Full Form of GTO, Gate Turn-Off Thyristor (GTO)

A unique kind of thyristor, such as a gate turn-off thyristor (GTO), is a high-power (1200V AC, for example) semiconductor device. General Electric was the one who created it. GTOs are automated switches that may be turned on and off by their gate lead, when compared to typical thyristors.

Description 

Switches with a standard silicon thyristor (controlled rectifier) are not entirely controllable (a "fully controllable switch" can be turned on and off at will). The gate lead can only be used to turn on thyristors; it cannot be used to turn them off.

A gate signal turns on thyristors, however even after the gate signal is deleted or reverse biassed, the thyristor stays in the on state until a turn-off condition is met. When a thyristor is activated, or "fired," it functions like a typical semiconductor diode.

A gate signal can activate the GTO, and a gate signal with a negative polarity can turn it off. A "positive current" pulse between the gate and cathode terminals turns the device on. There will be a little voltage between the terminals because the gate-cathode functions like a PN junction. GTOs, which are less reliable than an SCR, need to have a little amount of positive gate current maintained even after the turn-on occurrence (thyristor).

A "negative voltage" pulse between the gate and cathode terminals turns the device off. About one-third to one-fifth of the forward current is "taken" and utilised to create a cathode-gate voltage, which in turn causes the forward current to decrease and the GTO to turn off (transitioning to the "blocking" state).

Long switch-off times are a problem with GTO thyristors; once the forward current drops, there is a long tail period during which a residual current flows until all of the device's charge has been removed. This limits the switching frequency to a maximum of around 1 kHz. However, it should be noted that a GTO's turn-off time is around ten times faster than that of an equivalent SCR.

GTO thyristors are often built from a large number (hundreds or thousands) of tiny thyristor cells connected in parallel to help with the turn-off operation.

Negative Bias

There are GTO thyristors with and without the ability to block reverse current. Because a long, low-doped P1 area is required for reverse blocking, the forward voltage loss is increased.

Symmetrical GTO thyristors, also known as S-GTO thyristors, are GTO thyristors that may block reverse voltage. The values for forward and reverse blocking voltage are frequently equal. Current source inverters are the typical application for symmetrical GTO thyristors.

Asymmetrical GTO thyristors, also known as A-GTO thyristors, are more prevalent than symmetrical GTO thyristors and are GTO thyristors incapable of blocking reverse voltage. Their reverse breakdown rating typically ranges from tens to hundreds of volts. When a reverse conducting diode is applied in parallel (as in voltage source inverters, for instance), or when reverse voltage would never happen, A-GTO thyristors are utilised.

A reverse conducting diode and GTO thyristors can be produced in the same package. Reverse Conducting GTO Thyristor is the term given to this.

Safe Working Environment

The GTO thyristor needs external devices (called "snubber circuits") to shape the turn-on and turn-off currents in order to prevent device degradation, unlike the insulated gate bipolar transistor (IGBT).

The gadget has a maximum dI/dt rating that restricts the growth of current during turn-on. This is done so that the device's main body can switch on before the maximum current is used. The part of the device closest to the gate contacts will overheat and melt if this rating is exceeded.

Turn-on dI/dt is a less severe limitation with GTO thyristors than it is with conventional thyristors because of the way the GTO is built from numerous small thyristor cells in parallel. The rate of dI/dt is typically reduced by adding a saturable reactor (turn-on snubber). On GTO-based circuits, reset of the saturable reactor typically imposes a minimum off time requirement.

The device's forward voltage must be restricted during turn-off until the current stops flowing. Typically, the cap is 20% or less of the forward blocking voltage rating. 

Due to the high voltage and current concentrated on a tiny area of the device, if the voltage increases too quickly during turn-off, only a section of the device will turn off and the GTO will fail, frequently explosively. 

Significant snubber circuits have been installed all around the device to reduce voltage spikes when it turns off. On GTO-based circuits, resetting the snubber circuit typically imposes a minimum on time requirement.

In DC motor chopper circuits, the minimum on and off time is managed by utilising a variable switching frequency at the lowest and maximum duty cycle. This can be seen in traction applications, where the frequency ramps up as the motor starts, then maintains a constant value over the most of the speed ranges, until falling to zero at maximum speed.

Applications

1. Traction, high-power inverters, and variable-speed motor drives are the principal uses. Integrated gate-commutated thyristors (IGCT), an evolution of the GTO, and insulated-gate bipolar transistors (IGBT), members of the transistor family, are progressively taking the place of GTOs.
2. In the starter circuits for fluorescent lamps, they are also utilised.

Conclusion

A three terminal power semiconductor is referred to as a gate turn-off thyristor (GTO). GTOs are members of the four-layer thyristor family. Additionally, GTOs are part of a class of power semiconductor devices with complete control over on- and off-states via the control terminal (gate).

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