MODULE I Structure and working of N-channel enhancement type

POWER ELECTRONIC DEVICES

Various Power Electronic Devices
1. MOSFET (Metal Oxide Semiconductor Field Effect Transistor)
2. IGBT (Insulated Gate Bipolar Transistor)
3. GTO (Gate Turn Off Thyristor)
4. LASCR (Light Activated Silicon Controlled Rectifier)
5. SCS (Silicon Controlled Switch)
6. UJT (Uni Junction Transistor) Body (substrate)
7. DIAC (Diode for Alternating Current) • The body is made up of P-type material, two N+ -type materials are
8. TRAIC (Triode for Alternating Current diffused at the top of the body. So, a depletion layer will be formed
9. SCR (Silicon controlled Rectifier) in the PN junction.
Source (S)
MOSFET • Terminal where current enters. (electron flow from source todrain).
• It is a device used to switch or amplify voltages in circuits. Drain (D)
• They are voltage-controlled devices with three terminals, • Terminal where current exit.
Source (S), Drain (D) and Gate (G). Gate (G)
• Based on operation they classified into,
• Terminal that controls the flow of current between the source and
a) Enhancement mode MOSFET (E-MOSFET) drain.
b) Depletion mode MOSFET (D-MOSFET).
• It is separated from semiconductor material by thin insulating layer
E-MOSFET
usually made of silicon dioxide (SiO2).
 When there is no voltage across gate terminal, device does not
conduct. (normally OFF condition). Channel
 When there is maximum voltage across gate, it shows • When voltage is applied to gate, it creates an electric field that
enhanced conductivity. attracts or repels charge carriers in the semiconductor material
 They are widely used due to their ability to be easily switched forms a conduction channel between source and drain.
on and off. WORKING
D-MOSFET • When a +ve voltage is applied to the gate, relative to the source, it
 When there is no voltage across gate, device shows maximum creates an electric field in the oxide layer.
conductivity. (normally ON condition). • This electric field attracts electrons from N-type substrate towards
 When thereis voltage across gate, then conductivity decreases. the surface, forming an electron rich channel between the source
 They are less common than E-MOSFETs. and drain. Thus, regulating conductivity between source and drain.
• Based materials used to construction classified into, • Higher gate voltage increases electron concentration and enhances
a) N-channel MOSFET (NMOS) conductivity.
b) P-channel MOSFET (PMOS) Threshold voltage
 In NMOS, current flows from Drain to Source while in PMOS, • It is the minimum voltage required on the gate to initiate electron
current flows from Source to Drain. accumulation and turn on the MOSFET.
• In general, there are four different types of MOSFETs, • When gate voltage is below threshold voltage, MOSFET is off
1) N-channel depletion mode
and gate voltage is above threshold voltage, MOSFET is on.
2) N-channel enhancement mode
3) P-channel depletion mode
4) P-channel enhancement mode. IGBT
• It is a bipolar device with an insulated gate terminal.
• It has three terminals, Emitter (E), Gate (G) and Collector (C).
• It is the combination of the characteristics of both MOSFET and BJT.
• It has high efficiencyand fast switching capability.

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Structure and working of IGBT • By increasing the VG, which increases the number of charges.
• When the VG exceeds the threshold voltage, which eventuallyforms
a layer in the upper P-region.
• This layer form N-channel that shorts N-drift region and N+ region.
• Hence the electrons flow from N+ region into N- drift region.
• While the holes from the collector are injected from the P+ region
into the N- drift region.
• Due tothe excess of both electrons and holes in the drift region, its
conductivity increases and starts the conduction of current. Hence
the IGBT switches ON.

GTO
• It is made of four layers to form a PNPN structure. • It is a special type of semiconductor device in the family of SCRs.
• It ishigh power device (e.g 1200V AC) invented by General Electric.
• The collector (C) is attached to P layer while emitter (E) is
• GTOs, as opposed to normal thyristors, are fully controllable
attached between P and N layers.
switches which can be turned on and off by their gate lead.
• A P+ substrate is used and an N- layer is placed top of it to form a
PN junction J1.
• Two P regions are fabricated on top of these N- layer to form
junction J2.
• The P region is designed in such a waytoleave a path in the middle
for the gate (G).
• Two N+ regions are diffused over the top of each P regions as
shown in figure.
• The emitter is directly attached to the N+ region while the gate is
insulated using a SiO2 layer.
• The P+ layer inject holes into N- layer called injector layer while
N- layer called drift layer.
• The P layer above is known as the body of IGBT.
Turning off capacity
WORKING
• Unlike standard SCRs, GTO have a gate terminal that allows for
• The collector and emitter are used for the conduction of current turning off the device by applying a negative pulse to the gate.
while gate is used for controlling the IGBT. • When a negative pulse is applied to gate, it will neutralize thecharge
• Its working is based on the biasing of gate-emitter terminals and carriers in the device and will get turned off.
collector-emitter terminals.
• collector-emitter is connected to VCC such that Collector kept at a LASCR
+ve voltage than the emitter. • LASCR is a semiconductor device that turns ON when it is exposed
• The J1 becomes forward biased and J2 becomes reverse biased. to light.
• At this point, there is no voltage at the gate. • It is a type of thyristor which is triggered by photons present in the
• Due to reverse J2, the IGBT remains off and no current flow light rays.
between collector and emitter. • It is a three-terminal device, consists of cathode, anode and gate
terminal.

SCS
• SCS are four terminal devices.
• Terminals are Anode, Cathode, Gate Anode and Gate Cathode.
• Applying a gate voltage VG positive than the emitter, -ve charges • One gate is used to turn on SCR and other gate is for turn off or
will accumulate right beneath the SiO2 layer. commutation.

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 VI Characteristics of UJT

Turn on
• By applying a positive pulse to the Gate Anode (GA) terminal.
Turn off
• By applying a negative pulse to the Gate Cathode (GK) terminal.

UJT
• UJT is a three-terminal semiconductor device that has only one PN
junction.
• Terminals are emitter (E), and two bases (B1 and B2).
Cut-off Region
• Thepart of the characteristics curve where the unijunction transistor
does not get sufficientvoltage to turn on is called the cut-off region.
In this region, the unijunction transistor remains off-state.
Negative Resistance Region
• In the negative resistance region, the unijunction transistor (UJT)
receives enough voltageto turn on.
• When we increase the voltage applied to the emitter terminal, it
WORKING attains its peak value (VP) after a certain time.
• When a positive voltage (VE) is applied at the emitter as shown in • After this point, the voltage drops across the device starts
figure, the PN-junction remains to reverse-biased till the input decreasing, and this reduction stops at a point of voltage VV, this
voltage is less than the internal voltage (Vi). point is called the valley point.
• Although, the current through the device still increasing. Therefore,
the resistance of the UJT is found to be negative in this region so it
is called the negative resistance region.
Saturation Region
• The saturation region is the area on the characteristics curve of the
unijunction transistor where the voltage and current increase if the
emitter voltage is increased.

Applications of UJT

• When the applied voltage becomes greater than Vi, the PN- • It is used as a relaxation oscillator.
junction becomes forward-biased. • It is used as a voltage regulator.
• Thus, the holes started to move from the positive terminal (B2) to • It is used for the generation of sawtooth waveforms.
the negative terminal (B1). • It is most widelyused as a triggering device for SCRs.
• Theaccumulation of holes in the emitter to B1 region decreases the • It may also be used as a phase control circuit, etc.
resistance of the n-type semiconductor bar.
• Consequently, the internal voltage drop from the emitter to B1 is DIAC
decreased andhence the emitter current IE is increased. • DIAC is given bythe symbol of two Diodes connected in parallel and
• With the accumulation of a largenumber of holes, a condition of opposite to one another and has two terminals.
saturation will eventually be reached. • Since the DIAC is bidirectional, we can't name those terminals as
• In this condition, the emitter current is limited by the emitter power anode and cathode, the terminals of DIAC are simply called A1 and
supply, and the unijunction transistor is nowsaid to be in the ON A2 or MT1 and MT2 where MT stands for Main terminals.
state.

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 • When the reverse voltage exceeds break over voltage (VBO),
avalanche breakdown happens in reverse biased junction and
conduction is occurs.
• Hence the conduction will be possible in both directions.
VI characteristics of DIAC
• Initially, the resistance of the DIAC will be higher because of the
Reverse Bias junction between the layers.

Structure and working of DIAC

• The above diagram shows the typical construction of the DIAC.

• DIAC has two terminals namely MT1 and MT2 and it can deliver
current flow in both directions.
• DIAC is made of a five-layered structure; the layers closer to the
terminalsare the combination of both positive and negative layers. • So, there will be small leakage current flowing through the DIAC,
WORKING it is mentioned as the blocking state in the curve.
• Consider the MT1 terminal to be positive, then the P1 layer near • Once the applied voltage reaches the breakdown voltage the
MT1 will be activated, so the conduction will be taking place in the resistance of the DIAC drops abruptly and then it starts conducting
order of P1-N2-P2-N3. which leads to a sharp decrease in voltage and the current starts
increasing, which is mentioned as a conduction state in the curve.

TRIAC
• TRIAC is a three-terminal semiconductor switching device that is
used for controlling current flow in a circuit.
• It is one of the most important members of the thyristor family; it is
a bidirectional device that can pass the current in both forward and
reverse direction, which means that they can conduct in both the
conditions of the gate signal, positive and negative.

• Consider the MT1 terminal to be positive, then the P1 layer near

MT1 will be activated, so theconduction will be taking place in the
order of P1-N2-P2-N3.
• When the current is flowing from MT1 to MT2 the junction between
P1-N2 and P2-N3 are Forward Biased and the junction between N2-
P2 is reverse biased.
• When the reverse voltage exceeds break over voltage (VBO),
avalanche breakdown happens in reverse biased junction and
conduction is occurs.
• Similarly, if we consider MT2 terminal to be positive, then the P2
Structure and working of TRIAC
layer near MT2 will be activated and the conduction will be taking
• Figure shows the structure of the TRIAC, it is a five-layer device
place in the order of P2-N2-P1-N1.
that consists of six doping regions.
• The current will be flowing from MT2 to MT1 and the junctions
between P2-N2 and P1-N1 are forward biased and the junction
Between N2- P1 is reverse biased.

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 • Here charge carriers injected from P2 to N4 and current flows from
MT1 to MT2.
•

• The gate terminal is designed in a way to have ohmic contact with

both Nand P regions, which helps the device to get triggered with
both positive and negative polarities.
• If the gate current ishigh, a very small amount of voltage is enough
to turn on the TRIAC. • But th
WORKING Mode 4 : MT1= +ve, Gate= +ve
• As the TRIAC is bidirectional and has an ability to get turned on • When MT1 is +ve with respect to MT2, the current will flow in
with both the polarities tothe gate pulse it can operate in four the same path of P2-N1-P1-N3.
different types of modes of operation as listed below • During this operation, the junction between the layers P2-N1 and
Mode 1 : MT2= +ve, Gate= +ve P1-N3 are forward biased but N1-P1 is reverse biased.
• In this mode, the applied voltage at MT2 is positive with respect to • When a positive signal is applied to the gate, the junction N1-P1
MT1, the current will flow in the path of P1-N1-P2-N2.
occurs breakdown and device turns on.
• During this operation, the junction between the layers P1-N1 and
P2-N2 are forward biased but N1-P2 is reverse biased. • Here charge carriers injected from P2 to N2 and current flows from
• When a positive signal is applied to the gate, the junction N1-P2 MT1 to MT2.
occurs breakdown and device turns on. VI characteristics of TRIAC
• Charge carriers injected from P2 to N2 and current flows from MT2
to MT1.

SCR
Mode 2 : MT2= +ve, Gate= -ve
• A SCR is a 3-terminal, 4-layer semiconductor current controlling
• In this mode, the applied voltage is the same i.e. MT2 is positive
device.
with respect to MT1, the current will flow in the same path of P1- • It is mainly used in the devices for the control of high power.
N1-P2-N2. • It is made up of a silicon material which controls high power and
• During this operation, the junction between the layers P1-N1 and converts high AC current into DC current (rectification).
P2-N2 are forward biased but N1-P2 is reverse biased. • Hence, it is named as silicon-controlled rectifier.
• When a negative signal is applied to the gate, the junction N1-P2 • It is a unidirectional current controlling device.
occurs breakdown and device turns on. • Just like a normal p-n junction diode, it allows electric current in
• Here charge carriers injected from P2 to N4 and current flows onlyone direction and blocks electric current in another direction.
from MT2 to MT1.
Mode 3 : MT1= +ve, Gate= +ve
• In this mode, the applied voltage polarities are swapped i.e. MT1 is
positive with respect to MT2, the current will flow in the path of
P2-N1-P1-N3.
• During this operation, the junction between the layers P2-N1 and
P1-N3 are forward biased but N1-P1 is reverse biased.
• When a negative signal is applied to the gate, the junction N1-P1
occurs breakdown and device turns on.

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 Structure and working of SCR resistance to the current and the SCR is said to be in the off state.
• A normal p-n junction diode is made of two semiconductor layers Forward conduction mode
namely P-type and N-type. However, a SCR diode is made of 4 • In this mode of operation, the anode An SCR can be brought from
semiconductor layers of alternating P and N type materials that blocking mode to conduction mode in two ways: Either byincreasing
form two structures namely; NPNP or PNPN. the voltage between anode and cathode beyond the breakover
• An SCR conducts when a gate pulse is applied to it, just like a voltage, orby applying a positive pulse at the gate.
diode. • Once the SCR starts conducting, no more gatevoltage is required to
• In addition, it has three junctions labelled as J1, J2 and J3 and maintain it in the ON state.
three terminals Anode, Cathode and a Gate. • The minimum current necessaryto maintain the SCR in the ON state
on removal of the gate voltage is called the latching current.
Reverse blocking mode
• When a negative voltage is applied to the anode and a positive
voltage to the cathode,the SCR is in reverse blocking mode, making
J1 and J3 reverse biased and J2 forwardbiased.
• The device behaves as two diodes connected in series.
• A small leakage currentflow. This is the reverse blocking mode.
SCR turn on methods (SCR triggering)
VI characteristics of SCR & working 1) Forward voltage triggering
• The V-I characteristics of SCR is shown in the below figure. 2) Temperature triggering
3) dv/dt triggering
4) Light triggering
5) Gate triggering.
Forwardvoltage triggering
• Forward voltage triggering involves applying a sufficient voltage
across the SCR’s anode and cathode to initiate conduction.
• Here gate is open.
• It is a general term indicating the voltage required for SCR to start
conducting.
Temperature triggering
• This type of triggering is also known as Thermal Triggering as the
SCR is turned by heating it.
• The horizontal line in the figure represents the amount of voltage
applied across the SCR whereas the vertical line represents the • The reverse leakage current depends on the temperature. If the
amount of current flows in the SCR. temperature is increased to a certain value, thenumber of hole-pairs
• Here VA = Anode voltage, IA = Anode current, +VA = Forward also increases.
anode voltage, +IA = Forward anode current, -VA = Reverse anode • By increasing the temperature at junction J2, the width of the
voltage, +IA= Reverse anode current. depletion layer decreases.
• The V-I characteristics of SCR is divided into three regions: • At a particulartemperature, the reverse bias of the junction breaks
a) Forward blocking region down and the device starts toconduct.
b) Forward conduction region dv/dt triggering
c) Reverse blocking region. • In forward blocking state i.e., anode is more positive than cathode,
the junctions J1 andJ3 are forward biased and the junction J2 is
Forward blocking mode
reverse biased.
• In this mode of operation, the anode is given a positive voltage while
• So, the junction J2 behaves as a capacitor (J1 and J3 as conducting
the cathode is given a negative voltage, keeping the gate at zero
plates with a dielectric J2) due to the space charges in the depletion
potential i.e. disconnected.
region.
• In this case junction J1and J3 are forward-biased, while J2 is reverse-
biased, allowing only a small leakage current from the anode to the • If the rate of change of the applied voltage is large , then the flow of
cathode. charging current will increase, which causes theSCR to turn on
without any gate voltage.
• When the applied voltage reaches the breakover value for J2, then J2
undergoes avalanche breakdown. At this breakover voltage J2 starts
Light triggering
conducting, but below breakover voltage J2 offers very high
• An SCR turned ON by light radiation is also called as Light
Activated SCR (LASCR).

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 • Hence, Light Triggering is also known as Radiation Triggering. In Snubber circuit
this method, light rays with appropriate wavelength and intensity are  The main purpose of Snubber Circuit is to prevent the unwanted
allowed tostrike the junction J2. triggering (false triggering) of SCR or thyristor due to high rate of
• The bombarded energy particles from the light (neutrons or photons) rise of voltage. This is commonlyknown as dv/dt triggering.
causes to break electron bonds as result, new electron – hole pairs • Thus, we need tohave some arrangement to protect SCR from such
are formed in the device. undesirable turning.
• As the number of charge carriers are increased, there is an • Application of Snubber Circuit prevents from such spurious
instantaneous increase in the flow of current, causing the SCR to triggering of SCR. Thus, it is basicallydv/dt protection of SCR.
turn ON.
Gate triggering
• As This is most common and most efficient method to turn ON the
SCR.
• When the SCR isforward biased, a sufficient positive voltage at the
gate terminal injects some electrons into the junction J2.
• This results in an increase in thereverse leakage current and hencethe
breakdown of junction J2 occurs even at a voltage lower than the
VBO.
SCR turn off methods • Circuit consisting of series combination of resistance and
• The turn OFF process of an SCR is called Commutation. capacitance in parallel with SCR as shown in figure.
• Generally, they are:
• During the steady state position, the value of dv/dt is zero as there
a) Natural commutation (Line commutation)
b) Forced commutation (External commutation) is no change in voltage. So, the current through the capacitor is zero
and it acts like an open circuit.
Natural commutation • At the time of switching, dv/dt is generated across the circuit and
• It occurs in AC circuits i.e. when supply voltage is AC. the current will pass through the capacitor unit.
• Due to the placement of the capacitor in the circuit, during changing
of dv/dt, the capacitor conducts and the power is dissipated across
the resistor. In this way, it protects the thyristor from unwanted
triggering during switching.
Specifications of SCR
• SCRs are semiconductor devices with specific specifications that
• Due to this, SCR turns off when negative voltage appears across define their performance characteristics.
the SCR. As there are no special circuits needed to turn off the SCR  Forward Voltage Drop (VF):
(thyristor). The voltage drop across the SCR when it is in the conducting
• It relies on natural characteristics of circuit, such as natural zero- state.
crossing of current or voltage. This type of commutation is known  Forward Current (IF):
as natural commutation. The maximum continuous forward current the SCR can handle.
Forced commutation
• It applied to DC circuits.  Gate Trigger Current (IGT):
The minimum current required to turn on the SCR.
• Forced Commutation is achieved byreverse biasing SCR device or
byreducing SCR current below the holding current value.  Gate Trigger Voltage (VGT):
• Commutating elements such as inductance and capacitance are The minimum voltage required to turn on the SCR.
used here.  Holding Current (IH):
It is smallest current below which SCR in blocking mode. It is
related with turn off process of SCR.
 Latching Current (IL):
The minimum current required to keep the SCR in conduction
mode. It is related with turn on process of SCR.
IL > IH
 Reverse Recovery Time (tRR):
The time it takes for the SCR torecover from the conducting state
 Forced commutation is applied to Choppers and Inverters. to the blocking state when the forward current becomes zero.
 Class A, Class B, Class C, Class D and Class E are example of  Operating Temperature Range:
forced commutation. The range of temperatures within which the SCR can operate
reliably.

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 • A freewheeling diode, also known as a commutating or flyback
MODULE II
diode, is an essential component in controlled rectifiers to serve
CONTROLLED RECTIFIERS the following purposes:
• Controlled rectifiers (AC to DC converters ) are circuits which converts
a) Inductive Load Protection:
AC to DC with precise control by using thyristors or SCR.
• Unlike uncontrolled rectifiers, controlled rectifiers offer the ability • When an inductive load is connected to the rectifier, it stores
to vary output voltage and current by adjusting the firing angle of energy during the conduction period of the semiconductor device.
the semiconductor switches. • When the semiconductor device turns off, this stored energy
attempts to maintain the current flow, causing a voltage spike. The
freewheeling diode provides a path for this energy to circulate,
protecting the semiconductor device from voltage spikes.
b) Commutation:
• By providing a low-resistance path for the current when
semiconductor device turns off allows a smooth transition of
current from the conducting semiconductor device to the
Firing Angle and conduction angle freewheeling diode, minimizing voltage stress on the device and
• In controlled rectifiers, the firing angle and conduction angle are preventing potential damage.
crucial parameters that determine the timing and duration of the c) Improved Efficiency:

controlled rectification process. • By preventing voltage spikes and aiding in the commutation
process, the freewheeling diode contributes to the overall
1. Firing Angle:
efficiency of the controlled rectifier circuit.
• It is the angle in electrical degrees at which the thyristor or
semiconductor device is triggered to start conducting during each Types of Controlled Rectifiers
half-cycle of the AC input. The firing angle, often denoted by α • Controlled rectifiers are classified into two types
(alpha). I) Single-phase controlled rectifiers
• By adjusting the firing angle, the rectifier can control the amount of II) Three-phase controlled rectifiers.
power delivered to the load, thus regulating the output voltage and • The single-phase and three-phase rectifiers are further classified
current. as,
 Half-wave-controlled rectifiers
 Full-wave controlled rectifiers.
Half-wave Controlled Rectifier
• A half-wave-controlled rectifier converts only either the positive
or negative half-cycle of ac input supply into dc. The circuit
diagram of a half-wave-controlled rectifier is similar to a half-
2.Conduction Angle:
wave uncontrolled rectifier except the diode is replaced by a
• The conduction angle, often denoted by δ (delta), corresponds to the
thyristor.
duration or angle of time during which the thyristor or
semiconductor device conducts current in each half-cycle of the AC
input.
• As the firing angle increases, the conduction angle decreases,
leading to less time during which the device conducts current,
resulting in lower average output power.
Conduction angle = 1800 – firing angle

Freewheeling diode Full-wave Controlled Rectifier

• A full-wave controlled rectifier converts the full cycle (both
positive and negative half-cycle) of ac supply into dc.
• There are two types of full-wave rectifiers, center-tapped (mid-
point-controlled rectifier) and bridge rectifier. The center-tapped
uses two thyristors whereas the bridge uses four thyristors.

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 1) Single-phase Half Wave Controlled Rectifier with R Load Calculation of Average Load Voltage and Current with R load
• When the SCR is forward-biased and the gate terminal is not • From the above waveform, the load voltage appears only from α to π
triggered, there will be no conduction due to the reverse biasing of in the period of 2π during the first cycle of the input supply. From
the inner junction of SCR. Hence, the entire supply voltage appears this, the average value of load voltage and the load current is derived
across the SCR. as,
• If the gate terminal of SCR is triggered with an adequate pulse, then
the width of the depletion layer (reverse biased inner junction) starts
reducing and finally disappears and SCR starts conducting. As soon
as the conduction starts, the voltage across SCR drops to a very
small value and the supply voltage appears across the load.

• Therefore, from the above equation, the average output voltage

• The magnitude of the conduction current depends upon the instant across the load can be varied by varying the firing angle α. The
when it is triggered i.e., firing angle ‘α’, and the load resistance R. maximum output voltage across the load is obtained when firing
• Since the circuit does not contain any energy storing elements, the angle α = 0.
load current will be in phase with voltage and becomes zero 2) Single-phase Half Wave Controlled Rectifier with RL Load
instantaneously with the voltage at zero crossing. • When the load is complex i.e., when the load contains any energy
• During the negative half cycle of supply voltage, the SCR is reverse- storage elements like inductor (RL load), the situation becomes
biased, so there will be no conduction even if the gate is triggered. different from that of pure resistive load.
Hence, the entire supply voltage appears across SCR. • Here the load current will be out of phase with load voltage.

• In the above waveforms, the load current and voltage are zero from
0 to α. When SCR is triggered by giving gate signal at α.
• The entire supplyvoltage except for drop across SCR will be applied
across the load (from ωt = α to ωt = π). At ωt = π, the phase reversal
takes place and the negative half-cycle of the input supply will start.
• Due to the negative half-cycle, the SCR will be reverse biased and
will be turned OFF at ωt = π. From ωt = π to ωt = 2π, the load current
and voltage will be zero.
• Again, when the positive half cycle starts i.e., from ωt = 2π, SCR • At ωt = α, when SCR is triggered, it starts conducting. The load
will be forward biased but it will not be switched ON until it is voltage will now become equal to the supply voltage; whereas, the
triggered i.e. until ωt = (2π + α). load current increases at a slow rate because of the presence of an
inductor.

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 • At ωt = π, the supply voltage becomes zero but the load current will • At ωt = α, the thyristor is triggered and the supply voltage appears
not become zero instantaneously because of the load inductor. across the load from ωt = α to ωt = π.
• The load current after ωt = π slowly decreases due to which the load • With a freewheeling diode, the thyristor will not be able to conduct
voltage still follows the supply voltage even in the negative cycle beyond π. At ωt = π the supply voltage, as well as the load voltage,
upto ωt = β (extinction angle) due to energy stored in inductor. becomes zero, but the load current does not become zero
• At some angle ωt = β (extinction angle), the load current becomes instantaneously because of the presence of an inductor.
zero and hence the load voltage. • At ωt = π the thyristor becomes reverse biased, whereas the
Calculation of Average Load Voltage and Current with RL load freewheeling diode becomes forward biased.
• The average value of load voltage and load current is derived as, • Hence after ωt = π, the load current will be transferred from thyristor
to the freewheeling diode.
• During the negative half cycle of supply voltage, the freewheeling
diode action takes place and no power will be returned to the source.
• In practice, half-wave-controlled rectifiers are not generally used,
since they cannot produce continuous load current and a large ripple
will be present in the output voltage.
Single-Phase Bridge controlled rectifier
• A single phase bridge rectifier needs 4 thyristors.
3) Single-phase Half Wave Controlled Rectifier with RL load and • This configuration leads to two quadrant operation. Such a rectifier
Freewheeling diode is called the two-quadrant converter or fully-controlled rectifier.
• From the above rectifier operation with inductive load,we notice • Manytimes, the bridge circuit is modified byreplacing two thyristors
that, the load inductor stores energy in the initial part of the positive by two diodes.
half-cycle of the applied voltage and maintains the conduction of • This configuration leads to one quadrant operation (operation is
current during the negative half-cycle by releasing the stored energy restricted to first quadrant). Such a rectifier is called the one-
for a substantial period. quadrant rectifier or a semi-controlled rectifier or half controlled
• In order to stop the negative voltage wave to reach the load or to rectifier.
prevent the instantaneous value of the load voltage to become 4) Single phase bridge rectifier (fully controlled) with R load
negative, a free-wheeling or flywheel diode (FD) is connected
across the load.

• In this circuit, diagonally opposite pair of thyristors are made to

conduct, and are commutated, simultaneously.
• During the first positive half cycle, SCR1 and SCR3 are forward
biased and if they are triggered simultaneously, the current flows
through the load via thyristor SCR1-load-SCR3-source.
• Thus, during positive half cycle, thyristors SCR1 and SCR3 are
conducting.
• During the negative half cycle of the ac input, thyristors SCR2 and
SCR4 are forward biased and if they are triggered simultaneously, the
current flows through the load via SCR2-load-SCR4-source.
• SCR1, SCR3 and SCR2, SCR4 are triggered at the same firing angle
α in each positive and negative half cycles of the supply voltage
respectively.
• When the supply voltage falls to zero, the current also becomes zero.

10
 • SCR1 and SCR3 are turned off due to line or natural commutation and
current is transferred from SCR1 and SCR3 to SCR2 and SCR4.

Calculation of Average Load Voltage and Current with R load

• From the above waveform, the load voltage appears from α to π and
Calculation of Average Load Voltage and Current with RL load
π+α to 2π. in of the input supply. From this, the average value of load • From the above waveform, the load voltage appears only from α to
voltage and the load current is derived as, 2π + α of the input supply.
• Since α to 2π + α is equal to α to π + α and π + α to 2π + α, both are
equal, the average value of load voltage and the load current is
derived as,

5) Single-Phase Bridge controlled rectifier with RL load

• The circuit arrangement of a single-phase bridge rectifier is shown

in figure with an inductive load.
• During the positive half cycle, SCR1 and SCR3 are forward biased
and when these they are simultaneously fired at ωt =𝛼, the load is 6) Single-Phase Bridge controlled rectifier with RL load and
connected to the input supply through SCR1 and SCR3. freewheeling diode
• Due to the inductive load, thyristors SCR1 and SCR3 continue to
conduct beyond ωt=π, even though the input voltage is already
negative.
• During negative half cycle of input voltage, SCR2 and SCR4 are
forward biased and when these they are simultaneously fired, the
load is connected to the input supply through SCR2 and SCR4.

11
  In this circuit, two thyristors are connected to the Centre-
tapped secondary of a transformer. The input signal is coupled
through thetransformer to the Centre-tapped secondary.
 When terminal A is positive w.r.t. midpoint N of the
transformer secondary, point B will have negative polarity
w.r.t. mid-point.
 Here, SCR1 conducts when it is fired at an angle α. And the
currentcontinues to flow up to angle π.
 When the supply voltage reverses its polarity and thyristor
SCR1 gets turned off by natural commutation.
 During the negative half cycle of ac supply, the terminal B of
the transformer secondary is positive w.r.t. to mid-point N.
 Now SCR2 gets turned on when it is gated.
 Usually, the firing angles for the two thyristors are taken to be
equalso as to avoid unequal distribution of load current in the
two halvesof input cycle.
• From the above rectifier operation with inductive load, we
Single phase bridge rectifier (semi controlled)
notice that, the load inductor stores energy in the initial part of
 In a semi or half-controlled rectifier, one quadrant operation
each half- cycles of the applied voltage and maintains the
is obtained i.e., only positive voltage and current will be
conduction of currentduring the next half-cycles by releasing the obtained at the output.
stored energy for a substantial period.  This type of rectifier uses two thyristors and two diodes for
• In order to stop the negative voltage wave to reach the load or to the rectification process.
prevent the instantaneous value of the load voltage to become  Here we have two types of configurations
negative, a free-wheeling or flywheel diode (FD) is connected  Symmetrical
across the load.  Asymmetrical.
8) Single phase bridge rectifier (semi controlled) with RL load
7) Single-Phase Centre-tapped controlled rectifier with R
load

12
• In this circuit, two thyristors SCR1, SCR2 and two diodes D1,
D2 are used instead of four thyristors.
• During the positive half cycle of input signal, SCR1 and D2 are
forward biased but does not conduct until a gate pulse is applied
to the SCR1.
• When a gate is applied to SCR1 at ωt = α, it gets turn on.
• When SCR1 and D2 is ON, the input voltage is applied to the
load but due to the inductor present in the load, the current
through loadbuilds up.
• During negative half cycle, SCR2 and D1 is forward biased,
SCR1 and D2 is reverse biased.
• When SCR2 and D1 is ON, the input voltage is applied to the
load but due to the inductor present in the load, the current
through loadbuilds up (in the same direction).
• The current shifts its path from SCR1-load-D2 to D1-load-SCR2
in the case of symmetrical and from SCR1-load-D2 to SCR2-
load- D1in asymmetrical configuration.
• This process repeats for each cycle of input signal.

9) Three phase-controlled bridge rectifier

• In this circuit, SCR1, SCR3 and SCR5 form the positive group
andSCR4, SCR6 and SCR2 form the negative group.
• SCR1, SCR3, SCR4 and SCR6 produces the full wave
rectifiedoutput of Vab across the load.
• SCR3, SCR5, SCR6 and SCR2 produces the full wave • Here average output voltage can be varied by varying the
firingangle.
rectifiedoutput of Vbc across the load.
• For the firing angle <900, the circuit works as rectifier.
• SCR1, SCR5, SCR4 and SCR2 produces the full wave
rectifiedoutput of Vca across the load. • For the firing angle >900, the circuit works as line
commutatedinverter.
• All these three outputs are given simultaneously to the load and
getssuperimposed on each other to get the final output.

13
 MODULE III Advantages of D.C. choppers:
1. Smooth control
DC CONVERTER AND AC CONVERTER
2. Fast response
DC chopper (DC-DC converter)
3. High efficiency
• A DC chopper, also known as a DC-DC converter, is an electronic
4. Fast regeneration.
circuit that is used to control or regulate the output voltage of a direct
current (DC) power source. Types of choppers based on the average DC output
• The main purpose of a DC chopper is to convert a fixed DC voltage • Based on the average DC output voltage, choppers are classified
into a variable DC voltage by chopping or switching the input in to two types.
voltage. 1- Step down chopper (Buck converter)
2- Step up chopper (Boost converter)
Buck converter (Step down chopper)
• Step-down chopper works as a step-down transformer on DC
current.
• Here average output voltage of the chopper described above will
be less than the input voltage. (Vin >Vo and Iin<I0)

Principle of DC Chopper
• The basic principle of operation of a DC chopper involves rapidly
switching the input voltage ON and OFF to achieve the desired
output voltage.
• Normally the circuit is nothing but a switch connected in series with
the supply and the load.
• The output voltage is maintained by the controller by varying the
duty cycle.
• It consists of is an inductor, a semiconductor switch (MOSFET),
a diode, and a capacitor.
• Power MOSFET used as controllable switch and it is controlled
by pulse width modulation.
Working
• There are two modes of operation of the Buck converter.
• Mode I: Switch 1 is ON and Diode FD is OFF
Duty cycle • Mode II: Switch 1 is OFF and Diode FD is ON.
• The control circuit determines the ON and OFF times of the switch, Mode I:
known as the duty cycle. The duty cycle is the ratio of the time the • In this mode of operation, switch S1 is in closed condition, Thus,
switch is ON to the total time of one complete cycle (ON + OFF). switch S1 allows the flow of current through it.
Duty cycle (D) = [TON /T] Where T = TON + TOFF
Therefore, the duty cycle (D) = [TON / TON + TOFF]

• When a constant dc voltage is applied as input, then the current

flows through closed switch S1 whereas the diode FD is in reverse
biased condition.

14
• Due to this current flow the inductor “L” store energy in the form of Mode I:
a magnetic field. • In this mode of operation, switch S is in closed condition i.e. ON
• The capacitor is connected as shown in the circuit diagram. The state, and diode D is in open condition i.e. OFF state.
current flows through the capacitor also and hence the capacitor • Thus, switch S allows the flow of current through it.
store the charge. • All the current will flow through the closed path including inductor
• The voltage across the capacitor appears across the load and is equal L, switch S, and back to the dc input source and inductor “L” store
to the output voltage Vo. energy.
Mode II:
• When S1 is open, then the inductor acts as the source. Hence diode
FD becomes closed.
• Here, the inductor releases the energy stored in the previous mode
and during releasing of energy, the polarity of the inductor gets
reversed which causes the freewheeling diode (FD) to come in
forward biased condition and allows the flow of current.
• The flow of current will continue until the stored energy in the
inductor gets completely collapsed and the freewheeling diode (FD) Mode II:
comes in reverse biased condition.
• In this mode of operation, switch S is in open condition i.e. OFF
• Instantly, the switch S1 will get closed. In this way, the cycle will be
state and diode D is in closed condition i.e. ON state.
continuing.
• Thus, switch diode D allows the flow of current through it,
• We know that the value of duty cycle D varies between 0 and 1. For
whereas switching S blocks the current flow through it.
this range of D, the output voltage is lower than the input voltage.
• The inductor store energy , the inductor acting as the source when
Hence in this way buck converter steps down the input voltage.
the switch S is open.
• The output voltage equation is given by the formula:
• During releasing of energy stored in the inductor, the polarity of
the inductor gets reversed which causes the diode D to come in
forward biased condition.
Where:
• So it allows the flow of current in the circuit through diode D.
Vout is the output voltage.
• The released energy is ultimately dissipated in the load resistance
D is the duty cycle of the chopper.
which helps to maintain the flow of current in the same direction
Vin is the input voltage.
through the load and also steps up the output voltage.
• In a buck converter, the output voltage is directly proportional to the
duty cycle and the input voltage. • The output voltage can be approximated by the formula:
Boost converter (Step up chopper)
• Step-up chopper works as a step-up transformer on DC current.
• It operates by varying the amount of time in which the inductor
receives energy from the source.
Problems
Q1- A step down chopper has input voltage of 440V and output
voltage of 220V. If the total time period of thyristor chopper is
300μs, then compute its duty cycle (D) and turn ON time (TON).
Solution:-
Given that Vin =440V, Vo=220V, T=300 μs
a) Since Vout = D.Vin
Duty cycle (D) = Vout / Vin
= 220 / 440 = 0.5 %
• There are two modes of operation of the Boost converter. b) Since Duty cycle (D) = [TON /T]
• Mode I: Switch S is ON and Diode D is OFF ON time (TON) = DT
• Mode II: Switch S is OFF and Diode D is ON. = 0.5 x 300 μs = 150 μs

15
 Q2- A step down chopper has input voltage of 660V and output • As TON is varied, the width of the pulses is varied, and hence
voltage of 220V. If the total time period of chopper is 450 μs, then constant frequency system is also known as Pulse-width Modulation
compute its duty cycle (D) and turn ON time (TON). (PWM) Control.
Solution:-
Given that Vin =660V, Vo=220V, T=450 μs
a) Since Vout = D.Vin
Duty cycle (D) = Vout / Vin
= 220 / 660 = 1/3 %
b) Since Duty cycle (D) = [TON /T]
ON time (TON) = DT
= 1/3 x 450 μs = 150 μs.
• As shown in the waveforms, the total time period (T) of the
Q3- A step up chopper has input voltage of 220V and output voltage chopper is maintained constant; whereas, TON is varied.
of 660V. If the total time period of thyristor-chopper is 300μs, then • In figure (i), TON is 25% of T and in figure (ii), TON is 50% of T.
compute its duty cycle (D) and turn ON time (TON). 2. Variable Frequency System
Solution:- • In this method, VL can be controlled by controlling frequency.
Given that Vin =220V, Vo=660V, T=300 μs Since frequency is varied, the time period, T also varies.
• Hence, variable frequency is obtained either by varying TON or
TOFF and keeping the other constant.
a) Since
• Hence, as the time period varies, the frequency also varies and
output voltage also varies.
(1-D) = Vin / Vout =220 / 660 = 1/3
Therefore, D = 1- (1/3) = 2/3 %
b) Since Duty cycle, D = TON/T
TON = DT
= 2/3 x 300 μs = 200 μs.

Q4- A step up chopper has input voltage of 220V and output voltage
of 440V. If the total time period of chopper is 400μs, then
compute its duty cycle (D) and turn ON time (TON).
• Since frequency is varied, the variable frequency system is also
Solution:-
known as Frequency Modulation Control.
Given that Vin =220V, Vo=660V, T=400 μs
Types of choppers based on quadrant operation
• Based on quadrant operation chopper may be classified into five
a) Since different types:
(1-D) = Vin / Vout =220 / 440 = 0.5 i) Class-A
Therefore, D = 1- (0.5) = 0.5 % ii) Class-B
b) Since Duty cycle, D = TON/T iii) Class-C
TON = DT iv) Class-D
= 0.5 x 400 μs = 200 μs v) Class-E
Class-A chopper
Control strategies of choppers • It is a single quadrant chopper whose operation is restricted in first
1- Constant-Frequency System quadrant of Vo - Io plane.
• In this method, load voltage VL is controlled by maintaining • It is also called as step-down chopper as average output voltage is
frequency constant. less than the input DC voltage.
• As the frequency is kept constant, the time period also remains
• Used to control the speed of DC motor drive.
constant; whereas, the ON time (TON) of the chopper is varied.

16
 Working
• When the chopper is ON, Vo is zero but the load voltage E drives the
current through the inductor L and the chopper, L stores the energy
during the time TON of the chopper.
• When the chopper is off , VO =( E+ L . di/dt ) will be more than the
source voltage Vs .
• Diode D2 will be forward biased and begins conducting and hence
the power starts flowing to the source.
• No matter the chopper is on or off the current I0 will be flowing out
of the load and is treated negative . Since VO is positive and the
current I0 is negative.
Working • So, power flows from load to source and operation of type-B chopper
• When the chopper is ON, Vo = Vs as a result and the current flows is restricted in second quadrant of Vo – Io plane.
in the direction of the load. Class-C chopper
• Io flows from source to load and inductor stores energy.
VL = L di/dt.

• But when the chopper is OFF, Vo is zero but Io continues to flow in

the same direction through the freewheeling diode FD.
• Thus, average value of voltage and current say Vo and Io will be • Class C chopper consists of two chopper switches CH1, CH2 and two
always positive. diodes D1, D2.
Class-B chopper • This chopper is obtained by combining Class A and Class B choppers
• In type B or second quadrant chopper the load must always contain in parallel.
a DC source E. • Load voltage is always positive because of presence of freewheeling
• Here load voltage is always positive but load current is negative. diode and load current can be negative or positive.
• Power flow is always from load to source. • It can operate first as well as second quadrant.
• It is also called as step-up chopper as average output voltage is • It also called as two quadrant Class A chopper.
greater than the input DC voltage. • Used for motoring and regenerative breaking of DC motor.
• Used for regenerative breaking of DC motor. Working
First Quadrant Operation:
• To obtain a first quadrant operation switch CH1 is turned ON due to
which load gets connected across the dc supply source.
• Once the CH1 is turned ON, the output voltage will become equal to
the source voltage (i.e., Vo = Vdc) and the load current Io starts flowing
in the forward direction storing energy in the inductor as shown below
in figure 1.

17
 • Now when CH1 is turned OFF, the inductor reverses polarity to • Once the two switches are turned ON, the load starts receiving
make the load current continue. power from the source, and thus load current will flow in the circuit
• The diode D1 freewheel the realized energy from the inductor as through the path Vdc, CH1, load, CH2, and back to the Vdc as
shown above in figure 2. shown below.
• It can be seen that during the discharge period of the inductor also
the load current remains in the same direction i.e., forward direction.
• Thus, the output voltage Vo and output current Io will be positive and
hence the chopper operates in the first quadrant.
Second Quadrant Operation:
• For the second quadrant operation of a class-c chopper, the load
must contain a dc source.
• The inductance in the load stores energy during this period. The
• Now the CH2 is turned ON and the EMF (E) in the load forces a
diodes D1 and D2 will remain in OFF-state since they are reverse-
current in the reverse direction through the inductor and CH2 as
biased.
shown below in figure-1.
• We can see that the output current Io and output voltage Vo remain
positive during the whole period, and hence a first quadrant
operation of a class-D chopper is obtained.
Fourth Quadrant Operation:
• Now when the switches S1 and S2 are turned OFF. The load gets
disconnected from the source and the load current stops flowing
• When CH2 is turned OFF, the stored energy in the inductor starts in the circuit.
realizing by forcing a current in the reverse direction through diode • But at the same time, the inductive nature of the load doesn’t allow
D2 to the supply. When D2 conducts, the output voltage will become a sudden drop in load current, and hence a huge amount of voltage
equal to the source voltage (i.e., Vo will be positive). is induced in the inductor in the reverse direction.
• It can be seen that the direction of the output current is in the reverse • Thus, the polarity of the inductor gets reversed and the inductor
direction i.e., the output current Io will be negative. But the output starts discharging in the reverse direction. This causes the diodes
voltage remains positive as same as the first quadrant operation. D1 and D2 to forward bias and they start conducting.
Hence the chopper operates in the second quadrant. • The energy stored in the inductor is realized gradually through the
Class-D chopper load, D2, Vdc, D1, and back the load as shown below.

• Here, the direction of the output current Io has not changed but the
polarity of output voltage Vo is reversed. Thus, output current Io
remains positive whereas output voltage Vo will be negative and
hence a fourth quadrant operation of the chopper is obtained.
Class-E chopper
• Here load current is always positive and load voltage can be
• It can operate in all four quadrants.
negative.
• Uses in four quadrant reversible DC motor drive.
• It can operate first as well as fourth quadrant.
• It is also known as two quadrant Class B chopper.
• Used for power control and signal process applications.
Working
First Quadrant Operation:
• In the first quadrant operation of a class-D chopper, CH1 and CH2
are turned ON.

18
 Third Quadrant Operation:
• For third quadrant operation, CH1 is kept off, CH2 is kept ON and
CH3 is operated and the polarity of emf E in load must be reversed.
• When CH3 is ON, load gets connected to source and hence load
voltage is equal to source voltage. Hence, vo is assumed negative.
• It may be seen that io is flowing in the direction opposite to shown
in the circuit diagram and hence negative.
First Quadrant Operation: • Now, when CH3 is turned OFF, the negative load current free
• For first quadrant operation, CH4 is kept ON, CH3 is kept OFF and wheels through the CH2 and D4.
CH1 is operated. • In this manner, vo and io both are negative. Hence, the chopper
• When both CH1 & CH4 are ON simultaneously, hence the output
operates in third quadrant.
voltage becomes equal to the source voltage (vo = vs).
• Now the load current flows from source to load as shown by the
direction of Io.

Fourth Quadrant Operation:

• To obtain fourth quadrant operation, CH4 is operated while keeping
CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be
• When CH1 is switched OFF, the load current free wheels through reversed in this case too like third quadrant operation.
CH4 and D2. During this period, the load voltage and current • When CH4 is turned ON, positive current flows through CH4, D2,
remains positive. L and E. Inductance L stores energy during the time CH4 is ON.
• Thus, both the output voltage vs and load current io are positive and • When CH4 is made OFF, current is fed back to the source through
hence, the operation of chopper is in first quadrant. It may be noted diodes D2, D3. Here load voltage is negative but the load current is
that, Class-E chopper operates as a step-down chopper in this case. always positive.
Second Quadrant Operation: • This leads to chopper operation in fourth quadrant. Here, power is
• To obtain second quadrant operation, CH2 is operated while keeping fed back to the source from load and chopper acts as a step-up
the CH1, CH3 & CH4 OFF. chopper.
• When CH2 is ON, the DC source in the load drives current through
CH2, D4, E and L. Inductor L stores energy during the ON period
of CH2.
• When CH2 is turned OFF, current is fed back to the source through
D1, D4. It should be noted at this point that (E+Ldi/dt) is more than
the source voltage Vs.
• As load voltage Vo is positive and Io is negative, it is second
quadrant operation of chopper. Since, the current is fed back to the
Applications of choppers
source, this simply means that load is transferring power to the
• Power Supplies
source.
• Motor Control.
• For second quadrant operation, load must contain emf E as shown
• Renewable Energy
in the circuit diagram. In second quadrant, configuration operates as • Battery Charging Systems
a step-up chopper.
• Electric Vehicles
• DC-DC Converters in Power Electronics
• Lighting Control
• Uninterruptible Power Supplies (UPS)
• Aerospace Applications
• Medical Devices

19
 • At some time instant t = t1, the conducting thyristor P1 is force
CYCLOCONVERTER commutated and the forward biased thyristor N2 is fired to turn it
• Cycloconverter is device which converts input AC power at one ON.
frequency to output power at a different frequency. • During the period N2 conducts, the load voltage is negative because
• The output frequency is more than the input frequency for this O is positive & A is negative this time. The load or output voltage
cycloconverter. traces the negative envelop of the supply voltage.
• At t = t2, N2 is force commutated and P1 is turned ON. The load
voltage is now positive and follows the positive envelop of the
supply voltage.
• During the negative half cycle of input supply voltage, At t = π,
terminal N is positive with respect to terminal M, both SCRs P2 &
Types N1 are therefore forward biased from t = π to t = 2π.
• Single Phase to Single Phase Cycloconverter: This type of • At t = π, N2 is force commutated and forward biased SCR P2 is
cycloconverter converts single-phase AC input to single-phase AC turned ON. The load voltage is positive and follows the positive
output at a different frequency. envelop of supply voltage.
• Three Phase to Single Phase Cycloconverter : These cycloconverters • In this manner, SCRs P1, N2 for the first half cycle; P2, N1 in the
convert three-phase AC input to single-phase AC output at a different second half cycle and so on are switched alternately between
frequency. positive and negative envelops at a high frequency.
• Three Phase to Three Phase Cycloconverter: These devices convert • This results in output frequency fo more than the input supply
three-phase AC input to three-phase AC output at different frequency fs.
frequencies.
Single phase to single phase step up cyclo converter
(midpoint type)

• The circuit consists of a single-phase center-tapped transformer and

four thyristors.
• Two of these thyristors P1 & P2 are for positive group.
• Other two thyristors N1 & N2 are for negative group.
• Load is connected between secondary winding mid-point O and
terminal A.
• The thyristors are triggered depending upon the polarities of points
M and N of the transformer.
Working
• During the positive half cycle of input supply voltage, thyristors P1 &
N2 are forward biased for t = 0 to t = π.
• As such SCR P1 is fired to turn it ON at t = 0 such that load voltage
is positive with terminal A positive and O negative.
• The load voltage, thus, follows the positive envelop of the input
supply voltage.

20
 MODULE IV
INVERTERS, POWER SUPPLIES AND ELECTRICAL
DRIVES
Inverter
• An inverter is a device used to obtain ac power of desired voltage
and frequency from a dc power.
• The inverters achieve this by using thyristors with forced • As the current increases and reaches its positive maximum value, the
commutation or other semiconductor devices like BJT, MOSFET, voltage across the capacitor becomes equal to supply voltage Vdc.
IGBT, etc. • Now, the current starts decreasing after reaching its positive
maximum value but the voltage across the capacitor does not
decrease.
• Instead of decreasing it increases further and reaches a value higher
Classification of inverters than Vdc, and the capacitor retains this voltage for some time.
• Based on circuit configuration, inverters are classified into, • At t = t2, thyristor T1 is turned OFF when the current reaches zero by
I.Series Inverters natural commutation, but still, the capacitor holds the voltage (VC +
II.Parallel Inverters Vdc) in it.
III.Bridge Inverters. Mode II :
• Based on input source they classified into, • This mode starts from instant t2 when thyristor T1 is commutated and
I.Voltage Source Inverter (VSI) it remains in OFF state for a sufficient period of time (t2 to t3).
II.Current Source Inverter (CSI). • Hence, in this mode, both the thyristors T1 and T2 are in OFF-state
• Based on output voltage and current phases, they classified into, and the capacitor voltage is maintained at a constant value of (VC +
Vdc), and the load current IL remains zero in this mode i.e., from t2
I.Single phase inverter
to t3.
II. Three phase inverter.
Single phase Series Inverter
• The inverter in which the commutating elements L and C are
connected in series with the load to form an under damped circuit is
called a series inverter.
• A series inverter employs class-A commutation or resonant
commutation since the current decays to zero naturally by load Mode III :
commutation but not by forced commutation. • In this mode of operation, thyristor T2 is triggered at instant t3 since
the positive polarity of the capacitor appears across the anode of T2
and it starts conducting.
• As thyristor T2 conducts, the load current starts flowing in the
negative direction through the path C+ → L → T2 → R → C– as
shown below.
Working
Mode I:
• Mode I starts when thyristor T1 is triggered at instant t = t0 by
applying gate pulses to it.
• As T1 is triggered, it starts conducting and the load current flows
through the path Vdc+ → T1 → L → C → R → Vdc– as shown
• Now, the capacitor starts discharging and the load current IL flows in
below.
the reverse direction and reaches its negative maximum value.
• Initially, the capacitor is charged to a negative voltage -VC, but once
• Then, load current starts decreasing and becomes zero at t3, due to
T1 is triggered capacitor starts charging to positive voltage with
this thyristor T2 gets turned OFF at t3.
upper plate positive and lower plate negative.
• Then after capacitor again charges to negative voltage -VC as shown
in the waveform.

21
• Again, after maintaining a certain amount of time delay, thyristor T1 Mode-II :
is triggered and the cycle repeats. • With the end of mode I, mode-II begins and once the thyristor T2 is
triggered, T1 is reversed biased with respect to the voltage across
the capacitor C, i.e., a voltage equal to -2Vdc appears across
thyristor T1.
• So, T1 gets commutated when this reverse voltage is applied for a
sufficient time. Now, thyristor T2 alone will be in conduction and
the load current flows through inductor L and thyristor T2 as shown
above. During this, a voltage equal to 2Vdc appears across the
primary of the transformer and across the capacitor with reverse
polarities.

Single phase parallel inverter

Mode-III :
• The circuit consists of two thyristors (T1 and T2), a center-tapped
• This mode begins when the thyristor T1 is triggered again. Now, due
transformer, a commutating capacitor (C), and an inductor (L).
to the conduction of T1, the thyristor T2 is reversed biased with
• The load is connected to the secondary of the transformer. The dc respect to the voltage of the commutating capacitor i.e., a voltage
power input is given between center tap primary and common equal to -2Vdc appears across thyristor T2.
cathode through thyristors as shown below.
• When this voltage is applied for a sufficient time thyristor T2 gets
commutated and the whole process will be repeated.

Working
Mode-I :
• In this mode, thyristor T1 is triggered by giving a gate signal, and
the load current starts flowing through inductor L and thyristor T1
as shown below.
• A voltage equal to 2Vdc appears across the primary winding of the
transformer. Due to this, the capacitor also charges to a voltage equal
to 2Vdc with polarities as shown above. VSI (Voltage Source Inverter)
• This mode ends when the thyristor T2 is triggered. • A Voltage Source Inverter is an inverter where the input DC voltage
is fed by a stiff or constant DC voltage having negligible or zero
impedance. It is also called Voltage Fed Inverter (VFI).
• In VSI input voltage is kept constant, Here, we connect a DC source
in parallel with large capacitor that maintained Input voltage
constant and output voltage is independent of load.

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 CSI (Current Source Inverter) • Here assumed that, each thyristor conducts for the duration its gate
• A Current Source Inverter is an inverter where the input DC current signal is present and is commutated when this signal is removed. The
is fed by a stiff or constant but adjustable current source of high gating signal for thyristor T1 and thyristor T2 are ig1 and ig2
impedance.It is also called Current Fed Inverter (CFI). respectively.
• VSI can be converted into CSI, by connecting large series inductance • Load RL is connected between point A and B. Point A is always
that maintained input current constant. considered as +ve w.r.t point B. If the current flow in this direction
we assume that current is +ve.
• Similarly, if current from B to A the current is considered -ve.
• Due to inductive load the output voltage waveform is just similar to
that of R-load. However, the output current wave form is not similar
to the output voltage waveform.
Working
• The opération of Half Bridge Inverter is divided into four modes.
1. Mode I: T1 on
2. Mode II: D2 on
3. Mode III: T2 on
4. Mode IV: D1 on
Mode I :
• In this duration, we give gate pulse to thyristor T1. So T1 get turned
on at and current flow from the upper half of supply voltage and
current follow the path Vs/2 (Upper Supply) -T1 -load -back to Vs/2.
• In this mode inductor store energy and output current increases
exponentially as a function of time from zero to its positive max value
(Imax) and induced voltage across inductor.
• This time output voltage is also positive because point A is positive
(+ve) w.r.t point B.

Single Phase Half Bridge wave Inverter (with RL load) Mode II:
• It is a half bridge inverter circuit consist of two thyristors T1 & T2 • After some time , inductor dissipated energy and changes its
and two feedback diode D1 & D2, each diode is connected in anti- polarity.
parallel with each thyristor and three wire DC source that provide • So, inductor slowly release its energy through the D2 diode. D2
balance DC voltage at source. diode is conduct this time and current follow the path, Load - lower
half of supply (Vs/2) - D2 - back to Load at this time interval the
energy release by inductor feedback to the lower half supply.

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• At this mode output current is positive but gradually decreases from
Imax to zero because of energy dissipated by the inductive load. The
Output voltage is negative (-Vs/2) this time because point B is
positive w.r.t. A.
Mode III :
• At this time instant, thyristor T2 get turned on and current flow in a
lower half of the circuit and follow the path Vs/2 (lower supply) -
load - T2- back to Vs/2. Working
• So, the direction of current is reverse because point B is positive • The opération of full Bridge Inverter is divided into four modes.
w.r.t. A and inductor store energy in reverse direction from (-Imax) 1. Mode I: T1, T2 on
to zero.
2. Mode II: D3, D4 on
• At this time output voltage across load is negative (-Vs/2).
3. Mode III: T3, T4 on
4. Mode IV: D1, D2 on
Mode I:
• In this mode, we give firing pluses to thyristor T1 and T2, they will
start conduct. So current flow from Vs (supply voltage) - T1 - load -
T2 - back to Vs.
• In this mode inductor store energy and output current increases
Mode IV: exponentially as a function of time from zero to its positive max value
• After some time, inductor dissipated energy and changes its (Imax) and induced voltage across inductor.
polarity. • This time output voltage is also positive because point A is positive
• Therefore, T2 is turned off due to inductive load and D1 turned on. (+ve) w.r.t point B.
And current flow from load - D1- Vs/2 (Upper supply) - back to the Mode II:
load. • After time instant inductor dissipated energy and we know that when
• Here energy release by the inductor fad back to the upper part of inductor dissipated energy it changes its polarity. So, inductor slowly
supply voltage Vs/2. release its energy through the D2 diode.
• This time point A is positive w.r.t point B. So, output voltage is • D2 diode is conduct this time and current follow the path, Load - D3
positive Vs/2 and output current decreases exponentially from its - supply Vs - back to Load. At this time interval the energy release by
negative max value (-Imax) to zero. inductor feedback to the supply.
• At this mode output current is positive but gradually decreases from
Imax to zero because of energy dissipated by the inductive load. The
Output voltage is negative (-Vs) this time because point B is positive
w.r.t. A.

Single Phase full Bridge Inverter (with RL load)

• Single phase full Bridge Inverter circuit basically consist of four
Thyristor (T1to T4) and four diode (D1to D4) these diodes are called
feedback diode. Each diode is connected in anti-parallel with each
thyristor. A voltage source and a RL load.

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 Mode III: • If the same switches are off at the same time the state is taken as 0.
• At this time we give firing pulse to thyristor T3 and T4, they turned • The output is produced in states 1-6.
on and current flow from Vs supply - T3 - Load - T4 - back to Vs. • In states 7 and 8no line voltage is produced and the line currents
• So, the direction of current is reverse because point B is positive w.r.t. freewheel to either the upper or lower freewheeling diode.
A and inductor store energy in reverse direction from zero to (-Imax).
• At this time output voltage across load is negative (-Vs).
Mode IV:
• At time instant inductor dissipated energy and it changes its polarity.
Therefore, T3 and T4 is off and D1and D2 turned on. And current
flow from load - D1- Vs Supply - D2 - back to load.
• Here energy release by the inductor fad back supply voltage Vs This
time point A is positive w.r.t point B. So, output voltage is positive
Vs and output current decreases exponentially from its negative max
value (-Imax) to zero.

Three phase bridge inverter

• Three phase inverter are used in delivering larger load in high power
application.
• It can be configured using 6 thyristors and 6 diodes.
• Control signal for conduction can be applied for 120 degree
conduction or 180 degree conduction.
Pulse width modulation (PWM)
• Pulse Width Modulation (PWM) is a technique that modifies the duty
cycle of a signal to control the amount of power sent to a device.
• The duty cycle can be defined as the amount of time a signal is ON
over an interval or period of time.
• In simple terms, PWM is a way of digitally encoding analog signal
levels. This digital signal is represented by two states: active high (‘1’)
and active low (‘0’).
• PWM inverters operate by taking a DC voltage input and using a
• Let us consider every thytistor conducts for 180. switch to produce an output that resembles an AC waveform.
• The load can be connected in either star or delta. • The switch is turned on and off at a high frequency. The width of these
• The thyristos T1 –T6 work as switching devices S1 – S6. pulses is modulated to adjust the harmonic content of the output
• When 2 switches, one upper and lower conduct at the same time to waveform, thus making it more or less like a sine wave, depending on
produce an output voltage +/- Vs it is taken as state 1. the application.

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 Types of PWM Inverters • Inverters and motor control systems, where a high-quality output
I. Single Pulse Width Modulation: waveform is crucial for lowering harmonic interference and raising
II. Multiple Pulse Width Modulation: system efficiency, are two common applications for this technology.
III. Sinusoidal Pulse Width Modulation (SPWM):
Single-Pulse Width Modulation (PWM)
• A single pulse is produced at each switching cycle in single-pulse
width modulation.
• The average power applied to the load is controlled by varying the
pulse’s width.
• Single-PWM is straight forward and simple to use, although it could
have a larger harmonic content and be unsuitable for applications
that need precision control at low power levels.

Switched Mode Power Supply (SMPS)

• Switched Mode Power Supply (SMPS) is a compact and efficient
electrical device that converts AC voltage into regulated DC voltage.
• It employs high-frequency switching techniques, utilizing transistors
and capacitors, to regulate voltage output. SMPS operates in two
stages: first, it rectifies AC to DC, then it uses a switching regulator
to maintain a stable output voltage. T
• his design offers higher efficiency, lighter weight, and smaller size
Multiple-Pulse Width Modulation (MPWM) compared to traditional linear power supplies.
• Multiple pulses are generated during each switching cycle in • SMPS finds widespread use in electronic devices, from computers to
multiple-pulse width modulation. televisions, due to its ability to provide reliable and consistent power
• This method seeks to lower harmonic distortion while raising the while minimizing energy wastage and heat dissipation.
output waveform’s general quality. Types of SMPS
• Two-level and three-level MPWM are frequently used; in these  Buck Converter: Efficiently steps down voltage while increasing
implementations, the number of pulses in each cycle is increased to current, commonly used in applications requiring lower output
improve waveform fidelity, which makes MPWM especially helpful voltage than input.
in high-power applications.
 Boost Converter: Raises voltage while reducing current, ideal for
systems requiring higher output voltage than input.
 Buck-Boost Converter: Versatile in stepping up or stepping down
voltage, adapting to fluctuating input voltages.
 Flyback Converter: Utilizes energy storage in a transformer for
voltage regulation, suitable for low-power applications like battery
chargers and LED drivers.
 Multi-Output Converter: Provides multiple regulated outputs from
a single input, catering to systems requiring various voltage levels,
such as in telecommunications and industrial equipment.
 Push-Pull Converter: Utilizes a transformer operating in both
forward and reverse modes, minimizing power loss and increasing
Sinusoidal Pulse Width Modulation (SPWM) efficiency.
• The process of altering pulse width to resemble a sinusoidal waveform
is known as sinusoidal pulse width modulation.
• Through the use of a sine wave reference to alter the pulse widths,
SPWM minimizes harmonic distortion and yields a smoother output.

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 Block diagram Output Rectifier and filter
• The output rectifier converts the high-frequency AC voltage from
the DC-DC converter into a pulsating DC voltage.
• This rectification process ensures that current flows predominantly
in one direction through the load.
• Most commonly, output rectification is achieved using diodes
arranged in a bridge configuration similar to the input rectifier.
However, in high-power applications, semiconductor devices such
as MOSFETs or IGBTs might be used for rectification due to their
lower voltage drops and higher efficiency.
• The output filter is employed to smooth out the pulsating DC voltage
Input Rectifier and filter produced by the rectifier and reduce any remaining ripple or noise.
• The input rectifier converts the alternating current (AC) voltage from • Typically, the output filter consists of capacitors and sometimes
the mains power supply into pulsating direct current (DC). inductors.
• This is typically achieved using a diode bridge rectifier, which • Servo voltage stabilizer
consists of four diodes arranged in a bridge configuration.
• The output of the rectifier is a pulsating DC voltage with significant
ripple due to the periodic charging and discharging of the output
capacitor during each half-cycle of the input voltage.
• The input filter, typically comprised of capacitors and sometimes
inductors, is used to smooth out this pulsating DC voltage and reduce
ripple.
High frequency switch
• High-frequency switch, typically a MOSFET or IGBT, rapidly
alternates on/off states to regulate voltage or current. Working
• During the on-state, energy from the input source is stored in an • The electronic control circuit detects the voltage dip and voltage rise
inductor or transformer, and when off, it's released to the output. by comparing the input with built-in reference voltage source.
Operating at frequencies from kHz to MHz, it allows for smaller, • When the circuit finds the error, it operates the motor that in turn
lighter components. moves the arm on the autotransformer.
Control circuitry • This could feed the primary of buck boost transformer such that a
• Control circuitry adjusts the duty cycle for desired output regulation. voltage across the secondary should be the desired voltage output.
This high-frequency operation improves efficiency by minimizing • Most servo stabilizers use embedded microcontroller or processor for
energy losses. the control circuitry to achieve intelligent control.
• However, it necessitates EMI mitigation techniques. Overall, the • These stabilizers can again be categorized into single-phase, three-
high-frequency switch is pivotal in SMPS, facilitating efficient phase balanced type or three-phase unbalanced units.
power conversion in compact designs. • In single phase type, a servo motor coupled to the variable transformer
Output transformer achieves the voltage correction.
• The primary function of the output transformer is to transform the • In case of a three-phase balanced type, a servo motor is coupled with
voltage level of the DC signal. three auto transformers such that stabilized output is provided during
• In isolated SMPS topologies like flyback and forward converters, the fluctuations by adjusting the output of the transformers.
transformer is used to step up or step down the voltage as required by • In an unbalanced type of servo stabilizers, three independent servo
the load. motors coupled with three auto transformers and they have three
• The output transformer provides electrical isolation between the separate control circuits.
input and output sides of the SMPS.
Uninterruptible Power Supply (UPS)
• This isolation is essential for safety reasons, especially in • Uninterruptible Power Supply or Uninterruptible Power Source is an
applications where there may be differences in ground potential to electrical apparatus that provides emergency power to a load when the
protect sensitive components or operators. input power source or mains power fails.

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• It typically consists of a battery or flywheel backup power source, • This is what happens when a plugged-in laptop keeps on running
along with various components to regulate the flow of electricity and without interruption when mains power fails.
protect connected devices from power surges or fluctuations. • Line interactive type UPS
• UPS systems are commonly used to ensure continuous operation of
critical equipment such as computers, servers, networking hardware,
and other sensitive electronic devices.
• A UPS differs from an auxiliary or emergency power system or
standby generator in that it will provide near-instantaneous
protection from input power interruptions, by supplying energy
stored in batteries, supercapacitors, or flywheels.
Types
• Commonly UPS are three types,
1. Off line UPS
2. On line UPS
3. Line interactive type UPS
4. Off line UPS • A line-interactive UPS maintains the inverter in line and redirects the
battery's DC current path from the normal charging mode to supplying
current when power is lost.
• In this smart design, the battery-to-AC power inverter is always
connected to the output of the UPS.
• When the input AC power is normal, the inverter of the UPS is in
reverse operation and provides battery charging.
• Once the input power fails, the transfer switch will open and the power
will flow from the battery to the UPS output. This design offers
• In this type of UPSs the load is directly connected to the incoming additional filtering and yields reduced switching transients since the
AC power supply. inverter is always on and connected to the output.
• When this mains supply fails or goes below a minimum level, the Electric Drives
offline UPS blocks the incoming AC mains and deliver power to the • Electric Drives are electromechanical systems designed to control the
load via internally connected battery using DC-AC inverter motion of electrical machines.
circuitry. • It is considered an important component of various industrial
• Although all manufacturer tries to minimize the switching delay but processes equipment as it helps in easy optimization of motion
it could be as long as 25 milliseconds. controlling.
• Generally for home desktops we use offline UPS. • It is regarded as a complicated control system that controls the rotation
On line UPS shaft of the motor.
• A typical drive system has single or multiple electric motors along
with a controlling system by which the rotation of the motor shaft is
controlled.
• The major components that constitute the electric drives are electric
motor, energy transmitting device, working machine.
• This type of UPSs always deliver power to the load via battery using Classification of Electric Drives
DC-AC inverter. Therefore in these UPSs no switching mechanism • Majorly electric drives are classified into two categories namely,
required, and hence transfer time has no role during power failure. i. DC Drives:
• To maintain the charge of the battery, a battery charging unit is • The DC drives are the ones that where the motive power that excites
incorporated in the system. the system is DC in nature.
• So when mains supply fails, UPS continues to deliver power to the • Their main applications are involved in adjustable speed drives and
load using battery, however, charging of battery stops. position control.

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 • Here dc motors are used along with the power electronic converters. 1. Power Source:
ii. AC Drives: • This unit is responsible for providing the power which is needed by
• The operation of AC drives is based on the AC type of supply input. the system to do the desired operation.
• These are lightweight than dc drives. 2. Power Controller or Converter:
• AC drives can be of two namely induction motor drives and • This unit is responsible for the conversion of supplied input electrical
synchronous motor drives. energy into a form that can drive the motor (generally mechanical
energy).
• The power controller controls the power input provided to the motor
that can be handled by the same. Basically, this control is necessary
because power flowing through the system decides the torque-speed
characteristics which the load requires.
• When transient operations like starting, braking, etc. within the
system take place, this unit helps in limiting the current to specific
levels so that voltage overloads or dips can be avoided.
3. Control Unit and Sensor Unit:
• This unit performs the action of controlling the power converter
according to the provided input as well as the feedback signal
obtained from the load under the closed-loop operation.
• Basically, the control unit works in conjunction with the sensor unit
which actually senses the voltage or current signal as feedback to
have the proper operating conditions.
• The sensing unit is responsible for sensing the current or speed of the
motor. It protects as well as provides closed-loop operation.
4. Electric Motor:
• This mainly converters the applied energy into mechanical motion.
• Mostly DC motors used in the electric drive systems are in series,
shunt, or compound form while AC motors used are slip ring
induction motors. Sometimes stepper motors or brushless DC motors
are also used in special cases.
5. Load:
• The load which is the part of the system is specified according to the
torque/speed characteristics of the system such as pumps, machines,
etc.
Block Diagram of Electric Drives • The electric motor and load operate in compatibility with each other
in terms of torque-speed characteristics.

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