Thyristors wo examples.docx

Thyristors - Thyristors are constructed using four semiconductor layers (pnpn). - Thyristors include the 4-layer diode, the silicon-controlled rectifier (SCR), the diac, the triac, silicon-controlled switch (SCS) and etc. - They act as open circuits capable of withstanding a certain...

Thyristors - Thyristors are constructed using four semiconductor layers (pnpn). - Thyristors include the 4-layer diode, the silicon-controlled rectifier (SCR), the diac, the triac, silicon-controlled switch (SCS) and etc. - They act as open circuits capable of withstanding a certain rated voltage until they are triggered. When triggered, they turn on and become low-resistance current paths and remain so, even after the trigger is removed, until the current is reduced to a certain level or until they are triggered off, depending on the type of device. - The Four Layer Diode - Also known as a **Shockley diode** or a **Silicon Unilateral Switch (SUS)**, is a type of *thyristor* which is a class of devices constructed of four semiconductor layers. - SUS Operation - **Breakover Voltage** -- the 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 (off) to forward-conducting region (on). - Firing an SUS 1. By supplying a voltage in excess of its breakover voltage. When VAK surpasses the breakover voltage the SUS saturates and enters the conduction stage. It will remain open unless the anode current IA drops below the holding current IH value. 2. By excessive voltage rise in the bias voltage VAK or dv/dt triggering - dv/dt triggering - At forward-blocking region, two outer junctions (1 & 3) are forward biased but the inner junction (2) is reverse biased. - The reversed biased junction 2 behaves like a capacitor because of the space-charge present there. - A current will flow in the device to charge the device capacitance. - If the charging current becomes large enough, density of moving current carriers in the device induces switch-on. This method of triggering is not desirable because high charging current (lc) may damage the thyristor.