The Designer's Guide Community Forum https://designers-guide.org/forum/YaBB.pl Modeling >> Passive Devices >> constant current charging capacitor https://designers-guide.org/forum/YaBB.pl?num=1472668491 Message started by A.D.J on Aug 31st, 2016, 11:34am

 Title: constant current charging capacitor Post by A.D.J on Aug 31st, 2016, 11:34am I was trying to use constant I to charge one capacitor. The charging process is based on the equation: I*t=CV.If C=1pF, t=8us, I=10nA, so V=I*t/C=80mV. The schematic simulation is very closer to 80mV. However, if changing the C=100fF, the expected V=8us*10nA/100fF=800mV. The simulation result is less than 800mV. I have no idea what results in the voltage reduction. Thank you.--------------------------Is it related to capacitor leakage current issue?I read a article talked about the leakage current I is proportional to the  charges on the capacitor plate. For smaller capacitance, we got higher charge, so the leakage current is bigger. As a result, the final charged voltage could be smaller than expected. Is that reasonable?Thank you.

 Title: Re: constant current charging capacitor Post by davidshw on Aug 31st, 2016, 6:25pm to aid convergence, the simulator will add a resistor parallel with capacitor, which may cause leakage. you can try to set the gmin a smaller value to reduce the leakage..option gmin=1e-15

 Title: Re: constant current charging capacitor Post by Geoffrey_Coram on Sep 16th, 2016, 10:14am Or, if you don't want to play with gmin (which could affect other parts of your circuit), you could put a very large resistor in yourself.  Or, some simulators have a "switch" that you can have short to ground for the time=0 solution and then open up for t > 0.