5 March 2020

Gate Turn off Thyristor ( GTO ) - Interview Question Answer



1
State the full form of GTO.


Gate turn off thyristor


2
Describe the main disadvantage of the SCR.


Disadvantage of SCR
  • The SCR is turned 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 or applying reverse voltage across it for sufficient time. This is serious disadvantage of the SCR.


3
How many terminals the GTO consists of?


Three terminals – Anode, cathode and Gate

4
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.


5
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 )


6
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


7
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.


8
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.


9
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.


10
Explain the term : Anode junction in the GTO


Anode junction
  • The junction between n base and P+ anode is called as anode junction.


11
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.


12
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.


13
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 turned on performance.


14
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.

15
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.

16
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.

17
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


18
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


19
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


20
Why the turn on time of the GTO is small as compared to conventional SCR?


Due to narrow emitter width

21
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.


22
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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