Construction of Thyristor or Construction of SCR

As we now know that a thyristor is a four-layered and three-junction device. The four layers being p-n-p-n, i.e. two p-type and two n-type arranged in alternate p and n layers.

Practically, the layers are pnpn+, i.e. the doping concentration of the two n-type layers are different while it is the same for both p-type layers. But we will simply use p-n-p-n construction.

Thyristors have two following shapes.

    1. Stud Type Thyristor
    2. Capsule (or disc) Type Thyristor
stud type thyristor
Stud Type Thyristor
capsule type thyristor
Capsule Type Thyristor

We use all the power electronics devices as Switches. Hence, the thyristor is also nothing more than a switching device.

Junctions of Thyristor

The four layers of the thyristor form three junctions. We name them as J1, J2, and J3 as shown below in the diagram of the thyristor.

Terminals of Thyristor

Thyristor has three terminals.
Anode: The terminal connected to the outer p – layer is called Anode.

Construction of Thyristor
Construction of Thyristor

Gate: It is the terminal connected to the inner p – layer.
Cathode: It is connected to the outer n – layer.

Points to Remember:

  1. Practically, the manufacturer keeps the gate terminal near the cathode terminal as shown in the above figure.
  2. We use the threaded stud for tightening the thyristor to a heat sink using a nut.
  3. Thyristor gets much heated during its operation hence for better cooling, we have to use a heat sink fastened to the threaded stud of the thyristor.

Rating of Thyristor

Voltage – up to 10KV
Current – up to 3000A
Power – up to 30MW (10KV × 3000A)

Which type of Switch is Thyristor?

  •  It is a semi-controlled switch (we have already discussed it under the topic ‘types of switches’).
  •  In the thyristor, we use the gate terminal to turn it on but we cannot turn it off using the gate because thyristor is a semi-controlled switch.

What is the direction of flow of current in thyristor?

  • Remember that the thyristor is similar to a Diode (power diode) or we can say that thyristor = diode + gate terminal
    i.e. a thyristor is equivalent to a diode which can be turned on using the third terminal i.e. the gate terminal.
  • Hence, in a Thyristor current can only flow from Anode to Cathode similar to the unidirectional flow of current in diode.
  • Remember, current can never flow from cathode to anode in a thyristor (we will use this important concept later)

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