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AN3180

Zero-ripple current phenomenon: practice

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Zero-ripple current phenomenon: practice

Before giving details of the practical realization of a coupled inductor able to provide ripple

steering, it is useful to draw some conclusions of considerable practical interest from the

theoretical analysis carried out in the previous sections:

The fundamental point is that to achieve zero-ripple current in a coupled inductor with

low sensitivity to parameter spread, a high-leakage magnetic structure is needed,

which is contrary to the traditional transformer design practice. Then, in the important

case of inductors realized with a gapped ferrite core plus bobbin assembly, the usual

concentric winding arrangement is not recommended, although higher leakage

inductance values can be achieved by increasing the space between the windings.

However, it is difficult to obtain a stable value, because it depends on parameters (such

as winding surface irregularities or spacer thickness) which are difficult to control. Other

methods are used, such as placing the windings on separate core legs (if using an EE

core) or side-by-side on the same leg. The latter arrangement is possible with both EE

and pot cores (see Figure 8). They permit much more stable leakage inductance

values, because they are related to the geometry and the mechanical tolerances of the

bobbin, which are much better controlled. At this point there is the practical issue of

finding suitable bobbins for such arrangements: large production quantities could make

the case for a custom product, but using something readily available in the market is a

good choice anyway. Slotted bobbins, like those illustrated in Figure 9, are quite

commonly available from several manufacturers; they easily lend themselves to a side-

by-side winding arrangement, and they are considered here.

Figure 8.

Examples of high-leakage magnetic structures (cross-section)

The fundamental action of the smoothing transformer is to split up the current into its

DC component, which flows in the DC winding, and its AC component, which flows in

the AC winding. In this way the total rms current in the windings is unchanged, hence

the total copper area needed to handle the two currents separately is quite close to that

of a single inductor carrying the total current. As compared to a single inductor, no core

size increase is typically expected because of insufficient winding window area, except

for a few marginal cases where the slight decrease of window area due to slotting

becomes critical or where the DC winding has many more turns. It is also worth noting

that the DC winding can be made with a single wire, as its residual AC current is low;

only the AC winding is made with litz or multi-stranded wire. This minimizes the

additional cost of the added winding.

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