Ee 328 Lecture 1
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Transcript of Ee 328 Lecture 1
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Introduction
EE328 Power Electronics
Dr. Mutlu BOZTEPE
Department of Electrical and Electronics Engineering - Ege
University
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Course Book
“Power Electronics”
Daniel W. Hart
McGraw-Hill, 2011
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Power electronics
Power electronics circuits convert electric power
from one form to another using electronic devices.
Conversion is done using electronic switches,
capacitors, magnetics, and control systems
Applications of power electronics range from high-
power conversion equipment such as dc power
transmission to everyday appliances, such as
cordless screwdrivers, power supplies for
computers, cell phone chargers, and hybrid
automobiles.
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Scope of power electronics milliWattsgigaWatts
Power Level (Watts) Example System
0.1-10 •Battery operated equipment
10-100 •Satellite power systems
•Offline flyback power supply
100-1kW •Computer power supply
•Blender
1-10kW •Electronic welding machine
10-100 kW •Electric car
•Eddy current braking
100kW-1MW •Micro-SMES (Supeconducting Magnetic Energy Storage)
10MW-100MW •Magnetic aircraft lunch
•Big locomotives
•Power distribution
100MW-1GW •Power plant
>1GW •High Voltage DC Transmission (HVDC)
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Interdisciplinary nature of
power electronics
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Areas of applications
High frequency power
conversion
DC/DC, inverters
Low frequency power
conversion
Line rectifiers
Distributed power
systems
Power devices
Power transmission
HVDC
HVAC
Power quality
Power factor correction
Harmonic reduction
Passive filtering
Active filtering
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Some applications Heating and lighting control
Induction heating
Flourescent lamp ballast
Motor driver
Battery chargers
Electric vehicles, regenerative breaking
Switching power supplies
Uninterruptible power supplies (UPS)
Electric power transmission
Automotive electronics (Ignition, alternators)
Energy storage (Flywheel,SMES, super capacitor)
Power conditioning for alternative power sources: Solar
cells, Fuel cells, Wind turbines)
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Induction heating
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Maglev Train
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Electric car
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Renewable energy
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Conversion clasification
ac input/dc output (rectifier)
Full wave rectifier
dc input/ac output (inverter)
220VAC/50Hz inverter with battery input
dc input/dc output (converter)
Voltage regulator
ac input/ac output (converter)
Dimmer, speed control of induction machine
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A converter can operate as a rectifier or
an inverter, depending on the direction of
average power P.
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Multistep conversion
Power conversion can be a multistep process involving
more than one type of converter.
For example, an ac-dc-ac conversion can be used to
modify an ac source by first converting it to direct current
and then converting the dc signal to an ac signal that has
an amplitude and frequency different from those of the
original ac source
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Power electronics concept The purpose is to supply 3 V to a load resistance.
One simple solution is to use a voltage divider
Problem 1: the power absorbed by the 2RL resistor is
twice as much as delivered to the load and is lost as
heat, making the circuit only 33.3 percent efficient.
Problem 2: if the value of the load resistance changes,
the output voltage will change unless the 2RL
resistance changes proportionally.
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more desirable design solution:
Adding a switch which is opened and closed
perodically.
if the switch is closed for one-third of the period,
the average value of vx (denoted as Vx) is one-
third of the source voltage.
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Instantaneous power absorbed by the switch is the
product of voltage and current.
When the switch is open, power absorbed by it is
zero because the current in it is zero.
When the switch is closed, power absorbed by it is
zero because the voltage across it is zero.
Since power absorbed by the switch is zero for both
open and closed conditions, all power supplied by
the 9V source is delivered to RL, making the circuit
100% efficient.
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But the output is not pure dc!
However, the voltage waveform vx can be expressed as a
Fourier series
To create a 3-Vdc voltage, vx is applied to a low-pass filter.
An ideal low-pass filter allows the dc component of voltage
to pass through to the output while removing the ac terms,
thus creating the desired dc output.
If the filter is lossless, the converter will be 100 percent
efficient.
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In practice, the filter will have some losses and will
absorb some power.
Additionally, the electronic device used for the switch will
not be perfect and will have losses. However, the
efficiency of the converter can still be quite high (more
than 90 percent).
A feedback control system would detect if the output
voltage were not 3 V
and adjust the closing
and opening of the
switch accordingly
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Electronic Switches
Have two states: ON and OFF
Ideal switch: either switch current or switch
voltage is zero, making the power absorbed
by it is zero.
Real switches absorb some power.
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Diode Simplest electronic switch
Uncontrollable
on and off conditions are determined by voltages
and currents in the circuit.
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Reverse recovery An important dynamic characteristic of a nonideal diode
is reverse recovery current.
When a diode turns off, the current in it decreases and
momentarily becomes negative before becoming zero.
The time trr is the reverse recovery time, which is usually
less than 1 us.
Silicon carbide (SiC) and
schottky diodes have very little
reverse recovery effect.
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Mercury arc rectifier
Semiconductor rectifier
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Thyristor (SCR) and GTO
Controllable diode with three terminal
SCR (Silicon Controlled Rectifier)
GTO (Gate turnoff thyristor)
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Triacs and MCT
Triac: Two back-to-bact thyristor
MCT (Mos conttrolled thyristor)
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Thyristor (SCR), GTO, Triac
Thyratron
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Transistors
Unlike the diode, turn-on and turnoff of a
transistor are controllable.
Types:
MOSFET (Metal Oxide Semiconductor Field
Effect Transistor)
BJT (Bipolar Junction Transistor)
IGBT (Isolated Gated Bipolar Transistor)
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MOSFETS
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BJT
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IGBT
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Power MOSFET, IGBT
Vacuum tube
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Switch selection
The selection of a power device for a
particular application depends not only on the
required voltage and current levels but also
on its switching characteristics.
Transistors and GTOs provide control of
both turn-on and turnoff
SCRs of turn-on but not turnoff
and diodes of neither.
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Switch selection Switching speeds and the associated power losses are
very important in power electronics circuits.
The BJT is a minority carrier device have minority
carrier storage delays.
The MOSFET is a majority carrier device that does not
have minority carrier storage delays
Therefore, MOSFET has advantage in switching speeds.
Generally has lower switching losses and is preferred
over the BJT.
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Example 1-1
When S1 is opened, S2 must be close in order
to provide current path.
When S2 is opened, S1 is closed.
Switching
frequency
is 200 kHz
Select
switching
devices?
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The operating points are on the
positive i and v axes
S1 must turn off when
and must turn on when
So,the device used for S1 must
provide control of both turn-on
and turnoff.
BJTcharacteristic matches the
requirement.
But a MOSFET would be a good
choice because of the required
switching frequency, simple gate-
drive requirements, and relatively
low voltage and current
requirement (24 Vand 2 A).
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The operating points are on the
positive current axis and negative
voltage axis.
Therefore, a positive current in S2
is the requirement to turn S2 on,
and a negative voltage exists
when S2 must turn off.
This matches a diode operation
and no other control is needed for
the device
A diode is an appropriate choice
for S2.
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Switch implementation using a
MOSFET and a diode
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Switch implementation using
two MOSFET
(Synchronous rectification)
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Computer Simulation
Spice (developed at the university of California at
Berkeley)
Pspice (Commercially available version of spice)
OrCad capture - Cadence
PSIM - Powersim
Proteus – Labcenter
etc.