Designing SEPIC Using LM3478

(Continued)

The duty cycle of a SEPIC is given by:

In the above equation, V

MOSFET, Q, and V

diode.

POWER MOSFET SELECTION

As in boost converter, the parameters governing the selec-

tion of the MOSFET are the minimum threshold voltage,

V

Q

mum drain to source voltage, V

voltage in a SEPIC is given by:

V

The selected MOSFET should satisfy the condition:

V

The peak switch current is given by:

The rms current through the switch is given by:

POWER DIODE SELECTION

The Power diode must be selected to handle the peak

current and the peak reverse voltage. In a SEPIC, the diode

peak current is the same as the switch peak current. The

off-state voltage or peak reverse voltage of the diode is V

IN

+V

current is equal to the output current. Schottky diodes are

recommended.

SELECTION OF INDUCTORS L1 AND L2

Proper selection of the inductors L1 and L2 to maintain

constant current mode requires calculations of the following

parameters.

Average current in the inductors:

I

Peak to peak ripple current, to calculate core loss if neces-

sary:

Maintaining the condition I

∆i

mode yields:

10135544

FIGURE 12. Typical SEPIC Converter

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