SESSION:22

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1)       For High voltage applications will you prefer MOSFET or IGBT?

For High voltage applications we have to use IGBT. Because MOSFETs are low voltage devices. i.e., Their voltage rating is lesser than IGBT. General rule is MOSFETs are suitable for applications which have breakdown voltage less than 250V. The IGBTs are suitable for applications which have breakdown voltage up to 1000V.

2)       For High frequency applications will you prefer MOSFET or IGBT?  Why?

For High frequency applications, MOSFET is the right choice of the device.

Because MOSFET has low switching losses compare to that of IGBT. General rule of thumb is for low-frequency applications having frequency range up to 20 kHz, we have to use IGBT. For high frequency applications having frequency range of more than 200 kHz, we have to use MOSFET.

3)       Draw the buck topology?

In most of the power electronics interview questions, you are going to face this basic question. Not only buck converter, the interviewer may ask you to draw other fundamental topologies too

So you should be familiar with other basic topologies like Flyback, Boost , Buck-boost.

4)       Which is the good choice among the 2 options mentioned below to get the 12V as an output? Why?

Option I: Input voltage = 48V

Option II: Input voltage = 24V

We know that
Duty cycle D = V_out / V_Input

For case I,
D = 12/48 = 0.25

For case II,
D = 12/24 = 0.5

So for case II, the duty cycle is more than case I.
It means device is turned ON more time in the second case.

5)       What is effect of having more duty cycle and less duty cycle?

If the duty cycle D is greater than 0.5, the core would not be completely demagnetized at the end of the off-period, and a DC magnetization of the core would build up, resulting in core saturation.

6)       To design the buck converter, what are basic & essential information (parameters) we need to get from the Customer?

We need the following inputs from the customer,

·         Output Voltage V_OUT

·         Input Voltage V_IN

·         Output Current (load current) I_OUT

·         Maximum Ripple voltage allowed at the output side

·         Efficiency of the converter

7)       How to select the inductor for the buck converter?

Once the topology is finalized in the process of power supply design, the next step is selecting the Inductor.

We know the following basic equation

Voltage across Inductor VL = L di/dt

Rewriting the above equation as follows

L = VL.dt/di

VL = Maximum voltage appears across the Inductor

dt= The time during which the maximum voltage appears across the inductor

di = Ripple current

VL The maximum voltage appears across the inductor when the power switch is turned on. During that time the voltage will be
VL = VOUT - VIN

dt = The time during which the maximum voltage appears across the inductor

= D/FS

= Duty cycle/Switching frequency

di = Ripple current

= Generally it is assumed in between 10% to 30% of the load current

By knowing all these values we can get the required inductor value for the buck converter.

8)       What are the various parameters we have to consider, while selecting IGBT?

In IGBT datasheet, we have to focus on following parameters

·         Collector to emitter voltage V_CES

·         Collector Current I_C

·         Collector Dissipation P_C

·         Junction Temperature T_J

9)       For Selecting MOSFET, what are the major parameters we have to consider in the datasheet?

In MOSFET datasheet, we have to focus on following parameters:

·         Drain to Source Voltage V_DSS

·         On state Drain to Source Resistance R_DS(ON)

·         Reverse Recovery Current I_RR

·         Reverse Recovery Charge Q_RR

·         Gate Charge Q_G

·         Body Diode Characteristics

·         ID at ambient temperature

·         Channel power dissipation

·         Channel temperature

10)    What are the control strategies of chopper?

The control strategies of chopper are

·         Pulse width modulation PWM (Variable TON, Constant frequency)

·         Frequency modulation (Constant TON or TOFF, Variable frequency)

·         Current Limit Control (CLC)

11)    What is Universal Motor?

It is defined as a motor which can be operated either on DC or single-phase AC supply at approximately the same speed and output. The universal motor is built exactly like a series DC motor. But a series DC motor cannot be run as a universal motor, even though both motors look the same internally and externally.

We cannot use these motors in the industrial applications due to the low efficiency (25% -35%). It has high starting torque and a variable speed characteristic. It runs at dangerously high-speed on no load.

12)    What is meant by PMDC?

PMDC stands for Permanent Magnet DC Motor

A  Permanent Magnet DC Motor is similar to an ordinary dc shunt motor except that its field is provided by permanent magnets instead of salient-pole wound field structure. There are three types of permanent magnets used for such motors namely

a)      Alnico Magnets

b)      Ceramic magnets

c)      Rare-earth magnets

The major advantages are low noise, small size, high-efficiency, low manufacturing cost.

13)    Why is the efficiency of transformers more than that of other rotating machines?

There are no moving parts in transformer hence the friction and mechanical losses are absent in transformer. Hence efficiency of the transformer is more than of other rotating machines.

14)    Define leakage inductance.

Inductance offered by the winding due to the leakage flux associated with it is called leakage inductance. It is the ratio of leakage flux linkages with the winding to the current passing through the winding.

15)    What material is used in the transformer core?

High grade silicon steel (CRGO-Cold Rolled Grain Oriented) laminations are used for the construction of the core.