AN628 APPLICATION NOTE

20/35

if referred to the magnetic path, can be rewritten :

where :

Ae = effective area of the core section.

∆B = deviated magnetic flux density.

H = magnetic field strength.

The ratio between the ferrite and the air path magnetic permeability, depends on the ferrite materials. Core ma-

terials for power application (such as B50/51), have a initial permeability value about 2500 times that of air. This

means that, above a certain air-gap length percentage, it is possible to neglect the leff (length of the core) sim-

plifing the calculation e.g. if a 1% of air-gap length, respect to the core lenght value is used, the error introduced

is about 4%.

Rewriting

equating to and simplifing

Because:

and finally:

This simplified relation is much easier to use than the complete one:

Air-gap on ferrites databook). If the check is too close the rated limit, an increase of the lgap (gap lengh) and a

2

sidered for the wire selection.

P

Ot

Ae I

eff

H

B

∆

t

on

-------

⋅⋅ ⋅

=

P

Ot

Ae I

eff

H

B

∆

t

on

-------

⋅⋅ ⋅

=





P

Ot

Ae I

gap

H

gap

B

∆

t

on

-------

⋅⋅

⋅

≈

P

Ot

LI

Lt

I

L

∆

⋅⋅

t

on

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

=

Ae I

gap

H

gap

B

∆

⋅⋅

⋅

LI

Lt

I

L

∆

⋅⋅

≈

I

⋅

NI

B

∆

⋅

≈µ

0

H

∆

⋅

=

Ae N

µ

0

H

∆

⋅⋅

⋅

II

L

∆

⋅

≈

H

∆

N

I

L

∆

I

gap

----------

⋅

≈

N

LI

gap

⋅

µ

0

Ae

⋅

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

≈

N

L

µ

o

------

I

gap

Ae

π

4

--- I

gap

⋅

+





2

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

I

eff

µ

0

Ae

⋅

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

+

⋅

≈