Power

1. POWER SEMICONDUCTOR DEVICES
THYRISTORS
A Thyristors is one of the most important types of
power semiconductor devices.They are used
extensively in power electronics circuits. The word
thyristor’ is originated from Greek word. The word “thy”
means switch. The total word thyristor indicates that the
switch belongs to a transistor family. The
Development of thyristor has revolutionized the use of
semiconductors for power control. The thyristor was first
invented by Bell Laboratory in U.S.A. in the year 1948
and it (100V,10A) was first introduced in the market by
General Electric Company in the year 1957. However,
commercially it started becoming available after 1960.
Since this time there has been a rapid growth in the
control equipment making use of thyristors.

2. Initially however for a period of 5-6 years the failure
rate of the equipments hampered the growth of
thyristorised equipment in industry. This however did
not cause any break in the building up of know-how and
technical literature has been piling up since, at a rate of
about 200 reported articles per year on various aspects
of this technology.It will not be wrong to say that the
present state of thyristor technology has been a result
of about ten years efforts in all directions including
development in semiconductor and printed circuits
technology. Today single thyristor is Capable of
handling maximum of 25MW [5000V, 5000A].

3. TERMINAL CHARACTERISTICS OF THYRISTORS
The thyristor has four layers PNPN semiconductor
switching device. It has three terminals: Anode,
Cathode and gate
When the anode voltage is made positive with respect
to the cathode. The junction ‘J1’ and ‘J3’ are forward
biased while junction ‘J2’is reverse biased. A very
small leakage current will flow through the device. The
thyristor is then said to be in the forward blocking
state or off state condition. If the voltage at anode is
continuously increased, the breakdown of reverse
bias junction ‘J2’ occurs due to high voltage gradient
and the device is switched on. The voltage drop would
be due to ohmic drop in the four layers and it is small,
typically 1v. In the on-state, the anode current is
limited by an external impedance or a resistance,

4. ,As shown in fig.1(a). The anode current must be more
than ‘Latching Current’for on-state condition.If the anode
current does not fall below the ‘Holding Current’the
thyristor remains on.The latching current is usually double
the holding current but both are low, even much less
than 1 of the full load rated value. ‘Latching Current’is
the minimum anode current required to maintain the
thyristor in the on-state immediately after a thyristor has
been turned on and gate signal has been removed
Figure-1 Thyristor Symbol and Three P-N Junctions.

5. Two Transistor Model of Thyristor.
The regenrative or latching action due to positive
feedback can be demonstrated by using a two
transistor model of thyristor. A thyristor can be
considered as two complementry transistors, one
PNP transistor ‘Q1’ and other NPN-transistor ‘Q2’ as
shown in fig. If a positive potential is applied to the
gate electrode G when the anode A is positive with
respect to cathode K, then transistor ‘Q2’ is turned on
and starts to conduct. Since the collector current of
‘Q2’ is the base current of ‘Q1’, Q1 also starts to
conduct. The collector current of ‘Q1’ is also the base
current of ‘Q2’ Hence as long as the anode is
positive, each transistor maintains the other in
conducting state by regenrative action.The
regenrative action of the thyristor operation can be
explained mathematically

6. Figure-2 Two Transistor Model of
Thyristor

7. When Q1 and Q2 have a very small forward bias the
value  1 and 1 + 2 is small and hence IA is small.
The sum of 1 + 2 can be made to approach momentarily
by injecting a short duration positive current at the gate and
thyristor will be on due to execessive anode current IA
, which is clear from the above mathematical expression.
The PN junction of the thyristor is capacitive because of the
deplection layer during blocking. Whenever there is rapid
rate of change the forward anode to cathode voltage (dv /dt),
the charging current i = c dv / dt may, attain to sufficient,
magnitude with the leakage current that there may be
possibility of turning on of the thyristor. The dv / dt turn on of
the thyristor is avoided due to the following reasons.
Turning on by providing signal from gate takes negligible
power to turn on the thyristor. The spreading of charged
carriers over the entire area of thyristors take some time and
as a result , localised over heating with in the crystal may
reduce the life of the thyristor, of course,

8. there is no possibility of destruction of thyristor by the
principle of voltage gradient firing of thyristor. It is known
that in case of a transistor,
Ic = IE +ILeakage Ic = IE +ICBO where ICBO is
the leakage current of colector- base current and
  Ic / IE , For Q1 IE = IA
and collector current is Ic1
Ic1 = 1 IA +ICBO1, Similarly for transistor ‘Q2’
Ic2 = 2 IK +ICBO2
IA = IC1+IC2  1 IA +ICBO1 + 2 IK +ICBO2 where IK = IA +IG
IA = 1 IA +ICBO1 + 2 (IA +IG ) +ICBO2  1 IA +ICBO1 + 2
IA +2IG +ICBO2
1 IA +2 IA + ICBO1 +2IG +ICBO2  IA (1 +2 ) + ICBO1
+2IG +ICBO2
IA -IA (1 +2 ) = ICBO1 +2IG +ICBO2  IA[1 - (1 +2 ) ] =
ICBO1 +2IG +ICBO2
IA = ICBO1 +2IG +ICBO2 / 1 - (1 +2 )

9. THYISTOR TURN-ON.
A thyristor is turned on by increasing the anode
current. This can be accomplished in one of the
following ways.
1. Temperature Triggering.
If the temperature of a thyristor is high, there will be an
increase in the number of electron hole pairs, which
would increase the leakage current. This increase in
currents would cause‘1’ and ‘2’ to increase. Due to
regenrative action,(1 +2 ) may tend to be unity and
the thyristor may be turned on. This type of turn-on
may cause thermal runaway and is normally avoided.
2. Light Triggering.
When light is thrown on the gate-cathode junction
through a light window, the electron-hole pairs will
increase ( free charge carriers electrons and holes are
generated ). If the intensity of this light exceeds a
certain value, the thyristor is turned on.Such a thyristor
is known as light activated SCR (LASCR).

10. 3. Forward Voltage Triggering.
When the forward anode to cathode is greater than the
forward breakdown voltage VBO.
Sufficient leakage current will flow to initiate
regenrative turn-on. This type of turn-on may be
destructive and should be avoided.
4. dv / dt Triggering.
With forward voltage across the anode and cathode of
a thyristor, the two junctions are forward biased but the
inner junction J2 is reverse biased.This junction has
the characteristics of a capacitive due to charges
existing across the junction.If the entire anode to
cathode forward voltage Va appears across junction J2
and the charge is denoted by ‘q’ than a charging
current ‘I’ given by equation
 Gate Triggering. The gate triggering is the most common
method of turning on the SCRs, because this method lends itself
accurately for turning on the SCRs at the desired instant of time

11. I = (dq / d t),  d (Cj , Va )/ d t  Cj dVa / d t + Va
dCj / d t
As Cj, the capacitive of junction ‘J2’ is almost constant,
the current is given by
i = Cj dVa / d t
If the rate of rise of forward voltage ‘dVa / d t’ is high,
the charging current plays the role of gate current and
turns on the thyristor even when gate signal is zero.
Such phenomena of turning on a thyristor, called ‘dVa
/ d t’ turn-on, must be avoided as it leads to false
operation of the thyristor circuit. For controllable
operation of the thyristor, the rate of rise of forward
anode to cathode voltage ‘dVa / d t’ must be kept below
the specified rated limit. Typically ‘dV / d t’ are 20-500v
/ sec. False turn-on of a thyristor can be prevented by
using a snubber circuit in parallel with the device.

12. GATE CURRENT.
If a thyristor is forward biased, the injection of gate
current by applying positive gate voltage between the
gate and cathode terminals would turn on the thyristor.
As the gate current is increased, the forward blocking
voltage is decreased.
The following points should be considered in designing
the gate control circuit:
1. The gate signal should be removed after the
thyristor turned on. A continuous gating signal would
increase the power loss in the gate junction.
2. While thyristor is reverse biased. There should be no
gate signal; otherwise, the thyristor may fail due to an
increased leakage current.
3. The width of gate pulse tG must be longer than the
time required for the anode current to rise to the
holding current value IH. In practice, the pulse width tG
is normally made more than the turn on time ton of the
thyristor

14. FIRING CIRCUITS FOR THYRISTORS
An SCR can be switched from off-state to on-state in
several ways; these are forward voltage triggering,
dv / dt triggering, temperature triggering, light
triggering and gate triggering. The gate triggering is
the most common common method of turning on the
SCRs, because this method lends itself accurately for
turning on the SCRs at the desired instant of time.
MAIN FEATURES OF FIRING CIRCUITS
The most common method for controlling the onset
of conduction in an SCR is by means of gate voltage
control. The gate control circuit is also called firing, or
triggering circuit. These gating circuits are usually low
power electronics circuits. A firing circuit should fulfil
the following two functions.

15. If power circuit has more than one SCR, the firing
circuit should produce gating pulses for each SCR at
the desired instant for proper operation of the power
circuit. These pulses must be periodic in nature and
the sequence of firing must correspond with the type of
thyristorised power controller. For example, in a single
phase converter using two SCRs, the triggering circuit
must produce one firing pulse in each half cycle ; in a
3-phase full converter using six SCRs, gating circuit
must produce one trigger pulse after every 60 degree
interval
The control signal generated by a firing circuit may not
be able to turn –on an SCR. It is therefore common to
feed the voltage pulses to a driver circuit and then to
gate-cathode circuit. A driver circuit consists of a pulse
amplifier and a pulse transformer

16. A firing circuit scheme, in general consists of the components shown
in above fig. . A regulated DC power supply is obtained from an
alternating voltage source. Pulse generator, supplied from both
AC and DC sources, gives out voltage pulses which are then fed
to pulse amplifier for their amplification. Shielded cables transmit
the amplified pulses to pulse transformers. The function of pulse
transformer is to isolate the low voltage gate-cathode circuit from
the high voltage anode-cathode circuit

17. Types of Thyristor Firing Circuits
1. Resistance Firing Circuit
2. RC Firing Circuit
3.UJT Firing Circuit
4.Pulse Transformer Firing Circuit
Resistance Firing Circuit
Resistance triggering circuit is the simplest and the
most economical method.This however, suffer from a
limited range of firing angle control (0 to 90 degree),
great dependence on temperature and differnce in
performance between individual SCRs
 R C FIRING CIRCUITS
The limited range of firing angle control by resistance
firing circuit can be overcome by RC firing circuit.
The firing angle control range from 0 degree to 180
degree

18. Types of Thyristor Firing Circuits
 UJT triggering circuits.
Resistance and RC triggering circuits give prolonged
pulses. As a result, power dissipation in the gate circuit
is large. This difficulty can be overcome by UJT
triggering circuits.

19. RESISTANCE FIRING CIRCUITS
Theory of operation
As shown in the circuit, R2 is the variable resistance, R
is the stabilizing resistance. In case R2 is zero, gate
current may flow from source, through load, R1, Diode
D, and gate to cathode. This current should not
exceed permissible gate current . This current can be
limit with the value of R1

20. OPERATION OF RESISTANCE FIRING CIRCUITS
 It is thus seen that function of R1 is to limit the gate
current to a safe value as R2 is varied.
 Resistance R should have such a value that maximum
voltage drop across it does not exceed maximum
possible gate voltage

21. R C FIRING CIRCUITS
The limited range of firing angle control by
resistance firing circuit can be overcome by
RC firing circuit.

22. Theory of operation of RC Firing
Circuit
Fig illustrates RC triggering circuit.
 By varying the value of R, firing angle can be
controlled from 0 to 180 degree.
 In the negative half cycle, C charges through D2 .
This capacitor voltage remains constant at –Vm until
supply voltage attains zero value.
 When capacitor charges to positive voltage equal to
gate trigger voltage Vgt, SCR is fired and after this,
capacitor holds to a small positive voltage.
 Diode D1 is used to prevent the breakdown of
cathode to gate junction through D2 during the
negative half cycle.

23. Unijunction Transistor (UJT).
It is a three terminal device . The device input, is
called the emitter, has a resistance which rapidly
decreases when the input voltage reaches a certain
level. This is termed a “negative resistance
characteristics’’.
three terminals called the Emitter (E), Base-one(B1)
and Base-two(B2). It is made up of an N-type base to
which P-type emitter is embedded. P-type emitter is
heavily doped and N-type base is lightly doped

24. UJT Equivalent Circuit & Characteristics Curve

25. UJT Firing Circuit
 The unijunction transistor is a highly efficient switch ;
its switching time is in the range of nanoseconds.
Since UJT exhibits negative resistance
characteristics,
 Fig. (a) shows a circuit diagram with UJT working in
the oscillator mode. The external resistances R1 R2
are small in comparison with the internal resistances
RB1, RB2 of UJT bases

26. Operation of UJT Firing Circuit
In Fig. (a), when source voltage VBB is applied, capacitor C
begins to charge through R exponentially towards VBB, During
this charging, emitter circuit of UJT is an open circuit. The
capacitor voltage vC, equal to emitter voltage vE, is given by
VC = VE = VBB( 1 – e-t/RC)
The time constant of the charge circuit is 1 = RC
When this emitter voltage vE (or vC) reaches the
peak-point voltage VP (=  VBB + VD), the unijunction
between E – B1 breaks down. As a result, UJT turns
on and capacitor C rapidly discharges through low
resistance R1 with a time constant t2 = R1C. Here t2 is
much smaller than t1. When the emitter voltage
decays to the valley-point voltage VV, UJT turns off

27. Pulse Transformer Firing Circuit
Sometimes pulse transformers are used in firing
circuits for thyristors and GTOs, for isolation between
the gate circuit and the load circuit. The main reason
for this is that the load may use a high voltage ac
supply, and the firing circuit may use a low voltage.
The transformer generally used arc either l:l two-
winding, or l'l:l three-winding types. These have
transformers have
a low winding resistance, and a low leakage
resistance. The pulse transformer provides electrical
isolation as it transfers a pulse from the primary 1o the
secondary coil. The secondary coil of the pulse
transformer is connected directly between the gate
and the cathode, or may have series resistor, or a
series diode to prevent reverse gate current.
.

28. There are various ways of connecting the pulse
transformer to trigger the thyristor. Figure shows the
basic pulse transformer coupling to drive a single
thyristor

29. A pulse at the output of the pulse generator is given
to the primary of the pulse transformer, this is
transmitted faithfully at its secondary terminal through
the resistor R to the gate of the thyristor. Figure 3.19
shows another way of using a pulse transformer to
drive an anti-parallel pair of thyristors.

30. Here a three-winding transformer provides complete
isolation and the pulse generator must supply enough
energy to trigger both thyristors. Note the black dots
on the primary and secondary windings. These dots
are used to indicate the polarity of the windings.
Transformer polarity is defined as the relative
direction of the induced voltages in the primary and
secondary windings with respect to the winding
terminals. The dot is used to indicate which windings
have the same instantaneous polarity

31. Pulse Transformers
Pulse transformers are used quite often in firing circuits
for ,SCRs and GTOs. This transformer has usually two
secondaries. The turns ratio from primary to the two
secondaries is 2:1:1 or 1:1:1. These transformers are
designed to have low winding resistance, low leakage
reactar~ce and Iow interwinding capacitance. The
advantages of using pulse transformers in triggering
semiconductor devices are:
(a) They provide isolation of low voltage firing circuit
from high voltage anode-cathode power circuit and
(a) The trigger pulse can be coupled to one or more
devices from the same trigger source by means of pulse
transformer.
A square pulse at the primary terminals of a pulse
transformer may be transmitted at its secondary
terminals faithfully as a square wave or it may be

32. transmitted as a derivative of the input waveform.
A general layout of the trigger circuit using a pulse
transformer is shown in Fig. 2 Here, R1 limits the current
in the primary circuit of pulse transformer. In practice,
trigger pulses are preferred due to the following reasons:

33. (a) This pulse waveform is suitable for injecting a large
charge in the gate circuit for reliable turn on.
(b) The duration of this pulse is small, and therefore, no
significant heating of the gate circuit is observed.
(c) The fact stated (b) as mentioned permits Va to be
raised to a suitable high value so that a hard drive of
SCR is obtained. A device with a hard drive can
withstand high di/dt at the anode circuit, which is
desirable.

34. SNUBBER CIRCUIT
The circuit used to prevent unwanted dv /dt triggering
of SCR is called Snubber circuit
 For di/dt protection inductor is connected in series

35. THYRISTOR COMMUTATION TECHNIQUES
The gate has no control over thyristor once its turns on . It
can be turn off by reducing its forward anode current to a
level below the holding current.There are mainly two
types of commutation
 Natural Commutation
 Forced Commutation
 In Phase controlled Rectifiers Thyristor turned off
automatically due to natural behaviour of input supply
after half cycle.
 In Choppers, and Inverters input supply voltages are DC.
In these circuits Thyristor will turned off by applying
following forced commutation techniques
 Voltage Commutation
 Current Commutation
These techniques will be discussed in Chopper and
Inverters

36. Choppers
Introduction
 To produce quality goods in any industry, the
processes necessarily require the use of variable
speed drives.
 Variable speed d.c. and a.c. drives are being in-
creasingly used in all industries. These drives and
processes take power from d.c. voltage sources.
 In many cases, conversion of the d.c. source voltage
to different levels is required. For example, subway
cars, trolley buses, or battery operated vehicles
require power from a fixed voltage d.c. source.
However, their speed control requires conversion of
fixed voltage d.c. source to a variable-voltage d.c.
source for the armature of the d.c. motor.

37. Generally Following Methods are available for
obtaining variable DC from fixed DC voltages
Resistance control
In this method, a variable resistance is inserted between
the load and the source. This method is highly wasteful of
energy. Also, for a given output voltage, different values of
resistances are needed for different values of load current.
This method is still used for older traction installations
 Motor-generator set
Separate generator excitation gives a voltage which
can be varied from zero to rated value with either polarity.
The set-up is bulky, costly, slower in response, and less
efficient because of the generator field time-constant
 A.C. link chopper (inverter-rectifier)
In this method, the d.c. is first converted to a.c, by an
inverter (d.c. to a.c. converter) . The obtained a.c: is then
stepped up or down by a transformer and then rectified
back to d.c. by a rectifier.

38. D.C. chopper (d.c. to d.c. power converters)
 A d.c. chopper is a static device (switch) used to obtain
variable d.c. voltage from a source of constant d.c.
voltage. Therefore, chopper may be thought of as d.c.
equivalent of an a.c. transformer . The d.c. chopper offers
following advantages as compared to previous methods.
 Greater efficiency,
 Faster Response,
 Lower Maintenance,
 Small Size,
 Smooth Control,
 Solid-state choppers due to various advantages are
widely used in trolley cars, battery-operated vehicles,
traction-motor control, control of a large number of d.c.
motors from a common d.c. bus with a considerable
improvement of power factor,

39. Types of DC Chopper
According to the output voltages DC Chopper
are classified as follows
 Step Down Chopper
 Step Up Chopper
 Step Up-Down Chopper

40. Principle of Step Down Chopper
The output of step down chopper is less than the input
voltage.Figure . illustrates the principle of a chopper.
The chopper is represented by an SCR inside a dotted
square. It is triggered periodically and is kept
conducting for a period TON and is blocked for a period
TOFF. The chopped load voltage waveform is shown.
The output can be controlled either by current limit
control or time ratio control

41. CONTROL STRATEGIES OF CHOPPERS
The average value of output voltage. Vo can be
controlled by periodic opening and closing of the
switches. The two types of control strategies for
operating the switches are employed in d.c.
choppers. They are:
1. Time-ratio Control (TRC). and
2. Current Limit Control.
Time-Ratio Control (TRC)
In the time-ratio control, the value of TON /T is
varied. This is effected in two ways.
They are variable frequency operation and constant
frequency operation.

42. Constant Frequency System
In this scheme, the on-time Ton is varied but chopping
frequency f (or chopping period T) is kept constant.
Variation of Ton means adjustment of pulse width, as
such this scheme is also called pulse-width-
modulation scheme..
 Fig. illustrates the principle of pulse-width modulation.
Here chopping period T is constant. In Fig.(a), Ton =
1/4 T so that  = 25%. In Fig.(b), Ton = 3/4T so that
= 75%. The output voltage V0 can be varied
between zero and source voltage VS

43. Variable Frequency System
 In this scheme, the chopping frequency f (or chopping
period T ) is varied and either (i) on-time Ton is kept
constant or (ii) off-time TOFF is kept constant. This
method of controlling ‘’ is also called frequency-
modulation scheme

44. Principles of Step up Chopper
 In this chopper average output voltage VO is more than
the input voltage VO  VS .
 Large inductor L in series with source voltage VS is
essential
 When the chopper is on, inductor stores energy .
 When the chopper CH is off, as the inductor current
cannot die down instantaneously, this current is forced
to flow through the diode and load .
 As a result, voltage across the load, given by
 VO = VS + L (di/dt), exceeds the source voltage VS.
 In this manner, the circuit of Fig. (a) acts as a step-up
chopper and the energy stored in L is released to the
load.

45. Circuit Diagram and Waveform of Step
UP Chopper

46. CHOPPER COMMUTATION CIRCUITS
There are three types of Chopper
Commutations
 Voltage Commutated Chopper
 Current Commutated Chopper
 Load Commutated Chopper

47. Voltage Commutated Chopper
As shown Fig. The chopper feeds a constant current
load (highly inductive load). To start with the capacitor
is pre-charged with lower plate positive by closing the
switch shown in the Figure. The operation can be
explained using 4 modes.

48. Designing of commutation circuit
The following formulas are used to calculate the
values of Capacitor and Inductors
 where tC = Device turn off time,
 IO = Output current and
 E = Supply voltage.

49. CURRENT COMMUTATED CHOPPER (CCC)
The circuit and modes of CCC are shown in Fig.. In
current commutated chopper circuit, an inductor is
connected in series with the capacitor Tm is the main
SCR and TA is the auxiliary SCR. To start with the
capacitor is pre charged with top positive and bottom
negative. The details of commutation can be explained
with the following modes

50. CURRENT COMMUTATED CHOPPER (CCC)

51. Designing of commutation circuit
The following formulas are used to calculate the values
of Capacitor and Inductors
 where tC = Circuit turn off time,
 ICP = Peak values of capacitor current and
 VS = Supply voltage.

52. LOAD COMMUTATED CHOPPER
The load commutated chopper (LCC) uses four SCRs.
They are triggered in pairs. Initially the capacitor is
charged with a — and b +. The working of LCC can be
explained with the following modes. The circuit and
modes of LCC are shown in Fig.

53. SWITCHING- MODE REGULATORS
DC chopper can be used as switching-mode
regulators to convert a DC voltage, normally
unregulated DC output voltage. The regulation is
normally achieved by PWD at a fixed frequency and
the switching device is normally a power BJT, or
MOSFET

54. Topologies of switching
regulators
(1) Buck Regulators
(2) Boost Regulators
(3) Buck-Boost Regulators

55. Buck Regulators.
In a buck regulator, the average output voltage VO, is less
than the input voltage ‘VS’

56. BOOST REGULATORS
In a boost regulator, the output voltage is greater than the
input voltage . A boost regulator using a power ‘MOSFET’
as shown in fig. .The circuit operation can be divided
into two modes

57. Buck-Boost Regulators
A buck-boost regulator provides an output voltage that
may be less than or greater than the input voltage—
hence the name "buck-boost"; the output voltage polarity
is opposite to that of the input voltage. This regulator is
also known as an inverting regulator

58. CYCLOCONVERTERS
 A device which converts input power at one frequency to
output power at a different frequency with one-stage
conversion is called a cycloconverter.
 A cycloconverter is thus a one-stage frequency changer.
 Basically, cycloconverters are of two types, namely :
(i) Step down cycloconverters
(ii) Step up cycloconverters
 In step-down cycloconverters, the output frequency f0 is
lower than the supply frequency fS fo < fS .
 In step-up cycloconverters, fQ >fs.
At present, the applications of cycloconverters include
the following :
(i) Speed control of high-power ac drives
(ii) Induction heating
(iii) For converting variable-speed alternator voltage to
constant frequency output voltage for use as power
supply in aircraft or shipboards

59. Single-phase to Single-phase Step-up Cyclocbnverter
For understanding the operating principle of step-up
device, the load is assumed to be resistive for simplicity.
It should be noted that a step-up cycloconverter requires
forced commutation. The basic principle of step-up
device is described here first for mid-point and then for
bridge-type cycloconverters

60. . Waveforms for Step-up Cyclocbnverter

61. Single-phase to Single-phase Step-down Cycloconverter
A step-down cycloconverter does not require forced
commutation. It requires phase-controlled converters
connected as shown in Fig. .1. These converters need
only line, or natural, commutation which is provided by ac
supply. Both mid-point and bridge-type cycloconverters
are described in what follows

62. INVERTERS
Introduction
 It is DC to AC converter
 An Inverter enables one to convert a supply of dc input
voltage to a symmetrical ac output voltage of a desired
magnitude and frequency
 This output voltage may be fixed or variable, at a fixed or
variable frequency.
 The variable output voltage can be obtained either by
varying the input dc voltage, keeping the inverter at a
constant gain or by varying the gain of the inverter
 The gain of the inverter is defined as the ratio of the ac
output voltage to the dc input voltage

63. Although the output voltage waveforms of an ideal
inverter should be sinusoidal, in practice, they are non-
sinusoidal and contain certain harm
For low- and moderate-power applications these
harmonics may be acceptable, but for high-power
applications, low distorted sinusoidal waveforms are
required
These harmonic contents of the output voltage can be
reduced or minimized by using high-speed switching
power semiconductor devices
Applications of inverters are: variable speed ac motor
control, induction heating, standby/uninterrupted power
supplies, etc

64. Classification of Inverters
There are many ways to classify inverters, but they
are broadly divided in to two types, viz., single-
phase, and three-phase; these are further classified
according to the taxonomy of Figure .1.

65. Classification of Inverters
In amplifier-type inverters, transistors are used as
amplifiers and due to high power dissipation in the
device itself, they generally give a low efficiency
In saturated-type inverters, devices are used as a
switch; therefore, they show high efficiency
Transistors, and IGBTs are generally used in low-
and medium-power requirements
Thyristors, and GTOs are used in high-power
applications.
These inverters can also be classified into two
groups: voltage-driven inverters and current-driven
inverters

66. VOLTAGE CONTROL IN SINGLE-PHASE INVERTERS
AC loads may require constant or adjustable voltage at
their input terminals. When such loads are fed by
inverters, it is essential that output voltage of the
inverters is so controlled as to fulfill the requirement of
AC loads
The various methods for the control of output voltage of
inverters are as under
External control of ac output voltage
External control of dc input voltage
Internal control of inverter.
The first two methods require the use of peripheral
components whereas the third method requires no
peripheral components. These methods are now briefly
discussed

67. External Control of dc Input Voltage
In case the available voltage source is ac, then dc
voltage input to the inverter is controlled through
fully-controlled rectifier,
Through an uncontrolled rectifier and a chopper,
Through an ac voltage controller & an uncontrolled
rectifier
If available voltage is dc, then dc voltage input to the
inverter is controlled by means of a chopper

68. External Control of ac Output Voltage
There are two possible methods of external control of
ac output voltage obtained from inverter output
terminals.
AC voltage control
Series-inverter control
AC voltage control In this method, an ac voltage
controller is inserted between the output terminals of
inverter and the load terminals as shown in Fig. 1. The
voltage input to the ac load is regulated through the
firing angle control of ac voltage controller

69. Series-inverter control
This method of voltage control involves the use of two or
more inverters in series. Fig. (a) illustrates how the
output voltage of two inverters can be summed up with
the help of transformers to obtain an adjustable output
voltage.

70. Internal Control of Inverter
Output voltage from an inverter can also be adjusted by
exercising a control within the inverter itself. The most
efficient method of doing this is by pulse-width
modulation control used within an inverter. This is
discussed briefly in what follows
Pulse width modulation control.
In this method, a fixed dc input voltage is given to the
inverter and a controlled ac output voltage is obtained by
adjusting the on and off periods of the inverter
components. This is the most popular method of
controlling the output voltage and this method is termed
as pulse-width modulation (PWM) control.
The output voltage control with this method can be
obtained without any additional components
With this method, lower order harmonic can be
eliminated or minimised along with its output voltage
control. As higher order harmonics can be filtered easily,
the filtering requirements are minimised.

71. FORCE-COMMUTATED THYRISTOR INVERTERS
For low-and medium-power applications, inverters
using transistors, GTOs and IGBTs are becoming
increasingly popular. However, for high-voltage and
high-current applications, thyristors are more suitable.
In voltage fed inverters, thyristors remain forward
biased by the dc supply voltage. This entails the use of
forced commutation for inverter circuits using thyristors

72. Modified McMurray Half-bridge Inverter
It consists of main thyristors Tl, T2 and main diodes Dl,
D2. The commutation circuit consists of auxiliary
thyristors TA1, TA2, auxiliary diodes DA1, DA2 ;
damping resistor Rd, inductor L and capacitor C

74. Principle of Phase Controlled Converters
 The simplest form of controlled Rectifier circuit is
consist of single thyristor feeding DC power to a
resistive load R, as shown in fig 1(a).
 During positive half cycle of input voltage , the
thyristor anode is positive with respect of its cathode
and the thyristor is said to be forward biased. When
T1 is fired at t = , T1 conducts and input voltage
appears across the load.
 When the input voltage starts to be negative at, t =
, the thyristor anode is negative with respect to
cathode and T1 is said to be reverse biased and it is
turned off.

75. Circuit Diagram and Waveforms with Resistive
Load

76. Single Phase Half Wave Circuit with RL Load
As shown in fig. At t = , thyristor is turned on by
gating signal. The load voltage Vo at once brcomes
equal to source voltage Vs as shown. But the
inductance L forces the load or output current Io to
rise gradually. After some time, Io reaches maximum
value and then begins to decrease. At t = , Vo is
zero but Io is not zero because of the load
inductance L. After t = , SCR is subjected to
reverse anode voltage but it will not be turned off as
load current Io is not less than the holding current.At
some angle , Io reduces to zero and SCR is
turned off as it is already reverse biased. After t =
, Vo = 0 and Io = 0.

77. Single Phase Half Wave Circuit with RL Load