pe lecture 1

7/23/2019 PE Lecture 1

http://slidepdf.com/reader/full/pe-lecture-1 1/31

Power ElectronicsWinter 2012

Lecture 1

Dr. Walid Atef Omran



About me

Current: Assistant professor at Ain-Shams University

Part time lecturer at the GUC

Past:

Planning engineer at the Independent Electricity System Operator –

Canada

International TA developer at the University of Waterloo – Canada

Education

PhD title: “Performance Analysis of Grid Connected Photovoltaic

Systems” from the University of Waterloo – Canada

2



Contact Information

Dr. Walid Atef Omran

GUC email: [email protected]

Personal email: [email protected]

Office: C3 – 214

Office hours: Wednesdays (you have to book an appointment)

3

mailto:[email protected]














About You

4



About the Course

This course will introduce you to the fundamentals of powerelectronics.

The main focus of this course is to discuss the operational aspects

of different power electronic switches and power electronicconverters.

The course is of great importance in practical life due to the wide

application of power electronics in different engineering areas.

5



References

Denis Fewson, Introduction to Power Electronics

Ned Mohan, Power Electronics: A First Course

B.K. Bose, Modern Power Electronics and AC Drives

M.H. Rashid, Power Electronics Handbook

B.M. Bird, K.G. King, D.A. Pedder, An Introduction to Power

Electronics

Cyril W. Lander, Power Electronics 6



Course Outline

Part A: Power Electronic Switches and Power losses

Part B: Rectifiers

Part C: Inverters

Part D: DC Choppers

7



Course Logistics

Lectures: Wednesdays 1st lecture, H12

Grading Scheme:

8

Final Exam45%

MidtermExam20%

Project5%

Quizzes15%

Assignments15%



Recommendations

Attending the lectures and tutorials is a must in this course toavoid facing difficulties in understanding the material.

Taking notes is highly recommended because you will need to

remember what I say in the lecture.

Please be on time because lectures will start on time.

Watch out for Due Dates . Assignments and reports should besubmitted on time.

Please contact me as soon as you face any problem related to the

course. 9



Today’s Lecture

I ntroduction

What is Power Electronics

Power electronic converters

Power Electronic Devices

Uncontrolled power electronic Switches

Diodes

Semi-controlled power electronic Switches Thyristors

10



Introduction

Power electronics is an established technology that bridges the powerindustry with its need for fast controllers.

The main function of power electronics is to condition the power from

a supply to suit the needs of the load.

Power electronics emerged in the late 1950s when the silicon controlled

rectifier, known as thyristor, was developed.

The thyristor, like other power electronic devices, is used as anelectronic switch that is much faster than the mechanical switch.

The power electronic switch is the building block of power electronic

converters.11



Power Electronic Converters

There are 4 main types of power electronic converters: AC to DC converters (Rectifiers)

Changes from fixed AC voltage

to DC voltage with variable

magnitude.

DC to AC converters (Inverters)

Changes from fixed DC voltage

to AC voltage with variable

magnitude and frequency.

12



Power Electronic Converters

There are 4 main types of power electronic converters: DC to DC converters (DC choppers)

Changes from fixed DC voltage

to DC voltage with variable

magnitude.

AC to AC converters (AC choppers)

Changes from fixed AC voltage with

to AC voltage with variable

magnitude and frequency.

13



14





Power electronic devices operate as electronic switches and can be categorized into three main categories:

1. Uncontrolled electronic switches that turn on and off based on the

conditions of the circuit (power diodes).

2. Semi-controlled electronic switches that turn on based on the

conditions of the circuit and in the presence of an external signal,

but they turn off based on the conditions of the circuit (thyristors).

These devices can be also turned off by the aid of external circuits

called commutation circuits.

3. Controlled electronic switches that turn on and off based on the

conditions of the circuit and on the external signal (e.g., IGBT,

MOSFET, GTO).15



Power Diodes

Symbol and Structure:

A power diode is a p-n junction that is capable of handling highvoltages and currents.

It conducts if the voltage of the anode is higher than the cathode

and is turned off when the current passing from the anode to thecathodes falls to zero and the voltage is reversed.

ON: v AK > 0

OFF: v AK < 0 & i AK = 0 16



Power Diodes

Static I-V Characteristics:Under steady state conditions, the diode

has two main states:

1) Reverse blocking state: In this case v AK < 0

The p-n junction is reverse biased.

The diode is in the OFF state.

A very small leakage current will flow.

If the voltage is increased till the value

VRBD, reverse breakdown (avalanche breakdown)

will occur and diode will be damaged.

17

Actual characteristics



Power Diodes

Static I-V Characteristics:Under steady state conditions, the diode

has two main states:

2) Forward conducting state: In this case v AK > 0

The p-n junction is forward biased.

The diode is in the ON state.

The current increases exponentially as the

voltage increases.

The forward drop voltage, V Df , that depends

on the type of diode (V Df ≈ 0.2V – 1.5V)

.

18




Power Diodes

Linearized model:

The diode can be represented by a battery and a resistance during

the ON state and by an open circuit in the OFF state.

19

Linearized characteristics

OFF stateON state



Power Diodes

Ideal model:

In the ideal case, the diode is represented as a switch, i.e., it is a

short circuit in the ON state and open circuit in the OFF state.

20

Ideal characteristics

OFF stateON state



Thyristors (Silicon controlled

Rectifiers – SCRs)

Symbol and Structure:

The thyristor has the highest power rating and the lowest

frequency of operation among all power electronics devices.

It is a 4 layers, 3 junctions, 3 terminals, current controlled (iG),

latching device (latching: the device requires an external signal toturn on and then the signal can be removed).

ON: v AK > 0 & iG > 0

OFF: v AK < 0 & i AK ≈ 0 (i AK < I H ) 21





Static I-V Characteristics:Under steady state conditions, the thyristor

has three main states:

1) Reverse blocking state:

In this case v AK < 0

J1 & J3 are reverse biased while

J2 is forward biased.

The thyristor is in the OFF state.



VRBD, reverse breakdown (avalanche breakdown)

will occur and the thyristor will be damaged.

22








2) Forward blocking state:

In this case v AK > 0 & iG = 0

J1 & J3 are forward biased while

J2 is reverse biased.

The thyristor is in the OFF state.



VFBO, forward breakdown will occur and

thyristor will start to conduct without a gate signal. This decreases the

lifetime of the thyristor. 23








3) Forward conducting state:

In this case v AK > 0 & iG > 0

J1, J2 & J3 are forward biased.

The thyristor is in the ON state.

The current increases exponentially as

the voltage increases.

There will be forward drop voltage, V T

(V T ≈ 1.2V – 2.2V)

24






Important parameters: Latching current ( I L): is the anode current that must be reached for the

thyristor to start conducting before the gate signal is removed (i.e., if

i AK < I L the thyristor will be turned off if the gate signal is removed).

Holding current ( I H ): is the minimum anode current required to keepthe thyristor in the ON state (i.e, if the thyristor is already conducting

and i AK is decreased below I H , then the thyristor will be turned off).

Forward breakover voltage (V FBO

): is the forward voltage at which the

thyristor starts to conduct when there is no gate signal.

Reverse breakdown voltage (V RBD): is the reverse voltage which will

cause the thyristor to conduct in the reverse direction and avalanche

breakdown occurs (thyristor is damaged). 25

h ( l ll d





Thyristor turn on: There are 3 methods to turn on the thyristor:

1. Gate turn on: this method requires a current signal at the gate. The

gate signal can be a single short pulse, a train of short pulses or a

single long pulse based on the type of application and load.

26

t

iG

Single short

pulse

Train of short

pulses

Single long

pulse

Shapes of gate pulses

Th i (Sili ll d





Thyristor turn on: There are 3 methods to turn on the thyristor:

2. Breakover voltage turn on: in this case the thyristor forward voltage

across the thyristor reaches the VFBO and the thyristor is turned on

without a gate signal.

3. dv/dt turn on: in the forward blocking state, J2 is a reversed p-n

junction which can be considered as a capacitor with the depletion

region representing the insulator between the two plates. Thus, if the

rate of change of voltage is high enough, the charging current passingthrough the thyristor can cause it to turn on without a gate signal.

27

Th i (Sili ll d





Thyristor turn off: The thyristor is turned off when i AK is reduced below I H . There

are 2 main methods for turn off:

1. Natural commutation: where the current flowing in the circuit in

which the thyristor is installed decreases naturally below I H (as in thecase of AC circuits).

2. Forced commutation: in cases where the current flowing in the circuit

doesn’t decrease naturally below I H (as in the case of DC circuits),

thus, an auxiliary circuit is used to force the current flowing throughthe thyristor to decrease below I H . This circuit is called the

“commutation circuit”.

28

Th i (Sili ll d





Linearized model:

The thyristor can be represented by a battery and a resistance

during the ON state and by an open circuit in the OFF state.

29

Linearized characteristics

OFF stateON state

Th i t (Sili t ll d





Ideal model:

In the ideal case, the thyristor is represented as a switch, i.e., it is

a short circuit in the ON state and open circuit in the OFF state.

30

Ideal characteristics

OFF stateON state



Other Types of Thyristors

The Gate Turn Off Thyristor (GTO) is atype of thyristor that can be turned on by

a current signal in the gate and can be turned

off by applying a reverse current signal at the gate.

The Triac is composed of two thyristors back to back with one

gate. Thus the current can flow in both directions in case of AC

circuits.

31

pe lecture 1

Documents