Datasheet

LT3758/LT3758A
20
3758afd
ApplicAtions inForMAtion
L
LK
is the leakage inductance of the primary winding, which
is usually specified in the transformer characteristics. 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
∆V
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 absolute
maximum ratings are not exceeded.
Flyback Converter: Output Diode Selection
The output diode in a flyback 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
RRM
using:
V
RRM
>
N
S
N
P
• V
IN(MAX)
+ V
OUT
The power dissipated by the diode is:
P
D
= I
O(MAX)
• V
D
and the diode junction temperature is:
T
J
= T
A
+ P
D
• R
θJA
The R
θJA
to be used in this equation normally includes the
R
θJC
for the device, plus the thermal resistance from the
board to the ambient temperature in the enclosure. T
J
must
not exceed the diode maximum junction temperature rating.
Flyback Converter: Output Capacitor Selection
The output capacitor of the flyback 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
OUT
and ESR
COUT
.
The RMS ripple current rating of the output capacitors
in discontinuous operation can be determined using the
following equation:
I
RMS(COUT),DISCONTINUOUS
≥ I
O(MAX)
•
4 − (3 • D2)
3 • D2
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