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Multiple choice question for engineering

Set 1

1. An ideal power diode must have
a) low forward current carrying capacity
b) large reverse breakdown voltage
c) high ohmic junction resistance
d) high reverse recovery time

View Answer

Answer: b [Reason:] Large reverse breakdown voltage is desirable whereas others will increases the losses.

2. To make a signal diode suitable for high current & high voltage carrying applications with minimum losses, ________
a) a lightly doped n layer is grown between the two p & n layers
b) a heavily doped n layer is grown between the two p & n layers
c) a lightly doped p layer is grown between the two p & n layers
d) a heavily doped p layer is grown between the two p & n layers

View Answer

Answer: a [Reason:] The above process simply the one used to manufacture power diodes.

3. Power diode is __________
a) a three terminal semiconductor device
b) a two terminal semiconductor device
c) a four terminal semiconductor device
d) a three terminal analog device

View Answer

Answer: b [Reason:] It has two terminals anode and cathode same as that of a ordinary diode. In fact, a power diode is nothing but a signal diode with a extra layer.

4. The V-I Characteristics of the diode lie in the
a) 1st & 2nd quadrant
b) 1st & 3rd quadrant
c) 1st & 4th quadrant
d) Only in the 1st quadrant

View Answer

Answer: b [Reason:] First in the forward region & Third in the reverse biased mode.

5. Which of the following is true in case of a power diode with R load?
a) I grows almost linearly with V
b) I decays almost linearly with V
c) I is independent of V
d) I initial grows than decays

View Answer

Answer: a [Reason:] R load therefore V and I are linear and in phase.

6. A diode is said to be reversed biased when the
a) cathode is positive with respect to the anode
b) anode is positive with respect to the cathode
c) cathode is negative with respect to the anode
d) both cathode & anode are negative

View Answer

Answer: a [Reason:] K is positive w.r.t the A when the device is reversed biased.

7. A diode is said to be forward biased when the
a) cathode is positive with respect to the anode
b) anode is positive with respect to the cathode
c) anode is negative with respect to the anode
d) both cathode & anode are positive

View Answer

Answer: b [Reason:] A is positive w.r.t the K when the device is forward biased.

8. In case of an ideal power diode, the leakage current flows from
a) anode to cathode
b) cathode to anode
c) in both the directions
d) leakage current does not flow

View Answer

Answer: d [Reason:] Leakage current does not flow in IDEAL diode.

9. The peak inverse current IP for a power diode is given by the expression
a) IP=t + di/dt
b) IP=t * log ⁡i
c) IP=t * di/dt
d) IP=t * ∫ t*i dt

View Answer

Answer: c [Reason:] The leakage current is the reveres recovery time (t) into the rate of change of current.

10. A power diode with small softness factor (S-factor) has
a) small oscillatory over voltages
b) large oscillatory over voltages
c) large peak reverse current
d) small peak reverse current

View Answer

Answer: b [Reason:] Peak reverse current is independent of S-factor smaller the value of S-factor larger the oscillatory over voltage.

Set 2

1. For a power transistor, if the forward current gain α = 0.97, then β = ?
a) 0.03
b) 2.03
c) 49.24
d) 32.33

View Answer

Answer: d [Reason:] Use the relation α = β/(β+1).

2. The power electronics devices have a very high efficiency because
a) cooling is very efficient
b) the devices traverse active region at high speed & stays at the two states, on and off
c) the devices never operate in active region
d) the devices always operate in the active region

View Answer

Answer: b [Reason:] They are efficient due to their higher transition speeds.

3. For a power transistor, which of the following relations is true?
a) Ie>Ic>Ib
b) Ib>Ic>Ie
c) Ic>Ie>Ib
d) Ie=Ib

View Answer

Answer: a [Reason:] Practically speaking Ie = Ib+Ic. Ie is the highest as it is the sum of the collector and base currents. The base current is the smallest.

4. High frequency operation of any device is limited by the
a) forward voltage rating
b) switching losses
c) thermal conductivity
d) heat Sink arrangements

View Answer

Answer: b [Reason:] Lower the switching losses higher the frequency of operation of the device.

5. The instantaneous power loss during the delay time of a transistor is given by
a) Ic Vce
b) Ib Vbe
c) Ic Vbe
d) Ib Vce

View Answer

Answer: a [Reason:] During the delay time only the collector current flows & base to emitter voltage is zero.

6. For a power transistor, the average power loss during the delay time can be given by the equation
a) Ic * Vc
b) 1/T * ∫Td (Ic Vce) dt
c) Ic * dVc/dt * T
d) 1/T * ∫(Td *Tr) (Ic Vc) dt

View Answer

Answer: b [Reason:] During the delay time only, the collector current flows & base to emitter voltage is zero. Hence the average power can be found, simply by integrating it over the total delay time & dividing by the base time period.

7. A 1mv of i/p gives an output of 1V, the voltage gain as such would be
a) 0.001
b) 0.0001
c) 1000
d) 100

View Answer

Answer: c [Reason:] 1V/1mv = 1000.

8. Which of the following relations is true for a BJT?
a) Ic ≈ Ie
b) Ib ≈ Ic
c) Ie ≈ Ib
d) Ib ≈ Ie ≈ Ic

View Answer

Answer: a [Reason:] The collector & emitter current differ only by the base current, which is very very small.

9. Choose the correct statement
a) A transistor will remain on as long the the base current is applied
b) A transistor remains on after a high to low pulse is applied at the base
c) A transistor will remain on as long the the collector current is applied
d) A transistor remains on after a high to low pulse is applied at the collector

View Answer

Answer: a [Reason:] Unlike the thyristor devices, all the transistor family devices remain in the conducting state as long as the firing pulses are applied. This is a very important property of the transistor devices.

10. Let’s say that a transistor is operating at the middle of the load line, then a decrease in the current gain would
a) move the Q point up
b) move the Q point down
c) result in to & fro motion of the Q point
d) not change the Q point

View Answer

Answer: b [Reason:] The current gain would decreases the collector current, shifting the Q point below.

Set 3

1. Which of the following devices does not belong to the transistor family?
a) IGBT
b) MOSFET
c) GTO
d) BJT

View Answer

Answer: c [Reason:] GTO is gate turn off transistor, it belongs to the Thyristor family. All the other devices belong to the transistor family.

2. A power transistor is a
a) three layer, three junction device
b) three layer, two junction device
c) two layer, one junction device
d) four layer, three junction device

View Answer

Answer: b [Reason:] It has three layers p-n-p or n-p-n forming two p-n junctions.

3. In a power transistor, ____ is the controlled parameter.
a) VBE
b) VCE
c) IB
d) IC

View Answer

Answer: d [Reason:] The collector current is the controlled parameter.

4. A power transistor is a _________ device.
a) two terminal, bipolar, voltage controlled
b) two terminal, unipolar, current controlled
c) three terminal, unipolar, voltage controlled
d) three terminal, bipolar, current controlled

View Answer

Answer: d [Reason:] Power transistor is simply many BJT’s connected in series parallel on a single silicon chip for power applications. It is a three terminal, bipolar, current controlled device.

5. In a power transistor, _________ is the controlling parameter.
a) VBE
b) VCE
c) IB
d) IC

View Answer

Answer: c [Reason:] The base current controls the collector current. Hence, the base current Ib is the controlling parameter.

6. In a power transistor, the IB vs VBE curve is
a) a parabolic curve
b) an exponentially decaying curve
c) resembling the diode curve
d) a straight line Y = IB

View Answer

Answer: c [Reason:] The B-E junction of a BJT resembles a p-n junction diode, hence the curve.

7. For a power transistor, if the base current IB is increased keeping VCE constant, then
a) IC increases
b) IC decreases
c) IC remains constant
d) none of the mentioned

View Answer

Answer: a [Reason:] Ic is directly proportional to Ic.

8. The forward current gain α is given by
a) IC/IB
b) IC/IE
c) IE/IC
d) IE/IB

View Answer

Answer: b [Reason:] Collector current by emitter current is the current gain, its value is close to one but never greater than.

9. The value of β is given by the expression
a) IC/IB
b) IC/IE
c) IE/IC
d) IE/IB

View Answer

Answer: a [Reason:] Collector current by the base current is beta, its value is in the range 50 to 300.

10. A power BJT is used as a power control switch by biasing it in the cut off region (off state) or in the saturation region (on state). In the on state
a) both the base-emitter & base-collector junctions are forward biased
b) the base-emitter junction is reverse biased, and the base collector junction is forward biased
c) the base-emitter junction is forward biased, and the base collector junction is reversed biased
d) both the base-collector & the base-emitter junctions are reversed biased

View Answer

Answer: a [Reason:] When base-emitter & base-collector junctions are forward biased only than both the p-n junctions are forward biased and the device is on.

Set 4

1. For a transistor, the safe operating area (SOA) is a plot of
a) Ib versus Vce
b) Ib versus Ic
c) Ic versus Vce
d) Ic versus time

View Answer

Answer: c [Reason:] For reliable operation the collector current & voltage must remain within the SOA curves.

2. The forward safe operating area (FSOA) pertains to the operation when
a) the device is fired at a 50% Duty cycle
b) the device is forward-biased
c) the device is operated on AC
d) the device is operated on DC

View Answer

Answer: b [Reason:] The FSOA is for forward biased operations. The FSOA is plotted for AC as well as DC for different duty cycles. Hence, option (b) is the most appropriate choice.

3. The SOAs are plotted always on a _________ scale
a) time
b) frequency
c) logarithmic
d) polynomial

View Answer

Answer: c [Reason:] The scale is always logarithmic, irrespective of the type of device.

4. As the FSOA increases, the pulse width
a) decreases
b) increases
c) remains constant
d) vanishes

View Answer

Answer: b [Reason:] On reduced pulse width values, the devices can operated on higher voltages & currents.

5. The SOAs provided by the manufacturers are for
a) single pulse operation & a particular temperature
b) multi pulse operation & all the temperature
c) all the conditions
d) a particular duty cycle operation

View Answer

Answer: a [Reason:] The manufacturer specifies the SOAs only for single pulse DC operation & a particular temperature (usually 20Degree Centigrade Scale). For actual operations, The SOA’s have to be modified using the thermal impedance charts.

6. A device is operating at Ic = 4A & Vce = 50V. For the device to operate at Ic = 20A (Without damaging),
a) voltage should be increased
b) voltage should be reduced
c) voltage can be kept constant
d) current has to increased further

View Answer

Answer: b [Reason:] For safe operation, the values should be within the limits. P = V.I – with increase in one of the values, another value should decrease.

7. For a BJT, find the maximum power dissipation when the device is safely operated at Vce = 90V and Ic = 0.5A
a) 40 Watts
b) 35 Watts
c) 45 Watts
d) 30 Watts

View Answer

Answer: c [Reason:] P=90*0.5=45Watts.

8. The SOA for a MOSFET is plotted for
a) Id versus Vds
b) Ig versus Id
c) Ig versus Vds
d) Id versus Vgs

View Answer

Answer: a [Reason:] It is a plot of drain current vs drain to source voltage.

9. The SOA for an IGBT is plotted for
a) Ic versus Vge
b) Ig versus Ic
c) Ig versus Vce
d) Ic versus Vce

View Answer

Answer: d [Reason:] It is a plot of collector current vs collector to emitter voltage.

10. For MOSFET’s SOA, as the pulse width goes on increasing, the maximum voltage rating ____ & current rating ____
a) is constant, increases
b) increases, decreases
c) decreases, is constant
d) constant, decreases

View Answer

Answer: c [Reason:] Refer MOSFET’s SOA.

Set 5

1. Shown below is the diagram of an ideal super diode. When the input voltage Vin is negative, then the output voltage Vout = ?
power-electronics-mcqs-types-power-diodes-1-q1
a) Vout/Vin
b) Vin
c) 0
d) Vin * Rl

View Answer

Answer: c [Reason:] When the input voltage is negative, there would be a negative voltage on the diode so it works like an open circuit. Hence no current flows through the load and Vout is zero.

2. In order to reduce the reverse recovery time of the diodes, __________ is carried out.
a) shortening of the length of the device
b) platinum & gold doping
c) antimony doping
d) adding an extra silicon layer

View Answer

Answer: b [Reason:] Platinum & gold doping improves the performance of the devices.

3. Which of the following diodes uses a metal-semiconductor junction?
a) General purpose diodes
b) Fast recovery diodes
c) Schottky diode
d) None of the mentioned

View Answer

Answer: c [Reason:] Schottky diode uses a Al-Semiconductor junction.

4. Which of the below mentioned statements is false regarding Schottky diodes?
a) Schottky diodes have a Al-Silicon junction
b) There is no storage of charges in a Schottky diode
c) The majority charge carriers in a Schottky diode are holes
d) Schottky diodes can be switched off faster than a p-n junction diode of the same rating

View Answer

Answer: c [Reason:] The majority charge carriers in a Schottky diode are electrons not holes.

5. A Schottky diode _____
a) has current flow due to holes only
b) has no reverse recovery time
c) has large amount of storage charges
d) has zero cut-in voltage

View Answer

Answer: b [Reason:] Due to the metal-silicon junction there are no stored charges hence, no reverse recovery time, due to which the switching is faster.

6. Which of the following are/is the majority charge carriers in a Schottky diode?
a) Holes
b) Electrons
c) Both holes & Electrons carry equal current
d) None of the mentioned

View Answer

Answer: b [Reason:] The metal has no holes hence major(almost full) current flows due to the electrons only.

7. In a Schottky diode, the silcon is usually
a) N-type
b) P-type
c) un-doped semiconductor
d) silicon is not used

View Answer

Answer: a [Reason:] Usually only n-type silicon is used because the p-type has certain limitations.

8. As compared to a p-n junction diode(of the same rating), a Schottky diode has ___________
a) higher cut-in voltage
b) lower reverse leakage current
c) higher operating frequency
d) higher switching time

View Answer

Answer: c [Reason:] Due to the metal-silicon junction there are no stored charges, hence no reverse recovery time due to which the switching is faster.

9. A Schottky diode has __________
a) a gate terminal
b) aluminum-silicon junction
c) platinum gold junction
d) germanium-Arsenide junction

View Answer

Answer: b [Reason:] Schottky diode uses a Al-Semiconductor junction.

10. A Schottky diode can be switchd off much faster than an equivalent p-n junction diode due to its
a) higher operating frequency
b) no recombination of charges
c) more compact structure
d) None of the mentioned

View Answer

Answer: b [Reason:] Due to the metal-silicon junction there are no stored charges, hence no reverse recovery time due to which the switching is faster.