LT3758EDD View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LT3758EDD Datasheet PDF : 36 Pages
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LT3758/LT3758A
Applications Information
Flyback Converter: Transformer Design for
Discontinuous Mode Operation
The transformer design for discontinuous mode of opera-
tion is chosen as presented here. According to Figure 8,
the minimum D3 (D3MIN) occurs when the the converter
has the minimum VIN and the maximum output power
(POUT). Choose D3MIN to be equal to or higher than 10%
to guarantee the converter is always in discontinuous
mode operation. Choosing higher D3 allows the use of
low inductances but results in higher switch peak current.
The user can choose a DMAX as the start point. Then, the
maximum average primary currents can be calculated by
the following equation:
ILP(MAX )
=
ISW(MAX
)
=
POUT(MAX )
DMAX • VIN(MIN)
•
h
where h is the converter efficiency.
If the flyback converter has multiple outputs, POUT(MAX)
is the sum of all the output power.
The maximum average secondary current is:
ILS(MAX )
=
ID(MAX
)
=
IOUT(MAX
D2
)
where
D2 = 1 – DMAX – D3
the primary and secondary RMS currents are:
ILP(RMS) = 2 • ILP(MAX) •
DMAX
3
ILS(RMS) = 2 • ILS(MAX) •
D2
3
According to Figure 8, the primary and secondary peak
currents are:
ILP(PEAK) = ISW(PEAK) = 2 • ILP(MAX)
ILS(PEAK) = ID(PEAK) = 2 • ILS(MAX)
The primary and second inductor values of the flyback
converter transformer can be determined using the fol-
lowing equations:
LP
=
D
2
MAX
•
V
2
IN(MIN)
•
2 • POUT(MAX) • f
h
LS
=
D22 • (VOUT +
2 • IOUT(MAX)
VD)
•f
The primary to second turns ratio is:
NP = LP
NS LS
Flyback Converter: Snubber Design
Transformer leakage inductance (on either the primary or
secondary) causes a voltage spike to occur after the MOS-
FET turn-off. This is increasingly prominent at higher load
currents, where more stored energy must be dissipated.
In some cases a snubber circuit will be required to avoid
overvoltage breakdown at the MOSFET’s drain node. There
are different snubber circuits, and Application Note 19 is
a good reference on snubber design. An RCD snubber is
shown in Figure 7.
The snubber resistor value (RSN) can be calculated by the
following equation:
RSN
=
2
•
V
2
SN
−
VSN
•
VOUT
•
NP
NS
I2SW(PEAK) • LLK • f
where VSN is the snubber capacitor voltage. A smaller
VSN results in a larger snubber loss. A reasonable VSN is
2 to 2.5 times of:
VOUT • NP
NS
3758afd
19
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