Datasheet

LT3957A
17
3957afa
APPLICATIONS INFORMATION
The primary winding peak current is the switch current
limit (typical 5.9A). 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 efficiency.
Based on the preceding equations, the user should design/
choose the transformer having sufficient 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 dissi-
pated. 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, Zener clamp, etc.), and Application Note 19
is a good reference on snubber design. An RC snubber
circuit can be connected between SW and GND to damp
the ringing on SW pins. The snubber resistor values should
be close to the impedance of the parasitic resonance. The
snubber capacitor value should be larger than the circuit
parasitic capacitance, but be small enough to keep the
snubber resistor power dissipation low.
If the RC snubber is insufficient to prevent SW pins over-
voltage, the RCD snubber can be used to limit the peak
voltage on the SW pins, which is shown in Figure 6.
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
•ƒ
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
SN
) can be determined using the following equation:
C
CN
=
V
SN
ΔV
SN
•R
SN
•ƒ
where ΔV
SN
is the voltage ripple across C
SN
. A reasonable
ΔV
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)
.
A Zener clamp can also be connected between SW and
GND to ensure SW voltage does not exceed 40V.
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
.