2. Text Book
“Power Electronics”, by Daniel Hart; McGraw-Hill
Int’l Edition
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3. Power Electronics
Electrical Power Processing is
multidisciplinary and deals with the
conversion and processing of
electrical power using power
electronics.
Power electronics circuits convert
electric power from one form to
another using electronic devices.
Conversion is done using electronic
switches, passive energy storage
components (capacitors &
inductors), and control systems.
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4. Power Electronics
Applications of power electronics range from high-power
conversion equipment to everyday appliances, i.e.
Heating and lighting control
Induction heating
Fluorescent 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)
etc..
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5. Converter Classification
The objective of a power electronics circuit is to
match the voltage and current requirements of the
load to those of the source.
Power electronics circuits convert one type or level of
a voltage or current waveform to another and are
hence called converters.
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6. Converter Classification
ac input/dc output – rectifier
-Half-wave rectifier, full-wave rectifier, etc.
dc input/ac output – inverter
-Half-bridge inverter, full-bridge inverter, multilevel
inverter, etc.
dc input/dc output - dc-dc converter
-Buck, boost, buck-boost, etc.
ac input/ac output - ac-ac converter
-Matrix converter, cycloconverter, etc.
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7. 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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8. Power Electronics Concepts
Example: to supply 3 V to a load resistance from a 9V
battery.
One simple solution is to use a voltage divider.
Drawbacks:
Poor efficiency - power absorbed by the 2RL resistor is
twice as much as delivered to the load.
Poor regulation - if the value of the load resistance
changes, the output voltage will change.
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9. Power Electronics Concepts
Alternative solution:
Adding a switch which is opened and closed
periodically.
If the switch is closed for one-third of the period, the
average output voltage is one-third of the source
voltage.
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10. Power Electronics Concepts
Advantages of alternative solution:
Ideal switch consume no power and hence all power
supplied by battery is delivered to load making it 100
percent efficient.
when switch is closed, voltage is zero.
when switch is open, current is zero.
Output voltage can be regulated by controlling duty
cycle.
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11. Power Electronics Concepts
Drawback of alternative solution:
The output is not a pure dc voltage. The voltage
waveform containing a dc term (the average value)
plus sinusoidal terms at frequencies that are
multiples of the pulse frequency (harmonics).
To create a 3-V dc voltage, vx is applied to a low-pass
filter.
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12. Power Electronics Concepts
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%
efficient.
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%).
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13. Power Electronics Concepts
A feedback control system would detect if the output
voltage were not 3 V and adjust the closing and
opening of the switch accordingly.
Controller
Inductor and
capacitor
Electronic
switch
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14. Electronic Switches
Two states:
on - ideally being a short circuit with voltage is
zero
off - ideally being an open circuit with current is
zero.
Real switches absorb some power.
Diode
Thyristor
Transistor
on
off
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15. Diode
Uncontrollable
on and off conditions are determined by voltages and
currents in the circuit
forward-biased (on) when the current id is positive
and reverse biased (off) when vd is negative.
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16. Diode
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.
This phenomenon may become important in high-
frequency applications.
Silicon carbide (SiC) and schottky diodes
have very little reverse recovery,
resulting in more efficient circuits,
especially in high-frequency
high-power applications.
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17. Thyristor
Family of three-terminal devices - Controllable
diodes:
silicon-controlled rectifier (SCR)
Triac
gate turn-off thyristor (GTO)
MOS-controlled thyristor (MCT)
Thyristors are capable of large currents and large
blocking voltages for use in high-power applications,
but switching frequencies cannot be as high as when
using other devices such as MOSFETs.
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18. Silicon-Controlled Rectifier (SCR)
For the SCR to begin to conduct, it must have a gate
current applied while it has a positive anode-to-
cathode voltage.
After conduction is established, the gate signal is no
longer required to maintain anode current.
The SCR will continue to conduct as long as
the anode current remains
positive and above
minimum value called the
holding level.
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19. Gate Turn-off Thyristor (GTO)
Like the SCR, is turned on by a short-duration gate
current if the anode-to-cathode voltage is positive.
GTO can be turned off with a negative gate current.
Suitable for some applications where control of both
turn-on and turnoff of a switch is required.
The negative gate turnoff current can be of brief
duration (a few microseconds),
but its magnitude must be
very large compared to the
turn-on current.
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20. Triac
The triac is functionally equivalent to two
antiparallel SCRs (in parallel but in
opposite directions).
is a thyristor that is capable of conducting
current in either direction.
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21. MOS-Controlled Thyristor (MCT)
Functionally equivalent to the GTO but without the
high turn-off gate current requirement.
The MCT has an SCR and two MOSFETs integrated
into one device. One MOSFET turns the SCR on, and
one MOSFET turns the SCR off.
The MCT is turned on and off by establishing the
proper voltage from gate to
cathode, as opposed to
establishing a gate current
in the GTO.
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22. Transistor
Transistors are operated as switches in power
electronics circuits.
Drive circuits are designed to have the transistor either
in the fully on or fully off state.
Types of transistors:
MOSFET (Metal Oxide Semiconductor Field Effect
Transistor)
BJT (Bipolar Junction Transistor)
IGBT (Insulated Gate Bipolar Transistor)
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23. MOSFET
Voltage-controlled device.
A sufficiently large gate-to-source voltage will turn the
device on.
Have on-state resistances as low as a few milliohms.
Fast switching transition and used in converters
operating into the megahertz range.
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24. BJT
Current-controlled device.
On state is achieved by providing
sufficient base current to drive the
BJT into saturation.
Zero base current results in an off
transistor.
To carry high collector current, high
base current is needed to drive BJT
into saturation.
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25. Insulated Gate Bipolar Transistor (IGBT)
Integrated connection of a
MOSFET and a BJT.
Voltage-controller device.
Drive circuit for the IGBT is
like that of the MOSFET,
while the on-state
characteristics are like those
of the BJT.
IGBTs have replaced BJTs in
many applications.
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26. Emerging Transistors
Silicon-Carbide (SiC) power MOSFET
Enhancement-mode Gallium Nitride (GaN) power
FETs
Improve power efficiency and increase power density.
Characteristics:
Fast switching – minimized switching loss
Low reverse recovery – minimized switching loss
Low Rds(on) – minimized conduction loss
Low Qg – minimized gate power consumption
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27. Switch Selection
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Depends on :
voltage level
current level
switching
characteristics
(operating point,
turn-on & turnoff
characteristics)
28. Computer Simulation
OrCAD Pspice (used in the text book)
Demo Ver. :
http://www.cadence.com/products/orcad/pages/do
wnloads.aspx#demo
PSIM
Matlab Simulink
PLECS
SABER
Proteus
etc..
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