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DESIGN OF HIGH EFFICIENCY STEP-DOWN CONVERTER FOR ELECTRO

CHEMICAL ENERGY CONVERSION

Dr.R.Samuel Rajesh Babu ,

Associate Professor/EIE Department,Sathyabama Uniersity.

!r.samuelrajeshbabu"#mail.com

Abstract Recently the research nterest n !"#er electr"ncs nter$ace $"r $%el cell has

ncrease&' An& the statc characterstcs "$ $%el cells sh"# ("re than a )*+ &$$erence n

the "%t!%t ,"ltae .et#een n"-l"a& t" $%ll-l"a& c"n&t"ns' Th%s ne,ta.le &ecrease/ #hch

s ca%se& .y nternal l"sses/ re&%ces the %tl0at"n "$ $%el cells at l"# l"a&s' F%el cell s a

electr"che(cal enery c"n,ers"n &e,ce #hch &rectly !r"&%ces electrcty' Th%s/ a

ste!-&"#n c"n,erter s re1%re& t" trans$er $r"( a hh n!%t ,"ltae t" an

e$$cent an& c"nstant l"# le,el ,"ltae' Mantann hh e$$cency %n&er a #&e

l"a&n rane s (!"rtant n $%el cell'

The a( "$ ths #"r2 s t" &esn a ste!&"#n c"n,erter #th DCM "!erat"n

%n&er lht l"a& n"t "nly can reach hh e$$cency n hea,y-l"a&/ .%t als" has

reat (!r",e(ent n lht-l"a& e$$cency' The "%t!%t acc%racy an& the "%t!%t r!!les

".tane& are .etter than "ther c"n,erters %n&er lht-l"a& an& as ""& as a n"r(al

PWM c"n,erter %n&er hea,y-l"a&'Crc%t s(!lcty an& hh rela.lty are the (a3"r

a&,antaes "$ the !r"!"se& c"n,erter' 4sn ths techn1%e/ the %tl0at"n $act"r an&

e$$cency "$ the $%el cell s ncrease&' An& ths !a!er/ $"c%ses "n the s(%lat"n an&

(!le(entat"n "$ DC-DC c"n,erter $e& $%el cell' Val&at"n "$ the !r"!"se& ("&el s

,er$e& thr"%h s(%lat"n'

5ey#"r&s-F%el cell/ Ste!&"#n c"n,erter/ PWM

I. I$%R&DU'%I&$

%he last fe( years has (itnesse! a #ro(in# interest in the fiel! of fuel cell technolo#y. %he

stimulus for this interest is proi!e! by the fact that )uel 'ells are a source of clean an!

efficient source of electricity. %hey hae a (i!e ran#e of applications ran#in# from the

transportation in!ustry to utility in!ustries. Enironmental concerns about #lobal (armin# has

risen to become an important issue proi!in# the impetus to re!uce ' &*emissions (hich is one

of the many a!anta#es of usin# a )uel 'ell. )uel cells hae became a ery actie research fiel!to!ay an! the nee! of step+!o(n conerters is reuire! for ali!ation an! system inte#ration.

)uel cells are a source of D' olta#e but majority of applications both in!ustrial an!

!omestic reuire A' supply an! hence inerter circuit fe! by a fuel cell is employe!. -ultileel

inerter is a promisin# topolo#y that enables us to #enerate output (aeforms close to that of a

sine (ae. i#h efficiency an! lo( electroma#netic interference !ue to lo( freuency control

metho! maes this setup perfectly suitable for hi#h po(er motor !ries. Besi!es -0I uses

separate D' sources to synthesi1e an output that is far closer to sinusoi!al (aeform. %hus

better uality outputs can be achiee!. %he step+!o(n conerter is reuire! to transfer ener#y

from a hi#h input olta#e to an efficient an! constant lo( leel. -aintain hi#h

efficiency un!er a (i!e loa!in# ran#e is important in fuel cell.

Pulse (i!th mo!ulate! D'+D' conerters for )uel cell employin# semicon!uctor po(er

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the po(er semicon!uctor !eices nee!s to be increase!, but this results in increase! s(itchin#

losses an! electroma#netic interference. %o era!icate these problems, soft s(itchin# metho!s are

presente! an! inesti#ate!. 3ero olta#e s(itchin# 435S6 an! 3ero current s(itchin# 43'S6

techniues are t(o conentionally employe! soft + s(itchin# metho!s. %hese techniues lea! to

either 1ero olta#e or 1ero current s(itchin#, si#nificantly !ecreasin# the s(itchin# losses an!

increasin# the reliability for the )uel cells.

II )UE0 'E00 -&DE0

)uel cell mo!el is a electrochemical ener#y conersion !eice (hich !irectly pro!uces

electricity, (ater an! heat by processin# hy!ro#en an! o2y#en. 7enerally D' olta#e

#enerate! by a fuel cell stac aries (i!ely, lo( in ma#nitu!e an! the olta#e is bet(een *85

an! 985 at full loa!. %he step+!o(n conerter is responsible for absorbin# po(er from the fuel

cell, match the fuel cell ripple current specifications an! shoul! not con!uct any ne#atie

current.

%he fuel cell !irectly conerts chemical ener#y into electrical ener#y. It reacts (ith

hy!ro#en an! o2y#en to pro!uce electricity, (ater an! heat, accor!in# to the follo(in# oerall

chemical reaction.

**:&* *&:Electricity:eat

%he follo(in# fi#ure ; illustrates the principle of operation of a fuel cell

)i#.; )uel cell Structure

)uel cell consists of an ano!e, catho!e an! an electrolyte proi!in# con!uctin# me!ium

for current to flo(. %he actual reaction tain# place is reerse electrolysis. In reerse

electrolysis hy!ro#en an! o2y#en are recombine! an! electric current is pro!uce! (hich flo(s

throu#h the electrolyte.

%he a!anta#es of fuel cell are

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*. A re!uce! moin# part ensures hi#h reliability an! a lon# life.

>. 3ero emission can be achiee! because pure (ater is the only by+pro!uct (hen hy!ro#en is

the fuel. %hus they are ery useful in case of ehicular emission control.

A. Characteristics of Fuel Cell

%he #eneral 5+I polari1ation cure of a fuel cell sho(s a re!uction of the fuel cell

olta#e (ith loa! current !ensity. %his unaoi!able !ecrease is !ue to internal losses (hich in

turn affects the utili1ation factor of fuel cell at lo( loa!s.%his olta#e re!uction is mainly !ue to

three losses. At lo( current !ensities the loss is mainly !ue to the slu##ishness of reactions

tain# place at the electro!es, (hich is terme! as actiation loss.

&hmic losses are cause! by the flo( of electrons throu#h the electrolyte an! throu#h the

electro!es. %he electrolyte shoul! only transport ions throu#h the cell, ho(eer a small amount

of fuel !iffusion an! electron flo( occurs. &hmic losses are essentially linear, that is

proportional to the current !ensity. Decreasin# the electro!e separation an! enhancin# the ioniccon!uctiity can re!uce the ohmic losses. %he final loss component is the #as transport loss at

hi#her current !ensities. As the reactant is consume! at the electro!e, the concentration of the

surroun!in# material re!uces because not enou#h reactants an! pro!ucts are bein# transporte!

to an! from the electro!es. %he output olta#e !ecreases (ith the !ecrease in the concentration.

III BU'? '&$5ER%ERS U$DER 0I7% A$D EA5@ 0&AD '&$DI%I&$

A. Losses of PWM converters under light-load

)i#ure * sho(s the schematic of a P- step !o(n conerter an! the (aeforms. In stea!y+state con!ition, the leel of in!uctor current 4I06 (ill be constant. %he P- controller is!esi#ne! to maintain the constant I0 leel un!er stea!y state con!ition, (hen the conerteris operatin# un!er li#ht+loa! con!ition, the I0 leel (ill be lo(er an! some part of I0 mi#ht belo(er than 1ero. %his part is a hu#e (aste un!er li#ht+loa!, it is the only !efect that P-conerters operatin# un!er li#ht+loa!.

)i# * P- Step !o(n conerter an! aeform.

%he s(itchin# freuency of a conentional P- conerter is usually about *88?1 to;-1, no matter un!er heay+loa! or li#ht+loa!.%his freuency is not a problem (hen theconerter operates un!er heay loa! but in the case of li#ht loa! the s(itchin# loss

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B. Losses of PFM converters under heavy-load

)i#ure > sho(s the P)- Step !o(n conerter an! (aeforms. A P)- Step !o(nconerter is usually use! for operatin# un!er li#ht+loa!, because P)- conertersoperate in D'- mo!e an! hae lo(er s(itchin# freuency. %hese t(o characteristics

mae P)- conerters ery efficient un!er li#ht+loa!. But a conentional P)-conerter operatin# un!er heay loa! is not as efficient as P- conertor operatin# un!erheay loa!because of the lar#e ener#y nee!e! for heay+ loa!.

It is because the char#e leel of a conentional P)- is constant. )or a (i!e+ran#e P)- conerter, the char#e leel is more for li#ht+loa!. D'- mo!e also limits theoutput po(er of a conentional P)- conerter, so the conentional P)- conerters arenot usually use! f o r heay+ loa! con!ition.

)i# > P)- Step !o(n conerter an! aeform.

C.The Proposed Technique For uality !n P"# Converters

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%he simulation is !one usin# -atlab simulin/ an! results are presente!. Scope is connecte!

to !isplay the output olta#e.

)i# Simulate! !ia#ram of step+!o(n conerter

)i# H5olta#e an! 'urrent output of step+!o(n 'onerter

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)i#

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)i# ;8 5olta#e an! current output of fuel cell

)i# ;; -easurement of Po(er, 5olta#e from )uel cell

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)i# ;* -easurement of 'urrent, %orue from )uel cell

%hepropose! conerter maes the D'- mo!e be much easier to implement in a P- D'+

D' step+!o(n conerter. It also re!uces the output ripple an! increases the output accuracy.

%he pea efficiency of a conerter usin# this techniue is C.9>= un!er heay+ loa! an!

C8.CC= un!er li#ht+loa!. -a2imum output ripple is only ;.*m5, (hich is 8.;*= of the output

olta#e. %he conerter ripple is less than ; = at !ifferent olta#e leels. Un!er these

con!itions, the step+!o(n conerter oerall efficiency is aroun! C=. %he step+!o(nconerter

operates from heay+loa! to li#ht+ loa! (ith hi#h efficiency, hi#h accuracy, lo( output

ripple an! stable output olta#e.

5 EPERI-E$%A0 RESU0%S

%he Step+!o(n 'onerter is !eelope! an! teste! in the laboratory. %hee2perimental set+up

consists of po(er supply circuit, #ate !rier circuit an! D'+D' step+!o(n conerter circuit.

%he po(er supply circuit (ith input olta#e of **85 A' an! it is conerte! into D' supply of

95 an! ;*5 by conerter circuit. ere the sin#le phase rectifier circuit an! bri!#e rectifier

circuit is use! .%he filter circuit is use! to re!uce the !istortions an! noises in the circuit. %he

filter circuit is obtaine! by connectin# capacitance in parallel to the output of rectifier circuit.

%he #ate an! !rier circuit consists of rectifier bri!#es an! it is use! to #ie #ate pulse for the

s(itches in step+!o(n conerter circuit. %he Step+!o(n conerter is !esi#ne! by usin#

-&S)E% s(itch, at a particular time the #ate pulse is #ien to the s(itch to #et the !esire!

output olta#e. Pulses reuire! by the -&S)E%s are #enerate! by usin# a A%-E0microcontroller

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PARAMETER VAL4E

Resonant

In!uctor

;88F

Resonant

'apacitor

**88F),>5

0oa!

Resistance

*8G

Dio!e I$88H

-&S)E% IR)8/;95,988mA,981

-icrocontroller A%

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)i# ; E2perimental Set Up of Step+!o(n conerter in %op 5ie(

)i# ;9 &scillo#ram of Sine (ae for reference input

)i# ; &scillo#ram of Driin# pulses

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)i# ;< &utput olta#e (ith s(itchin# losses

)i# ;C &utput olta#e (ithout s(itchin# losses

5I '&$'0USI&$

In this paper, the approach to inte#rate fuel cell (ith step !o(n conerter is propose!. %he

propose! step !o(n conerter is operate! in D'- mo!e (ith constant on+time control to

achiee the !esire! P'- operation. )uel cell is a electrochemical ener#y conersion !eice

(hich !irectly pro!uces electricity. %he step !o(n conerter erifies that the D'- techniue

proi!es an e2act output olta#e, lo( output ripple an! hi#h efficiency from ;mA to 988mA

loa!in#. )uel cell !ynamic phenomena analysis in !ifferent physical !omains hae been

performe! an! !iscusse!. ith the mo!el pre!icte! stac olta#e alue, the fuel cell stac

po(er output is emulate! by step !o(n conerter. %he conerter is !esi#ne! to meet the real

fuel cell !ynamic responses. %he simulation results are presente! an! !iscusse! (ith har!(are.

%he conerter output has a ery #oo! accuracy compare! to the mo!el pre!icte! alue.%he

propose! conerter has a!anta#es lie hi#h efficiency, hi#h po(er !ensity, lo( E-I, re!uce!

s(itchin# stresses, hi#h circuit efficiency an! stable output olta#e.As a result the step !o(n

conerter !eliers selectable output olta#es an! can be euippe! (ith !ifferent fuel cell

systems. %hus, the oerall fuel cell is ali!ate!.

RE)ERE$'ES

J;K . @an, '.Pi, .0i, an! R.0iu, yna#ic dead-ti#e controller for synchronous$uc% C-C converters&Electronics 0etters, ol. , no. *, pp. ;+;9, Lun.

*8;8.

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"ith lo" output voltage ripple& in Proc. IEEE 'onference on In!ustrial

Electronics an! Applications, Lul. *8;8, pp. ;>H>+;>H, Apr. *88C.

JK -. D. -ulli#an, B. Broach, an! %. . 0ee, MA >-1 lo(+olta#e buc conerter

(ith improe! li#ht loa! efficiency,N in Proc. !((( !nternational )olid- )tate

Circuits Conference, )eb. *88H, pp. 9*

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