Instrumentation power electronics
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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.


T hree phase bridge rectifier1

Three Phase Bridge Rectifier


End of lectures 11 12

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End of Lectures-11-12


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