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Re: [Help-gnucap] Coupled inductors
From: |
Al Davis |
Subject: |
Re: [Help-gnucap] Coupled inductors |
Date: |
Thu, 7 Aug 2003 22:54:38 -0600 |
User-agent: |
KMail/1.5.3 |
On Wednesday 30 July 2003 05:58 am, devik wrote:
> I've read about problem with coupled inductors. Can
> someone tell me more ?
> I tested config with single transformer and it is
> not working (switched supply) once I add Kx.
>
> What exactly is the problem and is it tought to solve it ?
Multiple coupled inductors don't work in Gnucap. Simple pairs
of coupled inductors work fine.
In Spice, multiple coupled inductors work, but at a huge cost in
performance for any circuit containing any inductors.
Spice uses a subcircuit model for inductors. Each one has an
internal node. The subcircuit consists of 4 resistors (2 are
negative) and a capacitor. Coupling of inductors in Spice is
done with capacitors between the internal node of one inductor,
to the other.
Gnucap uses a subcircuit model only for coupled inductors, but
it is a different model. It is based on current controlled
voltage sources. It does not add internal nodes, so is more
efficient. The down side is that it doesn't work for multiple
couplings.
I have been meaning to change this for some time, but have had
too much pressure on other things. The plan is to use the
Spice type subcircuit when it is needed, and stick to the
current based model for non-coupled and singly coupled
inductors.
The performance hit of the Spice subcircuit model is severe for
the large circuits you get with layout extractions for signal
integrity. This hit is much larger than should be expected for
the increased node count and component count.
> (Problem is that Spice3f5 fails to converge on some simple
> Kx circuits too).
Linear inductors do not iterate, therefore cannot have
convergence problems. There have been some recent reports of
convergence problems in some circuits that have inductors. The
convergence problem is due to something other than the
inductors. It is only a coincidence that a circuit with
convergence problems has some inductors. This paragraph
applies to both Gnucap and Spice.
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