1
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What is full form of SCR?
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Silicon
controlled rectifier
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2
|
Which are the different methods for fabrication of the
thyristor?
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Fabrication
of the thyrister
- All diffused
- Alloy diffused
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3
|
Describe the main difference between planner diffused and alloy
diffused SCR.
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|
Planner
diffused SCR
- All the
junctions are diffused.
- More silicon
requires per ampere current carrying capacity
- Applicable for
low power
Alloy
diffused SCR
- The inner
junction is diffused whereas two outer junctions are alloy on it.
- Applicable for
high power
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4
|
What do you mean by the two transistor analogy of the SCR?
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Two
transistor analogy
- The SCR is
considered as two transistor analogy in which the base of the one transistor
is connected to the collector of the second transistor and vice versa.
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5
|
Explain the following terms : Forward blocking state, Reverse
blocking state and Forward break over voltage
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Forward
blocking state
- When anode is
made positive with respect to cathode, the outer junction J1 and J3 becomes
forward biased whereas inner junction J2 remains reverse biased.
- This is
called as forward blocking state of the SCR.
Reverse
blocking state
- When anode is
made negative with respect to cathode, the outer junction J1 and J3 becomes
reverse biased whereas inner junction J2 becomes forward biased.
- This is
called as reverse blocking state of the SCR.
Forward
break over voltage
- The anode is
made positive with respect to cathode and gate is kept open circuited. As the
anode to cathode voltage increases, at certain voltage avalanche breakdown of
the junction J2 occurs.
- This voltage is called as forward break over voltage.
The voltage across SCR is 1 – 2 volt during this state.
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6
|
Describe the effect of temperature on the depletion layer of
the SCR.
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As the
temperature increases, the width of the depletion layer of the SCR decreases
and vice versa.
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7
|
Describe the disadvantages of the DC gate triggering.
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Disadvantages
of DC gate triggering
- There is no
isolation between control and power circuit
- Due to
continuous dc signal, gate causes more power loss
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8
|
Describe the disadvantages of the AC gate triggering.
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Disadvantages
of AC gate triggering
- Step down
transformer requires
- Reverse
voltage applied across gate – cathode junction during negative half cycle
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9
|
Which is the most popular method of triggering of the SCR?
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|
Gate pulse
triggering
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10
|
Which type of gate triggering of an SCR has negligible power
loss?
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|
Pulse gate
triggering
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11
|
Describe the advantages of gate pulse triggering.
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|
Advantages
of gate pulse triggering
- It does not
require continuous signal.
- Low gate
losses
- It provides
isolation between control and power circuit.
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12
|
Why the SCR is switch on due to sudden dv / dt across it?
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|
- There is
always capacitance at junction J2 due to charges exist on its both sides in
the SCR. When a sudden forward voltage is suddenly applied to the SCR, the
SCR is switched on due charging current.
- The charging current is equal to Ic
= C (dv / dt)
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13
|
Explain the term : ON
state voltage, Voltage safety factor, Gate trigger voltage
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|
ON
state voltage
- It is a
voltage between anode to cathode terminal during ON state of the semi
conductor device with specified junction temperature.
Voltage
safety factor
- The SCR
operating voltage is kept less than to its peak reverse voltage in order to
prevent it from damage due to uncertain conditions.
- The voltage
safety factor is defined as Vf
= Peak reverse voltage / ( √2 × RMS input voltage )
Gate
trigger voltage
- It is minimum
gate voltage require to produce gate current.
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14
|
Describe the importance of the I2t rating of the
SCR.
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Importance
of I2t rating
- The SCR is
protected by fast acting fuse during overload or fault conditions.
- It
measures the thermal energy that can be absorbed by the fuse for a short time
before fault is cleared.
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15
|
Which type of power losses occurs in the SCR?
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Power
losses in the SCR
- Switching losses : ( 1 )
Turn on loss and ( 2 ) Turn off loss
- Forward
conduction loss
- Forward
blocking loss
- Reverse
blocking loss
- Gate power
loss
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16
|
What do you mean by the static characteristic of the SCR?
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|
Static
characteristic
- It shows
relation between anode voltage and anode current for different value of gate
current.
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17
|
What do you mean by the dynamic characteristic of the SCR?
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|
Dynamic
characteristic
- It is a
switching characteristic of the SCR.
- It includes turned on and turned off
characteristics.
- The dynamic characteristic shows the relation between anode
voltage and current with respect to time.
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18
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State the turn on and turn off period of the SCR.
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SCR
switching time
- Turn on time : ( 1 ) Delay
time ( 2 ) Rise time and ( 3 ) Spread time
- Turn off time : ( 1 )
Reverse recovery time and ( 2 ) Gate recovery time
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19
|
Define : Delay time, rise time and spread time
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Delay
time
- It is time
duration at which the gate current reaches 90% of its maximum value and anode
current 10% of its maximum value.
OR
- It is time
duration at which anode voltage reaches 90% of its maximum value.
Rise
time
- It is time
duration at which the anode current increases from 10% to 90% of its maximum
value.
OR
- It is time
duration at which the anode voltage drops from 90% to 10% of its maximum
value.
Spread
time
- It is time
duration at which the forward blocking voltage falls from 10% of its maximum
value to on state voltage drop of the SCR.
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