introduction power electronics

8/10/2019 Introduction Power Electronics

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Power Electronics

1.1 Introduction

1.2 What Is Power Electronics?

1.3 Why Power Electronics?

1.4 Power Semiconductor Switches

1.5 Power Losses in Real Switches

1.6 Types of Power Electronics Circuits Device

1.7 Applications of Power Electronics

Introduction to Power electronics

Applications of solid-state electronics in the electrical power field are steadily increasing, and a

course in power electronics is now a common feature of many electrical engineering

technology curricula. The term power electronics has been used since the 1960s, after the

introduction of the silicon controlled rectifier (SCR) by General Electric. Power electronics has

shown rapid growth in recent years with the development of power semiconductor devices thatcan switch large currents efficiently at high voltages. Since these devices offer high reliability

and small size, power electronics has expanded its range and scope to such applications as

lighting and heating control, regulated power supplies, variable-speed DC and AC motor drives,

static VAR compensation, and high- voltage DC transmission systems.

What Is Power Electronics?

The broad field of electrical engineering can be divided into three major areas: electric power,

electronics, and control. Power electronics deals with the application of power semiconductor

devices, such as thyristors and transistors, for the conversion and control of electrical energy at

high, power levels. This conversion is usually from AC to DC or vice versa, while the parameters

controlled are voltage, current, or frequency. For example, simple rectification from AC to DC is

power conversion, but if voltage level adjustment is applied to rectification, both conversion

and control of electrical power are involved. Therefore, power electronics can be considered to

be an interdisciplinary technology involving three basic fields, power, electronics, and control,

as shown in Figure below.


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Figure 1: Power electronics: a combination of power, electronics and control

This book will cover the use of power semiconductor devices in applications such as

rectification, inversion, frequency conversion, DC and AC drives, and power supplies. In power

electronics, devices such as diodes, transistors, thyristors, and triacs are used mainly as

switches to perform the on-off action that is basic to power electronics circuits.

Why Power Electronics?

Transfer of electrical power from a source to a load can be controlled either by varying the

supply voltage (by using a variable transformer) or by inserting a regulator (such as a rheostat,

variable reactor or switch). Semiconductor devices used as switches have the advantage of

being relatively small, inexpensive, and efficient, and they can be used to control power

automatically. An additional advantage of using a switch as a control element (compared to

using adjustable resistance provided by rheostat or potentiometer) is shown in the following

section.

History of power electronics

The history of power electronics began with the introduction of the mercury arc rectifier in

1900. Then the metal tank rectifier, grid-controlled vacuum-tube rectifier, ignitron, phanotron,

and thyratron were introduced gradually.

These devices were applied for power control until the 1950s. The first electronics revolution

began in 1948 with the invention of the silicon transistor at Bell Telephone Laboratories by

Bardeen, Brattain, and Schockley.

Most of today's advanced electronic technologies are traceable to that invention. Modern

microelectronics evolved over the years from silicon semiconductors. The next breakthrough, in

1956, was also from Bell Laboratories: the invention of the PNPN triggering transistor, which

was defined as a thyristor or silicon-controlled rectifier (SCR).

The second electronics revolution began in 1958 with the development of the commercial

thyristor by the General Electric Company. That was the beginning of a new era of power


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electronics. Since then, many different types of power semiconductor devices and conversion

techniques have been introduced.

The micro electronics revolution gave us the ability to process a huge amount of information at

incredible speed. The power electronics revolution is giving us the ability to shape and control

large amounts of power with everincreasing efficiency.

Due to the marriage off power electronics, the muscle with microelectronics, the brain, many

potential applications of power electronics are now emerging, and this trend will continue.

Within next 30 years power electronics will shape and condition the electricity somewhere in

the transmission line between its generation and all its users. The power electronics revolution

has gained momentum since the late 1980s and early 1990s.

1. A Rheostat as a Control Device

Figure below shows a rheostat controlling a load. When Rx is set at zero resistance, full power isdelivered to the load. When Rx is set for maximum resistance, the power delivered is close to

zero. For maximum power transfer to the load, Rx must equal RL. Under this condition, the

rheostat consumes as much power as the loadthe efficiency of conversion is only 50%.

Moreover, the rheostat must be physically larger than the load to dissipate additional heat.

Figure 2: A rheostat controlling a load

In applications where the power to be controlled is large, the efficiency of conversion is

important. Poor efficiency means large losses, an economic consideration, and it also generates

heat that must be removed from the system to prevent overheating.

Example1.1: A DC source of 100 V is supplying a 10resistive load. Find the power delivered to

the load (PL), the power loss in the rheostat (PR), the total power supplied by the source (PT),

and the efficiency , if the rheostat is set at:

a) 0

b) 10

c) 100

solution


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a) Voltage across the load

Power supplied to the load

Power dissipated in the rheostat

Power supplied by the source

Efficiency

VL = 100 V

PL = 1002/10 = 1 KW

PR= 0 W

PT= PL + PR= 1 KW

= PL/PTx100 = 100%

b) Voltage across the load




Efficiency

VL = 10x100/20=50V

PL = 502/10 = 250W

PR= 250 W

PT= PL + PR= 500W

= PL/PTx100 = 50%

c) Voltage across the load




Efficiency

VL = 10x100/110=9V

PL = 92/10 = 8.1W

PR= 91*19/100=82.8W

PT= PL + PR= 90.9W

= PL/PTx100 = 8.9%

It is clear from this example that the efficiency of power transfer from the source to the load is

very poornote that it is only 50% in case (b).

2. A Switch as a Control Device

In Figure 1.3, a switch is used to control the load. When the switch is on, maximum power is

delivered to the load. The power loss in the switch is zero since it has no voltage across it.

When the switch is off, no power is delivered to the load. Again, the switch has no power loss

since there is no current through it. The efficiency is 100% because the switch does not waste

power in either of its two positions.

The problem with this method is that unlike a rheostat, a switch cannot be set at intermediatepositions to vary the power. However, we can create the same effect by periodically turning the

switch on and off. Transistors and SCRs used as switches can be automatically turned on and off

hundreds of times a second. If we need more power, the electronic switch is set on for longer

periods and off for shorter periods. When less power is needed, it is set off longer.

Figure 3: A Switch controlling a load Switch ON Switch OFF


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Example1.2. A DC source of 100 V is supplying a 10resistive load through a switch. Find the

power supplied to the load (PL), the power loss in the switch (Ps), and the total power supplied

by the source (PT), if the switch is:

a) Closed

b) Open

c) Closed 50% of the time

d) Closed 20% of the time

Solution

a)

With the switch closed,

voltage across the load

power delivered to the load

power loss in the switch

power supplied by the source

VL = 100 V

PL = 1002/10 = 1KW

Ps = 0 W

Pr =1K W

b) With the switch open,

voltage across the load

power delivered to the load



VL = 0 V

PL = 0W

Ps = 0 W

Pr =0W

c)

With the switch closed 50% of the time

average voltage across the load

average power delivered to the load



VL = 50 V

PL = 502/10 = 250W

Ps = 0 W

Pr = 250 W

d) With the switch closed 20% of the time,

average voltage across the load

average power delivered to the load



VL = 20 V

PL = 202/10 = 40 W

Ps = 0 W

Pr = 40 W

As this example shows, all the power supplied by the source is delivered to the load. The

efficiency of power transfer is 100%. Of course, in this example the switch is assumed to be

ideal, but when we use a transistor as a switch, the result is very close to ideal circuit operation.


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Power Losses in Real Switches

An ideal switch is shown in Figure below. The power loss generated in the switch is the product

of the current through the switch and the voltage across the switch. When the switch is off,

there is no current through it (although there is a voltage Vs across it), and therefore there is no

power dissipation. When the switch is on, it has a current (Vs/RL) through it, but there is no

voltage drop across it, so again there is no power loss. We also assume that for an ideal switchthe rise and fall time of the current is zero. That is, the ideal switch changes from the off state

to the on state (and vice versa) instantaneously. The power loss during switching is therefore

zero.

Figure 4: Power losses in an ideal switch

Unlike an ideal switch, an actual switch, such as a bipolar junction transistor, has two major

sources of power loss: conduction loss and switching loss.

Conduction Loss

Switching Loss

APPLICATIONS OF POWER ELECTRONICS

The demand for control of electric power for electric motor drive systems and industrial

controls existed for many years, and this led to early development of the Ward-Leonard system

to obtain a variable dc voltage for the control of dc motor drives. Power electronics have


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revolutionized the concept of power control for power conversion and for control of electrical

motor drives.

Power electronics combine power, electronics, and control. Control deals with the steady-state

and dynamic characteristics of closed-loop systems. Power deals with the static and rotating

power equipment for the generation, transmission, and distribution of electric power.

Electronics deal with the solid-state devices and circuits for signal processing to meet the

desired control objectives.

Power electronics may be defined as the applications of solid-state electronics for the control

and conversion of electric power. The interrelationship of power electronics with power,

electronics, and control is shown in Fig. below. Power electronics is based primarily on the

switching of the power semiconductor devices. With the development of power semiconductor

technology, the power-handling capabilities and the switching speed of the power devices have

improved tremendously. The development of microprocessors/microcomputer technology has

a great impact on the control and synthesizing the control strategy for the powersemiconductor devices.

Modern power electronics equipment uses (1) power semiconductors that can be regarded as

the muscle, and (2) microelectronics that has the power and intelligence of a brain. Power

electronics have already found an important place in modern technology and are now used in a

great variety of high-power products, including heat controls, light controls, motor controls,

power supplies, vehicle propulsion systems, and high-voltage direct current (HVDC) systems, it

is difficult to draw the boundaries for the applications of power electronics.

Figure 5: Relationship of power electronics to power, electronics, and control


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Power Semiconductor Switches

Power semiconductor switches are the most important elements in a power electronics circuit.

The major types of semiconductor devices used as switches in power electronics circuits are:

Diodes

Bipolar junction transistors (BJT)

Metal-oxide semiconductor field-effect transistors (MOSFET) Insulated-gate bipolar transistors (IGBT)

Silicon controlled rectifiers (SCR)

Triacs

Gate-turnoff thyristors (GTO)

MOS-controlled thyristors (MCT)

In power electronics, these devices are operated in the switching mode. These switches can be

made to operate at high frequencies to convert and control electrical power with high

efficiency and high resolution. The power loss in the switch itself is very small since either the

voltage is nearly zero when the switch is on or the current is nearly zero when the switch is off.We will treat these switches as ideal (the limitations of an actual switch are covered in the next

section).

An ideal switch satisfies the following conditions/characteristics:

1. It turns on or turns off in zero time.

2. When the switch is on, the voltage drop across it is zero.

3. When the switch is off, the current through it is zero.

4. It dissipates zero power.

In addition, the following conditions are desirable:

5.

When on, it can carry a large current.6. When off, it can withstand high voltage.

7. It uses little power to control its operation.

8. It is highly reliable.

9. It is small in size and weight.

10. It is low in cost.

11. It needs no maintenance.

Power semiconductor devicesSince the first thyristor of silicon-controlled rectifier (SCR) was developed in late 1957, there

have been tremendous advances in the power semiconductor devices. Until 1970, the

conventional thyristors had been exclusively used for power control in industrial applications.

Since 1970, various types of power semiconductor devices were developed and became

commercially available.

These (Power semiconductor devices) can be divided broadly into five types:


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1) Power diodes,

2) Thyristors,

3) Power bipolar junction transistors (BJTs),

4) Power MOSFETs, and

5) Insulated-gate bipolar transistors (IGBTs) and static induction transistors (SITs).

The thyristors can be subdivided into eight types:

i. Forced-commutated thyristor,

ii. Line-commutated thyristor,

iii. Gate-turn-off thyristor (GTO),

iv. Reverse-conducting thyristor (RCT).

v. Static induction thyristor (SITH),

vi. Gate-assisted turn-off thyristor (GATT).

vii. Light-activated silicon-controlled rectifier (LASCR), and

viii. MOS-controlled thyristors (MCTs).

Power diodes are of three types:

i. general purpose,

ii. high speed (or fast recovery), and

iii. Schottky.

General-purpose diodes are available up to 3000 V, 3500 A, and the rating of fast-recovery

diodes can go up to 3000 V, 1000 A. The reverse recovery time varies between 0.1 and 5 /s.

The fast-recovery diodes are essential for high-frequency switching of power converters. A

diode has two terminals: a cathode and an anode.

Schottky diodes have low on-state voltage and very small recovery time, typically nanoseconds.

The leakage current increases with the voltage rating and their ratings are limited to 100 V,

300A.

A diode conducts when its anode voltage is higher than that of the cathode; and the forward

voltage drop of a power diode is very low, typically 0.5 and 1.2 V. If the cathode voltage is

higher than its anode voltage, a diode is said to be in a blocking mode.

A thyristor has three terminals: an anode, a cathode, and a gate. When a small current ispassed through the gate terminal to cathode, the thyristor conducts, provided that the

anode terminal is at a higher potential than the cathode.

Once a thyristor is in a conduction mode, the gate circuit has no control and the thyristor

continues to conduct. When a thyristor is in a conduction mode, the forward voltage drop is

very small, typically 0.5 to 2 V. A conducting thyristor can be turned off by making the potential

of the anode equal to or less than the cathode potential.


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Power MOSFETs are used in high-speed power converters and are available, at a relatively low

power rating in the range of 1000 V, 50 A at a frequency range of several tens of kilohertz.

IGBTs are voltage-controlled power transistors. They are inherently faster than BJTs, but still

not quite as fast as MOSFETs. However they offer far superior drive and output characteristics

to those of BJTs. IGBT are suitable for high voltage, high current, and frequencies up to 20 kHz.

IGBT are available up to 1200 V, 400 A.

A SIT is a high-power, high-frequency device. It is essentially the solid- state version of the

triode vacuum tube, and is similar to a JFET. It has a low-noise, low-distortion, high audio-

frequency power capability. The turn-on and turn-off times are very short, typically 0.25 S. The

normally on-characteristic and the high on-state drop limit its applications for general power

conversions. The current rating of SITs can be up to 1200 V, 300 A, and the switching speed can

be as high as 100 kHz. SITs are most suitable for high-power, high-frequency applications (e.g.,

audio, VHF/UHF, and microwave amplifiers). The ratings of commercially available power

semiconductor devices are shown in Table below where the on-voltage is the on-state voltage

drop of the device at the specified current.


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Control characteristics of power devices

The power semiconductor devices can be operated as switches by applying control signals to

the gate terminal of thyristors (and to the base of bipolar transistors).


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Figure 6: Control Characteristics of Power switching Devices


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The required output is obtained by varying the conduction time of these switching devices.

Figure 10 shows the output voltages and control characteristics of commonly used power

switching devices. Once a thyristor is in a conduction mode, the gate signal of either positive or

negative magnitude has no effect and this is shown in Fig. l0 a. When a power semiconductor

device is in a normal conduction mode, there is a small voltage drop across the device. In the

output voltage waveforms in Fig. 10, these voltage drops are considered negligible unless

specified.

The power semiconductor switching devices can be classified on the basis of:

1. Uncontrolled turn on and off (e.g., diode)

2. Controlled turn on and uncontrolled turn off (e.g., SCR)

3. Controlled turn on and off characteristics (e.g., BJT, MOSFET, GTO, SITH, IGBT, SIT: MCT)

4. Continuous gate signal requirement (BJT, MOSFET, IGBT, SIT)

5. Pulse gate requirement (e.g., SCR, GTO, MCT)

6. Bipolar voltage-withstanding capability (SCR, GTO)7. Unipolar voltage-withstanding capability (BJT, MOSFET, GTO, IGBT, MCT)

8. Bidirectional current capability (TRIAC, RCT)

9. Unidirectional current capability (SCR, GTO, BJT, MOSFET, MCT IGBT SITH, SIT, diode).

introduction power electronics

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