In a SEPIC converter, selecting the proper coupled inductor is a critical step. The first decision is to select the correct switching type of operation for the application, discontinuous conduction mode (DCM) or continuous conduction mode (CCM). CCM is selected for this design to minimize the RMS currents, maximize full load efficiency while minimizing load voltage ripple. On the other hand, DCM might be chosen if good light load efficiency is required.

A rule of thumb is to use 20 to 40% of the input current for K_IND, which represents the peak to peak inductor ripple current, as computed with the power-balance equation where η is the estimated efficiency.

The following value is a good estimate of the efficiency:

The input DC current is calculated as follows:

The inductor ripple current can be calculated with the following equation:

Finally, the coupled inductance value is given by:

Where D is the duty cycle and f_SW is the switching frequency.

Choosing a higher K_IND leads to:

• Lower recommended inductance
• Higher current ripple
• Higher EMI
• Faster transient performance
• Larger output capacitance for a desired output ripple

The lower current ripple typically reduces EMI, increases the maximum output current, reduces the needed output capacitance, and slows the transient response. The right tradeoff must be chosen by the designer.

It is worth mentioning that in a coupled inductor, the mutual inductance forces the ripple current to be split equally between the two coupled inductors and therefore the inductance required in a coupled inductor is half of what might be needed with two separate inductors. In addition, differently from a Flyback coupled inductor in which the coupling factor needs to be as high as possible, in a SEPIC it is preferable to have 10%-15% of leakage.

In this case, an inductor of 4.7 uH was chosen.