The rise time of the SCR
is reduced by applying steep and high current gate pulses.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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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