Switching Characteristics of GTO

Switching characteristics of a Gate Turn Off Thyristor or GTO comprises of dynamic characteristics during Turn ON and Turn OFF process. It basically represents the variation in anode voltage Va and anode current Ia when either positive or negative gate signal Ig is applied.

Turn On Characteristic of GTO:

Turn on process in GTO is similar to that of conventional thyristor. Gate turn on time of GTO is composed of delay time, rise time and spread time just like a normal thyristor. Besides, the tun on time can be decreased by increasing the forward gate current. Figure below shows the switching characteristics of a GTO.



A steep fronted gate pulse is applied in the above figure to turn on the GTO. It should be noted here that the gate drive can be removed once the anode current Ia reaches above the latching current. However, some manufacturers suggests to not to remove the gate drive current to eliminate the possibility of unwanted turn off of gate turn off thyristor (GTO). Therefore, even when the GTO is turned on, a small amount of positive gate current is continuously applied. This small positive gate current is called “Back Porch Current”. This is shown in the above of Ig Vs. time (t) graph.

Turn Off Characteristic of GTO:

Unlike turn on characteristics, turn off characteristics of a GTO is different from an SCR. Before the initiation of turn off process, GTO carries anode current Ia in forward direction. As soon as the negative gate current is applied at t=0, turn off process begins. The rate of rise of gate current depends on the circuit inductance and anode voltage. The very first step during turn off process is the removal of stored charges by the negative gate current. Stored charges here mean, the excess charges i.e. hole in p+ layers.  The time elapsed in removing the stored charges is called the Storage Period (ts). During this period, the anode voltage and current will remain unchanged.

Once the stored charges are removed, the anode current will fall rapidly and hence, the anode voltage starts rising. As can be seen from the figure that after storage time (ts) time, the anode current Ia starts to fall rapidly till a certain value and then changes its rate of fall abruptly. This time during which anode current falls rapidly is called the Fall Time (tf). This time is measured from the instant gate current is maximum negative to the instant anode current falls to its tail current. Kindly refer the figure to have the better understanding. The fall time is generally of the order of 1 micro second.

At the end of the fall time (tf), there is a spike in the anode voltage due to abrupt change in rate of fall of anode current Ia. After storage time and fall time, the anode voltage and current stars moving toward their turn off values i.e. rated anode voltage and zero respectively. The total time elapsed to reach anode voltage and current to their turn off values is called the Tail Time (tt). After tail time, anode current becomes zero but the anode voltage undergoes a transient overshoot due to the presence of resistance (Rs) and capacitance (Cs) and then stabilizes to its off-state value i.e. source voltage applied to anode circuit. Here, Rs and Cs are the Snubber Circuit parameters. The duration of tail time (tt) depends on the characteristics of the device.

From the above discussion, it may be said that the total turn off time of a GTO comprises of three times: Storage Time (ts), Fall Time (tf) and Tail Time (tt). So, turn off time (tq) of a Gate Turn Off Thyristor (GTO) may be written as

tq = ts + tf + tt

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