LT3957
17
3957f
APPLICATIONS INFORMATION
According to the Absolute Maximum Ratings table, the SW
voltage Absolute Maximum value is 40V. Therefore, the
maximum primary to secondary turns ratio (for both the
continuous and the discontinuous operation) should be.
N
P
N
S
≤
40V − V
IN(MAX)
k•V
OUT
According to the preceding equations, the user has relative
freedom in selecting the switch duty cycle or turns ratio to
suit a given application. The selections of the duty cycle
and the turns ratio are somewhat iterative processes, due
to the number of variables involved. The user can choose
either a duty cycle or a turns ratio as the start point. The
following trade-offs should be considered when select-
ing the switch duty cycle or turns ratio, to optimize the
converter performance. A higher duty cycle affects the
fl yback converter in the following aspects:
• Lower MOSFET RMS current I
SW(RMS)
, but higher
MOSFET V
SW
peak voltage
• Lower diode peak reverse voltage, but higher diode
RMS current I
D(RMS)
• Higher transformer turns ratio (N
P
/N
S
)
It is recommended to choose a duty cycle between 20%
and 80%.
Flyback Converter: Maximum Output Current
Capability and Transformer Design
The maximum output current capability and transformer
design for continuous conduction mode (CCM) is chosen
as presented here.
The maximum duty cycle (D
MAX
) occurs when the converter
has the minimum V
IN
:
D
MAX
=
V
OUT
•
N
P
N
S
⎛
⎝
⎜
⎞
⎠
⎟
V
OUT
•
N
P
N
S
⎛
⎝
⎜
⎞
⎠
⎟
+ V
IN(MIN)
Due to the current limit of its internal power switch, the
LT3957 should be used in a fl yback converter whose maxi-
mum output current (I
O(MAX)
) is less than the maximum
output current capability by a suffi cient margin (10% or
higher is recommended):
I
O(MAX)
<
V
IN(MIN)
V
OUT
•D
MAX
•5A− 0.5 • ΔI
SW
(
)
The transformer ripple current ΔI
SW
has a direct effect on
the design/choice of the transformer and the converter’s
output current capability. Choosing smaller values of
ΔI
SW
increases the output current capability, but requires
large primary and secondary inductances and reduce the
current loop gain (the converter will approach voltage
mode). Accepting larger values of ΔI
SW
allows the use
of low primary and secondary inductances, but results
in higher input current ripple, greater core losses, and
reduces the output current capability.
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the primary
winding, the primary winding inductance can be calculated
using the following equation:
L =
V
IN(MIN)
ΔI
SW
•ƒ
•D
MAX
The primary winding peak current is the switch current
limit (typical 5.9A). The primary and secondary maximum
RMS currents are:
I
LP(RMS)
≈
P
OUT(MAX)
D
MAX
•V
IN(MIN)
• η
I
LS(RMS)
≈
I
OUT(MAX)
1−D
MAX
where η is the converter effi ciency.
Based on the preceding equations, the user should de-
sign/choose the transformer having suffi cient saturation
and RMS current ratings.
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.
