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LT3757 데이터시트(PDF) 21 Page - Linear Technology

부품명 LT3757
상세설명  Boost, Flyback, SEPIC and Inverting Controller
PDF  36 Pages
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제조업체  LINER [Linear Technology]
홈페이지  http://www.linear.com
Logo LINER - Linear Technology

LT3757 데이터시트(HTML) 21 Page - Linear Technology

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LT3757

3757fb
applicaTions inForMaTion
Flyback Converter: Output Diode Selection
Theoutputdiodeinaflybackconverterissubjecttolarge
RMScurrentandpeakreversevoltagestresses.Afast
switchingdiodewithalowforwarddropandalowreverse
leakageisdesired.Schottkydiodesarerecommendedif
theoutputvoltageisbelow100V.
Approximatetherequiredpeakrepetitivereversevoltage
ratingVRRMusing:
V
N
N
V
V
RRM
S
P
IN MAX
OUT
>
+
•
(
)
Thepowerdissipatedbythediodeis:
PD=IO(MAX)•VD
andthediodejunctiontemperatureis:
TJ=TA+PD•RθJA
TheRθJAtobeusedinthisequationnormallyincludes
theRθJCforthedevice,plusthethermalresistancefrom
theboardtotheambienttemperatureintheenclosure.TJ
mustnotexceedthediodemaximumjunctiontemperature
rating.
Flyback Converter: Output Capacitor Selection
Theoutputcapacitoroftheflybackconverterhasasimilar
operationconditionasthatoftheboostconverter.Referto
theBoostConverter:OutputCapacitorSelectionsection
forthecalculationofCOUTandESRCOUT.
TheRMSripplecurrentratingoftheoutputcapacitors
indiscontinuousoperationcanbedeterminedusingthe
followingequation:
I
I
D
RMS COUT DISCONTINUOUS
O MAX
(
),
(
) •
( • )
•
≥
−
4 3 2
3 DD2
Flyback Converter: Input Capacitor Selection
Theinputcapacitorinaflybackconverterissubjectto
alargeRMScurrentduetothediscontinuousprimary
current.Topreventlargevoltagetransients,usealow
ESRinputcapacitorsizedforthemaximumRMScurrent.
TheRMSripplecurrentratingoftheinputcapacitorsin
discontinuous operation can be determined using the
followingequation:
I
P
V
RMS CIN DISCONTINUOUS
OUT MAX
IN MIN
(
),
(
)
(
) •
≥
h
••
( •
)
•
4 3
3
−
D
D
MAX
MAX
SEPIC CONVERTER APPLICATIONS
TheLT3757canbeconfiguredasaSEPIC(single-ended
primaryinductanceconverter),asshowninFigure1.This
topologyallowsfortheinputtobehigher,equal,orlower
thanthedesiredoutputvoltage.Theconversionratioas
afunctionofdutycycleis:
V
V
V
D
D
OUT
D
IN
+
=
−
1
incontinuousconductionmode(CCM).
InaSEPICconverter,noDCpathexistsbetweentheinput
andoutput.Thisisanadvantageovertheboostconverter
forapplicationsrequiringtheoutputtobedisconnected
fromtheinputsourcewhenthecircuitisinshutdown.
Comparedtotheflybackconverter,theSEPICconverter
hastheadvantagethatboththepowerMOSFETandthe
outputdiodevoltagesareclampedbythecapacitors(CIN,
CDCandCOUT),therefore,thereislessvoltageringing
acrossthepowerMOSFETandtheoutputdiodes.The
SEPICconverterrequiresmuchsmallerinputcapacitors
thanthoseoftheflybackconverter.Thisisduetothefact



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