Silicon Controlled Rectifier (SCR):The silicon controlled rectifier is a semiconductor device, which is used for changing the alternating current into direct current and during this time it also controlled the power feeding to the load side. Essentially, it is schockety diode but if we use it as a schockety diode then its uses are limited. Therefore the silicon control rectifier is a progression form of schockely diode by adding a gate in schockety diode.It was developed in 1957 by power electronics engineers. Some peoples say the thyristor and silicon controls are synonymous but some peoples say the silicon controlled is proper subset, of the set of thyristors. Asper the Bill Gutzwillert the term silicon controlled is earlier and the term thyristor is later but if we compared the silicon controlled with triac which is also a thyristor then it would be cleared, that the silicon controlled is unidirectional device and triac is bidirectional device. The physical, equivalent schematic and schematic symbol diagram is shown is figure 1
The silicon controlled rectifier consists of three PN junctions which are labeled by J1, J2, J3 and four semiconductor material layers, which are joined in the form of NPNP and PNPN. In PNPN type structure the anode terminal is connected at p-type material layer and cathode terminal is connected at N-type material layer. Similarly, the gate terminal of the silicon controlled rectifier is connected at P-type material layer which is nearest to the cathode type material layer. The semiconductor material layers are formed by the diffusion of lightly doped semiconductor material into highly doped semiconductor material. The whole construction and junction diagram is shown in figure 2.
The silicon controlled rectifier works in three mode of operations
Forward Blocking Mode:In this mode of operation, the anode terminal of silicon control rectifier is connected to positive terminal of the power supply and cathode terminal is connected at the negative terminal of the power supply. While the zero voltage is an applied at gate terminal. In this condition the junction J1, J2 would be forward biased and junction J3 would be reverse biased then there would be a small current flow from anode to cathode that is called leakage current. Due to this leakage current the voltages across anode and cathode increased until the breakdown the occurred between anode and cathode. The voltage at which the breakdown is occurred is called breakdown voltage and this mode is called forward blocking mode.
Reverse Blocking Mode: In this mode of operation the anode terminal is connected to the negative terminal of the power supply and cathode terminal is connected to positive terminal to the power supply. In this mode of operation, the current also can’t flows from anode to cathode and block the voltage this silicon control rectifier. Normally the forward blocking voltages and reverse blocking voltages are almost same.
Forward Conduction Mode: In this mode of operation the anode terminal is connected at positive terminal of power supply, cathode terminal is connected at negative terminal of power supply and positive pulse voltages are applied at gate terminal. When these positive voltages are applied at gate terminal then the silicon control rectifier is forward biased and current flows from anode to cathode. This mode is called forward conduction mode. Once it is forward biased then there is no need of any voltages at gate terminal. It would be in on state until it would be turned off. Two ways are applied for turned off this silicon control rectifier either the gate current is reduced by the holding current or anode and cathode are shorted through any transistor or push button.
There are four main types of silicon controlled rectifier,
1 Reverse Conducing Thyristor or SCR (RCT)
2 Gate Assisted Turn off Thyristor or SCR (GATT)
3 Gate Turn off Thyristor or SCR (GTO)
4 Asymmetric Thyristor or SCR
Reverse Conducting SCR (RCT):Normally, the thyristor conducts in forward direction but this type of thyristor conducts in reverse direction because this have an integrated diode for this conduction but in reverse direction there is no any facility of control the current or power. When it is conducting in reverse direction then this device and integrated diode do not conduct simultaneously therefore there is no so much heat produced so the both device and integrated diode can be cooled at same time. This type of silicon control rectifiers is often used in frequency changers and switching inverters.
Gate Assisted Turn Off SCR (GATT): This type of thyristor is used in that places where the fast turn off process is required because sometimes the negative voltage is applied at gate terminal for reducing the anode to cathode voltage. This negative voltage drains the minority carriers in N-type region and ensured gate to cathode junction do not forward biased. The structure of this type is almost same only differ the cathode strip is sometimes used for increasing the gate control of this type.
Gate Turn off SCR (GTO):This type of thyristor is sometimes referred to as gate turn off switch and this type usually count the pure thyristor type device. For turn off this type only negative voltage is applied at gate terminal and no need to turn off the anode to cathode voltages.
Asymmetric Thyristor or SCR:This type of thyristor is used in that places where the circuit does not see any reverse voltages and do not required any rectifier capability. Therefore, in this type of thyristors the junction J2 is so much thinner than the others two and resulted the N-type region provides very low Von voltage which improves the turn on and turn off time of thyristor.
The silicon controlled rectifier is used in different applications some which are listed below.
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