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L's and C's and Bsource

kevinv

HI All,

I was really excited once I saw that the spectre L and C elements support poly coeff's and can be used to model Voltage and Current variations...  BUT  I was VERY dissapointed to see that these variations are POLARITY dependant... 

 Meaning-  if you try to model inductor current saturation effects using the Coeff's matrix...  -  you get the wrong result if the current happens to be going the "wrong" direction through the inductor.

 So-  I thought about using a bsource instead...   

1)  Am I missing something wrt the Coeff's issue?

2)  If I switch to using a bsource-  is there a schematic element for it?  I don't see one.

 

Thanks!

  • Andrew Beckett

    Many years ago the resistor model added a parameter "symmetric" which allows the non-linearity to be symmetric (see "spectre -h resistor"). Note that this must be used with care because you can end up with non-causal/non-physical behaviour when using this.

    Capacitors and Inductors don't have this option though. In practice the voltage dependency wouldn't be symmetric - it would be dependent upon the voltage across the device, or the voltage relative to some third (e.g.) bulk node. So I'm not convinced it makes much sense to have this symmetric option there (the same is true for resistors, but it got requested so much that it was added, despite the fact that it's not really physical).

    You can use a bsource. There is not a direct component for it - you could use a copy of the analogLib cap or ind, and change the c or l parameter in the CDF to not have parseAsNumber=yes or parseAsCEL=yes - I think that would do it (you can use a capacitor or inductor in spectre in lieu of the bsource in c or l mode). This would stop ADE thinking that the expressions you use are referencing design variables.

    Regards,

    Andrew.

  • kevinv

    Many thanks for the reply.

     I understand your comments if they relate to integrated (on-chip) elements but are you claiming that even external components are not symmetric?

     For example-  are the saturation effects in an external (discrete) inductor dependant on current flow direction?   I can't think of a reason that this would be but I've certainly been wrong before   :)

     

     

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