STMicroelectronics A 200 W ripple-free input current PFC pre-regulator with the L6563S EVL6563S-200ZRC EVL6563S-200ZRC Data Sheet
Product codes
EVL6563S-200ZRC
AN3180
Zero-ripple current phenomenon: practice
Doc ID 17273 Rev 1
13/39
4
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:
steering, it is useful to draw some conclusions of considerable practical interest from the
theoretical analysis carried out in the previous sections:
1.
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
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
). 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
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
, are quite
commonly available from several manufacturers; they easily lend themselves to a side-
by-side winding arrangement, and they are considered here.
by-side winding arrangement, and they are considered here.
Figure 8.
Examples of high-leakage magnetic structures (cross-section)
2.
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.
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.