Silicon controlled rectifier
Tutorial on power electronics
Silicon Controlled Rectifier
A silicon controlled rectifier or semiconductor controlled rectifier is a four-layer solid current -controlling device. The name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor.
SCRs are mainly used in electronic devices that require control of high voltage and power. This makes them applicable in medium and high AC power operations such as motor control function.
An SCR conducts when a gate pulse is applied to it, just like a diode. It has four layers of semiconductors which form two structures namely; NPNP or PNPN. In addition, it has three junctions labeled J1, J2 and J3 and three terminals (anode, cathode and a grid). An SCR is shown schematically as shown below.
The anode connects to type P, cathode to type N and gate to type P as shown below.
In an SCR, the intrinsic semiconductor is silicon into which the required dopants are infused. However, the doping of a PNPN junction depends on the SCR application.
Operating modes in SCR
OFF state (direct blocking mode) - Here the anode receives a positive voltage, the gate receives a zero voltage (disconnected) and the cathode receives a negative voltage. As a result, the junctions J1 and J3 are in forward bias while J2 is in reverse bias. J2 reaches its avalanche value of breakdown and begins to conduct. Below this value, the resistance of J1 is significantly high and is therefore said to be in the off state.
ON state ( driver mode) - An SCR is brought to this state either by increasing the difference ofpotential between the anode and cathode above the avalanche voltage or by applying a positive signal to the gate. Immediately the SCR starts to conduct, the gate voltage is no longer needed to maintain the ON state and is therefore deactivated by -
Reverse blocking - This compensates for the forward voltage drop. This is because a lightly doped region in P1 is required. It is important to note that the tension values of the front and rear lock are equal.