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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
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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 

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 

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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