Download Acta Innovations • ISSN 2300-5599 • 2015 • no. 17: 22

ActaInnovations•ISSN2300-5599•2015•no.17:22-29•22
NataliaPragłowska-Ryłko
CracowUniversityofTechnology,InstituteofElectrotechnicsandComputerScience,
FacultyofElectricalEngineeringandComputerScience
24WarszawskaStreet,31-155Cracow,[email protected]
COMPARISIONOFBUCK-CONVERTERPARAMETERSQUALITYDEPENDINGONCOILSELECTION
Abstract
Buck-Converters belong to DC/DC converters group. One of the main problems during the design stage of
DC/DCconverteristhepropercomponentselection.Theaimofthisarticleistodescribeselectionoftheoptimal inductor for the Buck-Converter, enabling to minimize the device output overvoltage, while getting
possiblyhighdeviceefficiency.Inordertoverifythequalityparameters,authormadeanefficiencycomparison
ofthebuckconverter,dependingonusedinductor.Conclusionsfromthispaperallowforoptimalselectionof
coilsfortheBuck-Converterdependingongivencriteria.
Keywords
Buck-Converter,Step-downConverter,DC/DCPowerConverter,Overvoltage,Coilselection
Introduction
Buck-Converter, known also as Step-down Converter, is the basic part of power section in many electronic
devices.ItisalsocalledStep-downConverter.Suchconvertersarecommonlyusedinvarioustypesofpower
sectionsofmodernelectronicdevices,suchasnotebooksorsmartphones.Onecanfindinliteraturetwotopologies of Step-down Converters: synchronous and nonsynchronous [2]. The synchronous converter has an
additionalswitchingkeyinparallelbranch.Inthatcasethetransistorsworkcomplementarywhatmeanswhen
one transistor is switched on, the second one is switched off and conversely. However, due to existence of
gate-sourcecapacity,itisneededtoincludeadditionaldeadtime,necessaryforproperswitchingcontrol.Often,however,nonsynchronousStep-downConvertersareused.
Whatismore,Buck-Converterisapartofbuckfamily.Inthisgrouponecanfind:buck,buck-boostandbuckboost-buck Converters [1]. Buck-Converters work in continuous and discontinuous mode. Generally BuckConverter is used to reduce the input voltage. However this converter can adjust output voltage range from
zero to the input voltage value. The main impact on work of this device has proper selection of elements in
particularcoil,capacitorandtwoswitches:diodeandtransistor.Properselectionisunderstoodbysmallsize
andimprovingqualityofBuck-Converter.Themostimportantcharacteristicsofthebuckconverterareinput
and output voltage, nominal current and switching frequency. It is also important to define the quality requirements regarding tolerance to voltage and output current ripples [3]-[5]. The designer must also pay
attentiontoeconomicfactors,suchasobtainingapossiblysmallsizeoftheconverter,highefficiencyandlow
prices.
In this article author presents research results of coil selection impact on mentioned parameters quality of
Buck-Converter.Thisisasadevelopmentofconsiderationsmadein[6].IntheliteratureonecanfindtheoreticalthesisaboutworkofBuck-Converter.Inthispaperauthorincludedonlypracticalteststocheckrealworkof
Buck-Converterwithdifferentinductors(infigure1markedasL1)andtheirinfluenceonefficiencyandovervoltage.
Fig.1.Step-downConvertercircuitdiagram.L1–testedcoil,C1,C2–capacitors,T–MOSFETtransistor,D1–Schottkydiode,
Load–resistor,PWM–Pulse-WidthModulation
Source:Author’s
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ImpactofcoilselectiononsizeandpriceofBuck-Converter
In this thesis author analysed work of Buck-Converter with six different inductor models: solenoid (wounded
copper wire without any core) as a reference and five inductors with core. Author used few core coil types:
DTP, DTMSS, SMD and two bobbin coils. All used coils have the same inductance 330 µH but different core
materials,shapesandphysicalparametersasshownbelowinfigure2.
Fig.2.DifferentcoiltypesusedinBuck-Converterresearch,fromleft:aircoil,DTMSScoil,biggerbobbincoil,DTPcoil,
smallerbobbincoilandSMDcoil
Source:Author’s
Table1presentsdimensionsandpricesoftestedinductors(showninfigure2),whichaffecttheeconomicaspect.Allinductorshavedifferentshapesasshowninfigure2,sooccupiedspaceintable1isanapproximate
value.Theaircoilwasareferenceinductor,whichisfreeofthecoreinfluence.
Table1.Coildimensionsandprices
Coilsymbol
Solenoid
DTMSS-27/0,33/2,0
DSz-20/330/6,3
DTP-17,5/0,33/2,8
DSz-14/330/2,2-V
DSMD-10/330/0,5
Height
[mm]
34
27
18
21
15
5.4
Thickness
[mm]
8
15
20
9
14
9
Outsidediameter[mm]
34
27
20
21
14
10
Occupiedspace
3
[cm ]
9.3
10.9
5.7
4.0
2.3
0.3
Price
[PLN]
7.00
14.00
4.00
4.20
2.50
2.00
Source:http://www.feryster.com.pl/polski/index.php?lang=pl.
Thegraphsinthispaperusesthefollowingshortcuts:
§ S–solenoid(withoutcore),
§ DTMSS–DTMSS-27/0.33/2.0(DTMSS-typereactorcoretypeRTMSS),
§ DSz20–coilDSz-20/330/6.3(biggercoilofDSztypewithcoretypeRSZ),
§ DTP–coilDTP-17.5/0.33/2.8(DTP-typecoilwithcoretypeRTP),
§ DSz14–coilDSz-14/330/2.2-V(smallercoilofDSztypewithcoretypeRSZ),
§ SMD–coilDSMD-10/330/0.5(inductorSMDwithferritecoreNi-ZnRSMDE6H).
The use of an appropriate ferromagnetic core increases the self-inductance of the coil, so that you can get
smallersizescoilssimilarelectricalpropertiesassolenoid[10].Thesizeoftheelementsisimportantfordesigningprintedcircuitboardsforsmallsizedevices.Whileanalysingeconomicfactorsthemostoptimalintermsof
sizeandpriceisSMDcoil.Incontrast,thebiggestandthemostexpensiveisDTMSScoil.DTPcoiliswidelyused
inBuck-Convertersbutasonecanseeisneitherthecheapestnorthesmallestone.Anotherideaforachieving
smallerdevicesizeispresentedin[7].Thisarticlecanhelptoimproveideaofusingtwoinductorsinsteadof
onecoilinclassicBuck-Converters.
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Toshowelectricalparameterspracticaltestsweremade.Foundedtestscenarioswereimplementedusinglogic
analysercontrollingworkofconvertergatingtransistorandadjustablepowersupply,whichwassettinginput
voltage.Withthehelpofmeasuringequipment,whichincludedadigitaloscilloscopeanddigitalmeters,author
made practical measurements, in order to investigate the effect of inductor selection on the device parameters.
Inthisarticlethefollowingshortcutsareused:
ɳ-efficiency[%]
f–transistorswitchingfrequency[kHz]
I–Buck-Converterinputcurrent[A]
VGS–gate-sourcetransistorvoltage[V]
Vout–Buck-Converteroutputvoltage[V]
Vmax–maximumovervoltageamplitude[V]
Vmin–minimumovervoltageamplitude[V]
ImpactofcoilselectiononefficiencyinBuck-Converter
To show the impact of coil selection on tested device efficiency, the following tests were made. Figure 3a
showstheefficiencychangesasafunctionofcurrentforallanalysedcoils.Measurementsweremadeforfollowingcurrentvalues:0.5A,1A,1.5Aand2A,thefrequencyof20kHzandadutycycleof50%,withaloadof
5Ω.Figure3bshowstheefficiencychangesasafunctionoffrequency.Theauthorchoseloadvalue10Ω,input
currentof0.5A,dutycycleof50%andfrequencymeasurementvaluesat:20kHz,50kHz,100kHz,200kHz,
300kHzand500kHz.
Fig.3.Efficiencychanges:a)asacurrentfunctionwith10Ωload,b)asafrequencyfunctionwith5Ωload,whereη–efficiency[%]
Source:Author’s
Fromfigure3aonecanseethatwiththecurrentalsoincreasesefficiency.Italsocanbeseenthatthegreatest
efficiency in analysed current range are bobbin coil DSz-20, DTP and DTMSS. The lowest efficiency was
achieved for the SMD coil. Figure 3b reflecting the efficiency evolution as a function of frequency, confirms
earlierfindingsthatasignificantdifferenceinthecoilselectionisnoticeableforfrequenciesuptoabout100
kHz.Asbefore,thehighestefficiencywasobtainedforbobbincoiltypeDSz-20,andthelowestforthesolenoid
andtheSMDcoil.
Thefollowingfiguresillustratetheefficiencychangesasafunctionoffrequencyfortheresistanceloadof10Ω,
inputcurrent0.9Aanddutycycleof75%(figure4a)andtheloadresistanceof5Ω,theinputcurrent0.9Aand
dutycycleof75%(figure4b).Inordertomaintaintransparencyandforanalysisusedthreeselectedcoils:solenoid,bobbinDSz-20andSMDcoil,whichhadanaverage,thelargestandthesmallestefficiency.Thefrequency
inallgraphsisshowninlogarithmicscale.Measurementpointsatgraphsformfigures4aand4bwereselected
forfrequencyvaluesequal10kHz,20kHz,30kHz,50kHz,75kHz,100kHz,150kHzand200kHzand500kHz.
ActaInnovations•ISSN2300-5599•2015•no.17:22-29•25
Fig.4.Efficiencychangesasafrequencyfunctionwithload:a)10Ω,b)5Ω,Source:Author’s
Figures4aand4bshowtheefficiencyinfrequencyfunctionforthesamecurrentsandvariousloadresistances.
InFigure4befficiencyisrelativelylowercomparingtotheefficiencyfromFigure4a,duetolowerinputvoltage,
andthuscoupledwithitvoltageVGS,whichisrequiredtoachievethesameinputcurrent.InFigure4aonecan
seeasignificantdropofefficiencyforswitchingfrequencyof75kHz.Figure4bshowsthattheminimumefficiencyoccursatafrequencyof200kHz.Changeoftheminimumefficiencypointisdependentonbothcurrent
and duty cycle. This phenomenon occurs regardless of the type of coil used in the inverter and is connected
withtheresonance[7].
ImpactofcoilselectiononovervoltageinBuck-Converter
One of the most important features that define the quality of the output voltage of Step-down converter is
outputvoltagepeaksvalues.Overvoltagehasnegativeimpactondeviceworkandmaycausefasterdestruction
and affect on electromagnetic compatibility. This test was designed to check the effect of coil selection on
output voltage peaks depending on the frequency and current. Presented test results refer to the converter
showninFigure1.
Thefirstanalysewasmadejustforsolenoidwhichdoesnothavecoreinfluenceandcanbeusedasthereference inductor. In figure 5a one can see output voltage changes in function of time which coming from
oscilloscopemeasurements.Comparisonwasmadefortwocurrentvaluesof0.5Aand1A.Infigure5bauthor
presentsoutputvoltagesignalforalltestedcoilsinafunctionoftime.Thesignalswereobservedforswitching
frequencyof20kHz,dutycycleof50%andBuck-Converterinputcurrentequal0.5A.
Fig.5.Outputvoltagetimechangesa)fordifferentcurrentvalueswiththeusageofsolenoid,b)fordifferentcoils
Source:Author’s
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Analysingfigure5aonecanseethedependenceofvoltagepeaksfromcurrentvalue.Forlesscurrentvaluethe
biggest voltage peaks appear with delay. It can be also seen that bigger current value causes elongation of
transientstate.Sizeofovervoltagewillbediscussedinthefollowingparagraphs.Ingraphfromfigure5bone
canseethatcoilselectionaffectsonovervoltageamplitudeandthetimeofovervoltageoccurrence.Tocheck
differencesbetweenovervoltagepeaksthefollowinganalysiswasmade.
Thebelowmeasurementsweremadeforfrequencyequal100kHz.Graphsfromfigures6(aandb)showminimumandmaximumvoltagechangesinfunctionofinputcurrentfordutycycle50%.Measurementsweremade
forcurrentvaluesof0.2A,0.4A,0.8A,1Aand1.2A.
Fig.6.Outputvoltagepeaksfordifferentcurrentswiththeusageofsolenoid,fora)minimumvoltagepeaks,b)maximum
voltagepeaks
Source:Author’s
Graphs in figures 7 (a and b) show minimum and maximum voltage changes in function of input current for
dutycycle75%.Measurementsweremadeforcurrentvaluesof0.4A,0.8Aand1.2A.
Fig.7.Outputvoltagepeaksfordifferentcurrentswiththeusageofsolenoid,fora)minimumvoltagepeaks,b)maximum
voltagepeaks
Source:Author’s
Analysingthegraphsabove(infigures6a,band7a,b)itcanbeconcludedthattheincreaseofinputcurrent,
significantlyfloatsonthevalueoftheoutputvoltagepeaks,whichisparticularlyvisibletothedutycycleequal
50%.Moreover,theloadvoltagepeaksincreaseexponentiallyduringcurrentchanges.Inthecaseoftheduty
ActaInnovations•ISSN2300-5599•2015•no.17:22-29•27
cycleof75%overvoltagechangesarelessnoticeable,butthetrendisthesame.Tocompareovervoltagesize,
abovefigureshavethesamerangeofy-axis.
Figures8aand8bshowstheoutputvoltageamplitudechangesasafunctionoffrequency.Duetotheasymmetry of voltage peaks, the author made a distinction between positive and negative peaks. Measurement
pointswereselectedforfrequencyvaluesequal20kHz,50kHz,100kHzand200kHz.
Fig.8.Peakvalueschanges:a)maximum,b)minimum,asafunctionoffrequencyforinputcurrentof0.5A,Source:Author’s
Accordingtotheoreticalconsiderationsfrom[5],[11]-[13],forthelowerfrequenciesarevoltagepeakshave
bigger values. In the case of a positive voltage peaks the author measured values from about 1.1 to 1.35 V,
whileinthecaseofanegativepeak,recordedvaluesfrom-1to-1.3Volts.Forthelowerfrequencythesmallest
overvoltageoccurredforthebobbincoiltypeDSz-20whilethebiggestovervoltageappearedforsolenoidand
DTMSScoil.
Figure9aand9bshowtheoutputvoltagepeaksforcurrentequal1Aanddutycycleof50%.Forthiscaseto
maintaintheparametersofvoltageandcurrent,themeasurementsweremadeonlyfortwofrequencyvalues:
50kHzand100kHz.
Fig.9.Peakvalueschanges:a)maximum,b)minimum,asafunctionoffrequencyforinputcurrentof1A
Source:Author’s
Analyzingthefigures9aand9bonecanseethatbothpositiveandnegativeovervoltagetheworstresultswere
obtainedforthesolenoid,whilethebestresultsforbobbincoils:DSz-14andDSz-20.Despitethelimitednumber of measurement results, this study gives important information, because it allows observing the scale of
overvoltagegrowthduringcurrentincrease.Inthisstudy,appropriatecoilselectionaffectsonthedevicequali
ActaInnovations•ISSN2300-5599•2015•no.17:22-29•28
ty in terms of overvoltage for lower frequencies. With increasing frequency, these differences become less
visible.
Summaryandconclusions
ThearticlepresentsstudiesresultsofusingdifferentinductortypesimpactonBuck-Converterparameters.All
presentedresultscomefromrealtests.Authorobtainedtheinfluenceofcoilselectiononoutputvoltagecharacteristicsandefficiencychanges,dependingonfrequencyandcurrent.Thispaperalsoshowshowtheproper
coilselectionhelpstoreduceappearingovervoltageontheoutputoftheBuck-Converter.
Itishardtoindicatejustonecoilthatgivesthebestresults.Thefirstimportantproblemistoreducesizeof
power supply section. Clients usually want to have possibly cheap and small devices. Designers try to make
deviceswithhighefficiencyandlimitedovervoltage.Tominimizeconvertersizeandreduceprice,authorrecommendusingSMDcoil.Thiscoilisnoticeablycheaperthanothercoilsandtakesthesmallestsurface.Inthe
otherhandthebestresult,inthecaseofovervoltage,wecangetusingbiggerbobbincoil(DSz-20).Thisbobbin
coilalsoallowsachievingthebiggestefficiencyandhasmediumsize.PresentedtestscanbeusedduringprojectingStep-downConverters.
References
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