ALSTOM B5EC HENF105077R1 Normal thyristors
B5EC HENF105077R1 Normal thyristors (silicon-controlled rectifiers) are not fully controllable switches (a "fully controllable switch" can be turned on and off at will). Thyristors can only be turned on using the gate lead, but cannot be turned off using the gate lead. Thyristors are switched on by a gate signal, but even after the gate signal is de-asserted (removed, reverse biased), the thyristor remains in the on state until a turn-off condition occurs (which can be the application of a reverse voltage to the terminals or a decrease of the forward current below a certain threshold value known as the "holding current"). Thus, a thyristor behaves like a normal semiconductor diode after it is turned on, or "fired".
B5EC HENF105077R1 The GTO can be turned on by a gate signal and can also be turned off by a gate signal of negative polarity.
Turn on is accomplished by a "positive current" pulse between the gate and cathode terminals. As the gate-cathode behaves like PN junction, there will be some relatively small voltage between the terminals. The turn-on phenomenon in GTO is, however, not as reliable as an SCR (thyristor), and small positive gate current must be maintained even after turn on to improve reliability.
Turn off is accomplished by a "negative voltage" pulse between the gate and cathode terminals. Some of the forward current (about one-third to one-fifth) is "stolen" and used to induce a cathode-gate voltage, which in turn causes the forward current to fall and the GTO will switch off (transitioning to the "blocking" state).
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