lesson 5 - buck converter 2011-07-30

7/27/2019 Lesson 5 - Buck Converter 2011-07-30

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1

Introduction to Power Electronics

Buck Converter


2/16

Operations

Mode of Operation

Continuous mode

Discontinuous mode


3/16

3

Continuous Mode


4/16

Buck Converter Operation(Continuous Mode)

On State

Off State


5/16

Relationship of Vs and Vo(Continuous mode)

DVV

T

tV

tt

tVV

tVttV

tL

Vt

L

VV

didi

tL

Vdt

L

Vdi

tL

VVt

L

Vdt

L

Vdi

dt

diLV

so

ons

offon

onso

onsoffono

offo

onos

offon

offoL

off

onos

onLL

on

L

)(

0

0

equationsbothcombining

time;offduring

time;onduring

;

On State

Off State

0 offon dIdI


6/16

Inductor Selection(Continuous Mode)

f

RDL

Lf

D

R

Lf

D

RVi

Lf

D

RVTD

L

V

R

Vi

iii

Lf

D

RVTD

L

V

R

Vi

iii

o

ooo

LL

o

oo

LL

2

1

2

11

02

11

conditionboundaryAt

2

111

2

1

2

2

111

2

1

2

min

min

min

max

max

max

fi

DVTD

di

Vt

di

VL

fi

DVVDT

di

VVt

di

VVL

tL

Vdi

tL

VVdi

dt

diLV

L

o

off

ooff

off

o

L

os

on

oson

on

os

offo

off

onoi

on

L

)1()1(

time;offduring

time;onduring

;

for a given load Rfor a given change in IL


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Ripple Voltage due to ESR

C

iTV

Ti

CV

dttiC

V

Lc

L

c

t

t

8

24

1

)(1 2

1

esrCoRiV

Ripple due to ESR:


8/16

Capacitor Selection

C

iTV

Ti

CV

dttiC

V

Lc

Lc

t

t

8

24

1

)(1 2

1

Calculating for C given the desired

ripple voltage due to capacitor:

o

o

o

o

oo

Lo

Lo

t

t

VVLf

DC

LCf

D

V

V

TDLV

CTV

iC

TV

Ti

CV

dttiC

V

C

QVCVQ

2

2

8

1

8

1

)1(8

8

222

11

)(1

;

2

1


9/16

Sizing the Inductor

2max

2

maxmax

2

min

2

max3

1

dcdcrms

rms

iiiI

iiiiI

DID

iiI dcave

2

minmax

ISwitch

Idiode

Iinductor

ISwitch

IDiode

IInductor

DiD

iiI

DCave

11

2

minmax


10/16

Design Procedure

Given:

Vs, Vo, Po, F, Vripple

Calculate1. Duty Cycle

2. Minimum Inductance

3. Maximum ESR

4. Minimum Capacitance5. Device Ratings

Diode Rating

Switch Rating

Inductor Current

s

o

V

VD

)1( DII od

C

oesr

i

VR

fi

DVL o

1min

o

o

V

VLf

DC

28

1

DII os

2max

2

maxmax

2

min

2

max3

1

ooL

L

iiiI

iiiiI

* Assuming switch and diode use Mosfet, replace Io with IL


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11

Discontinuous Mode


12/16

Relationship of Vs and Vo(Discontinuous mode)

TL

Vdt

L

Vdi

DTL

VVdi

oL

off

oson

time;offduring

time;onduring

2

8

02

2

2

1

2

1

;2

1

1

2

1

2

1

2

2

max

max

maxmax

RT

LDD

RT

LD

RT

LD

R

VDT

L

V

R

VDI

R

VDII

TTIDTII

oo

oL

oLo

LLo

R

Lf

DD

D

V

V

RT

LDD

D

D

V

V

D

D

V

V

DVDV

VDVDV

VDVV

TL

VDT

L

VV

didi

s

o

s

o

s

o

os

oos

oos

oos

offon

8

2

2

8

0

0

0

0

0

2

2


13/16

Duty Cycle and Inductance

(Discontinuous mode)

8.0

8.0

8.0

8.0

8.0

s

o

s

o

s

o

s

o

V

VD

D

V

V

DD

D

V

V

D

D

D

D

V

V

s o

peak

s o

peak

V VL DT

I

V VL D

I f


14/16

Design Example #1

Specs:

Vis=12V

Vo=5V

Fsw=100kHzPo=10W

Calculate:

D, L, C, IL, Isw, Id


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Answer

D=


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16

END

lesson 5 - buck converter 2011-07-30

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