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LT3757EDD#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36
LT3757/LT3757A
19
3757afd
applicaTions inForMaTion
Flyback Converter: Transformer Design for
Discontinuous Mode Operation
The transformer design for discontinuous mode of opera-
tion is chosen as presented here. According to Figure 8,
the minimum D3 (D3
MIN
) occurs when the converter
has the minimum V
IN
and the maximum output power
(P
OUT
). Choose D3
MIN
to be equal to or higher than 10%
to guarantee the converter is always in discontinuous
mode operation (choosing higher D3 allows the use of low
inductances, but results in a higher switch peak current).
The user can choose a D
MAX
as the start point. Then, the
maximum average primary currents can be calculated by
the following equation:
I
LP(MAX)
=I
SW(MAX)
=
P
OUT(MAX)
D
MAX
• V
IN(MIN)
• h
where h is the converter efficiency.
If the flyback converter has multiple outputs, P
OUT(MAX)
is the sum of all the output power.
The maximum average secondary current is:
I
LS(MAX)
=I
D(MAX)
=
I
OUT(MAX)
D2
where:
D2 = 1 – D
MAX
– D3
the primary and secondary RMS currents are:
I
LP(RMS)
= 2 • I
LP(MAX)
•
D
MAX
3
I
LS(RMS)
= 2 • I
LS(MAX)
•
D2
3
According to Figure 8, the primary and secondary peak
currents are:
I
LP(PEAK)
= I
SW(PEAK)
= 2 • I
LP(MAX)
I
LS(PEAK)
= I
D(PEAK)
= 2 • I
LS(MAX)
The primary and second inductor values of the flyback
converter transformer can be determined using the fol-
lowing equations:
L
P
=
D
2
MAX
• V
2
IN(MIN)
• h
2 • P
OUT(MAX)
• f
L
S
=
D2
2
• (V
OUT
+ V
D
)
2 • I
OUT(MAX)
• f
The primary to second turns ratio is:
N
P
N
S
=
L
P
L
S
Flyback Converter: Snubber Design
Transformer leakage inductance (on either the primary or
secondary) causes a voltage spike to occur after the MOS-
FET turn-off. This is increasingly prominent at higher load
currents, where more stored energy must be dissipated.
In some cases a snubber circuit will be required to avoid
overvoltage breakdown at the MOSFET’s drain node. There
are different snubber circuits, and Application Note 19 is
a good reference on snubber design. An RCD snubber is
shown in Figure 7.
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
LT3757/LT3757A
20
3757afd
applicaTions inForMaTion
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 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.
LT3757/LT3757A
21
3757afd
applicaTions inForMaTion
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
Flyback Converter: Input Capacitor Selection
The input capacitor in a flyback converter is subject to
a large RMS current due to the discontinuous primary
current. To prevent large voltage transients, use a low
ESR input capacitor sized for the maximum RMS current.
The RMS ripple current rating of the input capacitors in
discontinuous operation can be determined using the
following equation:
I
RMS(CIN),DISCONTINUOUS
≥
P
OUT(MAX)
V
IN(MIN)
• h
•
4 − (3 • D
MAX
)
3 • D
MAX
SEPIC CONVERTER APPLICATIONS
The LT3757 can be configured as a SEPIC (single-ended
primary inductance converter), as shown in Figure 1. This
topology allows for the input to be higher, equal, or lower
than the desired output voltage. The conversion ratio as
a function of duty cycle is:
V
OUT
+ V
D
V
IN
=
D
1− D
in continuous conduction mode (CCM).
In a SEPIC converter, no DC path exists between the input
and output. This is an advantage over the boost converter
for applications requiring the output to be disconnected
from the input source when the circuit is in shutdown.
Compared to the flyback converter, the SEPIC converter
has the advantage that both the power MOSFET and the
output diode voltages are clamped by the capacitors (C
IN
,
C
DC
and C
OUT
), therefore, there is less voltage ringing
across the power MOSFET and the output diodes. The
SEPIC converter requires much smaller input capacitors
than those of the flyback converter. This is due to the fact
that, in the SEPIC converter, the inductor L1 is in series
with the input, and the ripple current flowing through the
input capacitor is continuous.
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LT3757EDD#PBF

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Manufacturer:
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Description:
Switching Voltage Regulators Boost, Fly, SEPIC & Inv Cntr
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