LTSpice simple OPT convertion to UL

[Elerion]

Hi everyone.
I know many of you will find this question trivial, but I haven't see the "trick" yet (and all disscusions I found on this forum are much more complex, not touching this particular matter)


Suppose we model a simple SE OPT 4k:8 as two inductor coupled by a simple K1 L1 Ls 1 command.
L1 = 20 H,
Ls = 40mH.
Ignore transformer parasitics.

If we want to convert it to an UL OPT (suppose 50% UL, for simplicity), we can do:
L1 = 5 H,
L2 = 5 H,
Ls = 40mH

Inductor in series add, so now the primary (L1+L2) is 10 H, not 20 H.
And, if we keep L1=10H and L2=10H, so the sum is 20H, obviously, the secondary has to go up to maintain the same Z ratio.

How should the inductances be modeled, so they are as close as possible to real OPT (again ignoring, just for now, the DCR and capacitance)?


Thank you.

 

[tikiroo]

On a common core, inductance is related to the square of the turns. So with two 5H primaries in series, the total inductance is 20H (twice the turns = 4x the inductance)

 

[MarekH]

That sounds like a single ended output transformer answer only.

For completeness, a push-pull output transformer would need four inductors on the primary as it'd have five circuit connections to make:- 2x normal non u/l taps; 2x u/l taps; one centre tap for B+.

kind regards
Marek

 

[Elerion]

Ok, maybe I didn't explain myself very well.
I know that for two 5H primaries in series, wound in the same coil, the total inductance is 20H. But, from the LTSpice schematic point of view, there're two coils in series, each of 5 H.
The total inductance of two coils in series (from basic electronics) is the sum of both, 5H + 5H = 10H.

 

[artosalo]

You already said the answer:

... two inductor coupled by a simple K1 L1 Ls 1 command.

In case of UL OPT, both primaries (and secondary of course) must be "tied" together with K1-command.