AN4205

Input stage of the AC/DC converter

Doc ID 023929 Rev 1

9/23

From Equation 4, we can determine the number of secondary turns:

We can also calculate the auxiliary needed to supply the driver. The driver used is the

UC3842 which is driven by 15 V.

Equation 16

Knowing the primary inductance, the inductance of a single turn can be calculated:

Equation 17

Now the air gap length is known:

Equation 18

From the datasheet of the core K1 = 42.2 and K2 = -0.701

Before proceeding to calculate the wire dimension, we can verify if the selected core is

acceptable (safety condition) with regard to thermal operation.

Using Ampere's law we can calculate the B field:

Equation 19

so we can neglect the last term in Equation 19.

Equation 20

This result confirms the accuracy of the hypothesis about

N

P

155

≈

N

PS

28 155

()

≈

N

P

N

Aux

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

V

fl

V

Aux

V

F diode

,

+

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

100

15

1

+

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

6.25

==

=

N

Aux

25

=

L

P

N

=

A

L

L

P

N

P

-------

85nH

==

2

2

l

g

A

L

K

1

------





1K

2

⁄

=

l

g

0.3mm

=

N

Hdl

°

ΣH

H

H

+

B

μ

0

B

μ

fe

+

==

=

=

N

B

μ

0

=

B194mT

=