A - Setting Initial State

Appendix overview

This appendix includes the following sections:

• Save and load bias point
• Setpoints
• Setting initial conditions

Save and load bias point

Save Bias Point and Load Bias Point are used to save and restore bias point calculations in successive PSpice simulations.

Saving and restoring bias point calculations can decrease simulation times when large circuits are run multiple times and can aid convergence. If the circuit uses high gain components, or if the circuit’s behavior is nonlinear around the bias point, this feature is not useful.

Save/Load Bias Point affect the following types of analyses:

• transient
• DC
• AC

Save bias point

Save bias point is a simulation control function that allows you to save the bias point data from one simulation for use as initial conditions in subsequent simulations. Once bias point data is saved to a file, you can use the load bias point function to use the data for another simulation.

To use save bias point

1. In the Simulation Settings dialog box, click the Analysis tab.
2. Under Options, select Save Bias Point.
3. Complete the Save Bias Point dialog box.
4. Click OK.

Load bias point

Load bias point is a simulation control function that allows you to set the bias point as an initial condition. A common reason for giving PSpice A/D initial conditions is to select one out of two or more stable operating points (set or reset for a flip-flop, for example).

To use load bias point

1. Run a simulation using the Save Bias Point option in the Simulation Settings dialog box.
2. Before running another simulation, click the Analysis tab in the Simulation Settings dialog box.
3. Under Options, select Load Bias Point.
4. Specify a bias point file to load. Include the path if the file is not located in your working directory, or use the Browse button to find the file.
5. Click OK.

Setpoints

Pseudo-components that specify initial conditions are called setpoints. These apply to the analog portion of your circuit.

Figure A-1 Setpoints.

The example in Figure A-1 includes the following:

Using IC symbols sets the initial conditions for the bias point only. It does not affect the DC sweep. If your circuit design contains both an IC symbol and a NODESET symbol for the same net, the NODESET symbol is ignored.

To specify the initial condition, edit the value of the VALUE property to the desired initial condition. PSpice A/D attaches a voltage source with a 0.0002 ohm series resistance to each net to which an IC symbol is connected. The voltages are clamped this way for the entire bias point calculation.

NODESET1 is a one-pin symbol which helps calculate the bias point by providing a initial guess for some net. NODESET2 is a two-pin symbol which helps calculate the bias point between two nets. Some or all of the circuit’s nets may be given an initial guess. NODESET symbols are effective for the bias point (both small-signal and transient bias points) and for the first step of the DC sweep. It has no effect during the rest of the DC sweep or during the transient analysis itself.

Unlike the IC pseudo-components, NODESET provides only an initial guess for some net voltages. It does not clamp those nodes to the specified voltages. However, by providing an initial guess, NODESET symbols may be used to break the tie (in a flip-flop, for instance) and make it come up in a desired state. To guess at the bias point, enter the initial guess in the Value text box for the VALUE property. PSpice A/D attaches a voltage source with a 0.0002 ohm series resistance to each net to which an IC symbol is connected.

These pseudo-components are netlisted as PSpice A/D .IC and .NODESET commands. Refer to these commands in the PSpice A/D Reference Guide for more information. Setpoints can be created for inductor currents and capacitor voltages using the IC property described in Setting initial conditions (below).

Setting initial conditions

The IC property allows initial conditions to be set on capacitors and inductors. These conditions are applied during all bias point calculations. However, if you select the Skip Initial Transient Solution check box in the Transient Analysis Setup dialog box, the bias point calculation is skipped and the simulation proceeds directly with transient analysis at TIME=0. Devices with the IC property defined start with the specified voltage or current value; however, all other such devices have an initial voltage or current of 0.

Applying an IC property for a capacitor has the same effect as applying one of the pseudo-components IC1 or IC2 across its nodes. PSpice A/D attaches a voltage source with a 0.002 ohm series resistance in parallel with the capacitor. The IC property allows the user to associate the initial condition with a device, while the IC1 and IC2 pseudo-components allow the association to be with a node or node pair. See Setpoints for more information about IC1 and IC2.

In the case of initial currents through inductors, the association is only with a device, and so there are no corresponding pseudo-components. The internal implementation is analogous to the capacitor. PSpice A/D attaches a current source with a 1 Gohm parallel resistance in series with the inductor.