AN2435

Sepic topology for PFC converter

11/25

The main conclusion of Equation 23 and Equation 24, is that the same equations can be

used for coupled and uncoupled inductors. If no current flows on the secondary side, voltage

Equation 25

As the same voltage is applied, simultaneously, to both inductors in a sepic converter,

Equation 25 may be simplified below as:

Equation 26

zero and no current flows on the secondary side. In the actual circuit only the high frequency

components of the inductor L2 current are attenuated. Once Equation 26 is verified,

Equation 24 may be simplified as:

Equation 27

Therefore, for a sepic converter with coupled inductors, infinite inductance at the input may

be seen theoretically once Equation 26 is satisfied. In the actual circuit, due to imperfect

matching with Equation 26 and due to the fact that the voltages applied to the two inductors

are not exactly the same (because of the voltage ripple on capacitor C1), a large input

inductance may be seen at the converter input. This input inductance helps the line filter in

eliminating the switching frequency component of the input current.

Figure 7.

Coupled inductor of a sepic converter

Figure 8.

Model of two coupled inductors

V

2

V

2

1

nV

1

1

•

n

L

M

L

M

L

LK1

+

--------------------------

•

V

1

•

=

=

=

n

L

M

L

M

L

LK1

+

--------------------------

•

1

=

L

EQ1

L

M

L

LK1

+

=

L

EQ2

∞

=

⎩

⎨

⎧

1

+

-

+

-

+

-

1

+

-

1 : n