Thyristor Topics

• neutron irradiation
• avalanche breakdown
• television receivers
• thyristors
• poor candidates
• frequency applications
• heavy metals
• grid power
• input ac
• ac supply
• charge carriers
• output dc
• breakdown voltage
• transformer cores
• scrs
• dc supply
• mosfets
• pulse width
• conduction
• anode

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Thyristor, Thyristor

As J 2 is reverse biased, no conduction takes place (Off state). Now if V AK is increased beyond the breakdown voltage V BO of the thyristor, avalanche breakdown of J 2 takes place and the thyristor starts conducting (On state).

By selecting an appropriate value of V G , the thyristor can be switched into the on state suddenly.

Gate trigger current varies inversely with gate pulse width in such a way that it is evident that there is a minimum gate charge required to trigger the thyristor.

Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined.

Such fast thyristors are made by diffusing into the silicon heavy metals ions such as gold or platinum which act as charge combination centres. Alternatively, fast thyristors may be made by neutron irradiation of the silicon.

The precise switching point was determined by the load on the output DC supply as well fluctuations on the input AC supply. They proved to be unpopular with the AC grid power supplier companies because the simultaneous switching of many television receivers, all at approximately the same time, introduced asymmetry into the supply waveform and, as a consequence injected DC back into the grid with a tendency towards saturation of transformer cores and overheating. Thyristors were largely phased out in this kind of application by the end of the decade.

One major problem associated with SCRs is that they are not fully controllable switches. The GTO (Gate Turn-off Thyristor) and IGCT are two related devices which address this problem. In high-frequency applications, thyristors are poor candidates due to large switching times arising from bipolar conduction. MOSFETs, on the other hand, have much faster switching capability because of their unipolar conduction (only majority carriers carry the current).

Source: Wikipedia > Thyristor