AN2435 datasheet(10/25 Pages) STMICROELECTRONICS

10 / 25 page

Sepic topology for PFC converter

AN2435

10/25

Figure 6.

Input current in TM for sepic PFC

1.3

Coupled inductor sepic converter and ripple steering

If the two inductors are wounded in the same magnetic core, it is possible to reuse the

equations used in Section 1.2. Moreover, the current ripple in the input inductor L2 can

theoretically be reduced to zero, simply by selecting the correct turn ratio. A model of two

coupled inductors is shown in Figure 8. Llk1 and Llk2 are the leakage inductances, LM is the

magnetizing inductance and n is the turn ratio.

The equations that describe the coupled inductor model of Figure 8 are given below.

Equation 22

In the first two sub equations of Equation 22, the derivate of the magnetizing current (IM) can

be substituted with the value given in sub-equation 3.

Considering that the same voltage is applied to both inductors we have:

Equation 23

Equation 24

1 t

() L

1 t

()

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

M t

()

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

•

2 t

() L

2 t

()

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

M t

()

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

•

---------

--------

2 t

()

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

•

⎩

⎪

⎨

⎪

⎧

1 t

() L

EQ1

1 t

()

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

•

2 t

() L

EQ2

2 t

()

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

•

⎩

⎪

⎨

⎪

⎧

EQ1

•

–

() L

•

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

EQ2

•

–

() L

•

–

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

⎩

⎪

⎨

⎪

⎧

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