What is a GTO?
If you already know how the SCR works,then you would be aware of the fact that an SCR can be turned on by applying the gate current but as soon as it turns on,it seems that turning the SCR off without an external commutation circuit is practically impossible.Let me tell you it is possible with the help of a modified version of an SCR that can be turned off without the help of an additional external circuit to turn it off.And the device is none other than GTO(gate turn off thyristor).
Now as the name suggests,you would have correctly figured out,GTO has the ability to turn off with the help of external gate voltage.
NOTE:If you aren’t familiar with the concept of an SCR and thyristors.Check out this amazing post on SCR.
SYMBOL AND OPERATION
Differentiating GTO(gate turn off thyristor) from traditional SCR symbol is pretty simple. A GTO has additional arrow pointing inwards and outwards as shown in the figure indicating it can be turn on and off by changing the biasing accordingly.
How does a GTO turn on and off?
Turning on process of a GTO is pretty similar to that of an SCR that we discussed earlier with the help of two transistor analogy in this post on SCR.
Assume that the GTO has to be latched in conducting state,the two transistors Q1 and Q2 are in saturation mode.Applying positive gate voltage to the base of transistor Q2 resulted in saturation of Q2 and the collector current of Q2 acts as base of Q1 which saturated the transistor Q1 and collector current of Q1 acts base current for Q2.The two transistors are in an infinite loop of saturation.
Now to break this loop,we need to somehow decrease the amount of current reaching the base of transistor Q2 which will push the transistor Q2 out of saturation ultimately resulting of other transistor coming out of saturation state too and the conduction stops.For this we apply negative gate to cathode voltage so that amount of current reaching the base of transistor Q2 decreases significantly.What happens exactly is the when base is negative with respect to cathode,electrons get attracted to negative terminal applied at gate and a reverse bias begins to form at the base terminal which restricts further current and thus turning off the GTO.
CONSTRUCTION OF GTO
The GTO has a short circuited anode structure (i.e. small n+ regions are built on p+ region at regular intervals) that come in contact with the n- region.The n+ region at the cathode terminal is heavily doped to give high emitter efficiency for better current conduction.This significantly reduces the breakdown voltage of junction that forms between gate and cathode.The p region at the gate is blended between low level of doping and high doping level regions.
Why a GTO has graded p-region?
High doping level ensures great turn-off characteristics and low doping level ensures high emitter efficiency.Since both the characteristic is desired for an ideal GTO to perform well.The doping level is graded for ensuring better turn off characteristic and emitter efficiency.
The n- region decides the forward voltage blocking capacity of a GTO. Doping level is kept low for high voltage blocking capacity and vice versa.
APPLICATIONS OF GATE TURN OFF THYRISTORS
- Application where high speed switching is the need.
- Application where high forward voltage blocking capacity is the need.
- UPS(uninterrupted power supply).
- Dc motor drives.