“100 μF” Capacitors

I have recently been designing and building circuits with very small (1206 and 0805) 100 μF capacitors. I got into a discussion about whether these are really up to spec or if they’re much, much lower. The reason for the discussion is graphs like this:

Schematic representation of change in capacitance with applied voltage. ( GFDL, Elcap, Jens Both (Frank Murmann at de.wikipedia https://commons.wikimedia.org/wiki/File:Delta-Cap-versus-Spannung-english.svg)

Actual data (from Murata, via Maxim) seems to be even less flattering for capacitors:

The problem is largest with the high value, small size capacitors as they’re restricted to the highest possible permittivity ceramics regardless of poor specifications in other areas.

So how bad is it really?

First, on to the test equipment. Here’s a picture of my trusty old capacitance meter measuring a nice, uncontroversial, high quality 47 μF metal can aluminium electrolytic capacitor, while also hooked up to my ‘scope.

The meter specifies a maximum bias of 2.8V, but few details on whether this will b reached. It seems the bias is between about 0.6 and 2.4V with a mean of about 1.5.

And here are the values I get from the nominally 100 μF capacitors:

• Samsung 1210, X5R, 6.3V, 20%: 72 μF
• Murata 1206, X5R, 4V, 20%: 73 μF
• Murata 0805, X5R, 4V 20%: 60 μF
• Some random SMD ali electrolytic: 105 μF

The results are not all that great. As a saving grace, the capacitors are to buffer out the very high internal resistance of the CR2032 coin cell, and you need the most capacitance when the voltage of the cell is lowest, which is what you get. Nonetheless even at the lowest operable voltage (1.8V), the capacitors are still quite substantially below the nominal rating.

Still, they’re not as bad as I thought they might be from (1/5th of the value!) the Murata data. However the graphs look quite steep, so at the nominal cell voltage (3V), the capacitance might be more like 1/3rd of the rated value. Interestingly that would make the density almost exactly match the top end 0805 4 V, 33 μF tantalum capacitors.

I think for my application, it’s a net win (just about) for ceramics.