AN1606 APPLICATION NOTE

10/18

Figure 9. Connection Diagram for the Coupled Inductor

Realizing the inductor in this manner improves common mode rejection and avoids the effect of the difference

between drain capacitance of the PowerMOSFETs. In order to simplify the model, assume a near unity coupling

factor and the equivalent circuit is shown in Figure 9b.

The inductance is proportional to the square of the number of turns. For the two windings it will be:

f)

and

The required number of turns for a given inductance on the same core is the same as it is for one winding or

two windings. The only difference is that the two windings are separated into two sections. For simplicity we can

design the coupled inductor using the same criteria as for a standard inductor - core size, number of turns, and

size of copper wire.

For the core, the preferred design is a gapped ferrite core set.

The size of the core can be chosen considering the maximum current Ipk. that, for the 800W target's parameters

can exceed 14A (placing Ipk. = 15A).

g)

Where:

For the 800W application, the nominal current ripple has been chosen around 25%. This fixes the boost induc-

tance value L=450

µH.

L

i

Mains

Inductor

Z

in

i

v

Z

in

i

v

Leq.

Equivalent circuit.

Mains

N

′

N

2

----

=

N

′ total

N

2

----

N

2

----

+

=

V

cor e

V

core "" min

,

≥

KL I

2

peck

(mm

⋅⋅

=

K1.4 10

4

lcore

lgap

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

⋅⋅

=