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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 . What is power electronics?. A more exact explanation :

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instrumentation power electronics

Instrumentation & Power Electronics

Lecture 11 & 12

Introduction to Power Electronics

what is power electronics
What is power electronics?

1) Definition

Power Electronics:is the electronics applied to conversion and control of electric power.

what is power electronics1
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
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 devices1
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
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.
diodes1
Diodes
  • Following figure shows the i-v characteristics of the diode.
diodes2
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.
diodes3
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
Types of Diodes
  • Depending on the application requirement various types of diodes are available.
    • Schottky Diode
    • Fast Recovery Diode
    • Line Frequency Diode
types of diodes1
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 diodes2
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 diodes3
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
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 load1
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
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 load1
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 load2
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 openS1 energy stored in inductor (=0.5Li2) will be transformed into high reverse voltage across diode and switch.
example 1
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=2μs
example 11
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 12
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=2μs
freewheeling diode
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 diode1
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 diode2
Freewheeling Diode
  • Mode 2 starts when the S1 is opened and the load current starts to flow through Dm.
freewheeling diode3
Freewheeling Diode
  • The waveform of the entire operation is given below.
line frequency diode rectifier
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
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 rectifier1
Single Phase Half Wave Rectifier
  • Although output voltage is D.C, it is discontinuous and contains Harmonics.
single phase full wave rectifier
Single Phase Full Wave Rectifier
  • Each half of the transformer with its associated acts as a half wave rectifier.
single phase full wave rectifier1
Single Phase Full Wave Rectifier
  • Instead of using centre-tapped transformer we could use four diodes.
t hree phase bridge rectifier
Three Phase Bridge Rectifier
  • Three Phase bridge rectifier is very common in high power applications.
  • It can operate with or without transformer and give six-pulse ripple on the out.
end of lectures 11 12
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End of Lectures-11-12
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