Electronic – Weird LTspice XVII behaviour

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I was designing a circuit, when I noticed some unexpected behaviour regarding \$V_{GS}\$ of the PMOS in LTspice XVII.
I have reduced the circuit in order to point out where the problem lies.
D3 was originally connect to another piece of circuit, but as shown below, the weird behaviour can still be reproduced with an unconnected D3.
Few side notes:

  • For quick reference: the BZX84C10L is a 10V zener
  • The picture rendered a false spacing in the word "ti me", the real equation for
    R1 is R={if(time>2,1G,if(time<1,1G,1u))})

enter image description here

The picture shows 2 simulations.

  • The left simulation is with the unconnected D3.
  • In the right simulation D3 has been 'delete'd.

Why does deleting the unconnected D3 give different simulation results?

Best Answer

The difference in simulation results is due different DC solutions using different methods to find an operating point.

For an unconnected diode, it fails finding an operation point using Direct Newton Iteration.

WARNING: Node NC_02 is floating.

WARNING: Less than two connections to node NC_01. This node is used by D1.
WARNING: Less than two connections to node NC_02. This node is used by D1.
Direct Newton iteration failed to find .op point. (Use ".option noopiter" to skip.)

When deleting this unconnected diode, it succeeds finding the DC operation point with Direct Newton Iteration.

For an unconnected resistor (with a given value), it also succeeds finding an operation point with Direct Newton Iteration.

WARNING: Node NC_02 is floating.

WARNING: Less than two connections to node NC_01. This node is used by R1.
WARNING: Less than two connections to node NC_02. This node is used by R1.
Direct Newton iteration for .op point succeeded.

Conclusion
Having unconnected components in your circuit will not give direct errors or warnings in LTspice XVIII (only warnings in the Spice error logs), but may end up in complete different simulation results.

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