Power Electronics - GTO(Gate Turn OFF Thyristor)

Quiz

Conventional thyristors(CTs) are nearly ideal switches for their use in Power Electronic applications. These can easily be turned on by positive gate current. Once in the on-state, gate loses control. CTs are now be turned OFF by expensive and bulky commutation circuitry. This shortcoming of thyristors limit their use up to about 1kHz applications. These drawbacks in thyristors has led to the development of GTOs.

A GTO is a more versatile power-semiconductor device. It is like a CT but with added features in it. A GTO can easily be turned OFF by a negative gate pulse of appropriate amplitude. It is a fully controlled device as we can turn-ON by a positive gate pulse and turn-OFF by a negative gate pulse at its gate cathode terminals.

GTO - Basic Structure

A GTO is pn pn, there terminal device with anode(A), cathode(K) and gate(G). The four layers are p⁺ n p⁺n⁺. In CT, anode consists of p⁺ layer, but in a GTO anode is made up of n⁺ type fingers diffused into p⁺ layer.

Unlike a thyristor, it can be turned OFF simply by applying a negative gate pulse of appropriate magnitude between the gate and cathode terminals. Therefore, for the applications where the supply is DC, such as in a chopper and inverter, GTO is preferred because this does not require any forced commutation circuit for its turn OFF. So, it becomes more compact and economical.

The negative gate current required to turn OFF a GTO is 15% - 25% of the anode current at the time of commutation, which is very high.

Basic circuit of Operation

A basic gate drive circuit for a GTO is shown below. For turning-ON a GTO, first transistor TR1 is turned on, this in turn switches on TR2 to apply a positive gate current pulse to turn on GTO. For turning OFF the GTO, the turn-OFF circuit should be capable of outputting a high peak current. Usually, a thyristor is used for this purpose. Turn OFF process is initiated by gating thyristor T1. When T1 is turned on, a large negative gate current pulse turns OFF the GTO.

Dynamic Characteristics of GTO

During turn ON an turn OFF processes, a GTO is subjected to different voltages across it and different current through it. The time variations of the voltage across a GTO and the current through it during turn-on and turn-off processes is given by the dynamic or switching characteristics of a thyristor.

GTO Turn ON characteristics

The turn ON process for GTO is similar to that of normal thyristor, that is, when GTO is forward biased and a positive gate pulse is applied between the gate and cathode it will be turned ON. But there is a finite transition time to switch from forward OFF state to forward ON-state for a thyristor. This finite transition time is known as turn ON time.

The turn ON time t_turn-ON is the sum of the delay time t_d, rise time t_r and spread time t_p and it can be expressed as −

$\mathrm{t_{turn-ON}\:=\:t_d\:+\:t_r\:+\:t_p}$

Turn - OFF of GTO

The turn OFF characteristics of a GTO are different from those of a normal thyristor or SCR. It can be turned OFF by applying a negative pulse between the gate and cothode terminals. The turn-OFF time of the GTO consists of three time intervals, that is, storage time t_s, fall time t_f and tail time t_t and it can be expressed as −

$\mathrm{t_{q}\:=\:t_s\:+\:t_f\:+\:t_t}$

Storage time (t_s)

During the storage period, the anode current (I_a) and anode voltage (equal to ON-state voltage drop) remain constant. The excess charge (holes in p-base) are removed by negative gate current during this period, that is, it prepares the GTO for turning OFF

Fall time (t_f)

During fall time the anode current falls rapidly and the anode voltage rises. The fall period is measured from the time the instant gate current is at a maximum negative to the instant the anode current falls to its tail current.

Tail time (t_t)

During tail time, the anode current and voltage keep moving toward their turn -OFF values. After t_t, the anode current reaches zero and the anode voltage V_a undergoes a transient overshoot due to the presence of R_s and C_s(where R_s and C_s are snubber circuit parameters).

Typical Ratings

Following are typical voltage, current and frequency of ratings of GTO −

V = 5000 volts
I = 3000 Amperes
f = 2kHz

Advantages of GTO's over Thyristor

Following are the advantages of GTO's over the thyristor −

GTO has faster switching speed compared to thyristor.
GTO has more di/dt rating at turn ON compared to thyristor
The surge current capability is analogous with an SCR.
GTO circuit has lower size and weight as compared to thyristor circuit.
Due to elimination of commutation choke, GTO circuit has reduced acoustical and electromagnetic noise.
Elimination of commutating circuit components in forced commutation.
Improved efficiency of converter circuit

Advantages of GTO's over BJT

Following are the advantages of GTO's over Bipolar junction transistor −

GTO has higher blocking voltage capability.
The ratio of maximum(peak) current rating to average current is high.
The ratio of maximum(peak) surge current rating to average current is high and it is about 10:1
High ON - state current gain and it value is about 600.
The pulse width of gate signal is short
Under surge condition, GTO goes into deeper saturation due to regenerative action.

Applications of GTO

Following are the various application of the GTO −

High performance in DC drives
High performance AC drive where the field oriented control or vector control are used specially in rolling mills, process control industry, machine tools control and robotics.
Electric traction drive system
Variable voltage variable frequency inverter fed AC drives.

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