Beware of Inrush Current: Why More Capacitors Aren't Always Better

When you design a product and if you are not really sure if you need an extra capacitor for one of the IC or a module. You add it for extra measure and maybe it’s also recommended by the app note or the reference design.

It’s not common to see issues arising from adding extra caps to the board, it is generally regarded as not a problem. The argument goes like this, its better to have and not need than to need and not have the capacitors. The issue that is overlooked is the problem of inrush current.

This problem occurs only when you turn on the device. For a short amount of time when the capacitors are charging the power supply can see it as a short for a brief period of time. Imagine what happens to the traces on the board.

You designed the board traces with load requirements you had and now due to inrush currents, they might be carrying more current than intended.

This leads to heating of the trace and some products can fail after some use. It could be very hard to detect the cause, as this issue is hard to detect. Especially if you don’t know where to look.

Sometimes, less is indeed more.