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LT3957A

Boost, Flyback, SEPIC and Inverting Converter with 5A, 40V Switch

Linear Technology 

LT3957A

APPLICATIONS INFORMATION

The RMS ripple current rating of the output capacitors

in continuous operation can be determined using the

following equation:

IRMS(COUT),CONTINUOUS ≈ IO(MAX) •

DMAX

1− DM AX

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 continuous

operation can be determined using the following equation:

IRMS(CIN),CONTINUOUS ≈

POUT(MAX) •

VIN(MIN) • η

1− DM AX

DMAX

SEPIC CONVERTER APPLICATIONS

The LT3957A 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 = D

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 current through inductor

L1 (which is series with the input) is continuous.

18

SEPIC Converter: Switch Duty Cycle and Frequency

For a SEPIC converter operating in CCM, the duty cycle

of the main switch can be calculated based on the output

voltage (VOUT), the input voltage (VIN) and the diode

forward voltage (VD).

The maximum duty cycle (DMAX) occurs when the converter

has the minimum input voltage:

DMAX

=

VOUT + VD

VIN(MIN) + VOUT

+

VD

SEPIC Converter: The Maximum Output Current

Capability and Inductor Selection

As shown in Figure 1, the SEPIC converter contains two

inductors: L1 and L2. L1 and L2 can be independent, but can

also be wound on the same core, since identical voltages

are applied to L1 and L2 throughout the switching cycle.

For the SEPIC topology, the current through L1 is the

converter input current. Based on the fact that, ideally, the

output power is equal to the input power, the maximum

average inductor currents of L1 and L2 are:

IL1(MAX

)

=

IIN(MAX

)

=

IO(M

AX)

•

DMAX

1− DM AX

IL2(MAX) = IO(MAX)

Due to the current limit of its internal power switch, the

LT3957A should be used in a SEPIC converter whose

maximum output current (IO(MAX)) is less than the output

current capability by a sufficient margin (10% or higher

is recommended):

IO(MAX) < (1−DMAX ) • (5A − 0.5 • ΔISW )

The inductor ripple currents ΔIL1 and ΔIL2 are identical:

ΔIL1 = ΔIL2 = 0.5 • ΔISW

The inductor ripple current ΔISW has a direct effect on the

choice of the inductor value and the converter’s maximum

output current capability. Choosing smaller values of ΔISW

requires large inductances and reduces the current loop

gain (the converter will approach voltage mode). Accepting

larger values of ΔISW allows the use of low inductances,

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