Gate Turn-Off Thyristor

Gate Turn-Off (GTO) Thyristor

A Gate Turn-Off (GTO) thyristor is a four-layer, three-terminal semiconductor device with a pnpn structure. It is similar to a conventional thyristor or SCR (Silicon-Controlled Rectifier), but with an added capability of being turned off by a negative gate pulse.

To turn on a GTO, a positive gate current pulse is applied, similar to a regular thyristor. However, what sets the GTO apart is its ability to be turned off by a negative gate pulse of appropriate amplitude. This characteristic makes GTOs more versatile than conventional thyristors in applications such as inverters and choppers.

Unlike regular thyristors that require forced commutation circuits to turn them off, GTOs can be turned off without the need for additional circuitry. This advantage allows for more compact and cost-effective inverter designs when using GTO devices.

To turn off a GTO, a negative gate current pulse is required. The magnitude of this negative pulse needs to be a significant percentage (typically 20 to 30%) of the anode current before commutation. For instance, an 800 A GTO would require a negative current pulse of around 200 A peak to turn it off.

Initially, GTOs faced certain performance issues, which hindered their commercial use after their development in the late 1960s. However, with advancements in technology, modern GTOs have shown improved performance, leading to their widespread adoption in various commercial inverter applications.

Gate Turn Off Thyristor Construction & Circuit Diagram

The Gate Turn Off Thyristor (GTO) is a type of power semiconductor device that can be turned on and off by applying gate signals. It consists of four layers of alternating P-type and N-type semiconductor material, forming three junctions. The structure of a GTO is similar to that of a conventional thyristor, but with an added N-type region near the anode.

The main components of a GTO include the anode, cathode, and gate terminals. The anode and cathode terminals are connected to the main power circuit, while the gate terminal controls the switching operation. The gate region is made up of a heavily doped N+ region.

The GTO operates in four modes: forward blocking, forward conduction, reverse blocking, and reverse conduction. In the forward blocking mode, no current flows through the device, and it acts as an open circuit. When a positive voltage is applied between the anode and cathode, the GTO enters the forward conduction mode, allowing current to flow.

To turn off the GTO, a negative gate pulse is applied to the gate terminal, which triggers the regenerative turn-off process. This process involves injecting charge carriers into the P-base region from the gate terminal, neutralizing the excess charge carriers present during forward conduction and turning off the device.

Structure of a GTO Thyristor

The structure of the GTO allows for effective control of the device's switching characteristics, making it suitable for applications in power electronics, such as motor drives, inverters, and high-power switching circuits.

Gate Turn Off Thyristor Symbol

Circuit symbol of a GTO Thyristor

In circuit diagrams, a GTO (Gate Turn-Off thyristor) can be represented by various symbols, depending on the level of detail required or the conventions used. Show in fig likely shows different representations of a GTO, each serving a specific purpose.

The symbol labeled (i) represents a basic GTO symbol, which consists of an anode, a cathode, and a gate terminal. This symbol is commonly used to depict the GTO device itself without emphasizing specific characteristics.

The symbol labeled (ii) indicates the gate current direction for turning the GTO on and off. The inward arrow signifies that gate current is applied to turn the GTO on, while the outward arrow suggests that gate current flows out to turn it off.

Now, symbol (iii) is described as being useful for drawing circuit configurations involving GTOs. Without a specific visual reference, it's difficult to provide a precise interpretation. However, a probable explanation could be that symbol (iii) represents a more detailed representation of a GTO, highlighting specific internal connections or features relevant to circuit configuration.

To gain a better understanding of the symbol (iii) and its purpose, it would be helpful to refer to the accompanying text or diagram in the source material or provide further details or context about the circuit configurations mentioned.

Gate Turn Off Thyristor Device Description

Equivalent Circuit of a GTO Thyristor

Thyristors, specifically silicon-controlled rectifiers (SCRs), are not fully controllable switches. They can be turned on using the gate lead, but once turned on, they cannot be turned off using the gate lead alone. They remain in the on state until a turn-off condition occurs, such as applying a reverse voltage or reducing the forward current below a certain threshold called the "holding current." After turning on, a thyristor behaves like a normal semiconductor diode.

In contrast, a Gate Turn-Off Thyristor (GTO) can be turned on and off using a gate signal. To turn on a GTO, a positive current pulse is applied between the gate and cathode terminals. Turn-on is not as reliable as with an SCR, so a small positive gate current must be maintained even after turn-on to improve reliability.

To turn off a GTO, a negative voltage pulse is applied between the gate and cathode terminals. Some of the forward current is "stolen" to induce a cathode-gate voltage, causing the forward current to decrease and the GTO to switch off, transitioning to the "blocking" state.

GTO thyristors have longer switch-off times compared to SCRs. After the forward current decreases, there is a long tail time during which residual current continues to flow until all remaining charge is taken away. This limits the maximum switching frequency of GTOs to around 1 kHz. However, GTOs have a turn-off time approximately ten times faster than that of comparable SCRs.

To enhance the turn-off process, GTO thyristors are often constructed from multiple small thyristor cells connected in parallel. This configuration helps with faster turn-off and allows for higher voltage blocking capabilities.

On the other hand, a distributed buffer gate turn-off thyristor (DB-GTO) is a type of thyristor that features additional PN layers in the drift region. These additional layers help reshape the field profile and increase the voltage that the thyristor can block in the off state. Compared to a conventional thyristor with a PNPN structure, a DB-GTO thyristor has a PN–PN–PN structure, which improves its voltage blocking capabilities.

Gate Turn-Off  Thyristor Reverse Bias 

Reverse bias refers to the application of a voltage across a semiconductor device in a direction opposite to its normal operation. In the case of GTO (Gate Turn-Off) thyristors, reverse bias refers to the voltage applied in the reverse direction, across the anode and cathode terminals of the device.

GTO thyristors can be classified into two types based on their reverse blocking capability: symmetrical GTO (S-GTO) thyristors and asymmetrical GTO (A-GTO) thyristors.

1. Symmetrical GTO (S-GTO) thyristors: These devices are capable of blocking reverse voltage. The reverse blocking voltage rating and forward blocking voltage rating are typically the same. S-GTO thyristors are designed with a long, low-doped P1 region, which adds to the forward voltage drop. They are commonly used in applications such as current source inverters.
2. Asymmetrical GTO (A-GTO) thyristors: These thyristors lack the ability to block reverse voltage effectively. They typically have a reverse breakdown rating in the tens of volts. In applications where reverse voltage would never occur or where a reverse conducting diode is used in parallel (such as voltage source inverters, switching power supplies, or DC traction choppers), A-GTO thyristors are commonly employed.

In addition, GTO thyristors can be manufactured with a built-in reverse conducting diode in the same package. These devices are known as Reverse Conducting GTO (RCGTO) thyristors. The reverse conducting diode allows current to flow in the reverse direction when the GTO thyristor is in the off-state. RCGTO thyristors find applications in various power electronic systems where bidirectional current flow is required, such as in AC motor drives.

Gate Turn Off Thyristor Safe Operating Area

The safe operating area (SOA) is an important concept in the operation of power electronic devices such as the GTO thyristor. It defines the limits within which the device can safely operate without being damaged or causing malfunctions.

For GTO thyristors, the SOA is defined by two main considerations: the maximum rate of change of current (dI/dt) during turn-on and the maximum voltage rate of change (dv/dt) during turn-off.

During turn-on, the dI/dt rating of the device determines how quickly the current can rise. Exceeding this rating can lead to overheating and damage to the device. To control dI/dt, snubber circuits are typically used. These circuits help shape the turn-on current waveform and prevent excessive dI/dt. While GTO thyristors are constructed from multiple small thyristor cells in parallel, reducing the impact of dI/dt compared to regular thyristors, the use of snubber circuits is still necessary. The reset of the saturable reactor in the snubber circuit often imposes a minimum off-time requirement on GTO-based circuits.

During turn-off, the forward voltage of the GTO thyristor must be limited until the current decreases to a safe level. If the voltage rises too quickly, not all of the device will turn off, and this concentrated high voltage and current can cause the GTO to fail, sometimes explosively. Snubber circuits are employed to restrict the rise of voltage during turn-off and protect the device. The reset of the snubber circuit may impose a minimum on-time requirement on GTO-based circuits.

In DC motor chopper circuits, which use GTO thyristors, the minimum on and off times are managed by employing a variable switching frequency. The switching frequency is typically highest at the lowest and highest duty cycles, while it remains constant over most of the speed ranges. For example, in traction applications, the frequency may ramp up as the motor starts, then stay constant during most speed ranges, and finally drop back down to zero at full speed.

By considering the safe operating area and implementing appropriate snubber circuits and control strategies, the GTO thyristor can be operated reliably and efficiently in various applications.

Gate Turn Off Thyristor Characteristics

Static VI characteristic of  a GTO

The static V-I characteristics of a GTO (Gate Turn-Off) thyristor show that the latching current for large power GTOs is typically several amperes (such as 2A), whereas conventional thyristors of the same rating have a latching current of 100-500 mA. If the gate current is insufficient to turn on the GTO, it functions like a high voltage, low gain transistor with a significant anode current. Consequently, this results in noticeable power loss in such circumstances.

Gate Turn Off Thyristor Switching Performance

The basic gate drive circuit shown in Figure utilizes a two-transistor configuration to turn on a GTO (Gate Turn-Off Thyristor) and a thyristor (T1) to turn it off. 

To turn on the GTO, the first transistor (TR1) is turned on, which subsequently switches on the second transistor (TR2). This allows a positive gate-current pulse to be applied, turning on the GTO.

For turning off the GTO, the turn-off circuit employs the thyristor T1. When T1 is turned on, it generates a large negative gate current pulse, which turns off the GTO.

The passage describes the gate turn-on and gate turn-off processes in a Gate Turn-Off Thyristor (GTO). 

Gate turn-on: To turn on the GTO, transistor TR1 is turned on, which subsequently switches on TR2. This applies a positive gate-current pulse to turn on the GTO. The turn-on time for a GTO is similar to that of a conventional thyristor and consists of delay time, rise time, and spread time. Increasing the forward gate current can decrease the turn-on time for a GTO.

Gate turn-off: The turn-off characteristics of a GTO are different from those of a Silicon-Controlled Rectifier (SCR). The turn-off process is initiated by gating thyristor T1, which generates a large negative gate current pulse to turn off the GTO. The turn-off time (tq) is divided into three periods: the storage period (ts), the fall period (tf), and the tail period (tt). 

During the storage period, the GTO carries a steady current (Ia), and the anode voltage (Va) remains constant. The initiation of the turn-off process occurs when a negative gate current starts to flow, and the rate of rise of this gate current depends on the gate circuit inductance and the gate voltage applied. In the storage period, excess charges in the p-base region are removed by the negative gate current, preparing the GTO for turn-off.

After the storage period, the anode current starts to fall rapidly, and the anode voltage begins to rise. The fall period (tf) is the interval during which the anode current falls rapidly, typically of the order of 1 microsecond. The fall period is measured from the instant the gate current is maximum negative to the instant the anode current falls to its tail current.

At the time t = ts + tf, there is a spike in voltage due to the abrupt change in current. After that, the anode current and voltage continue to move towards their turn-off values for a time t1 called tail time. The tail time is followed by a transient in the anode voltage due to the presence of the snubber circuit (Rs,Cs), and then the voltage stabilizes to its off-state value, which is equal to the source voltage applied to the anode circuit. The turn-off process is complete when the tail current reaches zero.

To reduce the overshoot voltage and tail current, the size of the snubber circuit (Cs) can be increased, but this comes at the cost of snubber losses. The duration of the tail period (tt) depends on the characteristics of the GTO device.

Basic gate-drive & voltage - current waveform during turn-off a GTO

Gate Turn Off Thyristor advantages and disadvantages

GTO has the following advantages over an SCR

(1) GTO has faster switching speed.

(ii) Its surge current capability is comparable with an SCR

(iii) It has more di/dt rating at turn-on. (iv) GTO circuit configuration has lower size and weight as compared to SCR circuit unit.

(v) GTO unit has higher efficiency because an increase in gate-drive power loss and on-state loss is more than compensated by the elimination of forced commutation losses.

(vi) GTO unit has reduced acoustical and electromagnetic noise due to elimination of commutation chokes.

A GTO has the following disadvantages as compared to a conventional thyristor

1.  Magnitude of latching and holding currents is more in a GTO.
2. On state voltage drop and the associated loss is more in a GTO.
3. Due to the multi cathode structure of GTO, triggering gate current, is higher than that required for a conventional SCR.
4. Gate drive circuit losses are more 
5. Its reverse-voltage blocking capability is less than its forward-voltage blocking capability. But this is no disadvantage so far as inverter circuits are concerned.

Gate Turn Off Thyristor Application

The main applications are in

1. GTOs are also used in variable-speed motor drives, high-power inverters, and traction systems.
2. GTOs are being replaced by newer technologies such as integrated gate-commutated thyristors (IGCT) and insulated-gate bipolar transistors (IGBT), which offer improved performance and efficiency. 
3. GTOs are also used in the starter circuits for fluorescent lamps.
4.  High-performance drive systems, such as the field-oriented control scheme used in rolling mills, robotics and machine tools  
5. Traction purposes because of their lighter weight
6. Adjustable-frequency inverter drives. 

Gate Turn Off Thyristor Summary

The Gate Turn-Off (GTO) thyristor is a powerful semiconductor device that has revolutionized power electronics. Its ability to switch off the current flow, coupled with high voltage handling and surge current capabilities, makes it an ideal choice for a wide range of applications. From power transmission and distribution systems to motor drives and electric traction, the GTO thyristor continues to play a vital role in modern power systems, offering efficient and reliable control over power flow. As technology advances, it will be fascinating to witness further developments and improvements in this remarkable semiconductor device.At present, GTOs with ratings up to 2500 V and 1400 A are available,seen for RCT.