Silicon
controlled rectifier
Which are the different methods for fabrication of the
thyristor?
Fabrication
of the thyrister
All diffused
Alloy diffused
Describe the main difference between planner diffused and alloy
diffused SCR.
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
What do you mean by the two transistor analogy of the SCR?
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.
Explain the following terms : Forward blocking state, Reverse
blocking state and Forward break over voltage
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.
Describe the effect of temperature on the depletion layer of the
SCR.
As
the temperature increases, the width of the depletion layer of the SCR
decreases and vice versa.
Describe the disadvantages of the DC gate triggering.
Disadvantages of
DC gate
triggering
There is no isolation
between control and power circuit
Due to continuous DC
signal, gate causes more power loss
Describe the disadvantages of the AC gate triggering.
Disadvantages of
AC gate
triggering
Step down transformer
requires
Reverse voltage applied
across gate – cathode junction during negative half cycle
Which is the most popular method of triggering of the SCR?
Gate pulse triggering
Which type of gate triggering of an SCR has negligible power
loss?
Pulse gate triggering
Describe the advantages of gate pulse triggering.
Advantages of
gate pulse
triggering
It does not require
continuous signal.
Low gate losses
It provides isolation
between control and power circuit.
Why the SCR is switch on due to sudden dv / dt across it?
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)
Explain the term : ON
state voltage, Voltage safety factor, Gate trigger voltage
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.
Describe the importance of the I2t rating of the SCR.
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.
Which type of power losses occurs in the SCR?
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
What do you mean by the static characteristic of the SCR?
Static
characteristic
It shows relation between
anode voltage and anode current for different value of gate current.
What do you mean by the dynamic characteristic of the SCR?
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.
State the turn on and turn off period of the SCR.
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
Define : Delay time, rise time and spread time
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.
You may also like :
No comments:
Post a Comment