EEE Unit 2 Important Questions with solutions
Draw the static V.I characteristics of PN junction diode.
VI characteristics of P-N junction diodes is a curve between
the voltage and current through the circuit. Voltage is taken
along the x-axis while the current is taken along the y-axis.
The above graph is the V-I characteristics curve of the P-N
junction diode. With the help of the curve, we can understand
that there are three regions in which the diode works, and they
are:
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Zero bias
Forward bias
Reverse bias
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EEE Unit 2 Important Questions with solutions
Discuss briefly about the operation of single phase
full bridge rectifiers and half bridge with its output
wave forms.
Rectification methods to convert AC (Alternating Current) to
DC (Direct Current) include full-wave rectification and halfwave rectification. In both cases, rectification is performed by
utilizing the characteristic that current flows only in the
positive direction in a diode.
Full-wave rectification rectifies the negative component of the
input voltage to a positive voltage, then converts it into DC
(pulse current) utilizing a diode bridge configuration. In
contrast, half-wave rectification removes just the negative
voltage component using a single diode before converting to
DC.
Afterward, the waveform is smoothed by
charging/discharging a capacitor, resulting in a clean DC
signal.
From this, it can be said that full-wave rectification is a more
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EEE Unit 2 Important Questions with solutions
efficient method than half-wave rectification since the entire
waveform is used.
Also, a ripple voltage that appears after smoothing will vary
depending on the capacitance of this capacitor and the load.
Given the same capacitance and load, ripple voltage is smaller
with full-wave rectification than haif-wave rectification. Of
course it goes without saying that the smaller the ripple
voltage the better the stability.
Explain the operation of common emitter
configuration with circuit diagram ,draw its input
and output characteristics.(common base)
Output characteristics is the plot between collector-emitter
voltage (VCE) and the collector current (IC) at different constant
values of base current (IB)
Output resistance is defined as the ratio of the change in collectoremitter voltage (ΔVCE) to the change in collector current
(ΔIC) at a constant base current IB.
Initially with the increase in VCE, the collector current increase
almost linearly this is because the junction is not reverse biased.
When the supply is more than required to reverse bias the base collector junction IC increase very little with VCE.
The reciprocal of slope of the linear part of the curve gives the
value of output resistance.
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EEE Unit 2 Important Questions with solutions
Input characteristics diagram
Output characteristics diagram
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EEE Unit 2 Important Questions with solutions
Explain the construction and working principle of
JFET and draw its output characteristics and
transverse.
JFET or Junction Field Effect Transistor is one of the simplest
types of field-effect transistor. Contrary to the Bipolar
Junction Transistor, JFETs are voltage-controlled devices. In
JFET, the current flow is due to the majority of charge
carriers. However, in BJTs, the current flow is due to both
minority and majority charge carriers. Since only the majority
of charge carriers are responsible for the current flow, JFETs
are unidirectional. The first working model of junction fieldeffect transistors was made in 1953.
In an N-channel JFET, the material is of P-type, and the
substrate is N-type, while in a P channel JFET the material is
of N-type, and the substrate used is p-type. JFET is made of a
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EEE Unit 2 Important Questions with solutions
long channel of semiconductor material. Ohmic contacts are
provided at each end of the semiconductor channels to form
source and drain connections. A P-type JFET contains many
positive charges, and if the JFET contains a large number of
electrons, it is called an N-type JFET.
JFET Operation
Let us understand the working of JFET by comparing it to a
garden hose pipe. Water flows smoothly through a garden
hose pipe if there is no obstruction, but if we squeeze the pipe
slightly, the water flow slows down. This is precisely how a
JFET works. Here the hose is analogous to JFET, and the
water flow is equivalent to a current. By constructing the
current carrying-channel according to our needs, we could
control the current flow.
When no voltage is applied across the source and gate, the
channel is a smooth path for the electrons to flow through.
When the polarity that makes the P-N junction reverse biased
is applied, the channel narrows by increasing the depletion
layer and could put the JFET in the cut-off or pinch-off
region.
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EEE Unit 2 Important Questions with solutions
Comparison between BJT and FET.
Explain the construction of IGBT in a circuit
diagram and draw its characteristics.
The good characteristics of MOSFET and BJT technologies
are integrated into Insulated Gate Bipolar Transistor (IGBT).
The p+ injecting layer forms a pn junction with the drain layer
and injects minority carriers into it. The n type drain layer
may have two different doping levels. The lightly doped n
region is called drain drift region. Doping level and width of
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EEE Unit 2 Important Questions with solutions
this layer sets the forward blocking voltage of the device. A
large number of basic cells are grown on a single silicon wafer
and connected in parallel to form a complete IGBJ device.
When gate-emitter voltage is less than threshold voltage, no
inversion layer is formed and the device is in OFF state. When
gate-emitter voltage exceeds the threshold, an inversion layer
fonns in the p type body region under the gate. This inversion
layer shorts the emitter and drain drift layer and an electron
current flows from the emitter through this channel to the drift
region.
This causes hole injection from pT collector to drain drift
region. Few holes recombine with the electrons arriving at the
drain drift region through the channel. The rest of the holes
cross drift region to reach p type body where they are
collected by the source metallization.
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EEE Unit 2 Important Questions with solutions
VI characteristics:
Transfer characteristics
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EEE Unit 2 Important Questions with solutions
Explain the working principle of SCR and draw its
static characteristics.
Working of Silicon Controlled Rectifier(SCR):
Depending on the biasing given to the SCR, the operation of
SCR is divided into three modes. They are
1. Forward blocking Mode
2. Forward Conduction Mode and
3. Reverse Blocking Mode
Forward blocking mode:
In this mode of operation, the Silicon Controlled Rectifier is
connected such that the anode terminal is made positive with
respect to cathode while the gate terminal kept open. In this
state junctions J1 and J3 are forward biased and the junction
J2 reverse biased.
Due to this, a small leakage current flows through the SCR.
Until the voltage applied across the SCR is more than the
break over voltage of it, SCR offers a very high resistance to
the current flow. Therefore, the SCR acts as a open switch in
this mode by blocking forward current flowing through the
SCR as shown in the VI characteristics curve of the SCR.
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EEE Unit 2 Important Questions with solutions
Forward Conduction Mode
In this mode, SCR or thyristor comes into the conduction
mode from blocking mode. It can be done in two ways as
either by applying positive pulse to gate terminal or by
increasing the forward voltage (or voltage across the anode
and cathode) beyond the break over voltage of the SCR.
Once any one of these methods is applied, the avalanche
breakdown occurs at junction J2. Therefore the SCR turns into
conduction mode and acts as a closed switch thereby current
starts flowing through it.
Note that in the VI characteristic figure, if the gate current
value is high, the minimum will be the time to come in
conduction mode as Ig3 > Ig2 > Ig1. In this mode, maximum
current flows through the SCR and its value depends on the
load resistance or impedance.
It is also noted that if gate current is increasing, the voltage
required to turn ON the SCR is less if gate biasing is
preferred. The current at which the SCR turns into conduction
mode from blocking mode is called as latching current (IL).
And also when the forward current reaches to level at which
the SCR returns to blocking state is called as holding current
(IH). At this holding current level, depletion region starts to
develop around junction J2. Hence the holding current is
slightly less than the latching current.
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EEE Unit 2 Important Questions with solutions
Reverse Blocking Mode
In this mode of operation, cathode is made positive with
respect to anode. Then the junctions J1 and J3 are reverse
biased and J2 is forward biased. This reverse voltage drives
the SCR into reverse blocking region results to flow a small
leakage current through it and acts as an open switch as
shown in figure.
So, the device offers a high impedance in this mode until the
voltage applied is less than the reverse breakdown voltage
VBR of the SCR. If the reverse applied voltage is increased
beyond the VBR, then avalanche breakdown occurs at
junctions J1 and J3 which results to increase reverse current
flow through the SCR.
This reverse current causes more losses in the SCR and even
to increase the heat of it. So there will be a considerable
damage to the SCR when the reverse voltage applied more
than VBR.
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EEE Unit 2 Important Questions with solutions
List various types of power converters
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The Power Electronic Converter can be classified into.
six types:
Diode Rectifier.
AC to DC Converter (Controlled Rectifier)
DC to DC Converter (DC Chopper)
AC to AC Converter (AC voltage regulator)
DC to AC Converter (Inverter)
Static Switches.
What is commutation?
To generate motion in a three-phase linear motor there must
be switching between the phases to energize appropriate
windings. The process of switching between the phases is
called commutation.
What are line and natural commutations?
Natural or Line commutation is a Class-F SCR commutation
technique in which, a thyristor is turned off due to natural
current zero and voltage reversal after every half cycle.
What are various forced commutations?
The forced commutation can be classified into different
methods as follows:
Class A: Self commutated by a resonating load.
Class B: Self commutated by an LC circuit.
Class C: Cor L-C switched by another load-carrying SCR.
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EEE Unit 2 Important Questions with solutions
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