Feature: Circuit design


Figure 1: Main thyristor Thymain


is turned off by a


control pulse at gate of an auxiliary thyristor Thyaux


across two silicon diodes. For both current directions, the total voltage drop can be about 2.8V, making this solution of limited practical use. When thyristors became widespread, the two diodes in the


Graetz bridge were replaced by two thyristors connected through their cathodes. Te voltage drop across the AC switch thus came to about 2.1V. Connecting thyristor cathodes simplifies controlling the switch,


AC and DC thyristor circuits


By Marián Štofa, Slovak University of Technology, Bratislava, Slovakia


two complementary semiconductor substrates (NPNP and PNPN) in parallel, with a shared common gate electrode. With the advent of the triac, AC switching became even simpler


A


and more efficient. For switching AC circuits, within both half- periods of the AC voltage, a thyristor can be connected between the DC terminals of a Graetz diode bridge, where the AC terminals of the bridge then represent a bidirectional thyristor, with its anode connected to the bridge’s positive terminal. Tis approach, however, means there’s an additional voltage drop of about 1.4V


36 February 2021 www.electronicsworld.co.uk


thyristor is in essence a four-layer NPNP switching device actively used in the positive region of its anode-cathode voltage. It is a five-layer device that conducts current in both directions, with control pulses of both polarities. It can be considered as connecting


since the common cathodes serve as a reference terminal for the control circuit. Te voltage drop in a triac is lower than that of a thyristor bridge


– as low as 1.2V. Note, however, that triacs are manufactured with a rather small current rating, of about 30A, in contrast to today’s thyristors with their kiloampere and kilovolt ratings. AC switching with thyristors can be the fully-on/fully-off type


with turn-on close to the zero-crossing of the AC voltage. Also, power can be controlled in a continuous way – by turning on the thyristor at a phase angle of between 0o


and 80o within a half


period of the AC voltage. Tis is probably why a thyristor in the US is also called a silicon-controlled rectifier, or SCR.


Thyristors in DC circuits In DC circuits, once turned on, a thyristor remains in that state regardless of the presence of a control pulse. To turn it off, it is necessary to force its anode-cathode current to drop to zero for a sufficiently long time, typically tens of µs. Teoretically, it would be enough to decrease the anode-cathode current below this value, called a “hold current”, IH


. Te hold current is typically a fraction


of a thyristor’s nominal current – about a hundredth of it. Tere’s a class of thyristors that can be turned off through the


gate, called “gate turn-off” (GTO) thyristors. Compared to classical thyristors, GTO thyristors have some drawbacks, mainly the anode-cathode voltage drop in the on state of around 3.4V – twice that of the common thyristor. Further, the negative gate current necessary to turn off the GTO is high, about a quarter of the actual anode current. Also, the positive gate current necessary to turn on a GTO is tenfold that of a comparable thyristor. Moreover, a lower value of positive current must continuously flow through a GTO’s gate to maintain its on state.


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