Question about capacitor

[hayowazzup]

Hi, I wanted to ask the following questions about capacitors...

1. What will happen if the cap is charging and discharging very quickly, Will this lead to the change in the cap's impedance and Will the rate still maintain constant and linear?

2. Will capacitors always follow the equation "I = C * dv/dt" for its rate of charging and discharging?

3. Say 10pF cap, it's impedance is very high to ac, does it mean it cannot handle a quick charging and discharging rate?

 

[RCinFLA]

All capacitors have parasitic lead inductance (even surface mount parts). They also have an internal resistance. This resistance may be non-linear and a function of frequency.

Everything is relative to its use. For 2 GHz RF circuits we use 10 pF for RF bypassing caps. Object is to series resonate with parasitic series inductance.

 

[Miguel Gaspar]

1. a quick charge and discharge implies a big dv/dt and a big current according I = C*dv/dt, which implies more heat that affects the capacitance and the rate can't be linear.

2. Yes but the temperature coeficient positive is not equal to the negative in a capacitor. Wich affects the performanace of it. If you need a capacitor that is necesarynot be affected by temeperature you need a NPO capacitor.

3. in AC the impedance depends of the frequency Xc = 1/(2*pi*f*c). Again a quick charging depends of the amount of current.

 

[ark5230]

1. What will happen if the cap is charging and discharging very quickly, Will this lead to the change in the cap's impedance and Will the rate still maintain constant and linear?

It works as expected. Impedance will not be affected. Linearity of rate needs clarification. So is the impedance issue, while charging and discharging, it looks like transient phenomenon.

2. Will capacitors always follow the equation "I = C * dv/dt" for its rate of charging and discharging?

Yes it has to.

3. Say 10pF cap, it's impedance is very high to ac, does it mean it cannot handle a quick charging and discharging rate?

As is seen from other post, lower capacitances are used at higher frequencies which mean faster charging and discharging rates so no issue. It handles fast changes.

 

[hayowazzup]

hi, thanks for the reply.
Also, will capacitor with smaller capacitance have bigger transient?

 

[ark5230]

Transients, in addition to selected characteristics, depend on inductances contributed by conductors, this becomes important in fast switching and low L low C circuits.