LT3758 LINER [Linear Technology], LT3758 Datasheet - Page 20

• Manufacturer Part Number: LT3758
• Description: High Input Voltage, Boost, Flyback, SEPIC and Inverting Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3758
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 higher switch peak current.
The user can choose a D
maximum average primary currents can be calculated by
the following equation:
where η is the converter effi ciency.
If the fl yback 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:
According to Figure 8, the primary and secondary peak
currents are:
20
OUT
I
I
I
I
I
I
LP(PEAK)
LS(PEAK)
LP MAX
LS MAX
LP RMS
LS RMS
). Choose D3
(
(
(
(
)
)
)
)
MAX
=
=
=
=
= I
= I
I
I
2
2
D MAX
SW MAX
SW(PEAK)
D(PEAK)
(
•
•
– D3
I
I
LP MAX
LS MAX
(
MIN
(
(
)
IN
MIN
=
)
= 2 • I
to be equal to or higher than 10%
=
I
and the maximum output power
) )
OUT MAX
)
= 2 • I
) occurs when the the converter
MAX
D
•
•
MAX
D
(
2
D2
LS(MAX)
D
as the start point. Then, the
P
3
LP(MAX)
MAX
OUT MAX
3
•
)
V
IN MIN
(
(
)
)
•
η
OUT(MAX)
The primary and second inductor values of the fl yback
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 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 (R
following equation:
where V
V
2 to 2.5 times of:
SN
L
L
R
N
N
V
P
S
results in a larger snubber loss. A reasonable V
SN
P
S
OUT
=
=
=
N
=
D
D
SN
S
2
2
2
2
•
MAX
L
L
N
2
2
•
•
is the snubber capacitor voltage. A smaller
I
P
S
P
•(
•
OUT MAX
V
P
V
OUT MAX
2
•
I
O
SN
2
(
V
U U T
SW PEAK
2
(
IN MAX
−
+
(
(
)
V
V
•
SN
)
D
f
•
)
•
)
)
f
•
V
•
L
η
OUT
SN
LK
) can be calculated by the
•
•
N
N
f
P
S
SN
3758f
is