At the breakover voltage the value of the thyristor anode current is called the

At the breakover voltage the value of the thyristor

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At the breakover voltage the value of the thyristor anode current is called the latching current (I L ) . Breakover voltage triggering is not normally used as a triggering method, and most circuit designs attempt to avoid its occurrence. When a thyristor is triggered by exceeding VBO, the fall time of the forward voltage is quite low (about 1/20th of the time taken when the thyristor is gate-triggered). However, a thyristor switches faster with VBO turn-ON than with gate turn-ON, so permitted di/dt for breakover voltage turn-on is lower. 11/18/2019 94 Forward Voltage Triggering
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With forward voltage across anode & cathode of a thyristor, two outer junctions (A & C) are forward biased but the inner junction (J2) is reverse biased. The reversed biased junction J2 behaves like a capacitor because of the space- charge present there. As p-n junction has capacitance, so larger the junction area the larger the capacitance. If a voltage ramp is applied across the anode-to-cathode, a current will flow in the device to charge the device capacitance according to the relation: 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 (Ic) may damage the thyristor. 11/18/2019 95 Dv/dt Triggering
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During forward blocking, most of the applied voltage appears across reverse biased junction J2. This voltage across junction J2 associated with leakage current may raise the temperature of this junction. With increase in temperature, leakage current through junction J2 further increases. This cumulative process may turn on the SCR at some high temperature. High temperature triggering may cause Thermal runaway and is generally avoided. 11/18/2019 96 Temperature Triggering
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In this method light particles ( photons ) are made to strike the reverse biased junction, which causes an increase in the number of electron hole pairs and triggering of the thyristor. For light-triggered SCRs, a slot (niche) is made in the inner p- layer. When it is irradiated, free charge carriers are generated just like when gate signal is applied b/w gate and cathode. Pulse light of appropriate wavelength is guided by optical fibers for irradiation. If the intensity of this light thrown on the recess exceeds a certain value, forward-biased SCR is turned on. Such a thyristor is known as light-activated SCR (LASCR). Light-triggered thyristors is mostly used in high-voltage direct current (HVDC) transmission systems . 11/18/2019 97 Light Triggering
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An easy method to switch ON a SCR into conduction is to apply a proper positive signal to the gate. This signal should be applied when the thyristor is forward biased and should be removed after the device has been switched ON. Thyristor turn ON time should be in range of 1-4 micro seconds , while turn-OFF time must be between 8-50 micro seconds .
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