Transcript What is power electronics? - Dr. Imtiaz Hussain

Instrumentation
&
Power Electronics
Lecture 11 & 12
Introduction to Power Electronics
What is power electronics?
1) Definition
Power Electronics: is the electronics applied to conversion and
control of electric power.
Power
input
Electric
Power
Converter
Control
input
Power
output
What is power electronics?
A more exact explanation:
The primary task of power electronics is to process and control
the flow of electric energy by supplying voltages and currents in
a form that is optimally suited for user loads.
Power Electronic Devices
• The power Electronic devices provides the
utility of switching.
• The flow of power through these devices can
be controlled via small currents.
• Power electronics devices differ from ordinary
electronics devices in terms of their
characterittics.
Power Electronic Devices
• Power Semiconductor Devices can be
classified into three groups according to their
degree of controllability.
– Diodes (on and off controlled by power circuit)
– Thyristors (latched on by control signal but must
be turned off by power circuit)
– Controllable Switches (turned on and off by
control signal)
Diodes
• When the diode is forward biased it begins to conduct
with only a small voltage across it.
• When the diode is reversed biased only a negligibly
small leakage current flow through the device until the
reverse breakdown voltage is reached.
• In normal operation reverse bias voltage should not
reach the breakdown rating.
Diodes
• Following figure shows the i-v characteristics of
the diode.
Diodes
• In view of very small
current in blocking
state
and
small
voltage in conducting
state , the i-v
characteristics of the
diode
can
be
idealized.
Diodes
• At Turn on, the diode can be considered as an ideal switch.
It turns on rapidly compared to the transients in the power
circuit.
• However, at turn off, the diode current reverses for a
reverse recovery time as shown in following figure.
• The reverse recovery current can lead to overvoltage in
inductive circuits.
Types of Diodes
• Depending on the application requirement various
types of diodes are available.
– Schottky Diode
– Fast Recovery Diode
– Line Frequency Diode
Types of Diodes
– Schottky Diode
– These diodes are used where a low forward voltage drop
(typically 0.3 v) is needed.
– These diodes are limited in their blocking voltage
capabilities to 50v- 100v.
Types of Diodes
– Fast Recovery Diode
– These diodes are designed to be used in high frequency
circuits in combination with controllable switches where
a small reverse recovery time is needed.
– At power levels of several hundred volts and several
hundred amperes such diodes have trr rating of less than
few microseconds.
Types of Diodes
– Line Frequency Diode
– The on state of these diodes is designed to be as low as
possible.
– As a consequence they have large trr, which are
acceptable for line frequency applications.
Diode With RC Load
• Following Figure shows a diode with RC load.
• When switch S1 is closed at t=0, the charging current that
flows through the capacitor is found from
Diode With RC Load
• Following Figure shows a diode with RC load.
• When switch S1 is closed at t=0, the charging current that
flows through the capacitor is found from
Diode With RL Load
• Following Figure shows a diode with RL load.
• When switch S1 is closed at t=0, the current through the
inductor is increased
Diode With RL Load
• Following Figure shows a diode with RL load.
• When switch S1 is closed at t=0, the current through the
inductor is increased.
Diode With RL Load
• The waveform shows when t>>T, the voltage across
inductor tends to be zero and its current reaches
maximum value.
• If an attempt is made to
open S1 energy stored in
inductor (=0.5Li2) will be
transformed into high
reverse voltage across
diode and switch.
Example#1
• A diode circuit is shown in figure, with R=44Ω and C=0.1μF. The
capacitor has an initial voltage Vo=220 v. If S1 is closed at t=0
determine:
– Peak Diode Current
– Energy Dissipated in resistor
– Capacitor voltage at t=2 μs
Example#1
• A diode circuit is shown in figure, with R=44Ω and C=0.1μF. The
capacitor has an initial voltage Vo=220 v. If S1 is closed at t=0
determine:
– Peak Diode Current
Example#1
• A diode circuit is shown in figure, with R=44Ω and C=0.1μF. The
capacitor has an initial voltage Vo=220 v. If S1 is closed at t=0
determine:
– Energy Dissipated in resistor
– Capacitor voltage at t=2 μs
Freewheeling Diode
• If switch S1 is closed a current is established through the
load, and then, if the switch is open, a path must be
provided for the current in the inductive load.
• This is normally done by
connecting a diode Dm, called
a freewheeling diode.
Freewheeling Diode
• The circuit operation is divided into two modes.
• Mode 1 begins when the switched is closed.
• During this mode the current voltage relation is
Freewheeling Diode
• Mode 2 starts when the S1 is opened and the load current
starts to flow through Dm.
Freewheeling Diode
• The waveform of the entire operation is given below.
Line Frequency Diode Rectifier
• In most power Electronic systems, the power
input is in the form of a 50Hz or 60Hz sine wave
ac voltage.
• The general trend is to use inexpensive diode
rectifiers to convert ac into dc in an uncontrolled
manner.
Single Phase Half Wave Rectifier
• A single Phase half wave rectifier is the simplest
type and is not normally used in industrial
applications.
Single Phase Half Wave Rectifier
• Although output voltage is
D.C, it is discontinuous and
contains Harmonics.
Single Phase Full Wave Rectifier
• Each half of the transformer
with its associated acts as a
half wave rectifier.
Single Phase Full Wave Rectifier
• Instead of using centretapped transformer we
could use four diodes.
Three Phase Bridge Rectifier
• Three Phase bridge rectifier is very common in high power
applications.
• It can operate with or without transformer and give sixpulse ripple on the out.
Three Phase Bridge Rectifier
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END OF LECTURES-11-12