Instrumentation &

Power Electronics

Lecture 11 & 12Introduction to Power Electronics

What is power electronics?

1) Definition

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

Electric Power Converter

Power output

Power input

Control input

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 centre-

tapped 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 six-

pulse ripple on the out.

Three Phase Bridge Rectifier

END OF LECTURES-11-12

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