LT3757 datasheet(21/38 Pages) AD

21 / 38 page

LT3757/LT3757A

Rev. G

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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 VRRM using:

VRRM >

• VIN(MAX)+ VOUT

The power dissipated by the diode is:

PD = IO(MAX) • VD

and the diode junction temperature is:

TJ = TA + PD • RθJA

The RθJA to be used in this equation normally includes

the RθJC for the device, plus the thermal resistance from

the board to the ambient temperature in the enclosure.

TJ must not exceed the diode maximum junction tem-

perature 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 COUT and ESRCOUT.

The RMS ripple current rating of the output capacitors

in discontinuous operation can be determined using the

following equation:

IRMS(COUT),DISCONTINUOUS ≥ IO(MAX) •

−(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 fol-

lowing equation:

IRMS(CIN),DISCONTINUOUS ≥

POUT(MAX)

VIN(MIN) • η

•

−(3 •DMAX)

3 •DMAX

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:

VOUT + VD

VIN

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 (CIN,

CDC and COUT), 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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