Power electronics Viva voce or interview questions part-6

 Q-126 What is pinch off voltage?

Ans The voltage across gate to source at which the drain to source current becomes zero is called pinch off voltage.

Q-127 In which region does the MOSFET used as a switch?

Ans In the linear region.

Q-128 Which parameter defines the transfer characteristics?

Ans The Tran conductance Gm=Id/Vgs

Q-129 Why are MOSFET’s mainly used for low power applications?

Ans MOSFET’s have high on state resistance Rds. Hence for higher currents; losses in the MOSFET’s are substantially increased. Hence MOSFET’s are substantially increased. Hence, MOSFET’s are mainly used for low power applications.

Q-130 How is MOSFET turned off?

Ans To turn off the MOSFET quickly, the negative gate current should be sufficiently high to discharge gate source input capacitance.

Q-131 What are the advantages of vertical structure of MOSFET?

Ans *On state resistance of MOSFET is reduced.

*Width of the gate is maximized. Hence, gain of the device is increased.

Q-132 What are the merits of MOSFET?

Ans * MOSFET’s are majority carrier devices.

* MOSFET’s have positive temperature coefficient, hence their paralleling is easy.

*MOSFET’s have very simple drive circuits.

*MOSFET’s have short turn on and turn off times; hence they operate at high frequencies.

*MOSFET’s do not require commutation techniques.

*Gate has full control over the operation of MOSFET.

Q-133 What are demerits of MOSFET?

Ans *On state losses in MOSFET are high.

*MOSFET’s are used only for low power applications.

*MOSFET’s suffer from static charge.

Q-134 What are the applications of MOSFET?

Ans *High frequency and low power inverters.

*High frequency SMPS.

 *High frequency inverters and choppers.

 *Low power AC and DC drives.

Q-135 What is IGBT?

Ans Insulated gate bipolar transistor is the latest device in power Electronics .It is obtained by combining the properties of BJT And MOSFET.

Q-136 In what way IGBT is more advantageous than BJT and MOSFET?

Ans *It has high input impedance of the MOSFET and has low on-state voltage drop.

*The turn off time of an IGBT is greater than that of MOSFET.

*It has low onstage conduction losses and there is no problem of second Breakdown as in case of BJT.

*It is inherently faster than a BJT.

Q-137 What are on state conduction losses? How is it low in IGBT?

Ans A high current is required to break the junctions in BJT. This results in On state conduction losses. The conduction losses in IGBT are proportional To duty cycle of the applied voltage. By reducing the duty cycle conduction losses can be reduced.

Q-138 What is second breakdown phenomenon?

Ans As the collector voltage drops in BJT there is an increase in collector Current and this substantially increase the power dissipation. This Dissipation is not uniformly spread over the entire volume of the device but is concentrated in highly localized regions where the local temperature may grow and forms the black spots. This causes the destruction of BJT. This is second breakdown.

Q-139 What is switching speed?

Ans The time taken to turn on or turn off a power device is called switching Speed.

Q-140 Can we observe the transfer and collector characteristics of IGBT on CRO?

Ans No. Because the waveform which is to be observed on the CRO should Vary with respect to time otherwise we can see only a straight line on the CRO.

Q-141 What is punch through IGBT?

Ans The IGBT’s which have n+ buffer layer present are called punch through IGBT.They have asymmetric voltage blocking capabilities and have faster turn off times. Hence they are used in choppers and inverters.

Q-142 What is non-punch through IGBT?

Ans The IGBT’S without n+ buffer layer are called non-punch through IGBT’s. They have symmetric voltage blocking capabilities and are used for rectifier applications.

Q-143 What are merits of IGBT?

Ans *The drive is simple.

*Onstage losses are reduced.

*No commutation circuits are required.

*Gate has full control.

*Switching frequencies are higher.

*It has flat temperature coefficient.

Q-144 What are demerits of IGBT?

Ans *They have static charge problems.

*They are very costly.

Q-145 What are the applications of IGBT’s?

Ans *Ac motor drives. (Inverters)

*Dc to Dc power supplies. (Choppers)

*UPS systems.

*Harmonic compensators.

Q-146 Why is silicon used in all power semiconductor devices and why not Germanium?

Ans The leakage current in silicon is very small compared to germanium. The germanium is also more sensitive compared to silicon.

Q-147 What is pinch off voltage?

Ans When Vge is made negative, electrons in the n-channel get repelled Creating a depletion region resulting in a narrower effective channel. If Vge is made negative enough so as to completely eliminate the channel (High resistance, low current state), that value is called the pinch off Voltage.

Q-148 What is threshold voltage?

Ans Threshold voltage is the voltage Vge at which IGBT begins to conduct.

Q-149 How is IGBT turned off?

Ans An IGBT can turn off by discharging the gate by means of short circuiting it to the emitter terminal.

Q-150 What is the rating of IGBT?

Ans The current rating can be up to 400A, 1200V with switching frequency of 20KHz.