AN2435

Sepic topology for PFC converter

7/25

Taking into account that input voltage is now the rectified main, Equation 1 and Equation 2

can be rewritten as follows:

Equation 7

Due to transition mode operation, the MOSFET is switched on as soon as the diode current

falls to zero. This means that:

Equation 8

From and Equation 8, the MOSFET peak current equation may be derived:

Equation 9

Equation 10

Equation 11

function of

ϑ:

Equation 12

Figure 5 shows the switching frequency versus

θ for two different input voltages. To

calculate input current averaged over one switching cycle, Equation 13, the charge balance

on capacitor C1, is used:

Equation 13

I

ϑ

,

()

I

L10

V

()

L

1

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

tI

ϑ

,

()

,

•

I

L20

V

()

L

2

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

t

•

+

=

+

=

I

L10

I

L20

–

0

=

I

()

1

L

1

------

1

L

2

------

+

⎝⎠

⎛⎞ t

()

•

2

•

V

ac

•ϑ

()

sin

•

=

t

()

I

PK

ϑ

()

sin

•

1

L

1

------

1

L

2

------

+

⎝⎠

⎛⎞

2V

ac

ϑ

()

sin

•

•

•

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

L

e

I

PK

•

2V

ac

•

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

T

ON

=

=

=

T

()

T

ON

2V

ac

ϑ

()

sin

•

•

•

V

O

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

=

f

()

1

T

ON

T

()

+

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

1

T

ON

1

2V

ac

ϑ

()

sin

•

•

V

O

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

+

⎝⎠

⎜⎟

⎛⎞

•

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

=

=

I

()

1

2

---

V

()

L

1

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

T

ON

•

•

+

⎝⎠

⎛⎞ T

ON

•

I

()

1

2

---

V

()

L

2

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

T

ON

•

•

+

⎝⎠

⎛⎞ T

OFF

•ϑ

() Q ϑ

()

=

=