spice sim. of a variable cap

hi everybody,
how could i simulate a variable capacitance whose value could be varied in a specified manner (in spice). i want to run a transient simuation where the capacitance of a cap. would have to vary (this is for an energy harvester ckt.)

A voltage controlled capacitance is a valid component in spice. But only the capacitor terminal voltage is allowed. The reason is that the transient equivalent circuit of the capacitor is depending on the integration algorithm and it is more difficult to built them for a control voltage not the capacitor terminals. So you have to use a AHDL description.

hi rfsystem,
could you explain how i could use ahdl with spice.(any freeware ahdl)
i use ltspice for my spice simulations.
thanks

I did not use free spices for up to 15 years. I think Xpice have some AHDL extension. You have to search for the capability of a dot expression. The current of a cap is expressed then as function of a voltage derivative.

The reason today not have the general controlled cap in spice point back to the early 70s. The transient simulation in spice is not more than the time point specific solution to a nonlinear DC system. So every capacitor and inductor is substituted by a DC equivalent circuit. The construction of the equivalent circuits and there parameters are depending on the integration algorithm. Then this circuit is loaded into the matrix before solving them. The matrix loading is much more difficult with general controlled components (eg consider multivoltage depend parameters). So the number of types stops in the mid 70s. Today AHDL spice implementation are in principle 2-kernel solutions. The first is the spice kernel, the second implement general integro/differential equations. The interconnection is general via sources. Time step organisation have to look at both domains. A mixed (A/D) kernel is similar organisized.