1
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State the full form of GTO.
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Gate turn off
thyristor
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2
|
Describe the main disadvantage of the SCR.
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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.
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3
|
How many terminals the GTO consists of?
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Three
terminals – Anode, cathode and Gate
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4
|
Describe the significance of two arrows in the gate terminal
of the GTO.
OR
How the GTO is switched on and off?
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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.
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5
|
Describe the main disadvantage of the GTO.
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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 )
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6
|
Describe the effect of n+ layer near cathode
terminal in the GTO structure.
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Effect
of n+ layer near cathode terminal
- Increases
emitter efficiency
- Reduces the
breakdown voltage of the junction J3
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7
|
Give reason : The doping level of the P type gate layer is
kept optimum.
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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.
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8
|
Describe the significance of highly inter digitated gate –
cathode junction of the GTO.
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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.
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9
|
Describe the significance of doping level and width of n type
base in the GTO.
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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.
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10
|
Explain the term : Anode junction in the GTO
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Anode
junction
- The junction
between n base and P+ anode is called as anode junction.
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11
|
Describe the significance of high doping level in the p+
anode layer in the GTO.
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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.
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12
|
What do you mean by the anode shorted GTO structure?
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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.
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13
|
Describe the significance of anode shorted GTO structure?
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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.
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14
|
Why the n type buffer layer is added between anode and n-
layer in the GTO?
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The buffer
layer with anode shorted structure increases the emitter efficiency of the
anode short.
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15
|
Whether the latching current and forward leakage current is
same for GTO and SCR?
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The latching
current and forward leakage current is considerable higher for GTO as
compared to that of SCR for same rating.
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16
|
What is asymmetric GTO?
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Asymmetric
GTO
- The GTO can
block very small reverse voltage due to anode short structure. This is called
as asymmetric GTO.
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17
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How the internal regeneration is reduced in the GTO?
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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
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18
|
Describe the effect of low internal generation in the GTO.
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Effect
of Low Internal Generation
- Increases the
holding current and latching current
- High on state
voltage drop
- High power
loss
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19
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Why the current gain of the NPN and PNP transistor is kept
large in the conventional SCR?
|
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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
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20
|
Why the turn on time of the GTO is small as compared to
conventional SCR?
|
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Due to narrow
emitter width
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21
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Compare the symmetrical GTO and asymmetrical GTO.
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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.
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22
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Describe the advantages of GTO over conventional thyristor.
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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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