LT3757 Datasheet, PDF (21/36 Pages) Linear Technology – Boost, Flyback, SEPIC and Inverting Controller

LT3757
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_IN
V_D
=
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
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