Perfectly coupled inductors in (LT-)spice

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I want to simulate a transformer using coupled inducors
in Spice (LTspice). I found some unexpected behaviour at
very high frequencies. My observation can be boild down to the
circuit given below: An AC source delivers a voltage. A resistor
converts the voltage to a current through a transformer (perfect coupling K=1).
The transformer is secondary shorted (by a 0-Volt AC souce, yes that works) and
I measure the secondary side current. ( My original application is understanding
and simulation of current transformes). At frequencies above 100MHz the current
drops, as if the coupling would not be perfect or as if an additional series
inductance of approx 2nH would be involved (see Mag/Phase Plot).

Has someone an explanation for that. Is there someone a good explanation what
spice does here. I am noramlly very very satisfied with spice results especially
because I can do simulations of very (unrealistically) broad frequancy ranges
and I can get an eye on every single cause/effect.

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What I found out now:
Standard inductars have a default series resistance
of 1mOhm. But that info disappears for coupled
Inductors. Perhaps some other "hidden default"?

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Look in the Control Panel -> Hacks.

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Quote:
Hacks

I always dreamed to become a hacker.

I looked up that (thanks very much, seem
some interesting options anywhere).
But I didn't find a cure. Any specific
recommandation?

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Ask on the LTspice list. You will find good help there.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Jim:
do you have a specific URL. Searching the web
showeed many hits, but I didn'n know how to choose from.
And I found no list that looked "official".

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The official list is hosted at Yahoo.

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ossi wrote:
Quote:
Hacks

I always dreamed to become a hacker.

I looked up that (thanks very much, seem
some interesting options anywhere).
But I didn't find a cure. Any specific
recommandation?


I'm assuming jayjay1974 is referring to the setting "Always default inductors to Rser=0".

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Yes, the official LTspice list is a Yahoo Group. Its pretty easy to use. Folks are as "friendly" there as here. Same dedication for not doing poster's "homework". And, a real variety of experience levels. There is a "list mom" named Helmut who is very helpful.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Quote:
I'm assuming jayjay1974 is referring to the setting "Always default inductors to Rser=0".

This optuion helps for uncoupled inductors. I did'nt
know it existed. Fron now on I don't have to set
Rser=0 for each individual inductor, so very good help.

Unfortunately that doesn't help in my case.

I had problems yesterday to register at yahoo.
Will try again today.

Thanks to all!

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Quote:
Ask on the LTspice list. You will find good
help there.

I even had not to ask :) , there was an interesting
discussuion of the problem here:

http://tech.groups.yahoo.com/gro...

Conclusion: The effect (problem) is known, there
is no easy workaround.

Since the problem is not really relevant for
simulation of real circuits its acceptable.

Thanks for the help.

And btw: I hate captchas here!

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Ossi,

Have You tried the simplified approach based on ideal transformer where the secondary side is "reflected " to the primary for an equivalent circuit?

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@ignoramus
Yes I tried that. First it didn't work because
I didn't know that LTspice adds a small Rser
in series to an inductor.
Setting Rser=0 explicitely or setting the
appropriate option makes it work perfectly.