1. Author: VirgileValente
ArizonaState University
April 22nd, 2015
Content
1. Summary
2. Terminology
3. Topologies
4. Switch Selection
5. Control and Load
6. Filtering
7. Efficiency
8. Glossary
9. Test Set-Up
10. References
Summary
Many systemsrequire thatthe primarysource of voltage be regulatedand convertedtoothervoltages
for differentcomponents. Buck converterscanbe usedwhenefficiency,size,orweightrequirements
are mandated.Buckconvertersare switch-modepowersuppliesusedtostepdowna highvoltage toa
lowervoltage efficiently.The DC-DCswitchmode buckconverterstepsdowna DCinputvoltage toa
desiredDCoutputvoltage using anactive device, aswitch,thattoggles onandoff to maintainan
average value of output voltage,hence the term‘switchmode’.The inputvoltage isregulatedbya
controllerthatimplements andadjusts pulse-widthmodulation tothe switch.The ratioof on-to-off time
of the switch isvaried bythe controllertoregulate the outputvoltage. The outputvoltage of anideal
buck converterisequal tothe productof the switchingdutycycle of the PWM signal andthe supply
voltage. Thisisthe basicpremise of howa buck converterworks.
The followingnoteswill introduce commonterminology,thenthe twocommonbuckconverter
topologies;asynchronousandsynchronous,andgive detailsastohow theyfunctionandwhich
parametershave the mosteffectonefficiency. Variousapplicablecomponentsandcontrol methods as
2. well asbasicfiltering are alsopresented inrelationtobuckconverters.The efficiencyof the buck
convertersisbrokendownanddetailed,asitis paramountto thisformof powersupply.
There are many factors and parametersto considerwhenplanningabuckconverter, andwiththese in
mind,the noteswill guide youthroughthe process of basicbuckconverterdesign.Finallythe notes
presentthe ON SemiconductorCapstone Team’sdesignandselectionprocessusedto designandbuild
asynchronousandsynchronousbuckconverters.
Terminology
Input Range
The range of inputvoltage the device canhandle tofunction
effectivelyatfull load.
Load Regulation
Load regulationisthe change inoutputvoltage overthe specified
change in outputload,expressedinpercentage.Asthe outputload
changes,the outputvoltage shouldremainconsistent(typically
millivoltscale).
To the right,the Load Regulationof the TI TPS65251 Buck Converter
isshownto have a maximumchange of approximately5mV in
outputvoltage,a0.05% change overthe specifiedrange of output
load[1].
Line Regulation
Line regulationisthe change inoutputvoltage fora givenchange in
inputvoltage,alsoexpressedaspercentage (typicallymillivolt
scale).
To the right,the Line Regulationof TITPS65251 BuckConverteris
shownto have lessthan10mV increase inoutputvoltage overa
large range of inputvoltage,approximately0.05% [1].
Input and Output Ripple and Noise
Inputand outputripple pertainstothe amountof voltage or
currentdrop at the inputor outputbetweenswitchingcycles.A waveformappearsfromthe switching
of the device whichgivesaslightlyinconsistentvoltage orcurrentvalue atthe inputor output.
Figure 1 - Load Regulation [1]
Figure 2 - Line Regulation [1]
3. At the input,capacitorsare usedto filterthe inputcurrentsothe current fromthe hostsource is
approximatelyanaverage current.These inputcapacitorshowevercancause an inputripple due to
parasiticequivalentseriesresistance (ESR) andparasiticequivalentseriesinductance (ESL) of the
capacitor. The ESR and ESL are definedinthe glossary.
The inputripple currentistypicallyestimatedbydividingloadcurrentby2:
𝐼 𝑅𝑖𝑝𝑝𝑙𝑒 =
𝐼 𝐿𝑜𝑎𝑑
2
The inputcapacitor mustbe selectedbasedonthe calculatedinputripplecurrent;howeverthe input
voltage ripple requirementisnotas stringentasthe outputvoltage ripple requirement.The input
voltage ripple canbe definedandchosenbased onspecificneedandfunction. A helpful guidefromthe
EEtimesoncalculatingappropriate capacitorvaluescanbe foundinthe Referencessection [2].
The worst case ripple currentoccurs whenthe
dutycycle is 50% as demonstratedbythe
graph on the right;
For a DC signal,the smallerthe ripple,the
betterthe voltage regulation.The output
voltage typicallyrisesduringthe onstate and
fallsduringthe off state of the device.The
ripple observedonthe inputand outputis
typicallyatthe converter’sswitchingfrequency
and maylooksimilartothe graphbelow:
The inputand output
ripple waveformswithVin
= 12V and Iout= 30A for
MaximIntegrated
MAX5060 buckboard. The
inputwaveformispurple,
outputwaveformisteal
[3]. A nominal mV change
inripple iseffective for
steadyline regulation.
Figure 3 - Inductor Ripple Current vs Duty Cycle
Figure 4 - Input and Output Ripple Waveforms
4. The effectsof ESR and Capacitance on outputripple asillustratedbyTexasInstruments [4]:
Difference onoutputvoltage ripplebasedon capacitortypes [5],as the diagrambelow illustrates,
ceramiccapacitors offerthe lowestESRandESL:
Figure 7 - Ripple Based Capacitor Comparison
Efficiencyat Full Load
Operatingat100% load conditionsat25°C, the ratio of powerdeliveredtopowersuppliedforthe
device.
Figure 5 - Output ESR Figure 6 - Output Capacitance
5. Temperature Drift
As ambienttemperature changes,thisisthe associatedchange in voltage,expressedaspercentage of
the nominal.
SwitchingFrequency
Switchingfrequencyisthe nominal frequencyof operationof the switchingcircuitinside the Buck
converter. The frequencyrepresentshow manytimespersecondthe switchdevice switches on/off.
Topologies
There are twotypesof Buck Converters;SynchronousandAsynchronous
Asynchronous Buck Converter
The main componentsof anasynchronousbuck
converterconsistof:
Switch(S)
Diode (D)
Filter:
o Inductor(L)
o Capacitor(C)
Controller
A typical asynchronousbuckconverterisshowntothe
right.The device typicallyusesa transistoranda Diode as
switches.These are the twomainswitchesthat
control powerto the load.The highside switchis
controlledusingPWM. The diagramto the right
describesitsstates.
As the switchis turnedON Vinchargesthe inductor,
capacitor andsuppliesthe loadcurrent.Once it
reachesitssetoutputvoltage,the control circuitry
turns the switchoff.Thisdisruptsthe currentflowing
throughthe inductor,and withoutapath forthe
current,the inductorwill resistthischange creatinga
large voltage spike.Toavoidthistroublesome spike,
a path isprovidedbythe bottomside diode forthe
inductorcurrentto continue flowinginthe same direction.Asthe topside switchisturnedoff,the
inductorvoltage reversesitspolarityforwardbiasingthe diode onandallowingthe currentflow.When
Figure 8 - Asynchronous Buck
Figure 9 - ON/OFF Switch State Diagram
6. the outputvoltage dropsbelowasetpoint,the control circuitrywill turnthe top side switchbackON
and the cycle repeatstoregulate the outputvoltage toa setvalue.
The diode’sforwardvoltage dropandcharacteristiclossesinanasynchronousbuckcanaccount for
almost50% of the total losses.Thisbuckconverterdesignisasynchronousbecausethe low side (diode)
switchingisindependentof the highside switching.
Synchronous Buck Converter
A synchronousbuckconverterconsistsof the same
maincomponents asan asynchronousbuckexcept
the diode issubstitutedforanotherswitchdevice.
The synchronoustermreferstothe concurrentand
complementarynature of the switches.
The high-side switch(S1) andlow-side switch(S2) are
controlledusingPWMbythe control circuitry.The
lowside switchisconsideredasthe synchronous
switchand the highside isreferredtoasthe
switchingorcontrol switch.The lowside switchdoes
not turnon automaticallyhoweverandisdrivensuchthatit
isthe complement of the highside switch.Thismeans
that wheneverone of these switchesison,the otheris
off and vice-versa. The highside switchremains
responsible forthe inductorcurrenthowever,but
usinga switchon the lowside decreasesthe amountof
losscomparedto a diode andthereforincreasesefficiency.
Thisis because usingatransistornegatesthe forwardvoltage dropof the diode,onlyasmall impedance
ispresent,butthe significance ismore closesydetailedinthe Efficiencysection.
The current duringthe charge (closedhighside switch/openlow side switch) anddischarge (openhigh
side switch/closedlowsideswitch)cyclesfollow these outlinedpathsinaDC-DC buckconverter:
Figure 10 - Synchronous Buck
Figure 11 - HighSide (S1) and Low Side (S2) PWM Cycle
7. Figure 12 - Current paths during charge (a) and discharge (b) [13]
To ensure bothswitchesare notturnedon simulatenously,deadtime,orafixeddelaycanbe
introducedbefore aswitchisturnedon.If bothswitchesare simultaneouslyon,shootthroughoccurs.
Shootthroughcan occur whenthe bothswitchesare eitherfullyorpartiallyturnedon,providingapath
for currentto “shootthrough” fromVinto GND. Figure 7 shows typical PWMcyclesof bothswitches,
notice thatthe PWMcyclesare notexact recipricalsandillustratesome deadtime toavoidshoot
through.
Althoughasynchronousbucktopologyislesscomplicatedandrequiressmaller,relativelyinexpensive
ICs forcontrol,the efficiencylossesdue to the diode canbe substantial,whichisdetailedinthe
Efficiencysection.Synchronousbucktopologyoptimizesthe overall conversionefficiencyhowever;
more complicateddrive circuitryisrequiredtocontrol the switches,increasingcomplexityandcosts.
Switch Selection
There are three mainchoicesof switchestoconsiderforimplementationinabuck converter;BJT,
MOSFET, or IGBT. Each switchprovidesmanydifferentadvantagesanddisadvantagesandare more
favorable forcertainapplications.Todeterminethe bestapplicable switchforthe buckconvertera
varietyof factorsare investigated.Here isabrief breakdownof eachswitch, anda final comparison
table to highlightwhichfeaturesare mostdesirable.
BJTs – BipolarjunctionTransistorsare characterizedbylinearcurrenttransferfunctionbetweenthe
collectorcurrentandthe base current.BJTs are current controlleddevicesthatcanreadilybe usedas
switches.BJTsthereforrequireaconstantcurrentto remaininthe on-state,andtherefortypically
exhibitmoderateswitchinglossescomparedtoMOSFETs.BJTsofferfastswitchingspeeds,andcan
switchfasterthanMOSFETs due to lesscapacitance at the base control pin;howeverthe losses
associatedwiththe currentneedsmakesthemlessefficient.
IGBTs – The InsulatedGate BipolarTransistorisa minority-carrierdevice withhighinputimpedance
and large bipolarcurrent-carryingandlow-saturation-voltage capability.Itismeantto combine the best
attributesof bothMOSFETs and BJTs.It has a large current-voltageoperatingboundarybefore itfailsor
experience breakdown. IGBTshave lowon-state voltage dropsdue toconductivitymodulationandhave
8. superioron-state currentdensity. IGBTsare alsobettersuitedforsoftswitchingdue toreducedtail
current. IGBTs exhibitconductionlossesthatare dictatedbytheirvoltage fromthe CollectortoEmitter,
typicallyavalue ( 𝑉𝐶𝐸(𝑜𝑛)) of 1V to 4V.
Conduction Loss of an IGBT: 𝑃𝑐𝑜𝑛𝑑 = 𝐼𝑉𝐶𝐸(𝑜𝑛)
Switching losses for IGBTs are comparable to that of MOSFETs of similar performance, although
IGBTs can have higher delay time, rise time and fall time, which can amount to higher losses.
IGBTs are generallymore favorable formore high voltage,highcurrentandlow switchingfrequency
applications.
MOSFETs – Metal oxide semiconductorfield-effecttransistorsare ideal forpowerswitchingcircuits
as opposedtoBJTs as theydo notrequire a continuousflow of currenttoremaininthe on-state.
MOSFETs can also offerhigherswitchingspeeds,lowerpowerlosses,loweron-resistances,andreduced
susceptibilitytothermal runaway.AsMOSFETscan switchat higherspeeds,theyalsoexhibitlower
switchinglossesthanBJTsbecause asMOSFETs switchfromON/OFFstates,theypassthroughitslinear
region.Duringthistime inthe linearregion,itconsumesmuchhigherpowerthanwhenitisfullyON or
OFF.Therefore,the fasteritswitchesbetweenthe ON/OFFstatesthe lessthe lossbecause itspendsless
time initslinearregion. Fasterswitchingalsoenablesthe use of smallerinductors,whichalsoreduces
losses.
Figure 13 - Turn On Switching Loss (Left) and Turn Off Switching Loss (Right)
Figure x. Turn on Switching Loss (Left) and Turn off Switching Loss (Right).
ConductionlossesinMOSFETsare directlydependenton 𝑅 𝐷𝑆(𝑜𝑛) values, which for low current
applications can be in the low milliohm range, amounting to smaller losses than that of IGBTs.
The conduction loss of a MOSFET can be determines by:
9. Conduction Loss of a MOSFET: 𝑃𝑐𝑜𝑛𝑑 = 𝐼2
𝑅 𝐷𝑆(𝑜𝑛)
MOSFETs are generallymore favorable forlow voltage,low currentandhighswitchingfrequency
applications–ideal fora buck converter.
Comparisonof switches:
Table 1- Comparison of Switches
BJT MOSFET IGBT
Control Method Currentcontrolled. Output
iscontrolledbycontrolling
base current
Voltage controlled.Output
iscontrolledbycontrolling
gate voltage
Voltage controlled.Output
iscontrolledbycontrolling
gate voltage
Temperature
Coefficient
Negative Positive Positive
Parallelingand
Drive Circuitry
Difficult Easy Easy
SwitchingLosses Medium Low Low to Medium
ConductionLosses Low Medium Low to Medium
Applications HighPower Low Power Mediumto HighPower
CurrentRating High Low Veryhigh
Voltage Rating High Low Veryhigh
Switching
Frequency
Low High Mediumto High
Colorsbasedondesirability,redisleastdesirable,greenismostdesirable. Thistable wasdesignedto
helpthe Teamdecide onan appropriate switchbasedonthe loadrequirements.
Switch Selection Conclusion
In summarythere are advantagestoall three devices,andeachdevice maybe more appropriate based
on the applicationof the converterorrelative tothe requiredload.AlthoughIGBTsexhibitlow to
mediumswitchingandconductionlosses,they are more ideal forhighpowerapplications withhigh
currentand voltage.IGBTsexhibitslowerswitchingfrequenciesthanMOSFETshowever,andatlower
voltages,MOSFETsprove to be more efficientastheydonot exhibitadiode likevoltage dropsimilarto
IGBTs. BJTs are also ideal forhighpowerapplications,howeverastheyare current controlled,aconstant
currentis requiredtokeeptheminonstate,due to thistheycan exhibitmoderate switchinglossesat
highfrequenciesbutare greatat lowfrequencies. Forsynchronousbuckapplications,drivecircuitryis
essential,forthisreasonMOSFETsare typicallyused.
10. An importantparametertoconsiderfora MOSFET is the Gate Capacitance (𝑄 𝐺).Thisparameterisof
primaryinterestalongwithon-resistance (Rdson).The MOSFETmusthave a 𝑄 𝐺 withinthe range of the
DC-DC converter.
Basedon these criteria,the Teamchose the ON SemiconductorNTMFS4927NT1G MOSFET for the buck
converter.The NTMFS4927N has a lowRdsonvalue of 7.3 mOhms to minimizeconductionloses.Italso
has lowcapacitance to minimizedriverlossesandoptimizedgate charge tominimizeswitchinglosses.It
has a highbreakdownvoltage of 30V, can handle highswitchingspeedsandcomesina convenient
package that will be easytoapply to our PCB design.These parameterswere ideal forthe teambuck
converterdesign.Specificcharacteristicsforthe MOSFETcan be foundfromON Semiconductor data
sheetinthe Reference section[6].
Control and Load
The most commontechnique tocontrol switchmode powersuppliesisPulse-width-Modulation(PWM).
There are twomethodsof control for DC-DCBuck Converters,VoltageMode of Control (VMC) and
CurrentMode of Control (CMC). A controllerunittypicallycomparesandassessesthe signalsof the buck
converterat variousstages,andcontrolsthe switchsignal basedonthose signals.The switchisdirectly
operatedbya gate-driver,partof the controllerwhichturnsthe switchon/off.
Voltage Mode of Control - VMC
The voltage mode of control usesvoltage feedbackfromthe outputof the buckconverterasthe input.
It containsonlya single feedbackloopmakingiteasiertodesignandimplement.
Figure 14 - Voltage Mode Control Loop
In thismethod,the control voltage (𝑉𝐶𝑜𝑛) isgeneratedand comparedwiththe rampvoltage (𝑉𝑅𝑎𝑚𝑝)
and the switchingsignal (q) issentbasedonthe followingconditions [7]:
If 𝑉𝑅𝑎𝑚𝑝 < 𝑉𝐶𝑜𝑛; q = 1 (switchclosed)
If 𝑉𝑅𝑎𝑚𝑝 > 𝑉𝐶𝑜𝑛; q = 0 (switchopen)
11. Current Mode of Control - CMC
In the currentmode of control there are typicallytwofeedbackloops:acurrentfeedbackloop,anda
voltage feedbackloop,withthe currentonthe inductoristypicallyusedasa feedbackstate.
Figure 15 - Current Mode Control Loop
At the start of the switchingcycle,anSRflip-flopisusedtosetq=1, effectivelyclosingandturningonthe
MOSFET switch.Duringthisinterval,the switchcurrentandinductorcurrentincrease linearlyandthe
inductorcurrent(𝐼 𝐿) is comparedto the control signal (𝐼 𝑅𝑒𝑓) fromthe controller. When 𝐼 𝐿becomes
greaterthan 𝐼 𝑅𝑒𝑓, the outputof the comparatorgoeshighand resetsthe flip-flop(q=0) effectively
openingandturningoff the switch.The processisthenrepeatedateachclock cycle as the switchis
turnedback on[7]. Thisis the basiccontrol methodthat controlsthe gate driverwhichopensandcloses
the switchor gate.The PWMsignal producedisa by thisswitchingistherebycontrolledfromthe
controller.
There are three thingstoconsiderforcurrentmode control:
1. Currentmode operation –Ideallythe converteris onlydependentonthe dc or average inductor
current.
2. Modular gain – This isdependentonthe effectiveslopeof the ramppresentedtothe
modulatingcomparatorinput.A unique characteristicequationformodulatorgainappliesto
each operatingmode.
3. Slope compensation –Thisis dependentonthe relationshipof the average current tothe value
of currentat the time whenthe sample istaken.
Featured Controller
The appropriate controllerforabuck convertermaydifferbasedonthe selectedload.The controllerthe
Team selectedforthe buckconverteristhe NCL30105D PWM CurrentMode ControllerforLED
ApplicationsfromON Semiconductor.The NCL30105D has an effectivedimmingfeatureforLEDs,which
the buck converterwill powerasa representative load.AlthoughCMCismore involvedthanVMC,for
12. LED applications,currentcontrol ismore practical. Datasheetforthe NCL30105D can be foundinthe
Reference section[8].
Load
The load fora buckconvertercan come withvariouspowerrequirements,buttypicallydemands high
efficiency.Buckconvertersare designedbe efficientpowersupplies,andare usedtoreduce losses,
where significantboostsinefficiencycansave quantifiableamountsof poweroverotherpower
supplies.
To provide anappropriate loadtoillustrate efficiency,the TeamselectedLED’sas theyare efficientand
have specificpowerrequirements.The LEDsselectedwere CREEXPLs,theyare powerful 3WLEDs witha
typical forwardvoltage of 2.95V and forwardcurrentof 1000mA. The Team decided touse three XPLsin
seriesasa load.DatasheetforCREE XPLcan be foundinReference section[9].
Calculations
The NCL30105D has an adjustable off time forstability.The off time durationforthe controllerissetby
a resistorto groundon Pin1 of the controller.Correctoff time isimportanttoavoidshootthrough,
where bothswitchesare simultaneouslyinthe ON state.First,the off time ( 𝑡 𝑜𝑓𝑓) requiredforour
applicationwasdetermined:
𝑡 𝑜𝑓𝑓 = (1 −
𝑉𝐿𝐸𝐷
𝑉𝑖𝑛
) ∗ 𝑇𝑆 = (1 −
9
12
) ∗ 10𝑢𝑠 = 2.5𝑢𝑠
Basedon providedequationsandourLED loadvoltage requirements,the resistorvalue (𝑅 𝑡𝑜𝑓𝑓) wasthen
calculated:
𝑅 𝑡𝑜𝑓𝑓[ 𝑘Ω] =
𝑡 𝑜𝑓𝑓[ 𝑢𝑠] − 0.1214
0.1864
=
2.5 − 0.1214
0.1864
= 12.76𝑘Ω
Additional LEDandControllerParameters:
LED Controller ParametersDerivation
*IndicatesInternal Constantsof the Controller
Parameters Symbol Unit Value
SwitchingFrequency fs kHz 100
SwitchingPeriod Ts us 10
PWMOff Time toff us 2.5
Off Time Resistor Rtoff kΩ 12.76
CurrentSensingResistor Rsense mΩ 50
MinimumPeakLED Current IpkLED(min) mA 500
MaximumPeakLED Current IpkLED(max) mA 1500
MinimumCurrentSensingVoltage Vsense(min) mV 10
13. CurrentSensingThresholdVoltage* VILIM V 1
Vsense AmplifierGain Avsense 14
SSTART Voltage toCurrentSet Point
Ratio* Iratio 3
MinimumSSTARTVoltage VSSTART(min) V 1.5
MaximumSSTARTVoltage VSSTART(max) V 4.5
SoftStart Duration tSSTART ms 15
SoftStart Current ISSTART uA 30
SoftStart Capacitor CSSTART nF 150
VCCDecouplingCapacitor CVCC nF 100
Efficiency
There are twomajor sourcesof lossthat can affectefficiencyinbuckconverters:
Conductionlosses –Powerdissipated asheat
Conductionlossesoccuranywhere thereisresistance,andparticularlyinthe inductor
Switchinglosses –Powerdissipatedfromswitchingcomponent
Refertothe Glossaryforterms.
Asynchronous Buck Converter Losses
The total lossesassociatedwithatypical asynchronous
buck converterare due to:
InputCapacitorLosses:
o ESR
o ESL
o PowerDissipated
o 𝑃 𝐷𝑖𝑠𝑠𝑖𝑝𝑎𝑡𝑒𝑑 = 𝐼 𝑅𝑖𝑝𝑝𝑙𝑒
2
∗ 𝐸𝑆𝑅
MOSFET Losses:
o Conduction - Rdson(mOhms)
o Switching–Gate Charge (nC)
o Powerdissipatedinthe MOSFETamounts
to bothconduction losses (Rdson) and
switchinglosses(Gate Charge)
o 𝑃 𝐷𝑖𝑠𝑠𝑖𝑝𝑎𝑡𝑒𝑑 = 𝑃𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖 𝑜 𝑛 + 𝑃𝑆𝑤𝑖𝑡 𝑐ℎ𝑖𝑛𝑔
o 𝑃𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑜𝑛 = 𝐼 𝐿
2
∗ 𝑅 𝑑𝑠𝑜𝑛 ∗ 𝐷
𝐼 𝐿 = Load Current
Figure 16 - Example Asynchronous Buck Converter Losses
[10]
14. D = Duty Cycle
o 𝑃𝑆𝑤𝑖𝑡 𝑐ℎ = (𝑉 ∗
𝐼 𝐷
2
) ∗ ( 𝑇𝑜𝑛 ∗ 𝑇𝑜𝑓𝑓) ∗ 𝐹𝑠𝑤 + ( 𝐶 𝑜𝑠𝑠 ∗ 𝑉2 ∗ 𝐹𝑠𝑤)
𝑇𝑜𝑛 = Time on
𝑇𝑜𝑓𝑓 = Time off
𝐹𝑠𝑤 = SwitchingFrequency
𝐶 𝑜𝑠𝑠 = Capacitance
Diode Losses:
o Switching–Negligible
o ForwardVoltage Drop(𝑉𝐷)
o 𝑃 𝐷𝑖𝑠𝑠𝑖𝑝𝑎𝑡𝑒𝑑 = 𝑉𝐷 ∗ 𝐼 𝐷
o 𝐼 𝐷 = 𝐼 𝐿 ∗ (1 − 𝐷)
InductorLosses:
o ESR
o ESL
o Hysteresis
o Eddy Current
o SkinEffect
o 𝑃 𝐷𝑖𝑠𝑠𝑖𝑝𝑎𝑡𝑖𝑜𝑛= 𝐼 𝐿
2
∗ 𝐸𝑆𝑅
OutputCapacitorLoss:
o ESR
o ESL
o 𝑃 𝐷𝑖𝑠𝑠𝑖𝑝𝑎𝑡𝑒𝑑 = 𝐼 𝑅𝑖𝑝𝑝𝑙𝑒
2
∗ 𝐸𝑆𝑅
Basedon the total losses,the efficiencycanbe calculatedbydividingthe outputpowerof the buckby
the outputpowerplusthe losspower.
Synchronous Buck Converter Losses
The total lossesassociatedwithatypical synchronous
buck converterare due to:
InputCapacitorLosses:
o ESR
o ESL
o 𝑃 𝐷𝑖𝑠𝑠𝑖𝑝𝑎𝑡𝑒𝑑 = 𝐼 𝑅𝑖𝑝𝑝𝑙𝑒
2
∗ 𝐸𝑆𝑅
MOSFET Losses(HighSide):
o Conduction – Rdson
o Switching–Gate Charge
o 𝑃 𝐷𝑖𝑠𝑠𝑖𝑝𝑎𝑡𝑒𝑑 = 𝑃𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖 𝑜 𝑛 + 𝑃𝑆𝑤𝑖𝑡 𝑐ℎ𝑖𝑛𝑔
o 𝑃𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑜𝑛 = 𝐼 𝐿
2
∗ 𝑅 𝑑𝑠𝑜𝑛 ∗ 𝐷
𝐼 𝐿 = Load Current
D = Duty Cycle
11%
29%
45%
14%
1%
AsynchronousBuck
Converter Losses
Input
Capacitor
MOSFET
Diode
Inductor
Output
Capacitor
Figure 17 - Example Asynchronous Buck Converter Loss
Percentages [10]
Figure 18 - Example Synchronous Buck Converter Losses [10]
15. o 𝑃𝑆𝑤𝑖𝑡 𝑐ℎ = (𝑉 ∗
𝐼 𝐷
2
) ∗ ( 𝑇𝑜𝑛 ∗ 𝑇𝑜𝑓𝑓) ∗ 𝐹𝑠𝑤 + ( 𝐶 𝑜𝑠𝑠 ∗ 𝑉2 ∗ 𝐹𝑠𝑤)
𝑇𝑜𝑛 = Time on
𝑇𝑜𝑓𝑓 = Time off
𝐹𝑠𝑤 = SwitchingFrequency
𝐶 𝑜𝑠𝑠 = Capacitance
MOSFET RectifierLoss(LowSide):
o Conduction – Rdson – Verylow almost
negligible
o 𝑃𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑜𝑛 = 𝐼 𝐿
2
∗ 𝑅 𝑑𝑠𝑜𝑛 ∗ (1 − 𝐷)
o Switching–VeryLow almostnegligible
InductorLosses:
o ESR
o ESL
o Hysteresis
o Eddy Current
o SkinEffect
o 𝑃 𝐷𝑖𝑠𝑠𝑖𝑝𝑎𝑡𝑒𝑑 = 𝐼 𝐿
2
∗ 𝐸𝑆𝑅
OutputCapacitorLoss:
o ESR
o ESL
o 𝑃 𝐷𝑖𝑠𝑠𝑖𝑝𝑎𝑡𝑒𝑑 = 𝐼 𝑅𝑖𝑝𝑝𝑙𝑒
2
∗ 𝐸𝑆𝑅
Efficiency Conclusions
Both asynchronousandsynchronousbuck
converterspresenthigh(>90%) efficiency
ratingsas powersupplies.Close inspection
showsthat the synchronoustopologygenerates
nearlyhalf the overlossescomparedtothe
asynchronoustopology [10].
It isclear that the lossesfromthe diode inan
asynchronousbuckconverteramounttoa
significantportionof the total losses,
accountingforalmosthalf of the total lossesas
showninFigure 17.
In the synchronousbuckconverter,alow side
switchor MOSFET rectifierlossessubstantially
lesspowerthana diode. Itisapparentthat synchronousrectificationcanincrease apowerconverter’s
efficiencysignificantly,inthe examplesof Figure 8andFigure 10, an almost4% increase isexpected.
Thisincrease can become substantialwhenconsideringthe relative lossesassociatedwithlarge power
applicationsandthe periodof the use of the converter.
20%
51%
2%
25%
2%
Synchronous Buck
Converter Losses
Input Capacitor
MOSFET
MOSFET
Rectifier
Inductor
Figure 19 - Synchronous Buck Converter Loss Percentages
[10]
0.00
0.50
1.00
1.50
Asynchronous Synchronous
Comparison of Total Losses (W)
Figure 20 - Comparison of Total Losses
16. Overall efficiencyof buckconverterscaneasily be measuredand calculatedusingthe followingequation
relatingPowerouttoPowerin:
𝑛 =
𝑃𝑜𝑢𝑡
𝑃𝑖𝑛
=
𝑉𝑜𝑢𝑡 𝐼 𝑜𝑢𝑡
𝑉𝑖𝑛 𝐼𝑖𝑛
Filtering
Filteringisnecessarytosmooth andcleanup the outputand inputvoltagesof the buckconverter.
Ripple currentFiltering helpsremove noise atthe inputandoutputof the buck converter,suchas the
harmonicscreatedbyPWM at the outputwhichneedtobe filteredout.Differentarrangementsof
capacitors,inductorsandresistorsare usedto create differenttypesof filters. Dependingonthe chosen
valuesandarrangementof these components, alow-passorhigh-passfiltercanbe created,whichcan
allow loworhighfrequencysignalstopass. The use of resistorsinthese filtersshouldbe avoideddue to
theirlarge impedance andrelativelylarge losses. Forbuckconverterapplications,onlylightfilteringis
neededatthe input,assumingastable powersource,andlow passfilteringistypicallyusedatthe
output,to stabilize and removethe PWMharmonics.
Capacitor Selection
At the inputa capacitor issufficient tosmooththe input
signal fromthe powersupply. Inputfilteringcan reduce the
reflectedinputripple currentandreducesbothpeakcurrent
drawnfrom the inputsupplyandradiatednoise toother
elementsof the system.The inputcapacitorshouldhave a
ripple-currentratingnearthe selectedswitching frequency.
The inputcapacitor ripple currentcanbe approximatedas
[13]:
Inductor Selection
The low passfilterusedonthe outputeffectivelyallows alow frequencysignaltopassand attenuates
highfrequencysignals. The followingdiagramsillustrateboththe ideal andreal modelsof anLC filter.
Figure 21 - Input Capacitor
17. Figure 22 - Output Filter Circuit
Againthe ESR and ESL valuesdirectlyaffectthe outputsignal,asdescribedinthe ripple frequency
section[1].
The followingequationcalculatesthe frequencyof the filteringcomponents basedonthe cutoff
frequency(𝑓𝑐):
𝑓𝑐 =
1
2𝜋√ 𝐿 ∗ 𝐶
Thisfrequencyshouldbe significantlylowerthanthe switchingfrequencyof the convertertoadequately
attenuate highfrequencynoise andripple.
An appropriate inductorshouldhave low DCresistance(DCR),andshouldmeetthe peakcurrent
requirement.Itmustalsobe designedtooperate atthe desiredswitchingfrequency.
The lossescausedbythe inductor in the low passfilter,due toESR, hysteresisandEddycurrentsas
describedinthe efficiencysectioncanamounttomeasurable losses. Itisdifficulttoimprove upon
inductorlossesastheyare such inexpensive partsthatimprovementsinefficiencyare oftennegatedby
the cost. One wayto lowerinductorlosseswouldbe toreduce ESRand Eddycurrent lossesbyusing
bettercore material andimprovingthe structure of the inductor.Special windingandbraidingusinglitz
wire,aninsulatedthingauge wire couldhelpreducethose losses.
Testing Procedures
For both the Asynchronous and the Synchronous board designs testing parameters include:
Efficiency
Line Regulation
Load Regulation
Inductor Parasitic
Current sensor power loss
18. Low side switch loss
Efficiency
The efficiency of the systemis determined by the ratio of power out vs power in. To test
these values, a signal generator is connected to the input of the board to power it. Voltage and
current meters are connected to both the input and output sides of the board. The input side
measures the power coming from the signal generator, while the output will measure voltage
across the load and current from the inductor. After measurements are taken, efficiency will be
determined using the formula:
𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦% =
𝐼𝑛𝑝𝑢𝑡 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 ∗ 𝐼𝑛𝑝𝑢𝑡 𝐶𝑢𝑟𝑟𝑒𝑛𝑡
𝑂𝑢𝑡𝑝𝑢𝑡 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 ∗ 𝑂𝑢𝑡𝑝𝑢𝑡 𝐶𝑢𝑟𝑟𝑒𝑛𝑡
Current sensor power loss
The voltage across the current sensor will be measured with a voltmeter using an
allotted test point. Current is calculated using ohms law. The product of voltage and current
across current sensor will determine power loss of the current sensor.
Low side switchloss
For the low side switch, an oscilloscope can be used to measure the signal across the
switch. These measurements give the voltage curve of the low side switch. Dividing these
measurements by the known current sensor resistance gives the current curve of the switch.
The area where the two signals are both on as well as the switching frequency will determine
the switching loss.
Glossary
MOSFET:
Rdson – The inherentonresistance betweenthe drainandthe source of the MOSFET duringthe
on state.The Rdsonof NTMFS4927N [6]:
19. Figure 24 - NTMFS4927N Rdson Characteristics
Gate Charge – The total Gate Charge (Qg) is a functionof the gate to source voltage.Asthe gate-
source voltage increases,the total Qgincreases.Itisa capacitance that buildsupbetweenthe
gate as currentflowsintoit.Qgsis the gate-source charge,Qgdisthe gate-drainor“Miller
charge” [11].
Figure 23 - NTMFS4927N MOSFET
Rdson
20. Inductor/Capacitor[1]:
ESR - Equivalentseriesresistance –Aninherentresistance value thatisparasitic.Inductorshave
resistance inherentinthe metal conductor,the DCparameterinSMPS designisanimportant
parameter.Canbe modeledasa resistorinserieswithaninductor.
ESL – Equivalentseriesinductance –The inductance builtupinherentlyfromthe component.A
parasiticinductance value.
DCR – DC Resistance,inherentresistanceof the metal incomponent.
Hysteresis –The historyof the inputandits current
state create a dependence of the output thataffects
the value of itsfuture state.Valueschange and
predictedfuture valuesare differentbasedonthe
increase ordecrease of the inputand itshistory.
Hysteresisof aferroelectricmaterial. Disthe electric
displacementfield,Eiselectricfield.Thiscurve forms
a hysteresisloopbasedonthe directionof the
values.
Eddy Currents – Faraday’slawof inductionstatesthatcircularelectriccurrentsare induced
withinconductorsbya changingmagneticfield.Eddycurrentsgenerate resistive losses.
Diode:
ForwardVoltage – The ‘threshold’orforwardvoltage potential requiredforthe diode tobe
turnedon andconduct duringforwardbias.Thisvoltage isdifferentfordifferenttypesof diode
materials.
Figure 25 - Gate-to-Source Voltage vs Total Gate Charge
Figure 26 - Gate-Source Voltage vs Charge
Figure 27 - Hysteresis of Ferroelectric
Material
![well asbasicfiltering are alsopresented inrelationtobuckconverters.The efficiencyof the buck
convertersisbrokendownanddetailed,asitis paramountto thisformof powersupply.
There are many factors and parametersto considerwhenplanningabuckconverter, andwiththese in
mind,the noteswill guide youthroughthe process of basicbuckconverterdesign.Finallythe notes
presentthe ON SemiconductorCapstone Team’sdesignandselectionprocessusedto designandbuild
asynchronousandsynchronousbuckconverters.
Terminology
Input Range
The range of inputvoltage the device canhandle tofunction
effectivelyatfull load.
Load Regulation
Load regulationisthe change inoutputvoltage overthe specified
change in outputload,expressedinpercentage.Asthe outputload
changes,the outputvoltage shouldremainconsistent(typically
millivoltscale).
To the right,the Load Regulationof the TI TPS65251 Buck Converter
isshownto have a maximumchange of approximately5mV in
outputvoltage,a0.05% change overthe specifiedrange of output
load[1].
Line Regulation
Line regulationisthe change inoutputvoltage fora givenchange in
inputvoltage,alsoexpressedaspercentage (typicallymillivolt
scale).
To the right,the Line Regulationof TITPS65251 BuckConverteris
shownto have lessthan10mV increase inoutputvoltage overa
large range of inputvoltage,approximately0.05% [1].
Input and Output Ripple and Noise
Inputand outputripple pertainstothe amountof voltage or
currentdrop at the inputor outputbetweenswitchingcycles.A waveformappearsfromthe switching
of the device whichgivesaslightlyinconsistentvoltage orcurrentvalue atthe inputor output.
Figure 1 - Load Regulation [1]
Figure 2 - Line Regulation [1]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)