Silicon Controlled Rectifier - Power Electronics

What is Silicon Controlled Rectifier?

Silicon controlled rectifier is General Electric's trade name for a type of thyristor. Silicon controlled rectifier or semiconductor-controlled rectifier is a four-layer solid state current-controlling device.

These SCRs will be widely used in electronic devices which have to control high voltage and power. It can also be applicable in medium and high AC power operations like motor control function.

SCR will be carried out when a gate pulse is applied to it, just like a diode. It will have four layers of semiconductors which form two structures namely; NPNP or PNPN. Additionally, it will have three junctions called as J1, J2 and J3 and three terminals (anode, cathode and a gate). Below is the diagrammatic representation of an SCR.

scr.jpg

Anode will be connected to the P-type, cathode to the N-type and the gate to the P-type as shown in below figure.

pnpn_junction.jpg

In an SCR, necessary dopants will be infused to the intrinsic semiconductor which is silicon. But doping a PNPN junction will be depending on the SCR application.

Modes of Operation in SCR

  • OFF state (forward blocking mode) – Here, a positive voltage will be assigned to an anode, zero voltage will be assigned to the gate (disconnected) and negative voltage will be assigned to the cathode. As a result, Junctions J1 and J3 will be in forward bias whereas J2 will be in reverse bias. J2 will be reaching its breakdown avalanche value and will start to perform. Below this value, resistance of J1 will be considerably high and is it will be in the off state.
  • ON state (conducting mode) − SCR will be in ON state either by rising the potential difference between the anode and cathode above the avalanche voltage or by applying a positive signal at the gate. Instantly, SCR will start performing and gate voltage will not be needed anymore for maintaining the ON state. Thus, it will be switched off by
  • Lessening the current flow through it to the lowest value known as holding current
  • Making use of a transistor which is placed across the junction.
  • Reverse blocking – reverse blocking will compensate the drop in forward voltage and this is due to the fact that a low doped region in P1 is required. Remember that the voltage ratings of forward and reverse blocking will be equal.

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