Thyristors Hard

Thyristors

• Thyristors are constructed using four semiconductor layers (pnpn).
• They include devices such as the 4-layer diode, SCR, diac, triac, SCS, and others.
• Thyristors act as open circuits, capable of withstanding a certain rated voltage until triggered.
• Once triggered, they turn on and become low-resistance current paths, remaining so until the current is reduced to a certain level or until they are triggered off.

Four Layer Diode (Shockley Diode or Silicon Unilateral Switch)

• A type of thyristor constructed from four semiconductor layers.
• SUS Operation:
  • Breakover Voltage: minimum forward voltage at which the SUS starts conducting heavily.
  • Holding Current: minimum anode current required to maintain the SUS in the on state.
  • Switching Current: the value of the anode current at the point where the SUS changes from the forward-blocking region to the forward-conducting region.

Firing an SUS

• Two methods to fire an SUS:
  • Supplying a voltage in excess of its breakover voltage.
  • Excessive voltage rise in the bias voltage VAK or dv/dt triggering.
• When VAK surpasses the breakover voltage, the SUS saturates and enters the conduction stage.
• The SUS will remain on unless the anode current IA drops below the holding current IH value.

dv/dt Triggering

• At forward-blocking region, the two outer junctions (1 & 3) are forward biased, but the inner junction (2) is reverse biased.
• The reverse-biased junction 2 behaves like a capacitor due to the space-charge present.
• A current will flow in the device to charge the device capacitance.