Gate Turn Off Thyristor

The thyristor has always been limited by the fact that it is a semi-controlled device, despite being widely used in high power applications. It could be turned ON by applying a gate signal, but to turn it OFF, a commutation circuit must interrupt the main current. Because there is no natural current zero (as in AC circuits) in DC to DC and DC to AC conversion circuits, this presents a serious thyristor deficiency. Therefore, by ensuring the turn OFF mechanism through the gate terminal, the development of the Gate Turn off Thyristor (GTO) addresses the primary issue with the thyristor.

Basics of Gate Turn Off Thyristor

A Gate Turn off Thyristor or The GTO is a three terminal, bi-polar semiconductor switching device (current controlled minority carrier). The terminals are anode, cathode, and gate, just like in a traditional thyristor, as shown in the figure below. It can turn the gate off, as the name suggests.

These have the ability to both turn ON and turn OFF the main current using a gate drive circuit. The GTO can be put into conduction mode with a small positive gate current and turned off with a small negative gate pulse. The gate in the figure below has two arrows, which distinguish the GTO from a typical thyristor. This shows that the gate terminal’s current is flowing in both directions.

To turn the GTO off, a sizable gate current is needed. A GTO rated at 4000V and 3000A, for instance, might require -750A of gate current to turn it off. Consequently, the typical GTO turn-off gain is low, falling between 4 and 5. GTOs are used in low power applications because of this significant negative current.

On the other hand, the GTO exhibits a small ON state voltage drop while in the conduction state, just like a thyristor. In addition to having higher voltage and current ratings than power transistors, the GTO switches items more quickly than a thyristor. Today’s market offers a number of GTO variations with symmetric and asymmetric voltage capabilities. Symmetric GTOs (S-GTOs) are GTOs with identical forward and reverse blocking properties. Although they are a little slower, these are used in current source inverters. Due to their lower ON state voltage drop and stable temperature characteristics, asymmetric GTOs (A-GTOs) are typically used.

The reverse voltage capability of these asymmetrical GTOs is respectable (typically 20 to 25 V). These are used in situations where either a reverse conducting diode is connected across the circuit. Or the reverse voltage across it would never occur. This article only discusses asymetric GTOs.

Gate Turn Off Thyristor Construction

Take a look at the GTO structure below, which is almost identical to the thyristor. Like a typical thyristor, it is a four layer, three junction P-N-P-N device. To achieve high emitter efficiency, the n+ layer at the cathode end is heavily doped in this. As a result, the junction J3’s breakdown voltage, which is typically between 20 and 40 volts, is low.

The doping level of the p type gate is highly graded. Because it should be low to maintain high emitter efficiency while it should be high to have good turn OFF properties. To maximize the ability to turn off the current, the gate and cathodes should also be highly interdigitated with different geometric forms.

Anode junction refers to the intersection of the P+ anode and N base. To obtain the higher efficiency anode junction and good turn ON properties, a heavily doped P+ anode region is necessary. However, such GTOs have issues with their ability to turn OFF. As shown in the figure, this issue can be resolved by adding N+ layers that are heavily doped at regular intervals within the P+ anode layer. Thus, at junction J1, this N+ layer directly contacts the N layer. As a result, there is no hole injection from the P+ anode and the electrons move directly from the base N region to the anode metal contact. A GTO structure with anode shorts is what this is known as.

These anode shorts cause the GTO’s reverse blocking capacity to drop to junction j3’s reverse breakdown voltage, which quickens the turn-off mechanism. But as there are more anode shorts, the anode junction’s efficiency declines, which affects the GTO’s ability to turn on. For a good turn ON and OFF performance, careful thought must be given to the density of these anode shorts.

Principle of Operation

The GTO’s turn-on process is comparable to that of a typical thyristor. The hole current injection from the gate forward biases the cathode p-base junction when the anode terminal is made positive with respect to the cathode by applying a positive gate current. As a result, electrons are emitted from the cathode and travel to the anode terminal. This triggers the injection of holes into the base region from the anode terminal. Up until the GTO enters the conduction state, these holes and electrons are continuously injected.

When using a thyristor, the cathode area near the gate terminal is first turned ON to initiate conduction. Thus, the remaining area enters the conduction by plasma spreading. In contrast to a thyristor, a GTO has narrow cathode elements that are tightly interconnected to the gate terminal, resulting in a very large initial turned-on area and a restrained amount of plasma spreading. As a result, the GTO enters the conduction state quickly.

A reverse bias is applied at the gate to turn off a conducting GTO by making the gate negative in relation to the cathode. Through the gate, which prevents the injection of electrons from the cathode, a portion of the holes from the P base layer are extracted. In response, the gate extracts more hole current, which causes the cathode’s electrons to be suppressed more strongly. Eventually, the gate cathode junction undergoes reverse bias due to the voltage drop across the p base junction, turning off the GTO.

The p-base region is gradually depleted during the hole extraction process, which squeezes the conduction area. The anode current flows through isolated regions during the course of this process, creating filaments with a high current density. Localized hot spots are created as a result, and if the filaments are not quickly put out, they could harm the device.

These filaments are quickly extinguished by using a strong negative gate voltage. The cathode current has stopped, but the anode to gate current is still flowing because of the stored charge in the N base region. This phenomenon, known as a tail current, degrades exponentially as the number of extra charge carriers is decreased through recombination. The device maintains its forward blocking characteristics once the tail current has been reduced to a leakage current level.

V-I Characteristics

GTO operates similarly to a thyristor when turned ON. Therefore, the characteristics of the first quadrant resemble those of a thyristor. The device operates in forward blocking mode when the anode is made positive in relation to the cathode. The GTO enters the conduction state when a positive gate signal is applied. According to the figure, the GTO has much higher latching and forward leakage currents than a thyristor. If the anode current is greater than the holding current level, the gate drive can be removed.

However, it is advised to keep the positive gate drive at a value greater than the maximum critical gate current during conduction. This is because, as was mentioned above, the cathode is divided into small finger elements to aid in the turn-OFF process. As a result, the anode current briefly drops below the holding current level, forcing a large anode current to flow rapidly back into the GTO. This has the potential to be harmful. Therefore, during the conduction state, some manufacturers advise using a continuous gate signal.

Reverse gate current, which can be driven in either a step- or ramp-style, can be used to turn off the GTO. It is possible to turn off the GTO without changing the anode voltage. The figure’s dashed line depicts the i-v trajectory during an inductive load’s turn-off. It should be noted that GTO can only block a rated forward voltage during the turn OFF. Either a recommended value of resistance must be connected between the gate and cathode, or a small reverse bias voltage (typically -2V) must be maintained on the gate terminal in order to prevent dv/dt triggering and protect the device during turn OFF. As a result, the GTO maintains during the turn-OFF state and prevents the gate cathode junction from becoming forward biased.

The blocking capacity in a reverse biased GTO condition depends on the type of GTO. According to the figure, an asymmetric GTO has a low reverse blocking capability. While a symmetric GTO has a high reverse blocking capability. It has been found that under reverse biased conditions,. The anode short structure causes GTO to begin conducting in the opposite direction at a low reverse voltage (between 20 and 30 V). As long as the gate is negatively biased and the operation takes a short time,. The device is not destroyed by this mode of operation.

Gate Turn Off Thyristor Applications

Excellent switching characteristics, the lack of a commutation circuit requirement, maintenance-free operation,. And other benefits make GTOs more popular than thyristors in many applications. In inverters and choppers, it serves as the primary control mechanism. Several of these programs include

• AC drives
• AC stabilizing power supplies
• DC drives or DC choppers
• Induction heating
• DC circuit breakers
• And other low power applications