Introduction

EE328 Power Electronics

Dr. Mutlu BOZTEPE

Department of Electrical and Electronics Engineering - Ege

University

Course Book

“Power Electronics”

Daniel W. Hart

McGraw-Hill, 2011

Power electronics

Power electronics circuits convert electric power

from one form to another using electronic devices.

Conversion is done using electronic switches,

capacitors, magnetics, and control systems

Applications of power electronics range from high-

power conversion equipment such as dc power

transmission to everyday appliances, such as

cordless screwdrivers, power supplies for

computers, cell phone chargers, and hybrid

automobiles.

Scope of power electronics milliWattsgigaWatts

Power Level (Watts) Example System

0.1-10 •Battery operated equipment

10-100 •Satellite power systems

•Offline flyback power supply

100-1kW •Computer power supply

•Blender

1-10kW •Electronic welding machine

10-100 kW •Electric car

•Eddy current braking

100kW-1MW •Micro-SMES (Supeconducting Magnetic Energy Storage)

10MW-100MW •Magnetic aircraft lunch

•Big locomotives

•Power distribution

100MW-1GW •Power plant

>1GW •High Voltage DC Transmission (HVDC)

Interdisciplinary nature of

power electronics

Areas of applications

High frequency power

conversion

DC/DC, inverters

Low frequency power

conversion

Line rectifiers

Distributed power

systems

Power devices

Power transmission

HVDC

HVAC

Power quality

Power factor correction

Harmonic reduction

Passive filtering

Active filtering

Some applications Heating and lighting control

Induction heating

Flourescent lamp ballast

Motor driver

Battery chargers

Electric vehicles, regenerative breaking

Switching power supplies

Uninterruptible power supplies (UPS)

Electric power transmission

Automotive electronics (Ignition, alternators)

Energy storage (Flywheel,SMES, super capacitor)

Power conditioning for alternative power sources: Solar

cells, Fuel cells, Wind turbines)

Induction heating

Maglev Train

Electric car

Renewable energy

Conversion clasification

ac input/dc output (rectifier)

Full wave rectifier

dc input/ac output (inverter)

220VAC/50Hz inverter with battery input

dc input/dc output (converter)

Voltage regulator

ac input/ac output (converter)

Dimmer, speed control of induction machine

A converter can operate as a rectifier or

an inverter, depending on the direction of

average power P.

Multistep conversion

Power conversion can be a multistep process involving

more than one type of converter.

For example, an ac-dc-ac conversion can be used to

modify an ac source by first converting it to direct current

and then converting the dc signal to an ac signal that has

an amplitude and frequency different from those of the

original ac source

Power electronics concept The purpose is to supply 3 V to a load resistance.

One simple solution is to use a voltage divider

Problem 1: the power absorbed by the 2RL resistor is

twice as much as delivered to the load and is lost as

heat, making the circuit only 33.3 percent efficient.

Problem 2: if the value of the load resistance changes,

the output voltage will change unless the 2RL

resistance changes proportionally.

more desirable design solution:

Adding a switch which is opened and closed

perodically.

if the switch is closed for one-third of the period,

the average value of vx (denoted as Vx) is one-

third of the source voltage.

Instantaneous power absorbed by the switch is the

product of voltage and current.

When the switch is open, power absorbed by it is

zero because the current in it is zero.

When the switch is closed, power absorbed by it is

zero because the voltage across it is zero.

Since power absorbed by the switch is zero for both

open and closed conditions, all power supplied by

the 9V source is delivered to RL, making the circuit

100% efficient.

But the output is not pure dc!

However, the voltage waveform vx can be expressed as a

Fourier series

To create a 3-Vdc voltage, vx is applied to a low-pass filter.

An ideal low-pass filter allows the dc component of voltage

to pass through to the output while removing the ac terms,

thus creating the desired dc output.

If the filter is lossless, the converter will be 100 percent

efficient.

In practice, the filter will have some losses and will

absorb some power.

Additionally, the electronic device used for the switch will

not be perfect and will have losses. However, the

efficiency of the converter can still be quite high (more

than 90 percent).

A feedback control system would detect if the output

voltage were not 3 V

and adjust the closing

and opening of the

switch accordingly

Electronic Switches

Have two states: ON and OFF

Ideal switch: either switch current or switch

voltage is zero, making the power absorbed

by it is zero.

Real switches absorb some power.

Diode Simplest electronic switch

Uncontrollable

on and off conditions are determined by voltages

and currents in the circuit.

Reverse recovery An important dynamic characteristic of a nonideal diode

is reverse recovery current.

When a diode turns off, the current in it decreases and

momentarily becomes negative before becoming zero.

The time trr is the reverse recovery time, which is usually

less than 1 us.

Silicon carbide (SiC) and

schottky diodes have very little

reverse recovery effect.

Mercury arc rectifier

Semiconductor rectifier

Thyristor (SCR) and GTO

Controllable diode with three terminal

SCR (Silicon Controlled Rectifier)

GTO (Gate turnoff thyristor)

Triacs and MCT

Triac: Two back-to-bact thyristor

MCT (Mos conttrolled thyristor)

Thyristor (SCR), GTO, Triac

Thyratron

Transistors

Unlike the diode, turn-on and turnoff of a

transistor are controllable.

Types:

MOSFET (Metal Oxide Semiconductor Field

Effect Transistor)

BJT (Bipolar Junction Transistor)

IGBT (Isolated Gated Bipolar Transistor)

MOSFETS

BJT

IGBT

Power MOSFET, IGBT

Vacuum tube

Switch selection

The selection of a power device for a

particular application depends not only on the

required voltage and current levels but also

on its switching characteristics.

Transistors and GTOs provide control of

both turn-on and turnoff

SCRs of turn-on but not turnoff

and diodes of neither.

Switch selection Switching speeds and the associated power losses are

very important in power electronics circuits.

The BJT is a minority carrier device have minority

carrier storage delays.

The MOSFET is a majority carrier device that does not

have minority carrier storage delays

Therefore, MOSFET has advantage in switching speeds.

Generally has lower switching losses and is preferred

over the BJT.

Example 1-1

When S1 is opened, S2 must be close in order

to provide current path.

When S2 is opened, S1 is closed.

Switching

frequency

is 200 kHz

Select

switching

devices?

The operating points are on the

positive i and v axes

S1 must turn off when

and must turn on when

So,the device used for S1 must

provide control of both turn-on

and turnoff.

BJTcharacteristic matches the

requirement.

But a MOSFET would be a good

choice because of the required

switching frequency, simple gate-

drive requirements, and relatively

low voltage and current

requirement (24 Vand 2 A).

The operating points are on the

positive current axis and negative

voltage axis.

Therefore, a positive current in S2

is the requirement to turn S2 on,

and a negative voltage exists

when S2 must turn off.

This matches a diode operation

and no other control is needed for

the device

A diode is an appropriate choice

for S2.

Switch implementation using a

MOSFET and a diode

Switch implementation using

two MOSFET

(Synchronous rectification)

Computer Simulation

Spice (developed at the university of California at

Berkeley)

Pspice (Commercially available version of spice)

OrCad capture - Cadence

PSIM - Powersim

Proteus – Labcenter

etc.