What is a Gate Turn Off Thyristor & Its Working

A solid-state semiconductor device like a thyristor is not a completely controlled switch. The operation of this can be done like this, it can be switched ON through a gate terminal however cannot switch OFF using a gate terminal. When the thyristor switch is turned ON and it will not turn OFF even if we detach the gate pulses. So there is no control to deactivate the thyristor switch. Once the main current is interrupted then the switch will be turned off. Thus, it is difficult to utilize in the application wherever the main supply should not disrupt. Like the conversion of DC-AC & DC-DC circuit. An expensive, as well as a bulky communication circuit, must be used to deactivate the thyristor. To overcome this problem, the GTO (Gate Turn-Off Thyristor) device is used. It is a current-controlled device similar to a normal thyristor. This article discusses an overview of a Gate Turn off Thyristor.

What is a Gate Turn Off Thyristor?

The term GTO stands for “Gate Turn off Thyristor”. It is a bipolar semiconductor switching device that includes three terminals namely anode, cathode & gate like a conventional thyristor. It has the capacity of gate turn off. This device is used to turn ON and OFF the main current supply through a gate drive circuit. The basics of Gate Turn-Off Thyristor are discussed below.

The Gate Turn-Off Thyristor symbol is shown below. To activate the GTO into the mode of conduction, a small positive gate current is required as well as through a negative pulse on the gate terminal; and it is capable of being switched off. In the following image, it includes double arrows on it which differentiate the thyristor from the ordinary thyristor. These arrows mainly specify the flow of current in the bidirectional throughout the gate terminal.

To deactivate the GTO, it uses a high gate current. Alternatively, in the conduction state, thus thyristor works like a normal thyristor including a small ON condition voltage drop. The switching speed of this gate turn-off thyristor is faster as compared to normal thyristor and also it has high current and voltage ratings as compared with power transistors.

At present, different types of GTOs are obtainable in the market including the capabilities of symmetric & asymmetric voltage. The symmetric GTOs are nothing but a GTO that has the capabilities of identical forward as well as reverse blocking which are applicable in current source inverters, however, these are fairly slow. Asymmetric GTOs (A-GTOs) are mostly applicable because of their lower ON-state voltage drop as well as constant temperature characteristics.

Construction Gate Turn Off Thyristor

The structure of the gate turn off thyristor is similar to a normal thyristor because it includes 3-junctions and 4- PNPN layers. A GTO is a three-terminal PNPN device like anode, cathode, and gate. In this kind of thyristor, the anode terminal is composed of a p+ layer through n+ type fingers diffused within it.

The N+ layer of this thyristor is doped highly to get high emitter efficiency and it provides a cathode terminal. Thus, the junction like J3 breakdown voltage is low and the typical breakdown voltage value ranges from 20 to 40V. The P-layer doping level must be low to maintain excellent emitter efficiency. Similarly, to have a good switch OFF properties, the region doping must be high.

The anode junction can be defined as the junction among the P+ anode as well as N base is known as anode junction. The high-efficiency anode junction can be obtained through a P+ anode region which is heavily doped so that the properties of a good switch ON can be achieved. But, the switch OFF capabilities are influenced through such GTOs.

So, this issue can be solved by initiating N+ layers which are heavily doped at normal intervals within the P+ anode layer. So at junction J1, this N+ layer will make direct contact through the N layer. So, the electrons can be moved from the base region to anode metal contact from the P+ anode without causing hole-injection, so this is known as a GTO structure with anode shorted.

Because of these anode shorts, the GTO’s reverse blocking capacity can be reduced toward the reverse breakdown voltage of the J3 junction & therefore the turn OFF device can be increased. But, using several anode shorts, the anode junction’s efficiency can be reduced & therefore the GTO’s switch ON performance can be degraded. So, careful considerations must be taken regarding the anode shorts density for a good switch ON/OFF performance.

Principle of Operation

The GTO’s principle of operation is the same as a conventional type thyristor. Once the positive gate current is applied to make the anode terminal positive to the cathode terminal, then the electrons can be generated from the cathode terminal to the anode. So, this induces the hole-injection with the help of an anode terminal in the base region. These electrons as well as holes-injection continuous till the gate turn off thyristor enters into the conduction region.

In thyristor, at first, the conduction begins through switch ON the region of cathode contiguous to the gate terminal. Thus, the remaining region comes into the conduction through plasma spreading.
Not like a thyristor, gate turn off thyristor includes narrow cathode elements which are interdigitated heavily through gate terminal, thus early turned ON region is extremely large & plasma spreading is little. Therefore, the gate turns off thyristor comes into the conduction region very fast.

At the gate terminal, a reverse bias can be applied to switch OFF a conducting thyristor by making the gate terminal negative as compared with the cathode. In the P-layer, a fraction of the holes can be extracted using the gate terminal to hold back the electrons injection from the cathode terminal.

In reply to this, an extra hole current can be removed by the gate terminal which results in more control of electrons from the cathode terminal. Finally across the p-base junction, the voltage drop can cause reverse bias in the cathode junction of the gate & therefore the thyristor will be deactivated.

Throughout the process of hole extraction, the area of the p-base is slowly exhausted so that the conduction region can be squeezed. As this procedure continues, then the anode current supplies in remote areas by forming filaments with high current density. So, this can cause limited hot spots which can damage the device if not these filaments are extinguished rapidly.

During the high negative gate voltage application, these filaments are extinguished quickly. Because of the stored charge in the N base region, the anode terminal to gate current flows continuously although the cathode current is stopped. So, this is known as a tail current which decomposes exponentially when the surplus charge carriers are decreased through the recombination procedure. When the tail current level is decreased to a leakage current level, then the device keeps the characteristics of forwarding blocking.

V-I Characteristics

The Gate Turn Off thyristor V-I characteristics are related to a CT or conventional thyristor. The GTO’s latching current is more than a CT. For GTO, the latching current is 2A whereas, for a CT, it ranges from 100 mA – 200 mA. The V-I characteristics of GTO are shown below.

The above characteristics mainly include four regions or modes like forward blocking, forward conduction, reverse blocking & reverse conduction.

In the first mode like forwarding blocking, the voltage is applied across the thyristor without applying the +ve gate signal. Therefore, it does not conduct in this mode. But, there is a little leakage current which is very much higher as compared to a thyristor’s leakage current. Actually, in this mode, the gate turns off thyristor works like a transistor with high voltage & low gain which means, the anode current is low. In this mode, the GTO simply blocks the rated forward voltage when the gate terminal is biased negatively to the cathode.

When a positive gate signal is given with appropriate amplitude to the GTO, then it moves into the mode of forwarding conduction. Similarly, whenever a reverse voltage is to this thyristor, then it blocks the reverse voltage up to a limit but as soon as the reverse voltage reaches a critical value, called the reverse break over-voltage, the GTO starts conducting in the reverse direction.

This mode of operation does not destroy the device if the gate is negatively biased and the time of this operation is small. In reverse biased condition, the blocking capacity mainly depends on the GTO type. A symmetric type includes a high reverse blocking capability whereas an asymmetric type includes a small reverse blocking capacity that ranges from 20-30 V.

Advantages

The advantages of gate turn off thyristor include the following.

• The GTO has outstanding switching characteristics
• The configuration of the GTO circuit has less weight and size than the thyristor circuit unit.
• A commutation circuit is not required, hence cost, weight and size can be reduced.
• The switching speed of GTO is high as compared with SCR.
• Less maintenance
• The current surge capacity is similar to an SCR.
• The blocking voltage capacity of GTO is high
• di/dt ratings are more at turn ON
• Efficiency is high

Disadvantages

The disadvantages of gate turn off thyristor include the following.

• The associated loss, as well as ON-state voltage drop, is more
• The structure of GTO is multi-layered, so the gate triggering current value is high as compared to the conventional thyristor.
• High losses of Gate drive circuit
• The voltage drop of ON state across the gate turn off thyristor is more.
• The latching & holding current’s magnitude is high as compared to SCR
• The latching current value is 2A whereas, for an SCR, it ranges from 100 mA to 500 mA.
• As compared with SCR, the triggering current of GTO is high

Applications

GTO is used in many applications because of many benefits as compared to another thyristor like outstanding switching characteristics, less maintenance and no require of commutation circuit, etc. The applications of gate turn off thyristor include the following.

• In choppers as well as inverters, It is used as the main control device.
• AC drives
• DC drives
• DC circuit breakers
• DC choppers otherwise DC drives
• Induction heating
• Used in traction applications because of less weight
• Low power applications
• AC stabilize power supplies
• It is used in inverters, SVCs (static VAR compensators)
• Used in drive systems like rolling mills, machine tools & robotics.

Thus, this is all about an overview of gate turn-off thyristor (GTO) like construction, working, advantages, disadvantages, and its applications. This device is from the family of Thyristor as well as belongs to a power semiconductor devices group. This device can control by turn ON/OFF the states with the help of the gate or control terminal. Here is a question for you, what are the different types of thyristors available in the market?