chapter 5_for students

8/7/2019 Chapter 5_for students

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DC CHOPPER(DC TO DC CONVERSION)


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Learning Obj ectives

To understand the operation andcharacteristics of DC chopper.

To examine the performance parametersof the DC chopper.

To describe the principles of the operationof DC chopper topologies.

To learn the techniques of simulating theDC chopper circuits.


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DEFINITION: DC to DC converter is used to change the dc voltagefrom one level to another.

In this case, the dc input voltage is fixed and the level of the dcoutput voltage depends upon the converters topology.

DC to DC converters are important in portable electronic devices

such as cellular phones and laptop computers, which are suppliedwith power from batteries.

The dc output voltage can be higher or lower than the inputvoltage.

A very simple scheme that illustrates the principle is shown in Figurebelow.

In this case, the dc voltage applied to the resistor is controlled via aswitch, which is usually a power semiconductor device such as anSCR, a BJT, a MOSFET, an IGBT, etc.

Introduction


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Introduction Let us assume that the time p eriod of a switching frequency f is T such

that

The switch is closed for a fraction of the time period T and is kept

open for the remainder period. Let us say that the switch is turned onat t = 0 and remains on for t = Ton, where Ton is called the ON time

which is some fraction of T such as and . D is usually

referred to as the duty cycle.

The output voltage obtained by opening and closing of the switch.

The time during which the switch remains closed is customarily

referred to as the OFF time (period). We can express the off time in

terms of the duty cycle as

f T

1!

TT

D ON! 10 ee D

T)D(TTT ONOFF !! 1


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The output voltage with such a high ripple content may besatisfactory for electric heaters, light-dimming circuits, etc., but it iscertainly not suitable for the operation of amplifiers and other circuitsrequiring almost constant dc voltage.The h ig h voltage ri pp le can b e controlled by p lacing a ca p acitor

across t he load. The capacitor is large enough so that its voltage does not have any

noticeable change during the time when the switch is OFF.Better circuit can b e develo p ed by including an inductor, w h ic h is inseries wit h the switc h wh en t h e switc h is ON ( closed ) to limit t h ecurrent in rus h .

However, this creates another problem. Since the current in theinductor cannot change suddenly, we have to provide at least onemore switch, such a freewheeling diode, to provide a path for theinductor current when the switch is OFF (open).

Introduction


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For a good DC-to-DC converter may have an inductor, a ca p acitor, and a freew heeling diode, and an electronic switc h .

There are th ree typ es o f b asic DC c h o pp ers to p ologies :(a) Buck C onverter (lowering the output voltage, step-

down application),(b) Boost C onverter (raising the output voltage, step-up

application), and(c) Buck-Boost C onverter (lowering or raising the

output voltage, step-down or step up application).

DC c ho pp er To p ologies


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BUCK CONVERTER

Mode 1: when switch is closed (ON-state)

Mode 2: when switch is open (OFF-state)


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BUCK CONVERTER

Waveform of voltages and currents in a buck converter


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BUCK CONVERTERMode 2: Wh en switc h is o p ened (OFF-state )by using Kirchhoffs Voltage Law (KVL),


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Ch ange in inductor Ch ange in inductor current when switc h + current when switc h = 0is closed is o p ened

io

OFFLONL

DVV

0ii

!@!((

BUCK CONVERTER

For stead y-state o p eration


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BUCK CONVERTER

Average, Maximum & Minimum Inductor C urrent

Average inductor current = A verage current in R

R

VII o

RL !!


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Maximum current;

Minimum current;

Inductor current ripp le;

BUCK CONVERTER



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BUCK CONVERTER

C ontinuous C onduction Mode

The limit of continuous current operation is reached, when theminimum inductor current never falls to zero.

This is the minimum inductor current to ensure continuousmode of operation.Normall y L is c h osen to b e >> Lmin


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BUCK CONVERTER

O ut p ut Voltage Ripp le & Ripp le FactorThe output voltage will raise and fall as a result of the outputcapacitor charging and discharging.

O ut p ut voltage ripp le

Ripp le factor


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A buck converter is supplied from a 5 0 V battery source.Given L = 4 00uH, C=100uF, R=20 , fs=20 kHz and D=0. 4Calculate;

i. The output voltageii. The maximum and minimum inductor currentiii. The ripple of inductor currentiv. The output ripplev. The ripple factor.

Exam p le 1


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Solution 1


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Exercise 1

Design a Buck converter to produce an output voltage of18 V across a 10 ohm load resistor. The output voltageripple must not be exceed 0. 5% . The dc supply is 48 V.

Design for continuous inductor current.

Assuming switching frequency is 4 0 kHz, inductor 2 5% larger than the minimum.


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BOOST CONVERTER

Mode 2: when switch is opened (OFF-state)



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BOOST CONVERTER

Waveform of voltages and currents in a boost converter


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BOOST CONVERTER

Anal ysis o f Boost C onverterMode 1: Wh en switc h is closed (ON -state )by using Kirchhoffs Voltage Law (KVL),


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BOOST CONVERTERMode 2: Wh en switc h is o p ened (OFF-state )by using Kirchhoffs Voltage Law (KVL),


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BOOST CONVERTER


OFFLONL

ii !(( 0


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BOOST CONVERTER

Average, Maximum & Minimum Inductor C urrentThe average current in the inductor is determined byrecognizing that the power supplied by the source must besame as the power absorbed by the load resistor

RD1V

R

D1V

IV

IIButR

VIV

power Out putpower In put

2

2i

2

i

Li

Li

2o

ii

!

!

!!

!

Thus, the average inductor current,

RD1V

I2

dL !


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Maximum current;

Minimum current;


BOOST CONVERTER



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BOOST CONVERTER



This is the minimum inductor current to ensure continuousmode of operation.Normall y L is c hosen to b e >> Lmin


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BOOST CONVERTER



Ripp le factor


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Exam p le 2

A boost converter is supplied by a 12 V DC source and theoutput voltage is 3 0 V. The circuit is connected to a load withresistance of 5 0 and the operating switching frequency is 2 5 kHz. Determine;i. The duty cycle of the converter

ii. The minimum value of inductance to ensure continuousconduction mode of operationiii. The minimum and maximum values of inductor current if theinductor used have a value of 120 uH.


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Solution 2


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Exercise 2

Design a boost converter that will have anoutput of 4 0 V from a 1 5 V source. Design for continuous inductor current and an outputripple voltage less than 1. 5% . The load is aresistance of 8 0 Ohm. Assume idealcomponents and switching frequency is 3 0 kHz


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BUCK-BOOST CONVERTER

Mode 2: when switch is opened (OFF-state)



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Waveform of voltages and currents in a buck boost converter



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Anal ysis o f Buck b oost C onverterMode 1: Wh en switc h is closed (ON -state )by using Kirchhoffs Voltage Law (KVL),



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Mode 2: Wh en switc h is o p ened (OFF-state )by using Kirchhoffs Voltage Law (KVL),



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OFFLONL ii !(( 0

*

Note: The Output of buck-boost converter can be either higher or lower than input- If D > 0 .5 , output is h ig h er th an inp ut- if D < 0 .5 , output is lower than inp ut

* Output voltage is alwa ys negative



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Average, Maximum & Minimum Inductor C urrentAssuming no power loss in the converter, power absorbed bythe load must equal power supplied by the source, i.e.

ii

2o

ino t

IVR

V

PP

!

!

DII

DTIT1

dtIT

1I

Li

L

T

0 Li

ON

!@

!

!

But average source current is related to average inductor current


RD1DV

DVP

R DVV

I

R

VDIV

2

i

i

o

i

2

o

L

2

o

Lid

!!!@

!


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Maximum current;

Minimum current;





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This is the minimum inductor current to ensure continuousmode of operation.Normall y L is c h osen to b e >> Lmin



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Ripp le factor



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Exam p le 3

A buck-boost converter supplies power to a load having 1. 5 and 0. 8 mH. The source voltage is 5 0 V DC and the load

voltage is 75 V. If the on time is 1. 5 ms, find;i. The converter switching frequency,ii. The average value of the input current,iii. The maximum and minimum inductor current,iv. The peak-to-peak ripple in the inductor current


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Solution 3

chapter 5_for students

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