Application description

AN1889

12/33

Equation 13

Before carrying on with the wire dimension calculation, we can preliminary verify if the

selected core is acceptable from a thermal point of view during its normal operation.

Through the Ampere's law we can calculate the B field in normal operation:

Equation 14

Being

permeability in the air) we can neglect the last item in the Equation 14:

Equation 15

From the ferrite datasheet the power dissipation is 150 mW/cm3, knowing that the total

Equation 16

This value allows the designers to operate in safe condition since we have to add also the

losses due to the winding with a further consequent increase in temperature.

The next step is the wire dimension calculation, from both primary and secondary side. First

we must assume the maximum losses on the copper. Normal practice is to choose a

dissipation level comparable with the one achieved in the core, practically we can chose 1W.

From the Joule's law we can calculate the resistance of both primary and secondary

winding.

Equation 17

Equation 18

Ω cm), and the average

Equation 19

N

P

N

Aux

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

V

fl

V

Aux

V

F diode

,

+

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

500

15

1

+

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

31

N

Aux

5

=

⇒

=

=

=

N

Hdl

H

H

H

B

µ

0

-----l

g

B

µ

fe

-------l

fe

+

=

+

=

∑

=

∫°

=

N

B

µ

0

-----l

g

B

170mT

=

⇒

=

P

fe

∆T

R

th

--------

∆T18° C

=

⇒

=

R

P

P

CU

2I

2

PMRS

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

R

P

3.46

Ω

=

⇒

=

R

s

P

CU

2I

2

SMRS

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

R

s

0.0174

Ω

=

⇒

=

A

PCU

ρ

100

N

P

•

L

t

•

R

P

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

4.08

10

4

–

d

p

0.022 cm

[]

=

⇒

•

=

=