Basics of Power Electronics

Dr.Mohamed Ansari M.Nainar

Sr.Lecturer, ME

BN-1-026, COE

UNITEN, Malaysia

Tel Ext: 2213

Email: ansari@uniten.edu.my

ELECTRO-MECHANICAL

SYSTEMS (EEEB413)

Book Reference

Theodore Wildi, Electrical Machines, Drives, and Power Systems, IE,

6th Edition, Prentice Hall

Basics of Power Electronics

Electronic systems Power technology

Solid-state electronic device for control and

conversion of electric power or Semi-

conducting switching devices (eg: Diodes,

Thyristors and Transistors)

Advances in Power electronics: GTOs, BPT,

MOSFETs, IGBTs.

Conversion of AC power to DC power and

vice-versa (eg.rectifiers, inverters, converters)

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Potential Level Example1

Potential level w.r.t

reference potential,

T1(0V)

Potential level of T2

is 80V w.r.t T1

Potential level of T3

w.r.t T1 and T2 based

on time factor, t1tn.

Battery

AC source

Potential level of terminals 1,2 and 3

4

Potential Level Example 2

Potential level w.r.t

reference potential,

T3(0V)

Potential level of T1

is w.r.t T3

Potential level of T2

w.r.t based on time

factor, t1tn.

Potential level of terminals 1,2 and 3

Battery

AC source

5

Voltage across some circuit elements

Potential at Source

Potential across a

switch (E1 = E2)

Potential across a

Resistor E3 = E4 = IR

Potential across a coil

or Inductance

E=L(dI/dt)

Potential across a

Capacitor,E=const; I=0

Potential level of some circuit elements

Switch

Resistor

Inductor

Capacitor

6

Basic electronic components

DIODE

THYRISTOR

GTO Thyristor

TRANSISTOR

BPT

MOSFET

IGBT

7

The Diode and Diode Circuits

A diode is an electronic

device possessing two

terminals, A and K

Rules:R1 R4 R1 : E= 0, then Open switch (OS)

R2 : Inv.Voltage (E2) - OS

R3 : For.Voltage (E1>0.7V) Closed switch

(CS)

R4 : If I=0 for 10sec, - OS

A perfect diode is a normally open

switch when A becomes +ve, then

it behaves as a CS and when I

drops to 0 then it behaves as an

OS again.

Basic rules governing diode behaviour

Diode

8

Main characteristics of a diode

Peak Inverse Voltage Maximum inverse voltage before the diode breaks down.

The peak inverse voltage (PIV) ranges from 50V to 4000V, depending on the

construction.

If the rated PIV is exceeded, the diode begins to conduct in reverse and in many

cases, is immediately destroyed.

Maximum Average Current Average current limit for a diode to carry.

Max. current ranges from few 100 mA to over 4000 A.

Nominal current rating depends upon the temperature of the diode, which depends

upon the mountings and cooling systems.

Maximum Temperature

Voltage across a diode X current it carries = Power loss (Heat)

The temp. of diode must not exceed the permissible limit.

Si diodes : -50 0C to +200 0C.

Heat sink, fans, oil or deionized water (For cooling of diodes) 9

Thyristors A thyristor is an electronic switch

similar to a diode, but wherein the

instant of conduction can be

controlled.

Like diode, a thyristor possesses an

anode and a cathode, in addition a

third contact terminal called a gate

is present.

If the gate is connected to the

cathode, the thyristor will not

conduct, even if the anode is

positive. (Blocked)

Conditions: a. The anode must be positive

b. A current I must flow into the gate for

at least a few microseconds.

Thyristor Conditions

Thyristor

10

GTO Thyristor A GTO thyristor is similar to a thyristor , both can be controlled

by the gate current and voltage.

To initiate conduction, anode voltage must be positive

w.r.t.cathode. Under this condition, thyristor may be turn-on by

any of the following techniques:

Gate Triggering

High Voltage

Thermals

Light

dV/dt

GTO Thyristor

GTO Thyristor Properties 11

Transistors A transistor is a semiconductor device used to amplify and switch electronic

signals.

It is made of a solid piece of

semiconductor material, with at least

three terminals for connection to an

external circuit. A voltage or current

applied to one pair of the transistor's

terminals changes the current flowing

through another pair of terminals.

Because the controlled power can be

much more than the controlling (input)

power, the transistor provides

amplification of a signal.

FET Field Effect Transistor

BPT Bipolar Junction Transistor

MOSFET Metal oxide

semiconductor Field Effect Transistor

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Transistors

High switching speed as compared with thyristors.

Extensively employed in dc-dc and dc-ac converters.

Have certain limitations and are restricted to some applications

(e.g. High voltage, high current).

Characteristics and ratings of each type should be carefully

examined to suit a particular application.

BPT Bipolar Junction Transistor

13

Transistors

A power MOSFET is a voltage-controlled device only a very

small gate current is required

Very high switching speed (the switching times are of the order

of nanoseconds)

Applications in low power high-frequency converters.

Problems of electrostatic discharge and require special care in

handling.

MOSFET Metal oxide

semiconductor Field Effect Transistor

MOSFET MOSFET Properties 14

Transistors Combines the advantages of BPTs and MOSFETs

High input impedance, like MOSFETs

Low on-state conduction losses, like BPTs

A voltage-controlled device similar to a power MOSFET

Lower switching and conduction losses

Ease of gate drive, peak current capability and ruggedness

Inherently faster than a BPT

The current rating can be up to 400A, 1200V

Medium-power applications such as d.c. and a.c. motor drives and

power supplies.

IGBT-Insulated Gate Bi-polar Jn Transistor

IGBT Properties

IGBT 15

Basic electronic circuits

RECTIFIER (AC- DC)

Single-Phase Bridge Rectifier

Three-Phase 3Pulse Rectifier

Three-Phase 6Pulse Rectifier

INVERTER (DC-AC)

PWM Inverter

CONVERTER (DC-DC)

DC Choppers

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Rectifiers A rectifier is an electrical device that converts alternating current (AC) to direct current (DC),

a process known as rectification.

Rectifiers have many uses including as

components of power supplies and as detectors

of radio signals.

Rectifiers may be made of solid state diodes,

vacuum tube diodes, mercury arc valves, and

other components.

(b) (c)

(a) Single-phase bridge rectifiers

(b) Three-phase, 3 pulse rectifier

(c) Three-phase, 6 pulse rectifier

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Rectifiers

(a) Single Phase Bridge Rectifier

Ed = 0.90 E ..(1)

Where, Ed = DC voltage of the Rectifier

E = Supply AC line voltage (V)

0.9 = constant (22 )/

Id = Ed /R..(2)

Where, Id = Rectified current

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Rectifiers (i) Rectifier with Inductive Filter

(ii) Rectifier with Capacitive Filter

Ed = 0.90 E ...(1)

Id = Ed /R..(2)

The peak-to-peak ripple,

Ripple = 5.5 P/f.WL(3)

Where, WL is the energy stored in the

Inductor,

P is the dc power drawn by the load [W],

f is the frequency of the 3ph, 6-pulse

source [Hz]

5.5 is the coefficient factor of units

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Rectifiers (b) Three-Phase, 3 Pulse (Bridge)Rectifier

Ed = 0.675 E ..(1)

Where, Ed = DC voltage of the 3

Pulse Rectifier

E = Supply AC line voltage (V)

0.675= constant [3/(2 )]

Id = Ed /R..(2)

Where, Id = Rectified current

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Rectifiers (c) Three Phase, 6 Pulse Rectifier

Ed = 1.35 E ..(1)

Where, Ed = DC voltage of the 6-

Pulse Rectifier

E = Supply AC line voltage (V)

1.35 = constant (32 )/

Id = Ed /R..(2)

Where, Id = Rectified current

The peak-to-peak ripple,

R = [0.17 P]/f.WL..(3)

Where, WL is the energy stored in

the Inductor,

P is the dc power drawn by the

load [W],

F is the frequency of the 3ph, 6-

pulse source [Hz]

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PROBLEMS

1. An AC source has an effective voltage of 240 V, 60Hz. The

load draws a DC current of 30A.

Calculate: (a) The DC voltage across the load.

(b) The average DC current in each diode.

2. We need to build a DC power supply of 110V,10A using a

single-phase bridge rectifier and an inductive filter. The P2P

current ripple should be about 15%. If a 50Hz AC source is

available, calculate the following values:

(a) The effective AC supply (Source)

(b) The energy stored in the inductor

(c) The inductance of the inductor

(d) The P2P current ripple

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PROBLEMS

3. A 3-phase bridge rectifier has to supply power to a 360kW, 240

V DC load. If a 600V, 3-phase, 60 Hz feeder is available,

calculate the following:

a. Voltage rating of the 3-ph transformer

b. DC current per diode

c. PIV across each diode

d. P2P ripple in the o/p voltage and its frequency.

e. Calculate the inductance of the choke required, if the P2P ripple is 5 %.

f. Does the presence of the choke modify the P2P ripple in the o/p

voltage EKA?

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Rectifier (Power circuits) with Thyristers

1. Controlled Rectifier supplying a

passive load

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2. Controlled Rectifier supplying a

active load

Inverter (Power circuits) with Thyristers

3. Line-Commutated inverter

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4. AC Static Switch

Converter(Power circuits) with Thyristers 5. Cycloconverter

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Converter(Power circuits) with Thyristers

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6. 3-phase, 6 pulse controllable converter

Converter(Power circuits) with Thyristers

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6. 3-phase, 6 pulse controllable converter

Converter (DC-DC)

DC to DC converters

Based on I/O voltage levels (Buck convertor or Boost Converter)

Based on output voltage and current (Class A, Class B, Class C and Class D)

Based on circuit operation (One quadrant, Two quadrant or Three quadrant)

Based on commutation (Natural or Forced)

Duty Cycle (D) = Ta/T

T is the period of one cycle

Ta is the on time of the switch

DC Choppers

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Converter

Switching Losses:

DC Choppers

EA = D EH

EB = (1-D) EH

ELL = EA - EB

= DEH - (1-D) EH

ELL = EH (2D 1)

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PROBLEMS

4. The switch opens and closes at a frequency of 20Hz and

remains closed for 3ms per cycle. A DC ammeter is connected

in series with load Eo indicates a current of 70 A.

a. If ammeter is connected in series with the source, what current will it

indicate?

b. What is the average current per pulse?

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Inverter An inverter is an electrical device that converts DC) to AC; the converted AC

can be at any required voltage and

frequency with the use of appropriate

transformers, switching, and control

circuits.

Types of Inverters:

- 2 quadrant

- 4 quadrant

PWM Pulse Width Modulated

Inverter

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PROBLEMS

5. We wish to generate a 3-phase, 245V, 60Hz source using the

converter of Fig.21.93. The DC supply voltage EH is 500V and

the carrier frequency fc is 540Hz: Determine the following

a. The peak value of the fundamental voltage between terminal L and

floating neutral N of the load.

b. The period T of the triangle wave and the corresponding angular

interval, in degrees

c. The PWM program

d. The waves shapes of the PWM voltage between terminals A and Y

during one cycle.

e. The waveshapes of the PWM voltage between terminals A-Y, B-Y,

and C-Y.

f. The waveshapes of the PWM voltages between terminals A-B, B-C,

and C-A.

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SUMMARY

We have studied the following sub-topics:

1. Introduction to Power Electronics and their devices

2. Electronic components such as Diode, Thyristor

and Transistor

3. Power electronic circuits such as Rectifier circuit,

Convertors and Inverters.

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