POWER ELECTRONICS INTRODUCTION
ECEN405
Definition
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Power electronics refers to control and conversion of
electrical power by power semiconductor devices
wherein these devices operate as switches.
•Advent of silicon-controlled rectifiers, abbreviated as
SCRs, led to the development of a new area of
application called the power electronics.
•Prior to the introduction of SCRs, mercury-arc rectifiers
were used for controlling electrical power, but such
rectifier circuits were a part of industrial electronics and
the scope for applications of mercury-arc rectifiers was
limited.
•Once the SCRs were available, the application area
spread to many fields such as drives, power supplies,
aviation electronics, high frequency inverters and power
electronics originated.
•
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Power Electronics
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Power electronics has applications that span the whole
field of electrical power systems, with the power range
of these applications extending from a few VA/Watts
to several MVA / MW.
The main task of power electronics is to control and
convert electrical power from one form to another. The
four main forms of conversion are:
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
Rectification referring to conversion of ac voltage to dc
voltage (Rectifier),
DC-to-AC conversion (Inverter),
DC-to DC conversion (Chopper/SMPS/Joule Thief) and
AC-to-AC conversion (Cyclo-converter).
Power Processor as a Combination of
Converters
• Most practical topologies require an energy
storage element, which also decouples the input
and the output side converters
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AC Motor Drive
• Converter 1 rectifies line-frequency ac into dc
• Capacitor acts as a filter; stores energy; decouples
• Converter 2 synthesizes low-frequency ac to motor
• Polarity of dc-bus voltage remains unchanged
– ideally suited for transistors of converter 2
Rectification
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Rectifiers can be classified as uncontrolled and controlled rectifiers, and the
controlled rectifiers can be further divided into semi-controlled and fullycontrolled rectifiers. Uncontrolled rectifier circuits are built with diodes, and
fully-controlled rectifier circuits are built with SCRs. Both diodes and SCRs are
used in semi-controlled rectifier circuits.
There are several rectifier circuit configurations. The popular rectifier
configurations are listed below.

Single-phase semi-controlled bridge rectifier,

Single-phase fully-controlled bridge rectifier,
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Three-phase three-pulse, star-connected rectifier,
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Double three-phase, three-pulse star-connected rectifiers with inter-phase
transformer (IPT),
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Three-phase semi-controlled bridge rectifier,

Three-phase fully-controlled bridge rectifier and

Double three-phase fully-controlled bridge rectifiers with IPT.
Apart from the configurations listed above, there are series-connected and
12-pulse rectifiers for delivering high power output.
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Power rating of Rectifiers
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Power rating of a single-phase rectifier tends to be lower than 10
kW.
Three-phase bridge rectifiers are used for delivering higher power
output, up to 500 kW at 500 V dc or even more.
For low voltage, high current applications, a pair of three-phase,
three-pulse rectifiers interconnected by an inter-phase
transformer(IPT) is used. For a high current output, rectifiers with IPT
are preferred to connecting devices directly in parallel.
There are many applications for rectifiers. Some of them are:



Variable speed dc drives,
Battery chargers,
DC power supplies and Power supply for a specific application like
electroplating
DC-TO-AC CONVERSION
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The converter that changes a dc voltage to an alternating voltage is called an
inverter. Earlier inverters were built with SCRs.
Since the circuitry required to turn the SCR off tends to be complex, other
power semiconductor devices such as bipolar junction transistors, power
MOSFETs, insulated gate bipolar transistors (IGBT) and MOS-controlled
thyristors (MCTs) are used nowadays.
Currently only the inverters with a high power rating, such as 500 kW or
higher, are likely to be built with either SCRs or gate turn-off thyristors(GTOs).
There are many inverter circuits and the techniques for controlling an inverter
vary in complexity.
Some of the applications of an inverter are listed below:

Emergency lighting systems,

AC variable speed drives,
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Uninterrupted power supplies, and

Frequency converters.
Wikipedia
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DC-TO-DC CONVERSION
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When the SCR came into use, a dc-to-dc converter circuit was called a
chopper. Nowadays, an SCR is rarely used in a dc-to-dc converter. Either a
power BJT or a power MOSFET is normally used in such a converter and this
converter is called a switch-mode power supply (SMPS). A switch-mode power
supply can be of one of the types listed below:

Step-down switch-mode power supply,
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Step-up chopper,
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Fly-back converter and
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Resonant converter.
The typical applications for a switch-mode power supply or a chopper are:
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DC drive

Battery charger and

DC power supply.
AC-TO-AC CONVERSION
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A cycloconverter or a cycloinverter converts an ac voltage, such as the mains
supply, to another ac voltage.
The amplitude and the frequency of input voltage to a cycloconverter tend to
be fixed values, whereas both the amplitude and the frequency of output
voltage of a cycloconverter tend to be variable. On the other hand, the circuit
that converts an ac voltage to another ac voltage at the same frequency is
known as an ac-chopper.
A typical application of a cycloconverter is to use it for controlling the speed
of an ac traction motor and most of these cycloconverters have a high power
output, of the order a few megawatts and SCRs are used in these circuits.
In contrast, low cost, low power cycloconverters for low power ac motors are
also in use and many of these circuit tend to use triacs in place of SCRs. Unlike
an SCR which conducts in only one direction, a triac is capable of conducting in
either direction and like an SCR, it is also a three terminal device.
It may be noted that the use of a cycloconverter is not as common as that of
an inverter and a cycloinverter is rarely used.

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Power Processing - Summary
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Control is Invariably Required
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High Efficiency is Required
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Power Conversion choices
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Ideal Switch
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Selecting a converter
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Option 1
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Using SPDT (Single Pole Double Throw) Switch
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Addition of Low Pass Filter
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Addition of Control System for regulation of output voltage
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So..
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What does Power Electronics offer?
Boost Converter
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Output DC voltage greater
than its input DC voltage
Since Power is conserved, the
output current is lower than
input
Also known as stepup
converter or Joule thief
Without Boost, Toyota Prius
needs 418 cells (168) –
boosts 202V to 500V
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Single Phase Inverter
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Linear Power Supply
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Transformer used for
electrical isolation and
step down
Rectifier converts AC –
DC
Filter capacitor
All ok but Transformer is
normally large and
heavy
Low efficiency
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Switch-Mode Power Supply
• Transistor as a switch
• High Efficiency
• High-Frequency Transformer
Basic Principle of Switch-Mode Synthesis
• Constant switching frequency
• pulse width controls the average
• L-C filters the ripple
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Switching Classification
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Line Frequency (naturally commutated) converters,
where the utility line voltages present at one side
are used to turn off and turn on the switches
Switching (forced commutated) converters, where the
switching frequency is higher to the line voltage. In
this converter, if input appears as voltage source,
then the output will be current source or vice versa
Resonant or quasi-resonant converters, where
controllable switches turn on or off at zero voltage
or zero current
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
Applications?
Source- Inst. Of Industrial Productivity 2012
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Application in Adjustable Speed Drives
•
•
Conventional drive wastes energy across the throttling
valve to adjust flow rate
Using power electronics, motor-pump speed is adjusted
efficiently to deliver the required flow rate
Scope and Applications
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Interdisciplinary Nature of Power Electronics
Concept Quiz #1
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In wind turbines, the role of power electronics is to
transfer power from the generator, that produces
voltages at variable amplitudes and variable
frequencies, to the utility-grid with voltages at a
constant amplitude and a constant frequency.
A.
False
B.
True
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Concept Quiz #2
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Compact Fluorescent Lamps (CFL) require power
electronics to convert the line-frequency ac voltage
into dc voltage that is applied to the lamp.
A.
True
B.
False
Concept Quiz #3
A power supply draws power from the utility input and is
designed to be 90% efficient. It is designed to supply a
maximum output power of 1kW, beyond which its
components are overloaded.
If the energy efficiency of the power electronic converter
within this power supply is improved to be 95% ,
calculate the ratio of the power-loss in this supply at this
efficiency, as a ratio of that at the previous efficiency.
A.
B.
C.
~0.947
~0.474
0.5
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Clicker Question
A power supply draws power from the utility input and is designed to
be 90% efficient. The maximum output power it can supply to a
load, without overheating, is 1kW.
In the same package size, if the energy efficiency of the power
electronic converter within this power supply is improved to be 95%,
what is the maximum output power that it can be loaded to?
Since the size of the package in both cases is the same, assume that the
maximum allowed power dissipation within this power supply, without
overheating it, remains unchanged.
A.
B.
C.
~2.1 kW
~1.06 kW
~1.56 kW
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