LT3757 LINER [Linear Technology], LT3757 Datasheet - Page 20

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

applicaTions inForMaTion
LT3757
where V
V
2 to 2.5 times of:
L
which is usually specified in the transformer character-
istics. L
inductance with the secondary windings shorted. The
snubber capacitor value (C
the following equation:
where ∆V
∆V
D
Flyback Converter: Sense Resistor Selection
In a flyback converter, when the power switch is turned
on, the current flowing through the sense resistor
(I
I
Set the sense voltage at I
the SENSE current limit threshold with a 20% margin. The
sense resistor value can then be calculated to be:
0
LK
SN
SN
SENSE
SN
R
is the leakage inductance of the primary winding,
SENSE
C
results in a larger snubber loss. A reasonable V
should be higher than the sum of V
V
SENSE
is 5% to 10% of V
CN
OUT
) is:
N
LK
=
SN
S
= I
SN
•
can be obtained by measuring the primary
∆
is the snubber capacitor voltage. A smaller
N
=
is the voltage ripple across C
V
LP
P
I
SN
LP PEAK
80
V
(
•
SN
mV
R
CN
)
•
f
SN
LP(PEAK)
. The reverse voltage rating of
CN
) can be determined using
to be the minimum of
SN
CN
and V
. A reasonable
IN(MAX)
SN
is
.
Flyback Converter: Power MOSFET Selection
For the flyback configuration, the MOSFET is selected with
a V
reflected secondary voltage and the voltage spike due to
the leakage inductance. Approximate the required MOSFET
V
BV
where:
The power dissipated by the MOSFET in a flyback con-
verter is:
P
C
The first term in this equation represents the conduction
losses in the device, and the second term, the switching
loss. C
usually specified in the MOSFET characteristics.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
equation:
T
T
temperature rating. It is recommended to measure the
MOSFET temperature in steady state to ensure that absolute
maximum ratings are not exceeded.
J
DC
RSS
must not exceed the MOSFET maximum junction
DC
V
rating using:
J
FET
DS PEAK
= T
• f / 1A
rating high enough to handle the maximum V
DSS
(
RSS
= I
A
> V
+ P
2
is the reverse transfer capacitance, which is
M(RMS)
)
DS(PEAK)
=
FET
V
IN MAX
• θ
(
• R
JA
DS(ON)
= T
)
+
V
A
+ P
SN
+ 2 • V
FET
• (θ
2
DS(PEAK)
JC
+ θ
CA
• I
)
L(MAX)
IN
, the
3757fb
•