STMicroelectronics A 200 W ripple-free input current PFC pre-regulator with the L6563S EVL6563S-200ZRC EVL6563S-200ZRC Data Sheet

   

   

   

14/39

 Doc ID 17273 Rev 1

3. 

As a result of the aforementioned current splitting action, the same happens to the 
magnetic flux: the AC winding is responsible for generating the AC component of the 
flux and the DC winding for its DC component, but their sum is the same as in a single 
inductor carrying the total current. This means that no core size increase is expected 
because of flux density limitations (core saturation) or core loss reasons. Also the core 
size selection approach is the same.

To complete the picture, an important property of the “gapped ferrite core plus bobbin” 
assembly should be recalled. The reluctance of the leakage flux path is constant for a given 
core geometry and is independent of the gap thickness, l

gap

: essentially, it is a function of 

the physical dimensions of the core and the distance between the windings. This implies 
that the leakage inductance of the primary winding, Ll1, depends only on the turn number 
N

1

 and does not change if l

gap

 is adjusted. However, the total primary inductance L

1

= L

l1

+ 

L

M

 is strongly affected by the gap thickness, but it is only the magnetizing inductance L

M

 that 

changes. With reference to the zero-ripple current condition in 

:

once the ferrite core and the associated bobbin are defined, it is thereby possible to control 
LL

l1

 and L

M

 separately, acting on N

1

 and l

gap

 respectively, then the secondary turn number 

N

2

 is determined so that the ratio n = N

2

/N

1

 meets the condition in 

.

         

         

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1

L

L

L

L

L

L

1

1

1

M

1

M

=

−

=

+

l

l