1

Power Electronic SystemsPower electronics refers to control and conversion of electricalpower by power semiconductor devices wherein these devices

operate as switches. Advent of silicon-controlled rectifiers, abbreviated as SCRs, led to the development of a new field of

application called the power electronics. Before SCRs, mercury-arc rectifiers were used for controlling electrical power, but such rectifier

circuits were part of industrial electronics and the scope for applications of mercury-arc rectifiers was limited. The application

spread to many fields such as drives, power supplies, aviation electronics, high frequency inverters and power electronics

Tasks of Power Electronics Rectification referring to conversion of ac voltage to dc voltage

DC-to-AC conversion DC-to DC conversion AC-to-AC conversion

http://www.powerdesigners.com/InfoWeb/resources/pe_html/ch01/ch01_p1.htm

2

ConvertersElectronic power converter is the term that is used to refer to a power electronic circuit that converts voltage and current from one form to

another.

• Rectifier converting an ac voltage to a dc voltage• Inverter converting a dc voltage to an ac voltage• Chopper or a switch-mode power supply that

converts a dc voltage to another dc voltage• Cycloconverter and cycloinverter converting an ac

voltage to another ac voltage.

3

RectifiersRectifiers may be classified as uncontrolled and controlled rectifiers. Controlled rectifiers can be further divided into semi-controlled and fully-controlled rectifiers. Uncontrolled rectifier circuits are built with

diodes, and fully-controlled rectifier circuits are built with SCRs. Both diodes and SCRs are used in semi-controlled rectifier circuits.

• Single-phase semi-controlled bridge rectifier

• Single-phase fully-controlled bridge rectifier

• Three-phase three-pulse, star-connected rectifier

• Double three-phase, three-pulse star-connected rectifiers with inter-phase transformer (IPT)

• Three-phase semi-controlled bridge rectifier

• Three-phase fully-controlled bridge rectifier

• Double three-phase fully-controlled bridge rectifiers with IPT.

4

DC to AC ConversionThe converter that changes a DC to AC is called an inverter. Earlier

inverters were built with SCRs. Since the circuitry required to turn the SCR off tends to be complex, other power semiconductor devices such as

bipolar junction transistors, power MOSFETs, insulated gate bipolar transistors (IGBT) and MOS-controlled thyristors (MCTs) are used

nowadays. Currently only the inverters with a high power rating, such as 500 kW or higher.

• Emergency lighting systems

• AC variable speed drives

• Uninterrupted power supplies

• Frequency converters.

5

DC to DC ConversionWhen the SCR came into use, a dc-to-dc converter circuit was called a

chopper. Nowadays, an SCR is rarely used in a dc-to-dc converter. Either a power BJT or a power MOSFET is normally used in such a converter and this converter is called a switch-mode power supply.

• Step-down switch-mode power supply

• Step-up chopper

• Fly-back converter

• Resonant converter.

6

AC to AC Converter

• A cycloconverter or a cycloinverter converts an ac voltage, such as the mains supply, to another ac voltage. The amplitude and the frequency of input voltage to a cycloconverter tend to be fixed values, whereas both the amplitude and the frequency of output voltage of a cycloconverter tend to be variable.

• Tthe circuit that converts an ac voltage to another ac voltage at the same frequency is known as an AC-chopper. A typical application of a cycloconverter is to use it for controlling the speed of an ac traction motor and most of these cycloconverters have a high power output, of the order a few megawatts and SCRs are used in these circuits. In contrast, low cost, low power cycloconverters for low power ac motors are also in use and many of these circuit tend to use TRIACS in place of SCRs.

• Unlike an SCR which conducts in only one direction, a TRIACS is capable of conducting in either direction and like an SCR, it is also a three terminal device. It may be noted that the use of a cycloconverter is not as common as that of an inverter and a cycloinverter is rarely used.

7

Applications of Power Electronics• In a conventional car, power electronics applications

are a major area of future expansion. • Look inside the audio system, for example; the

amplifiers in today’s car stereos are usually capable of delivering 40 W or more. But a 12 V supply applied to an 8 Ohm speaker produces 18 W output at best.

• To solve this power supply problem, designers use a boost converter (DC to DC Converter) to provide higher voltage power to the amplifier circuit. This allows car amplifiers to generate the same audio output power as home stereos.

8

Automobile’s Ignition System

• Another universal power electronics application is the automobile’s ignition system.

• Thousands of volts are required to ignite the fuel-air mixture inside a cylinder so that internal combustion can occur.

• Today’s cars employ all-electronic ignition systems, which have replaced the traditional spark plugs with boost converters coupled to transformers.

9

• We are curious about new electric and hybrid cars, in which the primary electrical system is dominated by power electronics. Electric cars offer high performance, zero tailpipe emissions, and low costs, but are still limited in range by the need for batteries.

• Hybrid car designs use various strategies to combine both an engine and electrical elements to gain advantages of each.

• Inverters and DC-DC converters rated for many kilowatts serve as primary energy control blocks. See http://www.howstuffworks.com/hybrid-car2.htm.

Hybrid Cars

10

Diodes

11

Zener Diodes

12

Semiconductor Switches

• Four-Level Diode• Thyristor SCR• Gate-Turnoff Thyristors (GTOs)• BJT Power Transistors

13

Semiconductor Switches

14

Silicon Controlled RectifiersThe basic purpose of the SCR is to function as a switch that canturn on or off small or large amounts of power. It performs thisfunction with no moving parts that wear out and no points that

require replacing. There can be a tremendous power gain in the SCR; in some units a very small triggering current is able to switch several hundred amperes without exceeding its rated abilities. The

SCR can often replace much slower and larger mechanical switches.

15

AC to DC Conversion: Half-Wave Rectifier

16

Full Wave Rectifier

17

Figure 12.1

Classification of Power Electronic DevicesThe following is taken from Principles and Applications of Electrical Engineering by G. Rizzoni, McGraw Hill

18

Table 12.1

Power Electric Circuits

19

Figure 12.2

AC-DC Converter Circuit and Waveform

20

Figure 12.3

AC-AC Converter Circuit and Waveform

21

Figure 12.4

DC-DC Converter Circuit and Waveform

22

Figure 12.17, 12.18

Rectifier Connected to an Inductive Load

Operation of a Freewheeling Diode

23

Figure 12.20, 12.21

Three-Phase Diode Bridge Rectifier

Waveforms and Conduction Times of Three-Phase Bridge

Rectifier

24

Controlled Rectifier Circuit

Half-Wave Controlled Rectifier

Waveforms

Figure 12.25, 12.26

25

Figure 12.34, 12.35

DC Motor Step-Down Chopper (Buck Converter)

mmaa wTIE ×=×

26

Figure 12.41, 12.42

Half-Bridge Voltage Source Inverter

Half-Bridge Voltage Source Inverter Waveforms

27

PID ControllersPID stands for Proportional, Integral, Derivative. One form of controller

widely used in industrial process is called a three term, or PID controller. This controller has a transfer function:

A proportional controller (Kp) will have the effect of reducing the rise time and will reduce, but never eliminate, the steady state error. An integral

control (KI) will have the effect of eliminating the steady-state error, but it may make the transient response worse. A derivative control (KD) will have the effect of increasing the stability of the system, reducing the

overshoot, and improving the transient response.

∫ ++=

++=

dttdeKdtteKteKtu

sKs

KKtG

DIp

DI

pC

)()()()(

termderivative a and n term,integratioan term,alproportion a provides controller The

)(

28

Proportional-Integral-Derivative (PID) Controllerkp

ki/s

kis+

+

+

e(t) u(t)

sKsKsK

sEsUsG

sEsKs

KKsU

KKKtytrte

tseKsteKteKtu

IPDPID

DI

p

DIP

DIp

++==

++=

−=

++=

2

)()()(

)()()(

ly.respective gains,feedback derivative and integral,al,proportion theare and ,, output; system theand

signal reference ebetween therror theis )()()(

)()()()(

29

Time- and s-domain block diagram of closed loop system

PIDController System

r(t) e(t) u(t)

-+R(s) E(s) U(s)

y(t)

Y(s)

)()(1)()(

)()()(

sGsGsGsG

sRsYsG

PIDsys

PIDsys

+==

30

PID and Operational AmplifiersA large number of transfer functions may be implemented using

operational amplifiers and passive elements in the input and feedback paths. Operational amplifiers are widely used in control systems to

implement PID-type control algorithms needed.

31

Figure 8.5

Inverting amplifier

1

2

)()(

RR

tVtV

S

o −=

32

Figure 8.30

Op-amp Integrator

sCRsZsZ

sVsV

sGFSs

out 1)()(

)()(

)(1

2 −=−==

)(1 sZ

)(2 sZ

33

Figure 8.35

Op-amp DifferentiatorThe operational differentiator performs the differentiation of the input signal.

The current through the input capacitor is CS dvs(t)/dt. That is the output voltage is proportional to the derivative of the input voltage with respect to time, and

Vo(t) = _RFCS dvs(t)/dt

sCRsZsZ

sVsV

sG SFS

o _)()(

)()(

)(1

2 =−==

)(2 sZ

)(1 sZ

34

Linear PID Controller

vo(t)

R1

R2

vs(t)

C1 C2

Z2(s)

Z1(s)

( )( )

122121

22112

2121

2211212

21

2211

;1;;)(

111

)()(

)(

CRKCR

KCR

CRCRKPs

KsKsKsG

sCR

sCR

CRCRsCR

sCRsCRsCR

sVsV

sG

DIIPD

PID

S

o

−=−=+

−=++

=

++

+=

++−==

35

Tips for Designing a PID ControllerWhen you are designing a PID controller for a given system, follow the

following steps in order to obtain a desired response.

• Obtain an open-loop response and determine what needs to be improved

• Add a proportional control to improve the rise time • Add a derivative control to improve the overshoot • Add an integral control to eliminate the steady-state error • Adjust each of Kp, KI, and KD until you obtain a desired overall

response.

• It is not necessary to implement all three controllers (proportional, derivative, and integral) into a single system, if not needed. For example, if a PI controller gives a good enough response, then you do not need to implement derivative controller to the system.

36

The popularity of PID controllers may be attributed partly to their robust performance in a wide range of operation conditions and partly to their

functional simplicity, which allows engineers to operate them in a simple manner.

( ) ( )( )

plane- hand-left in the anywhere locatedbecan that zeros twoandorigin at the pole oneith function w transfer a

introduces PID y theAccordingl ./ and ;/ Where

)(

3231

2132

3

212

33

21

s

KKbKKas

zszsKs

bassKs

KsKsKsK

sKKtGC

==

++=

++=

++=++=

37

Design of Robust PID-Controlled SystemsThe selection of the three coefficients of PID controllers is basically a search

problem in a three-dimensional space. Points in the search space correspond to different selections of a PID controller’s three parameters. By choosing different points of the parameter space, we can produce different step responses for a

step input.The first design method uses the ITAE performance index in Section 5.9 and the optimum coefficients of Table 5.6 for a step input or Table 5.7 for a ramp input. Hence we select the three PID coefficients to minimize the ITAE performance index, which produces an excellent transient response to a step. The design

procedure consists of the following three steps.