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Current in de-coupling capacitor

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engr_joni_ee

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Hi, I have seen de-coupling capacitors in the power rails in micro-electronics PCB. Normally there are 0603 SMD packages which I guess perform better and low ESR at higher frequencies. I have seen their voltage rating and capacitance as main selection parameters for examples 100 nF 35 V SMD 0603 capacitor. How about the current consideration if they are used in the power supplies having 12 V for example that is supposed to deliver 1 Amp.

I mean do I only need to select de-coupling capacitors with their capacitance and voltage rating regardless of which current is the power rail is delivering ?
 

Hi,

It´s a DC supply, thus it´s DC voltage.
Now capacitors block DC current. Thus the "1A" is meaningless for the capacitors.

The job of the capacitors is to stabilize the voltage in case the load draws (short) current pulses.
Especially for didgtal circuits, but also for fast analog circuits you can expect the load to draw currents with high dI/dt.
Now, because the wiring (traces) have inductive character, a fast rising current causes the voltage to drop across the wiring. The voltage at the point of load is not stable, maybe even drops below load device specifications.

So the capacitor acts like a small reservoir of power, just for very sharp change (in the ns range) of currents.
But all currents flow in a loop, thus you not only have to care for low impedance path between capacitor and VCC of the load, but also from load to GND of the capacitor. Here often a solid GND plane is the way to go.

For sure there are applications with (extremely) high ripple current, then you have to select a suitable capacitor.


Klaus
 

You care about ESR and ESL because these degrade the "HF short circuit" qualities that the decoupling is supposed to exhibit.
 




Capacitor technology matters :

1669138736855.png


OSCON in above is Polymer Tantalum.


The above references might help.


Regards, Dana.
 

Datasheets specify capacitor AC (ripple) current rating. You should determine the AC current fundamental frequency and magnitude exposed to the capacitors and check the rating.

In case of buck converters, the largest ripple current is flowing at the input side.
 

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