Power electronics Viva voce or interview questions part-5

 Q-101 Why does high power dissipation occur in reverse blocking mode?

Ans High power dissipation occurs because as voltage increases beyond Vbr current increases rapidly.

Q-102 Why shouldn’t positive gate signal be applied during reverse blocking Mode?

Ans If we apply positive gate signal J3 becomes forward biased. Reverse leakage current increases and Thyristor gets damaged due to large power dissipation.

Q-103 Explain reverse current Ire?

Ans When cathode voltage is positive, J2 is forward biased; J1 and J3 are reverse biased. The thyristors will be in reverse blocking state and reverse leakage current Ire flows.

Q-104 What happens when gate drive is applied?

Ans When gate drive is applied avalanche breakdown occurs at J2 causing excessive flow of charges and hence current surge. This turns the SCR into conduction state faster i.e. the Thyristor turns on at lower and lower anode to cathode voltages, which are less than Vbo.

Q-105 Differentiate between holding and latching currents

Ans Holding current is the minimum amount of current below, which SCR does not conduct. It is associated with the presence of gate terminal and concerns turn off condition. Latching current is the minimum amount of current required for the SCR to conduct. It is associated with absence of gate terminal and concerns turn on process. It is greater than holding current.

Q-106 Why is dv/dt technique not used?

Ans As this causes false triggering even when gate or voltage Vak is not applied, dv/dt technique is not used. Snubbed circuit, which is combination of a C, avoids this and R .The capacitor is placed in parallel with SCR.

Q-107 What sided?

Ans At the time of turn on, anode current increases rapidly. This rapid variation is not spread across the junction area of the thyristors. This creates local hotspots in the junction and increases the junction temperature and hence device may be damaged. This is avoided by connecting an inductor in series with an SCR.

Q-108 Why should the gate signal be removed after turn on?

Ans This prevents power loss in the gate junction.

Q-109 Is a gate signal required when reverse biased?

Ans No, otherwise SCR may fail due to high leakage current.

Q-110 What are different types of firing circuits to trigger SCR?

Ans *R firing circuit.

*RC firing circuit.

*UJT firing circuit.

*Digital firing circuit.

Q-111 What type of triggering is used in SCR?

Ans Pulse triggering.

Q-112 What is offset current?

Ans When anode voltage is made positive, J1 and J3 are forward biased, J2 is reverse biased. The Thyristor is in forward blocking or off state condition and the leakage current is known as offset current Io.

Q-113 What are the advantages of SCR?

Ans *Very small amount of gate drive is required since SCR is regenerative device.

*SCR’s with high voltage and current ratings are available.

*On state losses are reduced.

Q-114 What are the disadvantages of SCR’s?

Ans *Gate has no control once the SCR is turned on.

*External circuits are required to turn off the SCR.

*Operating frequencies are very low.

*Snubber (RC circuits) is required for dv/dt protection.

Q-115 What are applications of SCR?

Ans *SCR’s are best suitable for controlled rectifiers.

*AC regulators, lighting and heating applications.

*DC motor drives, large power supplies and electronic circuit breakers.

Q-116 What is the difference between an IGBT and SCR?

Ans IGBT comprises of a BJT and a MOSFET where as an SCR comprises of two BJT’s.

Q.117 Can we replace a SCR by a microprocessor by writing a program to exhibit characteristics of SCR?

ans. No, we can verify or test the working of SCR using microprocessor but we cannot replace it practically.

Q-117 What are MOSFET’s?

Ans Metal oxide silicon di-oxide field effect transistor is a voltage-controlled device. The parts of MOSFET are gate, drain and source.

Q-118 What is the difference between MOSFET and BJT?

Ans The MOSFET is a voltage controlled device where as BJT is a current controlled device.

Q-119 What is the difference between JFET and MOSFET?

Ans There is no direct contact between the gate terminal and the n-type channel of MOSFET.

Q-120 What are the two types of MOSFET?

Ans *Depletion MOSFET

 - N channel in p substrate.

 -P channel in n substrate.

*Enhancement mosfet

 -virtual n channel in p substrate

 -Virtual p channel in n substrate

Q-121 What is the difference between depletion and enhancement MOSFET?

Ans The channel in the centre is absent for enhancement type MOSFET but the channel is present in depletion type MOSFET.The gate voltage can either be positive or negative in depletion type MOSFET’s but enhancement MOSFET responds only for positive gate voltage.

Q-122 How does n-drift region affect MOSFET?

Ans The n- drift region increases the onstage drop of MOSFET and also the thickness of this region determines the breakdown voltage of MOSFET.

Q-123 How are MOSFET’s suitable for low power high frequency applications?

Ans MOSFET’s have high on state resistances due to which losses increase with the increase in the power levels. Their switching time is low and hence suitable for low power high frequency applications.

Q-124 What are the requirements of gate drive in MOSFET?

Ans *The gate to source input capacitance should be charged quickly.

 *MOSFET turns on when gate source input capacitance is charged to sufficient level.

 *The negative current should be high to turn off MOSFET.

Q-125 What is rise time and fall time?

Ans The capacitor Cgs charges from threshold voltage to full gate voltage Vgsp. The time required for this charging is called rise time. During this period, drain current rises to full value. The capacitor Cgs keeps on discharging and its voltage becomes equal to threshold voltage Vt.The time required for this discharge Cgs from Vgsp to Vt is called fall time.