Transcript power electronics

POWER ELECTRONICS
EE‐312
Engr.Talha Ahmed Khan
Introduction to Power Electronics
• Power Electronics = Power + Control + Electronics
• Control deals with the steady state and dynamic
characteristics of closed loop systems.
• Power deals with the static and rotating power
equipment for generation, transmission and
distribution of electrical energy.
• Electronics deals with the solid state devices and
circuits for signal processing to meet the desired
control objectives.
• A system in a steady state has numerous
properties that are unchanging in time.
• For example: The flow of fluid through a tube, or
electricity through a network, could be in a
steady state because there is a constant flow of
fluid, or electricity. Conversely, a tank which is
being drained or filled with fluid would be an
example of a system in transient state, because
the volume of fluid contained in it changes with
time.
• Dynamic characteristics tell us about how well
a sensor responds to changes in its input. For
dynamic signals, the sensor or the
measurement system must be able to respond
fast enough to keep up with the input signals.
• Solid-state electronics are those circuits or
devices built entirely from solid materials and
in which the electrons, or other charge carriers,
are confined entirely within the solid material.
Common solid-state devices
include transistors, microprocessor chips,
and DRAM.
• Power Electronics Def: The applications of solid
state electronics for the control and conversion of
electric power.
• – Primarily based on the switching of power
semiconductor devices.
• Modern power Electronics uses – Power
semiconductors that can be regarded as the
muscle
• – Microelectronics that have the power and
intelligence of brain
• Power Electronics have revolutionized the
concept of power control for power conversion
and for control of electrical motor drives.
• • Applications:
• – Heat controls
• – Light controls
• – Motor controls
• – Power supplies
• – And many more … (Table 1.1)
• • History Milestones:
• – 1948 Invention of Transistors
• – 1956 Invention of SCR (PNPN) or Thyristor
• – 1958 Commercialization of SCR
• Power Electronics Revolution is giving us the
ability to shape and control large amounts of
power with ever increasing efficiency.
• 1.2 Power Semiconductor Devices
• Till 1970s, Thyristor(SCR) was the major
device used for power control in industrial
appliances.
• After 1970s, a lot of power semiconductor
devices were developed.
• Major classification:
• – Power Diodes
• – Power Transistors
• – Thyristors
Power Semiconductor Diodes
• Power diode plays important role in power electronics
circuits for conversion of electric power.
• Diode acts as a switch to perform various functions
• – Switches in rectifiers
• – Free wheeling in switching regulators
• – Charge reversal of capacitors
• – Energy transfer between components
• – Voltage isolation
• – Energy feedback from load to power source
• A flyback diode (sometimes called a snubber
diode, freewheeling diode, suppressor diode,
or catch diode) is a diode used to eliminate
flyback, which is the sudden voltage spike
seen across an inductive load when its supply
voltage is suddenly reduced or removed.
Charge Reversal
• When the polarity of the battery is such that
electrons are allowed to flow through the
diode, the diode is said to be forward-biased.
Conversely, when the battery is “backward”
and the diode blocks current, the diode is said
to be reverse-biased. A diode may be thought
of as like a switch: “closed” when forwardbiased and “open” when reverse-biased.
• Power diodes have
• – similar V‐I characteristics as ordinary signal
diode
• – More power, voltage and current handling
capabilities
• The Static induction thyristor (SIT, SITh) is a
thyristor with a buried gate structure in which the
gate electrodes are placed in n-base region. Since
they are normally on-state, gate electrodes must
be negatively biased to hold off-state.
• A gate turn-off thyristor (GTO) is a special type of
thyristor, a high-power semiconductor device.
GTOs, as opposed to normal thyristors, are fully
controllable switches which can be turned on and
off by their third lead, the GATE lead.
N = 2 for Si diode,n=2 for Ge.
Power Transistors
• – Power Electronics deals mostly with the
transistor operation as a switch
• – Controlled‐on & Controlled‐off characteristics
• – For Switching, Transistor is operated in
saturation region ↓ on‐state Voltage drop
• – Can be operated at much speeds as compared
to thyristors.But lower power ratings
• – Used in dc‐dc and dc‐ac convertors
Quiz 1
1.Define the following.
• Fast recovery diodes
• Schottky Diode
• Silicon Carbide Diodes
2.Classify the power transistors.
THYRISTOR
• A Thyristor (silicon controlled rectifier or SCR)
is a little like a transistor. When a small current
flows into the GATE (G), this allows a larger
current to flow from the ANODE (A) to the
CATHODE (C). Even when the current into the
gate stops the thyristor continues to allow
current to flow from anode to cathode.
Function of the Gate Terminal
• The thyristor has three p-n junctions (serially
named J1, J2, J3 from the anode).
• When the anode is at a positive potential VAK with
respect to the cathode with no voltage applied at
the gate, junctions J1 and J3 are forward biased,
while junction J2 is reverse biased. As J2 is reverse
biased, no conduction takes place (Off state).
Now if VAK is increased beyond the breakdown
voltage VBO of the thyristor,avalanche
breakdown of J2 takes place and the thyristor
starts conducting (On state).
• If a positive potential VG is applied at the gate
terminal with respect to the cathode, the
breakdown of the junction J2 occurs at a lower
value of VAK. By selecting an appropriate value
of VG, the thyristor can be switched into the
on state suddenly.
Thyristor Turn-on