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LT3757 датащи(PDF) 19 Page - Linear Technology

номер детали LT3757
подробное описание детали  Boost, Flyback, SEPIC and Inverting Controller
PDF  36 Pages
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производитель  LINER [Linear Technology]
домашняя страница  http://www.linear.com
Logo LINER - Linear Technology

LT3757 датащи(HTML) 19 Page - Linear Technology

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LT3757

3757fb
applicaTions inForMaTion
Flyback Converter: Transformer Design for
Discontinuous Mode Operation
Thetransformerdesignfordiscontinuousmodeofopera-
tionischosenaspresentedhere.AccordingtoFigure8,
the minimum D3 (D3MIN)occurswhentheconverter
hastheminimumVINandthemaximumoutputpower
(POUT).ChooseD3MINtobeequaltoorhigherthan10%
toguaranteetheconverterisalwaysindiscontinuous
mode operation (choosing higher D3 allows the use
oflowinductances,butresultsinahigherswitchpeak
current).
TheusercanchooseaDMAXasthestartpoint.Then,the
maximumaverageprimarycurrentscanbecalculatedby
thefollowingequation:
I
I
P
D
V
LP MAX
SW MAX
OUT MAX
MAX
IN MIN
(
)
(
)
(
)
(
)
•
•
=
=
h
where
histheconverterefficiency.
Iftheflybackconverterhasmultipleoutputs,POUT(MAX)
isthesumofalltheoutputpower.
Themaximumaveragesecondarycurrentis:
I
I
I
D
LS MAX
D MAX
OUT MAX
(
)
(
)
(
)
=
=
2
where:
D2=1–DMAX–D3
theprimaryandsecondaryRMScurrentsare:
I
I
D
LP RMS
LP MAX
MAX
(
)
(
)
•
•
= 2
3
I
I
D
LS RMS
LS MAX
(
)
(
)
•
•
= 2
2
3
AccordingtoFigure8,theprimaryandsecondarypeak
currentsare:
ILP(PEAK)=ISW(PEAK)=2•ILP(MAX)
ILS(PEAK)=ID(PEAK)=2•ILS(MAX)
Theprimaryandsecondinductorvaluesoftheflyback
convertertransformercanbedeterminedusingthefol-
lowingequations:
L
D
V
P
f
L
D
V
P
MAX
IN MAX
OUT MAX
S
O
=
=
2
2
2
2
2
•
•
•
•
•(
(
)
(
)
h
UUT
D
OUT MAX
V
I
f
+ )
•
•
(
)
2
Theprimarytosecondturnsratiois:
N
N
L
L
P
S
P
S
=
Flyback Converter: Snubber Design
Transformer leakage inductance (on either the primary
orsecondary)causesavoltagespiketooccurafterthe
MOSFETturn-off.Thisisincreasinglyprominentathigher
loadcurrents,wheremorestoredenergymustbedis-
sipated.Insomecasesasnubbercircuitwillberequired
toavoidovervoltagebreakdownattheMOSFET’sdrain
node.Therearedifferentsnubbercircuits,andApplication
Note19isagoodreferenceonsnubberdesign.AnRCD
snubberisshowninFigure7.
Thesnubberresistorvalue(RSN)canbecalculatedbythe
followingequation:
R
V
V
V
N
N
I
L
f
SN
SN
SN
OUT
P
S
SW PEAK
LK
=
−
2
2
2
•
•
•
•
•
(
)



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