EEE30001-Thyristors.pdf - EEE30001 Analogue Electronics 2 Thyristors Cishen Zhang Professor of Electrical and Electronic Engineering School of Software

EEE30001-Thyristors.pdf - EEE30001 Analogue Electronics 2...

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EEE30001 Analogue Electronics 2 Thyristors Cishen Zhang Professor of Electrical and Electronic Engineering School of Software and Electrical Engineering
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Thyristors 2 What are Thyristors A family of devices known as thyristors are constructed of four semiconductor layers ( pnpn ). Thyristors include the 4-layer diode, the silicon-controlled rectifier (SCR), the diac, the triac, and the silicon-controlled switch (SCS). Basic structure SCR Diac Triac
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Thyristors 3 What are Thyristors A family of devices known as thyristors are constructed of four semiconductor layers ( pnpn ). Thyristors include the 4-layer diode, the silicon-controlled rectifier (SCR), the diac, the triac, and the silicon-controlled switch (SCS). Thyristor packages
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Thyristors 4 What are Thyristors Thyristors are designed for high power appliations and can be used to control the amount of ac power to a load, e.g. in lamp dimmers, motor speed controls, ignition systems, and charging circuits.
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SCR is a three-terminal (Gate, Anode, Cathode) device with three pn junctions (J1, J2, and J3) I A I GT physical structure Thyristors 5 schematic symbol Silicon controlled rectifier (SCR)
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The SCR can be viewed as a two-transistor equivalent: Thyristors 6 Silicon controlled rectifier (SCR)
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Thyristors 7 Silicon controlled rectifier (SCR) SCR off When the gate current I G is zero, the device acts as a 4- layer diode in the off state. In this state, the very high resistance between the anode and cathode can be approximated by an open switch.
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Thyristors 8 When a positive pulse of current (trigger) is applied to the gate, both transistors turn on (the anode must be more positive than the cathode). I B2 turns on Q 2 , providing a path for I B1 into the Q 2 collector, thus turning on Q 1 The collector current of Q 1 provides additional base current for so that stays in conduction after the trigger pulse is removed from the gate. Silicon controlled rectifier (SCR) SCR triggered on
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Thyristors 9 Q 2 sustains the saturated conduction of Q 1 by providing a path for I B1 . In turn, Q 1 sustains the saturated conduction of Q 2 by providing I B2 . Thus, the device stays on (latches) once it is triggered on. In this state, the very low resistance between the anode and cathode can be approximated by a closed switch Silicon controlled rectifier (SCR) SCR stays after trigger pulse
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Thyristors 10 Silicon controlled rectifier (SCR) forward-breakover voltage An SCR can also be turned on without gate triggering by increasing the anode-to-cathode voltage to a value exceeding the forward-breakover voltage V BR(F)
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Thyristors 11 The forward-breakover voltage decreases as I G is increased above 0 V. Eventually, a value of is reached at which the SCR turns on at a very low anode-to-cathode voltage. So the gate current controls the value of forward breakover voltage, V BR(F), required for turn-on.
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