Discussion in 'General Electronics Chat' started by AllenK, Feb 3, 2012.

1. ### AllenK Thread Starter New Member

Feb 3, 2012
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Hello everyone,

First post, hoping to learn and contribute to this amazing forum.

I am currently an electrical engineering student and im having trouble understanding the concept of load capacitance. More specifically, having enough current to over come the capacitance to run your load.

I understand the capacitance of your load effects the bandwidth of your system. Im really just having trouble understanding the statement " you must have enough input current to over come your loads capacitance" A real world analogy would be greatly appreciated.

I hope that makes sense.

Thank you very much !

2. ### steveb Senior Member

Jul 3, 2008
2,433
469
For starters, consider what happens when a voltage output drives a purely capacitive load. In the first instant, the capacitor can look like a short circuit. This is related to the bandwidth issues you understand. If your circuit has a particular bandwidth, then there is an associated minimum rise time that the source might need to switch or change with. A capacitor has current

$i=C {{dV}\over{dt}}$

which means that this rise time implies a particular derivative (slope) in voltage. Since this slope (risetime) might be reasonably large, any large capacitance will require a large current during the switching time.

Try to get this concept straight in your mind before proceeding to real world examples. However, a real world example is driving a MOSFET with a digital signal. MOSFETs have gate capacitance, hence fast switching of a MOSFET requires drivers that can provide high current, at least initially as that capacitance gets charged up.

3. ### AllenK Thread Starter New Member

Feb 3, 2012
8
0
Thank you, but i dont really understand your last statement regarding mosfets. Current doesnt flow into the Gate of a mosfet so im having trouble understanding what you mean by

" hence fast switching of a MOSFET requires drivers that can provide high current, at least initially as that capacitance gets charged up"

anyway you could go into that a tiny bit more.
Thanks.

4. ### steveb Senior Member

Jul 3, 2008
2,433
469
I'm heading to bed now, but tomorrow I can do a little better if someone else doesn't get to it first.

Just to expand slightly, it's not quite correct to say that current doesn't flow into a gate on a MOSFET. Yes, the input resistance is ultra-high, but only for DC. There is an effective input capacitance on the gate that needs to get charged up. This is exactly to the point of your question about load capacitance, and the need to be able to supply the necessary current. Look up the input capacitance on a datasheet for a power mosfet. The capacitance is relatively large by switching standards.

Also, to expand on why this is important. Imagine if you try to drive the gate with a voltage source that is limited in current, like a low power opamp, for instance. The output drive will become current limited for a brief instant while the capacitance loads down the output. Hence, it will take much more time to charge the capacitance and switching time will be very long.