LT3757 LINER [Linear Technology], LT3757 Datasheet - Page 19

• Manufacturer Part Number: LT3757
• Description: Boost, Flyback, SEPIC and Inverting Controller
• Manufacturer: LINER [Linear Technology]

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 (D3
has the minimum V
(P
to guarantee the converter is always in discontinuous
mode operation (choosing higher D3 allows the use
of low inductances, but results in a higher switch peak
current).
The user can choose a D
maximum average primary currents can be calculated by
the following equation:
where h is the converter efficiency.
If the flyback converter has multiple outputs, P
is the sum of all the output power.
The maximum average secondary current is:
where:
D2 = 1 – D
the primary and secondary RMS currents are:
OUT
I
I
I
I
LP MAX
LS MAX
LP RMS
LS RMS
(
(
(
(
). Choose D3
)
)
)
)
=
=
= 2
= 2
I
I
MAX
D MAX
SW MAX
(
•
•
I
I
LP MAX
LS MAX
(
– D3
MIN
(
(
)
IN
=
)
and the maximum output power
to be equal to or higher than 10%
MIN
=
I
)
OUT MAX
)
MAX
D
•
•
) occurs when the converter
MAX
D
(
2
as the start point. Then, the
D
D
P
3
MAX
2
OUT MAX
3
•
)
V
IN MIN
(
(
)
)
•
h
OUT(MAX)
According to Figure 8, the primary and secondary peak
currents are:
I
I
The primary and second inductor values of the flyback
converter transformer can be determined using the fol-
lowing equations:
The primary to second turns ratio is:
Flyback Converter: Snubber Design
Transformer leakage inductance (on either the primary
or secondary) causes a voltage spike to occur after the
MOSFET turn-off. This is increasingly prominent at higher
load currents, where more stored energy must be dis-
sipated. 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 (R
following equation:
L
L
R
N
N
LP(PEAK)
LS(PEAK)
P
S
SN
P
S
=
=
=
=
D
D
2
2
2
2
MAX
L
L
2
2
•
= I
= I
•
I
P
S
•(
•
OUT MAX
V
P
SW(PEAK)
D(PEAK)
V
OUT MAX
2
•
I
O
SN
2
(
V
U U T
SW PEAK
2
(
IN MAX
−
+
(
(
)
V
= 2 • I
V
•
SN
= 2 • I
)
D
f
•
)
•
)
)
f
•
V
•
LS(MAX)
L
h
OUT
SN
LK
LP(MAX)
) can be calculated by the
•
•
N
N
f
P
S
LT3757

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