8
Ifahigherinductancevalueisselectedtheoutput
ripplevoltageisreduced,howeverasthecurrentrating
dropsthesizeoftherequiredinductorisincreased.
Ontheotherhand,ifalowerinductancevalueis
selectedthesizeoftherequiredinductorisreduced
while peak current is increased, thus slightly
increasingthelosswhenaloadisapplied.
WhiletheMD3221Nincorporatesanover-current
protectionfunction,theinductorselectedmustbesuch
thatmagneticsaturationdoesnotoccurattheover-
currentdetectionpoint.
Asamagneticfieldisgeneratedinthevicinityofthe
inductor,componentlayoutandpatterndesignmust
besuchastoensurethattheydonotaffectthe
controlcircuit.Toroidalorclosedmagneticcircuitpot
typeinductorsarerecommendedforapplicationslikely
to be affected by magnetic field leakage and
radiatednoise.
◆
SelectingtheOutputCondenser(C5)
Anoutputcondenserisrequiredtoreduceoutputripple
andthusobtainastableDCvoltage.Theconverter
outputripplevoltageisdeterminedbyΔIand
condenserimpedance.Theoutputcondenseris
selectedinrelationtotheimpedance.Selecttheoutput
condenserbased on thedesiredoutput ripple
voltageusingthefollowingequation.
Zc :Condenserimpedance[Ω]
Vrip:Outputripplevoltage[V]
ΔI:30%ofmaximumoutputcurrent[A]
Thedesiredripplevoltageisobtainedifacondenser
withanimpedancelessthanthatcalculatedwiththe
aboveequationisselectedfromthecatalog.
In addition to a low impedance, the output
condensermustalsohaveareasonablecapacitance.
Controlwillreadilybecomeunstableandampgain
adjustmentwillprovedifficultifcapacitanceislow.
Aluminumelectrolyticcondensersandfunctional
high-polymer electrolytic condensers are of
sufficientlyhighcapacitancesothatitneednotbe
consideredwhenselectingonthebasisofthe
equationabove.Theuseofsuchcondensers,rather
than ceramic or film condensers, is therefore
recommended.Ontheotherhand,concurrentuseof
ceramicorfilmcondensersiseffectiveineliminating
highfrequencynoise.
◆SelectingtheInputCondenser(C1)
Asalargeripplecurrentflowsintheinputcondenser
itisnecessarytoconsidertheallowablevalueforripple
current.Selectaninputcondenserwithanallowable
ripplecurrentexceedingthevaluecalculatedinthe
equationbelow.
Irip :Allowableripplecurrent[A]
VO:Outputvoltage[V]
Vi :Inputvoltage[V]
D:Dutyratio
IO:Outputcurrent[A]
DistheratiobetweentheONintervalandthe
OFFinterval,andIripisatamaximumwhenthis
value is 0.5. The capacitance of the input
condenserneednotbeparticularlyhigh,however
careisrequiredifaceramicorfilmcondenseris
used.Aslargeripplevoltagesaregeneratedinthe
inputcondenserduringcharginganddischarge,a
particularly low input voltage may result in
operation becoming unstable. As this ripple
voltageisreturnedtotheinputlineothercircuits
poweredfromthesamelinemaybeaffected.
Interferencemayoccur,particularlywhenusing
multipleDC-DCconverters,andinsuchcasesan
inductorofafewμHisinsertedinthestagebefore
theinputcondensertoeliminatetheproblem.
Vrip
ZC[Ω]
ΔI
VO
D= Vi
Irip D(1−D)× IO[A]
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