Basic electronics interview questions PDF

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Basic electronics interview questions...

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Difference between UPS and INVERTER UPS: The one power cable of your UPS is fixed in the power socket of your wall when the power is on. So, the UPS keeps on getting a regular power supply from the mains when the power is on. This AC power received from the mains power source continuously keeps on being converted to DC. This DC power keeps on charging the battery of the UPS in a continuous charging mode. So, the battery of the UPS is always kept charged during the time when the power is there in the mains. The output from the battery goes to the Sine wave inverter of the UPS. It converts DC to AC and this feeds the equipment. This makes it very clear that the power to your electrical gadget which is connected to the UPS is always supplied from the battery. Due to the power always being drawn from the battery to the electrical equipment, there is no time lag in case of electricity failure and the flow of the mains power stops. It is only the battery of the UPS which stops from being charged since there is no power in the mains. However, the backup power from the battery of the UPS keeps on being supplied uninterrupted till the battery is discharged and is no more able to supply the power to the equipment. This is why we find that the backup power of the battery of the UPS is of very short period ranging from 15 – 20 minutes upward. The more is this backup time of the UPS battery, the more will be the cost of the UPS. Otherwise also, the circuitry of the UPS is expensive making the UPS even of a small backup time more costly. Inverter: As far as the power to the inverter is concerned, like UPS it also comes from the mains for the inverter battery to get charged when there is no power cut off. The main difference between the inverter and the UPS lies in the fact, that in an inverter, the power is directly sent to the output which is connected by wiring to various appliances. At the same time the AC is also converted to DC and this DC is constantly charging the battery. On the other hand, the power to the appliances is not directly sent to the output but it goes from the battery of the UPS which keeps on getting discharged. A sensor and relay mechanism checks whether the mains is ON or OFF in an inverter. When the mains get switched off, the relay mechanism triggers to switch from mains to inverter. Rest is same like the UPS. Because of this sensor and relay, there is a gap between triggering.

What is Switch-mode power supply? The electronic power supply integrated with the switching regulator for converting the electrical power efficiently from one form to another form with desired characteristics is called as Switch-mode power supply. It is used to obtain regulated DC output voltage from unregulated AC or DC input voltage.

Switch mode power supply

Similar to other power supplies, switch-mode power supply is a complicated circuit that supplies power from a source to loads. switch-mode power supply is essential for power consuming electrical and electronic appliances and even for building electrical and electronic projects.

Topologies of Switch Mode Power Supply There are different types of topologies for SMPS, among those, a few are as follows ● ● ● ●

DC to DC converter AC to DC converter Fly back converter Forward converter

[Q-2] 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 ●

Answer: We know that Duty cycle D = Vout / VInput

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For case I D = 12/48 = 0.25

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For case II D = 12/24 = 0.5

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So for case II, the duty cycle is more than case I. It means device is turned ON more time in the second case. while designing SMPS we have to consider the principle mentioned in Q-3.

[Q-3] What is effect of having more duty cycle and less duty cycle? ( continuation of above question) 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. Relate this answer the previous question.... [Q-4] 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) IOUT Maximum Ripple voltage allowed at the output side Efficiency of the converter

[Q-6] 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 VCES Collector Current IC Collector Dissipation PC Junction Temperature Tj [Q-6] 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 RDS(ON) Reverse Recovery Current IRR Reverse Recovery Charge QRR Gate Charge QG Body Diode Characteristics ID at ambient temperature Channel power dissipation Channel temperature [Q] What are the advantages of freewheeling diode in rectifier circuit? 1. The input power factor is improved. 2. It prevents the output voltage from becoming negative. 3. The Load current waveform is improved. What is a freewheeling or fly back diode?

Freewheel or Fly back diodes are used across inductive components such as coils to prevent voltage spikes when the power is turned off to the devices. When power to inductive loads such as coils and inductors is turned off, there is a sharp voltage spike. The direction of this voltage is opposite to the applied voltage in accordance with Lenz’s Law. When a current flows through the coil of a relay, the coil gets electromagnetically charged. The energy is stored in the magnetic field around the coil. When the power supply to the coil is interrupted and the current in the coil tends to decrease, the magnetic field discharges causing a surge in the voltage.

The voltage, thus induced, can jump across the contacts of relays connected to the coils. The sparks and arcing produced can affect the life of the contacts. The voltage spikes can also damage electronic components like transistors which may be driving the relay coils. Freewheel diodes are connected in reverse bias vis-� -vis the supply voltage. Hence, when the voltage spike appears in the opposite direction, they are short-circuited through the diode. The voltage spike is thus short-circuited across the coil. This protects the connected circuits.

[Q-1] What is holding current in SCR? It is the minimum current required to hold the SCR in forward conduction state. When the forward current becomes less than holding current, SCR turns from forward conduction state to forward blocking state. [Q-2] What is latching current in SCR? It is the minimum current required to latch(turn on) the SCR from forward blocking state to forward conduction state. [Q-3] What is the different turn on methods of SCR? Forward voltage triggering Gate Triggering dv/dt triggering Temperature triggering Light triggering [Q-4] What is snubber circuit? The snubber circuit is used for the dv/dt protection of the SCR. It is a series combination of a resistor and a capacitor in parallel with the SCR. [Q-5] What is hard switching of the thyristor? When gate current is several times higher than the required gate current, the SCR is said to be hard fired. It reduces the turn ON time and enhances the di/dt capability. [Q-6] What is firing angle? The angle between the zero crossing of the input voltage and the instant the SCR is fired is called as delay angle or firing angle. [Q-7] What is meant by SOA? SOA - Safe Operating Area determines the voltage and current boundary within which the Power Device can be operated without destructive failure. [Q-8] What are the main components used for isolating the Power Circuits, Power Semiconductor from the low-power circuit? Opto-Couplers, Transformers

[Q-9] Name some of the current controlled (current driven) devices... SCR, GTO, GTR [Q-10] Name some of the voltage driven ( Voltage controlled) devices IGBT, MCT, IGCT, SIT [Q-11] What is duty cycle? It is the ratio of the ON time of the chopper to total time period of the chopper. D = Ton / [Ton + Toff] [Q-12] Can fuses with an AC voltage rating be used in a DC applications? Fuses must be rated for the voltage AC or DC in which they will be used. Generally, fuses have a DC voltage rating that is half of the maximum AC voltage rating. [Q-13] What are the characteristics of ideal Opamp? Infinite open loop voltage gain Infinite input impedance Zero output impedance Infinite Bandwidth Zero offset voltage [Q-14] 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. ie, Their voltage rating is lesser than IGBT. ● General rule is MOSFETs are suitable for applications which has breakdown voltage less than 250V. ● The IGBTs are suitable for applications which has breakdown voltage upto 1000V. [Q-15] 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 upto 20kHz, we have to use IGBT. ● For high frequency applications having frequency range of more than 200kHz, we have to use MOSFET. Q: What is meant by CMOS device? Complementary metal–oxide–semiconductor (CMOS) is a technology for constructing integrated circuits. Two important characteristics of CMOS devices are : high noise immunity and low static power consumption. Significant power is only drawn while the transistors in the CMOS device are switching between on and off states. CMOS also allows a high density of logic

functions on a chip.The phrase "metal–oxide–semiconductor" is a reference to the physical structure of certain field-effect transistors, having a metal gate electrode placed on top of an oxide insulator, which in turn is on top of a semiconductor material. Aluminum was once used but now the material is polysilicon. Q :) Explain why and how a MOSFET works. MOSFET is a metal oxide semiconductor Field effect transistor, its unidirectional device like, formed by four terminals GATE,SOURCE,DRAIN AND SUSTRATE. GATE is the control signal, depends upon voltage applied at gate terminal MOSFET work on three regions saturation, cutoff and active region. active region is used for amplification, cut-off and saturation region used for switching operation, mostly digital ckt design Enhancement-mode, n-channel MOSFET · Cutoff, subthreshold, or weak-inversion mode When VGS < Vth: · Triode mode or linear region (also known as the ohmic mode) When VGS > Vth and VDS < ( VGS - Vth ) · Saturation or active mode When VGS > Vth and VDS > ( VGS - Vth ) Body effect :The body effect describes the changes in the threshold voltage by the change in the source-bulk voltage. Q: ) What are advantages of BJT over MOSFET? BJTs have some advantages over MOSFETs for at least two digital applications. · Firstly, in high speed switching, they do not have the "larger" capacitance from the gate, which when multiplied by the resistance of the channel gives the intrinsic time constant of the process. The intrinsic time constant places a limit on the speed a MOSFET can operate at because higher frequency signals are filtered out. Widening the channel reduces the resistance of the channel, but increases the capacitance by exactly the same amount. · The second application where BJTs have an advantage over MOSFETs stems from the first. When driving many other gates, called fanout, the resistance of the MOSFET is in series with the gate capacitances of the other FETs, creating a secondary time constant. Q: ) What isProcess variation Process variation is the naturally occurring variation the attributes of transistors (length, widths, oxide thickness) when integrated circuits are fabricated. It

becomes particularly important at smaller process nodes ( ( VGS - Vth ) Q: )What are the different regions of operation in a mos transistor? Explain. · Cutoff, subthreshold, or weak-inversion mode When VGS < Vth: According to the basic threshold model, the transistor is turned off, and there is no conduction between drain and source. In reality, the Boltzmann distribution of electron energies allows some of the more energetic electrons at the source to enter the channel and flow to the drain, resulting in a subthreshold current that is an exponential function of gate–source voltage. While the current between drain and source should ideally be zero when the transistor is being used as a turned-off switch, there is a weak-inversion current, sometimes called subthreshold leakage. · Triode mode or linear region (also known as the ohmic mode) When VGS > Vth and VDS < ( VGS - Vth ) The transistor is turned on, and a channel has been created which allows current to flow between the drain and the source. The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. The current from drain to source is modeled as:  where μn is the charge-carrier effective mobility, W is the gate width, L is the gate length and Cox is the gate oxide capacitance per unit area. · Saturation or active mode : When VGS > Vth and VDS > ( VGS - Vth ) The switch is turned on, and a channel has been created, which allows current to flow between the drain and source. Since the drain voltage is higher than the gate voltage, the electrons spread out, and conduction is not through a narrow channel but through a broader, two- or three-dimensional current distribution extending away from the interface and deeper in the substrate. The onset of this region is also known as pinch-off to indicate the lack of channel region near the drain....