LECTURE 1 (Ch. 1)

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LECTURE 1 (Ch.
1)

INTRODUCTION

ECE 452

Power Electronics

1

Application of Power Electronics


In early days, control of the electric power was
achieved with electric machinery.



Power electronics have revolutionized the
concept of power control for power conversion
and for control of electrical motor drives.



Power electronics combine power, electronics,
and control.

2


Control deals with the steady
-
state and dynamic
characteristics of closed
-
loop system.



Power deals with static and rotating power
equipment.



Electronics deals with the solid
-
state devices and
circuits for signal processing to meet the desired
control objectives.

3


Therefore, power electronics is defined as the
applications of solid
-
state electronics for control
and conversion of electric power.



Power electronics is based on switching of the
power semiconductor devices.



It covers a variety of switching circuits.

4

History of Power Electronics


The history of power electronics began with
introduction of the mercury arc rectifiers in
1900.



Devices which were based on the mercury arc
valve technology were used until 1950.



The
first

electronic revolution began in 1948
with the invention of the silicon transistor at
Bell Labs.

5


Most of today's advanced electronic technologies
are based on the transistor concept.



The next breakthrough was invention of
Thyristor

(SCR) in 1956, which is a PNPN
triggering transistor.




The
second

revolution began in 1958 with
development of the commercial
thyristor

by GE.


6


That was the beginning of a new era of power
electronics.


7

Power Semiconductor Devices


Since the first
thyristor

was developed in 1957,
there have been tremendous advances in the
power semiconductor devices.



Until 1970, the conventional
thyristors

had been
exclusively used for power control applications.



Since 1970 many types of power semiconductor
devices were developed.

8

9

10

Control Characteristics


The power semiconductor devices can be
operated as switches by applying a control
signals to gate.

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12


Power semiconductor switching devices can be
classified on the basis of:



Uncontrolled turn on and off (diodes)



Controlled turn on and uncontrolled turn off (SCR)



Controlled turn on and off (BJT, MOSFET, GTO, IGBT)



Continuous gate signal requirement (BJT, MOSFET,
IGBT)

13


Pulse gate requirement (SCR, GTO)



Bipolar voltage
-
withstanding capability (SCR, GTO)



Unipolar

voltage withstanding capability (BJT,



MOSFET, GTO)



Bidirectional current capability (TRIAC)



Unidirectional current capability (SCR, GTO, BJT,
MOSFET, DIODE)

14

Characteristics and Specification
of Switches


There are many types of power switching
devices.




Each has its own advantages and disadvantages
for an application.

15

Ideal Switches


In the on
-
state: carry high forward current,


low forward voltage drop, and low resistance



In the off
-
state: withstand a high voltage, low


leakage current, and high resistance



During turn
-
on and turn
-
off process


instantaneously turn on and off

16


Low gate power for turn on and off



Controllable turn on and off



Turn on and off require a small pulse



High
dv
/
dt

&
di
/
dt



Low thermal impedance

17



Sustain any fault current (i
2
t)



Equal current sharing for parallel operation



Low price

18

Characteristics of Practical Devices


During the turn
-
on and turn
-
off process a
practical device requires:



a finite delay time




rise time




storage time



fall time

19

20

Types of Power Electronic CKTs


For control of electric power or power
conditioning, the conversion of electric power
from one form to another is necessary.




Switching characteristics of the power devices
permit this conversion.

21


Power electronics circuits can be classified into
six types:



Diode rectifiers



Ac
-
dc converters (controlled rectifier)



Ac
-
ac converters (ac voltage controllers)



Dc
-
dc converters (dc choppers)



Dc
-
ac converters (inverters)



Static switches

22

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25

Design of Power Electronics
Equipment


The design is divided into four parts:



Design of power circuits



Protection of power devices



Determination of control strategy



Design of logic and gating circuits

26


In the chapters that follow, we will describe
various types of power electronic circuits.



In analysis, the power devices are assumed to be
ideal switches.



The effect of circuit resistance and source
inductance is ignored.



Ignoring these parameters will simplify the design
steps, but it is very useful to understand operation
of the circuit and establish the control strategy.

27

Determining the RMS Value


The RMS value of current should be known for
determination of conduction losses and current
rating of the device.



The RMS value of a current waveform is:




Also:



T
rms
dt
i
T
I
0
2
1
2
)
(
2
)
2
(
2
)
1
(
2
...
n
rms
rms
rms
dc
rms
I
I
I
I
I




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29

Peripheral Effects


Operations of power converters are mainly
based on the switching of power semiconductor
devices.




As a result, converters introduce current and
voltage harmonics into the supply system and on
the output of the converters.



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These can cause problems of distortion of the
output voltage, harmonic generation into the
supply system, and interference with the
communication and signaling circuits.




Therefore, it is normally necessary to introduce
filters on the input and output of a converter
system to reduce the harmonic level.



31


The following figure shows the block diagram of
a generalized power converter.



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