LT3758 LINER [Linear Technology], LT3758 Datasheet - Page 21

• Manufacturer Part Number: LT3758
• Description: High Input Voltage, Boost, Flyback, SEPIC and Inverting Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
L
which is usually specifi ed in the transformer character-
istics. L
inductance with the secondary windings shorted. The
snubber capacitor value (C
the following equation:
where ΔV
ΔV
D
Flyback Converter: Sense Resistor Selection
In a fl yback converter, when the power switch is turned on,
the current fl owing through the sense resistor (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:
Flyback Converter: Power MOSFET Selection
For the fl yback confi guration, the MOSFET is selected with
a V
refl ected secondary voltage and the voltage spike due to
the leakage inductance. Approximate the required MOSFET
V
where
The power dissipated by the MOSFET in a fl yback con-
verter is:
The fi rst term in this equation represents the conduction
losses in the device, and the second term, the switching
loss. C
usually specifi ed in the MOSFET characteristics.
LK
DC
SN
SN
I
BV
V
P
C
R
DC
SENSE
C
DS(PEAK)
FET
RSS
is the leakage inductance of the primary winding,
SENSE
rating using:
should be higher than the sum of V
CN
DSS
is 5% to 10% of V
rating high enough to handle the maximum V
RSS
=
= I
LK
• f/1A
SN
Δ
= I
> V
=
2
V
M(RMS)
is the reverse transfer capacitance, which is
can be obtained by measuring the primary
I
is the voltage ripple across C
LP
SN
LP PEAK
DS(PEAK)
= V
80
V
(
•
SN
mV
R
IN(MAX)
CN
• R
)
•
f
DS(ON)
+ V
SN
LP(PEAK)
. The reverse voltage rating of
CN
SN
+ 2 • V
) can be determined using
to be the minimum of
2
DS(PEAK)
SN
CN
. A reasonable
and V
• I
SENSE
L(MAX)
IN(MAX)
IN
, the
) is:
•
.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
equation:
T
temperature rating. It is recommended to measure the
MOSFET temperature in steady state to ensure that absolute
maximum ratings are not exceeded.
Flyback Converter: Output Diode Selection
The output diode in a fl yback converter is subject to large
RMS current and peak reverse voltage stresses. A fast
switching diode with a low forward drop and a low reverse
leakage is desired. Schottky diodes are recommended if
the output voltage is below 100V.
Approximate the required peak repetitive reverse voltage
rating V
The power dissipated by the diode is:
and the diode junction temperature is:
The R
R
to the ambient temperature in the enclosure. T
exceed the diode maximum junction temperature rating.
Flyback Converter: Output Capacitor Selection
The output capacitor of the fl yback converter has a similar
operation condition as that of the boost converter. Refer to
the Boost Converter: Output Capacitor Selection section
for the calculation of C
The RMS ripple current rating of the output capacitors
in discontinuous operation can be determined using the
following equation:
J
θJC
T
P
T
I
V
must not exceed the MOSFET maximum junction
RMS COUT DISCONTINUOUS
J
J
D
RRM
for the device, plus the thermal resistance from the board
= T
= T
θJA
= I
(
RRM
>
A
O(MAX)
A
to be used in this equation normally includes the
+ P
N
N
+ P
S
P
),
using:
•
FET
D
V
• R
• V
IN MAX
• θ
(
D
θJA
JA
)
= T
OUT
+
V
A
OUT
and ESR
≥
+ P
I
O MAX
FET
(
• (θ
COUT
)
•
JC
4
.
+ θ
−
3 D D 2
LT3758
( •
•
3
CA
J
D
)
must not
2
)
21
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