• 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
• 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
• 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
• 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
• 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 .
You've reached the end of your free preview.
Want to read all 138 pages?
- Summer '11