Series vs Parallel Windings

Part 21 · Voltage Regulation · page 3 of 3

Key point: two equal windings, and you pick the hookup. Parallel keeps \(N\) and \(L_m\), and doubles the copper. Series aiding doubles \(N\) and makes \(L\) ×4. That is not two loose chokes in parallel.

Beads follow the copper. Each winding here is 6 V class until you series them.

1
Parallel keeps Lm.
Parallel, dots together. 6 turns, 12 µH, 1.5 A each. Neq is 6 turns. Lm is 12.0 µH. I rating is 3.00 A. Voltage class is 6 V. Two separate chokes in parallel would read 6 µH. These share flux, so Lm stays 12. DCR halves. That is the 6 V catalog hookup.
2
Series aiding is L ×4.
Series, dots aiding. Neq is 12 turns. Lm is 48.0 µH. I rating is 1.50 A, one winding’s copper. Voltage class is 12 V. One bead goes through A, then B. Same core, twice the turns, four times L. That is the 12 V hookup of a 6–6 part.
3
More current does not cut L.
Back to parallel. Raise I each to 3.0 A. Lm is still 12.0 µH. I rating is 6.00 A. Still 6 V class. You bought current. You did not buy a smaller inductor.
4
Opposing dots cancel the flux.
Series opposing. I each back to 1.5 A. Neq reads ≈ 0 (cancel). Lm reads ≈ leakage. I rating is 1.50 A. Voltage class says don’t. Amp-turns fight. That is a shorted turn in spirit. Read the winding diagram before you solder “12 V.”
5
The secondary uses the same rule.
Series aiding again. 6 turns, 12 µH, 1.5 A. Neq is 12 turns. Lm is 48.0 µH. 12 V class. Two secondaries: series for a higher rail, parallel for current. Isolation still holds if you do not jumper the barrier.

Leakage of a series stack, pinout, and safety spacing are not drawn. The dots on the datasheet are the whole trick.

Two equal windings on one core

Beads follow the copper. Series aiding: N×2, L×4. Parallel: Lm stays L. Opposing: flux cancels.

Turns each L each µH I each A

N_eq—

L_m,eq—

I rating—

Voltage class—