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Nexxim Simulator >
Nexxim Component Models >
Capacitors >
Capacitor, Expression-Based (Netlist Only)

Capacitor, Expression-Based (Netlist Only)

For the expression-based capacitor, the charge is defined by an expression.

Expression-Based Capacitor Netlist Syntax

The equation-based capacitor has the format:

Cxxx n1 n2 [modelname] Q='expression' [CTYPE=val]
[instance_parameter=val] ...

The expression (enclosed in single quotes) defines the capacitor charge Q as a function of voltages in the circuit. The equivalent capacitance is then obtained using the differential equation:

The capacitance calculation is controlled by the CTYPE parameter. If Q is a function only of its terminal voltages V(n1, n2), set CTYPE=0 (the default). Otherwise, set CTYPE=1.

Note

The expression-based capacitor and its corresponding model are available for use in netlists, but are not supported in the Components window of the Designer Schematic Editor.

An alternative syntax will be accepted:

Cxxx n1 n2 [modelname] [C=]'expression' [CTYPE=val]
[instance_parameter=val] ...

Specifying the capacitance directly in the expression can lead to formulations that do not conserve charge, especially if the relationship between capacitance and voltage is nonlinear.

Note

If a capacitance expression contains a voltage dependency, and the expression is not in the form Q=’expression’, Nexxim issues the following warning: capacitance based expression worsens convergence. Replace with charge based expression if possible.

**Expression-Based Capacitor Instance Parameters**
Instance Parameter	Description	Unit	Default
CTYPE	0: Charge is a function of terminal voltages only 1: Charge is a function of one or more voltages other than the terminal voltages	None	0
DTEMP	Difference between capacitor and circuit temperatures	Celsius	0.0
M	Multiplier to simulate parallel capacitors	None	1.0
SCALE	Scale factor	None	1.0
TC1	1st-order temperature coefficient	°K^-1	0.0
TC2	2nd-order temperature coefficient	°K^-2	0.0

Expression-Based Capacitor Netlist Example

C21 54 55 Q='cos(V(54,55))' CTYPE=0

The example above calculates the capacitance as the derivative of the Q-expression with respect to V(54,55).

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