Debugging LTSpice Zener Diode Bug – Solutions and Tips

debuggingdiodesltspicezener

I'm trying to design a circuit using a zener diode but am having issues getting it to work right.
Working zener diode circuit

This circuit works fine and show Vout = 6.198V. However, if I swap their positions –
Dysfunctional zener diode circuit

Here, Vout = 200.364V. Why is this happening? Why does the anode of the zener diode need to be referenced to ground? I expected it to be 200-6.2 = 193.8V

Best Answer

From the LTspice documentation on .op:

There is no guarantee that the operating point of a general nonlinear circuit can be found with successive linear approximations as is done in Newton-Raphson iteration.

This circuit is non-linear and LTspice fails to detect by itself that Newton iteration fails. Disabling Newton iteration by .options NoOpIter forces LTspice to try different methods for operating point calculations. In this case it opts for Gmin stepping instead and finds the correct result:

Gmin stepping succeeded in finding the operating point.

Semiconductor Device Operating Points:
                        --- Diodes ---
Name:      d1
Model: bzx84b6v2l
Id:     -9.69e-04
Vd:     -6.18e+00
Req:     1.36e+01
CAP:     4.80e-09
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