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LT3758

3758f

APPLICATIONS INFORMATION

ΔIL requires large inductances and reduces the current

loop gain (the converter will approach voltage mode).

Accepting larger values of ΔIL allows the use of low in-

ductances, but results in higher input current ripple and

greater core losses. It is recommended that χ falls in the

range of 0.2 to 0.6.

Figure 9. The Switch Current Waveform of the SEPIC Converter

3758 F09

ISW =

ISW(MAX)

ISW

DTS

ISW(MAX)

where

LMAX

LRMS

LMAX

()

() •

where

LMAX

()

Based on the preceding equations, the user should choose

the inductors having sufﬁcient saturation and RMS cur-

rent ratings.

In a SEPIC converter, when the power switch is turned on,

the current ﬂowing through the sense resistor (ISENSE) is

the switch current.

Set the sense voltage at ISENSE(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:

SENSE

SW PEAK

()

SEPIC Converter: Power MOSFET Selection

For the SEPIC conﬁguration, choose a MOSFET with a

VDC rating higher than the sum of the output voltage and

input voltage by a safety margin (a 10V safety margin is

usually sufﬁcient).

The power dissipated by the MOSFET in a SEPIC con-

verter is:

PFET = I2SW(MAX) • RDS(ON) • DMAX

+ 2 • (VIN(MIN) + VOUT)2 • IL(MAX) • CRSS • f/1A

The ﬁrst term in this equation represents the conduction

losses in the device, and the second term, the switching

loss. CRSS is the reverse transfer capacitance, which is

usually speciﬁed in the MOSFET characteristics.

For maximum efﬁciency, RDS(ON) and CRSS should be

minimized. From a known power dissipated in the power

Given an operating input voltage range, and having chosen

the operating frequency and ripple current in the induc-

tor, the inductor value (L1 and L2 are independent) of the

SEPIC converter can be determined using the following

equation:

IN MIN

MAX

()

.•

•

For most SEPIC applications, the equal inductor values

will fall in the range of 1μH to 100μH.

By making L1 = L2, and winding them on the same core, the

value of inductance in the preceding equation is replaced

by 2L, due to mutual inductance:

IN MIN

MAX

()

•

This maintains the same ripple current and energy storage

in the inductors. The peak inductor currents are:

IL1(PEAK) = IL1(MAX) + 0.5 • ΔIL1

IL2(PEAK) = IL2(MAX) + 0.5 • ΔIL2

The RMS inductor currents are:

LRMS

LMAX

()

() •

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