Variable capacitor

Stan Zurek, Variable capacitor, Encyclopedia Magnetica, E-Magnetica.pl

Go to: LTspice.

Variable capacitance can be created by using the LTspice in-built variables for capacitors: $Q$ - electric charge (C), and $x$ - voltage across the capacitor (V).¹⁾

"Linear" model (valid only for Vmax < V capacitor!)

LTspice directive with parameters:

.params LINEAR=1 C0=10n Csat=1n Vtra=10 Vth=15 Vmax=Vth+0.5*Vtra

And instead of the value of the capacitor use:

Q=x*{C0}-0.5*x**2*({C0}-{Csat})/{Vmax}

Non-linear tanh model

LTspice directive with parameters:

.params LINEAR=1 C0=10n Csat=1n Vtra=10 Vth=15 Vmax=Vth+0.5*Vtra

And instead of the value of the capacitor use:

Q=x*({C0+Csat})/2+({Csat-C0})/4*{Vtra}*ln(cosh((x-{Vth})*2/{Vtra}))

Definitions of variables for the non-linear tanh model²⁾

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Performance of the two models - note that the “linear” model fails if voltage on capacitor exceeds the Vmax parameter (arrow); decreasing current through the capacitors indicate their decreasing values with increased DC voltage (the AC component is the same, so for a fixed capacitor the AC component in the current would also be the same)

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

References

^1),
2) Reiner Bidenbach, RAQ Issue 192: How to Use LTspice Simulations to Account for the Effect of Voltage Dependence, AnalogDialogue, {accessed 2021-08-17}

Encyclopedia Magnetica™

Table of Contents

Variable capacitor

"Linear" model (valid only for Vmax < V capacitor!)

Non-linear tanh model

References