SIMPLIS Zener Model

Any SPICE Zener model installed in the SIMetrix library can be converted for use in SIMPLIS. When a Zener is placed on a SIMPLIS schematic, a model parameter extraction routine is invoked to Converting SPICE Models for use in SIMPLIS - Overview. During the model parameter extraction process, SIMetrix/SIMPLIS automatically runs several SPICE simulations on the SPICE model and extracts the SIMPLIS model parameters. After the Piecewise Linear (PWL) model parameters have been extracted, the resulting Zener model will run in SIMPLIS.

The extracted Zener model can then be used to create a model for one of two Zener configurations:

Configuration Description
Single A single Zener
Back-to-back Zeners Two Zeners in series but with opposite polarities, creating a bidirectional clamp.

These Zener configurations are more than a convenience; SIMPLIS will run faster with these models and run into fewer problems with errors.

In this topic:

Extracting the Zener Parameters

When you place a Zener symbol on a schematic, the Extract Zener Parameters dialog opens for you to edit the default test conditions. The default test conditions are defined using the command shell menu File > Options > SIMPLIS Options.... For additional information, see SIMPLIS Zener Options.

The following table describes the Extract Zener Parameters dialog test conditions.

Extracted Zener Model

Test Condition Default Value Units Description
SPICE Model BZX79-4V7   The SPICE model used to extract SIMPLIS parameters.
Model type Extracted   Invokes the model parameter extraction algorithms.
Test current 5m A The peak forward and reverse current used for curve fitting. The algorithm fits a straight line between 50% and 100% of this value.
Model temperature 50 °C Temperature used for all extraction simulations.
Number of Segments 3   Zener Diodes may have two segments - representing Zener Resistance and Off Resistance or three segments with the third segment representing the transition between the Zener and Off states.
Initial Condition 2 Off Sets initial conducting segment.
Limit maximum off resistance Checked none Limits the off resistance for the diode. For some SPICE models, this will produce a SIMPLIS model which runs faster.
Maximum off resistance 100Meg W The maximum off resistance of the diode. This value is used only if the "Limit maximum off resistance" checkbox is Checked.

SIMPLIS Zener Model

SIMPLIS extracts a model based on the parameters in the Extract Zener Parameters dialog. Although the Zener model is internally saved as ASCII text, the following illustration shows the model in a schematic form.

Below is a schematic view of the Zener model:

The Zener models this circuit element Schematic
!R_D_POWER: Zener modeled by PWL Resistor

User-defined Models

The user-defined model uses parameters entered directly in the Edit Zener Parameters dialog without invoking the model extraction algorithms. A Zener can be switched from an extracted model to a user-defined model at any point; however the extracted parameters are by default copied over to the user-defined parameters, replacing any user-entered values. You can disable this behavior in the SIMPLIS Options by clearing the check box labeled Automatically copy extracted parameters to User-defined parameter. You can access these options from the command shell menu File > Options > SIMPLIS Options.... For more information, see SIMPLIS Zener Options.

User-defined Zener Model

Parameters Default Value Units Description
Label: USER_LABEL   Any descriptive text. Cannot contain whitespace characters.
Model type: User-defined   The values in this dialog are used literally.
Zener voltage: 4.7 V The Zener voltage. The Zener diode effectively turns on in the reverse direction at this voltage.
Zener resistance: 3 Ω The Zener resistance at voltages higher than the Zener voltage.
Forward voltage: 750m V Zener forward voltage drop. The Zener diode effectively turns on in the forward direction at this voltage.
Forward resistance: 10m Ω The Zener resistance at voltages higher than the Forward voltage.
Off Resistance: 1G Ω The resistance of the Zener at voltages less  than the Forward voltage.

User-defined model

Models these circuit elements User-defined Schematic
!R_D_POWER: Zener modeled by PWL Resistor

Manually Generate and Customize Zener Models

You can customize or manually generate your own Zener models using a parameter string with multiple PARAM_NAME=PARAM_VALUE key-value pairs. The parameter names and their functions are described in the Zener Model Parameters section below. You can interpret the SIMPLIS parameter values from device datasheet specifications and curves.

You can compose the parameter string in a text editor, spreadsheet, or script. The order of the parameter names in the parameter string and the capitalization of the parameter names are irrelevant.

You can include a PROTECTED=1 key-value pair to prevent from extracting a model and overwriting your manually generated parameters. The PROTECTED=1 key-value pair is not used in the simulation.  

Note: When you click on the device after adding the PROTECTED=1 key-value pair, the following message box appears to warn you that this is a hand-edited model.

To customize or generate your own Zener model, follow these steps:

  1. Create a parameter string of multiple PARAM_NAME=PARAM_VALUE key-value pairs using your preferred text editor, spreadsheet or script.
  2. Add the PROTECTED=1 key-value pair to the parameter string.
  3. Extract a Zener model and place it on a schematic.
  4. Right click on the symbol and select Edit/Add Properties....
  5. Double click on the PARAM_VALUES property.
    Result: The Edit Property dialog opens. At this point, you can change individual parameters in the Value box, or replace the entire default properties with the parameter string that you created in Step 2.
  6. To replace the entire string, follow these steps:
    1. Click in the Value box and type Ctrl A to select all of the existing parameter string, and press Delete.
    2. Copy the parameter string you completed in Step 2 and paste into the Value box.
    3. Click Ok.
  7. To change the name of your customized model, double click the VALUE property in the Edit Properties dialog, and change the name in the Value box.
  8. To return to the schematic, click Ok.
Alternately you can write the PARAM_VALUES property to the symbol using the Prop command in the command line with the following syntax:   
prop PARAM_VALUES parameter_string
where parameter_string is the set of key-value pairs that you created in Steps 1 and 2 above.
Important: Since parameter_string contains spaces, the entire string must be enclosed in double quotes.

Zener Model Parameters

The following tables detail the parameters which define the electrical behavior of the Zener model. Several other parameters in the PARAM_VALUES property have no effect on the electrical behavior of the model. These parameters are used to populate the Extract Zener Parameters dialog box.

Note: The default values are unlikely to appear in an extracted model. If these parameter values appear in your design, there has been an error in composing the parameter string.

Conduction Model

Zeners are modeled in SIMPLIS with Piecewise Linear (PWL) resistors. The PWL segments are represented by X,Y points

  • The points are defined with the voltage on the X-axis and  the current on the Y-axis.
  • The resistance is the inverse of the slope of any segment.
  • The subscripts on IDx and VDx indicate the point pair location from lowest voltage, i.e.  reverse biased, to highest voltage, i.e. forward conduction.

Conduction Parameters

Parameter Names Default Value Description
NUMSEG   3 Number of segments in the Zener model.
  • Valid NUMSEG are 2 , 3, 4, 5, 6
V0 ID0 1.123456789 X-Y point definitions for Zener:
  • X-axis is the Anode-Cathode voltage  in volts (V).
  • Y-axis is the Zener forward current in Amps (A).
  • Points with subscripts greater than the NUMSEG parameter are ignored.
V1 ID1 1.123456789
V2 ID2 1.123456789
V3 ID3 1.123456789
V4 ID4 1.123456789
V5 ID5 1.123456789
V6 ID6 1.123456789

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