Introduction to Power Electronics lecture1

17/03/2014

Dr. Kamal Ramadan; University of Misruta 1

Misruta University Faculty of Engineering

Electrical & Electronic Engineering Dept. M.Sc. Program

EE627 Advanced Power

Electronics Lecture 1

General Introduction

17/03/2014 Dr. Kamal Ramadan; Misruta University,

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Syllabus: 1. Converters:

3-phase full-wave controlled bridge converter operation, load side, supply side quantities, i/p power factor, shunt capacitor compensation, inverter mode operation-single phase rectifiers with motor load

1. Fully controlled rectifier drives Operation, 3-phase rectifiers with

freewheeling and regeneration.


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3. Inverters:

3-phase step-wave

5. Inverters circuits:

Skelton inverter circuit, operation, modes of operation, analysis, and measurement of harmonic distortion

6. 3-Phase pulse width modulation

7. Controlled Inverter Circuits

Sinusoidal pulse width modulation, pwm voltage waveforms applied to balanced 3-phase resistive & inductive load.

Syllabus cont.:


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8. DC choppers Drives:

Class A chopper, class B chopper, class C two quadrant operation chopper, class D chopper, class E four quadrant chopper.

9. Cycloconverters:

single phase, phase-controlled dual cycloconverter

10 Power Electronics applications:

Facts devices, SVC, Tcsc, Multilevel inverters, switched mode power supplies.

Syllabus cont.:


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Percentage Component

Evaluation

60 Final exam

Course Works

40

15 Midterm 1

15 Midterm 2

10 Assignments


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Grading

Instructor: Dr. Kamal Ramadan Email: [email protected] Office hours: Mon 3:00 - 4:00 pm Textbook: Erickson and Maksimovic, Fundamentals of Power Electronics, second edition, Springer, ISBN 0-7923-7270-0. Denis Fewson; Introduction to

Power Electronics; London 9 Sydney ~ Auckland; Co-published in the USA by Oxford University Press, Inc., New York


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mailto:[email protected]



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INTRODUCTION TO POWER ELECTRONICS

SYSTEMS

1. Definition of Power Electronics

DEFINITION:

To convert, i.e to process and control the flow of

electric power by supplying voltages and currents

in a form that is optimally suited for user loads.

1.1 Introduction to Power Processing

DC-DC conversion: Change and control voltage magnitude

AC-DC rectification: Possibly control dc voltage, ac current

DC-AC inversion: Produce sinusoid of controllable

magnitude and frequency

AC-AC cycloconversion: Change and control voltage

magnitude and frequency 17/03/2014

Dr. Kamal Ramadan; Misruta University, 2014

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Control is invariably required


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High efficiency is essential

• High efficiency leads to low power loss within converter

• Small size and reliable operation is then feasible

• Efficiency is a good measure of converter performance


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Rating Vs Applications


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Basic block diagram

Building Blocks: – Input Power, Output Power – Power Processor – Controller 17/03/2014


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2. Power Electronics (PE) Systems Goals: To convert electrical energy from one form to another,(i.e. from the source to load) with:

– highest efficiency, – highest availability – highest reliability – lowest cost, – smallest size – least weight.


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3. Static applications Involves non-rotating or moving mechanical components.

Examples: DC Power supply, Un-interruptible power supply, Power generation and transmission (HVDC), Electroplating, Welding, Heating, Cooling, Electronic ballast


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4. Drive applications Intimately contains moving or rotating components such as motors.

Examples: Electric trains, Electric vehicles, Air-conditioning System, Pumps, Compressor, Conveyer Belt (Factory automation).


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Application examples Static Application: DC Power Supply


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Drive Application: Air-Conditioning System


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5. Power Electronics Converters

AC to DC: RECTIFIER

DC to DC: CHOPPER

DC to AC: INVERTER


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6. Current PE issues a. Energy scenario Need to reduce dependence on fossil fuel: – coal, natural gas, oil, and nuclear power resource. Depletion of these sources is expected. Tap renewable energy resources: – solar, wind, fuel-cell, ocean-wave Energy saving by PE applications, examples: – Variable speed compressor air-conditioning system: 30% savings compared to thermostat-controlled system. 17/03/2014


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– Lighting using electronics ballast boost efficiency of fluorescent lamp by 20%.

b. Environment issues Nuclear safety. – Nuclear plants remain radioactive for thousands of years. Burning of fossil fuel – emits gases such as CO2, CO (oil burning), SO2, NOX (coal burning) etc.


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– Creates global warming (green house effect), acid rain and urban pollution from smokes. Possible Solutions by application of PE. Examples: – Renewable energy resources. – Centralization of power stations to remote non-urban area. (mitigation). – Electric vehicles.


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7. PE growth PE rapid growth due to: – Advances in power (semiconductor) switches – Advances in microelectronics (DSP, VLSI, microprocessor/microcontroller) – New ideas in control algorithms – Demand for new applications PE is an interdisciplinary field: – Digital/analogue electronics – Power and energy – Microelectronics


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– Control system – Computer, simulation and software – Solid-state physics and devices – Packaging – Heat transfer

8. Power semiconductor devices (Power switches):

Power switches: work-horses of PE systems.


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Operates in two states: – Fully on. i.e. switch closed. Conducting state – Fully off , i.e. switch opened. Blocking state Power switch never operates in linear mode.

Can be categorised into three groups: 1. Uncontrolled: Diode 2. Semi-controlled: Thyristor (SCR). 3. Fully controlled: Power transistors: e.g. BJT, MOSFET, IGBT, GTO, IGCT


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Photos of Power Switches


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9. Power Diode

When diode is forward biased, it conducts current

with a small forward voltage (Vf) across it (0.2-3V)

When reversed (or blocking state), a negligibly

small leakage current (A to mA) flows until the

reverse breakdown occurs.

Diode should not be operated at reverse voltage

greater than Vr 17/03/2014


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Reverse Recovery

When a diode is switched quickly from forward to reverse bias, it continues to conduct due to the minority carriers which remains in the p-n junction.


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When reversed (or blocking state), a negligibly

small leakage current (A to mA) flows until the

reverse breakdown occurs.

Diode should not be operated at reverse voltage

greater than VR


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Softness factor, Sr

This factor given by:

o

rtt

ttS

1

12

For Snap-off: Sr=0.3

For Soft-off: Sr=0.8


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10. Types of Power Diodes

a. Line frequency diode (general purpose): • On state voltage: very low (below 1V) • Large trr (about 25 s) (very slow response) • Very high current ratings (up to 5kA) • Very high voltage ratings(5kV) •Used in line-frequency (50/60Hz) applications such as rectifiers


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b. Fast Recovery Diode • Very low trr (< 1 s). • Power levels at several hundred volts and

several hundred amps • Normally used in high frequency circuits

c. Schottky Diode • Very low forward voltage drop (typical 0.3V) • Limited blocking voltage (50-100V) • Used in low voltage, high current application

such as switched mode power supplies.


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11. Thyristor (SCR)


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If the forward break-over voltage (Vbo) is exceeded, then SCR “self-triggers” into the conducting state.

The presence of gate current will reduce Vbo.


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“Normal” conditions for thyristors to turn on: – the device is in forward blocking state (i.e Vak is positive) – a positive gate current (Ig) is applied at the gate

Once conducting, the anode current is latched. Vak collapses to normal forward volt-drop, typically 1.5-3V.

In reverse -biased mode, the SCR behaves like a diode. 17/03/2014


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Thyristor Conduction

Thyristor cannot be turned off by applying negative gate current. It can only be turned off if Ia goes negative (reverse) This happens when negative portion of the of

sine-wave occurs (natural commutation), 17/03/2014


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Another method of turning off is known as “forced commutation”, The anode current is “diverted” to another

circuitry.

Types of Thyristors a. Phase controlled – rectifying line frequency voltage and current for ac and dc motor drives – large voltage (up to 7 kV) and current (up to 4 kA) capability – low on-state voltage drop (1.5 to 3V)


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b. Inverter grade – used in inverter and chopper. – quite fast. Can be turned-on using “force-commutation” method. c. Light activated – Similar to phase controlled, but triggered by pulse of light. – Normally very high power ratings d. Triac – Dual polarity thyristors


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12. Controllable switches (Power Transistors) Can be turned “ON” and “OFF” by relatively

very small control signals. Operated in SATURATION and CUT-OFF

modes only. No “linear region” operation is allowed due to

excessive power loss. In general, power transistors do not operate

in latched mode.


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Traditional devices:

Bipolar Junction Transistors (BJT),

Metal Oxide Silicon Field Effect Transistor (

MOSFET),

Insulated Gate Bipolar transistors (IGBT),

Gate turn-off Thyristors (GTO)

Emerging (new) devices:

Gate Controlled Thyristors (GCT).


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13. Bipolar Junction Transistor (BJT)


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Ratings: Voltage: VCE < 1000, Current: IC <400A. Switching frequency up to 5 kHz. Low on-state voltage: VCE(sat) : 2-3V Low current gain ( < 10): Need high base current to obtain reasonable IC .

Expensive and complex base drive circuit. Hence not popular in new products.


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14. BJT Darlington Pair

Normally used when higher current gain is required 17/03/2014


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15. Metal Oxide Silicon Field Effect Transistor (MOSFET)


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Ratings: Voltage VDS<500V, current IDS<300A. Frequency f >100 kHz. For some low power devices (few hundred watts) may go up to MHz range.

Turning on and off is very simple. – To turn on: VGS =+15V – To turn off: VGS =0 V Gate drive circuit is simple


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16. MOSFET characteristics

Basically low voltage device. High voltage device are available up to 600V but with limited current. Can be paralleled quite easily for higher current capability.

Internal (dynamic) resistance between drain and source during on state, RDS(ON) , limits the power handling capability of MOSFET. High losses especially for high voltage device due to RDS(ON) .


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Dominant in high frequency application (>100kHz).

Biggest application is in switched-mode power supplies.


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JFET – Junction Field Effect Transistor MOSFET -Metal Oxide Semiconductor Field Effect Transistor n-channel MOSFET (nMOS) & p-channel MOSFET (pMOS)

The MOS Transistor


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n+ n+

p-substrate

Field-Oxide

(SiO 2 )

p+ stopper

Polysilicon

Gate Oxide

Drain Source

Gate

Bulk Contact

CROSS-SECTION of NMOS Transistor

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MOS transistors Symbols


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D

S

G

D

S

G

G

S

D D

S

G

NMOS Enhancement NMOS

PMOS

Depletion

Enhancement

B

NMOS with Bulk Contact

Channel

17. Insulated Gate Bipolar Transistor (IGBT)


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Combination of BJT and MOSFET characteristics. – Gate behaviour similar to MOSFET - easy to turn on and off. – Low losses like BJT due to low on-state Collector-Emitter voltage (2-3V). Ratings: Voltage: VCE<3.3kV, Current,: IC<1.2kA currently available. Latest: HVIGBT 4.5kV/1.2kA. Switching frequency up to 100kHz. Typical applications: 20-50kHz.


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18. Gate turn-off Thyristor (GTO)

Behave like normal Thyristor, but can be turned off using gate signal 17/03/2014


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However turning off is difficult. Need very large reverse gate current (normally 1/5 of anode current).

Gate drive design is very difficult due to very large reverse gate current at turn off.

Ratings: Highest power ratings switch: Voltage: Vak<5kV; Current: Ia<5kA. Frequency<5kHz.

Very stiff competition: Low end-from IGBT. High end from IGCT


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19. Insulated Gate-Commutated Thyristor (IGCT)

Among the latest Power Switches.

Conducts like normal thyristor (latching), but can be turned off using gate signal, similar to IGBT turn off; 20V is sufficient.


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Power switch is integrated with the gate-drive unit.

Ratings: Voltage: Vak < 6.5kV; Current: Ia < 4 kA. Frequency < 1 kHz. Currently 10kV device is being developed. Very low on state voltage: 2.7V for 4 kA device


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Introduction to Power Electronics lecture1

Engineering

reverse breakdown occurs

reverse voltage greater

blocking state

power electronics

power transistors

power switches

anode current

kamal ramadan