State the
full form of GTO.
Gate
turn off thyristor
Describe the main disadvantage of the SCR.
Disadvantage of SCR
The SCR is turn on by the gate pulse. When the gate signal is removed, the SCR does not turn off but remains in the ON condition. The SCR is turned off only by reducing the forward current below the holding current. This is serious disadvantage of the SCR.
How many
terminals the GTO consists of?
Three terminals – Anode, cathode and Gate
Describe the significance of two arrows in the
gate terminal of the GTO.
OR
How the
GTO is switched on and off?
GTO terminals
The GTO consists of two arrows in the gate terminal.
The positive pulse is applied at the inward arrow
towards gate terminal in order to turn the GTO on.
The negative pulse is applied at the outward arrow at
gate terminal in order to turn off the GTO.
Describe
the main disadvantage of the GTO.
Disadvantage of GTO
The main disadvantage of the GTO is that it has high
turn off current gain. The turn off current gain lies in the range of 4 to 5. (
It means that if the anode current is 100 amp, the turn off current is in the
range of 20 to 25 ampere )
Describe
the effect of n+ layer near cathode terminal in the GTO structure.
Effect of n+ layer near cathode terminal
Increases emitter efficiency
Reduces the breakdown voltage of the junction J3
Give reason : The doping level of the P type gate layer is kept
optimum.
Doping of gate layer
The turn off time of the GTO is kept small by keeping
the doping level of the P type gate layer high. Whereas the emitter efficiency
is increased by keeping the doping level of the P type gate layer low.
Therefore the doping level of the P type gate layer is kept optimum by
considering the turn off time and emitter efficiency.
Describe
the significance of highly inter digitated gate – cathode junction of the GTO.
Inter digitated gate –
cathode junction
It means that if the GTO has maximum current capacity
1000 ampere, there are 1000 cathode segments connected to common point.
Describe
the significance of doping level and width of n type base in the GTO.
Significance of
doping level and width of n type base layer
The maximum forward blocking capacity of the GTO is
determined by doping level and width of the n type base. The width of the n
type base layer is kept high and doping level is kept low in order to keep the
forward blocking voltage in terms of kV.
Explain
the term : Anode junction in the GTO
Anode junction
The junction between n base and P+ anode is
called as anode junction.
Describe the significance
of high doping level in the p+ anode layer in the GTO.
Significance of
high doping level in the p+ anode layer
If the doping level of the p+ anode layer
is kept higher, it will result in good turn on characteristic as well as high
efficiency of the anode junction. However the GTO has poor turn on
characteristics and high power losses due to high doping level.
What do you mean by the anode shorted GTO
structure?
Anode shorted GTO
structure
The heavily doped n+ layers are introduced
in the p+ layer. The n+ layer makes metallic contact with
anode therefore it is called as anode shorted GTO structure. The density of the
anode short is determined by considering the turn on as well as turns off
performance of the GTO. To reduce the tail current faster.
Describe
the significance of anode shorted GTO structure?
Significance of
anode shorted GTO structure
The anode shorted GTO structure reduces the reverse
blocking capacity of the GTO and it also degraded turn on performance
Why the n
type buffer layer is added between anode and n- layer in the GTO?
The buffer layer with anode shorted structure
increases the emitter efficiency of the anode short.
Whether
the latching current and forward leakage current is same for GTO and SCR?
The latching current and forward leakage current is
considerable higher for GTO as compared to that of SCR for same rating.
What is
asymmetric GTO?
Asymmetric GTO
The GTO can block very
small reverse voltage due to anode short structure. This is called as
asymmetric GTO.
How the
internal regeneration is reduced in the GTO?
The current gain of the PNP transistor is reduced in
order to reduce the internal regeneration in the GTO. The current gain is
reduced by
Anode shorted GTO structure
Diffusing gold or heavy metal to reduce carrier life.
Describe
the effect of low internal generation in the GTO.
Effect of low
internal generation
Increases the holding current and latching current
High on state voltage drop
High power loss
Why the
current gain of the NPN and PNP transistor is kept large in the conventional
SCR?
The current gain of the NPN and PNP transistor is kept
large in order to
Increases gate sensitivity during turn on
Minimize on state voltage drop
Why the
turn on time of the GTO is small as compared to conventional SCR?
Due to narrow emitter width
Compare
the symmetrical GTO and asymmetrical GTO.
Symmetrical GTO
There is no anode short structure.
Slow switching speed as compared to asymmetrical GTO.
The N base is doped with heavy metal in order to
reduce turn off time.
High on state voltage drop as compared to asymmetrical
GTO.
It has considerable reverse blocking voltage.
Asymmetrical GTO
It consists of anode short structure.
Fast switching speed as compared to symmetrical GTO.
Low on state voltage drop as compared to symmetrical
GTO.
The diode is connected in series with GTO in order to
block reverse voltage.
Describe
the advantages of GTO over conventional thyristor.
Advantages of GTO
over conventional thyristor
Fast switching speed
High surge current capability
Large forward voltage blocking capability
Large current capability
The size and weight of the GTO inverter is approximately
60% to that of SCR inverter.
No force commutation circuit
High efficiency due to compensation of forced
commutation power loss over on state power loss and gate drive power loss in
the GTO.
It has very low reverse voltage blocking capability due to anode shorted structure.
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