4 March 2020

Silicon Controlled Rectifier ( SCR ) - Interview Question Answer - 2


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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