Silicon Controlled Rectifier - Interview Question Answer - 1

1	What is full form of SCR?
	Silicon controlled rectifier
2	Which are the different methods for fabrication of the thyristor?
	Fabrication of the thyrister All diffused Alloy diffused
3	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
4	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.
5	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.
6	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.
7	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
8	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
9	Which is the most popular method of triggering of the SCR?
	Gate pulse triggering
10	Which type of gate triggering of an SCR has negligible power loss?
	Pulse gate triggering
11	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.
12	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)
13	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.
14	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.
15	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
16	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.
17	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.
18	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
19	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.

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