Soft-chargingOperationofSwitchedCapacitorConverters
RobertPilawa-Podgurski,Yutian Lei
UniversityofIllinoisUrbana-Champaign
ECE598RPP,April7th,2016
Outline
§ Introductionofsoft-chargingconcept
§ Soft-chargingoperationwithaninductor
§ Example2-to-1converter
§ Frequencydependent behavior
§ ZCSresonantoperationalsopossible
§ ExtendingtoanarbitrarySCtopology
§ AnalysisofSCconvertersforsoft-charging operation
§ Losslessregulation
§ Concludingremarks.
SoftChargingOperation
n
n
Controlledchargingcurrent
Recoverenergythatisnormallydissipated
ReviewofSoft-chargingConcept
§ Toimprovetheenergyutilizationofcapacitors,
capacitorvoltageripplemustincrease.
§ Powerlossincreasesduetohighcapacitorvoltage
mismatch
§ Outputvoltagerippleincreases
§ Aninterfacingelementisneeded
§ Toaccommodatethechangeincapacitorvoltage
§ Toshieldtheoutputfromseeingthelargeripple
§ Inductoralonecanaccomplishthis
Soft-chargingwithanInductor
Aninductoractsasacontrolledcurrentsource.
2-to-1
converter:
Phase1:
Phase2:
Soft-chargingwithanInductor
Aninductoractsasacontrolledcurrentsource.
2-to-1
converter:
Capacitor
current:
Simulation parameters
Conventional
Soft-charging
Cfly
10µF
Loadcurrent (A)
2.00
2.00
Cout
100µF
CapacitorRMScurrent (A)
3.01
2.00
R on
10mΩ
Capacitor Maximumcurrent (A)
9.3
2.09
fsw
500kHz
L1
0.1µH
FrequencyDependentBehavior
FrequencyDependentBehavior
fsw =1/2 fcrit
fsw =fcrit
fsw >fcrit
Full-waveresonant
Half-waveresonant(ZCS)
Soft-charging
FrequencyDependentBehavior
§ Soft-chargingoperationcanbeachievedwithan
inductor
§ AchievesFSLatmuchlowerswitchingfrequency.
§ Desirableforheavyload.
§ Switchingfrequencyislowerboundedbytheresonant
frequency.
§ ZCSresonantoperation
§ Canalsobeachievedwithaninductor.
§ Canbeviewedasaspecialcaseofsoft-charging.
§ Hashigherconductionlossduetosinusoidal current
waveform.
§ Desirableatlightload.
FundamentalTrade-offs
§ SCconverters
'
§ 𝑓"#$% = ()*+
§ Alwaysatrade-off
between𝑓"#$%,𝑅./and𝐶
§ Soft-chargingSCconverters
'
§ 𝑓"#$% = () 1+
§ Thecriticalfrequencyis
independentof𝑅./
§ Theinductorintroducesanadditionaldesigndimensionin
whichtheconvertercanbeoptimized.
RequirementofSoft-chargingOperation
§ Soft-chargingoperationisdemonstratedwitha2-to-1
SCconverter.HowaboutotherSCconverters?
§ Requirementofsoft-charging
1. Theoutputmustchangeinstantaneouslytoaccommodate
capacitorvoltagemismatch.->Handledbytheinductor
2. KVLmustbesatisfiedfortheinternalparallelconnectionof
capacitors.->Notapparentforcomplextopologies
§ Needageneralmethodthatcanbeappliedtoany
givenSCtopology.
AnalysisofSCConverters
§ Dicksonconverterasanexample.
§ Forsimplicity,useanidealcurrentsourceastheload.
§ ThegoalistofindifKVLforthecapacitornetworkissatisfiedduring
thephasetransitions.
Phase1:
Phase2:
AnalysisofSCConverters
§ AKVLequationcanbewrittenforeachloop.
§ Matrixrepresentation:reducedloopmatrix
Phase1:
Phase2:
AnalysisofSCConverters
AnalysisofSCConverters
§ Let’sexamine
(Constantinput voltage)
(Periodic steady-state)
§ Howabout?
ForconventionalSCconverter, becauseofoutput capacitance:
Forsoft-charging converter, becauseoftheconstantcurrentloadandtheremoval
oftheoutput capacitance,theoutput voltageisabletochangeinstantaneously
during phasetransitions.Thequantitieshereonlyconcernsthechangeinvoltage
withineachphase.Thus,
AnalysisofSCConverters
§ ConventionalSC
Remember:
Wecanfindthebasisofvectorsthatspanthecommonnullspace
ofA1andA2.However,thecommonnullspace isempty.
ItmeansthatKVLcannotbesatisfiedduringphasetransitionfora
conventionalSCconverter.Capacitorcharginglossisinevitable.
§ Soft-chargingSC
AnalysisResult
§ FortheDicksonconverter,
§ ThisisthechangeinthecapacitorvoltagesineachphasetosatisfyKVL.
§ Alsoneedthechargeflowvector,obtainedfromKCL:
3
§ Capacitorvaluescanbeobtainedby𝑐$ = 564
§ Finallywehave
7,4
SimulationResult
§
§
Dicksonconverter
cannotachieve
completesoftcharging.
Howeveritcan
approachcomplete
soft-charging by
makingC2 much
largerthanC1 andC3
SimulationResult
§ Capacitorcurrent:
CommonSCTopologies
§ Series-parallel:
§ Ladder:
§ Fibonacci:
§ Doubler:
AnalysisResultofCommonTopologies
§ Series-parallel:
1
𝑪= 1
1
§ Fibonacci:
1
𝑪= 1
1
§ Ladder:
1
𝑪 = −2
1
§ Doubler:
1
𝑪= ∞
∞
AnalysisResultofCommonTopologies
§ Series-parallel:
§ Ladder:
§ Fibonacci:
§ Doubler:
AnalysisofSCConverters
§ CommonSCtopologies
§ Series-parallelandFibonacci convertercan
achievecompletesoft-charging.
§ Dicksonconvertercanapproachcompletesoftchargingbyresizingthecapacitors.
§ Doublerandladderconverteronlyyieldslimited
benefitfromsoft-chargingoperation.
§ ThemethodcanbeappliedtoanySCtopology
anddeterminesifsoft-charging ispossible.
RegulationofSCConverters
§ Regulationofoutputvoltageisachievedbymodulatingthe
outputimpedance.
§ ModulateRout bychanging
§
§
§
§
Switchingfrequency
Dutyratio
PulsedensityinPFM
Switchingconductance
§ Regardlessofmethod,theoperationislossy.
RegulationofSoft-chargingDicksonConverter
§ Withasecond-stagebuck
converter, wecanregulatethe
output voltage.
§ Withjustaninductor andexisting
switches,need tointroduce a
regulation phase.
Originaltwophases
Regulationphase
RegulationofSoft-chargingDicksonConverter
§ Onecompleteswitchingcycle:
§ Output voltagecanberegulatedbyvaryingthe
relativeduring ofthephases.
§ Similartopulse widthmodulation.
§ Noincreaseinswitchingfrequency.
CaseStudy:an8-to-1DicksonSCconverter
§ 12GaN switches
§ 7flyingcapacitorssizedaccording to
theprecedinganalysisresultto
approachcompletesoft-charging
ExperimentalResults
§ Measuredefficiencyatfixed
conversionratio.
§ Soft-charginghashigherefficiency
forloadcurrentlargerthan1A.
§ Measuredefficiencywithoutput
voltageregulatedat4Vandload
currentat1.2A.
§ Efficiencyismaintainedasvoltage
convectionratioincreases.
ConcludingRemarks
§ Thesoft-chargingconverterprototypeachieveshigherefficiencyand
higherpowerdensitysimultaneously,comparedtoconventionalSC
converters,duetobetterutilizationofpassivecomponents.
§ Itisalsoabletoregulatetheoutputvoltagelosslessly.
Comparison among Converter Topologies
Features
Dickson
SC
Yes
Buck
No
Soft-charging
Dickson
Yes
Capacitor
Large
None*
Small
Inductor
None
Large
Small
No
Yes
Yes*
Low voltage switches
Lossless regulation
§ Topologytrade-offs
§ Thebestconverterdependsontheapplicationrequirement
§ FullICintegration->ConventionalSC
§ Fullrangeregulation->Buck
§ Highconversionratio,highefficiencyandpowerdensity->Soft-chargingSC