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Pulse Width Modulation
Pulse-width modulation (PWM) uses a rectangular pulse wave whose
pulse width is modulated resulting in the variation of
the average value of the waveform.
The general purpose of Pulse Width Modulation is to control power
delivery, especially to inertial electrical devices.
The on-off behavior changes the average power of signal.
Output signal alternates between on and off within a specified
period.
A secondary use of PWM is to encode information for transmission.

Duty Cycle
The Duty Cycle is a measure of the time the
modulated signal is in its “high” state.
It is generally recorded as the percentage of the
signal period where the signal is considered on.
Period (T)
Duty Cycle
(D)
VL
VH
On Off
%
100


Period
Time
On
Cycle
Duty

Signal Average Value
Average value of a pulse waveform f(t) with period T, low value VL, high
value VH can be found as:
In general VL is 0; Vavg = Duty cycle * VH
The average value of the signal is directly dependent on the duty cycle D.
0
1
( )
T
y f t dt
T
    L
H
avg V
D
V
D
V 



 1
Period
(T)
Duty
Cycle (D)
VL
VH
On Off

Analog Generation of PWM
The Intersective Method: Allows for
analog creation of PWM signal
through noting intersections
between a sawtooth trigger signal
and a reference sinusoid.
Length of pulses is dependent
upon intersection of reference
sinusoid and trigger signal.
When sinusoid is greater than
trigger signal, PWM pulse is
switched to on/high position,
otherwise it is switched to off/low.

Advantages of using PWM
Average value proportional to duty cycle
Low power used in transistors used to switch the signal, and fast switching
possible due to MOSFETS and power transistors at speeds in excess of 100 kHz
To fulfill partial power requirements, variable resistance devices such as rheostats were
used to control the current entering a device (e.g. sewing machines)
Alleviates the problem of high heat losses through resistive elements at
intermediate voltage points

PWM Applications
Controlling the brightness of LED by adjusting the duty cycle
With an RGB (red green blue) LED, you can control how much of each
of the three colors you want in the mix of color by dimming them with
various amounts.
Pulse width modulation can be used to control the angle of a servo
motor attached to something mechanical like a robot arm.
Also used to drive the DC fans of our computers and laptops at
different speed.
Servos have shaft that turns to specific position based on its control
line.

Modulatio
n
In power electronics, modulation refers to the process of
manipulating the shape or characteristics of a signal to
achieve a desired output. This can be done in various ways,
including changing the amplitude, frequency, or phase of the
signal.
One common application of modulation in power electronics
is in the generation of pulse width modulation (PWM)
signals, which are used to control the power delivered to a
load.
Other types of modulation techniques used in power
electronics include amplitude modulation (AM), frequency
modulation (FM), and phase modulation (PM). These
techniques are used in various applications, such as in motor
control, switching power supplies, and audio amplifiers.

commutations, modlation, types of modulation and which tye is the best to se and wy

Modulati
on
Types of Modulation Techniques in power electronics:
There are many types of modulation and some of them are:
1- Sinusoidal pulse width modulation (SPWM)
2- Modified pulse width modulation (MPWM)
3- Space vector modulation (SVM)
4- Delta modulation (DM)
5- Specific harmonic elimination (SHE)
6- Wavelet modulation (WM).

Pulse Width
Modulation
(PWM)
• Pulse-width modulation (PWM) uses a rectangular pulse wave whose
pulse width is modulated resulting in the variation of the average value
of the waveform.
• The general purpose of Pulse Width Modulation is to control power
delivery, especially to inertial electrical devices.
• The on-off behavior changes the average power of signal.
• Output signal alternates between on and off within a specified period.
• A secondary use of PWM is to encode information for transmission.
• PWM inverters eliminate lower order harmonics and decrease the
THD content in the output AC voltage.

Duty cycle of
(PWM)
• D = Ton/ T is called the duty cycle & is the ratio of the pulse width
Ton to the time period T of the waveform
• It is generally recorded as the percentage of the signal period where the
signal is considered on.
• The complement of the duty ratio, D’ is defined as (1 – D)
D ’ = Toff / T Þ Toff = D’ T = (1-D)T
D = Ton/ T Þ Ton = DT & D + D’ =
1 , Ton + Toff = T
• The DC component of output voltage Vo is
vo
Vin
TOff
DT T
TON
t
(1-D)T
v
2
1
Vo

Sinusoidal
Pulse Width
Modulation
(SPWM)
• The term SPWM stands for “Sinusoidal pulse width modulation” is a
technique of pulse width modulation used in inverters.
• It is basically a sinusoidal function of its angular position with respect to
a reference sine wave resulting in a reduction in the harmonic content.
• The control function consists of a sinusoidal wave obtained from an
oscillator of variable amplitude A and of fundamental output inverter
frequency f = 1/T as well as a triangular wave of fixed amplitude Ap and
frequency fp .

Sinusoidal
Pulse Width
Modulation
(SPWM)
• The biased triangular waveform is reversed in polarity at the end of each
half-cycle of the output voltage.
• It is easily established from the figure that the number of gate pulses
(sinusoidally modulated) per half-cycle is
• The amplitude of the fundamental voltage can be controlled by varying
amplitude A of the sine wave over the range 0 < A < Amax where Amax =
2AP.

Modified
Sinusoidal
Waveform
PWM
• A modified sinusoidal PWM waveform is used for power
control and optimization of the power factor.
• The main concept is to shift the current delayed on the grid
to the voltage grid by modifying the PWM converter.
• Consequently, there is an improvement in the efficiency of
power as well as optimization in power factor.

Introduction
• The turn OFF process of an SCR is called Commutation.
Commutation means changing the state of a thyristor from its
forward conducting state to forward blocking state.
• To turn OFF a conducting SCR properly, the following
conditions must be satisfied:
1. The anode or forward current of SCR must be reduced to zero
or below the level of holding current and then,
2. A sufficient reverse voltage must be applied across the SCR to
regain its forward blocking state.
• The commutation methods are classified based on
1. The way anode current is reduced to zero
2. The configuration of the commutating circuits.
• One method of classification is as follows:
1. Natural or line commutation.
2. Load Commutation or Self Commutation.
3. Forced commutation.

NATURAL COMMUTATION
Requires AC voltage at Requires DC
voltage
Input.
External components are not required.
Used in controlled rectifiers, AC voltage
controller.
SCR turns off due to negative supply voltage.
No power loss takes place during
commutation.
Zero Cost.
FORCED COMMUATION
Requires DC voltage at Requires DC
voltage
Input.
External components are required.
Used in choppers, inverters, etc.
SCR turns off due to current and voltage
both.
Power loss takes place during commutation.
Significant Cost

Natural Commutation
Muhammad Furqan Ashraf
401640

Natural Commutation
• The process of turning OFF SCR
• The transition of an SCR from a forward conduction state to a blocking state is
turned off SCR.
Condition For commutation:
1. The Anode current must be lesser than the holding current.
2. Applied Reverse Voltage across SCR
• The naturally turning OFF SCR is called Natural Commutation.
A H
I < I

Natural Commutation
• In A.C circuit, the current always passes throw zero for every half cycle.
• As the current passes through natural zero, a reverse voltage will simultaneously
appear across the device.
• This will turn OFF the device immediately.
• This process is called as natural commutation, since no external circuit is required
for this purpose.
• This method is only applicable for A.C supply.

Class F Commutation
• In the class F thyristor commutation technique, AC voltage is used for supply,
during the positive half cycle of this supply, the load current will flow.
• If the load is highly inductive, the current will flow until the energy stored in the
inductive load is dissipated.
• During the negative half cycle as the load current becomes zero, then SCR will
turn OFF
• If voltage exists for the period of rated turn-off time of the device, the negative
polarity of the voltage across the outgoing SCR will turn it off.
• Here, the duration of the half cycle must be greater than the off time of the
thyristor.

Class A: Self Commutated by a Resonating Load
• Class A commutation of thyristor is a type of forced commutation and is also referred to as Load
Commutation.
• Class A commutation is a sub-classification of forced commutation sometimes called self or resonant
commutation. To commutate the thyristor, two necessary actions must be considered:
 IA > IH i.e., anode current must be less than holding current.
 The potential at anode must be lower than the cathode.
In the circuit given below it is clearly shown that load i.e.,
R is connected serially with the commutating components i.e.,
L and C. Generally, when values of R, L, and C are low then in that case,
the elements are arranged serially along with SCR as shown above.

Class A: Self Commutated by a
Resonating Load
On the contrary when the load
resistor possesses a high value then
it is connected in parallel across the
capacitor while this parallel
combination is connected in series
with the inductor.

Working of Class A Commutation
On applying an external dc input signal, the current starts flowing
through the circuit. In order to turn on the SCR in the circuit, a gate
trigger pulse is required. So, simultaneously gate signal is applied
that will put the thyristor in forward conduction mode. We have
already discussed the same that gate pulse is necessarily required to
turn on the SCR. Hence, after the SCR gets on, the forward current
that flows through the SCR begins to charge the capacitor. At the
same time, the inductor connected in the circuit stores energy.
We know that it is the property of the inductor, that it opposes the
change in current. So, once the capacitor gets charged up to the
peak of the supply input, the polarity of the inductor connected in
the circuit will get reversed and now the inductor will oppose any
further flow of current through it. As the inductor does not further
allow the flow of current, the output current starts to decrease and
reaches zero.

Necessary Condition
• We know that the current flowing through the circuit is given as:
• Further, writing the above equation in Laplace transform,
• R + Ls + 1/sC = Z that corresponds to the overall impedance of the circuit due to serially
connected elements.
• On rearranging,

• On simplifying,
• The characteristics equation for the above equation is given as:
• However, the standard characteristics equation is given as:
• : ξ corresponds to the damping ratio and
• ωn denotes the natural angular frequency
• On comparing the above two equations, we will get,

• Here,
• For underdamped system, ξ < 1
• This is the condition for an underdamped system.
• The resonant frequency is given as:
• The maximum conduction time of SCR is given as:
• For α as firing angle,

INTRODUCTION
Class B: self Commutation by an L-C Circuit
• The major difference between the class A and class B
thyristor commutation techniques is that the LC is
connected in series with thyristor in class A, whereas in
parallel with thyristor in class B.
• Before triggering on the SCR, the capacitor is charged up.
• If the SCR is triggered or given triggering pulse, then the
resulting current has two components.
• The constant load current flowing through the R-L load is
ensured by the large reactance connected in series with
the load which is clamped with freewheeling diode.
• If sinusoidal current flows through the resonant L-C circuit,
then the capacitor C is charged.

INTRODUCTION
Class B: self Commutation by an L-C Circuit
• The total current flowing through the SCR becomes zero
with the reverse current flowing through the SCR opposing
the load current for a small fraction of the negative swing.
• If the resonant circuit current or reverse current becomes
just greater than the load current, then the SCR will be
turned OFF.
• This commutation technique is mostly used for chopper
circuits
.

Class C
Commutation
A sub-classification of forced commutation in
which the device is commutated by transferring
the load current of the main thyristor to another
thyristor in the circuit is Class C Commutation of
the Thyristor. Another name for this type of
commutation is complementary commutation.
This commutation technique shows
high reliability and suits operations at
frequencies below 1000 Hz.

Class C
Commutation
• 2 SCRs will be used.
• One is main while the other is auxiliary.
• Both may act as main SCRS carrying load
current and they can be designed with four
SCRS with load across the capacitor by using
a current source for supplying an integral
converter.
• If the thyristor 2 is triggered, then the
capacitor will be charged up.
• If the thyristor 1 is triggered, then the
capacitor will discharge and this discharge
current of C will oppose the flow of load
current in 2 as the capacitor is switched
across 2 via 1.

Class C
Commutation
• Mainly used in single-phase inverters with
centre tapped transformers.
• It is useful even at frequencies below 1000 Hz.

Working of
Class C
Commutation
To understand the operation of class c commutation, there are
three modes of operation.
Both the SCRs of the circuit carry load current but not
simultaneously. Let us understand each
mode of operation separately.
Mode 0: This mode of operation corresponds to the initial
state of the circuit when both the
thyristors are in off state and so the voltage across the
capacitor is also 0.
This means, T1 = off; T2 = off; VC = 0
Mode I: In this mode of operation, the circuit is provided with
dc supply input and along with that thyristor T1 is triggered
with a gate signal. Due to this, T1 will come in conducting
state. This leads to two currents one will be the load current
while the other will be the charging current of the capacitor
to flow through the whole circuit.

Working of
Class C
Commutation
The load current and capacitor current are
given as:
While the charging current will be:
Hence, the overall current that flows
through SCR T1 will be the sum of load
current
and charging current. Thus, is given as:

Working of
Class C
Commutation
Due to the flow of the charging current, the capacitor gets
charged up to the peak of the
supply input according to the polarity as shown in the above
figure.
However, the charging current reduces to 0, once the
capacitor gets fully charged up to the supply input, and thus
the only current that continues to flow through T1 is the load
current. Even when the moment C is holding the charge, T1
continues to remain in conducting state. Hence,
T1 = on; T2 = off; VC = V
Now, our attempt should be to commutate T1 and this is
explained in the next mode of
operation.
Mode II: In this state of operation, T2 is provided the gate
pulse that triggers it and this leads
to turning off T1. Now, the question arises, how this actually
happens?

Working of
Class C
Commutation
So, this mode of operation is such that by providing a
triggering pulse to T2 the path becomes short-circuited. Once
this happens, the polarity of the charge stored by the
capacitor reverse biases the thyristor T1.
This reverse biased condition leads to turning off of the
thyristor T1. So, when T2 is in
conducting state
The current flowing through R-T2 will be given as:
The load current flowing through RL-C-T2 charges the
capacitor and its value will be given as:
Thus, the overall current flowing through thyristor T2,

Working of
Class C
Commutation
Hence, mode II operation provides,
T1 = off; T2 = on; VC1 = -V
Now, the next mode of operation of this commutation is based
on turning on the main
thyristor while turning off the auxiliary one.
Mode III: In order to turn off T2, T1 is triggered using gate
pulse. Once T1 starts conduction then the polarity existing
across C reverse biases T2 due to which T2 stops conduction
and the current flows through T1 like we have discussed in
mode I operation.
Hence, the mode III
operation will lead
cause T1 = on; T2 =
off; VC = V

Class E
Commutation
Class-E Commutation is one of the
forced commutation method to turn off an SCR /
Thyristor.
An external current pulse is used in this technique
to commutate SCR. This is the reason, Class-E
commutation is also known as External Pulse
Commutation.
This external current pulse is obtained form a
separate voltage s
The most important condition for reliable
commutation of SCR by this technique is that, the
magnitude of current pulse must be more than the
load current.ource.

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