Today’s Qspice update, Mike introduces two new instance parameters for capacitors—VsatOffset and SatAsym—to extend voltage-dependent capacitor modeling flexibility.
*10/24/2026 Extended the utility of the saturating capacitor.*
When modeling Cdg or Coss, capacitance is commonly treated as a constant once the body diode conducts. Some prefer to maintain a constant capacitance when Vds < 0V or extend the flat region slightly into negative voltages before diode conduct. These two new parameters provide much greater flexibility for modeling this behavior.
As usual, unofficial device guide is updated if more information is required, including the explanation of Vsat, SatFrac and Csat.
https://github.com/KSKelvin-Github/Qspice/tree/main/Guideline#guideline
Capacitor - voltage dependent example.qsch (2.2 KB)
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Hello Kelvin,
I noticed a wiered current on capacitor when a Vsat value is specified:
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I recall that I asked Mike why, and he told me to measure the nonlinear capacitor with a small resistor, as I(Cn) can return error in the current monitoring for a nonlinear capacitor.
Here, if you monitor I(R1) or add a 0V voltage source for current monitoring, both return correct values. This is possibly related to SPICE using Modified Nodal Analysis (MNA). In SPICE, you might notice that current reports can exhibit oscillating spikes for devices sharing the same loop (seems violate mesh analysis), but this is possible to happen in SPICE.
I don’t understand the math behind it; I just took the advice from the expert. I think @frank.wiedmann has a better understanding of this topic, since the first time I learnt about this was from his post.
Persistent Bug - Warning: Singular matrix. Check node B - QSPICE - Qorvo Tech Forum
CapVsatCurrent.qsch (2.2 KB)
