Silicon Controlled Rectifier wo examples.docx

Silicon Controlled Rectifier SCR is a four-layer PNPN device similar to the four-layer diode except with three terminals: anode, cathode and gate. - SCR in Normal Operation \(a) The supply voltage is generally much less than breakover voltage value at IG=0. \(b) The SCR is turned on by passing...

Silicon Controlled Rectifier SCR is a four-layer PNPN device similar to the four-layer diode except with three terminals: anode, cathode and gate. - SCR in Normal Operation \(a) The supply voltage is generally much less than breakover voltage value at IG=0. \(b) The SCR is turned on by passing an appropriate amount of gate current (a few mA) and not by breakover voltage. \(c) When SCR is operated from a.c. supply, the peak reverse voltage which comes during negative half-cycle should not exceed the reverse breakdown voltage. \(d) When SCR is to be turned OFF from the ON state, anode current should be reduced to holding current. \(e) If gate current is increased above the required value, the SCR will close at much reduced supply voltage. - SCR as a Switch The SCR has the following advantages over a mechanical or electromechanical switch: \(a) It has no moving parts. Consequently, it gives noiseless operation at high efficiency. \(b) The switching speed is very high up to 10^9^ operations per second. \(c) It permits control over large current (30--100 A) in the load by means of a small gate current (a few mA). \(d) It has small size and gives trouble free service. - Turning off an SCR - Anode current interruption: - A method in which the anode current is suppressed from going through the anode to the cathode terminal of the SCR. - The series switch in part (a) simply reduces the anode current to zero by cutting off the anode to cathode path and this causes the SCR to turn off. The parallel switch in part (b) routes part of the total current away from the SCR, thereby reducing the anode current to a value less than holding current. - Forced Commutation: - requires momentarily forcing current through the SCR in the direction opposite to the forward conduction so that the net forward current is reduced below the holding value. - The basic circuit, as shown in the figure, consists of a switch (normally a transistor switch) and a capacitor. - SCR Characteristics and Ratings - **Forward Breakover Voltage (V~BR(F)~)** -- the voltage at which the SCR enters the forward-conduction region. - **Holding Current (I~H~)** -- the value of the anode current below which the SCR switches from the forward-conducting region to the forward-blocking region. - **Gate Trigger Current (I~GT~)** -- the value of the gate current necessary to switch the SCR from forward-blocking region to forward-conducting region. - **Gate Trigger Voltage (VGT)** -- Voltage across the gate-cathode junction that turns the SCR on. - **Average Forward Current (I~F(AVG)~)** -- maximum continuous anode current the SCR can withstand in forward-conducting region. - **Peak Reverse Voltage (PRV)** -- the maximum reverse voltage that can be applied to an SCR without conducting in a reverse direction. - **Circuit Fusing Rating** -- the product of the square of forward surge current and the time of duration of the surge