The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Modeling >> Passive Devices >> Coupled inductors
https://designers-guide.org/forum/YaBB.pl?num=1031471485

Message started by Lieutenant Columbo on Sep 8^th, 2002, 12:51am

Title: Coupled inductors
Post by Lieutenant Columbo on Sep 8^th, 2002, 12:51am

We are having a problem with "time step too small" in Spectre and I was wondering if you may have any pointers.

The circuit is using a package model with R, L, C, and K (mutual inductance) and a bondwire model with R, L, and K. The transient simulation will complete with the package model only, but dies with the addition of the bondwire model. If I just take out the mutual inductance from the bondwire model the simulation completes again. One thing that is unique about the mutual inductance in the bondwire model is that there is much more coupling between multiple paths because many of the bondwires are connected to a common ground plane in the package and we are modeling the different currents in the plane also.

Title: Re: Coupled inductors
Post by Ken Kundert on Sep 8^th, 2002, 12:57am

I suspect that the problem is that when adding the additional coupling, you have added a very high frequency unstable pole. Unstable poles are one of the main reasons why Spectre will stop due to `time step too small', and Spectre's careful timestep control makes it more likely that Spectre will nurture unstable modes. Furthermore, any round-off error when describing the inductance matrix of a large coupled inductor will cause it to lose its passivity.

I suggest that you set the spectre option `diagnose' to `yes' and rerun the simulation and note any messages that would indicate the solution is growing without bound. If you have matlab, you could enter your inductance matrix and make sure it is positive definite. If it is not, it will cause the circuit to be unstable. Finally, you should consider increasing the resolution with which you specify the inductor values and the coupling factors.

The test for passivity of a two-terminal LTI inductor is that its inductance is positive [desoer and kuh]. The passivity criteria for a nonlinear two-terminal current-controlled inductor is that L(i) > 0 and dL/di > 0 for all i [desoer and kuh]. The passivity criteria for a four-terminal LTI inductor is that |M| < 1 [desoer and kuh]. Finally, the criteria for a 2n-terminal LTI current-controlled inductor is that L is symmetric and positive semi-definite [chua class notes]. For a flux-controlled inductor, the reciprocal inductance matrix must be symmetric and PSD.