20
|
By which method the rise
time of the SCR is reduced?
|
|
The rise time of the SCR is reduced by
applying steep and high current gate pulses.
|
21
|
Why the turn on losses is
the highest during rise time of the SCR?
|
|
The turn on losses is highest during rise
time because high anode voltage and large anode current occurs simultaneously
during this period.
|
21
|
Why an inductor is placed
in series with anode circuit in the SCR?
OR
How the SCR is protected
by rate of rise of anode current?
|
|
The SCR is protected against rate of
rise of anode current by placing inductor is in series with anode circuit.
|
22
|
Describe the adverse
effect of inductor if an inductor is placed in the anode circuit in order to
limit the rate of rise of anode current in the SCR?
|
|
An inductor in the anode circuit
increases reverse voltage transients during turn off process.
|
23
|
Describe the effect of
magnitude of the gate current pulse on switching time of the SCR.
|
|
Effect
of gate current pulse
- The turn on time of the SCR is reduced by
higher value of gate current pulse.
|
24
|
Describe the condition
necessary for gate pulse width in order to turn on the SCR.
|
|
The gate pulse width should be taken
as equal to greater as the SCR turn on time.
|
25
|
Describe the function of
the parallel resistor across gate cathode junction of the SCR.
|
|
Function
of shunt resistor across gate – cathode junction
- The parallel resistor is connected across
gate – cathode junction of the SCR in order to increase thermal stability of
the SCR.
- When the SCR is in the blocking state, the shunt resistor bypasses
the leakage current across junction J2.
|
26
|
Describe the effect of
capacitor across gate – cathode junction of the SCR.
|
|
Effect
of shunt capacitor
- The shunt capacitor across gate – cathode
junction of the SCR is connected in order to improve the dv / dt capability.
- However
it increases the delay time and consequently di / dt of the SCR is lowered.
- When
the SCR is switched on, the gate acts as voltage source and charging of the
capacitor is done.
- The capacitor charging current provides gate current even
after anode current stops to flow resulting turn off time increases.
.
|
27
|
What do you mean by the
converter controlled thyristors?
|
|
Converter
grade thyristors
- The thyristors which have large turn
off time is called as converter grade thyristors.
- They are also called as
slow switching or phase controlled thyristors.
|
28
|
What do you mean by the
inverter grade thyristors?
|
|
Inverter
grade thyristors
- The thyristors which have low turn off
time is called as inverter grade thyristors.
- They are also called as fast
switching thyristors.
|
29
|
Describe the advantages
and disadvantages of Negative gate – cathode voltage in order to turn on the
SCR.
|
|
Advantages
- Forward break over voltage increases.
- dv / dt withstand capability
increases.
Disadvantages
- Higher negative voltage requires to
turn on the SCR
|
30
|
Why the switching time is
lower in the fast switching SCR?
|
|
The fast switching SCR has lower
switching time due to gold doping in it.
|
31
|
Give the examples of
light triggering semi conductor devices.
|
|
Light triggering semi conductor device
- Light activated silicon controlled
rectifier ( LASCR )
- Light activated silicon controlled
switch ( LASCS )
|
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