LT3757 datasheet(20/36 Pages) LINER

20 / 36 page

LT3757

3757fb

applicaTions inForMaTion

whereVSNisthesnubbercapacitorvoltage.Asmaller

VSNresultsinalargersnubberloss.AreasonableVSNis

2to2.5timesof:

OUT

•

LLK is the leakage inductance of the primary winding,

whichisusuallyspecifiedinthetransformercharacter-

istics. LLK can be obtained by measuring the primary

inductance with the secondary windings shorted. The

snubbercapacitorvalue(CCN)canbedeterminedusing

thefollowingequation:

∆

•

where∆VSNisthevoltagerippleacrossCCN.Areasonable

∆VSNis5%to10%ofVSN.Thereversevoltageratingof

DSNshouldbehigherthanthesumofVSNandVIN(MAX).

Flyback Converter: Sense Resistor Selection

Inaflybackconverter,whenthepowerswitchisturned

on, the current flowing through the sense resistor

(ISENSE)is:

ISENSE=ILP

SetthesensevoltageatILP(PEAK)tobetheminimumof

theSENSEcurrentlimitthresholdwitha20%margin.The

senseresistorvaluecanthenbecalculatedtobe:

SENSE

LP PEAK

(

)

Flyback Converter: Power MOSFET Selection

Fortheflybackconfiguration,theMOSFETisselectedwith

aVDCratinghighenoughtohandlethemaximumVIN,the

reflectedsecondaryvoltageandthevoltagespikedueto

theleakageinductance.ApproximatetherequiredMOSFET

VDCratingusing:

BVDSS>VDS(PEAK)

where:

DS PEAK

IN MAX

(

)

(

)

ThepowerdissipatedbytheMOSFETinaflybackcon-

verteris:

PFET=I2M(RMS)•RDS(ON)+2•V2DS(PEAK)•IL(MAX)•

CRSS•f/1A

Thefirstterminthisequationrepresentstheconduction

lossesinthedevice,andthesecondterm,theswitching

loss.CRSSisthereversetransfercapacitance,whichis

usuallyspecifiedintheMOSFETcharacteristics.

FromaknownpowerdissipatedinthepowerMOSFET,its

junctiontemperaturecanbeobtainedusingthefollowing

equation:

TJ=TA+PFET•θJA=TA+PFET•(θJC+θCA)

TJ must not exceed the MOSFET maximum junction

temperaturerating.Itisrecommendedtomeasurethe

MOSFETtemperatureinsteadystatetoensurethatabsolute

maximumratingsarenotexceeded.

LT3757 Datasheet(PDF) 20 Page - Linear Technology

LT3757 Datasheet(HTML) 20 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL