AN3180 Datasheet, PDF (16/39 Pages) STMicroelectronics – A 200 W ripple-free input current

Zero-ripple current phenomenon: practice
AN3180
Equation 17
N₂
=
N_{1 L}
L
− L_LK
where, in this case, L is the measured inductance of the AC winding. Add 5 % to the
result, to account for the slight coupling improvement that there is as the DC winding is
entirely in place, and round up to the next integer. Let this number be N₂
7. After removing the layer used for the preliminary measurement and the core, wind the
N₂ turns of the DC winding. Do not permanently fix the end of the wire because N₂
should be greater than the value that meets the zero-ripple condition and some turns
need to be taken out in the following step
8. Reassemble the core and adjust the air gap so as to get the required value L1.
Measure the leakage and the magnetizing inductances (see Appendix B) and check if
(7) is met. If not, remove one turn and repeat the step until the condition in Equation 7
is close to being met
9. Connect the coupled inductor to the converter, power it on and measure the ripple on
the DC winding. It should be quite small. If it looks like that in Figure 11 the turn number
of the DC winding has been reduced too much. If it is not possible to add more turns to
the DC winding, either the air gap must be reduced, if possible (paying attention to
saturation!), or the turns number of the AC winding must be reduced. If the ripple looks
like that in Figure 12, where the ripple is 180° out-of-phase, there are still too many
turns on the DC winding and they should be further reduced. Also in this case it is
possible to fine tune by adjusting the gap, if possible. Only now can the DC winding
wire end be permanently fixed to the bobbin pin. Record the final values of N₁, N₂ and
l_gap.
Figure 11. Partial ripple cancellation: still
under compensated
Figure 12. Partial ripple cancellation:
overcompensated
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DFZ LQGLQJFXUUHQW
GFZ LQGLQJFXUUHQW
DFZ LQGLQJFXUUHQW
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JDWHGULYH
8QGHUFRPSHQVDWHGNQH 
GFZLQGLQJZLWKWRRIHZWXUQV
!-V
2YHUFRPSHQVDWHGNQH !
GFZLQGLQJZLWKWRRPDQ\WXUQV
!-V
Note that the first four steps of this procedure are the same as those for a conventional
inductor. Starting from step 5, the procedure becomes empirical and quite tricky and might
also be time-consuming. Fortunately, this work needs to be carried out only for the first
prototype.
There is still an important practical issue to consider. When the two half cores are
assembled, even if they are kept together well, they are typically quite loose inside the
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