A thyristor is a semiconductor device that acts as a bistable switch, meaning it can be turned on and off under specific conditions. Thyristors are widely used in power control applications due to their ability to handle high currents and voltages. The turning on and off processes involve several stages and are typically controlled by triggering signals. Let’s explore how a thyristor gets turned on and turned off:
Turning On (Firing):
- Forward Biasing:
- To initiate the turning-on process, a forward voltage is applied across the anode and cathode terminals of the thyristor.
- The anode is at a higher potential than the cathode, creating a forward bias condition.
- Gate Signal (Triggering):
- The thyristor has an additional terminal called the gate.
- To turn the thyristor on, a short-duration pulse or a continuous signal is applied to the gate terminal.
- This gate signal triggers the turning-on process by injecting minority carriers into the semiconductor layers.
- Injection of Minority Carriers:
- The gate signal causes the injection of minority charge carriers (holes for an NPNP thyristor or electrons for a PNPN thyristor) into the region near the junction between the anode and the cathode.
- Regeneration Process (Avalanche Breakdown):
- The injected carriers initiate a regenerative process called avalanche breakdown.
- Avalanche breakdown rapidly increases the number of charge carriers, reducing the resistance across the thyristor and allowing a large current to flow.
- Latching:
- Once the avalanche breakdown occurs, the thyristor latches into the on-state, and the gate signal is no longer needed to sustain conduction.
- The thyristor remains conducting until the anode-cathode voltage drops to zero or a reverse voltage is applied.
Turning Off:
- Reverse Biasing:
- To turn off the thyristor, the anode voltage is made negative with respect to the cathode, creating a reverse bias condition.
- Reducing Current Below Holding Current:
- The anode current is gradually reduced below a critical level known as the holding current.
- Below the holding current, the thyristor cannot sustain conduction, and it enters the off-state.
- Natural Turn-Off:
- The thyristor naturally turns off when the anode-cathode voltage reverses during the AC cycle or when the external circuit reduces the current below the holding current.
- Forced Turn-Off (Gate Pulse):
- In some cases, a reverse gate pulse can be applied to force turn off the thyristor by neutralizing the stored charge within the device.
In summary, turning on a thyristor involves forward biasing, applying a gate signal, initiating avalanche breakdown, and latching into conduction. Turning off can be achieved by reducing the current below the holding current or applying a reverse gate pulse. The natural turn-off occurs when the anode-cathode voltage reverses or the external circuit reduces the current.
