Datasheet

LT3757/LT3757A
19
3757afd
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
MIN
) occurs when the converter
has the minimum V
IN
and the maximum output power
(P
OUT
). Choose D3
MIN
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 a higher switch peak current).
The user can choose a D
MAX
as the start point. Then, the
maximum average primary currents can be calculated by
the following equation:
I
LP(MAX)
=I
SW(MAX)
=
P
OUT(MAX)
D
MAX
• V
IN(MIN)
• h
where h is the converter efficiency.
If the flyback converter has multiple outputs, P
OUT(MAX)
is the sum of all the output power.
The maximum average secondary current is:
I
LS(MAX)
=I
D(MAX)
=
I
OUT(MAX)
D2
where:
D2 = 1 – D
MAX
– D3
the primary and secondary RMS currents are:
I
LP(RMS)
= 2 • I
LP(MAX)
•
D
MAX
3
I
LS(RMS)
= 2 • I
LS(MAX)
•
D2
3
According to Figure 8, the primary and secondary peak
currents are:
I
LP(PEAK)
= I
SW(PEAK)
= 2 • I
LP(MAX)
I
LS(PEAK)
= I
D(PEAK)
= 2 • I
LS(MAX)
The primary and second inductor values of the flyback
converter transformer can be determined using the fol-
lowing equations:
L
P
=
D
2
MAX
• V
2
IN(MIN)
• h
2 • P
OUT(MAX)
• f
L
S
=
D2
2
• (V
OUT
+ V
D
)
2 • I
OUT(MAX)
• f
The primary to second turns ratio is:
N
P
N
S
=
L
P
L
S
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
SN
) can be calculated by the
following equation:
R
SN
= 2 •
V
2
SN
− V
SN
• V
OUT
•
N
P
N
S
I
2
SW(PEAK)
• L
LK
• f