power electronics
Asst. lecture:-ola hussein
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• 1.1.CONCEPT OF POWER ELECTRONICS:
•
•
Power electronics deals with conversion and control of
electrical power with the help of electronic switching
devices. The magnitude of power may vary widely, ranging
from a few watts to several giga watts. Power electronics
differs from signal electronics, where the power may be from
a few nanowatts to a few watts, and processing of power may
be by analog (analog electronics) or digital or switching
devices (digital electronics).
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1.3.ADVANTAGES AND DISADVANTAGES OF
POWER
ELECTRONIC CONVERTERS
• (i)High efficiency due to low loss in power-semiconductor
devices.
• (ii)High reliability of power-electronic converter systems.
• (iii)Long life and less maintenance due to the absence of any
moving parts.
• (iv)Fast dynamic response of the power-electronic systems as
compared to electromechanical converter systems.
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Chapter Two
POWER SEMICONDUCTOR DEVICES
• 2.1 Introduction
• the power semiconductor devices can be classified as under :
• (a) Diodes : These are uncontrolled rectifying devices. Their
on and off states are controlled by power supply.
• (b Thyristors : These have controlled turned-on by a gate
signal. After thyristors are turned-on, they remain latched-in
on-state due to internal regenerative action.
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2.2. CHARACTERISTICS OF POWER DIODES
• Power diode is a two-layer, two-terminal, p-n semiconductor
device. It has one p-n junction formed by alloying, diffusing
or epitaxial growth. The two terminals of diode are called
anode and cathode, Fig. 2.1 (a). Two important
characteristics of power diodes are now described.
• 2.2.1. Diode V-I Characteristics
• When anode is positive with respect to cathode, diode is said
to be forward biased.
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2.2.2Diode
Reverse
Recovery
Characteristics
• After the forward diode current decays to zero, the diode
continues to conduct in the reverse direction because of the
presence of stored charges in the two layers. The reverse
current flows for a time called reverse recovery time trr The
diode regains its blocking capability until reverse recovery
current decays to zero.
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2.3POWER TRANSISTORS
• Power diodes are uncontrolled devices. In other words,
their turn-on and turn-off characteristics are not under
control. Power transistors, however, possess controlled
characteristics. These are turned on when a current
signal is given to base, or control, terminal. The
transistor remains in the on-state so long as control
signal is present. When this control signal is removed, a
power transistor is turned off.
•
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DC/DC CHPPER
• In many industrial applications, it is required to convert a
fixed-voltage dc source into a variable-voltage dc source.
• A dc chopper converters is directly changed voltage from dc
to dc and is also known as a dc-to-dc converter. A chopper
can be considered as dc equivalent to an ac transformer with
a continuously variable turns ratio like a transformer, it can
be used to step-down or step-up a dc voltage source.
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• The dc to-dc converters are consists of two types:
a) AC Link Chopper. In the ac link chopper, dc is first
converted to ac by an inverter (dc to ac converter). AC is
then stepped-up or stepped-down by a transformer which is
then converted back to dc by a diode rectifier, Fig. 4.1 (a).
As the conversion is in two stages, dc to ac and then ac to
dc, ac link chopper is costly, bulky and less efficient.
•
b)DC Chopper. A chopper is a static device that converts
fixed dc input voltage to a variable dc output voltage
directly, Fig. 4.1 (b). A chopper may be thought of as dc
equivalent of an ac transformer since they behave in an
identical manner.
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• 4.2 Principle of step-down operation:
•
• The practical devices have a finite voltage ranging from 0.5
to 2V,and for the sake of simplicity we shall neglect the
voltage drops of these power semiconductor devices.
• A chopper is a high speed on/off semiconductor switch. It
connects source to load and disconnects the load from source
at a fast speed. In this manner, a chopped load voltage as
shown in Fig, 4.2 (b) is obtained from a constant dc supply of
magnitude VS. In Fig, 4.2 (a),chopper is represented by a
switch SW inside a dotted rectangle, which may be turned-on
or turned-off as desired.
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• 4.3 Control Techniques:
• Two different strategies called constant {frequency system
and variable frequency system as detailed below:
• 1. 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. 4.3 illustrates the principle of pulse-width modulation.
Here chopping period T is constant. In Fig. 4.3 (a), Ton
=(1/4)T so that α = 25%. In Fig. 4.3 (b), Ton =(3/4)T so that α
= 75%. Ideally α can be varied from zero to unity.
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• Frequency modulation scheme has some disadvantages as
compared to pulse width modulation scheme. These are as
under:
• (i) The chopping frequency has to be varied over a wide
range for the control of output voltage in frequency
modulation.Filter design for such wide frequency variation
is, therefore, quite difficult.
• (ii) For the control of α frequency variation would be wide.
As such there is a possibility of interference with signaling
and telephone lines in frequency modulation scheme.
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Fig.4.3 Principle of pulse-width modulation
(constant T)
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• 4.4 Principle of step-up operation:
• Fig.4.5(a) illustrates an elementary form of a step-up
chopper. ln this chopper, a large inductor L in series with
source voltage VS is essential as shown in Fig.4.5 (a). When
the chopper CH is on, the closed current path is as shown in
Fig.4.5(b) and inductor stores energy during Ton period.
When the chopper CH is off, as inductor current cannot die
down instantaneous, this current is forced to now through the
diode and load for a time Toff, Fig.4.5 (c).
• As the current tends to decrease, polarity of the emf induced
in L is reversed as shown in Fig. 4.5 (c). As a result, voltage
across the load, given by Vo = Vs+ L (di/dt), exceeds the
source voltage Vs
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• During the time Toff
when chopper is off, the energy
released by inductor to the load is Woff= (voltage across L)
(average current through L) Toff
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Fig. 4.5 (a) Step-up chopper (b) L stores energy (c) L. di/dt is
added to Vs (d) voltage and current waveforms.
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• Example.4.1.For the basic dc to dc converter of
Fig.4.2(a),express the following variables as functions of Vs.,
R and duty cycle α in case load is resistive:
• (a) Average output voltage and current.
• (b) Output current at the instant of commutation.
• (c) Average and rms freewheeling diode currents.
• (d) Rms value of the output voltage.
• (e) Rms and average thyristor currents.
• (f) Effective input resistance of the chopper and
• Solution. For a resistive load, output or load current
waveform is similar to load voltage waveform.
•
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• Example 4.2. For type A chopper, dc source voltage = 230 V,
load resistance
• = 10 Ω. Take a voltage drop of 2V across chopper when it is
on. For a duty cycle of 0.4, calculate
• (a) average and rms values of output voltage and
• (b) chopper efficiency.
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•
• Example 4.3. A step-up chopper has input voltage of 220 V
and output voltage 0f 660 V. If the conducting time of
thyristor-chopper is 100µs.Compute the pulse width of output
voltage. In case output-voltage pulse width is halved for
constant frequency operation, find the average value of new
output voltage.
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• Furthermore, by employing high switching frequencies, the
sizes of the power transformer and associated filtering
components in the SMPS are dramatically reduced in
comparison to the linear. For example, an SMPS operating at
20kHz produces a 4 times reduction in component size, and
this increases to about 8 times at 100kHz and above. This
means an SMPS design can produce very compact and light
weight supplies. This is now an essential requirement for the
majority of electronic systems. The supply must slot into an
ever shrinking space left for it by electronic system
designers.
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• The designer can select the switching frequency by choosing
the values of R and C of frequency oscillator transistor. The
transistor switching loss increases with the switching
frequency and as a result the efficiency decreases. In
addition the core loss of inductors limits the highfrequency operation. Control voltage vc, is obtained by
comparing the output voltage with its desired value. The
vcr can be compared with a saw tooth voltage vr, to
generate the PWM control signal for the dc converter.
There are four basic topologies of switching regulator:
• 1. Buck regulators. 2. Boost regulators.
• 3. Buck-boost regulators. 4. Cuk regulator.
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• 4.6.2 Boost Regulators:
•
• In a boost regulator the output voltage is greater than the
input hence the name "boost." A boost regulator is
shown in Fig.4.9(b).
• A boost regulator can step up the output voltage without a
transformer. Due to a single transistor. it has a high
efficiency. The input current is continuous however high
peak current has to flow through the power transistor. The
output voltage is very sensitive to changes in duty cycle
α and it might be difficult to stabilize the regulator.
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• 4.6.3 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 opposite to
that of the input voltage. This regulator is also known as an
inverting regulator. The circuit arrangement of a buck-boost
regulator is shown in Figure 4.9(c).
• A buck-boost regulator provides output voltage polarity
reversal without a transformer. It has high efficiency.
Under a fault condition of the transistor, the di/dt of the fault
current is limited by the inductor L, and will be VS/L.
Output short-circuit protection would
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Fig.4.9(a) Buck Regulator
Fig.4.9(b) Boost Regulator
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Fig.4.9(c) Buck Boost Regulator
Fig.4.9(d) Cuk Regulator
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• Chapter Five
• Dc To Ac Converters (Inverters)
• 5.1 Introduction:
A device that converts dc power into ac power at desired
output voltage and frequency is called an inverter.Some
industrial applications of inverters are for adjustable-speed ac
drives, induction heating, stand by air-craft power supplies,
UPS (uninterruptible power supplies) for computers, hvdc
transmission lines etc.The input may be a battery, fuel cell,
solar cell, or other dc source .The typical single phase
outputs are (1) 120 V at 60H2, (2) 220 V at 50 Hz, and
(3)115V at 400 Hz.
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• 5.2 Single Phase Bridge Voltage Source Inverters:
• Single-phase bridge inverters are of two types, namely (i)
single-phase half-bridge inverters and (ii) single-phase fullbridge inverters. Basic principles of operation of these two
types are presented here.
• Power circuit diagrams of the two configurations of singlephase bridge inverter, as stated above, are shown in Fig. 5.1
(a) for half-bridge inverter and in Fig.5.2(b) for full-bridge
inverter.In these diagrams, the circuitry for turning-on or
turning-off of the thyristors is not shown for simplicity. The
gating signals for the thyristors and the resulting output
voltage waveforms are shown in Figs. 5.1 (b) and 5.2 (b) for
half-bridge and full-bridge inverters respectively.
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Fig. 5.1. Single-phase half-bridge inverter
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Fig. 5.2. Single-phase full-bridge inverter
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• 5.3 Performance Parameters:
• The output of practical inverters contain harmonics and the
quality of an inverter normally evaluated in terms of the
following performance parameters.
• Harmonic factor of nth harmonic (HFn). The harmonic
factor (of the nth harmonic).which is a measure of
individual harmonic contribution is defined as
• where (Vo1 is thc rms value o[ the fundamental
component and Von is the rms value of nth harmonic
component.
• Total harmonic distortion (THD):The total harmonic
distortion. which is measure o[ closeness in shape between
a waveform
and its fundamental
component defined as
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•
• Lowest order harmonic (LOH):The LOH is that harmonic
component whose frequency is closest to the fundamental
one, and its amplitude is greater than or equal to 3% of
the fundamental component.
• 5.4 Steady-state Analysis of Single-phase Inverter :
• Figs.5.1(b) and 5.2 (b) reveal that load voltage waveform
does not depend on the nature of load. The load voltage is
given by
• The load current is, however, dependent upon the nature of
load. Let the load, in general, consist of RLC in series.The
circuit model of single-phase half-bridge or full-bridge
inverter is as shown in Fig.5.3(a).
•
•
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Fig. 5.3. Load voltage and current waveforms.
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• 5.4 Three Phase Bridge Inverters:
• A basic three-phase inverter is a six-step bridge inverter. It
uses a minimum of six thyristors. In inverter terminology, a
step is defined as a change in the firing from one thyristor to
the next thyristor in proper sequence. For one cycle of 360o ,
each step would be of 60° interval for a six-step inverter. This
means that thyristors would be gated at regular intervals of
60°in proper sequence so that a 3-phase ac voltage is
synthesized at the output terminals of a six-step inverter.
• Fig.5.4(a) shows the power circuit diagram of a three-phase
bridge inverter using six thyristors and six diodes.
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Fig. 5.4. Three-phase bridge inverter using (a) thyristors (b ) IGBTs
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• 5.4.1. Three-phase 180 Degree Mode VSI:
• In the three-phase inverter of Fig. 5.4, each SCR conducts for
180° of a cycle. Thyristor pair in each arm, i.e.Tl, T4 ; T3, T6
and T5,T2 are turned on with a time interval of 180°. It
means that T1 conducts for 1800 and T4 for the next 1800 of a
cycle. Thyristors in the upper group, i.e. T1, T3, T5 conduct
at an interval of 120°. It implies that if T1 is fired at wt = 0°,
then T3 must be fired at wt =1200 and T5 at wt =240°.Same
is true for lower group of SCRs.On the basis of this firing
scheme, a table is prepared as shown at the top of Fig. 5.5. In
this table, first row shows that T1 from upper group conducts
for 180°, T4 for the next 1800 and then again T1 for 1800 and
so on.
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Fig. 5.5. Voltage waveforms for 1800 mode 3‫االلكتروني‬
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phase VSI
Fig. 5.5. Voltage waveforms for 1200 mode 3phase VSI
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Fig. 5.6. Pertaining (a) 1800 mode (b) 1200 mode
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• (ii) In case inverter supplies power to a magnetic circuit, such
as an induction motor, the voltage to frequency ratio at the
inverter output terminals must be kept constant. This avoids
saturation in the magnetic circuit of the device fed by the
inverter.
• The various methods for the control of output voltage of
inverters are as under:
• (a) External control of ac output voltage
• (b) External control of dc input voltage
• (c) Internal control of inverter.
• The first two methods require the use of peripheral
components whereas the third method requires no peripheral
components.
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•
• 5.5.1. External Control of ac Output Voltage:
• There are two possible methods of external control of ac
output voltage obtained from inverter output terminals.
• These methods are:
(a)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. 5.7. The voltage input to the
ac load is regulated through the firing angle control of ac
voltage controller. This method gives rise to higher
harmonic content in the output voltage; particularly when
the output voltage from the ac voltage controller is at low
level.
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Fig. 5.8. Series inverter control of two inverters.
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• 5.5.2. External Control of dc Input Voltage:
• In case the available voltage source is ac, then dc voltage
input to the inverter is controlled through a fully-controlled
rectifier, Fig.5.9(a) ; through an uncontrolled rectifier and a
chopper, Fig. 5.9 (b); or through an ac voltage controller and
an uncontrolled rectifier, Fig. 5.9(c).If available voltage is dc,
then dc voltage input to the inverter is controlled by means of
a chopper as shown in Fig.5.9 (d).
• Input voltage-control techniques shown in Fig. 5.9, in which
dc voltage input to inverter is controlled by means of
components external to the inverter, has the following main
advantage.
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Fig. 5.9. External control of dc input voltage to
inverter.
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• 5.5.3. 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.
• 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.
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• • The amplitude of the harmonics can be reduced by using
the pulse width modulation(PWM) technique.
• • The basic concept of the PWM method is the division of the
on-time into several on and off periods with varying duration.
• • The rms value of the ac voltage is controlled by the on-time
of the switches.
• • The most frequently used PWM technique is sinusoidal
pulse width modulation.
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• • The frequency of the reference sine wave determines the
frequency of the generated ac voltage and the amplitude of
the ac voltage can be regulated by the variation of the
reference signal amplitude.
• • The amplitude of the fundamental component of the ac
voltage is:
• Where m is the modulation index.
• Some different PWM techniques are:
• (a)Single-pulse modulation(Fig.5.12(a) .(b)Multiple-pulse
modulation (Fig5.12(b). (c) Sinusoidal-pulse modulation
(Fig.12(c) .
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(a) Single-pulse modulation (SPM)
(b) Multiple-pulse modulation (MPM)
(c )sinusoidal pulse width modulation (SPWM)
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• Example5.2. The single-phase half-bridge inverter has a
resistive load of R=2.4 Ω and thc dc input voltage is Vs
=48V. Determine (a) the rms output voltage at the
fundamental frequency V01.(b) the output power (c) the
average and peak currents of each transistor. (d) the peak
reverse blocking voltage and current of each transistor, (e)
the THD. (f) the HF and of the LOH. Repeat for bridge
inverter.
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• Example5.3. A single-phase half-bridge inverter has load R
=2Ω and dc source voltage Vs/2 =115V.
• (a) Sketch the waveforms for vo, load current i01 currents
through thyristor 1 and diode1and voltage across thyristor TI.
Harmonics other than fundamental component are
neglected.Indicate the devices that conduct during different
intervals of one cycle.
(a)Find the power delivered to load due to fundamental
current.
(b)Calculate THD and HF.
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