Datasheet

LT3757/LT3757A
20
3757afe
For more information www.linear.com/LT3757
applicaTions inForMaTion
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
where V
SN
is the snubber capacitor voltage. A smaller
V
SN
results in a larger snubber loss. A reasonable V
SN
is
2 to 2.5 times of:
V
OUT
• N
P
N
S
L
LK
is the leakage inductance of the primary winding,
which is usually specified in the transformer character-
istics. L
LK
can be obtained by measuring the primary
inductance with the secondary windings shorted. The
snubber capacitor value (C
CN
) can be determined using
the following equation:
C
CN
=
V
SN
ΔV
SN
• R
CN
• f
where ∆V
SN
is the voltage ripple across C
CN
. A reasonable
∆V
SN
is 5% to 10% of V
SN
. The reverse voltage rating of
D
SN
should be higher than the sum of V
SN
and V
IN(MAX)
.
Flyback Converter: Sense Resistor Selection
In a flyback converter, when the power switch is turned
on, the current flowing through the sense resistor
(I
SENSE
) is:
I
SENSE
= I
LP
Set the sense voltage at I
LP(PEAK)
to be the minimum of
the SENSE current limit threshold with a 20% margin. The
sense resistor value can then be calculated to be:
R
SENSE
=
80mV
I
LP(PEAK)
Flyback Converter: Power MOSFET Selection
For the flyback configuration, the MOSFET is selected
with a V
DC
rating high enough to handle the maximum
V
IN
, the reflected secondary voltage and the voltage spike
due to the leakage inductance. Approximate the required
MOSFET V
DC
rating using:
BV
DSS
> V
DS(PEAK)
where:
V
DS(PEAK)
=
V
IN(MAX)
+
V
SN
The power dissipated by the MOSFET in a flyback con-
verter is:
P
FET
= I
2
M(RMS)
• R
DS(ON)
+ 2 • V
2
DS(PEAK)
• I
L(MAX)
•
C
RSS
• f /1A
The first term in this equation represents the conduction
losses in the device, and the second term, the switching
loss. C
RSS
is the reverse transfer capacitance, which is
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
J
= T
A
+ P
FET
• θ
JA
= T
A
+ P
FET
• (θ
JC
+ θ
CA
)
T
J
must not exceed the MOSFET maximum junction
temperature rating. It is recommended to measure the
MOSFET temperature in steady state to ensure that abso-
lute maximum ratings are not exceeded.
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