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, IGBT’s.

• Conversion of AC power to DC power and

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

3

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, t1…tn.

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, t1…tn.

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 10µsec, - 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

12

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

16

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

17

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 (2√2 )/π

Id = Ed /R………………..(2)

Where, Id = Rectified current

18

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

19

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/(π√2 )]

Id = Ed /R………………..(2)

Where, Id = Rectified current

20

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 (3√2 )/π

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

22

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?

23

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

24

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?

26

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

27

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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