Wednesday, June 11, 2014

Capacitance Loss

I didn't learn this one myself the hard way, but that's only luck. I very well could have.

I use 100nF (why doesn't the nanofarad get any respect?) capacitors all over the place on my boards, mostly as "filter" or "bypass" caps. They sit next to each Vcc pin of each digital IC, and the way I have heard it, they act like a little bitty power supply right next to where you need it. If the part needs a bit more power than average, it will suck charge off that capacitor before it sucks current off the Vcc rail. The part is protected from its own variations in power, and the rail (and therefore the other parts) are protected from this part.

I also use much larger caps as specified for regulators and other such power supply and sensor analog devices. The sensors want a much smoother power supply, and the regulators and such use them for feedback. These tend to be 1μF or even larger. Some parts call for as much as 10μF. The USB spec says no more than 10μF equivalent on a device across Vcc to ground, since too large of a value will draw a large inrush current as those capacitors are initially charged.

I tend to use ceramic caps for all of these, because I am obsessed with board space. Besides, a ceramic cap is pretty close to ideal. No polarization to worry about, etc.

Our good friend over on the EEVBlog demonstrates an issue which does bite ceramic caps -- capacitance change with applied voltage. I had heard of this before, but effectively ignored it as something I couldn't do anything about. Ignore the accent and presentation quirks, I am sure I would sound even worse.
He takes a 10μF 6.3V 0805 ceramic capacitor and demonstrates how to measure it with an oscilloscope and an RC circuit. With a 0-5V square wave, he demonstrates that the cap has its rated 10μF. But, when he changes the signal to 5V-6V, the capacitance drops to less than 5μF.

At first I breathed a sigh of relief when I saw the first demo. No capacitance loss even with a 5V signal. But then I thought about how the second demo applies to my circuits. A bypass cap is run with a large DC bias -- the Vcc voltage. This is directly relevant to my interests.

I guess all I can say is that the designs I am following call for a certain size capacitor. When these were tested by their original manufacturer, they either took these effects into account, or didn't, and just put in a larger cap than they needed when the circuit didn't originally work. If the design turns out to call for only 3μF, they specify a 10μF cap knowing that it will still have at least 3μF under the prevailing conditions.

More details at .

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