Power Electronics

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Nov 24, 2013 (3 years and 8 months ago)

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POWER ELECTRONICS

ECE 105 Industrial
Electronics

Engr. Jeffrey T. Dellosa

College of Engineering and Information Technology

Caraga State University

Ampayon, Butuan City

1

Power Electronics


Introduction





Power electronics is the technology of converting electric
power from one form to another using power
semiconductor devices based circuitry.




It incorporates concepts from analog circuits, electronic
devices, control systems, power systems, magnetics, and
electric machines.

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Power Electronics

3




The converter enables either the following:




DC
-
DC: conversion




AC
-
DC: rectification




DC
-
AC: inversion




AC
-
AC:
cycloconversion

Power Electronics




In the power converter, the
power semiconductor devices

function as switches, which operate statically, that is,
without moving contacts.




The
time durations
, as well as the
turn ON

and
turn OFF
operations

of these switches are controlled in such a way
that an electrical power source at the input terminals of
the converter appears in a different form at its output
terminals.

4

Power Electronics




Here power converter high conversion efficiency


is
essential!







High efficiency leads to low power loss within converter.
Efficiency is a good measure of converter performance.



Hence, a goal of current converter technology is to
construct converters of small size and weight, which
process substantial power at high efficiency.



5

Power Electronics


Components used in power electronics circuitry are:



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Power Electronics


Rapid development of power semiconductor devices led to
significant improvement in,


Speed


Power capability


Efficiency


Hence increase the range of applications


DC Servo control


AC motor control


Sophisticated power supplies (switching
-
mode,
uninterruptible)


High power DC transmission

7

Power Electronics


Often power loss in power semiconductor device (when
viewed as an ideal switch) is based on the following:














Thus an ideal power semiconductor device is characterized by


zero resistance during ON
-
state
,
infinite resistance during OFF
-
state
,
zero transient time from ON to OFF

and vice
-
versa.



Practical power semiconductor device has limited voltage and
current handling capability, an ON
-
resistance greater than zero and
finite switching times.

8

Power Electronics


Power Electronics Devices





Power Bipolar Transistors (BJTs)


Power Metal Oxide Semiconductor Field Effect
Transistors (MOSFETs)


Insulated Gate Bipolar Transistors (IGBTs)


Thyristors


Gate Turn
-
Off Thyristors (GTOs)


Power Diodes

9

Power Electronics

10

Power Electronics

11

Power Electronics

12

Alternatively power semiconductor devices can be classified into 3
groups according to their degree of controllability.




Power Diodes
-

ON and OFF states controlled by the power cct.


Thyristors

-

Latched ON by a control signal but must be turned
OFF by the power cct.



Controllable Switches
-

Turned ON and OFF by control signals.


The controllable switches include

i)

BJTs

ii)

MOSFETs

iii)

Gate Turn
-
OFF Thyristors (GTOs)

iv)

Insulated Gate Bipolar Transistors (IGBTs)

Power Electronics


Power Diodes



13

The circuit symbol for the diode and its steady
state v
-
i characteristics are as shown.

Power Electronics


Power Diodes



14

Power Electronics


Thyristors



15

The circuit symbol for the thyristor and its steady state v
-
i characteristics
are as shown.

Power Electronics


Thyristors



16

In its
OFF state
, the thyristor can block a forward polarity voltage
and not conduct, as is shown by the off
-
state portion of the i
-
v
characteristic.

The
thyristor

can be triggered into the
ON state

by applying a pulse of
positive gate current for a short duration provided that the device is in
its forward
-
blocking state.


The resulting i
-
v relationship is shown by the ON state portion of the
characteristics shown.
The forward voltage drop in the ON state is only a
few volts (typically 1
-
3V depending on the device blocking voltage rating).


Power Electronics


Power BJTs


17

The circuit symbol for the BJTs and its steady state v
-
i
characteristics are as shown.

Power Electronics


Power BJTs


18

As shown in the i
-
v characteristics, a sufficiently large base current
results in the device being fully ON. This requires that the control
circuit to provide a base current that is sufficiently large so that

where h
FE

is the dc current gain of the device


FE
h
C
I
B
I

BJTs are current
-
controlled devices, and base current must be
supplied continuously to keep them in the ON state: The dc
current gain hFE is usually only 5
-
10 in high
-
power transistors.


BJTs are available in voltage ratings up to 1400V and current
ratings of a few hundred amperes.

Power Electronics


Power BJTs


19


BJT has been replaced by MOSFET in low
-
voltage (<500V)
applications



BJT is being replaced by IGBT in applications at voltages above
500V


Power Electronics


Power MOSFETs


+
-
i
D
Gate(G)
Source(S)
V
DS
V
GD
+
-
+
-
Drain(D)
V
GS
20

The circuit symbol for the MOSFETs and its steady state v
-
i
characteristics are as shown.

Power Electronics


Power MOSFETs


21


Power MOSFET is a voltage controlled device.




MOSFET requires the continuous application of a gate
-
source voltage
of appropriate magnitude in order to be in the ON state.


The switching times are very short, being in the range of a few tens of
nanoseconds to a few hundred nanoseconds depending on the device
type.

Power Electronics


Power MOSFETs


22

Power Electronics


IGBTs


23

The circuit symbol for the IGBTs and its steady state v
-
i characteristics
are as shown.

Power Electronics


IGBTs


24


The IGBT has some of the advantages of the MOSFET, & the BJT
combined.



Similar to the MOSFET, the IGBT has a high impedance Gate,
which requires only a small amount of energy to switch the device.



Like the BJT, the IGBT has a small ON
-
state voltage even in
devices with large blocking voltage ratings (for example, V
ON

is 2
-
3V
in a 1000
-
V device)..



Power Electronics


IGBTs


25

Power Electronics


Several Applications of Power Electronics



26

A laptop computer power supply system

.

Power Electronics


Several Applications of Power Electronics



27

An electric vehicle power and drive system
.

Power Electronics


Transient Protection of Power Devices

dt
dv
28

Snubber circuit limits


as well as voltage and peak current in a switching device to safe
specified limits!

dt
dv
,

dt
di



Switching device’s


rating is significant during the switching device (e.g. thyristor) turn
-
OFF process. Voltage can increase very rapidly to high levels. If the
rate rise is excessive, it may cause damage to the device.

Power Electronics


Transient Protection of Power Devices

29

Power Electronics


Transient Protection of Power Devices

30

Power Electronics


Transient Protection of Power Devices

31

Power Electronics


Transient Protection of Power Devices

32

Power Electronics


Transient Protection of Power Devices

33

Power Electronics


Transient Protection of Power Devices

34

Power Electronics


Power and Harmonics in Non
-
sinusoidal
Systems

35

Non
-
sinusoidal waveforms are waveforms that are not sine waves.


,




Non
-
sinusoidal waveforms can be described as being made of
harmonics

(multiple sine waves of different frequencies).


Thus for a waveform whose fundamental frequency is

, than second
harmonic has a frequency 2


and so on
.

Waveforms occurring at frequencies of 2

, 4

, 6

, … are called even
harmonics;

Those occurring at frequencies of 3

, 5

, 7

, ... are called odd
harmonics.

Power Electronics


Power and Harmonics in Non
-
sinusoidal
Systems

36

Thus for the circuit shown (a
non
-
sinusoidal system
),

expressing the
circuit’s voltage and current as Fourier series:

,




Power Electronics


Power and Harmonics in Non
-
sinusoidal
Systems

37

,




Power Electronics


Power and Harmonics in Non
-
sinusoidal
Systems

38

,




Expression for average power becomes

So power is transmitted to the load only when the Fourier series of v(t)
and i(t) contain terms at the same frequency.

Eg. if the
voltage

&
current

both contain
3rd harmonic
, then they lead to
the average power

Power Electronics


Power and Harmonics in Non
-
sinusoidal
Systems

39

,




With the rms voltage defined as

Inserting Fourier series into the above, an expression of rms voltage
for non
-
sinusoidal voltage waveform

Notice harmonics always increase rms value & increased in rms values


increased
losses!

Power Electronics


Power and Harmonics in Non
-
sinusoidal
Systems

40

,




Notice harmonics always increase rms value & increased in rms values


increased losses!

For efficient transmission of energy from a
source to a load, it is desired to maximize
average power, while minimizing rms current
and voltage (and hence minimizing losses).


Power factor is a figure of merit that measures
how efficiently energy is transmitted. It is
defined as

Power Electronics


Basic Magnetics

41

,




Inductance (measured in Henry) is an effect which results from the
magnetic field that forms around a current carrying conductor.


Inductance can be increased by looping the conductor into a coil which creates a
larger magnetic field.

An inductor is usually constructed as a coil of copper wire, wrapped
around a core either of air or of ferrous material.

Core materials with a higher permeability than air confine the magnetic field closely to
the inductor, thereby increasing the inductance.

Inductors come in many shapes. Most are constructed as enamel coated wire wrapped
around a ferrite bobbin with wire exposed on the outside, while some enclose the wire
completely in ferrite and are called "shielded".


Some inductors have an adjustable core, which enables changing of the inductance.
Small inductors can be etched directly onto a printed circuit board by laying out the
trace in a spiral pattern.

Power Electronics


Basic Magnetics

42

,




Current flowing through an inductor creates a magnetic field which
has an associated electromotive force (
emf
).

This inductor’s

emf

opposes the change in applied voltage.

The resulting current in the inductor resists the change but does rise!



An inductor resists changes in current.



An ideal inductor would offer no resistance to direct current; however, all real
-
world
inductors have non
-
zero electrical resistance.

In general, the relationship between v(t) across an inductor with
inductance L and i(t) passing through it is described by the
differential equation:

The inductor is used as the energy storage device in power electronics circuitries.


Power Electronics


Basic Magnetics

43

Transformers

--

widely used in low
-
power electronic ccts for voltage step
-
up or step
-
down, & for isolating DC from two ccts while maintaining ac continuity.


--

consists of 2 windings linked by a mutual magnetic field. When one
winding, the primary has an ac voltage applied to it, a varying flux is
developed; the amplitude of the flux is dependent on the applied voltage
and number of turns in the winding.


Mutual flux linked to the secondary winding induces a voltage whose
amplitude depends on the number of turns in the secondary winding.


Power Electronics


Basic Magnetics

44

Mutual magnetic flux coupling is accomplished simply with an air core

but considerably more effective flux linkage is obtained with the use of
a core of iron or ferromagnetic material with higher permeability than
air.

The relationship between
voltage, current, & impedance

between the
primary & secondary windings of the transformer may be calculated
using the following relationships.

Power Electronics


Basic Magnetics

45

The basic phase relationship between the signals at the transformer
input & output ports may be
in
-
phase
, or
out
-
of
-
phase
. Conventionally,
the ports that are in
-
phase 1, and 3, are marked by dot notation as
shown.

Power Electronics


Basic Magnetics

46

EXAMPLE