Datasheet

LT3958
17
3958f
APPLICATIONS INFORMATION
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
LT3958 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
• 3.3A −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 cur-
rent 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. It is recommended to choose a
ΔI
SW
higher than 0.6A.
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 4A). 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
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 (such as RC
snubber, RCD snubber, etc.) and Application Note 19 is
a good reference on snubber design. An RCD snubber is
shown in Figure 6.