is used for low power SCRs (and all the junctions are diffused). Themesa

type construction is used for high power SCRs. In this case, junction J2 is obtained by the diffusion

method and then the outer two layers are alloyed to it, since the PNPN pellet is required to handle

large currents. It is properly braced with tungstenormolybdenum plates to provide greater mechanical

strength. One of these plates is hard soldered to acopperstud, which is threaded for attachment ofheat

sink. The doping of PNPN will depend on the application of SCR, since its characteristics are similar to

those of the thyratron. Today, the term thyristor applies to the larger family of multilayer devices that

exhibit bistable state-change behaviour, that is, switching either ON or OFF.

The operation of a SCR and otherthyristorscan be understood in terms of a pair of tightly coupled bipolar

junction transistors, arranged to cause the self-latching action:

[edit]Modes of operation

In the normal "off" state, the device restricts current to theleakage current. When the gate-to-

cathodevoltage 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 current. 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 isbelow the holding current, the device will remain in the "off" state.

If the applied voltage increases rapidly enough, capacitive couplingmay 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 a snubber. "dv/dt

triggering" may not switch the SCR into full conductionrapidly 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 breakdown

voltage(also called as break over 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. This device is generally used in switching applications.

[edit]Reverse Bias

SCR are available with or without reverse blocking capability. Reverse blocking capability adds to the

forward voltage drop because of the need to have a long, low doped P1 region. Usually, the reverse

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blocking voltage rating and forward blocking voltage rating are the same. The typical application for

reverse blocking SCR is in current source inverters.

SCR incapable of blocking reverse voltage are known as asymmetrical SCR, abbreviated ASCR.

They typically have a reverse breakdown rating in the 10's of volts. ASCR are used where either a

reverse conducting diode is applied in parallel (for example, in voltage source inverters) or where

reverse voltage would never occur (for example, in switching power supplies or DC traction

choppers).

Asymmetrical SCR can be fabricated with a reverse conducting diode in the same package. These

are known as RCT, forreverse conducting thyristor.

[edit]Thyristor turn on methods

1. forward voltage triggering

2. gate triggering

3. dv/dt triggering

4. temperature triggering

5. light triggering

Forward voltage triggering occurs when the anode-cathode forward voltage is increased with the gate

circuit opened. This is known as avalanche breakdown, during which junction j2 will breakdown. At

sufficient voltages, the thyristor changes to its on state with low voltage drop and large forward

current. In this case, J1 and J3 are already forward biased.

[edit]Application of SCRs

SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage,

is demanded. Their operation makes them suitable for use in medium to high-voltage AC powercontrol applications, such as lamp dimming, regulators and motor control.

SCRs and similar devices are used for rectification of high power AC inhigh-voltage direct

currentpower transmission. They are also used in the control of welding machines, mainly MTAW

and GTAW processes.

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