Dynamic characteristics of a GTO
Dynamic characteristics of a GTO:
- The switching characteristics of a GTO and refers to the resistive dc load switching circuit shown on the right hand side. When the GTO is off the anode current is zero and VAK = Vd.
- To turn on the GTO, a positive gate current pulse is injected through the gate terminal. A substantial gate current ensures that all GTO cathode segments are turned on simultaneously and within a short time.
- There is a delay between the application of the gate pulse and the fall of anode voltage, called the turn on delay time td. After this time the anode voltage starts falling while the anode current starts rising towards its steady value IL.
- Within a further time interval tr they reach 10% of their initial value and 90% of their final value respectively. tr is called the current rise time (voltage fall time). Both td and maximum permissible on state diA/dt are very much gate current dependent.
- High value of I gM and dig/dt at turn on reduces these times and increases maximum permissible on state A di/dt . It should be noted that large value of ig (IgM) and dig/dt are required during td and tr only. After this time period both vg and ig settles down to their steady value.
- A minimum ON time period tON (min) is required for homogeneous anode current conduction in the GTO. This time is also necessary for the GTO to be able to turn off its rated anode current.
- To turn off a GTO the gate terminal is negatively biased with respect to the cathode. With the application of the negative bias the gate current starts growing in the negative direction. However, the anode voltage, current or the gate voltage does not change appreciably from their on state levels for a further time period called the storage time (ts).
- The storage time increases with the turn off anode current and decrease with digQ/dt. During storage time the load current at the cathode end is gradually diverted to the gate terminal.
- At the end of the storage time gate current reaches its negative maximum value IgQ. At this point both the junctions J2 & J3 of the GTO starts blocking voltage. Consequently, both the gate cathode and the anode cathode voltage starts rising towards their final value while the anode current starts decreasing towards zero.
- At the end of current fall time “tf” the anode current reaches 10% of its initial value after which both the anode current and the gate current continues to flow in the form of a current tail for a further duration of ttail. A GTO is normally used with a R-C turn off snubber.
- Therefore, VAK does not start to rise appreciably till tf. At this point VAK starts rising rapidly and exceeds the dc voltage Vd (VdM) (due to resonance of snubber capacitor with di/dt limiting inductor) before setting down at its steady value Vd .
- A GTO should not be retriggered within a minimum off period off (min) to avoid the risk of failure due to localized turn ON. GTOs have typically low turn off gain in the range of 4-5.